11.3 Learning Goals Define front. Contrast warm and cold fronts. Discuss the weather conditions associated with high-pressure and low-pressure systems. Describe the characteristics of clouds and predict the weather based on the appearance of clouds.

11.3 Air masses and fronts An air mass is a large body of air with consistent temperature and moisture characteristics throughout. Two air masses that affect the United States are the continental polar air mass and the maritime tropical air mass. Changing conditions and global winds cause these air masses to move.

11.3 Fronts A cold front occurs when cold air moves in and replaces warm air.

11.3 Fronts A warm front occurs when warm air moves in and replaces cold air.

11.3 Fronts On a weather map, a cold front is shown using a line marked with triangles. The triangles point in the direction the front is moving. A warm front is shown using a line marked with semicircles.

11.3 Low- and high-pressure areas When a cold front moves into a region and warm air is forced upward, an area of low pressure is created near Earth’s surface at the boundary between the two air masses. A center of high pressure tends to be found where a stable cold air mass has settled in a region.

11.3 Clouds A cloud is a group of water droplets or ice crystals that you can see in the atmosphere.

11.3 Types of clouds Different conditions cause different clouds. Cumuliform clouds include: cirrocumulus altocumulus cumulus cumulonimbus

11.3 Cloud formation cirrostratus altostratus stratus nimbostratus Stratiform clouds form when a large mass of stable air gradually rises, expands, and cools. Stratiform clouds include: cirrostratus altostratus stratus nimbostratus

11.3 Cloud formation Sometimes a cloud formation combines aspects of both cumuliform and stratiform clouds. We call these clouds stratocumulus clouds.

11.3 Cloud formation Cirrus clouds are thin lines of ice crystals high in the sky, above 6,000 meters. They are just a thin streak of white across a blue sky.

11.3 Thunderstorms Thunderstorms occur because of convection in the atmosphere. The downdraft and updraft form a type of convection cell called a storm cell within the cloud.

11.3 Lightning Lightning is a bright spark of light that occurs within a storm cloud, between a cloud and Earth’s surface, or between two storm clouds. It occurs when a spark travels between negative and positive charges.

11.3 Hurricanes A hurricane is a tropical cyclone with wind speeds of at least 74 miles (119 km) per hour. The Saffir-Simpson Hurricane Scale is one scale used for rating hurricanes.

11.3 Tornadoes A tornado, like a hurricane, is a system of rotating winds around a low-pressure center. As the rotating wind pattern narrows and lengthens, it forms a funnel cloud.

11.3 Funnel clouds When updrafts in a storm cell reach high speed, they begin to rotate. As the diameter of the rotation narrows and extends downward, a funnel cloud takes shape.