Time Labels on Satellite Images and Weather Maps

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. 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 Coordinates (UTC). 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.

To figure out the local time at which an image was recorded, you have to know:

  1. the number of time zones between your location and Greenwich, England; and

  2. whether you're on standard time or daylight savings time.
Pacific Standard Time (PST), for example, which people on the West Coast use from late October to early April, is eight hours behind Greenwich, so:

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

From early April to late October, 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, and:

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: (Of course, 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 the date has to be adjusted in addition to the hour to get the local date and time.)

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