Introductory Meteorology

Does Anybody Really Know What Time It Is?


It's critical that you understand universal time conventions and be able to convert between universal time (aka UTC, GMT, or Z-time) and local time zones within the United States. You will use this skill throughout the course, so make sure you are comfortable making such conversions before moving on.


"Does anybody really know what time it is? Does anybody really care...?"

Those words come from this section's theme song--"Does Anybody Really Know What Time It Is" by Chicago (that's right, this section has a theme song). Well, I can tell you that meteorologists must know what time it is, and they definitely care about time. Weather is a global phenomenon, and since our world is sliced into individual time zones, meteorologists need a universal standard to keep it all straight.

That standard is Greenwich Mean Time (GMT). "Greenwich" refers to the English village of Greenwich, a borough of London, through which the Prime Meridian (zero degrees longitude) passes. The advantage of adhering to one time standard is that observers all over the world can record weather conditions in Greenwich time. Such a universal time system is indispensable for synchronizing when weather observations are collected. If observers worldwide were to record observations in local time, then interpretation would become much more complicated and confusing. Ultimately, it's important to remember that GMT is a time zone, just like any other.  It just happens to be the time zone at Greenwich, England, along the Prime Meridian.

GMT goes by a couple of other aliases--"Zulu time" (often shortened to Z-time), or UTC (Coordinated Universal Time). "Zulu" is a funny sounding name, but it's the U.S. Navy's and our civil aviation's version of GMT. The bottom line is that if you see time expressed as GMT, Z-time, or UTC, they're all referring to the same thing--the time in Greenwich, England. Most often, we'll use UTC or Z-time in this course. Meteorologists universally use this time to synchronize the times of weather observations and forecasts, so it's important for us to be able to convert from UTC to other local time zones, as well as from other local time zones to UTC.

You can convert to Local Time at any location by referring to a map of world time zones (zones are labeled along the bottom of the map). That's a pretty "busy" map, so let's streamline our discussion a bit. Focus your attention on the map of standard time zones for a large portion of the Western Hemisphere (shown below). Further note that each time zone is labeled with its corresponding time difference from Greenwich, England (expressed in hours UTC). How does this map work?

See caption.
The standard time zones of a large portion of the Western Hemisphere and their corresponding time differences from Greenwich, England (expressed in hours UTC). Let’s assume that it’s 1500 hours in Greenwich (alternatively, 15 UTC or 15Z). On a 12-hour clock, it would be 3 P.M. in Greenwich. At any rate, you can see, across the top of the map, the corresponding local times for each of the represented time zones. For example, at 15 UTC (1500 hours in Greenwich), it’s UTC - 5 hours, or 1000 hours (10 A.M.) local time in the eastern United States (Eastern Standard Time). Larger image of time zone map.
Credit: David Babb

First, we're using the military's 24-hour clock system. For this system, 0000 hours ("zero hundred hours") corresponds to local midnight, and 1200 hours ("12 hundred hours") represents local noon. Okay, let’s assume that it’s 1500 hours in Greenwich (alternatively, 15 UTC, 15Z or 15 GMT...take your pick!). On a 12-hour clock, the local time in Greenwich would be 3 P.M. At any rate, you can see, across the top of the colorful map above, the corresponding local times for each of the represented time zones. For example, at 15 UTC (1500 hours in Greenwich), it’s 1000 hours (10 A.M.) local time in the eastern United States (Eastern Standard Time is UTC - 5 hours), and 0600 hours (6 A.M.) local time in Alaska (Alaska Standard Time is UTC - 9 hours).

On the flip side, if you lived in Chicago, Illinois and it was 9 A.M. local time (0900 hours), and you wanted to convert to UTC, you would simply add 6 hours because Central Standard Time (where Chicago is located) is 6 hours behind UTC. So, 0900 hours + 6 hours = 1500 hours, or 15 GMT (or 15 UTC or 15Z).

Ultimately, converting from UTC to local time (or the other way) is really no different than figuring out what time it is in California if you live in, say, New York. If it's 5 P.M. local time in New York, we have to subtract 3 hours to get the local time on the West Coast in California, so we know its 2 P.M. local time in California. Converting to or from UTC is no different: It's just addition or subtraction. You have to figure out how many hours difference there is between whatever location you're interested in and UTC.

Many of the time-zone boundaries are parallel to longitude lines, although, for convenience, there are several exceptions (Alaska, for example). Each time zone spans approximately 15 degrees of longitude, which is the longitudinal distance that the Earth rotates in one hour. Of course, you must adjust for Daylight Saving Time during the warmer months (from the second Sunday in March to the first Sunday in November in the United States). While 15 UTC corresponds to 10 A.M. Eastern Standard Time (EST) in New York City, from early March to early November it's 11 A.M. Eastern Daylight Time (EDT) in the New York (Eastern Daylight Time is 4 hours behind GMT). So, when Daylight Saving Time is in effect, the difference between GMT and time zones in the U.S. is one hour less than what's indicated on the map above. By the way, it is bad form to say "Daylight Savings Time." Save yourself the trouble, and don't put the "s" on the end of "saving."

Please note that the International Date Line zig-zags across the Pacific Ocean in an attempt not to inconvenience local time keeping (traveling westward across the date line results in the calendar advancing one day). For convenience, the abrupt zig-zag in the International Date Line south of Siberia allows Alaska's long Aleutian Island chain to be in the same time zone as the rest of the state (Alaska Standard Time, AST, is 9 hours behind UTC).

Now that you know how time conversions work, the best way to really get comfortable with knowing what time it is anywhere in the world is to do some practicing. Make sure to spend some time on the Key Skill questions and the Quiz Yourself tool below.

Key Skill...

Time conversions are one of the most basic and important early skills that you must learn in this course. You must understand the concept of UTC and know how to convert it to a location's local time (as well as convert a location's local time back to UTC). You really need to know this, because this time convention is going to show up over and over again throughout the semester (not to mention on quizzes and lab assignments).

Here are a few examples for you to try (you'll likely need to refer to the map of time zones above)... 

Example #1:

Say that it starts raining at your house in Denver, Colorado, and the time is 20Z on June 23. What was the local time in Denver when the rain started?

Click for answer...

Answer: We notice from the map above that Denver is located in the UTC-7 time zone. However, since Daylight Saving Time is in effect (in June), Denver is only 6 hours behind UTC. So, if we subtract 6 hours from 20Z, we get 1400 local daylight time on June 23 (or 2:00 P.M. on June 23). Note that when talking about local time, we DO NOT have the "Z" or UTC designator (because we have converted from that time zone). When talking about local time, you should typically say "Local Standard Time" (LST) or "Local Daylight Time" (LDT).

Example #2:

You pull up a weather map on your favorite smartphone app at 10:35 P.M. local time on December 18 in New York, NY. What time stamp would be on this image if it was expressed in Z-time?

Click for answer...

Answer: We notice from the map above that New York is located in the UTC-5 time zone, meaning that New York is 5 hours behind UTC. So to convert from local time to UTC, we need to add 5 hours. 10:35 P.M. can also be written as 2235 hours on a 24-hour clock, so 2235 + 5 hours = 0335Z. Since we crossed over local midnight when making our conversion, we also need to increment the date by one. Therefore, the time stamp on the image would be 0335Z on December 19.

Example #3:

You're vacationing on big island of Hawaii, and your plane lands at 03Z on January 3. What local time is this (in Hilo, Hawaii)?

Click for answer...

Answer: We notice from the map above that Hawaii is located in the UTC-10 time zone. So, we must subtract 10 hours from 03Z, which gives us 1700 local standard time on January 2 (or 5:00 P.M. on January 2). Notice that we have to subtract a day because we passed 0000 (local midnight) when converting.

Quiz Yourself...

Think you understand how to convert between local time and "Z-time"? Take this self-quiz below to see how you do. Select whether you want to practice converting local time to GMT or GMT to local time (or "Either"). Then hit the "Quiz me" button. Use the provided drop-down menus to fill in the missing time and date. Click "Submit" to check your answer.  Good luck! If you've got the hang of it, you should be able to get the correct answer each time!