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 2024-03-01.

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	3 3 2 3 4 3 4 4 5% 6% 63%	78958
	Night	3 3 1 3 3 2 3 4 4 6% 67%	80522
Transparency	Day	21% 4 6% 15% 28% 26%	78685
	Night	18% 4 7% 16% 29% 27%	80317
Seeing	Day	5% 9% 22% 31% 22% 11%	14035
	Night	4 3 16% 37% 33% 7%	24307

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 5% 3 5% 4 3 4 3 4 6% 61%	2858
	11th to 20th	Day	3 4 1 4 4 4 4 4 5% 6% 63%	2860
	21st to end	Day	10% 7% 2 5% 4 3 4 4 5% 7% 50%	3146
February	1st to 10th	Day	3 6% 2 4 3 2 3 3 4 6% 62%	1788
	11th to 20th	Day	5% 6% 2 4 4 3 3 4 5% 5% 60%	1850
	21st to end	Day	3 6% 2 5% 5% 2 3 4 5% 7% 58%	1570
March	1st to 10th	Day	3 4 2 4 4 3 4 4 5% 8% 59%	1959
	11th to 20th	Day	4 5% 2 4 4 2 3 5% 5% 6% 60%	1980
	21st to end	Day	3 3 2 3 3 2 4 4 4 5% 65%	2178
April	1st to 10th	Day	2 2 2 3 4 3 3 4 5% 7% 66%	1980
	11th to 20th	Day	2 3 1 3 3 2 3 3 3 5% 73%	2082
	21st to end	Day	2 2 1 3 3 2 3 4 5% 6% 70%	2340
May	1st to 10th	Day	3 2 1 2 3 2 3 3 3 7% 70%	2340
	11th to 20th	Day	2 2 1 3 3 1 3 3 4 6% 71%	2301
	21st to end	Day	2 3 1 2 3 1 3 3 3 4 75%	2530
June	1st to 10th	Day	1 1 1 2 2 2 3 2 4 6% 76%	2340
	11th to 20th	Day	2 1 1 1 1 1 2 3 3 85%	2340
	21st to end	Day	1 1 1 2 3 2 3 3 4 4 75%	2340
July	1st to 10th	Day	1 2 2 3 5% 3 5% 5% 6% 6% 61%	2340
	11th to 20th	Day	4 3 2 5% 6% 5% 7% 8% 7% 10% 43%	2340
	21st to end	Day	5% 5% 3 6% 8% 6% 7% 9% 9% 10% 32%	2545
August	1st to 10th	Day	4 4 2 4 6% 3 6% 5% 6% 8% 52%	2328
	11th to 20th	Day	3 4 2 4 6% 4 6% 7% 8% 8% 49%	2299
	21st to end	Day	5% 4 2 4 5% 3 5% 6% 7% 8% 51%	2371
September	1st to 10th	Day	4 3 2 4 5% 3 6% 5% 6% 7% 55%	1980
	11th to 20th	Day	3 1 1 2 3 2 3 3 5% 6% 72%	1980
	21st to end	Day	3 2 1 2 3 3 2 3 4 6% 72%	1980
October	1st to 10th	Day	3 3 1 3 3 3 3 4 4 6% 68%	1962
	11th to 20th	Day	2 1 1 2 2 1 2 3 3 4 80%	1800
	21st to end	Day	1 1 1 2 2 2 3 3 3 5% 77%	1980
November	1st to 10th	Day	1 2 1 2 3 2 4 4 4 5% 72%	1826
	11th to 20th	Day	2 3 1 2 3 1 3 3 4 6% 73%	1710
	21st to end	Day	5% 4 1 3 3 2 3 3 3 6% 67%	1710
December	1st to 10th	Day	5% 6% 2 3 4 3 4 4 3 7% 60%	1710
	11th to 20th	Day	4 5% 3 5% 6% 3 4 4 7% 9% 50%	2169
	21st to end	Day	7% 7% 3 6% 4 3 4 3 4 6% 54%	3146
January	1st to 10th	Night	2 5% 2 4 4 3 3 4 5% 6% 62%	4097
	11th to 20th	Night	2 3 1 3 3 2 3 3 5% 7% 68%	4110
	21st to end	Night	9% 8% 3 5% 4 3 4 3 4 6% 51%	4521
February	1st to 10th	Night	2 6% 2 4 3 2 3 3 4 6% 66%	2390
	11th to 20th	Night	3 4 2 3 3 3 3 4 4 6% 65%	2218
	21st to end	Night	4 5% 2 3 4 2 4 5% 4 6% 61%	1884
March	1st to 10th	Night	2 3 2 4 4 2 4 3 5% 7% 63%	2005
	11th to 20th	Night	4 5% 2 3 2 2 3 3 5% 6% 64%	1980
	21st to end	Night	3 2 1 3 3 2 4 4 6% 7% 65%	2178
April	1st to 10th	Night	2 2 2 3 3 2 4 3 5% 6% 69%	1980
	11th to 20th	Night	1 2 1 3 2 2 3 3 4 5% 73%	1821
	21st to end	Night	1 2 1 2 2 1 3 3 4 5% 78%	1620
May	1st to 10th	Night	1 2 1 2 3 1 2 3 4 5% 74%	1620
	11th to 20th	Night	1 2 1 3 2 1 2 2 3 4 79%	1594
	21st to end	Night	2 2 1 1 1 1 1 2 3 5% 82%	1746
June	1st to 10th	Night	1 1 1 1 2 1 3 3 3 5% 79%	1620
	11th to 20th	Night	1 1 1 2 1 2 2 2 3 85%	1620
	21st to end	Night	1 2 1 2 2 1 3 3 3 4 78%	1620
July	1st to 10th	Night	2 1 1 3 3 3 5% 5% 5% 7% 64%	1620
	11th to 20th	Night	2 2 1 3 5% 2 4 6% 7% 9% 60%	1620
	21st to end	Night	3 4 2 5% 6% 4 6% 7% 7% 12% 44%	1773
August	1st to 10th	Night	1 2 1 3 4 3 3 5% 7% 11% 61%	1611
	11th to 20th	Night	2 2 1 3 4 3 4 5% 7% 11% 57%	1602
	21st to end	Night	2 2 1 3 4 3 5% 5% 7% 8% 59%	1978
September	1st to 10th	Night	2 2 1 4 4 3 4 4 6% 9% 60%	1980
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 5% 77%	1980
	21st to end	Night	2 1 1 2 2 2 2 3 3 4 78%	1980
October	1st to 10th	Night	3 2 1 2 4 2 3 3 4 5% 71%	1998
	11th to 20th	Night	2 1 1 2 2 1 2 2 3 4 81%	2160
	21st to end	Night	1 2 1 2 2 1 2 2 3 5% 79%	2376
November	1st to 10th	Night	1 2 1 2 2 2 3 3 4 6% 73%	2248
	11th to 20th	Night	1 2 1 2 2 2 3 3 4 5% 75%	2470
	21st to end	Night	4 3 2 3 2 2 3 3 4 6% 68%	2470
December	1st to 10th	Night	5% 3 1 3 3 3 3 4 4 6% 63%	2470
	11th to 20th	Night	5% 5% 2 5% 5% 4 5% 5% 6% 8% 52%	3041
	21st to end	Night	5% 6% 2 3 4 3 4 5% 4 5% 58%	4521

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	24% 2 7% 19% 33% 14%	2859
	11th to 20th	Day	23% 4 6% 28% 24% 15%	2860
	21st to end	Day	32% 6% 5% 16% 30% 11%	3146
February	1st to 10th	Day	24% 3 6% 15% 26% 26%	1788
	11th to 20th	Day	27% 3 5% 16% 27% 23%	1860
	21st to end	Day	25% 5% 6% 15% 28% 22%	1570
March	1st to 10th	Day	23% 4 5% 16% 30% 22%	1959
	11th to 20th	Day	24% 4 5% 15% 33% 20%	1980
	21st to end	Day	21% 4 5% 17% 38% 16%	2178
April	1st to 10th	Day	18% 3 5% 18% 34% 23%	1980
	11th to 20th	Day	15% 2 3 13% 35% 32%	2085
	21st to end	Day	14% 3 4 15% 36% 28%	2340
May	1st to 10th	Day	16% 3 5% 15% 32% 29%	2340
	11th to 20th	Day	15% 3 5% 13% 33% 30%	2304
	21st to end	Day	14% 2 3 11% 32% 38%	2530
June	1st to 10th	Day	11% 2 4 7% 32% 44%	2340
	11th to 20th	Day	7% 1 2 4 22% 64%	2340
	21st to end	Day	13% 2 4 8% 22% 51%	2340
July	1st to 10th	Day	20% 5% 7% 12% 19% 36%	2276
	11th to 20th	Day	29% 10% 15% 17% 17% 11%	2271
	21st to end	Day	37% 13% 16% 16% 12% 7%	2404
August	1st to 10th	Day	26% 8% 10% 16% 18% 23%	2313
	11th to 20th	Day	27% 8% 10% 19% 19% 16%	2299
	21st to end	Day	26% 8% 9% 17% 26% 15%	2371
September	1st to 10th	Day	25% 6% 9% 15% 22% 24%	1980
	11th to 20th	Day	13% 4 6% 12% 34% 31%	1980
	21st to end	Day	15% 2 5% 11% 31% 35%	1980
October	1st to 10th	Day	18% 3 6% 17% 23% 33%	1962
	11th to 20th	Day	10% 2 3 9% 34% 42%	1800
	21st to end	Day	12% 2 5% 13% 32% 35%	1980
November	1st to 10th	Day	14% 3 4 18% 35% 25%	1826
	11th to 20th	Day	14% 2 3 17% 31% 32%	1710
	21st to end	Day	21% 2 4 16% 31% 26%	1710
December	1st to 10th	Day	25% 4 7% 17% 23% 23%	1710
	11th to 20th	Day	31% 4 8% 21% 19% 17%	2168
	21st to end	Day	31% 3 5% 18% 30% 12%	3146
January	1st to 10th	Night	22% 3 8% 15% 38% 14%	4097
	11th to 20th	Night	16% 3 7% 31% 29% 13%	4110
	21st to end	Night	34% 5% 5% 13% 30% 13%	4521
February	1st to 10th	Night	21% 3 7% 16% 28% 26%	2390
	11th to 20th	Night	21% 4 6% 18% 28% 23%	2219
	21st to end	Night	24% 4 6% 14% 31% 22%	1884
March	1st to 10th	Night	21% 3 6% 16% 31% 23%	2005
	11th to 20th	Night	21% 3 5% 15% 34% 22%	1980
	21st to end	Night	17% 4 9% 18% 34% 18%	2178
April	1st to 10th	Night	16% 3 6% 15% 36% 23%	1980
	11th to 20th	Night	15% 2 4 11% 34% 34%	1821
	21st to end	Night	10% 2 5% 13% 37% 32%	1620
May	1st to 10th	Night	13% 4 5% 15% 30% 34%	1620
	11th to 20th	Night	11% 2 4 12% 36% 35%	1595
	21st to end	Night	8% 2 3 12% 35% 40%	1746
June	1st to 10th	Night	10% 1 3 6% 28% 51%	1620
	11th to 20th	Night	8% 1 3 4 20% 65%	1620
	21st to end	Night	12% 2 4 5% 19% 58%	1620
July	1st to 10th	Night	18% 5% 8% 10% 20% 39%	1575
	11th to 20th	Night	18% 7% 16% 20% 24% 14%	1566
	21st to end	Night	27% 10% 18% 21% 15% 9%	1674
August	1st to 10th	Night	17% 7% 14% 16% 20% 26%	1602
	11th to 20th	Night	19% 7% 12% 23% 18% 20%	1602
	21st to end	Night	20% 6% 10% 21% 26% 17%	1978
September	1st to 10th	Night	19% 6% 11% 18% 21% 24%	1980
	11th to 20th	Night	11% 3 6% 13% 31% 37%	1980
	21st to end	Night	12% 2 5% 11% 32% 38%	1980
October	1st to 10th	Night	17% 3 5% 17% 23% 35%	1998
	11th to 20th	Night	10% 2 3 10% 28% 46%	2160
	21st to end	Night	10% 2 5% 12% 29% 41%	2376
November	1st to 10th	Night	14% 2 4 16% 36% 27%	2248
	11th to 20th	Night	13% 3 4 15% 31% 34%	2470
	21st to end	Night	20% 3 4 14% 31% 28%	2470
December	1st to 10th	Night	20% 5% 6% 17% 25% 26%	2470
	11th to 20th	Night	30% 4 7% 17% 26% 17%	3041
	21st to end	Night	27% 4 7% 18% 29% 15%	4521

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% 27% 31% 19% 3	270
	11th to 20th	Day	10% 5% 25% 32% 21% 6%	270
	21st to end	Day	5% 9% 28% 28% 21% 9%	297
February	1st to 10th	Day	6% 13% 27% 33% 15% 6%	348
	11th to 20th	Day	8% 16% 33% 26% 15% 2	455
	21st to end	Day	8% 23% 33% 24% 11% 2	382
March	1st to 10th	Day	6% 21% 31% 29% 10% 2	421
	11th to 20th	Day	8% 16% 26% 28% 17% 5%	420
	21st to end	Day	6% 23% 34% 23% 11% 3	462
April	1st to 10th	Day	6% 13% 33% 27% 14% 7%	420
	11th to 20th	Day	5% 21% 33% 24% 12% 4	412
	21st to end	Day	4 18% 34% 24% 13% 8%	420
May	1st to 10th	Day	4 16% 31% 28% 15% 5%	423
	11th to 20th	Day	2 12% 28% 33% 17% 8%	448
	21st to end	Day	4 12% 31% 30% 17% 7%	485
June	1st to 10th	Day	1 8% 25% 34% 22% 10%	450
	11th to 20th	Day	3 5% 27% 35% 17% 12%	450
	21st to end	Day	1 1 23% 38% 24% 12%	450
July	1st to 10th	Day	1 1 14% 39% 30% 14%	446
	11th to 20th	Day	3 2 5% 32% 34% 24%	444
	21st to end	Day	5% 2 5% 25% 34% 28%	487
August	1st to 10th	Day	4 2 7% 32% 36% 19%	447
	11th to 20th	Day	3 3 10% 27% 37% 20%	437
	21st to end	Day	4 2 6% 29% 34% 25%	495
September	1st to 10th	Day	4 4 12% 33% 30% 18%	450
	11th to 20th	Day	4 3 16% 37% 28% 11%	450
	21st to end	Day	4 9% 18% 31% 25% 13%	450
October	1st to 10th	Day	4 8% 23% 29% 24% 11%	432
	11th to 20th	Day	3 4 19% 32% 27% 14%	270
	21st to end	Day	1 7% 20% 31% 29% 11%	297
November	1st to 10th	Day	4 7% 19% 38% 25% 8%	270
	11th to 20th	Day	3 8% 20% 30% 27% 13%	270
	21st to end	Day	6% 9% 23% 32% 23% 8%	270
December	1st to 10th	Day	7% 7% 19% 33% 29% 5%	270
	11th to 20th	Day	2 7% 29% 36% 21% 5%	270
	21st to end	Day	11% 12% 20% 34% 21% 2	297
January	1st to 10th	Night	5% 5% 18% 42% 23% 8%	760
	11th to 20th	Night	6% 4 21% 38% 26% 5%	760
	21st to end	Night	10% 5% 21% 39% 22% 4	836
February	1st to 10th	Night	5% 5% 25% 37% 24% 4	760
	11th to 20th	Night	4 5% 26% 41% 21% 2	744
	21st to end	Night	7% 7% 24% 41% 17% 3	628
March	1st to 10th	Night	4 7% 28% 40% 20% 2	722
	11th to 20th	Night	6% 5% 19% 37% 28% 5%	720
	21st to end	Night	3 7% 24% 40% 22% 3	792
April	1st to 10th	Night	3 5% 25% 43% 22% 3	720
	11th to 20th	Night	3 8% 27% 37% 23% 3	619
	21st to end	Night	1 5% 29% 38% 23% 4	540
May	1st to 10th	Night	2 5% 18% 46% 27% 2	540
	11th to 20th	Night	2 2 18% 41% 33% 4	531
	21st to end	Night	3 3 18% 44% 28% 5%	584
June	1st to 10th	Night	2 2 12% 43% 38% 3	540
	11th to 20th	Night	1 1 13% 45% 33% 6%	540
	21st to end	Night	1 1 8% 36% 48% 5%	540
July	1st to 10th	Night	2 3 31% 56% 8%	525
	11th to 20th	Night	2 1 1 20% 58% 18%	522
	21st to end	Night	3 2 2 17% 60% 17%	591
August	1st to 10th	Night	2 1 20% 56% 19%	537
	11th to 20th	Night	3 1 2 19% 61% 14%	534
	21st to end	Night	3 1 3 16% 56% 22%	714
September	1st to 10th	Night	3 1 4 24% 53% 14%	720
	11th to 20th	Night	2 1 7% 31% 51% 9%	720
	21st to end	Night	1 2 13% 34% 40% 10%	720
October	1st to 10th	Night	4 2 15% 39% 34% 6%	720
	11th to 20th	Night	2 2 12% 42% 36% 7%	720
	21st to end	Night	2 5% 14% 35% 34% 11%	792
November	1st to 10th	Night	2 3 13% 44% 32% 7%	740
	11th to 20th	Night	2 2 16% 40% 31% 7%	760
	21st to end	Night	6% 4 18% 38% 29% 6%	760
December	1st to 10th	Night	7% 3 16% 41% 30% 4	760
	11th to 20th	Night	9% 4 16% 44% 24% 3	760
	21st to end	Night	8% 5% 21% 44% 19% 4	836

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