Beaver Meadow Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Beaver Meadow 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	17% 13% 5% 10% 5% 4 4 3 5% 6% 27%	127003
	Night	15% 15% 6% 12% 6% 4 4 3 4 5% 25%	123016
Transparency	Day	56% 6% 10% 18% 9% 1	130595
	Night	59% 5% 10% 16% 8% 1	127510
Seeing	Day	26% 19% 29% 17% 8% 2	15857
	Night	24% 19% 28% 20% 8% 1	25189

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	11% 22% 12% 24% 9% 4 3 3 2 2 7%	3597
	11th to 20th	Day	9% 23% 12% 24% 8% 7% 3 2 2 2 7%	3865
	21st to end	Day	15% 17% 8% 19% 9% 7% 7% 3 3 3 9%	4466
February	1st to 10th	Day	14% 23% 7% 17% 10% 5% 4 3 3 3 12%	1800
	11th to 20th	Day	12% 18% 9% 19% 10% 4 4 3 3 4 14%	1753
	21st to end	Day	17% 18% 5% 15% 8% 5% 3 3 4 5% 18%	1575
March	1st to 10th	Day	14% 14% 10% 14% 6% 4 3 2 3 5% 25%	4904
	11th to 20th	Day	10% 18% 8% 9% 4 4 4 3 4 6% 30%	4980
	21st to end	Day	17% 18% 6% 11% 5% 4 3 3 4 5% 24%	5482
April	1st to 10th	Day	17% 19% 5% 8% 4 4 4 3 4 5% 27%	2400
	11th to 20th	Day	18% 16% 2 8% 4 3 4 3 4 5% 32%	2374
	21st to end	Day	21% 14% 4 8% 4 4 4 3 4 5% 32%	2570
May	1st to 10th	Day	26% 10% 3 7% 5% 4 4 3 5% 5% 27%	5925
	11th to 20th	Day	20% 10% 3 8% 6% 4 4 4 5% 7% 29%	6318
	21st to end	Day	22% 9% 2 6% 4 4 4 4 5% 6% 32%	6949
June	1st to 10th	Day	19% 9% 2 5% 5% 4 4 4 6% 6% 35%	2800
	11th to 20th	Day	17% 6% 2 7% 6% 5% 5% 5% 7% 8% 33%	2800
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 36%	2800
July	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 7% 10% 38%	6360
	11th to 20th	Day	14% 7% 2 7% 4 5% 5% 5% 7% 9% 35%	6360
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	6543
August	1st to 10th	Day	19% 6% 2 7% 4 4 4 4 6% 8% 35%	4234
	11th to 20th	Day	16% 8% 3 8% 7% 6% 7% 5% 7% 9% 24%	4174
	21st to end	Day	15% 6% 2 7% 4 5% 5% 4 6% 8% 38%	4378
September	1st to 10th	Day	17% 7% 2 6% 4 4 5% 4 5% 8% 37%	2400
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 6% 8% 41%	2357
	21st to end	Day	19% 8% 2 8% 5% 5% 5% 4 6% 7% 31%	2110
October	1st to 10th	Day	22% 8% 3 7% 4 4 4 4 5% 5% 34%	2000
	11th to 20th	Day	22% 14% 4 8% 6% 3 4 3 4 5% 27%	2000
	21st to end	Day	25% 18% 3 8% 4 4 4 3 4 5% 23%	2200
November	1st to 10th	Day	15% 18% 4 9% 4 4 5% 3 4 5% 29%	1783
	11th to 20th	Day	14% 25% 6% 9% 4 3 4 4 5% 6% 21%	1600
	21st to end	Day	17% 25% 7% 10% 5% 4 4 3 4 5% 16%	1600
December	1st to 10th	Day	17% 28% 13% 17% 4 3 3 2 3 3 8%	2600
	11th to 20th	Day	18% 22% 6% 18% 6% 4 3 3 4 4 12%	2986
	21st to end	Day	18% 24% 8% 14% 7% 4 3 3 4 4 11%	3960
January	1st to 10th	Night	12% 21% 9% 24% 12% 4 3 2 2 2 9%	6349
	11th to 20th	Night	7% 20% 13% 26% 11% 6% 4 2 2 2 5%	6017
	21st to end	Night	13% 19% 9% 20% 11% 6% 6% 3 3 3 9%	6490
February	1st to 10th	Night	14% 22% 6% 17% 10% 6% 4 3 3 3 13%	2600
	11th to 20th	Night	11% 17% 7% 20% 9% 6% 5% 3 4 4 14%	2533
	21st to end	Night	15% 17% 5% 19% 7% 4 4 3 5% 4 16%	2055
March	1st to 10th	Night	16% 15% 6% 12% 8% 6% 3 2 4 5% 24%	4919
	11th to 20th	Night	13% 16% 7% 10% 4 3 4 3 4 6% 30%	4979
	21st to end	Night	15% 14% 6% 13% 6% 4 3 3 4 5% 27%	5450
April	1st to 10th	Night	18% 16% 4 9% 5% 4 3 3 4 6% 27%	2000
	11th to 20th	Night	18% 13% 4 7% 4 4 4 3 5% 5% 33%	1975
	21st to end	Night	16% 12% 4 7% 4 4 5% 4 5% 7% 33%	1845
May	1st to 10th	Night	25% 12% 4 9% 4 4 4 3 4 4 26%	4013
	11th to 20th	Night	17% 9% 3 8% 5% 4 4 4 5% 6% 33%	3575
	21st to end	Night	21% 11% 3 7% 3 3 4 3 5% 7% 34%	3926
June	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 4 7% 7% 35%	1600
	11th to 20th	Night	13% 7% 2 7% 4 4 5% 4 6% 9% 39%	1600
	21st to end	Night	15% 7% 2 6% 4 4 4 5% 6% 6% 41%	1600
July	1st to 10th	Night	14% 7% 2 6% 5% 4 4 4 6% 9% 39%	3600
	11th to 20th	Night	12% 8% 2 5% 4 4 4 4 6% 8% 43%	3600
	21st to end	Night	12% 6% 2 6% 3 3 4 4 5% 7% 47%	4305
August	1st to 10th	Night	14% 8% 3 7% 4 4 4 4 6% 8% 38%	2915
	11th to 20th	Night	17% 9% 3 9% 4 5% 4 4 5% 7% 34%	2875
	21st to end	Night	10% 7% 2 5% 4 4 5% 4 7% 8% 44%	3487
September	1st to 10th	Night	15% 8% 2 6% 3 3 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 6% 49%	2039
	21st to end	Night	18% 9% 2 6% 5% 4 5% 4 6% 7% 34%	2288
October	1st to 10th	Night	19% 8% 3 6% 4 4 5% 3 6% 6% 37%	2400
	11th to 20th	Night	21% 13% 4 7% 4 4 4 3 5% 7% 29%	2400
	21st to end	Night	25% 15% 3 7% 4 4 4 3 4 6% 25%	2640
November	1st to 10th	Night	15% 17% 3 8% 4 4 4 3 4 5% 33%	2557
	11th to 20th	Night	15% 20% 4 10% 5% 5% 4 4 5% 6% 22%	2800
	21st to end	Night	15% 23% 8% 13% 6% 4 4 3 4 5% 17%	2800
December	1st to 10th	Night	15% 27% 12% 17% 6% 4 3 2 3 3 8%	4580
	11th to 20th	Night	17% 26% 9% 15% 6% 4 3 2 3 3 12%	5208
	21st to end	Night	16% 24% 9% 13% 6% 5% 4 3 4 4 11%	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	82% 4 5% 5% 3	3598
	11th to 20th	Day	84% 3 6% 4 2	3899
	21st to end	Day	77% 5% 8% 8% 2	4466
February	1st to 10th	Day	77% 4 9% 8% 2	1800
	11th to 20th	Day	74% 3 7% 11% 5%	1762
	21st to end	Day	69% 3 11% 12% 5% 1	1575
March	1st to 10th	Day	62% 4 11% 18% 5%	4907
	11th to 20th	Day	55% 4 17% 15% 7% 2	4980
	21st to end	Day	61% 4 6% 19% 8% 1	5482
April	1st to 10th	Day	59% 4 9% 16% 10% 1	2400
	11th to 20th	Day	53% 6% 9% 19% 11% 2	2374
	21st to end	Day	55% 5% 9% 17% 13% 2	2570
May	1st to 10th	Day	58% 5% 8% 16% 9% 3	5934
	11th to 20th	Day	54% 8% 8% 19% 11% 1	6322
	21st to end	Day	51% 7% 11% 19% 10% 2	6949
June	1st to 10th	Day	47% 7% 11% 20% 14% 1	2800
	11th to 20th	Day	47% 8% 13% 21% 11% 1	2800
	21st to end	Day	44% 7% 12% 22% 13% 1	2800
July	1st to 10th	Day	41% 6% 12% 26% 14% 1	6290
	11th to 20th	Day	42% 8% 13% 21% 15% 1	6285
	21st to end	Day	40% 7% 11% 26% 16%	6356
August	1st to 10th	Day	46% 7% 13% 21% 13%	4221
	11th to 20th	Day	53% 9% 13% 20% 5%	4213
	21st to end	Day	42% 7% 10% 31% 9% 1	4378
September	1st to 10th	Day	44% 7% 11% 26% 11% 1	2400
	11th to 20th	Day	41% 5% 11% 27% 14% 1	2357
	21st to end	Day	52% 7% 10% 19% 11% 1	2110
October	1st to 10th	Day	44% 6% 12% 23% 14% 1	3260
	11th to 20th	Day	50% 4 13% 14% 15% 4	3240
	21st to end	Day	51% 4 13% 22% 8% 1	3538
November	1st to 10th	Day	56% 7% 10% 17% 9% 2	1783
	11th to 20th	Day	63% 5% 11% 13% 6% 2	1600
	21st to end	Day	70% 4 11% 11% 4 1	1600
December	1st to 10th	Day	83% 3 6% 6% 2	2600
	11th to 20th	Day	75% 6% 9% 7% 2	2986
	21st to end	Day	77% 4 8% 7% 4	3960
January	1st to 10th	Night	80% 5% 5% 6% 4	6349
	11th to 20th	Night	83% 4 7% 4 2	6058
	21st to end	Night	78% 5% 7% 7% 3	6490
February	1st to 10th	Night	75% 5% 8% 8% 4	2600
	11th to 20th	Night	71% 5% 9% 10% 4	2535
	21st to end	Night	68% 5% 8% 12% 6%	2055
March	1st to 10th	Night	62% 5% 11% 16% 6% 1	4929
	11th to 20th	Night	54% 5% 14% 18% 6% 4	4980
	21st to end	Night	57% 5% 9% 17% 11% 2	5450
April	1st to 10th	Night	58% 5% 10% 17% 9% 1	2000
	11th to 20th	Night	53% 5% 10% 19% 12% 1	1975
	21st to end	Night	49% 5% 12% 19% 13% 2	1845
May	1st to 10th	Night	59% 6% 8% 14% 9% 4	4020
	11th to 20th	Night	50% 8% 12% 20% 10%	3576
	21st to end	Night	48% 6% 12% 19% 13% 2	3927
June	1st to 10th	Night	43% 7% 13% 22% 14% 1	1600
	11th to 20th	Night	41% 7% 15% 22% 15% 1	1600
	21st to end	Night	40% 8% 15% 22% 13% 2	1600
July	1st to 10th	Night	42% 6% 14% 23% 14% 1	3560
	11th to 20th	Night	38% 7% 14% 26% 15% 1	3552
	21st to end	Night	36% 6% 12% 25% 17% 3	4177
August	1st to 10th	Night	42% 6% 14% 26% 10% 1	2904
	11th to 20th	Night	47% 7% 13% 25% 7%	2902
	21st to end	Night	35% 7% 14% 33% 10% 1	3487
September	1st to 10th	Night	38% 6% 13% 28% 13% 2	2000
	11th to 20th	Night	35% 6% 13% 30% 16% 1	2039
	21st to end	Night	48% 7% 12% 21% 12% 1	2288
October	1st to 10th	Night	45% 7% 8% 23% 17% 1	3920
	11th to 20th	Night	49% 4 12% 16% 13% 5%	3899
	21st to end	Night	49% 5% 15% 23% 9% 1	4252
November	1st to 10th	Night	52% 6% 10% 20% 10% 3	2557
	11th to 20th	Night	61% 5% 12% 13% 6% 3	2800
	21st to end	Night	67% 6% 12% 11% 4	2800
December	1st to 10th	Night	81% 5% 7% 5% 1	4580
	11th to 20th	Night	77% 4 6% 8% 4	5208
	21st to end	Night	77% 5% 7% 8% 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	34% 26% 29% 9% 2 1	200
	11th to 20th	Day	33% 34% 25% 6% 2	265
	21st to end	Day	31% 31% 28% 9% 2	330
February	1st to 10th	Day	39% 31% 24% 7%	300
	11th to 20th	Day	36% 31% 29% 3 1	300
	21st to end	Day	41% 27% 26% 6%	339
March	1st to 10th	Day	32% 27% 26% 11% 3	500
	11th to 20th	Day	33% 25% 31% 9% 3	500
	21st to end	Day	35% 23% 29% 10% 4	550
April	1st to 10th	Day	35% 22% 29% 9% 4 1	500
	11th to 20th	Day	35% 24% 24% 10% 5% 2	492
	21st to end	Day	31% 20% 30% 15% 3 1	500
May	1st to 10th	Day	32% 18% 29% 13% 6% 2	503
	11th to 20th	Day	24% 16% 31% 21% 7% 2	527
	21st to end	Day	21% 16% 30% 20% 10% 3	573
June	1st to 10th	Day	22% 18% 33% 18% 6% 2	530
	11th to 20th	Day	20% 13% 32% 22% 12% 1	530
	21st to end	Day	18% 16% 38% 19% 6% 2	530
July	1st to 10th	Day	14% 11% 30% 28% 12% 4	525
	11th to 20th	Day	14% 10% 30% 28% 15% 3	524
	21st to end	Day	13% 10% 37% 24% 12% 3	576
August	1st to 10th	Day	14% 11% 26% 28% 15% 6%	526
	11th to 20th	Day	17% 9% 26% 27% 14% 6%	519
	21st to end	Day	16% 11% 30% 25% 12% 6%	583
September	1st to 10th	Day	20% 12% 26% 21% 15% 6%	530
	11th to 20th	Day	12% 9% 25% 23% 22% 8%	530
	21st to end	Day	20% 18% 25% 22% 11% 4	530
October	1st to 10th	Day	25% 13% 28% 18% 12% 4	530
	11th to 20th	Day	31% 20% 29% 15% 5% 1	530
	21st to end	Day	35% 22% 26% 15% 2	583
November	1st to 10th	Day	28% 23% 28% 14% 6% 1	382
	11th to 20th	Day	30% 31% 29% 9% 2 1	200
	21st to end	Day	35% 27% 30% 8% 1	200
December	1st to 10th	Day	40% 23% 28% 8% 2	200
	11th to 20th	Day	37% 32% 23% 8% 2	200
	21st to end	Day	37% 25% 30% 6% 3	220
January	1st to 10th	Night	28% 31% 28% 10% 2	800
	11th to 20th	Night	30% 31% 25% 12% 2	800
	21st to end	Night	24% 32% 30% 12% 2	880
February	1st to 10th	Night	31% 30% 30% 10% 1	800
	11th to 20th	Night	29% 32% 29% 8% 2	783
	21st to end	Night	31% 30% 27% 10% 1	660
March	1st to 10th	Night	26% 28% 29% 14% 3	800
	11th to 20th	Night	30% 24% 28% 14% 4	800
	21st to end	Night	24% 24% 32% 15% 5% 1	880
April	1st to 10th	Night	29% 21% 31% 18% 2	800
	11th to 20th	Night	30% 17% 29% 19% 5% 1	791
	21st to end	Night	23% 16% 28% 27% 7%	645
May	1st to 10th	Night	24% 12% 26% 27% 9% 1	586
	11th to 20th	Night	15% 10% 37% 31% 6%	394
	21st to end	Night	16% 10% 30% 30% 13% 1	433
June	1st to 10th	Night	15% 10% 34% 30% 11% 1	400
	11th to 20th	Night	19% 10% 29% 33% 9%	400
	21st to end	Night	13% 9% 36% 33% 9%	400
July	1st to 10th	Night	9% 6% 27% 39% 18% 1	390
	11th to 20th	Night	11% 4 33% 35% 15% 2	388
	21st to end	Night	12% 6% 29% 35% 18% 1	599
August	1st to 10th	Night	11% 5% 28% 33% 21% 3	597
	11th to 20th	Night	14% 7% 22% 30% 24% 3	594
	21st to end	Night	12% 9% 26% 31% 20% 2	820
September	1st to 10th	Night	19% 9% 23% 30% 17% 2	800
	11th to 20th	Night	15% 7% 22% 29% 24% 3	800
	21st to end	Night	22% 13% 21% 27% 15% 3	800
October	1st to 10th	Night	23% 12% 28% 21% 16% 1	800
	11th to 20th	Night	28% 18% 28% 21% 5% 1	800
	21st to end	Night	35% 16% 31% 16% 3	880
November	1st to 10th	Night	26% 19% 29% 20% 6%	789
	11th to 20th	Night	28% 24% 28% 13% 6% 1	800
	21st to end	Night	28% 28% 26% 14% 4	800
December	1st to 10th	Night	33% 29% 26% 10% 3	800
	11th to 20th	Night	32% 27% 28% 11% 1	800
	21st to end	Night	30% 29% 29% 11% 1	880

Page generated at 2025-10-31 02:02:35. Version ac07

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