The other day a commenter posted a simple question: what time zone is “Z”? To which, Personal Hurricane Center.com contributor “MetWiz” responded:
Z is Zulu or UTC (Universal Time Code), GMT (Greenwich Mean Time). All are used in most science applications. The NHC issues advisories at 3:00 AM UTC/Z/GMT, 9:00AM, 3:00PM and 9:00PM.
Eastern time is currently four hours behind. After standard time transition Sunday, it’ll go to five hours. Central time is five hours behind going to 6, etc. etc. So, the NHC advisory statements would come out at 11:00 PM EDT, 5:00 AM EDT, 11:00 AM EDT and 5:00 PM EDT.
When he writes something like 28/1500Z it means the 28th day of the current month (today), at 16:00 Z/UTC/GMT or 4:00PM. Minus the 4-hours for EDT, it’s 12:00 PM EDT. CDT would be 11:00 AM CDT.
By the way, thanks, Metwiz.
I figured I’d take some time to try and explain this a bit deeper.
First, I apologize for throwing out the time zone codes assuming everyone knew what I was referring to (as of this morning, I’m conciously providing two time zones, “UTC” – standard in the meteorological community, and local time for the area of the storm or the location it may impact).
Let’s start with the basics. As “MetWiz” wrote, advisories on tropical cyclones are issued at 3:00 AM, 9:00 AM, 3:00 PM and 9:00 PM Zulu Time (Z). Remember “Ghostbusters” when Dana is hovering above the bed and Peter asks to speak to Dana. She responds, “There is no Dana, only Zulu”.
Wait, she said Zuul – that’s right. Well, pretend she said Zulu. And now you’ll remember what time zone “Z” is.
Basically, Zulu, Coordinated Universal Time (UTC) and Greenwhich Mean Time (GMT) are all the same with some slight variations. UTC and Z are the most commonly used meteorological time codes with Z being the dominant code in official statements from weather offices. And they never change, regardless of the season (there is no “standard” or “daylight savings time”).
Let me use it in context to demonstrate: when I mention a satellite image or a forecast model, I commonly use the date and time stamp such as 30/0135Z. That is the time the below satellite image of Typhoon Mirinae (Santi) was captured:
IR image of Typhoon Mirinae captured at 30/0135Z
You’ll likely notice in the top-left that the date and time are spelled out moreso than I do, listing the capture time as 10/30/09 0135Z.
Now, thankfully because this image was captured at 1:35 AM Z, we don’t have to do any quick math. But, what if the image time read 10/30/09 1335Z? This is considered “military time”. Time is most always listed in military format to save space having to use AM or PM. Why? Well, consider that the National Weather Service alone transfers hundreds of gigabytes of data every day. If you can avoid using those two letters each time, you save yourself considerable file space.
The best way to determine real time from the military format is to subtract 12; i.e., 13:35Z would be 1:35 PM Z. 23:35Z would equate to 11:35 PM Z. Get the idea?
Back to time zones: all official data will list time in either the Z or UTC time code. So, how do you convert that to your local time or the local time of a community being affected by a cyclone?
I found The World Clock – Time Zones which should make it very easy for us to determine local time based off Z or UTC. You can also use their converter.
Here’s a quick rundown, though, of the most common time zones I use on this website:
- EST (Eastern Standard Time) or EDT (Eastern Daylight Time) – Typical of cyclones affecting the east coast of the United States. EST is 5 hours behind Z/UTC. So, 12:00 PM Z would be 7:00 AM EST or 8:00 AM EDT.
- CDT (Central Daylight Time) or CST (Central Standard Time) – for cyclones affecting the United States Gulf coast. CST/CDT is always one hour behind EST/EDT. The satellite image above would correspond to October 29, 9:35 PM EDT or 8:35 PM CDT.
- PST (Pacific Standard Time) or PDT (Pacific Daylight Time) – Typical of cyclones affecting the Pacific coast of Mexico, PST is 8 hours behind Z/UTC; PDT is 7 hours behind Z/UTC. The satellite image above would correspond to 5:35 PM PDT.
- HST (Hawaii Standard Time) or HDT (Hawaii Daylight Time) – for tropical cyclones in the North Central Pacific. HST is always 10 hours behind Z/UTC. So the satellite image above was taken at 3:35PM yesterday (October 29).
- PHT (Philippine Standard Time) – for tropical cyclones threatening the Philippines. PHT is always 8 hours ahead of Z/UTC. So the satellite image above was taken at October 30, 9:35 AM PHT. PHT is constant throughout the year.
I will add to this list as time permits. If you do not see your time zone here, feel free to use The World Clock – Time Zones. I will also try to be more clear when I use times in my articles.
