17.1 – Air Masses and Fronts An air mass is a huge body of air that has similar temperature, humidity, and air pressure throughout.

Types of Air Masses Scientists classify air masses according to temperature and humidity. There are four major types of air masses that influence weather in N. America.

Maritime Tropical – Warm, humid air masses that form over the oceans and Gulf of Mexico They travel up from the south bringing warm, humid air.

Bring showers and thunderstorms in the summer. Bring snow in the winter.

2. Maritime Polar Cool, humid air masses that form over the North Pacific and Atlantic Oceans. They affect the West Coast more than the East Coast.

They bring rain, and cool temps to the west coast.

3. Continental Tropical Hot, dry air masses that form only in summer over areas in the SW U.S. and N. Mexico. They move NE, bringing hot dry weather.

4. Continental Polar Large air masses that form over N. Canada and Alaska. Bring cool, dry air to much of North America.

FRONTS As huge masses of air move across land and water, they bump into each other.

When 2 masses with different temps and density collide, they do not mix. The area where the air masses meet and do not mix becomes a “front”

A front forms because one air mass is less dense than the other air mass. There are 4 types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts.

Cold Fronts

Cold fronts form when a rapidly moving cold air mass runs into a slowly moving warm air mass. Often results in the formation of clouds, rain, and/or snow. Also, thunderstorms.

Warm Fronts Forms when a rapidly moving warm air mass collides with a cold air mass. Can cause rain or fog. Slower moving than cold front.

Stationary Fronts Forms when a warm air mass and a cold air mass meet and no movement occurs. Neither air mass has enough force to move the other.

Occluded Front Forms when a warm air mass is caught between two cooler air masses. The warm air mass gets lifted and cut off from the ground by the two cooler air masses.

17.2 - Storms A swirling center of low air pressure is called a cyclone. As warm air at the center of the cyclone rises, cooler air blows toward this from areas with higher pressure.

The winds swirl in a counter-clockwise fashion b/c of the Coriolis Effect. Cyclones and low pressure areas are associated with storms and precipitation.

Anticyclones are the opposite of cyclones. They are areas of high pressure that are associated with dry and clear weather. Air blows from high pressure areas to low pressure areas.

Kinds of Storms A storm is a violent disturbance in the atmosphere. Storms involve sudden changes in air pressure, which cause rapid air movements.

Hurricanes A hurricane is a tropical storm that has winds of 74mph or greater. They begin over warm water as a low pressure area.

The center of a hurricane is a ring of clouds surrounding a quiet “eye” When the eye arrives, the winds grow calm and the sky may clear.

Hurricanes can last for over a week. They get their energy from the warm, humid air at the ocean’s surface. When they get over land, they begin to lose energy.

Tornadoes form in low, heavy cumulonimbus clouds. They form when cold, dry air mass from Canada collides with a warm, humid air mass from the Gulf of Mexico.

During a tornado, the safest place to be is in the basement of a well-built building.