TIME KEEPING
Weather constantly changes, with weather systems forming and dying; they move and affect many areas of the country in succession. To keep track of the location and movement of these systems, we must measure the weather elements AT THE SAME TIME and make maps and charts that reflect the weather conditions at a given time.
Because of the size of these systems, observers around the world have to coordinate their efforts. At what time should we make our observations? What do we even mean by the time anywhere in the world? The current weather maps that you will use in this course are identified in "Z time".
CIVIL TIME ZONES
For centuries, humans measured their activities by the daily motions of the sun. Local noon was a convenient reference, marking the time when the sun would be overhead locally. As a result, locations even several miles displaced in the cast-west direction could have different local times. With more rapid long distance transportation and communication made available by the railroads and telegraphy after the American Civil War, travel east or west meant that a person's local time kept constantly changing. To reduce the large number of locally observed times, the railroad companies pushed for the simplified standardized time keeping scheme we currently use. Time zones were initially instituted in the U.S. and Canada. The concept of international time zones was started in November 1884 at the International Meridian Conference in Washington DC. Because The Old Royal Observatory in Greenwich, England had instituted the best early astronomical time determinations, the meridian of longitude passing through this observatory became the Prime Meridian and serves as the world-wide standard for time keeping.
METEOROLOGICAL TIME KEEPING
Since the collection and exchange of weather information are of international concem, use of a single worldwide time system is needed so all weather observers around the world can take measurements at the same time, providing a "snapshot" of the weather. The times for essentially all meteorological reports are given according to the Universal Time Coordinated system (UTC), the current name for Greenwich Mean Time (GMT). Often, a single letter, Z, phonetically called "Zulu" is used because this letter is used to identify the Greenwich time zone (centered on the Greenwich Prime Meridian).
By international agreement, surface weather observation times are minimally 0000 Z, 0600 Z, 1200 Z and 1800 Z, with upper air measurements at 0000 Z and 1200 Z each day. In the U.S., surface observations are taken hourly (at the top of the hour). Radar summary charts are also hourly at 35 minutes past the hour.
UTC is the system used to indicate time in meteorology and is
recommended for all general timekeeping applications. Time on most weather maps is given
given in Coordinated Universal Time, (as in the example below).
Need help with UTC conversions? connect to this worksheet!