9.1: Meteorological Reports and Observations

Reading METARs

The World Meteorological Organization (WMO) is a branch of the United Nations, and it sets global weather observation standards for countries worldwide. Weather systems span countries and continents, so weather observations must 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 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 are compiled from 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, and weather conditions such as precipitation or thunderstorms near the station. SPECI is a special non-routine METAR issued 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 the METAR code can be easily found online.

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

This example is a METAR from Honolulu International Airport. It is decoded in the steps below:

1. “PHNL” is the ICAO airport code for Honolulu International.
2. 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.
3. The “05012KT” wind report indicates that winds are blowing at 12 knots from 50°, roughly from the northeast.
4. The “10SM” indicates that visibility is 10 statute miles, which means the runway is clear. If visibility is 10 statute miles or greater, it is always reported as just “10SM”.
5. 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.
6. The “31/17” indicates that the temperature is 31°C with a dew point of 17°C. If there is an "M" located next to the temperature and/or dew point number, it indicates a "minus" sign. For example: 03/M02 would indicate an air temperature of 3°C and a dew point temperature of -2°C.
7. The “A3006” gives the station air pressure at 30.06 in Hg (inches of Mercury, this unit is still primarily used in aviation).
8. The “RMK” denotes the remarks section of a METAR report where additional remarks and information about the weather are provided.
9. The “AO2” code indicates that the airport observing site is automated and equipped with 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 include non-automated remarks.
10. The “SLP180” gives sea-level pressure as 1018.0 mb.
11. And the “T03060167” provides more accurate temperature and dew-point readings. It reports that the temperature is 30.6°C and the dew point is 16.7°C.

Weather Observation Locations

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

Although surface weather observations are important, they tell only part of the 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 occurring at or near the ground is strongly influenced by conditions aloft.

Observations of the upper atmosphere are taken by radiosonde packages, which are carried aloft by weather balloons. These radiosonde observations (RAOBs) provide upper-air soundings, providing information on 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 measure 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 collected, organized, tested, and stored on computers at government centers such as the European Center for Medium-Range Weather Forecasts (ECMWF). Other data-collection systems include weather radar (such as the NEXRAD network in the United States), satellite data, vertical wind profilers, and Radio Acoustic Sounding Systems (RASS).

The following figures show the locations of ECMWF data collected over a 6-hour period. Figure 9.1.1 shows surface observation locations located on land, Figure 9.1.2 shows surface observations over the ocean, and Figure 9.1.3 shows the upper air sounding observational network.

Figure 9.1.1: Surface observation locations for temperature, humidity, winds, clouds, precipitation, pressure, and visibility. The markers are densely clustered across major land regions, including North America, Europe, Asia, Australia, and parts of South America and Africa, with fewer points over remote land areas and oceans. The overall pattern indicates widespread global weather station coverage with higher concentrations in regions associated with greater data availability or monitoring activity. (CC-BY-NC-SA 4.0; Roland Stull, via Practical Meteorology).

Figure 9.1.2: Buoy and ship-based observation platforms for surface temperature and winds over the ocean. The pattern indicates a global network of atmospheric data sources used for weather analysis or forecasting, with denser coverage where observation infrastructure or reporting activity is more frequent. (CC-BY-NC-SA 4.0; Roland Stull, via Practical Meteorology).

Figure 9.1.3: Upper air observation and sounding locations for temperature, pressure, and humidity aloft. Each point marks a station where radiosondes are released to measure atmospheric conditions, including temperature, humidity, pressure, and wind, at different heights. Launch sites are distributed across many countries on multiple continents, with higher concentrations in regions with established meteorological infrastructure and fewer sites in sparsely populated areas. The overall pattern illustrates the global coverage of the upper-air observing network used for weather forecasting and atmospheric analysis (CC-BY-NC-SA 4.0; Roland Stull, via Practical Meteorology).

Sea-Level Pressure Adjustment

Weather stations exist at many altitudes. Because air pressure decreases with altitude, weather stations in cities at high elevations will report lower air pressure than those 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 sea-level pressure. If this were not done, pressure differences between nearby surface stations would be dominated by their elevation differences, and a surface pressure map would more closely resemble an elevation map than an atmospheric pressure map. By correcting for altitude differences, synoptic maps show mean sea-level pressure, which is used to depict low- and high-pressure centers near the surface.