All Weather News

Driving Temperatures UP With Down Sloping Winds

27 Nov 2017, 7:17 pm

Denver’s record high temperatures Sunday and Monday had a few factors that gave us very warm conditions. Denver hit a high temperature of 74° Sunday afternoon, breaking the old record of 72° set last in 1998.  Monday was even warmer as the record high of 74° for the date set back 1950 was surpassed. Denver set a new all time record high for the entire month of November with a high of 81°.

Plenty of sunshine had a hand in warming very dry air in place.  Air with lower amounts of humidity will heat and cool quicker than humid air and generally have a greater range between morning lows and afternoon highs.  Monday started off very warm, with a low temperature just before sunrise of 47°.


Denver, Colorado

A strong southwesterly wind brought relatively warmer air from the desert southwest, up and over the Rockies only to descend even warmer as it spilled onto the plains of Colorado.

Downslope or “foehn winds” are fairly common along the Rockies and are often called  “Chinook” winds or even “snow eater” because they can melt and evaporate snow rapidly with very warm and dry air.

When moist winds from the west are forced to rise over the Colorado Rockies, the air cools, moisture in the air is condensed and falls out as precipitation.  The phase change from water vapor to liquid or frozen water releases heat into the air.  As stated, when the air is forced to rise over higher elevations it expands and cools.  That cooling is somewhat hindered by the latent heat released by condensation.

The air pushed up the windward side of the mountains will cool at the moist adiabatic lapse rate of around 3.5 °F / 1000 ft although this lapse rate varies depending on how much moisture there is. The moisture removed from the air will leave it much drier for it’s trip to lower elevations on the leeward side of the mountains. The dried air then descends on the leeward side of the mountains, warming at the dry adiabatic rate of 5.5 °F / 1000 ft.

This just means that the heat added to the air on its way up through condensation will be added to the warmth as it descends to lower elevations.


Colorado Temperatures Monday Afternoon

Air temperatures west of the Rockies can be much colder than locations east on the plains once the downslope is underway.  When the record temperature of 81° was reached in Denver the dew point temperature was 10° and the relative humidity was just 7%.  At that same moment in Grand Junction, Co. west of the Rockies the temperature was 57° with a dew point temperature of 27° and the relative humidity was 31%.  The air west of the mountains was the same air that ended up east on the plains only much drier and warmer because of the heat released into it through condensation of humidity.

A very warm start to the day in Denver also helped in attaining a new all time record for the Mile High City. The stronger winds Sunday night created turbulence on the plains preventing the normal nocturnal temperature inversion from forming near Denver, allowing night-time temperatures to remain elevated.

 

Meteorologist Mike Morrison

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