Prioritize...
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 and vice versa. You will use this skill throughout the course, so make sure you are comfortable making such conversions before moving on.
Read...
"Does anybody really know what time it is? Does anybody really care...?"
Those words come from this section's theme song, a classic from the musical vault—"Does Anybody Really Know What Time It Is" by Chicago. 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?
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 at 15Z for each of the represented time zones. For example, at 15Z (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 only 4 hours behind UTC). So, when Daylight Saving Time is in effect, the difference between UTC 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."
Want to see a few quick examples of time conversions between UTC and local time zones? Check out the short video (4:07) below:
Click here for a transcript of the video.
Let’s do some sample time conversions between Universal Time and local time zones in the U.S. Let’s start in Cincinnati, Ohio, which is in the eastern time zone. According to our time zone map, the eastern time zone is 5 hours behind UTC, so we have to subtract 5 hours to make the conversion. If we imagine that it’s 13Z on January 5, which means Daylight Saving Time is not in effect, we subtract 5 hours from 1300Z, and that gives us 0800 hours on a 24-hour clock, which is 8 A.M. Eastern Standard Time. If we were doing the same conversion from 13Z, but it was on June 5, when Daylight Saving Time is in effect, there would be a slight difference. We would start the same way, subtracting 5 hours to get 0800 hours, but because Daylight Saving Time is in effect, local clocks have jumped an hour ahead, so we add that hour to get 0900 hours, or 9 A.M. Eastern Daylight Time. In effect here, during Daylight Saving Time, we’re really subtracting 4 hours instead of 5 when Daylight Saving Time is in effect.
Let’s go over to the Central Time Zone and St. Louis, Missouri. According to our time zone map, the central time zone is 6 hours behind UTC, so we have to subtract 6 hours to make the conversion. If it’s 04Z on February 10, when Daylight Saving Time is not in effect, we subtract 6 hours from 0400Z. In doing so, we have to cross midnight local time so the date will change. It’s 4 hours to get back to midnight, and we still have to subtract 2 more hours for a total of 6. That gives us 2200 hours, or 10 P.M. Central Standard Time on February 9 in St. Louis. If we do the same conversion on June 10 when Daylight Saving Time is in effect, remember that we have to add in 1 hour, which gives us 11 P.M. Central Daylight Time on February 9. So, effectively, during Daylight Saving Time, we only have to subtract 5 hours to make our conversion for St. Louis.
If we’re in the mountain time zone at Salt Lake City, our time zone map says that mountain time is 7 hours behind UTC, so we have to subtract 7 hours to make the conversion. If it’s 1900Z on December 20, there’s no Daylight Saving Time in effect. We subtract 7 hours from 19Z to get 1200 hours, or 12 noon Mountain Standard Time. If we do the same conversion in July when Daylight Saving Time is in effect, again, we start the same way, but we have to add in the hour for Daylight Saving Time, which gives us 1300 hours on a 24-hour clock, or 1 P.M. Mountain Daylight Time. Effectively, we only have to subtract 6 hours during Daylight Saving Time to make this conversion.
Remember that Z-time, or UTC time, is universal. So, if it’s 15Z, that converts to 10 A.M. Eastern Standard Time, 9 A.M. Central Standard Time, 8 A.M. Mountain Standard Time, and 7 A.M. Pacific Standard Time. All of these local times occur at 15Z.
Finally, what if we need to convert the other way – from local time to UTC? Let’s do a quick example at Cincinnati, which is 5-hours behind UTC during standard time in the eastern time zone. If it’s 7 A.M. Eastern Standard Time on January 15, then we need to add 5 hours to local time to make the conversion. That’s 0700 hours plus 5 hours to get to 1200 hours, or 12Z. If we had to make the same conversion at 7 A.M. Eastern Daylight Time in June, then we end up having to add an hour less to make the conversion, and 0700 hours plus 4 hours gives us 11Z.
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...
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?
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 designation (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?
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)?
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 GMT? 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. Make it a goal to get at least five in a row correct. If you can get five in a row, you've likely got the hang of things!