V.N. Ivanov

To form a net of optical observing stations it-s important to emphasize most important parameters and have a comparison instrument. In case of regular satellite observations these parameters are geographical position and average cloud cover. Cloudiness is a variable and badly predictable atmospheric agent. Because of the seasonal effects average cloudiness gives general idea of climate characteristics of local area. This information is useful for choosing place for new observation station and determining time gaps between observations. Cloudiness data was taken from European Centre for Medium-Range Weather Forecasts (ECMWF) archive. Night was treated as "clear" if maximum cloudiness during whole night was below 50%. This value is based on real statistic of ISON project observatories - 21 participants and 11 partners. For each observatory the amount of clear nights was calculated. It differs from month to month. Having the ability to use several observatories simultaneously we can have 13 clear nights each month as minimum. This means observations once in two days. The best months give 20-25 clear nights. Maximum time gap between clear nights was also calculated. For more than a half of stations it doesn-t exceed two weeks through the whole year. Seven of them have gap less than a week. Satellite propagation algorithm gives good precision for times about a week too. So it is able to catch satellite after the gap (in case other observing conditions are satisfied). That-s why ISON project can ignore cloudiness factor. Point by point calculations are not suitable when choosing place for new observing station. Such information is better to be presented graphically. So for the whole year a graphic of average night cloud cover was prepared. Seasonal effects take place. The most "clear" regions are in north Africa, Arabia and North Hindustan. Similar time gaps graphic reveal similar regularity. This factor-s top is North Africa and Arabia. Problem of regular astronomic observations can be solved with the help of spread net of optical stations. Average cloud cover can be used as indicator of observational potential of area and can be used as numerical criteria of comparison of observatories. Obtained results can be used to simulate the observability of specific satellites during the year.

 

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