Black River Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black River Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	13% 10% 4 8% 5% 5% 5% 4 6% 7% 34%	126841
	Night	14% 11% 4 8% 5% 4 4 4 5% 7% 34%	124923
Transparency	Day	47% 7% 13% 20% 12% 1	130413
	Night	47% 6% 13% 21% 12% 2	129421
Seeing	Day	16% 11% 29% 25% 13% 5%	16820
	Night	18% 10% 30% 28% 12% 1	26411

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	11% 15% 6% 13% 5% 5% 4 4 5% 5% 27%	3697
	11th to 20th	Day	13% 18% 6% 14% 8% 8% 5% 4 4 5% 14%	3690
	21st to end	Day	19% 15% 7% 11% 5% 5% 6% 3 4 5% 19%	4466
February	1st to 10th	Day	15% 15% 5% 13% 8% 6% 6% 4 5% 5% 20%	1800
	11th to 20th	Day	14% 15% 5% 11% 7% 5% 4 4 6% 5% 25%	1833
	21st to end	Day	14% 16% 5% 13% 6% 5% 4 4 4 4 26%	1650
March	1st to 10th	Day	15% 12% 5% 10% 7% 4 4 3 4 6% 31%	4660
	11th to 20th	Day	11% 13% 4 9% 5% 5% 5% 4 6% 8% 31%	4980
	21st to end	Day	17% 14% 4 10% 5% 5% 5% 3 5% 6% 26%	5465
April	1st to 10th	Day	16% 12% 4 8% 5% 4 4 4 5% 7% 30%	2290
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 4 6% 6% 36%	2572
	21st to end	Day	17% 9% 3 8% 6% 5% 4 4 5% 6% 35%	2600
May	1st to 10th	Day	16% 9% 3 8% 5% 4 4 4 5% 7% 36%	5862
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 5% 7% 34%	5861
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 4 6% 40%	6616
June	1st to 10th	Day	9% 6% 3 6% 6% 4 5% 4 7% 9% 41%	2956
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 9% 42%	3000
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 8% 40%	3000
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 4 8% 10% 38%	6482
	11th to 20th	Day	7% 5% 2 7% 4 4 4 4 7% 9% 46%	6360
	21st to end	Day	6% 5% 2 6% 4 4 5% 5% 7% 10% 46%	6452
August	1st to 10th	Day	12% 5% 3 6% 5% 5% 6% 5% 7% 9% 39%	4234
	11th to 20th	Day	8% 6% 3 7% 5% 6% 6% 5% 8% 10% 38%	4174
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 9% 40%	4675
September	1st to 10th	Day	9% 6% 3 6% 5% 5% 5% 4 7% 7% 42%	2275
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 3 6% 8% 48%	2200
	21st to end	Day	15% 7% 2 8% 4 5% 4 4 7% 8% 35%	2200
October	1st to 10th	Day	12% 7% 2 7% 5% 4 5% 4 5% 7% 41%	2104
	11th to 20th	Day	18% 8% 3 8% 5% 5% 4 3 4 8% 34%	2000
	21st to end	Day	19% 10% 3 7% 5% 5% 5% 3 5% 7% 30%	2073
November	1st to 10th	Day	13% 8% 2 9% 6% 5% 4 4 6% 8% 34%	1852
	11th to 20th	Day	13% 12% 4 9% 7% 5% 4 4 6% 6% 29%	1843
	21st to end	Day	16% 13% 6% 9% 6% 4 4 4 5% 6% 26%	1680
December	1st to 10th	Day	15% 14% 4 10% 5% 5% 5% 4 7% 7% 24%	2680
	11th to 20th	Day	18% 20% 5% 8% 4 4 4 4 5% 5% 22%	2896
	21st to end	Day	25% 21% 7% 11% 6% 4 3 2 4 5% 14%	3663
January	1st to 10th	Night	13% 12% 5% 11% 5% 4 4 3 5% 6% 32%	6488
	11th to 20th	Night	11% 15% 6% 14% 8% 7% 5% 4 6% 5% 19%	6418
	21st to end	Night	16% 16% 8% 10% 5% 4 5% 3 4 5% 23%	6490
February	1st to 10th	Night	14% 17% 5% 11% 7% 6% 5% 4 4 5% 22%	2600
	11th to 20th	Night	14% 14% 5% 10% 6% 4 4 4 5% 6% 28%	2450
	21st to end	Night	17% 11% 4 11% 7% 5% 5% 3 5% 5% 27%	1980
March	1st to 10th	Night	14% 12% 3 8% 5% 4 4 3 4 6% 35%	5192
	11th to 20th	Night	9% 8% 3 7% 5% 5% 5% 4 6% 9% 38%	4980
	21st to end	Night	15% 9% 4 8% 5% 4 4 3 5% 5% 37%	5467
April	1st to 10th	Night	19% 11% 3 7% 5% 3 3 3 5% 7% 33%	2127
	11th to 20th	Night	19% 7% 3 7% 4 5% 3 3 6% 7% 37%	1777
	21st to end	Night	16% 8% 3 6% 5% 4 4 4 7% 7% 36%	1800
May	1st to 10th	Night	18% 9% 3 8% 4 4 4 4 5% 7% 34%	4033
	11th to 20th	Night	14% 9% 3 7% 5% 4 5% 3 5% 6% 39%	4032
	21st to end	Night	16% 6% 2 5% 3 4 4 4 6% 7% 44%	4220
June	1st to 10th	Night	12% 5% 2 6% 4 4 4 5% 6% 9% 45%	1439
	11th to 20th	Night	9% 5% 2 6% 4 5% 4 4 6% 10% 44%	1400
	21st to end	Night	8% 5% 2 5% 5% 5% 4 4 5% 8% 49%	1400
July	1st to 10th	Night	7% 5% 2 7% 4 4 5% 4 7% 11% 45%	3477
	11th to 20th	Night	7% 4 2 6% 4 4 4 4 6% 10% 48%	3617
	21st to end	Night	6% 4 2 6% 4 5% 5% 4 7% 8% 49%	4396
August	1st to 10th	Night	9% 6% 2 7% 3 4 4 4 6% 8% 46%	2915
	11th to 20th	Night	9% 7% 3 7% 4 4 4 4 5% 7% 45%	2875
	21st to end	Night	7% 6% 2 6% 5% 5% 5% 4 6% 8% 47%	3223
September	1st to 10th	Night	10% 6% 2 7% 4 4 5% 4 5% 7% 46%	2101
	11th to 20th	Night	9% 5% 2 5% 3 3 4 4 4 7% 55%	2200
	21st to end	Night	16% 6% 2 6% 4 4 5% 3 5% 7% 42%	2200
October	1st to 10th	Night	13% 7% 3 6% 4 3 4 3 5% 8% 46%	2295
	11th to 20th	Night	19% 8% 2 8% 4 4 4 3 5% 6% 37%	2400
	21st to end	Night	21% 9% 3 7% 4 4 5% 3 5% 7% 32%	2765
November	1st to 10th	Night	13% 9% 3 7% 4 4 4 3 5% 7% 41%	2666
	11th to 20th	Night	14% 11% 3 9% 5% 5% 5% 4 6% 7% 30%	2774
	21st to end	Night	19% 14% 4 8% 6% 4 4 4 5% 5% 28%	2940
December	1st to 10th	Night	12% 15% 5% 11% 6% 5% 5% 4 6% 8% 24%	4720
	11th to 20th	Night	14% 18% 5% 10% 6% 5% 4 3 6% 6% 24%	5513
	21st to end	Night	23% 19% 6% 10% 5% 5% 4 3 4 5% 16%	7553

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	57% 7% 11% 15% 7% 2	3698
	11th to 20th	Day	70% 7% 9% 8% 6% 1	3690
	21st to end	Day	66% 6% 12% 9% 6% 1	4466
February	1st to 10th	Day	64% 6% 10% 14% 6%	1800
	11th to 20th	Day	58% 7% 10% 16% 8% 1	1842
	21st to end	Day	61% 5% 11% 13% 7% 2	1650
March	1st to 10th	Day	55% 4 14% 17% 9% 1	4660
	11th to 20th	Day	49% 6% 15% 20% 8% 1	4980
	21st to end	Day	56% 7% 12% 15% 9% 1	5465
April	1st to 10th	Day	52% 6% 12% 19% 9% 1	2290
	11th to 20th	Day	47% 6% 12% 19% 15% 2	2572
	21st to end	Day	49% 7% 13% 17% 12% 2	2600
May	1st to 10th	Day	46% 7% 12% 17% 13% 5%	5897
	11th to 20th	Day	46% 8% 12% 20% 12% 2	5865
	21st to end	Day	43% 7% 9% 19% 18% 5%	6631
June	1st to 10th	Day	38% 8% 14% 25% 15% 1	2956
	11th to 20th	Day	37% 8% 14% 25% 15% 2	3000
	21st to end	Day	38% 8% 14% 24% 15% 1	3000
July	1st to 10th	Day	37% 8% 15% 24% 14% 1	6413
	11th to 20th	Day	32% 7% 14% 31% 16% 1	6285
	21st to end	Day	31% 8% 15% 25% 19% 1	6268
August	1st to 10th	Day	38% 9% 15% 23% 14% 1	4221
	11th to 20th	Day	37% 9% 15% 28% 9% 1	4213
	21st to end	Day	37% 8% 14% 29% 11% 1	4674
September	1st to 10th	Day	38% 9% 14% 25% 13% 1	2275
	11th to 20th	Day	32% 7% 14% 33% 13% 1	2200
	21st to end	Day	44% 8% 14% 23% 11% 1	2200
October	1st to 10th	Day	32% 6% 11% 33% 16% 2	3429
	11th to 20th	Day	42% 5% 11% 24% 17%	3240
	21st to end	Day	46% 6% 14% 22% 12%	3323
November	1st to 10th	Day	47% 8% 12% 23% 9% 1	1852
	11th to 20th	Day	54% 7% 14% 16% 9%	1843
	21st to end	Day	58% 6% 13% 18% 5%	1680
December	1st to 10th	Day	57% 8% 19% 12% 4	2680
	11th to 20th	Day	62% 6% 12% 13% 8%	2892
	21st to end	Day	74% 6% 7% 7% 5%	3663
January	1st to 10th	Night	52% 6% 12% 20% 8% 2	6488
	11th to 20th	Night	64% 7% 11% 12% 6% 1	6418
	21st to end	Night	62% 5% 10% 15% 7%	6490
February	1st to 10th	Night	62% 7% 10% 14% 7% 1	2600
	11th to 20th	Night	56% 6% 11% 18% 9% 1	2452
	21st to end	Night	57% 7% 11% 17% 7% 1	1980
March	1st to 10th	Night	49% 5% 15% 20% 10% 1	5192
	11th to 20th	Night	40% 6% 18% 23% 11% 1	4980
	21st to end	Night	47% 6% 12% 20% 14% 1	5467
April	1st to 10th	Night	50% 5% 11% 20% 12% 2	2127
	11th to 20th	Night	45% 6% 10% 20% 18% 2	1777
	21st to end	Night	45% 7% 12% 18% 15% 4	1800
May	1st to 10th	Night	48% 7% 10% 16% 13% 6%	4060
	11th to 20th	Night	44% 7% 12% 23% 12% 2	4033
	21st to end	Night	36% 8% 12% 23% 15% 6%	4227
June	1st to 10th	Night	35% 7% 11% 27% 19% 2	1439
	11th to 20th	Night	34% 8% 14% 24% 18% 2	1400
	21st to end	Night	33% 8% 13% 25% 20% 1	1400
July	1st to 10th	Night	32% 6% 15% 26% 17% 4	3439
	11th to 20th	Night	30% 7% 13% 30% 18% 1	3572
	21st to end	Night	30% 8% 14% 24% 23% 2	4267
August	1st to 10th	Night	34% 7% 15% 25% 16% 2	2904
	11th to 20th	Night	35% 7% 15% 29% 11% 2	2902
	21st to end	Night	34% 8% 13% 30% 14% 1	3223
September	1st to 10th	Night	35% 7% 14% 28% 14% 2	2101
	11th to 20th	Night	28% 7% 13% 33% 18% 1	2200
	21st to end	Night	41% 7% 11% 28% 11% 1	2200
October	1st to 10th	Night	25% 5% 10% 32% 26% 1	3750
	11th to 20th	Night	43% 5% 11% 23% 17%	3899
	21st to end	Night	43% 5% 12% 26% 13% 1	4465
November	1st to 10th	Night	42% 6% 13% 25% 12% 1	2666
	11th to 20th	Night	51% 7% 15% 18% 8% 1	2774
	21st to end	Night	57% 6% 15% 17% 5%	2940
December	1st to 10th	Night	57% 7% 19% 14% 3	4720
	11th to 20th	Night	59% 6% 12% 14% 9% 1	5516
	21st to end	Night	69% 7% 10% 9% 4	7553

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	20% 19% 40% 15% 5% 1	330
	11th to 20th	Day	21% 20% 42% 14% 3 1	318
	21st to end	Day	25% 16% 35% 18% 5% 1	330
February	1st to 10th	Day	22% 21% 40% 13% 4 1	300
	11th to 20th	Day	25% 18% 35% 19% 2	300
	21st to end	Day	18% 24% 41% 14% 2	249
March	1st to 10th	Day	24% 17% 39% 16% 4 1	376
	11th to 20th	Day	21% 15% 33% 23% 7% 1	500
	21st to end	Day	25% 14% 33% 23% 5% 1	550
April	1st to 10th	Day	20% 16% 36% 19% 7% 1	500
	11th to 20th	Day	24% 15% 29% 19% 8% 5%	492
	21st to end	Day	21% 11% 33% 24% 9% 3	500
May	1st to 10th	Day	19% 15% 29% 23% 12% 3	503
	11th to 20th	Day	18% 8% 28% 25% 15% 6%	527
	21st to end	Day	15% 8% 21% 31% 17% 8%	573
June	1st to 10th	Day	11% 8% 33% 30% 15% 4	686
	11th to 20th	Day	10% 8% 32% 32% 14% 4	730
	21st to end	Day	11% 8% 27% 34% 17% 4	730
July	1st to 10th	Day	8% 5% 24% 31% 21% 11%	721
	11th to 20th	Day	11% 4 25% 33% 18% 8%	718
	21st to end	Day	7% 3 22% 35% 23% 9%	591
August	1st to 10th	Day	8% 5% 17% 31% 25% 14%	526
	11th to 20th	Day	10% 5% 18% 31% 23% 13%	519
	21st to end	Day	13% 5% 20% 27% 24% 11%	583
September	1st to 10th	Day	12% 7% 24% 24% 22% 12%	530
	11th to 20th	Day	9% 5% 16% 25% 29% 16%	530
	21st to end	Day	16% 9% 24% 24% 17% 9%	530
October	1st to 10th	Day	14% 10% 25% 24% 19% 7%	530
	11th to 20th	Day	17% 14% 36% 24% 7% 1	530
	21st to end	Day	22% 16% 38% 19% 5% 1	456
November	1st to 10th	Day	19% 18% 34% 22% 7% 1	330
	11th to 20th	Day	23% 24% 30% 16% 7% 1	305
	21st to end	Day	25% 20% 37% 15% 3	220
December	1st to 10th	Day	22% 19% 35% 18% 5% 1	220
	11th to 20th	Day	25% 23% 33% 15% 3 1	220
	21st to end	Day	30% 17% 35% 15% 3	267
January	1st to 10th	Night	23% 14% 38% 20% 4 1	1050
	11th to 20th	Night	23% 16% 35% 21% 4 1	1017
	21st to end	Night	18% 13% 36% 26% 6%	880
February	1st to 10th	Night	22% 12% 42% 21% 2	800
	11th to 20th	Night	21% 15% 35% 25% 4	783
	21st to end	Night	22% 13% 41% 21% 3	660
March	1st to 10th	Night	19% 13% 36% 26% 5%	800
	11th to 20th	Night	17% 12% 37% 27% 6% 1	800
	21st to end	Night	22% 10% 31% 29% 7% 1	880
April	1st to 10th	Night	21% 14% 35% 25% 4	737
	11th to 20th	Night	20% 11% 32% 25% 10% 2	593
	21st to end	Night	19% 8% 31% 32% 9% 1	600
May	1st to 10th	Night	18% 6% 30% 32% 13% 1	600
	11th to 20th	Night	18% 5% 28% 35% 13% 1	591
	21st to end	Night	12% 4 24% 35% 22% 2	565
June	1st to 10th	Night	14% 4 25% 39% 19%	400
	11th to 20th	Night	13% 4 26% 40% 17% 1	400
	21st to end	Night	10% 4 21% 46% 19% 1	400
July	1st to 10th	Night	7% 1 16% 43% 29% 4	522
	11th to 20th	Night	10% 2 17% 41% 25% 4	582
	21st to end	Night	7% 2 19% 44% 25% 3	657
August	1st to 10th	Night	7% 3 15% 40% 32% 4	597
	11th to 20th	Night	7% 2 17% 34% 35% 4	594
	21st to end	Night	10% 3 18% 37% 28% 4	660
September	1st to 10th	Night	10% 5% 23% 33% 24% 5%	749
	11th to 20th	Night	10% 3 17% 34% 30% 6%	800
	21st to end	Night	17% 6% 21% 32% 19% 5%	800
October	1st to 10th	Night	15% 8% 27% 30% 19% 2	800
	11th to 20th	Night	22% 12% 30% 28% 8% 1	800
	21st to end	Night	24% 10% 36% 25% 5%	880
November	1st to 10th	Night	18% 13% 32% 25% 11% 1	820
	11th to 20th	Night	19% 16% 37% 21% 7% 1	840
	21st to end	Night	25% 14% 36% 20% 5%	840
December	1st to 10th	Night	21% 15% 40% 20% 4	840
	11th to 20th	Night	23% 16% 40% 18% 3	919
	21st to end	Night	30% 12% 34% 21% 3	1155

Page generated at 2026-01-14 01:14:32. Version ac07

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