In order for meteorologists to communicate about data, it is necessary to have a global reference time for all meteorological data. This reference system is called Coordinated Universal Time (UTC). UTC code appears on all the imagery in this instruction and the time shown represents the time at Greenwich, England (the prime meridian).

To convert the UTC time code (Greenwich time) to the local time of the area in the image, you need to know the time of year and the difference (in hours) between Greenwich time and local time.

The following example uses the UTC code on the adjacent imagery (13:15:00). This means the image was sensed by the satellite at 13 hours 15 minutes and 00 seconds (Greenwich time). The area covered in the imagery is the southeastern U.S. Determine the local time in Dallas, TX and Miami, FL when this image was taken.

Time Conversion Procedure

1. First think of 13:15:00 as a time on a 24-hour clock. Translate this clock value into either an A.M. or P.M. value. Here, 13:15:00 would be 1:15 P.M.
2. Use the date label on the imagery (17-Mar-98) to determine whether Daylight Savings Time or Standard Time was in force. (Standard Time is used from roughly late October to early April while Daylight Savings Time is used the rest of the year.) For our example image, Standard Time was in force.
3. Determine the time zones for Miami and Dallas. Miami is on Eastern Standard Time while Dallas is on Central Standard Time.
4. Using the adjacent table, locate the number of hours to subtract from 1:15 P.M. (time from step 1). (Greenwich time is ahead of all U.S. time zones, so you always subtract hours from UTC codes for U.S. locations.) For Dallas the number of hours to subtract is 6 while for Miami it is 5.
5. Subtract the numbers to determine local time. For Dallas, 1:15 P.M. minus 6 hours gives a local time of  7:15 A.M. For Miami, 1:15 P.M. minus 5 hours gives a local time of  8:15 A.M.

Time Zone	Standard (hours)	Daylight (hours)
Eastern	5	4
Central	6	5
Mountain	7	6
Pacific	8	7
Alaskan	9	8
Hawaiian	10	10

You can see why one universal time label simplifies scientific communications for all time zones around the world.