Boolean: Values that can only be true or false, e.g. thunder, hail, etc. Either it did thunder on a particular day (true) or it did not (false).
Crepuscular Rays: Rays of sunlight shining through gaps in clouds. They are most common near sunset when the sun is just above the horizon. Normally they radiate downwards towards the surface. With very dense cumulus clouds upward radiation visible against the blue of the sky can sometimes be seen.
Glaciation: where the supercooled water droplets that form clouds are converted to ice crystals, causing the edge of the cloud to have a fibrous appearance. High in the troposphere where the air is very clean, lacking particles of dust, salt, etc. water does not freeze at 0 o Celsius - temperatures below -30 o are required.
Temperature Inversion: a base of a layer in the atmosphere above which the temperature increases with height. Temperature inversions are common at ground level after cold clear calm nights, and help to explain why radiation fog often fills valleys and not nearby hills
Oktas: A unit of measurement for recording cloud cover. One okta is one eighth of the sky. The sky is sectioned into areas between the 8 main compass points: N, NE, E, SE, S, SW, W and NW - if a section is more than half covered it is recorded as 1 okta.
Relative Humidity is the percentage of the possible moisture content that can be held in vapour form (invisible) in a mass of air for a specific temperature. The higher the temperature the more moisture can be stored. When relative humidity reaches 100% saturation occurs and cloud or fog can form.
Snow Falling: Snow or sleet (rain mixed with wet snow) must be observed to fall at some point during the calendar day .
Snow Lying: This is recorded if more than or equal to 50% of an exposed level grass surface is coverd by snow at 0900 UTC/GMT on the day of observation. It excludes locations where drifting may have occurred.
Tropopause: the atmospheric boundary between the troposphere (the lowest layer of the atmosphere) and the stratosphere. The tropopause marks to upper limit of normal cloud formation, as beyond it temperature does not decrease with height so convection cannot take place. The tropopause is at about 17km altitude neat he equator but falls to below 11km at the poles, and during winter it is much lower than this.
Virga are those lineated trails that can occur beneath certain types of cloud. They are caused by falling precipitation, usually snow or ice crystals. When they fall into the dry air below the cloud they eventually disappear due to evaporation or sublimation.
Wind Chill represents the extra heat lost from exposed skin because of the strength of the wind. Often wind chill is expressed as an apparent temperature, which is the temperature still air would have to be to cause the same heat loss. Wind Chill can also be expressed more directly as the number of kilocalories of heat lost per square metre of exposed skin per hour.