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Air Masses, Weather Fronts and Severe Weather Air Mass Large volume of air with similar temperature and humidity that can cover entire continents or oceans Several types that each have specific characteristics Types of Air Masses mP- maritime Polar: cold air masses that form over the oceans near the poles Cold and Moist Air cP- continental Polar: cold air masses that form over land near the poles Cold and Dry Air Types of Air Masses Cont’d mT- maritime Tropical: warm air mass that is formed over the oceans near the equator Warm and Moist Air cT- continental Tropical: warm air mass that is formed over land near the equator Warm and Dry Air Effects of Physical Features Physical features of the Earth, such as, lakes, valleys, and mountains can affect the movement of air masses Atmospheric Lifting Type #2 Orographic Lifting An air mass is pushed upward over an obstacle, such as a mountain range. Atmospheric Lifting of Air Masses over a Mountain As the air mass is pushed over the mountain, the air mass will lose moisture in the form of clouds, rain, or snow Atmospheric Lifting Type #2 Atmospheric Lifting Type #2 Atmospheric Lifting The clouds dump their load of rain on the windward slope The down slope (leeward) air will be dry = rain shadow Rain shadow- Rain shadow Western Kansas is so dry only certain short grasses can grow; not enough rain for trees Rain Shadow; Big Hatchet Mt, New Mexico. Notice how dry the land is Weather Fronts This is the boundary between two air masses with different temperatures. Weather fronts are the site where most weather takes place. There are four types of weather fronts. Cold Front When a cold air mass moves into a warm air mass. Cold, denser air forces the warm air up where it cools and condenses, forming clouds Cold Front: Warm air is quickly pushed upward, cooling, condensing moisture into cumulus or cumulonimbus clouds Cold Front Notice how steep the angle is between the two air masses Typically brings sudden, heavy rains and storms Cumulonimbus Clouds of a Cold Front Cumulonimbus: Lake Superior Warm Front WARM FRONT: when a warm air mass moves into a colder,denser air mass. Warm air rides up and over the colder air Warm front Warm Front Notice the angle of slope between the two air masses. Warm Front The weather during a WARM FRONT starts with cirrus clouds about 24-48 hours before the rain begins Cirrus clouds are “at the front of the front” Warm Front As the warm front approaches, heavier clouds form over the cold air These are stratus clouds. Warm Front Warm fronts bring light rain or snow that can last for several hours or days Stationary Fronts Two air masses come together but neither one is pushed out Can last several days Stratus and nimbostratus clouds produce steady rain Stationary Front Occluded Front A mass of warm air gets trapped between two masses of cold air Warm air gets squeezed out Produces stratocumulus clouds and heavy rain or snow Severe Weather Thunderstorms Cold Fronts Begins with humid air rising, cooling, and condensing into a single cumulus cloud. Cloud builds as they are “fueled” by warm, moist air from below. Thunderstorms The droplets of water grow larger until they are so big that they fall as rain. Cumulonimbus clouds: L Superior Thunderstorms (con’t) Lightning may be associated with T.storms Hits Earth 100 times per second Over 200 people in the US die each year from lightning. Lightning Lightning is the cause of thunder Returns nitrogen to the soil Frequently causes forest fires Lightning Formation During thunderstorms strong updrafts cause molecules to bump together and their charges to separate Negative charges concentrate at the base of the clouds Lightning Formation The ground has lost electrons, giving it a positive (+) charge Opposites attract: The (-) from the cloud base is “pulled” to the (+) First downward movement of (-) is called a leader Lightning Formation When the (-) and (+) touch, they create a conductive path to the ground. The other (-) rush down this path = lightning Lightning on L.Superior Lightning on L.Michigan Lightning strike Tornadoes A funnel shaped cloud that extends downward from a cumulonimbus cloud. Called a tornado only once it hits the ground. Winds inside travel at up to 800km per hour in counterclockwise direction. Tornado: February 25, 2008 Hurricanes Caused when a warm, vertical wind mixes with a storm and causes the storm to turn inward and begin spin. Wind speeds of up to 180miles per hour. Hurricanes grow as more moist air rises into it. Strong winds rotate around a calm, low pressure zone in the middle, which is the eye of the storm. Hurricane Mitch Requirements Tropical Depressionwind speeds <45 mph Tropical Storm Wind speeds >45 mph but <74 mph Hurricane Wind speeds >74 mph