There are two primary networks for surface observations in the United States.
The Cooperative Observer Program (COOP)
Established in 1890, the COOP is the largest and oldest official network in the U.S. with 11,000 volunteers recording and submitting observations across the country.
Instruments are provided by the National Weather Service. Records include max/min temperature, liquid equivalent precipitation, and snowfall. However, these observations are not visible in “real time”, which means instantly. Instead, at the end of each month, records are sent to the local weather service offices who then send these observations to the National Climatic Data Center (NCDC). The NCDC does some quality control on the daily records, to some extent.
Let’s talk about what considerations we must take when using COOP data. Each site is vulnerable to changes in characteristics (e.g., urban geography developments), bias in observation time, and changes in instrumentation. For example, in the mid 1980s, thermometers with liquid-in-glass max/min that were located in wooded Cotton Region Shelters were swapped out for thermistor-based max/min temperature systems (MMTS) in plastic shelters. This introduced a few degree bias changes.
The automated surface observing system (ASOS)
The ASOS group, differs from COOP in that it provides real-time data for weather forecasting and aviation needs. Measurements are more frequent and include temperature, relative humidity, pressure, wind speed and direction, rainfall, visibility cloud ceiling and type of precipitation. The sites are majorly housed at airports and are cared for by the Federal Aviation Administration (FAA) and the National Weather Service (NWS).
Before these automatic readings, manual observations known as “first order weather stations” existed, with records dating back to the 1930s. Sensors were installed starting in the 1960s, but the equipment wasn’t fully automatic until 1991.
First order weather stations and modern ASOS stations differ in how observations were recorded— manually versus automatic. Additionally, instrument changes in the 1990s included swapping out rain gauges with a heated tipping bucket, which appears to under-measure liquid precipitation. Also, in the 1980’s and 1960’s, different temperature sensors were swapped out— again introducing an inconsistency. Changes in location was another big development in ASOS, as they relocated many stations from the downtown to regional airports.
So what instrumentation are these observation networks using? If you’ve taken all the considerations into account (e.g., research footprint, necessary parameters, etc.) you can get a pretty good guess. Check out Climate Change 101: Instrumentation to get a feel for what goes into a weather/climate station.