Brooklin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brooklin Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	18% 8% 2 6% 4 4 4 3 5% 7% 38%	148897
	Night	19% 9% 3 6% 4 4 4 3 5% 6% 37%	142879
Transparency	Day	46% 5% 13% 22% 13% 2	152421
	Night	48% 4 11% 21% 14% 2	147445
Seeing	Day	24% 15% 33% 20% 7% 1	18361
	Night	25% 15% 29% 23% 8% 1	26604

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	15% 9% 3 8% 5% 5% 5% 3 6% 6% 37%	4661
	11th to 20th	Day	17% 11% 3 5% 3 4 3 3 4 6% 41%	4650
	21st to end	Day	11% 10% 3 6% 3 4 4 3 5% 6% 45%	5320
February	1st to 10th	Day	16% 11% 3 6% 5% 4 5% 5% 5% 7% 34%	1890
	11th to 20th	Day	14% 9% 2 7% 5% 4 5% 3 6% 6% 38%	1952
	21st to end	Day	14% 10% 3 8% 5% 4 4 3 5% 5% 38%	1730
March	1st to 10th	Day	25% 10% 2 6% 5% 5% 4 3 4 5% 30%	4639
	11th to 20th	Day	21% 10% 3 6% 4 4 4 3 5% 5% 35%	5090
	21st to end	Day	29% 8% 3 6% 4 3 4 3 5% 6% 29%	5587
April	1st to 10th	Day	22% 10% 3 6% 4 3 4 3 4 6% 35%	2478
	11th to 20th	Day	20% 9% 2 6% 4 3 3 2 4 6% 38%	2681
	21st to end	Day	22% 9% 2 6% 4 4 4 3 5% 6% 36%	2730
May	1st to 10th	Day	28% 11% 3 7% 4 3 4 3 5% 6% 28%	7639
	11th to 20th	Day	17% 8% 3 8% 5% 5% 4 4 5% 7% 35%	7625
	21st to end	Day	19% 8% 3 7% 4 5% 4 4 5% 7% 33%	9133
June	1st to 10th	Day	22% 8% 2 7% 5% 4 4 3 6% 7% 33%	3150
	11th to 20th	Day	19% 8% 2 6% 4 4 3 4 4 7% 39%	3150
	21st to end	Day	25% 8% 2 6% 4 4 3 4 5% 7% 32%	3150
July	1st to 10th	Day	16% 8% 2 7% 5% 4 5% 4 7% 9% 33%	8503
	11th to 20th	Day	14% 8% 2 5% 3 3 4 4 6% 9% 42%	8280
	21st to end	Day	17% 6% 2 5% 3 3 4 3 5% 8% 43%	8763
August	1st to 10th	Day	16% 7% 3 7% 4 4 5% 4 6% 8% 36%	6015
	11th to 20th	Day	17% 7% 3 7% 5% 4 5% 5% 6% 8% 35%	5953
	21st to end	Day	13% 5% 2 5% 3 3 4 3 6% 8% 48%	6594
September	1st to 10th	Day	13% 5% 2 5% 4 3 4 3 5% 8% 49%	2403
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 4 5% 54%	2310
	21st to end	Day	18% 6% 2 5% 4 3 4 3 4 8% 43%	2310
October	1st to 10th	Day	16% 6% 2 5% 4 3 4 4 4 7% 43%	2244
	11th to 20th	Day	19% 6% 2 6% 3 4 4 3 5% 7% 42%	2030
	21st to end	Day	22% 9% 3 5% 4 4 3 3 5% 6% 38%	1985
November	1st to 10th	Day	17% 7% 2 6% 5% 4 3 4 5% 7% 41%	1851
	11th to 20th	Day	20% 6% 2 5% 3 3 4 4 5% 7% 42%	1853
	21st to end	Day	21% 9% 2 7% 4 4 3 2 4 5% 40%	1680
December	1st to 10th	Day	18% 10% 2 4 3 3 3 3 4 7% 43%	2559
	11th to 20th	Day	20% 8% 2 5% 3 3 3 3 4 5% 43%	2688
	21st to end	Day	20% 12% 3 6% 3 3 3 2 4 5% 37%	3621
January	1st to 10th	Night	12% 11% 4 8% 5% 4 4 3 4 6% 38%	8234
	11th to 20th	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 32%	8235
	21st to end	Night	11% 11% 3 6% 4 4 4 4 5% 7% 41%	8859
February	1st to 10th	Night	17% 10% 3 7% 5% 4 5% 4 5% 6% 35%	2730
	11th to 20th	Night	13% 9% 3 7% 5% 4 4 4 5% 6% 39%	2551
	21st to end	Night	15% 9% 3 7% 5% 4 3 4 5% 5% 40%	2076
March	1st to 10th	Night	27% 10% 2 7% 5% 3 3 3 4 5% 32%	5420
	11th to 20th	Night	18% 11% 3 7% 5% 4 5% 4 6% 7% 31%	5089
	21st to end	Night	27% 9% 2 6% 4 4 4 4 5% 6% 30%	5587
April	1st to 10th	Night	24% 11% 2 6% 5% 4 4 3 3 5% 34%	2139
	11th to 20th	Night	21% 8% 3 7% 4 4 4 3 5% 7% 34%	1886
	21st to end	Night	24% 9% 2 6% 4 4 4 3 4 6% 35%	1890
May	1st to 10th	Night	30% 9% 3 6% 4 3 3 3 4 6% 29%	5259
	11th to 20th	Night	22% 8% 3 6% 4 4 4 3 6% 7% 32%	5235
	21st to end	Night	16% 10% 4 8% 5% 5% 4 3 5% 7% 34%	5094
June	1st to 10th	Night	23% 9% 2 6% 4 3 5% 4 6% 7% 31%	1470
	11th to 20th	Night	19% 8% 3 6% 4 3 5% 4 5% 7% 37%	1470
	21st to end	Night	26% 11% 2 5% 4 4 4 3 5% 6% 30%	1470
July	1st to 10th	Night	15% 9% 4 9% 6% 5% 5% 4 6% 8% 29%	4451
	11th to 20th	Night	18% 10% 2 5% 3 4 4 4 5% 8% 37%	4680
	21st to end	Night	17% 6% 2 5% 3 4 4 4 6% 8% 42%	5252
August	1st to 10th	Night	19% 10% 3 8% 4 4 4 4 5% 7% 31%	4135
	11th to 20th	Night	18% 9% 3 7% 4 4 4 4 6% 6% 34%	4092
	21st to end	Night	15% 6% 2 5% 4 4 5% 4 6% 8% 42%	4531
September	1st to 10th	Night	15% 6% 1 4 3 3 4 3 6% 7% 47%	2212
	11th to 20th	Night	15% 6% 2 5% 3 2 3 3 4 6% 51%	2310
	21st to end	Night	20% 5% 2 4 3 3 4 3 5% 7% 44%	2310
October	1st to 10th	Night	18% 6% 2 5% 4 3 4 3 5% 6% 43%	2375
	11th to 20th	Night	19% 6% 2 5% 3 4 4 3 4 6% 43%	2437
	21st to end	Night	23% 8% 2 5% 4 4 3 3 4 6% 37%	2850
November	1st to 10th	Night	19% 6% 2 5% 4 4 5% 3 4 6% 43%	2667
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	2765
	21st to end	Night	19% 8% 3 5% 4 4 4 3 5% 6% 40%	2940
December	1st to 10th	Night	19% 8% 3 5% 3 4 4 3 6% 7% 38%	4846
	11th to 20th	Night	23% 8% 2 4 3 3 3 3 4 5% 42%	5741
	21st to end	Night	23% 10% 3 6% 4 3 3 2 4 5% 38%	7591

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	49% 3 11% 25% 11% 1	4662
	11th to 20th	Day	47% 4 10% 24% 13% 2	4680
	21st to end	Day	42% 3 10% 22% 18% 5%	5320
February	1st to 10th	Day	49% 6% 11% 18% 13% 3	1890
	11th to 20th	Day	46% 5% 13% 20% 14% 2	1960
	21st to end	Day	47% 5% 11% 20% 12% 4	1730
March	1st to 10th	Day	57% 4 8% 10% 15% 6%	4643
	11th to 20th	Day	51% 4 16% 19% 10% 2	5090
	21st to end	Day	56% 3 6% 17% 15% 3	5587
April	1st to 10th	Day	51% 5% 10% 16% 15% 3	2478
	11th to 20th	Day	46% 4 11% 22% 15% 3	2681
	21st to end	Day	49% 4 9% 19% 16% 4	2730
May	1st to 10th	Day	58% 5% 12% 18% 7% 1	7651
	11th to 20th	Day	48% 5% 15% 21% 9% 2	7629
	21st to end	Day	49% 6% 12% 20% 13% 1	9134
June	1st to 10th	Day	50% 7% 12% 21% 10%	3150
	11th to 20th	Day	46% 6% 11% 20% 15% 2	3150
	21st to end	Day	51% 7% 12% 18% 10% 2	3150
July	1st to 10th	Day	48% 4 14% 24% 10%	8433
	11th to 20th	Day	40% 4 14% 25% 16% 1	8205
	21st to end	Day	40% 4 13% 28% 14% 1	8568
August	1st to 10th	Day	46% 4 19% 21% 8% 1	6002
	11th to 20th	Day	47% 5% 18% 20% 10%	5992
	21st to end	Day	35% 5% 15% 27% 18% 1	6594
September	1st to 10th	Day	34% 7% 13% 27% 18% 1	2403
	11th to 20th	Day	34% 5% 11% 26% 21% 3	2310
	21st to end	Day	41% 5% 10% 26% 17% 2	2310
October	1st to 10th	Day	46% 4 12% 23% 13% 1	3582
	11th to 20th	Day	34% 4 9% 33% 21%	3270
	21st to end	Day	35% 3 17% 23% 18% 3	3185
November	1st to 10th	Day	44% 5% 14% 22% 14% 1	1851
	11th to 20th	Day	42% 5% 9% 23% 18% 3	1853
	21st to end	Day	48% 4 15% 22% 9% 1	1680
December	1st to 10th	Day	43% 4 15% 24% 13% 1	2559
	11th to 20th	Day	46% 4 13% 16% 16% 6%	2688
	21st to end	Day	51% 4 11% 19% 12% 3	3621
January	1st to 10th	Night	47% 3 12% 24% 13% 1	8234
	11th to 20th	Night	55% 3 11% 18% 11% 1	8280
	21st to end	Night	43% 4 8% 24% 18% 4	8859
February	1st to 10th	Night	50% 5% 12% 18% 13% 2	2730
	11th to 20th	Night	46% 4 11% 21% 16% 2	2554
	21st to end	Night	46% 5% 10% 21% 14% 4	2076
March	1st to 10th	Night	57% 2 6% 13% 13% 8%	5429
	11th to 20th	Night	50% 5% 20% 15% 8% 1	5090
	21st to end	Night	56% 3 6% 16% 17% 2	5587
April	1st to 10th	Night	54% 4 8% 14% 16% 3	2139
	11th to 20th	Night	50% 4 10% 19% 15% 2	1886
	21st to end	Night	51% 4 9% 18% 14% 3	1890
May	1st to 10th	Night	57% 5% 12% 15% 11%	5265
	11th to 20th	Night	50% 6% 13% 21% 9% 1	5236
	21st to end	Night	49% 5% 11% 21% 12%	5095
June	1st to 10th	Night	52% 7% 9% 22% 8% 1	1470
	11th to 20th	Night	46% 6% 14% 20% 12% 2	1470
	21st to end	Night	55% 5% 9% 19% 9% 2	1470
July	1st to 10th	Night	53% 4 12% 22% 8%	4411
	11th to 20th	Night	48% 3 12% 25% 12%	4632
	21st to end	Night	41% 4 15% 27% 12% 1	5133
August	1st to 10th	Night	54% 4 13% 21% 8%	4123
	11th to 20th	Night	49% 4 16% 21% 9%	4119
	21st to end	Night	40% 5% 15% 23% 17% 1	4531
September	1st to 10th	Night	36% 6% 11% 29% 17% 1	2212
	11th to 20th	Night	36% 4 11% 26% 21% 2	2310
	21st to end	Night	41% 5% 9% 27% 17% 2	2310
October	1st to 10th	Night	42% 4 13% 24% 16% 1	3817
	11th to 20th	Night	35% 3 9% 29% 23% 1	3936
	21st to end	Night	40% 3 10% 29% 15% 3	4600
November	1st to 10th	Night	42% 6% 12% 23% 16% 1	2667
	11th to 20th	Night	43% 5% 11% 22% 16% 2	2765
	21st to end	Night	46% 6% 13% 19% 14% 1	2940
December	1st to 10th	Night	47% 4 10% 27% 12%	4846
	11th to 20th	Night	46% 3 12% 20% 18% 2	5742
	21st to end	Night	50% 4 11% 18% 16% 2	7591

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	28% 25% 30% 13% 3	220
	11th to 20th	Day	34% 20% 33% 11% 2 1	220
	21st to end	Day	22% 24% 36% 15% 2	308
February	1st to 10th	Day	27% 23% 30% 15% 4	430
	11th to 20th	Day	25% 30% 31% 11% 2 1	484
	21st to end	Day	26% 22% 33% 16% 3	446
March	1st to 10th	Day	28% 22% 34% 14% 1	540
	11th to 20th	Day	27% 23% 31% 15% 3	540
	21st to end	Day	32% 22% 27% 14% 5% 1	594
April	1st to 10th	Day	33% 21% 31% 10% 4 1	540
	11th to 20th	Day	28% 20% 35% 12% 4	532
	21st to end	Day	25% 17% 36% 16% 4 1	540
May	1st to 10th	Day	31% 17% 33% 15% 3 1	543
	11th to 20th	Day	19% 20% 35% 20% 4 2	567
	21st to end	Day	25% 14% 37% 17% 6% 1	706
June	1st to 10th	Day	23% 12% 33% 23% 8% 1	780
	11th to 20th	Day	24% 12% 31% 21% 10% 2	780
	21st to end	Day	27% 8% 31% 25% 8% 1	780
July	1st to 10th	Day	22% 7% 35% 25% 9% 1	645
	11th to 20th	Day	20% 7% 35% 27% 9% 2	564
	21st to end	Day	19% 7% 35% 27% 9% 3	620
August	1st to 10th	Day	16% 7% 32% 30% 13% 2	566
	11th to 20th	Day	17% 8% 30% 29% 14% 4	559
	21st to end	Day	16% 10% 33% 28% 10% 3	627
September	1st to 10th	Day	16% 7% 38% 26% 10% 2	570
	11th to 20th	Day	17% 11% 31% 23% 10% 7%	570
	21st to end	Day	21% 10% 32% 23% 10% 4	570
October	1st to 10th	Day	19% 15% 28% 25% 11% 2	531
	11th to 20th	Day	23% 15% 32% 20% 9% 1	429
	21st to end	Day	30% 16% 31% 17% 5% 1	462
November	1st to 10th	Day	26% 18% 30% 20% 5% 1	425
	11th to 20th	Day	28% 18% 34% 14% 3 1	430
	21st to end	Day	33% 20% 29% 14% 4	430
December	1st to 10th	Day	28% 20% 33% 15% 5% 1	351
	11th to 20th	Day	32% 30% 26% 10% 2	220
	21st to end	Day	37% 19% 26% 15% 2	242
January	1st to 10th	Night	26% 23% 32% 16% 3	840
	11th to 20th	Night	30% 21% 30% 15% 4	840
	21st to end	Night	21% 27% 30% 18% 3	924
February	1st to 10th	Night	26% 22% 33% 16% 3	840
	11th to 20th	Night	22% 28% 30% 15% 3 1	823
	21st to end	Night	25% 18% 34% 19% 4	692
March	1st to 10th	Night	26% 19% 35% 18% 2	840
	11th to 20th	Night	24% 20% 31% 19% 6%	840
	21st to end	Night	25% 17% 32% 20% 5% 1	924
April	1st to 10th	Night	32% 14% 33% 16% 4	840
	11th to 20th	Night	26% 14% 33% 23% 4	642
	21st to end	Night	28% 14% 27% 23% 7% 1	630
May	1st to 10th	Night	28% 9% 31% 24% 7% 1	630
	11th to 20th	Night	26% 8% 33% 26% 7%	622
	21st to end	Night	25% 7% 28% 30% 9% 1	540
June	1st to 10th	Night	28% 7% 21% 33% 8% 2	420
	11th to 20th	Night	23% 5% 25% 33% 13% 1	420
	21st to end	Night	34% 4 17% 33% 12% 1	420
July	1st to 10th	Night	23% 4 22% 39% 12%	410
	11th to 20th	Night	23% 1 23% 37% 15% 1	408
	21st to end	Night	20% 3 23% 39% 13% 2	513
August	1st to 10th	Night	21% 4 23% 37% 14% 1	628
	11th to 20th	Night	20% 4 22% 39% 13% 1	622
	21st to end	Night	19% 4 23% 35% 18% 1	693
September	1st to 10th	Night	17% 4 29% 34% 15% 2	835
	11th to 20th	Night	19% 8% 25% 27% 17% 4	840
	21st to end	Night	22% 8% 27% 28% 13% 2	840
October	1st to 10th	Night	22% 11% 25% 25% 15% 1	840
	11th to 20th	Night	22% 15% 29% 22% 11% 1	813
	21st to end	Night	29% 16% 30% 20% 5%	880
November	1st to 10th	Night	26% 16% 27% 23% 7% 1	820
	11th to 20th	Night	27% 19% 28% 19% 7% 1	859
	21st to end	Night	28% 22% 29% 15% 6%	950
December	1st to 10th	Night	30% 18% 33% 14% 4	950
	11th to 20th	Night	31% 25% 27% 14% 3	950
	21st to end	Night	30% 21% 30% 15% 3	1026

Page generated at 2026-10-11 05:06:32. Version ac07

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