Redistribution of Heat

Key idea: Planets are not evenly heated by the Sun. The uneven heat is redistributed my moving air masses, water vapor and ocean currents (on Earth anyway). The non-uniform heating and the redistribution of heat produce global circulation of the atmosphere, global climate and local weather patterns.

South Pole of Mars Astronomical tidbit: The Martian poles are colder than the rest of Mars because the incoming radiation from the Sun at the poles is not as direct as it is everywhere else on the planet. The poles are covered with permanent ice caps, although the size of the each of the caps changes with the Martian seasons. The poles of Earth are also covered permanently with ice. The Martian ice caps are composed primarily of frozen carbon dioxide ("dry ice"), but the Northern polar cap contains water ice as well. This photograph of the South pole of Mars was taken by a Viking orbiter during the Viking mission to Mars in the late 1970's. The ice cap was imaged when its size was near the seasonal minimum. The diameter of the ice cap in the photo is approximately 250 miles.

You have learned in a previous lesson that planetary atmospheres and surfaces are heated by electromagnetic radiation from the Sun. The heating is not even, however. Heating at each point on the surface of a planet depends on the inclination of that part of the surface with respect to the Sun.

If the Sun is shining straight down on the surface (when the Sun is directly overhead), then the maximum amount of solar energy reaches the surface in any given area. The solar flux in this case is maximum. If the surface is angled with respect to the direction to the Sun, then less energy from the Sun reaches the surface in any given area. This leads to an uneven distribution of heat.

The Earth rotates once every 24 hours. The rotation ensures that heating is relatively uniform at any particular latitude. Over the whole range of latitudes, however, there is a large difference in the amount of solar flux received in an average day. The average solar flux is greatest at the equator of a planet and least near the poles. In general, heat is therefore redistributed from lower latitudes (latitude at the equator is equal to 0 degrees) to higher latitudes (latitude at the poles is equal to 90 degrees). On Earth, the non-uniform heating leads to three climate zones: the tropical zone from 0-30 degrees North and South latitude, the temperate zone from 30-60 degrees North and South latitude, and the polar zone from 60-90 degrees North and South latitude.

Demonstration: Shine a flashlight on a blackboard (or wall) at different angles, and observe how the light intensity on the board decreases as the angle increases. Also, shine a flashlight on a large ball or globe and observe where the intensity is largest and smallest. Rotate the ball as the Earth spins on its axis and observe that the highest intensity is always in the vicinity of the "equator."

The uneven heat energy from the Sun is redistributed by moving air masses, water vapor and ocean currents. Warm air masses and ocean currents carry heat energy from low to high latitudes, and warm water at low latitudes evaporates and carries heat energy with it, releasing the energy at higher latitudes in the form on condensation. The uneven Solar heating and the redistribution of heat energy leads to global circulation of the atmosphere, global climate and local weather patterns. The weather therefore results from the response of a planet and its atmosphere to uneven heating from the Sun and the attempt to move that heat from hotter areas to cooler areas!!!

On Earth, the prevailing circulation patterns have trade winds blowing from the East (easterly) in the lower latitudes, westerly winds blowing in the middle latitudes, and easterly winds blowing in the high latitudes. This circulation pattern helps to redistribute the uneven heat received from the Sun, and is also the result of the Coriolis force which is present due to the rapid rotation of the Earth. Because the prevailing winds blow from the west in the middle latitudes, it is faster to fly from the West Coast of the United States to the East Coast than it is in the other direction!!!

Ocean currents follow global atmospheric circulation patterns very closely. The currents are generated by the friction which results from winds blowing over the water.

Global temperatures on Earth generally correlate with the climate zones, but other factors such as proximity to large bodies of water, elevation, and wind and ocean currents also determine the typical temperatures at specific locations.

Question: How do you suppose all of these factors account for the typical temperatures and climate in your location???

The Coriolis force is almost absent on Venus because the planet rotates very slowly (one rotation every 243 days). Because of this, atmospheric circulation on Venus is less complex than it is on Earth.

The bright and dark bands on Jupiter are thought to indicate a complex circulation pattern which redistributes heat received from the Sun and heat from within the planet itself. The Jovian heat redistribution takes place in the presence of a very strong Coriolis force (the giant Jupiter rotates once every 10 hours). It is the atmosphere of Jupiter itself which is not evenly heated by the Sun. Jupiter and the other gas giant planets do not have actual surfaces, and consist a thick gases around a dense core.


Great Red Spot of Jupiter Astronomical tidbit: This photograph shows a number of light and dark bands of Jupiter. Just above the center of the photograph is the Great Red Spot of Jupiter. It is believed that the bands and spots are all part of a global circulation pattern on Jupiter which redistributes heat from the Sun and from within the planet in the presence of a strong Coriolis force. Some of the cloud details in the photograph are only 100 miles wide. The photograph was taken by the Voyager 1 spacecraft during its Jupiter encounter on February 25, 1979. The atmospheric circulation is captured in remarkable detail. The Great Red Spot was first observed in the 1600's. It is approximately 15,000 miles long and 7,000 miles wide. Its width is larger than the diameter of the Earth!!!


Last updated: November 17, 1999
Joe Twicken / joe@nova.stanford.edu
Rob Wigand