Weather-current state of the atmosphere,

The Causes of Weather

Climate-average weather over a long period of time (30 years or more)

What we now know:

Difference between weather and climate.

We know the atmosphere redistributes heat around the world to bring it into balance.

The Causes of Weather

– Weather is part of a constant redistribution of Earth’s heat energy

What we know about weather:• High pressure air masses sink/low pressure air

masses rise

• Air moves from high pressure to low pressure to create wind

• High and low pressure air masses are created by differences in pressure, temperature, density

• Different types of air masses affect weather in the United States

Different Air MassesForm Over DifferentRegions

The Causes of Weather

– An air mass has similar temperature and moisture content as the area over which it formed.

We Know Some Air Masses Are More Stable Than Others

The Causes of Weather

We Know That Air Masses Move To Transfer Heat, Including Latent Heat

The Causes of Weather

– An air mass exchanges heat or moisture with the surface over which it travels until its characteristics are about the same as the new surface over which it is traveling.

We Know a Front Separates Two Different Air Masses That Are Colliding

Weather Systems

• That the two air masses are of different densities caused by differences in temperature, pressure, and humidity

• That the interaction between the colliding air masses can bring dramatic changes in weather

Weather Systems

• Cold, dense air displaces warm air and forces the warm air up along a steep front.

• Clouds, showers, and sometimes thunderstorms are associated with cold fronts.

• A cold front is represented on a weather map as a solid blue line with blue triangles that point in the direction of the front’s motion.

We Know About Cold Fronts:

We Know About Warm Fronts:

Weather Systems

• Advancing warm air displaces cold air.

– The warm air develops a gradual frontal slope rather than a steep boundary.

– A warm front is characterized by extensive cloudiness and light precipitation

– On a weather chart, a warm front appears as a solid red line with regularly spaced, solid red semicircles pointing in the direction of the front’s motion.

About Stationary Fronts:

Weather Systems

– A stationary front is the result of two air masses meeting and neither advancing into the other’s territory, stalling the boundary between them.

– Stationary fronts seldom have extensive cloud and heavy precipitation patterns.

– A stationary front is represented on a weather map by a combination of short segments of cold- and warm-front symbols.

We Know the Earth Rotates• East to West (counterclockwise)

• How does the rotation of Earth affect the movement of air?

Earth’s Rotation Creates the Coriolis EffectCoriolis effect- causes moving particles such as air to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere.

-combines with the heat imbalance found on Earth to create distinct global wind systems that transport colder air to warmer areas and warmer air to colder areas.

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Weather Systems

Global Wind Systems

Weather Systems

Global Wind Systems• Three basic wind systems, in each hemisphere.

Weather Systems

The trade winds- flows at 30° north and south latitude, where air sinks, warms, and returns to the equator in a westerly direction (convection current)

• Around 30° latitude the sinking air associated with the trade winds creates a belt of high pressure that in turn causes generally weak surface winds.

Global Wind Systems

Weather Systems

• Near the equator, an area of low pressure is created over a large area called the doldrums.

• This area is characterized by a band of cloudiness and occasional showers.

Global Wind Systems

Other Wind Zones

Weather Systems

– Prevailing westerlies, flows between 30° and 60° north and south latitude in a circulation pattern opposite that of the trade winds.

– The prevailing westerlies are responsible for much of the movement of weather across the United States and Canada.

– The polar easterlies, lies between 60° latitude and the poles. In both hemispheres, the polar easterlies are characterized by cold air.

Jet Streams: high-altitude, westerly winds

Weather Systems

– The polar jet stream separates the polar easterlies from the prevailing westerlies.

– The subtropical jet stream is located where the trade winds meet the prevailing westerlies.

Jet Streams

Large-Scale Weather Systems

Weather Systems

– The position of the jet stream varies, and it can split into different branches and later reform into a single stream.

– The jet stream represents the strongest core of westerly winds.

– Weather systems generally follow the path of the jet stream.

– The jet stream affects the intensity of weather systems by moving air of different temperatures from one region to another.

Pressure Systems• At Earth’s surface, rising air is associated with

low pressure and sinking air is associated with high pressure.

Weather Systems

• Rising or sinking air, combined with the Coriolis effect, results in the formation of rotating low- and high-pressure systems in the atmosphere.

• Air in these systems moves in a general circular motion around either a high- or low-pressure center.

Pressure Systems

High-Pressure Systems

Weather Systems

– In a high-pressure system, air sinks, so that when it reaches Earth’s surface it spreads away from the center.

– The Coriolis effect causes the overall circulation around a high-pressure center to move in a clockwise direction in the northern hemisphere.

– High-pressure systems rotate in a counterclockwise direction in the southern hemisphere.

Low-Pressure Systems

Weather Systems

– In a low-pressure systems, air rises, causing an inward net flow toward the center and then upward.

– In contrast to air in a high-pressure system, air in a low-pressure system in the northern hemisphere moves in a counterclockwise direction.

– This movement is reversed in the southern hemisphere.

Low-Pressure Systems: The centers of all winter storms are areas of low pressure systems.

Weather Systems

• A COLD, dry Canadian air mass moves south and interacts with a warm, moist air mass moving north from the Gulf of Mexico to form a front. Winter storms usually form along a stationary front.

• As the atmosphere tries to even out the pressure difference between the cold and warm air masses, an area of lower pressure develops along the front .

• This creates wind (air blowing from high pressure towards low pressure) to try and move enough air to even out the pressure difference.

• As the air moves toward the low-pressure area, it has nowhere to go but up into the colder regions of the atmosphere (LIFT). Water vapor in the air condenses (MOISTURE) and the condensed water falls as snow to the north of the storm, where temperatures are colder,

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