To understand and predict the weather, we first must measure it. In-situ or direct weather instruments must physically touch, or be exposed to, the air being measured. Examples include thermometers (temperature), barometers (pressure), hygrometers (humidity), anemometers (wind speed), wind vanes (wind direction), pyranometers (solar radiation), and rain gauges (precipitation).
Remote sensors infer the weather conditions by detecting the characteristics of waves propagating from distant regions. The waves can be electromagnetic (light, infrared, microwaves, etc.) or sound. Active remote instrument systems such as radar (RAdio Detection And Ranging) transmit their own waves toward the object and then receive the signal bounced back to the sensors. Passive ones, such as some satellite sensors, receive waves naturally emanating from the object.
Clouds, precipitation, and air molecules can totally or partially absorb electromagnetic radiation (Fig. 8.1a), scatter it into many directions (Fig. 8.1b), or reflect it (Fig. 8.1c). Objects also emit radiation (Fig. 8.1d) according to Planck’s law. Interactions of radiation with the Earth, air, and clouds create the signals that satellites and radar use.
This chapter covers the basics of weather satellites and radar. Other remote-sensor systems, not covered here, include lidar (LIght Detection And Ranging), and sodar (SOund Detection And Ranging).
- 8.1: Radiative Transfer for Satellites
- Weather satellites have passive sensors called radiometers that measure upwelling electromagnetic radiation from the Earth and atmosphere. Infrared (IR, long-wave) and microwave radiation are emitted by the Earth, ocean, atmosphere, clouds, and the sun (see the Radiation chapter). Visible light (short-wave or solar radiation) is emitted by the sun and reflected and absorbed by the Earth system. Additional portions of the electromagnetic spectrum are useful for remote sensing.