Temperature Chapter



Atmospheric Forces

We constantly attempt to control the motion of objects around us. We push chairs, open doors, lift books, and throw balls. These actions require the application of a force. Applying a force to an object often moves the object. Wind is air in motion that arises from a combination of forces. Violent, destructive winds result from a complex interplay of different forces, just as gentle summer breezes.

Weather involves the wind, or the lack of wind, and thus forces. Complex interactions link winds, temperature, pressure, moisture, clouds, and precipitation to one another. These connections result in forces and provide the energy to drive various atmospheric circulations. The purpose of this chapter is to explain the forces of nature responsible for the winds and to illustrate how to use weather maps to interpret wind direction and speed. This chapter is an introduction to atmospheric dynamics.

In introducing atmospheric dynamics we will make use of simple diagrams, or models. While simple, and sometimes unrealistic, these models provide a conceptual view of how the atmosphere works. We will use these models to explain observations of the ‘real world.’ We will also begin to explore the coupling between circulations of the upper troposphere and the surface wind. The different spatial scales of weather systems are also defined in this chapter. Detailed discussions and examples of these scales of motion are the topics of following Chapters.

1 Magnitude and Direction of Forces

Weather results from the interaction of many forces. When discussing these interactions we are concerned with the direction and magnitude, or strength, of the force exerted on an object. These two factors, represented in diagrams by arrows, determine what effect the force has on the object. The arrow points in the same direction as the force. The length of the shaft represents the magnitude.

Two or more forces can act to pull or push at the same point on a body. A single force, the net force or resultant, which reproduces the same effect on the body as the separate forces, can replace these forces. Suppose we apply two forces to a body that will only act to move the object. If the two forces act in the opposite direction and with different magnitudes, the object will move in the direction of the stronger force (as in a tug-of-war). A single force can represent these two forces whose direction is the same as the stronger force and whose magnitude is the difference between the two forces (Figure 6.1).

Force exerted on an object equals the body’s mass times the acceleration that is produced. Force has a direction and a magnitude.

Forces often act at an angle to each other. In this situation we need an approach to determine the magnitude and direction of the net force. One way is to graphically construct a parallelogram (Figure 6.2) using two forces to represent two of the sides. The diagonal of the parallelogram represents the net force of the two given forces. The length of the diagonal represents the magnitude of the combined forces. The direction of the net force is along the diagonal and away from where the two forces are applied.

2 Laws of Motion

Applying a force often results in movement. When an object is changing its position with reference to another body, we say it is in motion. As with force, motion has a magnitude and a direction. The speed of the object, the distance traveled in a given amount of time, is the magnitude of the motion.

Wind Gust is an abrupt and momentary increase in the wind speed.

Wind is air in motion. Weather reports include wind speed and direction. Wind speed is reported on weather maps in knots--one nautical mile per hour (equivalent to 1.1508 statute miles per hour or 0.5144 meters per second). If the wind speed is strong (greater than 15 knots) and highly variable, the weather report will include wind gusts, the maximum observed wind speed.

Wind direction is reported as the direction from which the wind is blowing. It is reported with respect to compass directions or the number of degrees east of north. A north wind blows from the north to the south (Figure 6.3). Windward refers to the direction the wind is coming from, while leeward denotes the direction the wind is blowing towards. The prevailing wind direction of a region is the most frequently observed wind direction during a given period of time.

Acceleration is the time rate of change of velocity. An object that is accelerated under goes a change in speed and/or a change in direction of motion.

Wind that changes speed or direction has undergone acceleration. If a body increases its speed at a constant rate, it undergoes a uniform acceleration. A free falling body is a good example of an object that undergoes uniform acceleration. If we neglect air resistance, a falling body undergoes an acceleration of 9.8 meters per second for each second it falls. The object’s velocity increases 9.8 meters per second after each passing second. This downward acceleration results from the earth’s gravitational pull and is called the acceleration of gravity. Galileo discovered that the acceleration of two falling objects is the same, even if their weights are very different. Astronaut David Scott demonstrated this on August 2, 1971 by dropping a hammer and a feather at the same moment while on the moon. The moon has no atmosphere so there was no significant friction from air that would affect the fall of the feather more than the hammer in our atmosphere. The objects hit the ground at the same time, though the acceleration was less than 9.8 meters per second per second.

In the seventeenth century Sir Isaac Newton established the fundamental laws which describe the motion of bodies: the law of inertia, the law of momentum, and the law of reaction.

1 Newton's First Law: Law of Inertia

Newton’s first law of motion states that a body at rest tends to stay at rest while a body in motion tends to stay in motion traveling at a constant speed and in a straight line until acted upon by an outside force. If a bus suddenly starts the passengers lurch backward. The passengers are initially at rest and tend to remain at rest when the bus first starts. The moving bus is exerting a force that eventually gets the passengers moving at the same speed. If the bus suddenly stops, the passengers surge forward. They tend to remain in motion. If the bus makes a quick, sudden right turn the passengers, who want to continue traveling in a straight line, are crammed towards the left side of the bus. The resistance of an object to changing its velocity is called inertia.

Air at rest will tend to stay at rest until a force puts it in motion. We will discuss the forces that generate wind in the next section.

2 Newton's Second Law: Law of Momentum

The momentum of an object is its mass multiplied by its velocity. Two objects can have the same speed, but the one with more mass has greater momentum. Newton’s second law states: When a force acts on a body, the body’s momentum is changed by an amount that is proportional to the applied force and the amount of time the force acts on the body. Hitting a baseball with a bat is good example of this law. If you want to hit the ball far, you have to swing the bat hard to increase the force applied to the ball on contact. You also want to follow through with your swing to keep the bat in contact with the ball as long as possible.

Applying a force to an object changes its momentum by changing the speed at which it travels. A light breeze will not move a sailboat as fast as a strong steady wind. Back to our bus for a final example, as the bus accelerates, its momentum, and the momentum of the passengers inside, increases.

3 Law of Reaction

Firing a gun moves a bullet forward, but there is an equal force in the backward direction referred to as the ‘kick.’ Newton’s third law states that for every action (force) there is an equal and opposite reaction (force). When a cup of coffee is placed on a table, a downward force is exerted on the table because of gravity. The table exerts an equal and opposite force in order to support the cup.

3 Forces that Move the Air

There are different forces that act to move air (Figure 6.4). The weight of air is a force that always acts downward, towards the center of the earth. Other forces are the pressure gradient force, Coriolis force, friction, and centripetal force. We must examine these four forces to determine which direction the wind will blow and which direction storms will move.

1 Pressure Gradient Force

Spray paint cans exhibit warning labels that the contents are under pressure. When the nozzle is squeezed, or the sides punctured, the large pressure difference between the air and the inside of the can force the contents out of the can. Generating pressure differences is also important in flying and sailing (Box 7.1) Pressure differences exert a force and when not balanced by other forces cause movement. The force that results from pressure differences in a fluid such as our atmosphere is called the pressure force or the pressure gradient force. Air moves because of a pressure gradient force. The existence of a pressure gradient force is essential for sustaining winds.

The pressure gradient force (PG) always acts from high pressure towards low pressure. Its magnitude is equal to the pressure gradient, or the rate of change in pressure with distance at a specific time divided by the air density.

[pic]

When pressure changes rapidly over a small distance, the pressure gradient force is large. Strong winds result from large pressure gradients.

On the surface weather map, measured atmospheric pressure at the surface is converted to sea level pressure and analyzed. As noted in Chapter 1, when comparing the pressure of two cities it is important to reference the pressure measurements to the same altitude. If the pressures are not referenced to the same altitude, when analyzing differences in pressure between the two cities, we will just see differences in the altitude of the cities, since atmospheric pressure always decreases with altitude. Measured surface pressure is adjusted to what the pressure would be if the city were at sea level. To accomplish this we need to determine the pressure that would be exerted by a column of air that extends from the location of the weather station too sea level, and add this pressure to the observed pressure (Figure 6.5). The web page discusses how these corrections are made. On average, this correction is approximately 1 mb for each location 10 m of altitude

Isobars of constant sea level pressure are drawn at 4 mb intervals starting at 1000 mb. Figure 6.6 is an example of a surface weather map. The winds near Chicago are greater than those winds near Oklahoma City. The isobars are spaced closer together over the Great Lakes than over Oklahoma. Since the pressure difference between any two isobars is fixed, the closer the isobars the larger the pressure gradient, the stronger the pressure gradient force, and the greater the wind speed.

Isobaric map is commonly used to study the weather above the surface. This constant pressure chart includes information on the temperature, wind speed and direction, humidity, and the altitude at a given pressure.

The surface map plots atmospheric pressure adjusted to sea level. Another type of weather map plots the altitude of a given pressure surface. This map is commonly used when analyzing the weather above the surface and is called a constant-pressure chart or isobaric chart. Constant-pressure charts are commonly drawn for 850, 700, 500, 300, 250, and 100 millibars. Figure 6.7 is an example of a 500 mb map corresponding to November 10, 1976. The units of altitude are called geopotential meters and are nearly equivalent to geometric meters measured with a ruler. Isobaric maps are useful for portraying horizontal pressure gradients. The spacing between the lines of constant height (isoheight) portray the pressure gradient force. To see this consider Figure 6.8. The colored surface is a constant pressure surface of 850 mb. Anywhere on this surface the pressure is 850 mb. The altitude of this 850 mb surface varies, the altitude is higher at City A than at City B. Above this figure is an analysis of the pressure gradient force along this 850 mb surface. Atmospheric pressure always decreases with increasing altitude, so the steeper the pressure surfaces the greater the pressure gradient force. The magnitude of the pressure gradient force is weak near City B and stronger near City A. Also shown in Figure 6.8 are constant-height lines of the 850 mb pressure surface. The magnitude of the pressure gradient force is proportional to the spacing of the contour lines. The closer the constant-height lines the greater the pressure gradient force. The direction of the pressure gradient force is perpendicular to the lines of constant-height, and point towards lower heights.

To summarize, on an isobaric map the lines of constant-height of the pressure surface indicate the direction of the pressure gradient force and the magnitude. Referring back to Figure 6.7, the stronger winds are located in regions where the spacing of the constant-height lines is a minimum, as expected because this is where the pressure gradient force is the largest. However, the winds are not in the direction of the pressure gradient force! The winds, in general, are blowing to the right of the pressure gradient force, and are nearly parallel to the isoheight lines! This suggests that there must be more than one force acting on the winds.

2 Coriolis force

Coriolis force is an apparent force used to describe the observed deflection of moving objects caused by the observer’s moving frame of reference. The magnitude is proportional to the wind speed and the latitude. It always acts to the right of the wind.

Newton’s laws of motion are defined with respect to a fixed reference frame. However, Earth is moving and so we must include an apparent forces that accounts for our moving reference frame. This apparent force is called the Coriolis force. The magnitude of the Coriolis force is proportional to the speed of the wind. If the wind speed is zero, there is no relative motion and the Coriolis force is zero. An object’s inertia increases with speed, so a larger force is required to change its direction of travel. The Coriolis force increases with increasing wind speed. The Coriolis force acts perpendicular to the direction of motion (to the right of the wind in the Northern Hemisphere) and therefore cannot change the wind speed. The Coriolis force cannot generate a wind, it can only change its direction.

The magnitude of the Coriolis force depends also on of latitude. At the equator the Coriolis force is zero and it increases towards the poles. To conceptualize this consider launching a rocket from city X to city Y (Figure 6.9). As the rocket heads northward towards Y the earth is moving beneath it. Yet, the rocket will land to the east of Y! Both cities make one complete revolution each 24 hours. Each city is moving eastward at an angular velocity of 15( per hour (360( in 24 hours). X is closer to the equator where the latitude belt is largest. Thus, while the two cities have the same angular velocity, X is moving eastward faster, because it has more distance to travel in the same amount of time (Figure 6.10). The launched rocket carries an eastward velocity component as it streaks towards Y, a city that is moving eastward at a slower rate than the rocket. Because of this difference in latitude, the launched rocket lands off course, to the east of Y and appears to have been deflected to the right of its motion. If the rocket were launched from Y to X, it would still appear to veer to the right of its path.

We have to consider the Coriolis effect only when dealing with winds that travel over long distances. It takes only a few seconds for a thrown ball to reach its destination. During this time the earth has hardly moved and the deflection of the ball’s path is insignificant. Since we do not have to adjust for the Coriolis force when playing football, soccer, tennis, or Ping-Pong, there is no reason to assume that the Coriolis force has anything to do with the spinning motion of a flushed toilet. However, play catch on a rapidly revolving merry-go-round, with one person in the center and the other on the outer edge. When thrown, the ball will appear to curve away from its intended target.

3 Centripetal force

Air often travels in a curved path and is therefore, according to Newton's First Law, being accelerated. An acceleration involves a change in speed, a change in direction, or a change in speed and direction. According to Newton’s first law a net force must be at work causing the air parcels to turn. This net force is called a centripetal force. When the centripetal force is removed from a body, or reduced in magnitude, the object ceases traveling along a curved path and moves in a straight line because of inertia.

The direction of the centripetal force is always towards the center of curvature (centripetal means seeking a center) perpendicular to the direction of motion. A centripetal force simply states that a net force points radially inward along a curved path.

4 Friction

Boundary layer is the atmospheric layer that is within 1 to 4 kilometers of the surface.

Friction opposes, or decelerates, the wind. Friction is largest in the atmospheric boundary layer, the air layer nearest the ground. Friction always slows down the wind, and therefore also reduces the Coriolis force. The roughness of the surface and the speed of the wind determine the magnitude of frictional force. The force of friction over still water or an ice pond is small. Trees in a forest strongly oppose the wind, so the frictional force of forests is large. While the wind may be blusterous over an ice pond, it will be calmer in the woods.

We shall consider the effects of friction when dealing with the air within 20 to 200 meters of the surface. This layer is called the surface layer; it is here that frictional drag is important in determining wind direction and speed. In the surface layer friction acts to slow the wind, and thus always acts opposite to the wind direction. Its magnitude increases with increasing wind speed.

Friction near the surface not only slows the wind, it mixes the air. This mixing produces turbulence. Air motions that are turbulent are highly erratic, and therefore difficult to predict.

We shall now make observations of winds in the atmosphere. We will then explain these observations by applying what we have learned about forces. Remember that only a pressure gradient force can generate a wind. Friction can slow the wind speed. The Coriolis forceacts to change the direction of the wind, but has no effect on the wind speed.

5 Hydrostatic Balance

As we've seen, horizontal differences in pressure generate a horizontal pressure gradient force. Powerful hurricane force winds are generated with pressure changes of only 50 mb over a distance of a few hundred miles. Atmospheric pressure decreases with altitude generating a vertical pressure gradient. Near the earth's surface the pressure decreases by about 10 mb for every 100 meter increase in elevation, a much stronger pressure gradient than in a hurricane! Why don’t we have strong vertical winds as a result of this strong vertical pressure gradient? Because motion results from the imbalance of forces. The weight of air is a force that always acts downwards, towards the center of the earth, and usually balances the upward pressure gradient force. This is referred to as hydrostatic balance (Figure 6.11). The atmosphere is in approximate hydrostatic balance, explaining why vertical motions are typically small.

6 Air motion and Energy balance

The thickness of a layer between two fixed pressure levels increases as the air is warmed.

An energy imbalance can lead to a force that generates a wind. Energy imbalances are the impetus of atmosphere and ocean circulations. Consider the air between two pressure surfaces, say 850 mb and 300 mb (Figure 6.12). Initially the isobars are parallel, and the vertical separation between the surface is 700 meters. Now we heat the column of air. Since we are concerned with the air between the 850 and 300 mb layers, the vertical pressure difference has not changed; it is still 550 mb. Heating the air lowers the density and therefore, to maintain equality, the vertical height difference between the two pressure surfaces must increase. Because atmospheric pressure decreases with altitude, an outward pressure gradient force is generated near 300 mb and an inward pressure gradient force near 850 mb.

7 Observations of Upper Air Winds

Let’s return to Figure 6.7 and make three observations.

1. The winds are fastest where the spacing of the isolines is smallest. This is expected, as it is in these regions where the pressure gradient force is largest. But the wind is not blowing in the direction the pressure gradient force is acting. Another force must be acting to change the wind direction.

1. The winds blow parallel to the isolines of constant height. Imagine putting your back to the wind, low heights, and thus low pressures, are always to your left (since we are in the Northern Hemisphere). This relationship between wind and pressure is called Buys-Ballot’s Law.

2. The winds, while meandering, are blowing counterclockwise around the North Pole.

To explain the first two observations consider a simplified situation demonstrated in Figure 6.13. The isolines of constant height are parallel to one another with lower heights at the top of the page. By our observations, the wind is blowing parallel to these lines as drawn. The wind is moving in a straight line parallel to these isolines. Also shown in the figure are the forces acting on a parcel of moving air. There are only two forces acting: the pressure gradient force and the Coriolis force. The pressure gradient force acts towards low pressure perpendicular to the lines of constant height while the Coriolis force acts to the right and perpendicular to the wind direction. The two forces are acting in opposite directions and are of equal magnitude. A wind that results from a balance between the pressure gradient and Coriolis forces is called a geostrophic wind. When forces are in balance, there is no net force. How does the air move if the two forces are balanced?

To answer this question imagine an air parcel released from rest as in Figure 6.14. Initially, since the parcel is at rest, the Coriolis force is zero. The pressure gradient results in a force that accelerates the parcel of air towards lower pressures. Initially the parcel moves in the direction of the pressure gradient force(perpendicular to the isoheights. Once the parcel begins to move the Coriolis force acts to deflect the parcel’s path to the right, assuming we are in the Northern Hemisphere. As the wind speed increases because of the pressure gradient force, the Coriolis force strengthens and moves the parcel causing the parcel to travel in a curved path. Eventually the Coriolis force and the pressure gradient force are equal and acting in opposite directions. Although the net force acting on the parcel of air is zero, the parcel continues to move because of Newton’s First Law of Motion. The parcel is in motion and tends to remain in motion. Lower heights are to the left when you face downwind.

As observed in Figure 6.7 the winds blow parallel to the isoheights. The winds change direction to remain parallel to the constant-height lines. According to Newton’s first law of motion, forces are acting to change the direction of the wind. To explain this consider a simplified situation represented by Figure 6.15. In this case the lines of constant-height are curved and the pressure gradient and Coriolis force are not in balance. The net force is the centripetal force, which results in changing the wind direction. The wind is now three forces involved, the pressure gradient, Coriolis and centripetal forces. This type of balance results in a gradient wind.

When the Coriolis force is greater than the pressure gradient force, the wind curves clockwise (in the Northern Hemisphere). The centripetal force points towards regions of higher pressure. When the pressure gradient force exceeds the Coriolis force, the air curves counterclockwise in the Northern Hemisphere (clockwise in the Southern Hemisphere). In this case, the centripetal force points towards regions of lower pressure. Thus, in the Northern Hemisphere, winds flow clockwise around regions of high pressure and counterclockwise in regions of low pressure, in agreement with our observations.

Why, in general, do the global winds in the midlatitudes blow from west to east? We can explain this third observation by noting that the polar regions are colder than the tropics and conceptualize this using a simple model (Figure 6.16). In this model we examine a vertical slice of the atmosphere between the North Pole and 30(N latitude. The polar region is colder and so the thickness between the 500 mb and 850 mb surface must be greater in the warmer sub-tropical regions. As a result, the 500 mb pressure surface is at a higher altitude at 30N than near the pole, and a pressure gradient force acts from the right side of the conceptual diagram to the left. To determine the wind direction along the 500 mb pressure surface we apply the Buys-Ballot’s Law. To do this, imagine floating in a balloon on the 500 mb pressure surface. Extend your arms up from your sides and point to lower pressure with your left hand. The wind will be at your back, or in this example into the page(the wind blows counterclockwise around the pole.

The geostrophc wind is proportional to the tilt of the pressure surfaces. The greater the tilt the stronger the geostrophic wind. Figure 6.16 shows that the thickness between pressure surfaces in warm air are greater than in cold air. The pressure surfaces tilt with increasing altitude. So, the geostrophic wind increases with altitude when the temperature changes horizontally. The relationship between the vertical changes in the geostrophic wind and the horizontal temperature gradient is called the thermal wind relationship. The thermal wind of a layer represents the difference between the geostrophic winds at the top and bottom of a layer. The geostrophic wind increases with altitude when a strong horizontal temperature gradient exists. In the vicinity of fronts, where the temperature gradients are strong, we will observe strong winds aloft.

8 Observations of Wind near the Surface

Consider the surface map shown in Figure 6.6. The winds tend to blow counter-clockwise around regions of low pressure and clockwise around the high. The fastest wind speeds are located in regions where the spacing of the isobars is a minimum, which are around low pressure systems. We also observe that the winds blow across the isobars.

Friction causes the winds at the surface to cross the isobars and flow inward towards a low pressure region and outward from a high pressure region. To explain this consider a simplified situation depicted in Figure 6.17. In the absence of friction we have a geostrophic wind. The pressure gradient and Coriolis forces are in balance and the wind flows parallel to the isobars. Friction acts to reduce the wind speed. Depending on the surface, friction slows the wind by 25% to 75% of what the speed would be in the absence of friction. Slowing the wind has no effect on the pressure gradient force; however, by slowing the wind the Coriolis force is reduced, since the magnitude of the Coriolis force is dependent on the wind speed. With a weaker Coriolis force the pressure gradient force pulls the wind toward the low pressure region. A new balance is established between the pressure gradient, Coriolis and friction forces. The new balance has cross-isobaric flow. The angle at which the winds cross the isobars depends on the type of surface and the latitude. Over open water, where friction is low, the winds typically cross the isobars at an angle between 15( and 30(. Over land the angle is usually between 25( and 50(.

Friction is important in slowing and turning the wind. Friction is also important in shaping many of the earth’s geological features. Dunes of deserts and ripples in sand are examples of how the wind and friction combine to sculpt the land. Wind speed and duration are also important in generating waves (Box 6.2).

9 Convergence and Divergence

At the surface the wind in the Northern Hemisphere flows counterclockwise around a low pressure region and clockwise around a high. Unlike the upper air, the wind crosses the isobars and flows inward towards lower pressures. Because of friction, air in the surface layer cannot spiral around low pressure centers, but moves inward to fill them. In both hemispheres air flows inward, or converges towards a low pressure region. Surface winds flow outward or diverge from a high pressure region. Cyclonic circulation refers to convergent air flow around a region of low pressure. Cyclonic flow is counterclockwise in the Northern Hemisphere and clockwise in the Southern. Anticyclonic circulation refers to diverging air flow around a high pressure system.

1 Coupling of upper and lower air

The importance of divergence and convergence of air cannot be over stressed. Developing winter storms are marked on maps by a low pressure at the surface. Air is moving in to 'fill-in the low pressure', this is convergence. By converging air molecules over a given region, we are adding molecules to our column and so the pressure should increase. So how does the low pressure continue to decrease, 'develop', 'spin-up', or 'deepen'? Because divergence is occurring above the surface low. Divergence above removes air molecules from our column of air, dropping the pressure at the surface. If the upper level divergence is removing molecules faster than the low level convergence, the pressure at the surface will decrease even further. The pressure gradient force will increase, which increases the wind speed at the surface and causes the air to rush in faster towards the low pressure center. How can air in the upper tropopause diverge if there is no friction? One way is to change the speed of the wind. If the wind near the tropopause where to slow down, air molecules would diverge more slowly, causing the pressure at the surface to rise. Conversely, if the wind speed where to increase, divergence would be more rapid and the atmospheric pressure at the surface would drop.

Chapter 7 will discuss in detail the relationships between air flow near the surface and winds higher in the troposphere. For now it is useful to consider one simple diagram. Figure 6.18 presents a top and a side view of surface high and low pressure regions in the Northern Hemisphere. Surface air that diverges from a high pressure region is replaced by air above that sinks. Sinking motion, or subsidence, is unfavorable for cloud formation. High pressure systems are often cloud free. Daily surface weather observations show this (Figure 6.6). Air converging in towards the low pressure rises. This is favorable for cloud formation and explains why approaching low pressure weather systems are often accompanied by clouds and precipitation. How do regions of high and low pressures form? A good example is the sea breeze.

2 Putting it all together - the Sea Breeze

If you have spent much time at the beach during the summer you've probably noticed that at around 3:00 p.m. there often is a strong steady breeze blowing in from the water. This steady wind, the sea breeze, is a result of the uneven heating during the daytime between the land and the adjacent water. At night the wind often reverses direction and blows from the land to the water (a land breeze). Land and sea breezes are referred to as direct thermal circulations. Let's examine the sea breeze.

During the day the land, which has a low specific heat and is a poor conductor (Chapther 2), heats much more quickly than water. As the land warms up, the air next to it heats by conduction and rises, warming the air above the land by convection (Chapter 2). As the air rises it generates a pressure gradient, and thus a pressure gradient force, generating the thermal circulation. To understand this, let's first imagine that the land, sea and the air above them are at the same temperature and that the isobars are parallel. As pressure is defined as the weight of the air molecules above us (Chapter 1), the isobars must decrease in magnitude with increasing altitude (Figure 6.19). In this simplified model, the temperature of the ocean and the air above the ocean are not changing (Chapter 3) and thus initially no air molecules are moving. As air over the land warms (due to absorption of solar energy, and conduction to a thin air layer above), the air rises and there is a vertical displacement of air molecules to a higher altitude. These rising thermals of air change the initial isobars.

For example, the height of the 980 mb isobar in Figure 6.19 must increase if we are putting air molecules above this height. The result is that above the surface, the isobars begin to slope upward. Now, at a given altitude (say 100 meters), the pressure is no longer the same over the ocean as it is over land, it is higher over land. Note, at this time the surface pressures over the land and ocean are the same as we have not transported molecules horizontally. But now we have a horizontal pressure gradient above the surface and thus a pressure gradient force generating a wind from over the land out towards the ocean. This wind removes air molecules from over the land lowering pressure at the surface. The air that moves out over the ocean increases the surface pressure over the water. At the surface over land low pressure is developing while over the ocean high pressure is developing, generating a horizontal pressure gradient force at the surface acting from over the ocean towards the land. Air moves toward the land creating a sea breeze. To replace this surface air that is moving from over the water towards land, air sinks from above, completing the circulation. This sinking air moves air molecules towards the surface, causing the pressure above the surface to lower as the molecules from above descend. Notice the slope of the pressure surfaces in Figure 6.19. As the temperature difference between the land and water increases throughout the afternoon (Chapter 3), the circulation increases in strength and winds pick-up reaching a maximum in the middle to late afternoon. Over land the distance between two isobars (i.e., 980 and 960 mb) is greater than over the ocean. This difference is what keeps the circulation moving and is due to the air over land being warmer than the air over the ocean.

If you are not at the beach to feel the sea breeze, you can monitor its existence and intensity by analyzing satellite imagery (Figure 6.20). A rising parcel of air expands, and cools and the relative humidity increases(conditions favorable for the formation of clouds (Chapter 4). For this reason, the upward branch of the sea breeze is often visible from satellite pictures (Chapter 5) in the form of cumulus clouds. During the day, the upward branch moves inland and is an indication of the strength of the sea breeze. If the atmospheric conditions are favorable for the formation of thunderstorms, the sea breeze may provide just enough lifting to cause thunderstorms to develop.

Whenever large land and water bodies are adjacent to one another, sea breezes may develop and may cause thunderstorms. Florida's abundant summertime rainfall is a result of sea breezes. One sea breeze front advances from the east and one from the Gulf of Mexico side.

The important concept is that heating (or cooling) of a column of air leads to horizontal differences in pressure, generating a pressure gradient force, which causes the air to move and a circulation to develop. During the evening, the land cools faster than the water and the process is reversed (Figure 6.21). The net result is a land breeze, surface winds blow from the land out to sea.

10 Scales of Motion

Atmospheric motions span an enormous range of space and time. The size of an atmospheric weather system is related to how long it exists. Small swirls of wind may last for a few seconds while a hurricane may last several days. In general, as the size of the phenomena increases so does its life span. The size and life spans of different atmospheric phenomena are shown in Figure 6.22. Meteorologists refer to weather systems with reference to three primary size categories:

Microscale is generally applied to circulations that are less than 1 km in size.

Mesoscale systems, such and thunderstorms, range in size between 1 km and 1,000 km.

Macroscale systems, like winter storms, are larger than 1,000 km.

11 Summary

The wind is air in motion. Newton’s three laws of motion describe the physical laws that govern the movement of objects, including air. Bodies at rest tend to stay at rest until acted upon by a force. The important forces at work in the atmosphere are:

I. Pressure Gradient Force(directed from higher pressure towards lower pressure at right angles to the isobars. Contour lines on a constant pressure chart, or isobaric chart, tell us the altitude above sea level at which one obtains a given pressure reading (i.e., 500 mb isobaric map). Cold air aloft is normally associated with low heights as indicated by hydrostatic balance. The pressure gradient force acts perpendicular to lines of constant height and from high heights towards low heights. The greater the change in pressure over a given distance, the stronger the pressure gradient force.

II. Coriolis Force(describes an apparent force that is due to the rotation of the earth. The Coriolis force acts to the right in the Northern Hemisphere and to the left in the Southern hemisphere. As a result of the Coriolis force, winds blow counter clockwise around a low pressure system and clockwise around a high in the Northern Hemisphere. The Coriolis force is zero at the equator and increases in magnitude as the poles are approached. This force is zero if the velocity of the object is zero and increases as the velocity increases.

III. Friction(always acts in the direction opposite to movement. The magnitude of this force depends on the type of surface. Friction is small over a frozen lake and strong in a forest.

IV. Centripetal force(the direction of the centripetal force is always towards the center of the curvature and increases as the speed of the object increases.

Winds arise as a result of a combination of these forces. Geostrophic winds arise from the balance between pressure gradient and Coriolis forces. Geostrophic winds blow parallel to the isoheights, and in the northern hemisphere lower pressure is to the left. Near the surface, friction reduces the wind speed, which reduces the Coriolis force. The weaker Coriolis force no longer balances the pressure gradient force and so the wind blows across the isobars toward lower pressure. In the northern hemisphere, surface winds blow counterclockwise and into a low, and flow out of a high in a clockwise direction. A gradient wind results from a balance between the pressure gradient, Coriolis, and centripetal forces. Like the geostrophic wind, the gradient wind flows parallel to isolines of pressure altitude.

Convergence and divergence of air can occur anywhere in the atmosphere. The development of a major storm requires convergence near the surface and divergence above.

1 Terminology

You should understand all of the following terms. Use the glossary and this Chapter to improve your understanding of these terms.

2

Acceleration

Anticyclonic flow

Boundary layer

Buys-Ballot’s law

Convergence

Coriolis force

Cyclonic flow

Divergence

Force

Friction

Geostrophic wind

Gradient wind

Hydrostatic balance

Inertia

Isobaric map

Leeward

Newton’s laws of motion

Pressure gradient force

Prevailing wind

Sea breeze

Thermal wind

Wind gust

Windward

3 Review Question

1. Why might a sea breeze develop on one summer day and not another?

1. The sea breeze is one example of a circulation caused by differential heating between two regions. Give examples of other circulations that are driven by differential heating.

2. Describe the pressure gradient and Coriolis forces.

3. Why is friction important in weather systems?

4. Draw a picture of the forces acting on the winds about a low pressure region in the Southern Hemisphere.

5. Draw the wind flow around an anticyclone in the Southern and Northern Hemisphere.

6. What would happen to converging air at the surface if convergence also occurred in the upper troposphere?

7. Explain the relationship between the pressure gradient force and the slope of isobaric surfaces.

8. Is the following statement true: For a fixed pressure gradient the geostrophic wind will be stronger at 45(N than at 55(N. Justify your answer.

9. What is a wind gust? Why is gust important to report?

10. Explain why the Coriolis force does not influence how your bath tub drains.

11. Does Buys-Ballot’s law have to be modified to apply to winds in the Southern Hemisphere?

4 Web Activities

Web activities related to subjects in the book are marked with subscript (. Activities include:

Explorations of pressure gradient, Coriolis, and frictional forces on wind speed and direction

Wind rose

Contouring upper air weather maps

Practice multiple choice exam

Practice true/false exam

Box 7.1 Planes and pressure differences

Airplane wings have a particular shape. They are designed this way so they can lift the plane off the ground. The forces responsible for lifting the plane off the ground are produced by the wind flow over the wing. You can demonstrate this by shaping a piece of cardboard like a wing, securing it to a table and blowing air over it (See below). The cardboard will lift off the table!

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Bernoulli's Principle (named after the Swiss scientist Daniel Bernoulli) states that when a fluid in steady flow speeds up, the pressure lowers and where the fluid slows down the pressure is high. As the air flows over the wing it speeds up as it travels over the top of the wing, lowering the pressure causing a pressure difference between the bottom and top of the wing, resulting in an upward force called the lift. There is also a drag force acting to slow-down the moving wing. The net force, if the wing is designed correctly, lifts the wing and the plane off the ground. The key to designing wings is reducing the drag while increasing the lift.

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Box 7.2 Wind generated waves

The storm shown in Figures 6.6 and 6.7 represent a significant maritime event. On November 10, 1975 the 729-foot Great Lakes freighter Edmund Fitzgerald sank in Lake Superior. Can you determine the direction of the surface winds over Lake Superior from Figure 6.6?

In this chapter we have emphasized how friction at the ground modifies the wind. The wind also modifies the condition of the surface. A good example of this is wind generated waves. Waves form as the wind’s energy is transferred into the water. The size of a wind generated wave depends on:

1. The wind speed: The stronger the winds, the larger the force and thus the bigger the wave. The wind must also be steady - a constant wind speed.

1. The duration of the winds: The longer the wind blows over the open water, the larger the waves.

2. The fetch: This is the distance of open water over which the wind blows. The larger the fetch the larger the waves.

Figure B6.1 below plots observations of wind speed and wave height observed on Lake Superior on this fateful day. Also shown is the theoretical maximum wave height for different wind speeds. In this particular storm not only where the wind speeds large, but also the fetch was long for this lake. Figure B6.2 shows the path of the Edmund Fitzgerald (thin black line) and the wind direction (heavy blue lines) at the time the great ship sank. Chapter 10 describes the storm that generated these wind.

High waves are not only hazardous to shipping, but are also perilous to coasts. Waves can cause flooding, crush buildings, and scour the soil from under structures. The rapid rise in sea level associated with a storm is called the storm surge.

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