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Cloud cover values only vary by 3% from year-to-year averages, whereas the local, day-to-day variability in cloud amounts typically rises to 30% over the globe. Land is generally covered by 10-15% less cloud than the oceans, because the seas are covered with water, allowing for more evaporation .
They forecast the cloud cover, transparency and astronomical seeing, parameters which are not forecast by civil or aviation forecasts. [1] They forecast hourly data, but are limited to forecasting at most 48 hours into the future. Each individual chart provides data for only a 9 mile radius, and so are essentially point forecasts.
Cloud-cover symbols used on weather teleprinters and aviation reports in the US [6] In the early 20th century, it was common for weather maps to be hand drawn. The symbols for cloud cover on these maps, like the modern symbols, were drawn inside the circle marking the position of the weather station making the measurements.
This allows map users to analyze patterns in atmospheric pressure, temperature, wind speed and direction, cloud cover, precipitation, and other parameters. [1] The most common station plots depict surface weather observations although upper air plots at various mandatory levels are also frequently depicted.
One type of map shows where VFR (visual flight rules) are in effect and where IFR (instrument flight rules) are in effect. Weather depiction plots show ceiling height (level where at least half the sky is covered with clouds) in hundreds of feet, present weather, and cloud cover. [27] Icing maps depict areas where icing can be a hazard for flying.
Weather maps are created by plotting or tracing the values of relevant quantities such as sea level pressure, temperature, and cloud cover onto a geographical map to help find synoptic scale features such as weather fronts. The first weather maps in the 19th century were drawn well after the fact to help devise a theory on storm systems. [2]
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OpenWeatherMap is an online service, owned by OpenWeather Ltd, that provides global weather data via API, including current weather data, forecasts, nowcasts, and historical weather data. The company provides a minute-by-minute hyperlocal precipitation forecast.