Unit: Weather and Water
Convection
Text: Weather and Climate (Section 3, page 14-19)
Global Winds and Local Winds
Why air moves
Differences in air pressure create the movement of air, or wind. The bigger the difference in air
pressure, the faster the wind.
Air Rises at the Equator and Sinks at the Poles
The Earth heats unequally, which creates differences in air pressure. The equator receives more
direct solar energy (like our 90º angle in the Solar Angle Lab) than other latitudes. The air has;
more energy, which means these air molecules have more volume per mass (lower in density) and
will RISE. This rising air creates LOW pressure below. The air then moves toward the Poles
where it is cooler (like the 0º angle in the Solar Angle Lab). The cold air around the Poles is more
dense and sinks, creating HIGH pressure onto the Pole area. This cool, high density, high pressure
air is drawn toward the warmer, low pressure areas toward the equator. A convection cell is
formed.
Draw a convection cell
Pressure Belts Are Found Every 30º
Pressure belts are found at the equator (0º), 30º, and 60 º, 90 º (both north and south). These
bands of high and low pressure separate the winds, creating convection cells.
0º (equator): Low pressure belt (warm air rising, low density molecules)
30º (N, S): High pressure belt (cool air falling, high density molecules)
60º (N, S): Low pressure belt (warm air rising, low density molecules)
The Coriolis Effect: the apparent curving of the path of a moving object from an otherwise straight
path due to the Earth’s rotation
Unit: Weather and Water
Global Winds
Polar easterlies
Prevailing winds that blow from east to west between 60 º and 90 º latitude in both hemispheres.
Polar easterlies can bring cold arctic air over the US
Westerlies
Prevailing winds that blow from west to east between 30 º and 60 º latitude in both hemispheres
Westerlies can bring moist air over the US, pretty cold moist air
Trade winds
Prevailing winds that blow northeast from 30 º north latitude to the equator and that blow
southeast from 30 º south latitude to the equator
The Doldrums
Located around the equator, this region has little wind due to warm, rising air
The Horse Latitudes
Located around the 30 º (North and South) latitude, this region has little wind due to sinking air
which creates high pressure as it comes down
Most of the world’s deserts are around this latitude due the dry, sinking air
Unit: Weather and Water
Jet Streams: Atmospheric Conveyer Belts
A narrow belt of strong winds that blow in the upper troposphere and lower stratosphere
It doesn’t follow regular paths like the other global winds. Jet streams can pick up weather from
one area and drop it off in another area, making weather predictions difficult.
Local Winds
These generally short breezes are caused by pressure differences that geographic features create
Sea and Land Breezes (Figure 6)
Sea Breeze (occurs during the day)
Land Breeze (occurs during the night)
Mountain Breezes and Valley Breezes
Created by geographic differences, these breezes are similar to the sea and land breezes. The
mountain slopes are warmed and bring up the valley breeze during the day. At night, the slopes
cool and a breeze rushes down the mountain slopes into the valley, producing a mountain breeze.
Question
You are stuck on a deserted island and you want to leave. You build a sailboat to leave the island
but you have to choose the best time to leave.
1. What time should you leave the island?____________
2. Explain why you chose this time.
3. Describe how this weather pattern occurs.