Charles Douglas Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Charles Douglas Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	12% 7% 2 6% 4 4 4 3 5% 7% 45%	130945
	Night	12% 7% 3 6% 4 4 4 3 5% 6% 47%	130159
Transparency	Day	37% 6% 12% 26% 17% 3	134556
	Night	37% 5% 11% 25% 18% 3	134626
Seeing	Day	15% 9% 28% 27% 16% 4	17348
	Night	15% 9% 31% 32% 13% 1	27324

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	14% 11% 3 9% 4 4 4 3 4 5% 39%	5289
	11th to 20th	Day	6% 8% 3 9% 5% 7% 6% 3 5% 6% 42%	5310
	21st to end	Day	15% 13% 3 7% 4 4 4 3 4 4 41%	5812
February	1st to 10th	Day	15% 12% 4 8% 6% 3 3 3 4 6% 37%	1890
	11th to 20th	Day	15% 9% 3 7% 5% 5% 3 3 4 6% 40%	1799
	21st to end	Day	14% 9% 2 7% 4 3 3 3 5% 7% 42%	1695
March	1st to 10th	Day	14% 10% 3 5% 3 2 2 2 3 5% 51%	4936
	11th to 20th	Day	10% 6% 2 4 4 3 3 3 4 7% 54%	4980
	21st to end	Day	15% 11% 3 7% 5% 4 4 4 5% 8% 34%	5462
April	1st to 10th	Day	16% 8% 2 6% 4 4 3 3 5% 6% 43%	2355
	11th to 20th	Day	16% 8% 3 6% 4 4 3 4 5% 5% 42%	2374
	21st to end	Day	19% 8% 3 8% 6% 4 3 3 5% 7% 34%	2460
May	1st to 10th	Day	14% 7% 2 6% 5% 4 4 4 6% 7% 40%	5862
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 8% 43%	5862
	21st to end	Day	19% 6% 2 5% 4 4 4 3 5% 7% 41%	6840
June	1st to 10th	Day	8% 4 2 5% 4 4 4 5% 7% 10% 48%	2800
	11th to 20th	Day	8% 5% 2 6% 4 5% 4 5% 6% 9% 47%	2800
	21st to end	Day	6% 4 1 5% 4 4 4 5% 8% 9% 51%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 5% 9% 47%	6159
	11th to 20th	Day	7% 6% 2 6% 4 4 5% 4 6% 11% 45%	5900
	21st to end	Day	7% 5% 2 6% 4 5% 6% 5% 9% 11% 41%	6422
August	1st to 10th	Day	5% 4 1 5% 4 4 4 4 6% 10% 54%	4234
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 7% 48%	4171
	21st to end	Day	5% 4 1 4 4 4 4 4 7% 9% 54%	4316
September	1st to 10th	Day	11% 4 2 5% 4 3 4 3 5% 7% 53%	2400
	11th to 20th	Day	10% 4 2 4 3 3 4 4 6% 8% 50%	2300
	21st to end	Day	9% 5% 2 5% 3 4 4 3 4 7% 55%	2140
October	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 5% 6% 55%	2000
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 5% 54%	2000
	21st to end	Day	18% 8% 3 6% 3 3 3 2 4 5% 46%	2200
November	1st to 10th	Day	11% 7% 2 4 3 3 3 3 5% 7% 53%	2058
	11th to 20th	Day	14% 7% 2 5% 3 3 3 3 4 6% 50%	1791
	21st to end	Day	17% 9% 3 7% 3 3 3 3 4 5% 44%	1680
December	1st to 10th	Day	7% 7% 3 6% 4 3 3 3 4 7% 53%	2690
	11th to 20th	Day	11% 7% 4 7% 4 3 4 2 5% 5% 48%	3088
	21st to end	Day	18% 20% 6% 11% 4 4 3 3 3 4 24%	4070
January	1st to 10th	Night	13% 11% 4 12% 4 4 3 2 3 4 38%	7634
	11th to 20th	Night	9% 8% 3 9% 5% 5% 3 3 3 5% 46%	7680
	21st to end	Night	14% 11% 4 8% 4 4 4 3 4 5% 39%	8402
February	1st to 10th	Night	13% 10% 4 7% 6% 4 4 4 6% 6% 38%	2730
	11th to 20th	Night	13% 9% 3 8% 4 4 4 3 4 7% 41%	2599
	21st to end	Night	13% 9% 3 6% 3 3 3 3 4 5% 46%	1950
March	1st to 10th	Night	16% 10% 3 5% 3 3 3 2 4 4 46%	4919
	11th to 20th	Night	10% 6% 2 4 3 3 3 3 5% 7% 55%	4980
	21st to end	Night	14% 8% 4 7% 5% 4 5% 4 5% 7% 37%	5467
April	1st to 10th	Night	12% 7% 2 4 3 4 4 3 5% 7% 49%	2043
	11th to 20th	Night	14% 7% 2 6% 3 3 3 3 5% 6% 47%	1975
	21st to end	Night	16% 7% 3 6% 5% 3 3 3 5% 7% 41%	1936
May	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 4 7% 43%	4053
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 6% 8% 46%	4053
	21st to end	Night	15% 6% 2 6% 4 3 4 3 5% 6% 48%	4032
June	1st to 10th	Night	7% 3 2 4 3 3 4 5% 7% 9% 53%	1600
	11th to 20th	Night	5% 4 2 4 4 4 4 4 7% 13% 49%	1600
	21st to end	Night	4 3 1 3 3 4 4 4 7% 9% 58%	1600
July	1st to 10th	Night	9% 5% 2 6% 4 3 3 3 6% 9% 48%	3800
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 4 6% 10% 50%	4077
	21st to end	Night	6% 4 2 5% 4 4 5% 5% 9% 13% 44%	4449
August	1st to 10th	Night	2 1 1 3 3 3 3 4 6% 9% 64%	2926
	11th to 20th	Night	11% 6% 2 5% 3 2 4 3 5% 6% 52%	2888
	21st to end	Night	4 3 1 4 4 4 4 4 5% 7% 60%	3581
September	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 6% 56%	2000
	11th to 20th	Night	9% 4 2 4 3 4 3 3 5% 8% 54%	2096
	21st to end	Night	8% 4 2 4 4 4 3 3 4 7% 57%	2239
October	1st to 10th	Night	8% 5% 2 4 3 3 4 3 4 5% 59%	2400
	11th to 20th	Night	13% 6% 1 4 3 2 3 3 4 5% 56%	2400
	21st to end	Night	17% 6% 2 5% 3 3 3 2 4 4 51%	2640
November	1st to 10th	Night	11% 5% 2 4 3 3 3 2 3 5% 58%	2460
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 52%	2819
	21st to end	Night	16% 9% 3 6% 3 3 3 2 4 4 48%	2940
December	1st to 10th	Night	8% 6% 2 5% 4 4 3 3 5% 6% 53%	4720
	11th to 20th	Night	14% 9% 3 6% 3 3 4 3 4 6% 45%	5339
	21st to end	Night	20% 17% 4 9% 4 4 3 2 3 4 28%	7132

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	47% 5% 11% 25% 11% 2	5290
	11th to 20th	Day	43% 4 10% 25% 14% 4	5310
	21st to end	Day	48% 4 11% 18% 17% 3	5812
February	1st to 10th	Day	48% 4 12% 23% 11% 1	1890
	11th to 20th	Day	47% 5% 11% 21% 14% 2	1808
	21st to end	Day	42% 5% 11% 23% 16% 3	1695
March	1st to 10th	Day	39% 3 7% 24% 22% 4	4936
	11th to 20th	Day	31% 5% 9% 22% 26% 7%	4980
	21st to end	Day	47% 7% 16% 23% 5%	5464
April	1st to 10th	Day	41% 6% 11% 24% 16% 2	2355
	11th to 20th	Day	43% 5% 13% 23% 15% 1	2374
	21st to end	Day	50% 6% 11% 22% 10%	2460
May	1st to 10th	Day	42% 8% 13% 24% 13% 1	5900
	11th to 20th	Day	38% 7% 14% 24% 15% 3	5866
	21st to end	Day	42% 7% 10% 28% 13% 1	6854
June	1st to 10th	Day	28% 10% 17% 29% 15% 1	2800
	11th to 20th	Day	32% 9% 14% 27% 16% 2	2800
	21st to end	Day	27% 8% 16% 30% 15% 4	2800
July	1st to 10th	Day	34% 6% 12% 26% 19% 2	6094
	11th to 20th	Day	32% 8% 15% 29% 14% 1	5831
	21st to end	Day	31% 10% 17% 32% 9% 1	6240
August	1st to 10th	Day	25% 7% 17% 29% 21% 1	4220
	11th to 20th	Day	34% 7% 16% 28% 15% 1	4210
	21st to end	Day	24% 7% 15% 36% 15% 4	4315
September	1st to 10th	Day	30% 6% 13% 26% 19% 6%	2400
	11th to 20th	Day	31% 6% 11% 26% 24% 2	2300
	21st to end	Day	29% 5% 10% 29% 23% 3	2140
October	1st to 10th	Day	18% 3 6% 21% 40% 11%	3260
	11th to 20th	Day	22% 3 5% 23% 40% 8%	3240
	21st to end	Day	36% 4 8% 24% 25% 3	3538
November	1st to 10th	Day	31% 5% 12% 28% 19% 4	2058
	11th to 20th	Day	36% 5% 10% 25% 21% 4	1791
	21st to end	Day	44% 4 9% 27% 16% 1	1680
December	1st to 10th	Day	33% 5% 12% 34% 16% 1	2690
	11th to 20th	Day	39% 4 13% 30% 12% 1	3085
	21st to end	Day	64% 5% 11% 13% 6% 1	4070
January	1st to 10th	Night	51% 4 11% 24% 10% 1	7635
	11th to 20th	Night	42% 4 8% 25% 17% 4	7680
	21st to end	Night	48% 4 12% 17% 15% 3	8402
February	1st to 10th	Night	46% 5% 12% 21% 13% 3	2730
	11th to 20th	Night	44% 5% 11% 21% 17% 3	2600
	21st to end	Night	39% 5% 9% 22% 19% 5%	1950
March	1st to 10th	Night	41% 4 7% 21% 23% 4	4919
	11th to 20th	Night	30% 4 7% 23% 29% 7%	4980
	21st to end	Night	45% 8% 13% 23% 9% 2	5467
April	1st to 10th	Night	35% 5% 11% 24% 21% 4	2043
	11th to 20th	Night	37% 5% 9% 26% 18% 5%	1975
	21st to end	Night	42% 6% 10% 26% 15% 1	1936
May	1st to 10th	Night	40% 8% 14% 23% 14% 1	4080
	11th to 20th	Night	36% 7% 13% 23% 17% 5%	4054
	21st to end	Night	36% 7% 10% 25% 22% 1	4040
June	1st to 10th	Night	24% 9% 16% 31% 17% 3	1600
	11th to 20th	Night	26% 7% 14% 28% 21% 3	1600
	21st to end	Night	20% 7% 16% 33% 20% 5%	1600
July	1st to 10th	Night	31% 6% 13% 26% 22% 2	3759
	11th to 20th	Night	27% 8% 14% 34% 15% 2	4026
	21st to end	Night	27% 8% 18% 37% 10% 1	4320
August	1st to 10th	Night	16% 7% 21% 30% 23% 3	2916
	11th to 20th	Night	32% 5% 11% 30% 21% 1	2915
	21st to end	Night	23% 6% 12% 37% 17% 4	3581
September	1st to 10th	Night	30% 5% 11% 24% 23% 7%	2000
	11th to 20th	Night	28% 6% 12% 28% 23% 4	2096
	21st to end	Night	27% 5% 9% 26% 29% 4	2239
October	1st to 10th	Night	17% 3 6% 22% 43% 10%	3920
	11th to 20th	Night	22% 3 7% 24% 29% 16%	3899
	21st to end	Night	34% 4 7% 29% 24% 3	4252
November	1st to 10th	Night	30% 4 11% 27% 22% 5%	2460
	11th to 20th	Night	33% 4 10% 25% 23% 5%	2819
	21st to end	Night	41% 4 9% 27% 16% 3	2940
December	1st to 10th	Night	32% 5% 12% 35% 16% 1	4720
	11th to 20th	Night	40% 4 12% 27% 15% 1	5341
	21st to end	Night	60% 5% 10% 17% 6% 1	7132

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	21% 15% 37% 21% 5%	330
	11th to 20th	Day	25% 18% 41% 14% 2	330
	21st to end	Day	18% 16% 45% 17% 3 1	363
February	1st to 10th	Day	25% 17% 40% 14% 4	330
	11th to 20th	Day	24% 19% 37% 14% 5% 1	315
	21st to end	Day	21% 24% 38% 14% 2	249
March	1st to 10th	Day	18% 16% 46% 18% 3	300
	11th to 20th	Day	28% 13% 34% 21% 4	300
	21st to end	Day	32% 17% 32% 13% 5% 1	330
April	1st to 10th	Day	22% 16% 34% 21% 7%	455
	11th to 20th	Day	22% 15% 35% 21% 6% 1	492
	21st to end	Day	23% 14% 33% 21% 8% 1	560
May	1st to 10th	Day	17% 12% 35% 26% 9% 1	703
	11th to 20th	Day	18% 10% 28% 26% 15% 3	725
	21st to end	Day	17% 9% 26% 25% 17% 6%	789
June	1st to 10th	Day	7% 4 20% 34% 26% 8%	730
	11th to 20th	Day	10% 3 24% 35% 23% 5%	730
	21st to end	Day	5% 5% 31% 37% 17% 4	730
July	1st to 10th	Day	12% 2 14% 28% 30% 14%	721
	11th to 20th	Day	6% 2 18% 36% 30% 9%	718
	21st to end	Day	9% 2 16% 32% 31% 10%	794
August	1st to 10th	Day	7% 2 18% 39% 28% 5%	726
	11th to 20th	Day	8% 3 18% 34% 31% 7%	715
	21st to end	Day	9% 2 11% 32% 34% 11%	587
September	1st to 10th	Day	11% 3 13% 37% 27% 8%	530
	11th to 20th	Day	7% 3 14% 37% 32% 7%	430
	21st to end	Day	11% 7% 23% 31% 21% 8%	330
October	1st to 10th	Day	12% 6% 29% 30% 15% 7%	330
	11th to 20th	Day	13% 14% 39% 24% 8% 1	330
	21st to end	Day	27% 16% 30% 21% 5%	363
November	1st to 10th	Day	16% 12% 38% 23% 7% 4	330
	11th to 20th	Day	16% 16% 33% 24% 8% 2	330
	21st to end	Day	26% 12% 31% 23% 7%	330
December	1st to 10th	Day	13% 16% 39% 23% 6% 2	330
	11th to 20th	Day	19% 16% 38% 20% 6% 1	330
	21st to end	Day	22% 21% 40% 16% 1	363
January	1st to 10th	Night	16% 12% 40% 25% 7%	1050
	11th to 20th	Night	17% 15% 41% 22% 4 1	1050
	21st to end	Night	16% 13% 40% 28% 3	1155
February	1st to 10th	Night	17% 13% 42% 24% 3	1050
	11th to 20th	Night	18% 16% 36% 27% 3	1005
	21st to end	Night	20% 13% 45% 20% 2	630
March	1st to 10th	Night	17% 14% 40% 24% 5%	600
	11th to 20th	Night	20% 10% 35% 29% 6% 1	600
	21st to end	Night	27% 10% 36% 23% 4	660
April	1st to 10th	Night	17% 11% 37% 31% 4	600
	11th to 20th	Night	19% 11% 31% 30% 8%	593
	21st to end	Night	19% 8% 36% 29% 8%	600
May	1st to 10th	Night	16% 6% 32% 38% 8%	600
	11th to 20th	Night	16% 7% 25% 34% 17% 1	591
	21st to end	Night	13% 5% 28% 33% 19% 2	650
June	1st to 10th	Night	8% 3 17% 44% 27% 2	600
	11th to 20th	Night	7% 2 24% 44% 22% 2	600
	21st to end	Night	4 4 25% 48% 18% 1	600
July	1st to 10th	Night	9% 2 12% 42% 32% 4	585
	11th to 20th	Night	5% 1 15% 45% 32% 2	582
	21st to end	Night	9% 1 12% 41% 35% 2	657
August	1st to 10th	Night	5% 2 12% 48% 32% 1	597
	11th to 20th	Night	6% 2 14% 44% 31% 3	594
	21st to end	Night	7% 1 11% 39% 39% 4	660
September	1st to 10th	Night	13% 2 14% 41% 27% 3	600
	11th to 20th	Night	10% 2 17% 39% 27% 6%	600
	21st to end	Night	8% 4 21% 41% 23% 4	639
October	1st to 10th	Night	10% 4 26% 37% 20% 3	800
	11th to 20th	Night	15% 7% 36% 31% 10%	800
	21st to end	Night	20% 11% 33% 26% 10%	880
November	1st to 10th	Night	13% 8% 35% 34% 10%	820
	11th to 20th	Night	14% 12% 36% 29% 9%	971
	21st to end	Night	20% 11% 36% 27% 5%	1050
December	1st to 10th	Night	17% 11% 39% 27% 5% 1	1050
	11th to 20th	Night	20% 10% 37% 29% 4	1050
	21st to end	Night	21% 15% 36% 26% 3	1155

Page generated at 2026-02-13 00:42:14. Version ac07

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