24 Jul 2018

How hot can it get?

This question is often asked these days where many regions around the world suffer from heat stress periods and drought.
There are two aspects dictating the answer to this question: On the one hand, the current weather and air temperature are driven by the large scale meteorological circulation systems. How hot or how cold it gets mainly depends on where the air masses originate from: moderate ocean regions or hot and arid land masses, e.g. in Central Europe from the African continent.

On the other hand, the other aspect that modifies the local air temperature is the microclimate generated through the nearby environment, like Urban Areas, forests, and/or other landscapes. These environments can easily lead to an increase or decrease of 4 to 5 degrees in air temperature and even more in perceived temperature.

It is well known that in long lasting heat waves, the indoor temperature increases day after day because buildings receive and store more energy during the day than can be released during the night. Therefore, there is a daily net increase in energy up to a certain point of equilibrium.
The same is true for the outdoor environment, including artificial surfaces such as roads or pavement but also park areas or grassland.

The figure below shows the simulated evolution of the near-surface temperature (5 cm above ground) of an asphalt road during the last 10 days of a 20 day heat stress period. It can be observed that from day 1 to 4, the peak temperature of the surface still rises to approx. 308 K (=34,85 °C), but then stays constant over the last 7 days. In other words: Even if the heat wave will continue, the air above the road will not get any hotter, because an equilibrium temperature is reached near 308 K. (For a different street with different coloured asphalt or a different vertical construction, a different equilibrium will exist).

But what about parks and grassland? They are more complicated as the water availability plays a major role in the thermal performance.