Global Wind Currents

What do wind patterns have to do with oceans?

CURRENTS

Let’s review: Warm air is less dense then cold air (because the heating makes the molecules

move faster and farther apart) Warm air rises Cool air flows in and pushes up the warm air Where is the warmest air on Earth?

equator Where is the coldest air on Earth? N and S poles

Wind patterns on a spherical earth

When air is heated it expands, becomes less dense and rises

Cool air is more dense and exerts more pressure on the surroundings

Fluids always flow from a region of higher pressure to a region of lower pressure.

Cool air will flow towards and under warm air, pushing it up and away.

At the equator the surface and the air is warmer than at any other latitude

At the poles the coldest densest air exists.

The cold air from the poles moves along the earth’s surface toward the warmer regions, pushing the warm air ahead of it.

If the earth were a small sphere the wind patterns may look like this. However, because of its size, the air that is heated and rises at the equator heads northward or southward but cools before it can reach the poles.

By the time the air reaches north or south latitudes of about 30 degrees it has cooled so much and become so dense that it begins to drop back to Earth’s surface.

For the cold air leaving the poles, once it has reached a latitude of 60 degrees it has warmed enough to start to rise

The result is 3 closed patterns of air movement in each Hemisphere.

They are referred to as

Polar, Ferrel & Hadley cells.

Besides the global wind patterns moving from the equator north and south and from the poles to the equator the Earth is also rotating.

When air moves north or south due to convection it also moves East.

The influence of Earth’s rotation on air is called the CORIOLIS EFFECT.

Coriolis Effect on Canadian weather systems

In the central part of the Northern Hemisphere, including most of Canada and the United States, the Coriolis effect causes winds to blow from west to east.

These winds are known as the Prevailing Westerlies

They are the reason weather systems in Canada generally move out of the west and toward the east.

Global Wind Patterns

Contrary to the Northern Hemisphere in the Southern Hemisphere the Coriolis effect causes winds to move from east to west.

Resulting in a Global Pattern with 6 distinct regions

Jet StreamsJet streams are ribbons of extremely fast moving air near the troposphere. The Troposphere is the lowermost portion of Earth's atmosphere. It is the densest layer of the atmosphere and contains approximately 75% of the mass of the atmosphere and almost all the water vapour and aerosol.

- they are caused by the contact between warm and cold air

- they are found at boundaries between the polar and temperate zones and between temperate zones and tropical zones

- Wind speeds in a jet stream can vary from 100 km/h to 300 km/h

- they are thousands of kilometers long, a few hundred kilometers wide and two or three kilometers thick.

- In general the weather north of a jet stream is cold, while the weather south of a jet stream is warmer

Jet Stream

Location of the Jet Stream in the Winter

Location of the Jet Stream in the Summer

Based on the previous two diagrams that showed the general location of the jet stream in winter and summer how can these be used to explain our seasonal temperature changes here in Nova Scotia?

AIR MASSES

Although air is usually in motion, due to convection and the Coriolis effect, large portions of air often remain in nearly the same place long enough to take on the temperature and moisture characteristics of the land or ocean below.

Since these characteristics vary greatly, the differences between land and ocean can have a significant influence on the properties of the air above.

When this occurs, a large portion of the air is nearly uniform in temperature and humidity, we have an AIR MASS.

Air Masses are classified as CONTINENTAL or MARITIME, depending on whether they form over land or ocean. Continental air masses that form over land tend to be drier than Maritime air masses, so this classification serves as an indicator for how much moisture is in the air

Air masses are also classified as POLAR or TROPICAL depending on their temperature

maritime polarmPmaritime

polar mP

continental polar cP

maritime tropical mT

maritime tropical mT

continental tropical

cT

Characteristics of Air Masses

Continental Polar - Cool and Dry

Maritime Polar - Cool and Moist

Continental Tropical - Warm and Dry

Maritime Tropical - Warm and Moist

High and Low Pressure Systems

Any region where air is descending (and therefore exerting more pressure on the region below) is called a high pressure system

High pressure systems occur when an air mass forms over cold ground, the air cools by conduction and convection. As the air cools it becomes more dense and settles lower to the ground, creating more pressure. As the cool air continues to settle the system draws in more air from above and also pushes air out towards the lower pressure perimeter.

A low pressure system exists where rising air is leaving dense air below. Intense heating of the ground can create a low pressure system

The hot ground heats the air by conduction, then the heated air rises, creating a zone of low pressure. As the air rises it pulls in more air beneath it.

End