Branched Oak Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Branched Oak Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	11% 7% 2 7% 4 4 4 3 5% 7% 46%	148661
	Night	9% 6% 2 6% 4 4 4 3 5% 7% 49%	143149
Transparency	Day	38% 3 13% 27% 17% 3	152313
	Night	35% 3 12% 27% 20% 3	147565
Seeing	Day	14% 8% 30% 29% 15% 3	18442
	Night	13% 8% 33% 34% 11% 1	28198

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	11% 11% 4 12% 7% 5% 5% 3 4 5% 33%	5289
	11th to 20th	Day	6% 8% 3 8% 6% 6% 6% 4 5% 5% 42%	5310
	21st to end	Day	5% 11% 4 10% 4 4 5% 3 5% 6% 42%	5812
February	1st to 10th	Day	9% 11% 5% 8% 6% 4 4 4 6% 7% 35%	1890
	11th to 20th	Day	9% 10% 3 8% 4 4 4 4 5% 8% 42%	1933
	21st to end	Day	9% 7% 3 6% 4 3 4 3 5% 7% 49%	1730
March	1st to 10th	Day	13% 9% 3 6% 3 2 3 2 3 4 54%	4909
	11th to 20th	Day	10% 7% 2 5% 4 3 3 3 4 6% 53%	5090
	21st to end	Day	17% 11% 3 8% 4 4 4 4 6% 7% 32%	5583
April	1st to 10th	Day	15% 9% 2 5% 4 3 4 3 5% 6% 45%	2331
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 4 5% 7% 43%	2665
	21st to end	Day	19% 7% 2 7% 4 4 4 3 5% 7% 38%	2730
May	1st to 10th	Day	12% 6% 2 7% 4 4 4 4 5% 7% 47%	7617
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 49%	7627
	21st to end	Day	18% 7% 3 6% 4 4 5% 3 5% 7% 37%	8392
June	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 6% 9% 50%	2861
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 6% 9% 48%	2940
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 6% 8% 47%	2801
July	1st to 10th	Day	9% 4 2 5% 4 5% 5% 4 7% 9% 46%	7680
	11th to 20th	Day	6% 4 2 5% 3 3 4 3 5% 8% 57%	7680
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 44%	8337
August	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 6% 9% 48%	6014
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 7% 8% 39%	5948
	21st to end	Day	8% 4 2 6% 4 4 5% 4 6% 7% 49%	6551
September	1st to 10th	Day	12% 4 2 4 4 3 3 3 4 6% 53%	2376
	11th to 20th	Day	11% 5% 2 5% 3 4 4 3 5% 7% 51%	2310
	21st to end	Day	12% 5% 1 4 3 3 3 3 5% 6% 57%	2288
October	1st to 10th	Day	12% 6% 2 5% 4 3 4 2 4 5% 53%	2082
	11th to 20th	Day	10% 5% 2 6% 3 2 3 3 4 4 58%	2000
	21st to end	Day	13% 7% 2 6% 4 3 3 2 4 6% 49%	2184
November	1st to 10th	Day	8% 6% 1 6% 3 3 3 2 6% 6% 55%	1853
	11th to 20th	Day	9% 6% 2 7% 4 3 3 3 5% 7% 51%	1890
	21st to end	Day	12% 8% 3 6% 4 3 3 3 5% 7% 45%	1717
December	1st to 10th	Day	5% 6% 2 6% 4 3 4 3 6% 6% 55%	2690
	11th to 20th	Day	8% 7% 3 9% 5% 4 4 4 6% 7% 44%	3088
	21st to end	Day	15% 10% 3 12% 5% 4 4 3 4 5% 34%	4463
January	1st to 10th	Night	9% 9% 3 14% 6% 5% 4 3 6% 6% 34%	7634
	11th to 20th	Night	5% 6% 2 7% 5% 5% 6% 4 6% 7% 47%	7680
	21st to end	Night	7% 10% 5% 10% 4 3 3 3 4 5% 45%	8402
February	1st to 10th	Night	10% 10% 3 7% 5% 4 5% 4 5% 5% 42%	2730
	11th to 20th	Night	8% 8% 3 7% 5% 5% 5% 4 5% 7% 43%	2592
	21st to end	Night	8% 7% 3 6% 4 3 4 3 4 6% 53%	2076
March	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 4 5% 54%	5170
	11th to 20th	Night	10% 5% 2 5% 3 3 3 2 4 6% 58%	5090
	21st to end	Night	14% 8% 3 7% 4 4 4 4 6% 8% 38%	5588
April	1st to 10th	Night	13% 7% 2 6% 4 3 3 3 5% 7% 47%	2287
	11th to 20th	Night	11% 7% 3 6% 4 4 4 3 6% 7% 47%	1901
	21st to end	Night	16% 7% 2 5% 4 4 4 2 5% 7% 45%	1890
May	1st to 10th	Night	12% 6% 2 6% 3 3 3 3 4 6% 52%	5266
	11th to 20th	Night	11% 4 1 5% 3 3 4 3 5% 7% 54%	5265
	21st to end	Night	18% 7% 2 6% 3 3 4 3 5% 6% 43%	5793
June	1st to 10th	Night	6% 4 1 5% 5% 3 5% 4 7% 9% 52%	1754
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 10% 52%	1680
	21st to end	Night	4 4 1 4 3 4 4 4 6% 8% 58%	1808
July	1st to 10th	Night	7% 4 2 6% 4 4 4 4 5% 9% 51%	5308
	11th to 20th	Night	4 3 2 4 3 3 4 3 5% 7% 61%	5307
	21st to end	Night	7% 4 2 6% 4 4 5% 4 6% 9% 48%	5770
August	1st to 10th	Night	4 3 1 4 4 3 4 3 6% 9% 59%	4156
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 6% 9% 47%	4118
	21st to end	Night	5% 4 2 5% 4 4 4 4 6% 10% 54%	4594
September	1st to 10th	Night	8% 4 1 4 3 3 3 3 4 8% 60%	2240
	11th to 20th	Night	10% 3 1 5% 2 3 3 4 5% 7% 56%	2310
	21st to end	Night	10% 6% 2 4 3 3 3 3 4 5% 58%	2288
October	1st to 10th	Night	12% 5% 2 4 3 3 5% 3 5% 5% 52%	2296
	11th to 20th	Night	11% 6% 2 5% 3 2 3 3 3 5% 58%	2400
	21st to end	Night	14% 5% 2 4 3 2 3 3 4 5% 54%	2654
November	1st to 10th	Night	8% 3 2 4 4 3 3 3 4 8% 58%	2665
	11th to 20th	Night	8% 6% 2 5% 4 3 3 3 5% 6% 54%	2730
	21st to end	Night	11% 6% 2 6% 3 4 3 3 5% 6% 52%	2898
December	1st to 10th	Night	4 5% 2 5% 4 4 4 3 5% 7% 58%	4720
	11th to 20th	Night	9% 6% 2 7% 4 3 3 3 5% 8% 49%	5339
	21st to end	Night	15% 12% 4 10% 4 4 3 3 4 5% 37%	6750

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	55% 2 10% 21% 11% 1	5290
	11th to 20th	Day	44% 1 14% 20% 18% 2	5310
	21st to end	Day	45% 2 13% 25% 14% 1	5812
February	1st to 10th	Day	47% 4 15% 22% 11% 1	1890
	11th to 20th	Day	42% 3 12% 24% 16% 3	1942
	21st to end	Day	36% 3 13% 27% 18% 3	1730
March	1st to 10th	Day	38% 1 8% 27% 19% 7%	4909
	11th to 20th	Day	34% 1 8% 27% 25% 5%	5090
	21st to end	Day	51% 2 21% 18% 5% 2	5585
April	1st to 10th	Day	41% 2 13% 26% 15% 2	2331
	11th to 20th	Day	42% 2 12% 25% 15% 3	2665
	21st to end	Day	46% 3 10% 27% 13% 1	2730
May	1st to 10th	Day	38% 2 8% 28% 21% 2	7655
	11th to 20th	Day	37% 3 9% 23% 24% 4	7631
	21st to end	Day	48% 3 10% 25% 14% 1	8405
June	1st to 10th	Day	31% 4 17% 33% 13% 2	2861
	11th to 20th	Day	32% 4 13% 32% 17% 2	2940
	21st to end	Day	33% 5% 14% 29% 14% 4	2801
July	1st to 10th	Day	33% 4 15% 32% 15%	7616
	11th to 20th	Day	26% 4 12% 31% 21% 5%	7611
	21st to end	Day	35% 6% 17% 28% 14% 2	8155
August	1st to 10th	Day	31% 5% 14% 29% 19% 2	6000
	11th to 20th	Day	42% 6% 16% 24% 12% 1	5987
	21st to end	Day	33% 5% 17% 26% 14% 5%	6550
September	1st to 10th	Day	32% 3 9% 31% 19% 5%	2376
	11th to 20th	Day	33% 3 12% 31% 20% 1	2310
	21st to end	Day	30% 2 8% 32% 26% 2	2288
October	1st to 10th	Day	22% 2 6% 21% 46% 4	3394
	11th to 20th	Day	22% 2 12% 31% 26% 8%	3240
	21st to end	Day	34% 2 12% 28% 22% 3	3511
November	1st to 10th	Day	30% 2 14% 30% 22% 3	1853
	11th to 20th	Day	34% 3 12% 29% 21% 1	1890
	21st to end	Day	40% 2 11% 30% 15% 2	1717
December	1st to 10th	Day	29% 3 18% 31% 18% 1	2690
	11th to 20th	Day	40% 2 19% 28% 10% 1	3085
	21st to end	Day	54% 2 10% 24% 9% 1	4463
January	1st to 10th	Night	51% 2 12% 19% 15% 1	7635
	11th to 20th	Night	36% 2 12% 28% 20% 2	7680
	21st to end	Night	43% 2 12% 20% 20% 2	8402
February	1st to 10th	Night	44% 3 13% 22% 14% 3	2730
	11th to 20th	Night	41% 3 13% 25% 15% 3	2593
	21st to end	Night	33% 3 9% 27% 23% 5%	2076
March	1st to 10th	Night	35% 2 7% 28% 25% 3	5170
	11th to 20th	Night	30% 2 7% 24% 30% 8%	5090
	21st to end	Night	45% 2 19% 26% 7% 1	5588
April	1st to 10th	Night	37% 3 10% 29% 18% 3	2287
	11th to 20th	Night	37% 3 11% 27% 19% 3	1901
	21st to end	Night	40% 4 12% 26% 17% 1	1890
May	1st to 10th	Night	35% 2 8% 27% 25% 3	5293
	11th to 20th	Night	32% 3 11% 23% 25% 6%	5266
	21st to end	Night	43% 3 12% 25% 17% 1	5805
June	1st to 10th	Night	28% 5% 17% 32% 17% 2	1754
	11th to 20th	Night	27% 4 14% 29% 21% 5%	1680
	21st to end	Night	24% 4 14% 33% 21% 3	1808
July	1st to 10th	Night	31% 5% 13% 33% 15% 3	5265
	11th to 20th	Night	23% 4 11% 32% 23% 7%	5256
	21st to end	Night	32% 6% 17% 26% 18% 1	5641
August	1st to 10th	Night	22% 4 16% 35% 19% 4	4146
	11th to 20th	Night	34% 5% 21% 27% 12% 1	4145
	21st to end	Night	27% 4 16% 34% 13% 6%	4594
September	1st to 10th	Night	24% 4 12% 30% 23% 7%	2240
	11th to 20th	Night	28% 3 12% 33% 20% 4	2310
	21st to end	Night	30% 3 6% 29% 28% 5%	2288
October	1st to 10th	Night	22% 2 6% 18% 46% 6%	3751
	11th to 20th	Night	23% 2 7% 31% 29% 8%	3899
	21st to end	Night	32% 2 8% 32% 24% 2	4278
November	1st to 10th	Night	27% 3 13% 28% 25% 4	2665
	11th to 20th	Night	32% 2 11% 31% 20% 4	2730
	21st to end	Night	34% 2 13% 28% 20% 3	2898
December	1st to 10th	Night	28% 2 15% 35% 19% 1	4720
	11th to 20th	Night	35% 2 15% 32% 15% 1	5341
	21st to end	Night	51% 2 10% 27% 9% 1	6750

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	18% 13% 38% 21% 8% 2	330
	11th to 20th	Day	18% 16% 36% 22% 6% 1	330
	21st to end	Day	14% 9% 46% 22% 9% 1	363
February	1st to 10th	Day	15% 18% 42% 21% 4	330
	11th to 20th	Day	15% 13% 39% 25% 8% 1	330
	21st to end	Day	12% 18% 44% 21% 4 1	273
March	1st to 10th	Day	17% 11% 44% 22% 5% 1	330
	11th to 20th	Day	24% 10% 35% 22% 9% 1	330
	21st to end	Day	29% 13% 36% 18% 3	363
April	1st to 10th	Day	24% 18% 39% 16% 3	351
	11th to 20th	Day	20% 17% 35% 21% 7%	703
	21st to end	Day	23% 14% 33% 20% 8% 2	750
May	1st to 10th	Day	18% 11% 33% 26% 10% 3	753
	11th to 20th	Day	18% 9% 32% 25% 13% 4	775
	21st to end	Day	20% 5% 29% 30% 14% 3	844
June	1st to 10th	Day	8% 6% 22% 37% 22% 5%	780
	11th to 20th	Day	10% 6% 33% 31% 17% 4	780
	21st to end	Day	9% 5% 33% 35% 16% 2	780
July	1st to 10th	Day	9% 3 18% 36% 26% 7%	771
	11th to 20th	Day	5% 2 21% 38% 26% 9%	768
	21st to end	Day	8% 2 18% 40% 27% 5%	849
August	1st to 10th	Day	9% 3 20% 39% 23% 5%	776
	11th to 20th	Day	8% 4 17% 42% 25% 3	765
	21st to end	Day	11% 3 18% 39% 24% 5%	842
September	1st to 10th	Day	13% 3 24% 36% 19% 4	426
	11th to 20th	Day	13% 2 26% 36% 19% 4	360
	21st to end	Day	18% 5% 30% 30% 13% 5%	354
October	1st to 10th	Day	19% 5% 26% 29% 15% 5%	330
	11th to 20th	Day	11% 12% 44% 22% 9% 1	330
	21st to end	Day	22% 12% 38% 18% 8% 1	363
November	1st to 10th	Day	18% 12% 37% 23% 8% 2	330
	11th to 20th	Day	14% 12% 44% 23% 6% 1	330
	21st to end	Day	22% 13% 32% 23% 9% 1	330
December	1st to 10th	Day	9% 12% 43% 28% 7% 2	330
	11th to 20th	Day	17% 12% 39% 24% 8%	330
	21st to end	Day	20% 13% 40% 22% 4 1	363
January	1st to 10th	Night	12% 12% 36% 32% 8%	1050
	11th to 20th	Night	14% 13% 36% 30% 7%	1050
	21st to end	Night	14% 9% 41% 30% 5% 1	1155
February	1st to 10th	Night	15% 10% 41% 29% 4	1050
	11th to 20th	Night	13% 11% 41% 32% 4	958
	21st to end	Night	13% 13% 43% 26% 4	692
March	1st to 10th	Night	15% 13% 38% 27% 7%	698
	11th to 20th	Night	20% 7% 39% 26% 6% 1	630
	21st to end	Night	21% 10% 36% 28% 4	693
April	1st to 10th	Night	17% 13% 40% 24% 6%	630
	11th to 20th	Night	15% 16% 38% 27% 4	623
	21st to end	Night	20% 9% 33% 30% 7%	630
May	1st to 10th	Night	17% 8% 34% 31% 10%	630
	11th to 20th	Night	14% 10% 31% 33% 12%	621
	21st to end	Night	17% 6% 30% 34% 12% 1	683
June	1st to 10th	Night	8% 4 24% 46% 17% 1	630
	11th to 20th	Night	7% 6% 30% 43% 13% 1	630
	21st to end	Night	6% 4 32% 43% 15%	630
July	1st to 10th	Night	7% 2 18% 43% 30% 1	615
	11th to 20th	Night	4 2 15% 50% 28% 1	612
	21st to end	Night	4 1 16% 50% 28% 1	690
August	1st to 10th	Night	7% 2 17% 49% 23% 2	627
	11th to 20th	Night	7% 2 18% 50% 23% 1	624
	21st to end	Night	9% 2 19% 44% 24% 2	693
September	1st to 10th	Night	9% 1 28% 39% 22% 1	630
	11th to 20th	Night	11% 3 29% 40% 17%	630
	21st to end	Night	13% 4 29% 35% 18% 1	624
October	1st to 10th	Night	16% 7% 29% 33% 13% 2	696
	11th to 20th	Night	13% 11% 37% 31% 8%	800
	21st to end	Night	18% 10% 31% 31% 10%	894
November	1st to 10th	Night	11% 10% 35% 35% 9% 1	1025
	11th to 20th	Night	12% 11% 40% 30% 7%	1050
	21st to end	Night	15% 11% 34% 31% 9% 1	1050
December	1st to 10th	Night	13% 11% 40% 30% 6%	1050
	11th to 20th	Night	15% 12% 34% 32% 6%	1050
	21st to end	Night	18% 11% 33% 33% 6%	1155

Page generated at 2026-09-26 03:18:40. Version ac07

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