Does Anybody Really Know What Time It Is?

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman


It is absolutely imperative that you understand about various universal time conventions, and more specifically, how to convert times listed in GMT to local time and vice versa. You will use this skill numerous times throughout this course. You should be comfortable making such conversions before moving on.


Please note the four-digit number followed by "Z" in the second line on the lower-left-hand corner of this most recent map of surface station models. The "Z" is short for Zulu, the U.S. Navy's and our civil aviation's version of Greenwich Mean Time (GMT). Weatherwise, "Z-time" is a 24-hour clock system that serves as the standard by which all meteorological observations are synchronized each hour around the world.

For the record, "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.

The U.S. Navy's and our civil aviation's slant on the GMT system (Zulu Time or "Z-Time") is not the only variation on the GMT theme. Some weather maps you will encounter on the Web will be tabbed with "UTC," which translates as Universal Time Code. Don't get flustered when you see this reference; remember, it's nothing more than an alias for GMT. For the most part, we'll use UTC or Zulu Time ("Z-Time") in this course.

Time zone map for a large portion of the Western Hemisphere
The standard time zones of a large portion of the Western Hemisphere and their corresponding time differences from Greenwich, England (here expressed in hours UTC). Let’s assume that it’s 1500 hours local time (alternatively, 15 UTC or 15Z) in Greenwich. On a 12-hour clock, it would be 3 P.M. local time. Ate 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 1000 hours or 10 A.M. local time in the eastern United States (Eastern Standard Time), and 0600 hours or 6. A.M. local time in Alaska (Alaska Standard Time). Larger image of time zone map.
Credit: David Babb

Whatever the time convention happens to be on any map that you access on the Internet, 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 lot of information! 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 above). 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 local time 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 or 10 A.M. local time in the eastern United States (Eastern Standard Time), and 0600 hours or 6. A.M. local time in Alaska (Alaska Standard Time).

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). While 15 UTC corresponds to 10 A.M. E.S.T. in New York City, from early March to early November it's 11 A.M. E.D.T. (Eastern Daylight Time) in the Big Apple. By the way, it is bad form to say "Daylight Savings Time," which Hale Stone spouts much too often while he's "on air."

Please note that the International Date Line zigzags 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 zigzag in the International Date Line south of Siberia allows the long Alaska's Aleutian Island chain to be in the same time zone as the rest of the state (Alaska Standard Time, A.S.T., is nine hours behind Greenwich time).

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 GMT and know how to convert it to a location's local time. You really need to know this -- I mean really know it. You are going to have quiz questions, project questions, and lab after lab that require this skill.  Learn it now, and you will save yourself a lot of missed points in the future.

Here are a few more examples... 

Example #1:

You are looking at a map of station models that has a time stamp of 20Z on June 23. At what local time (you live in Denver, CO) was this map of observations valid?

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Answer: We notice from the map above that Denver is located in the UTC-7 time zone. Also, we note that Daylight Saving Time is in effect, making Denver 6 hours behind GMT. So, if we subtract 6 hours from 20Z, we get 1400 LDT 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 say Local Standard Time (LST) or Local Daylight Time (LDT).

Example #2:

A radar image is collected at 10:35 p.m. on December 18 in New York, NY (actually the New York City radar site is located in Upton, NY). What time stamp would be on this image (radar images are always time-stamped in "Z-time")?

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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 GMT. So to convert from local time to GMT, we need to add 5 hours. Since we cross the midnight local timeline, we also need to increment the date by one. Therefore, the time stamp on the radar image would be 0335Z on December 19.

Example #3:

A typhoon warning is issued for the big island of Hawaii starting at 03Z on October 14. 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 (note that Hawaii does not observe Daylight Saving Time ). So, if we subtract 10 hours from 03Z, we get 1700 LST on October 13 (or 5:00 p.m. on October 13). Notice that we have to subtract a day because we passed 0000 when subtracting.

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!

Explore Further...

The International Date Line is also a meteorological milepost of sorts. That's because hurricanes in the central Pacific that churn westward across the International Date Line during summer and autumn change designation to typhoons. No matter if a Pacific storm goes by "hurricane" or "typhoon," responsibility for tracking it (and issuing warnings) falls to the Joint Typhoon Warning Center in Hawaii. JTWC also tracks and issues warnings for tropical cyclones in the Indian Ocean ("tropical cyclone" is the generic name given to tropical storms and hurricanes that form in the Indian Ocean).

Like Shakespeare's witty quip, "A rose by any other name would smell as sweet," the change in title is strictly cultural. Indeed, the title assigned to the most powerful tropical tempest, be it hurricane, typhoon or tropical cyclone (Indian Ocean), reflects the culture of the region in which the storm finds itself.

As you might imagine, quirky things happen at The International Date Line. In late August 1994, Hurricane John (path of Hurricane John), the longest-lived named hurricane in modern history, crossed the Date Line and became "Typhoon John," only to re-curve eastward and cross the Date Line in early September to become "Hurricane John" once again. Here is a satellite image of Hurricane John at 0308Z on August 23, 1994, showing the storm's impressive eye. At the time, the storm was due south of the Hawaiian Islands.