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 2025-04-26.

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% 32%	113895
	Night	15% 10% 4 8% 5% 5% 5% 4 5% 7% 33%	114656
Transparency	Day	48% 7% 13% 20% 11% 1	117533
	Night	47% 7% 13% 21% 12% 1	119181
Seeing	Day	19% 10% 29% 25% 12% 5%	15043
	Night	19% 7% 30% 30% 12% 1	24500

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	13% 10% 4 13% 7% 5% 6% 4 5% 6% 25%	3597
	11th to 20th	Day	10% 16% 6% 17% 10% 7% 5% 3 4 4 16%	3911
	21st to end	Day	16% 16% 6% 13% 6% 5% 5% 4 4 5% 18%	4466
February	1st to 10th	Day	15% 16% 4 12% 7% 6% 5% 4 4 5% 22%	1800
	11th to 20th	Day	12% 14% 6% 12% 7% 6% 4 3 5% 6% 24%	1753
	21st to end	Day	17% 14% 5% 9% 6% 4 5% 4 4 5% 25%	1541
March	1st to 10th	Day	19% 9% 4 9% 5% 4 4 4 5% 5% 32%	4932
	11th to 20th	Day	11% 12% 4 8% 5% 5% 5% 4 6% 8% 31%	4980
	21st to end	Day	18% 11% 3 9% 5% 4 4 3 4 5% 33%	5465
April	1st to 10th	Day	17% 13% 4 8% 6% 4 5% 3 5% 7% 29%	2360
	11th to 20th	Day	19% 9% 3 8% 4 4 4 3 5% 6% 35%	2453
	21st to end	Day	20% 8% 3 7% 6% 4 4 3 5% 6% 33%	2535
May	1st to 10th	Day	18% 8% 3 8% 4 4 4 4 6% 8% 33%	4108
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 4 5% 8% 32%	4366
	21st to end	Day	12% 6% 2 5% 4 4 5% 3 5% 8% 46%	4837
June	1st to 10th	Day	14% 7% 2 6% 6% 4 5% 4 6% 8% 37%	2660
	11th to 20th	Day	13% 5% 2 6% 5% 4 5% 4 8% 11% 38%	2660
	21st to end	Day	14% 5% 2 6% 5% 5% 5% 4 7% 9% 39%	2761
July	1st to 10th	Day	10% 4 2 6% 5% 5% 6% 5% 8% 10% 40%	4440
	11th to 20th	Day	10% 4 2 5% 4 5% 5% 4 8% 9% 45%	4440
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 9% 41%	4503
August	1st to 10th	Day	17% 6% 3 6% 4 4 5% 4 7% 8% 38%	4104
	11th to 20th	Day	10% 7% 3 8% 5% 5% 6% 5% 7% 10% 35%	4044
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 42%	4354
September	1st to 10th	Day	13% 6% 2 7% 4 4 5% 4 6% 7% 42%	2280
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 6% 8% 46%	2166
	21st to end	Day	14% 6% 3 8% 5% 5% 5% 4 6% 9% 36%	2052
October	1st to 10th	Day	18% 9% 3 6% 4 4 4 4 5% 7% 35%	1900
	11th to 20th	Day	20% 7% 3 8% 6% 6% 4 4 5% 7% 30%	1900
	21st to end	Day	23% 11% 3 8% 6% 6% 4 4 5% 6% 22%	2090
November	1st to 10th	Day	14% 9% 4 7% 6% 5% 5% 4 6% 7% 33%	1691
	11th to 20th	Day	15% 11% 4 10% 6% 5% 6% 5% 6% 7% 25%	1600
	21st to end	Day	18% 15% 4 9% 6% 5% 6% 4 6% 7% 21%	1600
December	1st to 10th	Day	19% 18% 5% 10% 6% 7% 6% 3 6% 6% 15%	2600
	11th to 20th	Day	21% 16% 4 10% 6% 4 4 3 5% 5% 21%	2986
	21st to end	Day	27% 16% 6% 12% 5% 4 4 3 4 4 14%	3960
January	1st to 10th	Night	14% 11% 5% 11% 6% 6% 6% 4 6% 6% 24%	6349
	11th to 20th	Night	13% 13% 6% 14% 10% 7% 6% 4 5% 6% 16%	5969
	21st to end	Night	16% 13% 5% 12% 6% 5% 5% 4 5% 6% 22%	6490
February	1st to 10th	Night	13% 15% 5% 10% 7% 5% 5% 4 5% 6% 25%	2600
	11th to 20th	Night	14% 14% 5% 10% 7% 5% 5% 4 5% 5% 26%	2533
	21st to end	Night	14% 12% 4 10% 6% 4 6% 4 5% 6% 29%	2082
March	1st to 10th	Night	16% 10% 4 8% 5% 6% 5% 4 5% 5% 31%	4908
	11th to 20th	Night	10% 8% 3 7% 5% 5% 5% 5% 6% 9% 37%	4979
	21st to end	Night	21% 8% 2 6% 4 4 4 3 4 5% 39%	5467
April	1st to 10th	Night	18% 10% 3 9% 5% 4 5% 4 5% 6% 31%	2040
	11th to 20th	Night	17% 7% 3 7% 5% 5% 5% 3 5% 6% 38%	1912
	21st to end	Night	16% 9% 3 6% 4 4 4 4 6% 7% 36%	1761
May	1st to 10th	Night	18% 7% 3 6% 4 4 4 3 5% 6% 40%	2833
	11th to 20th	Night	15% 8% 3 6% 4 4 5% 4 6% 8% 37%	2527
	21st to end	Night	10% 4 2 4 4 4 4 3 6% 8% 51%	2738
June	1st to 10th	Night	12% 6% 2 5% 5% 5% 4 4 6% 8% 42%	1520
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 5% 7% 9% 43%	1520
	21st to end	Night	11% 6% 2 5% 5% 4 5% 4 5% 7% 46%	1385
July	1st to 10th	Night	6% 5% 2 5% 4 3 4 4 6% 9% 52%	2506
	11th to 20th	Night	12% 3 1 4 3 4 4 3 6% 8% 51%	2520
	21st to end	Night	7% 4 2 6% 3 4 5% 4 5% 8% 52%	3045
August	1st to 10th	Night	10% 5% 2 7% 5% 4 4 3 6% 7% 47%	2825
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 5% 8% 50%	2785
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 47%	3270
September	1st to 10th	Night	9% 5% 1 7% 4 4 4 4 6% 7% 48%	1900
	11th to 20th	Night	10% 5% 1 4 3 3 3 3 5% 7% 54%	2011
	21st to end	Night	15% 6% 3 7% 5% 4 4 3 5% 7% 42%	2128
October	1st to 10th	Night	15% 8% 3 6% 4 4 5% 4 6% 6% 40%	2280
	11th to 20th	Night	18% 8% 3 9% 5% 4 4 4 5% 7% 35%	2280
	21st to end	Night	20% 10% 2 8% 5% 4 5% 4 6% 7% 29%	2508
November	1st to 10th	Night	16% 8% 2 7% 5% 4 4 3 6% 6% 38%	2601
	11th to 20th	Night	16% 10% 3 9% 6% 5% 5% 4 5% 7% 30%	2800
	21st to end	Night	20% 12% 4 7% 6% 5% 4 4 5% 7% 26%	2800
December	1st to 10th	Night	18% 15% 5% 11% 6% 6% 5% 4 6% 6% 18%	4580
	11th to 20th	Night	17% 17% 4 11% 6% 5% 4 4 5% 6% 21%	5208
	21st to end	Night	25% 14% 6% 11% 5% 4 4 3 4 5% 16%	6996

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	54% 8% 14% 14% 9%	3598
	11th to 20th	Day	69% 6% 10% 11% 4	3945
	21st to end	Day	65% 6% 11% 9% 7% 1	4466
February	1st to 10th	Day	62% 6% 10% 17% 4 1	1800
	11th to 20th	Day	59% 5% 12% 15% 7% 1	1762
	21st to end	Day	59% 7% 9% 16% 7% 1	1541
March	1st to 10th	Day	52% 6% 15% 15% 13% 1	4935
	11th to 20th	Day	49% 6% 17% 17% 8% 2	4980
	21st to end	Day	51% 5% 11% 19% 13% 1	5465
April	1st to 10th	Day	55% 4 11% 18% 11% 1	2360
	11th to 20th	Day	50% 5% 10% 21% 14% 1	2453
	21st to end	Day	52% 5% 11% 18% 11% 3	2535
May	1st to 10th	Day	47% 7% 12% 19% 14% 2	4120
	11th to 20th	Day	48% 9% 13% 20% 9% 1	4370
	21st to end	Day	35% 7% 13% 24% 17% 3	4837
June	1st to 10th	Day	44% 7% 11% 22% 15% 1	2660
	11th to 20th	Day	37% 8% 13% 25% 16% 1	2660
	21st to end	Day	40% 7% 13% 24% 15% 1	2761
July	1st to 10th	Day	36% 7% 15% 25% 15% 2	4370
	11th to 20th	Day	32% 7% 14% 35% 12% 1	4365
	21st to end	Day	37% 10% 14% 21% 18% 1	4359
August	1st to 10th	Day	43% 8% 14% 21% 14% 1	4091
	11th to 20th	Day	40% 8% 16% 25% 10% 1	4083
	21st to end	Day	35% 8% 13% 33% 11% 1	4354
September	1st to 10th	Day	39% 7% 14% 28% 11% 1	2280
	11th to 20th	Day	35% 6% 14% 30% 15% 1	2166
	21st to end	Day	44% 7% 13% 26% 9%	2052
October	1st to 10th	Day	36% 7% 16% 27% 13% 1	3160
	11th to 20th	Day	39% 6% 15% 26% 14%	3140
	21st to end	Day	53% 7% 14% 20% 6%	3428
November	1st to 10th	Day	48% 8% 13% 21% 9% 1	1691
	11th to 20th	Day	55% 8% 14% 15% 9% 1	1600
	21st to end	Day	60% 7% 15% 15% 3	1600
December	1st to 10th	Day	66% 7% 14% 9% 4	2600
	11th to 20th	Day	62% 8% 13% 9% 8%	2986
	21st to end	Day	72% 6% 6% 9% 6%	3960
January	1st to 10th	Night	54% 9% 12% 19% 6% 1	6349
	11th to 20th	Night	63% 9% 11% 13% 4	6010
	21st to end	Night	59% 7% 12% 11% 10% 1	6490
February	1st to 10th	Night	58% 5% 11% 19% 6% 1	2600
	11th to 20th	Night	57% 6% 11% 17% 9% 1	2535
	21st to end	Night	53% 6% 14% 19% 8%	2082
March	1st to 10th	Night	51% 6% 12% 17% 11% 2	4918
	11th to 20th	Night	41% 6% 19% 21% 11% 2	4980
	21st to end	Night	46% 5% 10% 19% 18% 2	5467
April	1st to 10th	Night	52% 5% 10% 18% 14% 2	2040
	11th to 20th	Night	48% 5% 9% 21% 15% 2	1912
	21st to end	Night	45% 6% 12% 19% 15% 2	1761
May	1st to 10th	Night	44% 6% 13% 18% 16% 4	2840
	11th to 20th	Night	44% 8% 16% 19% 11% 2	2528
	21st to end	Night	29% 7% 14% 25% 22% 3	2739
June	1st to 10th	Night	39% 6% 10% 25% 18% 2	1520
	11th to 20th	Night	36% 5% 13% 23% 22% 2	1520
	21st to end	Night	36% 6% 13% 26% 18% 1	1385
July	1st to 10th	Night	29% 5% 15% 26% 22% 3	2466
	11th to 20th	Night	30% 6% 13% 34% 16% 1	2472
	21st to end	Night	29% 9% 14% 30% 16% 2	2948
August	1st to 10th	Night	35% 6% 13% 29% 15% 2	2814
	11th to 20th	Night	32% 6% 15% 33% 12% 1	2812
	21st to end	Night	33% 7% 14% 31% 14% 1	3270
September	1st to 10th	Night	33% 7% 15% 29% 15% 1	1900
	11th to 20th	Night	30% 5% 10% 36% 19% 1	2011
	21st to end	Night	42% 6% 12% 28% 12% 1	2128
October	1st to 10th	Night	33% 6% 10% 27% 24% 1	3800
	11th to 20th	Night	42% 6% 13% 27% 12%	3779
	21st to end	Night	39% 7% 16% 26% 11% 1	4120
November	1st to 10th	Night	46% 5% 12% 24% 12% 1	2601
	11th to 20th	Night	51% 7% 14% 18% 10% 1	2800
	21st to end	Night	56% 7% 17% 16% 4	2800
December	1st to 10th	Night	62% 8% 12% 14% 4	4580
	11th to 20th	Night	61% 7% 11% 13% 8%	5208
	21st to end	Night	68% 7% 11% 9% 5%	6996

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	19% 20% 30% 25% 6% 2	200
	11th to 20th	Day	23% 19% 35% 15% 7% 2	275
	21st to end	Day	24% 15% 37% 18% 5% 1	330
February	1st to 10th	Day	25% 15% 42% 14% 3	300
	11th to 20th	Day	26% 17% 42% 13% 2	300
	21st to end	Day	31% 18% 32% 17% 2	265
March	1st to 10th	Day	21% 15% 36% 21% 7%	500
	11th to 20th	Day	20% 15% 36% 22% 6% 1	500
	21st to end	Day	29% 11% 32% 20% 7% 1	550
April	1st to 10th	Day	24% 15% 33% 20% 7% 1	500
	11th to 20th	Day	28% 13% 31% 18% 7% 3	492
	21st to end	Day	24% 11% 33% 20% 9% 3	480
May	1st to 10th	Day	23% 8% 29% 24% 10% 6%	463
	11th to 20th	Day	18% 5% 28% 29% 14% 6%	487
	21st to end	Day	15% 7% 26% 29% 16% 7%	529
June	1st to 10th	Day	15% 8% 30% 31% 13% 3	490
	11th to 20th	Day	14% 10% 24% 30% 18% 3	490
	21st to end	Day	16% 7% 26% 32% 14% 5%	591
July	1st to 10th	Day	10% 4 21% 32% 25% 8%	485
	11th to 20th	Day	14% 4 22% 30% 21% 9%	484
	21st to end	Day	9% 2 24% 37% 18% 9%	532
August	1st to 10th	Day	13% 4 21% 28% 22% 12%	486
	11th to 20th	Day	10% 5% 20% 28% 25% 13%	479
	21st to end	Day	11% 6% 22% 28% 21% 11%	539
September	1st to 10th	Day	14% 6% 23% 28% 17% 12%	490
	11th to 20th	Day	10% 4 20% 27% 21% 18%	490
	21st to end	Day	14% 8% 28% 27% 16% 8%	490
October	1st to 10th	Day	20% 9% 28% 22% 15% 6%	490
	11th to 20th	Day	19% 11% 37% 22% 9% 2	490
	21st to end	Day	28% 10% 31% 24% 6% 1	539
November	1st to 10th	Day	20% 17% 29% 24% 9% 1	287
	11th to 20th	Day	22% 17% 34% 21% 6% 2	200
	21st to end	Day	24% 15% 39% 20% 3	200
December	1st to 10th	Day	27% 13% 37% 18% 6% 1	200
	11th to 20th	Day	30% 20% 26% 20% 5%	200
	21st to end	Day	29% 14% 39% 15% 3	220
January	1st to 10th	Night	23% 12% 36% 23% 6% 1	800
	11th to 20th	Night	26% 12% 32% 25% 5%	800
	21st to end	Night	20% 7% 37% 27% 7%	880
February	1st to 10th	Night	21% 12% 40% 24% 3	800
	11th to 20th	Night	22% 12% 37% 25% 4	783
	21st to end	Night	19% 11% 41% 24% 4	660
March	1st to 10th	Night	20% 11% 36% 26% 7%	800
	11th to 20th	Night	21% 13% 36% 24% 6% 1	800
	21st to end	Night	21% 10% 32% 27% 9% 1	880
April	1st to 10th	Night	24% 10% 33% 27% 5%	800
	11th to 20th	Night	20% 9% 35% 27% 8% 1	728
	21st to end	Night	21% 6% 30% 31% 11%	587
May	1st to 10th	Night	18% 4 28% 33% 15% 2	570
	11th to 20th	Night	16% 3 30% 36% 15% 1	393
	21st to end	Night	11% 4 27% 37% 21%	411
June	1st to 10th	Night	14% 3 28% 39% 16% 1	380
	11th to 20th	Night	13% 4 27% 38% 17% 1	380
	21st to end	Night	11% 6% 20% 45% 18% 1	380
July	1st to 10th	Night	6% 2 23% 44% 22% 2	370
	11th to 20th	Night	11% 1 21% 43% 22% 2	368
	21st to end	Night	8% 2 18% 46% 25% 2	595
August	1st to 10th	Night	10% 2 14% 41% 31% 2	567
	11th to 20th	Night	8% 1 15% 42% 31% 3	564
	21st to end	Night	12% 3 20% 36% 27% 3	708
September	1st to 10th	Night	12% 4 22% 36% 21% 6%	760
	11th to 20th	Night	13% 2 18% 32% 29% 5%	760
	21st to end	Night	18% 4 21% 32% 21% 4	760
October	1st to 10th	Night	19% 5% 25% 29% 19% 2	760
	11th to 20th	Night	21% 7% 28% 32% 11% 1	760
	21st to end	Night	25% 8% 30% 31% 6%	836
November	1st to 10th	Night	21% 10% 26% 31% 11% 1	780
	11th to 20th	Night	22% 10% 33% 26% 8% 2	800
	21st to end	Night	27% 9% 32% 25% 7%	800
December	1st to 10th	Night	27% 9% 36% 22% 6%	800
	11th to 20th	Night	22% 12% 38% 23% 4	800
	21st to end	Night	31% 8% 35% 23% 3	880

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