WEATHER 

Weather and climate are the most common terms that are used daily. We check for local weather reports to plan our days, but not climate reports. There is still a lot of confusion over the difference between the two.

1. Weather

The weather reflects short-term conditions of the atmosphere. Any casual remarks about the atmospheric conditions of a certain place at a certain time are about the weather. It is never static/constant, so cannot be generalized. In the same geographical landmass, it may be sunny in one part and a few miles away it may be raining. It is important to know that any place can be subjected to random changes in weather at any instant.

2. Climate

Climate is the average daily weather for an extended period at a certain location. The average atmospheric conditions of an area over a considerable time are considered as climate. For climatic averages, a minimum period of 35 years is desirable. This involves the systemic observation, recording and processing of the various elements of climate like rainfall, temperature, humidity, winds, clouds, pressure, and sunshine before any standardization of the climatic averages can arrive.

Differences between weather -climate

• The changeability in the weather and climate of a country differs. The climate of temperate latitudes is far more variable than that of tropical. There is a strong influence of weather and climate on life forms including physical characteristics, mental alertness, the food we eat, dressing style, etc, They even control the life expectancy of organisms. For example; death rates are normally high in tropical regions and low in deserts because germs are not easily transmitted in regions with high temperatures and low humidity.
• Meteorological Stations or weather bureaux are present all over the earth, including oceans with the most sophisticated weather instruments to gather a wide range of data as raw materials for the construction of weather maps or synoptic charts.
• Professional meteorologists can forecast the weather accurately from local observations. A casual glance at the sky will be sufficient for a weather expert to sum up the conditions of the atmosphere. Modern air transport, military operations, geographical expeditions, and games are often considered based on meteorological reports.

3. Elements of Weather and Climate 

Weather is composed of different elements, and the way they interact with each other to create different atmospheric conditions or weather events.

1. Temperature: Temperature is a very important element of climate and weather. Temperature is a measurement of the amount of kinetic energy present in the air, which manifests itself physically through the experience of heat or cold. The instrument used for measuring temperature is a THERMOMETER, which is made up of a long narrow glass tube and filled with mercury or alcohol. It works on the principle that mercury expands when heated and contracts when cooled.

Scales that are typically used to measure temperature are Celsius, Fahrenheit, and Kelvin. In Fahrenheit (F^o) the freezing point is 32^o F and the boiling point is 212^o F. For most scientific reasons Centigrade or Celsius scale is used. Its freezing point is 0 C and its boiling point is 100^o C. The mean daily temperature of India is 90 ^oF or 33^oC. For rapid conversion of one scale to into another, the following formula is used:

                      To obtain Fahrenheit = (1.8xC^o)+32^o

       For example, To convert 30 C into Fahrenheit:   (1.8x30)+32 ^oF=54 + 32 =86 ^oF

                                To obtain Centigrade = (F^o-32)/1.8

       For example, to convert 90 F into centigrade:   (90-32)/1.8=58/1.8= 32^oC

A temperature taken in open daylight is very high because it measures the direct insolation of the sun. It is also known as the temperature in the sun. For agricultural purposes, earth temperatures are taken at various depths in the ground. To assess possible damages done by ground frosts to crops in temperate latitudes grass temperatures are also taken.

The temperatures that we generally speak are shade temperatures i.e., the temperature of air. By placing the thermometers in a standard meteorological shelter known as STEVENSON SCREEN, we can measure the accurate temperature. This Stevenson screen consists of a white wooden box for about 4 feet above the ground level. The roof is double-layered with an intervening air space to exclude much of the direct rays of the sun. The sides of the box are Louvre-like Venetian Blinds to allow free circulation of the air. The Stevenson’s Screen normally carries maximum and minimum thermometers and dry and wet bulb thermometers. Larger ones may also contain a self-recording thermogram.

Maximum and minimum temperatures are measured by the maximum and minimum thermometers, they are either in the form of separate thermometers or joined in a U-shaped glass tube as in the six’s thermometer. The mercury in the closed glass tube expands when the temperature rises. The maximum thermometer records the highest temperature and the minimum thermometer records the lowest temperature reached during the day. This U-shaped maximum and minimum thermometer contain both alcohol and mercury.

Means Daily Temperature: The average of the maximum and minimum temperatures recorded during the day is known as the mean daily temperature. However, an accurate mean should be the average of 24 readings taken at hourly intervals.

The difference between the maximum and minimum temperatures of a day gives the diurnal range of temperature. The difference between the hottest month i.e. July in the northern hemisphere and the coldest month i.e. January in the northern hemisphere gives the annual range of temperature.

Isotherm:  A line drawn on a map or a chart joining points with the same temperature is called an isotherm or a simple temperature graph are temperature disturbance graph. Temperature decreases at the rate of 1^oF, drop in temperature for 300 feet ascent in altitude.

2. Rainfall: Rainfall along with other forms of precipitation like snow, sleet and hail is always measured by a metal instrument called a rain gauge. It consists of a copper cylinder with a metal funnel which leads into a copper container or a glass bottle. The hole in the funnel that leads down to the container is very small so that evaporation of the collected rain is minimized. The gauge should be at least 1 ft above the ground and firmly fastened to avoid splashing.

The measurement of the rainfall is done by removing the funnel and emptying the rain in the container into a graduated cylinder.

For meteorological recordings, the rain day is reckoned as a period of 24 hours with at least 0.01 inch or more rain being recorded if the amount exceeds 0.04Inch it is considered as a wet day.

The mean annual rainfall is obtained from the averages from of annual rainfall taken over a long period like 35 years. For plotting in rainfall maps, places having the same mean annual rainfall or joined by a line called isohyet.

3. Air Pressure: Air pressure is the result of the pressure created by the weight of the air in the Earth’s atmosphere. It is also called as barometric pressure, named after the instrument used to measure the air pressure. The pressure varies from place to place and from time to time.

The instrument used for measuring pressure is a barometer invented by the scientist Galileo and his assistant Torricelli. The ordinary mercury barometer consists of a long glass tube, sealed at the upper end and opened at the lower end. The lower end is inverted in a bowl of mercury whose surface is exposed to the air. Variations in the atmospheric pressure on the mercury surface are balanced by the column of mercury in the glass tube this gives the pressure of the air and can be quickly recorded on the scale of the glass tube. Mercury is chosen because it is the heaviest liquid known. If ordinary water is used, the corresponding column for normal atmospheric pressure would be 34ft at sea level, the mercury column is 29.9 inches or 76cm or 760mm. If the pressure increases the air pressing on the surface will force up the mercury column to about 31 inches. If the pressure decreases the mercury column will drop about 28 inches. A new unit known as millibar (MB) was adopted to measure the pressure. A normal atmospheric pressure equivalent to 14.7 Lb per square inch per weight or mercury column reaches 1013MB.

Isobars: Places of equal pressure are joined by lines called isobars, on maps.

In temperate latitudes, pressure changes are very rapid in the form of cyclones and anticyclones.

Air pressure varies with several factors.

As we move above the sea level the pressure decreases because there is less air above. The barometer is also sensitive to gravitational forces at different latitudes. Mercury also expands with the increase in temperature. For outdoor measurement of air pressure a portable but less accurate type of barometer called an aneroid barometer is used.

In aeroplanes a modified aneroid barometer a altimeter is used. As pressure decreases with altitude at an approximate rate of 1-inch drop in the mercury reading for every 900 feet ascent.

4. Winds: The moving air is called wind. The wind has both direction and speed. The wind is made up of a series of gusts and eddies that can only be felt but not seen. The major and extreme weather events like cold and warm fronts, clouds, thunderstorms and hurricanes are all driven by wind.

The instrument widely used for measuring wind direction is wind vane or weather cock. As wind direction is always blocked by trees and tall buildings, weather vanes are placed in an exposed position, to get a true direction. There is a vane on the top, which is free to move with the prevailing wind. The other part of the wind vane has four compass points which are stationary and show in which direction the wind is moving. Winds are always named from the direction they blow.

The direction of smoke drift and flag movements in fairly open spaces provide the most reliable indication of wind.

The speed of wind is usually measured by an Anemometer. It consists of three or four semi-circular cups attached to the ends of horizontal spokes mounted on a high vertical spindle. The speed recorded is not accurate because after the winds have abated, the rotation continues due to its own momentum. With some modifications, the anemometer can also record wind directions.

5. Sunshine: The amount of sunshine a region receives, depends on the seasons, a factor determined by latitude and by the position of the earth in its revolution around the sun. In the meteorological station, sunshine duration is recorded by a SUN-DIAL through which the sun’s rays are focused on a card for hours. A line is made on the card when it is sufficiently heated but not when the rays are fainted.

6. Humidity: Humidity is the amount of water vapour that is present in the atmosphere. It is the measure of the dampness of the atmosphere which varies greatly from place to place at different times of the day. The actual amount of water vapour present in the air, which is expressed in grams per cubic meter, is called absolute humidity.

Relative humidity: This is the ratio between the actual amount of water vapour and the total amount, that the air can hold at a given temperature, expressed in percentage.

Warm air can hold more water vapour than cold air. When the relative humidity reaches 100% the air is completely saturated. The temperature of the air is said to be at the due point. Further cooling will condense the water vapour into clouds or rain.

When the relative humidity is high- the air is moist as in equatorial regions, when it is low- the air is dry as in the deserts.

A hygrometer is an instrument used for measuring relative humidity. It is comprised of wet and dry bulb thermometers placed side by side in the Stevenson Screen. The dry bulb is the ordinary thermometer that measures the shade temperature. The wet bulb is kept wet by a wick that dips into a reservoir of distilled water. When the air is not saturated evaporation, which produces a cooling effect, takes place from the moist Vick, the wet bulb therefore always shows a lower reading than the dry bulb. The wet bulb always shows a lower reading than the dry bulb.

7. Clouds: Clouds are water droplets or water in different states like ice and snow crystals, which form after water vapour reaches condensation level and can no longer remain in gaseous form.

When air rises it is cooled by expansion. After the dew point has been reached, cooling leads to the condensation of water vapour in the atmosphere. Tiny droplets of water vapour which are too small to fall as rain or snow (less than 0.001cm) will be suspended in the air and float as clouds. Their form, shape, height and movements tell us about the sky conditions and the weather we are likely to experience. For meteorological purposes, the amount of cloud cover in the sky is expressed in eighths or octas.

Isonephs: On maps places with an equal degree of cloudiness are joined by lines known as isonephs.

 Clouds

The classification of clouds is based on a combination of form height and appearance.

1. High clouds- Mainly cirrus (Ci) of feathery form at 20 - 40,000 ft above ground.
2. Cirrus (Ci) – This looks fibrous and appears like wisps in the blue sky. It is often called mares or tails. It indicates fair weather and often gives a brilliant sunset.
3. Cirrocumulus (Cc) – This appears as white globular masses, forming ripples in a mackerel sky.
4. Cirrostratus (Cs) – This resembles a thin white sheet or veil. The sky looks milky and the sun or moon shines through it with a characteristic halo.
5. Medium Clouds – Middle height clouds at 7 – 20,000ft, mainly alto (Alt)
6. Altocumulus (Alt-Cu) – These are woolly, bump clouds arranged in layers and appear like waves in the blue sky. They indicate fine weather.
7. Altostratus (Alt-St) – These are denser greyish clouds with a watery look. They have fibrous structures through which the sun's rays shine faintly.
8. Low Clouds – Mainly stratus or sheet clouds below 7000ft.
9. Stratocumulus (St-Cu) – This is a rough bumpy cloud with waves more pronounced than in altocumulus. There is a huge difference between bright and shaded parts.
10. Stratus (St) – This is a very low cloud uniformly grey and thick which looks like a highland fog. It brings dull weather with a light drizzle; it reduces visibility to aircraft and is dangerous.
11. Nimbostratus (Ni St) – This is a dark dull cloud clearly layered also known as a rain cloud which brings continuous rain, snow or sleet.
12. Clouds with great vertical extent – Mainly cumulus or heap clouds with no definite height.
13. Cumulus (Cu) – This is a vertical cloud with a rounded top and horizontal base typical of humid tropical regions associated with uprising conventional currents. It is a fair weather cloud.
14. Cumulonimbus (Cu-Ni) – This is a large grown cumulus cloud extending a vertical height from 2000 – 30,000ft. This is frequently seen in tropical afternoons. It is also referred to as thunder clouds and brings conventional rain with lightning and thunder.

Other elements of weather include visibility. The elements affecting visibility include Haze caused by dust and smoke in an industrial area. The condensation of water vapour in the air causes small droplets of water to float about forming clouds at ground level is called mist. Fog-Water condensing on dust and other particles causes fog.