weather Flashcards
Stevenson screen
-This wooden box stands on four legs at a height of 121 cm to avoid ground heat affecting the thermometer bulbs at 125 cm
-They are painted white to reflect the sun’s rays
-Made of wood to prevent heat transfer
-The sides are slatted (louvred) to allow free movement of air
-The wood is double-layered with airspace for insulation
-The screen is placed on grass to reduce ground heat
-Daily readings are taken at the same time
wind direction
-Wind direction is measured with a wind vane
-Direction is the compass point from where the wind is blowing—south, north, north-east, etc.
-The unit of measurement for wind direction is compass direction
wind speed
-Anemometers measure wind speed
-Wind speed is measured in m/s or km/hr
-The anemometer has 3 or 4 cups on metal arms that spin freely on a 10 m vertical shaft
-The cups rotate quicker and the counter records more rotations with stronger winds
-Many digital anemometers send data directly to apps and computers for readings
-Wind vanes and anemometers are far from buildings and trees that could block airflow
-Buildings can form wind tunnels or delay airflow, affecting reading accuracy
precipitation
-Precipitation is any water that falls to earth—hail, mist, rain, sleet, or snow
-The unit of measure is millimetres (mm)
-A rain gauge is used to measure precipitation
-At the same time each day, any water that has collected is poured into a tapered measuring cylinder on a flat surface
-The water level is then read with the eye at the same level as the lowest part of the meniscus of the water
-Measurements are then recorded; too small a reading and it is recorded as ‘trace’
temperature
-Measurements are in Celsius ° C or Fahrenheit ° F, depending on location
-Temperature is measured using a thermometer or thermograph
-Shade temperature is measured, as air temperature is variable due to direct insolation and cloud cover
-Temperature range for 24 hrs—called the diurnal temperature range
humidity
-Humidity is the amount of water vapour in a given volume of air
-Hygrometers measure relative humidity with wet and dry bulb thermometers
-Relative humidity measures the amount of water vapour in the air compared to its maximum capacity at a given temperature
-Warm air holds more water vapour than cold air
-When the air is holding as much moisture as it can, it’s said to be saturated
-If the air is not saturated, water vapour evaporates from wet bulb muslin, cooling the bulb and lowering the mercury temperature
-If the air is saturated with vapour, evaporation is impossible, so both thermometers show the same temperature
pressure
-Air has weight and exerts pressure on the Earth’s surface
-Sea level pressure is approximately 1.03 kg/cm²
-Pressure varies with altitude and temperature
-The unit of measurement is millibars (mb)
-Lines of pressure on a map are called isobars
-A barometer measures air pressure, of which there are 3 types:
Mercury barometer
Aneroid barometer
A barograph
