Weather Forecasting
Weather is our most common topic of conversation, a factor in must of our agricultural, industrial and civic planning and a matter of constant concern- whether it is for planning a simple picnic, what we eat or wear, our work, our safety, etc. The study of the weather is called meteorology and the scientists who study weather are meteorologists. Accurate forecasting depends on knowing as much as possible about the total state of the atmosphere. The Meteorological department operates a far flung network of stations where such information is specially collected and sent to regional forecasting centers. Our Central Weather Observatory is at Pune; there are several regional observatories such as in New Delhi. Elements at higher altitude are sampled by radiosonde, weather balloons and satellites. Radars track patterns of storms and air masses. Synoptic or integrated surface reports and weather masses maps are made every six hours using conventional symbols agreed upon internationally.
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Because conditions around the globe affect local weather, international weather stations exchange information regarding weather using special communication network including undersea cable and satellite links.
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Two kinds of satellites watch weather from outer space- geostationary satellites, about 25,900 km above the equator that moves at the same speed as the earth’s rotation so that they always lie over the same place above the earth.
The polar orbit satellites travel in a north-south orbit so that the earth turns under them. These can observe most of the planet earth, including the Polar Regions that geostationary satellites cannot see. Satellites send raw date to weather stations via communication centers. They also collect date from ships, aircraft and island observatories.
About fifteen years ago five geostationary satellites went around the earth. They were, Meteostat (Europe, Insat (India), Goes West and Goes East of USA, Meteor of Russia and Himawari of Japan. The Meteor and NOAA of USA have Polar Orbits.
Weather Instruments
A meteorologist has to supplement the information from satellites with date read off weather instruments at different regional centers. You already know how to use a thermometer for reading temperature and a barometer to tell the atmospheric pressure. What special care would you take in placing a Six’s thermometer for temperature readings.
Measuring Wind Direction and Speed
Wind direction and speed are read off a wind vane and an anemometer respectively. The wind vane always points to the direction of the wind. The wind direction is shown on a dial that is connected to the wind vane.
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An anemometer is used to measure wind speed. It is recorded in kilometers per hour depending on the number of revolutions the cups make and is recorded no the meter.
Winds above the ground surface are measured with pilot balloons, radar tracking of balloon-borne targets, such as radiosonde and by satellites. Cloud motions are observed by satellite sensors. Aircraft pilots also report winds and turbulence observed in flight.
Measuring Humidity
Humidity is the amount of water vapor in the air per unit area. It can be expressed as absolute and relative humidity. The instrument used to measure it is a wet and dry bulb thermometer, or a hygrometer. The bulb of one thermometer is kept wet by wrapping it with a muslin cloth that is dipped in the cup of water at the base of the thermometer. As evaporation takes place, the temperature of the wet bulb drops sharply indicating low humidity. If the temperature does not drop much, the humidity is high. While absolute humidity is expressed as grams of water per cubic meter of air, relative humidity which is a more accurate way of expressing humidity is expressed as a percentage. It is a ratio between the total capacity of the air for holding moisture under a given temperature and the actual amount of moisture being carried by it.
Hydrographs
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Hydrographs are self recoding instruments for reading relative humidity. It has a sheet of human hairs that have been treated to remove oil. As the relative humidity increases, the hair increase in length and operate the recording mechanism. Knowledge of relative humidity is important in predicting precipitation and ice formation.
Measuring Precipitation
To measure precipitation, rain gauges of several types are used. They measure rainfall as well as snowfall and hail. A simple rain gauge has a mental container with a funnel and a measuring jar. The funnel fits well into the receiving vessel. Rain is collected in the vessel through the funnel and measured at the end of twenty four hours. It is measured in millimeters. Then centimeters of rain collected indicates that on level ground a layer of rain ten centimeters deep would remain if none of the water ran off or seeped into the ground. Snowfall and hail are measured after they melt.
Certain precautions are to be observed in using a rain-gauge. It is kept in an open, level area far from trees or buildings. It is usually placed some 30 cms above the ground level.