Berkshire School Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Berkshire School Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	16% 9% 3 8% 5% 4 5% 4 6% 7% 34%	131371
	Night	16% 9% 3 8% 5% 4 4 4 5% 6% 34%	130797
Transparency	Day	48% 6% 12% 21% 12% 2	134886
	Night	48% 6% 12% 20% 12% 2	135369
Seeing	Day	26% 17% 28% 18% 8% 2	16691
	Night	25% 16% 28% 23% 8% 1	25865

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% 12% 4 17% 6% 5% 5% 4 5% 5% 28%	4661
	11th to 20th	Day	14% 11% 4 10% 6% 4 4 4 4 5% 33%	4650
	21st to end	Day	14% 12% 5% 8% 5% 4 4 3 4 5% 36%	5304
February	1st to 10th	Day	15% 11% 3 9% 8% 5% 5% 4 5% 6% 30%	1904
	11th to 20th	Day	14% 9% 4 10% 6% 6% 5% 4 5% 6% 31%	2048
	21st to end	Day	16% 13% 4 8% 6% 5% 4 3 5% 5% 32%	1730
March	1st to 10th	Day	19% 13% 5% 9% 5% 4 3 3 4 5% 31%	4576
	11th to 20th	Day	13% 10% 3 8% 5% 5% 5% 4 5% 6% 36%	4792
	21st to end	Day	20% 9% 3 9% 5% 4 4 3 5% 6% 31%	6102
April	1st to 10th	Day	18% 10% 3 8% 4 5% 5% 3 4 6% 33%	2520
	11th to 20th	Day	15% 9% 3 7% 5% 5% 5% 3 5% 6% 38%	2446
	21st to end	Day	19% 7% 4 8% 5% 4 4 4 5% 6% 35%	2495
May	1st to 10th	Day	29% 11% 3 6% 4 4 4 3 5% 7% 24%	5886
	11th to 20th	Day	17% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	6308
	21st to end	Day	19% 9% 3 7% 4 4 5% 4 7% 8% 30%	6949
June	1st to 10th	Day	16% 7% 2 6% 5% 5% 6% 4 6% 10% 33%	2800
	11th to 20th	Day	17% 7% 2 5% 5% 5% 5% 4 7% 9% 34%	2800
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 7% 8% 34%	2800
July	1st to 10th	Day	11% 8% 3 8% 5% 5% 6% 4 7% 8% 34%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 9% 11% 33%	6360
	21st to end	Day	13% 7% 3 7% 4 4 5% 4 6% 7% 40%	6926
August	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 4 7% 8% 29%	4481
	11th to 20th	Day	12% 7% 3 7% 4 5% 5% 5% 7% 8% 37%	3850
	21st to end	Day	10% 6% 2 6% 4 5% 5% 5% 7% 9% 41%	4312
September	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 5% 7% 42%	2400
	11th to 20th	Day	12% 6% 3 6% 4 4 4 3 5% 6% 48%	2289
	21st to end	Day	17% 6% 2 5% 4 5% 5% 5% 6% 8% 38%	2200
October	1st to 10th	Day	21% 7% 3 7% 4 3 4 2 5% 5% 39%	2122
	11th to 20th	Day	15% 8% 3 8% 4 4 3 3 5% 7% 41%	2000
	21st to end	Day	22% 10% 3 7% 4 4 4 3 5% 5% 33%	1980
November	1st to 10th	Day	12% 9% 3 6% 5% 3 4 3 5% 6% 44%	1851
	11th to 20th	Day	16% 9% 3 6% 5% 3 5% 5% 5% 6% 38%	1890
	21st to end	Day	17% 11% 3 7% 4 5% 4 4 5% 5% 34%	1785
December	1st to 10th	Day	15% 10% 4 7% 6% 4 5% 4 6% 6% 34%	2680
	11th to 20th	Day	18% 11% 4 7% 4 4 4 3 5% 5% 35%	3066
	21st to end	Day	18% 13% 6% 10% 4 4 3 3 4 6% 29%	4048
January	1st to 10th	Night	12% 10% 4 17% 6% 5% 5% 4 6% 5% 27%	8234
	11th to 20th	Night	14% 11% 3 12% 7% 5% 5% 4 6% 7% 25%	8235
	21st to end	Night	13% 10% 3 7% 6% 7% 6% 3 6% 6% 33%	8861
February	1st to 10th	Night	16% 9% 4 7% 6% 5% 5% 5% 5% 6% 32%	2714
	11th to 20th	Night	13% 10% 4 9% 6% 5% 5% 5% 7% 7% 29%	2458
	21st to end	Night	17% 9% 4 9% 7% 4 5% 4 5% 6% 31%	2076
March	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 32%	5484
	11th to 20th	Night	15% 9% 4 7% 6% 5% 5% 4 5% 6% 34%	5381
	21st to end	Night	19% 12% 3 8% 5% 4 3 3 5% 5% 33%	5072
April	1st to 10th	Night	17% 11% 4 8% 6% 4 4 4 6% 6% 29%	2100
	11th to 20th	Night	19% 10% 3 6% 5% 4 3 3 5% 5% 36%	2036
	21st to end	Night	19% 10% 3 6% 5% 4 3 3 5% 6% 37%	1903
May	1st to 10th	Night	30% 12% 3 7% 4 3 4 3 4 5% 24%	4054
	11th to 20th	Night	18% 13% 4 8% 4 4 4 3 5% 7% 30%	3628
	21st to end	Night	21% 9% 4 8% 5% 4 4 3 5% 6% 31%	3926
June	1st to 10th	Night	17% 8% 3 6% 4 4 4 4 5% 7% 39%	1600
	11th to 20th	Night	15% 8% 2 6% 5% 4 5% 5% 6% 10% 34%	1600
	21st to end	Night	13% 6% 2 6% 5% 4 4 4 6% 8% 43%	1600
July	1st to 10th	Night	13% 7% 3 7% 5% 4 4 4 6% 8% 40%	3600
	11th to 20th	Night	14% 8% 2 8% 5% 4 5% 4 7% 9% 34%	3600
	21st to end	Night	11% 7% 3 6% 4 4 4 3 5% 6% 47%	3926
August	1st to 10th	Night	20% 8% 3 7% 5% 4 5% 4 6% 6% 33%	2637
	11th to 20th	Night	13% 7% 3 7% 4 4 5% 4 6% 7% 40%	3193
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 8% 49%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 5% 4 5% 6% 45%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 4 6% 53%	2110
	21st to end	Night	15% 9% 2 6% 4 3 4 3 5% 6% 43%	2200
October	1st to 10th	Night	19% 8% 2 6% 4 3 4 3 4 7% 40%	2275
	11th to 20th	Night	15% 7% 2 6% 4 5% 4 3 6% 6% 42%	2400
	21st to end	Night	20% 10% 3 6% 5% 4 4 2 5% 6% 35%	2860
November	1st to 10th	Night	14% 8% 3 6% 4 4 3 3 5% 6% 44%	2667
	11th to 20th	Night	17% 7% 3 7% 5% 4 5% 4 5% 6% 37%	2730
	21st to end	Night	13% 9% 4 7% 6% 5% 5% 4 6% 6% 35%	2833
December	1st to 10th	Night	18% 10% 3 8% 5% 5% 4 4 5% 8% 30%	4720
	11th to 20th	Night	23% 10% 3 7% 4 4 4 3 4 4 33%	5348
	21st to end	Night	18% 12% 6% 9% 5% 4 4 3 4 5% 29%	7150

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% 6% 10% 20% 7%	4662
	11th to 20th	Day	51% 6% 8% 19% 13% 3	4680
	21st to end	Day	51% 4 9% 21% 14% 1	5304
February	1st to 10th	Day	55% 5% 11% 17% 10% 1	1904
	11th to 20th	Day	53% 6% 8% 20% 12% 2	2057
	21st to end	Day	54% 4 10% 19% 12% 1	1730
March	1st to 10th	Day	56% 4 10% 14% 11% 5%	4579
	11th to 20th	Day	45% 7% 15% 22% 8% 3	4792
	21st to end	Day	52% 6% 5% 14% 18% 4	6102
April	1st to 10th	Day	53% 5% 8% 20% 12% 2	2520
	11th to 20th	Day	47% 5% 10% 21% 15% 2	2446
	21st to end	Day	49% 5% 9% 20% 14% 3	2495
May	1st to 10th	Day	61% 4 12% 14% 8% 1	5897
	11th to 20th	Day	53% 4 13% 19% 10% 1	6312
	21st to end	Day	49% 7% 11% 22% 10% 1	6949
June	1st to 10th	Day	46% 7% 14% 22% 10%	2800
	11th to 20th	Day	46% 5% 15% 22% 11% 1	2800
	21st to end	Day	47% 6% 14% 21% 11% 1	2800
July	1st to 10th	Day	46% 7% 13% 21% 13% 1	6290
	11th to 20th	Day	40% 8% 17% 23% 11% 1	6285
	21st to end	Day	42% 5% 13% 23% 16% 1	6728
August	1st to 10th	Day	50% 6% 15% 20% 8% 1	4467
	11th to 20th	Day	42% 6% 18% 27% 7%	3886
	21st to end	Day	37% 6% 14% 30% 12% 1	4312
September	1st to 10th	Day	41% 6% 11% 25% 15% 1	2400
	11th to 20th	Day	37% 5% 10% 24% 22% 1	2289
	21st to end	Day	43% 6% 11% 23% 14% 2	2200
October	1st to 10th	Day	43% 5% 8% 20% 21% 2	3460
	11th to 20th	Day	38% 5% 10% 23% 20% 4	3240
	21st to end	Day	38% 5% 13% 25% 15% 3	3180
November	1st to 10th	Day	41% 6% 13% 23% 14% 2	1851
	11th to 20th	Day	46% 6% 9% 21% 16% 2	1890
	21st to end	Day	51% 5% 13% 22% 8% 2	1785
December	1st to 10th	Day	49% 5% 16% 22% 7%	2680
	11th to 20th	Day	51% 5% 11% 18% 13% 1	3066
	21st to end	Day	56% 4 7% 16% 13% 4	4048
January	1st to 10th	Night	57% 6% 11% 18% 7% 1	8234
	11th to 20th	Night	55% 9% 11% 13% 9% 3	8280
	21st to end	Night	50% 6% 10% 16% 16% 2	8861
February	1st to 10th	Night	52% 5% 12% 19% 10% 2	2714
	11th to 20th	Night	49% 9% 11% 17% 13% 2	2460
	21st to end	Night	53% 6% 9% 20% 10% 3	2076
March	1st to 10th	Night	52% 5% 11% 17% 13% 2	5494
	11th to 20th	Night	48% 7% 15% 19% 8% 3	5382
	21st to end	Night	53% 6% 6% 14% 19% 3	5072
April	1st to 10th	Night	55% 5% 9% 18% 11% 2	2100
	11th to 20th	Night	50% 5% 10% 18% 16% 1	2036
	21st to end	Night	48% 4 10% 20% 15% 3	1903
May	1st to 10th	Night	63% 4 11% 13% 8% 1	4060
	11th to 20th	Night	55% 4 13% 17% 9% 2	3629
	21st to end	Night	53% 6% 12% 18% 10% 1	3927
June	1st to 10th	Night	44% 7% 11% 26% 12% 1	1600
	11th to 20th	Night	45% 6% 16% 23% 10%	1600
	21st to end	Night	39% 6% 17% 25% 13% 1	1600
July	1st to 10th	Night	41% 6% 16% 21% 15% 1	3560
	11th to 20th	Night	45% 8% 15% 21% 11% 1	3552
	21st to end	Night	37% 5% 14% 26% 17% 1	3809
August	1st to 10th	Night	50% 7% 14% 21% 8% 1	2627
	11th to 20th	Night	40% 6% 18% 28% 8%	3223
	21st to end	Night	32% 6% 14% 34% 13% 1	3586
September	1st to 10th	Night	39% 6% 12% 26% 16% 1	2000
	11th to 20th	Night	33% 4 12% 27% 24% 1	2110
	21st to end	Night	41% 5% 10% 25% 17% 1	2200
October	1st to 10th	Night	42% 6% 13% 23% 15% 1	3717
	11th to 20th	Night	35% 4 13% 25% 21% 3	3899
	21st to end	Night	37% 4 14% 29% 14% 1	4610
November	1st to 10th	Night	42% 4 11% 24% 18% 2	2667
	11th to 20th	Night	47% 6% 11% 18% 15% 3	2730
	21st to end	Night	46% 6% 16% 23% 8%	2833
December	1st to 10th	Night	52% 6% 13% 24% 6%	4720
	11th to 20th	Night	53% 6% 12% 13% 15% 1	5348
	21st to end	Night	56% 5% 8% 15% 13% 3	7150

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	35% 27% 26% 9% 4	220
	11th to 20th	Day	39% 26% 26% 7% 1	220
	21st to end	Day	31% 29% 26% 11% 3 1	275
February	1st to 10th	Day	39% 28% 26% 5% 1	344
	11th to 20th	Day	37% 26% 26% 9% 1	536
	21st to end	Day	31% 26% 29% 11% 2	446
March	1st to 10th	Day	34% 21% 31% 12% 1	540
	11th to 20th	Day	34% 23% 29% 12% 2 1	540
	21st to end	Day	39% 22% 26% 8% 3 2	594
April	1st to 10th	Day	39% 21% 26% 10% 5% 1	540
	11th to 20th	Day	32% 27% 24% 10% 5% 2	516
	21st to end	Day	29% 21% 30% 14% 5% 1	500
May	1st to 10th	Day	35% 21% 24% 14% 5% 2	503
	11th to 20th	Day	24% 20% 31% 20% 4 1	527
	21st to end	Day	21% 12% 32% 21% 11% 2	573
June	1st to 10th	Day	20% 14% 32% 22% 10% 1	530
	11th to 20th	Day	21% 13% 32% 23% 10% 1	530
	21st to end	Day	19% 16% 34% 19% 10% 2	530
July	1st to 10th	Day	16% 9% 35% 26% 12% 2	525
	11th to 20th	Day	14% 6% 31% 33% 13% 2	524
	21st to end	Day	13% 12% 35% 27% 11% 2	576
August	1st to 10th	Day	16% 10% 26% 31% 13% 4	526
	11th to 20th	Day	15% 8% 29% 27% 14% 7%	519
	21st to end	Day	14% 13% 30% 26% 12% 6%	583
September	1st to 10th	Day	20% 9% 25% 25% 15% 6%	530
	11th to 20th	Day	16% 11% 26% 22% 16% 10%	530
	21st to end	Day	22% 11% 33% 19% 12% 5%	530
October	1st to 10th	Day	28% 11% 25% 19% 12% 5%	530
	11th to 20th	Day	27% 16% 26% 22% 7% 2	530
	21st to end	Day	33% 15% 26% 18% 6% 1	462
November	1st to 10th	Day	26% 19% 26% 20% 9%	425
	11th to 20th	Day	33% 22% 25% 15% 5% 1	430
	21st to end	Day	36% 22% 24% 17% 2 1	325
December	1st to 10th	Day	30% 27% 28% 11% 4	220
	11th to 20th	Day	44% 23% 24% 8% 2	220
	21st to end	Day	33% 21% 31% 13% 2	242
January	1st to 10th	Night	27% 25% 29% 16% 2	840
	11th to 20th	Night	32% 21% 30% 15% 2	840
	21st to end	Night	27% 22% 30% 17% 3	924
February	1st to 10th	Night	27% 23% 30% 19% 2	840
	11th to 20th	Night	29% 24% 32% 14% 1	823
	21st to end	Night	32% 22% 29% 15% 3	692
March	1st to 10th	Night	30% 21% 31% 15% 3	840
	11th to 20th	Night	28% 24% 29% 16% 3	840
	21st to end	Night	30% 23% 26% 15% 5%	924
April	1st to 10th	Night	30% 21% 28% 17% 4	840
	11th to 20th	Night	28% 20% 27% 19% 5%	816
	21st to end	Night	27% 20% 29% 19% 4 1	703
May	1st to 10th	Night	31% 12% 31% 21% 6%	600
	11th to 20th	Night	23% 15% 33% 24% 4	421
	21st to end	Night	21% 8% 28% 30% 13%	433
June	1st to 10th	Night	21% 8% 29% 31% 11% 1	400
	11th to 20th	Night	19% 8% 27% 33% 14%	400
	21st to end	Night	16% 9% 26% 37% 12% 1	400
July	1st to 10th	Night	16% 4 23% 38% 17% 1	390
	11th to 20th	Night	15% 5% 22% 43% 14% 1	388
	21st to end	Night	13% 7% 23% 41% 17%	438
August	1st to 10th	Night	12% 4 21% 46% 17%	423
	11th to 20th	Night	15% 4 24% 32% 22% 2	786
	21st to end	Night	16% 5% 24% 35% 19% 2	880
September	1st to 10th	Night	17% 7% 27% 30% 17% 2	800
	11th to 20th	Night	16% 7% 23% 30% 20% 5%	800
	21st to end	Night	20% 9% 25% 31% 13% 2	800
October	1st to 10th	Night	24% 11% 22% 28% 13% 2	800
	11th to 20th	Night	21% 16% 27% 27% 9% 1	800
	21st to end	Night	29% 15% 28% 23% 6%	880
November	1st to 10th	Night	24% 17% 28% 24% 7% 1	820
	11th to 20th	Night	27% 19% 27% 19% 6% 1	840
	21st to end	Night	23% 20% 30% 22% 4 1	840
December	1st to 10th	Night	30% 21% 29% 16% 5%	840
	11th to 20th	Night	32% 20% 28% 17% 3	840
	21st to end	Night	33% 21% 30% 14% 2	924

Page generated at 2026-04-14 02:44:28. Version ac07

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