It is necessary to know the natural conditions in the higher atmosphere in addition to that near the ground level in order to make weather forecast. This is done by sending balloons carrying radiosonde, i.e. meteorological data recording instruments and radio transmitter, to high altitudes. A folded parachute with a container of radiosonde is tied to helium or hydrogen filled balloon. Meteorological instruments record the data such as atmospheric pressure, wind speed and direction, temperature, relative humidity etc and transmit it to the meteorological observatories by radio transmitter.
Nearly 650 meteorological observatories all over the world send radiosonde balloons twice a day precisely at 00:00 hrs and 12:00 hrs Greenwich Mean Time. As the forces responsible for ‘concocting’ weather (humidity, wind temperature, atmospheric pressure etc) are fluctuating unless all the countries ascertain atmospheric data at the same time it is not possible to construct the overall picture for forecasting weather on national or international scale.
Weather balloon (Wikipedia)
Weather forecasting (Wikipedia)