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 2024-10-25.

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%	101446
	Night	15% 9% 3 8% 5% 5% 4 4 5% 7% 35%	97311
Transparency	Day	48% 7% 13% 21% 11% 1	101212
	Night	48% 7% 13% 21% 11% 1	97164
Seeing	Day	19% 9% 29% 25% 13% 5%	14500
	Night	19% 7% 30% 30% 12% 1	23778

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% 12% 5% 9% 6% 4 5% 4 5% 6% 27%	2517
	11th to 20th	Day	11% 17% 6% 16% 9% 6% 5% 3 4 4 18%	2753
	21st to end	Day	21% 19% 6% 11% 6% 4 4 3 4 5% 17%	3124
February	1st to 10th	Day	15% 16% 4 12% 7% 6% 5% 4 4 5% 22%	1710
	11th to 20th	Day	12% 14% 6% 11% 6% 6% 4 3 5% 6% 26%	1663
	21st to end	Day	18% 14% 5% 9% 6% 4 5% 4 4 5% 25%	1466
March	1st to 10th	Day	20% 11% 4 8% 5% 4 4 4 4 5% 31%	3432
	11th to 20th	Day	11% 14% 5% 9% 5% 6% 5% 4 6% 6% 29%	3480
	21st to end	Day	16% 10% 3 8% 5% 4 4 4 5% 6% 36%	3815
April	1st to 10th	Day	17% 13% 3 8% 6% 4 5% 4 5% 7% 29%	2242
	11th to 20th	Day	20% 8% 3 7% 4 4 4 3 5% 6% 35%	2329
	21st to end	Day	21% 8% 3 7% 5% 4 4 3 5% 6% 33%	2470
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	24% 12% 3 8% 6% 5% 4 4 5% 6% 23%	2027
November	1st to 10th	Day	14% 9% 5% 7% 6% 4 5% 4 6% 7% 33%	1605
	11th to 20th	Day	15% 11% 4 10% 6% 5% 6% 5% 6% 7% 25%	1520
	21st to end	Day	19% 14% 4 8% 6% 6% 6% 4 6% 7% 22%	1520
December	1st to 10th	Day	16% 16% 6% 11% 6% 7% 6% 3 6% 6% 17%	1520
	11th to 20th	Day	19% 13% 4 10% 6% 5% 5% 4 6% 6% 22%	1906
	21st to end	Day	29% 18% 5% 12% 5% 3 4 3 4 4 12%	2772
January	1st to 10th	Night	18% 15% 5% 10% 6% 5% 5% 4 5% 6% 22%	4429
	11th to 20th	Night	15% 13% 6% 12% 9% 7% 5% 4 5% 6% 18%	4202
	21st to end	Night	21% 12% 5% 11% 7% 6% 5% 3 5% 5% 19%	4532
February	1st to 10th	Night	13% 15% 5% 10% 7% 5% 5% 4 5% 6% 25%	2470
	11th to 20th	Night	13% 14% 5% 9% 7% 5% 5% 4 5% 5% 27%	2403
	21st to end	Night	14% 12% 5% 10% 6% 4 6% 4 5% 5% 30%	1981
March	1st to 10th	Night	18% 10% 3 8% 6% 5% 5% 3 5% 5% 32%	3408
	11th to 20th	Night	13% 10% 3 8% 5% 5% 4 4 5% 7% 36%	3479
	21st to end	Night	17% 8% 2 5% 3 4 4 3 4 6% 44%	3817
April	1st to 10th	Night	18% 10% 3 8% 5% 4 4 4 5% 6% 32%	1938
	11th to 20th	Night	17% 7% 3 7% 5% 5% 5% 3 5% 6% 38%	1815
	21st to end	Night	16% 9% 3 6% 4 4 5% 4 6% 7% 36%	1710
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% 3 8% 5% 4 5% 4 6% 6% 29%	2435
November	1st to 10th	Night	17% 8% 2 7% 5% 4 4 4 6% 6% 38%	2467
	11th to 20th	Night	16% 10% 3 9% 6% 5% 5% 4 5% 7% 30%	2660
	21st to end	Night	20% 12% 4 7% 6% 5% 4 4 5% 7% 26%	2660
December	1st to 10th	Night	20% 15% 4 9% 5% 4 5% 4 5% 6% 22%	2660
	11th to 20th	Night	16% 16% 4 11% 6% 5% 5% 4 5% 6% 22%	3288
	21st to end	Night	30% 15% 5% 10% 6% 4 4 3 4 5% 14%	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% 7% 17% 12% 7% 1	2518
	11th to 20th	Day	68% 6% 10% 11% 4	2753
	21st to end	Day	69% 6% 10% 9% 5% 1	3124
February	1st to 10th	Day	62% 6% 10% 17% 4 1	1710
	11th to 20th	Day	58% 5% 13% 15% 8% 1	1672
	21st to end	Day	59% 7% 9% 16% 7% 1	1466
March	1st to 10th	Day	54% 7% 13% 17% 9% 1	3435
	11th to 20th	Day	52% 6% 13% 18% 9% 2	3480
	21st to end	Day	47% 5% 12% 19% 14% 2	3815
April	1st to 10th	Day	54% 5% 11% 18% 11% 1	2242
	11th to 20th	Day	49% 5% 11% 20% 15% 1	2329
	21st to end	Day	52% 5% 11% 18% 12% 3	2470
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	46% 6% 15% 19% 12% 1	1900
	11th to 20th	Day	52% 7% 12% 21% 8% 1	1900
	21st to end	Day	60% 6% 12% 16% 6%	2027
November	1st to 10th	Day	48% 8% 13% 21% 9% 1	1605
	11th to 20th	Day	55% 8% 14% 14% 8% 1	1520
	21st to end	Day	59% 7% 15% 15% 3	1520
December	1st to 10th	Day	65% 7% 13% 11% 4	1520
	11th to 20th	Day	60% 8% 16% 11% 6%	1906
	21st to end	Day	74% 6% 8% 10% 3	2772
January	1st to 10th	Night	59% 9% 12% 14% 5% 1	4429
	11th to 20th	Night	63% 9% 12% 12% 4 1	4202
	21st to end	Night	64% 8% 12% 11% 5% 1	4532
February	1st to 10th	Night	59% 5% 10% 19% 6% 1	2470
	11th to 20th	Night	56% 6% 11% 17% 9% 1	2405
	21st to end	Night	53% 5% 14% 19% 8% 1	1981
March	1st to 10th	Night	52% 6% 12% 22% 8% 1	3418
	11th to 20th	Night	45% 6% 15% 20% 12% 2	3480
	21st to end	Night	40% 5% 11% 22% 20% 2	3817
April	1st to 10th	Night	51% 5% 10% 18% 14% 1	1938
	11th to 20th	Night	48% 5% 9% 21% 15% 2	1815
	21st to end	Night	46% 6% 11% 19% 16% 2	1710
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	43% 7% 11% 22% 16% 1	2280
	11th to 20th	Night	49% 5% 12% 24% 9%	2280
	21st to end	Night	54% 6% 11% 17% 11% 1	2435
November	1st to 10th	Night	45% 5% 12% 24% 12% 1	2467
	11th to 20th	Night	51% 7% 14% 18% 10% 1	2660
	21st to end	Night	56% 7% 17% 16% 4	2660
December	1st to 10th	Night	60% 7% 11% 15% 6%	2660
	11th to 20th	Night	61% 6% 14% 14% 4	3288
	21st to end	Night	71% 7% 11% 8% 3	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	21% 16% 30% 26% 7% 2	180
	11th to 20th	Day	23% 18% 36% 14% 7% 2	247
	21st to end	Day	25% 13% 36% 20% 6% 1	297
February	1st to 10th	Day	24% 14% 44% 14% 3	270
	11th to 20th	Day	23% 16% 44% 14% 3	270
	21st to end	Day	32% 20% 31% 16% 2	240
March	1st to 10th	Day	21% 14% 37% 21% 8%	460
	11th to 20th	Day	21% 14% 36% 23% 6% 1	460
	21st to end	Day	27% 11% 33% 21% 7% 1	506
April	1st to 10th	Day	22% 14% 34% 21% 8% 1	460
	11th to 20th	Day	28% 12% 31% 18% 8% 4	452
	21st to end	Day	25% 11% 33% 20% 10% 3	460
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	29% 10% 30% 24% 6% 1	514
November	1st to 10th	Day	18% 17% 29% 24% 10% 1	261
	11th to 20th	Day	23% 14% 34% 22% 6% 2	180
	21st to end	Day	24% 10% 41% 22% 3	180
December	1st to 10th	Day	26% 12% 39% 18% 6% 1	180
	11th to 20th	Day	27% 20% 28% 20% 6%	180
	21st to end	Day	28% 14% 40% 16% 2	198
January	1st to 10th	Night	24% 11% 35% 24% 6% 1	760
	11th to 20th	Night	27% 11% 31% 25% 5%	760
	21st to end	Night	20% 7% 37% 28% 7%	836
February	1st to 10th	Night	20% 12% 41% 24% 3	760
	11th to 20th	Night	21% 12% 36% 25% 5%	743
	21st to end	Night	20% 11% 41% 23% 5%	628
March	1st to 10th	Night	20% 10% 36% 26% 7%	760
	11th to 20th	Night	22% 12% 36% 23% 6% 1	760
	21st to end	Night	21% 9% 33% 28% 9% 1	836
April	1st to 10th	Night	24% 10% 33% 28% 5%	760
	11th to 20th	Night	20% 9% 35% 26% 9% 1	691
	21st to end	Night	21% 6% 30% 31% 11%	570
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	26% 7% 29% 31% 6%	812
November	1st to 10th	Night	20% 10% 26% 31% 11% 1	740
	11th to 20th	Night	22% 9% 33% 26% 9% 2	760
	21st to end	Night	26% 8% 32% 26% 8%	760
December	1st to 10th	Night	28% 8% 36% 22% 6%	760
	11th to 20th	Night	22% 11% 39% 24% 4	760
	21st to end	Night	32% 8% 34% 23% 3	836

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