Brewster
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brewster clear sky chart from 2005-11-06 to 2026-03-03.

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% 7% 3 6% 4 4 5% 4 6% 7% 47%	134994
	Night	8% 9% 3 7% 4 4 5% 4 7% 8% 42%	134891
Transparency	Day	36% 7% 12% 22% 17% 7%	134722
	Night	41% 8% 12% 20% 13% 4	134741
Seeing	Day	10% 8% 24% 32% 20% 6%	15805
	Night	12% 4 19% 41% 21% 2	24937

By Month

Percentage Cloud Cover NW (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% 19% 6% 13% 6% 5% 6% 4 6% 7% 17%	4723
	11th to 20th	Day	9% 9% 3 7% 5% 4 4 4 9% 10% 36%	4716
	21st to end	Day	15% 16% 5% 9% 4 4 5% 3 6% 8% 23%	5186
February	1st to 10th	Day	8% 14% 4 9% 6% 6% 6% 4 7% 7% 28%	1793
	11th to 20th	Day	7% 14% 4 9% 4 5% 4 4 6% 8% 35%	1850
	21st to end	Day	5% 12% 4 9% 6% 5% 6% 6% 6% 8% 32%	1730
March	1st to 10th	Day	5% 7% 3 7% 5% 5% 6% 5% 7% 9% 40%	4539
	11th to 20th	Day	4 5% 2 6% 5% 5% 6% 4 6% 7% 48%	4797
	21st to end	Day	8% 8% 3 9% 6% 6% 6% 5% 8% 9% 32%	5881
April	1st to 10th	Day	8% 7% 2 7% 7% 6% 6% 6% 7% 8% 36%	2400
	11th to 20th	Day	5% 7% 2 6% 5% 6% 5% 5% 7% 9% 42%	2372
	21st to end	Day	5% 7% 2 7% 5% 5% 6% 5% 6% 9% 42%	2668
May	1st to 10th	Day	6% 5% 2 4 3 4 4 4 5% 7% 55%	6320
	11th to 20th	Day	5% 5% 3 8% 5% 5% 6% 4 7% 8% 43%	6277
	21st to end	Day	7% 6% 3 7% 5% 5% 5% 4 6% 8% 44%	6903
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 8% 42%	2800
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 5% 6% 8% 46%	2800
	21st to end	Day	4 4 1 5% 4 4 5% 4 6% 7% 56%	2800
July	1st to 10th	Day	1 1 1 2 2 2 3 2 4 5% 76%	6320
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 4 78%	6320
	21st to end	Day	2 2 1 3 2 2 3 3 5% 6% 71%	6874
August	1st to 10th	Day	2 2 1 2 2 3 3 3 5% 7% 70%	4541
	11th to 20th	Day	1 1 1 3 3 3 4 3 6% 8% 67%	4346
	21st to end	Day	3 4 2 5% 3 4 4 3 5% 6% 61%	4315
September	1st to 10th	Day	3 2 1 3 2 3 3 3 5% 7% 67%	2400
	11th to 20th	Day	4 3 2 5% 4 4 5% 4 6% 8% 55%	2382
	21st to end	Day	7% 4 1 4 4 4 4 3 6% 6% 57%	2180
October	1st to 10th	Day	4 3 1 4 3 3 4 3 6% 9% 60%	3280
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 5% 6% 9% 47%	3213
	21st to end	Day	8% 6% 3 7% 4 5% 5% 5% 6% 8% 43%	3200
November	1st to 10th	Day	12% 9% 4 7% 5% 6% 6% 5% 6% 9% 32%	1811
	11th to 20th	Day	10% 14% 4 9% 5% 5% 6% 4 6% 6% 32%	1680
	21st to end	Day	9% 14% 6% 10% 6% 5% 5% 4 6% 6% 28%	1680
December	1st to 10th	Day	8% 11% 4 11% 6% 6% 6% 4 8% 8% 29%	2700
	11th to 20th	Day	13% 23% 7% 12% 5% 4 4 3 5% 5% 20%	3105
	21st to end	Day	12% 21% 7% 13% 7% 6% 6% 4 6% 4 14%	4092
January	1st to 10th	Night	12% 23% 6% 10% 5% 5% 5% 4 6% 7% 18%	8172
	11th to 20th	Night	9% 9% 2 7% 4 4 5% 5% 10% 13% 34%	8182
	21st to end	Night	14% 15% 5% 9% 4 5% 5% 4 8% 9% 22%	8994
February	1st to 10th	Night	10% 13% 3 9% 6% 5% 5% 4 7% 8% 29%	2824
	11th to 20th	Night	9% 10% 4 8% 5% 5% 6% 5% 7% 9% 34%	2657
	21st to end	Night	7% 12% 4 8% 6% 5% 5% 4 7% 8% 33%	2076
March	1st to 10th	Night	5% 7% 2 7% 5% 5% 6% 4 7% 9% 42%	5400
	11th to 20th	Night	6% 9% 2 6% 4 5% 4 3 5% 7% 50%	5185
	21st to end	Night	9% 9% 3 8% 6% 6% 6% 5% 8% 10% 30%	5094
April	1st to 10th	Night	6% 7% 3 7% 6% 6% 6% 4 6% 9% 40%	2000
	11th to 20th	Night	5% 8% 3 6% 4 5% 4 4 6% 7% 48%	1979
	21st to end	Night	6% 6% 2 5% 4 4 4 4 7% 8% 50%	1727
May	1st to 10th	Night	3 4 2 4 2 3 4 4 7% 8% 59%	3640
	11th to 20th	Night	6% 6% 2 6% 3 4 5% 4 6% 8% 50%	3616
	21st to end	Night	6% 4 1 5% 4 3 4 4 7% 9% 53%	3972
June	1st to 10th	Night	7% 5% 1 5% 5% 4 4 5% 7% 8% 48%	1600
	11th to 20th	Night	5% 4 2 6% 4 4 5% 5% 6% 9% 51%	1600
	21st to end	Night	3 3 1 4 3 4 5% 3 5% 7% 61%	1600
July	1st to 10th	Night	1 2 2 1 2 1 2 3 5% 81%	3640
	11th to 20th	Night	1 1 1 1 2 2 2 3 4 83%	3640
	21st to end	Night	1 1 1 2 2 3 3 3 4 6% 73%	3969
August	1st to 10th	Night	2 2 1 4 3 3 4 3 5% 6% 68%	2609
	11th to 20th	Night	2 2 1 3 2 3 4 3 5% 9% 67%	2703
	21st to end	Night	4 3 1 4 4 4 3 3 5% 7% 64%	3604
September	1st to 10th	Night	2 2 1 4 2 2 4 3 6% 8% 67%	2000
	11th to 20th	Night	5% 3 2 4 3 3 4 3 5% 7% 61%	2017
	21st to end	Night	5% 4 1 3 3 3 4 3 6% 9% 59%	2217
October	1st to 10th	Night	4 3 1 3 3 4 4 5% 7% 7% 59%	3920
	11th to 20th	Night	4 5% 1 6% 4 4 5% 5% 6% 9% 49%	3941
	21st to end	Night	9% 7% 2 6% 5% 5% 4 4 6% 8% 44%	4600
November	1st to 10th	Night	15% 10% 3 6% 4 4 5% 5% 7% 7% 36%	2707
	11th to 20th	Night	11% 12% 3 7% 6% 5% 6% 5% 7% 7% 32%	2940
	21st to end	Night	12% 17% 4 9% 5% 4 4 5% 7% 6% 27%	2940
December	1st to 10th	Night	7% 10% 4 9% 5% 5% 5% 4 14% 9% 28%	4700
	11th to 20th	Night	13% 20% 8% 12% 7% 6% 5% 5% 5% 6% 14%	5320
	21st to end	Night	16% 21% 5% 11% 6% 5% 5% 4 6% 5% 16%	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	68% 9% 8% 11% 4	4723
	11th to 20th	Day	49% 11% 16% 19% 4 1	4740
	21st to end	Day	66% 6% 10% 13% 4	5187
February	1st to 10th	Day	57% 11% 13% 12% 5% 1	1793
	11th to 20th	Day	50% 8% 16% 17% 8% 1	1859
	21st to end	Day	50% 13% 14% 16% 7% 1	1730
March	1st to 10th	Day	38% 11% 15% 20% 16% 1	4539
	11th to 20th	Day	34% 7% 13% 33% 13% 1	4797
	21st to end	Day	42% 12% 19% 17% 9% 1	5881
April	1st to 10th	Day	41% 10% 18% 21% 9% 2	2400
	11th to 20th	Day	35% 8% 16% 22% 14% 4	2372
	21st to end	Day	35% 9% 14% 25% 14% 3	2668
May	1st to 10th	Day	26% 6% 10% 23% 31% 5%	6320
	11th to 20th	Day	32% 7% 15% 28% 16% 3	6280
	21st to end	Day	32% 9% 14% 28% 15% 2	6904
June	1st to 10th	Day	36% 7% 12% 21% 17% 6%	2800
	11th to 20th	Day	32% 7% 12% 25% 17% 5%	2800
	21st to end	Day	24% 6% 13% 27% 23% 7%	2800
July	1st to 10th	Day	11% 3 7% 20% 29% 30%	6251
	11th to 20th	Day	12% 2 4 19% 31% 32%	6245
	21st to end	Day	15% 2 5% 26% 31% 20%	6681
August	1st to 10th	Day	14% 3 7% 22% 36% 19%	4524
	11th to 20th	Day	16% 4 11% 35% 27% 7%	4388
	21st to end	Day	24% 5% 12% 27% 26% 7%	4314
September	1st to 10th	Day	16% 5% 12% 28% 30% 9%	2400
	11th to 20th	Day	25% 7% 13% 24% 24% 6%	2382
	21st to end	Day	27% 7% 16% 28% 16% 5%	2180
October	1st to 10th	Day	24% 5% 16% 36% 18% 2	3280
	11th to 20th	Day	35% 8% 17% 22% 17% 1	3213
	21st to end	Day	41% 7% 14% 23% 13% 2	3200
November	1st to 10th	Day	50% 10% 15% 15% 8% 1	1811
	11th to 20th	Day	55% 10% 13% 13% 8% 1	1680
	21st to end	Day	62% 10% 12% 10% 6% 1	1680
December	1st to 10th	Day	57% 10% 14% 16% 3	2700
	11th to 20th	Day	72% 9% 7% 8% 4	3108
	21st to end	Day	75% 8% 7% 8% 3	4092
January	1st to 10th	Night	69% 10% 10% 9% 2	8172
	11th to 20th	Night	49% 14% 14% 19% 4	8220
	21st to end	Night	67% 7% 12% 11% 3	8994
February	1st to 10th	Night	56% 10% 13% 13% 6% 2	2824
	11th to 20th	Night	48% 11% 16% 17% 7% 1	2659
	21st to end	Night	46% 12% 17% 16% 8% 1	2076
March	1st to 10th	Night	34% 12% 15% 24% 13% 1	5400
	11th to 20th	Night	36% 6% 14% 28% 15% 1	5186
	21st to end	Night	45% 13% 18% 19% 5% 1	5094
April	1st to 10th	Night	39% 10% 16% 25% 9% 2	2000
	11th to 20th	Night	32% 9% 15% 24% 18% 3	1979
	21st to end	Night	32% 8% 13% 28% 16% 3	1727
May	1st to 10th	Night	20% 7% 9% 25% 35% 4	3637
	11th to 20th	Night	29% 6% 13% 31% 19% 1	3616
	21st to end	Night	23% 8% 19% 33% 16% 1	3972
June	1st to 10th	Night	31% 8% 13% 24% 20% 5%	1600
	11th to 20th	Night	27% 7% 16% 26% 19% 5%	1600
	21st to end	Night	22% 7% 13% 28% 25% 6%	1600
July	1st to 10th	Night	10% 2 5% 22% 32% 29%	3600
	11th to 20th	Night	7% 2 5% 23% 30% 33%	3592
	21st to end	Night	13% 3 5% 23% 32% 23%	3857
August	1st to 10th	Night	18% 3 7% 23% 28% 22%	2601
	11th to 20th	Night	15% 5% 12% 35% 27% 6%	2727
	21st to end	Night	20% 4 10% 28% 28% 9%	3604
September	1st to 10th	Night	17% 5% 11% 27% 30% 11%	2000
	11th to 20th	Night	22% 6% 13% 25% 27% 8%	2017
	21st to end	Night	23% 8% 14% 32% 17% 6%	2217
October	1st to 10th	Night	24% 5% 12% 39% 18% 1	3920
	11th to 20th	Night	33% 8% 15% 26% 17% 1	3941
	21st to end	Night	40% 7% 13% 26% 12% 2	4600
November	1st to 10th	Night	47% 10% 18% 14% 9% 2	2707
	11th to 20th	Night	50% 13% 13% 15% 7% 2	2940
	21st to end	Night	59% 9% 13% 12% 6% 1	2940
December	1st to 10th	Night	56% 11% 15% 17% 2	4700
	11th to 20th	Night	76% 8% 8% 5% 3	5316
	21st to end	Night	72% 10% 6% 7% 4	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	25% 4 15% 40% 11% 5%	220
	11th to 20th	Day	22% 5% 15% 33% 20% 5%	220
	21st to end	Day	26% 2 14% 34% 19% 4	242
February	1st to 10th	Day	12% 5% 26% 38% 14% 4	333
	11th to 20th	Day	15% 5% 29% 31% 15% 4	428
	21st to end	Day	10% 7% 31% 36% 15% 2	446
March	1st to 10th	Day	12% 7% 29% 32% 15% 4	509
	11th to 20th	Day	13% 11% 25% 26% 19% 6%	500
	21st to end	Day	11% 13% 31% 28% 14% 3	550
April	1st to 10th	Day	14% 14% 27% 27% 14% 3	500
	11th to 20th	Day	8% 17% 30% 27% 13% 5%	492
	21st to end	Day	8% 14% 28% 28% 16% 6%	500
May	1st to 10th	Day	7% 13% 29% 32% 16% 3	503
	11th to 20th	Day	14% 9% 23% 29% 19% 6%	528
	21st to end	Day	11% 7% 25% 29% 20% 9%	573
June	1st to 10th	Day	9% 11% 28% 32% 15% 4	530
	11th to 20th	Day	8% 12% 31% 29% 16% 4	530
	21st to end	Day	5% 6% 23% 35% 25% 6%	530
July	1st to 10th	Day	5% 6% 20% 43% 22% 4	526
	11th to 20th	Day	3 8% 24% 34% 23% 8%	524
	21st to end	Day	3 4 21% 35% 28% 9%	575
August	1st to 10th	Day	5% 3 19% 34% 30% 9%	527
	11th to 20th	Day	2 3 18% 34% 30% 12%	517
	21st to end	Day	4 8% 20% 35% 25% 9%	583
September	1st to 10th	Day	4 5% 21% 30% 26% 15%	530
	11th to 20th	Day	5% 6% 20% 32% 27% 9%	530
	21st to end	Day	8% 8% 22% 33% 23% 5%	530
October	1st to 10th	Day	9% 7% 23% 31% 23% 7%	530
	11th to 20th	Day	10% 7% 24% 29% 22% 7%	507
	21st to end	Day	16% 4 23% 37% 16% 3	363
November	1st to 10th	Day	18% 6% 22% 36% 17% 1	307
	11th to 20th	Day	21% 5% 26% 23% 18% 6%	220
	21st to end	Day	19% 2 19% 36% 17% 6%	220
December	1st to 10th	Day	22% 5% 12% 31% 16% 13%	220
	11th to 20th	Day	20% 5% 21% 41% 11% 1	220
	21st to end	Day	17% 4 15% 41% 18% 5%	242
January	1st to 10th	Night	23% 5% 20% 35% 15% 1	840
	11th to 20th	Night	21% 3 16% 39% 18% 3	840
	21st to end	Night	20% 2 15% 39% 21% 4	924
February	1st to 10th	Night	16% 4 21% 43% 15% 2	840
	11th to 20th	Night	14% 2 22% 41% 18% 2	824
	21st to end	Night	11% 5% 24% 41% 17% 2	692
March	1st to 10th	Night	14% 3 20% 42% 19% 2	808
	11th to 20th	Night	10% 4 24% 43% 19% 1	800
	21st to end	Night	12% 6% 24% 40% 17% 1	880
April	1st to 10th	Night	10% 5% 21% 42% 21% 1	800
	11th to 20th	Night	8% 7% 26% 38% 19% 2	791
	21st to end	Night	7% 6% 27% 42% 16% 2	527
May	1st to 10th	Night	5% 4 29% 39% 23%	400
	11th to 20th	Night	11% 4 19% 39% 25% 2	394
	21st to end	Night	9% 4 16% 42% 25% 4	433
June	1st to 10th	Night	9% 4 25% 40% 21% 2	400
	11th to 20th	Night	6% 6% 25% 41% 21% 1	400
	21st to end	Night	3 4 21% 47% 23% 2	400
July	1st to 10th	Night	3 4 19% 44% 28% 2	390
	11th to 20th	Night	2 1 22% 47% 26% 1	388
	21st to end	Night	2 14% 48% 34% 2	438
August	1st to 10th	Night	2 2 13% 40% 38% 4	398
	11th to 20th	Night	2 1 10% 41% 40% 5%	470
	21st to end	Night	4 4 15% 41% 32% 3	876
September	1st to 10th	Night	3 3 14% 44% 30% 5%	800
	11th to 20th	Night	6% 3 14% 33% 37% 7%	800
	21st to end	Night	7% 3 16% 45% 26% 3	800
October	1st to 10th	Night	10% 4 15% 44% 25% 2	800
	11th to 20th	Night	9% 3 23% 41% 22% 3	800
	21st to end	Night	16% 4 20% 41% 17% 3	880
November	1st to 10th	Night	20% 4 19% 44% 14%	820
	11th to 20th	Night	18% 4 24% 37% 15% 2	840
	21st to end	Night	21% 3 18% 39% 14% 4	840
December	1st to 10th	Night	16% 2 17% 40% 21% 4	840
	11th to 20th	Night	22% 4 20% 40% 14% 1	840
	21st to end	Night	25% 2 18% 44% 10% 1	924

Page generated at 2026-02-27 22:40:18. Version ac07

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