Thread by @cristiproist

One of the most challenging consequences of climate change is the fact that we expect both more drought AND more flooding.

The basic reason for it is the same reason your glasses fog up and your house is dry in winter.

Strap in for a short thread.

1/11

The physical reason is something called the Clausius–Clapeyron relation (affectionally called C-C).

A complicated name (that I always misspell) for a very very simple concept:

*WARMER AIR CAN HOLD MORE WATER*

2/11

Imagine a bucket of air (or "parcel" in science speak). In this bucket of air, there is a certain amount of water vapor.

Turns out there is a maximum amount of water vapor this parcel can hold. Anything more turns from water vapor into liquid water (i.e. clouds and rain).

3/11

Claussius-Clapeyron says that the max amount of water vapor air can hold increases exponentially with temperature (at a rate of ~7% / degree Celsius)!

4/11

The opposite also holds: Colder air holds less water.

If you start out with a lot of water in the air bucket and cool it, eventually the max becomes less than the actual.

The excess water vapor (a gas) will fall out as liquid water (rain) or solid water (ice & snow).

5/11

This is what happens when you bring in cold glasses into a hot, humid room. The glasses cool the air around them until all of a sudden that air finds itself holding too much water vapor, which condenses into liquid.

6/11

It's also why your house is dry in winter.

The air outside is cold in winter (at least, in the midwest).

Even if it is maxed out on water vapor, the maximum limit is very low (by Clausius-Clapeyron).

7/11

When you bring the air inside, the actual amount of water doesn't change. What changes is how much water that air can hold.

All of a sudden it finds itself hot & very thirsty: it quickly sucks up all the moisture in the house (hence the dry skin)

8/11

The same thing happens with climate change. As the air warms, the maximum amount of water vapor it can hold all of a sudden increases, and the atmosphere finds itself much thirstier.

That thirstier atmosphere sucks water out of the soil very quickly.

9/11

At the same time, when conditions are right, and extra water vapor is pumped into a region - say, by a passing hurricane - it the air can hold more water vapor before it decides to dump it all as rain.

When it does rain, it pours! 10/11

That's the gist of it, at least in terms of the atmosphere.

It gets worse of course, as drier soils would be more prone to flooding even if the storms wouldn't get worse, but that's a thread for a hydrologist to write.

11/11 (end main thread. bonus below)

An interesting question that doesn't really affect the argument is what happens to the average "actual water vapor in the air"? Does it stay the same?

We think not.

bonus: 12/16

Over the oceans at least, what stays the same is relative humidity, i.e. the ratio of actual water to max possible water and, thus, the ratio of vapor "decificit" to max possible water vapor.

13/16

But even if the fractional deficit stays the same, the actual deficit is larger, since it's the same fraction of an exponentially larger limit.

14/16

Over land, however, the average amount of water does increase but not as much as over the oceans, and relative humidity decreases over land (for complicated reasons. Go ask @DrMichaelByrne).

15/16

@DrMichaelByrne And, if you're still here with me, here is a graph of how fast the max possible water vapor increases.

It's easier to read the blue line, which is in grams of water vapor per kg of air. Hartmann textbook, fig. 1.9.

16/16

@DrMichaelByrne P.S. can anyone tell I'm preparing slides for my climate change 101 class?