Weather Geek Speak: What is ‘Z’ time?

If you follow a meteorologist on social media, odds are they've talked about "Z" time. Here's how to find out what time they mean in your neighborhood.

When the weather starts to get exciting or perhaps even dangerous, social media feeds fill with professional and armchair meteorologists giving their thoughts on the upcoming storms. 

But the weather community is filled with jargon and acronyms that would make any government bureau proud.

We're here to help as translators.

What's with the "Z"?

One of the most common jargons you might see in a weather post is noting "Z" as a time reference – commonly paired with notes about computer forecast models, such as this example:

"The new 12Z models show a big increase in the storm threat – much larger than what the 0Z models had shown last night."

Think of Z like you would if you saw "GMT" or "UTC." "Z" time is short for "Zulu" time from the NATO Phonetic Alphabet and is set to the same time as global time standard "UTC" (Universal Time, Coordinated) -- which is the same time as the old Greenwich Mean Time, or GMT (Got all that?).

They're all generally considered the official World Clock time zone, from which all the other time zones are set. This time zone is based at the Prime Meridian - where 0 degrees longitude sits. This meridian cuts through Western Europe -- England and France included, which are the source of the time zone names -- and western Africa.

But the U.S. military takes a different naming convention. It begins at the Prime Meridian with the letter Z, and then each time zone is given a letter -- going east from UTC, it starts at ‘A’ and counts up through ‘M’ until it hits the International Dateline when it resets to ‘Y’ and counts backward to ‘N' and… I could do a whole separate story on that, so let's say it's complicated and check this map if you're curious.

(A fun fact with military jargon: The Pacific Time Zone is considered "Uniform Time," which speaks to the phonetic alphabet letter "U" assigned to the time zone and not, as you might expect, actual uniform time. Meanwhile, in Hawaii and some other Pacific Island nations, no matter what time the clock says, it's always Whiskey Time ("W"), while in parts of Russia and southeastern Asia, it's always Golf Time ("G"). )

Suffice to say: Z time = UTC Time = old GMT time. 

In the original example above, the Z time notes the time of day when these particular computer models were "initialized" or started.

Most of the leading global models run at least four times a day with forecasts that begin at 0Z, 06Z, 12Z and 18Z. (Some models run every three hours, and we have some now that run every hour).

So...what time is Z time? 

For Eastern Standard Time, Zulu time is five hours ahead (four hours during Daylight Saving Time). With Pacific Time, it's eight hours ahead (seven hours during Pacific Daylight Time). 

MORE: NOAA's Zulu Time Conversion Chart To American Time Zones

In this 12Z case, the model began at 8 a.m. Eastern Daylight Time and 5 a.m. in the Pacific Time Zone. Weather charts use the 24-hour clock, like the military, so you may see such times given as ‘18’Z or ’21’Z, which is the same as 6 p.m.'Z' and 9 p.m.'Z' times, respectively.

When Tomorrow Is Still Today

One tricky part is that meteorologists have to get used to subtracting a day when you work with a map initialized at ‘0Z.’ 

0:00 on the 24-hour clock is the equivalent to midnight, which becomes "tomorrow." If you're peeking at weather charts on your own, be careful as 0Z charts will be time-stamped as a day ahead for North America. 

For example, a computer model might show a map forecasting for 0Z on August 23, 2021. That forecast is late evening on August 22 in the United States. (Remember: Midnight Z time on a Thursday morning is 8 p.m. EDT on Wednesday.) Don't forget to adjust the date mentally, or you'll be a day off! The same goes for any 03Z or 06Z forecasts, depending on your time zone. 

A better way to quickly decode model times is thinking of 12Z as "morning run" and the 0Z as "evening run" with 18Z as midday and 06Z as the overnight run.

These conversions work for all other fields that use global time, though most use "UTC" instead of "Z."