Surface weather observation

Surface weather observation is the systematic collection of data about the atmosphere near the Earth's surface. This data is essential for forecasting weather patterns and issuing safety warnings worldwide.

Observers can gather this information manually by hand. They can also use automated weather stations that rely on computers. Some locations use a hybrid scheme. In these cases, human observers work alongside automated systems to improve accuracy. This constant stream of data helps meteorologists understand how the atmosphere behaves.

To make data useful, scientists use the International Standard Atmosphere, or ISA. The ICAO defines this as a model of how pressure, temperature, density, and viscosity change with altitude. This model helps experts reduce a station's local pressure to sea level pressure, known as SLP.

Reducing pressure to a sea level standard allows different locations to be compared easily on weather maps. Without this calibration, high and low pressure areas would depend on local geography rather than actual weather systems. This standardization is also vital for designing aircraft and calibrating flight instruments.

Weather observations include several specific physical measurements. Temperature measures the kinetic energy of matter and determines how heat flows between objects. In meteorology, thermometers are kept in the air but sheltered from direct sunlight. Most of the world uses the Celsius scale, though the United States often uses Fahrenheit. Another key metric is the dew point. This is the temperature at which air must cool for water vapor to condense into liquid dew. If the dew point is below freezing, it is called the frost point, where water vapor creates frost instead.

Wind and pressure are also critical components of a report. Wind speed is measured using anemometers, while direction is tracked with wind vanes or aerovanes. In the United States, average wind speed is a two-minute average, while other regions use a 10-minute average. Wind direction is measured in degrees, starting at 0 or 360 for North and increasing clockwise. For aviation, the altimeter setting is used. This is the local air pressure at mean sea level, which helps pilots calibrate their altimeters to show height above the ground.

History shows that weather recording has evolved from simple notes to complex technology. Reverend John Campanius Holm is credited with the first systematic observations in Colonial America. In 1644 and 1645, he recorded daily weather at a Swedish colony near the Delaware River without any instruments. Later, President George Washington kept a detailed weather diary at Mount Vernon during the late 1700s. In 1807, Dr. B. S. Barton requested that the Linnaean Society of Philadelphia maintain instrumented sites to build a climatological history. By 1926, the United States and the Caribbean had over 5,000 observing locations. The U.S. Navy began developing automated stations in 1939.

Modern aviation relies heavily on specialized reporting codes. Airports often use the METAR observing code to transmit data globally.

METAR reports are usually generated once every hour. However, if weather conditions change suddenly, they issue special reports called SPECIs. Each METAR report includes a four-character station identifier. The first letter indicates the region; for example, "P" is for the Pacific and "E" is for Europe. The second character often represents a country or state. In the U.S., the letters "C" or "K" are used to represent specific regions. This coding allows pilots to receive rapid, standardized updates on visibility and hazards.

Safety is the primary reason for detailed surface observations at airports. Pilots must monitor present weather, such as precipitation, squalls, or volcanic ash. Heavy precipitation can limit visibility during critical takeoff and landing phases. Freezing rain is particularly dangerous because it can deposit ice on aircraft wings. Observers also track snow depth and snowfall amounts. For airport technicians, hourly snowfall reports help determine how often runways must be plowed. By monitoring these variables, weather observation systems protect lives and ensure the smooth operation of global transportation networks.