Climate Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

29 January - 2 February 2018

ACCESSING AND INTERPRETING CLIMATE DATA

NOTE: This Supplemental Information is a repeat of that which appeared in last week's Supplemental Information...In Greater Depth file.

Our understanding of the climate system and climate variability depends on the quantitative measurement of a variety of atmospheric characteristics. These characteristics, which are often called "weather elements", include 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 become climate data that can be used to help establish how the planetary climate has varied during the instrumental era, roughly spanning the past 130 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 Centers for Environmental Information (NCEI), which was known as the National Climatic Data Center (NCDC) in Asheville, NC. NCEI 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 free from the NCDC website. You may access some of these climate data through the link to US Climatology on the RealTime Climate Portal. More recent climatological data can also be obtained from the "Climatology" or "Climate" section of the web page maintained by essentially all of the National Weather Service Forecast Offices. Using the interactive map locate the desired NWS Office and go to the Local Climate section once their homepage appears on the screen; a link to this climate section can be located in the column on the left of the homepage. Nearly all 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. These data are updated daily and appear on the "Observed Weather Reports" section of the Climate page.

Climate data for selected cooperative observer stations in the NWS office's area of responsibility are also available in the section of the Climate Page identified with a tab titled "NOWData" (NOAA Online Weather Data). Daily temperature extremes, 24-hr precipitation totals and snow data are available for the last two months.

WHAT IS RECORDED

Whereas a variety of weather information is collected at essentially all types of weather observing stations, the following list pertains to the summary of the day weather data collected at first-order stations and presented in the monthly climate summaries (such as that found on the "CF6" form):

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 -- Before October 2009, some stations that had sunshine recorders used to report the observed Minutes of Bright Sunshine 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. At the present time, no replacement to this sunshine recorder network is contemplated.
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 that currently cover the 1981-2010 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.

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