Chapter 7,

Weather and Climate

LESSON 3 PREDICTING WEATHER

OBJECTIVES • Describe high- and low- pressure systems

and the weather associated with each.

• Explain how technology is used to study weather.

MAIN IDEA

• To predict weather, scientists study air’s properties and movement.

VOCABULARY • isobar - lines that connect places with equal

air pressure

• air mass - a large region of the atmosphere in which the air has similar properties throughout

• front - the boundary between two air masses

• cold front - cold air moves in under a warm air mass

• warm front - warm air moves in over a cold air mass

• occluded front - a weather front where a cold front catches up with a warm front and then moves underneath the warm front producing a wedge of warm air between two masses of cold air

WHAT ARE HIGHS AND LOWS?

• Scientists predict weather by studying how wind moves, from areas of high pressure to areas of low pressure.

• A region’s air pressure is shown on weather maps which include isobars, measured in millibars, to show places with equal air pressure.

• A low pressure system is illustrated by an (L) and has isobar readings that decrease towards the center.

• A high pressure system (H) has, at its center, higher air pressure than its surroundings.

• Wind speed is fastest where air pressure differences are greatest.

• Closely spaced isobars show a large change over a small area, which indicates high wind speeds.

• Gentle winds are shown by widely spaced isobars.

AIR MOVEMENT AROUND HIGHS AND LOWS

• Air flows outward from the center of a high pressure system.

• The air leaving rotates clockwise in the Northern Hemisphere because of the Coriolis effect.

• In a low pressure system, the opposite occurs; air flows in and rotates counterclockwise.

• In the Southern Hemisphere the opposite occurs; the Coriolis effect bends moving air to the left.

AIR PRESSURE AND WEATHER • Fair weather is usually found in areas of high

pressure. • Clouds and precipitation are found in low-

pressure areas.

• A drop in air pressure is usually followed by storms and rain; this can be predicted by a drop in barometer readings.

QUICK CHECK Cause and Effect

What kind of weather would be caused by a drop in air pressure?

The types of weather caused by a drop in air pressure includes: clouds, rainfall (precipitation), and storms.

Critical Thinking In what direction would air move around a low in the Southern Hemisphere? Explain.

In the Southern Hemisphere air moves clockwise around a low pressure system. Air would move in toward the low and curve to the right. This is the opposite of the type of motion in the Northern Hemisphere.

WHAT ARE WEATHER FRONTS? • Weather conditions vary due to different air

masses.

• Each air mass has its own characteristics.

• Air masses that form over • land tend to be dry, • water – humid, • tropics – hot, • poles – cold.

• Global winds such as westerlies, west to east, move air masses.

• Air masses meet at a front. • One of the three basic types of moving fronts

is a cold front. Cold fronts usually have quick, heavy storms, followed by clear, cool, and dry weather.

• Warm fronts usually have light, steady precipitation, followed by warm and humid weather.

• In an occluded front the storm is usually brief.

INTERPRETING WEATHER MAPS • Factors that influence weather are air

pressure, humidity, and temperature.

• Scientists compile this and other information and use computers to analyze the information which is then summarized on weather maps.

• On a weather map look at the high and low air pressure systems.

• High means fair weather and lows mean precipitation. • Then look at the fronts. • Fronts always come out of lows. • In the Northern Hemisphere, fronts rotate

counterclockwise around a low. • If you are northeast of an approaching front, the

winds will blow from the southeast.

Cause and Effect Suppose the temperature dropped and a storm brought heavy rains. Which type of front might have caused this?

Cold fronts bring heavy storms.

QUICK CHECK

Critical Thinking Why does weather change along a front?

Weather changes along a front because one type of air mass is replaced by another air mass.

• Weather stations record temperature, wind direction, wind speed, and humidity.

• Weather balloons rise up 22 miles (35 km) into the stratosphere collecting data.

• Replacing balloons is expensive, so satellites are used.

HOW IS TECHNOLOGY USED TO STUDY WEATHER?

• Satellites take pictures of earth and weather conditions.

• Radar, using radio signals, pulses of energy, detects precipitation.

• Doppler radar measures wind speed. • Using all these instruments and data,

scientists form detailed pictures of weather conditions.

Cause and Effect What effect has technology had on weather prediction?

Technology allows us to be more accurate in predicting the weather. Satellites and radar provide data for large areas. Computers can process it, so scientists can predict weather more quickly and accurately.

QUICK CHECK

Critical Thinking Why is it better to obtain weather information from more than one source?

Additional sources provide a “backup” to ensure accuracy.