Etna
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Etna clear sky chart from 2005-11-05 to 2008-11-20.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	6% 5% 5% 5% 7% 53%	12201
	Night	6% 5% 6% 8% 51%	12085
Transparency	Day	9% 16% 23% 23% 11%	9097
	Night	11% 16% 25% 20% 8%	8913
Seeing	Day	28% 36% 23%	801
	Night	13% 26% 31% 22%	3656

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	6% 7% 6% 6% 9% 7% 39%	796
	Night	5% 6% 5% 5% 7% 7% 8% 9% 37%	1240
February	Day	6% 10% 5% 7% 7% 7% 7% 35%	748
	Night	6% 5% 8% 5% 8% 11% 39%	1031
March	Day	6% 10% 5% 5% 8% 6% 7% 13% 27%	1016
	Night	7% 5% 6% 11% 7% 9% 10% 31%	1024
April	Day	11% 8% 5% 5% 6% 7% 38%	1089
	Night	6% 7% 6% 5% 6% 8% 43%	886
May	Day	11% 5% 6% 5% 7% 46%	1270
	Night	6% 6% 5% 10% 53%	775
June	Day	6% 65%	1260
	Night	7% 67%	720
July	Day	5% 77%	1270
	Night	5% 79%	771
August	Day	5% 81%	1170
	Night	87%	868
September	Day	79%	1030
	Night	82%	944
October	Day	9% 5% 5% 7% 50%	920
	Night	6% 5% 6% 61%	1113
November	Day	8% 7% 5% 7% 6% 7% 39%	888
	Night	7% 5% 5% 8% 5% 7% 9% 40%	1413
December	Day	7% 8% 5% 6% 10% 8% 9% 10% 25%	744
	Night	6% 9% 5% 7% 8% 8% 8% 9% 27%	1300

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	18% 14% 27% 13%	536
	Night	18% 20% 24% 13%	812
February	Day	13% 21% 26% 9%	472
	Night	12% 19% 26% 13%	700
March	Day	18% 25% 16% 10%	586
	Night	20% 21% 21% 14%	555
April	Day	11% 31% 24% 12%	676
	Night	16% 32% 24% 12%	561
May	Day	11% 21% 25% 16%	943
	Night	13% 26% 29% 13%	603
June	Day	7% 17% 29% 29% 10%	1015
	Night	9% 19% 39% 24%	580
July	Day	9% 24% 26% 33%	868
	Night	6% 15% 22% 26% 26%	535
August	Day	6% 19% 41% 21%	1130
	Night	6% 31% 40% 14%	846
September	Day	10% 17% 33% 26%	1007
	Night	9% 16% 33% 27%	921
October	Day	7% 14% 28% 18% 5%	821
	Night	7% 14% 34% 17% 6%	1032
November	Day	10% 10% 13% 24% 11%	603
	Night	15% 11% 14% 19% 11%	1003
December	Day	13% 21% 21% 10%	440
	Night	13% 17% 20% 14%	765

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Day	30% 38% 17% 5%	52
	Night	21% 39% 24% 8%	361
February	Day	30% 29% 30%	72
	Night	15% 28% 25% 24%	297
March	Day	47% 28% 10%	76
	Night	23% 32% 31% 11%	352
April	Day	44% 30% 11%	76
	Night	21% 30% 28% 16%	332
May	Day	30% 33% 15%	84
	Night	7% 26% 31% 29%	214
June	Day	35% 36% 21%	85
	Night	5% 25% 45% 21%	167
July	Day	12% 49% 33%	77
	Night	5% 54% 36%	190
August	Day	17% 52% 25%	88
	Night	13% 41% 39%	298
September	Day	12% 31% 42% 7%	88
	Night	14% 28% 43% 7%	348
October	Day	23% 34% 18% 7%	91
	Night	9% 20% 30% 26% 7%	363
November	Day	41% 8% 33% 8%	12
	Night	14% 35% 26% 12% 5%	393
December	Night	26% 26% 24% 11%	341

Page generated at 2008-11-20 21:15:49. Version ab11