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9.6 Temperature, Pressure and Humidity

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Surface Temperatures

Surface air temperature is measured at a height between 1.25m and 2m above ground level. Surface air temperatures can differ considerably from ground temperatures. During hot days the ground temperature can be greater than surface air temperatures (i.e. asphalt roads), while on cold days the ground temperature can be significantly less than surface air temperatures (i.e. snow covered land).

The temperature of the earth's surface depends on the reflectivity, conductivity, specific heat and cloud cover. By day, cloud cover reflects the sun’s radiation back out into space resulting in a lower temperature underneath the clouds and on land surfaces. By night, clouds insulate land surface heat loss, by trapping in heat released from land surfaces (known as terrestrial radiation) producing warmer nights.

Temperature Scales

There are three temperature scales in common use:

The degree Celsius (symbol: °C) is an SI metric scale unit of temperature. The freezing point of water is designated at 0 °C and the boiling point at 100 °C. The degree Celsius scale is used throughout aviation.

The kelvin (K) temperature scale is an extension of the degree Celsius scale down to absolute zero, a hypothetical temperature characterised by a complete absence of heat energy. Temperatures on this scale are called kelvins, NOT degrees kelvin, kelvin is not capitalised, and the symbol (capital K) stands alone with no degree symbol.

The degree Fahrenheit (°F) non-metric temperature scale was devised and evolved over time so that the freezing and boiling temperatures of water are whole numbers, but not round numbers as in the Celsius temperature scale.

Celsius (°C)

  • water boiling: 100 degree
  • ice melting: 0 degree
  • absolute zero: -273.15 degree

Kelvins (K)

  • water boiling: 373.15 degree
  • ice melting: 273.15 degree
  • absolute zero: 0 degree

Fahrenheit (°F)

  • water boiling: 212 degree
  • ice melting: 32 degree
  • absolute zero: -459.67 degree