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 2024-01-14.

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	13% 8% 4 7% 6% 3 5% 5% 5% 7% 37%	74745
	Night	13% 9% 4 7% 6% 3 4 4 5% 7% 39%	75040
Transparency	Day	46% 6% 13% 23% 11% 1	74445
	Night	45% 6% 13% 23% 12% 1	74860
Seeing	Day	17% 10% 30% 26% 13% 4	14821
	Night	17% 9% 30% 32% 11% 1	24839

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	10% 11% 7% 7% 9% 4 6% 4 5% 5% 32%	1517
	11th to 20th	Day	9% 17% 7% 11% 8% 4 5% 5% 5% 5% 23%	1464
	21st to end	Day	10% 14% 7% 10% 10% 4 5% 4 6% 7% 24%	1749
February	1st to 10th	Day	8% 11% 5% 9% 8% 4 4 6% 6% 7% 32%	1620
	11th to 20th	Day	10% 11% 6% 9% 5% 3 5% 4 5% 7% 35%	1574
	21st to end	Day	14% 12% 5% 8% 7% 3 5% 4 5% 7% 31%	1440
March	1st to 10th	Day	14% 10% 5% 8% 6% 4 5% 4 4 5% 35%	1876
	11th to 20th	Day	15% 10% 4 6% 6% 3 4 4 5% 7% 35%	1980
	21st to end	Day	20% 11% 4 5% 5% 3 4 4 6% 7% 31%	2240
April	1st to 10th	Day	18% 10% 4 7% 6% 2 4 4 4 7% 34%	2160
	11th to 20th	Day	21% 11% 4 6% 5% 2 4 5% 4 7% 32%	2134
	21st to end	Day	20% 11% 3 7% 6% 4 5% 4 5% 6% 29%	2275
May	1st to 10th	Day	17% 7% 3 8% 7% 3 5% 5% 6% 7% 32%	2484
	11th to 20th	Day	16% 8% 4 5% 6% 3 5% 4 5% 7% 36%	2479
	21st to end	Day	15% 5% 3 6% 6% 4 5% 5% 5% 6% 39%	2725
June	1st to 10th	Day	12% 4 3 5% 4 3 5% 5% 5% 9% 45%	2520
	11th to 20th	Day	13% 7% 3 6% 5% 3 5% 5% 6% 9% 39%	2520
	21st to end	Day	10% 4 2 6% 6% 4 5% 6% 7% 9% 41%	2520
July	1st to 10th	Day	7% 5% 1 4 6% 3 5% 6% 6% 10% 47%	2520
	11th to 20th	Day	8% 4 1 4 5% 3 5% 6% 6% 8% 50%	2520
	21st to end	Day	7% 3 3 4 5% 4 6% 5% 8% 10% 47%	2743
August	1st to 10th	Day	9% 4 2 5% 6% 4 5% 6% 9% 9% 41%	2358
	11th to 20th	Day	13% 5% 3 6% 5% 3 5% 5% 5% 7% 43%	2301
	21st to end	Day	9% 4 2 5% 4 3 4 4 6% 8% 50%	2411
September	1st to 10th	Day	13% 5% 3 5% 4 2 4 4 5% 8% 47%	2160
	11th to 20th	Day	12% 5% 2 6% 5% 3 4 4 7% 7% 45%	2160
	21st to end	Day	14% 7% 2 5% 4 3 5% 5% 6% 8% 40%	1892
October	1st to 10th	Day	17% 5% 3 5% 4 2 5% 4 5% 6% 43%	1800
	11th to 20th	Day	15% 10% 3 6% 5% 3 4 4 5% 6% 37%	1800
	21st to end	Day	19% 12% 4 7% 4 2 3 4 4 6% 36%	1911
November	1st to 10th	Day	12% 9% 3 7% 7% 3 4 5% 5% 7% 37%	1631
	11th to 20th	Day	15% 13% 6% 8% 7% 3 5% 4 4 7% 29%	1520
	21st to end	Day	14% 10% 4 8% 5% 3 4 4 6% 6% 37%	1520
December	1st to 10th	Day	11% 13% 5% 11% 9% 4 6% 5% 4 5% 27%	1520
	11th to 20th	Day	11% 12% 5% 9% 7% 4 5% 5% 5% 7% 28%	1918
	21st to end	Day	20% 15% 8% 14% 7% 3 4 3 2 4 19%	2783
January	1st to 10th	Night	9% 11% 6% 9% 9% 4 5% 4 5% 6% 33%	2648
	11th to 20th	Night	12% 11% 6% 11% 9% 4 5% 5% 5% 5% 27%	2569
	21st to end	Night	13% 14% 6% 11% 7% 4 5% 5% 5% 5% 25%	2606
February	1st to 10th	Night	6% 11% 6% 10% 7% 5% 6% 4 5% 6% 33%	2340
	11th to 20th	Night	8% 9% 4 9% 6% 4 5% 5% 5% 7% 39%	2273
	21st to end	Night	14% 9% 4 7% 6% 4 5% 5% 5% 5% 36%	1816
March	1st to 10th	Night	13% 10% 4 6% 6% 3 4 4 5% 7% 37%	2098
	11th to 20th	Night	13% 10% 4 7% 5% 3 5% 4 4 7% 39%	1980
	21st to end	Night	18% 11% 4 7% 6% 3 4 3 4 8% 34%	2115
April	1st to 10th	Night	16% 10% 4 6% 4 3 4 4 5% 7% 37%	1800
	11th to 20th	Night	24% 9% 4 6% 4 1 6% 4 4 5% 33%	1775
	21st to end	Night	19% 8% 3 6% 5% 3 3 4 4 7% 37%	1683
May	1st to 10th	Night	20% 7% 3 5% 5% 3 4 5% 5% 7% 37%	1476
	11th to 20th	Night	18% 5% 2 5% 4 3 4 4 5% 8% 42%	1416
	21st to end	Night	13% 7% 3 5% 6% 3 6% 5% 5% 8% 40%	1551
June	1st to 10th	Night	10% 5% 2 5% 5% 3 4 6% 6% 8% 46%	1440
	11th to 20th	Night	12% 6% 3 5% 6% 2 4 4 5% 7% 46%	1440
	21st to end	Night	7% 4 2 5% 5% 3 5% 5% 6% 8% 51%	1440
July	1st to 10th	Night	6% 2 2 3 3 2 5% 4 7% 10% 57%	1440
	11th to 20th	Night	6% 3 2 4 4 2 4 5% 6% 10% 55%	1440
	21st to end	Night	5% 3 2 3 3 2 4 4 6% 10% 58%	1575
August	1st to 10th	Night	7% 4 2 4 4 3 5% 5% 6% 9% 50%	1578
	11th to 20th	Night	12% 4 2 4 4 2 3 5% 4 7% 53%	1600
	21st to end	Night	9% 4 2 4 5% 3 4 5% 4 7% 53%	1944
September	1st to 10th	Night	10% 5% 2 4 5% 3 4 4 4 6% 53%	1800
	11th to 20th	Night	11% 4 2 4 4 3 3 4 5% 8% 52%	1800
	21st to end	Night	14% 5% 2 4 5% 3 3 4 5% 6% 49%	2046
October	1st to 10th	Night	14% 6% 2 4 4 3 4 4 4 7% 48%	2160
	11th to 20th	Night	16% 8% 3 6% 4 3 3 4 5% 7% 41%	2160
	21st to end	Night	18% 9% 4 6% 5% 3 4 5% 4 6% 37%	2444
November	1st to 10th	Night	11% 9% 4 6% 5% 3 3 4 4 7% 43%	2444
	11th to 20th	Night	13% 10% 5% 8% 7% 4 5% 4 4 6% 34%	2660
	21st to end	Night	16% 9% 4 8% 6% 3 3 4 4 6% 37%	2660
December	1st to 10th	Night	12% 11% 5% 9% 7% 4 5% 5% 6% 7% 30%	2660
	11th to 20th	Night	12% 12% 5% 10% 8% 4 5% 4 5% 6% 28%	3279
	21st to end	Night	23% 15% 6% 10% 6% 3 4 3 4 5% 21%	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	55% 5% 14% 20% 6%	1518
	11th to 20th	Day	63% 5% 10% 13% 9%	1464
	21st to end	Day	59% 6% 14% 13% 7% 1	1749
February	1st to 10th	Day	51% 5% 15% 19% 9% 1	1620
	11th to 20th	Day	48% 6% 13% 23% 9% 1	1583
	21st to end	Day	54% 5% 10% 19% 11% 1	1440
March	1st to 10th	Day	51% 5% 14% 19% 9% 2	1876
	11th to 20th	Day	48% 5% 14% 21% 11% 1	1980
	21st to end	Day	53% 6% 12% 20% 8% 1	2240
April	1st to 10th	Day	51% 6% 11% 21% 11% 1	2160
	11th to 20th	Day	53% 4 9% 20% 12% 2	2134
	21st to end	Day	56% 6% 12% 17% 9% 1	2275
May	1st to 10th	Day	49% 7% 13% 21% 9% 1	2484
	11th to 20th	Day	46% 6% 13% 21% 12% 1	2483
	21st to end	Day	45% 7% 10% 23% 13% 2	2725
June	1st to 10th	Day	35% 7% 14% 29% 14% 2	2520
	11th to 20th	Day	41% 9% 13% 24% 13% 1	2520
	21st to end	Day	38% 8% 16% 26% 11% 2	2520
July	1st to 10th	Day	31% 8% 17% 29% 15% 1	2451
	11th to 20th	Day	30% 9% 14% 26% 19% 2	2445
	21st to end	Day	30% 9% 17% 28% 14% 2	2591
August	1st to 10th	Day	35% 9% 15% 30% 9% 2	2343
	11th to 20th	Day	40% 7% 13% 28% 11%	2301
	21st to end	Day	32% 6% 13% 29% 18% 1	2411
September	1st to 10th	Day	36% 6% 14% 29% 14% 1	2160
	11th to 20th	Day	37% 7% 15% 32% 9% 1	2160
	21st to end	Day	42% 6% 12% 31% 9%	1892
October	1st to 10th	Day	40% 6% 13% 28% 12% 1	1800
	11th to 20th	Day	45% 7% 14% 22% 12% 1	1800
	21st to end	Day	50% 5% 12% 24% 9%	1911
November	1st to 10th	Day	46% 6% 13% 19% 14% 2	1631
	11th to 20th	Day	56% 7% 11% 17% 8% 1	1520
	21st to end	Day	48% 5% 18% 21% 8% 1	1520
December	1st to 10th	Day	59% 7% 13% 15% 6%	1520
	11th to 20th	Day	54% 8% 17% 16% 5%	1915
	21st to end	Day	71% 4 8% 13% 4	2783
January	1st to 10th	Night	53% 5% 13% 20% 9% 1	2648
	11th to 20th	Night	58% 6% 14% 16% 6%	2569
	21st to end	Night	60% 5% 12% 16% 7% 1	2606
February	1st to 10th	Night	51% 5% 12% 22% 9% 1	2340
	11th to 20th	Night	45% 5% 14% 26% 9% 1	2274
	21st to end	Night	50% 5% 11% 22% 10% 2	1816
March	1st to 10th	Night	47% 7% 15% 23% 8% 1	2098
	11th to 20th	Night	46% 5% 12% 22% 13% 1	1980
	21st to end	Night	51% 5% 11% 22% 11% 1	2115
April	1st to 10th	Night	47% 5% 9% 21% 15% 2	1800
	11th to 20th	Night	52% 5% 8% 18% 14% 2	1775
	21st to end	Night	46% 6% 11% 20% 15% 2	1683
May	1st to 10th	Night	47% 6% 13% 20% 14% 1	1476
	11th to 20th	Night	39% 6% 13% 23% 16% 2	1417
	21st to end	Night	42% 8% 11% 21% 18% 1	1552
June	1st to 10th	Night	32% 8% 13% 29% 16% 2	1440
	11th to 20th	Night	38% 7% 13% 23% 18% 3	1440
	21st to end	Night	29% 8% 15% 30% 15% 3	1440
July	1st to 10th	Night	22% 6% 15% 36% 19% 2	1400
	11th to 20th	Night	24% 7% 17% 31% 19% 3	1392
	21st to end	Night	21% 7% 18% 31% 18% 5%	1487
August	1st to 10th	Night	30% 7% 15% 29% 16% 3	1569
	11th to 20th	Night	32% 7% 14% 28% 17% 1	1600
	21st to end	Night	30% 6% 15% 29% 17% 3	1944
September	1st to 10th	Night	32% 6% 10% 34% 18% 1	1800
	11th to 20th	Night	32% 6% 15% 31% 17% 1	1800
	21st to end	Night	36% 5% 13% 31% 14% 1	2046
October	1st to 10th	Night	37% 4 10% 33% 15% 2	2160
	11th to 20th	Night	43% 5% 13% 25% 13% 1	2160
	21st to end	Night	48% 6% 12% 21% 12% 1	2444
November	1st to 10th	Night	42% 5% 12% 24% 15% 2	2444
	11th to 20th	Night	53% 5% 12% 18% 10% 1	2660
	21st to end	Night	49% 5% 15% 23% 7% 1	2660
December	1st to 10th	Night	53% 7% 15% 20% 5%	2660
	11th to 20th	Night	56% 6% 14% 17% 6% 1	3281
	21st to end	Night	66% 5% 10% 14% 5%	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	16% 14% 32% 27% 10% 2	270
	11th to 20th	Day	23% 15% 34% 20% 6% 2	249
	21st to end	Day	19% 13% 36% 22% 8% 1	264
February	1st to 10th	Day	18% 15% 45% 16% 4 1	240
	11th to 20th	Day	17% 23% 37% 16% 7%	240
	21st to end	Day	23% 16% 40% 20% 1	198
March	1st to 10th	Day	19% 16% 38% 17% 9% 1	240
	11th to 20th	Day	22% 12% 36% 20% 7% 3	240
	21st to end	Day	27% 13% 29% 20% 10% 2	326
April	1st to 10th	Day	27% 13% 33% 21% 5% 1	420
	11th to 20th	Day	30% 14% 26% 22% 8% 1	412
	21st to end	Day	27% 11% 29% 22% 9% 2	535
May	1st to 10th	Day	20% 12% 33% 25% 9% 1	603
	11th to 20th	Day	21% 13% 26% 23% 14% 3	625
	21st to end	Day	18% 10% 27% 26% 15% 4	679
June	1st to 10th	Day	11% 8% 27% 33% 17% 4	630
	11th to 20th	Day	16% 7% 34% 29% 11% 3	630
	21st to end	Day	11% 6% 33% 28% 17% 6%	630
July	1st to 10th	Day	8% 3 21% 30% 26% 11%	621
	11th to 20th	Day	10% 6% 26% 32% 18% 9%	618
	21st to end	Day	6% 4 27% 37% 18% 8%	684
August	1st to 10th	Day	12% 3 21% 33% 25% 7%	626
	11th to 20th	Day	14% 5% 20% 30% 24% 7%	615
	21st to end	Day	13% 5% 24% 30% 21% 6%	530
September	1st to 10th	Day	14% 7% 27% 31% 15% 5%	450
	11th to 20th	Day	12% 5% 29% 31% 19% 4	450
	21st to end	Day	20% 11% 30% 25% 11% 3	312
October	1st to 10th	Day	19% 13% 29% 23% 14% 3	270
	11th to 20th	Day	22% 14% 35% 21% 7%	270
	21st to end	Day	30% 15% 32% 15% 5% 2	297
November	1st to 10th	Day	20% 13% 34% 23% 9% 1	270
	11th to 20th	Day	20% 17% 42% 17% 5%	270
	21st to end	Day	22% 13% 42% 16% 6%	270
December	1st to 10th	Day	18% 11% 40% 26% 5% 1	270
	11th to 20th	Day	19% 17% 39% 19% 6% 1	270
	21st to end	Day	26% 15% 40% 17% 2	297
January	1st to 10th	Night	15% 13% 35% 29% 8% 1	950
	11th to 20th	Night	18% 11% 35% 29% 7% 1	918
	21st to end	Night	19% 11% 34% 27% 7% 1	990
February	1st to 10th	Night	12% 14% 38% 29% 6% 1	900
	11th to 20th	Night	13% 14% 40% 25% 8%	879
	21st to end	Night	19% 16% 31% 28% 6%	632
March	1st to 10th	Night	17% 12% 35% 28% 8% 1	658
	11th to 20th	Night	21% 9% 35% 29% 7%	540
	21st to end	Night	22% 9% 31% 27% 9% 2	594
April	1st to 10th	Night	22% 12% 35% 26% 6%	540
	11th to 20th	Night	29% 9% 27% 28% 8%	533
	21st to end	Night	24% 8% 29% 29% 8% 1	540
May	1st to 10th	Night	22% 8% 32% 31% 6% 1	540
	11th to 20th	Night	20% 6% 29% 34% 10% 2	531
	21st to end	Night	17% 5% 24% 39% 14% 1	584
June	1st to 10th	Night	13% 4 25% 44% 15%	540
	11th to 20th	Night	15% 3 30% 40% 11%	540
	21st to end	Night	9% 4 21% 47% 18% 1	540
July	1st to 10th	Night	6% 2 22% 41% 27% 2	525
	11th to 20th	Night	7% 4 21% 46% 19% 3	522
	21st to end	Night	6% 2 18% 50% 21% 2	591
August	1st to 10th	Night	7% 2 15% 47% 26% 2	537
	11th to 20th	Night	13% 3 16% 37% 27% 4	534
	21st to end	Night	10% 3 22% 36% 26% 2	594
September	1st to 10th	Night	12% 4 23% 38% 18% 5%	540
	11th to 20th	Night	11% 3 28% 36% 18% 3	540
	21st to end	Night	16% 10% 28% 30% 15% 1	652
October	1st to 10th	Night	17% 8% 27% 31% 15% 2	720
	11th to 20th	Night	20% 12% 31% 30% 7%	720
	21st to end	Night	22% 10% 27% 32% 7% 1	792
November	1st to 10th	Night	17% 10% 34% 30% 9% 1	778
	11th to 20th	Night	19% 13% 36% 25% 7%	950
	21st to end	Night	20% 14% 33% 27% 6%	950
December	1st to 10th	Night	18% 10% 36% 28% 7% 1	950
	11th to 20th	Night	22% 10% 34% 26% 7% 1	950
	21st to end	Night	27% 12% 31% 25% 5%	1045

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