2019 – Posted in: NCERT – Tags: Atmospheric pressure, Humidity, Precipitation, Temperature, Weather and Climate, Wind

LOCAL WEATHER AND SKY

KEY TERMS THAT YOU KNOW

AIR: A gaseous mixture around the earth

RAIN: Water drop form clouds

DAILY MEAN TEMPERATURE: The average obtained by dividing the total of the maximum and the minimum temperature of a day by 2

CONSTELLATION: Small clusters of stars in the sky

Weather and Climate

Weather may be define as the state of the atmosphere at a given place and time. It is constantly changing, sometimes from hour to hour and at other times from day to day.

Climate is the sum of all statistical weather information about a large area or region. In other words, it is the ‘average weather’ as well as variations and extremes, which together characterize a place.

The basic elements of weather and the climate are same. The most important of these are temperature, humidity, precipitation, cloudiness, or sunshine, air pressure and winds

The study of weather is called METEOROLOGY and the scientists who study weather is called METEOROLOGISTS.

Temperature

Temperature is a measure of how hot or cold something is. It is expressed according to a comparative scale and shown in an thermometer.

The instrument which is used for measuring the air temperature is called a THERMOMETER. Since different substances react to temperature changing in the varying degrees, mercury or alcohol is used in thermometers. When heated, it expands more than glass and when cooled, it contracts more than the glass.

Temperature is indicated in degrees of Celsius or Fahrenheit. These are two type scales named after two scientists who devised them. In our country, we use the Celsius scale. It is a decimal scale on which the melting point of ice is at 0 and the boiling point of water at sea level pressure is 100.

These days weather and meteorological stations use an instruments called THERMOGRAPH which continuously records temperature.

Atmospheric pressure

It is the force exerted on a surface by the air above it as gravity pulls it to Earth. Atmospheric pressure is measured with the help of BAROMETER. Simple barometers use mercury to measure air pressure.

Wind direction and speed

The horizontal movements of air is called WIND, which always blows from high pressure to low pressure areas. Two basic measurements –direction and speed – are particularly significant to weather observes. A wind is named after the direction from which it blows. For example, winds blowing from the west are called WESTERLIES. The instrument used to find the wind called a WIND VANE. A wind vane always point to the direction of the wind.

Wind speed is measured by an instrument called ANEMOMETER. It is recorded in kilometers per hours. These days, weather stations use an instrument which recorded wind speed automatically, known as ANEMOGRAPH.

Humidity

Humidity can be expressed in two ways—absolute humidity and relative humidity.

Absolute humidity is the weight of water vapour in a given volume of air. It is expressed usually in grams per cubic of air.

Relative humidity is expressed in percentage. It is a ratio between the total capacity of the air for holding moisture under a given temperature and actual amount of moisture being carried by it.

The humidity of a place can be found out with the help of an instrument called Wet and Dry bulb Thermometer or hygrometer. If the temperature on the wet bulb drops sharply, the humidity is very low. On the other hand, if the temperature does not drop much, the humidity is high.

Precipitation

Precipitation takes place in several forms such as rainfall, snowfall, hail etc.

Rainfall, including other forms of precipitation, is measured with the help of a metallic instrument called a RAIN-GAUGE.

The rain gauge has a vessel and a funnel with a measuring glass. The circumference of the mouth of the funnel is equal to the base of the receiving funnel. The rain water is collected during a day (24 hours) is then measured. Rainfall is measured in millimeters.

Snowfall or hail is measured after melting it. The rain-gauge is kept in an open and level area away from trees and buildings.

The night sky

On a cloudless night, you can see countless stars. Some are bright while others are not so bright. Some of them appear in peculiar shapes. These groups of stars are called CONSTELLATIONS. There are many Constellations in the sky. Some of them are easy to locate. One of these is called the Sapta Rishi Mandal of the Great Bear. It is has seven stars.

The two stars in front of this constellation are called ‘pointers’ because they always point to the Pole Star which is relatively brighter. It always shines vertically above the North Pole. The position of the Sapta Rishi Mandal changes from time to time, but the pole Star always points to the north in the northern Hemisphere. We can thus determine the north in the Northern Hemisphere, with the help of the pole. Star on a cloudless night.

Source: NCERT

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