Basics of Weather and Climate

In this article, we will discuss some of the basic terms that you will encounter in Climatology. We will learn about the basic components of Weather and Climate, and find out how they differ.

Difference between Weather and Climate

Weather refers to the short-run atmospheric conditions involving heat, moisture and their motions for a given time and a specific area.

Weather keeps on changing. You might have heard people say - Weather is fine today, or It seems like bad weather is coming, or It’s about to rain.

On the other hand, Climate is an aggregation (average) of atmospheric conditions including heat, moisture and their motion over a long period of time and for a bigger area.

Note

The climate of temperate latitudes is far more variable than that of the tropics.

Elements of Weather & Climate

Temperature

Temperature is a physical quantity that expresses how hot or how cold something is.

Temperature of an object increases as it receives heat. Earth receives its heat majorly from the Sun. We will study about it in the topic of ‘Heat Budget of Earth’.

The instrument for measuring temperature - thermometer.

We always measure the shade temperatures, i.e. the temperatures of the air. To do so, we place the thermometer in a shelter known as the Stevenson Screen. It is done to exclude the intensity of the sun’s radiant heat.

Note

There are three very famous units that we use to measure temperature - Celcius/Centigrade (°C), Fahrenheit (°F), and Kelvin (K).

Fahrenheit = (1.8 x °C) + 32°

Eg. Let’s convert 10°C into Fahrenheit:
(1.8 x 10°C.) + 32°F = 18° + 32° = 50°F

Centigrade = (°F - 32) / 1.8

E.g. to convert 68°F into Centigrade:
(68° - 32°) / 1.8 = 36 / 1.8 = 20°C

Some temperature related terms

• Mean daily temperature - the average of maximum and minimum.

• Diurnal range of temperature - The difference between the maximum and minimum temperatures of a day.

• Annual range of temperature - The difference between the hottest month (i.e. June or July in the northern hemisphere) and the coldest month (i.e. January in the northern hemisphere).

Isotherms

Monthly mean temperatures are shown in temperature distribution maps as isotherms.

Atmospheric Pressure

Pressure is a force, measured in millibar (mb).

Air has weight and exerts a pressure on the earth’s surface. This is called atmospheric pressure.

The instrument for measuring pressure – barometer (invented by Galileo and Torricelli in 1643).

Atmospheric Pressure differs from one place to another and even at the same place it varies from time to time. Let us see some factors affecting Atmospheric Pressure:

• Atmospheric circulation: Average atmospheric pressure is equivalent to 1013 millibars. But it generally varies from 960 mb to 1,040 mb due to constant movement of air in the atmosphere. It can vary much more during the formation of cyclones and anticyclones.

• Altitude: As one ascends there is less air above and so the weight, or pressure is less.

Isobars

It is a line that joins the places of equal pressure on a map.

Wind

Wind is air in motion, and has both direction and speed.

They are majorly caused by the heat imbalance on the Earth and in the Atmosphere, that leads to pressure differences (i.e. formation of pressure gradient).

Winds are always named from the direction they blow. For example, winds blowing from the East may be termed easterlies.

Instruments for measuring wind speed and direction:

• The instrument used for measuring wind direction - wind vane or weather cock.

• The speed of wind - usually measured by an anemometer. With some modifications, the anemometer can also record wind directions.

Admiral Beaufort in 1805 devised a mechanism (Beaufort Wind Scale), using which we can assess the speed/strength of winds just by observing the way some objects are moved by a local wind.

Humidity

It is a measure of the dampness of the atmosphere.

The instrument for measuring relative humidity - hygrometer

But what is relative humidity?

Difference between Absolute and Relative humidities

• Absolute humidity - It is the actual amount of water vapour present in the air.

• Relative humidity - It is the ratio between the actual amount of water vapour and the total/maximum amount of water vapour the air can hold at a given temperature, expressed as a percentage.

Note

Relative humidity depends on the temperature.

You can infer it from the definition of relative humidity. It is because, the maximum amount of water vapour that the air can hold depends on the temperature.

Warm air can hold more water vapour than cold air. So, as you increase the temperature of air, its water vapour carrying capacity increases too and vice-versa.

If the relative humidity is low, then it means that the air is dry (e.g. in deserts).
If the relative humidity is high, then it means that the air is moist (e.g. in equatorial regions).

What happens at 100% relative humidity?

Relative humidity of 100% means that the air is completely saturated. It cannot hold any more water.

The air temperature at this point is called to be at dew-point. If now we cool the air any further, then the water vapour present in the air will condense into clouds or rain. (this is what actually happens as air moves up and gets cooled, leading to the formation of clouds and eventually rain)

Rainfall

Rainfall and other forms of precipitation (snow, sleet and hail).

They are measured by a rain gauge.

• Rain-day - a period of 24 hours with at least 0.01 inch or more rain being recorded.

• Wet day - a period of 24 hours with at least 0.04 inch or more rain being recorded.

Note

10 - 12 inches of snow is equivalent to 1 inch of rain.

Isohyet

It is a line that joins the places having the same mean annual rainfall on a map.