Phoenix Misses Record Low. Kindof. Sortof.

This morning, Phoenix hit 51F, just missing the record of 49F last set in 1965.  But as I mentioned in my previous post on the record high we hit (just four days ago!), the growth of the Phoenix urban heat island in the last 4 decades has a lot to do with how these records are set and not set. 

The Phoenix urban heat island has been measured many times, including by my son in a recent science project.  In that project we measured an urban warming of 8-10F around midnight vs. the outlying areas of the city.  We have not measured the daytime heat island effect, but others have, such that it is not hard to imagine that the record high set four days ago would not have been a record when the city was smaller.  Today, we can similarly argue that we could have easily had a record low last night, since we only missed by 2 degrees, had it not been for the heat island effect.  While 2 vs. 8-10 degrees seems compelling, we also had a lot of wind last night that tends to break up the urban heat island effect, so we can never be sure.

  • Darin

    Wouldn’t the heat island effect tend to increase the highs and decrease the lows? I thought the idea was that the thermal capacitance of the area was lower, meaning that less energy is stored in buildings and roads than in trees and grass. It can’t be that the heat island effect creates a general increase in temperature — since the amount of energy from the sun is the same. (It’s not heat from cars and lights and so on that makes the difference, is it?)

  • Stevo

    It’s a bit more complicated than that, and there are some parts that don’t appear to be fully understood.

    Part of the effect is due to increased absorption. While the incoming sunlight is the same, black tarmac and roofing absorbs a lot more of it than green grass or tree canopies. There is also the “canyon effect”, in which light reflects off high buildings giving it several chances to be absorbed, as opposed to flat ground where once it reflects it’s unlikely to hit anything else.

    There are also changes to the rate at which it is lost. Plants and moist soil lose a lot of heat by evaporation – water takes a lot of energy to turn it into vapour. Plants are especially effective, as they use this one-way water flow as their circulation system. If water is locked under concrete or drained away rapidly, you soon get a far more arid surface than in nature. There are also significant changes in micro-convection and the effect of wind. Heat is normally conducted only into the first few centimetres of air, but air micro-currents rapidly carry that away and replace it, speeding the process enormously. When nearby surfaces are at different temperatures you get convection. Urban surfaces tend to be more uniform on the micro-scale, and tall buildings shelter you from the wind on a larger scale. And grass and trees are flexible enough to move in the wind, which further enhances cooling efficiency.

    There are also more complicated processes involving the air well above ground level, such as inversion layers in the first few hundred metres, and different effects depending on whether it is cloudy, windy, or has recently rained. The air temperature changes rapidly with altitude near the surface, so you have to be careful to define exactly what is meant by “the surface air temperature”. The trend at two metres may not be the same as the trend at ten metres or ten centimetres.

    Direct heat energy from mankind is not usually significant, unless your thermometer is very close to the source. Putting thermometers next to air conditioning units and at sewage treatment plants could well cause such influences.

    Generally, the temperature difference between country and city is some fraction of a degree C during the day and several degrees C at night, both upwards. And because the temperature stations that go back a century or more were probably not in such urbanised environments originally, so one would expect there to be a rising bias to the temperature measurements.

    However, it is worth pointing out that much of the countryside has experienced land use changes too, from modern farming methods and the spread of agriculture for example. Corn instead of trees will affect the sunlight absorbed. Irrigation will affect evaporation. Big open fields instead of scrub or forest will affect convection and wind. These may affect rural temperatures systematically too. (And if there are systematic climatological changes in precipitation or windiness, or ecological changes due to the wildlife changing character these may affect natural surface temperatures independently of radiative heat balance. That’s not related to UHI, though, and shouldn’t be corrected for.)

    Despite this expectation of a warming effect, some of the climatologists have reported contrary results. Hansen has applied UHI corrections to data in which over 40% increase urban temperatures relative to the surrounding countryside, indicating that he has found measurements taken in cities are often cooler than they should be. Parker compared temperature trends on windy and non-windy nights (although there is dispute over whether his chosen threshold was appropriate) and found no difference, showing that there is no significant UHI trend.

    That urban heat islands exist is easily measurable (as in the above linked science project), and there has undoubtedly been a trend in urbanisation around many weather stations over the past century. How these can both be true, and yet no systematic trend in UHI result is a bit of a mystery.

  • Darin

    Sure, there must be a lot of I’m not considering. But it still seems strange to me that this effect changes the average temperature. However you define “surface” or “urban area” or whatever, I would think that the energy balance picture is unchanged. I guess what you’re saying is that, while the amount of energy “in” is the same, the amount “out” at a given temperature is lower — so the temperature rises. Is that a fair approximation? It’s non-intuitive to me, I admit.

  • Stevo

    A lot about this subject is non-intuitive. If it was all intuitive, we wouldn’t have to pay scientists to figure it out. 🙂

    Yes, that’s a fair summary. The energy flow in is also a little bit higher (for the same sort of reason a black surface left out in the sun gets hotter than a white or silver one), but the main reason for the effect is the energy flow out being smaller for a given temperature.

  • Darin

    At night, though, the black surface will get a little colder — it’s a better emitter of radiation, just as it’s a better absorber. This is why cooling fins, say on electronics, are painted black.

  • Stevo

    At night, it will cool faster. Whether it gets colder depends on where it starts from, and the temperature of the surroundings into which it is radiating.

  • Darin

    Yes, that’s right. My point is that the effect is symmetrical. So, if you use the “black surface” analogy to explain heat islands, my original question is valid: Shouldn’t it be colder at night?

  • Stevo

    Ah, right. In my May 28th post wasn’t using it to explain heat islands, just as a minor pedantry over whether the heat input was identical. The output side of the balance is more responsible for the effect, though.

    To some degree you are right, black surfaces cool faster at night. It’s not safe to assume the effect is symmetrical, though, because the incoming radiation is shortwave visible light, while the outgoing is longwave IR light (and while black surfaces tend to be darker at all wavelengths, that’s not necessarily so) and because the rate of heat flow depends on the temperature of the surroundings too, which are not necessarily symmetrically hotter/colder than the ground between day and night. If the warm body is mainly radiating towards surroundings that are also warm (because it is surrounded by tall buildings) then the heat loss will be slowed, but as the heating during the day is dominated by input from a single direction, the heating is not symmetrically reduced by the buildings.