Bonebrake Center of Nature and History
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bonebrake Center of Nature and History clear sky chart from 2005-11-06 to 2026-01-17.

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	12% 8% 3 6% 4 4 4 3 5% 7% 43%	127192
	Night	12% 8% 3 7% 4 4 4 3 5% 6% 44%	124733
Transparency	Day	39% 5% 11% 25% 17% 4	130788
	Night	39% 5% 10% 24% 18% 4	129213
Seeing	Day	18% 13% 28% 24% 13% 3	17159
	Night	18% 14% 32% 25% 11% 1	27044

By Month

Percentage Cloud Cover SE (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	13% 11% 4 10% 5% 4 3 3 4 5% 38%	3862
	11th to 20th	Day	9% 11% 3 8% 4 4 4 3 4 6% 45%	4134
	21st to end	Day	18% 8% 4 7% 3 3 3 3 4 4 41%	4488
February	1st to 10th	Day	15% 11% 4 8% 6% 4 4 3 5% 6% 35%	1800
	11th to 20th	Day	15% 10% 3 7% 4 4 4 3 5% 6% 39%	1881
	21st to end	Day	14% 11% 2 6% 4 5% 4 3 6% 6% 39%	1650
March	1st to 10th	Day	14% 12% 3 6% 4 3 3 2 4 4 45%	4788
	11th to 20th	Day	10% 7% 2 4 3 3 3 3 5% 7% 53%	4980
	21st to end	Day	12% 9% 3 6% 5% 6% 5% 4 6% 7% 37%	5617
April	1st to 10th	Day	12% 7% 2 6% 4 3 4 3 6% 7% 44%	2400
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 3 5% 6% 40%	2374
	21st to end	Day	19% 9% 3 7% 4 4 4 3 6% 6% 36%	2400
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 5% 7% 8% 30%	5448
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 35%	6130
	21st to end	Day	18% 8% 2 6% 4 4 4 3 4 7% 40%	6935
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 5% 8% 44%	2800
	11th to 20th	Day	6% 5% 2 5% 5% 4 5% 5% 7% 10% 48%	2800
	21st to end	Day	5% 4 2 4 4 4 5% 4 8% 10% 50%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 45%	6360
	11th to 20th	Day	5% 5% 2 7% 5% 5% 5% 4 8% 12% 41%	6094
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 41%	6422
August	1st to 10th	Day	4 4 1 4 4 4 4 4 7% 10% 53%	4234
	11th to 20th	Day	11% 6% 2 5% 5% 5% 5% 3 6% 8% 44%	3894
	21st to end	Day	8% 5% 2 5% 4 4 4 4 6% 9% 51%	4312
September	1st to 10th	Day	12% 5% 1 4 4 4 4 3 6% 8% 49%	2400
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 6% 7% 48%	2400
	21st to end	Day	10% 5% 2 6% 4 4 4 3 4 7% 51%	2130
October	1st to 10th	Day	9% 4 2 5% 2 4 3 3 4 7% 57%	2000
	11th to 20th	Day	14% 6% 2 5% 3 2 2 2 4 5% 55%	2000
	21st to end	Day	18% 8% 2 5% 3 3 3 2 4 5% 46%	2200
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 4 5% 53%	1947
	11th to 20th	Day	13% 8% 3 6% 3 3 4 3 3 7% 48%	1890
	21st to end	Day	14% 10% 4 6% 4 3 3 3 3 5% 44%	1774
December	1st to 10th	Day	9% 10% 3 6% 4 3 3 3 4 5% 49%	2690
	11th to 20th	Day	12% 11% 5% 8% 4 4 4 3 4 6% 41%	3088
	21st to end	Day	17% 20% 5% 8% 4 3 4 2 4 4 27%	4070
January	1st to 10th	Night	14% 10% 6% 12% 6% 4 4 3 4 4 33%	6318
	11th to 20th	Night	10% 9% 4 11% 5% 4 5% 3 3 4 41%	5979
	21st to end	Night	16% 11% 4 7% 4 4 3 3 4 5% 40%	6490
February	1st to 10th	Night	16% 11% 4 8% 4 3 3 3 5% 5% 37%	2600
	11th to 20th	Night	16% 8% 4 7% 4 4 3 2 4 6% 43%	2404
	21st to end	Night	15% 8% 3 7% 5% 4 3 3 5% 5% 41%	1980
March	1st to 10th	Night	16% 13% 3 6% 4 3 3 3 4 4 42%	5107
	11th to 20th	Night	11% 9% 3 5% 3 3 4 3 4 6% 49%	4980
	21st to end	Night	13% 10% 3 7% 5% 5% 5% 4 5% 7% 37%	5297
April	1st to 10th	Night	10% 5% 2 5% 4 4 3 3 6% 8% 49%	2000
	11th to 20th	Night	14% 9% 2 5% 4 3 3 3 4 6% 46%	1975
	21st to end	Night	18% 8% 3 7% 4 3 4 2 5% 6% 40%	2000
May	1st to 10th	Night	17% 9% 2 7% 4 4 4 3 6% 7% 37%	4433
	11th to 20th	Night	13% 9% 3 7% 4 4 4 3 5% 8% 41%	3780
	21st to end	Night	19% 8% 3 8% 5% 4 3 3 5% 6% 36%	3942
June	1st to 10th	Night	9% 4 1 6% 4 4 4 3 6% 9% 50%	1600
	11th to 20th	Night	6% 4 2 3 4 3 4 3 7% 11% 55%	1600
	21st to end	Night	3 3 1 3 3 4 3 4 6% 10% 60%	1600
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 9% 45%	3618
	11th to 20th	Night	4 4 1 5% 4 4 4 4 6% 9% 55%	3879
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 7% 10% 48%	4449
August	1st to 10th	Night	4 3 1 4 3 3 4 3 6% 9% 59%	2926
	11th to 20th	Night	9% 5% 2 7% 4 4 5% 4 6% 8% 48%	3156
	21st to end	Night	5% 4 1 4 2 3 4 3 5% 8% 60%	3586
September	1st to 10th	Night	11% 5% 2 4 4 3 4 3 5% 6% 53%	2000
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 6% 8% 51%	2000
	21st to end	Night	9% 5% 2 4 4 4 3 3 5% 7% 54%	2245
October	1st to 10th	Night	6% 4 1 4 3 2 3 3 4 7% 62%	2400
	11th to 20th	Night	14% 6% 2 4 3 3 3 2 3 5% 55%	2400
	21st to end	Night	18% 7% 2 5% 3 3 3 3 4 4 48%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 5% 56%	2570
	11th to 20th	Night	12% 7% 2 6% 3 3 4 2 5% 6% 49%	2730
	21st to end	Night	15% 10% 4 6% 4 3 3 2 4 5% 44%	2840
December	1st to 10th	Night	12% 9% 2 7% 3 3 3 2 5% 6% 46%	4720
	11th to 20th	Night	13% 10% 4 7% 4 4 4 3 4 5% 44%	5339
	21st to end	Night	20% 17% 6% 9% 5% 4 3 3 4 5% 26%	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	50% 3 9% 22% 14% 2	3863
	11th to 20th	Day	40% 4 7% 26% 18% 5%	4134
	21st to end	Day	46% 3 8% 15% 17% 11%	4488
February	1st to 10th	Day	51% 4 10% 22% 12% 1	1800
	11th to 20th	Day	46% 4 11% 20% 14% 5%	1890
	21st to end	Day	46% 4 7% 24% 14% 6%	1650
March	1st to 10th	Day	44% 3 6% 21% 16% 10%	4788
	11th to 20th	Day	32% 4 7% 24% 23% 9%	4980
	21st to end	Day	45% 6% 13% 27% 10% 1	5619
April	1st to 10th	Day	39% 4 12% 24% 20% 2	2400
	11th to 20th	Day	45% 5% 10% 21% 16% 3	2374
	21st to end	Day	48% 5% 11% 21% 13% 1	2400
May	1st to 10th	Day	48% 9% 12% 19% 12% 1	5483
	11th to 20th	Day	46% 7% 11% 21% 13% 1	6134
	21st to end	Day	45% 5% 8% 25% 16% 2	6949
June	1st to 10th	Day	37% 6% 14% 26% 15% 2	2800
	11th to 20th	Day	30% 6% 13% 30% 18% 3	2800
	21st to end	Day	27% 6% 19% 27% 16% 4	2800
July	1st to 10th	Day	36% 5% 12% 28% 18% 2	6291
	11th to 20th	Day	33% 7% 15% 31% 12% 2	6020
	21st to end	Day	33% 8% 17% 30% 11% 1	6240
August	1st to 10th	Day	25% 6% 17% 34% 19%	4220
	11th to 20th	Day	37% 5% 14% 26% 16% 2	3930
	21st to end	Day	29% 6% 15% 35% 12% 3	4311
September	1st to 10th	Day	33% 4 13% 27% 17% 6%	2400
	11th to 20th	Day	35% 5% 8% 27% 24% 1	2400
	21st to end	Day	34% 5% 7% 27% 25% 3	2130
October	1st to 10th	Day	17% 2 6% 19% 42% 13%	3260
	11th to 20th	Day	23% 2 6% 22% 37% 10%	3240
	21st to end	Day	30% 3 7% 32% 25% 4	3538
November	1st to 10th	Day	35% 3 9% 24% 22% 6%	1947
	11th to 20th	Day	38% 4 7% 23% 21% 7%	1890
	21st to end	Day	44% 3 8% 26% 18% 2	1774
December	1st to 10th	Day	38% 3 7% 37% 14% 1	2690
	11th to 20th	Day	46% 4 7% 23% 20% 1	3085
	21st to end	Day	61% 4 9% 14% 10% 1	4070
January	1st to 10th	Night	54% 5% 8% 18% 15% 1	6318
	11th to 20th	Night	45% 6% 6% 22% 19% 3	5979
	21st to end	Night	48% 4 7% 19% 18% 4	6490
February	1st to 10th	Night	48% 4 10% 22% 14% 2	2600
	11th to 20th	Night	45% 3 11% 22% 17% 3	2405
	21st to end	Night	45% 4 7% 22% 18% 5%	1980
March	1st to 10th	Night	47% 3 6% 22% 17% 5%	5107
	11th to 20th	Night	37% 4 8% 23% 18% 9%	4980
	21st to end	Night	45% 6% 13% 24% 12% 1	5297
April	1st to 10th	Night	33% 3 13% 28% 19% 3	2000
	11th to 20th	Night	39% 4 11% 25% 16% 5%	1975
	21st to end	Night	46% 4 9% 25% 15% 1	2000
May	1st to 10th	Night	45% 8% 14% 20% 12% 2	4463
	11th to 20th	Night	42% 6% 12% 24% 13% 3	3781
	21st to end	Night	48% 6% 10% 22% 12% 3	3950
June	1st to 10th	Night	31% 6% 16% 26% 19% 2	1600
	11th to 20th	Night	23% 6% 13% 33% 21% 4	1600
	21st to end	Night	19% 5% 18% 31% 20% 7%	1600
July	1st to 10th	Night	35% 7% 13% 23% 20% 2	3580
	11th to 20th	Night	26% 7% 15% 35% 15% 2	3833
	21st to end	Night	30% 7% 18% 32% 13% 1	4320
August	1st to 10th	Night	20% 6% 18% 33% 21% 1	2916
	11th to 20th	Night	34% 6% 17% 26% 15% 3	3186
	21st to end	Night	23% 5% 15% 38% 16% 3	3586
September	1st to 10th	Night	32% 4 9% 28% 22% 5%	2000
	11th to 20th	Night	31% 5% 9% 28% 26% 2	2000
	21st to end	Night	30% 4 9% 24% 29% 3	2245
October	1st to 10th	Night	14% 2 6% 17% 47% 13%	3920
	11th to 20th	Night	24% 2 6% 21% 32% 15%	3899
	21st to end	Night	32% 3 7% 27% 28% 4	4252
November	1st to 10th	Night	31% 3 10% 28% 22% 5%	2570
	11th to 20th	Night	36% 3 7% 25% 22% 7%	2730
	21st to end	Night	44% 3 7% 24% 20% 2	2840
December	1st to 10th	Night	40% 3 8% 31% 16% 1	4720
	11th to 20th	Night	44% 3 9% 23% 20% 2	5341
	21st to end	Night	62% 5% 9% 16% 7% 2	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	26% 26% 36% 10% 1	330
	11th to 20th	Day	30% 25% 38% 7%	318
	21st to end	Day	22% 33% 34% 10% 2	330
February	1st to 10th	Day	29% 25% 36% 10% 1	300
	11th to 20th	Day	34% 29% 28% 7% 1	300
	21st to end	Day	32% 26% 32% 9% 1	249
March	1st to 10th	Day	27% 22% 39% 11% 1	300
	11th to 20th	Day	35% 22% 31% 10% 2	300
	21st to end	Day	32% 25% 32% 10% 1	395
April	1st to 10th	Day	26% 20% 35% 14% 4	500
	11th to 20th	Day	28% 23% 29% 17% 4	492
	21st to end	Day	23% 22% 31% 19% 4 1	500
May	1st to 10th	Day	23% 20% 33% 20% 4 1	520
	11th to 20th	Day	19% 15% 32% 21% 11% 2	725
	21st to end	Day	16% 11% 35% 24% 11% 4	789
June	1st to 10th	Day	12% 5% 27% 31% 20% 5%	730
	11th to 20th	Day	9% 4 24% 41% 19% 3	730
	21st to end	Day	6% 6% 30% 37% 16% 3	730
July	1st to 10th	Day	12% 3 19% 30% 28% 8%	721
	11th to 20th	Day	5% 4 22% 39% 25% 6%	718
	21st to end	Day	10% 5% 17% 34% 28% 6%	794
August	1st to 10th	Day	8% 4 18% 42% 23% 4	726
	11th to 20th	Day	11% 4 20% 32% 25% 7%	552
	21st to end	Day	13% 2 15% 34% 27% 9%	583
September	1st to 10th	Day	12% 5% 17% 35% 24% 6%	530
	11th to 20th	Day	9% 3 22% 33% 25% 8%	530
	21st to end	Day	12% 11% 27% 27% 18% 5%	431
October	1st to 10th	Day	12% 10% 36% 28% 10% 4	330
	11th to 20th	Day	21% 15% 35% 22% 5% 1	330
	21st to end	Day	31% 18% 33% 16% 1	363
November	1st to 10th	Day	17% 18% 41% 18% 4 2	330
	11th to 20th	Day	23% 20% 36% 15% 5%	330
	21st to end	Day	30% 17% 36% 13% 5%	330
December	1st to 10th	Day	22% 26% 33% 13% 4 2	330
	11th to 20th	Day	25% 24% 35% 13% 2	330
	21st to end	Day	29% 25% 36% 8% 2	363
January	1st to 10th	Night	22% 23% 37% 16% 3	1050
	11th to 20th	Night	24% 27% 34% 12% 3	1031
	21st to end	Night	20% 23% 39% 17% 1	1100
February	1st to 10th	Night	24% 22% 37% 15% 1	1000
	11th to 20th	Night	24% 21% 40% 13% 2	849
	21st to end	Night	26% 24% 34% 15% 2	660
March	1st to 10th	Night	25% 18% 38% 18% 1	690
	11th to 20th	Night	25% 17% 37% 19% 3	600
	21st to end	Night	28% 17% 35% 16% 4	660
April	1st to 10th	Night	16% 20% 42% 21% 2	600
	11th to 20th	Night	20% 17% 33% 25% 4	593
	21st to end	Night	22% 15% 37% 20% 6%	600
May	1st to 10th	Night	17% 10% 37% 28% 7%	600
	11th to 20th	Night	19% 8% 33% 26% 13% 1	591
	21st to end	Night	13% 8% 32% 29% 16% 2	650
June	1st to 10th	Night	9% 5% 22% 36% 26% 3	600
	11th to 20th	Night	7% 4 26% 40% 22% 2	600
	21st to end	Night	4 3 27% 45% 20% 2	600
July	1st to 10th	Night	11% 1 14% 37% 30% 7%	585
	11th to 20th	Night	6% 2 19% 39% 30% 4	582
	21st to end	Night	6% 2 17% 43% 29% 3	657
August	1st to 10th	Night	6% 2 19% 42% 30% 2	597
	11th to 20th	Night	6% 3 18% 42% 28% 4	594
	21st to end	Night	7% 3 15% 38% 32% 5%	660
September	1st to 10th	Night	13% 3 18% 36% 26% 4	600
	11th to 20th	Night	12% 2 17% 39% 26% 6%	600
	21st to end	Night	12% 7% 25% 29% 22% 4	750
October	1st to 10th	Night	8% 7% 32% 29% 21% 3	800
	11th to 20th	Night	17% 13% 35% 26% 9%	800
	21st to end	Night	25% 14% 34% 21% 6%	880
November	1st to 10th	Night	15% 12% 42% 25% 7%	820
	11th to 20th	Night	20% 16% 35% 22% 6%	840
	21st to end	Night	23% 19% 34% 18% 5%	950
December	1st to 10th	Night	22% 19% 37% 18% 3	1050
	11th to 20th	Night	24% 16% 38% 19% 3	1050
	21st to end	Night	25% 23% 36% 15% 2	1155

Page generated at 2026-01-14 00:14:26. Version ac07

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