Measurement Times

 

Q: Why, and how, is time of day expressed in 24-hour format?

Q: When entering the time that data were collected, should I round it to the nearest hour or just drop the minutes? For example, should 1845 be entered as 1800 or as 1900?

Q: What time do I have to observe atmosphere measurements? Is data taken at times other than solar noon useful?

Q: What is UT or UTC (GMT) and why do we use it? How do I convert my local time to UT?

Q: How do I calculate solar noon?


Q: Why, and how, is time of day expressed in 24-hour format?

To avoid possible confusion from having to state whether a time is a.m. or p.m., a 24-hour clock is used in many countries. Even when a 12-hour clock is used informally, a 24-hour clock is often used when times are particularly important. The a.m. times are the same in a 12 or 24-hour clock, and the p.m. times are 12 hours more in the 24-hour clock. For example, 1:00 p.m. is 13:00. The 24-hour format also has the advantage of distinguishing between noon and midnight without confusion, 12:00 always means noon, and 00:00 (or sometimes 24:00) is midnight.  
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Q: When entering the time that data were collected, should I round it to the nearest hour or just drop the minutes? For example, should 1845 be entered as 1800 or as 1900?

Prior to September 2000, the minutes were dropped and just the hour was used; however, GLOBE now asks that you report the actual minutes as well. So in this example, the time should be entered as 1845.  
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Q: What time do I have to observe atmosphere measurements? Is data taken at times other than solar noon useful?

Most measurements can be observed anytime of the day but certain measurements need to be observed at a specific time of the day. Visit the GLOBE Teacher's Guide Search Tool In the search filter use the Documents and Investigations drop down menu and click on Atmosphere -> select the check box for Introduction and click the Apply Filter button. Select the Atmosphere Introduction PDF from the search results -> Scroll down to page 7. The chart at the bottom of the page shows the range of times of day for observing a complete set of Daily Atmosphere Measurements.  
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Q: What is UT or UTC (GMT) and why do we use it? How do I convert my local time to UT?

UT stands for Universal Time (also called UTC for Coordinated Universal Time). It is widely used as a scientific reference time. GLOBE schools span many different time zones. For example, 9:00 a.m. in Buenos Aires, Argentina is not the same time as 9:00 a.m. in Sydney, Australia. For this reason GLOBE schools report their measurement times in UT so that measurements from across the world are all in a common time frame and can be compared with each other. (UT is the time of the same zone that was previously called GMT for Greenwich Mean Time, but it is based on atomic measurements instead of Earth's rotation.)

To convert your local time to UT, you need to know which time zone you are in and whether or not daylight savings time (or summer time) is in effect. (Some sources for this information are given below.) The standard time difference between your time zone and UT is the time difference when it is not daylight savings time. It can be found in the table below, or from the source you used to find your time zone.

If you are west of the UT time zone (i.e. behind UT):

The actual time difference is the standard time difference except during daylight savings time, when one hour must be subtracted to get the actual time difference. UT is local time plus actual time difference.

If you are east of the UT time zone (i.e. ahead of UT):

The actual time difference is the standard time difference except during daylight savings time, when one hour must be added to get the actual time difference. UT is local time minus actual time difference.

If you are in the UT time zone:

Your local time is UT except during summer time (daylight savings time), when you need to subtract one hour to get UT.

If you are unable to determine which time zone you are in from a map, atlas, or local source, or don't know whether or not you have daylight savings time, you may be able to find the actual difference between your local time and UT directly. To do this, follow these steps:

Look up the current UT on the GLOBE website navigation bar (be sure to Refresh or Reload the page first), or a website such as http://tycho.usno.navy.mil/utclock.html, or http://wwp.greenwichmeantime.com. Look up your local time. Find the difference between UT and your local time. Add, if you are behind UT, (or subtract, if you are ahead of UT), this difference to any of your local measurement times to get the measurement times in UT.*

The following map is to help you identify which time zone you are in. The time zones shown on the map are listed in the table below the map with their most common names and abbreviations. For some areas, it may be difficult to determine which zone you are in from this map since it does not show much detail of any particular region. A larger map which shows more detail for some areas is available at http://www.travel.com.hk/region/timezone.htm. (If you know your position on the time zone map, or the name of your time zone, or the time difference from your time zone to UT, you can find the remaining information about your time zone in the table below the map.)

Time Zone Map

A list of most countries that have daylight savings time can be found at http://webexhibits.org/daylightsaving/g.html. Additional time zone maps can be found at: http://www.timeticker.com/, http://wwp.greenwichmeantime.com/info/world-time-zone-map.pdf, http://www.time.gov

*NOTES

Most time zones differ from UT by a whole number of hours. For example, Central European Time (CET), shown in yellow on the map, is one hour ahead of UT, Eastern European Time (EET) is two hours ahead of UT, and so on. But there are a few time zones, called non-standard time zones, whose time difference from UT includes a fraction of an hour. For example, Newfoundland Standard Time is 3 hours 30 minutes behind UT. If you find the time difference directly, you will not need to be concerned about whether or not it is daylight savings time, but if you change between it being in effect and not being in effect, you will need to find the time difference again. Unless you are in a non-standard time zone, this difference should be a whole number of hours. So, for example, if you get a difference of 3 hours and 55 minutes, then the time difference is actually 4 hours and due to time loading the page and/or inaccuracy in your local clock, an error of 5 minutes was introduced. If you are in a non-standard time zone, the time difference will be an exact half or quarter hour. If you have any difficulty or questions about converting your local time to UT, please contact the GLOBE Community Support Team.  
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Q: How do I calculate solar noon?

Solar noon is the time during the day when the sun appears to have reached its highest point in the sky. Astronomers refer to this time as local apparent noon. Usually, this is not the same time as "clock noon". The relationship between clock noon and solar noon depends on your location within your time zone and the time of year.

Solar noon always occurs half-way between local sunrise and sunset. Therefore, an easy way to calculate your local solar noon is to find a newspaper from a nearby town that gives the times of sunrise and sunset and to take the average of these. Remember to first convert all your times to 24-hour clock times by adding 12 to any PM times before calculating the average of the sunrise and sunset times. The result is the time of your local solar noon.

Here's how the math is done: Add the two times and divide by two. Remember, however, that doing math with time is a bit tricky because there are 60 minutes to an hour.

For example, adding 0:45 and 1:30 equals 2:15 (two and a quarter hours), not 1:75 (one and three-quarters hours). Here's the actual calculation:

45 minutes + 1 hour 30 minutes = 1 hour 75 minutes = 2 hours 15 minutes 75 minutes = 1 hour 15 minutes (convert 60 minutes to an hour and you have 15 minutes remaining).

Now let's look at dividing times.

Half of 25:18 is actually 12:39, not 12:59. Here's the actual calculation:

1. First, divide the hours by 2. So 25 divided by 2 equals 12 with 1 hour remaining. 2. Next, convert the remaining 1 hour to 60 minutes and add this to the 18 minutes. The sum of 60+18 gives you 78. 3. Now, divide 78 by 2. You get 39 minutes. Thus, the final result is 12:39.

NOAA has an online calculator that you can use at http://www.esrl.noaa.gov/gmd/grad/solcalc/

Remember to set the time zone to 0 or UTC in the tool above after you select your location and date so you can use the Solar Noon value directly. Otherwise you will need to convert this time to UTC before entering it on the data entry form.  
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