Long Mountain Observatory
Clear Sky Chart
History

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

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	13% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	126420
	Night	13% 9% 3 7% 4 4 4 4 5% 7% 40%	123646
Transparency	Day	43% 7% 11% 21% 15% 3	130145
	Night	41% 6% 11% 21% 17% 4	127991
Seeing	Day	19% 13% 23% 22% 15% 7%	15953
	Night	18% 10% 21% 28% 19% 3	25931

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	14% 15% 7% 12% 4 3 3 3 4 4 29%	4056
	11th to 20th	Day	15% 10% 7% 11% 3 3 3 3 3 4 37%	4060
	21st to end	Day	21% 10% 5% 6% 4 3 3 3 4 6% 35%	4741
February	1st to 10th	Day	19% 13% 4 8% 5% 4 4 2 4 4 33%	2000
	11th to 20th	Day	20% 12% 4 8% 6% 3 3 3 5% 4 31%	1948
	21st to end	Day	18% 11% 4 7% 5% 4 4 3 4 5% 34%	1650
March	1st to 10th	Day	18% 11% 4 7% 4 4 5% 3 5% 6% 34%	4480
	11th to 20th	Day	12% 9% 3 7% 4 4 3 3 4 5% 46%	4921
	21st to end	Day	14% 8% 3 6% 5% 4 4 3 5% 7% 41%	5508
April	1st to 10th	Day	13% 7% 3 6% 5% 5% 4 4 5% 8% 41%	2400
	11th to 20th	Day	13% 9% 2 6% 4 4 4 3 5% 7% 43%	2374
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 5% 7% 39%	2507
May	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 5% 7% 8% 28%	5862
	11th to 20th	Day	15% 10% 4 10% 6% 6% 6% 5% 6% 6% 27%	5861
	21st to end	Day	15% 9% 3 8% 5% 5% 5% 4 5% 6% 36%	6433
June	1st to 10th	Day	10% 6% 3 7% 5% 6% 6% 4 8% 8% 38%	2600
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 9% 10% 42%	2600
	21st to end	Day	6% 5% 2 6% 6% 5% 6% 6% 9% 12% 36%	2600
July	1st to 10th	Day	7% 5% 2 7% 5% 6% 6% 5% 9% 11% 37%	5900
	11th to 20th	Day	8% 6% 2 7% 6% 7% 7% 6% 9% 12% 30%	5900
	21st to end	Day	12% 8% 4 9% 7% 7% 7% 6% 8% 11% 22%	6424
August	1st to 10th	Day	6% 4 2 5% 5% 5% 6% 5% 8% 11% 42%	4181
	11th to 20th	Day	7% 5% 2 7% 5% 6% 6% 5% 8% 12% 36%	3849
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 7% 8% 44%	4312
September	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 5% 6% 7% 38%	2304
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 4 6% 7% 40%	2200
	21st to end	Day	13% 5% 3 7% 4 4 5% 4 6% 6% 44%	2200
October	1st to 10th	Day	10% 5% 2 5% 4 4 3 4 5% 5% 54%	2200
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 4 7% 54%	2150
	21st to end	Day	14% 7% 2 4 4 3 4 4 5% 6% 47%	2050
November	1st to 10th	Day	12% 7% 2 3 3 3 3 3 5% 6% 53%	1780
	11th to 20th	Day	12% 9% 2 5% 4 4 3 4 5% 6% 46%	1800
	21st to end	Day	14% 9% 2 6% 3 4 4 4 5% 6% 43%	1800
December	1st to 10th	Day	14% 9% 3 7% 5% 4 4 4 5% 5% 41%	2930
	11th to 20th	Day	15% 14% 5% 10% 6% 5% 5% 3 5% 4 28%	3373
	21st to end	Day	15% 12% 6% 8% 5% 4 4 3 6% 8% 29%	4466
January	1st to 10th	Night	15% 15% 5% 10% 3 3 3 2 4 5% 35%	5889
	11th to 20th	Night	17% 13% 4 7% 4 5% 2 2 3 4 40%	5900
	21st to end	Night	17% 13% 4 7% 4 3 3 3 4 5% 38%	6200
February	1st to 10th	Night	19% 10% 4 9% 4 4 3 3 5% 6% 33%	2400
	11th to 20th	Night	23% 10% 4 8% 5% 4 3 3 4 4 32%	2338
	21st to end	Night	22% 11% 3 6% 4 3 4 3 5% 6% 32%	1980
March	1st to 10th	Night	18% 14% 3 6% 4 4 4 3 5% 6% 32%	5373
	11th to 20th	Night	13% 6% 2 4 3 4 4 4 5% 7% 47%	5037
	21st to end	Night	14% 8% 3 9% 5% 5% 4 3 5% 6% 40%	5450
April	1st to 10th	Night	13% 6% 2 5% 4 5% 4 4 5% 7% 44%	2000
	11th to 20th	Night	15% 6% 3 5% 3 2 3 3 5% 7% 48%	1975
	21st to end	Night	13% 6% 2 6% 4 4 4 4 6% 8% 43%	1890
May	1st to 10th	Night	12% 10% 3 8% 6% 5% 6% 5% 7% 8% 31%	4033
	11th to 20th	Night	15% 9% 4 10% 6% 5% 5% 4 6% 7% 29%	4032
	21st to end	Night	13% 7% 2 7% 5% 5% 5% 4 6% 9% 35%	4421
June	1st to 10th	Night	6% 6% 3 7% 6% 6% 4 5% 6% 9% 43%	1800
	11th to 20th	Night	4 4 1 4 4 5% 3 4 7% 12% 51%	1800
	21st to end	Night	5% 5% 2 6% 5% 5% 5% 5% 7% 10% 45%	1800
July	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 5% 9% 11% 40%	4058
	11th to 20th	Night	7% 5% 2 6% 5% 5% 5% 5% 8% 15% 37%	4057
	21st to end	Night	8% 8% 3 8% 7% 6% 6% 5% 9% 12% 29%	4427
August	1st to 10th	Night	5% 4 2 5% 4 5% 5% 4 7% 10% 50%	2963
	11th to 20th	Night	7% 6% 2 5% 4 4 5% 5% 7% 10% 46%	3200
	21st to end	Night	5% 4 1 5% 3 4 4 4 6% 9% 55%	3586
September	1st to 10th	Night	10% 7% 2 6% 4 3 5% 4 6% 7% 46%	2075
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 6% 8% 45%	2200
	21st to end	Night	10% 5% 2 6% 4 4 4 3 5% 7% 50%	2200
October	1st to 10th	Night	10% 4 2 4 4 3 2 3 4 5% 59%	2200
	11th to 20th	Night	14% 5% 2 5% 3 4 3 2 4 5% 53%	2250
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 50%	2787
November	1st to 10th	Night	12% 7% 2 5% 4 3 3 3 4 6% 50%	2563
	11th to 20th	Night	13% 8% 2 5% 4 3 3 3 5% 7% 47%	2600
	21st to end	Night	15% 10% 3 5% 4 3 3 4 5% 6% 42%	2600
December	1st to 10th	Night	17% 13% 3 8% 4 4 3 3 4 5% 37%	4250
	11th to 20th	Night	19% 13% 4 8% 4 4 3 3 4 5% 35%	4822
	21st to end	Night	12% 14% 5% 9% 5% 5% 5% 4 5% 7% 29%	6490

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	56% 4 9% 15% 14% 2	4057
	11th to 20th	Day	51% 4 4 14% 25% 2	4060
	21st to end	Day	49% 4 7% 22% 13% 5%	4741
February	1st to 10th	Day	55% 4 7% 16% 17% 2	2000
	11th to 20th	Day	55% 5% 7% 16% 14% 3	1958
	21st to end	Day	52% 5% 6% 15% 17% 5%	1650
March	1st to 10th	Day	51% 5% 7% 13% 16% 8%	4480
	11th to 20th	Day	41% 4 6% 19% 21% 10%	4921
	21st to end	Day	44% 4 9% 27% 15% 1	5510
April	1st to 10th	Day	41% 6% 11% 22% 17% 3	2400
	11th to 20th	Day	41% 5% 8% 19% 24% 3	2374
	21st to end	Day	44% 5% 10% 26% 14% 1	2507
May	1st to 10th	Day	50% 9% 11% 20% 9% 1	5897
	11th to 20th	Day	54% 9% 12% 18% 7%	5865
	21st to end	Day	47% 7% 9% 21% 14% 2	6447
June	1st to 10th	Day	41% 8% 15% 23% 13% 2	2600
	11th to 20th	Day	33% 9% 19% 22% 15% 2	2600
	21st to end	Day	36% 10% 19% 20% 13% 3	2600
July	1st to 10th	Day	35% 11% 19% 23% 11% 1	5835
	11th to 20th	Day	40% 13% 22% 20% 4 1	5831
	21st to end	Day	51% 11% 17% 17% 3	6240
August	1st to 10th	Day	32% 9% 17% 32% 10% 1	4168
	11th to 20th	Day	36% 9% 19% 23% 12% 1	3885
	21st to end	Day	34% 8% 12% 31% 14% 1	4311
September	1st to 10th	Day	43% 6% 13% 18% 16% 5%	2304
	11th to 20th	Day	42% 7% 10% 21% 18% 2	2200
	21st to end	Day	40% 6% 10% 22% 18% 5%	2200
October	1st to 10th	Day	24% 4 7% 18% 34% 12%	3590
	11th to 20th	Day	23% 3 6% 24% 29% 15%	3481
	21st to end	Day	28% 4 9% 24% 25% 10%	3287
November	1st to 10th	Day	32% 4 9% 23% 26% 5%	1780
	11th to 20th	Day	39% 3 6% 22% 23% 6%	1800
	21st to end	Day	42% 4 7% 22% 21% 4	1800
December	1st to 10th	Day	44% 4 5% 20% 23% 3	2930
	11th to 20th	Day	58% 5% 6% 17% 12% 2	3370
	21st to end	Day	52% 5% 11% 19% 11% 2	4466
January	1st to 10th	Night	52% 4 8% 17% 17% 1	5889
	11th to 20th	Night	51% 3 5% 15% 21% 6%	5900
	21st to end	Night	50% 3 5% 16% 20% 6%	6200
February	1st to 10th	Night	52% 4 9% 17% 16% 2	2400
	11th to 20th	Night	57% 3 5% 16% 13% 5%	2339
	21st to end	Night	52% 4 8% 16% 15% 5%	1980
March	1st to 10th	Night	54% 4 7% 14% 17% 4	5373
	11th to 20th	Night	36% 4 7% 20% 19% 14%	5037
	21st to end	Night	46% 4 8% 25% 14% 2	5450
April	1st to 10th	Night	39% 4 11% 22% 20% 3	2000
	11th to 20th	Night	36% 5% 9% 22% 22% 6%	1975
	21st to end	Night	38% 6% 12% 28% 15% 1	1890
May	1st to 10th	Night	47% 9% 14% 20% 9% 1	4060
	11th to 20th	Night	53% 8% 13% 17% 9%	4033
	21st to end	Night	42% 9% 13% 22% 12% 1	4429
June	1st to 10th	Night	37% 7% 15% 26% 12% 2	1800
	11th to 20th	Night	24% 7% 22% 31% 15% 2	1800
	21st to end	Night	31% 9% 17% 28% 12% 4	1800
July	1st to 10th	Night	33% 11% 20% 23% 12% 1	4015
	11th to 20th	Night	32% 11% 27% 23% 5% 2	4006
	21st to end	Night	43% 11% 22% 18% 5% 1	4297
August	1st to 10th	Night	27% 7% 14% 37% 14% 1	2952
	11th to 20th	Night	31% 9% 23% 23% 13% 1	3230
	21st to end	Night	25% 7% 14% 36% 18% 1	3586
September	1st to 10th	Night	35% 7% 15% 21% 17% 5%	2075
	11th to 20th	Night	35% 8% 11% 23% 21% 1	2200
	21st to end	Night	34% 5% 10% 23% 23% 5%	2200
October	1st to 10th	Night	22% 3 6% 24% 29% 15%	3590
	11th to 20th	Night	23% 3 5% 24% 28% 17%	3658
	21st to end	Night	25% 3 6% 26% 28% 12%	4500
November	1st to 10th	Night	36% 3 7% 22% 24% 7%	2563
	11th to 20th	Night	37% 4 7% 20% 25% 8%	2600
	21st to end	Night	44% 3 6% 22% 20% 5%	2600
December	1st to 10th	Night	50% 4 5% 15% 24% 2	4250
	11th to 20th	Night	52% 4 6% 17% 18% 2	4824
	21st to end	Night	52% 6% 10% 20% 10% 1	6490

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	29% 29% 29% 10% 3	300
	11th to 20th	Day	34% 30% 22% 10% 3 1	300
	21st to end	Day	29% 28% 33% 7% 2	330
February	1st to 10th	Day	28% 30% 30% 10% 3	300
	11th to 20th	Day	44% 24% 26% 4 2	300
	21st to end	Day	37% 27% 29% 6%	249
March	1st to 10th	Day	32% 24% 31% 11% 2	300
	11th to 20th	Day	28% 25% 28% 14% 4 1	467
	21st to end	Day	27% 19% 34% 14% 5% 1	550
April	1st to 10th	Day	23% 22% 28% 17% 10%	500
	11th to 20th	Day	23% 17% 31% 20% 7% 2	492
	21st to end	Day	21% 15% 32% 21% 10% 1	500
May	1st to 10th	Day	20% 15% 32% 17% 11% 4	503
	11th to 20th	Day	16% 10% 30% 24% 13% 7%	527
	21st to end	Day	18% 8% 22% 26% 15% 12%	573
June	1st to 10th	Day	13% 6% 23% 26% 21% 11%	530
	11th to 20th	Day	6% 7% 17% 32% 23% 15%	530
	21st to end	Day	7% 5% 20% 30% 21% 17%	530
July	1st to 10th	Day	11% 4 14% 29% 24% 18%	525
	11th to 20th	Day	12% 3 10% 24% 29% 22%	524
	21st to end	Day	13% 4 11% 26% 29% 18%	576
August	1st to 10th	Day	13% 4 14% 28% 25% 17%	526
	11th to 20th	Day	11% 5% 12% 35% 28% 10%	519
	21st to end	Day	15% 5% 9% 24% 30% 18%	583
September	1st to 10th	Day	12% 4 13% 28% 30% 12%	530
	11th to 20th	Day	14% 4 13% 30% 26% 12%	530
	21st to end	Day	15% 7% 18% 28% 24% 7%	530
October	1st to 10th	Day	14% 5% 22% 30% 22% 7%	530
	11th to 20th	Day	16% 9% 29% 29% 15% 3	530
	21st to end	Day	23% 12% 31% 22% 9% 3	433
November	1st to 10th	Day	21% 13% 33% 24% 7% 2	306
	11th to 20th	Day	18% 20% 38% 20% 3	300
	21st to end	Day	23% 17% 35% 16% 7% 1	300
December	1st to 10th	Day	30% 18% 33% 13% 4 1	300
	11th to 20th	Day	30% 20% 27% 17% 5%	300
	21st to end	Day	27% 24% 31% 14% 3	330
January	1st to 10th	Night	26% 20% 30% 21% 5%	800
	11th to 20th	Night	29% 22% 27% 18% 4	800
	21st to end	Night	23% 20% 34% 20% 3	880
February	1st to 10th	Night	25% 18% 30% 24% 3	800
	11th to 20th	Night	32% 20% 28% 19% 2	783
	21st to end	Night	31% 20% 27% 20% 2	660
March	1st to 10th	Night	27% 15% 29% 26% 4	800
	11th to 20th	Night	24% 16% 30% 22% 7%	800
	21st to end	Night	23% 16% 28% 25% 8%	875
April	1st to 10th	Night	19% 16% 27% 28% 10% 1	600
	11th to 20th	Night	20% 11% 31% 27% 9% 2	593
	21st to end	Night	17% 9% 28% 34% 11% 1	600
May	1st to 10th	Night	16% 8% 29% 29% 18% 1	600
	11th to 20th	Night	16% 7% 21% 33% 19% 4	591
	21st to end	Night	11% 2 17% 35% 31% 5%	650
June	1st to 10th	Night	9% 2 15% 37% 32% 5%	600
	11th to 20th	Night	5% 1 13% 36% 34% 10%	600
	21st to end	Night	7% 2 12% 32% 39% 8%	600
July	1st to 10th	Night	11% 2 6% 29% 42% 11%	585
	11th to 20th	Night	10% 2 7% 26% 43% 12%	582
	21st to end	Night	10% 2 5% 32% 41% 11%	657
August	1st to 10th	Night	8% 2 6% 31% 45% 9%	597
	11th to 20th	Night	8% 2 7% 36% 41% 6%	594
	21st to end	Night	10% 1 7% 28% 44% 8%	660
September	1st to 10th	Night	13% 2 8% 32% 36% 8%	675
	11th to 20th	Night	14% 3 12% 28% 37% 5%	800
	21st to end	Night	12% 3 14% 34% 32% 5%	800
October	1st to 10th	Night	13% 5% 17% 32% 27% 8%	800
	11th to 20th	Night	18% 8% 22% 30% 20% 2	800
	21st to end	Night	19% 9% 25% 31% 14% 2	880
November	1st to 10th	Night	17% 11% 20% 33% 17% 2	789
	11th to 20th	Night	18% 15% 28% 27% 11% 1	800
	21st to end	Night	22% 15% 26% 27% 10% 1	800
December	1st to 10th	Night	29% 14% 26% 25% 6%	800
	11th to 20th	Night	24% 16% 27% 27% 6% 1	800
	21st to end	Night	27% 19% 25% 24% 5%	880

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