MadSci Network: Chemistry
Query:

Re: Why does the overnight low temperature not go below the dew point temp.

Area: Chemistry
Posted By: Steve Czarnecki, senior technical staff member, systems engineering, Lockheed Martin
Date: Thu Nov 6 18:53:53 1997
Area of science: Chemistry
ID: 877371363.Ch
Message:

Before I answer, a short preamble: throughout the late summer and fall, I'm fortunate to observe firsthand an atmospheric pheonomenon related to dew point: I awaken to bright morning sunshine on the hilltop where I reside, while the valley dwellers below are enshrouded in fog until midmorning. From my vantage point, the sky is blue and the air cool and crisp while the low ground is covered with a beautiful silvery blanket. Yet, in the valley below, the sky is gray and the air is sodden. The temperature in the valley is at the dewpoint mentioned during last night's weather forecast, while my location on the hill is a few degrees warmer.

The answer to your question rests in one of the great wonders of nature; in particular, I'm thinking of water's amazing ability to store and release heat. It is this property which gives rise to the prediction that the night time temperature will not fall below the dew point temperature, as well as the ability of this planet to sustain life.

As you know, water can exist in three forms, or "phases": liquid, solid or gas. The gaseous state of water has various names, including steam or vapor. It should be noted that in its gaseous state, water is invisible; the "clouds of steam" above a boiling pot or "vapor trails" behind a high flying jet are actually mists of tiny liquid water droplets, not water in the gaseous state. For simplicity, I'll simply refer to the gaseous state of water here as "water gas" -- the terminology is a bit odd (see my postscript below), but for the majority it hopefully avoids confusion with incorrect ideas.

Besides containing nitrogen, oxygen, argon, carbon dioxide, and a traces of a variety of other gases, air contains a substantial amount of water gas.

The ability of air to hold water gas depends on the air's temperature and pressure. At constant pressure, warm air can hold more water gas than cold air. When air is holding the maximum amount of water gas possible at a particular temperature it is "saturated". The dew point is the temperature to which a given parcel of air must be cooled to reach saturation, assuming the pressure remains constant and the amount of water in the air remains the same.

What happens on a clear, cloudless night is that after the sun goes down the air and earth radiate their heat into the cold night sky. As the air loses heat, its temperature drops. The coldest air sinks to the lowest points, such as to the valleys I mentioned above. As the air temperature drops, its capacity to hold water decreases. Eventually, the air's capacity to hold water diminishes to the point where it's just equal to the actual water content (which hasn't changed).

Now an amazing thing happens: the air continues to lose heat, but the temperature remains practically constant! The reason for this is that the air has become saturated with water gas, and further heat loss comes from water condensing from the gaseous state to the liquid state. The air can't cool any further without forcing out some of the water gas. This condensate is better known as fog, if it is formed throughout the body of the air and remains suspended, or as dew, if the condensation is happening at the interface between the air and other solid objects (e.g., on the grass).

I said "practically constant", because as some of the water condenses out of the air, the air can cool a little bit more, maintaining saturation conditions. But this futher incremental change is so tiny as to be unnoticeable in practice.

The key thing to keep in mind is that the amount of energy to convert a drop of water at say, 70 deg F, to the equivalent amount of gas at 70 deg F is many , many times more than the amount of heat needed to change the temperature of that liquid or gas by one degree. Therefore, once the dewpoint is reached and the atmosphere becomes saturated, the continued heat loss of the atmosphere serves almost exclusively to condense water, and almost not at all to change temperature.

The air's temperature doesn't change because the heat being radiated to space is coming from the water as it condenses. This is the same heat which was absorbed when this water evaporated.

It may seem odd that something can lose heat and not change temperature, but it's really quite commonplace (yet amazing). For example, as water freezes into ice, the mixture of ice and water remains at 32 deg F until all the water is frozen. The mixture is losing heat all the while, yet the temperature is not changing.

In the morning, when the sun comes up, it begins to heat the air. The air can warm quite rapidly, as there is no barrier to warming up above the dewpoint temperature. As the air warms, the dew evaporates, and any fog dissipates by evaporation, as well.

In other words, moisture in the atmosphere helps to moderate temperature; it's nature's "buffer" for storing heat. Without water's amazing capacity to store and release heat through phase transitions, this planet would be inhabitable. For example, the atmospheric moisture regulation of a rain forest is the difference between a lush Amazon jungle and the Saharan desert.

Regards,

Steve Czarnecki

P.S. "water gas" is also a term for a mixture of carbon monoxide and hydrogen produced by an industrial coal gasification process. That's something else entirely and has nothing to do with my explanation above.


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