York University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the York University Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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	15% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	144403
	Night	15% 10% 3 8% 5% 5% 5% 4 5% 7% 34%	137728
Transparency	Day	47% 7% 12% 21% 11% 1	147997
	Night	47% 7% 12% 21% 12% 1	142221
Seeing	Day	19% 11% 30% 24% 12% 5%	16833
	Night	19% 8% 30% 30% 12% 1	26065

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	17% 10% 4 13% 7% 5% 6% 4 5% 6% 22%	4661
	11th to 20th	Day	10% 18% 8% 17% 9% 7% 5% 3 4 4 14%	5103
	21st to end	Day	14% 15% 6% 14% 7% 6% 5% 4 5% 6% 19%	5780
February	1st to 10th	Day	15% 16% 4 12% 7% 6% 5% 4 4 6% 23%	1890
	11th to 20th	Day	13% 14% 6% 12% 7% 6% 4 3 5% 6% 24%	1843
	21st to end	Day	17% 14% 5% 9% 6% 5% 5% 4 5% 5% 25%	1616
March	1st to 10th	Day	19% 10% 4 9% 5% 4 4 4 5% 5% 32%	5042
	11th to 20th	Day	12% 12% 4 9% 5% 5% 5% 4 6% 8% 31%	5090
	21st to end	Day	18% 11% 3 9% 5% 5% 4 3 4 5% 33%	5586
April	1st to 10th	Day	18% 13% 4 8% 6% 4 5% 3 5% 7% 29%	2478
	11th to 20th	Day	20% 9% 3 8% 4 4 4 3 5% 6% 34%	2577
	21st to end	Day	20% 8% 3 7% 5% 4 5% 3 5% 6% 33%	2730
May	1st to 10th	Day	20% 9% 3 8% 4 4 4 3 5% 7% 33%	7642
	11th to 20th	Day	15% 9% 3 7% 5% 5% 5% 4 5% 8% 36%	8162
	21st to end	Day	18% 5% 2 5% 4 4 4 3 5% 7% 43%	9060
June	1st to 10th	Day	13% 7% 2 6% 6% 4 5% 4 6% 8% 38%	2940
	11th to 20th	Day	12% 4 2 7% 5% 4 5% 4 8% 11% 38%	2940
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	3053
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 10% 39%	8280
	11th to 20th	Day	7% 3 2 5% 4 4 5% 4 7% 11% 49%	8280
	21st to end	Day	9% 5% 2 6% 4 5% 6% 4 7% 9% 44%	8405
August	1st to 10th	Day	16% 6% 3 6% 4 4 5% 4 7% 8% 38%	4364
	11th to 20th	Day	10% 7% 3 9% 5% 5% 6% 5% 7% 10% 35%	4304
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 6% 9% 42%	4606
September	1st to 10th	Day	12% 6% 2 7% 4 4 5% 4 6% 7% 43%	2400
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 8% 46%	2280
	21st to end	Day	14% 6% 3 8% 5% 5% 5% 4 6% 9% 36%	2160
October	1st to 10th	Day	17% 9% 2 6% 4 4 4 5% 5% 7% 37%	2000
	11th to 20th	Day	20% 7% 3 8% 6% 5% 4 4 6% 7% 30%	2000
	21st to end	Day	23% 11% 3 8% 6% 6% 5% 5% 5% 6% 23%	2200
November	1st to 10th	Day	15% 9% 4 7% 6% 5% 5% 4 6% 7% 33%	1777
	11th to 20th	Day	15% 12% 4 10% 6% 5% 6% 4 6% 7% 26%	1680
	21st to end	Day	18% 15% 4 9% 6% 5% 6% 4 6% 7% 21%	1680
December	1st to 10th	Day	18% 17% 5% 10% 6% 7% 6% 3 5% 6% 16%	2680
	11th to 20th	Day	20% 16% 4 10% 6% 5% 4 3 5% 5% 22%	3066
	21st to end	Day	27% 17% 6% 12% 5% 4 4 3 4 4 14%	4048
January	1st to 10th	Night	14% 12% 5% 11% 6% 6% 6% 4 6% 6% 24%	8234
	11th to 20th	Night	12% 16% 6% 13% 10% 7% 5% 4 5% 5% 16%	7777
	21st to end	Night	13% 12% 5% 12% 7% 6% 6% 4 6% 7% 23%	8401
February	1st to 10th	Night	13% 15% 5% 10% 7% 5% 5% 4 5% 6% 27%	2730
	11th to 20th	Night	14% 14% 5% 10% 7% 5% 5% 4 6% 5% 26%	2663
	21st to end	Night	13% 12% 4 10% 6% 4 6% 4 5% 6% 29%	2183
March	1st to 10th	Night	16% 10% 4 8% 5% 6% 5% 4 5% 6% 31%	5018
	11th to 20th	Night	10% 9% 3 7% 5% 5% 5% 5% 7% 9% 36%	5089
	21st to end	Night	21% 8% 2 6% 4 4 4 3 5% 5% 39%	5588
April	1st to 10th	Night	18% 10% 3 8% 5% 4 5% 4 5% 6% 31%	2142
	11th to 20th	Night	16% 8% 3 7% 5% 5% 5% 3 5% 6% 38%	2009
	21st to end	Night	15% 8% 2 6% 4 4 5% 4 6% 7% 38%	1890
May	1st to 10th	Night	20% 7% 3 6% 5% 4 5% 3 5% 6% 36%	5257
	11th to 20th	Night	14% 8% 3 7% 4 5% 4 4 6% 7% 39%	4698
	21st to end	Night	18% 5% 2 5% 4 4 3 3 5% 6% 46%	5114
June	1st to 10th	Night	12% 6% 2 5% 5% 5% 4 4 6% 7% 44%	1680
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 5% 6% 10% 43%	1680
	21st to end	Night	10% 6% 2 5% 5% 4 5% 4 6% 7% 45%	1531
July	1st to 10th	Night	8% 5% 2 6% 4 3 4 4 6% 9% 49%	4640
	11th to 20th	Night	8% 4 1 4 3 4 4 4 6% 9% 52%	4680
	21st to end	Night	9% 4 2 5% 3 4 4 4 6% 8% 51%	5667
August	1st to 10th	Night	10% 5% 2 7% 5% 5% 4 3 6% 7% 47%	3005
	11th to 20th	Night	10% 4 2 5% 4 4 4 4 5% 8% 51%	2965
	21st to end	Night	10% 5% 2 6% 3 4 4 4 6% 8% 47%	3457
September	1st to 10th	Night	8% 5% 2 7% 4 4 4 4 6% 7% 49%	2000
	11th to 20th	Night	10% 4 1 4 3 3 3 3 5% 7% 55%	2117
	21st to end	Night	15% 6% 2 7% 5% 4 4 3 5% 6% 42%	2240
October	1st to 10th	Night	15% 7% 3 6% 4 4 4 4 6% 6% 42%	2400
	11th to 20th	Night	18% 8% 3 9% 5% 4 4 4 5% 7% 35%	2400
	21st to end	Night	19% 9% 2 8% 5% 4 5% 4 6% 6% 31%	2640
November	1st to 10th	Night	17% 8% 2 7% 5% 4 4 4 6% 6% 38%	2735
	11th to 20th	Night	16% 10% 3 9% 6% 5% 5% 4 5% 7% 30%	2940
	21st to end	Night	20% 12% 4 7% 6% 5% 4 4 5% 7% 25%	2940
December	1st to 10th	Night	17% 15% 5% 11% 6% 6% 5% 4 6% 6% 18%	4720
	11th to 20th	Night	16% 17% 4 11% 6% 5% 4 4 5% 6% 22%	5348
	21st to end	Night	25% 14% 6% 12% 6% 4 4 3 4 5% 16%	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	57% 8% 13% 13% 9%	4662
	11th to 20th	Day	70% 6% 9% 11% 4	5137
	21st to end	Day	62% 6% 12% 11% 8%	5780
February	1st to 10th	Day	61% 6% 10% 17% 5% 1	1890
	11th to 20th	Day	60% 5% 12% 15% 7% 1	1852
	21st to end	Day	59% 7% 10% 16% 7% 1	1616
March	1st to 10th	Day	52% 6% 15% 15% 12% 1	5045
	11th to 20th	Day	50% 6% 17% 17% 8% 2	5090
	21st to end	Day	52% 5% 10% 18% 13% 1	5586
April	1st to 10th	Day	55% 4 11% 18% 11% 1	2478
	11th to 20th	Day	50% 5% 10% 21% 14% 1	2577
	21st to end	Day	51% 5% 11% 19% 11% 3	2730
May	1st to 10th	Day	49% 7% 12% 17% 11% 4	7651
	11th to 20th	Day	44% 8% 12% 20% 14% 1	8166
	21st to end	Day	40% 5% 10% 28% 15% 2	9061
June	1st to 10th	Day	42% 7% 11% 22% 16% 1	2940
	11th to 20th	Day	38% 8% 13% 25% 15% 2	2940
	21st to end	Day	40% 7% 13% 24% 15% 1	3053
July	1st to 10th	Day	38% 8% 14% 25% 13% 1	8210
	11th to 20th	Day	28% 6% 12% 37% 16% 1	8205
	21st to end	Day	34% 8% 13% 28% 16%	8219
August	1st to 10th	Day	42% 8% 14% 21% 14% 1	4351
	11th to 20th	Day	40% 8% 15% 26% 10%	4343
	21st to end	Day	35% 8% 13% 33% 10% 1	4606
September	1st to 10th	Day	38% 7% 14% 29% 11% 1	2400
	11th to 20th	Day	34% 6% 14% 31% 15% 1	2280
	21st to end	Day	44% 7% 12% 26% 10%	2160
October	1st to 10th	Day	36% 7% 16% 27% 13% 1	3260
	11th to 20th	Day	39% 6% 15% 26% 14%	3240
	21st to end	Day	53% 7% 14% 20% 7%	3538
November	1st to 10th	Day	49% 8% 13% 21% 9% 1	1777
	11th to 20th	Day	54% 8% 14% 15% 9% 1	1680
	21st to end	Day	60% 7% 15% 15% 3	1680
December	1st to 10th	Day	66% 7% 14% 9% 4	2680
	11th to 20th	Day	62% 8% 13% 9% 8%	3066
	21st to end	Day	72% 6% 6% 9% 6%	4048
January	1st to 10th	Night	54% 9% 13% 18% 5%	8234
	11th to 20th	Night	64% 8% 10% 13% 5%	7818
	21st to end	Night	57% 7% 12% 12% 12% 1	8401
February	1st to 10th	Night	57% 5% 11% 20% 7% 1	2730
	11th to 20th	Night	57% 6% 12% 17% 9% 1	2665
	21st to end	Night	54% 6% 14% 18% 8%	2183
March	1st to 10th	Night	51% 6% 12% 17% 11% 2	5028
	11th to 20th	Night	42% 6% 19% 21% 11% 2	5090
	21st to end	Night	46% 5% 10% 19% 18% 2	5588
April	1st to 10th	Night	51% 5% 11% 18% 13% 2	2142
	11th to 20th	Night	48% 5% 8% 21% 15% 2	2009
	21st to end	Night	44% 6% 11% 20% 16% 2	1890
May	1st to 10th	Night	48% 6% 10% 16% 15% 5%	5264
	11th to 20th	Night	43% 7% 13% 20% 14% 2	4699
	21st to end	Night	38% 5% 12% 26% 17% 2	5115
June	1st to 10th	Night	37% 6% 10% 25% 19% 2	1680
	11th to 20th	Night	36% 6% 13% 23% 21% 2	1680
	21st to end	Night	37% 6% 13% 26% 18% 1	1531
July	1st to 10th	Night	32% 6% 16% 27% 17% 2	4600
	11th to 20th	Night	27% 6% 12% 33% 21% 1	4632
	21st to end	Night	31% 7% 12% 28% 21% 1	5538
August	1st to 10th	Night	36% 6% 13% 28% 15% 3	2994
	11th to 20th	Night	32% 6% 15% 33% 13% 1	2992
	21st to end	Night	32% 7% 15% 32% 13% 1	3457
September	1st to 10th	Night	33% 7% 15% 30% 14% 1	2000
	11th to 20th	Night	29% 5% 11% 37% 18% 1	2117
	21st to end	Night	42% 6% 12% 28% 12% 1	2240
October	1st to 10th	Night	32% 6% 10% 28% 24% 1	3920
	11th to 20th	Night	42% 6% 13% 27% 12%	3899
	21st to end	Night	39% 7% 16% 26% 11% 1	4252
November	1st to 10th	Night	45% 5% 12% 24% 12% 1	2735
	11th to 20th	Night	51% 7% 13% 18% 10% 1	2940
	21st to end	Night	56% 7% 17% 16% 4	2940
December	1st to 10th	Night	62% 8% 12% 14% 4	4720
	11th to 20th	Night	61% 7% 11% 13% 8%	5348
	21st to end	Night	68% 7% 10% 9% 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	20% 21% 29% 23% 6% 1	220
	11th to 20th	Day	24% 20% 34% 14% 6% 2	303
	21st to end	Day	25% 15% 36% 19% 5% 1	363
February	1st to 10th	Day	25% 15% 40% 16% 3 1	330
	11th to 20th	Day	25% 17% 41% 15% 2	330
	21st to end	Day	31% 18% 33% 17% 2	290
March	1st to 10th	Day	21% 15% 36% 21% 7%	540
	11th to 20th	Day	23% 16% 34% 21% 5% 1	540
	21st to end	Day	28% 13% 32% 19% 7% 1	594
April	1st to 10th	Day	25% 16% 33% 19% 7% 1	540
	11th to 20th	Day	29% 12% 31% 18% 7% 3	532
	21st to end	Day	25% 11% 33% 19% 9% 2	540
May	1st to 10th	Day	22% 12% 30% 22% 9% 5%	543
	11th to 20th	Day	17% 9% 30% 27% 12% 6%	567
	21st to end	Day	17% 9% 27% 26% 14% 6%	617
June	1st to 10th	Day	14% 8% 33% 29% 12% 3	570
	11th to 20th	Day	15% 11% 27% 28% 16% 3	570
	21st to end	Day	17% 7% 28% 30% 13% 4	683
July	1st to 10th	Day	10% 6% 22% 33% 22% 7%	565
	11th to 20th	Day	13% 5% 25% 30% 20% 8%	564
	21st to end	Day	9% 2 27% 36% 17% 8%	620
August	1st to 10th	Day	13% 5% 22% 29% 20% 11%	566
	11th to 20th	Day	9% 6% 23% 28% 23% 11%	559
	21st to end	Day	11% 7% 23% 29% 20% 10%	619
September	1st to 10th	Day	14% 6% 23% 27% 18% 12%	530
	11th to 20th	Day	10% 4 19% 28% 22% 18%	530
	21st to end	Day	15% 8% 28% 28% 15% 7%	530
October	1st to 10th	Day	19% 9% 26% 24% 15% 6%	530
	11th to 20th	Day	19% 12% 38% 21% 8% 2	530
	21st to end	Day	28% 11% 31% 24% 6% 1	583
November	1st to 10th	Day	21% 17% 30% 22% 9% 1	313
	11th to 20th	Day	22% 19% 33% 20% 5% 2	220
	21st to end	Day	25% 15% 39% 18% 3	220
December	1st to 10th	Day	25% 15% 36% 18% 5%	220
	11th to 20th	Day	30% 20% 26% 20% 5%	220
	21st to end	Day	30% 15% 38% 14% 2	242
January	1st to 10th	Night	23% 12% 36% 23% 5% 1	840
	11th to 20th	Night	26% 13% 31% 25% 5%	840
	21st to end	Night	20% 9% 37% 27% 7%	924
February	1st to 10th	Night	20% 12% 40% 25% 3	840
	11th to 20th	Night	22% 12% 37% 25% 4	823
	21st to end	Night	19% 12% 41% 23% 4	692
March	1st to 10th	Night	20% 11% 36% 26% 7%	840
	11th to 20th	Night	23% 13% 35% 23% 6% 1	840
	21st to end	Night	22% 10% 33% 26% 9% 1	924
April	1st to 10th	Night	24% 11% 33% 27% 5%	840
	11th to 20th	Night	20% 10% 35% 27% 8% 1	765
	21st to end	Night	20% 7% 31% 32% 11%	630
May	1st to 10th	Night	18% 5% 28% 32% 14% 2	630
	11th to 20th	Night	16% 4 31% 34% 14%	435
	21st to end	Night	12% 5% 28% 35% 19%	455
June	1st to 10th	Night	13% 3 28% 39% 16% 1	420
	11th to 20th	Night	13% 5% 29% 37% 15% 1	420
	21st to end	Night	11% 6% 21% 45% 16% 1	420
July	1st to 10th	Night	6% 3 22% 45% 22% 2	410
	11th to 20th	Night	11% 1 22% 43% 21% 2	408
	21st to end	Night	8% 3 19% 45% 23% 2	659
August	1st to 10th	Night	10% 2 13% 42% 31% 2	627
	11th to 20th	Night	8% 2 16% 42% 29% 3	624
	21st to end	Night	11% 3 20% 37% 26% 3	775
September	1st to 10th	Night	11% 4 23% 36% 21% 6%	800
	11th to 20th	Night	13% 2 17% 33% 30% 5%	800
	21st to end	Night	18% 4 21% 32% 21% 4	800
October	1st to 10th	Night	18% 5% 25% 30% 20% 3	800
	11th to 20th	Night	21% 7% 29% 32% 10% 1	800
	21st to end	Night	24% 8% 30% 32% 6%	880
November	1st to 10th	Night	20% 11% 26% 30% 10% 1	820
	11th to 20th	Night	22% 11% 33% 25% 8% 2	840
	21st to end	Night	27% 9% 32% 25% 7%	840
December	1st to 10th	Night	26% 9% 36% 23% 5%	840
	11th to 20th	Night	22% 12% 39% 23% 4	840
	21st to end	Night	31% 9% 35% 22% 3	924

Page generated at 2026-08-27 04:07:09. Version ac07

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