|Current & Forecast
The visible images display the Earth similarly to how humans see it with their eyes or how typical cameras view it. Clouds and snow appear bright white (high albedo/reflectance) but oceans and trees are much dimmer.
The infrared images display the Earth in a manner that correlates with temperature. Generally speaking, the warmer an object, the more infrared energy it emits (gives off). The satellite sensor measures this energy and calibrates it to temperature using a very simple physical relationship (Planck’s Law). In the real world, clouds that are very high in the atmosphere are generally quite cold (perhaps -50 °C) whereas clouds that are very near the earth’s surface can be quite warm (perhaps +5 °C). Likewise, the land may be even warmer than the lower clouds (perhaps +20 °C). Those colder clouds emit much less infrared energy than the warmer clouds and the land emits more than those warm clouds. The data measured by the satellite are calibrated and colorized according to the temperature with red shades representing higher (warmer) temperatures and blue shades representing lower (cooler) temperatures. If the temperature of the atmosphere decreases with height (which is typical), a user can get an idea of which clouds are high-level and which are low-level based on the cloud top temperature.
The water vapor images display the Earth in a manner that correlates to quantity of water vapor in the upper portions of the atmosphere (25,000 feet and higher in general). The actual numbers displayed on the water vapor images correspond to temperature but there is no direct relationship between these values and the temperatures of clouds (as is the case for infrared images) since this channel doesn’t really “see” clouds but “sees” high-level water vapor instead. The most useful information to be gained from the water vapor images is the locations of storm systems and the jet stream. The color scale on the images can be helpful in interpreting the water vapor images. In general, regions displayed in shades of red are VERY dry in the upper atmosphere and MAY correlate to crisp blue skies from a ground perspective. On the contrary, regions displayed in shades of blue or green are indicative of a significant amount of high-level moisture and may also indicate cloudiness. This cloudiness could be high-level cirrus types or serious storms. That determination can only be made when this image is used in conjunction with the other channels.
The visible images are very good at depicting the location of low clouds that won’t show up on infrared imagery. So if clouds show up in the visible and not the IR, they are likely low clouds or snow cover. But the visible image is not useful at night and thus can’t be used.
As mentioned, the basic IR image won’t depict low clouds. To resolve this problem, it’s best to look at multiple channels. The best solution is to difference the 3.9 and 11 micron channels. This highlights emissivity differences rather than temperature. Smaller droplets like those in fog will emit less whereas hogher clouds will emit more. Then by using the proper gray shading, the low clouds can be detected.
In the images on the Vis/Fog page, low clouds show up bright and higher clouds show up as dark. The ground shows up as gray. In addition, flight categories for various airports are plotted over the imagery as a secondary verification.