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 2025-06-10.

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	20% 8% 3 6% 4 4 4 3 5% 6% 37%	118833
	Night	20% 9% 3 6% 4 4 4 3 5% 6% 36%	119413
Transparency	Day	47% 5% 12% 21% 13% 2	122400
	Night	48% 5% 11% 21% 13% 2	124008
Seeing	Day	25% 16% 32% 19% 7% 1	16524
	Night	25% 15% 29% 23% 8% 1	24939

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	16% 8% 3 8% 5% 5% 5% 3 5% 6% 37%	3597
	11th to 20th	Day	15% 9% 3 5% 3 4 3 3 5% 6% 44%	3570
	21st to end	Day	14% 10% 3 7% 4 3 3 3 5% 6% 43%	4114
February	1st to 10th	Day	17% 11% 3 6% 5% 4 4 5% 5% 7% 34%	1800
	11th to 20th	Day	15% 9% 2 7% 5% 4 5% 3 6% 6% 38%	1857
	21st to end	Day	14% 10% 3 8% 5% 4 4 3 5% 5% 38%	1650
March	1st to 10th	Day	25% 10% 2 6% 5% 5% 4 3 4 5% 30%	4538
	11th to 20th	Day	21% 10% 3 6% 4 4 4 3 4 5% 35%	4980
	21st to end	Day	29% 8% 3 6% 4 3 4 3 5% 6% 29%	5466
April	1st to 10th	Day	22% 10% 3 6% 4 4 4 3 4 6% 34%	2360
	11th to 20th	Day	20% 9% 2 6% 4 3 3 3 4 6% 39%	2552
	21st to end	Day	22% 9% 1 6% 4 4 4 3 4 6% 37%	2600
May	1st to 10th	Day	30% 11% 4 7% 4 3 3 3 5% 5% 25%	5886
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 5% 6% 33%	5861
	21st to end	Day	21% 8% 3 6% 4 4 4 4 6% 8% 32%	7008
June	1st to 10th	Day	23% 8% 2 7% 5% 4 4 3 6% 7% 32%	2985
	11th to 20th	Day	20% 8% 2 6% 4 4 3 3 4 7% 39%	2850
	21st to end	Day	26% 8% 2 6% 4 4 3 4 5% 6% 31%	2850
July	1st to 10th	Day	18% 7% 3 7% 5% 4 5% 3 6% 7% 35%	4551
	11th to 20th	Day	17% 5% 2 5% 3 3 4 4 6% 9% 41%	4440
	21st to end	Day	22% 8% 2 5% 3 4 4 3 5% 8% 37%	4696
August	1st to 10th	Day	19% 7% 3 7% 4 3 4 3 6% 8% 35%	4105
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 5% 6% 7% 35%	4045
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 8% 49%	4532
September	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 4 9% 50%	2175
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 4 5% 54%	2090
	21st to end	Day	18% 7% 2 5% 4 3 4 3 4 8% 42%	2090
October	1st to 10th	Day	17% 7% 2 6% 4 4 4 4 4 7% 41%	2030
	11th to 20th	Day	19% 5% 2 6% 3 4 4 3 5% 7% 42%	1900
	21st to end	Day	22% 8% 3 5% 3 4 3 3 5% 6% 38%	1886
November	1st to 10th	Day	17% 7% 2 6% 5% 3 3 4 5% 6% 42%	1761
	11th to 20th	Day	20% 6% 2 4 3 3 4 4 5% 7% 41%	1765
	21st to end	Day	21% 9% 2 7% 4 3 2 2 4 5% 41%	1600
December	1st to 10th	Day	18% 10% 2 4 2 3 4 3 4 7% 43%	2483
	11th to 20th	Day	21% 8% 2 5% 3 3 3 3 4 5% 43%	2618
	21st to end	Day	21% 12% 3 6% 3 3 3 2 4 5% 37%	3542
January	1st to 10th	Night	12% 10% 3 8% 5% 4 4 3 5% 7% 39%	6349
	11th to 20th	Night	14% 11% 3 9% 5% 4 5% 3 5% 6% 34%	6315
	21st to end	Night	12% 10% 3 7% 4 4 4 4 5% 7% 39%	6840
February	1st to 10th	Night	18% 10% 3 7% 5% 4 5% 4 5% 6% 34%	2600
	11th to 20th	Night	14% 9% 3 7% 5% 4 4 4 5% 6% 38%	2427
	21st to end	Night	15% 9% 3 7% 5% 4 3 4 4 5% 40%	1980
March	1st to 10th	Night	27% 10% 2 7% 5% 3 3 3 4 5% 31%	5301
	11th to 20th	Night	18% 11% 3 7% 5% 4 5% 4 6% 7% 31%	4979
	21st to end	Night	27% 8% 2 5% 4 4 4 4 5% 6% 30%	5466
April	1st to 10th	Night	24% 10% 2 6% 5% 4 4 3 3 5% 34%	2037
	11th to 20th	Night	21% 8% 3 7% 4 4 4 3 6% 7% 34%	1795
	21st to end	Night	24% 9% 2 6% 4 4 3 3 4 6% 34%	1800
May	1st to 10th	Night	30% 11% 4 7% 4 3 3 3 4 6% 25%	4054
	11th to 20th	Night	23% 8% 3 7% 4 5% 4 3 6% 7% 30%	4033
	21st to end	Night	17% 10% 4 9% 5% 5% 4 3 5% 7% 31%	3902
June	1st to 10th	Night	24% 9% 2 6% 4 4 5% 3 6% 7% 30%	1399
	11th to 20th	Night	19% 8% 3 5% 4 3 5% 4 5% 7% 37%	1330
	21st to end	Night	27% 11% 2 5% 4 4 4 3 4 5% 30%	1330
July	1st to 10th	Night	18% 9% 4 7% 5% 5% 5% 4 7% 8% 27%	2395
	11th to 20th	Night	20% 8% 2 5% 4 5% 5% 4 5% 8% 34%	2520
	21st to end	Night	18% 8% 2 5% 4 4 4 4 6% 7% 36%	2823
August	1st to 10th	Night	24% 10% 3 8% 4 4 5% 4 5% 7% 28%	2825
	11th to 20th	Night	19% 9% 3 8% 4 4 5% 5% 6% 7% 31%	2782
	21st to end	Night	11% 6% 2 5% 4 4 5% 4 6% 8% 46%	3124
September	1st to 10th	Night	15% 6% 1 4 3 3 4 3 6% 7% 48%	2000
	11th to 20th	Night	16% 6% 3 5% 2 2 2 3 4 6% 50%	2090
	21st to end	Night	19% 5% 2 4 3 3 4 3 5% 7% 45%	2090
October	1st to 10th	Night	20% 6% 2 5% 4 3 4 3 5% 6% 41%	2149
	11th to 20th	Night	18% 6% 2 5% 3 4 4 3 5% 6% 43%	2280
	21st to end	Night	24% 8% 2 5% 4 4 3 2 4 6% 38%	2707
November	1st to 10th	Night	18% 6% 2 5% 5% 4 5% 3 5% 6% 43%	2537
	11th to 20th	Night	18% 6% 2 5% 4 4 4 3 5% 6% 42%	2633
	21st to end	Night	19% 7% 3 5% 4 4 4 3 5% 6% 40%	2800
December	1st to 10th	Night	19% 9% 3 5% 3 4 4 3 6% 7% 38%	4702
	11th to 20th	Night	24% 8% 2 4 3 3 3 3 4 5% 42%	5591
	21st to end	Night	23% 10% 3 6% 4 3 3 2 4 5% 38%	7428

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% 4 11% 27% 9% 1	3598
	11th to 20th	Day	42% 5% 13% 26% 12% 2	3600
	21st to end	Day	44% 4 10% 19% 17% 6%	4114
February	1st to 10th	Day	49% 6% 12% 17% 13% 3	1800
	11th to 20th	Day	47% 5% 13% 19% 14% 2	1865
	21st to end	Day	48% 5% 11% 20% 11% 5%	1650
March	1st to 10th	Day	57% 4 8% 10% 15% 6%	4542
	11th to 20th	Day	51% 4 16% 18% 10% 2	4980
	21st to end	Day	56% 3 6% 17% 15% 3	5466
April	1st to 10th	Day	51% 5% 9% 16% 15% 4	2360
	11th to 20th	Day	46% 5% 11% 22% 14% 3	2552
	21st to end	Day	49% 4 9% 19% 16% 4	2600
May	1st to 10th	Day	60% 6% 11% 16% 6% 1	5897
	11th to 20th	Day	51% 6% 12% 21% 7% 2	5865
	21st to end	Day	49% 7% 13% 18% 12% 1	7008
June	1st to 10th	Day	51% 8% 12% 20% 10% 1	2985
	11th to 20th	Day	46% 6% 11% 20% 15% 2	2850
	21st to end	Day	52% 8% 11% 17% 10% 2	2850
July	1st to 10th	Day	48% 6% 11% 24% 11%	4481
	11th to 20th	Day	40% 7% 17% 25% 11%	4365
	21st to end	Day	47% 6% 12% 22% 12% 1	4546
August	1st to 10th	Day	48% 5% 15% 25% 6%	4092
	11th to 20th	Day	48% 6% 17% 20% 9%	4084
	21st to end	Day	31% 6% 14% 27% 21% 1	4532
September	1st to 10th	Day	33% 7% 13% 27% 19% 1	2175
	11th to 20th	Day	34% 6% 12% 27% 19% 3	2090
	21st to end	Day	42% 5% 10% 26% 16% 1	2090
October	1st to 10th	Day	48% 4 12% 22% 12% 1	3368
	11th to 20th	Day	33% 4 9% 33% 22%	3140
	21st to end	Day	35% 3 18% 23% 18% 3	3086
November	1st to 10th	Day	43% 5% 14% 23% 14% 1	1761
	11th to 20th	Day	42% 5% 9% 22% 18% 3	1765
	21st to end	Day	48% 4 16% 22% 9% 1	1600
December	1st to 10th	Day	43% 4 15% 24% 13% 1	2483
	11th to 20th	Day	46% 4 13% 16% 16% 6%	2618
	21st to end	Day	51% 4 11% 19% 12% 3	3542
January	1st to 10th	Night	45% 4 13% 26% 11% 1	6349
	11th to 20th	Night	51% 4 13% 21% 9% 1	6360
	21st to end	Night	45% 4 8% 22% 16% 4	6840
February	1st to 10th	Night	52% 5% 12% 16% 13% 2	2600
	11th to 20th	Night	46% 4 12% 20% 15% 3	2430
	21st to end	Night	46% 5% 10% 20% 14% 4	1980
March	1st to 10th	Night	58% 2 6% 13% 13% 8%	5310
	11th to 20th	Night	50% 5% 21% 15% 8% 1	4980
	21st to end	Night	55% 3 6% 16% 17% 2	5466
April	1st to 10th	Night	54% 4 8% 14% 16% 3	2037
	11th to 20th	Night	49% 5% 10% 20% 15% 2	1795
	21st to end	Night	52% 4 9% 17% 14% 3	1800
May	1st to 10th	Night	60% 6% 11% 15% 8%	4060
	11th to 20th	Night	51% 7% 14% 19% 8% 1	4034
	21st to end	Night	51% 7% 12% 20% 10%	3903
June	1st to 10th	Night	54% 6% 10% 20% 8% 1	1399
	11th to 20th	Night	46% 7% 14% 20% 11% 2	1330
	21st to end	Night	57% 5% 9% 18% 9% 2	1330
July	1st to 10th	Night	54% 6% 11% 17% 11% 1	2355
	11th to 20th	Night	49% 5% 14% 24% 9%	2472
	21st to end	Night	47% 6% 13% 22% 12% 1	2733
August	1st to 10th	Night	57% 5% 12% 19% 7%	2813
	11th to 20th	Night	52% 6% 14% 20% 9%	2809
	21st to end	Night	36% 5% 12% 24% 21% 1	3124
September	1st to 10th	Night	36% 7% 12% 28% 17% 1	2000
	11th to 20th	Night	37% 4 12% 26% 19% 2	2090
	21st to end	Night	40% 5% 9% 28% 16% 1	2090
October	1st to 10th	Night	43% 4 13% 24% 15% 1	3591
	11th to 20th	Night	34% 3 10% 29% 23% 1	3779
	21st to end	Night	40% 3 10% 29% 15% 3	4457
November	1st to 10th	Night	42% 6% 12% 24% 15% 1	2537
	11th to 20th	Night	43% 5% 11% 22% 17% 2	2633
	21st to end	Night	46% 6% 13% 20% 14% 1	2800
December	1st to 10th	Night	47% 4 10% 28% 12%	4702
	11th to 20th	Night	46% 3 12% 20% 18% 2	5592
	21st to end	Night	50% 4 11% 18% 16% 1	7428

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	30% 25% 28% 15% 3	200
	11th to 20th	Day	35% 18% 33% 12% 2 1	200
	21st to end	Day	24% 23% 35% 16% 2	283
February	1st to 10th	Day	29% 24% 30% 14% 4	400
	11th to 20th	Day	27% 31% 31% 9% 2	449
	21st to end	Day	26% 22% 33% 16% 3	414
March	1st to 10th	Day	29% 22% 33% 14% 2	500
	11th to 20th	Day	27% 24% 30% 16% 3	500
	21st to end	Day	32% 20% 28% 14% 5% 1	550
April	1st to 10th	Day	33% 21% 31% 10% 4 1	500
	11th to 20th	Day	28% 21% 34% 13% 4	492
	21st to end	Day	25% 18% 37% 15% 4 1	500
May	1st to 10th	Day	31% 17% 32% 15% 4 1	503
	11th to 20th	Day	19% 20% 34% 21% 4 2	527
	21st to end	Day	26% 14% 36% 17% 7% 1	658
June	1st to 10th	Day	24% 12% 33% 22% 8% 1	725
	11th to 20th	Day	23% 11% 31% 22% 11% 2	680
	21st to end	Day	29% 8% 30% 24% 8% 2	680
July	1st to 10th	Day	24% 7% 34% 24% 10% 1	557
	11th to 20th	Day	19% 5% 35% 29% 10% 2	484
	21st to end	Day	19% 6% 36% 27% 10% 2	532
August	1st to 10th	Day	17% 7% 32% 29% 12% 1	486
	11th to 20th	Day	18% 8% 30% 28% 13% 3	479
	21st to end	Day	16% 10% 32% 28% 11% 2	539
September	1st to 10th	Day	16% 8% 37% 27% 10% 2	490
	11th to 20th	Day	18% 12% 30% 22% 11% 7%	490
	21st to end	Day	21% 10% 32% 23% 10% 4	490
October	1st to 10th	Day	19% 16% 28% 25% 11% 2	457
	11th to 20th	Day	23% 15% 32% 20% 9% 2	390
	21st to end	Day	30% 16% 31% 17% 6% 1	429
November	1st to 10th	Day	25% 18% 30% 21% 5% 1	395
	11th to 20th	Day	29% 19% 34% 15% 3 1	400
	21st to end	Day	33% 21% 28% 14% 4	400
December	1st to 10th	Day	28% 19% 33% 15% 5% 1	325
	11th to 20th	Day	33% 30% 27% 9% 3	200
	21st to end	Day	38% 18% 25% 16% 2	220
January	1st to 10th	Night	26% 24% 32% 15% 3	800
	11th to 20th	Night	30% 22% 30% 15% 4	800
	21st to end	Night	21% 27% 30% 18% 4	880
February	1st to 10th	Night	28% 21% 33% 15% 3	800
	11th to 20th	Night	23% 29% 31% 15% 2 1	783
	21st to end	Night	25% 19% 33% 19% 4	660
March	1st to 10th	Night	27% 19% 34% 18% 2	800
	11th to 20th	Night	24% 19% 32% 19% 6%	800
	21st to end	Night	25% 17% 32% 20% 5% 1	880
April	1st to 10th	Night	32% 14% 32% 17% 4	800
	11th to 20th	Night	25% 13% 34% 23% 5%	611
	21st to end	Night	28% 15% 27% 24% 7% 1	600
May	1st to 10th	Night	28% 9% 31% 25% 7% 1	600
	11th to 20th	Night	26% 8% 33% 26% 7% 1	592
	21st to end	Night	25% 7% 27% 31% 9% 1	514
June	1st to 10th	Night	30% 8% 22% 31% 9% 2	400
	11th to 20th	Night	23% 6% 24% 33% 13% 1	380
	21st to end	Night	35% 4 17% 33% 10% 1	380
July	1st to 10th	Night	24% 4 21% 38% 13%	370
	11th to 20th	Night	23% 2 22% 37% 15% 1	368
	21st to end	Night	21% 3 22% 40% 13% 2	463
August	1st to 10th	Night	21% 5% 24% 36% 13% 1	568
	11th to 20th	Night	21% 4 22% 38% 13% 1	562
	21st to end	Night	19% 4 22% 35% 19% 1	627
September	1st to 10th	Night	17% 4 29% 34% 14% 2	755
	11th to 20th	Night	21% 8% 25% 26% 16% 4	760
	21st to end	Night	21% 8% 28% 27% 14% 3	760
October	1st to 10th	Night	24% 10% 24% 26% 15% 1	760
	11th to 20th	Night	21% 16% 29% 22% 11% 1	760
	21st to end	Night	30% 16% 29% 19% 5%	836
November	1st to 10th	Night	25% 15% 28% 24% 7% 1	780
	11th to 20th	Night	27% 19% 28% 19% 6% 1	817
	21st to end	Night	28% 21% 29% 15% 6%	900
December	1st to 10th	Night	30% 18% 34% 14% 5%	900
	11th to 20th	Night	31% 24% 27% 15% 3	900
	21st to end	Night	30% 21% 30% 15% 4	973

Page generated at 2025-06-07 10:28:16. Version ac07

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