Earlier this week I was commenting on the very cool spring that Boise has been experiencing. Living in Boise I happen to know how built up the area has become over the past 20 years. That it is in a valley as well only makes the potential Urban Heat Island effect even greater. I have often wondered how much of the warming that Boise experiencing is due to the change in the area. Since I had done the analysis for Boise already, I decided to compare it to a region that was close area that was not an urban or agricultural area.
I ended up picking McCall, Idaho. This is one of the nearer locations that I know is a lower impact for the UHI.
McCall is even a little town, but having been there plenty of times I know that it isn’t very built up, at least not yet. It is a higher altitude that Boise so the temperatures are lower, but this is where anomaly is once again useful. The locations are near enough that major storm systems will effect both places in a similar manner, but a difference in the anomaly could be an indication of the UHI.
The very first thing I noticed is that McCall has been experiencing a much colder than usual year. It makes Boise look tropical so far and that is based on the temperature anomaly. The average high temperature should be over 10 °C by this time in April. The only day in the last month to even reach 10 °C was April first. Much like that was the warmest day in April for Boise.
This puts the average high temperature anomaly at -6.9 °C for the month of April. For the entire year that average anomaly for the high temperature is -3.7 °C. If this was the station nearest the North Pole and it was being interpolated out for 1200km in each direction it would count for a very cold spot on the Earth. I am guessing that it will be ignored as much as possible.
The average anomaly for the year is not quite that low as it is only -2.4 °C for the entire year. For the month of April the average anomaly is -4.7 °C. That is a huge deviation from normal. You can be sure that if McCall was experiencing this in reverse (this much warmer than average) it would be making news and used as evidence for global warming.
The real kicker is the trend in the anomaly. At the current rate of cooling it will be forming glaciers there by July.
Not only is the anomaly trending down, the average temperature trend for the month of April -0.04 °C/day. So on average in the month of April, McCall has been cooling down.
Now let’s look at how the anomaly has differed between the two locations. This is going to be ugly and show the UHI in very realtime.
I was stunned by the uniform anomaly difference for both the high and low temperature anomaly. The average difference for the year is -2.0 °C. This would indicate that Boise has a UHI of 2.0 °C. That is a huge value for an area like Boise.
Nor is it possible to say that McCall is not a valid comparison for Boise for the UHI. The mean temperatures for both locations show that the same weather events impact both places by a comparable amount.
It is hard to argue that the UHI result of 2.0 °C is invalid when both locations show response that is this well matched. Considering that there are stations in the Boise area that are part of the station measurement system (Caldwell, in an alley surrounded by concrete building on asphalt) it makes it even harder to trust any of the data from the station measurement method. The low impact data from McCall shows that even less densely populated place like Boise have a significant UHI.