Colour States are a system used for quickly showing meteorological conditions.

Meteorological colour states are determined by the relevant worst condition from the visibility

Visibility

In meteorology, visibility is a measure of the distance at which an object or light can be clearly discerned. It is reported within surface weather observations and METAR code either in meters or statute miles, depending upon the country. Visibility affects all forms of traffic: roads, sailing...

 and significant cloud height

Cloud base

This article refers to meteorology. For the airborne base of the TV series Captain Scarlet and the Mysterons, see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...

. In the USA and parts of Europe the lowest significant cloud layer is 5 okta

Okta

In meteorology, an okta is a unit of measurement used to describe cloud cover. Sky conditions are estimated in terms of how many eighths of the sky are obscured by cloud, ranging from completely clear, 0 oktas, through to completely overcast, 8 oktas...

s or more; in the United Kingdom, Belgium, France and the Netherlands it is three oktas or more. If visibility or cloud height measurements fall on a boundary (e.g. 5000 m visibility or 1500 ft cloud height) the colour state assumes the higher value in this case WHT.

If an airfield runway is unusable for reasons other than clouds or low visibility, e.g. ice or other obstructions, then the word BLACK is written in full and placed immediately before the actual colour state e.g. BLACKWHT.

The colour state may be appended to a METAR

METAR

METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....

report. A short period forecast called a TREND which covers the following two hours from the observation may also be added, often with reference to the colour state.

Colour state criteria

Colour state	BLU	WHT	GRN	YLO1	YLO2	AMB	RED
Cloud height in feet	2500	1500	700	500	300	200	<200
Visibility in metres	8000	5000	3700	2500	1600	800	<800