Fan Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fan Mountain Observatory clear sky chart from 2005-11-05 to 2008-11-27.

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	13% 5% 5% 5% 7% 42%	12225
	Night	14% 6% 5% 6% 44%	12164
Cloud Cover	Day	13% 5% 5% 5% 7% 42%	12244
	Night	14% 5% 6% 44%	12187
Transparency	Day	6% 17% 28% 20%	9066
	Night	5% 15% 28% 23%	9142
Seeing	Day	18% 27% 24% 15%	740
	Night	9% 26% 35% 17%	3627

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	14% 7% 5% 43%	833
	Night	16% 5% 6% 5% 6% 41%	1207
February	Day	13% 6% 6% 5% 43%	766
	Night	14% 5% 6% 5% 46%	1024
March	Day	12% 5% 5% 7% 46%	999
	Night	13% 5% 6% 7% 46%	998
April	Day	15% 5% 6% 5% 6% 5% 38%	1075
	Night	18% 5% 5% 7% 40%	904
May	Day	12% 5% 5% 9% 42%	1224
	Night	11% 5% 5% 8% 48%	821
June	Day	10% 5% 7% 7% 6% 9% 37%	1251
	Night	9% 8% 5% 8% 45%	718
July	Day	10% 6% 8% 6% 8% 10% 32%	1213
	Night	8% 7% 6% 8% 10% 39%	828
August	Day	10% 5% 5% 6% 6% 7% 12% 35%	1161
	Night	10% 6% 5% 9% 5% 6% 9% 35%	871
September	Day	13% 5% 7% 45%	1024
	Night	15% 6% 49%	952
October	Day	15% 6% 55%	923
	Night	14% 6% 52%	1111
November	Day	14% 5% 5% 7% 45%	1003
	Night	14% 5% 5% 6% 5% 43%	1443
December	Day	17% 5% 45%	753
	Night	16% 5% 5% 5% 5% 43%	1287

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	15% 7% 5% 43%	832
	Night	16% 5% 6% 5% 5% 41%	1208
February	Day	14% 5% 6% 43%	764
	Night	14% 6% 47%	1015
March	Day	11% 6% 5% 8% 46%	1022
	Night	14% 6% 7% 45%	1020
April	Day	15% 5% 5% 5% 5% 6% 6% 38%	1074
	Night	19% 5% 6% 40%	904
May	Day	12% 6% 5% 9% 43%	1223
	Night	11% 5% 5% 9% 48%	821
June	Day	10% 5% 7% 7% 6% 10% 36%	1260
	Night	9% 8% 5% 9% 44%	720
July	Day	10% 5% 7% 6% 9% 9% 34%	1213
	Night	8% 5% 7% 7% 7% 9% 39%	829
August	Day	10% 5% 5% 7% 6% 7% 11% 37%	1156
	Night	10% 5% 6% 7% 7% 6% 10% 34%	873
September	Day	13% 5% 5% 7% 46%	1023
	Night	15% 6% 50%	953
October	Day	15% 5% 56%	923
	Night	15% 6% 53%	1111
November	Day	14% 5% 5% 6% 46%	1001
	Night	15% 5% 5% 5% 5% 43%	1442
December	Day	17% 5% 45%	753
	Night	16% 5% 5% 43%	1291

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	13% 26% 29% 5%	565
	Night	16% 30% 23% 5%	831
February	Day	20% 32% 18%	573
	Night	15% 28% 28%	761
March	Day	9% 19% 29% 18% 8%	745
	Night	5% 18% 33% 18%	746
April	Day	5% 16% 27% 17% 8%	773
	Night	18% 21% 22% 9%	656
May	Day	6% 20% 34% 20%	901
	Night	6% 20% 34% 22%	624
June	Day	12% 29% 31% 12%	841
	Night	12% 24% 34% 13%	503
July	Day	9% 16% 29% 17%	654
	Night	10% 19% 29% 14%	476
August	Day	8% 15% 28% 17%	991
	Night	8% 13% 27% 19%	714
September	Day	5% 12% 20% 31%	842
	Night	12% 18% 33% 6%	805
October	Day	11% 29% 27% 6%	821
	Night	7% 28% 28% 5%	966
November	Day	18% 29% 19% 5%	795
	Night	13% 31% 20%	1095
December	Day	18% 29% 19% 5%	565
	Night	16% 27% 23%	965

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	Night	19% 38% 29%	330
February	Day	15% 15% 55% 5%	20
	Night	15% 44% 28%	290
March	Day	10% 32% 34% 13%	86
	Night	15% 38% 32% 5%	337
April	Day	8% 26% 24% 20% 7%	83
	Night	13% 26% 33% 12%	328
May	Day	28% 30% 21% 10%	88
	Night	27% 38% 18%	253
June	Day	20% 22% 37% 17%	79
	Night	13% 45% 35%	157
July	Day	6% 6% 22% 28% 25%	76
	Night	7% 41% 38%	229
August	Day	5% 18% 32% 34%	87
	Night	6% 36% 39% 5%	292
September	Day	8% 20% 28% 28%	80
	Night	12% 34% 32% 6%	329
October	Day	18% 36% 18% 11%	87
	Night	23% 34% 23%	343
November	Day	24% 31% 20%	54
	Night	9% 32% 37% 10%	401
December	Night	9% 31% 40% 7%	338

Page generated at 2008-11-27 13:47:40. Version ab11