Air Masses and Fronts

What is an Air Mass?

• Air masses are large bodies of air which have similar temperature and moisture characteristics.

• Air masses form when air stays over a region (called the source region) for several days.

• Air masses that form over water will be moist.

• Air masses that form over land will be dry.

Latitude determines temperatureMoisture determined by whether it forms

over oceans or continents

c continental = dry m marine = moist

A = Arctic

P = Polar

T = tropic

Continental Polar does not move west

Fronts – boundary that separates 2 air masses with different temperatures

Cold Front a. short-lived thunderstorms b. move faster than warm fronts

Warm Front – longer, steady precipitation

Stationary Front

• A stationary front occurs when the air masses on either side of the front are not moving toward each other.

Counter clockwise rotation into Low

Storm moves northeast due to prevailing winds

How Mid-Latitudes

Lows(occluded

fronts) Form

Counter-Clockwise

rotation

OCCLUDED FRONTWarm air mass gets caught between 2 colder air masses and is forced aloft

Occluded Front

Winds spiral clockwise out of the High and counterclockwise into the Low

Fronts are low pressure – lousy weatherAfter the front passes – happy weather

Weather Changes after Fronts Pass

• 2 TOTALLY different air masses so a change in weather

• Temperature changes• Wind direction changes• Wind velocity changes• Dew Point changes• Barometer falls when front approaches and

rises after it passes

BRRR!

WINDS FLOW COUNTERCLOCKWISE IN TO THE LOW PRESSURE SYSTEM – RISING AIR

WINDS FLOW CLOCKWISE OUT OF THE HIGH PRESSURE SYSTEM – SINKING AIR

Hurricanes – counterclockwise low pressure systems (lousy weather)

Weather in a High

• Clockwise Rotation

• Bright, Clear

• Happy Weather

Box 9.2

LAKE EFFECT - SNOW

Thunderstorm Facts

• At any given time there are an estimated 2000 thunderstorms in progress, mostly in     tropical and subtropical latitudes. - About 45,000 thunderstorms take place each day

• Annually, The U.S. experiences about 100,000 thunderstorms.

• About 16 million thunderstorms occur annually around the world!

• The lightning from these storms strikes Earth about 100 times each second

Lightning• Within the thunderstorm

clouds, rising and falling air causes turbulence which results in a build up of a static charge. The negative charges concentrate in the base of the cloud.

• This first, invisible stroke is called a stepped leader.

• As soon as the negative and positive parts of the stepped leader connect there is a conductive path from the cloud to the ground and the negative charges rush down it causing the visible stroke.

Tornadoes

• Area of quickly rotating air associated with a thunderstorm

• Some contain winds of over 300 miles per hour

• They can be as small as a hundred yards wide or larger than a mile wide

Development of a Funnel

Damage

Severe Weather Warnings

• Tornado, Severe Thunderstorm, and Flash Flood Warnings

• Severe weather is occurring

• If it is for your area, take shelter right away

• Don’t leave shelter until warning has expired

Severe Thunderstorm Warnings• A thunderstorm with winds of 58 mph or

greater

• Penny sized hail or larger

• Also contain lots of lightning

• Stay inside

• Keep off the phone

• Don’t take a shower or bath

• Stay away from windows

Tornado Warnings

• A tornado or funnel cloud has been detected

• Take shelter RIGHT AWAY!!!

• Get under something sturdy in your basement

• No basement? Go to a room that has no windows like a closet or bathroom

• In mobile homes, leave them and go to a storm shelter if available or lie flat in a ditch

Where we get data

• Weather Stations• Radar• Satellites

Doppler Radar

Next Generation Radar (Nexrad)