How to Tell Time Like a Meteorologist

You, Too, Can Predict the Weather

* Slowly Warming: Full Forecast | Later: Wildest Temperature Swings *

Want to predict the weather? Or at least understand some of the pictures and numbers meteorologists look at when making their forecasts? Most of this information is available on the Internet to anyone, free of charge. The tricky part is finding and making sense of the sometimes colorful maps and often confusing streams of numerical data. That's where we'll try to help, with this periodically appearing series on how You, Too, Can Predict the Weather.

A weather observation isn't very useful without knowing what time it was made, nor is a forecast map very helpful without knowing what time the forecast is for. So before we start looking at all those meteorological maps and numbers, we'd be wise to understand the zany way meteorologists keep time.

Keep reading to learn about meteorologists and the zany way they keep time. Graphic courtesy U.S. Department of Health and Human Services.

Almost all meteorological information is tracked and reported using a standard time called Coordinated Universal Time (UTC), which is also known as Zulu (Z) time (and used to be known as Greenwich Mean Time, or GMT). This is why you'll here meteorologists and weather enthusiasts talk to each other in what sounds like secret code -- "12Z NAM" or "0Z GFS," for example -- if you're lucky (or unlucky?) enough to listen in on such a conversation. "12Z" and "0Z" simply refer to particular times, while "NAM" and "GFS" are acronyms for computer weather models (a subject for a later post).

The UTC or Z day starts at 0000 (i.e., 00UTC or 00Z), which is midnight along the 0° longitude line (which runs through Greenwich, England) and 7 p.m. Eastern Standard Time the previous day, and counts upward to 2359 hours in military style. In other words, 00Z Monday is 7 p.m. Sunday (EST). To convert from UTC or Z to your local time, you subtract the number of hours shown for your time zone...

EST: Subtract 5 hours
CST: Subtract 6 hours
MST: Subtract 7 hours
PST: Subtract 8 hours

... which gives you your local time in military (24-hour) format. To make things even more confusing, you subtract one hour less during daylight saving time.

The four most meteorologically important times are 00Z, 6Z, 12Z and 18Z. These are the most common times that computer weather models are run and that their forecast maps show information for. Still not sure how to convert from UTC/Z to local time? Let's spell it out more clearly with this cheat sheet...

00Z minus 5 = 1900 hrs = 7 p.m. EST (8 p.m EDT) the previous day
06Z minus 5 = 0100 hrs = 1 a.m. EST (2 a.m. EDT)
12Z minus 5 = 0700 hrs = 7 a.m. EST (8 a.m. EDT)
18Z minus 5 = 1300 hrs = 1 p.m. EST (2 p.m. EDT)

See more about the history of UTC/GMT/Z here, and below for a full conversion table.

Conversion table courtesy National Weather Service Southern Region Headquarters.

By Dan Stillman |  March 20, 2009; 10:15 AM ET You, Too, Can Predict the Weather
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