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 2024-09-14.

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 6% 7% 33%	101006
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 31%	96895
Transparency	Day	49% 7% 13% 21% 10% 1	100767
	Night	53% 6% 12% 18% 9% 1	96754
Seeing	Day	20% 11% 28% 24% 12% 4	14391
	Night	23% 9% 30% 28% 10% 1	23321

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% 15% 5% 12% 6% 5% 6% 3 4 6% 21%	2517
	11th to 20th	Day	15% 13% 6% 10% 5% 5% 5% 4 6% 5% 25%	2520
	21st to end	Day	15% 17% 5% 9% 6% 5% 5% 4 6% 7% 22%	2772
February	1st to 10th	Day	15% 16% 4 10% 7% 5% 5% 3 4 5% 26%	1672
	11th to 20th	Day	14% 11% 4 11% 6% 6% 4 5% 5% 6% 27%	1761
	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 3 3 3 4 5% 6% 49%	2122
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 7% 41%	1980
October	1st to 10th	Day	19% 8% 3 7% 5% 5% 4 4 5% 5% 35%	1859
	11th to 20th	Day	18% 9% 3 8% 6% 7% 5% 4 6% 6% 29%	1764
	21st to end	Day	27% 10% 3 6% 4 4 4 4 4 6% 28%	1782
November	1st to 10th	Day	18% 10% 3 7% 4 4 4 4 5% 8% 32%	1671
	11th to 20th	Day	18% 11% 5% 8% 6% 4 5% 3 5% 7% 27%	1635
	21st to end	Day	22% 12% 4 8% 6% 4 6% 4 6% 6% 23%	1520
December	1st to 10th	Day	24% 16% 5% 9% 5% 5% 4 4 5% 5% 18%	1520
	11th to 20th	Day	20% 16% 4 9% 5% 4 4 5% 5% 5% 23%	1906
	21st to end	Day	27% 15% 4 12% 6% 4 4 2 4 4 19%	2772
January	1st to 10th	Night	17% 16% 6% 11% 6% 5% 5% 3 5% 5% 21%	4429
	11th to 20th	Night	12% 14% 5% 13% 8% 6% 5% 4 6% 6% 20%	4440
	21st to end	Night	17% 18% 5% 10% 7% 4 4 3 4 5% 24%	4884
February	1st to 10th	Night	14% 16% 6% 11% 7% 6% 5% 3 5% 5% 22%	2505
	11th to 20th	Night	15% 11% 3 10% 6% 6% 5% 4 5% 6% 28%	2304
	21st to end	Night	16% 15% 4 9% 5% 4 4 3 4 5% 31%	1884
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	16% 6% 2 5% 3 3 4 3 4 6% 49%	1915
	21st to end	Night	17% 6% 3 6% 3 4 4 3 5% 7% 42%	1980
October	1st to 10th	Night	18% 7% 2 7% 4 5% 5% 4 4 7% 36%	2099
	11th to 20th	Night	21% 7% 3 8% 4 4 4 3 5% 6% 34%	2196
	21st to end	Night	24% 9% 3 8% 4 5% 4 3 5% 5% 29%	2574
November	1st to 10th	Night	20% 11% 3 7% 5% 4 4 4 6% 5% 31%	2407
	11th to 20th	Night	19% 11% 4 8% 6% 4 4 3 5% 5% 31%	2541
	21st to end	Night	22% 14% 3 9% 5% 4 4 3 5% 5% 25%	2660
December	1st to 10th	Night	22% 16% 6% 9% 5% 4 5% 3 4 5% 19%	2660
	11th to 20th	Night	24% 15% 5% 9% 5% 4 3 3 5% 5% 23%	3288
	21st to end	Night	29% 16% 4 9% 4 3 3 3 4 4 21%	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	62% 8% 14% 13% 3	2518
	11th to 20th	Day	60% 6% 14% 13% 7%	2520
	21st to end	Day	60% 9% 13% 12% 5%	2772
February	1st to 10th	Day	61% 4 11% 18% 4 1	1672
	11th to 20th	Day	55% 6% 12% 17% 7% 1	1770
	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% 13% 31% 15% 1	2122
	21st to end	Day	42% 6% 15% 27% 9% 1	1980
October	1st to 10th	Day	49% 7% 13% 20% 10%	1859
	11th to 20th	Day	53% 8% 14% 19% 6%	1764
	21st to end	Day	57% 5% 13% 16% 7% 1	1782
November	1st to 10th	Day	49% 7% 15% 20% 8% 1	1671
	11th to 20th	Day	56% 5% 13% 15% 10%	1635
	21st to end	Day	61% 7% 14% 13% 5%	1520
December	1st to 10th	Day	66% 8% 10% 12% 3	1520
	11th to 20th	Day	62% 8% 13% 13% 5%	1906
	21st to end	Day	68% 7% 10% 10% 5%	2772
January	1st to 10th	Night	64% 7% 14% 11% 4	4429
	11th to 20th	Night	61% 8% 13% 12% 5% 1	4440
	21st to end	Night	64% 5% 13% 11% 6% 1	4884
February	1st to 10th	Night	64% 5% 10% 15% 6% 1	2505
	11th to 20th	Night	54% 6% 13% 18% 8% 1	2306
	21st to end	Night	56% 4 9% 18% 12% 1	1884
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	37% 5% 14% 32% 12% 1	1915
	21st to end	Night	42% 7% 11% 25% 14%	1980
October	1st to 10th	Night	48% 6% 11% 23% 12% 1	2099
	11th to 20th	Night	52% 6% 11% 23% 8%	2196
	21st to end	Night	57% 6% 13% 15% 9% 1	2574
November	1st to 10th	Night	53% 5% 13% 19% 9% 1	2407
	11th to 20th	Night	54% 5% 12% 18% 9% 1	2541
	21st to end	Night	62% 7% 11% 15% 5%	2660
December	1st to 10th	Night	67% 5% 12% 12% 3	2660
	11th to 20th	Night	64% 5% 9% 15% 6%	3288
	21st to end	Night	68% 5% 10% 13% 4	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	27% 12% 30% 23% 7% 1	180
	11th to 20th	Day	29% 14% 32% 19% 4 1	180
	21st to end	Day	21% 15% 32% 23% 8% 1	198
February	1st to 10th	Day	27% 21% 35% 14% 4	252
	11th to 20th	Day	20% 15% 42% 20% 2	368
	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	14% 9% 21% 23% 20% 13%	463
	21st to end	Day	17% 8% 28% 22% 19% 6%	450
October	1st to 10th	Day	22% 10% 25% 22% 15% 6%	450
	11th to 20th	Day	22% 13% 29% 22% 12% 2	432
	21st to end	Day	34% 11% 26% 21% 8%	396
November	1st to 10th	Day	26% 15% 26% 24% 9% 1	365
	11th to 20th	Day	24% 19% 31% 18% 7% 1	295
	21st to end	Day	26% 19% 26% 22% 6% 1	180
December	1st to 10th	Day	32% 10% 26% 23% 8% 1	180
	11th to 20th	Day	23% 15% 39% 18% 3 1	180
	21st to end	Day	31% 17% 27% 18% 6% 2	198
January	1st to 10th	Night	26% 14% 30% 26% 5%	760
	11th to 20th	Night	27% 10% 36% 22% 5%	760
	21st to end	Night	20% 9% 36% 28% 7% 1	836
February	1st to 10th	Night	21% 12% 36% 25% 5% 1	760
	11th to 20th	Night	20% 12% 39% 26% 4	743
	21st to end	Night	23% 10% 36% 28% 4	628
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% 35% 21% 3	736
	21st to end	Night	18% 6% 27% 30% 18% 2	720
October	1st to 10th	Night	21% 7% 28% 28% 15% 1	720
	11th to 20th	Night	23% 9% 29% 28% 11%	720
	21st to end	Night	26% 11% 31% 27% 5%	792
November	1st to 10th	Night	26% 12% 29% 24% 9%	740
	11th to 20th	Night	23% 14% 33% 20% 9% 1	760
	21st to end	Night	29% 12% 30% 20% 8%	760
December	1st to 10th	Night	32% 12% 32% 21% 4	760
	11th to 20th	Night	30% 13% 31% 19% 5% 1	760
	21st to end	Night	34% 14% 30% 18% 4	836

Page generated at 2024-09-11 10:00:01. Version ac07

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