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 2025-05-26.

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%	114107
	Night	10% 6% 2 6% 4 3 4 3 5% 7% 50%	115383
Transparency	Day	38% 3 13% 27% 17% 3	117800
	Night	35% 3 12% 28% 20% 3	119820
Seeing	Day	14% 8% 30% 29% 15% 3	16485
	Night	13% 8% 33% 34% 11% 1	26469

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	10% 11% 5% 13% 7% 6% 5% 4 4 6% 28%	4077
	11th to 20th	Day	7% 9% 3 8% 5% 5% 6% 4 5% 5% 43%	4080
	21st to end	Day	6% 9% 4 10% 5% 4 5% 3 5% 6% 42%	4488
February	1st to 10th	Day	10% 11% 5% 8% 6% 4 4 4 6% 7% 35%	1800
	11th to 20th	Day	9% 10% 3 8% 4 4 4 4 5% 8% 41%	1839
	21st to end	Day	9% 7% 3 6% 4 3 4 3 5% 7% 48%	1650
March	1st to 10th	Day	13% 9% 2 6% 3 2 2 2 3 4 54%	4802
	11th to 20th	Day	10% 7% 2 5% 4 3 3 3 4 6% 53%	4980
	21st to end	Day	17% 11% 3 8% 4 4 4 4 6% 7% 32%	5462
April	1st to 10th	Day	14% 8% 2 5% 4 3 4 3 5% 6% 46%	2220
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 4 4 7% 42%	2537
	21st to end	Day	19% 7% 2 7% 4 4 4 3 5% 7% 38%	2600
May	1st to 10th	Day	15% 7% 2 7% 4 5% 4 4 5% 8% 39%	4212
	11th to 20th	Day	12% 7% 3 6% 4 4 4 3 5% 6% 46%	4212
	21st to end	Day	14% 6% 3 6% 4 4 5% 4 6% 7% 40%	4543
June	1st to 10th	Day	6% 4 1 5% 4 4 5% 4 6% 10% 51%	2589
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 6% 9% 47%	2660
	21st to end	Day	6% 5% 2 5% 4 5% 5% 5% 6% 9% 48%	2535
July	1st to 10th	Day	13% 5% 2 5% 4 5% 4 4 6% 8% 45%	4120
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 8% 52%	4120
	21st to end	Day	4 3 1 5% 3 4 5% 4 7% 11% 52%	4465
August	1st to 10th	Day	7% 5% 2 5% 5% 5% 5% 5% 7% 10% 45%	4104
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 7% 8% 43%	4041
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 7% 53%	4505
September	1st to 10th	Day	12% 4 2 4 4 3 3 3 4 7% 53%	2150
	11th to 20th	Day	9% 5% 2 5% 3 4 4 3 5% 7% 54%	2090
	21st to end	Day	10% 4 1 4 3 3 3 3 5% 6% 59%	2090
October	1st to 10th	Day	12% 6% 2 5% 4 3 4 2 4 5% 53%	1978
	11th to 20th	Day	10% 5% 2 5% 3 2 3 3 4 4 58%	1900
	21st to end	Day	13% 7% 2 6% 4 3 3 2 4 6% 50%	2075
November	1st to 10th	Day	9% 6% 1 6% 3 3 3 3 6% 6% 54%	1763
	11th to 20th	Day	9% 6% 2 7% 4 3 3 3 5% 7% 51%	1800
	21st to end	Day	12% 8% 3 6% 4 3 3 3 5% 7% 46%	1635
December	1st to 10th	Day	5% 6% 2 5% 4 3 4 3 6% 6% 56%	2610
	11th to 20th	Day	8% 7% 3 9% 5% 4 4 4 5% 7% 44%	3008
	21st to end	Day	16% 10% 3 13% 5% 4 4 3 4 5% 33%	4367
January	1st to 10th	Night	9% 10% 4 16% 7% 5% 5% 4 6% 6% 29%	5888
	11th to 20th	Night	6% 8% 2 7% 5% 4 7% 4 5% 6% 46%	5900
	21st to end	Night	8% 9% 5% 10% 4 3 3 3 4 5% 46%	6490
February	1st to 10th	Night	10% 10% 3 7% 5% 4 5% 4 5% 5% 41%	2600
	11th to 20th	Night	8% 8% 3 7% 5% 5% 5% 4 5% 7% 43%	2466
	21st to end	Night	8% 7% 2 6% 4 3 4 3 4 5% 53%	1980
March	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 4 5% 54%	5057
	11th to 20th	Night	10% 5% 2 4 3 2 3 2 4 6% 58%	4980
	21st to end	Night	14% 9% 3 7% 5% 4 4 4 6% 8% 37%	5467
April	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 5% 7% 47%	2178
	11th to 20th	Night	11% 7% 3 6% 4 3 4 3 5% 7% 47%	1809
	21st to end	Night	16% 7% 2 5% 4 4 4 2 5% 6% 46%	1800
May	1st to 10th	Night	18% 7% 2 5% 4 3 3 2 4 6% 45%	2913
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 5% 7% 51%	2913
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 7% 44%	3140
June	1st to 10th	Night	6% 3 1 4 4 3 5% 4 7% 10% 54%	1587
	11th to 20th	Night	6% 4 2 5% 4 4 3 4 6% 10% 51%	1520
	21st to end	Night	3 3 2 4 4 4 4 4 7% 8% 59%	1636
July	1st to 10th	Night	8% 4 2 5% 4 3 4 3 5% 7% 54%	2850
	11th to 20th	Night	5% 3 1 3 3 3 4 3 6% 8% 61%	2850
	21st to end	Night	3 2 1 3 3 3 4 4 6% 10% 61%	3098
August	1st to 10th	Night	4 3 1 3 4 3 4 3 6% 10% 58%	2836
	11th to 20th	Night	12% 6% 1 4 3 3 3 3 5% 8% 51%	2798
	21st to end	Night	6% 4 2 5% 4 3 4 3 5% 8% 57%	3160
September	1st to 10th	Night	8% 4 1 4 3 3 3 2 4 7% 62%	2026
	11th to 20th	Night	8% 3 1 4 2 3 3 3 5% 7% 59%	2090
	21st to end	Night	8% 5% 1 3 3 3 3 3 4 5% 60%	2090
October	1st to 10th	Night	12% 5% 2 4 3 3 5% 3 5% 5% 52%	2181
	11th to 20th	Night	11% 6% 2 5% 3 2 2 3 3 5% 59%	2280
	21st to end	Night	13% 5% 2 4 3 2 3 3 4 5% 55%	2521
November	1st to 10th	Night	8% 4 2 4 4 3 3 3 4 7% 58%	2535
	11th to 20th	Night	8% 6% 2 5% 4 3 3 3 4 6% 54%	2600
	21st to end	Night	10% 6% 2 5% 3 3 3 3 5% 6% 53%	2760
December	1st to 10th	Night	5% 5% 2 5% 4 4 4 3 5% 7% 59%	4580
	11th to 20th	Night	9% 6% 2 8% 4 3 3 3 5% 8% 49%	5199
	21st to end	Night	15% 12% 4 10% 4 4 3 3 4 5% 36%	6605

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	58% 2 10% 22% 8% 1	4078
	11th to 20th	Day	44% 1 14% 21% 18% 2	4080
	21st to end	Day	44% 3 12% 25% 15% 1	4488
February	1st to 10th	Day	48% 4 15% 21% 11% 1	1800
	11th to 20th	Day	43% 3 13% 23% 16% 3	1848
	21st to end	Day	37% 3 13% 26% 18% 3	1650
March	1st to 10th	Day	37% 1 8% 27% 19% 7%	4802
	11th to 20th	Day	34% 1 7% 27% 25% 5%	4980
	21st to end	Day	52% 2 21% 18% 5% 2	5464
April	1st to 10th	Day	40% 2 13% 27% 16% 2	2220
	11th to 20th	Day	43% 2 12% 24% 15% 3	2537
	21st to end	Day	46% 3 10% 27% 13% 1	2600
May	1st to 10th	Day	44% 3 9% 26% 15% 3	4250
	11th to 20th	Day	40% 2 10% 25% 20% 1	4216
	21st to end	Day	44% 3 12% 27% 14% 1	4555
June	1st to 10th	Day	29% 4 17% 34% 14% 2	2589
	11th to 20th	Day	33% 4 13% 32% 15% 2	2660
	21st to end	Day	31% 5% 14% 31% 14% 4	2535
July	1st to 10th	Day	37% 3 13% 29% 17% 1	4056
	11th to 20th	Day	31% 4 12% 32% 19% 3	4051
	21st to end	Day	24% 6% 17% 29% 22% 3	4325
August	1st to 10th	Day	32% 5% 16% 29% 15% 3	4090
	11th to 20th	Day	39% 4 14% 24% 17% 1	4080
	21st to end	Day	30% 5% 14% 25% 19% 6%	4504
September	1st to 10th	Day	32% 3 9% 31% 19% 6%	2150
	11th to 20th	Day	31% 3 11% 32% 22% 2	2090
	21st to end	Day	28% 2 9% 33% 27% 2	2090
October	1st to 10th	Day	22% 2 6% 21% 46% 4	3290
	11th to 20th	Day	22% 2 11% 31% 26% 8%	3140
	21st to end	Day	33% 2 12% 28% 22% 3	3402
November	1st to 10th	Day	31% 2 14% 30% 21% 3	1763
	11th to 20th	Day	35% 3 12% 29% 21% 1	1800
	21st to end	Day	39% 2 11% 30% 15% 2	1635
December	1st to 10th	Day	29% 3 18% 31% 18% 1	2610
	11th to 20th	Day	40% 2 19% 28% 9% 1	3005
	21st to end	Day	55% 1 10% 24% 9% 1	4367
January	1st to 10th	Night	56% 2 10% 18% 13%	5888
	11th to 20th	Night	39% 1 13% 26% 18% 2	5900
	21st to end	Night	43% 2 11% 21% 21% 2	6490
February	1st to 10th	Night	45% 3 13% 22% 14% 3	2600
	11th to 20th	Night	41% 3 13% 24% 15% 3	2467
	21st to end	Night	34% 3 9% 26% 22% 6%	1980
March	1st to 10th	Night	34% 2 7% 29% 25% 3	5057
	11th to 20th	Night	30% 2 6% 24% 31% 8%	4980
	21st to end	Night	46% 2 19% 26% 6% 1	5467
April	1st to 10th	Night	37% 3 10% 28% 18% 3	2178
	11th to 20th	Night	37% 3 11% 28% 18% 3	1809
	21st to end	Night	40% 4 12% 26% 17% 1	1800
May	1st to 10th	Night	42% 2 10% 26% 18% 2	2940
	11th to 20th	Night	34% 3 12% 27% 23% 2	2914
	21st to end	Night	41% 2 14% 22% 20% 2	3149
June	1st to 10th	Night	26% 5% 17% 33% 17% 3	1587
	11th to 20th	Night	28% 4 14% 29% 20% 5%	1520
	21st to end	Night	23% 4 14% 35% 21% 3	1636
July	1st to 10th	Night	30% 4 13% 32% 18% 3	2805
	11th to 20th	Night	22% 4 14% 36% 21% 3	2796
	21st to end	Night	18% 5% 17% 32% 26% 2	2999
August	1st to 10th	Night	23% 4 18% 34% 19% 4	2826
	11th to 20th	Night	33% 3 16% 29% 17% 2	2825
	21st to end	Night	27% 4 15% 29% 17% 9%	3160
September	1st to 10th	Night	24% 3 11% 30% 24% 8%	2026
	11th to 20th	Night	25% 2 12% 35% 21% 4	2090
	21st to end	Night	28% 2 6% 31% 28% 5%	2090
October	1st to 10th	Night	22% 2 6% 18% 46% 6%	3636
	11th to 20th	Night	22% 2 7% 32% 29% 8%	3779
	21st to end	Night	31% 2 8% 32% 24% 2	4145
November	1st to 10th	Night	27% 2 13% 28% 25% 5%	2535
	11th to 20th	Night	32% 2 11% 31% 21% 4	2600
	21st to end	Night	33% 2 13% 28% 20% 3	2760
December	1st to 10th	Night	27% 2 15% 35% 19% 1	4580
	11th to 20th	Night	35% 2 15% 32% 15% 1	5201
	21st to end	Night	52% 2 10% 27% 9% 1	6605

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% 39% 19% 9% 2	300
	11th to 20th	Day	19% 14% 35% 23% 7% 1	300
	21st to end	Day	13% 8% 47% 23% 8% 1	330
February	1st to 10th	Day	16% 18% 41% 21% 4	300
	11th to 20th	Day	15% 13% 39% 24% 8% 1	300
	21st to end	Day	13% 18% 43% 21% 4 1	249
March	1st to 10th	Day	17% 10% 43% 23% 5% 1	300
	11th to 20th	Day	23% 9% 35% 22% 9% 1	300
	21st to end	Day	30% 12% 37% 17% 4	330
April	1st to 10th	Day	23% 16% 40% 18% 4	320
	11th to 20th	Day	20% 16% 35% 21% 7%	655
	21st to end	Day	23% 14% 33% 20% 9% 2	700
May	1st to 10th	Day	18% 10% 33% 25% 10% 3	703
	11th to 20th	Day	18% 9% 31% 25% 13% 4	725
	21st to end	Day	20% 5% 29% 29% 13% 3	760
June	1st to 10th	Day	7% 5% 22% 38% 23% 5%	680
	11th to 20th	Day	10% 6% 32% 32% 18% 4	680
	21st to end	Day	8% 4 32% 36% 18% 2	680
July	1st to 10th	Day	9% 3 17% 36% 27% 8%	671
	11th to 20th	Day	6% 2 22% 39% 25% 8%	668
	21st to end	Day	7% 2 18% 40% 29% 5%	739
August	1st to 10th	Day	8% 3 20% 38% 25% 6%	676
	11th to 20th	Day	8% 3 18% 42% 25% 3	665
	21st to end	Day	11% 3 18% 37% 25% 6%	734
September	1st to 10th	Day	13% 3 24% 36% 19% 4	360
	11th to 20th	Day	10% 2 25% 38% 20% 5%	300
	21st to end	Day	15% 5% 32% 31% 13% 4	300
October	1st to 10th	Day	20% 5% 26% 28% 16% 5%	300
	11th to 20th	Day	11% 13% 42% 23% 10% 1	300
	21st to end	Day	20% 12% 39% 19% 9% 2	330
November	1st to 10th	Day	19% 11% 35% 23% 8% 3	300
	11th to 20th	Day	15% 12% 43% 22% 6% 1	300
	21st to end	Day	19% 13% 33% 24% 10% 1	300
December	1st to 10th	Day	9% 12% 42% 28% 7% 1	300
	11th to 20th	Day	17% 11% 40% 25% 8%	300
	21st to end	Day	21% 14% 39% 22% 4 1	330
January	1st to 10th	Night	12% 12% 36% 32% 8% 1	1000
	11th to 20th	Night	14% 12% 36% 30% 7% 1	1000
	21st to end	Night	14% 9% 41% 30% 5% 1	1100
February	1st to 10th	Night	16% 10% 41% 29% 5%	1000
	11th to 20th	Night	13% 11% 41% 32% 4	912
	21st to end	Night	13% 14% 42% 26% 5%	660
March	1st to 10th	Night	15% 12% 38% 28% 7%	665
	11th to 20th	Night	21% 8% 38% 27% 6% 1	600
	21st to end	Night	22% 10% 36% 27% 4	660
April	1st to 10th	Night	16% 13% 41% 25% 6%	600
	11th to 20th	Night	15% 16% 38% 27% 4	593
	21st to end	Night	20% 9% 33% 30% 7% 1	600
May	1st to 10th	Night	17% 8% 33% 31% 11%	600
	11th to 20th	Night	14% 9% 31% 33% 12% 1	591
	21st to end	Night	17% 5% 31% 33% 13% 1	634
June	1st to 10th	Night	7% 4 24% 47% 17% 1	570
	11th to 20th	Night	7% 5% 30% 44% 13% 1	570
	21st to end	Night	4 4 32% 43% 16% 1	570
July	1st to 10th	Night	7% 2 17% 43% 30% 1	555
	11th to 20th	Night	4 1 15% 52% 26% 1	552
	21st to end	Night	4 1 15% 51% 28% 1	624
August	1st to 10th	Night	7% 2 16% 49% 24% 2	567
	11th to 20th	Night	7% 2 17% 50% 23% 1	564
	21st to end	Night	9% 1 20% 42% 25% 3	627
September	1st to 10th	Night	9% 1 27% 39% 23% 1	570
	11th to 20th	Night	10% 2 29% 40% 18% 1	570
	21st to end	Night	12% 5% 30% 36% 18% 1	570
October	1st to 10th	Night	16% 7% 29% 33% 14% 2	661
	11th to 20th	Night	13% 10% 37% 31% 9%	760
	21st to end	Night	17% 10% 30% 32% 10%	849
November	1st to 10th	Night	11% 10% 33% 36% 9% 1	975
	11th to 20th	Night	13% 11% 39% 29% 7%	1000
	21st to end	Night	14% 11% 35% 31% 9% 1	1000
December	1st to 10th	Night	13% 11% 40% 30% 6% 1	1000
	11th to 20th	Night	15% 12% 34% 33% 6%	1000
	21st to end	Night	18% 11% 32% 32% 6%	1100

Page generated at 2025-05-23 08:21:00. Version ac07

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