Have you ever noticed several numbers followed by the letters "Z" or "UTC" listed at the top or bottom of weather maps, radar, and satellite images? Ever wondered what these mean?
This string of numbers and letters is a timestamp. It tells when the weather product was issued or when its forecast is valid. Instead of local AM and PM hours, a type of standardized time called Z time is used.
Z time vs. Military time
The difference between Z time and military time is slight, and often confusing. Military time is based on a 24-hour clock which runs from midnight to midnight. Z, or GMT time, is also based on the 24-hour clock, however, its midnight is based on the local time at the 0° longitude prime meridian (Greenwich, England). This means that while the time 0000 always corresponds to midnight local time, 00Z is considered midnight in Greenwich, and will therefore correspond to different local times depending on the geographic time zone. For example, in the United States, 00Z refers to evening hours.
How to Calculate
Calculating Z time can be tricky. While it's easiest to use a table like this one provided by the NWS, here are some tips to figuring it out by hand:
Local time to Z time
- Convert the local time (12-hour) to military time (24-hour)
- Find your time zone "offset," or the number of hours your time zone is ahead of or behind local Greenwich Mean Time
Standard Time / Daylight Savings
Eastern: -5 hrs / -4 hrs
Central: -6 hrs / -5 hrs
Mountain: -7 hrs / -6 hrs
Pacific: -8 hrs / -7 hrs
- Add the time zone offset amount to the military time. This is the current Z time.
- Subtract the time zone offset amount from the Z time. This is the current military time.
- Convert the military time (24-hour) to local time (12-hour).
Remember, in the 24-hour clock 23:59 is the final time before midnight, and 00:00 starts the first hour of a new day.