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 2005-11-06 to 2026-07-16.

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% 10% 4 9% 5% 5% 5% 4 5% 7% 34%	137567
	Night	14% 11% 4 10% 5% 5% 4 3 5% 6% 33%	135351
Transparency	Day	48% 6% 11% 21% 13% 1	141109
	Night	50% 5% 11% 20% 12% 1	139886
Seeing	Day	17% 12% 28% 24% 14% 6%	18102
	Night	19% 9% 30% 30% 11% 1	27932

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 16% 7% 15% 7% 6% 5% 4 4 5% 17%	4691
	11th to 20th	Day	11% 14% 8% 19% 12% 12% 5% 3 4 3 9%	4710
	21st to end	Day	15% 17% 6% 13% 9% 5% 7% 3 3 4 17%	5252
February	1st to 10th	Day	12% 16% 5% 12% 8% 6% 6% 4 5% 5% 21%	1920
	11th to 20th	Day	13% 13% 6% 13% 9% 5% 5% 3 3 6% 25%	2048
	21st to end	Day	14% 15% 5% 11% 6% 5% 5% 4 5% 6% 24%	1730
March	1st to 10th	Day	14% 11% 4 10% 5% 4 5% 4 5% 6% 33%	4580
	11th to 20th	Day	11% 10% 4 8% 6% 5% 4 4 6% 8% 35%	5032
	21st to end	Day	17% 14% 5% 9% 5% 4 4 3 5% 5% 27%	5671
April	1st to 10th	Day	18% 13% 3 9% 5% 4 4 3 5% 6% 30%	2520
	11th to 20th	Day	16% 9% 3 7% 4 5% 5% 4 5% 7% 36%	2590
	21st to end	Day	16% 8% 3 8% 5% 4 5% 4 5% 7% 35%	2730
May	1st to 10th	Day	17% 8% 3 7% 4 5% 4 4 6% 7% 36%	7616
	11th to 20th	Day	13% 7% 3 9% 5% 5% 5% 4 6% 8% 36%	7626
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 7% 45%	8843
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 4 4 6% 9% 43%	2940
	11th to 20th	Day	11% 5% 2 6% 5% 4 4 4 6% 9% 43%	2940
	21st to end	Day	9% 5% 3 7% 5% 5% 4 4 7% 9% 42%	2940
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 5% 7% 10% 38%	6500
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 6% 10% 47%	6431
	21st to end	Day	8% 5% 2 6% 4 5% 4 4 7% 9% 47%	6843
August	1st to 10th	Day	12% 4 2 6% 5% 4 5% 4 6% 9% 43%	4234
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 4 6% 7% 45%	4174
	21st to end	Day	8% 5% 2 7% 4 5% 5% 4 7% 8% 43%	4449
September	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 5% 7% 44%	2334
	11th to 20th	Day	11% 5% 2 5% 3 4 4 3 5% 8% 49%	2200
	21st to end	Day	13% 7% 3 7% 5% 5% 4 4 6% 8% 38%	2200
October	1st to 10th	Day	14% 6% 2 7% 5% 3 4 3 4 7% 44%	2200
	11th to 20th	Day	18% 9% 3 8% 6% 4 5% 4 6% 6% 31%	1965
	21st to end	Day	20% 10% 4 8% 5% 5% 4 4 5% 6% 28%	1980
November	1st to 10th	Day	11% 12% 3 7% 5% 5% 5% 3 5% 6% 39%	1852
	11th to 20th	Day	14% 14% 5% 10% 6% 5% 5% 4 5% 7% 25%	1890
	21st to end	Day	17% 15% 5% 10% 5% 4 4 3 5% 5% 26%	1890
December	1st to 10th	Day	9% 18% 9% 14% 7% 6% 6% 3 5% 6% 17%	2888
	11th to 20th	Day	14% 22% 9% 11% 8% 5% 4 3 4 4 16%	3088
	21st to end	Day	24% 20% 7% 13% 5% 4 4 3 4 4 13%	4070
January	1st to 10th	Night	12% 16% 8% 17% 6% 5% 4 3 4 5% 19%	8232
	11th to 20th	Night	10% 16% 8% 19% 9% 10% 5% 4 4 4 11%	8280
	21st to end	Night	13% 15% 7% 14% 8% 7% 6% 3 5% 6% 17%	8956
February	1st to 10th	Night	14% 15% 5% 11% 9% 5% 5% 3 5% 5% 22%	2698
	11th to 20th	Night	12% 13% 6% 11% 7% 6% 5% 4 5% 5% 26%	2458
	21st to end	Night	17% 12% 5% 8% 5% 5% 4 4 6% 6% 28%	2076
March	1st to 10th	Night	14% 10% 3 9% 4 4 4 3 5% 7% 38%	5493
	11th to 20th	Night	11% 8% 3 6% 4 4 4 4 5% 8% 43%	5146
	21st to end	Night	16% 11% 5% 8% 4 4 4 3 4 6% 34%	5542
April	1st to 10th	Night	15% 11% 3 7% 5% 4 5% 4 4 6% 36%	2100
	11th to 20th	Night	19% 8% 3 7% 5% 4 4 3 5% 5% 37%	1978
	21st to end	Night	17% 7% 3 7% 4 4 4 3 6% 7% 39%	1890
May	1st to 10th	Night	15% 9% 3 7% 4 4 5% 4 5% 7% 38%	5237
	11th to 20th	Night	14% 7% 3 6% 4 4 4 3 5% 7% 43%	5234
	21st to end	Night	14% 6% 3 6% 4 4 4 3 5% 6% 46%	5317
June	1st to 10th	Night	11% 5% 2 5% 4 3 5% 4 6% 9% 46%	1680
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 8% 9% 43%	1680
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 6% 10% 45%	1680
July	1st to 10th	Night	8% 5% 2 5% 5% 4 4 4 6% 10% 46%	3698
	11th to 20th	Night	8% 4 2 5% 4 4 4 4 5% 8% 52%	3657
	21st to end	Night	10% 6% 2 6% 4 4 4 3 5% 8% 48%	4023
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 6% 9% 49%	2915
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 6% 8% 46%	2875
	21st to end	Night	8% 6% 3 8% 4 5% 3 4 6% 7% 46%	3445
September	1st to 10th	Night	9% 5% 3 5% 5% 5% 4 3 6% 7% 49%	2045
	11th to 20th	Night	10% 4 1 4 3 4 3 3 5% 7% 56%	2200
	21st to end	Night	14% 7% 3 6% 5% 4 4 3 4 6% 44%	2200
October	1st to 10th	Night	15% 5% 2 5% 4 4 4 3 6% 6% 48%	2200
	11th to 20th	Night	19% 10% 4 7% 5% 3 4 3 5% 6% 35%	2430
	21st to end	Night	21% 9% 3 7% 4 4 4 3 6% 6% 33%	2860
November	1st to 10th	Night	14% 9% 3 7% 4 4 4 3 5% 6% 41%	2666
	11th to 20th	Night	13% 15% 3 8% 5% 5% 6% 5% 6% 6% 27%	2730
	21st to end	Night	17% 15% 4 9% 4 4 5% 3 4 6% 27%	2730
December	1st to 10th	Night	12% 18% 8% 14% 6% 5% 4 3 4 5% 21%	4511
	11th to 20th	Night	15% 19% 7% 15% 8% 5% 4 3 4 5% 15%	5339
	21st to end	Night	24% 20% 7% 12% 5% 4 4 3 4 5% 14%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	68% 6% 11% 12% 2	4692
	11th to 20th	Day	80% 4 7% 5% 4	4710
	21st to end	Day	71% 4 12% 9% 4	5252
February	1st to 10th	Day	64% 6% 10% 14% 6%	1920
	11th to 20th	Day	63% 4 10% 14% 9% 1	2058
	21st to end	Day	61% 6% 10% 15% 7% 1	1730
March	1st to 10th	Day	51% 6% 14% 19% 10% 2	4580
	11th to 20th	Day	46% 6% 15% 22% 9% 1	5032
	21st to end	Day	58% 5% 10% 16% 10% 1	5673
April	1st to 10th	Day	56% 5% 10% 19% 10%	2520
	11th to 20th	Day	48% 5% 10% 20% 14% 2	2590
	21st to end	Day	48% 7% 11% 19% 13% 2	2730
May	1st to 10th	Day	46% 5% 11% 22% 14% 2	7651
	11th to 20th	Day	44% 7% 12% 22% 14% 1	7630
	21st to end	Day	39% 5% 9% 22% 22% 4	8857
June	1st to 10th	Day	36% 6% 12% 27% 16% 2	2940
	11th to 20th	Day	36% 7% 13% 24% 18% 1	2940
	21st to end	Day	36% 8% 13% 24% 17% 2	2940
July	1st to 10th	Day	39% 7% 14% 24% 15% 2	6431
	11th to 20th	Day	32% 6% 13% 29% 19% 1	6356
	21st to end	Day	32% 7% 14% 25% 21% 1	6648
August	1st to 10th	Day	36% 8% 13% 27% 16% 1	4221
	11th to 20th	Day	36% 7% 11% 31% 14% 1	4213
	21st to end	Day	36% 7% 13% 30% 13% 2	4448
September	1st to 10th	Day	38% 8% 12% 26% 15% 2	2334
	11th to 20th	Day	32% 6% 13% 30% 18% 1	2200
	21st to end	Day	42% 6% 13% 23% 15%	2200
October	1st to 10th	Day	30% 5% 8% 36% 20% 1	3590
	11th to 20th	Day	41% 5% 11% 27% 16%	3168
	21st to end	Day	48% 5% 12% 24% 11% 1	3180
November	1st to 10th	Day	46% 6% 13% 22% 13% 1	1852
	11th to 20th	Day	58% 5% 12% 18% 6% 1	1890
	21st to end	Day	60% 5% 13% 16% 5%	1890
December	1st to 10th	Day	67% 7% 15% 9% 2	2888
	11th to 20th	Day	73% 5% 10% 11% 2	3085
	21st to end	Day	76% 4 7% 9% 4	4070
January	1st to 10th	Night	67% 5% 10% 12% 6% 1	8233
	11th to 20th	Night	75% 4 10% 6% 5%	8280
	21st to end	Night	69% 4 11% 12% 4	8956
February	1st to 10th	Night	63% 6% 10% 15% 5% 1	2698
	11th to 20th	Night	59% 6% 9% 17% 8% 1	2459
	21st to end	Night	56% 6% 12% 17% 8% 1	2076
March	1st to 10th	Night	46% 5% 14% 25% 10% 1	5493
	11th to 20th	Night	38% 6% 14% 26% 15% 1	5146
	21st to end	Night	50% 5% 12% 19% 12% 1	5542
April	1st to 10th	Night	49% 6% 11% 19% 14% 1	2100
	11th to 20th	Night	48% 4 10% 19% 16% 2	1978
	21st to end	Night	44% 5% 14% 21% 13% 3	1890
May	1st to 10th	Night	45% 6% 12% 19% 17% 2	5264
	11th to 20th	Night	41% 5% 11% 24% 16% 2	5235
	21st to end	Night	39% 5% 9% 21% 22% 4	5327
June	1st to 10th	Night	34% 6% 10% 29% 18% 3	1680
	11th to 20th	Night	36% 7% 13% 26% 17% 2	1680
	21st to end	Night	33% 7% 14% 27% 17% 2	1680
July	1st to 10th	Night	32% 6% 14% 27% 16% 5%	3660
	11th to 20th	Night	30% 6% 13% 30% 20% 2	3612
	21st to end	Night	34% 7% 13% 21% 23% 2	3907
August	1st to 10th	Night	32% 6% 13% 29% 19% 1	2904
	11th to 20th	Night	35% 6% 10% 32% 15% 1	2902
	21st to end	Night	36% 6% 11% 30% 15% 2	3445
September	1st to 10th	Night	33% 7% 11% 29% 18% 2	2045
	11th to 20th	Night	27% 6% 12% 32% 22% 2	2200
	21st to end	Night	41% 6% 11% 25% 16% 1	2200
October	1st to 10th	Night	26% 4 10% 36% 23% 1	3590
	11th to 20th	Night	45% 4 10% 21% 20% 1	3966
	21st to end	Night	41% 4 10% 30% 15% 1	4610
November	1st to 10th	Night	45% 5% 10% 24% 15% 1	2666
	11th to 20th	Night	54% 7% 13% 18% 8% 1	2730
	21st to end	Night	58% 6% 12% 19% 5%	2730
December	1st to 10th	Night	66% 6% 13% 12% 3	4511
	11th to 20th	Night	71% 5% 10% 10% 4	5341
	21st to end	Night	74% 5% 8% 8% 5%	7150

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	22% 15% 35% 19% 8% 1	330
	11th to 20th	Day	18% 17% 44% 16% 4 1	330
	21st to end	Day	25% 16% 30% 23% 6%	363
February	1st to 10th	Day	22% 20% 34% 19% 3 2	330
	11th to 20th	Day	21% 19% 38% 18% 3	330
	21st to end	Day	26% 23% 38% 12% 2	273
March	1st to 10th	Day	22% 18% 41% 14% 5%	330
	11th to 20th	Day	23% 16% 33% 21% 7%	508
	21st to end	Day	26% 16% 31% 21% 5% 1	594
April	1st to 10th	Day	24% 13% 35% 21% 5% 1	540
	11th to 20th	Day	23% 20% 30% 16% 7% 5%	532
	21st to end	Day	20% 12% 32% 24% 11% 2	540
May	1st to 10th	Day	24% 15% 27% 23% 9% 2	543
	11th to 20th	Day	18% 13% 26% 25% 13% 6%	567
	21st to end	Day	15% 8% 27% 25% 17% 7%	770
June	1st to 10th	Day	11% 9% 29% 31% 16% 5%	780
	11th to 20th	Day	13% 8% 31% 27% 15% 4	780
	21st to end	Day	11% 8% 27% 33% 16% 6%	780
July	1st to 10th	Day	9% 4 20% 33% 22% 13%	771
	11th to 20th	Day	10% 7% 24% 28% 21% 10%	744
	21st to end	Day	8% 4 23% 33% 23% 10%	763
August	1st to 10th	Day	9% 4 18% 27% 24% 18%	526
	11th to 20th	Day	12% 4 16% 26% 32% 10%	519
	21st to end	Day	10% 4 21% 28% 24% 14%	583
September	1st to 10th	Day	15% 5% 20% 25% 21% 13%	530
	11th to 20th	Day	10% 5% 16% 23% 30% 17%	530
	21st to end	Day	16% 10% 21% 25% 17% 11%	530
October	1st to 10th	Day	16% 7% 28% 25% 17% 8%	530
	11th to 20th	Day	21% 16% 24% 28% 9% 1	480
	21st to end	Day	25% 15% 33% 22% 5% 1	363
November	1st to 10th	Day	17% 17% 35% 24% 6% 1	330
	11th to 20th	Day	22% 21% 32% 19% 5% 1	330
	21st to end	Day	23% 17% 41% 14% 5%	330
December	1st to 10th	Day	24% 21% 29% 18% 8% 1	330
	11th to 20th	Day	22% 22% 38% 14% 4	330
	21st to end	Day	26% 20% 35% 15% 4 1	363
January	1st to 10th	Night	20% 14% 35% 24% 6% 1	1050
	11th to 20th	Night	22% 14% 37% 22% 4 1	1050
	21st to end	Night	19% 12% 35% 29% 5%	1118
February	1st to 10th	Night	21% 13% 37% 25% 3	840
	11th to 20th	Night	20% 14% 37% 26% 4	823
	21st to end	Night	22% 14% 40% 20% 4	692
March	1st to 10th	Night	20% 11% 37% 24% 7% 1	840
	11th to 20th	Night	18% 15% 36% 26% 5% 1	840
	21st to end	Night	23% 10% 33% 27% 7%	903
April	1st to 10th	Night	19% 12% 37% 26% 5% 1	630
	11th to 20th	Night	24% 11% 33% 23% 7% 1	623
	21st to end	Night	21% 6% 35% 30% 7% 1	630
May	1st to 10th	Night	19% 7% 34% 32% 7%	630
	11th to 20th	Night	20% 5% 31% 31% 11% 1	621
	21st to end	Night	13% 3 27% 35% 20% 2	683
June	1st to 10th	Night	11% 5% 25% 44% 15% 1	630
	11th to 20th	Night	13% 3 28% 39% 16%	630
	21st to end	Night	9% 3 23% 43% 21% 1	630
July	1st to 10th	Night	8% 2 18% 41% 26% 4	615
	11th to 20th	Night	11% 2 17% 44% 23% 3	597
	21st to end	Night	8% 2 18% 46% 25% 2	657
August	1st to 10th	Night	9% 1 14% 45% 28% 3	597
	11th to 20th	Night	10% 3 15% 40% 27% 5%	594
	21st to end	Night	10% 3 17% 39% 28% 3	660
September	1st to 10th	Night	11% 3 19% 37% 27% 4	645
	11th to 20th	Night	12% 3 15% 35% 30% 4	800
	21st to end	Night	17% 5% 22% 30% 22% 5%	800
October	1st to 10th	Night	17% 5% 24% 33% 19% 1	800
	11th to 20th	Night	24% 13% 27% 28% 8% 1	800
	21st to end	Night	25% 10% 32% 27% 6%	880
November	1st to 10th	Night	20% 10% 32% 29% 8% 1	820
	11th to 20th	Night	20% 14% 37% 23% 5% 1	840
	21st to end	Night	25% 12% 34% 25% 4	840
December	1st to 10th	Night	23% 14% 34% 23% 6%	919
	11th to 20th	Night	23% 15% 36% 22% 3	1050
	21st to end	Night	31% 12% 35% 21% 2	1155

Page generated at 2026-07-13 03:14:51. Version ac07

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