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 2024-12-24.

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 7% 5% 5% 5% 4 6% 7% 34%	101747
	Night	16% 9% 3 7% 5% 4 4 4 5% 6% 35%	98385
Transparency	Day	48% 6% 12% 21% 12% 1	105280
	Night	47% 6% 12% 21% 12% 1	102941
Seeing	Day	26% 17% 29% 19% 8% 2	15035
	Night	24% 16% 28% 23% 8% 1	24148

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	13% 10% 3 7% 5% 6% 5% 4 5% 6% 35%	2517
	11th to 20th	Day	15% 11% 5% 10% 7% 5% 5% 4 4 5% 28%	2520
	21st to end	Day	20% 14% 7% 8% 5% 4 4 3 4 4 25%	2868
February	1st to 10th	Day	16% 12% 3 9% 8% 4 5% 4 5% 6% 29%	1722
	11th to 20th	Day	14% 9% 4 9% 6% 6% 5% 4 5% 6% 33%	1848
	21st to end	Day	16% 13% 4 8% 6% 5% 4 3 5% 5% 31%	1570
March	1st to 10th	Day	21% 16% 4 9% 4 3 3 2 4 4 29%	3116
	11th to 20th	Day	11% 12% 4 9% 5% 5% 6% 4 6% 6% 32%	3273
	21st to end	Day	20% 8% 3 9% 6% 4 4 3 5% 6% 33%	4166
April	1st to 10th	Day	18% 11% 4 8% 4 4 5% 3 4 6% 34%	2280
	11th to 20th	Day	15% 9% 3 7% 5% 5% 4 3 5% 5% 38%	2254
	21st to end	Day	19% 7% 4 8% 5% 4 4 4 5% 6% 34%	2370
May	1st to 10th	Day	25% 10% 3 6% 4 4 4 4 5% 7% 29%	4106
	11th to 20th	Day	20% 8% 3 8% 5% 5% 4 4 5% 7% 32%	4388
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 7% 9% 40%	4837
June	1st to 10th	Day	16% 7% 2 6% 5% 5% 6% 5% 6% 9% 33%	2660
	11th to 20th	Day	16% 7% 2 5% 5% 5% 5% 4 7% 9% 34%	2660
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 7% 8% 33%	2660
July	1st to 10th	Day	10% 7% 3 8% 5% 5% 6% 5% 7% 8% 36%	4440
	11th to 20th	Day	10% 6% 2 6% 5% 5% 6% 4 8% 11% 36%	4440
	21st to end	Day	13% 9% 4 8% 5% 5% 6% 4 7% 7% 33%	4814
August	1st to 10th	Day	18% 8% 3 8% 6% 5% 5% 4 7% 8% 29%	4343
	11th to 20th	Day	12% 7% 3 7% 4 5% 5% 5% 7% 8% 36%	3730
	21st to end	Day	11% 6% 2 6% 4 5% 5% 5% 7% 9% 41%	4180
September	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 5% 7% 41%	2280
	11th to 20th	Day	13% 6% 3 6% 4 4 4 3 5% 6% 47%	2175
	21st to end	Day	17% 6% 2 5% 4 5% 5% 5% 6% 7% 38%	2090
October	1st to 10th	Day	22% 7% 3 7% 5% 3 4 2 5% 5% 37%	2016
	11th to 20th	Day	14% 8% 3 8% 4 4 3 3 5% 6% 41%	1900
	21st to end	Day	23% 10% 3 7% 4 4 5% 3 5% 5% 32%	1881
November	1st to 10th	Day	12% 9% 3 6% 4 3 4 3 5% 6% 45%	1761
	11th to 20th	Day	17% 9% 3 6% 5% 3 5% 5% 5% 6% 38%	1800
	21st to end	Day	18% 10% 3 7% 4 4 5% 4 5% 5% 35%	1700
December	1st to 10th	Day	18% 10% 5% 8% 6% 4 5% 4 6% 6% 29%	1600
	11th to 20th	Day	20% 11% 4 6% 5% 4 3 3 5% 5% 35%	1986
	21st to end	Day	20% 14% 7% 10% 5% 4 4 3 4 6% 23%	2796
January	1st to 10th	Night	15% 10% 4 9% 5% 4 4 3 5% 5% 37%	4429
	11th to 20th	Night	14% 10% 4 10% 7% 5% 6% 4 6% 6% 28%	4440
	21st to end	Night	21% 15% 4 8% 6% 5% 4 2 5% 5% 25%	4786
February	1st to 10th	Night	17% 9% 4 7% 6% 5% 5% 5% 5% 6% 32%	2456
	11th to 20th	Night	13% 10% 4 9% 6% 5% 5% 5% 7% 6% 30%	2218
	21st to end	Night	18% 9% 3 8% 7% 4 5% 4 5% 5% 32%	1884
March	1st to 10th	Night	23% 12% 3 8% 5% 4 4 3 5% 5% 29%	3724
	11th to 20th	Night	11% 10% 4 8% 7% 6% 6% 4 5% 6% 33%	3681
	21st to end	Night	18% 12% 4 9% 5% 4 3 3 4 5% 34%	3466
April	1st to 10th	Night	16% 11% 4 8% 6% 4 4 4 6% 6% 30%	1900
	11th to 20th	Night	18% 10% 3 6% 5% 4 3 3 5% 5% 37%	1875
	21st to end	Night	19% 10% 3 6% 5% 4 3 3 5% 6% 37%	1808
May	1st to 10th	Night	24% 11% 3 7% 5% 3 5% 4 4 5% 29%	2834
	11th to 20th	Night	18% 10% 3 8% 5% 4 4 3 5% 7% 32%	2534
	21st to end	Night	16% 6% 2 6% 5% 4 4 4 6% 8% 40%	2738
June	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 5% 7% 40%	1520
	11th to 20th	Night	15% 7% 2 6% 5% 4 5% 5% 6% 9% 35%	1520
	21st to end	Night	13% 6% 2 6% 5% 4 4 4 6% 8% 42%	1520
July	1st to 10th	Night	11% 5% 3 7% 5% 4 5% 5% 6% 8% 40%	2520
	11th to 20th	Night	14% 7% 2 6% 5% 4 5% 4 7% 9% 38%	2520
	21st to end	Night	15% 9% 3 6% 4 4 5% 3 5% 6% 39%	2739
August	1st to 10th	Night	20% 8% 3 7% 5% 4 5% 4 5% 6% 32%	2556
	11th to 20th	Night	14% 8% 3 6% 4 4 5% 4 6% 7% 40%	3093
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 8% 48%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 4 5% 4 5% 6% 44%	1900
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 4 6% 53%	2004
	21st to end	Night	15% 9% 2 6% 4 3 4 4 5% 6% 43%	2090
October	1st to 10th	Night	19% 8% 2 6% 4 3 4 3 4 6% 38%	2161
	11th to 20th	Night	14% 7% 2 6% 4 5% 4 3 6% 6% 42%	2280
	21st to end	Night	20% 10% 3 6% 5% 4 4 2 5% 6% 34%	2717
November	1st to 10th	Night	13% 8% 3 6% 4 3 3 3 5% 7% 45%	2537
	11th to 20th	Night	18% 7% 3 7% 5% 4 5% 3 5% 6% 37%	2600
	21st to end	Night	13% 9% 4 7% 6% 5% 5% 4 6% 6% 35%	2698
December	1st to 10th	Night	18% 11% 3 8% 5% 5% 4 4 5% 7% 28%	2800
	11th to 20th	Night	20% 10% 4 8% 6% 5% 5% 4 5% 5% 30%	3428
	21st to end	Night	20% 12% 6% 10% 5% 4 4 3 5% 6% 25%	4933

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	46% 7% 10% 28% 7% 1	2518
	11th to 20th	Day	57% 7% 10% 20% 6% 1	2520
	21st to end	Day	63% 4 8% 17% 6% 1	2868
February	1st to 10th	Day	56% 6% 12% 16% 10% 1	1722
	11th to 20th	Day	52% 6% 8% 20% 13% 2	1857
	21st to end	Day	55% 4 9% 19% 12% 1	1570
March	1st to 10th	Day	59% 4 11% 15% 10% 1	3119
	11th to 20th	Day	48% 9% 13% 20% 8% 1	3273
	21st to end	Day	51% 6% 5% 15% 19% 4	4166
April	1st to 10th	Day	53% 5% 8% 21% 12% 2	2280
	11th to 20th	Day	47% 5% 9% 22% 15% 2	2254
	21st to end	Day	50% 5% 9% 19% 14% 4	2370
May	1st to 10th	Day	56% 4 12% 15% 11% 1	4120
	11th to 20th	Day	51% 4 15% 20% 8% 1	4392
	21st to end	Day	37% 7% 13% 27% 14% 1	4837
June	1st to 10th	Day	46% 7% 14% 22% 10%	2660
	11th to 20th	Day	46% 5% 15% 23% 11% 1	2660
	21st to end	Day	47% 6% 14% 21% 11% 1	2660
July	1st to 10th	Day	44% 7% 14% 21% 14% 1	4370
	11th to 20th	Day	40% 7% 17% 24% 12% 1	4365
	21st to end	Day	49% 6% 15% 16% 14% 1	4661
August	1st to 10th	Day	51% 6% 15% 20% 8% 1	4329
	11th to 20th	Day	42% 6% 18% 27% 6%	3766
	21st to end	Day	38% 6% 14% 30% 11% 1	4180
September	1st to 10th	Day	42% 6% 11% 25% 15% 1	2280
	11th to 20th	Day	38% 5% 10% 24% 22% 2	2175
	21st to end	Day	43% 6% 11% 23% 14% 2	2090
October	1st to 10th	Day	44% 5% 8% 20% 21% 2	3354
	11th to 20th	Day	37% 5% 10% 23% 21% 4	3140
	21st to end	Day	39% 5% 14% 25% 14% 3	3081
November	1st to 10th	Day	40% 6% 14% 24% 14% 2	1761
	11th to 20th	Day	46% 6% 9% 20% 17% 2	1800
	21st to end	Day	49% 5% 13% 22% 9% 2	1700
December	1st to 10th	Day	55% 5% 14% 16% 10%	1600
	11th to 20th	Day	51% 6% 13% 20% 10% 1	1986
	21st to end	Day	62% 5% 9% 15% 8%	2796
January	1st to 10th	Night	48% 6% 12% 23% 9% 2	4429
	11th to 20th	Night	51% 10% 11% 17% 9% 2	4440
	21st to end	Night	62% 5% 9% 16% 8% 1	4786
February	1st to 10th	Night	52% 5% 13% 18% 10% 2	2456
	11th to 20th	Night	49% 9% 11% 16% 14% 2	2220
	21st to end	Night	52% 6% 9% 20% 10% 3	1884
March	1st to 10th	Night	57% 5% 9% 19% 10% 1	3734
	11th to 20th	Night	48% 9% 12% 20% 9% 2	3682
	21st to end	Night	51% 6% 7% 14% 19% 4	3466
April	1st to 10th	Night	54% 5% 9% 19% 11% 2	1900
	11th to 20th	Night	49% 5% 10% 19% 17% 1	1875
	21st to end	Night	48% 4 10% 20% 14% 3	1808
May	1st to 10th	Night	57% 4 11% 17% 9% 1	2840
	11th to 20th	Night	53% 5% 15% 18% 9% 1	2535
	21st to end	Night	42% 6% 16% 21% 14% 1	2739
June	1st to 10th	Night	43% 7% 11% 26% 12% 1	1520
	11th to 20th	Night	44% 6% 16% 23% 10%	1520
	21st to end	Night	39% 6% 17% 25% 13%	1520
July	1st to 10th	Night	40% 7% 17% 21% 15% 1	2480
	11th to 20th	Night	41% 6% 16% 25% 11%	2472
	21st to end	Night	45% 6% 14% 18% 16% 1	2651
August	1st to 10th	Night	51% 7% 14% 21% 7% 1	2546
	11th to 20th	Night	41% 6% 18% 27% 7%	3123
	21st to end	Night	32% 6% 15% 34% 13% 1	3476
September	1st to 10th	Night	39% 6% 12% 26% 16% 1	1900
	11th to 20th	Night	33% 4 12% 27% 23% 1	2004
	21st to end	Night	41% 5% 10% 25% 18% 1	2090
October	1st to 10th	Night	43% 6% 13% 22% 15% 1	3603
	11th to 20th	Night	34% 4 13% 25% 22% 3	3779
	21st to end	Night	38% 4 15% 29% 14% 1	4467
November	1st to 10th	Night	41% 4 11% 23% 18% 3	2537
	11th to 20th	Night	47% 6% 11% 18% 16% 3	2600
	21st to end	Night	46% 6% 16% 23% 8%	2698
December	1st to 10th	Night	55% 5% 12% 19% 8%	2800
	11th to 20th	Night	53% 8% 13% 14% 12% 1	3428
	21st to end	Night	58% 7% 9% 17% 9%	4933

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% 24% 29% 10% 4 1	180
	11th to 20th	Day	36% 26% 28% 8% 1 1	180
	21st to end	Day	25% 29% 28% 13% 3 1	225
February	1st to 10th	Day	40% 28% 25% 5% 2	282
	11th to 20th	Day	36% 27% 26% 9% 2	456
	21st to end	Day	32% 24% 29% 13% 2	382
March	1st to 10th	Day	33% 22% 31% 14% 1	460
	11th to 20th	Day	33% 24% 28% 12% 2 1	460
	21st to end	Day	38% 21% 26% 9% 4 2	506
April	1st to 10th	Day	35% 22% 27% 11% 5% 1	460
	11th to 20th	Day	31% 27% 24% 11% 6% 2	452
	21st to end	Day	30% 20% 29% 15% 5% 1	460
May	1st to 10th	Day	34% 21% 24% 14% 5% 2	463
	11th to 20th	Day	24% 18% 32% 20% 5% 1	487
	21st to end	Day	20% 12% 32% 22% 12% 2	529
June	1st to 10th	Day	20% 13% 32% 22% 11% 1	490
	11th to 20th	Day	19% 13% 33% 23% 11% 1	490
	21st to end	Day	19% 17% 32% 19% 11% 2	490
July	1st to 10th	Day	15% 9% 35% 27% 13% 2	485
	11th to 20th	Day	14% 6% 32% 32% 13% 2	484
	21st to end	Day	13% 13% 34% 27% 12% 2	532
August	1st to 10th	Day	16% 10% 25% 31% 12% 4	486
	11th to 20th	Day	15% 9% 30% 26% 13% 7%	479
	21st to end	Day	15% 11% 29% 26% 13% 6%	539
September	1st to 10th	Day	20% 9% 24% 26% 15% 6%	490
	11th to 20th	Day	17% 12% 25% 20% 15% 10%	490
	21st to end	Day	22% 10% 33% 18% 12% 5%	490
October	1st to 10th	Day	29% 11% 26% 18% 12% 4	490
	11th to 20th	Day	25% 16% 27% 23% 7% 2	490
	21st to end	Day	34% 15% 25% 19% 6% 1	429
November	1st to 10th	Day	24% 18% 27% 21% 9% 1	395
	11th to 20th	Day	33% 21% 25% 15% 5% 1	400
	21st to end	Day	34% 21% 25% 18% 2 1	300
December	1st to 10th	Day	29% 28% 29% 11% 4	200
	11th to 20th	Day	43% 23% 25% 8% 2	200
	21st to end	Day	30% 23% 32% 14% 2	204
January	1st to 10th	Night	27% 23% 30% 17% 3	760
	11th to 20th	Night	31% 20% 31% 16% 2	760
	21st to end	Night	26% 23% 30% 18% 3	836
February	1st to 10th	Night	27% 21% 30% 19% 2	760
	11th to 20th	Night	28% 25% 32% 14% 1	743
	21st to end	Night	32% 23% 27% 15% 3	628
March	1st to 10th	Night	29% 22% 30% 16% 3	760
	11th to 20th	Night	26% 25% 31% 14% 4	760
	21st to end	Night	29% 22% 26% 16% 6%	836
April	1st to 10th	Night	28% 20% 29% 19% 4	760
	11th to 20th	Night	27% 20% 27% 19% 6%	751
	21st to end	Night	27% 19% 30% 19% 4 1	668
May	1st to 10th	Night	29% 12% 30% 22% 6%	570
	11th to 20th	Night	23% 15% 34% 25% 4	400
	21st to end	Night	19% 7% 29% 30% 14%	411
June	1st to 10th	Night	21% 8% 29% 30% 11% 1	380
	11th to 20th	Night	19% 7% 27% 33% 14%	380
	21st to end	Night	16% 8% 26% 37% 12% 1	380
July	1st to 10th	Night	16% 4 23% 38% 18% 1	370
	11th to 20th	Night	15% 4 22% 43% 15% 1	368
	21st to end	Night	14% 7% 21% 40% 18%	416
August	1st to 10th	Night	12% 4 22% 45% 16%	402
	11th to 20th	Night	16% 4 25% 32% 22% 2	746
	21st to end	Night	15% 5% 22% 36% 20% 2	836
September	1st to 10th	Night	17% 7% 28% 30% 17% 2	760
	11th to 20th	Night	16% 8% 23% 29% 19% 5%	760
	21st to end	Night	20% 9% 25% 30% 13% 2	760
October	1st to 10th	Night	25% 11% 22% 28% 12% 2	760
	11th to 20th	Night	20% 16% 26% 27% 9% 1	760
	21st to end	Night	29% 16% 27% 22% 6%	836
November	1st to 10th	Night	22% 18% 28% 24% 7% 1	780
	11th to 20th	Night	27% 19% 27% 20% 6% 1	800
	21st to end	Night	23% 20% 30% 22% 4 1	800
December	1st to 10th	Night	30% 22% 28% 16% 5%	800
	11th to 20th	Night	32% 20% 28% 17% 3	800
	21st to end	Night	32% 21% 31% 14% 2	851

Page generated at 2024-12-21 04:13:48. Version ac07

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