Doppler radar (or weather radar) tracks precipitation – rain, snow, hail, etc.  It calculates the motion and intensity of the precipitation, and also how likely it is to turn into a severe storm.

How it works:
A Dopper radar is an electronic instrument that sends out radio waves from its antenna.  Objects in the air like raindrops, snow crystals, etc. reflect some of the radio waves back to the antenna.  Then the instrument converts the reflected radio waves into pictures to show the location and intensity of the precipitation.

Radio waves bounce off all objects, including insects, buildings, trees and mountains.  Meteorologists have to take this into account when reading the radar pictures.

Weather balloons carry something called a radiosonde into the upper atmosphere.

Radiosondes measure atmospheric conditions such as barometric pressure, relative humidity, temperature, and the direction and speed of winds, and send the data back to an upper-air observation station on the ground via a radio.

The data collected helps to make long-term weather forecasts and to monitor atmospheric phenomena like ozone depletion.  Radiosondes can also be dropped out of aircraft – these are usually called dropsondes.

How it works:
A rubber or latex balloon, carrying a radiosonde, is filled with helium or hydrogen – gases that are lighter than air.  As the balloon rises, the pressure of the atmosphere decreases, causing the balloon to expand. At some point high up in the atmosphere, the balloon will break, causing the radiosonde to fall back down to the ground. If recovered, the radiosonde can be reconditioned and used again.