# Weather math

The National Weather Service has just issued a “heavy snow” warning for the Minneapolis Saint Paul Airport:

THE NATIONAL WEATHER SERVICE IN THE TWIN CITIESCHANHASSEN HAS

ISSUED AN AIRPORT WEATHER WARNING FOR…

MINNEAPOLISSAINT PAUL INTERNATIONAL AIRPORT MSP

* VALID FROM NOON TODAY TO 4 PM CST THIS AFTERNOON

FOR THE FOLLOWING THREATS…

* SNOW FALLING AT A RATE OF 34 INCH OR MORE PER HOUR

LIGHT SNOW WILL ENVELOP THE TWIN CITIES METRO AIRPORT SHORTLY BEFORE

THE NOON HOUR. SNOWFALL RATES OF UP TO THREE QUARTERS OF AN INCH

PER HOUR ARE LIKELY BETWEEN NOON AM AND 430 PM. SNOWFALL

ACCUMULATION OF NEAR AN INCH SHOULD BE EXPECTED.

This invites some math. If the rate of snowfall is “up to” 3/4 of an inch per hour, and the snow will last for four hours and the total accumulation is near an inch, for how long would the rate of snowfall be 3/4 of an inch per hour?

• Disco

Just because it falls at 3/4-in/hr does not mean it will ACCUMULATE at 3/4-in/hr.

• BenCh

Maybe it was suppose to say “3-4 inches,” not “3/4.”

• JSK

I’m hoping they really meant 34″ per hour. That would be a sight to behold.

• Jim Shapiro

That one looks like it was written by one of the chimps from the days of Soviet space launches.

• Eric

They use the rate as a way to communicate what viability will look like at the hardest point. Much like a thunderstorm, you may get 3″ an hour rate, but it might only occur for a couple minutes. The rest will be at lower rates. So in this case you might get 20 minutes at the 3/4″ rate, the remaining at 1/4″ an hour which will get you just over an inch.

• Pete

@Eric,

Actually, with your example, you’d get less half an inch, not over an inch.

(3/4 inch/hour) * (1/3 hour) + (1/4 inch/hour) * (2/3 hour) = (5/12 inch)

Which is exactly the point that people get confused by fractions, rates and addition.

• gml4

Doesn’t this call for a rousing game of SNINGO!

• Rick

They give a big range of time when it might happen, but it doesn’t mean that it will occur the whole time. They might just be noting that it could happen in that time, but might not be the whole time, similar to a severe thunderstorm warning.

34 inches of snow an hour would be awesome to watch. Especially from a tropical climate or in a really sturdy building.