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Climatic Data OnLine (CDO)

Global Climate Station Summaries (TD3505)
ISD Summary – Ceiling-Visibility (Frequency of Occurrence) Help

Definitions: Ceiling is defined here as the height above ground of the base of the lowest layer of clouds that when combined with any layers below it, accounts for more than half of the sky above the point of observation. Visibility is defined as the greatest horizontal distance that can be seen in half or more of the horizon circle.

Purpose: This summary shows the percent frequency of occurrence that weather station visibilities are greater than or equal to any of 16 selected visibility thresholds (given in miles) while ceilings are at or below any of 31 selected ceiling threshold values (given in feet). Ceiling values range from 0 to 20,000 feet and visibilities range from 0 to 7 miles. Users define period of record, station(s), month(s) and hour(s) to summarize.

Date Restrictions

Choosing Date Range gives the user the available data over the period of record (e.g. between the selected dates).
Choosing Selected Dates gives the user the available data for the individual dates selected. Summaries will be generated over the entire range of years, days, and hours selected for each month.

Information and Abbreviations: Please note that all times are UTC (Universal Time Coordinate) and must be adjusted for local time if desired. Values in the leftmost column in the table are ceiling height values given in feet. The top row of each table gives visibility values in number of miles. It should be noted that in the interest of operational significance to aviation, the visibility and ceiling thresholds are closer together as they approach 0.