Weather is the day to day change in the state of the Earth’s atmosphere. Every day, we get weather forecast through newspaper, radio and television. The weather experts predict about the possibility of rain, storm or thunder showers. Do you know how this information is obtained? Scientists have to study many aspects to make predictions about the weather. In fact, weather forecasting is the practical application of the knowledge gained through the study of weather behaviour, termed as meteorology. The weather depends mainly on atmosphere, i.e. pressure, wind direction, humidity, ambient temperature, cloud formation, rains, snowfall, etc. Weather forecasting is organised nationally by government agencies and is coordinated internationally by the ‘World Meteorological Organisation’ (WMO). There are three basic stages — observation, analysis and forecasting. Observation involves round-the-clock weather watching and gathering of meteorological data by land stations, balloon launching and also by using satellites. In analysis, information is coordinated at national centres and plotted in terms of weather maps and charts. Then in forecasting, predictions of future weather pattern are made by the ‘synoptic method’ — in which the forecaster applies his experience of evaluation of the past weather patterns of current situation. Computers at these centres analyse the data collected by different methods. Nowadays, the use of supercomputers have revolutionised the area of forecasting. An instrument named ‘anemometer’ is used to measure the speed and the direction of the wind, whereas another kind of instrument called ‘hygrometer’ is used to measure the humidity present in the air. Rain gauges measure the amount of rainfall, while sunshine recorders measure the duration of the sunshine. ‘Maximum-minimum thermometers’ give information about varying temperatures during the 24 hours of a day. The atmospheric pressure is measured with the help of barometers. Sudden drop in the atmospheric pressure indicates the possibility of a storm or hurricane. Gradual drop in the pressure, indicates the increase in the humidity and as such the possibility of rain. Easterly winds are also indicators of rain. Rise in the atmospheric pressure heralds fine weather.