Robert Brownlee Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert Brownlee Observatory 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	3 4 1 3 2 2 2 2 4 5% 72%	136609
	Night	3 5% 2 4 2 2 2 2 4 5% 70%	132674
Transparency	Day	15% 4 5% 15% 28% 33%	131695
	Night	17% 5% 5% 15% 26% 32%	128561
Seeing	Day	8% 11% 30% 34% 13% 5%	17295
	Night	7% 5% 17% 35% 28% 8%	27267

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	4 5% 2 3 2 2 2 1 4 5% 69%	4188
	11th to 20th	Day	5% 4 1 2 1 2 2 2 3 5% 73%	4315
	21st to end	Day	7% 9% 2 5% 3 3 3 3 5% 8% 53%	5016
February	1st to 10th	Day	7% 8% 3 3 3 2 3 2 5% 5% 59%	2000
	11th to 20th	Day	6% 8% 2 5% 3 2 2 3 5% 6% 58%	1950
	21st to end	Day	6% 11% 3 5% 3 3 3 2 4 6% 54%	1650
March	1st to 10th	Day	9% 10% 2 5% 3 3 3 3 5% 6% 51%	4518
	11th to 20th	Day	6% 9% 2 5% 3 3 3 3 6% 7% 54%	4749
	21st to end	Day	5% 7% 2 4 3 3 4 3 5% 7% 58%	5827
April	1st to 10th	Day	6% 5% 1 4 3 3 3 2 5% 6% 61%	2400
	11th to 20th	Day	3 4 2 3 2 3 3 2 4 5% 67%	2369
	21st to end	Day	2 5% 1 4 3 3 3 2 4 7% 67%	2400
May	1st to 10th	Day	2 5% 1 3 2 2 3 2 3 5% 71%	5781
	11th to 20th	Day	2 2 1 2 2 2 2 3 4 5% 76%	5841
	21st to end	Day	1 1 1 2 1 1 2 1 3 5% 81%	6423
June	1st to 10th	Day	1 2 1 3 2 2 2 2 4 6% 75%	2600
	11th to 20th	Day	1 1 1 1 1 1 3 4 87%	2623
	21st to end	Day	1 2 1 2 1 1 2 1 2 2 85%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 2 90%	6285
	11th to 20th	Day	1 1 1 1 2 2 2 3 4 82%	5880
	21st to end	Day	1 1 1 1 1 1 2 1 3 4 84%	6234
August	1st to 10th	Day	1 1 2 1 1 2 2 3 4 84%	5346
	11th to 20th	Day	1 1 1 1 1 1 3 91%	5364
	21st to end	Day	2 1 1 2 1 2 2 2 3 4 80%	5984
September	1st to 10th	Day	2 1 1 3 2 2 2 2 4 5% 76%	2400
	11th to 20th	Day	1 1 1 1 1 1 2 3 3 85%	2200
	21st to end	Day	1 1 1 3 2 1 2 2 3 4 81%	2200
October	1st to 10th	Day	1 2 1 3 2 2 2 2 4 5% 78%	3600
	11th to 20th	Day	1 1 1 1 1 1 1 3 6% 82%	3517
	21st to end	Day	2 2 1 3 2 2 3 2 4 5% 74%	3367
November	1st to 10th	Day	3 4 1 2 3 2 2 2 4 5% 71%	1854
	11th to 20th	Day	4 4 1 5% 2 2 3 3 4 6% 66%	1890
	21st to end	Day	5% 5% 1 4 3 3 4 2 5% 7% 61%	1890
December	1st to 10th	Day	3 3 1 3 3 3 3 4 6% 7% 64%	3040
	11th to 20th	Day	5% 6% 2 4 3 3 6% 5% 6% 8% 52%	3499
	21st to end	Day	6% 7% 2 5% 4 4 4 3 7% 9% 47%	4609
January	1st to 10th	Night	4 6% 1 3 2 2 3 2 3 6% 68%	5997
	11th to 20th	Night	3 4 2 4 2 2 2 2 3 5% 70%	5802
	21st to end	Night	7% 10% 2 5% 3 3 3 3 4 4 55%	5984
February	1st to 10th	Night	6% 10% 2 6% 3 3 2 3 4 5% 57%	2400
	11th to 20th	Night	6% 7% 2 5% 3 3 3 3 4 5% 59%	2338
	21st to end	Night	6% 11% 3 5% 3 3 3 3 5% 5% 53%	1980
March	1st to 10th	Night	6% 14% 4 8% 3 3 3 2 4 5% 46%	5376
	11th to 20th	Night	4 10% 3 6% 3 4 4 3 5% 6% 52%	5249
	21st to end	Night	2 10% 3 5% 4 3 4 3 5% 6% 56%	5143
April	1st to 10th	Night	4 8% 2 4 4 3 3 3 5% 6% 58%	2000
	11th to 20th	Night	3 5% 2 5% 3 2 3 2 4 5% 65%	1979
	21st to end	Night	2 6% 2 6% 3 2 3 2 4 5% 65%	2000
May	1st to 10th	Night	4 4 2 4 2 2 2 2 3 4 71%	4202
	11th to 20th	Night	1 3 1 3 2 2 2 2 4 5% 76%	4074
	21st to end	Night	1 1 1 3 2 2 2 2 3 5% 78%	4474
June	1st to 10th	Night	1 1 3 2 2 2 1 4 5% 78%	1800
	11th to 20th	Night	1 1 1 2 1 2 1 1 3 4 85%	1766
	21st to end	Night	1 1 1 1 1 2 2 2 3 86%	1600
July	1st to 10th	Night	1 1 1 1 2 94%	3640
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 86%	4100
	21st to end	Night	1 2 1 1 1 1 1 1 2 3 87%	4643
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	4486
	11th to 20th	Night	1 1 1 2 3 91%	4506
	21st to end	Night	1 2 1 2 1 1 2 1 2 3 84%	5015
September	1st to 10th	Night	2 2 1 2 2 1 1 1 2 4 81%	2000
	11th to 20th	Night	1 1 1 2 1 1 2 1 2 3 85%	2200
	21st to end	Night	1 1 1 2 2 2 2 2 2 3 84%	2200
October	1st to 10th	Night	1 2 1 2 1 1 1 1 2 3 84%	3600
	11th to 20th	Night	1 2 1 2 1 2 2 2 4 3 81%	3636
	21st to end	Night	2 4 1 4 2 2 2 2 2 4 76%	4437
November	1st to 10th	Night	3 4 2 3 3 3 2 2 3 5% 70%	2665
	11th to 20th	Night	3 5% 2 4 2 2 3 2 5% 6% 65%	2730
	21st to end	Night	4 6% 2 5% 4 3 4 3 5% 6% 58%	2730
December	1st to 10th	Night	3 5% 2 3 2 2 3 3 4 6% 67%	4370
	11th to 20th	Night	5% 6% 2 4 3 4 4 4 6% 8% 53%	4941
	21st to end	Night	4 6% 2 6% 3 3 5% 4 7% 10% 48%	6611

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	17% 4 4 20% 33% 22%	4189
	11th to 20th	Day	15% 3 9% 16% 21% 35%	4315
	21st to end	Day	27% 6% 6% 13% 24% 25%	5016
February	1st to 10th	Day	26% 4 6% 14% 25% 27%	2000
	11th to 20th	Day	26% 4 4 17% 27% 22%	1960
	21st to end	Day	30% 5% 7% 15% 21% 21%	1650
March	1st to 10th	Day	29% 9% 6% 16% 25% 14%	4518
	11th to 20th	Day	27% 5% 10% 23% 21% 13%	4749
	21st to end	Day	21% 10% 11% 29% 21% 9%	5827
April	1st to 10th	Day	23% 5% 8% 23% 27% 14%	2400
	11th to 20th	Day	20% 5% 6% 20% 29% 21%	2372
	21st to end	Day	19% 5% 7% 21% 36% 13%	2400
May	1st to 10th	Day	17% 5% 6% 21% 33% 17%	5781
	11th to 20th	Day	10% 5% 7% 23% 42% 13%	5844
	21st to end	Day	8% 5% 4 18% 46% 18%	6424
June	1st to 10th	Day	12% 3 6% 19% 37% 22%	2600
	11th to 20th	Day	6% 3 4 11% 30% 46%	2623
	21st to end	Day	8% 2 3 5% 23% 58%	2800
July	1st to 10th	Day	4 1 2 4 14% 77%	6217
	11th to 20th	Day	7% 3 3 8% 23% 55%	5811
	21st to end	Day	6% 2 4 14% 21% 53%	6053
August	1st to 10th	Day	8% 3 3 11% 35% 40%	3890
	11th to 20th	Day	4 2 2 13% 23% 57%	3880
	21st to end	Day	6% 2 2 5% 25% 60%	4311
September	1st to 10th	Day	10% 3 5% 8% 31% 44%	2400
	11th to 20th	Day	6% 2 3 10% 34% 46%	2200
	21st to end	Day	9% 2 4 13% 30% 43%	2200
October	1st to 10th	Day	10% 3 3 10% 19% 55%	3600
	11th to 20th	Day	7% 2 3 11% 33% 44%	3517
	21st to end	Day	14% 3 3 8% 31% 42%	3367
November	1st to 10th	Day	16% 2 5% 13% 31% 33%	1854
	11th to 20th	Day	19% 3 5% 13% 23% 37%	1890
	21st to end	Day	24% 3 6% 14% 22% 30%	1890
December	1st to 10th	Day	17% 3 6% 15% 28% 31%	3040
	11th to 20th	Day	26% 5% 8% 16% 29% 16%	3498
	21st to end	Day	28% 8% 10% 20% 23% 11%	4609
January	1st to 10th	Night	18% 4 5% 15% 33% 26%	5997
	11th to 20th	Night	16% 5% 8% 18% 20% 33%	5802
	21st to end	Night	30% 5% 7% 12% 22% 24%	5984
February	1st to 10th	Night	28% 4 5% 16% 20% 27%	2400
	11th to 20th	Night	25% 5% 6% 17% 23% 24%	2339
	21st to end	Night	32% 6% 4 15% 20% 24%	1980
March	1st to 10th	Night	34% 11% 5% 17% 16% 17%	5376
	11th to 20th	Night	28% 7% 12% 23% 22% 9%	5249
	21st to end	Night	24% 11% 13% 20% 24% 8%	5143
April	1st to 10th	Night	26% 5% 9% 22% 21% 17%	2000
	11th to 20th	Night	20% 6% 6% 21% 26% 21%	1979
	21st to end	Night	21% 5% 5% 21% 33% 14%	2000
May	1st to 10th	Night	19% 4 5% 27% 28% 18%	4199
	11th to 20th	Night	12% 5% 5% 27% 38% 13%	4075
	21st to end	Night	8% 7% 5% 22% 43% 15%	4474
June	1st to 10th	Night	10% 3 6% 21% 35% 25%	1800
	11th to 20th	Night	7% 3 4 13% 29% 45%	1766
	21st to end	Night	6% 1 3 7% 23% 60%	1600
July	1st to 10th	Night	2 1 3 17% 76%	3600
	11th to 20th	Night	4 2 3 11% 23% 57%	4046
	21st to end	Night	6% 1 3 13% 30% 47%	4512
August	1st to 10th	Night	5% 2 3 14% 36% 41%	3256
	11th to 20th	Night	3 1 3 12% 27% 53%	3256
	21st to end	Night	5% 1 2 5% 25% 62%	3608
September	1st to 10th	Night	9% 3 4 7% 31% 47%	2000
	11th to 20th	Night	7% 2 3 9% 36% 44%	2200
	21st to end	Night	9% 2 4 13% 29% 44%	2200
October	1st to 10th	Night	9% 1 3 9% 20% 58%	3600
	11th to 20th	Night	9% 2 3 11% 33% 42%	3636
	21st to end	Night	14% 3 2 7% 27% 47%	4437
November	1st to 10th	Night	16% 4 4 12% 28% 35%	2665
	11th to 20th	Night	19% 3 4 13% 24% 36%	2730
	21st to end	Night	25% 5% 6% 14% 20% 31%	2730
December	1st to 10th	Night	18% 3 4 10% 25% 39%	4370
	11th to 20th	Night	25% 7% 8% 16% 25% 19%	4941
	21st to end	Night	25% 8% 9% 23% 24% 11%	6611

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	21% 10% 30% 28% 9% 2	330
	11th to 20th	Day	14% 9% 31% 32% 13% 2	388
	21st to end	Day	14% 16% 30% 29% 10% 2	550
February	1st to 10th	Day	17% 17% 31% 23% 11% 2	500
	11th to 20th	Day	15% 16% 35% 28% 6% 1	495
	21st to end	Day	15% 20% 34% 25% 6%	414
March	1st to 10th	Day	20% 17% 34% 20% 7% 1	500
	11th to 20th	Day	18% 14% 35% 24% 7% 2	500
	21st to end	Day	13% 22% 35% 22% 7% 1	550
April	1st to 10th	Day	12% 15% 33% 25% 12% 2	500
	11th to 20th	Day	6% 25% 34% 23% 10% 2	492
	21st to end	Day	8% 18% 30% 28% 9% 6%	500
May	1st to 10th	Day	10% 16% 34% 27% 9% 4	503
	11th to 20th	Day	7% 15% 34% 29% 12% 4	528
	21st to end	Day	7% 11% 36% 29% 13% 5%	573
June	1st to 10th	Day	3 8% 29% 36% 17% 7%	530
	11th to 20th	Day	1 8% 35% 36% 15% 6%	530
	21st to end	Day	2 6% 30% 38% 16% 8%	530
July	1st to 10th	Day	1 3 22% 48% 17% 9%	526
	11th to 20th	Day	1 1 18% 47% 21% 12%	524
	21st to end	Day	2 2 19% 45% 18% 13%	575
August	1st to 10th	Day	2 2 28% 43% 15% 11%	527
	11th to 20th	Day	1 2 21% 45% 18% 13%	517
	21st to end	Day	2 3 23% 44% 19% 9%	583
September	1st to 10th	Day	3 4 27% 43% 17% 5%	530
	11th to 20th	Day	3 7% 31% 41% 15% 3	530
	21st to end	Day	2 10% 33% 35% 17% 3	530
October	1st to 10th	Day	4 10% 31% 39% 15% 1	530
	11th to 20th	Day	6% 8% 32% 37% 14% 2	530
	21st to end	Day	5% 9% 28% 37% 18% 3	467
November	1st to 10th	Day	6% 9% 25% 43% 15% 2	330
	11th to 20th	Day	8% 12% 28% 34% 14% 4	330
	21st to end	Day	7% 12% 31% 34% 14% 2	330
December	1st to 10th	Day	8% 10% 33% 33% 14% 2	330
	11th to 20th	Day	6% 11% 31% 33% 16% 3	330
	21st to end	Day	17% 15% 35% 21% 10% 3	363
January	1st to 10th	Night	15% 8% 23% 31% 18% 6%	840
	11th to 20th	Night	10% 6% 24% 37% 17% 6%	824
	21st to end	Night	14% 7% 26% 33% 17% 4	880
February	1st to 10th	Night	12% 8% 25% 32% 17% 6%	800
	11th to 20th	Night	11% 8% 25% 34% 18% 5%	784
	21st to end	Night	14% 10% 26% 31% 15% 4	660
March	1st to 10th	Night	14% 9% 24% 34% 17% 3	800
	11th to 20th	Night	13% 7% 21% 31% 21% 7%	800
	21st to end	Night	10% 8% 24% 36% 18% 4	880
April	1st to 10th	Night	10% 7% 18% 40% 21% 4	800
	11th to 20th	Night	7% 5% 25% 37% 21% 4	791
	21st to end	Night	8% 6% 18% 39% 23% 7%	800
May	1st to 10th	Night	8% 4 19% 41% 24% 4	650
	11th to 20th	Night	6% 3 18% 37% 31% 6%	591
	21st to end	Night	5% 4 17% 41% 27% 6%	650
June	1st to 10th	Night	1 3 13% 41% 38% 5%	600
	11th to 20th	Night	2 2 13% 44% 36% 4	566
	21st to end	Night	1 1 6% 36% 48% 9%	400
July	1st to 10th	Night	1 2 30% 55% 13%	390
	11th to 20th	Night	2 4 35% 52% 8%	582
	21st to end	Night	2 1 4 30% 51% 12%	727
August	1st to 10th	Night	1 1 5% 34% 44% 15%	796
	11th to 20th	Night	1 1 2 27% 52% 18%	792
	21st to end	Night	2 4 28% 49% 18%	880
September	1st to 10th	Night	2 1 4 33% 44% 16%	800
	11th to 20th	Night	2 1 9% 35% 41% 12%	800
	21st to end	Night	2 1 13% 36% 37% 11%	800
October	1st to 10th	Night	5% 2 13% 46% 28% 6%	800
	11th to 20th	Night	5% 4 16% 36% 31% 9%	800
	21st to end	Night	6% 4 21% 33% 25% 11%	880
November	1st to 10th	Night	6% 4 15% 41% 25% 8%	820
	11th to 20th	Night	8% 4 22% 34% 24% 8%	840
	21st to end	Night	8% 6% 23% 35% 22% 6%	840
December	1st to 10th	Night	10% 5% 24% 34% 21% 5%	840
	11th to 20th	Night	14% 5% 25% 34% 18% 4	840
	21st to end	Night	12% 9% 28% 30% 16% 4	924

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