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 19th 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 has published monthly and annual issues of Local Climatological Data for individual first-order stations and Climatological Data arranged by state. Printed copies of these publications are available at selected local libraries or resource centers; they are also available for free from the NCDC website. However, only annual issues of the Local Climatological Data documents are available in pdf format starting in 2018, while dynamically-generated monthly versions can be obtained from this website. Monthly and annual issues of the Climatological Data for each state are still being prepared and posted as pdf documents. You may access some of these climate data through the link to US Climatology on the RealTime Weather 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.
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):
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.
The annual summaries for the first-order stations include a section that includes 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 1990s, the normals referred to the 1961 to 1990 interval, whereas during the first decade of the 21st century, the 1971-2000 interval was used. Since July 2011, the normals for each station encompass the 1981-2010 interval.