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 2025-02-06.

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% 4 8% 5% 5% 5% 4 6% 7% 33%	108541
	Night	15% 11% 4 8% 5% 5% 4 4 5% 6% 34%	109811
Transparency	Day	47% 7% 12% 20% 12% 1	112170
	Night	48% 6% 11% 20% 13% 1	114310
Seeing	Day	17% 11% 28% 24% 15% 6%	15880
	Night	19% 9% 29% 29% 13% 1	25816

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% 17% 8% 19% 6% 5% 4 3 5% 4 18%	3617
	11th to 20th	Day	13% 17% 6% 14% 9% 6% 7% 4 3 4 16%	3620
	21st to end	Day	22% 17% 8% 13% 5% 4 4 3 3 4 17%	4403
February	1st to 10th	Day	15% 16% 5% 11% 7% 5% 4 3 5% 6% 22%	1772
	11th to 20th	Day	15% 12% 4 13% 8% 6% 4 3 4 6% 25%	1848
	21st to end	Day	17% 13% 4 9% 5% 4 4 4 6% 7% 26%	1570
March	1st to 10th	Day	20% 17% 3 10% 6% 4 3 3 4 5% 24%	3148
	11th to 20th	Day	13% 11% 4 7% 5% 5% 4 4 5% 7% 35%	3480
	21st to end	Day	20% 12% 4 9% 5% 4 4 3 5% 6% 27%	3815
April	1st to 10th	Day	15% 12% 3 8% 6% 5% 5% 4 6% 6% 30%	2244
	11th to 20th	Day	18% 8% 3 6% 5% 5% 4 4 5% 7% 35%	2356
	21st to end	Day	18% 9% 3 6% 5% 4 5% 4 5% 7% 35%	2470
May	1st to 10th	Day	18% 9% 3 7% 6% 4 5% 4 6% 7% 30%	4082
	11th to 20th	Day	17% 7% 2 6% 5% 4 5% 4 6% 8% 35%	4081
	21st to end	Day	9% 6% 2 7% 5% 4 5% 4 6% 7% 43%	4476
June	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 4 7% 7% 40%	2631
	11th to 20th	Day	10% 5% 2 7% 6% 5% 4 4 7% 9% 42%	2660
	21st to end	Day	11% 6% 2 5% 5% 6% 6% 5% 7% 8% 38%	2660
July	1st to 10th	Day	12% 6% 2 8% 5% 5% 5% 4 6% 10% 37%	4440
	11th to 20th	Day	7% 3 2 5% 5% 4 4 4 7% 10% 48%	4440
	21st to end	Day	6% 6% 2 8% 6% 6% 6% 5% 7% 8% 40%	4530
August	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 5% 6% 9% 44%	4104
	11th to 20th	Day	8% 5% 2 6% 4 4 6% 4 8% 10% 42%	4044
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 6% 10% 46%	4350
September	1st to 10th	Day	12% 6% 3 7% 4 4 4 3 6% 7% 45%	2169
	11th to 20th	Day	9% 6% 2 5% 3 4 5% 4 6% 8% 47%	2090
	21st to end	Day	15% 7% 2 5% 4 4 4 4 7% 6% 41%	2090
October	1st to 10th	Day	12% 7% 2 6% 5% 4 4 5% 5% 7% 44%	2081
	11th to 20th	Day	16% 8% 2 6% 5% 4 5% 3 6% 5% 39%	1900
	21st to end	Day	17% 10% 3 7% 5% 5% 5% 4 6% 6% 32%	1915
November	1st to 10th	Day	11% 9% 2 7% 4 4 3 3 6% 7% 43%	1762
	11th to 20th	Day	15% 13% 4 8% 5% 4 4 4 4 5% 32%	1800
	21st to end	Day	20% 15% 4 8% 5% 4 4 3 5% 6% 27%	1800
December	1st to 10th	Day	12% 14% 6% 12% 7% 6% 4 3 5% 6% 25%	2930
	11th to 20th	Day	18% 16% 7% 11% 5% 4 5% 3 5% 5% 21%	3181
	21st to end	Day	25% 20% 7% 11% 5% 4 4 3 3 4 14%	3982
January	1st to 10th	Night	14% 16% 7% 15% 6% 5% 5% 3 4 5% 20%	6348
	11th to 20th	Night	12% 18% 7% 15% 7% 6% 5% 4 4 5% 16%	6360
	21st to end	Night	20% 15% 6% 11% 5% 4 4 3 4 5% 22%	6540
February	1st to 10th	Night	18% 14% 5% 10% 7% 5% 3 3 5% 6% 23%	2517
	11th to 20th	Night	13% 14% 5% 8% 6% 5% 5% 4 5% 6% 29%	2218
	21st to end	Night	20% 13% 4 8% 5% 5% 3 3 5% 6% 29%	1884
March	1st to 10th	Night	19% 12% 4 7% 5% 4 3 3 4 5% 34%	3705
	11th to 20th	Night	11% 7% 2 5% 5% 5% 5% 4 5% 6% 45%	3480
	21st to end	Night	17% 9% 4 9% 5% 4 3 3 4 7% 34%	3817
April	1st to 10th	Night	15% 10% 3 6% 4 5% 4 4 6% 7% 35%	1952
	11th to 20th	Night	19% 7% 2 6% 5% 5% 4 4 5% 6% 36%	1772
	21st to end	Night	17% 8% 3 7% 4 4 4 4 4 7% 38%	1710
May	1st to 10th	Night	17% 8% 4 10% 6% 5% 5% 3 5% 7% 31%	2813
	11th to 20th	Night	18% 7% 2 6% 4 4 4 3 5% 7% 39%	2812
	21st to end	Night	11% 6% 2 5% 4 4 4 4 6% 9% 45%	3079
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 5% 7% 8% 46%	1545
	11th to 20th	Night	9% 4 2 6% 5% 4 5% 3 7% 9% 45%	1520
	21st to end	Night	10% 5% 2 6% 4 4 5% 5% 8% 8% 45%	1520
July	1st to 10th	Night	9% 4 1 5% 5% 4 5% 5% 7% 9% 48%	2530
	11th to 20th	Night	8% 4 1 5% 3 4 4 3 6% 11% 51%	2530
	21st to end	Night	7% 5% 2 6% 5% 5% 4 4 7% 9% 45%	3021
August	1st to 10th	Night	11% 7% 3 6% 4 4 4 4 6% 8% 43%	2825
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 48%	2785
	21st to end	Night	8% 5% 1 6% 4 4 4 4 6% 7% 50%	3302
September	1st to 10th	Night	12% 4 2 5% 4 4 3 3 5% 6% 51%	1991
	11th to 20th	Night	9% 5% 2 5% 3 4 3 3 4 7% 55%	2090
	21st to end	Night	13% 7% 2 6% 5% 4 5% 3 3 7% 45%	2090
October	1st to 10th	Night	12% 6% 2 7% 4 4 3 3 4 7% 48%	2098
	11th to 20th	Night	17% 9% 3 7% 5% 4 4 3 5% 5% 39%	2280
	21st to end	Night	20% 8% 3 6% 4 3 4 3 5% 6% 37%	2679
November	1st to 10th	Night	13% 8% 2 6% 4 4 4 3 5% 6% 44%	2536
	11th to 20th	Night	17% 12% 3 7% 4 4 5% 4 5% 7% 33%	2600
	21st to end	Night	19% 13% 4 9% 6% 4 4 3 6% 6% 26%	2600
December	1st to 10th	Night	13% 13% 5% 11% 6% 5% 5% 3 5% 7% 25%	4250
	11th to 20th	Night	17% 19% 7% 10% 5% 5% 4 3 4 5% 22%	5016
	21st to end	Night	26% 19% 6% 10% 5% 4 4 3 4 5% 14%	6996

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% 5% 9% 13% 4	3618
	11th to 20th	Day	72% 4 9% 6% 9% 1	3620
	21st to end	Day	71% 4 8% 10% 6%	4403
February	1st to 10th	Day	62% 7% 10% 14% 6% 1	1772
	11th to 20th	Day	61% 6% 8% 17% 8%	1858
	21st to end	Day	57% 6% 14% 15% 6% 2	1570
March	1st to 10th	Day	62% 6% 8% 18% 5%	3148
	11th to 20th	Day	45% 8% 14% 22% 9% 1	3480
	21st to end	Day	56% 8% 14% 17% 5%	3815
April	1st to 10th	Day	51% 8% 11% 16% 13% 1	2244
	11th to 20th	Day	48% 6% 11% 20% 12% 2	2356
	21st to end	Day	48% 7% 12% 20% 11% 2	2470
May	1st to 10th	Day	49% 9% 12% 20% 9%	4120
	11th to 20th	Day	43% 9% 14% 23% 10% 1	4085
	21st to end	Day	36% 9% 11% 18% 21% 4	4489
June	1st to 10th	Day	40% 8% 11% 24% 15% 2	2631
	11th to 20th	Day	37% 9% 13% 22% 17% 3	2660
	21st to end	Day	39% 10% 13% 19% 15% 4	2660
July	1st to 10th	Day	42% 6% 12% 21% 17% 3	4371
	11th to 20th	Day	30% 7% 12% 30% 19% 1	4365
	21st to end	Day	38% 9% 13% 18% 21% 2	4386
August	1st to 10th	Day	34% 8% 15% 26% 15% 1	4091
	11th to 20th	Day	34% 8% 13% 29% 15% 1	4083
	21st to end	Day	32% 7% 12% 34% 12% 3	4349
September	1st to 10th	Day	38% 7% 13% 22% 17% 3	2169
	11th to 20th	Day	32% 6% 13% 26% 20% 1	2090
	21st to end	Day	40% 6% 13% 23% 17% 1	2090
October	1st to 10th	Day	28% 5% 13% 34% 20% 1	3458
	11th to 20th	Day	38% 5% 11% 26% 20%	3140
	21st to end	Day	42% 5% 12% 25% 15% 1	3127
November	1st to 10th	Day	40% 6% 11% 26% 17% 1	1762
	11th to 20th	Day	53% 6% 11% 17% 11% 2	1800
	21st to end	Day	59% 6% 12% 17% 8%	1800
December	1st to 10th	Day	60% 6% 16% 15% 3	2930
	11th to 20th	Day	64% 6% 10% 15% 5%	3178
	21st to end	Day	74% 6% 7% 9% 3	3982
January	1st to 10th	Night	65% 7% 8% 16% 5%	6348
	11th to 20th	Night	71% 5% 10% 8% 7%	6360
	21st to end	Night	64% 5% 14% 9% 7%	6540
February	1st to 10th	Night	61% 6% 12% 14% 5% 1	2517
	11th to 20th	Night	55% 6% 10% 20% 8% 1	2219
	21st to end	Night	57% 6% 11% 17% 9% 1	1884
March	1st to 10th	Night	52% 6% 10% 24% 8% 1	3705
	11th to 20th	Night	36% 9% 15% 23% 16% 2	3480
	21st to end	Night	48% 8% 12% 23% 8% 1	3817
April	1st to 10th	Night	46% 7% 12% 19% 14% 2	1952
	11th to 20th	Night	47% 7% 10% 17% 18% 2	1772
	21st to end	Night	45% 7% 11% 19% 15% 2	1710
May	1st to 10th	Night	50% 9% 10% 18% 11% 2	2840
	11th to 20th	Night	42% 10% 12% 23% 12% 1	2813
	21st to end	Night	32% 10% 14% 23% 19% 2	3087
June	1st to 10th	Night	34% 7% 12% 25% 19% 3	1545
	11th to 20th	Night	34% 8% 13% 24% 18% 3	1520
	21st to end	Night	33% 9% 15% 23% 17% 2	1520
July	1st to 10th	Night	32% 7% 12% 26% 18% 6%	2490
	11th to 20th	Night	27% 6% 17% 28% 20% 2	2482
	21st to end	Night	33% 7% 17% 19% 18% 6%	2925
August	1st to 10th	Night	37% 7% 14% 24% 15% 2	2814
	11th to 20th	Night	33% 6% 13% 29% 17% 2	2812
	21st to end	Night	32% 7% 12% 28% 18% 2	3302
September	1st to 10th	Night	34% 6% 12% 25% 19% 4	1991
	11th to 20th	Night	29% 6% 10% 27% 26% 2	2090
	21st to end	Night	39% 6% 11% 24% 18% 2	2090
October	1st to 10th	Night	25% 4 8% 31% 31% 1	3501
	11th to 20th	Night	37% 4 10% 26% 22% 1	3779
	21st to end	Night	35% 4 8% 34% 17% 2	4405
November	1st to 10th	Night	40% 6% 12% 24% 17% 1	2536
	11th to 20th	Night	50% 6% 12% 19% 12% 2	2600
	21st to end	Night	57% 6% 11% 19% 6%	2600
December	1st to 10th	Night	57% 6% 12% 18% 6% 1	4250
	11th to 20th	Night	64% 5% 12% 10% 7% 1	5018
	21st to end	Night	73% 6% 9% 9% 4	6996

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% 20% 34% 21% 4 1	300
	11th to 20th	Day	21% 19% 44% 12% 3 1	300
	21st to end	Day	27% 13% 34% 19% 7%	330
February	1st to 10th	Day	25% 18% 38% 15% 3 1	285
	11th to 20th	Day	23% 19% 35% 20% 4	270
	21st to end	Day	24% 21% 40% 10% 4	225
March	1st to 10th	Day	25% 18% 33% 20% 4	285
	11th to 20th	Day	25% 12% 33% 22% 7% 2	460
	21st to end	Day	30% 15% 30% 18% 6% 1	506
April	1st to 10th	Day	19% 16% 30% 23% 10% 2	460
	11th to 20th	Day	22% 17% 30% 18% 9% 4	452
	21st to end	Day	22% 15% 31% 20% 10% 2	460
May	1st to 10th	Day	23% 12% 30% 22% 10% 3	463
	11th to 20th	Day	20% 9% 26% 24% 14% 7%	487
	21st to end	Day	11% 7% 26% 26% 18% 11%	529
June	1st to 10th	Day	11% 9% 28% 31% 17% 5%	651
	11th to 20th	Day	13% 6% 28% 28% 20% 5%	680
	21st to end	Day	13% 6% 26% 30% 18% 6%	680
July	1st to 10th	Day	11% 4 20% 30% 23% 13%	671
	11th to 20th	Day	8% 5% 23% 30% 21% 12%	668
	21st to end	Day	8% 4 21% 27% 27% 13%	570
August	1st to 10th	Day	8% 3 18% 28% 27% 16%	486
	11th to 20th	Day	9% 4 20% 30% 24% 12%	479
	21st to end	Day	9% 3 18% 28% 27% 15%	539
September	1st to 10th	Day	13% 4 19% 26% 24% 13%	490
	11th to 20th	Day	11% 2 15% 24% 31% 17%	490
	21st to end	Day	18% 9% 22% 24% 19% 9%	490
October	1st to 10th	Day	12% 7% 23% 30% 19% 9%	490
	11th to 20th	Day	18% 16% 30% 23% 10% 2	490
	21st to end	Day	22% 15% 36% 21% 5% 2	364
November	1st to 10th	Day	13% 15% 35% 22% 11% 4	300
	11th to 20th	Day	25% 17% 30% 18% 9% 1	300
	21st to end	Day	28% 15% 38% 17% 3	300
December	1st to 10th	Day	18% 22% 34% 17% 8%	300
	11th to 20th	Day	22% 19% 35% 18% 5%	300
	21st to end	Day	29% 20% 29% 18% 3	330
January	1st to 10th	Night	23% 15% 35% 22% 6%	1000
	11th to 20th	Night	26% 14% 37% 19% 4 1	1000
	21st to end	Night	21% 14% 37% 22% 4	904
February	1st to 10th	Night	25% 12% 34% 25% 3	781
	11th to 20th	Night	21% 14% 37% 25% 2	743
	21st to end	Night	27% 14% 37% 20% 2	628
March	1st to 10th	Night	22% 10% 35% 26% 7%	760
	11th to 20th	Night	17% 14% 38% 24% 6%	760
	21st to end	Night	23% 11% 32% 26% 8%	836
April	1st to 10th	Night	19% 13% 35% 28% 4 1	622
	11th to 20th	Night	21% 10% 36% 24% 8% 1	563
	21st to end	Night	20% 9% 36% 24% 10% 1	570
May	1st to 10th	Night	21% 7% 31% 31% 9% 1	570
	11th to 20th	Night	21% 4 29% 29% 16% 1	561
	21st to end	Night	12% 4 22% 39% 20% 3	617
June	1st to 10th	Night	11% 3 24% 44% 17% 1	570
	11th to 20th	Night	10% 4 24% 42% 19% 1	570
	21st to end	Night	11% 3 24% 38% 21% 2	570
July	1st to 10th	Night	7% 3 19% 39% 29% 4	555
	11th to 20th	Night	9% 3 15% 40% 28% 4	552
	21st to end	Night	7% 2 15% 42% 32% 2	624
August	1st to 10th	Night	7% 2 14% 43% 30% 4	567
	11th to 20th	Night	11% 2 15% 39% 29% 4	564
	21st to end	Night	9% 2 13% 37% 36% 4	627
September	1st to 10th	Night	13% 5% 16% 34% 28% 4	661
	11th to 20th	Night	9% 4 15% 34% 34% 4	760
	21st to end	Night	16% 6% 21% 33% 22% 3	760
October	1st to 10th	Night	15% 6% 25% 32% 20% 2	760
	11th to 20th	Night	21% 12% 29% 26% 10% 1	760
	21st to end	Night	23% 12% 32% 25% 8%	836
November	1st to 10th	Night	17% 10% 33% 27% 11% 1	780
	11th to 20th	Night	23% 14% 31% 25% 7% 1	800
	21st to end	Night	26% 13% 30% 25% 6%	800
December	1st to 10th	Night	23% 15% 35% 21% 5%	800
	11th to 20th	Night	26% 12% 35% 23% 4	885
	21st to end	Night	29% 14% 32% 23% 3	1100

Page generated at 2025-02-03 03:05:54. Version ac07

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