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Name: Marcelo
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Question:
What is the relation between dew point and minimum temperature? Could dew point be a reference for minimum temperature? I am a pilot and everyday we get weather information with the dew point temperature. Once I heard it could have relation with minimum temperature for that day. The METARs (Weather) info we receive as pilots are issued hourly.

I also found this text in the Google search: "It is suggested by several authors that Td remains relatively constant during the day (Dyer and Brown,

1977; Glassy and Running, 1994; and Running et al.,1987) and equal to nightly minimum temperature (Dyerand Brown, 1977; Running et al., 1987). As a result, managerial and operational level decision-makers tend to use minimum temperature as a surrogate for Td. However, Butler (1992) noted that these assumptions are, not correct. It is reported that the strength of the relationship between minimum temperature and Td depends on the climatic characteristics and the season (Running et al., 1987; Butler, 1992; Kimball et al., 1997). Kimball et al. (1997) indicated that the assumption of equivalency is not applicable in the absence of nighttime condensation."


Replies:
Marcelo,

The dew point is a temperature; it is the temperature at which a parcel of air cannot hold any more moisture; the parcel of air has reached its vapor capacity. Dew point is reached by either i) lowering the temperature of a parcel of air or ii) increasing the moisture content of a parcel of air. Pressure would also have an effect, but we (not pilots) are mostly concerned about the effects of temperature and moisture content at standard atmospheric pressures (pressures at Earth's surface).

Think about the best way to defog the windows in a car: blow warm air (warm air has a higher vapor capacity), with the air conditioner on (the air conditioner removes moisture from the air), and turn off the recirculation option (fresh air has a lower moisture content than the air inside the car). High temperatures and lower moisture contents reduce the relative humidity and moves the parcel of air away from its vapor capacity.

If the temperature during the day drops low enough, then the dew point temperature could be reached; that is why we commonly find moisture on surfaces in the morning. If the temperature does not drop low enough, then the parcel of air will not be saturated (that is, the parcel of air has not reached its vapor capacity), and condensation will not occur.

The dew point temperature only remains constant during the day if the vapor content of the parcel of air remains constant. I assume that planes fly great enough distances that they will fly through different parcels of air with different vapor contents and thus have different dew point temperatures.

Another way to think about dew point temperature is to consider where and when fog forms...

Leslie Kanat, Ph.D.
Professor of Geology
Department of Environmental Sciences


Marcelo,

Butler is correct (quote "these assumptions are not correct. It is reported that the strength of the relationship between minimum temperature and Td depends on the climatic characteristics and the season (Running et al., 1987; Butler, 1992; Kimball et al., 1997). Kimball et al. (1997) indicated that the assumption of equivalency is not applicable in the absence of nighttime condensation."

By no means does the minimum temperature reach down to dew point temperature on most nights in most locations. However, seasonally, (particularly early Spring and late Autumn), the minimum temperature reaches and goes slightly lower than the dew point on many nights.

It is true that dew point does remain fairly constant all day when a high system is sitting over you, particularly if you are near the middle of it, but it is not generally true otherwise.

So, using dew point to predict the minimum temperature at night does not work particularly well for most nights, unless it is clear that fog/condensation will form on a particular night.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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