Weather Chapter 21

Air Masses

How Air Moves Areas of high pressure to areas of low pressure Temperature and pressure differences on the surface create three wind belts in the Northern Hemisphere and three wind belts in the Southern Hemisphere The Coriolis effect makes these winds rotate

Air Mass: a large body of air throughout which temperature and moisture are similar

Formation of Air Masses If pressure differences are small, then the air will move slowly or remain stationary. If this happens, then the air will take on the characteristics (temperature and humidity) of that region.

Types of Air Masses Classified according to its source. (where it formed) Source for cold air masses is polar Source for warm air masses is tropical Air masses that form over oceans are called maritime Air masses that form over land are called continental

Air masses affecting North America

Air Mass Source Location Movement Weather cP Polar regions in Canada South-southeast Cold and dry mP Polar Pacific; polar atlantic Southeast; southwest-south Cold and moist cT U.S. southwest; Mexico North-northwest Warm and dry mT Tropical Pacific; tropical Atlantic Northeast; north-northwest Warm and moist A Arctic; northern Canada Very cold and dry

Section 2: Fronts Cold, dense air masses don’t mix well with less-dense, warm air masses. Thus a boundary is formed between the two. This is called a front. Why are there no fronts in the tropics?

Fronts form when one air mass collides into another air mass

Types of Fronts: Cold Front Cold air mass overtakes a warm air mass. The moving cold air lifts the warm air. If the warm air is moist, clouds form. Why do fast moving cold fronts usually produce heavy thunderstorms while slow moving cold fronts do not? A long line of heavy thunderstorms is called a squall line

Squall line circled in red

Warm front Warm air mass overtakes a cold air mass The less dense warm air rises over cooler air. The slope of the warm front is gentle and gradual. Because of this, clouds may extend far ahead of the front and usually produce less violent weather

Stationary Front A front in which air masses move very slowly or not at all

Occluded front Fast moving cold front overtakes a warm front and lifts the air completely off the ground

Front map symbols

Polar front: the boundary where very cold air near the poles meets tropical air masses near the equator Waves develop along these fronts. Waves are bends that form in cold or stationary fronts

Mid-latitude cyclone The waves are the beginning of mid-latitude cyclones which are areas of low pressure that are characterized by rotating wind that moves toward the rising air

Stages of a mid-latitude cyclone

Anticyclone Air sinks and flows outward from a center of high pressure Bring dry weather because of the sinking air.

Severe Weather Weather that may cause property damage or loss of life Includes: heavy rain, lightning, hail, strong winds, tornadoes

Thunderstorms A heavy storm that is accompanied by rain, thunder, lightning, and strong winds There are three stages

Stage 1: Cumulus Stage Warm, moist air rises, and water vapor within the air condenses to form a cumulus cloud. Latent heat causes the air to continue to rise adiabatically.

Stage 2: Mature Stage Dark, cumulonimbus cloud forms as air continues to rise. Has updrafts and downdrafts capable of producing heavy rain and hail.

Stage 3: Dissipating Stage Updrafts dissipates due to the strong downdrafts. No more latent heat is being released. The cloud cools and disappears.

Lightning Lightning is discharged electricity from clouds The released electricity heats up the air and expands rapidly. This produces the loud sound called thunder.

Causes of lightning In order for a cloud to discharge electricity, it must carry distinct electrical charges. The upper part of a cloud usually carries a positive charge (the rising water droplets) while the lower part carries a negative charge (the heavier ice particles)

Lightning strikes the ground when the negative charges in the cloud are attracted to the positive charges on the ground.

Stepped leader

Weather Instruments Weather observations are based on measurements from atmospheric pressure, humidity, temperature, wind speed, and precipitation

Measuring Lower Atmospheric Conditions Lower atmospheric conditions change drastically

Air Temperature Measured by thermometers usually in a liquid form such as mercury or alcohol Rising temperature causes the liquid to expand and rise while cooler temperatures causes the liquid to contract and drop. Electrical thermometers measure the flow of electricity through a material

Air Pressure The approach of a front is usually indicated by a drop in air pressure. Barometers measure atmospheric pressure

Wind Speed and Wind Direction Anemometer measures wind speed Wind vanes measure wind direction

Measuring Upper-Atmospheric Conditions Surface conditions are only part of the picture. The upper atmosphere gives a better understanding of local and global patterns

Radiosonde An instrument package carried by a helium-filled balloon that measures relative humidity, air pressure, and air temperature

Radar: Radio Detection And Ranging Uses radio waves to determine velocity and location of objects

Weather Satellites Provides weather information where observations cannot be made on the ground There are many types of images satellites can provide

Infrared: images made at night that reveal temperatures at the top of clouds

Visible Imagery: taken during the day

Water Vapor Imagery: combines Infrared and Visible to determine moisture

Computers The use of supercomputers are valuable because they help meteorologists understand and interpret weather data and for forecasting. Computers also store weather information for quick access

Forecasting Weather Weather maps allow scientists to understand current weather and to predict future weather events.

Plotting Temperature and Pressure Scientists use lines on weather maps to connect points of equal measurements. Lines of equal temperature are called isotherms Lines of equal pressure are called isobars Closer spaced lines indicate a rapid change. Widely spaced lines indicated a more gradual change H represents high pressure. L represents low pressure

Isotherm weather map

Isobar weather map

Weather Forecasts Forecasting the weather involves using supercomputers to plot the intensity and path of weather systems on maps. Studying the current maps with previous maps allow for more accurate weather forecasts. Meteorologists use different computer models to analyze weather data. Some are better at predicting certain types of weather than others. Temperature, wind direction, wind speed, cloudiness, and precipitation are more accurate than the timing of the event.

Types of Forecasts: Nowcasts Use radar and enable forecasters to focus on timing precipitation and tracking severe weather

Daily Forecasts: predict weather conditions for a 48 hour period

Extended Forecasts: looks ahead 3 to 5 days

Medium-range forecast: 3 to 7 days

Long-range forecasts: 7 or more days ahead; sometimes monthly or season

Severe Weather watches and warnings A primary goal in meteorology is to reduce the amount of destruction caused by severe weather. Meteorologists use watches and warnings to help Watch: conditions are ideal for severe weather Warning: severe weather has been spotted or is expected within 24 hours