Nashville Heat Island

[Coach B]

Yesterday evening I was wondering why in the world we had a freeze warning for Marshall county when we were in the 20s 11 or 12 days ago and have been below freezing 3 or 4 times already.  It seems that on calm and clear nights we are a good 5-10 degrees colder here than what is reported in Nashvegas and I'm only 30 miles or so from the Alabama line.   Do you think the lack of reporting stations south of Nashville is why they didn't know we have long ago had what appeared to be a widespread killing freeze, or do the forcasters assume everyone south of town is as warm as the Nashville Heat Island?

Has there always been this much difference between Nashville and the surrounding areas?  Some of you longtime weather guys fill me in.

[Thundersnow]

There are always going to be certain areas that get colder than surrounding areas.  It has to do with topography, etc.  You may be in one of those areas.  What they explained last night on the local weather was that the freeze warning areas had not experienced a wide-spread freeze yet, even though some locations in those areas did.

One thing to keep in mind... most of the reporting stations around the state are located in towns.  I suspect every little town is going to have its own heat island (though not to the extent of a bigger city like Nashville).  I suspect, for example, that Lewisburg would report in as slightly warmer than a lot of locations in rural Marshall County.

That's just a guess.  It's not a perfect science, by any means.

Even though I am in Davidson County, which did not have a freeze warning last night since we supposedly already had our first freeze a week ago, last night was my first real freeze (low of 30.2).  A week ago, the lowest temperature I had was 32.4, which was barely a light freeze at most.

So, even though you were in the 20s last week while I barely hit freezing, I supposedly had the freeze, and you didn't.  :roll:

[Coach B]

Using noaa's climate page I looked at the difference in temps from Nashville to my nearest reporting station-Lewisburg.  I used the period Dec 31 2000-Jan 31 2001.

For the period the recorded high temp averaged .41 degrees lower for Lewisburg.  Not much difference.

The interesting part is found in the low temps.  For the period Lewisburg averaged 4.84 degrees lower than Nashville.  Which is pretty significant considering Lewisburg is roughly 50 miles south of Nashville.   However, on the 18 nights with primarily calm and clear weather Lewisburg averaged 8.22 degrees colder and on nights that were primarily cloudy, windy, or had active weather Lewisburg was only .50 degrees colder.  

Bottom line to me is that on a clear and calm night if they are forcasting a low of 40 for Nashville then I better get ready for freezing temps.  However, if the weather is active in any way the forcast will be more accurate area-wide.  The forcasters clearly point out that there will be differences in temps from the Nashville reporting station to the rural areas, but I didn't realize there was that much difference

[Ron_Jarrell]

On clear windless nights radiational cooling is at the maximum. Forested and grassy areas release or radiate away heat more easily. That's why there is such a large difference on those clear windless nights. For an example of this, if you have a brick home wait a couple hours after daylight and feel the brick on your house. It will still be very warm from the day's sunlight and warmth. Asphalt and concrete have the same effects of holding heat.