8.2: Meteorological Reports and Observations

The World Meteorological Organization (WMO) is a branch of the United Nations and it sets global weather observation standards to be used by countries across the globe. Weather systems span countries and continents so weather observations have to be synchronized to get an accurate big-picture view (a synoptic view) of the weather at a given time. Upper air and surface observations are taken at specific times in Coordinated Universal Time (UTC) so that they can be coordinated simultaneously across different time zones. For example, most upper-air observations are taken at 00 and 12 UTC, but surface observations are typically taken more frequently. Synoptic weather maps put together these observations.

Observations are reported using internationally standardized codes. One of the most commonly used codes is METAR, which comes from the French MÉTéorologique Aviation Régulière, or Meteorological Terminal Aviation Routine. It is a summary of surface weather conditions reported at hourly or half hourly intervals depending on the station. METAR is provided by airport terminals for the purposes of aviation meteorology. In general, METAR includes reports of wind speed and direction, visibility, cloud layers at different levels of the atmosphere, surface temperature and dew point temperature, air pressure, as well as weather conditions such as precipitation or thunderstorms near the station. SPECI is a special non-routine form of METAR, provided when airport weather conditions change significantly. You are not required to translate or memorize METAR for the purposes of this class, but it is useful to recognize it because it is very commonly used. Translations for parts of METAR code can be easily found online.

METAR: PHNL 072153Z 05012KT 10SM FEW025 FEW050 BKN200 31/17 A3006 RMK AO2 SLP180 T03060167

Above is an example of METAR, taken from Honolulu International Airport. “PHNL” is the ICAO airport code for Honolulu International. The “072153Z” indicates that the report was given on the 7th of the month (August 7th) at 2153Z (Universal Time), which is 11:53 AM local time. The “05012KT” is the wind report, and indicates that winds are blowing at 12 knots from 50°, which is roughly from the northeast. The “10SM” indicates that visibility is 10 statute miles, which is another way of saying that visibility on the runway is clear. If visibility is 10 statute miles or greater, it is always reported as just “10SM”. The “FEW025 FEW050 BKN200” are reports of different cloud heights, which are important for aviation purposes. This report says that there are is a layer of a “few” clouds at 2,500 and 5,000 feet, with a layer of “broken” clouds at 20,000 feet. The “31/17” is telling us that the temperature is 31°C with a dew point temperature of 17°C. The “A3006” gives the station air pressure at 30.06 in Hg (inches of Mercury, this unit is still primarily used in aviation). The “RMK” denotes the remarks section of a METAR report where additional remarks and information about the weather are provided. The “AO2” is a code that indicates that the airport observing site is automated and contains a precipitation sensor. Some sites are not automated or do not have a precipitation sensor so there are different codes to denote this. Automated reports can also have non-automated remarks added to them. The “SLP180” gives the sea level pressure as 1018.0 mb. And the “T03060167” gives a more accurate temperature and dew point temperature reading. It reports that the temperature is actually 30.6°C and the dew point temperature is actually 16.7°C.

Weather Observation Locations

Surface weather observations include automated observations from Automated Surface Observing System (ASOS) sites in the United States, as well as hourly observations from airports around the world, reported as METAR. Manual and ship observations are also made at specific times. Surface observations of the weather are important for providing information about the weather conditions that we experience here on the ground. These conditions include temperature, dew point temperature, wind speed and direction, air pressure, cloud cover, visibility, and weather conditions such as precipitation or thunderstorms.

Although surface weather observations are important, they only tell a part of the full story. Just as ripples on the surface of a river can be a sign of what is happening below, surface observations can give an idea of what is happening above. Much of the weather that happens at or near the ground is strongly affected by conditions higher up in the atmosphere.

Observations of the upper atmosphere are taken by radiosonde packages, which are carried aloft by weather balloons. These radiosonde observations (RAOBs) provide soundings of the upper-air environment, giving information about temperature, humidity, and pressure at vertical levels throughout the atmospheric column. Recall from Chapter 5 that temperature and humidity aloft are usually plotted against pressure in a thermodynamic diagram called a Skew-T. Some radiosondes can infer wind data at different heights—these are called rawinsondes. When these instrument packages are dropped from an aircraft, they are called dropsondes. All of this data is accumulated, organized, tested, and stored in computers at governmental centers such as the European Centre for Medium-Range Weather Forecasts (ECMWF). Other data collecting systems include weather radar (such as the NEXRAD network in the United States) and satellite, as well as vertical wind profilers and Radio Acoustic Sounding Systems (RASS).

The following figures show locations of data collected by ECMWF over a 6-hour time period. The first figure shows surface observation locations located on land, the second shows surface observations over the ocean, and the third shows the upper air sounding observational network.

Surface observation locations for temperature, humidity, winds, clouds, precipitation, pressure, and visibility (CC BY-NC-SA 4.0).

Buoy observation platforms for surface temperature and winds over the ocean (CC BY-NC-SA 4.0).

Upper air observation and sounding locations for temperature, pressure, and humidity aloft. Stull Figure 9.4 (CC BY-NC-SA 4.0).

Sea-Level Pressure Adjustment

Weather stations exist at many altitudes. Because air pressure decreases with height, weather stations in cities at high elevations will report far lower air pressures than cities at lower elevations. Air pressure varies vertically far more than it does horizontally. To make air pressure comparable, the station pressure needs to be adjusted to the pressure it would have if the station were at sea level. If this were not done, pressure differences between nearby surface stations would be dominated by their difference in elevation, and a surface pressure map would more closely resemble a map of elevation rather than a map of atmospheric pressure. By correcting for altitude differences, synoptic maps show mean sea level pressure, which is used to show low and high pressure centers near the surface.