Every drop of water vapour in the column of air overhead, squeezed into a puddle — that is
precipitable water. It is the fuel supply for rain: how much the atmosphere could wring out.
This maps it over the Lower 48 from ERA5, day by day since 1 January 1940, for the nation,
the nine climate regions, all 48 states and 344 climate divisions — as a value, an anomaly, a percent of
normal, or an all-time rank.
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Moistest
Driest
All-time records
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Day by day · United States
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The atmosphere is holding more water · United States
Annual mean precipitable water, 1940 → present. Warmer air holds more moisture — about 7 % more per
degree Celsius (the Clausius–Clapeyron relation) — so this is one of the most direct fingerprints of a warming
atmosphere. The trend line is fitted from 1979, when satellite moisture soundings entered the reanalysis;
the 1940–1978 back extension is shown but is far more weakly constrained by observations.
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💧 What the numbers mean. A precipitable water of 1.5 inches means that if every bit of vapour
in the column above you condensed and fell, it would leave an inch and a half of water on the ground. Values over the
Lower 48 run from under 0.1 in on a bitter, dry winter day in the northern Rockies to about 2.5 in in a
tropical air mass over the Gulf Coast. It is potential, not rainfall — a storm typically precipitates only a
fraction of it, but recycles the column many times over.