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 2024-11-02.

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	19% 8% 2 6% 4 4 4 3 5% 7% 37%	101168
	Night	20% 9% 3 6% 4 4 4 3 5% 6% 36%	97813
Transparency	Day	46% 6% 13% 21% 13% 2	100930
	Night	49% 6% 12% 20% 12% 1	97672
Seeing	Day	25% 16% 32% 20% 7% 1	15789
	Night	26% 15% 28% 23% 8% 1	24095

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	18% 8% 2 7% 4 4 4 2 4 5% 40%	2517
	11th to 20th	Day	18% 8% 2 4 3 4 3 3 4 6% 44%	2520
	21st to end	Day	18% 10% 3 7% 4 3 3 3 5% 6% 37%	2870
February	1st to 10th	Day	17% 11% 3 6% 5% 3 4 5% 5% 7% 33%	1710
	11th to 20th	Day	14% 10% 2 8% 5% 4 5% 3 5% 6% 38%	1761
	21st to end	Day	14% 10% 3 8% 5% 5% 4 3 5% 5% 37%	1570
March	1st to 10th	Day	26% 11% 3 6% 5% 4 4 2 4 4 30%	3164
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 3 5% 6% 35%	3480
	21st to end	Day	28% 9% 3 6% 4 4 4 3 5% 5% 28%	3816
April	1st to 10th	Day	21% 10% 3 6% 4 4 4 3 4 6% 34%	2242
	11th to 20th	Day	20% 9% 2 6% 4 3 3 2 4 7% 39%	2423
	21st to end	Day	22% 9% 1 6% 4 4 4 3 4 6% 36%	2470
May	1st to 10th	Day	20% 9% 3 7% 4 3 4 4 6% 7% 32%	4106
	11th to 20th	Day	18% 7% 3 6% 4 4 4 4 6% 6% 36%	4081
	21st to end	Day	19% 7% 2 6% 4 4 4 4 6% 9% 35%	4882
June	1st to 10th	Day	23% 8% 2 7% 5% 4 4 3 6% 7% 32%	2850
	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	18% 7% 2 6% 5% 3 3 4 5% 6% 42%	1681
	11th to 20th	Day	21% 6% 2 4 3 3 4 4 5% 7% 41%	1677
	21st to end	Day	20% 9% 2 7% 4 3 2 2 4 5% 41%	1520
December	1st to 10th	Day	23% 9% 2 6% 4 4 4 3 4 6% 34%	1449
	11th to 20th	Day	24% 9% 2 6% 4 3 3 4 4 6% 36%	1668
	21st to end	Day	25% 10% 2 6% 3 4 3 3 4 6% 34%	2471
January	1st to 10th	Night	15% 9% 3 8% 4 4 3 3 4 6% 40%	4429
	11th to 20th	Night	14% 10% 3 9% 5% 4 4 3 5% 6% 37%	4440
	21st to end	Night	17% 12% 3 8% 4 4 4 3 5% 7% 33%	4784
February	1st to 10th	Night	18% 10% 3 7% 5% 4 5% 4 5% 6% 34%	2470
	11th to 20th	Night	14% 9% 3 8% 5% 4 3 4 5% 6% 38%	2303
	21st to end	Night	15% 9% 3 7% 5% 4 3 4 4 5% 39%	1884
March	1st to 10th	Night	29% 12% 3 6% 4 3 3 2 3 4 32%	3675
	11th to 20th	Night	15% 10% 3 8% 5% 5% 6% 5% 7% 7% 28%	3479
	21st to end	Night	28% 9% 3 6% 4 4 4 4 5% 5% 29%	3816
April	1st to 10th	Night	24% 10% 2 5% 5% 4 4 3 3 4 34%	1935
	11th to 20th	Night	21% 8% 3 7% 4 4 4 3 5% 7% 35%	1704
	21st to end	Night	24% 9% 2 6% 4 4 3 3 4 6% 35%	1710
May	1st to 10th	Night	24% 8% 3 6% 4 3 3 4 5% 8% 32%	2834
	11th to 20th	Night	19% 7% 3 7% 5% 6% 5% 4 7% 9% 28%	2813
	21st to end	Night	16% 8% 4 8% 5% 5% 5% 3 4 7% 35%	2723
June	1st to 10th	Night	25% 10% 2 6% 4 3 5% 3 6% 6% 30%	1330
	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% 7% 2 5% 5% 4 5% 3 4 6% 42%	2431
	11th to 20th	Night	19% 7% 2 5% 4 4 4 3 5% 6% 42%	2501
	21st to end	Night	18% 7% 3 5% 4 4 4 3 5% 6% 40%	2660
December	1st to 10th	Night	24% 9% 3 6% 4 4 4 3 5% 6% 32%	2726
	11th to 20th	Night	24% 9% 2 4 3 3 3 3 4 6% 39%	3531
	21st to end	Night	25% 9% 3 5% 3 3 3 3 4 5% 36%	5190

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% 14% 24% 9% 1	2518
	11th to 20th	Day	42% 6% 12% 25% 12% 2	2520
	21st to end	Day	48% 5% 14% 20% 12% 1	2870
February	1st to 10th	Day	50% 6% 12% 17% 13% 2	1710
	11th to 20th	Day	48% 5% 12% 19% 14% 1	1769
	21st to end	Day	48% 5% 12% 20% 10% 5%	1570
March	1st to 10th	Day	57% 4 9% 13% 13% 3	3168
	11th to 20th	Day	48% 5% 14% 19% 12% 2	3480
	21st to end	Day	56% 4 7% 14% 15% 4	3816
April	1st to 10th	Day	51% 5% 9% 17% 15% 4	2242
	11th to 20th	Day	45% 5% 11% 22% 15% 3	2423
	21st to end	Day	49% 4 9% 18% 16% 3	2470
May	1st to 10th	Day	49% 8% 14% 20% 8% 1	4120
	11th to 20th	Day	45% 7% 16% 22% 9% 1	4085
	21st to end	Day	45% 9% 14% 17% 14% 1	4882
June	1st to 10th	Day	51% 8% 12% 19% 9% 1	2850
	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	43% 6% 12% 23% 15% 2	2030
	11th to 20th	Day	42% 5% 12% 28% 12%	1900
	21st to end	Day	47% 4 12% 21% 14% 1	1886
November	1st to 10th	Day	44% 5% 14% 22% 14% 1	1681
	11th to 20th	Day	42% 6% 10% 22% 18% 3	1677
	21st to end	Day	47% 4 16% 22% 10% 1	1520
December	1st to 10th	Day	52% 6% 13% 19% 10% 1	1449
	11th to 20th	Day	51% 5% 11% 18% 13% 2	1668
	21st to end	Day	53% 6% 12% 18% 8% 3	2471
January	1st to 10th	Night	45% 5% 14% 25% 10% 2	4429
	11th to 20th	Night	49% 5% 14% 20% 11% 2	4440
	21st to end	Night	51% 6% 11% 21% 10% 1	4784
February	1st to 10th	Night	52% 5% 12% 17% 12% 2	2470
	11th to 20th	Night	47% 5% 11% 20% 15% 2	2306
	21st to end	Night	46% 5% 10% 20% 14% 4	1884
March	1st to 10th	Night	59% 3 8% 15% 12% 4	3684
	11th to 20th	Night	49% 7% 16% 15% 10% 2	3480
	21st to end	Night	58% 3 7% 14% 15% 2	3816
April	1st to 10th	Night	54% 4 8% 14% 16% 3	1935
	11th to 20th	Night	49% 5% 9% 20% 15% 2	1704
	21st to end	Night	51% 4 10% 17% 15% 3	1710
May	1st to 10th	Night	49% 8% 15% 19% 8% 1	2840
	11th to 20th	Night	48% 10% 16% 17% 7% 1	2814
	21st to end	Night	47% 9% 13% 20% 11% 1	2724
June	1st to 10th	Night	54% 7% 10% 21% 8% 1	1330
	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	44% 5% 11% 23% 14% 2	2149
	11th to 20th	Night	42% 5% 12% 27% 14% 1	2280
	21st to end	Night	50% 4 10% 21% 13% 1	2707
November	1st to 10th	Night	43% 6% 12% 23% 15% 1	2431
	11th to 20th	Night	44% 6% 11% 22% 17% 2	2501
	21st to end	Night	45% 6% 13% 19% 15% 2	2660
December	1st to 10th	Night	54% 5% 10% 20% 11%	2726
	11th to 20th	Night	48% 5% 11% 20% 15% 1	3532
	21st to end	Night	51% 6% 13% 18% 11% 2	5190

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	29% 24% 29% 14% 3	180
	11th to 20th	Day	37% 18% 32% 10% 2 1	180
	21st to end	Day	25% 22% 35% 16% 2	257
February	1st to 10th	Day	29% 24% 29% 14% 4	370
	11th to 20th	Day	26% 32% 31% 9% 2	413
	21st to end	Day	27% 22% 31% 17% 2	382
March	1st to 10th	Day	28% 21% 34% 15% 2	460
	11th to 20th	Day	26% 25% 30% 16% 3	460
	21st to end	Day	32% 21% 27% 14% 6% 1	506
April	1st to 10th	Day	33% 20% 31% 10% 4 1	460
	11th to 20th	Day	27% 21% 33% 13% 5%	452
	21st to end	Day	25% 18% 37% 15% 4 1	460
May	1st to 10th	Day	28% 18% 32% 16% 4 1	463
	11th to 20th	Day	18% 20% 35% 21% 4 1	487
	21st to end	Day	25% 14% 37% 17% 7% 1	610
June	1st to 10th	Day	24% 13% 32% 21% 8% 1	680
	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% 30% 17% 6% 1	429
November	1st to 10th	Day	25% 18% 30% 20% 5% 1	369
	11th to 20th	Day	30% 18% 32% 15% 3 1	370
	21st to end	Day	32% 21% 28% 14% 4	370
December	1st to 10th	Day	28% 19% 34% 13% 4 1	299
	11th to 20th	Day	35% 30% 26% 8% 2	180
	21st to end	Day	40% 20% 24% 15% 2	198
January	1st to 10th	Night	26% 24% 32% 16% 3	760
	11th to 20th	Night	30% 22% 30% 14% 3	760
	21st to end	Night	21% 27% 30% 18% 4	836
February	1st to 10th	Night	28% 21% 34% 15% 3	760
	11th to 20th	Night	23% 29% 30% 15% 3 1	743
	21st to end	Night	26% 19% 33% 18% 4	628
March	1st to 10th	Night	27% 19% 35% 18% 2	760
	11th to 20th	Night	24% 20% 32% 19% 6%	760
	21st to end	Night	25% 17% 32% 20% 5% 1	836
April	1st to 10th	Night	32% 14% 32% 18% 4	760
	11th to 20th	Night	25% 13% 33% 23% 5%	580
	21st to end	Night	28% 14% 27% 24% 7% 1	570
May	1st to 10th	Night	27% 9% 31% 25% 8% 1	570
	11th to 20th	Night	25% 8% 33% 26% 7% 1	562
	21st to end	Night	26% 7% 27% 30% 9% 1	488
June	1st to 10th	Night	30% 8% 23% 30% 8% 2	380
	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	26% 15% 26% 24% 7% 1	748
	11th to 20th	Night	28% 20% 26% 19% 7% 1	775
	21st to end	Night	27% 22% 29% 16% 6%	850
December	1st to 10th	Night	31% 19% 33% 13% 4	850
	11th to 20th	Night	31% 24% 27% 15% 3	850
	21st to end	Night	31% 21% 30% 14% 4	920

Page generated at 2024-10-30 13:19:22. Version ac07

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