Coordinated Universal Time (UTC)

For practical reasons many fields including meteorology and aviation use a single time reference called Coordinated Universal Time, abbreviated UTC. Coordinated Universal Time is essentially the same as Greenwich Mean Time (GMT), also called "Zulu" (Z) time. (The slight difference between UTC and GMT is unimportant for our purposes). UTC or GMT is the time at the Royal Observatory in Greenwich, England, located at the prime meridian (0^o longitude). The reason for using UTC is to avoid confusion with time zones; if the time of a weather report is given as 1630Z (read 16:30 Zulu or UTC), a person anywhere in the world can determine the local time of the report by adjusting the UTC time appropriately.

Local time is determined by subtracting the number of hours our time zone differs from the time in England. In the United States, local time is determined as follows:

EST = UTC - 5 (Eastern Standard Time)
CST = UTC - 6 (Centeral Standard Time)
MST = UTC - 7 (Mountain Standard Time)
PST = UTC - 8 (Pacific Standard Time)

When daylight savings time is in effect, a one hour adjustment must be made:

EDT = UTC - 4 (Eastern Daylight Time)
CDT = UTC - 5 (Centeral Daylight Time)
MDT = UTC - 6 (Mountain Daylight Time)
PDT = UTC - 7 (Pacific Daylight Time)

Example:

A time on a weather map is given as 18Z. (On weather maps, the minutes are often omitted, so you should read this time as 1800Z or UTC. Also note that a 24 hour clock is used). If you live in the Central time zone, and daylight savings time is in effect, the local time is

CDT = 1800 - 500 = 1300 or 1:00 p.m.

To convert from local time to UTC time, you would instead add the appropriate number of hours for your time zone.