SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

Monday, 12 February 2018

WEATHER RECORDS - A STATISTICIAN'S DELIGHT


Undoubtedly, the comment that "records were made to be broken" was made numerous times during 2007 when Barry Bonds broke Henry Aaron's career record of 755 home runs. Likewise, numerous record high temperatures were set, while some stations had the wettest month on record. What do these record weather events signify? One may think that some of these weather statistics are as obscure, detailed or complicated as some of the detailed baseball statistics that die-hard fans keep on their favorite player or club. While some of the weather records may appear to have little meaning, they still provide benchmarks to be used to gauge the unusual behavior or severity of a particular weather event, just as the baseball statistics are used to judge athletic achievement.

WHAT IS RECORDED

Detailed instrumental weather records have been collected and compiled in this country at many stations for more than a century. The National Weather Service and its predecessor, the U.S. Weather Bureau, have operated a network of weather observation stations and offices in or near many of the large cities in every state, commonwealth and territory under its jurisdiction. At many of these nearly 300 "first-order stations", systematic measurements are made of numerous weather elements, such as temperature, precipitation, humidity, air pressure, cloud cover, visibility, wind speed and wind direction. Some of these weather data are recorded hourly, while other data are recorded once a day at some fixed time. An additional cooperative observer network of approximately 8000 volunteer observers provides daily readings of such weather elements as daily maximum and minimum temperatures, and 24-hour precipitation totals. While the following weather information is collected at essentially all types of stations, the following discussion will pertain to the first order stations:

THE RECORDS

On each day, the daily weather data are compared with the long-term records that have been maintained at the first order weather station. At some stations, these daily weather records may extend back for more than a century. A record event report is made to the public if the day's temperature tied or exceeded the long-term record for that calendar date or season:

DAILY RECORD TEMPERATURES
The first and last records -- or simply, the daily record highs and lows -- are the most often reported, and are relatively obvious. The other records are reported less often and usually require some explanation. A record low maximum indicates that exceptionally cold conditions prevailed through the daylight hours, either because of a very cold air mass or because extensive cloud cover blocked solar heating. Record high minimums typically occur when a hot, humid air mass prevents overnight cooling, thereby keeping an elevated overnight minimum temperature.
MONTHLY AND SEASONAL RECORD TEMPERATURE EVENTS

Attention is given to those times when the maximum or minimum temperature exceeds the respective monthly temperature record. In addition, during meteorological spring (March, April, May), a low temperature record may be classified as "Low so late" or a record high temperature may be identified as "High so early". Similarly, during meteorological autumn (September, October, November) record temperatures could have the designations, "Low so early" or "High so late". No such designations are used during meteorological summer or winter other than monthly record exceeded.

ANNUAL RECORD TEMPERATURE EVENTS

Anytime the absolute lowest or highest temperature that was ever observed at the weather office is exceeded, the event would be identified as the "all time record low" or "all time high".

OTHER RECORD EVENTS

The public is usually informed when an exceptionally heavy precipitation event sets a daily, monthly or annual precipitation record. Such a record means that the amount of rain that has fallen on a given day, month or year is the greatest ever recorded for that given time interval. Snowfall records are similarly noted when applicable. However, in the latter case, the snowfall season used is from 1 July to 30 June rather than the calendar year as for total liquid equivalent precipitation.

On occasion, a station's record sea level pressure or record peak wind gust may be reported. Usually sufficient explanation is provided in these record event reports.

A CAUTION

While a record event that eclipses a century-old record may be newsworthy, one should realize that many of the weather stations may have moved from a city office to an airport office, resulting in a slightly different weather record. Changes in the types of thermometers also may pose a serious problem.

During the last several years, NOAA's National Centers for Environmental Information (formerly the National Climatic Data Center or NCDC) has undertaken a project called ThreadEx, for "threaded data extremes", in which climate bases have been assembled from the earlier 19th and 20th century data sets collected by the US Army Signal Service and the US Weather Bureau, predecessors to the current National Weather Service. The new ThreadEx records can be used by the public and researchers to study the longer-term climate extremes at a number of locations across the nation, especially where changes have occurred in station location. For access to the ThreadEx records for selected stations in your area, check http://threadex.rcc-acis.org/ for the daily temperature and precipitation records.

"NORMALS"

While the weather is never truly "normal", the term usually refers to a long-term average condition, typically of temperature or precipitation. By international convention, the normals are computed for a standard three-decade (30-year) interval. Every 10 years the interval is shifted forward in time by a decade, and a new set of 30-year averages computed. During the first decade of the 21st century (2001-2010), the normals referred to the 1971 to 2000 interval, and starting in July 2011, the normals that will be used for the rest of this current decade are for the 1981-2010 interval.


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Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2018, The American Meteorological Society.