Beacon Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Beacon Hill Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	127214
	Night	17% 10% 3 9% 6% 5% 5% 4 5% 6% 31%	124156
Transparency	Day	50% 6% 12% 20% 11% 1	130708
	Night	51% 6% 12% 19% 11% 1	128749
Seeing	Day	25% 17% 28% 18% 9% 3	16223
	Night	22% 13% 27% 28% 9% 1	25212

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% 10% 4 18% 9% 6% 5% 4 4 5% 23%	3605
	11th to 20th	Day	11% 12% 6% 14% 6% 5% 5% 4 5% 5% 26%	3570
	21st to end	Day	10% 13% 6% 11% 5% 6% 4 4 5% 5% 30%	3960
February	1st to 10th	Day	12% 12% 5% 10% 6% 6% 5% 4 6% 7% 27%	1764
	11th to 20th	Day	9% 13% 5% 12% 7% 6% 5% 5% 5% 5% 28%	1948
	21st to end	Day	16% 13% 4 9% 6% 4 4 4 5% 5% 30%	1650
March	1st to 10th	Day	19% 14% 4 12% 6% 5% 4 3 3 3 26%	4476
	11th to 20th	Day	14% 12% 5% 6% 5% 5% 5% 3 5% 7% 33%	4653
	21st to end	Day	22% 11% 3 9% 5% 5% 4 3 4 5% 29%	5959
April	1st to 10th	Day	20% 13% 3 7% 5% 4 4 3 5% 6% 32%	2400
	11th to 20th	Day	18% 11% 2 6% 5% 5% 4 4 4 6% 35%	2374
	21st to end	Day	22% 9% 4 8% 5% 3 5% 4 4 6% 30%	2590
May	1st to 10th	Day	34% 10% 3 8% 4 4 4 3 4 5% 21%	5886
	11th to 20th	Day	19% 10% 3 9% 6% 5% 5% 4 6% 7% 28%	6315
	21st to end	Day	19% 8% 2 7% 5% 4 5% 4 6% 8% 31%	6949
June	1st to 10th	Day	20% 8% 3 7% 6% 5% 5% 4 6% 7% 30%	2800
	11th to 20th	Day	18% 7% 2 6% 5% 5% 4 4 5% 9% 35%	2800
	21st to end	Day	20% 8% 2 7% 6% 6% 6% 4 6% 9% 28%	2800
July	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 5% 7% 10% 34%	6360
	11th to 20th	Day	12% 6% 2 6% 5% 6% 6% 5% 8% 10% 34%	6360
	21st to end	Day	16% 5% 2 5% 4 5% 4 4 7% 8% 40%	6926
August	1st to 10th	Day	15% 6% 2 7% 5% 6% 5% 5% 7% 8% 34%	4477
	11th to 20th	Day	16% 7% 3 8% 5% 5% 6% 4 6% 8% 32%	4060
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	4312
September	1st to 10th	Day	13% 5% 1 6% 3 4 4 4 6% 8% 46%	2400
	11th to 20th	Day	14% 6% 2 5% 4 3 4 2 5% 6% 48%	2251
	21st to end	Day	18% 6% 2 5% 4 4 3 4 6% 7% 42%	2200
October	1st to 10th	Day	22% 8% 2 6% 4 4 4 3 4 6% 37%	2170
	11th to 20th	Day	16% 9% 3 8% 4 5% 4 2 4 6% 38%	1920
	21st to end	Day	24% 12% 3 7% 4 5% 4 3 4 5% 29%	1980
November	1st to 10th	Day	13% 10% 3 6% 6% 5% 4 4 5% 6% 38%	1851
	11th to 20th	Day	16% 11% 3 7% 6% 4 4 4 6% 6% 33%	1890
	21st to end	Day	19% 11% 4 8% 5% 5% 5% 3 5% 5% 28%	1764
December	1st to 10th	Day	15% 16% 5% 14% 6% 5% 5% 4 5% 6% 18%	2680
	11th to 20th	Day	20% 13% 4 9% 5% 4 4 3 4 6% 28%	3066
	21st to end	Day	20% 12% 6% 12% 5% 4 4 3 4 4 27%	4048
January	1st to 10th	Night	12% 10% 5% 16% 9% 5% 5% 4 5% 6% 24%	6364
	11th to 20th	Night	9% 14% 5% 15% 8% 7% 7% 5% 5% 6% 20%	6315
	21st to end	Night	14% 11% 4 12% 7% 6% 5% 4 5% 5% 27%	6996
February	1st to 10th	Night	12% 12% 4 9% 6% 6% 5% 5% 7% 6% 26%	2633
	11th to 20th	Night	10% 11% 5% 10% 7% 6% 6% 5% 7% 6% 28%	2338
	21st to end	Night	12% 11% 3 9% 7% 6% 5% 4 6% 7% 30%	1980
March	1st to 10th	Night	20% 12% 3 8% 6% 5% 5% 3 5% 6% 26%	5364
	11th to 20th	Night	14% 13% 4 9% 6% 5% 4 3 4 6% 32%	5306
	21st to end	Night	21% 13% 3 8% 4 4 4 3 4 5% 30%	4971
April	1st to 10th	Night	20% 10% 3 8% 6% 4 5% 4 4 6% 31%	2000
	11th to 20th	Night	19% 12% 3 6% 5% 4 4 3 5% 5% 34%	1975
	21st to end	Night	21% 9% 3 6% 4 4 4 3 5% 7% 34%	1810
May	1st to 10th	Night	36% 10% 2 6% 4 4 4 3 4 5% 22%	4054
	11th to 20th	Night	17% 13% 4 9% 5% 5% 5% 3 4 5% 29%	3621
	21st to end	Night	20% 7% 3 8% 5% 5% 5% 4 5% 7% 32%	3926
June	1st to 10th	Night	20% 6% 3 6% 6% 5% 4 4 6% 9% 33%	1600
	11th to 20th	Night	19% 7% 2 5% 4 4 4 4 6% 6% 39%	1600
	21st to end	Night	19% 5% 2 6% 6% 4 5% 4 5% 7% 36%	1600
July	1st to 10th	Night	12% 5% 3 6% 5% 5% 6% 5% 8% 9% 38%	3600
	11th to 20th	Night	15% 7% 3 7% 5% 4 5% 4 6% 7% 37%	3600
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 5% 7% 47%	3926
August	1st to 10th	Night	20% 7% 3 7% 5% 4 3 3 4 6% 36%	2639
	11th to 20th	Night	18% 6% 2 6% 5% 4 4 4 6% 8% 38%	2970
	21st to end	Night	10% 5% 2 5% 3 5% 5% 3 7% 8% 47%	3586
September	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 48%	2000
	11th to 20th	Night	13% 5% 2 4 3 4 3 2 5% 7% 50%	2148
	21st to end	Night	16% 6% 2 5% 4 4 4 4 5% 7% 44%	2200
October	1st to 10th	Night	20% 7% 3 6% 4 4 3 3 6% 7% 36%	2228
	11th to 20th	Night	17% 9% 2 7% 4 4 5% 3 5% 7% 35%	2479
	21st to end	Night	22% 9% 3 7% 5% 5% 5% 3 6% 7% 29%	2860
November	1st to 10th	Night	15% 10% 3 7% 5% 5% 4 4 5% 6% 36%	2667
	11th to 20th	Night	16% 8% 3 6% 6% 5% 6% 4 5% 7% 33%	2730
	21st to end	Night	16% 12% 4 8% 6% 5% 4 3 5% 6% 31%	2852
December	1st to 10th	Night	15% 15% 5% 13% 7% 6% 6% 4 5% 6% 18%	4720
	11th to 20th	Night	21% 12% 4 9% 5% 5% 4 3 4 5% 28%	5348
	21st to end	Night	21% 14% 5% 9% 5% 5% 4 3 4 5% 25%	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	61% 6% 9% 19% 5%	3606
	11th to 20th	Day	58% 6% 9% 16% 8% 3	3600
	21st to end	Day	55% 5% 8% 16% 14% 2	3960
February	1st to 10th	Day	54% 7% 11% 17% 8% 2	1764
	11th to 20th	Day	55% 7% 11% 18% 9% 2	1957
	21st to end	Day	55% 6% 10% 16% 12% 2	1650
March	1st to 10th	Day	62% 4 7% 11% 9% 6%	4479
	11th to 20th	Day	50% 6% 15% 18% 9% 3	4653
	21st to end	Day	57% 3 7% 13% 17% 3	5959
April	1st to 10th	Day	54% 4 9% 19% 12% 3	2400
	11th to 20th	Day	50% 6% 10% 17% 15% 2	2374
	21st to end	Day	55% 5% 9% 17% 12% 2	2590
May	1st to 10th	Day	66% 6% 7% 15% 5% 1	5897
	11th to 20th	Day	54% 8% 14% 15% 9% 1	6319
	21st to end	Day	47% 9% 12% 22% 9% 1	6949
June	1st to 10th	Day	52% 7% 13% 17% 10%	2800
	11th to 20th	Day	46% 7% 12% 20% 14% 2	2800
	21st to end	Day	52% 8% 11% 18% 10% 1	2800
July	1st to 10th	Day	43% 7% 15% 22% 12%	6290
	11th to 20th	Day	41% 9% 15% 23% 11% 1	6285
	21st to end	Day	40% 6% 13% 30% 11%	6728
August	1st to 10th	Day	45% 9% 14% 21% 10%	4463
	11th to 20th	Day	48% 8% 15% 24% 6%	4099
	21st to end	Day	36% 7% 17% 25% 13% 1	4312
September	1st to 10th	Day	35% 7% 12% 30% 14% 1	2400
	11th to 20th	Day	37% 5% 10% 29% 18% 1	2251
	21st to end	Day	40% 6% 14% 26% 13% 2	2200
October	1st to 10th	Day	46% 6% 10% 22% 15% 1	3534
	11th to 20th	Day	44% 4 14% 25% 13%	3110
	21st to end	Day	43% 4 11% 28% 12% 1	3180
November	1st to 10th	Day	45% 7% 14% 20% 13%	1851
	11th to 20th	Day	50% 7% 13% 15% 14% 2	1890
	21st to end	Day	57% 5% 16% 17% 5%	1764
December	1st to 10th	Day	65% 7% 12% 12% 4	2680
	11th to 20th	Day	58% 5% 8% 17% 11% 1	3066
	21st to end	Day	60% 5% 6% 14% 13% 2	4048
January	1st to 10th	Night	59% 6% 15% 15% 6%	6364
	11th to 20th	Night	60% 7% 13% 11% 8% 1	6360
	21st to end	Night	57% 6% 9% 12% 13% 3	6996
February	1st to 10th	Night	53% 9% 11% 18% 9% 1	2633
	11th to 20th	Night	53% 6% 11% 19% 10% 1	2340
	21st to end	Night	50% 6% 14% 18% 9% 3	1980
March	1st to 10th	Night	57% 4 11% 13% 13% 3	5374
	11th to 20th	Night	52% 7% 15% 15% 10% 2	5307
	21st to end	Night	55% 3 7% 14% 17% 3	4971
April	1st to 10th	Night	53% 5% 9% 19% 13% 2	2000
	11th to 20th	Night	50% 6% 9% 21% 13% 1	1975
	21st to end	Night	50% 5% 10% 18% 14% 3	1810
May	1st to 10th	Night	64% 6% 9% 13% 7% 1	4060
	11th to 20th	Night	55% 7% 11% 18% 8% 1	3622
	21st to end	Night	50% 9% 13% 17% 10% 1	3927
June	1st to 10th	Night	47% 9% 13% 19% 12% 1	1600
	11th to 20th	Night	44% 8% 14% 21% 11% 2	1600
	21st to end	Night	46% 8% 13% 22% 10% 1	1600
July	1st to 10th	Night	39% 8% 19% 23% 11%	3560
	11th to 20th	Night	44% 8% 14% 23% 12%	3552
	21st to end	Night	35% 7% 14% 33% 10%	3809
August	1st to 10th	Night	49% 7% 12% 23% 9%	2629
	11th to 20th	Night	43% 8% 18% 21% 10%	2997
	21st to end	Night	31% 6% 21% 29% 13%	3586
September	1st to 10th	Night	34% 7% 14% 29% 14%	2000
	11th to 20th	Night	34% 6% 11% 30% 17% 1	2148
	21st to end	Night	38% 8% 12% 26% 15% 2	2200
October	1st to 10th	Night	50% 6% 11% 22% 10% 1	3644
	11th to 20th	Night	40% 5% 15% 23% 15% 2	4028
	21st to end	Night	44% 6% 11% 28% 10% 1	4610
November	1st to 10th	Night	47% 7% 12% 22% 11% 2	2667
	11th to 20th	Night	47% 7% 12% 20% 12% 2	2730
	21st to end	Night	53% 6% 14% 20% 7%	2852
December	1st to 10th	Night	63% 7% 14% 14% 3	4720
	11th to 20th	Night	58% 5% 9% 13% 14% 1	5348
	21st to end	Night	62% 4 8% 15% 9% 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	23% 25% 30% 16% 5% 1	202
	11th to 20th	Day	33% 21% 28% 14% 4 1	200
	21st to end	Day	25% 23% 30% 15% 7%	220
February	1st to 10th	Day	32% 27% 30% 10% 2	294
	11th to 20th	Day	30% 26% 28% 14% 2	496
	21st to end	Day	30% 21% 29% 15% 4	414
March	1st to 10th	Day	32% 17% 32% 16% 3	500
	11th to 20th	Day	32% 23% 28% 11% 5% 1	500
	21st to end	Day	34% 20% 25% 12% 6% 3	550
April	1st to 10th	Day	39% 17% 25% 12% 6% 1	500
	11th to 20th	Day	36% 23% 22% 12% 8% 1	492
	21st to end	Day	30% 24% 26% 13% 6% 1	500
May	1st to 10th	Day	36% 20% 25% 12% 6% 2	503
	11th to 20th	Day	24% 15% 33% 20% 6% 2	527
	21st to end	Day	20% 18% 32% 20% 9% 1	573
June	1st to 10th	Day	23% 15% 37% 16% 8% 1	530
	11th to 20th	Day	20% 17% 32% 20% 10% 2	530
	21st to end	Day	22% 16% 32% 19% 9% 2	530
July	1st to 10th	Day	15% 12% 32% 26% 11% 3	525
	11th to 20th	Day	15% 9% 32% 29% 11% 3	524
	21st to end	Day	16% 9% 38% 24% 11% 2	576
August	1st to 10th	Day	13% 12% 28% 24% 19% 4	526
	11th to 20th	Day	17% 11% 25% 23% 15% 8%	519
	21st to end	Day	12% 14% 26% 26% 15% 7%	583
September	1st to 10th	Day	16% 12% 26% 24% 15% 8%	530
	11th to 20th	Day	16% 11% 23% 20% 18% 12%	530
	21st to end	Day	20% 11% 27% 23% 12% 7%	530
October	1st to 10th	Day	26% 13% 22% 18% 15% 6%	530
	11th to 20th	Day	22% 17% 25% 21% 12% 2	486
	21st to end	Day	35% 16% 22% 20% 7%	462
November	1st to 10th	Day	27% 17% 22% 21% 12% 1	425
	11th to 20th	Day	28% 20% 27% 17% 8% 1	430
	21st to end	Day	36% 16% 27% 16% 6%	304
December	1st to 10th	Day	30% 17% 30% 15% 7% 1	220
	11th to 20th	Day	38% 19% 30% 10% 3	220
	21st to end	Day	33% 20% 23% 18% 5%	242
January	1st to 10th	Night	22% 18% 34% 22% 3	805
	11th to 20th	Night	27% 16% 29% 24% 4 1	800
	21st to end	Night	18% 16% 35% 26% 4	880
February	1st to 10th	Night	22% 18% 33% 24% 3	800
	11th to 20th	Night	22% 19% 35% 22% 3	783
	21st to end	Night	23% 17% 30% 27% 3	660
March	1st to 10th	Night	25% 15% 31% 26% 3	800
	11th to 20th	Night	25% 18% 28% 22% 6%	800
	21st to end	Night	28% 15% 26% 23% 7%	880
April	1st to 10th	Night	28% 18% 27% 23% 5%	800
	11th to 20th	Night	26% 20% 26% 22% 7%	791
	21st to end	Night	28% 17% 26% 22% 6%	610
May	1st to 10th	Night	29% 13% 28% 22% 8%	600
	11th to 20th	Night	19% 13% 29% 33% 6%	414
	21st to end	Night	19% 7% 27% 34% 12%	433
June	1st to 10th	Night	20% 8% 28% 34% 11%	400
	11th to 20th	Night	20% 10% 26% 33% 13%	400
	21st to end	Night	18% 6% 25% 39% 12% 1	400
July	1st to 10th	Night	16% 5% 19% 44% 16% 1	390
	11th to 20th	Night	15% 4 21% 39% 19% 2	388
	21st to end	Night	13% 5% 21% 44% 17%	438
August	1st to 10th	Night	17% 3 15% 40% 24%	425
	11th to 20th	Night	17% 5% 22% 34% 20% 3	651
	21st to end	Night	14% 6% 19% 36% 22% 2	880
September	1st to 10th	Night	16% 5% 24% 33% 20% 2	800
	11th to 20th	Night	16% 7% 19% 30% 24% 4	800
	21st to end	Night	18% 7% 25% 31% 17% 2	800
October	1st to 10th	Night	24% 11% 18% 28% 17% 2	800
	11th to 20th	Night	22% 11% 24% 30% 12% 1	800
	21st to end	Night	27% 13% 26% 26% 8%	880
November	1st to 10th	Night	22% 16% 25% 30% 7%	820
	11th to 20th	Night	22% 17% 28% 24% 8% 1	840
	21st to end	Night	24% 16% 26% 27% 7%	840
December	1st to 10th	Night	26% 15% 33% 21% 4	840
	11th to 20th	Night	28% 16% 28% 25% 4	840
	21st to end	Night	29% 16% 29% 22% 3	924

Page generated at 2025-12-30 01:14:07. Version ac07

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