Humidity, water vapor maps, and the tephigram

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Water Vapor Maps

Profiles of moisture and water vapor maps are equally important

in examining the effects of dampness on weather. How do we deal with humid air? First, we present a recap of the most significant variables used to gauge humidity:

1) Mixing ratio (r) mass of water vapour to that of dry air, given in g/kg. Lines of constant mixing ratio, go straight up steeply to the right on a tephigram.

2) Relative humidity (RH) the relative humidity is approximately the ratio of the air's actual mixing ratio to its mixing ratio if saturated at the same temperature and pressure. Then multiply by a hundred to express as a percent. This one requires data from both temperature and humidity recorders.

Water vapor maps display relative humidity most often. Use the mixing ratio passing through the isobar's intersection with dew point temperature and actual temperature isotherms respectively. If there is no difference between these two temperatures, RH is 100 percent.

3) Specific humidity (q) ratio of the mass of water vapor to the total mass, water included.

4) Absolute humidity actual density (in g/cubic meter) of the water vapour present in the air.

See also the list of thermodynamic temperature conversions used in tephigram analysis.

Meteorological Processes

We use the charts to follow these processes. You might want to print the tephigram or skew T diagram to help your understanding here:

Humidity Tephigram

Air receives heat from the sun or the warm ground below and cools down by various mechanisms. It moves uphill, downhill or out onto open water. It can mix with other air with different properties, receive rainfall from above, or encounters one of many other changes in circumstances. Water vapor maps help us mark off these effects. We call these responses meteorological processes, and you can sort them by type.

Adiabatic process: - where no heat is given nor received by the air. Its temperature changes only by compression or expansion.

Diabatic process: - opposite of adiabatic, heat flows into or out of air by sunlight or moving over cooler ground.

Isobaric process: - something that happens without changing the pressure (or elevation).

Pseudo-adiabatic process: - where water evaporates into or condenses out the air, such as clouds forming or dissipating. The latent heat from the water stays behind even if the water itself leaves or enters.

Through these processes, a specified property of the air is conserved. It does not change. Maybe temperature remains constant, but not always. In the process which creates warm dry Chinook winds, foehn winds, air goes over high terrain and comes down warm and dry. Neither temperature, relative humidity nor dewpoint is conserved.

But wet-bulb potential temperature remains constant,

barring any external influences which sometimes change the air. You can call this a pseudo-adiabatic process; the parcel's dry adiabat and mixing ratio line will always intersect on the same pseudo-adiabat. Even the (ordinary) potential temperature changes.

How do forecasters use the tephigram for these special cases, air near the ground? Potential temperature stays the same for elevation changes and you can move along an adiabat.

Surface heating or cooling, caused by incoming or outgoing radiation, means that although heat and temperature changes, our water vapor maps show that moisture remains constant. Dew point stays the same but relative humidity does not.

Air moving onto water: abrupt boundaries on water vapor maps, moisture changes, temperature usually changes, something nothing is conserved. What happens?

  1. Cold air over warm water - Air becomes warmer and more moist at lower levels. This can make it unstable (top-heavy) and stormy.
  2. Dry warm air over cool water - Air cools and humidity increases.
  3. Moist warm air over cool water - Air cools and fog may form.

Humidity Information


Rain falling through the air

What result do we get here?

Well, the dewpoint and the mixing ratio likely rise due to the additional moisture injected. Look for these changes on the maps. And evaporating that water transforms some of the air's heat energy, causing a temperature drop. The one variable that does not change in this case is the web-bulb potential temperature.

High Water Vapor Maps:
When cloud tops cool at night

The earth gives off infrared radiation all the time, thereby losing heat constantly. And it usually emerges from the highest object, clouds included. At night, when no incoming solar energy compensates for it, the cloud tops cool. Heat loss = diabatic modification. The cloud now becomes lopsided and turns over to bring nocturnal thunderstorms in some regions.

Also, clouds can spring up at night unexpectedly if air with high humidity prevents energy from escaping from below. Instead, the air cools at the top. If it cools enough, clouds appear seemingly out of nowhere. The forecasting meteorologist can study his water vapor maps more closely to handle this situation.

Go back from Water Vapor Maps to the Chasing Storms webpage.

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