NOAA Weather Stations

Determining Global Temperature

The U.S. Climate Reference Network consists of 114 stations, including this one in Tucson, Ariz.

High resolution (Credit: NOAA)

Q. Why is NOAA using fewer weather stations to measure surface temperature around the globe — from 6,000 to less than 1,500?

The physical number of weather stations has shrunk as modern technology improved and some of the older outposts were no longer accessible in real time.

However, over time, the data record for surface temperatures has actually grown, thanks to the digitization of historical books and logs, as well as international data contributions. The 1,500 real-time stations that we rely on today are in locations where NOAA scientists can access information on the 8th of each month. Scientists use that data, as well as ocean temperature data collected by a constantly expanding number of buoys and ships – 71 percent of the world is covered by oceans, after all – to determine the global temperature record.


Q. Could stations located in potentially warmer locations near buildings and cities influence temperature readings?

Yes. That is one reason why NOAA created the Climate Reference Network. These stations adhere to all of the established monitoring principles and are located in unpopulated areas. They are closely monitored and are subject to rigorous calibration procedures. It is a network designed specifically for assessing climate change.

An effort is also underway to modernize the Historical Climatology Network of over 1,000 long-term weather and climate stations nationwide. Stations in the Southwest are currently undergoing modernization and maintenance through this program. Managers of both of these networks work diligently to locate stations in pristine areas where the dynamics of the immediate region, like urbanization, are unlikely to change very much over the coming decades.

The U.S. Climate Reference Network consists of 114 stations, including this one in Capitol Reef National Park, Torrey, Utah.

The U.S. Climate Reference Network consists of 114 stations, including this one in Capitol Reef National Park, Torrey, Utah. The stations measure precipitation, solar radiation, ground surface temperature and solar winds to provide information on climate change over 50 years.

High resolution (Credit: NOAA)

Q. Does a station with good exposure record warmer temperatures than a station with poor exposure?

Not necessarily. Many local factors influence the observed temperature: whether a station is in a valley with cold air drainage, whether it’s a liquid-in-glass thermometer in a standard wooden shelter or an electronic thermometer in the new smaller and more open plastic shelters, whether the station reads and resets its maximum and minimum thermometers in the coolest time of the day in early morning or in the warmest time of the day in the afternoon, etc. But for detecting climate change, the concern is not the absolute temperature – whether a station is reading warmer or cooler than a nearby station placed on grass – but how that temperature changes over time. Because even monthly averaged temperatures at nearby stations with identical instrumentation and excellent siting can differ by a couple degrees due to local effects.

Q. Is there any question that surface temperatures in the United States have been rising rapidly during the last 50 years?

No. Even if NOAA did not have weather observing stations across the United States, the impacts of the warming are clear and present. For example, lake and river ice is melting earlier in the spring and forming later in the fall. Plants are blooming earlier in the spring. Mountain glaciers are melting. Coastal temperatures are rising. A multitude of species of birds, fish, mammals and plants are extending their ranges northward and, in mountainous areas, uphill toward cooler areas.

Q. What are some of the temperature discrepancies you found in the climate record and how have you compensated for them?

Weather station schematic.

NOAA is deploying a new network of stations called the U.S. Historical Climatology Network - Modernization. These stations maintain the same level of climate science quality measurements as the USCRN, but are spaced more closely and focus solely on temperature and precipitation.

High resolution (Credit: NOAA)

Over time, the thousands of weather stations around the world have undergone changes that often result in sudden or unrealistic discrepancies in observed temperatures requiring a correction. For the U.S.-based stations, we have access to detailed station history that helps us identify and correct discrepancies. Some of these differences have simple corrections.

The most important difference globally was the modification in measured sea surface temperatures. In the past, ship measurements were taken by throwing a bucket over the side, bringing some ocean water on deck and putting a thermometer in it. Today, temperatures are recorded by reading thermometers in the engine coolant water intake — this is considered a more accurate measure of ocean temperature. The bucket readings used early in the record were cooler than engine intake observations, so the early data have been adjusted warmer to account for that difference. This makes global temperatures indicate less warming than the raw data does.

The most important difference in the U.S. temperature record occurred with the systematic change in observing times from the afternoon (when it is warm) to morning (when it is cooler). This shift has resulted in a well-documented and increasing cool discrepancy over the last several decades and is addressed by applying a correction to the data.

Q. How do volunteers affect the data from the network?

NOAA’s National Weather Service field office personnel train volunteers and visits them regularly to provide support and ensure instrumentation is working correctly. If there are any inconsistencies or difficulties on the part of the volunteer, NWS personnel works with the volunteer to ensure that daily and monthly measurements are being taken correctly.
NOAA’s National Climatic Data Center also provides data checks of the measurements, looking for discrepancies. Some of the volunteers have been working with the National Weather Service for as many as 50 years and more, and provide a valuable service to the agency's and the nation's weather record.

Q. Are data and processing information available?

At NOAA, we have always made data regularly available to the public. The updated Global Historical Climatology Network and U.S. Historical Climatology Network station data used by NOAA and others have been available each month from the National Climatic Data Center Web site since the 1990s.

For more information:

Monitoring Global & U.S. Temperatures FAQ

Reliability of U.S. Temperature Record Talking Points

“On the reliability of the U.S. Surface Temperature Record,”
by Matthew J. Menne, Claude N. Williams, Jr., and Michael A. Palecki
Submitted to Journal of Geophysical Research Atmospheres
August 27, 2009; revised December 21, 2009 NOAA logo.