Weather 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 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 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: 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 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: 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 herehere Hail Video – click herehere

Weather 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 EF5 Joplin Tornado How a tornado forms! Moore Tornado 2013

Weather 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: 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 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.