Dorval
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dorval clear sky chart from 2005-11-06 to 2026-06-16.

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	16% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	137835
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 29%	134206
Transparency	Day	50% 6% 12% 21% 10% 1	141282
	Night	55% 5% 11% 18% 10% 1	138835
Seeing	Day	20% 12% 29% 24% 12% 3	16555
	Night	22% 10% 30% 28% 10% 1	25562

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	15% 18% 6% 13% 6% 5% 5% 3 4 6% 20%	4661
	11th to 20th	Day	12% 14% 6% 13% 6% 6% 5% 4 6% 5% 22%	4650
	21st to end	Day	10% 13% 5% 11% 9% 8% 6% 4 6% 6% 23%	5124
February	1st to 10th	Day	14% 15% 4 10% 7% 5% 5% 4 4 5% 27%	1848
	11th to 20th	Day	13% 12% 4 11% 7% 6% 4 4 6% 6% 27%	1953
	21st to end	Day	17% 16% 4 9% 7% 4 4 4 4 5% 27%	1730
March	1st to 10th	Day	19% 12% 5% 11% 5% 5% 5% 4 5% 5% 25%	4576
	11th to 20th	Day	10% 12% 4 10% 7% 5% 5% 4 6% 6% 31%	4800
	21st to end	Day	18% 9% 4 9% 5% 4 4 3 4 5% 35%	6098
April	1st to 10th	Day	21% 10% 4 8% 5% 4 4 3 5% 6% 31%	2520
	11th to 20th	Day	20% 8% 3 8% 4 3 4 3 5% 6% 36%	2494
	21st to end	Day	21% 9% 3 7% 4 5% 4 4 4 6% 33%	2687
May	1st to 10th	Day	22% 9% 4 9% 5% 5% 4 4 6% 7% 25%	8115
	11th to 20th	Day	19% 8% 3 8% 5% 4 4 4 6% 8% 33%	8221
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 7% 34%	9060
June	1st to 10th	Day	19% 8% 2 7% 5% 4 4 4 7% 7% 32%	2940
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	2870
	21st to end	Day	19% 8% 2 7% 5% 5% 5% 4 6% 7% 33%	2800
July	1st to 10th	Day	11% 7% 3 8% 6% 6% 6% 4 7% 8% 34%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 5% 6% 8% 42%	6360
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 9% 46%	6926
August	1st to 10th	Day	14% 6% 3 8% 5% 5% 5% 4 7% 9% 35%	4563
	11th to 20th	Day	12% 7% 3 7% 5% 4 5% 4 6% 8% 40%	4116
	21st to end	Day	15% 5% 2 5% 4 5% 5% 4 6% 9% 40%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 5% 4 6% 9% 44%	2400
	11th to 20th	Day	13% 4 2 4 4 4 3 4 5% 7% 50%	2315
	21st to end	Day	16% 6% 2 5% 4 4 4 4 6% 7% 41%	2200
October	1st to 10th	Day	18% 8% 3 6% 5% 5% 4 4 5% 5% 38%	2065
	11th to 20th	Day	17% 8% 3 7% 6% 6% 5% 4 5% 6% 32%	1959
	21st to end	Day	26% 10% 3 6% 3 4 4 4 5% 6% 29%	1980
November	1st to 10th	Day	18% 11% 3 8% 5% 4 4 4 4 8% 31%	1851
	11th to 20th	Day	18% 11% 5% 8% 6% 4 5% 3 5% 7% 28%	1807
	21st to end	Day	22% 13% 4 8% 6% 4 5% 4 5% 5% 22%	1680
December	1st to 10th	Day	20% 20% 6% 11% 5% 5% 4 4 5% 5% 16%	2680
	11th to 20th	Day	17% 13% 5% 10% 6% 6% 5% 5% 5% 5% 24%	3066
	21st to end	Day	22% 12% 4 11% 6% 5% 6% 4 5% 5% 19%	4048
January	1st to 10th	Night	14% 19% 8% 14% 6% 4 4 3 4 4 20%	8234
	11th to 20th	Night	12% 16% 6% 17% 8% 6% 5% 4 5% 5% 16%	8235
	21st to end	Night	11% 14% 5% 12% 9% 6% 5% 3 4 6% 25%	9057
February	1st to 10th	Night	13% 16% 6% 11% 6% 6% 5% 3 5% 5% 23%	2768
	11th to 20th	Night	15% 11% 4 10% 6% 5% 5% 4 5% 6% 28%	2552
	21st to end	Night	15% 15% 4 9% 5% 4 5% 3 4 5% 31%	2076
March	1st to 10th	Night	20% 13% 3 9% 5% 5% 4 3 4 4 30%	5484
	11th to 20th	Night	16% 14% 3 8% 5% 5% 4 4 6% 8% 29%	5376
	21st to end	Night	15% 12% 4 10% 6% 5% 5% 3 4 5% 32%	5075
April	1st to 10th	Night	23% 10% 3 8% 4 5% 4 3 5% 6% 30%	2100
	11th to 20th	Night	23% 10% 3 7% 4 4 4 3 4 6% 33%	2075
	21st to end	Night	21% 9% 3 8% 4 4 4 3 5% 6% 35%	1932
May	1st to 10th	Night	22% 9% 4 8% 5% 5% 4 4 6% 8% 25%	4826
	11th to 20th	Night	21% 8% 3 8% 5% 5% 5% 3 5% 7% 30%	4639
	21st to end	Night	18% 9% 3 8% 5% 4 5% 3 5% 7% 32%	5114
June	1st to 10th	Night	21% 8% 2 7% 5% 4 4 4 6% 8% 32%	1680
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 6% 8% 38%	1640
	21st to end	Night	18% 7% 2 8% 6% 5% 4 5% 6% 7% 34%	1600
July	1st to 10th	Night	13% 7% 3 8% 5% 5% 6% 4 7% 9% 34%	3600
	11th to 20th	Night	13% 6% 3 6% 5% 5% 5% 4 6% 8% 40%	3600
	21st to end	Night	10% 6% 2 7% 5% 5% 4 4 6% 8% 43%	3926
August	1st to 10th	Night	15% 8% 3 7% 4 4 4 4 6% 7% 38%	2586
	11th to 20th	Night	14% 7% 3 8% 4 5% 5% 4 5% 7% 39%	2894
	21st to end	Night	15% 6% 2 7% 4 4 5% 4 5% 8% 39%	3586
September	1st to 10th	Night	12% 5% 2 7% 5% 4 4 3 6% 7% 44%	2000
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 4 6% 50%	2084
	21st to end	Night	16% 6% 3 6% 3 4 4 3 5% 7% 42%	2200
October	1st to 10th	Night	18% 7% 2 7% 4 4 5% 4 4 7% 38%	2333
	11th to 20th	Night	20% 7% 3 8% 4 5% 4 3 5% 6% 36%	2440
	21st to end	Night	23% 9% 3 8% 4 5% 4 3 5% 6% 30%	2860
November	1st to 10th	Night	20% 10% 3 7% 5% 4 4 4 5% 6% 31%	2667
	11th to 20th	Night	19% 12% 4 8% 5% 4 4 3 5% 5% 32%	2809
	21st to end	Night	22% 14% 4 9% 6% 5% 4 3 5% 5% 24%	2940
December	1st to 10th	Night	18% 19% 7% 12% 6% 5% 5% 3 4 4 17%	4720
	11th to 20th	Night	23% 13% 5% 10% 5% 4 4 3 6% 5% 22%	5348
	21st to end	Night	25% 16% 4 9% 4 3 3 4 6% 6% 20%	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	65% 6% 11% 15% 3	4662
	11th to 20th	Day	63% 5% 9% 13% 10% 1	4680
	21st to end	Day	61% 6% 10% 12% 11% 1	5124
February	1st to 10th	Day	60% 4 10% 18% 5% 2	1848
	11th to 20th	Day	56% 6% 12% 18% 7% 1	1962
	21st to end	Day	59% 5% 10% 15% 9% 1	1730
March	1st to 10th	Day	59% 5% 9% 14% 10% 3	4579
	11th to 20th	Day	51% 6% 17% 14% 10% 1	4800
	21st to end	Day	51% 4 8% 18% 17% 2	6098
April	1st to 10th	Day	54% 5% 8% 19% 12% 2	2520
	11th to 20th	Day	49% 5% 9% 21% 16% 1	2494
	21st to end	Day	51% 5% 10% 18% 13% 3	2687
May	1st to 10th	Day	56% 6% 10% 18% 10% 1	8127
	11th to 20th	Day	48% 6% 13% 22% 9% 1	8225
	21st to end	Day	49% 6% 11% 22% 11% 1	9061
June	1st to 10th	Day	48% 8% 10% 20% 13% 1	2940
	11th to 20th	Day	43% 6% 11% 24% 14% 3	2870
	21st to end	Day	49% 7% 12% 20% 10% 1	2800
July	1st to 10th	Day	45% 7% 13% 26% 9%	6290
	11th to 20th	Day	38% 6% 14% 29% 13%	6285
	21st to end	Day	35% 6% 14% 37% 9%	6728
August	1st to 10th	Day	44% 7% 16% 24% 8%	4549
	11th to 20th	Day	41% 7% 16% 28% 7%	4158
	21st to end	Day	38% 7% 18% 22% 14%	4312
September	1st to 10th	Day	35% 8% 14% 30% 13%	2400
	11th to 20th	Day	34% 6% 13% 31% 15% 1	2315
	21st to end	Day	42% 6% 14% 26% 11% 1	2200
October	1st to 10th	Day	45% 5% 14% 27% 9%	3364
	11th to 20th	Day	43% 6% 18% 27% 6%	3162
	21st to end	Day	46% 4 19% 22% 7%	3180
November	1st to 10th	Day	51% 7% 14% 20% 8% 1	1851
	11th to 20th	Day	55% 5% 12% 16% 11%	1807
	21st to end	Day	62% 6% 13% 13% 5%	1680
December	1st to 10th	Day	69% 6% 9% 13% 2	2680
	11th to 20th	Day	61% 6% 11% 14% 8%	3066
	21st to end	Day	64% 6% 11% 14% 5%	4048
January	1st to 10th	Night	68% 4 10% 13% 4	8234
	11th to 20th	Night	69% 5% 9% 10% 6%	8280
	21st to end	Night	61% 4 9% 13% 12%	9057
February	1st to 10th	Night	63% 5% 10% 15% 6% 1	2768
	11th to 20th	Night	55% 5% 12% 19% 8%	2554
	21st to end	Night	56% 4 9% 18% 12% 1	2076
March	1st to 10th	Night	58% 4 7% 15% 11% 4	5494
	11th to 20th	Night	53% 5% 14% 16% 11% 2	5377
	21st to end	Night	54% 4 7% 20% 12% 2	5075
April	1st to 10th	Night	55% 3 9% 18% 13% 2	2100
	11th to 20th	Night	54% 4 9% 19% 12% 1	2075
	21st to end	Night	52% 4 8% 18% 15% 4	1932
May	1st to 10th	Night	56% 6% 11% 18% 8% 1	4832
	11th to 20th	Night	51% 7% 10% 22% 9% 1	4640
	21st to end	Night	50% 6% 8% 19% 15% 1	5115
June	1st to 10th	Night	49% 8% 10% 20% 12% 1	1680
	11th to 20th	Night	43% 7% 12% 21% 15% 2	1640
	21st to end	Night	47% 7% 13% 19% 12% 1	1600
July	1st to 10th	Night	44% 7% 17% 21% 10%	3560
	11th to 20th	Night	40% 6% 14% 27% 12%	3552
	21st to end	Night	38% 7% 14% 28% 13%	3809
August	1st to 10th	Night	45% 6% 14% 27% 9%	2576
	11th to 20th	Night	44% 6% 19% 22% 9%	2918
	21st to end	Night	41% 7% 15% 22% 14% 1	3586
September	1st to 10th	Night	38% 7% 12% 27% 15% 1	2000
	11th to 20th	Night	36% 5% 14% 32% 13% 1	2084
	21st to end	Night	42% 7% 11% 24% 15%	2200
October	1st to 10th	Night	44% 6% 13% 27% 10% 1	3814
	11th to 20th	Night	45% 5% 12% 29% 9%	3963
	21st to end	Night	48% 5% 16% 18% 11% 1	4610
November	1st to 10th	Night	53% 5% 12% 20% 8% 1	2667
	11th to 20th	Night	54% 5% 12% 18% 10% 1	2809
	21st to end	Night	63% 6% 11% 15% 5%	2940
December	1st to 10th	Night	71% 5% 11% 10% 3	4720
	11th to 20th	Night	63% 4 10% 12% 10%	5348
	21st to end	Night	65% 4 11% 12% 8%	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	27% 12% 31% 22% 6% 1	220
	11th to 20th	Day	28% 17% 34% 17% 4	220
	21st to end	Day	19% 17% 33% 21% 8% 1	242
February	1st to 10th	Day	24% 21% 36% 14% 5%	308
	11th to 20th	Day	21% 15% 41% 20% 3	440
	21st to end	Day	25% 15% 31% 26% 3	446
March	1st to 10th	Day	24% 15% 34% 22% 6%	540
	11th to 20th	Day	20% 18% 35% 19% 6% 1	540
	21st to end	Day	26% 16% 31% 16% 9% 2	594
April	1st to 10th	Day	28% 17% 28% 19% 8% 1	540
	11th to 20th	Day	25% 17% 28% 18% 9% 3	532
	21st to end	Day	25% 15% 31% 19% 9% 2	540
May	1st to 10th	Day	24% 11% 32% 23% 8% 2	543
	11th to 20th	Day	17% 11% 32% 23% 13% 4	567
	21st to end	Day	20% 11% 30% 25% 12% 3	617
June	1st to 10th	Day	21% 11% 25% 29% 12% 3	570
	11th to 20th	Day	13% 13% 27% 30% 15% 2	550
	21st to end	Day	20% 9% 30% 23% 13% 4	530
July	1st to 10th	Day	11% 9% 26% 29% 19% 5%	525
	11th to 20th	Day	12% 7% 29% 29% 18% 5%	524
	21st to end	Day	14% 6% 24% 34% 18% 4	576
August	1st to 10th	Day	14% 6% 20% 34% 20% 6%	526
	11th to 20th	Day	13% 5% 21% 33% 19% 9%	519
	21st to end	Day	14% 8% 26% 25% 19% 8%	583
September	1st to 10th	Day	12% 10% 25% 24% 20% 8%	530
	11th to 20th	Day	12% 8% 19% 24% 24% 13%	530
	21st to end	Day	17% 9% 27% 22% 19% 6%	530
October	1st to 10th	Day	20% 9% 27% 23% 15% 6%	530
	11th to 20th	Day	21% 13% 31% 22% 12% 2	507
	21st to end	Day	32% 11% 25% 23% 9%	462
November	1st to 10th	Day	28% 16% 25% 22% 8% 1	425
	11th to 20th	Day	24% 18% 31% 19% 7% 1	347
	21st to end	Day	29% 21% 25% 20% 5%	220
December	1st to 10th	Day	31% 10% 27% 22% 8% 1	220
	11th to 20th	Day	23% 16% 40% 17% 3 1	220
	21st to end	Day	28% 16% 30% 19% 5% 2	242
January	1st to 10th	Night	26% 13% 32% 25% 4	840
	11th to 20th	Night	26% 12% 35% 21% 5%	840
	21st to end	Night	20% 10% 35% 27% 6% 1	924
February	1st to 10th	Night	20% 12% 35% 26% 6% 1	840
	11th to 20th	Night	20% 12% 39% 26% 3	823
	21st to end	Night	22% 10% 36% 29% 4	692
March	1st to 10th	Night	25% 13% 34% 25% 4	840
	11th to 20th	Night	25% 12% 33% 24% 5%	840
	21st to end	Night	23% 11% 31% 27% 8% 1	924
April	1st to 10th	Night	28% 12% 29% 26% 5%	840
	11th to 20th	Night	27% 12% 30% 25% 6%	831
	21st to end	Night	26% 10% 30% 26% 8%	672
May	1st to 10th	Night	27% 8% 30% 26% 9% 1	482
	11th to 20th	Night	20% 7% 31% 33% 8% 1	414
	21st to end	Night	15% 8% 34% 33% 10%	455
June	1st to 10th	Night	21% 7% 30% 32% 9% 1	420
	11th to 20th	Night	19% 6% 28% 33% 14%	410
	21st to end	Night	20% 6% 27% 31% 15% 1	400
July	1st to 10th	Night	13% 4 25% 39% 18% 1	390
	11th to 20th	Night	10% 4 25% 42% 16% 2	388
	21st to end	Night	12% 3 24% 43% 16% 1	438
August	1st to 10th	Night	14% 4 17% 44% 20% 1	398
	11th to 20th	Night	13% 3 22% 35% 23% 3	597
	21st to end	Night	16% 3 22% 37% 20% 2	880
September	1st to 10th	Night	13% 5% 25% 38% 18% 1	800
	11th to 20th	Night	17% 5% 17% 35% 23% 3	800
	21st to end	Night	18% 6% 27% 30% 18% 2	800
October	1st to 10th	Night	20% 7% 28% 28% 16% 1	800
	11th to 20th	Night	22% 9% 30% 28% 10%	800
	21st to end	Night	25% 11% 31% 28% 5%	880
November	1st to 10th	Night	27% 13% 28% 24% 8%	820
	11th to 20th	Night	24% 14% 32% 21% 9% 1	840
	21st to end	Night	28% 13% 31% 19% 7%	840
December	1st to 10th	Night	31% 12% 32% 21% 4	840
	11th to 20th	Night	29% 15% 31% 19% 5% 1	840
	21st to end	Night	34% 13% 29% 19% 4	924

Page generated at 2026-06-13 02:31:42. Version ac07

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