Weather

• Stevenson screen


  A wooden box standing on four legs at a height of 121 cm to avoid heat radiated from the ground and to have the thermometer bulbs at a standard height of 125 cm
• Stevenson screen

  • Painted white to reflect the sun's rays
  • Made of wood to avoid the conduction of heat into it
  • The sides are slatted (louvered) to allow free movement of air
  • The roof is made of a double layer of wood with airspace between for insulation
  • Placed on a grass-covered surface, which reduces radiating ground heat
• Instruments found inside the Stevenson screen

  • Maximum-minimum thermometer (Six's thermometer)
  • Wet and dry bulb thermometer - hygrometer
• Instruments found outsid

  • Rain gauge
  • Wind vane
  • Anemometer
• Weather station layout

  • Barometers and barographs are kept away from strong air movements, direct sunlight and heat sources
  • Thermometers are kept away from buildings that may radiate heat
  • Wind vanes and anemometers are positioned in the open, away from trees or buildings, and away from the nearest obstacle by at least three times the height
  • The rain gauge must be in an open space with a distance from the nearest object twice its height
• Wind speed is measured with an anemometer
• Unit of measurement for wind speed

  m/s or km/hr
• Anemometer

  • Consists of 3 or 4 cups fixed on metal arms that rotate freely on a 10m vertical shaft
  • The stronger the wind, the faster the cups rotate, and more rotations are recorded on the counter
• Digital handheld anemometers need to be held into the oncoming wind and as the fan rotates, the number is shown on the screen
• Many anemometers are digital that transmit data directly to apps and computers to show readings directly
• Wind vanes and anemometers are placed well away from any buildings or trees that can interfere with air movement
• Buildings can create wind tunnels or slow airflow and therefore, affect the accuracy of any reading
• Precipitation


  Any water that falls to earth - hail, mist, rain, sleet, or snow
• Unit of measure for precipitation

  millimetres (mm)
• Using a rain gauge

  1. At the same time each day, any water that has collected is poured into the tapered measuring cylinder

  2. The water level is then read with the eye at the same level as the lowest part of the meniscus of the water

  3. Measurements are then recorded; too small a reading and it is recorded as 'trace'
• Temperature

  Measured in Celsius ° C, or Fahrenheit ° F depending on where you are in the world
• Thermometer

  Measures shade temperature, as air temperature is variable due to direct insolation and cloud cover
• Six's thermometer

  Houses a maximum and minimum thermometer in a U shape
• Temperature measurements

  1. Daily minimum temperature

  2. Daily maximum temperature

  3. Average temperature calculations for one day (maximum + minimum / 2 = mean daily temperature)

  4. Temperature range for 24 hrs - called the diurnal temperature range (minimum temp - maximum temp = diurnal range)

  5. Mean monthly temperature is found by recording daily mean temperature for one month and then dividing by the number of days in that month

  6. The sum of the mean monthly temperatures divided by 12 gives the mean annual temperature
• Thermograph

  Has an exposed bimetallic strip which deforms with a change in temperature, this change is transmitted to amplifying levers which trace a curve on a roll of graph paper, a vertical movement of 1 mm is equivalent to about 1°C
• Humidity

  The amount of water vapour in a given volume of air
• Wet and dry bulb thermometers

  Used to measure relative humidity and are called a hygrometer
• Relative humidity

  A measure of how much water vapour the air is holding in relation to the maximum amount of water vapour it could hold at a specific temperature
• Warm air can hold more water vapour than cold air
• When the air is holding as much moisture as it can, it's said to be saturated
• How a wet and dry bulb hygrometer works

  1. If the air is not saturated, water vapour evaporates from the wet bulb muslin, this evaporation cools the bulb and the mercury contracts and registers a lower temperature

  2. If the air is saturated with vapour, then evaporation is not possible, so both thermometers show the same temperature
• Air pressure

  Air has weight and exerts pressure on the Earth's surface, sea level pressure is approximately 1.03 kg/cm²
• Unit of measurement for air pressure

  millibars (mb)
• Isobars


  Lines of pressure on a map
• Types of barometers

  • Mercury barometer
  • Aneroid barometer
  • Barograph
• Mercury barometer

  A hollow tube with all the air extracted, the open end is then placed in a bath of mercury, mercury is forced up the tube by atmospheric pressure on the mercury in the bath
• Aneroid barometer

  Has a partly vacuumed, corrugated metal chamber inside with a strong metal spring that prevents it from collapsing, levers magnify changes in atmospheric pressure and a pointer moves across a calibrated scale to show atmospheric pressure
• Barograph

  A continuous reading of atmospheric pressure for one week, changes in pressures are traced on a rotating cylinder by a flexible arm
• Sunshine hours are measured by a Cambell-Stokes sphere
• Cambell-Stokes sunshine recorder

  A glass sphere partly surrounded by a metal frame, sunlight is concentrated through the sphere onto a recording card placed beneath the focal point, the length of the burn trace on the card shows the sunshine duration
• Using a sunshine recorder

  1. Placed in an open space, south-facing in northern hemisphere or north-facing in southern hemisphere

  2. Make sure the recorder is outside, unaffected by shade and has direct exposure to sun's rays

  3. At day's end, measure the length of the burn line, then convert to hours and finally calculate the time it was sunny

  4. Record every 24 hours at the same time every day and stop at sunset to record for the day

  5. Remove and replace the card (paper or sheet) each day into sunshine recorder
• Cloud cover

  Measured in units of oktas, each okta represents one eighth of the sky covered by cloud
• Cirrus

  High above 6 km, thin, white and made of ice crystals, forms narrow wisps, threads or feathers
• Cirrostratus

  High above 6 km, thin, white layers made of ice crystals with a wide horizontal spread, often covers whole sky
• Cirrocumulus

  High above 6 km, thin, white, heaped cloud with ice crystals