CH 20 Patterns Severe Weather AIR MASSESare immense

CH 20 Patterns & Severe Weather • AIR MASSES—are immense bodies of air. • Air Masses—are characterized by 2 things: 1. Temperature 2. Moisture Content They can be 1600 km (1000 miles) across!

4 Air mass letter symbols: • • (P) = polar, cool or cold (T) = tropical, warm or hot (c) = continental, land = dry (m) = maritime, ocean = moist • Choose 1 from P or T & 1 from c or m.

• What air mass usually brings us precip. ? • What 2 nd air mass affects Asheville? • What air mass affects the northwest? • What air mass causes lake effect snow? • What air mass causes Indian Summers?

`20. 2 FRONTS = are boundaries between 2 air masses! • Fronts are much narrower than air masses—usually 15 -200 km wide. • 4 TYPES OF FRONTS:

1. • • WARM FRONTS Warmer air moves into area of cooler air Symbol red line with semi-circles Gently sloped boundary & move slow Steady (long-term) light to moderate precipitation & warmer temperatures. • Cloud sequence—cirrus, cirrostratus, alto stratus, & then nimbostratus.

`2. COLD FRONTS • • Colder air moves into area of warmer air Symbol blue line with triangles Steeper sloped & move fast Heavy (short-term) precipitation, even severe thunderstorms or tornadoes • Cloud sequence—cumulus begin to vertically develop into cumulonimbus = anvil head clouds!

3. STATIONARY FRONTS • Movement is parallel to the line of the front. (no forward movement) • Surface position does not move • Symbol is blue triangle on one side and red semi-circles on the other side • Weather is similar to a warm front but usually last even longer!

`4. OCCLUDED FRONTS • Occur when an active (fast) cold front overtakes a warm front. • Symbol is either purple triangles & semi -circles or just purple semi-circles on the same side of the line • Weather is (at first) similar to a cold front and then ends up lasting longer with a steady rain.

Mid-Latitude Cyclones! • M-L C. are the main stormy weather producers for the United States. • M-L C. are large low pressure centers. • 6 Formation Steps: (page 569)

` • 1. Formation of a stationary front. • 2. Wave shape (indentation) forms. • 3. Counterclockwise flow (separation of front into 2 fronts (cold & warm). • 4. Cold front catches up to warm front & an occluded front forms. • 5. You now have 3 fronts. • 6. The low or cyclone weakens.

20. 3 Severe Weather!!!

Thunderstorms • Static electricity forms in cumulonimbus clouds. • High winds, hail, heavy rains, L & T. • Lightning bolt starts from ground goes up. • 5 -6 times hotter than sun. • One inch in diameter.

Tornadoes • • • Season is in spring & summer. Tornado alley—c. P & m. T clash. Unstable air—forms mesocyclone Doppler Radar- or visual. Fujita scale F 0 -F 5 Small intense storms, usually last for short time.

Hurricanes • • Season is in late summer & fall. Occur where water is warm. Form from tropical storms. Alternate male & female names. Some names (Katrina) retired. Saffir-Simpson scale Category 1 -5 Storm surge, winds, & flooding.

Sandy Oct. 2012

Katrina August 2005 Over 1, 800 people dead

Hurricane Floyd September 1999 • 52 deaths and 8, 000 homes destroyed