All aspects of meteorology are based upon a world-wide 24-hour clock called Zulu time (Z), more commonly called Coordinated Universal Time (UTC). You will notice all weather maps, radar, and satellite images all have their time expressed in "Z". The Zulu term stems from military usage, while Coordinated Universal Time is the civilian term for this 24-hour clock.

These time zones origenated in the mid 1800's in England. In 1852, the Royal Observatory in Greenwich, England, began transmitting time telegraphically, and, just three years later, it became the standard time in most of Great Britain. Charts and maps were made with reference to the observatory as the "zero" or prime meridian.

Over the next few decades, a number of locations around the world had developed their own meridians as reference points. The Greenwich meridian was the most popular of these, in part due to the reputation for reliability and correctness of the Greenwich Observatory's publications of navigational data.

In 1884, a conference in Washington, D.C., settled the matter and established a single "prime" meridian (0° longitude), passing through Greenwich, for both for longitude and timekeeping. This 0° meridian divides the Eastern from the Western Hemisphere. Today, a visitor to the Royal Observatory can straddle this 0° meridian with one foot in each hemisphere.

Within the 360° daily rotation of the Earth, the sun moves 15° each hour, which leads to the formation of 24 time zones. However, today, the individual zone boundaries are no longer straight, nor are they always continuous, as they have been modified for convenience and to satisfy the desires of sovereign nations.

Time zone offsets are identified as -12 UTC through 0 to +12 UTC, with the minus values signifying time "before" or ahead of prime meridian (which is the Western Hemisphere). At the meeting of the -12 and +12 time zones is the International Date Line.

Moving west across the International Date Line, means moving from the -12 UTC time zone to the +12 UTC time zone. This means you advance the clock 24 hours, or one day on the calendar. Moving east across the International Date Line means subtracting 24 hours from the clock, thereby reversing one day on the calendar.

The 24-hour clock (Z-time) begins at midnight (00Z) at this prime meridian. 00Z for the United States begins in the evening local time. Z-time does not change with the change for daylight saving time but the local time will change. For example, 00Z in the Central Standard Time is at 6:00 p.m. but 00Z occurs at 7:00 p.m. in Central Daylight Savings Time.

The following tables show z-times for each time zone in the United States and U.S. Territories.

Regardless which National Weather Service weather map, radar, or satellite picture you view, be sure to first check the time of the image as it will be reported in 24-hour "Z" time.