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 2025-02-21.

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% 33%	108598
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	109971
Transparency	Day	49% 7% 13% 21% 9% 1	112091
	Night	54% 6% 12% 18% 10% 1	114629
Seeing	Day	20% 12% 28% 24% 12% 4	14832
	Night	22% 9% 30% 28% 10% 1	23958

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% 19% 6% 12% 6% 5% 5% 3 4 5% 19%	3597
	11th to 20th	Day	13% 15% 7% 12% 5% 6% 5% 4 6% 5% 23%	3570
	21st to end	Day	12% 15% 5% 11% 6% 6% 5% 4 5% 6% 25%	3960
February	1st to 10th	Day	14% 15% 4 10% 7% 5% 5% 4 4 5% 26%	1760
	11th to 20th	Day	14% 11% 4 12% 7% 6% 4 4 5% 6% 26%	1857
	21st to end	Day	18% 16% 3 8% 7% 4 4 3 4 5% 27%	1570
March	1st to 10th	Day	21% 11% 4 10% 5% 5% 4 3 4 5% 26%	3116
	11th to 20th	Day	13% 15% 5% 11% 7% 5% 5% 4 5% 5% 27%	3280
	21st to end	Day	15% 7% 4 8% 4 4 4 3 5% 6% 39%	4162
April	1st to 10th	Day	21% 10% 4 7% 5% 4 4 3 5% 6% 32%	2280
	11th to 20th	Day	19% 8% 2 8% 4 3 4 3 5% 6% 37%	2254
	21st to end	Day	21% 10% 3 7% 4 5% 4 4 4 5% 33%	2431
May	1st to 10th	Day	19% 8% 3 9% 5% 5% 4 5% 5% 7% 30%	4360
	11th to 20th	Day	18% 8% 3 8% 5% 4 5% 3 6% 8% 33%	4398
	21st to end	Day	14% 6% 2 6% 4 5% 5% 4 7% 9% 37%	4837
June	1st to 10th	Day	20% 8% 2 7% 5% 4 4 4 7% 8% 31%	2660
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 4 6% 9% 39%	2660
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 4 5% 7% 33%	2660
July	1st to 10th	Day	11% 6% 3 7% 5% 6% 5% 4 6% 8% 38%	4440
	11th to 20th	Day	12% 6% 2 7% 4 5% 5% 5% 7% 9% 39%	4440
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 8% 45%	4814
August	1st to 10th	Day	14% 6% 3 8% 5% 5% 5% 4 7% 9% 33%	4423
	11th to 20th	Day	12% 7% 3 7% 5% 4 5% 4 6% 8% 40%	3988
	21st to end	Day	15% 5% 2 5% 4 5% 5% 4 6% 9% 41%	4180
September	1st to 10th	Day	11% 6% 3 5% 4 5% 5% 4 6% 8% 43%	2280
	11th to 20th	Day	14% 5% 2 4 4 4 4 4 5% 7% 49%	2199
	21st to end	Day	16% 6% 2 5% 4 4 4 4 6% 7% 41%	2090
October	1st to 10th	Day	19% 8% 3 7% 5% 5% 4 4 5% 5% 36%	1962
	11th to 20th	Day	18% 9% 3 7% 6% 7% 5% 4 6% 6% 31%	1861
	21st to end	Day	26% 10% 3 6% 4 4 3 4 5% 6% 30%	1881
November	1st to 10th	Day	18% 10% 3 7% 4 4 4 4 4 8% 32%	1761
	11th to 20th	Day	17% 11% 4 8% 6% 4 5% 3 5% 7% 29%	1721
	21st to end	Day	22% 13% 4 8% 6% 4 6% 4 6% 5% 22%	1600
December	1st to 10th	Day	21% 20% 6% 11% 4 5% 4 4 5% 5% 16%	2600
	11th to 20th	Day	17% 13% 4 10% 6% 6% 5% 5% 5% 4 24%	2986
	21st to end	Day	23% 12% 4 11% 6% 5% 6% 4 5% 5% 19%	3960
January	1st to 10th	Night	15% 21% 8% 13% 5% 4 4 3 4 4 19%	6349
	11th to 20th	Night	10% 16% 7% 16% 8% 6% 5% 4 6% 5% 17%	6315
	21st to end	Night	13% 17% 5% 12% 7% 5% 4 3 4 6% 24%	6996
February	1st to 10th	Night	14% 16% 6% 11% 6% 6% 5% 3 5% 5% 22%	2636
	11th to 20th	Night	15% 11% 4 10% 6% 6% 5% 4 5% 6% 28%	2428
	21st to end	Night	16% 15% 4 9% 5% 4 4 3 4 5% 31%	1885
March	1st to 10th	Night	21% 11% 3 7% 5% 5% 4 3 4 4 33%	3724
	11th to 20th	Night	19% 18% 4 9% 5% 4 3 3 4 5% 26%	3676
	21st to end	Night	13% 10% 4 9% 5% 4 4 3 4 5% 39%	3469
April	1st to 10th	Night	23% 10% 3 8% 3 5% 4 3 6% 5% 30%	1900
	11th to 20th	Night	23% 9% 2 6% 4 4 4 3 5% 6% 34%	1875
	21st to end	Night	21% 9% 3 8% 4 3 4 2 5% 6% 34%	1748
May	1st to 10th	Night	22% 9% 3 8% 4 5% 3 4 5% 7% 30%	2596
	11th to 20th	Night	23% 8% 2 6% 4 4 4 3 4 6% 36%	2495
	21st to end	Night	16% 7% 3 7% 5% 5% 4 3 5% 8% 37%	2738
June	1st to 10th	Night	21% 8% 2 7% 5% 4 5% 4 6% 8% 31%	1520
	11th to 20th	Night	16% 7% 3 6% 3 4 4 4 6% 8% 39%	1520
	21st to end	Night	18% 7% 2 7% 5% 5% 4 5% 6% 7% 33%	1520
July	1st to 10th	Night	14% 7% 3 8% 5% 5% 5% 4 7% 8% 34%	2520
	11th to 20th	Night	13% 5% 2 6% 5% 5% 5% 4 6% 9% 40%	2520
	21st to end	Night	12% 6% 2 7% 5% 5% 4 4 6% 8% 43%	2739
August	1st to 10th	Night	16% 8% 3 7% 4 4 4 4 6% 7% 38%	2506
	11th to 20th	Night	15% 7% 3 8% 4 5% 5% 4 5% 7% 38%	2803
	21st to end	Night	15% 6% 2 7% 4 4 5% 4 5% 8% 40%	3476
September	1st to 10th	Night	13% 6% 2 7% 5% 4 4 3 6% 7% 44%	1900
	11th to 20th	Night	15% 6% 2 5% 2 3 4 3 4 6% 50%	1980
	21st to end	Night	17% 6% 2 6% 3 4 4 3 5% 7% 42%	2090
October	1st to 10th	Night	18% 7% 2 7% 4 5% 5% 4 4 7% 36%	2216
	11th to 20th	Night	21% 7% 3 8% 4 4 4 3 5% 6% 35%	2318
	21st to end	Night	23% 10% 3 8% 4 5% 4 3 5% 6% 30%	2717
November	1st to 10th	Night	19% 11% 3 7% 5% 4 4 4 6% 6% 31%	2537
	11th to 20th	Night	19% 11% 4 8% 6% 4 4 3 5% 5% 32%	2675
	21st to end	Night	22% 14% 3 9% 6% 4 4 3 5% 5% 24%	2800
December	1st to 10th	Night	18% 19% 7% 12% 6% 5% 5% 3 4 4 16%	4580
	11th to 20th	Night	23% 13% 5% 10% 5% 4 3 3 6% 5% 22%	5208
	21st to end	Night	25% 16% 4 9% 4 3 3 4 6% 6% 20%	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	66% 7% 11% 14% 2	3598
	11th to 20th	Day	62% 5% 11% 14% 7%	3600
	21st to end	Day	58% 7% 11% 13% 9% 1	3960
February	1st to 10th	Day	61% 4 11% 17% 6% 2	1760
	11th to 20th	Day	57% 6% 12% 17% 7% 1	1866
	21st to end	Day	58% 5% 11% 15% 9% 1	1570
March	1st to 10th	Day	59% 6% 9% 18% 7% 1	3119
	11th to 20th	Day	57% 6% 12% 13% 11% 2	3280
	21st to end	Day	45% 5% 10% 21% 17% 2	4162
April	1st to 10th	Day	53% 6% 8% 20% 12% 2	2280
	11th to 20th	Day	48% 5% 9% 21% 16% 2	2254
	21st to end	Day	52% 5% 10% 17% 13% 3	2431
May	1st to 10th	Day	52% 7% 10% 20% 9% 1	4374
	11th to 20th	Day	48% 8% 15% 19% 9% 1	4402
	21st to end	Day	41% 8% 13% 23% 13% 2	4837
June	1st to 10th	Day	49% 8% 11% 19% 12% 1	2660
	11th to 20th	Day	43% 6% 11% 23% 14% 3	2660
	21st to end	Day	50% 8% 12% 20% 9% 1	2660
July	1st to 10th	Day	41% 7% 14% 28% 10% 1	4370
	11th to 20th	Day	39% 7% 16% 30% 8%	4365
	21st to end	Day	36% 7% 14% 33% 9%	4661
August	1st to 10th	Day	45% 7% 16% 23% 8%	4409
	11th to 20th	Day	41% 8% 17% 28% 7%	4030
	21st to end	Day	38% 8% 18% 22% 15%	4180
September	1st to 10th	Day	36% 8% 14% 28% 13%	2280
	11th to 20th	Day	35% 6% 14% 30% 15% 1	2199
	21st to end	Day	42% 6% 15% 26% 10% 1	2090
October	1st to 10th	Day	46% 5% 14% 27% 8%	3261
	11th to 20th	Day	43% 6% 18% 26% 6%	3064
	21st to end	Day	46% 4 20% 22% 7%	3081
November	1st to 10th	Day	49% 7% 14% 21% 8% 1	1761
	11th to 20th	Day	54% 5% 12% 16% 11%	1721
	21st to end	Day	61% 7% 14% 14% 4	1600
December	1st to 10th	Day	69% 7% 10% 13% 2	2600
	11th to 20th	Day	61% 6% 11% 15% 8%	2986
	21st to end	Day	64% 7% 11% 14% 5%	3960
January	1st to 10th	Night	69% 5% 10% 12% 3	6349
	11th to 20th	Night	67% 7% 10% 12% 4	6360
	21st to end	Night	62% 4 11% 11% 11% 1	6996
February	1st to 10th	Night	64% 5% 10% 15% 6% 1	2636
	11th to 20th	Night	55% 6% 13% 18% 8%	2430
	21st to end	Night	56% 4 9% 18% 12% 1	1885
March	1st to 10th	Night	54% 5% 9% 20% 9% 3	3734
	11th to 20th	Night	59% 5% 10% 14% 10% 1	3677
	21st to end	Night	47% 4 8% 22% 15% 3	3469
April	1st to 10th	Night	55% 4 9% 18% 13% 2	1900
	11th to 20th	Night	53% 5% 9% 20% 13% 2	1875
	21st to end	Night	52% 4 9% 18% 14% 4	1748
May	1st to 10th	Night	53% 7% 9% 19% 9% 3	2602
	11th to 20th	Night	47% 7% 12% 19% 12% 2	2496
	21st to end	Night	43% 7% 10% 20% 18% 1	2739
June	1st to 10th	Night	50% 8% 10% 20% 11% 1	1520
	11th to 20th	Night	42% 7% 12% 22% 15% 2	1520
	21st to end	Night	47% 7% 14% 19% 12% 1	1520
July	1st to 10th	Night	46% 7% 13% 20% 14%	2480
	11th to 20th	Night	39% 7% 16% 29% 9%	2472
	21st to end	Night	39% 8% 16% 24% 13%	2651
August	1st to 10th	Night	46% 7% 13% 26% 8%	2496
	11th to 20th	Night	44% 7% 19% 22% 8%	2827
	21st to end	Night	41% 7% 16% 22% 14% 1	3476
September	1st to 10th	Night	39% 7% 12% 26% 16% 1	1900
	11th to 20th	Night	36% 5% 14% 31% 13% 1	1980
	21st to end	Night	42% 7% 11% 25% 15%	2090
October	1st to 10th	Night	45% 6% 13% 27% 9%	3697
	11th to 20th	Night	45% 5% 12% 29% 9%	3841
	21st to end	Night	49% 5% 16% 18% 12% 1	4467
November	1st to 10th	Night	53% 5% 12% 19% 9% 1	2537
	11th to 20th	Night	54% 5% 12% 18% 10% 1	2675
	21st to end	Night	62% 6% 11% 15% 5%	2800
December	1st to 10th	Night	71% 5% 12% 10% 3	4580
	11th to 20th	Night	63% 4 10% 13% 10%	5208
	21st to end	Night	65% 4 11% 12% 8%	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	29% 13% 31% 22% 6% 1	200
	11th to 20th	Day	29% 14% 34% 19% 5% 1	200
	21st to end	Day	20% 16% 33% 22% 7% 1	220
February	1st to 10th	Day	26% 21% 35% 14% 4	280
	11th to 20th	Day	22% 16% 41% 19% 2	404
	21st to end	Day	26% 15% 28% 28% 3	382
March	1st to 10th	Day	23% 14% 33% 23% 7%	460
	11th to 20th	Day	18% 18% 36% 20% 7% 2	460
	21st to end	Day	26% 14% 31% 17% 10% 2	506
April	1st to 10th	Day	28% 14% 28% 20% 9% 1	460
	11th to 20th	Day	25% 16% 28% 19% 10% 3	452
	21st to end	Day	25% 15% 29% 20% 9% 2	460
May	1st to 10th	Day	22% 10% 32% 24% 9% 2	463
	11th to 20th	Day	17% 10% 31% 25% 13% 4	487
	21st to end	Day	17% 10% 30% 27% 13% 3	529
June	1st to 10th	Day	23% 10% 23% 28% 12% 3	490
	11th to 20th	Day	14% 11% 26% 32% 16% 2	490
	21st to end	Day	20% 9% 28% 24% 14% 4	490
July	1st to 10th	Day	11% 8% 25% 30% 19% 6%	485
	11th to 20th	Day	13% 7% 28% 30% 17% 5%	484
	21st to end	Day	14% 6% 23% 34% 18% 5%	532
August	1st to 10th	Day	16% 6% 21% 32% 20% 6%	486
	11th to 20th	Day	13% 5% 22% 34% 18% 9%	479
	21st to end	Day	14% 8% 26% 25% 19% 9%	539
September	1st to 10th	Day	12% 10% 26% 26% 19% 8%	490
	11th to 20th	Day	13% 9% 20% 23% 23% 13%	490
	21st to end	Day	17% 8% 27% 22% 20% 6%	490
October	1st to 10th	Day	21% 10% 27% 22% 14% 6%	490
	11th to 20th	Day	22% 13% 29% 22% 12% 2	469
	21st to end	Day	33% 11% 25% 21% 9%	429
November	1st to 10th	Day	26% 16% 25% 23% 9% 1	395
	11th to 20th	Day	23% 19% 31% 18% 7% 1	321
	21st to end	Day	28% 21% 26% 21% 5% 1	200
December	1st to 10th	Day	32% 10% 27% 22% 8% 2	200
	11th to 20th	Day	23% 16% 40% 18% 3 1	200
	21st to end	Day	29% 16% 28% 19% 6% 2	220
January	1st to 10th	Night	26% 13% 31% 25% 5%	800
	11th to 20th	Night	26% 11% 36% 22% 6%	800
	21st to end	Night	20% 9% 36% 28% 6% 1	880
February	1st to 10th	Night	21% 13% 36% 25% 5% 1	800
	11th to 20th	Night	20% 12% 39% 25% 4	783
	21st to end	Night	23% 10% 36% 28% 4	629
March	1st to 10th	Night	25% 11% 33% 27% 5%	760
	11th to 20th	Night	25% 12% 32% 25% 6%	760
	21st to end	Night	23% 10% 31% 28% 8% 1	836
April	1st to 10th	Night	27% 11% 29% 27% 5%	760
	11th to 20th	Night	26% 12% 31% 25% 6% 1	751
	21st to end	Night	26% 10% 30% 25% 9%	608
May	1st to 10th	Night	26% 7% 30% 26% 9% 1	436
	11th to 20th	Night	20% 6% 31% 33% 9% 1	374
	21st to end	Night	14% 6% 33% 35% 11%	411
June	1st to 10th	Night	22% 7% 28% 33% 9% 1	380
	11th to 20th	Night	19% 4 28% 34% 15%	380
	21st to end	Night	20% 6% 27% 32% 15% 1	380
July	1st to 10th	Night	14% 4 25% 39% 18% 1	370
	11th to 20th	Night	11% 4 25% 42% 17% 1	368
	21st to end	Night	12% 3 24% 43% 17% 1	416
August	1st to 10th	Night	15% 3 16% 45% 20% 1	378
	11th to 20th	Night	14% 4 22% 35% 23% 3	566
	21st to end	Night	16% 3 22% 37% 20% 2	836
September	1st to 10th	Night	13% 5% 25% 39% 18% 1	760
	11th to 20th	Night	18% 6% 17% 34% 22% 3	760
	21st to end	Night	18% 6% 27% 30% 18% 2	760
October	1st to 10th	Night	21% 8% 28% 28% 15% 1	760
	11th to 20th	Night	23% 9% 29% 28% 10%	760
	21st to end	Night	26% 12% 31% 27% 5%	836
November	1st to 10th	Night	26% 13% 28% 24% 8%	780
	11th to 20th	Night	23% 14% 33% 21% 9% 1	800
	21st to end	Night	29% 13% 30% 20% 8%	800
December	1st to 10th	Night	31% 12% 32% 21% 4	800
	11th to 20th	Night	30% 14% 31% 20% 5% 1	800
	21st to end	Night	33% 13% 29% 19% 5%	880

Page generated at 2025-02-18 03:26:30. Version ac07

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