Brewster Academy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brewster Academy clear sky chart from 2005-11-06 to 2026-04-02.

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	16% 8% 3 6% 4 4 4 4 6% 7% 37%	130993
	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 37%	130861
Transparency	Day	44% 6% 12% 22% 14% 2	134500
	Night	45% 6% 11% 21% 14% 2	135441
Seeing	Day	23% 17% 30% 20% 8% 2	16635
	Night	21% 14% 30% 26% 8% 1	25621

By Month

Percentage Cloud Cover NE (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	10% 8% 3 9% 7% 6% 5% 4 6% 6% 35%	4661
	11th to 20th	Day	12% 10% 4 7% 4 3 4 3 6% 7% 41%	4650
	21st to end	Day	10% 9% 3 6% 4 4 4 4 5% 6% 45%	5124
February	1st to 10th	Day	14% 8% 3 7% 4 5% 5% 4 6% 7% 37%	1874
	11th to 20th	Day	10% 8% 3 7% 7% 5% 5% 4 6% 7% 39%	2048
	21st to end	Day	15% 10% 3 7% 5% 6% 4 5% 5% 5% 36%	1730
March	1st to 10th	Day	18% 12% 3 8% 5% 5% 4 3 5% 5% 32%	4576
	11th to 20th	Day	16% 9% 3 7% 4 4 4 4 5% 6% 38%	4891
	21st to end	Day	24% 8% 2 5% 3 3 3 3 5% 8% 34%	5902
April	1st to 10th	Day	19% 9% 3 6% 4 3 4 4 5% 6% 38%	2412
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 6% 8% 39%	2418
	21st to end	Day	20% 7% 2 7% 5% 5% 4 3 5% 6% 36%	2600
May	1st to 10th	Day	31% 10% 4 7% 4 4 4 3 5% 5% 23%	5886
	11th to 20th	Day	17% 9% 4 8% 5% 5% 5% 4 5% 8% 30%	6086
	21st to end	Day	19% 6% 2 7% 4 5% 5% 4 6% 8% 34%	6949
June	1st to 10th	Day	20% 8% 2 6% 4 5% 5% 4 6% 8% 32%	2800
	11th to 20th	Day	17% 6% 2 5% 5% 5% 5% 4 6% 8% 39%	2800
	21st to end	Day	18% 8% 2 6% 4 5% 5% 4 5% 9% 34%	2800
July	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 5% 6% 10% 33%	6360
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 7% 10% 40%	6360
	21st to end	Day	14% 6% 2 6% 4 4 4 3 6% 8% 44%	6926
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 4 7% 8% 38%	4344
	11th to 20th	Day	14% 8% 3 7% 4 4 4 4 6% 8% 36%	4175
	21st to end	Day	9% 5% 2 5% 4 5% 4 4 7% 9% 47%	4327
September	1st to 10th	Day	12% 4 1 5% 4 4 5% 4 6% 7% 48%	2398
	11th to 20th	Day	12% 6% 2 3 3 3 3 3 4 6% 54%	2200
	21st to end	Day	15% 6% 2 6% 4 3 3 3 5% 7% 47%	2200
October	1st to 10th	Day	18% 8% 3 6% 4 4 4 3 5% 5% 40%	2200
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 6% 43%	1917
	21st to end	Day	22% 7% 2 6% 5% 4 3 4 4 6% 36%	1980
November	1st to 10th	Day	13% 7% 3 5% 3 3 4 3 5% 7% 45%	1851
	11th to 20th	Day	17% 6% 2 4 4 5% 4 4 5% 6% 44%	1890
	21st to end	Day	20% 8% 3 6% 4 4 4 4 6% 5% 36%	1864
December	1st to 10th	Day	14% 10% 3 7% 5% 5% 5% 4 7% 7% 32%	2680
	11th to 20th	Day	17% 8% 3 5% 3 3 4 3 6% 8% 42%	3066
	21st to end	Day	19% 11% 3 7% 4 4 4 4 5% 5% 34%	4048
January	1st to 10th	Night	12% 7% 3 8% 5% 5% 5% 4 5% 6% 39%	8234
	11th to 20th	Night	12% 10% 3 10% 5% 5% 5% 4 6% 7% 32%	8235
	21st to end	Night	13% 7% 2 6% 4 5% 5% 4 6% 7% 42%	9057
February	1st to 10th	Night	14% 9% 3 7% 5% 4 5% 4 6% 8% 36%	2742
	11th to 20th	Night	10% 9% 3 7% 5% 6% 5% 4 5% 8% 38%	2458
	21st to end	Night	12% 9% 3 6% 5% 4 5% 4 6% 7% 39%	2076
March	1st to 10th	Night	21% 11% 3 6% 5% 4 4 4 4 6% 32%	5484
	11th to 20th	Night	12% 10% 3 8% 5% 5% 6% 5% 5% 7% 34%	5288
	21st to end	Night	25% 8% 3 5% 3 4 4 4 6% 8% 32%	5270
April	1st to 10th	Night	20% 9% 3 7% 4 4 4 2 5% 6% 36%	2011
	11th to 20th	Night	18% 8% 3 7% 5% 5% 4 4 4 6% 37%	1931
	21st to end	Night	20% 8% 3 6% 5% 4 3 4 5% 5% 38%	1800
May	1st to 10th	Night	34% 10% 3 7% 4 4 4 3 5% 5% 22%	4054
	11th to 20th	Night	14% 8% 3 9% 7% 6% 6% 5% 6% 7% 28%	3851
	21st to end	Night	19% 7% 3 7% 5% 5% 5% 4 5% 7% 33%	3926
June	1st to 10th	Night	20% 8% 3 6% 4 4 4 4 6% 6% 35%	1600
	11th to 20th	Night	18% 6% 2 7% 4 4 4 3 6% 8% 38%	1600
	21st to end	Night	18% 6% 2 6% 5% 5% 4 3 6% 8% 38%	1600
July	1st to 10th	Night	13% 6% 2 6% 4 4 5% 5% 7% 10% 38%	3600
	11th to 20th	Night	11% 7% 2 9% 5% 5% 5% 4 6% 8% 38%	3600
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 6% 50%	3926
August	1st to 10th	Night	21% 7% 3 7% 5% 4 5% 4 4 6% 34%	2772
	11th to 20th	Night	13% 8% 3 8% 5% 5% 4 4 6% 7% 38%	2872
	21st to end	Night	9% 4 2 5% 4 4 4 4 6% 9% 50%	3567
September	1st to 10th	Night	12% 5% 2 4 3 3 4 3 5% 6% 52%	2001
	11th to 20th	Night	13% 6% 2 4 3 3 3 4 4 7% 52%	2200
	21st to end	Night	15% 7% 3 5% 3 3 3 3 5% 7% 46%	2200
October	1st to 10th	Night	18% 7% 2 7% 4 4 4 3 5% 6% 40%	2200
	11th to 20th	Night	15% 6% 3 6% 4 4 5% 4 5% 7% 40%	2477
	21st to end	Night	21% 8% 2 6% 4 4 4 3 5% 5% 36%	2860
November	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 5% 7% 43%	2667
	11th to 20th	Night	15% 8% 2 6% 5% 4 4 4 5% 6% 41%	2730
	21st to end	Night	15% 8% 3 6% 5% 5% 5% 4 5% 7% 39%	2754
December	1st to 10th	Night	14% 11% 3 9% 6% 5% 5% 3 5% 6% 34%	4720
	11th to 20th	Night	21% 8% 2 6% 4 4 4 3 4 6% 37%	5348
	21st to end	Night	19% 11% 3 8% 4 4 4 4 5% 6% 33%	7150

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	47% 5% 11% 23% 13% 1	4662
	11th to 20th	Day	42% 5% 12% 23% 15% 2	4680
	21st to end	Day	39% 5% 10% 25% 18% 3	5124
February	1st to 10th	Day	46% 6% 13% 21% 12% 3	1874
	11th to 20th	Day	43% 7% 12% 20% 15% 4	2057
	21st to end	Day	48% 5% 11% 19% 14% 2	1730
March	1st to 10th	Day	53% 5% 8% 14% 12% 8%	4579
	11th to 20th	Day	45% 8% 14% 18% 11% 3	4891
	21st to end	Day	49% 4 7% 17% 19% 4	5902
April	1st to 10th	Day	48% 5% 9% 19% 16% 4	2412
	11th to 20th	Day	43% 8% 11% 20% 16% 3	2418
	21st to end	Day	50% 6% 8% 17% 15% 3	2600
May	1st to 10th	Day	62% 8% 8% 14% 7% 1	5897
	11th to 20th	Day	51% 9% 13% 16% 9% 2	6090
	21st to end	Day	45% 9% 13% 20% 12% 1	6949
June	1st to 10th	Day	49% 7% 12% 21% 11% 1	2800
	11th to 20th	Day	42% 8% 12% 20% 15% 3	2800
	21st to end	Day	47% 9% 12% 19% 12% 1	2800
July	1st to 10th	Day	46% 7% 13% 21% 13% 1	6290
	11th to 20th	Day	38% 7% 15% 24% 14% 1	6285
	21st to end	Day	38% 6% 12% 28% 16% 1	6728
August	1st to 10th	Day	44% 8% 14% 24% 11%	4330
	11th to 20th	Day	44% 8% 16% 24% 8% 1	4214
	21st to end	Day	32% 6% 14% 28% 18% 2	4327
September	1st to 10th	Day	32% 7% 13% 28% 17% 1	2398
	11th to 20th	Day	31% 5% 10% 30% 22% 3	2200
	21st to end	Day	38% 6% 11% 25% 18% 2	2200
October	1st to 10th	Day	45% 6% 8% 23% 17% 2	3590
	11th to 20th	Day	39% 4 11% 26% 16% 4	3094
	21st to end	Day	37% 4 8% 30% 18% 3	3180
November	1st to 10th	Day	39% 5% 14% 24% 16% 2	1851
	11th to 20th	Day	39% 6% 11% 20% 21% 3	1890
	21st to end	Day	48% 5% 15% 20% 11% 1	1864
December	1st to 10th	Day	47% 6% 14% 24% 8% 1	2680
	11th to 20th	Day	41% 6% 10% 18% 21% 3	3066
	21st to end	Day	50% 6% 7% 18% 14% 5%	4048
January	1st to 10th	Night	44% 5% 12% 25% 12% 2	8234
	11th to 20th	Night	50% 6% 11% 17% 13% 3	8280
	21st to end	Night	42% 5% 7% 21% 21% 5%	9057
February	1st to 10th	Night	45% 6% 12% 21% 14% 2	2742
	11th to 20th	Night	43% 6% 11% 22% 15% 3	2460
	21st to end	Night	44% 5% 11% 23% 13% 4	2076
March	1st to 10th	Night	52% 5% 9% 15% 15% 5%	5494
	11th to 20th	Night	47% 7% 16% 17% 12% 2	5289
	21st to end	Night	50% 4 7% 14% 21% 5%	5270
April	1st to 10th	Night	49% 5% 10% 18% 15% 3	2011
	11th to 20th	Night	47% 6% 8% 21% 17% 1	1931
	21st to end	Night	47% 6% 11% 17% 17% 3	1800
May	1st to 10th	Night	63% 7% 9% 13% 7% 1	4060
	11th to 20th	Night	52% 10% 11% 17% 8% 2	3852
	21st to end	Night	49% 8% 14% 19% 9% 1	3927
June	1st to 10th	Night	49% 8% 12% 20% 12% 1	1600
	11th to 20th	Night	44% 9% 14% 18% 13% 2	1600
	21st to end	Night	44% 8% 14% 22% 10% 1	1600
July	1st to 10th	Night	39% 8% 17% 24% 12%	3560
	11th to 20th	Night	43% 7% 13% 21% 16%	3552
	21st to end	Night	34% 5% 13% 35% 13% 1	3809
August	1st to 10th	Night	51% 7% 11% 21% 9% 1	2762
	11th to 20th	Night	44% 8% 16% 20% 11%	2899
	21st to end	Night	30% 7% 17% 28% 17% 1	3567
September	1st to 10th	Night	32% 7% 13% 28% 19% 1	2001
	11th to 20th	Night	32% 6% 12% 28% 21% 2	2200
	21st to end	Night	37% 6% 12% 27% 17% 2	2200
October	1st to 10th	Night	42% 7% 12% 24% 13% 2	3590
	11th to 20th	Night	36% 5% 11% 26% 18% 4	4039
	21st to end	Night	40% 4 9% 31% 15% 2	4610
November	1st to 10th	Night	40% 6% 11% 25% 17% 2	2667
	11th to 20th	Night	42% 5% 11% 23% 17% 2	2730
	21st to end	Night	44% 6% 16% 23% 11% 1	2754
December	1st to 10th	Night	51% 4 12% 26% 7%	4720
	11th to 20th	Night	48% 4 12% 15% 19% 2	5348
	21st to end	Night	51% 4 8% 21% 14% 2	7150

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% 23% 32% 17% 3	220
	11th to 20th	Day	28% 24% 30% 15% 2 1	220
	21st to end	Day	21% 24% 35% 15% 5%	242
February	1st to 10th	Day	27% 23% 34% 13% 3	376
	11th to 20th	Day	26% 25% 34% 13% 2	536
	21st to end	Day	27% 23% 32% 15% 3	446
March	1st to 10th	Day	29% 22% 33% 14% 2	540
	11th to 20th	Day	28% 27% 29% 11% 4 1	540
	21st to end	Day	28% 24% 28% 13% 5% 2	594
April	1st to 10th	Day	32% 22% 27% 11% 6% 2	504
	11th to 20th	Day	30% 24% 25% 15% 5% 2	492
	21st to end	Day	27% 22% 29% 14% 6% 2	500
May	1st to 10th	Day	32% 18% 29% 16% 4 2	503
	11th to 20th	Day	21% 17% 34% 18% 8% 2	527
	21st to end	Day	21% 13% 34% 21% 9% 1	573
June	1st to 10th	Day	19% 14% 37% 20% 9% 2	530
	11th to 20th	Day	18% 16% 30% 24% 9% 3	530
	21st to end	Day	21% 13% 34% 21% 9% 2	530
July	1st to 10th	Day	18% 8% 33% 25% 12% 3	525
	11th to 20th	Day	15% 6% 32% 32% 12% 3	524
	21st to end	Day	14% 9% 36% 29% 11% 2	576
August	1st to 10th	Day	12% 10% 28% 31% 15% 3	526
	11th to 20th	Day	16% 10% 26% 25% 17% 6%	519
	21st to end	Day	12% 10% 29% 29% 15% 5%	583
September	1st to 10th	Day	16% 10% 25% 25% 17% 7%	530
	11th to 20th	Day	16% 10% 26% 20% 19% 9%	530
	21st to end	Day	16% 13% 29% 24% 12% 5%	530
October	1st to 10th	Day	25% 14% 21% 22% 13% 4	530
	11th to 20th	Day	22% 16% 28% 20% 12% 2	456
	21st to end	Day	32% 14% 28% 19% 5% 1	462
November	1st to 10th	Day	23% 18% 30% 22% 6% 1	425
	11th to 20th	Day	28% 21% 27% 18% 5% 1	430
	21st to end	Day	32% 20% 26% 17% 4	404
December	1st to 10th	Day	25% 25% 29% 18% 3	220
	11th to 20th	Day	33% 22% 31% 10% 3	220
	21st to end	Day	32% 19% 29% 16% 4	242
January	1st to 10th	Night	23% 19% 36% 21% 1	840
	11th to 20th	Night	25% 17% 33% 20% 3	840
	21st to end	Night	19% 22% 34% 21% 4	924
February	1st to 10th	Night	21% 16% 39% 21% 3	840
	11th to 20th	Night	21% 19% 37% 19% 3	823
	21st to end	Night	22% 16% 37% 21% 4	692
March	1st to 10th	Night	27% 17% 34% 20% 3	840
	11th to 20th	Night	24% 18% 32% 21% 5%	840
	21st to end	Night	26% 17% 31% 20% 6%	924
April	1st to 10th	Night	26% 16% 31% 22% 4	805
	11th to 20th	Night	22% 18% 33% 21% 6%	747
	21st to end	Night	26% 17% 27% 25% 5%	600
May	1st to 10th	Night	26% 11% 30% 28% 5%	600
	11th to 20th	Night	21% 13% 32% 27% 8%	514
	21st to end	Night	20% 8% 25% 37% 9%	433
June	1st to 10th	Night	20% 7% 31% 32% 10% 1	400
	11th to 20th	Night	19% 9% 30% 30% 12% 1	400
	21st to end	Night	17% 8% 28% 39% 9%	400
July	1st to 10th	Night	15% 4 26% 42% 14%	390
	11th to 20th	Night	14% 4 22% 39% 20% 1	388
	21st to end	Night	14% 5% 23% 42% 15% 1	438
August	1st to 10th	Night	16% 5% 21% 41% 18%	506
	11th to 20th	Night	14% 8% 27% 34% 16% 2	592
	21st to end	Night	15% 5% 25% 35% 18% 3	861
September	1st to 10th	Night	15% 7% 28% 31% 18% 1	800
	11th to 20th	Night	14% 7% 24% 32% 18% 5%	800
	21st to end	Night	16% 9% 29% 27% 17% 2	800
October	1st to 10th	Night	21% 11% 23% 28% 15% 1	800
	11th to 20th	Night	20% 14% 30% 24% 13% 1	800
	21st to end	Night	27% 16% 25% 25% 7%	880
November	1st to 10th	Night	20% 16% 30% 25% 7% 1	820
	11th to 20th	Night	22% 20% 29% 22% 6% 1	840
	21st to end	Night	23% 20% 30% 23% 4	840
December	1st to 10th	Night	25% 18% 33% 19% 5%	840
	11th to 20th	Night	26% 20% 30% 20% 3	840
	21st to end	Night	27% 19% 29% 22% 3	924

Page generated at 2026-03-30 02:18:40. Version ac07

Copyright © 2026 Attilla Danko.