Brown Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brown Observing Site clear sky chart from 2005-11-06 to 2026-02-16.

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	6% 6% 2 6% 4 4 4 3 5% 6% 55%	139360
	Night	8% 7% 2 6% 4 4 4 3 5% 6% 50%	138787
Transparency	Day	28% 6% 11% 20% 20% 14%	134143
	Night	33% 7% 12% 20% 17% 11%	135001
Seeing	Day	9% 8% 22% 29% 22% 9%	16015
	Night	11% 5% 20% 38% 22% 3	25459

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	11% 15% 5% 12% 8% 6% 6% 4 6% 8% 20%	4722
	11th to 20th	Day	8% 8% 3 9% 5% 4 5% 4 6% 8% 40%	4740
	21st to end	Day	9% 8% 4 11% 6% 5% 4 3 4 6% 40%	5187
February	1st to 10th	Day	11% 11% 4 9% 6% 6% 5% 4 4 7% 34%	1867
	11th to 20th	Day	10% 12% 3 8% 5% 4 3 5% 4 7% 38%	2001
	21st to end	Day	8% 13% 3 6% 6% 5% 6% 4 6% 7% 37%	1650
March	1st to 10th	Day	6% 10% 4 10% 6% 5% 5% 4 7% 7% 36%	4518
	11th to 20th	Day	5% 8% 3 7% 4 5% 4 3 5% 6% 49%	4797
	21st to end	Day	8% 9% 3 8% 6% 6% 6% 5% 7% 9% 33%	5748
April	1st to 10th	Day	10% 8% 3 5% 4 4 5% 4 7% 9% 42%	2400
	11th to 20th	Day	7% 7% 2 6% 5% 3 4 3 6% 8% 48%	2404
	21st to end	Day	8% 6% 2 5% 4 4 4 4 5% 7% 50%	2600
May	1st to 10th	Day	7% 5% 2 4 3 3 3 3 5% 6% 59%	5880
	11th to 20th	Day	6% 4 2 4 3 3 3 3 4 6% 61%	6027
	21st to end	Day	4 3 1 3 3 3 3 3 5% 7% 64%	6902
June	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 6% 9% 48%	2800
	11th to 20th	Day	6% 3 1 4 3 3 3 3 5% 7% 61%	2800
	21st to end	Day	3 2 1 2 2 2 3 2 5% 5% 73%	2800
July	1st to 10th	Day	2 1 1 2 2 2 2 2 3 4 80%	6319
	11th to 20th	Day	1 2 1 1 2 2 2 3 3 83%	6320
	21st to end	Day	2 2 1 3 2 2 3 3 5% 7% 70%	6880
August	1st to 10th	Day	2 2 1 4 3 3 3 3 5% 7% 67%	5963
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 4 82%	5785
	21st to end	Day	4 3 1 4 2 3 3 3 4 5% 69%	5984
September	1st to 10th	Day	2 2 1 2 2 2 3 3 4 5% 75%	2400
	11th to 20th	Day	4 3 1 4 3 3 2 3 4 6% 66%	2220
	21st to end	Day	5% 3 1 4 3 2 3 2 4 5% 67%	2200
October	1st to 10th	Day	5% 2 1 4 2 3 3 2 3 4 70%	3600
	11th to 20th	Day	6% 4 2 4 3 4 3 3 5% 7% 57%	3144
	21st to end	Day	6% 6% 2 7% 4 4 4 4 6% 8% 50%	3200
November	1st to 10th	Day	11% 8% 2 7% 5% 3 5% 4 5% 8% 43%	1854
	11th to 20th	Day	12% 9% 2 7% 6% 5% 5% 4 6% 8% 37%	1890
	21st to end	Day	10% 8% 2 10% 6% 5% 6% 4 6% 8% 35%	1858
December	1st to 10th	Day	8% 6% 2 8% 4 5% 5% 3 5% 8% 45%	2700
	11th to 20th	Day	15% 10% 5% 10% 7% 5% 6% 4 6% 6% 26%	3108
	21st to end	Day	14% 10% 5% 13% 7% 5% 5% 4 5% 7% 25%	4092
January	1st to 10th	Night	13% 14% 5% 12% 7% 6% 6% 4 5% 5% 21%	8172
	11th to 20th	Night	10% 9% 2 7% 5% 5% 4 3 5% 6% 44%	8220
	21st to end	Night	9% 7% 3 8% 5% 4 4 3 5% 7% 43%	8994
February	1st to 10th	Night	13% 11% 3 8% 5% 4 5% 4 5% 7% 35%	2748
	11th to 20th	Night	10% 9% 3 7% 4 5% 5% 4 6% 8% 38%	2398
	21st to end	Night	10% 11% 3 8% 5% 6% 6% 5% 6% 6% 36%	1980
March	1st to 10th	Night	9% 10% 3 7% 5% 4 4 3 6% 8% 42%	5376
	11th to 20th	Night	9% 11% 3 7% 4 4 4 3 4 6% 45%	5190
	21st to end	Night	9% 9% 2 8% 5% 5% 6% 4 7% 8% 37%	5213
April	1st to 10th	Night	10% 7% 2 5% 4 4 6% 4 6% 6% 46%	2000
	11th to 20th	Night	9% 6% 2 5% 4 5% 4 4 6% 6% 49%	1942
	21st to end	Night	7% 5% 2 5% 5% 3 5% 3 7% 7% 52%	1800
May	1st to 10th	Night	9% 5% 1 3 2 3 3 3 5% 7% 59%	4100
	11th to 20th	Night	5% 4 2 6% 4 4 4 3 5% 6% 57%	3873
	21st to end	Night	5% 3 1 4 3 3 3 3 4 5% 65%	3972
June	1st to 10th	Night	9% 5% 2 5% 5% 4 5% 3 6% 8% 49%	1600
	11th to 20th	Night	6% 4 1 4 2 3 3 3 5% 6% 62%	1600
	21st to end	Night	3 2 1 4 3 3 3 3 4 6% 70%	1600
July	1st to 10th	Night	2 1 1 2 2 2 3 3 3 4 78%	3640
	11th to 20th	Night	1 1 1 2 3 2 3 4 83%	3640
	21st to end	Night	1 2 1 4 4 4 5% 4 6% 8% 62%	3970
August	1st to 10th	Night	2 3 1 4 3 3 4 3 5% 7% 65%	3839
	11th to 20th	Night	1 1 1 2 2 2 2 3 5% 80%	4051
	21st to end	Night	5% 3 1 4 3 3 2 3 4 5% 69%	5012
September	1st to 10th	Night	3 2 1 3 2 2 3 2 4 5% 73%	2000
	11th to 20th	Night	5% 2 1 4 3 3 3 3 5% 6% 66%	2180
	21st to end	Night	6% 3 1 4 2 3 3 2 4 5% 68%	2200
October	1st to 10th	Night	4 3 1 3 3 3 3 3 3 5% 71%	3600
	11th to 20th	Night	6% 4 2 4 2 3 4 3 4 6% 62%	4004
	21st to end	Night	9% 6% 2 6% 3 4 4 4 5% 8% 49%	4600
November	1st to 10th	Night	12% 7% 2 6% 5% 4 4 3 5% 7% 45%	2665
	11th to 20th	Night	15% 7% 3 6% 5% 5% 4 4 5% 8% 39%	2730
	21st to end	Night	12% 8% 3 7% 4 3 4 3 5% 7% 44%	2756
December	1st to 10th	Night	9% 8% 2 6% 4 4 5% 3 4 7% 48%	4700
	11th to 20th	Night	15% 12% 4 10% 7% 6% 6% 4 6% 7% 24%	5316
	21st to end	Night	14% 11% 4 10% 7% 6% 5% 4 6% 6% 28%	7106

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	55% 15% 17% 11% 1	4723
	11th to 20th	Day	40% 10% 21% 24% 6%	4740
	21st to end	Day	45% 9% 15% 23% 7% 1	5187
February	1st to 10th	Day	48% 9% 20% 18% 4 1	1867
	11th to 20th	Day	45% 9% 19% 20% 8%	2011
	21st to end	Day	42% 9% 18% 20% 10% 1	1650
March	1st to 10th	Day	39% 11% 14% 24% 11% 2	4518
	11th to 20th	Day	33% 8% 11% 34% 13% 1	4797
	21st to end	Day	40% 11% 20% 19% 8% 2	5748
April	1st to 10th	Day	36% 8% 15% 24% 14% 3	2400
	11th to 20th	Day	32% 7% 13% 25% 18% 5%	2407
	21st to end	Day	31% 7% 13% 27% 18% 4	2600
May	1st to 10th	Day	22% 6% 8% 24% 33% 7%	5880
	11th to 20th	Day	23% 5% 7% 26% 30% 10%	6030
	21st to end	Day	17% 6% 11% 29% 28% 8%	6904
June	1st to 10th	Day	30% 8% 13% 23% 22% 4	2800
	11th to 20th	Day	21% 5% 9% 23% 30% 12%	2800
	21st to end	Day	12% 3 8% 18% 34% 24%	2800
July	1st to 10th	Day	10% 2 3 10% 23% 52%	6251
	11th to 20th	Day	9% 1 2 6% 22% 61%	6245
	21st to end	Day	14% 2 3 14% 34% 33%	6681
August	1st to 10th	Day	19% 3 5% 12% 26% 35%	4343
	11th to 20th	Day	9% 2 4 15% 36% 34%	4184
	21st to end	Day	14% 2 4 15% 36% 29%	4311
September	1st to 10th	Day	12% 3 5% 17% 31% 32%	2400
	11th to 20th	Day	18% 4 10% 25% 27% 16%	2220
	21st to end	Day	20% 3 6% 24% 34% 12%	2200
October	1st to 10th	Day	20% 3 7% 18% 39% 13%	3600
	11th to 20th	Day	27% 4 12% 29% 25% 4	3144
	21st to end	Day	31% 5% 13% 30% 20% 1	3200
November	1st to 10th	Day	39% 7% 17% 26% 12%	1854
	11th to 20th	Day	42% 10% 21% 22% 5%	1890
	21st to end	Day	43% 11% 24% 18% 4	1858
December	1st to 10th	Day	37% 9% 24% 26% 3	2700
	11th to 20th	Day	55% 11% 22% 10% 2	3108
	21st to end	Day	54% 11% 18% 15% 3	4092
January	1st to 10th	Night	57% 13% 18% 10% 1	8172
	11th to 20th	Night	39% 10% 18% 26% 8%	8220
	21st to end	Night	40% 9% 15% 23% 11% 2	8994
February	1st to 10th	Night	46% 10% 19% 21% 4	2748
	11th to 20th	Night	40% 9% 19% 21% 9% 1	2399
	21st to end	Night	42% 9% 16% 21% 11% 1	1980
March	1st to 10th	Night	35% 9% 12% 28% 14% 2	5376
	11th to 20th	Night	38% 5% 7% 29% 20% 1	5190
	21st to end	Night	37% 11% 18% 24% 9% 1	5213
April	1st to 10th	Night	35% 7% 12% 25% 18% 3	2000
	11th to 20th	Night	31% 7% 11% 26% 21% 4	1942
	21st to end	Night	29% 6% 12% 27% 23% 3	1800
May	1st to 10th	Night	21% 6% 10% 21% 32% 10%	4097
	11th to 20th	Night	26% 5% 7% 23% 32% 7%	3873
	21st to end	Night	20% 6% 8% 26% 30% 9%	3972
June	1st to 10th	Night	30% 9% 12% 21% 24% 3	1600
	11th to 20th	Night	22% 5% 8% 20% 33% 11%	1600
	21st to end	Night	16% 4 6% 17% 31% 26%	1600
July	1st to 10th	Night	11% 2 3 9% 20% 55%	3600
	11th to 20th	Night	7% 1 2 5% 20% 64%	3592
	21st to end	Night	19% 3 3 13% 30% 32%	3857
August	1st to 10th	Night	21% 2 5% 10% 23% 39%	2786
	11th to 20th	Night	10% 2 4 9% 27% 47%	2928
	21st to end	Night	14% 2 3 12% 32% 36%	3605
September	1st to 10th	Night	14% 3 4 13% 30% 37%	2000
	11th to 20th	Night	19% 4 8% 20% 25% 24%	2180
	21st to end	Night	19% 3 5% 20% 32% 21%	2200
October	1st to 10th	Night	18% 3 6% 17% 31% 25%	3600
	11th to 20th	Night	23% 4 8% 29% 31% 6%	4004
	21st to end	Night	32% 5% 10% 29% 21% 3	4600
November	1st to 10th	Night	38% 6% 13% 24% 18% 1	2665
	11th to 20th	Night	41% 7% 15% 27% 9%	2730
	21st to end	Night	39% 6% 19% 26% 9%	2756
December	1st to 10th	Night	36% 8% 26% 26% 5%	4700
	11th to 20th	Night	57% 9% 17% 14% 2	5316
	21st to end	Night	51% 12% 18% 14% 5%	7106

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	20% 10% 21% 27% 17% 4	220
	11th to 20th	Day	14% 8% 22% 29% 21% 7%	220
	21st to end	Day	11% 5% 20% 34% 23% 7%	242
February	1st to 10th	Day	14% 14% 28% 24% 15% 5%	355
	11th to 20th	Day	17% 13% 26% 26% 15% 3	516
	21st to end	Day	14% 13% 29% 26% 15% 4	414
March	1st to 10th	Day	12% 13% 31% 29% 12% 3	500
	11th to 20th	Day	11% 11% 29% 31% 14% 4	500
	21st to end	Day	13% 13% 31% 24% 17% 3	550
April	1st to 10th	Day	14% 14% 24% 22% 19% 7%	500
	11th to 20th	Day	9% 14% 32% 24% 16% 5%	492
	21st to end	Day	9% 14% 32% 25% 16% 4	500
May	1st to 10th	Day	6% 13% 29% 29% 17% 7%	503
	11th to 20th	Day	16% 7% 22% 29% 17% 9%	528
	21st to end	Day	9% 6% 17% 31% 22% 15%	573
June	1st to 10th	Day	8% 6% 21% 30% 23% 12%	530
	11th to 20th	Day	5% 11% 25% 35% 18% 6%	530
	21st to end	Day	4 4 23% 33% 24% 12%	530
July	1st to 10th	Day	2 2 20% 38% 29% 9%	526
	11th to 20th	Day	2 2 20% 39% 27% 9%	524
	21st to end	Day	3 2 15% 33% 35% 11%	575
August	1st to 10th	Day	4 3 21% 30% 33% 10%	527
	11th to 20th	Day	2 2 16% 33% 32% 16%	517
	21st to end	Day	4 3 18% 28% 29% 18%	583
September	1st to 10th	Day	5% 2 13% 27% 34% 19%	530
	11th to 20th	Day	5% 4 16% 30% 28% 17%	530
	21st to end	Day	4 5% 19% 30% 28% 13%	530
October	1st to 10th	Day	7% 8% 17% 25% 28% 15%	530
	11th to 20th	Day	11% 7% 21% 25% 24% 14%	422
	21st to end	Day	10% 9% 21% 32% 21% 6%	363
November	1st to 10th	Day	14% 8% 23% 29% 21% 5%	330
	11th to 20th	Day	15% 7% 22% 30% 19% 6%	330
	21st to end	Day	16% 6% 15% 34% 24% 6%	313
December	1st to 10th	Day	20% 4 21% 22% 19% 15%	220
	11th to 20th	Day	17% 7% 28% 26% 16% 5%	220
	21st to end	Day	20% 10% 24% 25% 16% 4	242
January	1st to 10th	Night	18% 9% 22% 30% 18% 2	840
	11th to 20th	Night	16% 6% 20% 33% 21% 3	840
	21st to end	Night	15% 4 19% 35% 24% 3	924
February	1st to 10th	Night	17% 7% 25% 34% 16% 2	840
	11th to 20th	Night	13% 5% 25% 38% 17% 1	804
	21st to end	Night	13% 7% 30% 36% 13% 1	660
March	1st to 10th	Night	15% 9% 29% 34% 13% 1	800
	11th to 20th	Night	14% 9% 22% 38% 15% 2	800
	21st to end	Night	14% 9% 28% 35% 13% 1	880
April	1st to 10th	Night	12% 9% 22% 39% 18% 2	800
	11th to 20th	Night	11% 8% 27% 36% 16% 2	754
	21st to end	Night	8% 8% 24% 44% 15% 2	600
May	1st to 10th	Night	8% 8% 26% 38% 19% 1	600
	11th to 20th	Night	8% 7% 25% 33% 24% 3	508
	21st to end	Night	7% 7% 19% 36% 27% 4	433
June	1st to 10th	Night	10% 5% 20% 38% 23% 4	400
	11th to 20th	Night	9% 7% 24% 38% 21% 2	400
	21st to end	Night	4 4 20% 43% 28% 2	400
July	1st to 10th	Night	2 1 16% 52% 27% 2	390
	11th to 20th	Night	1 1 15% 60% 22%	388
	21st to end	Night	3 13% 49% 32% 2	438
August	1st to 10th	Night	5% 2 14% 46% 32% 2	505
	11th to 20th	Night	1 2 13% 51% 32% 1	594
	21st to end	Night	3 3 14% 44% 32% 4	877
September	1st to 10th	Night	4 1 11% 43% 37% 5%	800
	11th to 20th	Night	5% 2 12% 39% 37% 5%	800
	21st to end	Night	8% 3 15% 36% 30% 8%	800
October	1st to 10th	Night	8% 3 19% 39% 27% 4	800
	11th to 20th	Night	9% 4 17% 41% 25% 5%	800
	21st to end	Night	11% 5% 20% 38% 23% 3	880
November	1st to 10th	Night	11% 6% 19% 38% 23% 2	820
	11th to 20th	Night	18% 6% 21% 35% 17% 2	840
	21st to end	Night	16% 3 19% 38% 21% 3	840
December	1st to 10th	Night	17% 6% 20% 36% 18% 3	840
	11th to 20th	Night	20% 7% 19% 36% 16% 2	840
	21st to end	Night	23% 8% 25% 31% 11% 1	924

Page generated at 2026-02-12 23:42:14. Version ac07

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