The Atmosphere in Motion

What is air pressure? It is the weight of the atmosphere above as it pushes down on earth’s surface.

Air pressure surrounds us from all sides.

Air pressure decreases as you go up in the atmosphere Air pressure decreases as you go up in the atmosphere. The balloon above will expand as a result.

We use a barometer to measure air pressure.

What are the 3 things that cause changes in air pressure? Elevation. Higher elevations exert less pressure. Humidity. More humid air exerts less pressure. Temperature. Warmer air exerts less pressure.

How do we plot pressure differences? By using isobars on a weather map

Wind blows from High to Low Pressure How does wind blow? Wind blows from High to Low Pressure

But the Coriolis Force causes wind to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

Will the Coriolis Force have an effect on your bathtub, toilet, or kitchen sink?

Absolutely NOT! Coriolis only takes effect when there are large distances, high velocities, and large time scales!

The Coriolis Effect is greatest near the poles and the least near the Equator. The effect is greater on fast moving objects. The effect does not depend on an object’s direction of movement. It is only noticeable for objects traveling great speeds over great distances.

This causes wind directions around high and low pressure systems to change. Northern Hemisphere Clockwise Counter-Clockwise

This causes wind directions around high and low pressure systems to change. Southern Hemisphere Clockwise Counter-Clockwise

Friction of wind with the ground will cause Coriolis Effect to be weakened. Winds will flow more perpendicular to isobars. Northern Hemisphere Clockwise Counter-Clockwise

The Jet Stream is a high altitude stream of air at the boundary between warm air and cold air.

Global Wind Patterns

Global Wind Patterns are the result of earth’s rotation and temperature (pressure) differences around the globe

If the Earth did not rotate, this is what the global wind patterns would look like. Winds would only respond to temperature (pressure) differences.

Because the Earth rotates, this is what the global wind patterns look like. Winds respond to temperature (pressure) differences and Earth’s rotation.

The rising motion of air at low pressure areas and descending motion of air at high pressure areas creates Hadley cells.

If the Earth did not rotate, one large circulation cell would occur in each hemisphere.

Because Earth rotates, the circulation pattern looks like this Because Earth rotates, the circulation pattern looks like this. We call this the Three-celled Circulation Model

Weaknesses of the Three-celled Model It assumes that surface winds are uniform. Surface winds can be determined by local High and Low pressure weather systems It assumes that all of the Earth’s surface is the same. In actuality, Earth has oceans and landmasses. It assumes that upper-level winds travel north or south. Actually, upper level winds are primarily westerly, except near the equator where the Coriolis effect is weak.

Prevailing winds are the result of the global circulation model. Horse Latitudes Doldrums Horse Latitudes

The Horse Latitudes, which have little or no wind, got their name because horses were dumped overboard to lighten the ship’s load and conserve fresh water. Horse Latitudes Doldrums Horse Latitudes

The Doldrums, which have little or no wind, got their name because sailors would get trapped there in between the trade winds for days or weeks. Hence, the saying “stuck in the doldrums.” Horse Latitudes Doldrums Horse Latitudes

El Niño occurs when the tropical easterlies (trade winds) die off about every 3-5 years. Go here for the latest information. Northeast Trade Winds Southeast Trade Winds

Tracking El Niño was the NOVA video that we saw in class Tracking El Niño was the NOVA video that we saw in class. Click here for the latest Topex-Poseidon El Niño Sea Surface Temperature Image. The warm flux of ocean water from the 1997-1998 El Niño

Local Wind Patterns The Indian Monsoon

Sea Breezes during the day Rising warm air over the land is replaced by cooler air from the ocean. The wind goes from the ocean to the land, High to Low.

Valley Breeze during the day Rising warm air over the valley flows uphill as a valley breeze.

Land Breezes during the night Rising warm air over the ocean is replaced by cooler air from the land. The wind goes from the land to the ocean, High to Low.

Mountain Breezes during the night Cool air at the top of the mountain sinks and causes a mountain breeze.

The Indian Monsoon Increased rainfall occurs over India in the Summer months. This is because the warmer continent creates rising air and a sea breeze pattern. Cooler, moist air from the Indian Ocean brings monsoon rains to the Indian landmass.

The Indian Monsoon Decreased rainfall occurs over India in the Winter months. This is because the colder continent creates sinking air and a land breeze pattern. Rain stays offshore over the warmer Indian Ocean.