Atmospheric Circulation Lab

Overview of Atmospheric Movements

Solar radiation supplies the energy for atmospheric motion. Uneven surface heating causes vertical air motion and horizontal pressure differences, creating thermal circulation. On a global scale, the effect of Earth's rotation produces three circulation cells, seven pressure belts and six wind belts.

On the 3D globe, drag to rotate and scroll to zoom. The meridional circulation cross section at the globe's edge always faces you.

Use ← → to change chapters and F for full screen during classroom demonstrations.

Thermodynamic circulation principle

Thermal circulation is driven by uneven surface heating and is the simplest form of atmospheric circulation. Follow the steps to see how it develops:

Warm → rising air → surface low pressure, high pressure aloftCold → sinking air → surface high pressure, low pressure aloft

Isobaric surfaces bulge upward at high pressure and downward at low pressure.

At the same location, surface and upper-level pressure anomalies have opposite signs.

Horizontally, air flows from high pressure to low pressure.

Vertically, surface pressure is always greater than pressure aloft. Here, 'high' and 'low' compare locations on the same horizontal plane.

urban heat island circulation

Concentrated urban population and industry release large amounts of waste heat. Concrete and asphalt absorb heat, while green space is scarce. Cities become warmer than their surroundings, creating an urban heat island. Air rises over the city and sinks over the suburbs. Surface winds blow from suburbs toward the city.

Surface: suburbs → cityAloft: city → suburbs; air rises over the city and sinks over the suburbs.

Industries producing heavy air pollution should be located outside the heat island circulation.

Urban green space works best inside the circulation, near the sinking-air zone to clean air flowing toward the city.

More green space and water, and lighter-colored roofs, can reduce the heat island effect.

sea ​​and land breeze

Land has a lower specific heat capacity, so it heats and cools faster than the ocean. During the day, warmer land creates surface low pressure and wind blows from sea to land: a sea breeze. At night, land cools faster, its pressure rises relative to the sea, and wind blows offshore: a land breeze.

Day: sea breeze · Night: land breezeSurface wind flows from the cooler, high-pressure side toward the warmer, low-pressure side.

Sea breezes keep coastal areas cooler and more humid on summer days.

Traditional fishers use the land breeze to sail out early and the sea breeze to return later in the day.

valley wind

During the day, slopes warm quickly. Air above the slopes is warmer than air over the valley at the same altitude and rises upslope, drawing in valley air: a valley breeze. At night, slopes cool quickly and cold air drains down into the valley: a mountain breeze.

Day: valley breeze · Night: mountain breeze

At night, cold air lifts warm, moist valley air, favoring clouds and nighttime rain.

Cold air pooling in valleys can create a temperature inversion that traps pollution. Heavily polluting industries should avoid valley locations.

Single-cell circulation

Assume two conditions: a uniform surface, no Earth rotation. Equatorial heating makes air expand and rise, then flow poleward aloft. Polar cooling makes air contract and sink, then return toward the equator near the surface. Each hemisphere has one closed thermal circulation cell.

Equator: heating → ascent → surface low pressurePoles: cooling → descent → surface high pressure

In the cross section at the globe's edge, particles rise at the equator and sink at the poles.

Surface air flows along meridians directly from the poles toward the equator, with no east–west deflection.

Earth actually rotates, so this single-cell circulation does not occur; see the next chapter.

Three-cell circulation

Earth's rotation produces the Coriolis effect (deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). Poleward air aloft becomes westerly near 30°. Air accumulates and sinks, splitting the single-cell model into three circulation cells.

Hadley cell: equatorial ascent → upper-level flow toward 30° → descent → surface return toward the equator.

Ferrel cell: some air descending at 30° moves poleward and is lifted where it meets cold polar air near 60°.

Polar cell: polar cooling and descent → surface flow toward 60° → ascent → upper-level return toward the pole.

Pressure and wind belts

Three-cell circulation produces seven pressure belts, six wind belts, with alternating high and low pressure and wind belts between them.

Equatorial low · Subtropical high · Subpolar low · Polar highWind moves from high to low pressure, deflecting right in the north and left in the south.

Seasonal movement

Pressure and wind belts shift with the subsolar point. In the Northern Hemisphere they generally move north in summer and south in winter, by about 5–10 degrees of latitude.

Mediterranean climate (continental west coasts near 30°–40°): the summer subtropical high brings hot, dry weather; winter westerlies bring mild, wet weather. See the live display for 38°N in the lower left.

Tropical savanna climate: alternating equatorial low pressure and trade winds produce distinct wet and dry seasons.

Distribution of sea and land and monsoon

Earth has both land and sea. Land is cold in winter and hot in summer, while oceans moderate seasonal temperatures. Pressure belts break into high- and low-pressure centers, also producing monsoons. The globe shows monthly mean surface wind and sea-level pressure from NCEP reanalysis for 1991–2020.

Cooling over Eurasia produces a strong Asian High (the Siberian High), interrupting the subpolar low-pressure belt. Over the oceans, the Aleutian Low and Icelandic Low.

Eastern China experiences a northwesterly monsoon with cold, dry weather; South Asia experiences a northeasterly monsoon.

Heating over Eurasia produces the Asian Low (Indian Low), interrupting the subtropical high-pressure belt. Over the oceans, the Hawaiian High and Azores High.

Eastern China experiences a southeasterly monsoon with hot, wet weather; South Asia experiences a southwesterly monsoon formed as Southern Hemisphere southeasterly trades cross the equator and turn right.

East Asian monsoon: land–sea heating contrastsSouth Asian monsoon: land–sea heating contrasts + seasonal shifts of pressure and wind belts

In-class testing