Dew point

[Ref Understanding Weather & Climate, Books a la Carte Edition (6th Edition) #ISBN-13: 978-0321773227 #ISBN-10: 0321773225 CH 5 Atmospheric Moisture, Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 12-13]

Dew point temperature (Dew point) – is the temperature at which saturation occurs (RH 100%) and it is a valuable indicator of the moisture content. When surrounding air drops below the dew point, the moisture in the air condenses.

In other words, dew point is the temperature at which the air can hold no more moisture. Indeed, when temperature and dew point are closer together, it is highly likely that fog, low clouds, and precipitation will form.

Dew point temperature always below or equal to the temperature (but not exceed)

  • About Dew point temperature (> Dew point)
    – air is unsatureated;
  • At the Dew point temperature (= Dew point)
    – air is saturated;
  • Below the Dew point temperature (< Dew point)
    – the removal of water vapor by condensation

Dew point v.s Frost point

Indeed, both “Dew point” and “Frost point” are saturation point, the only different is the temperature.


  1. Distinct altitude may have different Dew point temperature
  2. Frost point ≠ Dew point
    – Frost point for those saturation temperature below 0 °C (32 °F) 


Application of dew point – determine the height of the cloud base

Divide the temperature-dew point spread by the convergence rate (2.4ºC or 4.4 ºF), and multiply by 1000 –

Est Cloud base = (spread/2.4ºC)*1000 feet 

example: Temperature 85F; Dew point 71F 

[formula: F= (9C)/5 + 32]

85 F = 29.4 C

71 F = 21.7 C

Spread = 29.4 -21.7 = 7.7 C

Ans: Est Cloud base = (7.7/2.4)* 1000 = 3208 feet ≈ 3200 feet

#determine #calc #cloud #base


Weather Radar – RDR-4000 IntuVue™

Extension Reading

[P.131 – Dew Point serve as a predictor for Nighttime Minimum Temperatures]

Dew Point could be used for the prediction of the following morning’s low temperature
Dew point ≈ following morning’s minimum temperature – If no major wind shifts or other WX change are anticipated

However, both heavy cloud cover and strong winds inhibit a drop in air temperature, and their presence may keep minimum air temperature above the dew point

  • Cloud cover’s absorption and downward reradiation of longwave energy;
  • Strong winds cause vertical mixing

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