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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