Black Swamp Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Swamp Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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	14% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	131845
	Night	15% 10% 4 9% 5% 5% 4 4 5% 7% 34%	131445
Transparency	Day	48% 6% 12% 21% 12% 1	135430
	Night	48% 6% 11% 20% 13% 1	135919
Seeing	Day	17% 11% 28% 24% 15% 6%	17681
	Night	19% 9% 29% 29% 13% 1	27558

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	15% 17% 7% 17% 5% 5% 4 3 5% 5% 16%	4691
	11th to 20th	Day	12% 16% 6% 13% 9% 6% 6% 4 4 4 20%	4710
	21st to end	Day	19% 16% 6% 12% 7% 6% 6% 4 5% 4 16%	5704
February	1st to 10th	Day	14% 16% 6% 11% 7% 5% 5% 3 5% 6% 23%	1909
	11th to 20th	Day	15% 12% 5% 13% 7% 6% 4 3 4 6% 24%	2048
	21st to end	Day	16% 13% 5% 9% 6% 4 4 5% 6% 7% 26%	1730
March	1st to 10th	Day	18% 13% 5% 11% 6% 4 4 3 4 5% 27%	4623
	11th to 20th	Day	11% 11% 3 7% 5% 4 4 4 5% 6% 39%	5090
	21st to end	Day	18% 13% 4 9% 5% 5% 5% 3 6% 5% 25%	5586
April	1st to 10th	Day	16% 12% 3 8% 6% 5% 5% 4 6% 6% 29%	2480
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 4 5% 7% 34%	2607
	21st to end	Day	17% 9% 3 7% 5% 4 5% 4 5% 7% 35%	2730
May	1st to 10th	Day	22% 10% 3 7% 5% 4 5% 4 5% 6% 30%	5992
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 4 6% 8% 36%	5991
	21st to end	Day	15% 6% 2 7% 4 4 5% 4 6% 7% 39%	6576
June	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 4 6% 6% 41%	2770
	11th to 20th	Day	10% 5% 2 7% 6% 5% 4 4 7% 9% 41%	2800
	21st to end	Day	10% 6% 2 5% 5% 5% 6% 5% 7% 9% 39%	2800
July	1st to 10th	Day	9% 5% 2 7% 4 4 5% 4 6% 11% 41%	6360
	11th to 20th	Day	8% 4 2 6% 5% 5% 5% 5% 7% 10% 43%	6360
	21st to end	Day	9% 6% 2 8% 5% 5% 5% 5% 7% 10% 39%	6516
August	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 5% 6% 9% 45%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 4 6% 5% 8% 10% 42%	4174
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 6% 10% 46%	4487
September	1st to 10th	Day	11% 6% 2 6% 4 4 4 3 6% 7% 46%	2283
	11th to 20th	Day	9% 5% 2 5% 3 4 5% 4 6% 8% 49%	2200
	21st to end	Day	15% 7% 2 5% 4 4 4 4 6% 6% 41%	2200
October	1st to 10th	Day	11% 7% 2 5% 5% 4 4 4 5% 7% 45%	2190
	11th to 20th	Day	16% 8% 2 6% 5% 4 5% 3 6% 6% 40%	2000
	21st to end	Day	17% 10% 3 7% 5% 5% 5% 4 6% 6% 33%	2016
November	1st to 10th	Day	11% 9% 2 7% 4 4 4 3 6% 8% 43%	1852
	11th to 20th	Day	15% 13% 4 8% 5% 4 4 4 4 5% 33%	1890
	21st to end	Day	19% 16% 4 9% 5% 4 4 3 5% 6% 26%	1890
December	1st to 10th	Day	13% 14% 6% 12% 7% 6% 4 3 5% 6% 25%	3020
	11th to 20th	Day	18% 16% 7% 11% 5% 4 5% 3 5% 5% 22%	3266
	21st to end	Day	25% 20% 7% 11% 5% 4 4 3 3 4 14%	4070
January	1st to 10th	Night	14% 16% 6% 15% 6% 5% 5% 3 4 4 20%	8232
	11th to 20th	Night	10% 16% 7% 16% 7% 5% 5% 4 4 5% 20%	8280
	21st to end	Night	17% 14% 5% 10% 6% 5% 5% 4 5% 6% 22%	8465
February	1st to 10th	Night	17% 14% 5% 10% 7% 5% 3 3 5% 6% 25%	2709
	11th to 20th	Night	14% 14% 5% 9% 6% 5% 5% 4 5% 6% 28%	2458
	21st to end	Night	19% 13% 4 9% 5% 4 3 3 5% 6% 29%	2076
March	1st to 10th	Night	18% 11% 4 7% 5% 4 3 3 4 6% 35%	5450
	11th to 20th	Night	11% 7% 2 6% 5% 5% 5% 4 5% 7% 44%	5090
	21st to end	Night	16% 9% 4 9% 4 4 4 3 4 6% 35%	5588
April	1st to 10th	Night	17% 10% 3 6% 4 5% 4 4 6% 6% 34%	2158
	11th to 20th	Night	18% 7% 2 7% 5% 5% 4 4 6% 6% 36%	1960
	21st to end	Night	17% 9% 2 7% 4 4 4 4 4 7% 38%	1890
May	1st to 10th	Night	21% 10% 4 9% 5% 5% 5% 3 4 6% 29%	4123
	11th to 20th	Night	18% 7% 3 6% 4 4 3 3 5% 7% 39%	4122
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 8% 43%	4520
June	1st to 10th	Night	11% 5% 2 6% 4 4 4 4 6% 8% 46%	1626
	11th to 20th	Night	9% 4 2 6% 5% 5% 5% 3 6% 9% 45%	1600
	21st to end	Night	10% 4 2 6% 3 4 5% 5% 7% 9% 46%	1600
July	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 7% 9% 48%	3618
	11th to 20th	Night	8% 5% 2 6% 4 5% 4 4 5% 11% 45%	3617
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 6% 9% 46%	4335
August	1st to 10th	Night	10% 7% 3 6% 4 4 4 4 6% 8% 44%	2915
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 48%	2875
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 7% 50%	3407
September	1st to 10th	Night	11% 4 2 5% 3 4 3 3 5% 6% 52%	2096
	11th to 20th	Night	9% 5% 2 5% 3 4 3 3 4 7% 56%	2200
	21st to end	Night	13% 7% 2 6% 5% 4 5% 3 4 6% 45%	2200
October	1st to 10th	Night	11% 6% 2 7% 4 4 3 3 4 6% 49%	2209
	11th to 20th	Night	16% 9% 3 7% 5% 4 4 3 5% 5% 40%	2400
	21st to end	Night	20% 8% 3 6% 4 3 4 3 5% 6% 38%	2820
November	1st to 10th	Night	13% 8% 2 6% 3 4 4 4 5% 6% 45%	2666
	11th to 20th	Night	16% 11% 3 8% 4 4 5% 4 5% 7% 33%	2730
	21st to end	Night	19% 13% 4 8% 6% 4 4 3 5% 6% 26%	2730
December	1st to 10th	Night	13% 13% 5% 11% 6% 5% 5% 4 5% 7% 25%	4380
	11th to 20th	Night	17% 19% 7% 10% 5% 5% 4 3 4 5% 22%	5150
	21st to end	Night	26% 19% 6% 10% 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	69% 6% 11% 11% 3	4692
	11th to 20th	Day	68% 4 8% 7% 12%	4710
	21st to end	Day	70% 4 9% 10% 6%	5704
February	1st to 10th	Day	62% 6% 10% 15% 6% 1	1909
	11th to 20th	Day	62% 6% 8% 16% 7%	2058
	21st to end	Day	57% 6% 14% 15% 6% 2	1730
March	1st to 10th	Day	60% 5% 9% 18% 7% 1	4623
	11th to 20th	Day	44% 6% 14% 25% 9% 1	5090
	21st to end	Day	58% 6% 12% 16% 8%	5586
April	1st to 10th	Day	53% 7% 11% 16% 12% 1	2480
	11th to 20th	Day	48% 6% 11% 21% 12% 2	2607
	21st to end	Day	47% 7% 12% 21% 11% 2	2730
May	1st to 10th	Day	53% 7% 10% 17% 13%	6027
	11th to 20th	Day	43% 8% 14% 22% 12% 1	5995
	21st to end	Day	41% 7% 10% 21% 17% 3	6590
June	1st to 10th	Day	40% 8% 11% 23% 16% 2	2770
	11th to 20th	Day	38% 9% 13% 22% 16% 3	2800
	21st to end	Day	38% 10% 13% 20% 15% 4	2800
July	1st to 10th	Day	36% 6% 11% 27% 18% 2	6291
	11th to 20th	Day	34% 8% 14% 26% 17% 1	6285
	21st to end	Day	38% 7% 12% 22% 19% 2	6330
August	1st to 10th	Day	34% 8% 16% 27% 15% 1	4221
	11th to 20th	Day	34% 8% 13% 29% 15% 1	4213
	21st to end	Day	32% 7% 12% 34% 12% 3	4486
September	1st to 10th	Day	37% 7% 13% 23% 17% 3	2283
	11th to 20th	Day	32% 6% 12% 28% 20% 1	2200
	21st to end	Day	40% 6% 13% 22% 18% 1	2200
October	1st to 10th	Day	28% 5% 12% 34% 21% 1	3567
	11th to 20th	Day	38% 5% 10% 27% 20% 1	3240
	21st to end	Day	42% 5% 12% 25% 16% 1	3228
November	1st to 10th	Day	40% 6% 11% 26% 17% 1	1852
	11th to 20th	Day	53% 6% 11% 17% 11% 2	1890
	21st to end	Day	59% 5% 11% 17% 7%	1890
December	1st to 10th	Day	60% 6% 16% 15% 3	3020
	11th to 20th	Day	64% 6% 10% 15% 5%	3263
	21st to end	Day	74% 6% 8% 9% 4	4070
January	1st to 10th	Night	66% 6% 8% 15% 5%	8233
	11th to 20th	Night	66% 4 10% 10% 9% 1	8280
	21st to end	Night	61% 5% 14% 12% 7%	8465
February	1st to 10th	Night	60% 6% 12% 15% 6% 1	2709
	11th to 20th	Night	56% 6% 10% 19% 8% 1	2459
	21st to end	Night	56% 5% 12% 17% 9% 1	2076
March	1st to 10th	Night	50% 5% 10% 25% 9% 1	5450
	11th to 20th	Night	37% 7% 15% 26% 14% 1	5090
	21st to end	Night	48% 6% 11% 20% 13% 1	5588
April	1st to 10th	Night	48% 7% 11% 19% 13% 2	2158
	11th to 20th	Night	47% 6% 10% 16% 18% 2	1960
	21st to end	Night	45% 7% 11% 21% 15% 2	1890
May	1st to 10th	Night	55% 7% 8% 14% 12% 3	4150
	11th to 20th	Night	43% 8% 11% 23% 14% 1	4123
	21st to end	Night	37% 8% 12% 23% 18% 2	4528
June	1st to 10th	Night	34% 7% 12% 24% 19% 3	1626
	11th to 20th	Night	35% 8% 13% 25% 17% 3	1600
	21st to end	Night	32% 9% 16% 23% 18% 2	1600
July	1st to 10th	Night	31% 7% 12% 26% 19% 5%	3580
	11th to 20th	Night	33% 7% 16% 25% 17% 2	3572
	21st to end	Night	34% 6% 14% 23% 19% 5%	4208
August	1st to 10th	Night	37% 7% 14% 25% 16% 2	2904
	11th to 20th	Night	33% 6% 13% 29% 17% 1	2902
	21st to end	Night	32% 7% 12% 29% 18% 2	3407
September	1st to 10th	Night	33% 6% 12% 27% 18% 4	2096
	11th to 20th	Night	28% 6% 10% 29% 26% 2	2200
	21st to end	Night	39% 6% 11% 23% 19% 2	2200
October	1st to 10th	Night	25% 4 8% 30% 32% 1	3612
	11th to 20th	Night	37% 4 10% 26% 22% 1	3899
	21st to end	Night	35% 4 8% 34% 17% 2	4546
November	1st to 10th	Night	40% 6% 12% 25% 17% 1	2666
	11th to 20th	Night	49% 5% 12% 19% 12% 2	2730
	21st to end	Night	58% 6% 11% 19% 6%	2730
December	1st to 10th	Night	57% 6% 12% 18% 6% 1	4380
	11th to 20th	Night	64% 5% 12% 11% 7% 1	5152
	21st to end	Night	73% 6% 9% 9% 4	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% 19% 34% 19% 5% 1	330
	11th to 20th	Day	21% 21% 43% 11% 3 1	330
	21st to end	Day	26% 15% 33% 20% 7%	363
February	1st to 10th	Day	23% 17% 39% 17% 4 1	330
	11th to 20th	Day	23% 17% 33% 22% 4	330
	21st to end	Day	23% 22% 41% 12% 3	273
March	1st to 10th	Day	24% 17% 37% 18% 3	347
	11th to 20th	Day	27% 13% 33% 20% 6% 1	540
	21st to end	Day	27% 17% 32% 18% 6% 1	594
April	1st to 10th	Day	22% 16% 30% 21% 9% 1	540
	11th to 20th	Day	22% 18% 30% 18% 8% 3	532
	21st to end	Day	21% 15% 30% 20% 11% 2	540
May	1st to 10th	Day	24% 12% 31% 21% 10% 2	543
	11th to 20th	Day	20% 10% 27% 24% 13% 6%	567
	21st to end	Day	13% 7% 28% 25% 17% 10%	617
June	1st to 10th	Day	11% 9% 28% 30% 17% 5%	700
	11th to 20th	Day	13% 6% 28% 29% 19% 4	730
	21st to end	Day	13% 6% 25% 32% 18% 7%	730
July	1st to 10th	Day	10% 4 19% 31% 24% 12%	721
	11th to 20th	Day	9% 5% 23% 30% 21% 12%	718
	21st to end	Day	9% 4 20% 27% 28% 13%	616
August	1st to 10th	Day	7% 3 17% 28% 28% 17%	526
	11th to 20th	Day	9% 4 19% 29% 26% 13%	519
	21st to end	Day	9% 3 17% 28% 28% 15%	583
September	1st to 10th	Day	12% 5% 19% 26% 23% 15%	530
	11th to 20th	Day	10% 2 14% 24% 31% 19%	530
	21st to end	Day	18% 8% 21% 24% 20% 10%	530
October	1st to 10th	Day	12% 7% 22% 30% 20% 10%	530
	11th to 20th	Day	18% 15% 30% 25% 11% 2	530
	21st to end	Day	22% 14% 37% 21% 5% 2	399
November	1st to 10th	Day	15% 15% 35% 22% 10% 3	330
	11th to 20th	Day	25% 18% 31% 17% 8% 1	330
	21st to end	Day	27% 15% 38% 17% 3	330
December	1st to 10th	Day	19% 21% 33% 19% 8%	330
	11th to 20th	Day	22% 19% 35% 18% 5%	330
	21st to end	Day	30% 19% 30% 17% 3	363
January	1st to 10th	Night	23% 14% 35% 22% 6%	1050
	11th to 20th	Night	25% 15% 38% 19% 4 1	1050
	21st to end	Night	21% 15% 38% 22% 4	949
February	1st to 10th	Night	25% 12% 35% 25% 4	840
	11th to 20th	Night	21% 13% 38% 26% 2	823
	21st to end	Night	26% 14% 38% 20% 2	692
March	1st to 10th	Night	22% 11% 35% 25% 7%	840
	11th to 20th	Night	18% 15% 38% 23% 6%	840
	21st to end	Night	23% 12% 32% 25% 7%	924
April	1st to 10th	Night	21% 14% 35% 26% 3 1	688
	11th to 20th	Night	21% 10% 36% 24% 9% 1	623
	21st to end	Night	20% 10% 36% 25% 10%	630
May	1st to 10th	Night	21% 7% 33% 30% 8% 1	630
	11th to 20th	Night	20% 5% 30% 29% 15% 1	621
	21st to end	Night	13% 4 22% 40% 19% 2	683
June	1st to 10th	Night	12% 3 24% 44% 17% 1	600
	11th to 20th	Night	10% 4 24% 42% 19% 1	600
	21st to end	Night	11% 3 24% 39% 22% 2	600
July	1st to 10th	Night	7% 3 18% 40% 29% 4	585
	11th to 20th	Night	9% 3 15% 40% 28% 4	582
	21st to end	Night	8% 2 14% 42% 32% 2	657
August	1st to 10th	Night	7% 2 14% 44% 30% 4	597
	11th to 20th	Night	11% 2 15% 38% 30% 4	594
	21st to end	Night	8% 2 13% 38% 36% 4	660
September	1st to 10th	Night	13% 5% 16% 34% 28% 4	696
	11th to 20th	Night	9% 4 15% 34% 35% 4	800
	21st to end	Night	16% 6% 20% 33% 22% 3	800
October	1st to 10th	Night	14% 5% 25% 33% 21% 2	800
	11th to 20th	Night	20% 12% 29% 27% 11% 1	800
	21st to end	Night	23% 11% 32% 26% 7%	880
November	1st to 10th	Night	17% 10% 33% 28% 11% 1	820
	11th to 20th	Night	22% 14% 31% 25% 7% 1	840
	21st to end	Night	26% 13% 31% 24% 6%	840
December	1st to 10th	Night	23% 14% 36% 22% 5%	840
	11th to 20th	Night	25% 12% 36% 23% 4	929
	21st to end	Night	29% 14% 32% 22% 3	1155

Page generated at 2026-05-29 01:53:27. Version ac07

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