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 2024-01-31.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 34%	75060
	Night	16% 9% 4 7% 6% 3 4 4 5% 7% 35%	75083
Transparency	Day	48% 7% 12% 19% 12% 1	74771
	Night	48% 7% 12% 19% 12% 2	74893
Seeing	Day	17% 10% 27% 24% 15% 6%	14487
	Night	19% 9% 29% 29% 13% 1	24449

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	17% 15% 6% 11% 9% 4 4 4 4 6% 20%	1517
	11th to 20th	Day	18% 18% 7% 10% 8% 4 5% 4 4 5% 16%	1520
	21st to end	Day	18% 17% 8% 10% 6% 4 5% 4 4 5% 20%	1846
February	1st to 10th	Day	16% 15% 6% 10% 9% 4 5% 4 4 6% 21%	1636
	11th to 20th	Day	16% 11% 5% 11% 9% 5% 5% 4 4 6% 23%	1748
	21st to end	Day	18% 12% 6% 8% 7% 3 4 5% 6% 7% 25%	1480
March	1st to 10th	Day	17% 12% 5% 9% 8% 3 4 4 5% 5% 27%	1814
	11th to 20th	Day	16% 12% 4 7% 6% 3 3 4 4 7% 32%	1980
	21st to end	Day	24% 12% 4 7% 5% 3 4 4 5% 6% 25%	2178
April	1st to 10th	Day	15% 11% 4 7% 7% 4 5% 5% 6% 6% 31%	2125
	11th to 20th	Day	18% 8% 3 6% 6% 4 5% 5% 4 8% 35%	2230
	21st to end	Day	19% 8% 3 5% 6% 3 4 5% 5% 7% 34%	2340
May	1st to 10th	Day	17% 8% 3 7% 7% 4 5% 6% 5% 7% 31%	2340
	11th to 20th	Day	19% 7% 3 7% 6% 3 5% 6% 6% 6% 34%	2301
	21st to end	Day	9% 6% 3 6% 6% 4 6% 6% 7% 7% 40%	2531
June	1st to 10th	Day	11% 5% 2 6% 6% 4 6% 6% 6% 7% 40%	2492
	11th to 20th	Day	10% 4 2 6% 6% 4 4 5% 7% 9% 42%	2520
	21st to end	Day	11% 5% 3 5% 6% 4 6% 7% 7% 9% 39%	2520
July	1st to 10th	Day	9% 5% 2 6% 6% 3 5% 5% 6% 9% 44%	2520
	11th to 20th	Day	9% 4 3 4 5% 3 5% 5% 8% 9% 46%	2520
	21st to end	Day	8% 5% 2 6% 6% 3 5% 6% 7% 9% 42%	2583
August	1st to 10th	Day	7% 4 2 5% 5% 4 6% 6% 6% 9% 46%	2327
	11th to 20th	Day	8% 5% 2 5% 6% 4 6% 6% 7% 10% 40%	2303
	21st to end	Day	8% 5% 2 5% 5% 4 5% 6% 6% 10% 45%	2476
September	1st to 10th	Day	12% 6% 2 6% 5% 4 4 5% 6% 7% 43%	2055
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 5% 6% 8% 46%	1980
	21st to end	Day	14% 6% 2 4 5% 3 4 5% 7% 7% 43%	1980
October	1st to 10th	Day	12% 7% 3 5% 5% 3 4 5% 5% 7% 42%	1972
	11th to 20th	Day	16% 8% 3 5% 6% 3 4 4 6% 6% 38%	1800
	21st to end	Day	18% 10% 4 6% 6% 4 6% 5% 5% 6% 31%	1815
November	1st to 10th	Day	11% 8% 3 6% 4 3 4 4 6% 8% 43%	1672
	11th to 20th	Day	16% 12% 5% 7% 6% 3 4 5% 4 5% 34%	1710
	21st to end	Day	20% 14% 4 8% 5% 2 5% 4 5% 6% 27%	1710
December	1st to 10th	Day	16% 15% 8% 11% 8% 4 5% 4 4 5% 21%	1710
	11th to 20th	Day	14% 14% 7% 11% 6% 3 6% 5% 5% 6% 23%	2026
	21st to end	Day	22% 22% 8% 10% 7% 3 4 4 3 4 12%	2783
January	1st to 10th	Night	18% 13% 6% 9% 7% 4 4 4 5% 5% 25%	2648
	11th to 20th	Night	18% 15% 5% 11% 7% 4 5% 4 4 5% 22%	2660
	21st to end	Night	17% 16% 8% 10% 7% 3 4 4 4 5% 22%	2734
February	1st to 10th	Night	19% 14% 6% 9% 7% 4 4 4 5% 7% 23%	2322
	11th to 20th	Night	14% 14% 6% 8% 7% 3 5% 5% 5% 6% 27%	2098
	21st to end	Night	22% 12% 4 7% 6% 4 3 4 4 6% 29%	1776
March	1st to 10th	Night	17% 9% 4 7% 8% 4 4 4 4 6% 33%	2146
	11th to 20th	Night	15% 8% 2 5% 5% 3 5% 5% 5% 7% 40%	1980
	21st to end	Night	21% 9% 4 7% 5% 3 3 4 4 7% 34%	2178
April	1st to 10th	Night	15% 9% 4 5% 4 4 5% 5% 6% 7% 35%	1850
	11th to 20th	Night	18% 6% 3 6% 6% 4 5% 5% 5% 6% 36%	1678
	21st to end	Night	18% 8% 3 6% 5% 3 4 5% 4 8% 37%	1620
May	1st to 10th	Night	18% 8% 4 8% 5% 4 6% 5% 5% 7% 31%	1620
	11th to 20th	Night	18% 7% 3 5% 4 3 4 5% 5% 8% 38%	1592
	21st to end	Night	9% 6% 2 5% 4 3 6% 6% 6% 9% 45%	1744
June	1st to 10th	Night	10% 4 2 6% 4 3 5% 5% 6% 8% 46%	1464
	11th to 20th	Night	9% 4 2 5% 6% 3 6% 4 6% 8% 46%	1440
	21st to end	Night	10% 4 1 5% 5% 3 5% 6% 7% 9% 46%	1440
July	1st to 10th	Night	7% 4 2 4 6% 3 5% 5% 5% 8% 50%	1440
	11th to 20th	Night	8% 4 2 4 4 3 4 4 5% 10% 52%	1440
	21st to end	Night	6% 3 2 5% 5% 3 4 5% 7% 9% 50%	1731
August	1st to 10th	Night	6% 4 2 3 4 3 5% 6% 6% 10% 50%	1611
	11th to 20th	Night	8% 4 2 4 5% 3 5% 5% 6% 8% 49%	1598
	21st to end	Night	8% 4 1 5% 5% 3 4 5% 6% 7% 52%	1876
September	1st to 10th	Night	12% 5% 2 5% 4 3 4 4 5% 7% 49%	1886
	11th to 20th	Night	10% 5% 3 4 4 2 4 4 4 7% 54%	1980
	21st to end	Night	12% 6% 2 5% 6% 3 5% 4 3 7% 47%	1980
October	1st to 10th	Night	12% 6% 2 6% 5% 3 3 4 5% 7% 46%	1987
	11th to 20th	Night	17% 9% 3 6% 5% 3 4 4 5% 5% 38%	2160
	21st to end	Night	21% 8% 3 5% 5% 2 4 4 5% 6% 36%	2538
November	1st to 10th	Night	13% 8% 3 5% 4 3 4 4 4 7% 44%	2406
	11th to 20th	Night	17% 11% 4 7% 4 3 5% 4 5% 7% 33%	2470
	21st to end	Night	20% 12% 4 8% 6% 3 5% 4 5% 6% 27%	2470
December	1st to 10th	Night	17% 13% 6% 10% 7% 4 5% 4 5% 8% 22%	2470
	11th to 20th	Night	18% 14% 6% 8% 6% 4 5% 4 4 5% 25%	3166
	21st to end	Night	26% 19% 7% 9% 6% 3 4 4 4 5% 13%	4884

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	66% 6% 11% 13% 4	1518
	11th to 20th	Day	70% 6% 12% 7% 4	1520
	21st to end	Day	66% 6% 10% 12% 6%	1846
February	1st to 10th	Day	65% 6% 9% 13% 5% 1	1636
	11th to 20th	Day	63% 6% 9% 15% 7%	1758
	21st to end	Day	59% 6% 13% 15% 6% 2	1480
March	1st to 10th	Day	59% 6% 11% 19% 6%	1814
	11th to 20th	Day	52% 6% 12% 19% 10% 2	1980
	21st to end	Day	59% 6% 11% 19% 5%	2178
April	1st to 10th	Day	52% 7% 11% 17% 13%	2125
	11th to 20th	Day	48% 6% 11% 20% 13% 2	2230
	21st to end	Day	48% 8% 12% 20% 11% 3	2340
May	1st to 10th	Day	51% 7% 12% 18% 11% 1	2340
	11th to 20th	Day	49% 8% 10% 19% 13% 2	2305
	21st to end	Day	41% 9% 14% 22% 13% 2	2531
June	1st to 10th	Day	40% 8% 11% 24% 14% 2	2492
	11th to 20th	Day	37% 9% 13% 22% 17% 2	2520
	21st to end	Day	39% 11% 13% 19% 14% 4	2520
July	1st to 10th	Day	37% 8% 11% 22% 18% 4	2451
	11th to 20th	Day	32% 10% 14% 25% 17% 2	2445
	21st to end	Day	35% 9% 14% 22% 17% 3	2439
August	1st to 10th	Day	31% 10% 16% 26% 15% 1	2314
	11th to 20th	Day	37% 10% 14% 25% 14% 1	2303
	21st to end	Day	33% 8% 12% 24% 18% 4	2476
September	1st to 10th	Day	39% 7% 14% 20% 16% 3	2055
	11th to 20th	Day	34% 7% 13% 27% 19% 1	1980
	21st to end	Day	38% 6% 13% 24% 18% 1	1980
October	1st to 10th	Day	40% 7% 11% 25% 15% 2	1972
	11th to 20th	Day	46% 6% 11% 22% 15% 1	1800
	21st to end	Day	52% 7% 13% 17% 10% 1	1815
November	1st to 10th	Day	40% 6% 11% 25% 17% 1	1672
	11th to 20th	Day	53% 6% 12% 17% 12% 2	1710
	21st to end	Day	58% 6% 12% 16% 8%	1710
December	1st to 10th	Day	66% 6% 11% 12% 4	1710
	11th to 20th	Day	59% 9% 13% 15% 4	2023
	21st to end	Day	75% 7% 8% 6% 3	2783
January	1st to 10th	Night	61% 7% 9% 16% 8%	2648
	11th to 20th	Night	65% 6% 12% 12% 4	2660
	21st to end	Night	64% 5% 12% 13% 6% 1	2734
February	1st to 10th	Night	63% 6% 12% 14% 5% 1	2322
	11th to 20th	Night	57% 6% 10% 18% 9% 1	2099
	21st to end	Night	57% 5% 11% 17% 9% 1	1776
March	1st to 10th	Night	53% 6% 11% 20% 10% 1	2146
	11th to 20th	Night	44% 7% 13% 19% 14% 3	1980
	21st to end	Night	51% 5% 8% 24% 10% 1	2178
April	1st to 10th	Night	46% 7% 12% 19% 14% 2	1850
	11th to 20th	Night	47% 7% 10% 17% 18% 2	1678
	21st to end	Night	45% 6% 11% 20% 15% 2	1620
May	1st to 10th	Night	53% 7% 11% 16% 11% 2	1620
	11th to 20th	Night	45% 8% 10% 19% 18% 2	1593
	21st to end	Night	34% 9% 15% 25% 14% 3	1745
June	1st to 10th	Night	34% 7% 13% 24% 19% 3	1464
	11th to 20th	Night	34% 9% 13% 24% 17% 3	1440
	21st to end	Night	32% 10% 16% 23% 17% 2	1440
July	1st to 10th	Night	31% 8% 12% 24% 21% 3	1400
	11th to 20th	Night	28% 8% 16% 26% 20% 3	1392
	21st to end	Night	28% 8% 16% 25% 18% 4	1635
August	1st to 10th	Night	27% 9% 16% 29% 18% 2	1600
	11th to 20th	Night	31% 7% 16% 26% 16% 3	1598
	21st to end	Night	30% 8% 13% 23% 22% 4	1876
September	1st to 10th	Night	35% 7% 13% 24% 17% 4	1886
	11th to 20th	Night	30% 6% 10% 27% 25% 1	1980
	21st to end	Night	37% 7% 11% 25% 19% 2	1980
October	1st to 10th	Night	38% 6% 11% 24% 20% 1	1987
	11th to 20th	Night	47% 6% 9% 21% 16% 1	2160
	21st to end	Night	49% 5% 10% 23% 11% 2	2538
November	1st to 10th	Night	40% 6% 12% 24% 17% 1	2406
	11th to 20th	Night	50% 6% 13% 19% 12% 1	2470
	21st to end	Night	57% 6% 11% 19% 6%	2470
December	1st to 10th	Night	61% 7% 11% 15% 7%	2470
	11th to 20th	Night	60% 7% 13% 13% 6% 1	3168
	21st to end	Night	73% 7% 10% 8% 3	4884

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% 21% 31% 21% 4 1	270
	11th to 20th	Day	23% 18% 43% 12% 4 1	270
	21st to end	Day	31% 12% 30% 20% 7%	294
February	1st to 10th	Day	27% 18% 39% 15% 1	240
	11th to 20th	Day	24% 19% 37% 17% 3	240
	21st to end	Day	25% 22% 40% 11% 2	198
March	1st to 10th	Day	25% 17% 35% 19% 4	254
	11th to 20th	Day	26% 11% 33% 22% 7% 2	420
	21st to end	Day	31% 15% 29% 18% 7% 1	462
April	1st to 10th	Day	20% 15% 29% 24% 10% 2	420
	11th to 20th	Day	22% 17% 29% 18% 9% 4	412
	21st to end	Day	23% 14% 30% 19% 11% 2	420
May	1st to 10th	Day	24% 11% 29% 22% 10% 3	423
	11th to 20th	Day	21% 9% 27% 24% 13% 7%	447
	21st to end	Day	12% 7% 26% 26% 18% 12%	485
June	1st to 10th	Day	11% 7% 28% 32% 17% 5%	602
	11th to 20th	Day	13% 6% 28% 27% 21% 5%	630
	21st to end	Day	13% 6% 25% 31% 18% 7%	630
July	1st to 10th	Day	10% 3 19% 31% 24% 14%	621
	11th to 20th	Day	8% 5% 22% 31% 22% 12%	618
	21st to end	Day	9% 4 22% 26% 26% 13%	524
August	1st to 10th	Day	7% 3 16% 29% 27% 18%	446
	11th to 20th	Day	9% 5% 20% 29% 24% 13%	439
	21st to end	Day	9% 3 19% 27% 26% 15%	495
September	1st to 10th	Day	14% 4 19% 26% 24% 13%	450
	11th to 20th	Day	11% 2 16% 24% 30% 16%	450
	21st to end	Day	15% 8% 22% 25% 20% 9%	450
October	1st to 10th	Day	13% 7% 24% 30% 17% 9%	450
	11th to 20th	Day	18% 17% 32% 22% 10% 1	450
	21st to end	Day	23% 15% 36% 21% 3 2	330
November	1st to 10th	Day	13% 14% 35% 23% 11% 4	270
	11th to 20th	Day	24% 19% 29% 19% 10% 1	270
	21st to end	Day	27% 14% 37% 18% 3	270
December	1st to 10th	Day	19% 24% 33% 15% 9%	270
	11th to 20th	Day	21% 19% 35% 19% 6%	270
	21st to end	Day	27% 21% 30% 19% 3	297
January	1st to 10th	Night	23% 14% 35% 21% 6%	950
	11th to 20th	Night	26% 13% 37% 19% 4 1	950
	21st to end	Night	22% 14% 36% 23% 5%	858
February	1st to 10th	Night	26% 12% 35% 24% 3	720
	11th to 20th	Night	22% 14% 37% 25% 3	703
	21st to end	Night	27% 14% 38% 20% 2	592
March	1st to 10th	Night	22% 10% 34% 26% 7%	720
	11th to 20th	Night	18% 13% 38% 25% 6%	720
	21st to end	Night	23% 10% 32% 26% 9%	792
April	1st to 10th	Night	19% 13% 36% 28% 4 1	590
	11th to 20th	Night	21% 9% 35% 25% 9% 1	533
	21st to end	Night	21% 8% 35% 24% 11% 1	540
May	1st to 10th	Night	21% 7% 31% 30% 9% 1	540
	11th to 20th	Night	21% 4 29% 29% 16% 1	531
	21st to end	Night	13% 3 22% 39% 21% 3	584
June	1st to 10th	Night	11% 3 23% 45% 18% 1	540
	11th to 20th	Night	10% 4 24% 41% 19% 1	540
	21st to end	Night	11% 3 23% 39% 22% 2	540
July	1st to 10th	Night	7% 3 18% 38% 30% 4	525
	11th to 20th	Night	9% 3 15% 41% 28% 4	522
	21st to end	Night	7% 2 15% 43% 31% 2	591
August	1st to 10th	Night	7% 2 14% 43% 31% 4	537
	11th to 20th	Night	10% 2 15% 38% 30% 5%	534
	21st to end	Night	9% 2 13% 36% 36% 4	594
September	1st to 10th	Night	14% 4 15% 34% 28% 4	626
	11th to 20th	Night	10% 4 16% 34% 33% 4	720
	21st to end	Night	14% 6% 21% 34% 23% 3	720
October	1st to 10th	Night	15% 6% 25% 33% 20% 2	720
	11th to 20th	Night	21% 13% 29% 27% 9% 1	720
	21st to end	Night	25% 12% 32% 24% 7%	792
November	1st to 10th	Night	17% 10% 33% 27% 11% 1	740
	11th to 20th	Night	23% 14% 30% 25% 7% 1	760
	21st to end	Night	26% 12% 30% 26% 6%	760
December	1st to 10th	Night	23% 14% 36% 22% 5%	760
	11th to 20th	Night	26% 12% 36% 22% 4	840
	21st to end	Night	29% 14% 32% 22% 3	1045

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