B&W Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the B&W Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	11% 7% 3 7% 4 4 4 3 5% 7% 44%	131682
	Night	10% 7% 3 7% 4 4 4 3 5% 7% 47%	131087
Transparency	Day	38% 6% 13% 25% 16% 2	135320
	Night	35% 5% 13% 25% 19% 3	135513
Seeing	Day	15% 8% 30% 29% 15% 3	17759
	Night	14% 8% 32% 35% 11% 1	27753

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	10% 11% 5% 14% 7% 4 4 3 4 6% 31%	5289
	11th to 20th	Day	6% 9% 3 9% 6% 6% 6% 4 6% 6% 39%	5310
	21st to end	Day	6% 12% 4 11% 5% 6% 4 4 4 6% 37%	5812
February	1st to 10th	Day	9% 12% 5% 9% 5% 6% 5% 3 6% 7% 33%	1890
	11th to 20th	Day	10% 10% 3 9% 5% 4 4 3 5% 7% 39%	1871
	21st to end	Day	10% 7% 3 7% 4 4 4 3 5% 7% 45%	1730
March	1st to 10th	Day	15% 9% 3 5% 3 2 2 2 3 4 51%	4998
	11th to 20th	Day	12% 6% 2 5% 4 4 3 3 4 8% 50%	5090
	21st to end	Day	18% 11% 4 8% 4 4 5% 4 6% 7% 30%	5583
April	1st to 10th	Day	16% 8% 3 6% 4 4 3 3 5% 6% 43%	2378
	11th to 20th	Day	16% 7% 3 6% 4 4 4 3 5% 7% 41%	2601
	21st to end	Day	19% 7% 3 6% 4 4 4 3 5% 6% 37%	2730
May	1st to 10th	Day	12% 7% 3 7% 4 4 4 3 5% 6% 45%	5876
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 3 6% 50%	5862
	21st to end	Day	16% 7% 3 6% 4 4 5% 4 6% 7% 37%	6635
June	1st to 10th	Day	7% 5% 2 6% 4 5% 4 4 7% 10% 47%	2800
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 6% 10% 47%	2800
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 49%	2800
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 7% 9% 46%	5993
	11th to 20th	Day	7% 5% 2 7% 4 4 5% 4 6% 8% 48%	5900
	21st to end	Day	6% 4 2 5% 3 4 4 4 7% 10% 51%	6422
August	1st to 10th	Day	6% 4 2 6% 4 5% 6% 5% 7% 10% 46%	4234
	11th to 20th	Day	12% 6% 2 7% 4 4 5% 4 6% 8% 43%	4171
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 6% 9% 49%	4581
September	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 7% 54%	2332
	11th to 20th	Day	10% 5% 2 5% 3 4 3 3 5% 7% 53%	2200
	21st to end	Day	10% 4 2 5% 3 4 3 3 4 6% 56%	2200
October	1st to 10th	Day	14% 5% 2 4 3 3 3 3 4 7% 52%	2040
	11th to 20th	Day	11% 6% 2 5% 4 3 2 2 4 4 57%	2000
	21st to end	Day	13% 8% 3 6% 4 3 3 3 5% 6% 47%	2200
November	1st to 10th	Day	10% 6% 2 5% 3 3 3 3 4 6% 54%	1888
	11th to 20th	Day	10% 8% 2 7% 5% 3 4 4 4 6% 48%	1816
	21st to end	Day	13% 8% 4 6% 4 3 3 3 5% 7% 43%	1680
December	1st to 10th	Day	6% 5% 2 7% 5% 5% 4 3 5% 6% 52%	2690
	11th to 20th	Day	9% 9% 3 9% 6% 4 4 3 5% 6% 42%	3088
	21st to end	Day	17% 10% 4 13% 6% 4 4 3 4 5% 29%	4192
January	1st to 10th	Night	10% 9% 4 17% 8% 5% 4 3 4 6% 30%	7634
	11th to 20th	Night	5% 7% 3 7% 5% 5% 7% 4 7% 7% 43%	7680
	21st to end	Night	8% 11% 5% 10% 4 4 4 3 4 5% 40%	8402
February	1st to 10th	Night	11% 10% 4 9% 5% 5% 4 3 4 7% 38%	2730
	11th to 20th	Night	9% 8% 4 8% 5% 4 4 4 5% 7% 42%	2652
	21st to end	Night	8% 7% 3 7% 3 4 4 3 5% 5% 51%	2076
March	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 5% 5% 51%	5078
	11th to 20th	Night	9% 6% 2 5% 3 3 3 3 4 6% 56%	5090
	21st to end	Night	14% 9% 3 7% 4 4 4 4 5% 7% 37%	5588
April	1st to 10th	Night	13% 8% 3 5% 4 3 3 3 5% 7% 46%	2242
	11th to 20th	Night	11% 7% 2 6% 3 4 4 3 5% 7% 47%	1966
	21st to end	Night	16% 7% 2 6% 4 3 4 2 3 7% 44%	1890
May	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 5% 50%	4062
	11th to 20th	Night	14% 4 2 4 3 2 3 3 5% 7% 53%	4053
	21st to end	Night	18% 7% 2 5% 4 4 4 3 4 6% 43%	4224
June	1st to 10th	Night	5% 4 1 5% 3 4 5% 3 5% 9% 55%	1600
	11th to 20th	Night	6% 4 1 3 4 4 4 5% 8% 10% 51%	1600
	21st to end	Night	5% 3 1 4 4 3 4 4 6% 8% 56%	1600
July	1st to 10th	Night	7% 4 2 6% 4 3 4 4 6% 8% 53%	3978
	11th to 20th	Night	6% 4 2 5% 3 3 4 3 6% 8% 55%	4077
	21st to end	Night	5% 3 1 5% 4 4 4 4 6% 9% 54%	4449
August	1st to 10th	Night	4 3 2 4 4 3 4 3 6% 10% 58%	2926
	11th to 20th	Night	11% 5% 1 5% 3 3 4 3 5% 8% 52%	2888
	21st to end	Night	7% 4 2 4 3 3 4 3 6% 8% 57%	3325
September	1st to 10th	Night	9% 4 2 4 3 2 3 4 5% 7% 58%	2064
	11th to 20th	Night	9% 3 1 4 3 4 3 3 6% 7% 57%	2200
	21st to end	Night	8% 5% 2 4 2 4 3 3 5% 6% 59%	2200
October	1st to 10th	Night	13% 5% 2 4 3 2 3 4 5% 6% 52%	2338
	11th to 20th	Night	13% 5% 1 4 4 3 3 3 5% 6% 55%	2400
	21st to end	Night	14% 7% 2 4 2 3 4 3 4 5% 51%	2640
November	1st to 10th	Night	9% 4 2 4 3 3 4 3 5% 7% 56%	2625
	11th to 20th	Night	9% 7% 2 5% 4 3 3 3 5% 6% 53%	2801
	21st to end	Night	12% 8% 2 6% 4 3 3 2 5% 5% 50%	2940
December	1st to 10th	Night	5% 6% 2 6% 4 4 4 4 6% 7% 53%	4720
	11th to 20th	Night	10% 8% 3 7% 4 4 4 4 6% 7% 44%	5339
	21st to end	Night	17% 13% 5% 10% 4 4 3 3 4 5% 33%	7010

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	52% 6% 12% 19% 10% 1	5290
	11th to 20th	Day	44% 4 13% 20% 16% 3	5310
	21st to end	Day	47% 5% 15% 22% 11%	5812
February	1st to 10th	Day	48% 7% 15% 19% 9% 1	1890
	11th to 20th	Day	44% 6% 11% 24% 13% 3	1880
	21st to end	Day	38% 6% 13% 25% 16% 2	1730
March	1st to 10th	Day	38% 4 8% 27% 18% 6%	4998
	11th to 20th	Day	33% 5% 9% 26% 23% 4	5090
	21st to end	Day	50% 9% 20% 15% 5% 2	5585
April	1st to 10th	Day	42% 6% 12% 25% 13% 2	2378
	11th to 20th	Day	42% 6% 13% 23% 14% 3	2601
	21st to end	Day	46% 7% 11% 23% 13% 1	2730
May	1st to 10th	Day	37% 8% 11% 26% 16% 2	5914
	11th to 20th	Day	36% 6% 8% 23% 23% 4	5866
	21st to end	Day	43% 8% 11% 22% 15% 1	6649
June	1st to 10th	Day	31% 8% 17% 28% 15% 2	2800
	11th to 20th	Day	33% 7% 14% 30% 16% 1	2800
	21st to end	Day	31% 9% 15% 28% 14% 4	2800
July	1st to 10th	Day	33% 6% 13% 32% 15% 1	5929
	11th to 20th	Day	34% 7% 13% 28% 16% 2	5831
	21st to end	Day	27% 7% 19% 30% 16% 1	6240
August	1st to 10th	Day	30% 8% 15% 27% 17% 1	4220
	11th to 20th	Day	38% 7% 15% 25% 16%	4210
	21st to end	Day	32% 7% 14% 26% 16% 5%	4580
September	1st to 10th	Day	30% 6% 12% 29% 18% 5%	2332
	11th to 20th	Day	31% 6% 13% 31% 18% 1	2200
	21st to end	Day	29% 5% 9% 31% 25% 1	2200
October	1st to 10th	Day	20% 3 7% 19% 47% 3	3326
	11th to 20th	Day	21% 3 12% 30% 27% 7%	3240
	21st to end	Day	35% 4 9% 27% 22% 3	3538
November	1st to 10th	Day	31% 5% 13% 29% 19% 3	1888
	11th to 20th	Day	37% 5% 14% 25% 18% 1	1816
	21st to end	Day	41% 4 13% 28% 13% 1	1680
December	1st to 10th	Day	32% 5% 18% 29% 16% 1	2690
	11th to 20th	Day	43% 5% 17% 25% 10%	3085
	21st to end	Day	56% 5% 13% 19% 6% 1	4192
January	1st to 10th	Night	53% 6% 13% 16% 11% 1	7635
	11th to 20th	Night	37% 5% 12% 26% 19% 2	7680
	21st to end	Night	45% 5% 15% 18% 16% 2	8402
February	1st to 10th	Night	45% 6% 14% 21% 12% 3	2730
	11th to 20th	Night	40% 5% 15% 22% 15% 3	2653
	21st to end	Night	34% 5% 11% 25% 21% 4	2076
March	1st to 10th	Night	35% 5% 7% 28% 23% 3	5078
	11th to 20th	Night	29% 4 7% 27% 25% 7%	5090
	21st to end	Night	43% 8% 19% 22% 7% 1	5588
April	1st to 10th	Night	37% 6% 11% 26% 17% 3	2242
	11th to 20th	Night	36% 6% 12% 25% 17% 3	1966
	21st to end	Night	41% 6% 12% 23% 17% 1	1890
May	1st to 10th	Night	36% 6% 10% 26% 21% 2	4089
	11th to 20th	Night	31% 5% 11% 22% 26% 5%	4054
	21st to end	Night	41% 7% 13% 22% 16% 2	4232
June	1st to 10th	Night	25% 9% 14% 31% 17% 4	1600
	11th to 20th	Night	24% 9% 14% 30% 19% 4	1600
	21st to end	Night	23% 8% 15% 30% 22% 3	1600
July	1st to 10th	Night	29% 6% 13% 31% 18% 3	3935
	11th to 20th	Night	27% 6% 15% 29% 21% 2	4026
	21st to end	Night	25% 8% 17% 29% 20% 1	4320
August	1st to 10th	Night	22% 6% 15% 35% 20% 2	2916
	11th to 20th	Night	32% 5% 14% 29% 19% 2	2915
	21st to end	Night	26% 5% 16% 27% 18% 8%	3325
September	1st to 10th	Night	26% 6% 12% 28% 22% 6%	2064
	11th to 20th	Night	25% 6% 14% 32% 20% 3	2200
	21st to end	Night	26% 6% 8% 27% 29% 4	2200
October	1st to 10th	Night	20% 4 7% 17% 48% 4	3819
	11th to 20th	Night	22% 3 7% 29% 30% 9%	3899
	21st to end	Night	34% 4 9% 29% 23% 2	4252
November	1st to 10th	Night	27% 5% 14% 26% 23% 4	2625
	11th to 20th	Night	32% 4 13% 30% 18% 3	2801
	21st to end	Night	37% 4 12% 28% 18% 2	2940
December	1st to 10th	Night	30% 5% 14% 34% 16% 1	4720
	11th to 20th	Night	38% 5% 16% 28% 13%	5341
	21st to end	Night	54% 5% 11% 22% 8% 1	7010

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% 11% 36% 23% 8% 2	330
	11th to 20th	Day	18% 14% 38% 21% 8% 1	330
	21st to end	Day	14% 10% 45% 22% 7% 1	363
February	1st to 10th	Day	17% 13% 45% 20% 4	330
	11th to 20th	Day	18% 16% 38% 22% 5% 1	330
	21st to end	Day	15% 16% 40% 22% 7%	273
March	1st to 10th	Day	21% 11% 42% 21% 5%	330
	11th to 20th	Day	28% 9% 33% 20% 8% 1	330
	21st to end	Day	32% 12% 36% 17% 2 1	363
April	1st to 10th	Day	24% 16% 39% 18% 4	398
	11th to 20th	Day	21% 15% 36% 21% 7%	639
	21st to end	Day	24% 13% 33% 21% 8% 1	750
May	1st to 10th	Day	20% 9% 32% 26% 11% 2	709
	11th to 20th	Day	20% 7% 31% 26% 14% 2	725
	21st to end	Day	20% 6% 29% 29% 15% 2	789
June	1st to 10th	Day	8% 5% 22% 35% 25% 5%	730
	11th to 20th	Day	9% 4 29% 36% 18% 4	730
	21st to end	Day	10% 5% 33% 35% 14% 2	730
July	1st to 10th	Day	11% 1 15% 34% 30% 9%	721
	11th to 20th	Day	8% 2 22% 39% 22% 8%	718
	21st to end	Day	8% 2 17% 41% 27% 5%	794
August	1st to 10th	Day	8% 1 19% 43% 25% 4	726
	11th to 20th	Day	9% 2 16% 41% 27% 4	715
	21st to end	Day	10% 1 21% 38% 24% 6%	748
September	1st to 10th	Day	13% 3 23% 37% 19% 5%	462
	11th to 20th	Day	8% 2 30% 33% 19% 6%	330
	21st to end	Day	15% 6% 30% 29% 16% 4	330
October	1st to 10th	Day	20% 5% 27% 30% 13% 5%	330
	11th to 20th	Day	13% 14% 44% 19% 8% 1	330
	21st to end	Day	19% 13% 38% 19% 9% 1	363
November	1st to 10th	Day	18% 9% 37% 27% 7% 2	330
	11th to 20th	Day	17% 13% 40% 23% 7%	330
	21st to end	Day	23% 12% 33% 23% 8% 1	330
December	1st to 10th	Day	9% 12% 41% 30% 6% 2	330
	11th to 20th	Day	15% 14% 34% 28% 8% 1	330
	21st to end	Day	20% 12% 40% 23% 4 1	363
January	1st to 10th	Night	13% 10% 36% 34% 7% 1	1050
	11th to 20th	Night	14% 13% 37% 29% 7%	1050
	21st to end	Night	16% 7% 38% 32% 6%	1155
February	1st to 10th	Night	15% 10% 39% 30% 5%	1050
	11th to 20th	Night	13% 10% 42% 30% 5%	1018
	21st to end	Night	14% 9% 45% 26% 5%	692
March	1st to 10th	Night	15% 11% 38% 28% 6%	655
	11th to 20th	Night	19% 9% 35% 30% 7%	630
	21st to end	Night	23% 10% 39% 23% 5%	693
April	1st to 10th	Night	17% 14% 40% 24% 5%	630
	11th to 20th	Night	14% 14% 39% 29% 4	623
	21st to end	Night	21% 8% 34% 30% 7%	630
May	1st to 10th	Night	18% 8% 30% 35% 8%	603
	11th to 20th	Night	16% 9% 28% 37% 10% 1	591
	21st to end	Night	18% 5% 28% 33% 15%	650
June	1st to 10th	Night	8% 3 24% 49% 17%	600
	11th to 20th	Night	9% 2 30% 44% 15% 1	600
	21st to end	Night	6% 3 32% 45% 14% 1	600
July	1st to 10th	Night	8% 1 16% 43% 30% 2	585
	11th to 20th	Night	5% 1 19% 49% 26% 1	582
	21st to end	Night	5% 15% 53% 27% 1	657
August	1st to 10th	Night	5% 1 18% 49% 24% 2	597
	11th to 20th	Night	7% 2 16% 49% 25% 1	594
	21st to end	Night	10% 1 18% 42% 24% 3	660
September	1st to 10th	Night	10% 3 22% 40% 23% 2	600
	11th to 20th	Night	10% 2 30% 40% 18% 1	600
	21st to end	Night	13% 3 30% 36% 18% 1	600
October	1st to 10th	Night	14% 7% 31% 34% 12% 2	738
	11th to 20th	Night	14% 9% 36% 34% 7%	800
	21st to end	Night	19% 9% 31% 32% 9%	880
November	1st to 10th	Night	12% 9% 33% 36% 10% 1	985
	11th to 20th	Night	13% 11% 38% 31% 6%	1050
	21st to end	Night	17% 9% 33% 31% 9%	1050
December	1st to 10th	Night	13% 10% 39% 32% 6%	1050
	11th to 20th	Night	17% 9% 35% 33% 6% 1	1050
	21st to end	Night	20% 10% 32% 32% 4	1155

Page generated at 2026-04-29 01:21:42. Version ac07

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