When it rains, why “how much snow would this be” is a fun but terrible question


A measure of moisture, known as precipitable water, was extremely high along the East Coast Tuesday night when torrential rain fell. (WeatherBell.com)

No, the six inches of rain Baltimore received Tuesday would not have been 60 inches of snow.

Whenever we get torrential rain, it’s fun to ponder how much snow would’ve fallen.  But, turns out, it’s really an unanswerable question – even if tempting.

During winter, when 10 inches of snow falls, that usually equates to about an inch of rain.  In other words, if you took a 10-inch pile of snow and melted it, one inch of water would remain. (This conversion can vary depending on whether the snow is dry or wet.  It might take 20 inches of light, powdery snow to melt down into an inch of water but just 6 inches of heavy, wet snow.)

There’s the enticement, then, to apply this ten to one conversion when we’re doused by tropical downpours. Imagine Long Island buried under 130 inches of snow in less than 12 hours if that 13-inch deluge Wednesday morning had fallen as the white stuff. But it doesn’t work that way.

Because warm air is able to hold more water vapor than cold air (caution: some say this explanation is oversimplified), vastly more moisture is available for summer weather systems compared to winter weather systems.  That’s why practically all records for wettest days have occurred in summer and early fall.

We’re just not going to get the obscene precipitation amounts in winter that we get in summer.


Climatology of precipitable water – a proxy for moisture – at Dulles Airport. The red line (or 50th percentile) shows average moisture levels throughout the year, with much lower levels in winter (near 0.3″)  compared to summer (around 1.4″). (National Weather Service)

Even when it rains in winter, it’s still not possible to say how much snow would’ve fallen.  When it’s cold enough to snow, the air typically comes from a completely different source region (than when it rains) – usually the north or northwest – where the air is somewhat dry.  When it rains in winter, it’s usually because the air is streaming in from the south and east, with moist tropical and marine origins.

Think of it this way, when it rains, it rains (rather than snows) for a reason.  So we have to resist the urge to take rain events and  project them into epic snow amounts, however fascinating.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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