||This article needs additional citations for verification. (February 2012)|
In meteorology, a mesonet is a network of (typically) automated weather stations designed to observe mesoscale meteorological phenomena. Dry lines, squall lines, and sea breezes are examples of phenomena that can be observed by mesonets. Due to the space and time scales associated with mesoscale phenomena, weather stations comprising a mesonet will be spaced closer together and report more frequently than synoptic scale observing networks, such as ASOS. The term mesonet refers to the collective group of these weather stations, and are typically owned and operated by a common entity.
The distinguishing features that classify a network of weather stations as a mesonet are station density and temporal resolution. Depending upon the phenomena meant to be observed, mesonet stations will have a spatial spacing of 2 to 40 kilometres (1.2 to 25 mi) and report conditions every 1 to 15 minutes.
Why mesonets? 
Thunderstorms, squall lines, dry lines, and other mesoscale phenomena can cause weather conditions in a localized area to be significantly different from that dictated by the ambient large-scale condition. As such, meteorologists need to understand these phenomena in order to improve forecast skill. Observations are critical to understanding the processes by which these phenomena form, evolve, and dissipate.
The long-term observing networks (ASOS, AWOS, Coop), however, are too sparse and report too infrequently for mesoscale research. ASOS and AWOS stations are typically spaced 50 to 100 kilometres (31 to 62 mi) apart and report only hourly on most occasions. The Cooperative Observer Program (COOP) database consists of only daily reports. "Mesoscale" weather phenomena occur on spatial scales of tens to hundreds of kilometers and temporal (time) scales of hours. Thus, an observing network with finer temporal and spatial scales is needed for mesoscale research. This need led to the development of the mesonet.
Mesonet data is directly used by humans for decision making, but also boosts the skill of numerical weather prediction and is especially beneficial for short-range mesoscale models. Mesonets, along with remote sensing solutions, allow for much greater temporal and spatial resolution in a forecast model. In addition to meteorology and climatology users, transportation departments and utility companies also have a need for fine scale weather information. These organizations operate dozens of mesonets within the US and globally. Agricultural, environmental, and emergency management interests also are heavy users of mesonet information.
How do mesonets work? 
Mesonets were born out of the need to conduct mesoscale research. The nature of this research is such that mesonets, like the phenomena they are meant to observe, are short-lived. Long term research projects and non-research groups, however, have been able to maintain a mesonet for many years. For example, the U.S. Army Dugway Proving Ground in Utah has maintained a mesonet for many decades. The research-based origin of mesonets has led to the characteristic that mesonet stations tend to be modular and portable, able to be moved from one field program to another.
Whether the mesonet is temporary or semi-permanent, each weather station is typically independent, drawing power from a battery and solar panels. An on-board computer takes readings from several instruments measuring temperature, humidity, wind, soil temperature, and atmospheric pressure (or any other environmental variable deemed important to the mission of the mesonet). The computer periodically saves these data to memory and transmits the observations to a base station via radio, telephone, or satellite transmission. Advancements in computer technology and wireless communications in recent decades has made possible the collection of mesonet data in real-time. The availability of mesonet data in real-time can be extremely valuable to operational forecasters as they can monitor weather conditions from many points in their forecast area.
Early mesonets operated differently than modern mesonets. Each constituent instrument of the weather station was purely mechanical and fairly independent of the other sensors. Data were recorded continuously by an inked stylus that pivoted about a point onto a rotating drum covered by a sheath of graphed paper called a trace chart, much like a traditional seismograph station. Data analysis could occur only after the trace charts from the various instruments were collected.
One of the earliest mesonets operated in the summer of 1946 and 1947 and was part of a field campaign called The Thunderstorm Project. As the name implies, the objective of this program was to better understand thunderstorm convection.
The following table is an incomplete list of mesonets that have operated in the past and present.
|Years of operation||Name of Network, Place||Spacing||No. of Stations||Objectives|
|1946||The Thunderstorm Project, Florida||1 mile (1.6 km)||50||thunderstorm convection|
|1947||The Thunderstorm Project, Ohio||2 miles (3.2 km)||58||thunderstorm convection|
|Present||Dugway Proving Ground, Utah||9 miles (14 km)||26||air quality modeling|
|1991 - Present||Oklahoma Mesonet, Oklahoma||Varies||121||real-time observation|
|1994 - Present||Weatherbug (AWS), across United States||Varies||8,000||real-time observation|
|1999 - Present||West Texas Mesonet, Texas||Varies||63+||archive, real-time observation|
|2001 - Present||Iowa Environmental Mesonet, Iowa||Varies||469*||archive, real-time observation|
|2007 - Present||Kentucky Mesonet, Kentucky||Varies||62||archive, real-time observation|
|2008 - Present||Quantum Weather Mesonet, St. Louis metropolitan area, Missouri||Varies (average ~5 miles (8.0 km))||100||archive, real-time observation|
|Present||North Carolina ECONet, North Carolina||Varies||99||archive, real-time observation|
*Not all stations owned by network.
Although not labeled a mesonet, the Japan Meteorological Agency also maintains a nationwide surface observation network with the density of a mesonet. JMA operates AMeDAS, consisting of approximately 1,300 stations at a spacing of 17 kilometres (11 mi).
See also 
- Fujita, Tetsuya. "A Review of Researches on Analytical MesoMeteorology, Research Paper #8, February 1962
- Overview of The Thunderstorm Project
- Oklahoma Mesonet
- Earth Networks: Mesonet Solutions
- Texas Tech University: West Texas Mesonet
- Iowa Environmental Mesonet
- Kentucky Mesonet
- Quantum Weather
- North Carolina Mesonet (ECONet)
- Japan Meteorological Agency: Observations
- MADIS Meteorological Surface Integrated Mesonet Data Providers (data)
- Hydrometeorological Networks in the United States
- AMS Glossary definition of portable meteorological mesonet
- Salt Lake City NWS Mesonet Observations