UTC - Coordinated Universal Time
Map of World Time Zones. Source: NIST.
The use of UTC was brought about by the extensive use of computers in telecommunications networks around the world. Computers required a more precise time reference than GMT, which is based on solar time.
Old textbooks and meteorological documentation used to refer to the Greenwich Mean Time (GMT), also known as "Zulu" time (i.e. 18:00Z). The "Z" refers to "zero hour time zone", and has been in use since the second world war. "Zulu" is simply the standard phonetic word attributed to the letter "Z" in radio communications.
Because the atmosphere is constantly changing. It is impossible, even with present day super computer power, to analyze the changes in the atmosphere continuously.
In addition, meteorological organizations around the world limit the number of observations to the barest minimum they can afford. Meteorology is not different, in this respect, than any other of man's activities. We have to watch our budget, and try to get the most "bang for the buck"!
Thus, all over the world, a "snapshot" of all significant weather phenomena, as observed from the surface, is taken every hour, on the hour.
Every surface weather observer, or automatic weather observing station, around the world, take their observation at the same time. All are synchronized on UTC, the common world time line. All readings are taken at the same moment.
The atmospheric pressure is read exactly on the hour.
The wind reported is the one minute mean of direction and speed, immediately prior to reading the pressure.
The weather phenomena reported are those that were observed in the few minutes immediately preceding the hour.
When the weather situation changes significantly from the last regular hourly observation (i.e. rain begins), a special observation will be taken to report the change. Its time stamp will also use the UTC as reference.
On the other hand, radiosondes are much more expensive, per observation. Therefore, we limit ourselves to two standard flights per 24 hour period, one at 00:00 UTC, and another at 12:00 UTC.
Some countries with bigger budgets may send more than two scheduled radiosondes on a given day, if the meteorological situation warrants it. Hurricanes are an example of such a significant event.
In summary, regular "snapshots" of the atmosphere are synchronized on UTC, the common world time line reference.
Stay tuned, and keep a sharp weather watch!