Bear Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bear Mountain Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	16% 11% 4 9% 5% 5% 5% 4 5% 6% 30%	126990
	Night	15% 12% 4 10% 6% 5% 4 3 5% 6% 29%	123701
Transparency	Day	51% 7% 12% 19% 10% 1	130471
	Night	53% 8% 12% 17% 9% 1	128294
Seeing	Day	27% 20% 28% 16% 7% 2	16160
	Night	24% 18% 26% 23% 8% 1	25134

By Month

Percentage Cloud Cover NE (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% 15% 8% 21% 8% 4 4 3 3 4 21%	3597
	11th to 20th	Day	9% 13% 7% 19% 8% 4 4 3 5% 5% 22%	3570
	21st to end	Day	12% 16% 8% 12% 5% 5% 5% 3 4 4 25%	3960
February	1st to 10th	Day	13% 15% 5% 12% 7% 6% 4 4 6% 5% 23%	1785
	11th to 20th	Day	10% 14% 7% 15% 8% 5% 4 4 5% 5% 24%	1948
	21st to end	Day	15% 15% 7% 12% 6% 4 4 2 4 5% 26%	1650
March	1st to 10th	Day	18% 15% 6% 13% 5% 5% 4 3 4 4 23%	4476
	11th to 20th	Day	13% 14% 6% 9% 4 4 4 3 5% 6% 33%	4666
	21st to end	Day	19% 10% 4 10% 5% 5% 4 3 4 5% 30%	5970
April	1st to 10th	Day	18% 17% 4 8% 4 4 3 3 4 5% 30%	2400
	11th to 20th	Day	16% 14% 3 7% 5% 5% 4 3 4 5% 33%	2374
	21st to end	Day	21% 13% 4 8% 4 4 3 3 5% 5% 30%	2553
May	1st to 10th	Day	32% 12% 3 7% 4 4 4 3 5% 5% 20%	5932
	11th to 20th	Day	18% 10% 3 8% 5% 5% 5% 4 6% 7% 28%	6318
	21st to end	Day	19% 9% 3 8% 5% 4 5% 4 6% 7% 30%	6949
June	1st to 10th	Day	21% 8% 3 7% 5% 6% 5% 4 7% 7% 27%	2800
	11th to 20th	Day	20% 7% 2 6% 5% 5% 5% 4 6% 9% 31%	2800
	21st to end	Day	20% 8% 3 7% 5% 6% 5% 5% 5% 6% 29%	2800
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 5% 8% 9% 33%	6360
	11th to 20th	Day	10% 6% 2 7% 6% 6% 6% 5% 8% 9% 35%	6360
	21st to end	Day	14% 6% 3 6% 5% 4 5% 4 6% 7% 40%	6926
August	1st to 10th	Day	14% 8% 2 8% 5% 5% 6% 4 7% 7% 33%	4510
	11th to 20th	Day	13% 8% 3 8% 5% 5% 5% 5% 6% 9% 33%	3982
	21st to end	Day	13% 6% 2 5% 4 4 5% 5% 6% 9% 40%	4312
September	1st to 10th	Day	15% 5% 2 5% 4 5% 5% 4 5% 7% 43%	2400
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 5% 7% 49%	2275
	21st to end	Day	16% 7% 2 5% 4 4 5% 3 6% 7% 40%	2200
October	1st to 10th	Day	21% 8% 2 7% 4 4 4 3 5% 5% 34%	2137
	11th to 20th	Day	15% 12% 3 8% 5% 3 4 4 5% 7% 35%	1953
	21st to end	Day	22% 14% 3 7% 4 4 4 3 4 6% 28%	1980
November	1st to 10th	Day	13% 12% 4 8% 5% 4 4 4 5% 6% 35%	1851
	11th to 20th	Day	14% 15% 5% 9% 5% 4 4 3 4 6% 31%	1890
	21st to end	Day	16% 14% 5% 11% 5% 4 5% 3 4 5% 26%	1744
December	1st to 10th	Day	12% 18% 5% 17% 6% 5% 4 3 5% 6% 19%	2616
	11th to 20th	Day	14% 16% 7% 13% 6% 5% 4 3 4 5% 24%	2986
	21st to end	Day	14% 16% 6% 12% 6% 5% 4 3 4 4 25%	3960
January	1st to 10th	Night	10% 12% 9% 22% 7% 5% 3 3 3 4 21%	6349
	11th to 20th	Night	8% 16% 8% 19% 9% 6% 6% 4 5% 5% 15%	6315
	21st to end	Night	13% 13% 6% 13% 6% 6% 5% 4 5% 5% 23%	6996
February	1st to 10th	Night	13% 14% 6% 13% 8% 5% 5% 4 5% 5% 22%	2612
	11th to 20th	Night	10% 12% 6% 11% 9% 7% 6% 5% 6% 5% 23%	2338
	21st to end	Night	12% 14% 4 14% 7% 5% 3 3 4 5% 28%	1980
March	1st to 10th	Night	16% 11% 6% 13% 8% 5% 4 4 4 5% 24%	5364
	11th to 20th	Night	12% 16% 5% 8% 5% 4 4 3 4 6% 32%	5277
	21st to end	Night	18% 16% 5% 12% 5% 4 3 3 3 4 28%	4962
April	1st to 10th	Night	18% 14% 5% 8% 5% 4 3 3 5% 5% 29%	2000
	11th to 20th	Night	18% 14% 3 8% 5% 3 4 3 5% 6% 31%	1975
	21st to end	Night	19% 13% 4 7% 5% 3 4 3 5% 6% 32%	1845
May	1st to 10th	Night	31% 13% 3 8% 4 4 4 3 4 5% 20%	4037
	11th to 20th	Night	17% 13% 3 8% 6% 5% 5% 4 5% 6% 28%	3575
	21st to end	Night	21% 10% 2 8% 5% 5% 4 4 6% 6% 29%	3926
June	1st to 10th	Night	20% 8% 3 6% 4 5% 4 4 6% 6% 33%	1600
	11th to 20th	Night	20% 6% 2 5% 5% 4 4 4 6% 7% 36%	1600
	21st to end	Night	17% 7% 2 6% 6% 5% 5% 4 6% 6% 36%	1600
July	1st to 10th	Night	12% 7% 2 7% 5% 5% 6% 5% 7% 8% 36%	3600
	11th to 20th	Night	15% 8% 3 8% 6% 5% 5% 4 5% 8% 34%	3600
	21st to end	Night	11% 7% 2 6% 5% 5% 5% 4 5% 6% 45%	3926
August	1st to 10th	Night	22% 9% 4 6% 4 3 3 3 4 6% 35%	2606
	11th to 20th	Night	13% 6% 3 8% 4 5% 5% 3 5% 8% 40%	3060
	21st to end	Night	10% 7% 2 5% 3 4 4 4 7% 9% 44%	3586
September	1st to 10th	Night	15% 8% 2 4 3 3 4 3 4 6% 47%	2000
	11th to 20th	Night	16% 5% 2 5% 2 3 3 3 5% 6% 51%	2125
	21st to end	Night	16% 7% 2 5% 4 4 5% 3 5% 5% 44%	2200
October	1st to 10th	Night	21% 8% 3 7% 4 4 3 3 4 6% 36%	2260
	11th to 20th	Night	16% 10% 3 7% 5% 4 4 4 5% 5% 36%	2445
	21st to end	Night	21% 13% 4 8% 5% 5% 5% 3 4 5% 28%	2860
November	1st to 10th	Night	14% 14% 4 9% 5% 4 4 3 4 5% 35%	2667
	11th to 20th	Night	15% 14% 5% 9% 5% 4 4 4 4 6% 32%	2730
	21st to end	Night	13% 14% 5% 10% 6% 4 4 4 5% 6% 28%	2872
December	1st to 10th	Night	14% 19% 6% 16% 7% 5% 5% 3 4 5% 16%	4609
	11th to 20th	Night	17% 17% 5% 11% 6% 5% 4 3 4 4 25%	5208
	21st to end	Night	15% 17% 6% 12% 6% 6% 4 3 4 4 22%	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	65% 8% 9% 15% 3	3598
	11th to 20th	Day	62% 9% 7% 12% 8% 2	3600
	21st to end	Day	59% 7% 10% 13% 8% 2	3960
February	1st to 10th	Day	61% 7% 11% 14% 7% 1	1785
	11th to 20th	Day	60% 7% 9% 15% 8% 1	1957
	21st to end	Day	60% 5% 9% 14% 11% 1	1650
March	1st to 10th	Day	63% 6% 10% 9% 8% 4	4479
	11th to 20th	Day	50% 7% 15% 18% 8% 3	4666
	21st to end	Day	54% 8% 8% 12% 16% 2	5970
April	1st to 10th	Day	56% 5% 8% 19% 10% 2	2400
	11th to 20th	Day	51% 7% 8% 19% 13% 1	2374
	21st to end	Day	54% 6% 9% 16% 12% 2	2553
May	1st to 10th	Day	65% 6% 8% 14% 7% 1	5943
	11th to 20th	Day	53% 9% 14% 15% 8% 1	6322
	21st to end	Day	50% 9% 12% 21% 8% 1	6949
June	1st to 10th	Day	53% 8% 14% 17% 9%	2800
	11th to 20th	Day	50% 7% 12% 19% 11% 1	2800
	21st to end	Day	53% 8% 11% 18% 9% 1	2800
July	1st to 10th	Day	44% 7% 14% 23% 11%	6290
	11th to 20th	Day	41% 9% 15% 24% 11% 1	6285
	21st to end	Day	41% 7% 12% 30% 9%	6728
August	1st to 10th	Day	48% 9% 14% 19% 10%	4496
	11th to 20th	Day	45% 9% 18% 23% 5%	4021
	21st to end	Day	37% 8% 17% 26% 10% 1	4312
September	1st to 10th	Day	39% 8% 11% 28% 13%	2400
	11th to 20th	Day	35% 7% 10% 30% 18% 1	2275
	21st to end	Day	41% 7% 13% 25% 12% 2	2200
October	1st to 10th	Day	46% 7% 9% 23% 14% 1	3475
	11th to 20th	Day	45% 4 13% 24% 13% 1	3156
	21st to end	Day	41% 7% 12% 28% 11% 2	3180
November	1st to 10th	Day	47% 8% 13% 20% 12%	1851
	11th to 20th	Day	53% 6% 10% 15% 13% 2	1890
	21st to end	Day	59% 6% 15% 15% 5% 1	1744
December	1st to 10th	Day	66% 8% 13% 9% 3	2616
	11th to 20th	Day	60% 8% 10% 15% 7% 1	2986
	21st to end	Day	61% 5% 9% 13% 9% 3	3960
January	1st to 10th	Night	65% 7% 12% 12% 5%	6349
	11th to 20th	Night	65% 10% 9% 10% 5% 1	6360
	21st to end	Night	60% 6% 10% 11% 11% 2	6996
February	1st to 10th	Night	62% 8% 10% 13% 7% 1	2612
	11th to 20th	Night	57% 9% 11% 13% 9% 1	2340
	21st to end	Night	56% 7% 11% 17% 7% 2	1980
March	1st to 10th	Night	58% 6% 11% 13% 11% 1	5374
	11th to 20th	Night	49% 9% 17% 14% 9% 2	5278
	21st to end	Night	58% 7% 6% 12% 15% 2	4962
April	1st to 10th	Night	56% 6% 9% 17% 10% 1	2000
	11th to 20th	Night	52% 7% 10% 19% 12% 1	1975
	21st to end	Night	52% 7% 9% 18% 12% 3	1845
May	1st to 10th	Night	64% 8% 8% 12% 7% 1	4043
	11th to 20th	Night	54% 9% 12% 18% 7% 1	3576
	21st to end	Night	52% 9% 12% 16% 10% 1	3927
June	1st to 10th	Night	48% 9% 13% 18% 12% 1	1600
	11th to 20th	Night	47% 8% 16% 17% 12% 1	1600
	21st to end	Night	47% 9% 12% 22% 9% 1	1600
July	1st to 10th	Night	43% 7% 17% 22% 10% 1	3560
	11th to 20th	Night	47% 8% 13% 21% 11%	3552
	21st to end	Night	38% 7% 13% 30% 11% 1	3809
August	1st to 10th	Night	51% 7% 12% 20% 9% 1	2596
	11th to 20th	Night	41% 7% 20% 22% 9%	3087
	21st to end	Night	34% 8% 19% 28% 11% 1	3586
September	1st to 10th	Night	37% 7% 14% 28% 13% 1	2000
	11th to 20th	Night	34% 6% 12% 30% 18% 1	2125
	21st to end	Night	41% 7% 11% 24% 15% 2	2200
October	1st to 10th	Night	49% 7% 13% 20% 10% 1	3689
	11th to 20th	Night	44% 6% 12% 21% 15% 2	3981
	21st to end	Night	47% 7% 11% 27% 8% 1	4610
November	1st to 10th	Night	52% 6% 10% 19% 11% 2	2667
	11th to 20th	Night	52% 7% 10% 16% 11% 2	2730
	21st to end	Night	55% 8% 14% 17% 6%	2872
December	1st to 10th	Night	68% 9% 11% 11% 2	4609
	11th to 20th	Night	61% 9% 8% 10% 12%	5208
	21st to end	Night	63% 8% 8% 12% 5% 3	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	32% 30% 24% 13% 2	200
	11th to 20th	Day	34% 24% 30% 10% 2 1	200
	21st to end	Day	25% 32% 29% 9% 5%	220
February	1st to 10th	Day	40% 35% 20% 4	294
	11th to 20th	Day	35% 27% 26% 9% 2	496
	21st to end	Day	34% 25% 28% 11% 2	414
March	1st to 10th	Day	34% 24% 28% 12% 2	500
	11th to 20th	Day	33% 26% 26% 11% 3 1	500
	21st to end	Day	33% 27% 22% 10% 5% 2	550
April	1st to 10th	Day	42% 19% 23% 11% 4 1	500
	11th to 20th	Day	35% 26% 22% 10% 5% 2	492
	21st to end	Day	34% 22% 27% 11% 4 2	500
May	1st to 10th	Day	35% 23% 25% 10% 5% 2	503
	11th to 20th	Day	24% 18% 35% 17% 4 2	527
	21st to end	Day	20% 19% 33% 19% 6% 2	573
June	1st to 10th	Day	24% 20% 32% 16% 7% 1	530
	11th to 20th	Day	23% 18% 32% 18% 8% 1	530
	21st to end	Day	25% 16% 35% 15% 7% 1	530
July	1st to 10th	Day	20% 12% 34% 22% 10% 2	525
	11th to 20th	Day	14% 11% 40% 25% 9% 2	524
	21st to end	Day	17% 11% 39% 23% 8% 1	576
August	1st to 10th	Day	16% 12% 29% 26% 13% 4	526
	11th to 20th	Day	18% 13% 29% 21% 12% 6%	519
	21st to end	Day	17% 14% 28% 24% 10% 6%	583
September	1st to 10th	Day	16% 16% 27% 23% 12% 5%	530
	11th to 20th	Day	16% 14% 24% 20% 17% 9%	530
	21st to end	Day	20% 16% 28% 20% 12% 5%	530
October	1st to 10th	Day	26% 15% 26% 17% 13% 3	530
	11th to 20th	Day	26% 18% 26% 19% 9% 1	503
	21st to end	Day	34% 19% 26% 16% 3 1	462
November	1st to 10th	Day	29% 22% 23% 17% 8%	425
	11th to 20th	Day	29% 25% 24% 15% 6% 1	430
	21st to end	Day	38% 20% 30% 10% 2	284
December	1st to 10th	Day	33% 28% 25% 13% 1	204
	11th to 20th	Day	39% 31% 22% 8% 2	200
	21st to end	Day	35% 21% 30% 11% 3	220
January	1st to 10th	Night	26% 25% 32% 15% 3	800
	11th to 20th	Night	28% 24% 29% 17% 2 1	800
	21st to end	Night	21% 23% 33% 20% 3	880
February	1st to 10th	Night	25% 25% 34% 15% 1	800
	11th to 20th	Night	24% 28% 30% 16% 2	783
	21st to end	Night	25% 25% 30% 18% 2	660
March	1st to 10th	Night	28% 21% 30% 18% 3	800
	11th to 20th	Night	27% 24% 28% 17% 4	800
	21st to end	Night	29% 22% 24% 18% 6%	880
April	1st to 10th	Night	29% 23% 26% 19% 4	800
	11th to 20th	Night	26% 26% 23% 17% 7%	791
	21st to end	Night	30% 19% 24% 21% 5% 1	645
May	1st to 10th	Night	33% 15% 25% 19% 8%	596
	11th to 20th	Night	22% 15% 28% 29% 6%	394
	21st to end	Night	21% 11% 25% 31% 11% 1	433
June	1st to 10th	Night	20% 13% 27% 28% 12% 1	400
	11th to 20th	Night	21% 10% 21% 36% 11% 1	400
	21st to end	Night	17% 12% 27% 33% 11% 1	400
July	1st to 10th	Night	16% 5% 23% 43% 13% 1	390
	11th to 20th	Night	16% 5% 22% 38% 16% 2	388
	21st to end	Night	14% 5% 26% 38% 17%	438
August	1st to 10th	Night	15% 6% 19% 40% 20%	405
	11th to 20th	Night	16% 7% 23% 33% 18% 4	702
	21st to end	Night	16% 8% 20% 33% 21% 1	880
September	1st to 10th	Night	18% 9% 23% 32% 16% 2	800
	11th to 20th	Night	18% 8% 19% 30% 21% 4	800
	21st to end	Night	20% 11% 23% 28% 16% 2	800
October	1st to 10th	Night	27% 11% 19% 26% 15% 2	800
	11th to 20th	Night	24% 15% 24% 26% 11% 1	800
	21st to end	Night	30% 20% 24% 20% 6%	880
November	1st to 10th	Night	26% 21% 25% 21% 6% 1	820
	11th to 20th	Night	25% 22% 26% 19% 7% 1	840
	21st to end	Night	26% 25% 26% 17% 5% 1	840
December	1st to 10th	Night	29% 28% 24% 15% 4	809
	11th to 20th	Night	31% 25% 23% 18% 3	800
	21st to end	Night	30% 24% 28% 15% 3	880

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