Fronts Chapter 21 Section 2

Front: boundary between air masses of different densities. Fronts can be hundreds to thousands of kilometers long. Cooler air is denser and often does not mix with warmer air so a front forms.

Cold Front: Cold air mass moves into a warm air mass. It pushes underneath the warm air & lifts it. Lots of cold air arrives at once. Warm air cooling makes clouds & rain.

Fast moving cold fronts often form cumulonimbus clouds…can result in short lived violent storms. Squall line: line of heavy thunderstorms that form ahead of a fast cold front.

Cold front: shown as a blue line with triangles pointing toward warmer air.

Cold front moves in: sudden short rainstorm, then temperature drops.

Warm Front: Warm air moves over a cold air mass. Warm air rises as a gentle slope. Clouds can form way beyond the base of the front so warm fronts cause precipitation over large areas.

Warm fronts form clouds and rain over a long distance.

Warm front: shown as a red line with semicircles pointing toward colder air.

Warm front moves in: lots of rain, then warmer weather.

Stationary Front: Forms when two air masses move very slowly or not at all Air masses may move parallel to each other. Rain where fronts meet. Symbol combines cold & warm front symbols.

Occluded Front: Fast moving cold front completely overtakes a warm front and lifts it off the ground.

Polar Front In the main area of the US, between 60o and 30o N, weather is mainly caused by the polar front between two large wind belts.

Mid-latitude Cyclones Large, rotating storms often occur in the middle latitudes where the polar front meets warmer air.

Mid-latitude cyclone vs hurricane Mid-latitude cyclones are similar to hurricanes, but much larger and less intense.

Anticyclones Anticyclones blow outward from a high pressure area, the opposite of a cyclone.

Thunderstorms have three stages: Cumulus – warm, moist air rises (updraft) & forms cloud. Mature – cumulonimbus forms; air pulled downward by rain. Anvil appears. Torrential rain and hailstones. Dissipating – no more warm air rising, only downdraft. Water vapor decreases.

Lightning – go to NOAA for animations http://www.lightningsafety.noaa.gov/science/science- overview.shtml

Lightning an electrical spark in the atmosphere, or between the atmosphere and the ground. Electricity is a stream of moving electrons. Air is an insulator that keeps electricity from flowing easily. So the charge builds up until it can zap across.

Lightning: A charge is formed Particles of ice and hail bounce around in the cloud. Ice crystals become positively charged and rise to the top of the cloud. Hailstones become negative and drop to the bottom.

Lightning The charge on the bottom of the cloud attracts positive charges on the ground.

Lightning: between clouds, between bottom of cloud and ground.

Positive lightning can occur between the top of the cloud and the ground.

Positive lightning has farther to go; it is rare and stronger than negative lightning. Positive lightning strikes cause forest fires. Negative lightning doesn’t.

Forestry departments have machines and watchtowers to record lightning data and find fires early.

Hurricanes – most destructive storms Develop over tropical oceans. Winds greater than 120 km/hr, spiralling in towards the low pressure center.

Hurricanes cause huge storm surges. Most hurricane deaths are caused by drowning.

Hurricane Formation: Warm moist ocean air rises rapidly & condenses Hurricane Formation: Warm moist ocean air rises rapidly & condenses. Large amounts of latent heat are released, which strengthens the rising air.

Hurricane structure Thick cumulonimbus clouds spiral upward around a very low pressure center and cause the eyewall winds to increase speeds. The center (eye) is clear, sinking dry air.

Hurricane Ratings How do we rate them? The Saffir-Simpson scale (uses several factors: pressure, wind speed, storm surge) Categories are from 1-5 http://www.nhc.noaa.gov/a boutsshws.php

Alerts Hurricane conditions possible within 48 hours. Hurricane Watch Hurricane Warning Hurricane conditions possible within 48 hours. Hurricane conditions expected within 36 hours.

Hurricane History Hurricane Andrew, 1992 Hurricane Wilma, 2005

Tornadoes – highest speed winds destructive, rotating column of air with VERY high wind speeds - up to 400 km/h. If it touches the ground, it follows a haphazard (wandering) path.

Forms when a thunderstorm meets high horizontal winds.

Most common in Tornado Alley.

Most deaths are from being trapped in collapsing buildings and flying debris.

Alerts Tornado Watch Tornado Warning Weather conditions are favorable for severe thunderstorms that can produce tornadoes. Severe thunderstorms with tornadoes are imminent or occurring.

Tornado In Coconut Creek, Broward County, FL Jan 27, 2016