Most of the satellite images of the United States and the Eastern Pacific are brought to us by a pair of weather satellites called Geostationary Operational Environmental Satellites or GOES.   GOES satellites orbit the earth at an altitude of 22,000 miles above the equator once every 24 hours, keeping pace with the earth's rotation. Thus, the satellites appear to remain fixed above the same point on earth, which is why they are called "geostationary".    The U.S. has two GOES birds. GOES-East orbits above Brazil's Amazon Basin, from where it can see both the continental U.S. and the eastern tropical Atlantic Ocean. This satellite is primarily used for monitoring purposes for the Eastern half of the US.  GOES-West orbits near the equator over the eastern Pacific Ocean. GOES-West is the satellite forecasters in the Western U.S. rely on. It sends us pictures every 15 to 30 minutes on the location and movement of fog banks, thunderstorms and large storms over the data sparse Pacific Ocean.

For more in depth information on weather satellites and remote sensing, visit NOAASIS.

1)  How do I interpret visible satellite pictures?

      The pictures sent back to us from Weather Satellites are simply photographs taken from outer space. We get visible images during the daylight when the earth reflects sunlight back to outer space. Visible pictures show the amount of light reflected back to outer space. Thick water rich clouds, such as stratus (common along the North Coast during Summer months), show up as bright white on visible pictures. Snow pack over the Sierra Nevada during the Winter is another good reflector of sunlight. During the night, visible pictures are completely black as there is very little or no visible light to reflect!

2) How do I interpret infrared satellite pictures?

     Clouds and atmospheric gases such as water vapor radiate infrared energy to space. The amount of energy radiated by clouds and gasses relates directly with its temperature. This relationship, know as the Stephan-Boltzmann Law, allows us to "see" clouds at night. The atmosphere generally cools with height. A cloud that radiates low energy is higher in the troposphere than a cloud that radiates higher energy. On a standard linear enhancement curve, bright white represent cold ice-crystal clouds, while clouds colored with light shades of gray are warm water filled clouds in the mid and lower troposphere. There are a number of enhancement curves that color pictures of clouds based on temperature. These curves accentuate temperature ranges that allow us to discern high clouds from mid and low clouds. At the bottom of every infrared image there is a key telling you the temperature range for that color. For example, on the infrared pictures provided on this web site, red represents cloud tops with a temperature of -40C which in a standard atmosphere is about 40,000 ft. See example.

3) How do I interpret water vapor imagery?

     Water vapor in the mid and upper troposphere absorbs infrared energy at discrete wavelengths. Using this information, water vapor can act like a tracer of atmospheric circulation in the mid and upper troposphere. Dark regions on water vapor pictures are generally areas where the air is sinking and drying. Light colored regions are areas of rising motion and moisture. Water vapor images are useful in locating jet streams and short-wave troughs and ridges in the mid and upper troposphere, but are lousy when trying to see clouds near the ground such as stratus.

     Fog and stratus are almost impossible to see at night using standard Infrared imagery because stratus clouds and fog radiate at nearly the same temperature as the ground. In other words, there is no "thermal contrast". A new product developed recently subtracts infrared energy radiated at two different wavelengths. This allows us to see fog and low clouds at night. A good example is fog in the Central Valley during the Winter and stratus and fog along the California Coast during the Summer.

4)  Where can I find the time on satellite pictures? What time zone is this?

     The date and time stamps are located at the top or bottom of every image, along with other information. For example, G-10 IMG 01 3 Jun 00 TIME=00:30UTC RES=4km NWS/WR=SSD, is a typical date-time stamp which appears on satellite images. The date and time, colored blue in the above example, is in Universal Coordinated Time (UTC). UTC is also known as Greenwich Mean Time (GMT) or Zulu Time (Z). Along the West Coast, we are about 8 hours behind the UTC during Pacific Standard Time (PST). During Daylight Saving Time (PDT), from mid April through mid October, we are about 7 hours behind the UTC. In the example above, it's June 3rd 2000, 30 minutes past midnight in Greenwich England. In order to get local time, you subtract 7 hours to get June 2nd 2000,  5:30PM PDT.  The next calendar day has already started in Greenwich England! If you're still confused, check out our Time Conversion Tables or visit this link for more information about UTC.