Commentary: With forensic meteorology, timing is everything

Daily Record (Rochester, NY), May 22, 2008 by Kevin Williams

They say that "timing is everything."

Indeed.

While accurate and localized meteorological data is essential to any forensic investigation, understanding the elements of time associated with that data is equally important.

So here are some "timely" thoughts to consider:

When utilizing the official cooperative weather observer data that depicts once-daily observations you should be aware of the 24- hour time frame that is employed. Most of these volunteer observers do not use a midnight to midnight period to mark the day. Many instead, at their option, use a 24- hour day beginning at 7 a.m., local time.

Thus, the highest temperature you see reported at the daily 7 a.m. observation, more times than not, was registered the day before. This is because the high in this case refers to the highest temperature observed during the 24-hour period between 7 a.m. the day before and 7 a.m. the day of the observation. Likewise, total snowfall that was reported for the date could have actually fallen entirely on the prior day.

The first order weather stations across the country that are housed mostly at major airports all use a 24-hour clock that begins at midnight. So there is no confusion as to what day the high temperature occurred. But that doesn't mean there aren't issues.

The hourly observations at these major stations aren't actually taken on the hour. Data gathering begins at approximately 45 minutes past the hour with observations posted by 54 minutes past the hour. Thus, the 8:00 observation was actually filed at 7:54.

In some cases, such time disparities can prove significant. For example, precisely timing the onset of freezing rain can be critical in determining whether or not ice in existence at the time of an incident was newly formed or had previously existed for some hours or even days.

Another factor we all must deal with is the bi-annual change between Daylight Saving Time and Standard Time. This can be significant because certain observations are conducted in six hour intervals.

For example, snow depth is measured at 7 p.m. each day. But when we flip to Daylight Saving Time that measurement is pushed back to 8 p.m. (please note that many U.S. Government agencies, including the National Oceanic and Atmospheric Administration, refer to local time in military terms so that 7 p.m. is actually 1900 hours.) This could have a negative or positive impact on a case depending upon the time of an incident.

Finally, in order to coordinate weather observations and the computer model forecasts from which they are derived, a standard international measurement of time was necessary.

According to Weatherwise magazine, in 1878 a Scotsman by the name of Sir Sanford Fleming proposed a system of 24 time zones, a system that remains in use today. His idea stemmed from the rather chaotic method of time keeping that had existed. Previously, towns kept their own time based on high noon -- the point when the sun reached its highest point in the local skies.

Initially this new standard was called Greenwich Mean Time (GMT) for Greenwich, England whose zero degree longitude was used as the base. In 1972, GMT was replaced by UTC which stands for Coordinated Universal Time.

Later, it was changed to "Zulu Time" as the U.S. Armed Forces, working with member nations of the North Atlantic Treaty Organization, assigned phonetic letters to each of the 24 time zones.

For purposes of forensic research, Zulu Time is five hours ahead of Eastern Standard Time and four hours ahead of Eastern Daylight Saving Time.

The attorney needs to know that official weather documentation may only reference Zulu Time. You can tell whether or not this is the case because time stamps on official meteorological data will end with a "Z" to reflect Zulu time. Thus, if a Doppler radar image you possess shows a time of 2230Z, the actual Eastern Daylight time is 6:30 p.m.

If you would like further clarification on any of these points, I would be happy to discuss this column with any legal professional.