Z time (aka UTC time) is used in order for all meteorological measurements to be made at the same time. This is the basis for synoptic meteorology, to take all measurements at the same time to produce a snapshot of the state of the atmosphere. Z time is in reference to 0 ° longitude at Greenwich, England. Notice the time stamps on top of each of the four panel images in the link below (the first states 12Z FRI 5 JAN 01).
Click here for Z time example
The first frame states 12Z FRIDAY. The 12Z time will be in the early morning hours in the United States. Relative to Central Time, 12Z is at 6 a.m. in January (Central Standard Time) and 7 a.m. in July (Central Daylight Time). When it is noon in Europe, it is the early morning hours in the United States.
The second frame has a time stamp of 0Z SATURDAY. 0Z occurs in the late afternoon hours in the United States. Relative to Central Time, 0Z is at 6 p.m. (Central Standard Time) and 7 p.m. (Central Daylight Time). Many beginning analyzers are tricked into thinking 0Z Tuesday is on Tuesday. This is not the case in the United States. When it turns to Tuesday in Greenwich, England, it is still Monday afternoon in the United States. 0Z TUE on the chart occurs at 6 p.m. Monday (Central Standard Time).
The most common Z times you will come across are 0Z (late afternoon in US), 18Z (near noon in US), 12Z (morning in US) and 6Z (near midnight in US). It is a wise habit to look at the time stamp first on an image before interpretation. This prevents the unwanted task of interpreting old data.
Most of the major synoptic scale models will have time stamps of 12Z or 0Z since most models are run twice per day. Surface analysis charts are updated hourly or every 3 hours.
It is important to keep track of time stamps when comparing one model to another. Be sure you are comparing the same run for each model. The major synoptic models do not come out at the same time in the morning. For example, the Eta may be updated while the AVN is not. It is best to compare models that are from the same run.
The chart below has the conversions for Z time, CST and CDT in 3 hour increments. With practice, you will be able to convert any local time in your head to Z time almost instantaneously. A pneumonic device to remember the difference between CDT and CST is to remember that more "daylight" (D stands for "daylight") occurs in the summer. Therefore, during the warm half of the year it is CDT and during the cool half of the year it is CST.
There is a Z-time converted for several time zones given by the NWS. Go to http://www.srh.noaa.gov/jetstream/remote/radarfaq.htm#utc and read the information on Z-time.
Example Z-time questions and answers
1. Convert the following times from Central Daylight Time in the US to Z time.
Answers (13Z, 19Z, 23Z, 8Z)
2. Convert the following times from Z time to Central Standard Time in the US.
Answers (6am, 9pm, 12noon, 1am)
3. Convert the following Z times to Central Standard Time in the US.
Answers (6pm Sunday, 6am Wednesday, 3pm Friday, 8pm Friday)
*When it turns to 0Z in Greenwich, England, it is still the previous day's evening in the U.S.