DS WES SUPPLEMENTAL INFORMATION

DATASTREME WES SUPPLEMENTAL INFORMATION

DataStreme WES Week Twelve: 1-8 December 2008

ACCESSING AND INTERPRETING CLIMATE DATA

Our understanding of the climate system and climate variability depends on the quantitative measurement of a variety of weather elements including temperature, precipitation, humidity, air pressure, cloud cover, visibility, and wind speed and direction from locations around the world. Whereas some rudimentary weather instruments have been available for more than two centuries, detailed quantitative observations of the atmosphere did not commence until the mid to late 19^th century. When compiled, these data can be used to help establish how the planetary climate has varied during the instrumental era, roughly spanning the past 120 years.

Detailed instrumental weather records have been collected and compiled in this country at many stations for more than a century. The National Weather Service (NWS) 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 of numerous weather elements are made by professional observers. Some of these weather data are recorded hourly, whereas 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 a limited selection of weather elements such as daily temperature extremes, 24-hr precipitation totals, and snowfall/snow depth. Some of these stations also make observations of evaporation, soil temperature, and peak wind gusts.

Essentially all meteorological data collected in the United States by government-sponsored observation networks are stored in archival form by the National Climatic Data Center (NCDC) in Asheville, NC. NCDC publishes Local Climatological Data for individual first-order stations and Climatological Data arranged by state. Copies of these publications are available at selected local libraries or resource centers; they are also available for a modest fee from the NCDC website. You may access some of these climate data through the link to US Climatology on the DataStreme WES Website. Recent climatological data can also be obtained from the "Climatology" or "Climate" section of the web page maintained by essentially all National Weather Service Forecast Offices. Using the interactive map , locate the desired NWS Office and go to the climatology section (listed as "Climate, Local") once their homepage appears on the screen. Many of these NWS stations maintain Preliminary Climatological Data (or "CF6", formerly "F-6") for the current month on the Internet for the first-order stations in their area of responsibility. Climate data for selected cooperative observer stations in the area are also available in the section with a tab titled "NOWData" (NOAA Online Weather Data).

WHAT IS RECORDED

Whereas a variety of weather information is collected at essentially all types of stations, the following list pertains to the weather data collected at first-order stations and presented in the monthly climate summaries:

DAILY MAXIMUM TEMPERATURE -- The highest temperature recorded by a sheltered registering thermometer during a calendar day defined as midnight to midnight local standard time.
DAILY MINIMUM TEMPERATURE -- The lowest temperature recorded during a calendar day.
DAILY AVERAGE TEMPERATURE -- The arithmetic average of the observed maximum and minimum temperature for the calendar day.
24-HOUR ACCUMULATED (liquid equivalent) PRECIPITATION -- The total depth of daily rainfall and melted frozen precipitation accumulated in a gauge during a calendar day.
24-HOUR ACCUMULATED SNOWFALL -- The total depth of the snow that fell during a calendar day; the snow depth is measured on a snowboard.
SNOW COVER -- The average depth of accumulated snow on the ground measured at several representative points in the immediate area at a fixed observation time. Note that some stations may also include the water equivalent of the snow cover, obtained by melting a core of snow that has been collected.
HEATING DEGREE-DAYS (65 degrees F base) -- The arithmetic difference between the daily average temperature and the base temperature of 65 degrees Fahrenheit. One heating degree-day is given for each degree that the daily average temperature departs below this base temperature.
COOLING DEGREE-DAYS (65 degrees F base) -- The arithmetic difference between the daily average temperature and the base temperature of 65 degrees Fahrenheit. One cooling degree-day is given for each degree that the daily average temperature departs above this base temperature.
RESULTANT WIND SPEED AND DIRECTION -- The vector sum of the hourly wind vectors (speed and direction) divided by the number of observations, specified in terms of a resultant speed (in mph) and a resultant direction taken (in degrees) with respect to true north (e.g., 90=east wind and 360=north wind).
AVERAGE WIND SPEED -- The arithmetic average of the 24 hourly wind speed observations.
PEAK or MAXIMUM WIND SPEED and DIRECTION -- The maximum observed wind speed and the direction for a 5-second or 2-minute time interval during the day.
OTHER INFORMATION -- Some of the publications or sites include the following additional information in their monthly summaries:

Departures from Normal -- The arithmetic difference between the daily average temperature and the long-term average daily temperature.
Average Dewpoint and Average Wet Bulb Temperature -- The arithmetic average of the 24-hourly observed dewpoint and wet-bulb temperature for the calendar day.
Weather -- Those significant weather phenomena observed during the calendar day such as thunder, rain, snow, fog, smoke or tornadoes.
Sunshine Data -- Some stations report the Minutes of Bright Sunshine as observed by a sunshine recorder and the Percent Possible Sunshine, or the ratio of the observed minutes of bright sunshine during the day to the elapsed time between local sunrise and sunset.
Monthly tally of selected days that exceed certain thresholds -- The number of days during the month that are considered "rain days" when measurable rain (0.01 in. or greater) has fallen; "snow days" when snow (0.1 in. or greater) has fallen; and the number of days that the daily maximum and minimum temperatures have been above or below selected temperature thresholds (generally, 0 degrees, 32 degrees, and 90 degrees Fahrenheit).

Monthly summaries also include the monthly means of maximum and minimum temperature, determined from averaging the respective daily temperature extremes for each day in the month, as well as the average monthly temperature, representing the arithmetic average of the monthly average maximum and minimum temperatures. Monthly precipitation and snowfall totals represent the sum of the daily precipitation/snowfall totals in the month. Monthly totals of heating and cooling degree-days are also included which represent the sum of the daily number of degree-days in the month. Seasonal totals are also given for the official heating season that extends from 1 July to 30 June and the official cooling season from 1 January to 31 December. Comparisons are also provided for these monthly and seasonal values with the corresponding 30-year average values.

"NORMALS" AND COMPARATIVE DATA

The annual summaries for the first-order stations include the normals and the extremes for the various weather elements at that particular location. 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, normals are computed for a standard three-decade (30-year) interval. Every 10 years this interval is shifted forward in time by a decade, and a new set of 30-year averages are computed. During the 1980s, the normals referred to the 1951 to 1980 interval, whereas during the 1990s, the 1961-90 interval was used. Currently, the normals for each station encompass the 1971-2000 interval.

Annual summaries also include the extremes in temperature, precipitation, and snow that have been observed at that current observing site. Since many of the weather stations have moved from city offices to airport locations, the noted extremes may not be the all-time extremes for that city.