Dunham Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dunham Hill 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	15% 8% 6% 6% 5% 6% 5% 7% 30%	12267
	Night	18% 9% 6% 6% 5% 5% 7% 29%	12125
Cloud Cover	Day	16% 8% 5% 6% 5% 6% 5% 7% 30%	12285
	Night	19% 9% 6% 6% 5% 5% 6% 29%	12148
Transparency	Day	10% 17% 23% 14%	8077
	Night	9% 18% 22% 13%	7910
Seeing	Day	8% 27% 30% 17% 8%	775
	Night	11% 34% 30% 11%	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	20% 16% 13% 10% 9% 6% 9%	745
	Night	24% 15% 11% 10% 6% 5% 11%	1295
February	Day	20% 12% 11% 9% 8% 9% 5% 5% 10%	785
	Night	19% 12% 8% 10% 10% 5% 7% 13%	1005
March	Day	16% 12% 7% 8% 5% 5% 6% 24%	971
	Night	19% 9% 7% 6% 5% 6% 29%	1028
April	Day	16% 8% 6% 5% 5% 5% 36%	1121
	Night	19% 5% 5% 5% 6% 6% 36%	855
May	Day	16% 6% 7% 6% 6% 34%	1231
	Night	16% 6% 5% 6% 5% 9% 34%	815
June	Day	9% 5% 6% 7% 6% 5% 10% 37%	1250
	Night	9% 7% 8% 11% 39%	719
July	Day	9% 6% 6% 6% 10% 42%	1298
	Night	7% 6% 5% 11% 47%	745
August	Day	10% 7% 6% 5% 7% 8% 39%	1180
	Night	10% 5% 5% 8% 49%	855
September	Day	16% 5% 5% 6% 6% 41%	1008
	Night	17% 5% 7% 47%	966
October	Day	19% 7% 5% 6% 5% 6% 34%	914
	Night	20% 6% 5% 8% 35%	1120
November	Day	21% 14% 7% 6% 6% 7% 19%	1002
	Night	22% 15% 6% 6% 6% 6% 21%	1444
December	Day	22% 14% 15% 8% 5% 5% 5% 10%	762
	Night	26% 14% 13% 8% 5% 6% 5% 10%	1278

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	21% 17% 12% 8% 9% 6% 9%	744
	Night	24% 14% 11% 11% 6% 12%	1295
February	Day	20% 12% 10% 9% 7% 7% 5% 6% 11%	783
	Night	20% 11% 7% 9% 10% 5% 7% 13%	996
March	Day	17% 11% 8% 6% 5% 6% 5% 7% 23%	992
	Night	22% 8% 6% 6% 6% 6% 28%	1052
April	Day	16% 8% 5% 5% 5% 5% 5% 6% 36%	1120
	Night	19% 6% 5% 6% 36%	855
May	Day	18% 5% 6% 5% 5% 6% 35%	1230
	Night	17% 5% 6% 5% 5% 7% 35%	815
June	Day	10% 5% 5% 7% 5% 6% 9% 38%	1260
	Night	10% 6% 5% 8% 11% 40%	720
July	Day	9% 6% 6% 6% 10% 42%	1298
	Night	7% 6% 5% 10% 48%	746
August	Day	11% 5% 5% 6% 6% 8% 41%	1175
	Night	9% 5% 8% 49%	856
September	Day	16% 5% 6% 8% 40%	1007
	Night	17% 5% 7% 48%	967
October	Day	20% 8% 6% 5% 5% 35%	914
	Night	22% 5% 5% 5% 7% 35%	1120
November	Day	23% 14% 5% 6% 5% 7% 21%	1000
	Night	23% 15% 6% 6% 5% 6% 22%	1444
December	Day	22% 15% 13% 8% 6% 12%	762
	Night	26% 15% 12% 7% 5% 5% 5% 10%	1282

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	9% 18% 7% 5%	350
	Night	8% 21% 10%	630
February	Day	11% 13% 15%	388
	Night	14% 14% 15%	538
March	Day	14% 26% 16% 7%	583
	Night	10% 21% 25% 6%	647
April	Day	9% 17% 28% 19%	720
	Night	7% 21% 25% 23%	565
May	Day	9% 20% 30% 13%	830
	Night	5% 20% 29% 16%	555
June	Day	11% 22% 29% 13%	922
	Night	12% 29% 26% 15%	533
July	Day	12% 17% 26% 17% 6%	730
	Night	11% 21% 31% 20%	429
August	Day	8% 14% 22% 21%	982
	Night	6% 10% 24% 29%	724
September	Day	8% 14% 29% 18%	814
	Night	6% 15% 30% 20%	795
October	Day	6% 13% 21% 19%	719
	Night	6% 14% 26% 15%	889
November	Day	9% 15% 17% 8%	666
	Night	9% 17% 17% 6%	992
December	Day	13% 20% 12%	373
	Night	15% 21% 9%	613

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% 43% 22%	381
February	Night	19% 45% 19%	291
March	Day	11% 45% 17% 9%	51
	Night	18% 38% 25% 6%	325
April	Day	15% 28% 33% 14%	83
	Night	13% 33% 35% 8%	240
May	Day	8% 34% 24% 21%	106
	Night	7% 36% 33% 13%	253
June	Day	8% 31% 31% 14% 8%	167
	Night	31% 36% 26%	248
July	Day	6% 31% 32% 15% 10%	148
	Night	24% 42% 26%	231
August	Day	13% 35% 23% 12%	89
	Night	21% 51% 15%	260
September	Day	13% 31% 23% 13%	82
	Night	5% 17% 31% 30%	303
October	Day	12% 14% 36% 16%	49
	Night	9% 32% 29% 10%	341
November	Night	14% 34% 27% 5%	390
December	Night	17% 43% 22%	364

Page generated at 2008-11-27 13:47:36. Version ab11