Weather instruments at stations see only one spot. Satellites and radar see whole regions at once. Reading their images uses the ideas from sunlight in, infrared out.
Key idea
Visible satellite images show sunlight reflected by clouds and the ground, so they go dark at night; thick clouds look brightest. Infrared images show the infrared each surface gives off, which tells its temperature; they work day and night, and they are usually drawn so that the coldest, highest cloud tops look brightest. Radar sends out microwave pulses and listens for echoes from raindrops, hail and snowflakes: it shows where precipitation is falling and how heavy it is, and Doppler radar also shows motion toward or away from the radar.
Why it works
Clouds and the ground glow in the infrared, mostly near 10 μm, where the air is fairly transparent (the atmospheric window). A satellite measuring that glow can work out the temperature of whatever it sees: warm ground, or a cloud top. Since air cools with height, a colder top is a higher one, so a bright spot on an infrared image is a tall storm. Radar works the other way around: it makes its own signal. A pulse travels out at the speed of light, bounces off precipitation and comes back; the delay gives the distance, $d = ct/2$, and the strength of the echo gives the size and number of the drops. Cloud droplets are far too small to give a strong echo, which is why radar shows rain but not most clouds.
Worked example: How far is the storm?
A radar pulse returns 0.6 milliseconds after it was sent. How far away is the rain? ($c = 3 \times 10^8$ m/s)
- The pulse goes out and back, so the distance is half of $c \times t$.
- $d = 3 \times 10^8 \times 0.0006 \div 2 = 90{,}000$ m.
- The rain is about 90 km away.
Answer: About 90 km
Worked example: How high is the cloud top?
An infrared satellite reads a cloud top at −55 °C. The ground is 25 °C, and the air cools about 6.5 °C per km. About how high is the top?
- Difference: $25 - (-55) = 80$ °C.
- Height: $80 \div 6.5 \approx 12$ km: a towering thunderstorm.
Answer: About 12 km
Common mistake
Thinking satellite pictures show rain. Satellites mostly show clouds, and plenty of clouds produce no rain; radar shows where precipitation is actually falling. On an infrared image, remember the colors: the brightest clouds are the coldest, highest tops. Check: a visible image of the same storm at night is black, while the infrared image still shows it.
The U.S. radar network has about 160 Doppler radars (WSR-88D). The GOES weather satellites sit about 35,800 km above the equator, where an orbit takes exactly one day, so each one watches the same half of the planet all the time.