Weather
· Any casual remarks about the atmospheric conditions of a certain place at a certain time are about weather. It is never static, and thus cannot be generated.
· When we speak of climate we mean the average atmospheric
conditions of an area over a considerable time.
The Elements of Weather and Climate:
1. Rainfall:
· Rainfall including other forms of precipitations (snow, sleept and hail) is always measured by a metal instrument called a Rain Guage.
· For Plotting in rainfall maps, places having the same mean annual rainfall are joined by a line called an Isohyet.
2. Pressure:
· Air is made up of a number of mixed gases and has weight. It therefore exerts a pressure on the earth‟s surface which varies from place to place and from time to time. This force that presses on the surface of any object can be fairly accurately measured.
· The instrument for measuring pressure is a Barometer invented by the scientist Galileo and his assistant Torricelli in 1643.
· As Pressure is a force, it is more appropriate to measure it in terms of a unit of force. A new unit known as the Millibar (mb) was adopted by meteorological stations in 1914.
· On maps places of equal pressure are joined by lines called Isobars.
· Since the mercury Barometer that dips in liquid mercury is
inconvenient for outdoor measurement, a more portable but less accurate type known as the Aneroid Barometer is used.
· In aeroplanes, a modified type of aneroid Barometer called an Altimeter is used.
· As pressure decreases with latitudes at an approximate rate of 1 inch drop in mercury reading for every 900 feet ascent, the altimeter gives the reading in feet for height attained instead of Millibars or inches.
3. Temperature:
· Temperature is a very important element of climate and weather. The instrument for measuring temperature is the Thermometer.
· It works on the principle that mercury expands when heated and contracted when cooled.
· In °F,(Fahrenheit) the freezing point is 32° F. And the boiling point is 212° F. For most scientific purposes the Centigrade °C. scale is preferred. Its freezing point is 0° C. and its boiling point is 100° C.
· To obtain Fahrenheit = (1.8 * °C.)+32° F.
· Temperature is a very important element of climate and weather. The instrument for measuring temperature is the Thermometer.
· It works on the principle that mercury expands when heated and contracted when cooled.
· In °F,(Fahrenheit) the freezing point is 32° F. And the boiling point is 212° F. For most scientific purposes the Centigrade °C. scale is preferred. Its freezing point is 0° C. and its boiling point is 100° C.
· To obtain Fahrenheit = (1.8 * °C.)+32° F.
E.g to convert 20° C. into Fahrenheit:
(1.8*20° C.) +32° F. + 36° +32° =68° F
To obtain Centrigrade = (°F.-32)÷1.8
e.g. to convert 59°F into Centigrade:
(59°-32°)÷1.8=27÷1.8=15°C.
(1.8*20° C.) +32° F. + 36° +32° =68° F
To obtain Centrigrade = (°F.-32)÷1.8
e.g. to convert 59°F into Centigrade:
(59°-32°)÷1.8=27÷1.8=15°C.
4. Humidity:
· Humidity is a measure of the dampness of the atmosphere which varies greatly from place to place at different times of day. The actual amount of water vapour present in the air, which is expressed in grams per cubic metre, is called the Absolute Humidity but more important from the point of view
of weather studies is the Relative Humidity.
· This is the ratio between the actual amount of water vapour and the total amount the air can hold at a given temperature, expressed as a percentage.
· Humidity is a measure of the dampness of the atmosphere which varies greatly from place to place at different times of day. The actual amount of water vapour present in the air, which is expressed in grams per cubic metre, is called the Absolute Humidity but more important from the point of view
of weather studies is the Relative Humidity.
· This is the ratio between the actual amount of water vapour and the total amount the air can hold at a given temperature, expressed as a percentage.
· When the relative humidity reaches 100 percent, the air is completely saturated. The air temperature is said to be at Dew Point.
· Further cooling will condense the water vapour into clouds or rain.
· The instrument for measuring relative humidity is the Hygrometer.
· Further cooling will condense the water vapour into clouds or rain.
· The instrument for measuring relative humidity is the Hygrometer.
5. Winds:
· Wind is air in motion and has both direction and speed. Unlike other elements in climate such as rain, snow or sleet, winds are made up of a series of gusts and eddies that can only be felt but not seen.
· The instrument widely used for measuring wind direction is a Wind Vane or weather cock.
· Winds are always named from the direction they blow, an east wind is one that blows from east to west and a south-west wind is one that blows from the south-west.
· The speed of wind is usually measured by an Anemometer.
· Since an Anemometer is not easily available, a little practice of
local wind observations will help us to assess the speed of winds.
· By seeing the way some objects move, a great deal can be said about the strength of winds. The best guide is obtainable from the Beaufort Wind Scale which was devised by Admiral Beaufort in 1805 for estimating wind speed.
· Wind is air in motion and has both direction and speed. Unlike other elements in climate such as rain, snow or sleet, winds are made up of a series of gusts and eddies that can only be felt but not seen.
· The instrument widely used for measuring wind direction is a Wind Vane or weather cock.
· Winds are always named from the direction they blow, an east wind is one that blows from east to west and a south-west wind is one that blows from the south-west.
· The speed of wind is usually measured by an Anemometer.
· Since an Anemometer is not easily available, a little practice of
local wind observations will help us to assess the speed of winds.
· By seeing the way some objects move, a great deal can be said about the strength of winds. The best guide is obtainable from the Beaufort Wind Scale which was devised by Admiral Beaufort in 1805 for estimating wind speed.
6. Sunshine:
· In the meteorological station, sunshine duration is recorded by
a Sun-dial.
· On maps places with equal sunshine duration are joined by
isobels.
7. Clouds:
· When air rises, it is cooled by expansion. After dew-point has been reached cooling leads to condensation of water vapour in the atmosphere.
· In the meteorological station, sunshine duration is recorded by
a Sun-dial.
· On maps places with equal sunshine duration are joined by
isobels.
7. Clouds:
· When air rises, it is cooled by expansion. After dew-point has been reached cooling leads to condensation of water vapour in the atmosphere.
· Tiny droplets of water vapour which are too small to fall as rain or snow will be suspended in the air and float as Clouds.
· For Meteorological purposes the amount of cloud cover in the sky is expressed in Eights or Oktas.
· On maps places with an equal degree of cloudiness are joined by lines known as Isonephs.
· The classification of clouds is based on a combination of form, height and appearance.
· Four major cloud types and their variations can be recognised.
a) High Clouds: Mainly cirrus (Ci)of feathery form at 20-40,000 feet above ground.
i) Cirrus (Ci) This looks fibrous and appears like wisps in the blue sky: it is often called Mares Tails.
ii) Cirrocumulus (Cc) This appears as white globular masses, forming ripples in a „mackerel sky‟
iii) 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‟
b) Medium Clouds: Mainly alto(Alt) or middle height clouds at 7—20,000 feet
iv) Altocumulus(Alt-Cu) These are wooly, bumpy clouds arranged in layers and appearing like waves in the blue sky.
v) Altostratus (Alt-St) These are denser greyish clouds with a watery look.
c) Low Clouds: mainly stratus or sheet clouds below 7,000 feet.
vi) Stratocumulus (St-Cu) This is rough bumpy cloud with the waves more pronounced than in altocumulus.
vii) Stratus (St) This is a very low cloud, uniformly grey and thick, which appears like a Low Ceiling or highland fog. It reduces the visibility of air craft and is thus a danger.
viii) Nimbostratus (Ni-St) This is a dark dull cloud, clearly layered and is also known as a Rain Cloud. It brings continuous rain, snow or sleet.· For Meteorological purposes the amount of cloud cover in the sky is expressed in Eights or Oktas.
· On maps places with an equal degree of cloudiness are joined by lines known as Isonephs.
· The classification of clouds is based on a combination of form, height and appearance.
· Four major cloud types and their variations can be recognised.
a) High Clouds: Mainly cirrus (Ci)of feathery form at 20-40,000 feet above ground.
i) Cirrus (Ci) This looks fibrous and appears like wisps in the blue sky: it is often called Mares Tails.
ii) Cirrocumulus (Cc) This appears as white globular masses, forming ripples in a „mackerel sky‟
iii) 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‟
b) Medium Clouds: Mainly alto(Alt) or middle height clouds at 7—20,000 feet
iv) Altocumulus(Alt-Cu) These are wooly, bumpy clouds arranged in layers and appearing like waves in the blue sky.
v) Altostratus (Alt-St) These are denser greyish clouds with a watery look.
c) Low Clouds: mainly stratus or sheet clouds below 7,000 feet.
vi) Stratocumulus (St-Cu) This is rough bumpy cloud with the waves more pronounced than in altocumulus.
vii) Stratus (St) This is a very low cloud, uniformly grey and thick, which appears like a Low Ceiling or highland fog. It reduces the visibility of air craft and is thus a danger.
d) Clouds with Great Vertical extent: mainly cumulus or heat clouds with no definite height 2—30,000 feet.
ix) Cumulus (Cu) This is a vertical cloud with a rounded top and horizontal base, Typical of humid tropical regions, associated with uprising convectional
currents.
x) Cumulonimbus (Cu-Ni) This is, infact, an over grown cumulus cloud, extending for a tremendous vertical height from a base of 2000 feet to over 30,000 feet. Its cauliflower top often spreads out like an Anval. It is
also referred to as a Thunder Cloud and brings convectional ram. Accompanied by Lightening and Thunder.
8. Other Elements Pertaining to Visibility:
A) Haze : This is caused by smoke and dust particles in Industrial Areas or may be due to unequal refractions of light in air of different densities in the Lower Atmosphere.
B) Mist: The condensation of water vapour in the air causes small droplets of water to float about forming clouds at ground level mist. Unlike Haze, Mist occurs in wet air, When the relative humidity is over 75 percentage.
C) Fog: Ordinary fog is due to water condensing on dust and other particles like smoke from houses and factories. It only occurs in the lower strata of the atmosphere as a sort of dense ground cloud.
Generally speaking fogs are more common over seas than lands, and are most prevelant over coastal areas.
Source: Goh Cheng Leong's Certificate of physical and Human Geography