Life for meteorologists is a constant search for more and more data, all aimed at building an accurate numerical picture of the atmosphere in three dimensions. Nowadays satellites provide a great deal of the needed information, but the data from satellites alone is not sufficient.
Satellites cover a very large area, and show in great detail how temperature, cloud and other elements vary from one region to another, and also at the different levels in the atmosphere. But to get the best out of them, it is still desirable to have a good network of accurate instruments at ground level. The information from this network can then be used to interpret the satellite images more effectively.
It is relatively easy, albeit expensive, to establish a network of weather stations for making surface and upper-air observations over land. Data over the oceans, however, is a different matter. Strategically placed weather ships traditionally provided data of this kind, but dedicated weather ships are very expensive to operate, and there has been a tendency in recent years to phase them out.
Surface weather observations are regularly received from merchant ships plying the Atlantic. In addition, drifting ocean buoys are useful for this purpose. They measure air and sea temperature, pressure, humidity and other elements of interest to the forecaster, and the data, together with the position of the buoy, is relayed to forecasting offices around the world by satellite.
ASAP and AMDAR provide data about conditions in the upper atmosphere. ASAP in a meteorological context is not "as soon as possible", but stands for Automated Shipboard Aerological Programme. It describes a scheme whereby "ships of opportunity" are fitted with automated equipment to launch hydrogen-filled radiosonde balloons; the radiosonde is a small box containing instruments and also a transmitter, and as it floats upwards it relays, via satellite, information on temperature, pressure and humidity at various levels in the atmosphere.
AMDAR stands for Aircraft Meteorological Data Relay. The principle here is simple: the equipment comprises a small "black-box" fitted inside the aircraft, and connected to sensors on the airframe, which measure the outside temperature and the barometric pressure. It is also connected to the aircraft's navigational system which, combined with other data from the aircraft, allows it to estimate the speed and direction of the wind. In level flight ASDAR provides data readings every seven minutes; on ascent or descent, readings are taken more frequently to provide a picture of the vertical distribution of wind, temperature and pressure at the different levels.