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9.9 Temperature, Pressure and Humidity

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Diurnal Variation of Surface Air Temperature

In meteorology, diurnal means daily, especially pertaining to actions which are completed in 24 hours and are repeated every 24 hours; this can be seen in the diurnal temperature variation. An alternative word for diurnal in this sense is “diel”.

When surface air continues to warm by conduction throughout the day and the solar radiation is around its maximum, a peak temperature is reached approximately 2 pm to 3 pm. The lowest temperature is around sunrise, due to surface temperatures losing heat all night as terrestrial radiation. Ocean temperatures are very stable and don’t fluctuate in their temperatures the way land temperatures do.

Diurnal Variation of Solar Radiation: Plotted is the diurnal variation of incoming solar radiation at the earth’s surface.

  • It begins at sunrise, 6 am
  • It’s a maximum at noon
  • It shuts off at sunset, 6 pm

Diurnal Variation of Terrestrial Radiation: Plotted is the surface diurnal variation of earth-emitted terrestrial radiation. Its trend is similar to the diurnal solar radiation.

  • Minimum at sunrise, 6 am
  • Maximum at 3-4 pm

Diurnal Variation Daily: The combination of solar and terrestrial radiation determines whether the surface temperature rises, falls, or remains the same during the day, while giving cause to peak surface temperatures about 2pm to 3pm. Net radiation = incoming solar (insolation) - outgoing terrestrial (inferred):

  • If the net radiation > 0, surface warms (6 am - 3-4 pm)
  • if the net radiation < 0, surface cools (3-4 pm - 6 am)

This also explains why the warmest part of the year is in January/February, not on 21 December during the summer solstice.