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Chapter 12: Meteorology

Chapter worksheet


Ch. 12.1 The Causes of Weather

Air masses have different temperatures and amounts of moisture because of the uneven heating of Earth's surface.

What is Meteorology?

Meteorology is the study of physics, chemistry, and dynamics of atmospheric phenomena.
The Greek work meteoros, means high in the air and is the root work for meteorology.

Atmospheric phenomena are often classified as types of meteors:

  • Hydrometeors: cloud droplets & precipitation- rain, snow, sleet, and hail
  • Lithometeors: smoke, haze, dust, and other particles
  • Electrometeors: thunder and lightning

Meteorologists study these various meteors.

Weather vs. Climate

Weather- short-term variations in atmospheric phenomena that interact and affect the environment and life on Earth.
Climate- the long-term average of variations in weather for a particular area.
Weather data averages over 30 years are used to define an area's climate.

Heating Earth's Surface

In meteorology, a crucial question is how solar radiation is distributed around Earth.

Imbalanced Heating

Earth's axis of rotation is tilted relative to the plane of Earth's orbit; therefore the number of hours of daylight and amount of solar radiation is not the same around the planet.
Additionally, Earth is a sphere so the angle of the sun is different at locations around the planet.

Thermal Energy Redistribution

Earth's constant movement of air and oceans redistributes thermal energy around the world.

Air Masses

Air over a warm surface can be heated by conduction over thousands of kilometers for days or weeks resulting in the formation of an air mass.
Air mass- a large volume of air that has the same characteristics, such as humidity and temperature as its source region.
Source region- the area over which the air mass forms.

Types of Air Masses

In North America there are five typical air masses due to the source region being nearby.

Tropical Air Mass

Maritime (mT) forms over the oceans; creates hot, humid weather.
Continental (cT) forms over land; creates hot, dry weather.

Polar Air Masses

Maritime (mP) forms over the oceans; humid, warm.
Continental (cP) forms over land; dry, cool.

Arctic Air Masses

Arctic (A) forms above 60º N latitude. Extremely cold.

Air Mass Modification

Air masses do not stay in one place. When they move they transfer thermal energy. As the air mass moves from its source region it picks up characteristics of the new region. Example: lake-effect snow.

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Ch. 12.2 Weather Systems

Weather results when air masses with different pressures and temperatures move, change, and collide.

Global Wind Systems

If Earth did not rotate on its axis, two large convection currents would cover Earth. Cold, dense air at the pole would sink and flow towards tropics, forcing warmer air to rise.

Coriolis Effect- results in fluids and objects moving in an apparent curved path rather than a straight line. This causes moving air to curve to the right in the northern hemisphere and to the left in the southern hemisphere.
Together, the Coriolis effect and the heat imbalance on Earth create distinct global wind systems. These systems help to equalize the thermal energy on Earth.

Three basic systems (zones):

Polar easterlies: wind zones between 60º and the poles both North and South. Cold dense air at poles descend, Earth's spin deflects the winds in easterly direction. Cold and sporadic.

Polar front created at 60º N&S where polar easterlies meet the prevailing westerlies; an area of stormy weather.

Prevailing westerlies: wind systems located between 30º and 60º N&S. These winds are called westerlies since that is where they originate. These winds move much of the weather across the US and Canada.

Trade winds: winds between 30º N&S. Air sinks and moves towards equator where the warms and rises.

Horse Latitudes: near the 30º N&S latitude the sinking air creates an area of high pressure resulting in a belt of weak surface winds. Earth's major deserts are under these high-pressure areas.

Intertropical Convergence Zone: at equator where the north and south trade winds meet (converge) air is forced upward creating a low pressure area. This area can be large or small and creates cloudiness and thunderstorms which deliver moisture to many tropical forest.

Jet Stream

Wind is the movement of air from areas of high pressure to areas of low pressure.
A large temperature gradient in upper-level air combined with the Coriolis effect results in strong westerly winds called jet stream.
Jet Stream- a narrow band of fast wind. Occurs at wind zone boundaries; can have speed up to 400km/h at altitudes of 10.7-12.2km.

Position varies with the season and represents the strongest core of winds.

Types of Jet Streams

The major jet streams, called the polar jet streams, separate the polar easterlies from the prevailing westerlies in the northern and southern hemispheres; occur 40º-60º N&S.

Minor jet streams are the subtropical jet streams where the trade winds meet the prevailing westerlies; 20º-30º N&S.

Jet Streams and Weather Systems

Storms form along jets streams and generate large-scale systems that transport cold air toward the tropics and warm air toward the poles.


A collision of two air masses forms a front.
Front- a narrow region between two air masses of different densities. Fronts can form across thousand so kilometers of Earth's surface.

Cold Front: cold, dense air forces warm air up along a steep slope. As it rises, it cools and water vapor condenses. Intense precipitation and sometimes thunderstorms are common. A blue line with blue triangles represents a cold front; triangles point in the direction the front is moving.

Warm Front: warm air displaces cold air forming a gradual boundary slope; can cause widespread light precipitation. A red line with semicircles pointing in the direction the front is moving.

Stationary Front: when two air masses meet but neither advances; occurs between two modified air masses that have small temperature and pressure gradients between them. Air masses can move parallel to the front. Sometimes have light winds and precipitation. A line with alternating cold and warm front symbols pointing in opposite directions represents the front.

Occluded Front: a cold air mass moves so rapidly that it forces a warm air mass upward, the cold air continues on until it meets another cold air mass. Strong winds and heavy precipitation are common. A purple line with alternating purple triangles and semicircles pointing in the direction the front is moving.

Pressure Systems

Sinking or rising air, combined with the Coriolis effect, results in the formation of rotating high and low pressure systems in the atmosphere. Air in these systems move in a circular motion.

Low-pressure Systems

In low pressure systems, air rises. When air from outside the system replaces the rising air, it spirals inward toward the center and then upward in a counterclockwise direction (opposite in southern hemisphere). Associated with cloudy weather and precipitation.

High-pressure Systems

Sinking air moves away from the center; the air circulates in a clockwise direction in the northern hemisphere (opposite in souther hemisphere). Associated with fair weather.

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Ch. 12.3 Gathering Weather Data

Accuracy is critical in weather analysis and prediction.

Data from Earth's Surface

Two important factors in weather forecasting are the accuracy of the data and the amount of available data.

Temperature and Air Pressure

Thermometer- measures temperature using either the Fahrenheit or Celsius scale.
Liquid-in-glass thermometer: contains a column of alcohol sealed in a glass tube; the liquid expands when heated, causing the column to rise.
Bimetallic-strip thermometer: dial with a pointer; contains a strip of metal made from two different metals that expand at different rates when heated. The strip is long and coiled into a spiral, making it more sensitive.

Barometer- measures air pressure.
Some barometers have a tube of mercury upside down and submerged in a open container of mercury.

Aneroid barometer has a sealed metal chamber with flexible sides; chamber contracts or expands with changes in air pressure. A systems of levers connects the chamber to a pointer on a dial.

Wind Speed and Relative Humidity

Anemometer- measures wind speed. Wind speed is calculated using the number of revolutions of the cups over a given time.

Hygrometer- measures humidity. Uses techniques such as finding the temperature difference between the wet bulb and the dry bulb.

Automated Surface Observing System

In the late twentieth century development of reliable automated sensors and computer technology has made it possible to take "snapshots" of the atmosphere at one particular moment.

Automated Surface Observing System (ASOS) is a surface-weather observation network that gathers data 24 hours a day, every day. It began in later 1990s and provides essential weather data for aviation, forecasting, and research.

Data From the Upper Atmosphere

Weather largely a result of changes high in the troposphere. To make accurate forecasts, meteorologists must gather data up to 30,000 m.
Radiosonde- an instrument used for gathering upper-atmosphere data. Consists of a package of sensors and a battery-powered radio transmitter and is suspended from a balloon that is about 2 m in diameter.
Rawinsonde- a radiosonde that also measures wind direction and speed. radar + wind + radiosonde

GPS and the latest computer technology is used to track the information. The information is then used to forecast atmospheric changes that affect surface weather.

Weather Observation Systems

Exact locations for where precipitation falls is determined with the use of data from weather radars and weather satellites.

Weather Radar

Radar stands for radio detection and ranging. A radar system generates radio waves and transmits them through an antenna at the speed of light. Radars send a pulse and wait for the return before another pulse is sent. From the data, the distance to precipitation and its location can be determined.

Doppler Weather Radar

Doppler effect- the change in pitch or frequency that occurs due to the relative motion of a wave, such as sound or light, as it comes toward or goes away from an observer.

The NWS uses weather surveillance based on the Doppler effect of moving waves. Determines the speed at which precipitation moves toward or away from a radar station.
Doppler can also be used to determine wind speeds since the movement of precipitation is caused by wind.

Weather Satellites

Cameras mounted on weather satellites take images of Earth at regular intervals. Can be infrared, visible-light, or water-vapor imagery.

Infrared Imagery

Infrared imagery detects different thermal energy frequencies, which enables meteorologists to map either cloud cover or surface temperatures.

Meteorologists can determine a cloud's temperature, type, and its altitude. Strong thunderstorms appear as very cold areas on an infrared image. Infrared can be used to determine a storm's potential to produce severe weather since the strength of a thunderstorm is related to the altitude it reaches.

Visible-light Imagery

Digital photos are taken and sent to ground stations where the data is plotted on maps. These satellites track clouds but not precipitation. Cloud thickness can be determined by shading.

Combining radar and visible imagery data meteorologists can determine where clouds as well as precipitation are occurring.

Water-vapor Imagery

Water vapor cannot be photographed directly because it is an invisible gas, but it absorbs and emits infrared radiation at certain wavelengths.

Water-vapor imagery shows moisture in the atmosphere, not just cloud patterns.
Because air currents that guide weather systems are often defined by trails of water vapor, meteorologists can monitor the growth and change in storm systems even when clouds are not present.

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Ch. 12.4 Weather Analysis and Prediction

Surface Weather Analysis

Weather data is plotted on weather charts and maps and are accompanied by radar and satellite imagery.

Station Models

Station model- a record of weather data for a particular site at a particular time. A station model allows meteorologists to fit a large amount of data into a small space, as well as a universal way to communicate weather data.

Plotting Station Model Data

To plot data nationwide and globally, meteorologists use lines called isopleths that connect points of equal or constant values.
Isobars- lines of equal pressure, Isotherms- lines of equal temperature.

Interpreting Station Model Data

Inferences about weather, such as wind speed, can be made by studying isobars and isotherms on a map.
Isobars that are close together indicate a large pressure difference over a small area, which means strong winds; far apart indicate a small difference in pressure and light winds.
Isobars also indicate locations of high/low pressure systems. Combined with isotherms, meteorologists are able to identify fronts.

Types of Forecasts

Using data from different levels of the atmosphere, based on current and past weather conditions meteorologists can make forecasts. Two types: digital forecast and analog forecast.

Digital Forecasts

The atmosphere behaves like a fluid. Digital forecast- created by applying physical principles and mathematics to atmospheric variables and then making a prediction about how these variables will change over time.
The main method of used by meteorologists, and relies on numerical data.

Analog Forecasts

To ensure the accuracy of an analog forecast, meteorologists must find a past event that had similar atmosphere, at all levels and over a large area, to a current event.
Analog forecast- based on a comparison of current weather patterns to similar weather patterns from the past.
Analog forecasting is useful for conducting monthly or seasonal forecasts which are based mainly on the past behavior of cyclic weather patterns.

Short-Term Forecasts

The most accurate and detailed forecast because weather systems change directions, speeds, and intensities over time.
A one-to-three day forecast is usually accurate for temperatures, and for when and how much precipitation will occur but cannot pinpoint an exact temperature at a specific time.

Long-Term Forecasts

It is impossible for computers to model all of the different variables that affect the weather at a given time and place, all long-term forecasts are less reliable than short-term forecasts.

Meteorologists use changes in surface weather systems based on circulation patterns throughout the troposphere and lower stratosphere for four-to-seven day forecasts.
One-to-two week forecasts are based on changes in large-scale circulation patterns; they are vague and based mainly on similar conditions that have occurred in the past.

Forecasts for months and seasons are based mostly on weather cycles or patterns.

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Page last updated January 2, 2017.