This chapter contains instructions for identifying, recording, and reporting weather. For the purpose of this order, weather is a category of atmospheric phenomena that includes tornadoes, funnel clouds, waterspouts, thunderstorms, squalls, precipitation, and obscurations. The types of weather phenomena reported vary according to the type of station. Weather phenomena may be evaluated instrumentally, manually, or through a combination of instrumental and manual methods.

11-2.   PRECIPITATION

Precipitation is any of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The types of precipitation reported in surface observations are:

a.  Liquid Precipitation. Liquid precipitation is any form of precipitation that does not fall as frozen precipitation and does not freeze upon impact. Types of liquid precipitation are:

(1)  Drizzle (DZ). Drizzle is defined as fairly uniform precipitation composed exclusively of fine drops (diameter less than 0.02 inch/0.5 mm) very close together. Drizzle appears to float, following air currents, although unlike fog droplets, it falls to the ground.

(2)  Rain (RA). Rain is defined as precipitation of liquid water particles, either in the form of drops larger than 0.02 inch/0.5 mm., or smaller drops which, in contrast to drizzle, are widely separated.

b.  Freezing Precipitation. Any form of precipitation that freezes upon impact and forms a glaze on the ground or on exposed objects is called freezing precipitation. Types of freezing precipitation are:

(1)  Freezing Drizzle (FZDZ). Freezing drizzle is drizzle that freezes upon impact with the ground or other exposed objects.

(2)  Freezing Rain (FZRA). Freezing rain is rain that freezes upon impact with the ground or other exposed objects.

c.  Frozen Precipitation. Frozen precipitation is any form of precipitation that reaches the ground in solid form. Types of frozen precipitation are:

(1)  Snow (SN). Snow is composed of crystals, mostly branched in the form of six-pointed stars. At temperatures higher than about -5°C, the crystals are generally clustered to form snowflakes.

(2)  Small Hail and/or Snow Pellets (GS). Small hail or snow pellets are defined as white, opaque grains of ice. The pellets are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm). Snow pellets are brittle and easily crushed. When they fall on hard ground, they bounce and often break up.

(3)  Snow Grains (SG). Snow grains are defined as precipitation of very small, white, opaque grains of ice. When the grains hit hard ground, they do not bounce or shatter. They usually fall in small quantities, mostly from stratus type clouds, and never as showers.

(4)  Hail (GR). Hail is precipitation in the form of small balls or other pieces of ice falling separately or frozen together in irregular lumps. Hailstones consist of alternate opaque and clear layers of ice in most cases.

(5)  Ice Pellets (PL). Ice pellets are transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch/5 mm or less. The pellets usually rebound when striking hard ground, and make a sound on impact. There are two main types:

(a)  One type is composed of hard grains of ice consisting of frozen raindrops, or largely melted and refrozen snowflakes (formerly sleet). This type falls as continuous or intermittent precipitation.

(b)  The second type consists of snow encased in a thin layer of ice which has formed from the freezing, either of droplets intercepted by the pellets, or of water resulting from the partial melting of the pellets. This type falls as showers.

(6)  Ice Crystals (Diamond Dust) (IC). Ice crystals are unbranched and fall in the form of needles, columns, or plates. (Snow crystals are branched.) These are often so tiny that they seem to be suspended in the air. They may fall from a cloud or from clear air. The crystals are visible mainly when they glitter in the sunshine or other bright light (diamond dust); they may then produce a luminous pillar or other optical phenomena. This hydrometeor (rarely more than the lightest precipitation), which is frequent in polar regions, occurs only at very low temperatures in stable air masses.

d.  Unknown Precipitation. Unknown precipitation is the term used by automated weather observing systems to characterize precipitation of an unknown type that cannot be identified any further by the system.

An obscuration is any phenomenon in the atmosphere, other than precipitation, that reduces horizontal visibility. Except where noted, obscurations are reported when the prevailing visibility is less than 7 miles or considered operationally significant. The types of obscurations reported in surface observations are:

a.  Blowing Dust (BLDU). Blowing dust is dust raised by the wind to a height of 6 feet or more, sufficient to restrict horizontal visibility. When visibility decreases to 5/8 SM or less, this becomes a duststorm (DS). Note: see 11-4g.

b.  Blowing Sand (BLSA). Blowing sand is sand raised by the wind to a height of 6 feet or more, sufficient to restrict horizontal visibility. When visibility decreases to 5/8 SM or less, this becomes a sandstorm (SS). Note: see 11-4f.

c.  Blowing Snow (BLSN). Blowing snow is made up of snow particles raised by the wind to a height of 6 feet or more, sufficient to restrict horizontal visibility.

d.  Spray (PY). Spray is water droplets torn by the wind from a substantial body of water, generally from the crests of waves, and carried up a short distance into the air.

e.  Blowing Spray (BLPY). Blowing spray is made up of water droplets torn by the wind from a body of water, generally from the crest of waves, and carried up into the air to a height of 6 feet or more in such quantities that they reduce the horizontal visibility.

f.  Widespread Dust (DU). Dust consists of fine particles of earth or other matter raised or suspended in the air by a wind that may have occurred at or far away from the station. Dust gives a tan or gray tinge to distant objects. The sun's disk is pale and colorless, or has a yellow tinge through dust.

g.  Fog (FG). Fog is a visible aggregate of minute water particles (droplets) that is based at the earth's surface and reduces horizontal visibility to less than 5/8 SM (statute miles), and unlike drizzle, does not fall to the ground.

h.  Freezing Fog (FZFG). Freezing fog is a suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0°C, and visibility less than 5/8 SM, based at the earth's surface. A report of freezing fog does not necessarily mean that ice is forming on surfaces.

i.  Haze (HZ). Haze is made up of extremely small, dry particles suspended in the air, invisible to the naked eye and sufficiently numerous to give the air an opalescent appearance. This phenomenon resembles a uniform veil over the landscape that subdues all colors. Dark objects viewed through this veil tend to have a bluish tinge while bright objects, such as the sun or distant lights, tend to have a dirty yellow or reddish hue. When haze is present and the sun is well above the horizon, its light may have a peculiar silvery tinge. Haze particles may be composed of a variety of substances; e.g., dust, salt, residue from distant fires or volcanoes, and/or pollen. The particles, generally, are well diffused through the atmosphere.

j.  Mist (BR). Mist is a visible aggregate of minute water particles suspended in the atmosphere that reduces visibility to less than 7 SM but greater than or equal to 5/8 SM, and unlike drizzle, does not fall to the ground.

k.  Shallow (Ground) Fog (MIFG). Shallow ground fog is fog in which the visibility at 6 feet above the ground is 5/8 SM or more and the apparent visibility in the fog layer is less than 5/8 SM.

l.  Smoke (FU). Smoke is defined as small particles produced by combustion suspended in the air. This phenomenon may be present either near the Earth's surface or in the free atmosphere. When viewed through smoke, the disk of the sun at sunrise and sunset appears very red. The disk may have an orange tinge when the sun is above the horizon. Evenly distributed smoke from distant sources generally has a light grayish or bluish appearance. A transition to haze may occur when smoke particles have traveled great distances; for example, 25 to 100 miles or more, and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere.

m.  Volcanic Ash (VA). Volcanic ash consists of fine particles of rock powder that originate from a volcano and that may remain suspended in the atmosphere for long periods producing red sunsets and climatic modifications thousands of miles away. Volcanic ash is always reported when observed, no matter what the value of prevailing visibility.

a.  Well-Developed Dust/Sand Whirls (PO). Particles of dust or sand, sometimes accompanied by small litter, raised from the ground in the form of a whirling column of varying height with a small diameter and an approximately vertical axis.

b.  Squalls (SQ). A strong wind characterized by a sudden onset, in which the wind speed increases by at least 16 knots and is sustained at 22 knots or more for at least one minute.

c.  Tornado (+FC). A violent, rotating column of air touching the ground. It forms a pendant, usually from a cumulonimbus cloud, nearly always starts as a funnel cloud, and is accompanied by a loud roaring noise.

d.  Funnel Cloud (FC). A violent, rotating column of air which does not touch the surface. It is usually in the form of a pendant from a cumulonimbus cloud.

e.  Waterspout (+FC). A violent, rotating column of air that forms over a body of water, and touches the water surface.

f.  Sandstorm (SS). Particles of sand that are carried aloft by a strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground. Sandstorm is reported when visibility is reduced to between 5/8 and 5/16 statute mile. If visibility is less than 5/16 statute mile, then heavy sandstorm (+SS) is reported.

g.  Duststorm (DS). A severe weather condition characterized by strong winds and dust-filled air over an extensive area. Duststorm is reported when visibility is reduced to between 5/8 and 5/16 statute mile. If visibility is less than 5/16 statute mile, then heavy duststorm (+DS) is reported.

Present weather qualifiers fall into two categories: intensity or proximity, and descriptors. Qualifiers may be used in various combinations to describe weather phenomena. Details on the coding of qualifiers are contained in Chapter 15, Coding and Dissemination, and appendix E, METAR User Aids.

a.  Intensity. The intensity qualifiers are: Light (-), Moderate (No Entry), Heavy (+).

b.  Proximity. The proximity qualifier is "vicinity" (VC).

c.  Descriptors. The descriptors are: Shallow (MI), Partial (PR), Patches (BC), Low Drifting (DR), Blowing (BL), Shower or Showers (SH), Thunderstorm (TS), and Freezing (FZ).

When more than one type of weather and/or obscuration is reported at the same time, they shall be reported in the following order:

a.  Tornado, funnel cloud, or waterspout.

b.  Thunderstorms, with or without associated precipitation.

c.  Weather and obscurations in order of decreasing predominance, i.e., the most dominant type is reported first.

d.  From left to right in Figure 11-1, Present Weather.

When reference is made to phenomena not occurring at the point of observation, the following rules shall apply:

a.  Weather occurring at the airport shall be coded in the body of the report. Vicinity is defined as between 5 and 10 SM from the usual point of observation for all but precipitation and up to 10 SM from the usual point of observation for precipitation. (See paragraph 11-7b below.) Distant is defined as greater than 10 statute miles from the usual point of observation. With the exception of volcanic ash, low drifting dust, low drifting sand and low drifting snow, an obscuration shall be coded in the body of the report if the surface visibility is less than 7 miles or considered operationally significant. Volcanic ash shall always be coded when observed. MIFG, BCFG and PRFG may be reported when visibility is equal to or greater than 7 miles. Weather and/or obscurations observed but not occurring at the station or in the vicinity shall be coded in the remarks section.

b.  If precipitation is not occurring at the station or airport, but is within 10 miles of the usual point of observation, the phenomena shall be reported in the body of the report as "showers in the vicinity" (VCSH). For other than precipitation, (VCFG, VCBLSN, etc.), vicinity is 5 SM to 10 SM. Examples of how to use VC correctly are included in paragraph 15-14a(2), Intensity or Proximity Qualifier, and appendix E, METAR User Aids.

c.  If the phenomenon is not occurring at the usual point of observation but is affecting part of the operating areas of the airport, the phenomenon may be reported in remarks with the phrase "at the airport" (AT AP) appended, e.g., SHRA AT AP. "At the airport" includes runways, taxiways, ramps, terminals and/or adjacent areas. Buffer zones around the operating areas of the airport are not included in this area.

d.  Weather phenomena beyond 10 statute miles of the point of observation shall be coded as distant (DSNT) followed by the direction from the station. For example, lightning 25 statute miles west of the station would be coded as LTG DSNT W.