Urban heat Island and local microclimates

[PTPatrick]

The variation of low temps across the Denver Metro never ceases to amaze me. This is especially notable with strong high pressure radiational cooling events. Last night Most of the metro was forecast to be around 0. The airport, about 20 miles east of downtown got to -1. At my house near the capitol we only saw the low around 11. With a dewpoint of 4. West of I 25 in lakewood my friend only recorded 15. An 16 degree spread in 20 miles, but with very little actual elevation change.

This was the case last year when DIA dropped to -18...my house was around -6. The possible reasoning for dowtown and places like Boulder and Lakewood stays warmer are obviously urban heat island...but more likely slight downsloping wind effects from the foothills. Of course 50 miles north in the platt valley (not that low but bit lower than denver, the lows often bottom out -20 below in these situations. I have seen Greeley hit -30 while Downtown was around 8. I just find that amazing coming form Missisippi where radiational cooling events often yield very little difference between memphis and hattiesburg(about 200 miles).

The previous poster about Tallahassee FL brings to mind its odd temp anomalies. I can only figure in hindsight that Tally Airport was in a valley. It was not unusual in the years I went to school there (1999-2003) to see radiational cooling events with lows in the mid teens...while farther NORTH in Central GA or south central AL...would be in the 20s. In the 2003 freeze I think we got to 12 in Tally while many areas north were close to 20.

[Scott Patterson]

Nice topic and it is interesting.

I just find that amazing coming form Missisippi where radiational cooling events often yield very little difference between memphis and hattiesburg(about 200 miles).

Low humidity can make radiative cooling more pronouced.

Even Urban heat islands aside, there can be huge differences between locations due to location and wind drainage or what side of the mountain you are on.

Here are some of the more extreme examples.

Wind Drainageand Radiative cooling

Fruita Colorado and Colorado National Monument are only five miles apart.

Fruita is at 4530 feet elevation and Colorado National Monument is at 5780.

Compare the average night differences in winter:

http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?cocolo

http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?cofrui

The differences in average aren't that far, but the extremes are. Fruita has dropped below -30 several times and dropped to -36 on February 6 1989. That same day was only -12 at Colorado National Monument.

Here's an even more extreme example, this time in Utah.

Loa, Capitol Reef, and Hanksville all sit in one line along the same river. Loa is highest at 7050 feet. Capitol Reef is at 5510 feet and between the two. Hanksville is lowest at 4460 feet.

Loa:

http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?utloa

Capitol Reef:

http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?utcapi

Hanksville:

http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?uthank

Capitol Reef is well drained by winds and sits between the two locations which pool cool air. Loa, up river from Capitol Reef has dropped as low as -37. Hanksville, down river from Capitol Reef has dropped to -35. Capitol Reef has never seen a day lower than -9, something that happens at the other locations every year.

Here in the Yampa River Valley in NW Colorado, we also see huge changes in short distances. The river bottoms get much colder than the higher hillsides. For example, the Weather Station Craig 4SW is on the high hill south of town and at the Trapper Mine. Since 1977, the coldest it has gotten there officially is -41F, and in winter is the warmest of the locations in the area. At the Craig-Moffat Station which is just outside downtown and only seven miles away has gotten as low as -56 and has dropped to -50 on several occasions (last time in Feb 1989). It's at the airport and is the closest official station to town. Maybell, the next town down river is a bit colder still. In 1985, it hit -61 in 1985 and still remains the record low in Colorado. Surpisingly, it's considered a fairly low altitude in Colorado (5920 feet) and is not much higher than Denver.

Chinooks and Wind Drainage

As you know, you can see huge differences in temperatures in only a few miles on either side of the Continental Divide. Often Denver will be sitting with Chinook winds and +70 and places like here and Gunnison are sitting at -20. Closer to the divide there are still huge differences in a short distance.

Nedarland and Fraser are at almost the same exact eleavtion and only 15 miles apart. One is on the west side of the Continental Divide and one on the east. Check out the differences in temperatures between the two:

http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?cofras

http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?conede

Colorado also sees a lot of seasonal variations and percipitation ones too, but I guess that's another topic. Still it's interesting that on average December is our driest month. In Steamboat Springs, just to the east, it is the wettest month on average.

[JonathanBelles]

I live in a local microclimate that is also affected by a heat island. St. Pete being the heat island. I live in inland Southern Pinellas County between the Bayou and US-19. My temps here are generally 3-10°, sometimes up to 15°, cooler in the winter and up to 15° warmer in the summer because I am not affected as much be the GOM.

[Aslkahuna]

Here The Post routinely is much warmer during winter nights than downtown SV and we all pale in comparison to the areas down by the river 9 miles to the east. On a cold night the Post will be in the mid to upper 20's, I'll have high teens or maybe lower as will the fire station in town whereas down by the river they will be in the zero to -5F range. My back yard is open to cold air from the nearby wash while the front is not and I will have a 5 degree difference between the two sides of the house. Winds are another fun topic around here and of course the rain from our monsoon storms is very locally controlled.

Steve

[JBG]

The variation of low temps across the Denver Metro never ceases to amaze me. This is especially notable with strong high pressure radiational cooling events. Last night Most of the metro was forecast to be around 0. The airport, about 20 miles east of downtown got to -1. At my house near the capitol we only saw the low around 11. With a dewpoint of 4. West of I 25 in lakewood my friend only recorded 15. An 16 degree spread in 20 miles, but with very little actual elevation change.

I don't know that this is relevant, but isn't it often the case that arctic air spilling down from Canada settles in east of the Rockies, and the Rockies and adjacent front ranges often in the nominally "warm" Pacific air?

[PTPatrick]

Well...I would think it all depends on the jet stream. Denver and the front range gets its fair share of candadian arctic highs. Sometimes the cold air does go more to the great lakes...but I wouldnt say more. I am guessing that what you are refering to is when cold back doors in from the northest. Often artic cold air is somewhat shallow...so what you can end up with is the plains including places like Denver, Boulder, and Fort Collins can be very cold...while Foothil locations, higher mountains and west of the divide will be in warmer airmass. It is odd but it occasionally does happen...bucking the theory of higher up equals colder....I know a couple time last year where all the ski towns like aspen, vail and winter park were actually warmer than denver because the cold air was shallow. In fact I can remember being flabergasted while driving UP I-70 and watching the car thermometer climb...which normally is opposite.

[JBG]

I know a couple time last year where all the ski towns like aspen, vail and winter park were actually warmer than denver because the cold air was shallow. In fact I can remember being flabergasted while driving UP I-70 and watching the car thermometer climb...which normally is opposite.

I remember when the New York Times used to publish the prior day's weather map there was often a stationary front riding just east of Calgary, Billings, Casper and Denver. Often the first days of a cold spell would feature bitter temperatures in those locales and when the cold air settled in eastward the Front Range cities would poke into the warm air. It was rare that the cold air backed up as far west as Aspen.