Time and Weather

Objectives

Upon completion of this web page you will be able to:

1. Define what is meant by Coordinated Universal Time (UTC).
2. Convert local time to UTC.

Coordinated Universal Time

Since January 1, 1972, weather services have used Coordinated Universal Time (UTC) to identify the time and date of weather observations and weather forecasts. This procedure allows international exchange of weather data and weather forecasts without the need to convert local issue time to your local time. Anyone working in meteorology needs to be able to convert UTC times to their local time easily.

There is also a system of local time classification called zone time. This system uses capital letters to designate time centered on 15 degree longitude intervals. In this system UTC is designated as Z time or (in the phonic alphabet) ZULU time. In weather observations and on some weather charts, the letter Z is used to indicate UTC.

At one time meteorologists referred to UTC as Greenwich Mean Time (GMT). Although the term GMT is heard occasionally in the weather business today, the terms UTC and Z are in common usage (and are the correct phraseology).

Time Conversion

Meteorological time uses a 24-hour clock. That is, 0000 is 12 midnight; 0600 is 6 am; 1200 is 12 noon; and 1800 is 6 pm. A time of 2400 is not used. Time progresses from 2359 to 0000.

Conversion of UTC to local time is easy. To convert between UTC and local time, select the formula which corresponds to your time zone and time of year (standard time or daylight savings time).

For example, assume you live in the Central Time Zone and it is January. To convert 1200 UTC to CST you need to subtract 6 hours from 1200 to yield 0600 CST or 6 am. Similiarly, to convert 12 Noon CST to UTC you need to add 6 hours to 1200 to yield 1800 UTC.

Another aspect of the time conversion process is the day of the month. For example, 0300 UTC on the 10th of the month converts to 2100 CST on the 9th. Over the United States, the UTC day (0000 UTC or 12 midnight UTC) starts from 4 to 8 hours prior to the start of local calendar day. Keep this in mind as you convert UTC to your local time zone.

Exercise

Examine each line in the table, determine the correct time that will complete the fill in the empty box, and then move your cursor over the center of the box to reveal the correct answer.