Chapter 12 – Meteorology
Chapter 12 – Meteorology
12.1 – The Causes of Weather
- meteorology is the study of atmospheric conditions
Weather and Climate
- weather is the current state of the atmosphere
o these are short-term variations in the atmosphere
- long-term variations in weather for a particular area is called climate
o climate is usually averaged over 30 years
A Question of Balance
- the sun heats the surface of Earth
- it heats some parts of the Earth’s surface more direct then others (12.2)
- the tropics are warmer then the poles because the sun hits there almost straight on and the poles get hit by the sun at more of an angle
o the poles has the same amount of energy spread out over a larger area and this causes the solar radiation to be less intense
Balancing the Budget
- even though the tropics receive direct sunlight the still maintain a fairly constant temp
o this is because the heat is redistributed around the world
o the continual motion of air and water allows the heat to be redistributed around the Earth
Air Masses
- an air mass is a large body of air that takes on characteristics of the area over which it forms
o meteorologists call the region over which on air mass forms the source region
- air masses can form over land or water
- air masses take on the temp of where they form and those that form over land are drier then those formed over water
Classifying Air Masses
- fig 12-3
Source Regions
- all 5 main types of air masses can be found in North America
Air Mass Modification
- air masses do not stay in one place indefinitely
- the air mass will move and can take on new characteristics because the land below it is different then the source region
o when this happens, it is said to undergo air mass modification
- table 12-1
12.2 – Weather Systems
- Coriolis Effect
Global Wind Systems
- trade winds
- horse latitudes
- prevailing westerlies
- polar easterlies
Jet Streams
- Earth’s weather is strongly influenced by atmospheric conditions and events that occur at the boundaries between wind zones
- here is where narrow bands of fast moving winds called the jet stream occurs (fig 12-6)
- the most significant one, the polar jet stream, separates the polar easterlies from the prevailing westerlies
- the other one is the subtropical jet stream
Large-Scale Weather Systems
- disturbances form along the jet streams
- even though the jet streams are high in altitude, weather systems follow along them like a railroad track
- the jet stream also affects the intensity of the storms
Fronts
- in the middle latitudes, air masses with different characteristics sometimes collide and form a front
- a front is a narrow region separating two separate air masses of different densities
- the interaction of these fronts can bring dramatic changes in the weather
- fig 12-7 shows the 4 different types of air masses
Cold Fronts
- 12-7A
- this is where cold dense air displaces warm air and forces the warm air up along a steep front
- clouds, showers, and sometime t-storms happen along a cold front
Warm Fronts
- 12-7B
- this is where advancing warm air displaces cold air
- a warm front is characterized with extensive cloudiness and precipitation
Stationary Fronts
- 12-7C
- sometimes two air masses meet and neither advance upon one another
- when this occurs the front stalls
- this happens because the temps and pressures differences between the to fronts are small
- these seldom have extensive rainfall or clouds
Occluded Front
- 12-7D
- this is when a cold air mass is moving so rapidly it takes over a warm front
- the cold air wedges the warm air upward
- precipitation is common on both sides of this front
Pressure Systems
- rising air is associated with low pressure and sinking air is associated with high pressure
- rising or sinking air along with the Coriolis effect form a rotating low and high pressure systems
High Pressure Systems
- 12-8A
- in a surface high-pressure system, air sinks, so that when it reaches the Earth’s surface, it spreads away from the center
- the deflection of air to the right caused by the Coriolis effect makes the overall circulation of wind move in a clockwise direction in the northern hemisphere
Low Pressure Systems
- in a surface low pressure system air rises
- the rising air must be replaced by air outside the system, so the noet flow is inward and then upward
- the system moves in a counterclockwise direction in the northern hemisphere
- it is difficult for clouds to form when air is sinking, as it does in high pressure systems
o this is why high pressure systems are usually associated with fair weather
- low pressure systems are normally associated with clouds and precipitation
o a main producer of bad weather in the middle latitudes is a wave cyclone
o fig 12-9
12-3 – Gathering Weather Data
- meteorologists measure the atmospheric variables of temp, air pressure, wind, and relative humidity to make accurate weather forecasts
Surface Data
- common weather instruments are the thermometer and barometer
o barometers measure air pressure
o fig 12-10A-B
Other Surface Instruments
- an anemometer is used to measure wind speed (12-10C)
- a hygrometer measures relative humidity (12-10D)
Automated Surface Observing System
- for accurate weather observations, meteorologists gather weather data from many different locations
- the National Weather Service gathers this weather info from about 1700 different sites about every hour
Upper Level Data
- surface weather is important, but the weather we experience is mainly the result of what happens high in the troposphere
- balloon-borne packages are used to do this
o these are called radiosonde
- this device measures temp, air pressure, and humidity
- the readings are sent back to a ground station
Weather Radar
- this is used to pinpoint rainfall
- radar uses electromagnetic waves that ignore small cloud droplets, but bounce off larger droplets
- the data is then collected by antennae on the ground
Doppler Radar
- this takes advantage of the Doppler effect
o the Doppler effect is the change in wave frequency that occurs in energy, as that energy moves toward or away from an observer
o ex.) train horn
Weather Satellites
- cameras aboard a weather satellite takes photos of the Earth at regular intervals
- this data is sent back to ground stations and plotted on maps
- satellites track cloud cover, but combining this with radar meteorologist can determine cloud cover and rain
Infrared Imagery
- these detect differences in thermal energy
- fig 12-14
- infrared images detect the different temps of clouds
12.4 – Weather Analysis
- after meteorologists gather data, they plot on a map using station models
o a station model is a record of weather data for a particular site at a particular time
o fig 12-15
Surface Analysis
- to plot data nationwide or globally, meteorologists use isopleths
o these are lines that connect points of equal or constant values
o can represent pressure (isobars) or temperature (isotherms)
o theses are similar to contour lines in mapping
o fig 12-16
Short-Term Forecasts
- a reliable short-term forecast relies on data from different levels in the atmosphere
Digital Forecasts
- this forecast relies on numerical data analyzed by computers
Analog Forecasts
- this compares current weather patterns to ones in the past
- this is useful for predicting monthly or seasonal forecasts
Long-Term Forecast
- all forecasts become less reliable the longer they are
- 1-3 day are somewhat accurate, but cannot predict exact precipitation amount and temp
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