Omegadropwindsondes (ODW) are instrument packages that airplanes drop to measure meteorological conditions below flight level. As the instrument falls on its parachute, it measures temperature with a thermistor, relative humidity with a hydristor, and pressure with an aneroid cell. A truly accurate determination of pressure as a function of height requires downward integration of the hydrostatic equation from the known pressure and geometric altitude at flight level using density based upon measured temperature and humidity as a function of measured pressure. The "O" in ODW comes from the Omega navigation receiver that each instrument carries. The dropsonde moves horizontally with the wind it encounters as it falls. Thus, the difference between the top and bottom of a layer of horizontal position determined by Omega divided by the time required to fall through the layer is the average wind in the layer.

In this sketch, a WP-3D has just released an ODW. The thermodynamic sensors are visible on the bottom of the package. The aneroid, Omega receiver, and radio transmitter are inside the sonde's cylindrical metal body which serves as an antenna. Each sonde costs $600, weighs about 1.5 kg (3 lbs), and falls at 5 m/s (1000 ft/ min) when deployed.