Weather PatternsWeather Patterns• Air Mass: A large

body of air that has properties similar to the part of Earth’s surface over which it develops.

• Air masses cover thousands of square miles.

• Six major air masses affect weather in the Unites States.

High Pressure

•Winds blow away from a center of high pressure.

•These winds spiral clockwise in the northern hemisphere.

•High-pressure areas are associated with fair weather and are called anticyclones.

Low Pressure

•Winds blow into a low-pressure area in the northern hemisphere and swirl in a counter-clockwise direction.

•Low pressure systems are called cyclones and are associated with stormy weather.

Weather PatternsWeather Patterns•Air pressure is measured using a barometer.

•Weather has high- and low-pressure systems.

•Winds blow from areas of high to areas of low pressure.

Weather PatternsWeather Patterns

• Low pressure systems at Earth’s surface are regions of rising air.

• Clouds form when air is lifted and cools.

• Areas of low pressure usually have cloudy weather.

• Sinking motion in high-pressure air masses makes it difficult for air to rise and clouds to form.

• That’s why high pressure usually means good weather.

Weather Patterns: FrontsWeather Patterns: Fronts

• A boundary between two air masses of different density, moisture, or temperature is called a front.

• Cloudiness, precipitation, and storms sometimes occur at frontal boundaries.

• Four types of fronts include cold, warm, occluded, and stationary.

A cold front forms when a mass of cold air meets & pushes warm air up rapidly.. Violent storms are associated with a cold front. Fair, cool weather usually follows.

Cold Front

A warm front forms when a mass of warm air overtakes a cold air mass and moves over it. Rain and showers usually accompany a warm front. Hot, humid weather usually follows.

Warm Front

When a cold front overtakes a warm front, an occluded front forms.

As the warm air is pushed upward, the cold air meets cool air. An occluded front may also occur when cool air overtakes a cold front and warm air is pushed upward. An occluded front produces less extreme weather than a cold or warm front.

Occluded Front

When a warm air mass meets a cold air mass and no movement occurs, a stationary front forms. Rain may fall in an area for many days when a stationary front is in place.

Stationary Front

Weather Patterns: Severe Weather Patterns: Severe WeatherWeather• Thunderstorms can stall over

a region, causing flash flooding.

• Flash floods can occur with little warning.

• Strong Winds >89mph are classified as severe.

• Hail can destroy property and farm crops.

• Lightning

• Tornadoes

Weather Patterns: LightningWeather Patterns: Lightning• In a storm cloud, warm air is lifted rapidly as cooler air

sinks. This movement of air causes different parts of the cloud to become oppositely charged.

• When current flows between regions of opposite electrical charge, lightning flashes.

•occurs within clouds, b/w clouds, or from cloud to ground.

•Thunder results from rapid heating of air around a bolt of lightning. Temperature can reach 30,000°C!

•The extreme heat causes air around the lightning to expand rapidly, then quickly cools and contracts.

•rapid movement of molecules form sound waves heard as thunder.

•Thunder-snow video, click here

• Hail Video – click here

Weather Patterns: TornadoesWeather Patterns: Tornadoes• Tornado: A violently rotating column of air in contact

with the ground.

• Wind Shear: Wind at different heights blows in different directions and at different speeds. This can create a rotating column parallel to the ground.

• Updraft tilts the rotating column upward into the thunderstorm creating a funnel cloud.

• If the funnel comes into contact with the ground, it is called a tornado.

• Although tornadoes rarely exceed 200m in diameter and usually last only a few minutes, they often are extremely destructive.

2011 EF5 Joplin TornadoHow a tornado forms!

Moore Tornado 2013

Weather Patterns: HurricanesWeather Patterns: Hurricanes• Steered by surface winds, they can travel west, gaining

strength from the heat and moisture of warm ocean water.

• Damage: High winds, tornadoes, heavy rains, lightning, high waves, and flooding can destroy crops, demolish buildings, kill people and animals.

• When over water, the warm, moist air rises and provides energy for the storm.

• When it reaches land, its supply of energy disappears and the storm loses power.

Weather Patterns: BlizzardsWeather Patterns: Blizzards

The National Weather Service classifies a winter storm as a blizzard if:

1.The winds are 56 km/h.

2.The temperature is low.

3.The visibility is less than 400 m in falling or blowing snow.

4.These conditions persist for three hours or more.

Severe Weather SafetySevere Weather Safety• When severe weather threatens, the National Weather

Service issues a watch or warning.

• Watches are issued when conditions are favorable for severe thunderstorms, tornadoes, floods, blizzards, and hurricanes.

• METEROLOGISTS ARE ASKING YOU TO WATCH AND SEE WHAT WEATHER MAY FORM.

• When a warning is issues, severe weather conditions already exist.

• THEY ARE WARNING YOU TO GET AWAY FROM THE DANGEROUS WEATHER!!!

• You should take immediate action.