Passive sensors on weather satellites observe many different wavelengths of electromagnetic radiation upwelling from the Earth-ocean-atmospherecloud system. Imager sensors take high-quality digital photographs of clouds and air motion, for which the most important wavelength channels are visible, IR, and water-vapor. Sounder radiometers remotely probe different depths in the atmosphere.
For reflected visible sunlight and for wavelengths in IR windows, the satellite sees down to the highest cloud top, or to the ground if no clouds block the view. In IR-opaque regions, the satellite cannot see through the atmosphere, and instead measures radiation emitted from the air.
For this latter situation, atmospheric emissions at any one wavelength come from a broad range of altitudes, as defined by weighting functions. By observing many different wavelengths with weighting functions that peak at different altitudes, the data can be inverted to retrieve vertical profiles of temperature. Retrievals are difficult, but provide useful remotely-sensed soundings over the oceans having insufficient in-situ observations.
Two favorite satellite orbits are geostationary (high altitude) and sun-synchronous polar-orbiting (low altitude). Weather features and storms can be interpreted from satellite images and movie loops. In IR images, high (cold) clouds are often displayed with lighter grays and white, while lower clouds are darker grey.
Weather radars are active sensors that transmit strong pulses of microwaves. The radars measure the echoes that bounce back from precipitation. Stronger echoes (i.e., large scattering) show regions of more and larger rain and snow particles, which are used to estimate rainfall rate and storm intensity. Movement of these echoes show the storm track.
When the wind carries the hydrometeors in a radial direction relative to the radar, the echo frequency is shifted slightly due to the Doppler effect. Doppler radars analyze this radial velocity by measuring the phase shift of the echoes. Certain wind patterns yield characteristic signatures in the Doppler wind field, which are used to detect incipient tornadoes and other damaging winds.
Polarimetric weather radars transmit and receive microwaves with different polarizations (usually horizontal and vertical). Hydrometeors with different shapes return polarized signals differently, allowing better estimation of rainfall rates and precipitation type. Phased-array radars need no moving parts to scan the weather. Wind profilers measure horizontal wind speed and direction.