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 2024-11-01.

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 4 1 4 3 4 4 3 5% 6% 63%	101184
	Night	3 3 1 3 3 3 3 3 5% 6% 67%	98040
Transparency	Day	20% 4 7% 16% 27% 26%	100951
	Night	18% 4 7% 16% 28% 28%	97863
Seeing	Day	4 9% 22% 30% 23% 11%	14991
	Night	4 3 16% 37% 33% 7%	25155

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 6% 3 3 3 3 4 5% 61%	2858
	11th to 20th	Day	3 4 1 4 3 5% 4 3 5% 6% 63%	2860
	21st to end	Day	9% 8% 2 6% 3 3 4 3 5% 7% 50%	3146
February	1st to 10th	Day	3 6% 2 4 3 3 3 3 5% 6% 62%	1788
	11th to 20th	Day	5% 7% 2 5% 3 3 3 3 5% 5% 60%	1850
	21st to end	Day	3 6% 2 5% 4 3 3 4 5% 7% 58%	1570
March	1st to 10th	Day	2 5% 1 3 3 3 3 3 4 6% 66%	3411
	11th to 20th	Day	3 10% 3 6% 4 4 4 4 5% 6% 49%	3490
	21st to end	Day	2 5% 2 4 3 3 4 4 5% 6% 62%	3825
April	1st to 10th	Day	2 3 1 3 4 4 3 3 5% 6% 66%	2090
	11th to 20th	Day	2 3 1 3 2 3 2 2 3 5% 74%	2201
	21st to end	Day	1 3 1 3 3 2 3 3 5% 6% 70%	2470
May	1st to 10th	Day	2 2 1 2 2 2 2 1 3 5% 79%	4110
	11th to 20th	Day	2 3 1 4 3 3 3 3 5% 6% 66%	4071
	21st to end	Day	1 2 1 2 2 1 2 2 2 4 83%	4477
June	1st to 10th	Day	1 1 1 2 2 3 2 2 4 6% 76%	2470
	11th to 20th	Day	2 1 1 1 1 1 1 3 3 85%	2470
	21st to end	Day	1 2 1 3 3 3 3 3 4 4 73%	2470
July	1st to 10th	Day	1 2 1 2 3 3 4 3 5% 5% 71%	4110
	11th to 20th	Day	3 2 1 5% 5% 7% 9% 7% 8% 10% 42%	4110
	21st to end	Day	3 3 1 6% 6% 6% 7% 7% 8% 10% 42%	4454
August	1st to 10th	Day	3 4 2 7% 7% 6% 7% 5% 8% 10% 41%	4092
	11th to 20th	Day	2 3 1 4 4 5% 6% 6% 9% 8% 52%	4028
	21st to end	Day	4 3 1 5% 4 4 5% 5% 7% 8% 54%	4162
September	1st to 10th	Day	4 3 1 5% 4 4 5% 5% 6% 7% 55%	2090
	11th to 20th	Day	3 1 1 2 2 2 3 2 5% 6% 73%	2090
	21st to end	Day	3 2 1 2 3 3 2 2 4 5% 73%	2090
October	1st to 10th	Day	3 3 1 3 2 3 3 3 4 5% 68%	2070
	11th to 20th	Day	2 1 1 3 2 2 2 2 3 4 80%	1900
	21st to end	Day	1 1 3 2 2 3 3 3 5% 77%	2090
November	1st to 10th	Day	1 2 1 2 2 3 4 3 5% 5% 72%	1826
	11th to 20th	Day	2 3 1 2 3 2 3 2 5% 5% 73%	1710
	21st to end	Day	4 5% 1 3 2 3 3 2 3 6% 66%	1710
December	1st to 10th	Day	5% 6% 1 4 3 4 5% 3 3 7% 59%	1710
	11th to 20th	Day	4 6% 3 6% 5% 4 4 4 7% 8% 49%	2169
	21st to end	Day	7% 7% 3 7% 3 4 3 2 5% 6% 53%	3146
January	1st to 10th	Night	2 5% 2 4 3 4 3 3 5% 6% 62%	4097
	11th to 20th	Night	2 3 1 3 3 3 3 3 5% 7% 68%	4110
	21st to end	Night	9% 8% 3 5% 3 4 4 2 4 6% 51%	4521
February	1st to 10th	Night	2 6% 2 4 3 3 2 2 4 6% 65%	2390
	11th to 20th	Night	3 5% 1 4 3 3 3 3 5% 6% 64%	2218
	21st to end	Night	4 5% 2 3 3 4 4 4 5% 6% 60%	1884
March	1st to 10th	Night	2 3 1 4 3 3 3 3 5% 8% 67%	3461
	11th to 20th	Night	3 7% 2 7% 4 4 5% 3 6% 6% 52%	3490
	21st to end	Night	4 3 1 3 2 3 4 4 6% 8% 61%	3828
April	1st to 10th	Night	2 2 1 3 3 2 4 3 5% 6% 68%	2090
	11th to 20th	Night	1 2 1 3 3 2 2 2 4 5% 74%	1922
	21st to end	Night	1 2 1 2 2 2 3 2 4 5% 78%	1710
May	1st to 10th	Night	1 1 1 2 1 1 1 2 3 4 83%	2860
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 4 78%	2834
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	3110
June	1st to 10th	Night	1 1 2 2 2 3 2 3 5% 79%	1710
	11th to 20th	Night	1 1 2 1 1 2 1 2 3 85%	1710
	21st to end	Night	2 2 1 2 2 2 3 2 3 4 76%	1710
July	1st to 10th	Night	1 2 1 3 3 2 4 3 4 5% 72%	2860
	11th to 20th	Night	1 1 1 5% 4 5% 5% 5% 9% 11% 53%	2860
	21st to end	Night	2 3 1 4 4 4 5% 5% 9% 10% 52%	3109
August	1st to 10th	Night	1 3 2 5% 4 5% 5% 5% 11% 16% 43%	2847
	11th to 20th	Night	1 1 1 3 2 3 3 3 6% 9% 68%	2811
	21st to end	Night	1 2 1 4 3 4 4 4 6% 7% 65%	3505
September	1st to 10th	Night	2 2 1 4 3 4 5% 3 6% 9% 60%	2090
	11th to 20th	Night	2 2 2 2 2 2 2 3 5% 78%	2090
	21st to end	Night	2 1 1 2 2 2 2 2 3 4 78%	2090
October	1st to 10th	Night	3 2 1 3 3 2 3 3 4 5% 71%	2109
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 4 80%	2280
	21st to end	Night	1 2 1 2 2 2 2 2 3 5% 79%	2508
November	1st to 10th	Night	1 3 1 3 2 3 3 3 4 6% 73%	2254
	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 3 3 2 4 5% 68%	2470
December	1st to 10th	Night	5% 3 1 3 3 3 4 3 4 6% 63%	2470
	11th to 20th	Night	5% 5% 2 5% 4 5% 4 4 6% 8% 52%	3041
	21st to end	Night	5% 6% 1 4 4 4 4 4 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	18% 3 6% 23% 35% 15%	3411
	11th to 20th	Day	30% 5% 9% 22% 23% 12%	3490
	21st to end	Day	21% 4 6% 23% 37% 9%	3825
April	1st to 10th	Day	18% 3 5% 17% 34% 23%	2090
	11th to 20th	Day	15% 2 3 12% 34% 33%	2204
	21st to end	Day	14% 3 4 14% 36% 28%	2470
May	1st to 10th	Day	9% 2 5% 13% 37% 35%	4110
	11th to 20th	Day	16% 3 10% 9% 32% 31%	4074
	21st to end	Day	8% 1 2 7% 24% 57%	4477
June	1st to 10th	Day	11% 2 4 7% 33% 43%	2470
	11th to 20th	Day	7% 1 2 4 21% 65%	2470
	21st to end	Day	14% 3 4 8% 22% 49%	2470
July	1st to 10th	Day	14% 4 6% 9% 13% 56%	4046
	11th to 20th	Day	28% 8% 14% 20% 21% 9%	4041
	21st to end	Day	28% 10% 15% 14% 22% 10%	4314
August	1st to 10th	Day	32% 9% 15% 21% 11% 13%	4078
	11th to 20th	Day	22% 7% 10% 21% 24% 16%	4067
	21st to end	Day	24% 6% 9% 18% 26% 18%	4161
September	1st to 10th	Day	24% 6% 9% 14% 22% 25%	2090
	11th to 20th	Day	12% 4 5% 12% 34% 33%	2090
	21st to end	Day	15% 2 5% 11% 30% 37%	2090
October	1st to 10th	Day	18% 3 6% 16% 23% 35%	2070
	11th to 20th	Day	10% 2 4 9% 35% 40%	1900
	21st to end	Day	11% 2 5% 13% 32% 36%	2090
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	15% 4 6% 26% 30% 19%	3461
	11th to 20th	Night	28% 5% 8% 19% 25% 16%	3490
	21st to end	Night	18% 4 9% 24% 32% 12%	3828
April	1st to 10th	Night	17% 3 6% 14% 36% 24%	2090
	11th to 20th	Night	14% 2 4 10% 35% 35%	1922
	21st to end	Night	10% 2 5% 13% 37% 32%	1710
May	1st to 10th	Night	8% 2 3 11% 33% 43%	2860
	11th to 20th	Night	10% 2 7% 7% 35% 39%	2835
	21st to end	Night	5% 1 2 7% 25% 61%	3110
June	1st to 10th	Night	10% 1 3 6% 29% 51%	1710
	11th to 20th	Night	7% 1 3 4 20% 65%	1710
	21st to end	Night	13% 2 4 6% 19% 56%	1710
July	1st to 10th	Night	14% 3 5% 9% 13% 56%	2815
	11th to 20th	Night	20% 5% 12% 26% 25% 11%	2806
	21st to end	Night	20% 7% 19% 17% 22% 16%	3011
August	1st to 10th	Night	24% 7% 20% 23% 12% 15%	2838
	11th to 20th	Night	13% 5% 12% 22% 29% 19%	2838
	21st to end	Night	17% 4 8% 18% 28% 24%	3505
September	1st to 10th	Night	19% 6% 11% 17% 22% 25%	2090
	11th to 20th	Night	11% 3 5% 12% 30% 38%	2090
	21st to end	Night	11% 2 4 11% 31% 41%	2090
October	1st to 10th	Night	16% 3 5% 16% 22% 37%	2109
	11th to 20th	Night	11% 2 3 10% 30% 44%	2280
	21st to end	Night	10% 2 5% 11% 29% 43%	2508
November	1st to 10th	Night	14% 2 4 16% 36% 27%	2254
	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% 32% 24% 11% 1	382
March	1st to 10th	Day	6% 22% 31% 28% 11% 2	460
	11th to 20th	Day	8% 16% 27% 28% 17% 5%	460
	21st to end	Day	6% 23% 34% 22% 11% 3	506
April	1st to 10th	Day	6% 14% 32% 26% 15% 8%	460
	11th to 20th	Day	5% 20% 33% 24% 13% 5%	452
	21st to end	Day	3 17% 33% 24% 13% 9%	460
May	1st to 10th	Day	5% 15% 32% 27% 15% 6%	463
	11th to 20th	Day	2 12% 27% 33% 17% 9%	488
	21st to end	Day	4 11% 31% 29% 18% 7%	529
June	1st to 10th	Day	1 8% 23% 34% 23% 11%	490
	11th to 20th	Day	3 5% 29% 36% 17% 11%	490
	21st to end	Day	1 2 23% 38% 24% 12%	490
July	1st to 10th	Day	1 1 14% 38% 31% 15%	486
	11th to 20th	Day	3 2 5% 32% 35% 23%	484
	21st to end	Day	5% 2 5% 26% 34% 28%	531
August	1st to 10th	Day	4 2 7% 30% 37% 20%	487
	11th to 20th	Day	3 2 9% 27% 38% 20%	477
	21st to end	Day	4 2 7% 29% 34% 24%	539
September	1st to 10th	Day	3 3 11% 32% 32% 18%	490
	11th to 20th	Day	3 6% 18% 36% 26% 11%	490
	21st to end	Day	4 9% 17% 32% 26% 12%	490
October	1st to 10th	Day	4 8% 21% 28% 27% 12%	470
	11th to 20th	Day	2 5% 21% 32% 26% 14%	300
	21st to end	Day	1 7% 20% 30% 29% 12%	330
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% 8% 24% 41% 17% 3	628
March	1st to 10th	Night	4 7% 28% 41% 19% 2	760
	11th to 20th	Night	6% 5% 20% 37% 27% 5%	760
	21st to end	Night	4 7% 25% 40% 22% 3	836
April	1st to 10th	Night	3 5% 25% 43% 22% 3	760
	11th to 20th	Night	3 7% 27% 38% 23% 2	653
	21st to end	Night	1 5% 28% 38% 23% 4	570
May	1st to 10th	Night	2 5% 18% 48% 26% 2	570
	11th to 20th	Night	2 2 18% 42% 32% 4	561
	21st to end	Night	3 2 18% 44% 29% 4	617
June	1st to 10th	Night	2 2 11% 44% 38% 3	570
	11th to 20th	Night	2 1 14% 46% 32% 6%	570
	21st to end	Night	1 1 9% 36% 48% 5%	570
July	1st to 10th	Night	2 3 32% 55% 7%	555
	11th to 20th	Night	2 1 1 20% 58% 18%	552
	21st to end	Night	3 1 2 18% 60% 16%	624
August	1st to 10th	Night	2 2 20% 57% 19%	567
	11th to 20th	Night	3 1 2 19% 62% 14%	564
	21st to end	Night	3 1 3 17% 55% 21%	754
September	1st to 10th	Night	3 1 4 24% 54% 14%	760
	11th to 20th	Night	2 1 8% 32% 49% 8%	760
	21st to end	Night	1 2 13% 36% 39% 9%	760
October	1st to 10th	Night	4 2 15% 38% 35% 6%	760
	11th to 20th	Night	2 3 12% 41% 35% 6%	760
	21st to end	Night	2 4 14% 36% 34% 10%	836
November	1st to 10th	Night	2 3 13% 44% 32% 7%	742
	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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