NCDC May 2011 monthly state of the climate report — supplemental figures and information

June 15, 2011

Global surface temperature Anomalies - May 2011.

Global surface temperature Anomalies - May 2011.
High Resolution (Credit: NOAA)

Each month NOAA's National Climatic Data Center (NCDC) releases two summaries, one national and one global, of the previous month's climate. These reports include information on the temperature and precipitation levels experienced nationally and globally, providing useful information about these important climate variables in historical perspective. The reports also chronicle many significant weather and climate-related events that occurred during the month. This trusted source of information is used globally by industry and business, government agencies, academia, and members of the public to help inform decision making.

May 2011 global temperature anomalies

Supplemental highlights:

Global land surface dataset updated


Beginning with the April 2011 Global monthly State of the Climate Report, NCDC switched to an updated version (Version 3) of the land-surface temperature data as its official land-surface temperature reporting datasets. GHCN-M version 3 is used for NCDC climate monitoring activities, including calculation of global land surface temperature anomalies and trends. A more thorough description of this change, the motivation behind it, and the impact upon this and other reports can be found at this link.


What about uncertainty? Doesn't that matter?


Uncertainty is an important component to any measurement, including the global temperature. NCDC uses the reported value, rounded to the nearest 0.0°C, to determine its ranks. The value is the central, and most likely, value within a range defined by uncertainty ("plus or minus") estimates. The "plus" and the "minus" aspects of this range are equally likely, so for ranking purposes, the central value is used. Values near the ends of the uncertainty range (the full values of the "plus or minus" range) are much less likely than those near the center. Statistically, when comparing any given year on the graph to another, it is very unlikely — about 1 in 1,600 chance —. that the true value for one year would lie at the highest end of the uncertainty range and the true value for another year would lie at the lowest end of the uncertainty range. It is much more likely that the true value for any given year will be close to the central value. More about NCDC's uncertainty values.

Background Information

Why track temperature anomalies, not absolute temperatures?

Four climate monitoring stations in Asheville, NC area.

Four climate monitoring stations in Asheville, NC area.
High Resolution (Credit: NOAA)

Temperature anomaly refers to the difference from average. The global temperature is calculated using anomalies because they give a more accurate picture of temperature change.

If calculating an average temperature for a region, factors like station location or elevation affect the data, but when looking at the difference from the average for that same location, those factors are less critical. For example, while the actual temperature on a hilltop will be different than in a nearby valley on a given day or month, stations in both places will show a similar trend in temperature when the change in temperature compared to average for that station is calculated.

Using anomalies also helps minimize problems when stations are added to or removed from the monitoring network. The above diagram helps show that if a station were removed from the record or did not report data for some period of time, the average anomaly would not change significantly, whereas the overall average temperature could change significantly, depending on which station dropped out of the record.

Where does NOAA get its global data?

Global climate data animation.

Red dots show the stations that arrived at NCDC during a typical month. The arrival pattern (dots appearing in chunks over time) represents the fact that data are available when different nations' meteorological services distribute them.

Animation (Credit: NOAA)

NOAA's NCDC is the world's largest active archive of weather data. It houses data archives dating back to 1880 from all over the world. Each month, countries from all over the world send their land-based meteorological surface observations, meaning temperature and precipitation measurements, to NCDC to be added to the global record.

This information is sent through the World Meteorological Organization's (WMO) Global Telecommunication System (GTS), a coordinated system for the rapid collection, exchange and distribution of observation data from more than 200 countries around the world. The data are sent in a message format called CLIMAT (pronounced "KLEE-mat"). These messages are a summary of monthly weather data for a specific station. The CLIMAT message contains information of average pressure at station level, average air temperature, maximum air temperature, minimum air temperature, average vapor pressure, total precipitation, and total sunshine for a particular month.

These messages are typically sent to NCDC by the 8th of every month. NCDC uses the data to produce numerous climate publications, such as the monthly global state of the climate report. In the image, the red dots on the animation above show how a typical month's worth of data arrives at NCDC, in a day-by-day, country-by-country fashion. In a typical month, about 2,000 to 2,500 stations have arrived by the time NCDC makes its monthly analysis. Please refer to the WMO for detailed information about CLIMAT messages or the GTS.

Other useful links: