Boulder Knolls Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boulder Knolls Observatory clear sky chart from 2005-11-06 to 2024-11-12.

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	5% 4 1 3 3 3 3 3 4 6% 66%	105018
	Night	7% 6% 2 4 3 3 3 3 4 6% 59%	102979
Transparency	Day	19% 4 6% 17% 26% 27%	104789
	Night	25% 4 7% 16% 23% 25%	102799
Seeing	Day	7% 7% 26% 38% 17% 5%	15773
	Night	8% 3 11% 35% 35% 8%	26132

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	8% 4 1 4 3 2 3 2 4 5% 64%	2858
	11th to 20th	Day	8% 5% 2 4 3 3 3 3 5% 8% 56%	3029
	21st to end	Day	13% 6% 2 5% 3 4 4 3 5% 8% 47%	3498
February	1st to 10th	Day	10% 5% 2 6% 3 3 3 3 3 5% 58%	1900
	11th to 20th	Day	10% 5% 2 3 3 3 4 2 6% 7% 54%	1850
	21st to end	Day	11% 7% 3 5% 4 3 4 4 5% 6% 48%	1570
March	1st to 10th	Day	10% 5% 2 5% 4 4 5% 5% 9% 10% 40%	3138
	11th to 20th	Day	6% 4 2 4 3 3 4 4 5% 8% 58%	3325
	21st to end	Day	12% 6% 2 4 3 3 3 3 5% 7% 52%	4036
April	1st to 10th	Day	8% 6% 2 4 4 3 3 3 5% 7% 56%	2280
	11th to 20th	Day	5% 5% 1 4 3 3 3 3 5% 6% 63%	2249
	21st to end	Day	7% 5% 1 3 2 3 2 2 4 7% 64%	2280
May	1st to 10th	Day	5% 4 2 3 2 2 3 2 3 5% 69%	3986
	11th to 20th	Day	3 6% 1 5% 3 3 3 2 4 7% 61%	4071
	21st to end	Day	4 6% 2 5% 4 3 3 2 5% 7% 60%	4477
June	1st to 10th	Day	4 6% 2 5% 2 2 2 2 4 6% 63%	2470
	11th to 20th	Day	3 3 1 4 2 2 2 2 3 6% 73%	2470
	21st to end	Day	2 2 1 3 3 2 3 2 3 4 75%	2470
July	1st to 10th	Day	1 2 1 1 2 2 2 1 2 4 82%	4110
	11th to 20th	Day	1 2 1 2 3 3 3 3 4 6% 73%	4110
	21st to end	Day	1 1 1 2 2 2 2 2 4 5% 78%	4255
August	1st to 10th	Day	1 2 1 2 2 2 2 2 4 6% 79%	3783
	11th to 20th	Day	1 1 1 2 1 1 1 1 3 5% 84%	3724
	21st to end	Day	1 1 1 2 2 1 2 2 3 4 82%	4180
September	1st to 10th	Day	2 2 1 2 2 2 3 3 3 5% 75%	2267
	11th to 20th	Day	2 3 1 2 2 1 2 1 3 5% 79%	2090
	21st to end	Day	2 2 1 3 2 2 1 2 3 5% 76%	2090
October	1st to 10th	Day	2 2 1 2 2 2 2 1 3 4 78%	3490
	11th to 20th	Day	4 4 1 3 2 2 2 2 3 5% 73%	3453
	21st to end	Day	3 2 1 3 2 2 3 3 5% 7% 70%	3291
November	1st to 10th	Day	5% 3 1 2 2 3 2 2 5% 6% 68%	1764
	11th to 20th	Day	4 3 2 3 2 3 3 3 4 5% 69%	1719
	21st to end	Day	7% 5% 2 4 2 3 4 3 5% 8% 58%	1710
December	1st to 10th	Day	8% 5% 1 4 2 3 3 3 5% 8% 57%	1710
	11th to 20th	Day	14% 6% 2 5% 4 4 4 4 7% 7% 44%	2169
	21st to end	Day	12% 6% 2 6% 5% 6% 4 4 6% 6% 42%	3146
January	1st to 10th	Night	7% 5% 2 3 3 3 3 2 4 6% 62%	4097
	11th to 20th	Night	9% 6% 2 5% 4 4 3 3 5% 8% 51%	3942
	21st to end	Night	16% 8% 2 4 3 2 3 3 5% 6% 47%	4180
February	1st to 10th	Night	10% 6% 1 4 3 2 3 3 5% 5% 58%	2280
	11th to 20th	Night	10% 7% 2 5% 4 3 3 2 4 4 55%	2218
	21st to end	Night	13% 8% 2 5% 3 3 4 3 4 5% 49%	1884
March	1st to 10th	Night	17% 12% 3 6% 3 3 4 3 5% 7% 38%	3736
	11th to 20th	Night	8% 6% 2 5% 3 3 3 3 5% 7% 54%	3655
	21st to end	Night	12% 12% 2 4 4 3 3 3 5% 7% 44%	3612
April	1st to 10th	Night	9% 8% 3 5% 3 3 3 4 5% 6% 51%	1900
	11th to 20th	Night	8% 10% 2 5% 3 3 4 3 3 4 54%	1879
	21st to end	Night	8% 8% 3 5% 3 4 3 3 5% 5% 54%	1900
May	1st to 10th	Night	6% 9% 2 4 3 3 2 2 3 5% 62%	2974
	11th to 20th	Night	4 15% 4 7% 7% 6% 3 3 4 6% 41%	2834
	21st to end	Night	6% 14% 3 6% 4 4 2 3 3 5% 50%	3110
June	1st to 10th	Night	3 10% 4 6% 4 5% 4 3 4 8% 47%	1710
	11th to 20th	Night	2 6% 3 5% 3 4 3 3 5% 7% 60%	1710
	21st to end	Night	2 3 2 5% 3 4 2 3 4 7% 65%	1710
July	1st to 10th	Night	1 3 1 2 2 2 2 2 2 5% 78%	2860
	11th to 20th	Night	1 2 1 4 5% 5% 4 3 4 6% 66%	2860
	21st to end	Night	1 1 1 3 3 3 3 2 4 5% 74%	3297
August	1st to 10th	Night	1 2 1 2 2 2 2 2 3 7% 76%	3166
	11th to 20th	Night	1 2 1 2 2 2 2 3 4 6% 76%	3126
	21st to end	Night	1 2 1 3 4 3 2 1 3 5% 76%	3498
September	1st to 10th	Night	2 5% 2 4 2 3 3 2 4 6% 66%	1911
	11th to 20th	Night	4 4 1 3 3 2 2 2 3 4 73%	2090
	21st to end	Night	2 3 1 3 4 2 2 2 4 5% 71%	2090
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 4 77%	3490
	11th to 20th	Night	6% 10% 3 4 3 2 2 2 3 5% 61%	3469
	21st to end	Night	4 4 1 3 2 2 3 2 4 6% 68%	4271
November	1st to 10th	Night	5% 4 2 3 3 3 3 3 4 6% 66%	2535
	11th to 20th	Night	4 4 1 4 3 3 3 2 4 5% 67%	2483
	21st to end	Night	7% 4 2 4 4 4 4 4 5% 7% 55%	2470
December	1st to 10th	Night	9% 5% 1 5% 3 3 3 3 6% 7% 55%	2470
	11th to 20th	Night	13% 8% 2 6% 3 4 4 4 5% 8% 45%	3041
	21st to end	Night	10% 8% 3 8% 5% 4 5% 4 6% 6% 42%	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	23% 4 6% 20% 27% 20%	2859
	11th to 20th	Day	26% 5% 10% 26% 18% 15%	3029
	21st to end	Day	33% 6% 8% 12% 24% 17%	3498
February	1st to 10th	Day	30% 3 5% 13% 21% 28%	1900
	11th to 20th	Day	29% 4 7% 13% 27% 20%	1860
	21st to end	Day	36% 6% 6% 14% 20% 18%	1570
March	1st to 10th	Day	31% 9% 8% 22% 17% 12%	3138
	11th to 20th	Day	22% 7% 12% 27% 19% 12%	3325
	21st to end	Day	31% 6% 11% 32% 14% 8%	4036
April	1st to 10th	Day	28% 5% 9% 25% 22% 12%	2280
	11th to 20th	Day	22% 5% 8% 20% 28% 19%	2252
	21st to end	Day	21% 4 8% 25% 31% 11%	2280
May	1st to 10th	Day	18% 5% 7% 19% 41% 10%	3986
	11th to 20th	Day	18% 5% 7% 22% 38% 10%	4074
	21st to end	Day	21% 5% 6% 23% 40% 5%	4477
June	1st to 10th	Day	22% 3 6% 24% 30% 15%	2470
	11th to 20th	Day	15% 3 4 12% 33% 34%	2470
	21st to end	Day	13% 2 3 8% 27% 47%	2470
July	1st to 10th	Day	8% 1 3 6% 16% 66%	4046
	11th to 20th	Day	11% 3 9% 13% 32% 33%	4041
	21st to end	Day	9% 3 6% 23% 24% 34%	4121
August	1st to 10th	Day	10% 2 4 18% 40% 26%	3770
	11th to 20th	Day	7% 1 5% 13% 26% 48%	3760
	21st to end	Day	8% 2 3 8% 29% 51%	4179
September	1st to 10th	Day	13% 3 4 9% 35% 36%	2267
	11th to 20th	Day	10% 2 3 11% 37% 38%	2090
	21st to end	Day	13% 2 4 12% 33% 37%	2090
October	1st to 10th	Day	12% 2 4 10% 15% 57%	3490
	11th to 20th	Day	15% 2 5% 13% 26% 39%	3453
	21st to end	Day	15% 2 4 9% 31% 39%	3291
November	1st to 10th	Day	17% 4 5% 17% 22% 35%	1764
	11th to 20th	Day	19% 3 5% 13% 20% 39%	1719
	21st to end	Day	24% 4 7% 13% 20% 32%	1710
December	1st to 10th	Day	26% 4 9% 16% 16% 29%	1710
	11th to 20th	Day	37% 6% 9% 12% 19% 17%	2168
	21st to end	Day	40% 6% 7% 20% 16% 11%	3146
January	1st to 10th	Night	24% 5% 6% 14% 29% 22%	4097
	11th to 20th	Night	30% 6% 11% 23% 15% 16%	3942
	21st to end	Night	37% 5% 7% 9% 23% 18%	4180
February	1st to 10th	Night	29% 3 7% 13% 21% 28%	2280
	11th to 20th	Night	33% 4 5% 13% 21% 24%	2219
	21st to end	Night	37% 5% 5% 14% 21% 18%	1884
March	1st to 10th	Night	43% 8% 6% 16% 15% 12%	3736
	11th to 20th	Night	27% 7% 10% 26% 18% 12%	3655
	21st to end	Night	37% 8% 11% 22% 14% 8%	3612
April	1st to 10th	Night	32% 5% 9% 21% 19% 13%	1900
	11th to 20th	Night	31% 5% 8% 16% 22% 18%	1879
	21st to end	Night	31% 6% 7% 22% 22% 12%	1900
May	1st to 10th	Night	24% 6% 8% 22% 31% 9%	2974
	11th to 20th	Night	33% 9% 8% 18% 24% 8%	2835
	21st to end	Night	34% 7% 6% 22% 26% 5%	3110
June	1st to 10th	Night	30% 6% 8% 21% 21% 13%	1710
	11th to 20th	Night	22% 3 4 14% 29% 28%	1710
	21st to end	Night	19% 2 4 11% 28% 36%	1710
July	1st to 10th	Night	9% 1 4 7% 17% 62%	2815
	11th to 20th	Night	12% 3 11% 15% 37% 23%	2806
	21st to end	Night	12% 3 7% 24% 24% 30%	3194
August	1st to 10th	Night	10% 2 4 22% 42% 20%	3156
	11th to 20th	Night	9% 2 7% 15% 27% 40%	3156
	21st to end	Night	9% 2 7% 8% 28% 46%	3498
September	1st to 10th	Night	19% 3 5% 11% 29% 34%	1911
	11th to 20th	Night	17% 1 5% 8% 33% 36%	2090
	21st to end	Night	14% 3 5% 13% 36% 30%	2090
October	1st to 10th	Night	13% 2 4 10% 17% 54%	3490
	11th to 20th	Night	26% 3 4 9% 21% 37%	3469
	21st to end	Night	18% 2 5% 9% 28% 38%	4271
November	1st to 10th	Night	21% 4 5% 15% 19% 37%	2535
	11th to 20th	Night	21% 3 5% 14% 19% 39%	2483
	21st to end	Night	28% 4 8% 14% 15% 31%	2470
December	1st to 10th	Night	28% 4 8% 15% 17% 28%	2470
	11th to 20th	Night	37% 5% 8% 12% 20% 18%	3041
	21st to end	Night	38% 8% 9% 18% 14% 13%	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	17% 5% 30% 34% 13% 1	270
	11th to 20th	Day	14% 7% 26% 34% 17% 2	374
	21st to end	Day	12% 9% 30% 34% 13% 2	506
February	1st to 10th	Day	13% 11% 32% 29% 13% 3	460
	11th to 20th	Day	12% 15% 33% 30% 10% 1	455
	21st to end	Day	16% 17% 35% 25% 7% 1	382
March	1st to 10th	Day	13% 16% 34% 26% 10% 2	460
	11th to 20th	Day	14% 10% 34% 30% 11% 2	460
	21st to end	Day	11% 16% 35% 28% 8% 1	506
April	1st to 10th	Day	12% 11% 32% 30% 13% 2	460
	11th to 20th	Day	4 18% 35% 31% 10% 2	452
	21st to end	Day	8% 13% 31% 29% 16% 4	460
May	1st to 10th	Day	11% 11% 32% 29% 13% 3	463
	11th to 20th	Day	9% 7% 31% 37% 15% 2	488
	21st to end	Day	8% 11% 30% 34% 14% 2	529
June	1st to 10th	Day	2 5% 24% 40% 23% 5%	490
	11th to 20th	Day	3 3 26% 44% 18% 6%	490
	21st to end	Day	3 2 23% 45% 20% 8%	490
July	1st to 10th	Day	3 12% 49% 26% 10%	486
	11th to 20th	Day	2 9% 47% 29% 12%	484
	21st to end	Day	2 1 7% 50% 28% 12%	531
August	1st to 10th	Day	2 14% 49% 24% 10%	487
	11th to 20th	Day	2 1 10% 51% 23% 13%	477
	21st to end	Day	1 1 13% 51% 23% 10%	539
September	1st to 10th	Day	3 1 23% 47% 19% 8%	490
	11th to 20th	Day	3 5% 26% 45% 17% 3	490
	21st to end	Day	5% 6% 27% 40% 19% 5%	490
October	1st to 10th	Day	5% 7% 28% 39% 18% 4	490
	11th to 20th	Day	6% 4 32% 38% 18% 2	490
	21st to end	Day	4 5% 27% 38% 20% 6%	444
November	1st to 10th	Day	6% 7% 21% 38% 23% 5%	300
	11th to 20th	Day	5% 4 24% 40% 22% 4	273
	21st to end	Day	10% 7% 31% 33% 14% 5%	270
December	1st to 10th	Day	11% 6% 26% 34% 20% 3	270
	11th to 20th	Day	7% 3 31% 35% 19% 4	270
	21st to end	Day	21% 10% 22% 31% 13% 3	297
January	1st to 10th	Night	10% 3 19% 39% 22% 7%	760
	11th to 20th	Night	9% 4 16% 41% 23% 7%	760
	21st to end	Night	14% 3 16% 42% 21% 3	836
February	1st to 10th	Night	12% 4 16% 36% 25% 7%	760
	11th to 20th	Night	12% 4 18% 37% 23% 5%	744
	21st to end	Night	15% 6% 17% 38% 21% 4	628
March	1st to 10th	Night	13% 6% 17% 41% 21% 2	760
	11th to 20th	Night	12% 4 12% 39% 26% 7%	760
	21st to end	Night	10% 5% 16% 38% 26% 5%	836
April	1st to 10th	Night	13% 3 13% 42% 26% 4	760
	11th to 20th	Night	10% 4 16% 38% 27% 5%	751
	21st to end	Night	11% 4 10% 38% 31% 6%	760
May	1st to 10th	Night	10% 4 12% 37% 33% 5%	645
	11th to 20th	Night	9% 2 14% 34% 36% 6%	561
	21st to end	Night	10% 5% 12% 37% 33% 3	617
June	1st to 10th	Night	4 2 11% 40% 39% 4	570
	11th to 20th	Night	4 2 7% 40% 43% 4	570
	21st to end	Night	3 1 6% 37% 48% 5%	570
July	1st to 10th	Night	2 1 27% 58% 11%	555
	11th to 20th	Night	2 1 19% 66% 13%	552
	21st to end	Night	2 2 17% 61% 18%	737
August	1st to 10th	Night	2 1 2 25% 55% 16%	756
	11th to 20th	Night	1 2 17% 61% 18%	752
	21st to end	Night	2 1 17% 59% 21%	836
September	1st to 10th	Night	3 1 4 22% 56% 14%	760
	11th to 20th	Night	5% 1 4 32% 48% 10%	760
	21st to end	Night	3 1 7% 35% 42% 12%	760
October	1st to 10th	Night	5% 2 6% 43% 37% 8%	760
	11th to 20th	Night	8% 3 10% 38% 31% 10%	760
	21st to end	Night	7% 1 12% 34% 32% 13%	836
November	1st to 10th	Night	6% 3 10% 41% 31% 9%	780
	11th to 20th	Night	5% 1 13% 41% 29% 11%	764
	21st to end	Night	9% 4 15% 38% 26% 8%	760
December	1st to 10th	Night	11% 2 18% 41% 21% 7%	760
	11th to 20th	Night	14% 3 17% 41% 20% 4	760
	21st to end	Night	13% 5% 18% 38% 21% 6%	836

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