Goh Cheng Leong Chapter 13: Weather YouTube Lecture Handouts

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Goh Cheng Leong Chapter 13: Weather
  • Weather: Casual remark about atmospheric condition at certain place and time – never static and cannot be generalized
  • Climate: Average condition over a period of time around 35 years – systematic observation, recording and processing
  • Degree of variability differs from latitude to latitude – climate of British Isles is highly variable while that of Egypt is static
  • Direction of winds controlled the trading routes
  • Death rates are high in tropics and low in deserts as germs are transmitted easily in tropics due to high humidity
  • Weather Bureaux or Meteorological stations gather weather data for weather maps or synoptic charts – forecast the weather form local observatory – take precautions against frost, hail, heavy snowfall or drought & sailors are warned of oncoming typhoons

Elements of Weather and Climate

Rainfall: Form of precipitation measured by rain gauge – copper cylinder with metal funnel of 5 inch or 8 inch diameter – atleast 1 feet above ground and fastened to avoid splashing – placed away from tall building and high trees – measured by removing funnel and emptying it in graduated cylinder with 1.5 inch diameter. Taper measure tapers at bottom and gives 0.005 inch accuracy

Rain day – 24 hours with 0.01 inch rainfall

Wet day – 24 hours with 0.04 inch rainfall

10 - 12 inch of snow is equivalent to 1 inch of rainfall

Daily records are added at month end to find total records. Monthly total added to find annual rainfall

Mean annual rainfall is average taken over long period

Isohyets – lines joining places of equal rainfall

Rain Gauge

Pressure: Air is made of gases and has weight. Pressure is measured by barometer invented by Galileo and assisted by Torricelli in 1643 – long glass tube sealed at upper end and open at lower end in mercury – variation is balanced by column of mercury in glass tube

Hg Pressure Equal to Air Pressure and Atmospheric

For ordinary water – column of normal atmospheric pressure would be 34 feet.

At sea level – mercury column is 29.9 inch or 760 mm

If pressure increases, air pressure will force mercury upto 31 inches (HP) & when drops it can go to 28 inches (LP)

Unit of millibar adopted in 1914 by meteorological stations

Isobars – lines joining places of equal pressure

Sea level reading of 30 inch will be halved on mountains of 3.5 miles ASL – as one ascends there is less air above so weight or pressure is less.

Pressure is Unit of Force

Mercury also expands with heating so corrections required for altitude, latitude and temperature

Aneroid barometer – metal container with air driven out to form vacuum – no pressure inside and any pressure outside will cause lid to move inward and register high pressure on revolving dial. With pressure decrease – it will turn outwards

Pressure Unit of Force

Altimeter – modified aneroid barometer – pressure decreases at 1 inch drop in mercury reading for every 900 feet ascent – altimeter gives reading in height attained instead of millibars or inches and pilot can tell altitude ASL

Barogram – for continuous recording of pressure – self recording device

Temperature: measured by mercury or alcohol in thermometer – mercury expands when heated and contracts when cooled.

  • - freezing point at 32 and boiling point at 212
  • - freezing point at 0 and boiling point at 100
  • It measures degree of hotness – temperature in open daylight is high and measures direct insolation of sun
  • Thermometer is enclosed in glass tube and bulb embedded in paraffin wax so that they are less sensitive to temperature changes
  • To assess damage to crops in temperate latitude by frost – grass temperature is considered
  • Shade temperature is the temperature of the air
  • Stevenson՚s Screen – thermometer is placed in standard meteorological shelter – white wooden box – 4 ft above ground level on stilts – roof is double layered with intervention of air spaces to exclude direct heat – sides are lowered like venetian blinds to allow free circulation of air – one side is hinged to serve as door – carries maximum and minimum thermometer and wet and dry thermometer
  • Large Stevenson՚s screen can also have thermogram and hygrograms
Image Result for Stevensona Screen
  • Maximum (highest) and minimum (lowest) thermometer – U shaped glass tube as Six՚s thermometer – mercury expands as temperature rises – to reset swing hard or draw indicator by magnet
Maximum (Highest) and Minimum (Lowest) Thermometer

When temperature drops alcohol contracts and drags indicator towards the bulb by surface tension of the indicator – farthest from bulb gives reading of minimum temperature

Surface Tension of the Indicator

Mean daily temperature = average of minimum (current day) and maximum temperature (previous day)

Diurnal range of temperature = difference between maximum and minimum temperature of day

Annual range of temperature = difference between hottest and coldest month

Isotherms = line joining places with same temperature

Temperature decreases at for every 300 feet ascent

Humidity: It is the dampness in atmosphere

  • Absolute humidity – actual amount of water that is present in the air as grams per cubic meter
  • Relative humidity – ratio between actual amount of water vapor and total amount the air can hold at given temperature expressed as percentage
  • Warm air can hold more water vapor than cold air – so if it contains half the amount it would carry RH as 50 %
  • 80 % RH implies air contains 4 ⁄ 5th as much water vapor as it can carry
  • Saturated air is 100 % RH – temperature is said to be at dew point
  • High RH – air is moist and low RH – air is dry
  • Hygrometer measures RH and includes wet and dry bulb thermometer – dry bulb measures temperature in shade while wet bulb has wick dipped in distilled water – cooling effect from moist wick – RH is obtained as percentage.
Measuring Humidity

Wind: Air in motion with direction and speed and includes series of gust and eddies

Wind direction is measured by wind vane or weather clock – in exposed portion where wind is not blocked by tall buildings or trees – arrow moves with prevailing wind and compass moves with direction of wind

Wind Vane

Winds are named from direction where they blow as east wind blows from east to west

Woven cloth with tail fixed to top of high pole indicates wind direction

Anemometer – speed of wind with 3 - 4 semi-circular cups – concave side offers resistance to winds – horizontal spoke will rotate and move central rod which transmits velocity in miles per hour to a dial


Beaufort wind scale by Admiral Beaufort in 1805 to estimate wind speed

Beaufort Scale

Sunshine: It depends on latitude and position of earth in revolution around sun

Sunshine duration is recorded by sun dial (4 inch diameter) – sun rays are focused on sensitization cards

Isohels – lines joining places of equal sunshine

Clouds: After dew point cooling leads to condensation of water vapor in atmosphere – tiny drops will suspend as clouds – form, shape, height and movement tell us about the sky conditions

Cloud cover is expressed in eights or oktas – 2 ⁄ 8th is quarter covered and 8 ⁄ 8th is overcast

Isonephs: lines joining places of equal degree of cloudiness

Classification of Clouds

Cloud Classification

High Clouds – Cirrus (Ci) or feathery at 20 - 40,000 feet ASL

  • Cirrus (Ci) – fibrous, wisps in blue sky as mares tail – fair weather with brilliant sunset
  • Cirrocumulus (Cc) – globular mass as ripples in mackerel sky
  • Cirrostratus (Cs) – white sheet or veil with milky sky and sun and moon shines by halo

Medium Clouds – Alto at 7 - 20,000 feet ASL

  • Altocumulus (Alt-Cu) – wooly bumpy clouds in layers as waves in blue sky – fine weather
  • Altostratus (Alt-St) – denser, greyish watery look – fibrous and striated structure

Low Clouds – Below 7,000 ft ASL and mainly stratus or sheet

  • Stratocumulus (St-Cu) – rough bumpy with waves more than Alt-Cu – more contrast between bright and shaded part
  • Stratus (St) – very low cloud – grey and thick, dull weather with light drizzle
  • Nimbostratus (Ni-St) - dark dull colored, layered cloud or rain cloud – brings continuous rain, snow and sleet

Clouds with vertical extent – no definite height (2 to 30,000 feet)

  • Cumulus (Cu) – vertical cloud with rounded top and horizontal base – humid tropical region with uprising convectional currents – fair weather cloud
  • Cumulonimbus (Cu-Ni) – overgrown cumulus cloud – black and white globular mass – cauliflower top that spreads out like anvil – seen in tropical afternoons as thunderclouds – convectional rains with lightning and thunder

Other Elements of Pertaining to Visibility

Other Elements of Pertaining to Visibility
Haze Mist Fog
  • Haze: Due to smoke and dust in industrial area due to unequal refraction of light in air of different density in lower atmosphere – used in connection with reduction of visibility in low humidity regions (< 75 %) - haze – is present if visibility is less than 1.25 miles
  • Mist: Condensation of water vapor in air forms drops of water to float about forming clouds at ground level – reduces visibility to about 1000 meters, occurs in wet air with RH > 75 %
  • Fog: Water condensing on dust and other particles like smoke from houses or factories – occurs in lowers strata as dense ground cloud, visibility < 1000 meters – Black County & North England – smoky fog as smog with visibility < 220 yards
  • Fogs that occur on hills are called hill fogs – common in morning and disperse when sun rises
  • Temperate areas – hot days and clear nights – fog may result from cooling of land surface by radiation
  • Radiation fog or land fog – lower layers of air are chilled and water vapor in atmosphere is condensed

Sea fog – When cooling surface is over the sea or when a damp air stream is brought in contact with cold current as off Newfoundland – some sea fogs are light and shallow and mast of sea can be seen protruding above them

  • Fogs are more common over sea than land and most prevalent over coastal areas
  • Dry interiors witness mist or haze
  • Dense fogs in high and mid latitude rather than tropics

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