Just now in fact where I live, Detroit, MI. We had brutally hot weather. Then a storm. Now things are cool again.
I know that is what the weathermen say in any event. The upcoming storm is going to cool things down. That is what they say.
Specifically how does it cool things down? What is the mechanism involved (in plain English please )? And does the storm really cool things down? Or does it just coïncide with the cool weather? How does that work, then?
Rain is usually generated when a warm air front meets a cool air front. Since the warm air is lighter, it rises and cools in the higher altitude resulting in condensation forming and resulting in rain. Thus when a storm comes, it cools down.
What you call an “air front” is more commonly known as an “air mass”. The front is the boundary between air masses. Your explanation works fine if it’s the cold air mass that’s invading the territory previously occupied by a warm air mass, but what if the front were moving in the opposite direction? Shouldn’t you expect a temperature increase once the warm air mass takes over the territory formerly occupied by cold air?
Obviously there’s some air mass modification going on throughout the whole process, with all the water changing phase and the cloud cover affecting how much sunlight arrives at the surface. But an easier explanation for the OP’s association is that strong storms are more likely to be generated by an advancing cold front, while an advancing warm front might come and go without any precipitation whatsoever (if the atmosphere is stable in the warm air mass). Because the storms generated by cold fronts are more common and more memorable, you associate storms in general with a temperature decrease. Those rare occasions when an advancing warm front also has enough atmospheric instability to generate storms are not as easily called to mind, and the temperature increase left in their wake don’t form the same mental associations with storms in general.
Doesn’t explain why it gets cooler in an area with local convection (i.e. no front at all), like it does here in NE Florida in the summer (and did today, from 90 down to 76 for several minutes after a 30 min. long cloudburst). Likely a combination of the water itself being cooler by default (having fallen from a higher cooler altitude) along with the cooling effect of the subsequent evaporation and the sun being blocked.
Rain and storms are not quite the same thing, although they overlap. Also there are lots of ways either can occur.
Both cold and warm fronts carry rain, as do low pressure systems and cyclonic storms. The mechanisms for moisture transport and precipitation vary. But you do basically need to get moisture laden air to cool in some manner, and then the rain falls. Rain can become self sustaining once it starts (which was part of the idea about cloud seeding.)
So, rain is more likely when warm moist air is cooled. That can occur with uplifts from an oncoming front. It can also occur in the violent uplifts inside thunder-heads. Large wildfires can create enough uplift to make it rain. Cyclonic storms have massive uplifts, and can inject moist air into the atmosphere over vast distances making it rain thousands of kilometres away.
Any water evaporating will pull energy from the air - the latent heat of evaporation of water is significant. So rain will make things cooler as well. The muggy humid heat after is another matter.
A thunderstorm carries the rain droplets upwards in the uplifts where they cool significantly - which is why they may freeze and become hail. So thunderstorms can deliver cold rain, even in very hot climates.
So, yeah, rain usually cools things, but the reasons are manyfold. What happens in one region may not be relevant to another.
If the downpour is especially intense, then you might see something called entrainment, which carries the cooler air from high altitudes down to the surface with the heavy precipitation. This cold air hits the surface and spreads outward in what’s known as a gust front, so even in this case you aren’t able to say “no front at all”.
As for the energy transfer due to evaporation, it would be more obvious if a clear boundary could always be drawn between the material being cooled (the air parcel) and the material changing phase (the raindrops). In their liquid phase shouldn’t the falling raindrops not be included when determining its average internal energy of the air parcel, while after evaporation their gaseous phase counterparts are included in the air parcel? If the system is redefined midway through the process, it becomes less obvious why any comparison of the average internal energies (before and after) should be meaningful. Maybe I’m overthinking how air parcel is defined, and it really is just as simple as “liquid phase less energetic than gaseous phase”.
Because the rain is falling from a higher altitude, and it is cooler up there, the rain is usually much cooler than the streets, sidewalks, buildings, etc that it lands on, so it reduces their temperature. Also, if it is raining, the sun is obscured and that, in and of itself, will reduce the temperature, at least for a while.
there is also the problem of evaporation:
you need to give energy (hot) to water for transforming it in vapor, and when the first raindrops falls, they revert to vapor, thus taking energy from the air → cold.
In fact, in any significant rainstorm, the rain is actually frozen when it starts falling, and melts on the way down. So not only does it take energy away from the air to heat it up, it takes even more to melt it.
Yes, energy gets extracted from the surrounding air in order to turn the raindrop back into water vapor. But once it’s water vapor, doesn’t that make it part of the air, and hence something that needs to be accounted for when computing the temperature?
Cue up the song Particle Man by They Might Be Giants. “Is he a dot, or is he a speck? When he’s underwater does he get wet? Or does the water get him instead?”
Once the raindrop reverts to vapor, one might argue that it should be included in the air parcel again. Then an accounting of the average internal energy (before and after evaporation) is less straightforward than the analogous accounting when sweat evaporates from a person’s skin. I’m sure it eventually works out to be a temperature decrease, but the simplistic descriptions fail to address the crucial point of which particles are being counted in an air temperature reading.
Thanks Melbourne - this sounds like a good explanation of outflow. We see a lot of thunderstorms pass by on their way east off the Front Range of the Rockies. Do i remember correctly that the wind is going to flow counterclockwise in this here northern hemitypesphere?
If it is a cumulonimbus thunder storm there will often be a gust front of very cold high altitude air. Those towering thundeheads have enourmous convection currents bringing air down from the stratosphere.
Often summer thunderstorms will drop the local temperature here in Calgary dramatically from sweltering to fridgid. Unfortunately not much relief this summer for us thin blooded northerners.