Map Symbols for Weather and Clouds
A large S for sand or dust has a thicker arrow cutting across it for a strong dust storm, no arrow if the dust is suspended in the air without a wind, a vertical arrow if the dust is raised by the wind at the time of observation. The haze symbol, looking like infinity, is for a thin dry haze. The fog symbol is for a fog obscuring the sky. Two lines represent a light fog, and if the lines are broken the fog is patchy. The smoke symbol is used when visibility is restricted due to smoke. The drizzle, rain and snow symbols are used in multiple to indicate intensity. One is light, intermittent while four is heavy, continuous. A curved line beneath the symbol means that the precipitation is not reaching the ground (virga). The shower symbol is combined with a precipitation symbol to indicate rain or snow shower. The hail symbol may be combined with the thunderstorm symbol, for example, and a dot in the triangle represents sleet. Everywhere except in the U.S., sleet is hail or snow with rain; in the U.S., it is freezing rain with clear crystals (ice pellets).
A rain dot or snow asterisk can be used above a thunderstorm symbol to indicate a slight or moderate storm with rain or snow. If the lightning is given an extra zigzag, a heavy thunderstorm is indicated. The sandstorm symbol can be used if the thunderstorm is kicking up dust. Slight, moderate and heavy intensities are distinguished, as are intermittent or continuous precipitation. Drizzle is rain in tiny drops that cannot easily be visually distinguished, while rain is in visually evident drops. Water is also deposited from colloidal suspension in fogs, but this is reported under fogs.
Snow includes any kind of falling ice crystals, except for hail, which is specially distinguished and originates in thunderstorms. The symbol for hail is an equilateral triangle. Frozen raindrops are represented by the hail triangle with a dot in it. Freezing rain and freezing drizzle have their special symbols with the 'lazy S.' This precipitation falls as water, but freezes on contact with the earth surface.
Cloud cover at a station is represented inside the circle from which the wind speed arrow projects. Cloud cover is estimated in tenths of the sky covered from cloudless, 0/10 to overcast, 10/10. 'Sky obscured' means that the sky cannot be seen due to smoke, fog or other obstruction. A ceiling is quoted as the height of the cloud base when 6/10 or more of the sky is covered. Visibility is a 'hazy' concept; it is the greatest distance at which you can see what you need to see. It can be more precisely defined as the distance at which a specified black shape can be seen against the horizon by day. Visibility can be measured by some instrument over a fixed range, and this is then extrapolated as necessary. The best practical way is to have a series of targets at known distances, and to note the most distant one that can be distinctly seen.
This probably gives better practical results than any objective instrumental method, since it uses actual vision and actual targets. The weather scientist is not usually very concerned with cloud cover, ceiling and visibility, but many users are critically concerned with these factors.
Cloud symbols for upper and middle-level clouds are placed above the cloud-cover circle, and those for lower-level clouds below, so that many station reports contain three cloud levels. The symbols are generally divided into stratiform, cumuliform and cirriform clouds, which is usually the best classification. Stratiform clouds do not show convective behaviour and are formed by cooling of moist air. Cumuliform clouds exhibit convective behaviour, either the ground-based convection of cumulus clouds, or the upper-level convection of altocumulus, or even the mixing by turbulence at very high levels. Cirriform clouds show the typical features produced by ice crystals. Clouds are composed of colloidal water droplets or ice crystals, and for this reason are white. Middle clouds contain supercooled water, and sometimes ice crystals. Supercooled water in high-level clouds changes into larger ice crystals, which then precipitate, forming the typical cirrus shapes when blown by the winds. Supercooled water can occur at any level, but by -40 o C, the change to ice is almost irresistible.
Official cloud names are, unfortunately, based simply on appearances, while it would be much better if they reflected atmospheric conditions, such as humidity, instability, turbulence and convection. Names based on altitude are often misleading, since altitude is only one factor that affects cloud formation.