Satellite Movie of Earth's General Circulation.

The movie you will be watching today is a 3-month composite of thermal infrared images taken every 3 hours by a variety of Earth-orbiting weather satellites. Geostationary satellites orbit the Earth along the equator at high altitude with an orbital period that matches the Earth's rotation, so relative to the surface, they remain at a fixed longitude over the equator. Five of these satellites spaced roughly evenly around the world allow us to see almost everywhere on the planet. Polar orbiters are low-altitude satellites that orbit the Earth about every 90 minutes in an almost latitudinal direction at a fixed angle relative to the Sun. They pass almost directly over both the North Pole and the South Pole and see all other parts of the Earth at the same time of day, once each during the day and the night. They give us a better view of the polar regions than do the geostationary satellites, while the geostationary satellites allow us to see the rest of the world more often. Together the data from these satellites enable us to map changes in the global atmosphere and surface almost continuously.

Weather satellites take visible images that can help us differentiate thick from thin clouds, but they do not tell us how high the clouds are and they do not allow us to view the night side of the planet. Thus, the movie is instead made up of thermal infrared images taken at a wavelength where clouds absorb well but the clear atmosphere does not. It is thus a map of the temperature at which different locations on Earth emit radiation to space at different times. In clear locations, the radiation comes mostly from the surface and tells us the surface temperature. In cloudy locations, the radiation usually comes from close to the cloud top, so colder temperatures indicate high-altitude (often thick) clouds and warmer temperatures denote low-level clouds. The map is color-coded as follows: shades of white/gray for clouds, brown for the land surface, blue for the ocean surface, green for snow/ice surfaces. Since clouds (especially high ones) tend to form in regions of rising motion, while regions of sinking motion tend to be either cloud-free or contain only low-level clouds, clouds act as a tracer, or dye marker, of the atmospheric general circulation. A seasonal clock helps you mark the day and month on which a particular phenomenon occurs.

Since today is the final day of the atmospheric science section of the course, we'd like to try to synthesize everything that you have learned over the last few weeks. Watch the movie and see how many of the phenomena that we have discussed can be detected in the movie. To assist you, we will break into three groups; each group will be assigned a set of questions to answer:

Group I: Radiation and Thermodynamics.

  1. Can you see evidence of the diurnal cycle of temperature over tropical land masses? How does cloud activity correlate with time of day in these places, what phenomenon accounts for this, and why? Is there a similar diurnal cycle of temperature and cloudiness over the tropical oceans? Why might it be different than over land?

  2. Can you see any evidence of the seasonal variation of surface temperature from the beginning to the end of the movie? Where? For which type of surface is it most evident?

  3. Are the western or eastern sides of the oceans generally warmer? Do you see cloud differences that correlate with this?

Group II: Midlatitude dynamics.

  1. What is the prevailing wind direction in midlatitudes? Is it the same or different in the Northern and Southern Hemispheres? Why? Do clouds generally move faster at high or low altitudes? Why?

  2. Are the midlatitudes cloudier or clearer than the adjacent latitude zones (polar, subtropics)? What does this tell you about the sense of motion there, and what accounts for most of it?

  3. Is there evidence of synoptic-scale geostrophic motion in midlatitudes? Is it identical or different in the two hemispheres, and why?

Group III: Tropical general circulation.

  1. Is the region between 30 N and 30 S uniformly cloudy? If not, which latitudes are cloudier/clearer (or dominated by higher/lower altitude clouds), and why? Similarly, how does the cloudiness vary with longitude?

  2. What is the prevailing wind direction in the tropics? Why? Can you see evidence of low-level meridional motions that provide evidence for this?

  3. Can you detect the direction of the meridional wind in the tropical upper troposphere? What phenomenon does this reveal? What would the Coriolis force do to this wind, and what does that help explain?