Browns Summit
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Browns Summit clear sky chart from 2005-11-06 to 2025-03-12.

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	13% 6% 2 6% 4 4 4 4 6% 8% 44%	109383
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 46%	109642
Transparency	Day	38% 4 12% 23% 18% 5%	113033
	Night	38% 4 10% 21% 21% 6%	114131
Seeing	Day	15% 7% 23% 24% 20% 10%	15814
	Night	16% 7% 25% 32% 18% 2	25995

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	16% 5% 2 4 2 3 2 2 3 4 56%	4056
	11th to 20th	Day	11% 6% 2 5% 3 3 3 2 4 4 58%	4060
	21st to end	Day	15% 8% 2 6% 4 4 4 3 3 4 48%	4466
February	1st to 10th	Day	13% 6% 3 6% 4 4 4 4 5% 8% 43%	1800
	11th to 20th	Day	21% 7% 2 4 4 3 4 3 5% 5% 43%	1798
	21st to end	Day	18% 7% 2 6% 4 2 4 3 6% 6% 41%	1765
March	1st to 10th	Day	22% 6% 2 6% 3 3 3 3 5% 7% 39%	3545
	11th to 20th	Day	10% 4 1 4 3 3 4 3 5% 7% 55%	3491
	21st to end	Day	19% 8% 2 5% 3 3 3 3 5% 6% 42%	3815
April	1st to 10th	Day	10% 4 1 4 3 4 4 4 6% 8% 53%	2166
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 48%	2254
	21st to end	Day	14% 5% 2 5% 5% 4 4 4 5% 7% 45%	2344
May	1st to 10th	Day	13% 7% 2 8% 6% 5% 5% 5% 7% 10% 34%	4082
	11th to 20th	Day	19% 7% 3 6% 5% 4 5% 4 6% 7% 35%	4081
	21st to end	Day	7% 4 1 5% 5% 5% 5% 4 6% 10% 47%	4476
June	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 8% 10% 42%	2470
	11th to 20th	Day	8% 5% 2 6% 5% 4 6% 4 7% 11% 42%	2470
	21st to end	Day	7% 5% 1 6% 5% 5% 5% 5% 8% 11% 42%	2470
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 5% 8% 10% 38%	4120
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 5% 8% 13% 34%	4120
	21st to end	Day	12% 8% 3 9% 6% 6% 6% 5% 7% 10% 28%	4466
August	1st to 10th	Day	17% 8% 2 8% 5% 5% 6% 5% 7% 11% 27%	4104
	11th to 20th	Day	8% 6% 3 7% 6% 6% 6% 5% 8% 11% 36%	4017
	21st to end	Day	6% 3 2 5% 4 3 4 4 6% 9% 53%	4180
September	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 5% 8% 48%	2261
	11th to 20th	Day	14% 7% 2 6% 4 3 4 3 5% 8% 42%	2090
	21st to end	Day	21% 7% 2 6% 3 3 3 3 5% 6% 39%	2090
October	1st to 10th	Day	15% 6% 2 5% 3 3 3 2 4 6% 52%	1900
	11th to 20th	Day	13% 5% 1 5% 3 3 3 2 4 5% 56%	1900
	21st to end	Day	11% 4 2 5% 3 2 3 3 4 5% 57%	2090
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 5% 53%	1957
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 4 7% 52%	1910
	21st to end	Day	12% 5% 2 5% 5% 4 4 4 5% 6% 49%	1800
December	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 6% 52%	2930
	11th to 20th	Day	17% 5% 2 6% 3 4 4 3 5% 6% 45%	3373
	21st to end	Day	18% 6% 2 5% 3 4 3 3 5% 8% 42%	4466
January	1st to 10th	Night	13% 6% 2 4 3 3 3 3 4 5% 57%	5889
	11th to 20th	Night	12% 6% 2 5% 3 2 3 3 3 4 58%	5900
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 6% 48%	6490
February	1st to 10th	Night	15% 7% 2 6% 4 4 4 4 5% 6% 44%	2600
	11th to 20th	Night	20% 7% 2 4 3 3 3 4 5% 5% 43%	2483
	21st to end	Night	15% 6% 2 6% 4 4 3 4 5% 6% 45%	1864
March	1st to 10th	Night	22% 9% 3 6% 4 4 4 3 4 4 38%	3528
	11th to 20th	Night	12% 6% 2 4 3 3 4 3 4 6% 53%	3498
	21st to end	Night	19% 5% 2 4 3 3 3 3 4 6% 48%	3817
April	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	2014
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 6% 7% 49%	1875
	21st to end	Night	13% 6% 2 4 5% 4 4 4 5% 6% 46%	1852
May	1st to 10th	Night	13% 6% 3 7% 5% 4 4 3 5% 7% 43%	2813
	11th to 20th	Night	18% 8% 3 7% 4 3 3 3 5% 7% 40%	2812
	21st to end	Night	9% 4 2 6% 3 3 4 4 7% 11% 47%	3079
June	1st to 10th	Night	9% 4 2 7% 4 5% 6% 6% 7% 10% 39%	1710
	11th to 20th	Night	9% 5% 2 6% 5% 5% 5% 4 7% 11% 42%	1710
	21st to end	Night	7% 5% 1 6% 5% 5% 5% 5% 8% 11% 41%	1710
July	1st to 10th	Night	7% 6% 2 8% 7% 6% 6% 5% 9% 9% 34%	2840
	11th to 20th	Night	10% 7% 3 7% 5% 6% 7% 5% 7% 9% 35%	2840
	21st to end	Night	14% 9% 3 8% 5% 5% 5% 5% 7% 9% 29%	3087
August	1st to 10th	Night	19% 8% 3 8% 5% 5% 6% 5% 8% 9% 25%	2825
	11th to 20th	Night	8% 5% 2 6% 5% 5% 6% 4 8% 11% 41%	2785
	21st to end	Night	6% 4 2 6% 4 4 4 4 6% 8% 51%	3470
September	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 46%	1914
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 8% 43%	2090
	21st to end	Night	20% 8% 3 6% 4 4 5% 4 4 5% 38%	2090
October	1st to 10th	Night	15% 6% 2 5% 4 3 3 2 4 6% 49%	2279
	11th to 20th	Night	13% 5% 2 5% 3 3 2 2 4 5% 56%	2280
	21st to end	Night	13% 5% 1 4 3 4 3 2 3 5% 58%	2508
November	1st to 10th	Night	14% 5% 2 5% 3 3 3 2 4 5% 53%	2341
	11th to 20th	Night	13% 5% 2 5% 3 4 4 4 3 5% 52%	2487
	21st to end	Night	13% 4 2 5% 3 3 3 4 5% 6% 52%	2600
December	1st to 10th	Night	12% 6% 2 5% 3 4 3 3 4 5% 53%	4250
	11th to 20th	Night	19% 8% 3 7% 4 4 4 3 4 4 40%	4822
	21st to end	Night	19% 4 2 4 3 3 4 3 5% 7% 46%	6490

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	34% 2 4 22% 31% 8%	4057
	11th to 20th	Day	31% 3 4 18% 36% 8%	4060
	21st to end	Day	42% 3 4 23% 23% 5%	4466
February	1st to 10th	Day	41% 3 10% 22% 20% 5%	1800
	11th to 20th	Day	45% 3 6% 22% 18% 6%	1807
	21st to end	Day	43% 3 10% 16% 21% 6%	1765
March	1st to 10th	Day	46% 3 7% 22% 14% 8%	3545
	11th to 20th	Day	30% 3 8% 24% 23% 12%	3491
	21st to end	Day	44% 3 8% 21% 20% 3	3815
April	1st to 10th	Day	29% 3 10% 25% 24% 8%	2166
	11th to 20th	Day	35% 3 12% 22% 21% 7%	2254
	21st to end	Day	38% 4 11% 26% 17% 4	2344
May	1st to 10th	Day	45% 4 17% 21% 11% 2	4120
	11th to 20th	Day	48% 4 11% 26% 10% 1	4085
	21st to end	Day	32% 4 18% 24% 19% 3	4489
June	1st to 10th	Day	35% 5% 18% 27% 14% 1	2470
	11th to 20th	Day	35% 5% 18% 25% 15% 2	2470
	21st to end	Day	34% 7% 18% 25% 14% 2	2470
July	1st to 10th	Day	37% 11% 18% 19% 12% 3	4055
	11th to 20th	Day	39% 7% 24% 23% 6% 1	4051
	21st to end	Day	50% 9% 14% 17% 9%	4324
August	1st to 10th	Day	49% 7% 18% 20% 5%	4091
	11th to 20th	Day	38% 8% 22% 22% 9% 1	4056
	21st to end	Day	26% 6% 17% 35% 14% 2	4180
September	1st to 10th	Day	35% 5% 13% 23% 19% 4	2261
	11th to 20th	Day	41% 4 10% 21% 21% 3	2090
	21st to end	Day	47% 4 9% 20% 18% 3	2090
October	1st to 10th	Day	32% 4 8% 19% 27% 10%	3160
	11th to 20th	Day	22% 2 4 22% 25% 24%	3140
	21st to end	Day	24% 3 9% 25% 24% 15%	3428
November	1st to 10th	Day	35% 3 7% 23% 25% 7%	1957
	11th to 20th	Day	34% 2 8% 24% 26% 7%	1910
	21st to end	Day	35% 3 8% 23% 27% 5%	1800
December	1st to 10th	Day	33% 3 7% 22% 27% 8%	2930
	11th to 20th	Day	41% 3 10% 18% 23% 6%	3370
	21st to end	Day	41% 3 10% 27% 17% 1	4466
January	1st to 10th	Night	32% 2 5% 20% 31% 11%	5889
	11th to 20th	Night	33% 2 4 18% 35% 7%	5900
	21st to end	Night	39% 2 5% 22% 26% 5%	6490
February	1st to 10th	Night	41% 3 8% 23% 20% 4	2600
	11th to 20th	Night	43% 3 8% 17% 21% 7%	2485
	21st to end	Night	40% 3 9% 20% 21% 7%	1864
March	1st to 10th	Night	51% 3 6% 17% 18% 5%	3528
	11th to 20th	Night	34% 3 6% 18% 25% 15%	3498
	21st to end	Night	40% 3 7% 25% 20% 6%	3817
April	1st to 10th	Night	36% 3 10% 19% 26% 5%	2014
	11th to 20th	Night	35% 3 9% 23% 23% 8%	1875
	21st to end	Night	37% 4 12% 25% 18% 5%	1852
May	1st to 10th	Night	41% 5% 16% 24% 13% 2	2840
	11th to 20th	Night	46% 4 10% 22% 17% 1	2813
	21st to end	Night	30% 4 17% 30% 16% 3	3087
June	1st to 10th	Night	37% 7% 16% 24% 14% 1	1710
	11th to 20th	Night	36% 6% 17% 24% 15% 2	1710
	21st to end	Night	34% 9% 18% 22% 14% 3	1710
July	1st to 10th	Night	42% 11% 15% 17% 13% 2	2795
	11th to 20th	Night	44% 9% 23% 17% 6% 2	2786
	21st to end	Night	50% 8% 15% 19% 9%	2988
August	1st to 10th	Night	52% 8% 17% 16% 6% 1	2814
	11th to 20th	Night	34% 9% 22% 25% 10% 1	2812
	21st to end	Night	30% 6% 13% 32% 17% 2	3470
September	1st to 10th	Night	35% 6% 13% 23% 17% 6%	1914
	11th to 20th	Night	40% 5% 11% 20% 22% 2	2090
	21st to end	Night	49% 5% 8% 18% 16% 4	2090
October	1st to 10th	Night	39% 4 7% 19% 25% 7%	3799
	11th to 20th	Night	24% 1 6% 23% 28% 18%	3779
	21st to end	Night	24% 2 6% 27% 25% 16%	4120
November	1st to 10th	Night	36% 3 7% 20% 25% 9%	2341
	11th to 20th	Night	36% 2 7% 19% 26% 11%	2487
	21st to end	Night	33% 3 6% 25% 27% 6%	2600
December	1st to 10th	Night	34% 3 7% 22% 30% 5%	4250
	11th to 20th	Night	48% 3 7% 18% 16% 8%	4824
	21st to end	Night	38% 3 12% 24% 22% 2	6490

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	23% 17% 36% 17% 7%	300
	11th to 20th	Day	18% 18% 40% 20% 4	300
	21st to end	Day	18% 16% 42% 20% 4	330
February	1st to 10th	Day	16% 14% 42% 20% 7% 1	300
	11th to 20th	Day	33% 15% 37% 11% 3 1	300
	21st to end	Day	25% 14% 36% 20% 4	364
March	1st to 10th	Day	22% 11% 34% 24% 9% 1	500
	11th to 20th	Day	17% 11% 42% 22% 7% 1	464
	21st to end	Day	19% 15% 34% 26% 6% 1	506
April	1st to 10th	Day	17% 13% 32% 25% 12% 2	460
	11th to 20th	Day	13% 13% 28% 25% 18% 4	452
	21st to end	Day	18% 8% 28% 26% 18% 2	460
May	1st to 10th	Day	14% 14% 26% 27% 16% 4	463
	11th to 20th	Day	15% 7% 20% 24% 24% 11%	487
	21st to end	Day	16% 2 16% 27% 25% 14%	529
June	1st to 10th	Day	11% 3 14% 30% 29% 13%	490
	11th to 20th	Day	10% 3 17% 26% 27% 17%	490
	21st to end	Day	10% 2 12% 28% 36% 13%	490
July	1st to 10th	Day	10% 1 6% 25% 35% 23%	485
	11th to 20th	Day	5% 1 9% 21% 39% 24%	484
	21st to end	Day	9% 2 8% 23% 39% 18%	532
August	1st to 10th	Day	15% 2 12% 23% 27% 21%	486
	11th to 20th	Day	9% 2 9% 24% 33% 23%	479
	21st to end	Day	9% 2 7% 22% 32% 27%	539
September	1st to 10th	Day	13% 1 10% 21% 31% 23%	490
	11th to 20th	Day	17% 3 13% 18% 29% 20%	490
	21st to end	Day	27% 3 18% 25% 20% 7%	490
October	1st to 10th	Day	17% 5% 15% 27% 25% 11%	490
	11th to 20th	Day	13% 5% 23% 28% 24% 7%	490
	21st to end	Day	12% 7% 24% 31% 20% 5%	539
November	1st to 10th	Day	18% 7% 26% 29% 16% 5%	495
	11th to 20th	Day	15% 10% 34% 25% 14% 2	410
	21st to end	Day	9% 13% 33% 27% 15% 3	300
December	1st to 10th	Day	20% 14% 37% 21% 7% 1	300
	11th to 20th	Day	18% 10% 34% 25% 11% 2	300
	21st to end	Day	22% 12% 30% 22% 11% 2	330
January	1st to 10th	Night	19% 13% 37% 26% 4	800
	11th to 20th	Night	19% 11% 38% 27% 5%	800
	21st to end	Night	18% 10% 41% 28% 2	880
February	1st to 10th	Night	19% 11% 38% 29% 3	800
	11th to 20th	Night	25% 13% 40% 21% 1	783
	21st to end	Night	18% 13% 38% 27% 4	660
March	1st to 10th	Night	17% 14% 35% 29% 5%	800
	11th to 20th	Night	19% 12% 34% 27% 7% 1	767
	21st to end	Night	18% 8% 36% 31% 6%	836
April	1st to 10th	Night	14% 14% 36% 30% 7%	760
	11th to 20th	Night	15% 9% 32% 31% 14%	751
	21st to end	Night	15% 7% 29% 39% 10%	712
May	1st to 10th	Night	12% 7% 31% 35% 14% 1	570
	11th to 20th	Night	16% 4 21% 35% 22% 2	561
	21st to end	Night	15% 2 10% 36% 32% 5%	617
June	1st to 10th	Night	10% 2 15% 40% 31% 2	570
	11th to 20th	Night	10% 2 12% 36% 34% 6%	570
	21st to end	Night	9% 2 14% 35% 37% 3	570
July	1st to 10th	Night	11% 2 9% 30% 41% 6%	555
	11th to 20th	Night	7% 1 5% 32% 48% 7%	552
	21st to end	Night	10% 1 8% 38% 37% 7%	624
August	1st to 10th	Night	14% 2 8% 35% 34% 6%	567
	11th to 20th	Night	13% 2 9% 32% 39% 6%	564
	21st to end	Night	12% 1 7% 33% 39% 9%	830
September	1st to 10th	Night	13% 2 8% 31% 38% 8%	760
	11th to 20th	Night	17% 3 10% 28% 36% 5%	760
	21st to end	Night	23% 4 16% 29% 27% 1	760
October	1st to 10th	Night	17% 3 16% 30% 27% 6%	760
	11th to 20th	Night	17% 5% 26% 31% 19% 2	760
	21st to end	Night	14% 7% 25% 35% 18% 2	836
November	1st to 10th	Night	16% 6% 23% 36% 17% 2	780
	11th to 20th	Night	15% 8% 31% 34% 12% 1	800
	21st to end	Night	14% 8% 31% 33% 12% 1	800
December	1st to 10th	Night	21% 8% 33% 31% 7%	800
	11th to 20th	Night	18% 6% 37% 31% 8%	800
	21st to end	Night	23% 8% 28% 34% 7%	880

Page generated at 2025-03-09 08:08:49. Version ac07

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