Broken Oak Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Broken Oak Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	7% 5% 2 7% 5% 5% 6% 5% 7% 10% 40%	127389
	Night	8% 6% 2 6% 5% 5% 5% 4 7% 9% 43%	124613
Transparency	Day	36% 10% 16% 19% 13% 5%	131137
	Night	36% 9% 16% 19% 14% 6%	128933
Seeing	Day	9% 7% 21% 27% 21% 15%	17104
	Night	9% 6% 23% 32% 23% 7%	27195

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	8% 6% 2 5% 4 4 4 3 5% 8% 52%	4146
	11th to 20th	Day	20% 9% 3 8% 4 4 4 4 5% 7% 32%	4150
	21st to end	Day	12% 6% 1 5% 4 3 4 3 6% 9% 47%	4869
February	1st to 10th	Day	13% 5% 2 7% 4 4 4 4 5% 7% 44%	2000
	11th to 20th	Day	9% 4 2 5% 3 4 4 5% 4 6% 53%	1948
	21st to end	Day	8% 6% 2 6% 5% 4 4 4 5% 7% 49%	1706
March	1st to 10th	Day	7% 6% 2 7% 4 5% 5% 4 6% 9% 45%	4935
	11th to 20th	Day	5% 3 2 5% 3 3 3 3 6% 9% 58%	4980
	21st to end	Day	8% 5% 2 5% 4 4 4 4 6% 8% 50%	5465
April	1st to 10th	Day	7% 4 2 4 5% 4 6% 4 6% 8% 50%	2200
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 6% 7% 47%	2176
	21st to end	Day	4 3 1 5% 4 5% 5% 5% 6% 10% 52%	2423
May	1st to 10th	Day	3 3 1 5% 5% 5% 6% 5% 7% 11% 50%	5862
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	5861
	21st to end	Day	4 3 1 5% 5% 5% 5% 5% 8% 11% 49%	6433
June	1st to 10th	Day	11% 8% 3 9% 7% 7% 6% 6% 8% 11% 25%	2600
	11th to 20th	Day	6% 5% 3 9% 8% 8% 10% 7% 10% 10% 25%	2600
	21st to end	Day	7% 7% 3 9% 7% 7% 8% 7% 9% 10% 26%	2600
July	1st to 10th	Day	6% 6% 3 10% 8% 8% 9% 7% 11% 14% 18%	5900
	11th to 20th	Day	4 4 2 8% 7% 8% 9% 7% 10% 15% 27%	5900
	21st to end	Day	4 5% 2 8% 6% 7% 8% 7% 10% 14% 30%	6424
August	1st to 10th	Day	7% 5% 3 9% 7% 7% 7% 6% 9% 12% 29%	4234
	11th to 20th	Day	4 5% 2 7% 6% 7% 7% 6% 8% 13% 35%	3881
	21st to end	Day	8% 7% 3 9% 7% 8% 8% 6% 9% 12% 25%	3949
September	1st to 10th	Day	7% 7% 3 11% 10% 8% 9% 7% 9% 10% 20%	2200
	11th to 20th	Day	9% 7% 5% 11% 7% 7% 8% 5% 7% 9% 26%	2200
	21st to end	Day	8% 6% 3 8% 7% 7% 8% 5% 8% 8% 32%	2200
October	1st to 10th	Day	11% 6% 4 9% 6% 5% 5% 5% 6% 8% 34%	2200
	11th to 20th	Day	5% 5% 2 5% 5% 5% 4 4 6% 7% 52%	2025
	21st to end	Day	7% 5% 3 5% 3 4 5% 4 6% 9% 50%	2200
November	1st to 10th	Day	7% 5% 3 6% 4 4 4 3 5% 8% 50%	2057
	11th to 20th	Day	8% 6% 2 6% 5% 4 4 4 6% 7% 50%	2100
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 6% 8% 46%	1917
December	1st to 10th	Day	5% 4 2 4 4 3 4 4 7% 7% 56%	3020
	11th to 20th	Day	8% 6% 3 6% 4 4 4 4 6% 8% 46%	3463
	21st to end	Day	11% 7% 3 8% 5% 4 5% 4 6% 7% 40%	4565
January	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 5% 8% 53%	6019
	11th to 20th	Night	20% 10% 4 7% 5% 4 5% 4 6% 7% 29%	6030
	21st to end	Night	15% 6% 2 6% 4 4 4 3 6% 8% 41%	6325
February	1st to 10th	Night	11% 7% 2 6% 4 4 4 4 6% 7% 45%	2401
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 6% 8% 49%	2338
	21st to end	Night	8% 7% 3 7% 5% 4 4 3 5% 8% 47%	1921
March	1st to 10th	Night	12% 7% 3 7% 4 4 4 3 5% 7% 44%	4918
	11th to 20th	Night	5% 4 2 4 3 3 4 4 6% 9% 57%	4980
	21st to end	Night	9% 6% 2 6% 4 3 3 3 5% 8% 49%	5467
April	1st to 10th	Night	7% 5% 2 5% 4 3 4 4 8% 9% 51%	2200
	11th to 20th	Night	6% 5% 2 5% 4 3 3 4 6% 10% 52%	2173
	21st to end	Night	6% 3 1 3 4 4 5% 4 5% 8% 57%	1991
May	1st to 10th	Night	5% 4 1 5% 4 4 4 4 7% 10% 52%	4033
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 5% 9% 51%	4032
	21st to end	Night	3 3 1 5% 3 4 5% 4 7% 11% 55%	4421
June	1st to 10th	Night	9% 6% 3 6% 4 5% 6% 4 8% 11% 37%	1800
	11th to 20th	Night	5% 6% 2 6% 5% 6% 7% 6% 10% 13% 34%	1800
	21st to end	Night	6% 6% 2 8% 7% 6% 7% 6% 9% 11% 33%	1800
July	1st to 10th	Night	7% 7% 3 10% 7% 7% 8% 6% 10% 14% 20%	4058
	11th to 20th	Night	4 4 2 7% 6% 8% 8% 7% 12% 15% 27%	4057
	21st to end	Night	5% 7% 3 10% 8% 8% 8% 7% 10% 10% 24%	4427
August	1st to 10th	Night	6% 5% 3 8% 7% 6% 6% 6% 10% 12% 32%	2915
	11th to 20th	Night	4 4 2 5% 4 5% 6% 5% 9% 16% 39%	3134
	21st to end	Night	7% 7% 3 8% 6% 6% 7% 6% 9% 13% 28%	3916
September	1st to 10th	Night	7% 6% 2 11% 7% 7% 7% 5% 9% 11% 27%	2200
	11th to 20th	Night	9% 8% 3 9% 6% 7% 6% 5% 8% 10% 29%	2200
	21st to end	Night	8% 7% 3 8% 6% 5% 6% 6% 7% 9% 36%	2200
October	1st to 10th	Night	10% 7% 3 7% 5% 5% 5% 4 5% 9% 39%	2200
	11th to 20th	Night	5% 5% 2 4 3 3 4 3 6% 8% 56%	2372
	21st to end	Night	8% 6% 2 6% 4 4 3 3 5% 8% 50%	2640
November	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 5% 8% 52%	2461
	11th to 20th	Night	7% 7% 2 6% 4 4 4 4 4 7% 52%	2520
	21st to end	Night	8% 5% 3 8% 4 4 4 3 6% 8% 47%	2699
December	1st to 10th	Night	5% 4 2 4 3 4 4 4 5% 8% 58%	4380
	11th to 20th	Night	9% 7% 2 6% 3 3 4 3 5% 7% 50%	4952
	21st to end	Night	13% 8% 3 7% 5% 4 5% 4 6% 7% 39%	6633

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	30% 5% 9% 16% 27% 14%	4147
	11th to 20th	Day	52% 4 6% 16% 15% 8%	4150
	21st to end	Day	33% 5% 9% 26% 19% 8%	4869
February	1st to 10th	Day	37% 7% 9% 16% 19% 12%	2000
	11th to 20th	Day	31% 5% 9% 19% 24% 12%	1958
	21st to end	Day	33% 6% 11% 18% 21% 11%	1706
March	1st to 10th	Day	34% 8% 11% 20% 20% 8%	4935
	11th to 20th	Day	23% 5% 10% 22% 22% 18%	4980
	21st to end	Day	31% 5% 9% 27% 20% 8%	5465
April	1st to 10th	Day	29% 7% 13% 21% 21% 9%	2200
	11th to 20th	Day	34% 7% 10% 16% 23% 10%	2176
	21st to end	Day	25% 7% 14% 23% 24% 7%	2423
May	1st to 10th	Day	23% 10% 22% 27% 14% 4	5897
	11th to 20th	Day	32% 9% 16% 29% 12% 2	5865
	21st to end	Day	25% 9% 19% 31% 14% 2	6447
June	1st to 10th	Day	48% 13% 17% 16% 5%	2600
	11th to 20th	Day	46% 15% 20% 17% 3	2600
	21st to end	Day	48% 15% 20% 14% 3	2600
July	1st to 10th	Day	51% 15% 28% 6%	5835
	11th to 20th	Day	39% 17% 35% 9%	5831
	21st to end	Day	38% 16% 25% 20%	6240
August	1st to 10th	Day	42% 15% 29% 11% 3	4221
	11th to 20th	Day	35% 15% 24% 22% 4	3918
	21st to end	Day	46% 15% 23% 15% 1	3949
September	1st to 10th	Day	52% 18% 17% 10% 3	2200
	11th to 20th	Day	50% 13% 19% 13% 5%	2200
	21st to end	Day	45% 13% 14% 16% 10% 3	2200
October	1st to 10th	Day	57% 10% 11% 10% 7% 5%	3590
	11th to 20th	Day	24% 5% 9% 21% 34% 7%	3278
	21st to end	Day	28% 5% 8% 22% 30% 7%	3538
November	1st to 10th	Day	31% 7% 10% 21% 18% 12%	2057
	11th to 20th	Day	32% 6% 10% 18% 21% 11%	2100
	21st to end	Day	33% 6% 9% 22% 19% 10%	1917
December	1st to 10th	Day	26% 4 15% 26% 19% 10%	3020
	11th to 20th	Day	34% 6% 11% 19% 19% 11%	3460
	21st to end	Day	41% 7% 10% 21% 17% 5%	4565
January	1st to 10th	Night	30% 5% 8% 17% 29% 11%	6019
	11th to 20th	Night	53% 5% 9% 14% 14% 4	6030
	21st to end	Night	39% 6% 10% 20% 16% 8%	6325
February	1st to 10th	Night	36% 7% 10% 17% 18% 12%	2401
	11th to 20th	Night	32% 8% 10% 19% 22% 9%	2339
	21st to end	Night	34% 5% 13% 19% 22% 8%	1921
March	1st to 10th	Night	39% 7% 10% 17% 18% 9%	4918
	11th to 20th	Night	22% 7% 16% 21% 17% 18%	4980
	21st to end	Night	32% 6% 9% 27% 19% 6%	5467
April	1st to 10th	Night	26% 8% 19% 22% 18% 7%	2200
	11th to 20th	Night	26% 8% 14% 24% 20% 8%	2173
	21st to end	Night	24% 7% 15% 25% 24% 6%	1991
May	1st to 10th	Night	23% 11% 24% 27% 12% 4	4060
	11th to 20th	Night	28% 10% 18% 27% 15% 1	4033
	21st to end	Night	21% 9% 20% 34% 15% 1	4429
June	1st to 10th	Night	37% 13% 23% 22% 6%	1800
	11th to 20th	Night	36% 15% 27% 20% 2	1800
	21st to end	Night	40% 17% 23% 15% 4	1800
July	1st to 10th	Night	50% 17% 28% 5% 1	4015
	11th to 20th	Night	40% 19% 34% 7%	4006
	21st to end	Night	48% 16% 23% 13%	4297
August	1st to 10th	Night	40% 15% 31% 12% 2	2904
	11th to 20th	Night	30% 16% 30% 21% 3	3162
	21st to end	Night	44% 15% 26% 13% 3	3916
September	1st to 10th	Night	46% 18% 21% 12% 3	2200
	11th to 20th	Night	46% 13% 20% 16% 3 1	2200
	21st to end	Night	40% 13% 17% 19% 10% 2	2200
October	1st to 10th	Night	56% 10% 11% 12% 7% 4	3590
	11th to 20th	Night	22% 5% 10% 24% 33% 5%	3858
	21st to end	Night	31% 5% 8% 19% 31% 7%	4252
November	1st to 10th	Night	29% 6% 13% 21% 19% 11%	2461
	11th to 20th	Night	33% 5% 10% 20% 21% 10%	2520
	21st to end	Night	34% 5% 10% 23% 18% 9%	2699
December	1st to 10th	Night	24% 5% 16% 27% 15% 13%	4380
	11th to 20th	Night	34% 6% 11% 21% 17% 11%	4954
	21st to end	Night	43% 7% 12% 19% 15% 4	6633

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	10% 15% 46% 22% 7% 1	330
	11th to 20th	Day	12% 18% 37% 23% 8% 2	330
	21st to end	Day	15% 19% 45% 18% 3	363
February	1st to 10th	Day	13% 15% 32% 28% 11% 1	300
	11th to 20th	Day	14% 11% 38% 28% 7% 3	300
	21st to end	Day	10% 15% 30% 31% 13%	305
March	1st to 10th	Day	8% 17% 38% 27% 10% 1	500
	11th to 20th	Day	11% 13% 31% 29% 14% 2	500
	21st to end	Day	7% 13% 32% 32% 15% 2	550
April	1st to 10th	Day	11% 12% 32% 27% 16% 2	500
	11th to 20th	Day	9% 12% 32% 28% 17% 3	492
	21st to end	Day	5% 7% 26% 33% 23% 7%	500
May	1st to 10th	Day	6% 7% 23% 34% 21% 9%	503
	11th to 20th	Day	9% 4 20% 32% 21% 13%	527
	21st to end	Day	7% 3 13% 27% 31% 19%	573
June	1st to 10th	Day	13% 4 11% 26% 29% 17%	530
	11th to 20th	Day	6% 2 12% 25% 29% 26%	530
	21st to end	Day	6% 2 7% 20% 28% 37%	530
July	1st to 10th	Day	6% 1 8% 14% 28% 43%	525
	11th to 20th	Day	5% 1 5% 16% 28% 44%	524
	21st to end	Day	6% 1 9% 21% 27% 36%	576
August	1st to 10th	Day	7% 2 6% 17% 32% 36%	526
	11th to 20th	Day	6% 1 6% 13% 33% 41%	519
	21st to end	Day	9% 2 10% 24% 27% 28%	583
September	1st to 10th	Day	9% 2 8% 18% 32% 30%	530
	11th to 20th	Day	10% 2 11% 26% 31% 21%	530
	21st to end	Day	10% 3 9% 26% 31% 21%	530
October	1st to 10th	Day	13% 6% 22% 32% 22% 5%	530
	11th to 20th	Day	7% 6% 18% 35% 26% 7%	530
	21st to end	Day	8% 5% 23% 39% 20% 4	583
November	1st to 10th	Day	8% 7% 26% 35% 20% 4	535
	11th to 20th	Day	12% 7% 25% 36% 16% 4	540
	21st to end	Day	8% 11% 36% 30% 12% 4	357
December	1st to 10th	Day	9% 11% 38% 27% 11% 3	330
	11th to 20th	Day	11% 12% 35% 31% 11% 1	330
	21st to end	Day	8% 10% 36% 34% 10% 2	363
January	1st to 10th	Night	12% 12% 32% 33% 10%	840
	11th to 20th	Night	12% 11% 29% 37% 10%	840
	21st to end	Night	14% 15% 34% 31% 6%	924
February	1st to 10th	Night	13% 10% 32% 32% 13%	801
	11th to 20th	Night	10% 12% 31% 37% 10% 1	783
	21st to end	Night	10% 9% 37% 36% 8%	660
March	1st to 10th	Night	7% 10% 39% 34% 10%	800
	11th to 20th	Night	9% 10% 33% 35% 13%	800
	21st to end	Night	7% 11% 35% 36% 10%	880
April	1st to 10th	Night	7% 9% 35% 37% 12%	800
	11th to 20th	Night	8% 9% 35% 36% 11% 1	791
	21st to end	Night	6% 6% 29% 41% 18%	776
May	1st to 10th	Night	6% 3 20% 45% 24% 2	600
	11th to 20th	Night	6% 4 19% 40% 28% 3	591
	21st to end	Night	6% 2 14% 37% 35% 6%	650
June	1st to 10th	Night	11% 1 13% 31% 35% 9%	600
	11th to 20th	Night	8% 1 7% 25% 40% 20%	600
	21st to end	Night	8% 1 7% 18% 42% 23%	600
July	1st to 10th	Night	8% 1 5% 14% 43% 29%	585
	11th to 20th	Night	4 1 4 14% 45% 32%	582
	21st to end	Night	6% 5% 12% 49% 27%	657
August	1st to 10th	Night	7% 1 5% 12% 48% 27%	597
	11th to 20th	Night	6% 1 5% 9% 46% 34%	594
	21st to end	Night	12% 1 10% 19% 37% 21%	860
September	1st to 10th	Night	10% 1 10% 25% 37% 17%	800
	11th to 20th	Night	12% 2 13% 29% 34% 11%	800
	21st to end	Night	11% 2 13% 29% 38% 7%	800
October	1st to 10th	Night	14% 3 21% 37% 22% 3	800
	11th to 20th	Night	6% 5% 20% 38% 29% 2	800
	21st to end	Night	11% 5% 24% 37% 21% 2	880
November	1st to 10th	Night	10% 5% 29% 34% 20% 3	820
	11th to 20th	Night	10% 6% 27% 39% 16% 2	840
	21st to end	Night	9% 7% 32% 38% 14%	840
December	1st to 10th	Night	10% 7% 30% 34% 18% 1	840
	11th to 20th	Night	10% 9% 27% 39% 15% 1	840
	21st to end	Night	12% 9% 29% 35% 14% 1	924

Page generated at 2026-01-29 01:21:46. Version ac07

Copyright © 2026 Attilla Danko.