Many people who use Weather Forecast to decide if they can put out their washing for drying (or stargazing from directly above their head/zenith), also use the same logic to decide if they should go out for moon sighting or not (when it’s predicted to be visible)?

The above comparison is not correct since the distance to the horizon is usually much further away from the observation location than the horizon (where the weather forecast could be clear skies)!

The reason the actual weather may be much better at the horizon is because, cloudiness is a local phenomenon i.e. the whole region is never (or very rarely) cloudy at the same time, such that two locations, 10 – 20 miles apart may have totally different weather conditions (clear and cloudy).

The question arises, how far distance is your horizon (l) so you can estimate the weather forecast more accurately? The answer to that depends on both the height of your observation location (which is variable, h) and the radius of the earth (which is a constant, R = 3959 miles), as given in the formula below:

HorizonDiagramDistance to the horizon (l) = 1.22495 √h where h is in feet and l is in miles OR
Distance to the horizon (l) = 3.56972 √h where h is in meters and l is in kilometres (km)

Examples:
1. What is the distance to the horizon for someone at 10m height of location and 1.5m tall? Distance = 1.22495 x √(32.8 + 5) = 7.53 miles OR = 3.56972 √(10 + 1.5) = 12.11 km
2. What is the distance to the horizon for someone at 1000 ft height of location and 6ft tall? Distance = 1.22495 x √(1000 + 6) = 38.85 miles OR = 3.56972 √(305 + 1.8) = 62.52 km

The above example calculations are for York (height, 10 m) and Thirsk (height, 1000 ft) in North Yorkshire.

It is clear from the above calculations that the higher the location, the further away the horizon will be and hence the weather at that distance may not be as cloudy as the location of the observer.

Furthermore, when it’s partly cloudy, it has been found that the location near sunset may have a clear gap, allowing the moon to be easily seen, even though other parts of the skies are totally cloudy.

Therefore, never give up looking for the moon (until moon set times), even if the forecast is cloudy, since there may be a clear gap to see the moon through a break in the clouds (at the horizon)!

Note that it is advisable to use the first part of your postcode (eg. YO10) for the UK regional weather forecasts (which may include locations near your distant horizons).

Ref: How far away is the horizon? (PDF) | See also: How to Calculate the Distance to the Horizon

Related: First day's moon on UK horizon | Met Office UK weather map


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