Weather PatternsChapter 17

Air Masses•Air masses: a huge body of air with similar temperature, humidity, and pressure.•In America, we have four air masses that affect us. They are based on temperature and humidity.

Types of Air Masses•Tropical: form over the tropics, so they are warm and will have low air pressure.•Polar: form north/south of 50º latitude. So they are cold and will have high pressure.•Maritime: form over the oceans so they are humid.•Continental: form over the middle of continents (land) so they are dry.

Air Masses ComparedType Temp Humidity Pressure

Maritime tropical (mT)

Warm Humid Low

Maritime polar (mP)

Cold Humid High

Continental tropical (cT)

Warm Dry Low

Continental polar (cP)

Cold Dry high

Air Masses and Global Winds•In America air masses are pushed by the prevailing westerlies.•So they tend to move west to east.

Weather Fronts•Fronts: where two air masses meet and don’t mix.•Front is a military term where two armies meet to fight.•The collision of fronts often causes storms and changeable weather.•There are four kinds of fronts.

Cold Fronts•Cold fronts have cold, dense air.•They will slide under warm, less dense air.•So the warm air is pushed up, which causes it to cool.•This means it can hold less water vapor, so clouds form as it reaches its dew point.•This can cause rain and storms behind the front.•They move quickly, and bring cool, dry air after the front passes.

Cold Front Details

Warm Fronts•Moving warm air collides with cold air.•Tend to move more slowly than cold fronts.•Can bring cloudy skies, showers, and light rain preceding the front.•Weather will be warm and humid after it passes.

Warm Front Details

Stationary Fronts•Occur when cold and warm fronts meet and neither is strong enough to force the other to move.•It is a “standoff.”•The warm air will condense and form rain, fog, snow, or clouds.•Can cause many days of wet weather.

Occluded Fronts•Occur when a warm air mass is trapped between two cold air masses.•The warm air is pushed up when the two cold air masses meet.•It is cut off from the ground, or occluded.•Tend to produce strong thunderstorms.

Weather Map Symbols•These are the symbols you would see on a weather map.•The arrows or bumps point in the direction the front is moving.

Low Pressure Areas•Low pressure areas will have warm, rising air.•Cool air around it will spiral in counter clockwise (Coriolis effect).•As the air rises, it cools and reaches its dew point.•So clouds and rain will form.•Called cyclones from a Greek word meaning wheel.

High Pressure Areas•High pressure centers of dry air.•Spiral outward towards areas of lower pressure.•Coriolis effect causes them to spiral clockwise in the Northern Hemisphere.•Made of cool, falling air, so its relative humidity drops.•Produces clear, dry weather.•Called anticyclones.

Storms

Storms•A violent disturbance in the atmosphere.•Involve sudden changes in air pressure, which cause rapid air movement.

Thunderstorms•Heavy rainfall with thunder and lightning.•Form within cumulonimbus clouds.•Warm air rapidly rises, causing updrafts that bring moisture to the tops of the cloud.•The moisture condenses, and eventually forms rain.

Thunderstorm Stages

Thunder and Lightning•Friction between updrafts and downdrafts cause electrons to be stripped from some atoms and given to others. Just like walking across a carpet in your socks.•So the clouds develop charges.•Lightning is the spark when the discharge occurs.•Lightning is one big static electricity discharge!

Thunder• Lightning heats the air as it passes through

it.• This air is hotter than the sun, so it moves

rapidly.• This causes the loud, rumbling sound.• Light travels about a million times faster

than sound.• Sound travels roughly 1-km in 3 seconds or

1-mile in 5 seconds.• So if you see the flash of lightning, and it

takes 10 seconds for the thunder to reach you, the lightning was about 2 miles away.

Tornadoes•Rapidly rotating, funnel shaped cloud.•Most locally destructive of all storms.•Usually touch the ground for only a few minutes.•Wind speeds can be over 200mph!

Tornado Formation• Develop in thunderstorms.• If the rising updraft in a

thunderstorm changes direction, it can start to rotate.

• As this rotation tightens, it gets stronger but narrower.

Tornado Damage•Tornadoes are usually less than a 100 yards wide.•The damage they inflict is severe but localized.

Fujita Wind Damage Scale• F-5’s are the strongest, most

damaging tornadoes.

New enhanced scale is very similar.

Tornado Safety•Tornado Watch: conditions are favorable for tornadoes.•Tornado Warning: Tornado has been sighted. Take cover if it is in your area.•Basements are the best place if there is a tornado.

Tornado Alley•The area over which tornadoes occur most often in North America is called Tornado Alley. •It covers the Great Plains witch is between the Rocky Mountains and the Appalachian Mountains. It includes all or half of the 13 mid-west states.

Hurricanes• A tropical cyclone that occurs in the

Atlantic.• Also a generic term for low pressure

systems that develop in the tropics.• Starts as a tropical depression (winds

less than 39mi/hr).• Becomes a tropical storm and is given a

name when the winds exceed 39mi/hr.• Finally becomes a hurricane when the

winds reach 74mi/hr.

Hurricane Names• Since at least 1945, the US Navy and

later the Air Force started naming tropical cyclones.

• At first they used exclusively English female names, but since 1978 have started to alternate male and female names.

• Different areas of the world tend to use local names for their areas.

Atlantic Hurricane Names

How Hurricanes Form• Form in warm, tropical waters.• Water must be at least 80°F(27°C).• Needs warm, moist air and converging

winds.• Has a large difference in air pressure.• Formed by the heat energy and as long as

the water is warm are self-sustaining.• The moist, warm air circulates around a

well defined center.• The lower the pressure at the center, the

faster the winds will rush in to try to fill it.

Parts of a hurricane

Parts Continued• Eye: center of the hurricane. Weather

is calm, may be clear, and will have no rain.

• Winds will come from the opposite direction after the eye passes.

• Rain bands will move counter-clockwise around the eye. These bands with hurricane force winds can extend over 300 kilometers from the eye. So the storms can affect a wide area.

Hurricane Movement•Hurricanes are steered by the global winds.•So the storms in the tropics are steered to the west by the trade winds. When they get far enough north the westerlies take over and steer them east.•Once over land, they lose strength as they no longer have a source of warm water to draw energy from.•Friction with the land can slow the winds down also.

Hurricane Damage•Hurricane can cause enormous damage when they come ashore.•While high winds do a lot of damage, flooding is more serious.•Heavy rains cause flooding, especially if the hurricane is slow moving.•Storm surge is even more serious. It is a dome of water caused by low pressure and high winds. If it coincides with the high tide, many coastal areas will be devastated.

Katrina descends on Alabama

Katrina descends on Alabama