I. California K-6 Standards for Atmospheric Circulation -

3. Grade 6
   1. Energy in the Earth System
      1. Many phenomena on the Earth’s surface are affected by the transfer of energy through radiation and convection curents. As the basis for understanding this concept, students know:
         1. the sun is the major source of energy for phenomena on the Earth’s surface, powering winds, ocean currents, and the water cycle.

         2. convection currents distribute heat in the atmosphere and oceans.

 Back to Top

II. Goals of this module -

• To learn about the tools used to measure air pressure and wind.

• To learn the definitions of and proper spelling for scientific terms that are used commonly in newspapers, magazines, and books, and on television and radio programs to explain atmospheric circulation.

• To learn about the atmospheric circulation patterns that are created by variations in pressure on the surface of the Earth.

 Back to Top

III. Atmospheric (air) pressure -

• Objects resting on the surface of the Earth, due to their mass and the force of gravity, have a certain weight which pushes down and exerts pressure on the Earth’s surface. Likewise, air in the Earth’s atmosphere is being pulled toward the Earth by the force of gravity, and the weight of this air exerts a certain amount of pressure on everything below it.

• The force exerted over a given area by the weight of the overlying air is called the air pressure, and at sea level it equals 1.04 kg per square cm (14.7 lbs per square inch). On mountain tops at higher elevations, the air pressure is less because the amount of overlying air is less.

• Differences in air pressure, though not noticeable to humans, cause the atmosphere to move in the form of wind and are a significant factor in weather forecasting.

• Measuring air pressure - using a mercury barometer
  • A glass tube about 92 cm (36 in) long and sealed at one end is filled with mercury and inverted in a bowl of mercury. The mercury level in the tube drops to about 76 cm (30 in) and creates a vacuum at the top of the tube. the weight of the overlying air pushes down on the mercury in the bowl with enough force to support the weight of about 30 inches of mercury in the tube. As air pressure changes, the height of the column of mercury in the tube changes.

• Measuring wind -
  • Wind direction is determined with a wind vane. The wind direction is always stated in terms of the direction from which it is coming.

  • Prevailing wind is the general direction from which the wind usually comes.

  • Wind speed is measured with a cup anemometer. The spinning cups are attached to a rotating cable, which controls a dial on which the speed is read, much like a car speedometer.

 Back to Top

IV. Recording variations in air pressure on a map -

• Air pressure measurements for many locations are recorded on a map. Points of equal air pressure are connected with lines called isobars.

 Back to Top

V. Three factors affecting wind -

• Pressure gradient force -
  • Air always moves as wind from areas of high pressure toward areas of low pressure. The greater the pressure difference over a given distance (the pressure gradient), the higher will be the wind speed. Isobars that are close together mean high wind speeds; isobars that are far apart mean low wind speed. Without further modification the wind direction would be perpendicular to the isobars.

• Coriolis effect -
  • Coriolis effect occurs because the Earth is spherical and is rotating.

  • Speed of the earth's rotation is faster at the equator than at higher latitudes. Objects moving northward in the atmosphere of the northern hemisphere are moving eastward faster than the speed of their final destination; hence they seem to veer to the right or toward the east.

  • Objects moving southward in the atmosphere of the northern hemisphere are moving eastward slower than the speed of their final destination; hence they also seem to veer to the right or toward the west.

  • In the southern hemisphere, objects moving through the atmosphere will seem to veer toward the left.

• Friction with Earth's surface -
  • At high levels in the atmosphere, friction with the Earth's surface has no effect on the wind. Coriolis effect turns the higher level winds until the pressure gradient force is balanced by the force of the Coriolis effect and the wind is blowing parallel to the isobars. These winds, blowing free of the surface frictional forces, are referred to as the jet stream.

  • Along the Earth's surface, the wind is slowed down by friction as it blows across the surface and the pressure gradient force is never balanced by the force of the Coriolis effect, so the wind crosses the isobars at an angle as it moves from high pressure areas to low pressure areas.

 Back to Top

VI. General atmospheric circulation on a nonrotating Earth -

• On a nonrotating Earth there would be no Coriolis effect, so the atmosphere would circulate in one large cell in each hemisphere.

• At the Earth's poles, the air of the upper atmosphere is cold (hence heavy), so it would descend over the poles (causing high pressure) and spread out in all directions, flowing along the surface toward the equator.

• At the equator the surface air is hot (hence light), so it would rise over the equator (causing low pressure) and move into the upper atmosphere and toward the poles.

 Back to Top

VII. General atmospheric circulation on our rotating Earth -

• Coriolis effect causes global circulation to be divided into three cells of circulation in the northern hemisphere and three cells of circulation in the southern hemisphere with their boundaries generally at the equator and at 30°N and 60°N latitude and at 30°S and 60°S latitude. The 13 zones of surface atmospheric circulation created from the 6 circulation cells are shown below.

 Back to Top

VIII. Cyclones and anticyclones -

• Cyclones -
  • Around areas of low pressure in the northern hemisphere, wind blows across the isobars diagonally and inward toward the low pressure area in a counterclockwise direction.

  • These cyclonic winds cause convergence and uplift of the air at the point where the air pressure is lowest.

  • Rising air cools and may create stormy conditions.

  • In the southern hemisphere, cyclonic circulation is clockwise.

• Anticyclones -
  • Around areas of high pressure in the northern hemisphere, wind blows across the isobars diagonally and outward away from the high pressure area in a clockwise direction.

  • These anticyclonic winds cause divergence and descent of the air at the point where the air pressure is highest.

  • Descending air warms and usually creates good weather conditions.

  • In the southern hemisphere, anticyclonic circulation is counterclockwise.

 Back to Top

IX. Possible essay questions -

• Explain why air pressure exists on the Earth.

• Describe how a mercury barometer is made, and explain how it works.

• Explain why objects moving through the atmosphere of the northern hemisphere seem to turn toward the right.

• Draw a diagram that shows the general atmospheric circulation patterns on Earth, clearly label all the features shown in the diagram, and explain why Earth's atmospheric circulation occurs as it does.

• Diagram a cyclone in the southern hemisphere. Show isobars, wind direction arrows, and relative pressure in the center (high or low).

• Diagram an anticyclone in the northern hemisphere. Show isobars, wind direction arrows, and relative pressure in the center (high or low).

 Back to Top

X. Practice Questions

 Back to Index