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 2026-07-16.

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%	138063
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	134634
Transparency	Day	38% 5% 12% 22% 18% 5%	141669
	Night	38% 4 10% 21% 20% 6%	139100
Seeing	Day	15% 8% 23% 24% 20% 9%	17679
	Night	16% 7% 25% 32% 18% 2	27762

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% 7% 2 5% 3 3 3 3 4 5% 52%	5258
	11th to 20th	Day	11% 6% 2 5% 3 3 3 2 3 4 59%	5280
	21st to end	Day	16% 9% 3 6% 4 3 3 3 3 4 46%	5780
February	1st to 10th	Day	13% 6% 3 6% 4 4 4 4 5% 8% 43%	1890
	11th to 20th	Day	21% 7% 2 4 4 3 3 3 5% 5% 43%	1890
	21st to end	Day	18% 7% 2 6% 4 3 4 3 6% 7% 40%	1851
March	1st to 10th	Day	19% 7% 3 6% 3 3 3 3 5% 7% 42%	5045
	11th to 20th	Day	9% 5% 2 5% 4 3 3 3 5% 7% 54%	5090
	21st to end	Day	15% 7% 2 6% 4 4 3 3 4 6% 46%	5586
April	1st to 10th	Day	10% 4 1 4 3 4 4 3 6% 8% 52%	2394
	11th to 20th	Day	11% 5% 2 5% 4 4 3 3 5% 8% 49%	2494
	21st to end	Day	13% 5% 2 5% 5% 4 4 4 5% 7% 45%	2591
May	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 7% 10% 38%	7616
	11th to 20th	Day	17% 6% 2 6% 4 4 4 4 6% 7% 40%	7626
	21st to end	Day	15% 5% 2 7% 5% 5% 5% 4 6% 9% 35%	8391
June	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 4 8% 10% 42%	2730
	11th to 20th	Day	7% 5% 2 6% 5% 4 6% 4 7% 11% 42%	2730
	21st to end	Day	6% 5% 1 6% 5% 5% 5% 5% 8% 11% 43%	2730
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 39%	6030
	11th to 20th	Day	7% 5% 2 8% 6% 5% 6% 6% 9% 15% 31%	5965
	21st to end	Day	9% 6% 3 7% 6% 6% 6% 5% 7% 11% 34%	6424
August	1st to 10th	Day	17% 8% 2 8% 5% 5% 6% 5% 7% 11% 27%	4234
	11th to 20th	Day	8% 6% 3 7% 6% 6% 6% 5% 8% 11% 36%	4146
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 9% 53%	4312
September	1st to 10th	Day	10% 5% 2 5% 5% 3 5% 4 5% 8% 48%	2380
	11th to 20th	Day	13% 7% 2 6% 4 3 5% 3 5% 8% 43%	2200
	21st to end	Day	21% 7% 2 7% 3 4 4 3 5% 6% 38%	2200
October	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 4 6% 53%	2000
	11th to 20th	Day	13% 6% 1 5% 3 3 3 2 4 5% 56%	2000
	21st to end	Day	12% 4 2 5% 3 2 3 3 4 5% 57%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 5% 54%	2057
	11th to 20th	Day	13% 5% 2 4 3 3 3 4 4 7% 53%	2005
	21st to end	Day	12% 5% 1 5% 5% 4 4 4 5% 6% 49%	1890
December	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 6% 52%	3020
	11th to 20th	Day	17% 5% 2 6% 3 4 4 3 5% 6% 46%	3463
	21st to end	Day	18% 6% 2 5% 3 4 3 3 5% 8% 43%	4565
January	1st to 10th	Night	13% 6% 2 4 3 3 3 2 4 5% 55%	7635
	11th to 20th	Night	12% 6% 2 5% 2 2 3 2 3 4 59%	7680
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 6% 47%	8401
February	1st to 10th	Night	15% 7% 2 5% 4 4 4 4 5% 6% 45%	2730
	11th to 20th	Night	20% 7% 2 5% 3 3 3 3 5% 5% 43%	2610
	21st to end	Night	16% 6% 3 6% 4 4 3 4 5% 6% 45%	1954
March	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 4 4 45%	5028
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 5% 6% 55%	5090
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 7% 48%	5588
April	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	2226
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 6% 7% 49%	2075
	21st to end	Night	12% 6% 2 5% 5% 4 4 4 5% 7% 46%	2046
May	1st to 10th	Night	12% 6% 2 7% 5% 4 5% 4 7% 8% 41%	5237
	11th to 20th	Night	14% 6% 3 6% 4 3 4 3 5% 8% 43%	5234
	21st to end	Night	20% 7% 2 7% 4 4 4 3 6% 8% 36%	5761
June	1st to 10th	Night	9% 4 2 7% 4 5% 6% 6% 7% 10% 40%	1890
	11th to 20th	Night	9% 5% 2 6% 6% 5% 5% 4 7% 10% 42%	1890
	21st to end	Night	7% 5% 1 6% 5% 5% 4 5% 9% 11% 41%	1890
July	1st to 10th	Night	10% 6% 2 8% 6% 6% 6% 5% 8% 9% 33%	4148
	11th to 20th	Night	10% 7% 3 9% 6% 7% 8% 5% 7% 9% 28%	4102
	21st to end	Night	11% 8% 3 7% 5% 5% 6% 6% 8% 11% 31%	4427
August	1st to 10th	Night	19% 7% 3 8% 5% 5% 6% 5% 8% 9% 25%	2915
	11th to 20th	Night	8% 5% 2 6% 5% 5% 6% 4 8% 11% 40%	2875
	21st to end	Night	6% 4 2 6% 4 4 5% 4 6% 8% 50%	3580
September	1st to 10th	Night	10% 6% 2 6% 4 4 4 5% 6% 7% 47%	2015
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 8% 44%	2200
	21st to end	Night	20% 8% 3 6% 5% 4 5% 4 4 5% 38%	2200
October	1st to 10th	Night	15% 6% 2 5% 4 3 3 2 4 6% 50%	2399
	11th to 20th	Night	14% 5% 2 5% 3 3 2 2 4 5% 55%	2400
	21st to end	Night	13% 5% 1 4 3 4 3 2 3 5% 57%	2640
November	1st to 10th	Night	14% 5% 2 6% 3 3 3 2 4 5% 53%	2461
	11th to 20th	Night	12% 5% 2 5% 4 4 4 4 3 5% 53%	2612
	21st to end	Night	13% 4 2 5% 3 3 3 4 5% 6% 52%	2730
December	1st to 10th	Night	12% 6% 2 5% 3 4 3 3 4 5% 52%	4380
	11th to 20th	Night	19% 8% 3 7% 4 4 4 3 4 4 41%	4952
	21st to end	Night	19% 4 2 4 3 3 4 3 5% 7% 46%	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	37% 2 8% 21% 27% 6%	5259
	11th to 20th	Day	30% 3 4 14% 36% 13%	5280
	21st to end	Day	44% 2 3 23% 21% 7%	5780
February	1st to 10th	Day	41% 3 10% 22% 20% 5%	1890
	11th to 20th	Day	44% 3 6% 23% 18% 6%	1899
	21st to end	Day	44% 3 10% 16% 20% 6%	1851
March	1st to 10th	Day	43% 2 6% 19% 18% 11%	5045
	11th to 20th	Day	30% 4 7% 21% 22% 16%	5090
	21st to end	Day	41% 3 8% 22% 20% 6%	5586
April	1st to 10th	Day	29% 3 11% 25% 24% 8%	2394
	11th to 20th	Day	34% 3 12% 23% 21% 7%	2494
	21st to end	Day	38% 4 11% 27% 16% 3	2591
May	1st to 10th	Day	41% 5% 16% 23% 14% 2	7651
	11th to 20th	Day	43% 5% 12% 25% 14% 1	7630
	21st to end	Day	44% 6% 13% 19% 15% 3	8405
June	1st to 10th	Day	35% 5% 17% 27% 15% 2	2730
	11th to 20th	Day	34% 5% 19% 25% 15% 2	2730
	21st to end	Day	32% 7% 18% 26% 14% 1	2730
July	1st to 10th	Day	37% 11% 18% 20% 12% 2	5965
	11th to 20th	Day	39% 12% 27% 18% 4 1	5896
	21st to end	Day	42% 9% 17% 22% 9%	6240
August	1st to 10th	Day	50% 7% 18% 20% 5%	4221
	11th to 20th	Day	38% 8% 22% 22% 9% 1	4185
	21st to end	Day	27% 6% 17% 35% 14% 2	4312
September	1st to 10th	Day	34% 5% 14% 22% 19% 4	2380
	11th to 20th	Day	40% 4 11% 21% 22% 2	2200
	21st to end	Day	47% 4 10% 20% 17% 3	2200
October	1st to 10th	Day	32% 4 8% 20% 26% 10%	3260
	11th to 20th	Day	22% 2 4 22% 26% 23%	3240
	21st to end	Day	25% 3 8% 25% 25% 15%	3538
November	1st to 10th	Day	34% 3 7% 23% 26% 8%	2057
	11th to 20th	Day	33% 2 8% 25% 26% 6%	2005
	21st to end	Day	36% 3 8% 23% 27% 4	1890
December	1st to 10th	Day	34% 3 7% 22% 27% 7%	3020
	11th to 20th	Day	41% 3 10% 17% 23% 6%	3460
	21st to end	Day	41% 3 10% 27% 17% 1	4565
January	1st to 10th	Night	33% 2 10% 21% 25% 8%	7636
	11th to 20th	Night	32% 2 4 17% 36% 10%	7680
	21st to end	Night	41% 3 5% 22% 24% 6%	8401
February	1st to 10th	Night	41% 3 8% 23% 21% 4	2730
	11th to 20th	Night	44% 3 8% 18% 21% 6%	2612
	21st to end	Night	41% 3 8% 20% 21% 7%	1954
March	1st to 10th	Night	43% 3 5% 16% 23% 10%	5028
	11th to 20th	Night	30% 3 6% 18% 26% 18%	5090
	21st to end	Night	37% 3 7% 26% 22% 5%	5588
April	1st to 10th	Night	35% 4 11% 19% 25% 5%	2226
	11th to 20th	Night	33% 3 10% 23% 23% 7%	2075
	21st to end	Night	36% 5% 12% 25% 18% 4	2046
May	1st to 10th	Night	39% 6% 15% 22% 15% 2	5264
	11th to 20th	Night	40% 5% 12% 23% 18% 1	5235
	21st to end	Night	46% 5% 13% 22% 12% 2	5771
June	1st to 10th	Night	37% 7% 16% 23% 16% 2	1890
	11th to 20th	Night	37% 6% 17% 24% 14% 2	1890
	21st to end	Night	32% 10% 18% 23% 14% 3	1890
July	1st to 10th	Night	43% 10% 16% 17% 12% 1	4105
	11th to 20th	Night	49% 13% 20% 12% 4 1	4051
	21st to end	Night	43% 9% 18% 21% 9%	4297
August	1st to 10th	Night	52% 8% 17% 16% 6% 1	2904
	11th to 20th	Night	34% 9% 22% 24% 10% 1	2902
	21st to end	Night	30% 6% 13% 32% 17% 2	3580
September	1st to 10th	Night	34% 6% 14% 24% 17% 6%	2015
	11th to 20th	Night	39% 5% 11% 20% 22% 2	2200
	21st to end	Night	49% 5% 9% 17% 16% 4	2200
October	1st to 10th	Night	38% 4 7% 19% 25% 7%	3919
	11th to 20th	Night	24% 1 6% 23% 28% 18%	3899
	21st to end	Night	24% 2 6% 26% 25% 16%	4252
November	1st to 10th	Night	36% 3 7% 20% 25% 9%	2461
	11th to 20th	Night	35% 2 7% 19% 26% 11%	2612
	21st to end	Night	33% 3 6% 24% 27% 6%	2730
December	1st to 10th	Night	34% 3 7% 22% 29% 5%	4380
	11th to 20th	Night	48% 3 7% 17% 17% 8%	4954
	21st to end	Night	38% 3 12% 24% 22% 2	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	23% 18% 36% 17% 6%	330
	11th to 20th	Day	19% 19% 39% 19% 3	330
	21st to end	Day	18% 17% 42% 19% 3	363
February	1st to 10th	Day	16% 14% 43% 20% 6% 1	330
	11th to 20th	Day	32% 16% 37% 11% 3 1	330
	21st to end	Day	26% 15% 35% 19% 5%	394
March	1st to 10th	Day	21% 11% 34% 25% 9% 1	540
	11th to 20th	Day	19% 12% 40% 22% 7% 1	540
	21st to end	Day	17% 16% 35% 26% 6% 1	594
April	1st to 10th	Day	17% 12% 32% 26% 12% 2	540
	11th to 20th	Day	12% 12% 29% 26% 16% 4	532
	21st to end	Day	16% 8% 28% 28% 18% 3	540
May	1st to 10th	Day	14% 13% 28% 27% 14% 4	543
	11th to 20th	Day	14% 7% 22% 25% 23% 9%	567
	21st to end	Day	18% 4 18% 26% 22% 12%	617
June	1st to 10th	Day	10% 4 16% 30% 28% 13%	570
	11th to 20th	Day	9% 3 18% 28% 26% 16%	570
	21st to end	Day	9% 2 12% 28% 36% 14%	570
July	1st to 10th	Day	10% 1 6% 24% 35% 24%	565
	11th to 20th	Day	6% 1 9% 21% 39% 25%	544
	21st to end	Day	9% 2 8% 22% 40% 19%	576
August	1st to 10th	Day	17% 2 12% 23% 27% 20%	526
	11th to 20th	Day	10% 2 9% 24% 33% 23%	519
	21st to end	Day	9% 2 9% 23% 31% 26%	583
September	1st to 10th	Day	13% 1 11% 23% 30% 22%	530
	11th to 20th	Day	15% 3 12% 20% 29% 20%	530
	21st to end	Day	27% 3 18% 25% 20% 7%	530
October	1st to 10th	Day	16% 5% 16% 27% 24% 12%	530
	11th to 20th	Day	14% 5% 24% 27% 24% 6%	530
	21st to end	Day	14% 9% 23% 30% 19% 5%	583
November	1st to 10th	Day	17% 7% 27% 29% 15% 5%	535
	11th to 20th	Day	14% 11% 34% 25% 13% 2	445
	21st to end	Day	10% 15% 32% 26% 14% 3	330
December	1st to 10th	Day	21% 13% 37% 21% 7% 1	330
	11th to 20th	Day	19% 10% 33% 25% 10% 2	330
	21st to end	Day	22% 14% 31% 21% 10% 2	363
January	1st to 10th	Night	19% 13% 37% 26% 4	840
	11th to 20th	Night	19% 12% 38% 26% 5%	840
	21st to end	Night	18% 11% 41% 28% 2	924
February	1st to 10th	Night	19% 11% 38% 30% 3	840
	11th to 20th	Night	25% 13% 40% 21% 1	823
	21st to end	Night	19% 14% 38% 26% 4	692
March	1st to 10th	Night	16% 14% 36% 30% 4	840
	11th to 20th	Night	19% 13% 35% 26% 6%	840
	21st to end	Night	17% 9% 39% 30% 5%	924
April	1st to 10th	Night	13% 13% 37% 30% 7%	840
	11th to 20th	Night	14% 9% 33% 31% 13%	831
	21st to end	Night	14% 7% 30% 39% 11%	786
May	1st to 10th	Night	12% 7% 33% 34% 13%	630
	11th to 20th	Night	15% 5% 21% 36% 21% 2	621
	21st to end	Night	17% 2 12% 35% 29% 4	683
June	1st to 10th	Night	10% 2 15% 40% 30% 2	630
	11th to 20th	Night	10% 2 13% 37% 32% 5%	630
	21st to end	Night	8% 2 13% 36% 37% 3	630
July	1st to 10th	Night	11% 2 9% 30% 42% 6%	615
	11th to 20th	Night	8% 2 5% 31% 47% 8%	597
	21st to end	Night	9% 1 7% 36% 38% 8%	657
August	1st to 10th	Night	15% 2 8% 35% 34% 6%	597
	11th to 20th	Night	13% 2 8% 32% 39% 7%	594
	21st to end	Night	11% 1 8% 34% 38% 8%	874
September	1st to 10th	Night	13% 2 8% 33% 37% 7%	800
	11th to 20th	Night	16% 3 10% 29% 36% 5%	800
	21st to end	Night	23% 4 16% 30% 27% 1	800
October	1st to 10th	Night	17% 3 16% 30% 28% 6%	800
	11th to 20th	Night	17% 5% 25% 32% 19% 2	800
	21st to end	Night	14% 7% 25% 34% 17% 2	880
November	1st to 10th	Night	16% 6% 24% 36% 16% 2	820
	11th to 20th	Night	14% 10% 31% 33% 11% 1	840
	21st to end	Night	15% 8% 31% 33% 12% 1	840
December	1st to 10th	Night	20% 8% 34% 30% 7%	840
	11th to 20th	Night	18% 6% 37% 31% 7%	840
	21st to end	Night	22% 8% 30% 33% 7%	924

Page generated at 2026-07-13 03:14:47. Version ac07

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