What if rain grew more slowly than water vapor?

Warm air can hold more water than cool air.

So does a warmer world get lots more rain? Not as much as you might think!

When air warms up, it can hold more invisible water vapor, about 7 percent more for every degree Celsius, according to NOAA. You might expect rain to grow by the same amount, since what goes up must come down.

But climate models find that rain over the whole planet grows much more slowly, only about 1 to 3 percent per degree. So what happens when the air's water grows faster than the rain that empties it? In this lab you can test that in a model of the whole Earth's water cycle.

At near-constant relative humidity, the Clausius–Clapeyron relation makes atmospheric water vapor rise about 7% per kelvin of warming. Yet climate models simulate global mean precipitation rising only about 1–3% per kelvin (Siler et al., 2018).

This lab explores that mismatch. If the store of vapor grows faster than the flow that drains it, the average time a water molecule spends in the atmosphere must lengthen. How much, in a simple box model, and why does the rain lag?

There is always some water in the air, even on a sunny day.

Imagine it all raining down at once. It would cover Earth only about one inch deep.

Even clear air holds water vapor. The U.S. Geological Survey estimates that the air holds about 12,900 cubic kilometers of water at any moment. If it all fell at once, it would cover the globe only about 2.5 centimeters deep.

Yet a huge amount passes through. In the model, about 486 thousand cubic kilometers evaporate each year and the same amount falls as rain and snow. Because the store is small and the flow is big, water only stays in the air for about 9.7 days on average in the model.

USGS gives about 12,900 km³ of water in the atmosphere at any time, roughly 0.001 percent of Earth's water; about 90 percent of it comes from evaporation and 10 percent from plant transpiration. The model's yearly flows follow Trenberth et al. (2007): 486 thousand km³ evaporated, 113 falling on land, 40 returning in rivers.

Dividing store by flow gives a residence time of 12.9 ÷ 486 × 365.25 ≈ 9.7 days, consistent with the 8–10 days Siler et al. (2018) quote for an average molecule's trip through the cycle.

Olly warms the pretend world by 2 degrees.

First, rain grows as fast as the water in the air. Then Olly makes rain grow more slowly. Now water waits in the sky a bit longer.

The lab starts 2 °C warmer than today, with rain growing at the vapor rule's 7 percent per degree. The air holds about 14.8 thousand cubic kilometers of vapor, and about 556 thousand cubic kilometers evaporate and fall each year. Water still stays up about 9.7 days.

Now switch rain to about 2 percent per degree, near the middle of what climate models find. The air still holds 14.8, but only about 506 thousand cubic kilometers fall each year. With more water in the air and less flowing through, each bit of water stays up longer: about 10.7 days.

At ΔT = 2 °C the vapor store is 12.9 × 1.07² = 14.8 thousand km³ whatever the rain rate. At r = 7%/°C all flows scale by 1.145 (evaporation 556, land precipitation 129 thousand km³/yr), so residence time stays 9.7 days.

At r = 2%/°C the flows scale by only 1.040 (evaporation 506, land precipitation 118, rain +4 percent), while the store still rises 14.5 percent; residence time lengthens to 10.7 days. Push warming to 4 °C at 2 %/°C and it reaches 11.7 days. The model scales every flow by one factor and has no regions or seasons.

When water vapor turns into rain, it gives off heat.

The air has to send that heat away into space. It can only do that a little faster each time the world warms. So rain grows slowly.

Water soaks up energy when it evaporates and gives that energy back as heat when the vapor turns into cloud drops and rain. Many scientists explain the slow growth of rain using the air's energy budget: the extra heat from more rain must leave the air, mostly as invisible infrared light sent out toward space, and that cooling grows only a little as the world warms.

Other scientists explain it from the ocean surface, where temperature mostly controls how fast water evaporates. Both views agree on the result: global rain grows more slowly than the air's water.

In the global atmospheric energy budget, L·P = R_a − H: latent heat released by precipitation must be balanced by net radiative cooling R_a minus surface sensible heat H. In most simulations H changes little, so the rise in P is set largely by how fast radiative cooling increases, which is far below 7%/K (Siler et al., 2018).

Siler and colleagues also offer a surface view, deriving a Penman–Monteith form for ocean evaporation in which temperature is the main control and radiation and ocean heat uptake are secondary. The box model contains none of this physics: the rate r is simply a setting.

Scientists think more water in the air can make some rainstorms heavier.

They study clouds and rain carefully so towns can get ready.

The gap between vapor and rain may sound small, but scientists use it to explain bigger changes. NOAA explains that when rain does fall, the extra water vapor in warmer air can make it rain more heavily, which can lead to more flooding.

Researchers have also linked the gap to a slowing of some of the air's large circulation, and to changes in floods and droughts. A whole-planet box model can't show any of that: it only shows totals for the entire Earth.

Siler et al. (2018) note that the disparity between 7%/K vapor growth and 1–3%/K precipitation growth has been invoked to explain a slowdown of the atmospheric circulation (Held and Soden, 2006) and an increase in the frequency and intensity of floods and droughts (Allen and Ingram, 2002; Trenberth, 1999 and 2011). NOAA Climate.gov highlights the heavier-rain side: extra background vapor can intensify rainfall when storms form.

The box model has no storms, regions or circulation, so these consequences remain outside it; they are labeled ideas in this lab.