Blue Hills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Hills Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	2 3 1 4 3 3 4 3 5% 7% 64%	157728
	Night	2 3 1 3 3 3 3 3 5% 6% 68%	151028
Transparency	Day	19% 4 7% 14% 28% 28%	152598
	Night	16% 3 6% 15% 30% 30%	147159
Seeing	Day	4 10% 23% 30% 23% 10%	17503
	Night	4 3 16% 38% 33% 6%	27619

By Month

Percentage Cloud Cover SW (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	2 4 2 5% 4 4 4 3 5% 5% 62%	5318
	11th to 20th	Day	1 2 1 2 2 3 2 2 3 4 78%	5340
	21st to end	Day	6% 5% 1 5% 3 4 4 4 6% 6% 56%	5844
February	1st to 10th	Day	3 6% 2 4 2 3 3 3 5% 6% 63%	1977
	11th to 20th	Day	5% 7% 2 5% 3 3 4 3 6% 5% 58%	2050
	21st to end	Day	3 6% 2 5% 4 3 3 3 5% 7% 61%	1730
March	1st to 10th	Day	2 6% 2 3 2 3 3 3 4 6% 66%	5017
	11th to 20th	Day	3 8% 2 6% 5% 5% 5% 4 5% 6% 52%	5110
	21st to end	Day	2 4 2 4 3 3 4 3 4 6% 65%	5607
April	1st to 10th	Day	2 3 1 3 4 4 3 3 5% 6% 66%	2310
	11th to 20th	Day	2 3 1 3 2 3 3 2 3 5% 74%	2435
	21st to end	Day	1 3 1 3 3 2 3 3 5% 6% 70%	2730
May	1st to 10th	Day	3 4 2 4 3 3 3 2 3 5% 69%	7626
	11th to 20th	Day	1 2 1 2 2 2 2 3 4 5% 75%	7596
	21st to end	Day	1 1 1 2 2 2 2 2 3 4 80%	8370
June	1st to 10th	Day	1 1 1 2 2 3 3 2 4 6% 75%	2730
	11th to 20th	Day	2 1 1 1 1 1 1 3 4 85%	2730
	21st to end	Day	1 2 1 3 3 3 3 3 4 4 75%	2730
July	1st to 10th	Day	1 1 2 2 2 3 3 4 6% 75%	7649
	11th to 20th	Day	4 3 2 6% 5% 6% 7% 6% 8% 10% 43%	7650
	21st to end	Day	2 3 1 5% 5% 5% 6% 6% 8% 8% 52%	8304
August	1st to 10th	Day	2 2 1 4 5% 5% 6% 5% 7% 8% 54%	7549
	11th to 20th	Day	1 2 1 4 4 5% 6% 6% 10% 10% 51%	7565
	21st to end	Day	3 3 1 5% 4 5% 6% 5% 8% 10% 47%	7716
September	1st to 10th	Day	4 4 1 5% 4 5% 5% 5% 6% 7% 55%	2310
	11th to 20th	Day	3 1 1 2 2 2 3 3 5% 6% 73%	2234
	21st to end	Day	3 2 1 3 3 3 2 3 4 5% 71%	2200
October	1st to 10th	Day	2 3 1 3 2 3 3 3 3 4 72%	3553
	11th to 20th	Day	1 1 2 2 2 2 1 3 4 82%	3250
	21st to end	Day	1 1 2 1 2 2 2 4 5% 80%	3548
November	1st to 10th	Day	1 2 1 2 2 3 3 3 5% 5% 73%	2022
	11th to 20th	Day	2 3 1 3 3 2 3 3 5% 5% 71%	1890
	21st to end	Day	4 5% 1 4 2 3 3 2 3 7% 66%	1890
December	1st to 10th	Day	3 3 1 2 2 2 3 2 3 6% 73%	3040
	11th to 20th	Day	2 4 2 4 3 3 3 3 7% 11% 58%	3499
	21st to end	Day	5% 5% 2 5% 2 3 3 3 5% 8% 59%	4609
January	1st to 10th	Night	2 5% 2 5% 4 4 4 4 5% 6% 59%	7605
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 5% 79%	7650
	21st to end	Night	5% 5% 2 5% 3 4 4 3 5% 6% 58%	8370
February	1st to 10th	Night	2 5% 2 4 3 3 3 2 4 6% 66%	2641
	11th to 20th	Night	3 5% 2 4 3 4 3 3 5% 6% 62%	2458
	21st to end	Night	4 5% 2 3 3 3 3 3 4 7% 62%	2076
March	1st to 10th	Night	1 4 1 3 2 3 3 2 4 6% 68%	5095
	11th to 20th	Night	3 6% 2 6% 4 4 5% 4 6% 6% 53%	5110
	21st to end	Night	3 2 1 3 2 3 4 4 6% 8% 65%	5610
April	1st to 10th	Night	2 2 1 3 3 2 4 3 4 6% 69%	2310
	11th to 20th	Night	1 2 1 3 3 3 3 2 4 5% 73%	2127
	21st to end	Night	1 2 1 2 1 2 2 2 4 4 79%	1890
May	1st to 10th	Night	2 3 1 3 2 2 2 3 4 5% 71%	5321
	11th to 20th	Night	1 1 2 1 2 1 2 3 3 83%	5296
	21st to end	Night	1 1 1 1 1 2 2 2 3 86%	5837
June	1st to 10th	Night	2 1 1 2 2 2 3 2 3 5% 78%	1890
	11th to 20th	Night	1 1 1 1 1 2 2 2 3 85%	1890
	21st to end	Night	1 2 1 2 2 2 3 2 3 4 78%	1890
July	1st to 10th	Night	1 1 1 2 2 2 3 2 3 4 79%	5340
	11th to 20th	Night	2 3 2 6% 5% 5% 5% 5% 8% 10% 51%	5340
	21st to end	Night	2 3 1 4 3 4 4 4 8% 10% 57%	5805
August	1st to 10th	Night	1 2 1 4 3 3 4 4 8% 12% 59%	5273
	11th to 20th	Night	1 1 1 2 2 3 3 3 6% 10% 68%	5291
	21st to end	Night	1 2 1 4 3 4 5% 5% 8% 11% 55%	6514
September	1st to 10th	Night	2 2 1 4 3 4 5% 3 6% 9% 60%	2310
	11th to 20th	Night	2 2 2 2 2 2 2 3 5% 79%	2233
	21st to end	Night	2 1 1 2 2 2 2 2 3 4 78%	2200
October	1st to 10th	Night	2 1 1 2 2 2 2 2 3 4 78%	3632
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 5% 79%	3898
	21st to end	Night	1 1 1 2 1 2 2 2 3 5% 81%	4252
November	1st to 10th	Night	1 2 1 2 2 2 3 3 4 5% 75%	2492
	11th to 20th	Night	2 2 1 2 2 2 3 3 4 5% 74%	2730
	21st to end	Night	4 3 2 3 2 3 3 3 4 5% 67%	2730
December	1st to 10th	Night	3 2 1 2 2 2 3 3 5% 7% 72%	4370
	11th to 20th	Night	3 3 1 3 3 4 3 3 5% 7% 64%	4941
	21st to end	Night	3 4 1 3 3 3 4 3 4 5% 66%	6611

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	23% 3 7% 19% 35% 14%	5321
	11th to 20th	Day	12% 2 4 15% 24% 42%	5340
	21st to end	Day	26% 3 6% 9% 31% 24%	5844
February	1st to 10th	Day	22% 3 6% 15% 28% 26%	1977
	11th to 20th	Day	27% 3 5% 17% 26% 21%	2060
	21st to end	Day	23% 4 6% 16% 28% 24%	1730
March	1st to 10th	Day	19% 2 5% 18% 33% 22%	5017
	11th to 20th	Day	29% 4 8% 20% 25% 15%	5110
	21st to end	Day	19% 3 6% 23% 35% 14%	5607
April	1st to 10th	Day	19% 3 5% 16% 34% 24%	2310
	11th to 20th	Day	15% 2 3 12% 35% 33%	2438
	21st to end	Day	14% 3 4 14% 37% 27%	2730
May	1st to 10th	Day	19% 2 5% 12% 35% 28%	7626
	11th to 20th	Day	11% 2 6% 8% 32% 42%	7599
	21st to end	Day	9% 1 3 5% 32% 50%	8371
June	1st to 10th	Day	11% 2 4 7% 32% 44%	2730
	11th to 20th	Day	7% 1 2 4 22% 64%	2730
	21st to end	Day	13% 2 4 8% 21% 52%	2730
July	1st to 10th	Day	11% 2 5% 6% 16% 60%	7586
	11th to 20th	Day	31% 8% 14% 16% 22% 10%	7581
	21st to end	Day	24% 8% 12% 17% 18% 21%	8120
August	1st to 10th	Day	28% 7% 13% 21% 19% 12%	5978
	11th to 20th	Day	25% 9% 12% 23% 20% 11%	5964
	21st to end	Day	25% 7% 9% 23% 22% 13%	6055
September	1st to 10th	Day	24% 6% 9% 15% 23% 23%	2310
	11th to 20th	Day	12% 4 6% 12% 33% 33%	2234
	21st to end	Day	16% 3 5% 11% 30% 36%	2200
October	1st to 10th	Day	16% 2 5% 10% 18% 49%	3553
	11th to 20th	Day	8% 1 4 7% 47% 33%	3250
	21st to end	Day	8% 2 4 10% 32% 44%	3548
November	1st to 10th	Day	14% 3 4 17% 35% 27%	2022
	11th to 20th	Day	15% 2 4 17% 29% 32%	1890
	21st to end	Day	21% 2 5% 17% 31% 25%	1890
December	1st to 10th	Day	15% 2 4 16% 37% 25%	3040
	11th to 20th	Day	21% 3 6% 18% 38% 15%	3498
	21st to end	Day	24% 3 7% 24% 33% 10%	4609
January	1st to 10th	Night	23% 4 8% 14% 34% 16%	7605
	11th to 20th	Night	10% 2 4 17% 28% 39%	7650
	21st to end	Night	26% 3 6% 8% 29% 28%	8370
February	1st to 10th	Night	20% 2 7% 16% 29% 26%	2641
	11th to 20th	Night	22% 4 6% 19% 28% 21%	2459
	21st to end	Night	22% 3 6% 15% 30% 24%	2076
March	1st to 10th	Night	17% 2 6% 18% 32% 25%	5095
	11th to 20th	Night	26% 4 8% 18% 27% 16%	5110
	21st to end	Night	15% 4 7% 22% 38% 14%	5610
April	1st to 10th	Night	16% 3 6% 14% 36% 25%	2310
	11th to 20th	Night	15% 2 4 11% 35% 34%	2127
	21st to end	Night	9% 2 5% 12% 40% 31%	1890
May	1st to 10th	Night	15% 2 4 14% 32% 32%	5319
	11th to 20th	Night	7% 1 5% 7% 33% 47%	5297
	21st to end	Night	6% 1 2 5% 30% 55%	5838
June	1st to 10th	Night	11% 1 3 7% 28% 51%	1890
	11th to 20th	Night	7% 1 2 4 21% 65%	1890
	21st to end	Night	12% 2 3 6% 19% 59%	1890
July	1st to 10th	Night	11% 2 4 6% 17% 61%	5295
	11th to 20th	Night	26% 6% 11% 21% 23% 13%	5286
	21st to end	Night	19% 5% 14% 20% 19% 23%	5677
August	1st to 10th	Night	20% 6% 15% 22% 22% 16%	4168
	11th to 20th	Night	14% 6% 12% 28% 26% 14%	4168
	21st to end	Night	18% 4 9% 26% 24% 17%	5099
September	1st to 10th	Night	19% 6% 11% 17% 23% 24%	2310
	11th to 20th	Night	11% 3 5% 13% 30% 38%	2233
	21st to end	Night	12% 2 5% 11% 31% 40%	2200
October	1st to 10th	Night	11% 2 4 10% 22% 51%	3632
	11th to 20th	Night	11% 2 3 8% 41% 35%	3898
	21st to end	Night	8% 1 4 8% 31% 48%	4252
November	1st to 10th	Night	13% 2 4 15% 36% 30%	2492
	11th to 20th	Night	14% 3 5% 15% 31% 34%	2730
	21st to end	Night	20% 3 5% 14% 31% 27%	2730
December	1st to 10th	Night	13% 3 4 17% 36% 27%	4370
	11th to 20th	Night	20% 2 5% 15% 40% 18%	4941
	21st to end	Night	20% 3 6% 25% 33% 13%	6611

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	9% 11% 30% 30% 17% 3	330
	11th to 20th	Day	8% 5% 26% 35% 21% 5%	330
	21st to end	Day	4 10% 28% 29% 21% 7%	363
February	1st to 10th	Day	5% 12% 29% 34% 15% 5%	417
	11th to 20th	Day	8% 18% 33% 25% 14% 2	535
	21st to end	Day	7% 21% 31% 26% 13% 1	446
March	1st to 10th	Day	6% 24% 31% 26% 11% 2	540
	11th to 20th	Day	8% 16% 26% 27% 17% 5%	540
	21st to end	Day	6% 22% 32% 23% 13% 4	594
April	1st to 10th	Day	5% 14% 31% 26% 15% 8%	540
	11th to 20th	Day	5% 20% 34% 24% 13% 4	532
	21st to end	Day	4 18% 34% 24% 12% 8%	540
May	1st to 10th	Day	5% 14% 32% 26% 17% 5%	543
	11th to 20th	Day	2 14% 29% 32% 16% 8%	568
	21st to end	Day	4 12% 29% 30% 19% 7%	617
June	1st to 10th	Day	1 8% 24% 35% 23% 10%	570
	11th to 20th	Day	2 5% 30% 37% 16% 10%	570
	21st to end	Day	1 4 25% 36% 23% 11%	570
July	1st to 10th	Day	1 1 15% 38% 31% 14%	566
	11th to 20th	Day	4 2 6% 32% 35% 22%	564
	21st to end	Day	5% 2 7% 27% 34% 26%	619
August	1st to 10th	Day	4 2 7% 32% 36% 20%	567
	11th to 20th	Day	3 2 10% 28% 38% 20%	557
	21st to end	Day	4 2 7% 29% 34% 25%	627
September	1st to 10th	Day	3 4 14% 30% 32% 17%	570
	11th to 20th	Day	3 6% 18% 36% 27% 11%	543
	21st to end	Day	4 8% 19% 32% 25% 12%	530
October	1st to 10th	Day	4 8% 22% 28% 26% 12%	509
	11th to 20th	Day	3 6% 21% 32% 26% 12%	330
	21st to end	Day	1 7% 21% 29% 30% 12%	363
November	1st to 10th	Day	4 8% 18% 35% 26% 9%	330
	11th to 20th	Day	3 9% 21% 30% 25% 12%	330
	21st to end	Day	5% 8% 25% 33% 22% 7%	330
December	1st to 10th	Day	6% 6% 20% 34% 29% 5%	330
	11th to 20th	Day	2 6% 26% 36% 22% 8%	330
	21st to end	Day	10% 12% 23% 35% 19% 2	363
January	1st to 10th	Night	5% 5% 19% 41% 22% 7%	840
	11th to 20th	Night	5% 4 21% 41% 24% 5%	840
	21st to end	Night	9% 4 22% 40% 21% 3	924
February	1st to 10th	Night	5% 4 24% 40% 23% 4	840
	11th to 20th	Night	5% 6% 27% 41% 19% 2	824
	21st to end	Night	7% 7% 24% 42% 17% 3	692
March	1st to 10th	Night	4 7% 28% 42% 18% 1	840
	11th to 20th	Night	6% 6% 21% 37% 26% 4	840
	21st to end	Night	3 6% 24% 42% 22% 3	924
April	1st to 10th	Night	3 5% 25% 44% 21% 3	840
	11th to 20th	Night	3 7% 26% 40% 22% 2	723
	21st to end	Night	1 5% 29% 40% 21% 4	630
May	1st to 10th	Night	2 5% 17% 49% 25% 2	630
	11th to 20th	Night	2 3 19% 43% 30% 3	621
	21st to end	Night	2 2 18% 45% 28% 5%	683
June	1st to 10th	Night	2 2 11% 46% 36% 3	630
	11th to 20th	Night	1 1 13% 47% 32% 5%	630
	21st to end	Night	1 1 10% 38% 46% 5%	630
July	1st to 10th	Night	2 3 33% 54% 7%	615
	11th to 20th	Night	3 1 1 23% 55% 16%	612
	21st to end	Night	3 1 2 20% 58% 15%	690
August	1st to 10th	Night	2 2 22% 57% 17%	627
	11th to 20th	Night	3 2 21% 61% 13%	624
	21st to end	Night	3 1 4 18% 56% 20%	834
September	1st to 10th	Night	3 1 4 26% 52% 13%	840
	11th to 20th	Night	2 1 8% 32% 49% 8%	812
	21st to end	Night	2 2 13% 36% 39% 9%	800
October	1st to 10th	Night	4 2 14% 39% 34% 5%	800
	11th to 20th	Night	3 3 12% 42% 35% 6%	800
	21st to end	Night	2 4 13% 37% 33% 10%	880
November	1st to 10th	Night	2 3 13% 45% 31% 7%	820
	11th to 20th	Night	3 3 17% 40% 31% 7%	840
	21st to end	Night	5% 4 19% 39% 27% 5%	840
December	1st to 10th	Night	6% 3 16% 42% 31% 3	840
	11th to 20th	Night	8% 3 16% 44% 26% 3	840
	21st to end	Night	7% 5% 21% 45% 18% 4	924

Page generated at 2026-09-11 01:11:55. Version ac07

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