WXSIM and Heat Island index

Hi,

My studies of weather and climate, base on my own data, has made me think of Heat Island issues.

In Wxsim, lower right box, you have Heat Island: and I have “rural” and the number “36”.
The range goes from 0 to 100!

I have started comparing my data with other - official - stations. Recalculating data with resepct to altitude, et c is quite easy, and in
once case I got the same results as a big consulting company - in an insurance case.

But, what about Heat Island index? I have seen names like ‘rural’ being used in different places and words for dense cities et c, as is also used in combinations of names and numbers in Wxsim for heat island. However, I have not found any source for an official scale of numbers and names. Or in case of Wxsim if it is related to some coefficient. I have noteced articles that a change of some definitions of Heat Islands were made around 2011.

The thing is - I have not seen Swedish Met Office (SMHI) stating anything about where their stations are situated, i.e. their Heat Index in any kind of scale/range.

I appreciate any knowlede shared regarding this.

Ingemar

Hi Ingemar!

That particular scale is my own. Here’s what it means: first, urbanization has the well known effect of raising temperatures. Maybe a bit less obvious (but still well-studied) is that it raises nighttime temperatures more than daytime temperatures, especially in relatively clear, calm wind conditions. I’ve also noticed that the temperature drop in cities is more gradual, but also a bit more steady during the night, compared to outlying areas in the “countryside”, which tend to have a dramatic temperature drop around and shortly after sunset, and then gradually slowing during the night.

This is mostly related to the failure to form a significant temperature inversion over the slowly cooling concrete and pavement of urbanized areas, while open fields and grass in rural areas can develop strong nighttime inversions, which also tend to break up quite quickly around mid-morning. Not only is the diurnal temperature cirve of an urban area less extreme (lower amplitude) - it’s also a somewhat different shape (with some time-lag or phase-delay).

When I assign urban heat island effects, much of that decision is based on looking at the actual amount of urbanization (buildings, houses, parking lots, roads), but it’s also a result of testing and tweaking to try to match the customer’s weather station’s actual characteristics. I’ve found that another factor comes into play heavily: the elevation relative to surroundings. Local high-elevation spots, either because the ground is locally high OR because the thermometer is mounted high above the ground (on a roof, for example), behave a lot like more urbanized locations. For this reason, I’ve often given a remote, non-urbanized location a somewhat “urban” heat island effect, like 60 or 70, because the thermometer is relatively high up compared to most of the surrounding area.

In professional mode, it’s possible to play around with this number. However, my recommendation is always to keep it like it is unless I can revisit the situation, because I’ve already gone through the process of trying different values to see what works best.

So, again, it’s my own creation, and it matters a lot!

Tom

Thanks Tom! I totally agree with the concept of Heat Islands and the effects and causes - and how it differs from my rural site, and similarities in between those two types f ex in a nearby small town. And, just as you say, a valley or a sloop, or high up on a mountain matters. During hunting season I often take with me a small Raspberry Pi weather station with temp and pressure, but no wind dir or speed. It works on a battery up 8-10 hours, so I can compare my villige WS data with the around 70 fiex hunting positions “towers” et c around the 24 sq km hunting ground. Since winds are crucial for hunting I like to add a small windspeed and wind dir. Now I do this manually.

The big thing I saw was in 2009-2010-2011 when there was a deforesting around the village size about a US sq mile. Snow stayed on longer in spring, but it was a lot warmer during summertime, by now brush has grown and spruce and pine plants are bigger. so I cannot see the effect any longer 200 meters/220 yards away at my station. But still, If I put a thermometer out in the deforestation and compare it with the temps in a small forrest 50 yards by 300 years nearby it is quite different in temperature. The national board of forresting in Sweden says that swedish forrest grow about 30% faster now compared to a century ago, just because of warming of the ground which stays on longer even into the night, the effect stays on for 15 years, and comes back after the 40 year “cleaneaning up” i forrest vegetations - this effects around 10-12% of the whole of Sweden!

But, the main reason for my question was and is if there are som distinct tables for deciding on any scale what is a heat island or rural and there between, but I have not found such tables. I know that temperature differences are a big part of such a decision if heat island or not. Could be done, but prof. WS are not in aboundance and non-pro stations are often not correctly situated, or has less confident in their hardware. Well, I understand this a bigger discussion.

Got the idea of perhaps 10 people going from the center of a town in diverse directions out to countryside, farming landscape, and into a forrest. Perhaps it has already been done in heat island research?

Again, Thank you Tom for a very good explanation.

Ingemar