City Cooling Downdraft Towers

Cities are hot, hotter than the surrounding area for a number of reasons. They're often less reflective and lack the vegetation of other environments that can supply shade and some level of evaporative cooling via transpiration. In addition, there's often lots of heat-generating activities and localized atmospheric pollution that makes things worse. This leads to a heat island effect, hot air rises and pulls in air from around the city. This is no good however, because the air is pulled in at ground level, and that's already quite warm since it's the sun heating the ground that is the source of heat.

What we need is cold air, there's actually plenty of that. Take a look at an altitude/temperature graph <link> and you'll notice that the air a few meters up is meaningfully cooler. Go a few hundred metres up and you have a very big difference. So how do we get it down? The traditional method involves clouds and rain/hail. Falling rain evaporates to some extent, cooling the already cool air which increases in density and falls. This drags replacement air in from above. Take a look at a microburst <link> and you'll see that the air falls, and then spreads out in the opposite pattern of normal city heat island effects.

How do we make this happen on purpose? Big towers. Well, not wide or massive, just very tall. These are actually very cheap to build and are available as off-the-shelf engineering items for things like radio antennas. 500m is no problem at all and 1000m shouldn't be a massive challenge. All the things need to do is support a water pipe. We build a ring of these around a city centre and spray water into the middle. The goal should be to set up a gentle downdraft dragging cooling air downwards. The added advantage would be that the water should collect particulates and some pollutant gasses on the way down, although you'd have to make sure at least some of the water makes it to the ground in liquid form for this to have a significant cleaning effect.

This should pay for itself in lower electrical use city wide.