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What Do 12Z and 18Z Mean?

Short answer: 12Z and 18Z both hours (times) of the day expressed on a 24-hour clock, on local standard time in Greenwich, England. (The longitude in Greenwich is 0°; the "Z" attached to the hour refers to "zero" degrees longitude.)

12Z is 1200 hours, or 12 p.m. (noon), local standard time in Greenwich, while 18Z is 1800 hours, or 6 p.m., local standard time in Greenwich.

Pacific Standard Time (PST) is 8 hours behind standard time in Greenwich, so 18Z would be 10 a.m. PST and 12Z would be 4 a.m. PST. When we go on daylight savings time, we move our clocks forward an hour, so Pacific Daylight Time (PDT) is 7 hours behind standard time in Greenwich. Hence, 18Z would be 11 a.m. PDT and 12Z would be 5 a.m. PDT.

"Universal Time Coordinated" (UTC)

Satellite images and weather maps are usually labeled with the date and time at which the data were recorded. Since many images and maps cover regions large enough to span more than one time zone, it makes little sense to put the "local" time on an image. (See Appendix G: "Time Zones", on pp. 424-425 of our text, Meteorology by Danielson et al.) Instead, the time that appears on most images is an internationally agreed-upon reference time–namely, the local standard time in Greenwich, England.

This reference time was formerly called Greenwich Mean Time (GMT) but is now called Universal Time Coordinated (UTC) (translated literally from the French expression). The longitude of Greenwich, England is zero degrees–sometimes referred to as "Zulu"–and so the hour appearing on most satellite images often has a "Z" appended to it to identify it as UTC (for example, "12Z", or "00Z", or "1630Z"). The hours, sometimes including the minutes, are given on a 24-hour clock.

Converting UTC to Local Time

To figure out the local time at which an image was recorded, you have to know the number of time zones from your location to Greenwich, England, and you have to know whether you're on standard time or daylight savings time.

For example, Pacific Standard Time (PST), which people on the West Coast use from early November to early March, is eight hours behind Greenwich, so:
PST = UTC   -   08 or PST = UTC   -   0800
(if UTC includes only hours, no minutes)   (if UTC includes both hours and minutes)

(For example, if UTC is 15Z, then PST = 15 - 08 = 07, or 0700, or 7:00 a.m. PST.)

From early March to early November, the West Coast shifts to daylight savings time by moving the clock forward an hour, so Pacific Daylight Time (PDT) trails UTC by only seven hours:
PST = UTC   -   07 or PST = UTC   -   0700
People on the East Coast use Eastern Standard Time (EST) or Eastern Daylight Time (EDT), which are three hours ahead of the corresponding Pacific times, and so EST = UTC - 05 and EDT = UTC - 04.

Converting the Date

Since the local time in Greenwich, England is ahead of the local time everywhere in the Western Hemisphere (including North America), it follows that if it's early enough in the morning in Greenwich, England, then it's still the previous day in at least parts of the U.S., so sometimes the date, as well as the hour, has to be adjusted to get the local date and time.

For example, at 03Z (3:00 a.m. in Greenwich England), the local time on the West Coast of the U.S. is seven or eight hours behind (that is, 7 or 8 hours earlier, depending on the time of year). That would make the local time on the West Coast either 8:00 p.m. PDT or 7:00 p.m. PST–the previous evening! In such cases, the date has to be adjusted as well as the time of day.