IR or infrared satellite imagery is sort of a temperature map. The weather satellite detects heat energy in the infrared spectrum (infrared energy is invisible to the human eye). The satellite image displays objects(whether clouds, water or land surfaces) based on the temperature of the object. Warm temperatures appear in dark shades. Cold temperatures appear in light shades. A temperature scale(in degrees Celsius) is depicted to the left of the image.
The chief advantage of IR imagery is that it's not dependent on sunlight. Visible imagery(like the photos you take with a normal camera) relies on sufficient sunlight reflecting off a surface to be viewable. It's useless at night, but IR imagery relies on emitted heat energy(detectable day or night if you have the right equipment).
You can infer relative altitudes of clouds from their temperature. Since temperature, in general, decreases with increasing height, high altitude clouds will appear whiter than low altitude clouds.
Visible vs. IR — Why?
A visible satellite image is created by looking only at the visible portion of the light spectrum and is thus only really useful during daylight hours. The Infrared (IR) image comes from the satellite detecting heat energy in the infrared sepectrum and thus does not depend on visible light. For this reason we switch between the visible and IR images at 1500 GMT (8:00am PDT) and 0200 GMT (7:00pm PDT).
What are the symbols on the satellite image?
This is a three letter identifier for each station. Example: 'SFO' is San Fransisco. Use the station search engine to find the name of the plotted stations (select the "Call Sign" option on the search form before attempting a search).
Wind Speed and Direction
The wind at a given station is depicted with yellow wind barbs. The wind barb displays both speed and direction. The wind direction is given as the direction from which the barb points and wind speed is indicated by the number and size of the "barbs", where whole barbs indicate 10 knots. The key, provided below, shows the wind barb convention. In this example the wind is from the NNE at 5 knots.
The cloud cover is reported as a fractional coverage graphically depicted as filled wedges in a "pie". Cloud cover symbols are defined on the key, provided below.
Temperature at the reporting station in degrees Fahrenheit.
Current Weather Symbol
Current weather reported at the station. Symbols are defined on the key, provided below.