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 2024-07-08.

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	17% 8% 2 6% 4 4 4 4 6% 7% 38%	89455
	Night	17% 8% 3 6% 4 4 4 4 5% 7% 37%	89890
Transparency	Day	44% 7% 12% 21% 13% 2	89174
	Night	45% 7% 12% 21% 13% 2	89725
Seeing	Day	23% 16% 30% 20% 9% 2	14471
	Night	21% 14% 30% 26% 8% 1	23367

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	14% 9% 2 6% 5% 4 3 4 6% 7% 42%	2517
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 3 6% 6% 37%	2520
	21st to end	Day	16% 11% 3 6% 5% 4 4 4 5% 7% 35%	2772
February	1st to 10th	Day	15% 9% 3 7% 4 5% 4 4 6% 7% 36%	1694
	11th to 20th	Day	11% 8% 3 7% 6% 4 5% 4 6% 7% 39%	1848
	21st to end	Day	15% 10% 3 7% 5% 5% 4 5% 5% 5% 36%	1570
March	1st to 10th	Day	21% 15% 3 7% 5% 4 4 3 4 5% 29%	3116
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 5% 6% 7% 39%	3342
	21st to end	Day	20% 9% 3 7% 4 4 3 4 5% 8% 35%	4027
April	1st to 10th	Day	19% 9% 2 6% 4 3 4 4 5% 6% 38%	2280
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 6% 8% 39%	2296
	21st to end	Day	20% 7% 3 7% 5% 5% 5% 3 5% 6% 35%	2470
May	1st to 10th	Day	21% 11% 4 8% 5% 4 4 4 5% 6% 27%	4106
	11th to 20th	Day	18% 7% 3 8% 5% 4 5% 3 5% 8% 32%	4236
	21st to end	Day	14% 5% 2 5% 4 5% 5% 4 6% 8% 41%	4837
June	1st to 10th	Day	20% 8% 2 6% 5% 5% 5% 4 6% 8% 32%	2660
	11th to 20th	Day	17% 6% 2 5% 5% 5% 5% 4 6% 8% 39%	2660
	21st to end	Day	18% 8% 2 6% 5% 5% 5% 4 5% 9% 33%	2660
July	1st to 10th	Day	14% 7% 2 6% 5% 5% 4 5% 5% 8% 39%	2621
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 7% 8% 39%	2520
	21st to end	Day	14% 4 2 7% 5% 5% 5% 4 7% 9% 39%	2744
August	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 7% 8% 41%	2391
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 41%	2304
	21st to end	Day	11% 5% 2 5% 4 5% 4 4 6% 8% 46%	2390
September	1st to 10th	Day	13% 4 1 5% 4 4 4 4 6% 7% 47%	2159
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 6% 53%	1980
	21st to end	Day	14% 5% 2 6% 4 3 3 3 5% 7% 48%	1980
October	1st to 10th	Day	20% 8% 3 6% 4 4 4 3 5% 6% 38%	1980
	11th to 20th	Day	15% 7% 2 6% 5% 4 4 3 5% 6% 43%	1727
	21st to end	Day	24% 8% 2 6% 5% 4 4 4 4 5% 34%	1782
November	1st to 10th	Day	14% 7% 3 5% 3 3 4 3 4 8% 46%	1671
	11th to 20th	Day	19% 5% 2 4 4 5% 4 4 5% 6% 42%	1710
	21st to end	Day	20% 8% 3 6% 3 4 4 4 5% 6% 37%	1687
December	1st to 10th	Day	19% 10% 3 6% 5% 5% 5% 4 7% 7% 30%	1520
	11th to 20th	Day	20% 8% 2 5% 4 3 4 3 5% 6% 40%	1906
	21st to end	Day	23% 12% 3 6% 4 4 4 3 5% 5% 30%	2772
January	1st to 10th	Night	16% 7% 2 5% 4 4 4 3 5% 5% 43%	4429
	11th to 20th	Night	12% 8% 3 9% 5% 5% 5% 4 7% 8% 35%	4440
	21st to end	Night	20% 10% 3 7% 4 5% 5% 3 5% 6% 32%	4884
February	1st to 10th	Night	14% 10% 3 7% 5% 4 4 4 6% 8% 35%	2482
	11th to 20th	Night	11% 9% 3 7% 5% 5% 5% 4 5% 7% 39%	2218
	21st to end	Night	13% 9% 3 6% 5% 4 5% 4 6% 7% 39%	1884
March	1st to 10th	Night	24% 11% 3 6% 5% 4 4 3 4 5% 32%	3724
	11th to 20th	Night	12% 8% 3 8% 6% 6% 6% 5% 6% 6% 33%	3617
	21st to end	Night	23% 9% 3 5% 3 4 4 4 6% 8% 31%	3604
April	1st to 10th	Night	19% 9% 3 7% 4 4 4 2 5% 6% 38%	1900
	11th to 20th	Night	18% 8% 3 7% 5% 5% 4 3 4 6% 37%	1833
	21st to end	Night	20% 7% 3 6% 5% 4 3 4 5% 5% 38%	1710
May	1st to 10th	Night	25% 12% 3 7% 5% 4 4 3 5% 6% 24%	2834
	11th to 20th	Night	18% 7% 2 9% 7% 6% 6% 5% 7% 7% 26%	2687
	21st to end	Night	16% 6% 3 6% 4 4 4 3 6% 7% 40%	2738
June	1st to 10th	Night	20% 8% 3 6% 4 4 4 4 6% 6% 35%	1520
	11th to 20th	Night	18% 6% 2 7% 4 4 4 3 6% 8% 38%	1520
	21st to end	Night	18% 6% 2 6% 5% 5% 4 3 6% 8% 37%	1520
July	1st to 10th	Night	15% 5% 2 5% 5% 4 5% 4 6% 10% 38%	1504
	11th to 20th	Night	14% 7% 1 6% 3 4 5% 4 7% 9% 40%	1440
	21st to end	Night	12% 5% 2 5% 3 4 3 5% 6% 8% 46%	1574
August	1st to 10th	Night	13% 5% 2 6% 5% 4 4 4 5% 7% 44%	1528
	11th to 20th	Night	14% 6% 3 6% 5% 4 5% 3 6% 8% 40%	1596
	21st to end	Night	15% 5% 2 5% 3 4 3 3 5% 7% 48%	1962
September	1st to 10th	Night	13% 5% 2 4 3 3 5% 3 5% 6% 51%	1800
	11th to 20th	Night	14% 6% 2 4 3 3 3 4 4 7% 51%	1980
	21st to end	Night	14% 6% 2 5% 3 3 3 3 5% 6% 48%	1980
October	1st to 10th	Night	20% 7% 2 6% 4 5% 4 3 5% 6% 38%	1980
	11th to 20th	Night	15% 6% 3 6% 4 5% 5% 4 5% 7% 39%	2228
	21st to end	Night	22% 9% 2 6% 4 4 4 3 5% 5% 34%	2574
November	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 5% 7% 43%	2407
	11th to 20th	Night	16% 8% 2 6% 5% 4 4 4 4 6% 41%	2470
	21st to end	Night	15% 8% 3 5% 5% 4 4 4 5% 7% 40%	2491
December	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 34%	2660
	11th to 20th	Night	19% 8% 2 6% 4 4 4 4 5% 6% 37%	3288
	21st to end	Night	22% 11% 3 8% 5% 3 4 3 4 5% 33%	4884

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	40% 7% 11% 30% 12% 1	2518
	11th to 20th	Day	45% 7% 15% 23% 8% 2	2520
	21st to end	Day	48% 7% 12% 21% 10% 1	2772
February	1st to 10th	Day	47% 6% 14% 21% 10% 2	1694
	11th to 20th	Day	44% 7% 11% 19% 14% 4	1857
	21st to end	Day	49% 5% 12% 18% 14% 2	1570
March	1st to 10th	Day	57% 6% 9% 13% 12% 3	3119
	11th to 20th	Day	41% 10% 13% 17% 14% 5%	3342
	21st to end	Day	49% 4 8% 16% 19% 5%	4027
April	1st to 10th	Day	47% 5% 9% 19% 15% 4	2280
	11th to 20th	Day	42% 8% 12% 20% 15% 2	2296
	21st to end	Day	51% 6% 8% 17% 15% 3	2470
May	1st to 10th	Day	56% 10% 9% 16% 8% 1	4120
	11th to 20th	Day	48% 10% 15% 16% 10% 2	4240
	21st to end	Day	37% 9% 14% 23% 15% 1	4837
June	1st to 10th	Day	49% 8% 12% 21% 10% 1	2660
	11th to 20th	Day	42% 9% 12% 20% 15% 3	2660
	21st to end	Day	48% 9% 12% 18% 12% 1	2660
July	1st to 10th	Day	42% 8% 12% 22% 15% 1	2551
	11th to 20th	Day	41% 8% 15% 25% 11% 1	2445
	21st to end	Day	39% 9% 15% 23% 13% 1	2591
August	1st to 10th	Day	40% 9% 16% 23% 11% 1	2377
	11th to 20th	Day	38% 9% 16% 24% 11% 1	2304
	21st to end	Day	34% 8% 14% 24% 18% 2	2390
September	1st to 10th	Day	34% 8% 14% 27% 16% 1	2159
	11th to 20th	Day	33% 5% 9% 28% 22% 3	1980
	21st to end	Day	37% 6% 12% 25% 19% 2	1980
October	1st to 10th	Day	47% 6% 10% 21% 14% 2	1980
	11th to 20th	Day	41% 6% 12% 29% 10% 1	1727
	21st to end	Day	51% 6% 10% 19% 12% 2	1782
November	1st to 10th	Day	39% 5% 15% 24% 15% 2	1671
	11th to 20th	Day	40% 6% 12% 20% 19% 3	1710
	21st to end	Day	47% 5% 17% 19% 11% 1	1687
December	1st to 10th	Day	51% 7% 13% 18% 11% 1	1520
	11th to 20th	Day	45% 7% 10% 18% 18% 2	1906
	21st to end	Day	54% 7% 9% 18% 10% 1	2772
January	1st to 10th	Night	41% 6% 14% 26% 10% 3	4429
	11th to 20th	Night	44% 9% 15% 20% 11% 2	4440
	21st to end	Night	53% 7% 10% 18% 11% 1	4884
February	1st to 10th	Night	46% 7% 13% 20% 13% 2	2482
	11th to 20th	Night	43% 6% 11% 22% 15% 3	2220
	21st to end	Night	43% 6% 11% 23% 13% 4	1884
March	1st to 10th	Night	54% 5% 11% 17% 11% 2	3734
	11th to 20th	Night	46% 9% 11% 17% 14% 2	3618
	21st to end	Night	49% 5% 9% 13% 18% 5%	3604
April	1st to 10th	Night	48% 5% 10% 18% 16% 3	1900
	11th to 20th	Night	47% 6% 8% 21% 17% 1	1833
	21st to end	Night	47% 6% 11% 16% 16% 3	1710
May	1st to 10th	Night	57% 9% 9% 16% 8% 1	2840
	11th to 20th	Night	52% 11% 13% 15% 8% 2	2688
	21st to end	Night	40% 9% 17% 20% 12% 2	2739
June	1st to 10th	Night	48% 8% 12% 20% 11% 1	1520
	11th to 20th	Night	44% 8% 15% 17% 13% 2	1520
	21st to end	Night	45% 8% 15% 21% 10% 1	1520
July	1st to 10th	Night	38% 10% 16% 20% 14% 1	1464
	11th to 20th	Night	39% 7% 17% 26% 11%	1392
	21st to end	Night	34% 7% 16% 29% 13% 1	1486
August	1st to 10th	Night	38% 9% 13% 26% 14% 1	1518
	11th to 20th	Night	41% 9% 17% 19% 14%	1596
	21st to end	Night	35% 7% 13% 26% 18% 1	1962
September	1st to 10th	Night	32% 8% 14% 28% 18% 1	1800
	11th to 20th	Night	34% 6% 11% 27% 20% 2	1980
	21st to end	Night	35% 6% 12% 28% 17% 2	1980
October	1st to 10th	Night	47% 7% 10% 22% 12% 2	1980
	11th to 20th	Night	41% 8% 13% 26% 11% 1	2228
	21st to end	Night	50% 5% 11% 20% 11% 2	2574
November	1st to 10th	Night	40% 6% 11% 24% 16% 2	2407
	11th to 20th	Night	42% 6% 11% 23% 16% 2	2470
	21st to end	Night	43% 6% 16% 23% 11% 1	2491
December	1st to 10th	Night	51% 6% 13% 20% 10% 1	2660
	11th to 20th	Night	46% 6% 14% 17% 16% 1	3288
	21st to end	Night	55% 5% 9% 21% 10% 1	4884

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	26% 21% 31% 19% 3	180
	11th to 20th	Day	31% 20% 29% 17% 2 2	180
	21st to end	Day	17% 22% 38% 18% 6%	198
February	1st to 10th	Day	27% 22% 35% 13% 3	316
	11th to 20th	Day	25% 26% 33% 14% 2	456
	21st to end	Day	29% 21% 31% 17% 3	382
March	1st to 10th	Day	29% 19% 35% 15% 2	460
	11th to 20th	Day	27% 26% 30% 12% 3 1	460
	21st to end	Day	26% 22% 29% 15% 6% 3	506
April	1st to 10th	Day	30% 23% 27% 12% 6% 2	460
	11th to 20th	Day	28% 24% 26% 16% 5% 2	452
	21st to end	Day	27% 22% 29% 14% 6% 2	460
May	1st to 10th	Day	30% 18% 29% 16% 5% 2	463
	11th to 20th	Day	21% 16% 35% 18% 8% 2	487
	21st to end	Day	21% 13% 33% 22% 9% 2	529
June	1st to 10th	Day	18% 14% 37% 20% 10% 2	490
	11th to 20th	Day	17% 16% 30% 25% 9% 3	490
	21st to end	Day	22% 14% 33% 20% 9% 2	490
July	1st to 10th	Day	18% 8% 32% 26% 13% 3	474
	11th to 20th	Day	15% 6% 31% 32% 13% 3	444
	21st to end	Day	14% 6% 37% 29% 12% 2	488
August	1st to 10th	Day	13% 10% 28% 30% 16% 4	446
	11th to 20th	Day	16% 9% 28% 25% 17% 6%	439
	21st to end	Day	13% 9% 29% 29% 15% 5%	495
September	1st to 10th	Day	17% 9% 25% 25% 18% 7%	450
	11th to 20th	Day	19% 12% 27% 18% 16% 8%	450
	21st to end	Day	15% 13% 30% 24% 12% 6%	450
October	1st to 10th	Day	28% 13% 20% 23% 13% 3	450
	11th to 20th	Day	21% 15% 30% 21% 12% 2	390
	21st to end	Day	34% 12% 27% 21% 5% 1	396
November	1st to 10th	Day	21% 17% 29% 25% 7% 2	365
	11th to 20th	Day	29% 19% 26% 20% 5% 1	370
	21st to end	Day	30% 18% 29% 19% 5% 1	347
December	1st to 10th	Day	24% 26% 28% 18% 4 1	180
	11th to 20th	Day	31% 22% 33% 12% 3	180
	21st to end	Day	33% 20% 29% 15% 3 1	198
January	1st to 10th	Night	22% 18% 37% 22% 1	760
	11th to 20th	Night	25% 17% 33% 21% 3	760
	21st to end	Night	19% 22% 33% 22% 4	836
February	1st to 10th	Night	22% 17% 39% 21% 2	760
	11th to 20th	Night	20% 20% 38% 19% 3	743
	21st to end	Night	22% 17% 37% 20% 4	628
March	1st to 10th	Night	26% 16% 34% 20% 3	760
	11th to 20th	Night	23% 18% 33% 21% 5%	760
	21st to end	Night	25% 16% 32% 21% 7%	836
April	1st to 10th	Night	25% 16% 31% 24% 4	760
	11th to 20th	Night	22% 17% 33% 22% 6%	709
	21st to end	Night	25% 16% 27% 25% 6%	570
May	1st to 10th	Night	25% 11% 30% 29% 5%	570
	11th to 20th	Night	22% 13% 32% 26% 8%	488
	21st to end	Night	19% 8% 25% 37% 10%	411
June	1st to 10th	Night	19% 7% 32% 31% 9% 1	380
	11th to 20th	Night	19% 8% 29% 31% 12% 1	380
	21st to end	Night	17% 8% 28% 39% 8%	380
July	1st to 10th	Night	15% 3 26% 42% 14%	366
	11th to 20th	Night	16% 2 22% 39% 20% 1	348
	21st to end	Night	14% 5% 22% 42% 16% 1	394
August	1st to 10th	Night	15% 4 21% 41% 19%	454
	11th to 20th	Night	15% 7% 27% 35% 15% 2	532
	21st to end	Night	16% 4 24% 36% 18% 2	774
September	1st to 10th	Night	15% 6% 28% 32% 18% 1	720
	11th to 20th	Night	15% 8% 25% 30% 17% 4	720
	21st to end	Night	16% 9% 30% 26% 17% 2	720
October	1st to 10th	Night	23% 11% 23% 27% 14% 1	720
	11th to 20th	Night	19% 13% 31% 23% 13% 1	720
	21st to end	Night	28% 17% 25% 24% 6%	792
November	1st to 10th	Night	19% 14% 31% 26% 8% 1	740
	11th to 20th	Night	22% 19% 29% 22% 6% 1	760
	21st to end	Night	22% 19% 31% 23% 4	760
December	1st to 10th	Night	25% 17% 34% 19% 5%	760
	11th to 20th	Night	24% 20% 31% 21% 3	760
	21st to end	Night	26% 19% 30% 22% 2	836

Page generated at 2024-07-05 11:05:35. Version ac07

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