Urban Heat Island

General Discussions: ASK John: Urban Heat Island

By hens (Tom Stephens) on Monday, November 26, 2007 - 08:28 am: Edit Post


I have a theory that I'd like to run buy you....

A friend of mine has a farm north of Chippewa Falls, WI about 3 miles. He said recently (the last 10-15 years) they have had a lot less rain and especially storms in that timeframe.

My theory is that because our weather (severe mostly) comes from the west,and the Twin Cities urban area has grown pretty significantly in that time frame, when a storm approaches from the west, it splits and goes north and south of the urban area, due to the rising air from the heat island effect, akin to a hurricane, but clearly not the same dynamics. It has done this for quite some time, but since the area has expanded over the period of time I'm talking about the deflection is further north and south.

His farm now sits in a "shadow effect" for storms.

It's a long shot, but I'm thinking it might be a good paper subject for a budding geographer (I'm just an old one and not really interested in writing any more papers....)

Maybe when I quit my day job.....


By John Dee on Monday, November 26, 2007 - 12:10 pm: Edit Post


While I cannot back up the rain shadow occurring in your friends farm, it would most likely NOT be from the heat island effect. If anything, the rising air would lead to more instability and thus more clouds and thus better chances for rain. Sinking air is what would cause a drop in clouds/precip.

A more likely cause (if it really is happening) would be the fact that a rural area has less vegetation and more hard surfaces like roads, sidewalks and rooftops. These to not store water in them like vegetation and thus do not allow evapotranspiration (evaporation from vegetation) to occur and thus you would not have as much of a moisture flux into the atmosphere and thus inhibit cloud/precipitation formation. However, even that theory is really a stretch as I have never seen any literature to that effect.

The most likely scenario is just that his recollections of past weather are not as accurate as he thinks (unless he has hard data to back this up), or that this is just some sort of a temporarly bump in the road his specific area is going through.

Chippewa Falls is really pretty far away to be impacted by anything the Twin Cities is doing. The truth is, if Lake Superior was sitting where the Twin Cities is, you see very little, if any weather modification (lake effect snow, cooler temps in summer, warmer temps in winter, wind changes) where you are at from it and Lake Superior is just a wee bit bigger than the twin cities!


By FRNash/PHX, AZ on Monday, November 26, 2007 - 12:20 pm: Edit Post

John Dee:
"… a rural area has less vegetation and more hard surfaces like roads, sidewalks and rooftops."

Ooopsie! I'm sure you meant:

a rural area an urban area has less vegetation and more hard surfaces like roads, sidewalks and rooftops.

(A very interesting theory, though!)

By John Dee on Monday, November 26, 2007 - 12:24 pm: Edit Post

Yep, thanks for the correction.


By JR on Monday, November 26, 2007 - 09:22 pm: Edit Post

What about Chicago and it suburbs? (It's bigger then the Twin Cities.) Or would you contribute it more to Lake Michigan?

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