Sunny Brook observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunny Brook observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	13% 9% 3 8% 5% 4 4 4 5% 7% 38%	127794
	Night	12% 9% 4 9% 5% 4 4 4 5% 6% 38%	123843
Transparency	Day	45% 4 12% 26% 12% 1	131333
	Night	46% 4 12% 24% 13% 1	128378
Seeing	Day	17% 11% 30% 26% 13% 3	17577
	Night	17% 9% 30% 32% 11% 1	27511

By Month

Percentage Cloud Cover SE (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	8% 14% 7% 17% 7% 6% 4 2 4 5% 27%	3625
	11th to 20th	Day	6% 14% 7% 11% 6% 6% 8% 9% 8% 5% 21%	3620
	21st to end	Day	8% 16% 7% 11% 6% 3 3 3 4 6% 33%	4413
February	1st to 10th	Day	8% 12% 5% 10% 7% 5% 4 5% 6% 7% 32%	1800
	11th to 20th	Day	9% 11% 6% 9% 4 4 4 3 6% 7% 37%	1754
	21st to end	Day	12% 11% 4 9% 5% 4 4 3 5% 7% 33%	1605
March	1st to 10th	Day	14% 7% 2 5% 3 3 3 3 4 7% 48%	4682
	11th to 20th	Day	11% 9% 3 7% 4 4 4 4 6% 8% 41%	4980
	21st to end	Day	23% 11% 3 9% 5% 4 4 3 5% 5% 27%	5635
April	1st to 10th	Day	18% 12% 3 8% 5% 3 4 3 5% 6% 34%	2400
	11th to 20th	Day	20% 12% 3 7% 5% 3 4 3 4 6% 31%	2374
	21st to end	Day	20% 11% 3 8% 5% 5% 4 3 5% 6% 30%	2529
May	1st to 10th	Day	14% 6% 2 6% 4 4 3 3 4 6% 48%	6228
	11th to 20th	Day	14% 7% 3 6% 4 4 4 3 5% 7% 41%	6319
	21st to end	Day	19% 7% 2 7% 4 4 4 3 5% 6% 38%	6935
June	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 5% 9% 45%	2800
	11th to 20th	Day	14% 8% 3 6% 5% 5% 4 4 6% 9% 36%	2800
	21st to end	Day	12% 5% 2 7% 5% 5% 5% 5% 7% 8% 40%	2800
July	1st to 10th	Day	9% 5% 2 5% 5% 4 5% 5% 7% 10% 43%	6360
	11th to 20th	Day	8% 4 1 5% 3 3 4 4 6% 8% 55%	6360
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 9% 44%	6924
August	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 7% 7% 41%	4279
	11th to 20th	Day	12% 6% 2 6% 5% 5% 4 4 5% 7% 44%	4171
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 6% 8% 44%	4363
September	1st to 10th	Day	12% 6% 2 5% 4 3 4 3 5% 8% 48%	2400
	11th to 20th	Day	12% 6% 2 7% 4 4 4 4 7% 7% 43%	2400
	21st to end	Day	13% 7% 3 6% 4 3 5% 4 6% 7% 43%	2101
October	1st to 10th	Day	16% 5% 3 5% 3 3 4 3 5% 6% 46%	2000
	11th to 20th	Day	14% 11% 3 7% 4 5% 4 3 5% 6% 38%	2000
	21st to end	Day	17% 13% 4 8% 3 3 3 3 4 6% 37%	2123
November	1st to 10th	Day	12% 10% 3 9% 6% 4 4 4 5% 7% 37%	1806
	11th to 20th	Day	13% 14% 5% 9% 6% 4 4 4 4 6% 30%	1680
	21st to end	Day	13% 11% 4 10% 4 4 4 3 6% 7% 34%	1680
December	1st to 10th	Day	7% 10% 4 14% 11% 8% 6% 4 6% 5% 25%	2690
	11th to 20th	Day	12% 16% 5% 10% 6% 5% 5% 3 5% 6% 27%	3088
	21st to end	Day	19% 19% 8% 17% 6% 4 4 2 3 3 15%	4070
January	1st to 10th	Night	6% 13% 7% 15% 8% 5% 3 2 5% 5% 31%	6363
	11th to 20th	Night	7% 11% 5% 13% 7% 7% 9% 8% 6% 4 21%	6360
	21st to end	Night	9% 17% 5% 10% 5% 4 4 3 5% 5% 32%	6564
February	1st to 10th	Night	7% 12% 6% 12% 6% 6% 5% 3 5% 6% 33%	2600
	11th to 20th	Night	7% 9% 4 10% 6% 5% 5% 4 5% 7% 39%	2533
	21st to end	Night	13% 9% 3 8% 5% 5% 5% 3 5% 5% 37%	2025
March	1st to 10th	Night	11% 7% 3 6% 4 4 4 3 5% 7% 46%	5199
	11th to 20th	Night	10% 7% 2 8% 4 5% 5% 4 6% 9% 39%	4980
	21st to end	Night	25% 12% 3 10% 4 3 3 2 4 5% 28%	5293
April	1st to 10th	Night	16% 11% 4 6% 4 4 4 4 5% 7% 37%	2000
	11th to 20th	Night	22% 10% 3 6% 3 3 5% 3 5% 5% 35%	1975
	21st to end	Night	19% 8% 3 6% 5% 4 3 4 5% 7% 38%	1869
May	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 7% 47%	3674
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 7% 51%	3596
	21st to end	Night	15% 9% 3 7% 5% 4 4 4 4 6% 38%	3942
June	1st to 10th	Night	11% 5% 2 5% 5% 4 4 5% 6% 8% 45%	1600
	11th to 20th	Night	12% 7% 3 6% 5% 4 4 3 6% 7% 44%	1600
	21st to end	Night	8% 4 2 6% 5% 4 4 4 6% 8% 49%	1600
July	1st to 10th	Night	8% 3 1 5% 4 4 4 4 7% 9% 49%	3618
	11th to 20th	Night	6% 4 2 4 3 3 4 4 6% 9% 55%	3617
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 7% 9% 49%	3947
August	1st to 10th	Night	12% 5% 2 6% 4 5% 4 3 6% 7% 47%	2877
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 7% 51%	2888
	21st to end	Night	9% 6% 2 7% 5% 4 4 4 5% 7% 47%	3535
September	1st to 10th	Night	10% 5% 2 4 5% 4 4 3 4 6% 54%	2000
	11th to 20th	Night	10% 5% 2 6% 3 4 3 3 5% 8% 51%	2000
	21st to end	Night	13% 5% 2 5% 4 3 3 3 5% 5% 51%	2275
October	1st to 10th	Night	13% 6% 2 5% 3 4 3 4 4 7% 50%	2400
	11th to 20th	Night	16% 9% 3 6% 3 4 3 3 5% 7% 42%	2400
	21st to end	Night	18% 9% 3 6% 5% 3 4 3 4 6% 37%	2716
November	1st to 10th	Night	12% 10% 4 7% 4 4 3 3 4 7% 42%	2708
	11th to 20th	Night	12% 12% 4 9% 6% 5% 5% 3 4 6% 34%	2940
	21st to end	Night	15% 10% 4 10% 5% 4 4 3 4 6% 36%	2940
December	1st to 10th	Night	8% 9% 4 13% 8% 7% 5% 4 7% 7% 29%	4720
	11th to 20th	Night	11% 17% 7% 13% 6% 5% 5% 4 5% 4 24%	5339
	21st to end	Night	22% 16% 7% 15% 5% 4 3 3 3 4 17%	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	63% 2 10% 19% 6%	3626
	11th to 20th	Day	60% 5% 7% 16% 12%	3620
	21st to end	Day	54% 4 15% 19% 8%	4413
February	1st to 10th	Day	51% 5% 14% 18% 10% 1	1800
	11th to 20th	Day	47% 6% 12% 26% 9% 1	1763
	21st to end	Day	51% 5% 12% 21% 11% 1	1605
March	1st to 10th	Day	37% 3 17% 26% 14% 3	4682
	11th to 20th	Day	40% 3 15% 29% 11% 2	4980
	21st to end	Day	60% 4 10% 16% 11% 1	5637
April	1st to 10th	Day	51% 5% 10% 20% 12% 1	2400
	11th to 20th	Day	54% 4 10% 19% 12% 2	2374
	21st to end	Day	55% 5% 12% 18% 9% 1	2529
May	1st to 10th	Day	37% 4 11% 30% 16% 2	6267
	11th to 20th	Day	41% 5% 11% 25% 14% 3	6323
	21st to end	Day	47% 5% 8% 29% 11% 1	6949
June	1st to 10th	Day	35% 7% 13% 29% 15% 1	2800
	11th to 20th	Day	43% 8% 13% 23% 12% 1	2800
	21st to end	Day	40% 7% 15% 24% 11% 2	2800
July	1st to 10th	Day	36% 3 17% 33% 10% 1	6291
	11th to 20th	Day	29% 4 14% 36% 17% 1	6285
	21st to end	Day	38% 4 13% 28% 16% 1	6728
August	1st to 10th	Day	39% 5% 12% 28% 14% 1	4264
	11th to 20th	Day	40% 4 11% 33% 11%	4210
	21st to end	Day	38% 4 14% 30% 13% 1	4362
September	1st to 10th	Day	35% 6% 14% 31% 14% 1	2400
	11th to 20th	Day	38% 6% 16% 31% 8% 1	2400
	21st to end	Day	40% 5% 11% 32% 12%	2101
October	1st to 10th	Day	28% 3 8% 37% 24%	3260
	11th to 20th	Day	39% 4 11% 25% 19% 2	3240
	21st to end	Day	43% 3 13% 31% 10%	3413
November	1st to 10th	Day	47% 5% 13% 19% 13% 2	1806
	11th to 20th	Day	55% 6% 10% 18% 8% 1	1680
	21st to end	Day	50% 5% 16% 20% 7% 1	1680
December	1st to 10th	Day	60% 5% 13% 15% 6%	2690
	11th to 20th	Day	58% 5% 12% 19% 6%	3085
	21st to end	Day	77% 3 7% 10% 3	4070
January	1st to 10th	Night	57% 3 10% 22% 8%	6363
	11th to 20th	Night	62% 4 9% 13% 11%	6360
	21st to end	Night	54% 4 11% 22% 9%	6564
February	1st to 10th	Night	51% 5% 12% 21% 10% 1	2600
	11th to 20th	Night	45% 5% 14% 26% 9% 1	2534
	21st to end	Night	48% 5% 12% 23% 10% 2	2025
March	1st to 10th	Night	39% 4 14% 26% 14% 4	5199
	11th to 20th	Night	42% 4 15% 25% 14% 1	4980
	21st to end	Night	60% 3 10% 16% 11%	5293
April	1st to 10th	Night	47% 5% 10% 21% 16% 2	2000
	11th to 20th	Night	51% 5% 9% 18% 15% 2	1975
	21st to end	Night	46% 6% 11% 20% 15% 2	1869
May	1st to 10th	Night	38% 5% 10% 24% 22% 1	3698
	11th to 20th	Night	34% 3 12% 25% 21% 6%	3597
	21st to end	Night	46% 6% 8% 27% 12%	3950
June	1st to 10th	Night	33% 8% 13% 28% 16% 2	1600
	11th to 20th	Night	39% 6% 13% 22% 18% 2	1600
	21st to end	Night	32% 7% 15% 29% 15% 3	1600
July	1st to 10th	Night	32% 3 19% 32% 12% 1	3580
	11th to 20th	Night	27% 3 15% 37% 16% 1	3572
	21st to end	Night	32% 4 16% 29% 17% 2	3832
August	1st to 10th	Night	38% 4 12% 29% 15% 3	2868
	11th to 20th	Night	34% 5% 13% 31% 16% 2	2915
	21st to end	Night	37% 4 15% 28% 14% 2	3535
September	1st to 10th	Night	31% 5% 11% 35% 17% 1	2000
	11th to 20th	Night	32% 5% 16% 31% 16%	2000
	21st to end	Night	34% 5% 12% 32% 16% 1	2275
October	1st to 10th	Night	26% 2 6% 36% 26% 4	3920
	11th to 20th	Night	34% 3 11% 28% 22% 2	3899
	21st to end	Night	46% 3 11% 26% 14%	4376
November	1st to 10th	Night	42% 5% 12% 25% 14% 1	2708
	11th to 20th	Night	52% 5% 12% 19% 10% 2	2940
	21st to end	Night	51% 4 15% 22% 7% 1	2940
December	1st to 10th	Night	54% 5% 17% 20% 4	4720
	11th to 20th	Night	63% 4 10% 16% 6%	5341
	21st to end	Night	72% 3 7% 12% 5%	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	14% 14% 34% 26% 10% 2	303
	11th to 20th	Day	19% 15% 39% 21% 5% 1	300
	21st to end	Day	18% 15% 33% 24% 9% 1	330
February	1st to 10th	Day	19% 16% 44% 15% 4 1	300
	11th to 20th	Day	15% 24% 37% 18% 6%	300
	21st to end	Day	20% 15% 43% 20% 1	249
March	1st to 10th	Day	19% 15% 40% 18% 8% 1	300
	11th to 20th	Day	22% 13% 37% 20% 6% 2	300
	21st to end	Day	28% 12% 30% 21% 8% 2	400
April	1st to 10th	Day	27% 14% 33% 21% 5% 1	500
	11th to 20th	Day	30% 15% 26% 21% 7% 1	492
	21st to end	Day	28% 13% 29% 21% 8% 1	629
May	1st to 10th	Day	20% 12% 31% 26% 10% 1	703
	11th to 20th	Day	21% 13% 24% 25% 14% 3	725
	21st to end	Day	19% 10% 27% 25% 14% 4	789
June	1st to 10th	Day	12% 8% 27% 32% 17% 4	730
	11th to 20th	Day	18% 8% 33% 27% 11% 2	730
	21st to end	Day	14% 7% 34% 26% 15% 5%	730
July	1st to 10th	Day	9% 3 21% 30% 26% 10%	721
	11th to 20th	Day	10% 7% 25% 31% 18% 9%	718
	21st to end	Day	6% 3 27% 38% 18% 7%	794
August	1st to 10th	Day	12% 4 22% 32% 23% 7%	726
	11th to 20th	Day	14% 5% 21% 31% 23% 6%	715
	21st to end	Day	12% 6% 26% 30% 20% 6%	621
September	1st to 10th	Day	12% 8% 28% 30% 15% 6%	530
	11th to 20th	Day	11% 7% 28% 32% 18% 3	530
	21st to end	Day	18% 10% 28% 24% 15% 5%	376
October	1st to 10th	Day	17% 14% 30% 24% 12% 2	330
	11th to 20th	Day	23% 15% 36% 18% 6% 1	330
	21st to end	Day	27% 16% 33% 17% 5% 2	363
November	1st to 10th	Day	20% 16% 34% 22% 8% 1	330
	11th to 20th	Day	21% 18% 38% 18% 5%	330
	21st to end	Day	22% 15% 41% 17% 5%	330
December	1st to 10th	Day	17% 13% 40% 25% 5% 1	330
	11th to 20th	Day	20% 18% 38% 18% 5% 1	330
	21st to end	Day	26% 15% 38% 17% 3 1	363
January	1st to 10th	Night	14% 14% 35% 29% 8% 1	1006
	11th to 20th	Night	17% 12% 36% 28% 7% 1	1000
	21st to end	Night	19% 11% 35% 28% 7%	1100
February	1st to 10th	Night	13% 14% 37% 29% 6% 1	1000
	11th to 20th	Night	12% 14% 40% 25% 8%	979
	21st to end	Night	18% 15% 33% 28% 6%	705
March	1st to 10th	Night	17% 11% 36% 28% 7% 1	730
	11th to 20th	Night	20% 10% 35% 29% 6%	600
	21st to end	Night	23% 9% 31% 27% 8% 2	660
April	1st to 10th	Night	22% 12% 34% 26% 6%	600
	11th to 20th	Night	28% 9% 28% 28% 7%	593
	21st to end	Night	24% 8% 30% 29% 8% 1	600
May	1st to 10th	Night	21% 8% 32% 31% 7% 1	600
	11th to 20th	Night	20% 6% 28% 34% 11% 1	591
	21st to end	Night	18% 5% 25% 39% 13% 1	650
June	1st to 10th	Night	14% 4 26% 43% 14%	600
	11th to 20th	Night	16% 4 31% 39% 11%	600
	21st to end	Night	10% 4 22% 46% 17% 1	600
July	1st to 10th	Night	6% 2 21% 42% 27% 2	585
	11th to 20th	Night	7% 4 21% 46% 20% 3	582
	21st to end	Night	7% 2 18% 49% 22% 2	657
August	1st to 10th	Night	8% 3 14% 48% 25% 2	597
	11th to 20th	Night	13% 3 16% 39% 27% 4	594
	21st to end	Night	10% 3 21% 38% 26% 2	660
September	1st to 10th	Night	12% 5% 24% 37% 18% 4	600
	11th to 20th	Night	11% 3 27% 37% 20% 3	600
	21st to end	Night	14% 10% 26% 32% 17% 1	725
October	1st to 10th	Night	16% 8% 28% 32% 15% 2	800
	11th to 20th	Night	20% 12% 32% 30% 7%	800
	21st to end	Night	22% 10% 28% 32% 7%	880
November	1st to 10th	Night	17% 10% 34% 29% 8% 1	862
	11th to 20th	Night	19% 14% 35% 25% 7%	1050
	21st to end	Night	20% 15% 33% 26% 6%	1050
December	1st to 10th	Night	18% 10% 36% 28% 7% 1	1050
	11th to 20th	Night	21% 12% 34% 25% 7% 1	1050
	21st to end	Night	27% 12% 30% 26% 5%	1155

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