Moisture Content at Higher Elevations

[Kyle Brittain]

Why is it that air with dewpoint temperature of say, 15C in Denver, has more moisture than air with a dewpoint temperature of 15C in Oklahoma City?

Saturation vapour pressure or saturation mixing ratio increases as temperature increases. But what effect does elevation have these variables? Would this also mean that a temperature of say 25C with an RH of 50% would yield a different dewpoint temperature at two different elevations - say at one location with a station pressure of 975mb and one at 825mb?

When we hear of ideal ingredients for severe environments, the climatology differs from place to place. For instance, you generally want surface dews to be at least 20C for sigtor events in OK in springtime setups. However, you can get by with 15C dews in higher elevations (ie. over the high plains). Why is this?

 

[Jeff Duda]

Why is it that air with dewpoint temperature of say, 15C in Denver, has more moisture than air with a dewpoint temperature of 15C in Oklahoma City?

Saturation vapour pressure or saturation mixing ratio increases as temperature increases. But what effect does elevation have these variables? Would this also mean that a temperature of say 25C with an RH of 50% would yield a different dewpoint temperature at two different elevations - say at one location with a station pressure of 975mb and one at 825mb?

When we hear of ideal ingredients for severe environments, the climatology differs from place to place. For instance, you generally want surface dews to be at least 20C for sigtor events in OK in springtime setups. However, you can get by with 15C dews in higher elevations (ie. over the high plains). Why is this?

Dewpoint is not a true measure of atmospheric water vapor. Better measures of (true) water vapor content include vapor density, vapor pressure, specific humidity, and mixing ratio. Relative humidity is defined as the ratio of actual vapor pressure to the saturation vapor pressure value. The Clauseus-Clapeyron equation enables a computation of saturation vapor pressure as a function of temperature. Mixing ratio can be derived from vapor pressure. Specific humidity and mixing ratio are technically different quantities, but typically have very similar values. It would be a good idea to learn mixing ratio since it is used by modelers and analysts as the more common means of depicting moisture content. When an unsaturated air parcel is lifted adiabatically, its mixing ratio will not change, but its dewpoint will. This is because dewpoint is a function of pressure as well as of actual moisture content.

Saturation vapor pressure is a function only of temperature, and of nothing else (elevation plays no role). However, saturation mixing ratio is derived from saturation vapor pressure and thus is a function of pressure as well as temperature.

I wouldn't rely on using hard thresholds for dewpoint for severe storm forecasting, as there really is no one single threshold dewpoint value that always works or never works at any given location. You're better off looking at composite indices like CAPE and CIN since they incorporate the impacts of moisture content, which is what you want in the end anyway.

 

[Kyle Brittain]

When an unsaturated air parcel is lifted adiabatically, its mixing ratio will not change, but its dewpoint will. This is because dewpoint is a function of pressure as well as of actual moisture content.

Indeed, this is what I was referring to. When an unsaturated parcel rises, the temperature cools at the DALR (~10C/km) and the dewpoint also decreases at the dewpoint lapse rate (~2C/km), even while the mixing ratio stays the same. So if we have a dewpoint of 10C at sea level and a dewpoint of 10C 800mb, the latter airmass will have more moisture, and hence more theta e.

I found a web page that addresses the issue I'm getting at. The author gives several examples of high plains tornadoes in which the respective environments were shown to have high theta e despite lower mixing ratios. This is the result of elevation.

http://bangladeshtornadoes.org/elevated_heating/mix.html

Saturation vapor pressure is a function only of temperature, and of nothing else (elevation plays no role). However, saturation mixing ratio is derived from saturation vapor pressure and thus is a function of pressure as well as temperature.

It was my understanding that the reason boiling occurs at lower temperatures in higher elevations was directly attributable to lower saturation vapour pressures. For example, at sea level, water boils at 100C. At 800mb, it boils at ~93C.

Another indicator is the result of looking at a Skew-T. The saturation mixing ratio lines do not parallel the isotherms. If SMR was only influenced by temperature, why does its rate change with height compared to the isotherms? For example, looking at the skew-T, at 1000mb and a temp of 20C, the SMR is just over 15g/kg. At 800mb and 20C, the SMR is about 19g/kg.

Sample Skew-T for reference (on p2): http://www.atmos.washington.edu/~houze/301/Miscellaneous/Skew-T.pdf

I wouldn't rely on using hard thresholds for dewpoint for severe storm forecasting, as there really is no one single threshold dewpoint value that always works or never works at any given location. You're better off looking at composite indices like CAPE and CIN since they incorporate the impacts of moisture content, which is what you want in the end anyway.

Agreed, however I think using dewpoint (as opposed to RH or something else) gives a good first approximation as to how much moisture there is in the air, since higher Tds correspond to higher amounts of moisture, and lower Tds to lower, etc. Especially if you know what the "ballpark" is for severe setups in your local area (see article above) - for instance, a Td of 10C in OK in May would be pretty scant for moisture, but a Td of 10C in the front ranges of WY has been associated with numerous tornado events.

 

[Jeff Duda]

It was my understanding that the reason boiling occurs at lower temperatures in higher elevations was directly attributable to lower saturation vapour pressures. For example, at sea level, water boils at 100C. At 800mb, it boils at ~93C.

Water reaching boiling just means the liquid water molecules have attained enough energy to break free from the liquid state. The major resistance is provided by air pressure. At higher elevations there is less air mass to hold down the liquid water molecules, and thus it takes less energy for them to escape into the vapor state. Saturation vapor pressure is a quality of the atmosphere, not of liquid water. So saturation vapor pressure really has nothing to do with the dependence of boiling of water on elevation.