Section 17.1 Start Air Masses Are Bodies of Air Air Mass – A large volume of air in which temperature and humidity are nearly the same in different locations at the same altitude. An air mass forms when the air over a large region of Earth sits in one place for many days. The air gradually takes on the characteristics of the land or water below it. Earths surface can affect the air on Earth.

The Four Types of Air Masses Continental air masses form over land. Air becomes dry as it loses moisture. Maritime air masses form over water. Air becomes moist as it gains water vapor. Tropical air masses form near the equator. Air becomes warm as it gains energy from the warm land or water below. Polar air masses form far from the equator. Air becomes cool as it loses energy from the cold land or water below.

The Four Types of Air Masses

Movement of an Air Mass Air masses can travel away from the regions where they form. They move with the global pattern of winds. When an air mass moves to a new region, it carries along its characteristic moisture and temperature.

Cold Front Front – A boundary between air masses. A cold front forms when a cold air mass pushes a warm air mass and forces the warm air mass to rise. Cold fronts often produce cumulonimbus clouds, which can cause heavy storms. Cold Front

Warm Front A warm front forms when a warm air mass pushes a cold air mass and the warm air rises slowly over the cold air. As a warm front approaches, you may first see high cirrus clouds, then stratus clouds, then lower stratus clouds. Warm fronts often brings many hours of steady rain or snow. Warm Front

Stationary Front A stationary front occurs when 2 air masses push against each other without moving. A stationary front becomes a warm or cold front when one air mass advances. Stationary Front

High Pressure Systems Formed when air moves all the way around a high-pressure center. Air sinks and spreads out towards locations with low pressure. Generally brings clear skies and calm air or gentle breezes.

Low Pressure Systems A large weather system that surrounds a center of low pressure. Air circles and moves upward. The rising air produces stormy weather.

Section 17.2 Start Hurricanes form over warm ocean water Tropical storm – a low-pressure system that starts near the equator and has winds that blow at 65 km per hour. Hurricane – a tropical low-pressure system with winds blowing at speeds of 120 km per hour.

Formation of Hurricanes Hurricanes most often strike between the months of August and October. Ocean water has to be 80oF or higher for a hurricane to form. Tropical storms and hurricanes normally move west due to the trade winds. Hurricanes die out when they move over land.

Formation of Hurricanes The center (eye) of a hurricane is calm. The eye wall (surrounds the eye) has heavy rain and tremendous winds.

Effects of Hurricanes Hurricanes can lift cars, uproot trees, and tear the roofs off buildings due to strong winds. Heavy rain can also cause severe flooding. Storm surge – A rapid rise in water level in a coastal area that occurs when a hurricane pushes a high mass of ocean water, often leading to flooding and widespread destruction.

Winter storms produce snow and ice Winter storms are formed when 2 air masses collide (a cold, dry, dense air mass and a moist air mass). Blizzards – Blinding snowstorms with winds at least 56 km per hour and low temperatures. Lake-Effect Snowstorms – Cold air gains moisture and warmth as it passes over a lake and then moves over cold land where it cools again and snows.

Winter storms produce snow and ice Ice Storms – When rain falls onto freezing-cold ground and covers everything with heavy, smooth ice.

Section 17.3 Start Thunderstorms form from rising moist air Thunderstorm – A storm with lightning and thunder. Can form at a cold front or within an air mass.

Formation of Thunderstorms: Step 1 Rising humid air forms a cumulus cloud. The water vapor releases energy when it condenses into cloud droplets. This energy increases the air motion. The cloud continues building up into the tall cumulonimbus cloud of a thunderstorm.

Formation of Thunderstorms: Step 2 Ice particles form in the low temperatures near the top of the cloud. As the ice particles grow large, they begin to fall and pull cold air down with them. This strong downdraft brings heavy rain or hail – the most severe stage of a thunderstorm.

Formation of Thunderstorms: Step 3 The downdraft can spread out and block more warm air from moving upward into the cloud. The storm slows down and ends.

Effects of Thunderstorms Flash floods – heavy rainfall in the same area that can cause rivers to overflow their banks. Winds – winds as high as 170 mi/hr that can knock down entire forests. Hail – can wipe out entire fields, damage roofs, and kill livestock. Lightning – damage power lines and equipment, cause forest fires, and kill people.

Tornadoes form in severe thunderstorms Tornado – A violently rotating column of air stretching from a cloud to the ground. More tornadoes occur in North America than anywhere else in the world. Occur when warm air masses meet cold, dense air and form thunderstorms normally in the central plains of the United States.

Effects of Tornadoes Powerful winds can pick up or destroy everything in the path of a tornado. Most tornadoes are small and last only a few minutes. Larger tornadoes are less common but have stronger winds and last longer. The path of a tornado is unpredictable.