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 2024-01-31.

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	17% 9% 3 6% 6% 3 5% 5% 5% 7% 34%	74903
	Night	19% 10% 4 7% 6% 3 5% 4 5% 6% 32%	75220
Transparency	Day	33% 39% 9% 13% 6% 1	74608
	Night	36% 38% 8% 12% 6% 1	75049
Seeing	Day	20% 11% 28% 24% 12% 4	13501
	Night	23% 9% 30% 28% 10% 1	22631

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	19% 17% 5% 10% 8% 3 5% 4 4 4 20%	1517
	11th to 20th	Day	19% 14% 6% 9% 5% 4 5% 5% 4 4 24%	1520
	21st to end	Day	12% 14% 4 8% 7% 4 7% 5% 6% 7% 26%	1664
February	1st to 10th	Day	15% 15% 5% 9% 7% 4 6% 4 4 5% 25%	1584
	11th to 20th	Day	14% 11% 4 10% 7% 5% 4 6% 5% 6% 28%	1666
	21st to end	Day	19% 15% 4 7% 8% 3 5% 4 4 5% 27%	1480
March	1st to 10th	Day	16% 11% 5% 9% 6% 3 6% 5% 4 5% 30%	1797
	11th to 20th	Day	16% 11% 4 6% 6% 4 5% 5% 5% 6% 32%	1864
	21st to end	Day	19% 9% 4 7% 4 3 4 4 5% 6% 35%	2372
April	1st to 10th	Day	21% 9% 4 7% 6% 3 5% 4 5% 6% 32%	2160
	11th to 20th	Day	19% 7% 3 7% 5% 2 4 4 5% 6% 37%	2134
	21st to end	Day	21% 9% 3 6% 5% 3 5% 4 4 6% 33%	2303
May	1st to 10th	Day	18% 7% 4 7% 6% 4 5% 5% 4 6% 35%	2482
	11th to 20th	Day	18% 6% 3 6% 5% 3 4 4 5% 7% 39%	2478
	21st to end	Day	17% 6% 3 5% 6% 3 5% 5% 6% 9% 36%	2725
June	1st to 10th	Day	21% 8% 3 6% 5% 3 4 5% 7% 8% 31%	2520
	11th to 20th	Day	15% 6% 2 6% 6% 3 5% 5% 5% 9% 39%	2520
	21st to end	Day	18% 7% 3 6% 5% 3 6% 5% 5% 7% 33%	2520
July	1st to 10th	Day	11% 6% 3 6% 5% 3 5% 5% 6% 8% 44%	2520
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 7% 7% 10% 39%	2520
	21st to end	Day	12% 4 3 5% 6% 3 5% 6% 6% 9% 39%	2744
August	1st to 10th	Day	12% 6% 3 6% 5% 3 5% 6% 7% 9% 38%	2506
	11th to 20th	Day	10% 5% 3 5% 5% 3 5% 6% 6% 9% 43%	2278
	21st to end	Day	12% 5% 2 4 6% 4 5% 5% 6% 9% 42%	2376
September	1st to 10th	Day	11% 5% 3 4 5% 3 5% 6% 6% 8% 43%	2160
	11th to 20th	Day	15% 5% 2 3 4 3 3 4 5% 6% 49%	2084
	21st to end	Day	16% 6% 2 5% 5% 3 4 5% 6% 7% 41%	1980
October	1st to 10th	Day	19% 7% 4 6% 5% 4 4 5% 5% 5% 35%	1859
	11th to 20th	Day	18% 9% 3 6% 7% 5% 6% 5% 6% 6% 29%	1764
	21st to end	Day	27% 10% 3 5% 4 3 4 5% 5% 6% 28%	1782
November	1st to 10th	Day	18% 9% 3 6% 5% 3 4 6% 5% 8% 33%	1671
	11th to 20th	Day	18% 10% 5% 7% 7% 3 6% 4 5% 7% 27%	1635
	21st to end	Day	22% 11% 4 7% 7% 3 6% 5% 5% 6% 23%	1520
December	1st to 10th	Day	25% 14% 6% 8% 6% 4 5% 4 5% 5% 18%	1520
	11th to 20th	Day	21% 15% 5% 8% 6% 2 5% 5% 6% 5% 23%	1906
	21st to end	Day	27% 14% 4 11% 7% 3 4 3 4 4 19%	2772
January	1st to 10th	Night	18% 16% 7% 10% 7% 3 5% 4 4 5% 20%	2649
	11th to 20th	Night	18% 15% 6% 10% 7% 3 4 5% 5% 5% 22%	2660
	21st to end	Night	13% 13% 5% 8% 8% 4 5% 5% 4 6% 29%	2919
February	1st to 10th	Night	15% 15% 7% 11% 7% 4 6% 4 5% 5% 21%	2373
	11th to 20th	Night	16% 10% 4 9% 7% 4 6% 5% 5% 6% 29%	2179
	21st to end	Night	16% 13% 6% 8% 6% 3 5% 3 3 5% 32%	1776
March	1st to 10th	Night	17% 13% 5% 8% 6% 4 5% 4 5% 5% 28%	2150
	11th to 20th	Night	18% 12% 4 7% 6% 4 4 3 4 6% 31%	2092
	21st to end	Night	19% 9% 3 8% 5% 3 4 3 4 4 38%	1983
April	1st to 10th	Night	24% 8% 3 6% 5% 3 5% 5% 5% 5% 30%	1800
	11th to 20th	Night	23% 8% 3 6% 4 2 5% 4 5% 6% 35%	1775
	21st to end	Night	22% 9% 3 7% 5% 2 5% 3 5% 6% 33%	1656
May	1st to 10th	Night	22% 7% 4 6% 5% 3 3 3 4 6% 36%	1494
	11th to 20th	Night	20% 6% 3 5% 5% 3 5% 4 4 6% 38%	1415
	21st to end	Night	16% 5% 3 6% 5% 4 5% 4 5% 7% 40%	1550
June	1st to 10th	Night	22% 6% 3 6% 6% 3 4 5% 6% 8% 32%	1440
	11th to 20th	Night	16% 6% 3 6% 4 2 4 5% 6% 8% 39%	1440
	21st to end	Night	17% 7% 2 6% 6% 4 5% 5% 6% 7% 34%	1440
July	1st to 10th	Night	12% 4 2 5% 5% 3 6% 5% 6% 10% 42%	1440
	11th to 20th	Night	11% 5% 3 5% 6% 3 5% 6% 8% 10% 40%	1440
	21st to end	Night	12% 5% 3 5% 6% 4 5% 5% 6% 9% 41%	1574
August	1st to 10th	Night	14% 4 2 4 4 2 5% 5% 6% 9% 44%	1432
	11th to 20th	Night	13% 5% 3 6% 5% 4 5% 4 6% 7% 43%	1599
	21st to end	Night	14% 5% 2 5% 5% 3 5% 5% 5% 9% 43%	1980
September	1st to 10th	Night	13% 5% 2 6% 6% 3 4 4 6% 7% 43%	1800
	11th to 20th	Night	16% 6% 3 5% 3 2 4 4 4 6% 48%	1875
	21st to end	Night	17% 5% 3 5% 4 3 5% 4 4 7% 42%	1980
October	1st to 10th	Night	18% 7% 2 6% 5% 3 5% 5% 4 7% 36%	2099
	11th to 20th	Night	21% 7% 3 8% 5% 3 5% 4 4 6% 34%	2196
	21st to end	Night	24% 9% 3 7% 5% 4 4 4 5% 5% 29%	2574
November	1st to 10th	Night	20% 10% 4 7% 6% 3 4 5% 6% 6% 31%	2407
	11th to 20th	Night	19% 10% 4 7% 6% 3 5% 4 5% 5% 32%	2541
	21st to end	Night	23% 14% 3 8% 6% 3 4 4 5% 6% 25%	2660
December	1st to 10th	Night	23% 15% 7% 9% 6% 3 5% 5% 4 5% 19%	2660
	11th to 20th	Night	24% 14% 6% 8% 5% 3 4 3 5% 5% 23%	3288
	21st to end	Night	29% 16% 5% 8% 5% 2 3 4 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	42% 41% 7% 9% 2	1518
	11th to 20th	Day	40% 41% 6% 8% 5%	1520
	21st to end	Day	37% 39% 9% 8% 6%	1664
February	1st to 10th	Day	41% 36% 8% 12% 3 1	1584
	11th to 20th	Day	38% 40% 10% 9% 3 1	1675
	21st to end	Day	40% 37% 8% 11% 4	1480
March	1st to 10th	Day	38% 37% 8% 12% 4 1	1800
	11th to 20th	Day	34% 39% 11% 11% 5% 1	1864
	21st to end	Day	31% 38% 8% 12% 9% 2	2372
April	1st to 10th	Day	36% 37% 5% 14% 7% 1	2160
	11th to 20th	Day	30% 38% 6% 14% 10% 1	2134
	21st to end	Day	34% 38% 6% 11% 7% 3	2303
May	1st to 10th	Day	31% 38% 7% 12% 10% 2	2482
	11th to 20th	Day	29% 38% 8% 12% 10% 2	2482
	21st to end	Day	27% 39% 8% 13% 10% 2	2725
June	1st to 10th	Day	33% 40% 9% 11% 7% 1	2520
	11th to 20th	Day	28% 38% 8% 16% 8% 2	2520
	21st to end	Day	35% 40% 9% 11% 5% 1	2520
July	1st to 10th	Day	26% 41% 9% 17% 7%	2450
	11th to 20th	Day	27% 41% 11% 16% 6%	2445
	21st to end	Day	26% 41% 12% 14% 6%	2591
August	1st to 10th	Day	29% 39% 10% 16% 5%	2492
	11th to 20th	Day	22% 39% 14% 18% 7%	2278
	21st to end	Day	26% 39% 11% 14% 9%	2376
September	1st to 10th	Day	26% 39% 10% 17% 8%	2160
	11th to 20th	Day	25% 38% 8% 18% 10%	2084
	21st to end	Day	29% 40% 9% 16% 5%	1980
October	1st to 10th	Day	34% 39% 8% 13% 5%	1859
	11th to 20th	Day	36% 40% 8% 11% 4	1764
	21st to end	Day	36% 38% 9% 11% 5% 1	1782
November	1st to 10th	Day	27% 42% 10% 14% 6%	1671
	11th to 20th	Day	37% 41% 8% 7% 7%	1635
	21st to end	Day	38% 43% 8% 10% 1	1520
December	1st to 10th	Day	44% 43% 6% 6% 2	1520
	11th to 20th	Day	42% 35% 10% 10% 2	1906
	21st to end	Day	54% 29% 8% 7% 2	2772
January	1st to 10th	Night	42% 42% 6% 8% 2	2649
	11th to 20th	Night	41% 42% 7% 7% 3	2660
	21st to end	Night	39% 37% 8% 9% 6% 1	2919
February	1st to 10th	Night	43% 37% 7% 9% 4	2373
	11th to 20th	Night	37% 39% 10% 11% 3	2181
	21st to end	Night	40% 37% 7% 11% 5%	1776
March	1st to 10th	Night	39% 37% 7% 12% 5% 1	2160
	11th to 20th	Night	38% 37% 8% 12% 5%	2093
	21st to end	Night	32% 37% 6% 13% 9% 2	1983
April	1st to 10th	Night	35% 37% 7% 12% 7% 1	1800
	11th to 20th	Night	33% 36% 7% 14% 8% 1	1775
	21st to end	Night	33% 37% 6% 13% 8% 3	1656
May	1st to 10th	Night	34% 36% 4 11% 10% 5%	1494
	11th to 20th	Night	29% 39% 6% 11% 11% 4	1416
	21st to end	Night	28% 38% 8% 12% 13% 2	1551
June	1st to 10th	Night	34% 40% 7% 13% 5% 1	1440
	11th to 20th	Night	28% 39% 9% 14% 8% 2	1440
	21st to end	Night	32% 40% 9% 12% 7%	1440
July	1st to 10th	Night	26% 41% 9% 15% 8%	1400
	11th to 20th	Night	25% 41% 11% 14% 9%	1392
	21st to end	Night	26% 41% 11% 15% 6%	1486
August	1st to 10th	Night	26% 40% 11% 16% 7%	1422
	11th to 20th	Night	23% 38% 13% 16% 9%	1599
	21st to end	Night	27% 41% 10% 15% 7% 1	1980
September	1st to 10th	Night	28% 38% 8% 16% 9%	1800
	11th to 20th	Night	26% 38% 8% 20% 8%	1875
	21st to end	Night	29% 39% 8% 15% 9%	1980
October	1st to 10th	Night	32% 39% 7% 14% 8%	2099
	11th to 20th	Night	36% 38% 9% 13% 4	2196
	21st to end	Night	35% 39% 9% 11% 5% 1	2574
November	1st to 10th	Night	31% 42% 8% 13% 6%	2407
	11th to 20th	Night	32% 41% 8% 11% 6% 1	2541
	21st to end	Night	39% 41% 8% 10% 3	2660
December	1st to 10th	Night	45% 41% 7% 6% 1	2660
	11th to 20th	Night	44% 33% 8% 12% 3	3288
	21st to end	Night	54% 27% 8% 10% 2	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% 16% 32% 21% 8% 1	196
February	1st to 10th	Day	29% 21% 35% 12% 3	224
	11th to 20th	Day	22% 12% 42% 20% 3	333
	21st to end	Day	28% 13% 28% 28% 3	346
March	1st to 10th	Day	23% 13% 34% 23% 7%	420
	11th to 20th	Day	18% 17% 35% 21% 7% 2	420
	21st to end	Day	27% 12% 30% 18% 11% 2	462
April	1st to 10th	Day	27% 14% 29% 20% 9% 1	420
	11th to 20th	Day	25% 14% 27% 20% 10% 4	412
	21st to end	Day	25% 13% 29% 21% 9% 2	420
May	1st to 10th	Day	23% 10% 33% 22% 9% 3	423
	11th to 20th	Day	18% 10% 32% 24% 13% 4	447
	21st to end	Day	17% 9% 31% 26% 13% 3	485
June	1st to 10th	Day	24% 10% 24% 27% 12% 3	450
	11th to 20th	Day	14% 11% 25% 31% 16% 3	450
	21st to end	Day	19% 8% 28% 26% 15% 5%	450
July	1st to 10th	Day	11% 8% 24% 30% 21% 6%	445
	11th to 20th	Day	12% 6% 28% 30% 18% 5%	444
	21st to end	Day	14% 6% 24% 33% 18% 5%	488
August	1st to 10th	Day	15% 5% 22% 31% 21% 6%	446
	11th to 20th	Day	12% 5% 21% 34% 18% 10%	439
	21st to end	Day	14% 8% 27% 23% 20% 9%	495
September	1st to 10th	Day	12% 9% 25% 26% 19% 9%	450
	11th to 20th	Day	14% 9% 22% 23% 20% 12%	450
	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% 10% 35% 28% 7% 1	835
February	1st to 10th	Night	22% 12% 36% 25% 5% 1	720
	11th to 20th	Night	21% 11% 38% 26% 4	703
	21st to end	Night	23% 9% 36% 28% 4	592
March	1st to 10th	Night	24% 11% 33% 26% 5%	720
	11th to 20th	Night	26% 10% 33% 25% 6%	720
	21st to end	Night	24% 8% 30% 29% 9% 1	792
April	1st to 10th	Night	27% 11% 29% 28% 6%	720
	11th to 20th	Night	26% 11% 31% 26% 6% 1	711
	21st to end	Night	26% 10% 30% 25% 9%	576
May	1st to 10th	Night	26% 7% 30% 26% 10% 1	414
	11th to 20th	Night	20% 6% 32% 33% 8% 1	354
	21st to end	Night	15% 6% 32% 35% 12%	389
June	1st to 10th	Night	23% 6% 29% 32% 9% 1	360
	11th to 20th	Night	19% 4 28% 34% 15%	360
	21st to end	Night	18% 6% 27% 33% 16% 1	360
July	1st to 10th	Night	13% 3 26% 39% 19% 1	350
	11th to 20th	Night	11% 4 25% 42% 17% 1	348
	21st to end	Night	11% 3 24% 43% 17% 2	394
August	1st to 10th	Night	14% 3 16% 45% 21% 1	358
	11th to 20th	Night	14% 3 22% 36% 23% 3	535
	21st to end	Night	16% 3 23% 36% 20% 2	792
September	1st to 10th	Night	13% 4 24% 39% 18% 1	720
	11th to 20th	Night	18% 6% 18% 35% 21% 2	720
	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

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