Forecasting Weather http://www.usatoday.com/weather/basics/wworks0.htm

What are Fronts? A boundary between two air masses is called a __Front_________ The shape of the _Front_______ depends on the density of the air masses and their speed

When a front forms: Pressure falls when a front approaches (low pressure edges of air masses move in) Changes in wind direction occur. High pressure air moves Clockwise, low pressure air moves Counterclockwise Temperature changes occur Precipitation occurs and can be in the form of strong storms

Low pressure is associated with the formation of clouds and precipitation. Warm air rises and when it reaches a certain elevation it cools and become saturated. Clouds form and when they are saturated, it precipitates

There are four main types of fronts

Warm Front: Warmer air invades a cold air mass and is lifted over and replaces cooler air. Cirrus and stratus clouds are associated with warm fronts Gentle rain for days, slow clearing, and rising temperatures follow

Cold Front Cooler air invades a warm air mass and moves under the warm air and replaces it Cumulus clouds and thunderstorms produce HEAVY RAIN for a short period of time Cooler temperatures and clear and fair weather invade the area once the storms have passed

Stationary Front Occurs when pressure differences cause a warm front or cold front to stop moving It may remain in the SAME area for a few days. Weather conditions include light winds and precipitation across the entire frontal region.

Occluded Front Results from two cool air masses merging and forcing warmer air between them to rise Strong winds and heavy precipitation may occur

How do weather systems move? Weather systems move across North America from West to East.

Weather forecasting Making predictions about future weather based on weather data.

Weather Maps Provide data from stations all over the earth Help meteorologists forecast weather Data included: Temperature, air pressure, change in air pressure during the last three hours, wind speed, direction, dew point, visibility, cloud cover, cloud types and precipitation.

Isobars Isobars - pressure lines drawn on weather maps to connect places having equal air pressure. Isobars that are close together indicate a large pressure difference over a small area

Isotherms Isotherms are lines drawn to connect places having equal temperatures

High and Low Pressure Pressure is indicated in the middle of the isobars H stands for a high pressure center L stands for a low pressure center. Winds develop and move from high to low pressure areas.

Technology and Weather Forecasting Satellites photograph sections of the earth’s surface and show cloud coverage

Air Mass A large body of air that has the same properties as the surface over which it develops An air mass over the GULF of MEXICO would be warm and moist An air mass over CANADA would be cool and dry

Air masses Cool and dry --> high pressure, low humidity CANADA Warm and moist --> low pressure, high humidity GULF of MEXICO

Thunderstorms Occur inside warm moist air masses and fronts Result when warm moist air rises quickly and cools and condenses rapidly Heavy raindrops fall, dragging air with them, creating strong winds Produced by cumulonimbus clouds

Lightning Atoms of air which are caught in the strong winds lose and gain electrons, creating cloud regions which are positively or negatively charged Currents flow between these regions resulting in lightning

Thunder Rapid heating and cooling of air due to lightning (rapid expanding and contracting)

Tornado Forms along fronts Wind at different heights blows in different directions and at different speeds This along with a strong updraft produces a rotation

Hurricanes A low pressure area develops where the ocean water is very warm Trade winds blowing in opposite directions meet causing a swirl of air that is rotating As moist air rises over the low pressure system, it cools and condenses and begins to descend This pattern is repeated over and over

Clouds - condensed water vapor Recipe for a Cloud: Water vapor Condensation nuclei (dust, smoke, ice, salt) Relative humidity of 100% - saturation point or dew point

Types of Clouds: Stratus fair weather or precipitation Layers of sheets of clouds, can be dull gray blanket of clouds Low altitude clouds See pages 425-427

Types of Clouds: Cumulus White puffy clouds with flat bases Formed when moist air rises Fair weather or thunderstorms See pages 425-427

Types of Clouds: Cirrus Wispy High altitude Contain ice crystals Fair weather See pages 425-427