16.2 Weather Patterns

Air Mass A large body of air that contains properties similar to the part of the Earth’s surface where it developed.  Air masses developed over Land = Dry Water = Moist  Density, moisture and temperature are properties of air masses.  A change in weather is due to the movement of air masses.

Atmospheric Pressure Changes over the Earth’s surface.  Winds blow from high pressure to low pressure.  High pressure areas experience fair weather  Low pressure areas experience cloudy weather.  Falling temperature means precipitation is expected.

Fronts A boundary between two air masses is called a front. Types of fronts include cold, warm, occluded, and stationary.

Cold Front A cold front occurs when colder air advances toward warm air. Cold Fronts are shown on maps using blue lines and blue triangles. The triangles point to the direction the cold front is moving.

Warm Front Warm fronts occur when a warmer air mass moves toward an area of colder air. Warm fronts are shown on weather maps with the red lines and red semicircles. The semicircles point in the direction the front is moving.

Occluded Front An occluded front involves three air masses of different temperatures. An example of how an occluded front forms is when a cold air mass moves toward cool air with warm air in the middle. Occluded fronts are shown using purple lines with triangles and semicircles.

Stationary Front A stationary front occurs when a boundary between air masses stops advancing. A stationary front boundary is shown by alternating blue triangles (cold air) and red semicircles (warm air). The blue triangles point to the colder air mass and the red semicircles point to the warmer air mass.

Thunderstorms Severe thunderstorms occur when warm air masses collide with colder air masses. The warm less dense air is pushed up over the cooler air mass. The greater the temperature difference between the air masses, the more severe the storm can be.

Lightning When electric current flows between regions of opposite electrical charge, lightning flashes. These areas of opposite electrical charge can occur within a cloud, between clouds, or between a cloud and the ground.

Thunder and Lightning Thunder results from the rapid heating of air around a bolt of lightning. Lightning can reach temperatures of 30,000 degrees Celsius. The extreme heat causes air to expand rapidly then cools quickly and contracts. This rapid movement of air molecules forms sound waves and thunder is heard.

Tornados A tornado is a violently rotating column of air in contact with the ground. Winds at different heights blow in different directions and different speeds. This difference, called wind shear, creates a rotating column parallel to the ground. This rotating column is called a funnel cloud.

Hurricanes A hurricane is a large, swirling, area of low pressure that forms over the warm Atlantic Ocean. A hurricane turns heat energy from the warm ocean in to wind. A storm must have wind speeds higher than 119 kilometers per hour (75 mph) to be called a hurricane.

Blizzards Severe storms in winter are called blizzards. A blizzard occurs if winds are 56 km/h, (35 mph) the temperature is low, and visibility is less than 400 meters or a quarter of mile. These conditions must exist for more than three hours to be classified as a blizzard.

Severe Weather When severe weather occurs STAY INSIDE. The National Weather Service issues watches and warnings. Watches are issued when conditions are favorable for severe weather. Warnings are issued when severe weather conditions already exist.