Ville-Marie
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ville-Marie clear sky chart from 2005-11-06 to 2024-11-04.

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% 11% 4 9% 5% 5% 4 4 5% 6% 30%	101744
	Night	16% 13% 4 10% 6% 5% 4 3 5% 6% 28%	97324
Transparency	Day	55% 6% 11% 20% 8% 1	105268
	Night	57% 6% 11% 17% 8% 1	101889
Seeing	Day	22% 14% 31% 22% 9% 3	15157
	Night	24% 12% 30% 25% 9% 1	24107

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	13% 20% 8% 15% 6% 6% 5% 4 5% 5% 12%	2208
	11th to 20th	Day	10% 14% 6% 19% 9% 7% 5% 5% 4 5% 16%	2210
	21st to end	Day	15% 19% 10% 14% 7% 5% 5% 3 5% 5% 14%	3069
February	1st to 10th	Day	7% 17% 6% 13% 11% 8% 6% 4 5% 6% 18%	1710
	11th to 20th	Day	11% 19% 4 10% 8% 6% 5% 4 5% 7% 20%	1663
	21st to end	Day	14% 17% 4 11% 8% 5% 5% 4 4 5% 22%	1413
March	1st to 10th	Day	18% 14% 5% 10% 5% 4 4 4 5% 6% 25%	3381
	11th to 20th	Day	10% 16% 6% 16% 7% 4 4 3 4 5% 25%	3480
	21st to end	Day	12% 15% 5% 12% 6% 5% 4 3 4 5% 30%	3815
April	1st to 10th	Day	19% 15% 5% 9% 5% 4 4 4 5% 6% 26%	2270
	11th to 20th	Day	21% 11% 3 9% 5% 4 4 3 5% 5% 32%	2402
	21st to end	Day	21% 11% 3 7% 4 4 4 3 4 5% 34%	2470
May	1st to 10th	Day	18% 10% 3 7% 5% 5% 4 3 5% 7% 34%	4317
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 4 5% 7% 35%	4398
	21st to end	Day	18% 8% 3 6% 4 4 4 3 5% 6% 40%	4865
June	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 7% 31%	2850
	11th to 20th	Day	14% 6% 2 6% 6% 4 5% 5% 7% 7% 38%	2850
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	2850
July	1st to 10th	Day	17% 7% 2 7% 5% 4 5% 4 7% 8% 33%	4760
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 39%	4760
	21st to end	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	4877
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 8% 36%	4114
	11th to 20th	Day	19% 6% 3 6% 4 4 5% 4 6% 7% 37%	4044
	21st to end	Day	14% 6% 2 8% 4 4 4 4 5% 7% 42%	4445
September	1st to 10th	Day	18% 7% 2 7% 5% 4 5% 3 5% 7% 37%	2280
	11th to 20th	Day	19% 7% 2 6% 5% 4 4 4 4 7% 38%	2195
	21st to end	Day	24% 8% 2 8% 4 5% 4 3 4 6% 33%	2038
October	1st to 10th	Day	21% 13% 3 7% 4 4 4 3 5% 5% 32%	1900
	11th to 20th	Day	24% 16% 4 8% 5% 5% 3 4 4 5% 22%	1900
	21st to end	Day	26% 20% 3 10% 5% 4 3 3 4 5% 17%	2009
November	1st to 10th	Day	20% 15% 7% 8% 6% 4 4 2 4 5% 25%	1510
	11th to 20th	Day	19% 16% 5% 13% 7% 5% 4 3 5% 4 17%	1520
	21st to end	Day	23% 20% 7% 13% 7% 6% 4 2 3 4 10%	1520
December	1st to 10th	Day	16% 22% 8% 14% 6% 5% 4 4 3 4 15%	1520
	11th to 20th	Day	16% 18% 6% 12% 6% 4 6% 3 4 5% 19%	1700
	21st to end	Day	19% 19% 7% 19% 6% 4 4 2 3 3 13%	2431
January	1st to 10th	Night	14% 17% 8% 15% 7% 6% 5% 4 4 5% 15%	4748
	11th to 20th	Night	11% 15% 6% 16% 8% 7% 6% 5% 5% 4 17%	4760
	21st to end	Night	12% 22% 9% 12% 8% 7% 5% 4 4 4 13%	4582
February	1st to 10th	Night	11% 15% 5% 14% 9% 6% 4 4 5% 6% 20%	2470
	11th to 20th	Night	11% 17% 7% 12% 7% 6% 4 4 4 5% 23%	2403
	21st to end	Night	12% 16% 5% 13% 7% 6% 5% 3 5% 6% 22%	2041
March	1st to 10th	Night	13% 14% 5% 10% 5% 5% 5% 3 4 5% 32%	3454
	11th to 20th	Night	12% 19% 6% 13% 6% 5% 4 3 4 4 24%	3479
	21st to end	Night	14% 12% 5% 10% 6% 5% 4 3 4 5% 33%	3817
April	1st to 10th	Night	19% 14% 4 8% 6% 4 4 3 4 6% 28%	1909
	11th to 20th	Night	19% 11% 3 7% 4 4 4 3 5% 6% 33%	1744
	21st to end	Night	19% 11% 3 8% 4 4 4 2 5% 6% 35%	1710
May	1st to 10th	Night	17% 10% 3 9% 5% 6% 4 4 4 5% 32%	2622
	11th to 20th	Night	18% 7% 3 6% 4 5% 4 3 6% 8% 38%	2495
	21st to end	Night	17% 6% 2 6% 5% 4 5% 5% 6% 8% 37%	2707
June	1st to 10th	Night	17% 8% 2 8% 5% 5% 5% 5% 6% 7% 33%	1330
	11th to 20th	Night	20% 5% 2 4 4 4 3 3 6% 8% 42%	1330
	21st to end	Night	17% 6% 2 8% 4 4 5% 4 6% 8% 36%	1330
July	1st to 10th	Night	16% 6% 3 6% 5% 5% 5% 4 5% 6% 40%	2210
	11th to 20th	Night	15% 8% 3 6% 4 5% 5% 3 6% 7% 38%	2210
	21st to end	Night	14% 5% 2 6% 4 4 4 4 5% 7% 44%	2678
August	1st to 10th	Night	11% 5% 2 7% 6% 5% 5% 4 6% 8% 42%	2813
	11th to 20th	Night	16% 7% 3 5% 4 4 4 4 6% 8% 38%	2785
	21st to end	Night	16% 6% 2 7% 5% 4 4 3 5% 7% 43%	3178
September	1st to 10th	Night	17% 5% 2 5% 4 4 4 3 5% 7% 43%	1900
	11th to 20th	Night	19% 7% 2 5% 3 4 4 4 4 6% 43%	1983
	21st to end	Night	21% 8% 3 5% 3 3 3 3 4 7% 39%	2141
October	1st to 10th	Night	21% 11% 3 6% 4 4 4 3 5% 6% 33%	2280
	11th to 20th	Night	25% 15% 3 7% 5% 4 4 4 4 5% 24%	2280
	21st to end	Night	23% 17% 4 9% 5% 5% 4 3 4 4 22%	2586
November	1st to 10th	Night	19% 15% 5% 9% 5% 4 4 3 4 6% 26%	2641
	11th to 20th	Night	18% 17% 6% 13% 7% 5% 4 3 4 5% 18%	2660
	21st to end	Night	22% 19% 6% 12% 6% 5% 5% 4 4 4 14%	2660
December	1st to 10th	Night	17% 23% 7% 13% 6% 5% 5% 3 4 3 14%	2660
	11th to 20th	Night	16% 19% 6% 12% 8% 5% 5% 3 4 4 17%	3492
	21st to end	Night	19% 21% 6% 17% 6% 5% 4 2 3 4 13%	5236

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	73% 7% 12% 7% 1	2209
	11th to 20th	Day	71% 5% 8% 12% 4	2210
	21st to end	Day	72% 5% 9% 10% 4	3069
February	1st to 10th	Day	65% 8% 8% 12% 7%	1710
	11th to 20th	Day	64% 6% 9% 12% 9% 1	1672
	21st to end	Day	64% 6% 7% 15% 8%	1413
March	1st to 10th	Day	59% 6% 9% 19% 7% 1	3384
	11th to 20th	Day	63% 5% 8% 16% 8%	3480
	21st to end	Day	58% 4 8% 18% 10% 2	3815
April	1st to 10th	Day	59% 5% 8% 18% 8% 1	2270
	11th to 20th	Day	54% 4 8% 23% 10% 1	2402
	21st to end	Day	54% 5% 7% 17% 15% 2	2470
May	1st to 10th	Day	49% 7% 10% 22% 10% 1	4330
	11th to 20th	Day	47% 8% 12% 23% 9% 1	4402
	21st to end	Day	45% 6% 12% 19% 17% 1	4865
June	1st to 10th	Day	51% 7% 11% 21% 9% 1	2850
	11th to 20th	Day	43% 7% 12% 25% 12% 1	2850
	21st to end	Day	51% 7% 11% 22% 9% 1	2850
July	1st to 10th	Day	48% 5% 16% 24% 6%	4686
	11th to 20th	Day	41% 6% 16% 29% 7%	4679
	21st to end	Day	40% 6% 15% 32% 8%	4723
August	1st to 10th	Day	45% 6% 16% 27% 6%	4100
	11th to 20th	Day	45% 6% 15% 25% 9% 1	4083
	21st to end	Day	43% 6% 12% 30% 8% 2	4445
September	1st to 10th	Day	46% 8% 14% 22% 9%	2280
	11th to 20th	Day	47% 7% 16% 24% 6%	2195
	21st to end	Day	53% 5% 13% 22% 7%	2038
October	1st to 10th	Day	55% 4 10% 24% 6%	3160
	11th to 20th	Day	58% 5% 11% 19% 7%	3140
	21st to end	Day	68% 4 15% 8% 5%	3287
November	1st to 10th	Day	64% 5% 13% 14% 5%	1510
	11th to 20th	Day	70% 6% 9% 10% 5%	1520
	21st to end	Day	80% 5% 7% 6% 2	1520
December	1st to 10th	Day	74% 6% 8% 9% 3	1520
	11th to 20th	Day	68% 7% 7% 15% 2	1700
	21st to end	Day	77% 5% 6% 10% 2	2431
January	1st to 10th	Night	71% 7% 11% 8% 2	4748
	11th to 20th	Night	68% 7% 9% 13% 4	4760
	21st to end	Night	74% 5% 7% 8% 5%	4582
February	1st to 10th	Night	64% 6% 10% 13% 7%	2470
	11th to 20th	Night	64% 4 11% 14% 6%	2405
	21st to end	Night	62% 6% 9% 14% 8% 1	2041
March	1st to 10th	Night	56% 5% 10% 16% 11% 2	3464
	11th to 20th	Night	64% 6% 9% 14% 7% 1	3480
	21st to end	Night	55% 4 8% 17% 13% 3	3817
April	1st to 10th	Night	59% 4 8% 16% 12% 2	1909
	11th to 20th	Night	52% 4 8% 19% 14% 2	1744
	21st to end	Night	51% 5% 9% 15% 17% 2	1710
May	1st to 10th	Night	51% 9% 7% 19% 13% 1	2628
	11th to 20th	Night	43% 7% 11% 24% 14% 1	2496
	21st to end	Night	43% 7% 11% 21% 15% 3	2708
June	1st to 10th	Night	48% 7% 11% 21% 10% 2	1330
	11th to 20th	Night	41% 6% 12% 24% 15% 2	1330
	21st to end	Night	45% 7% 11% 22% 13%	1330
July	1st to 10th	Night	43% 6% 13% 26% 11%	2175
	11th to 20th	Night	45% 5% 16% 25% 9% 1	2168
	21st to end	Night	40% 5% 14% 26% 15%	2592
August	1st to 10th	Night	40% 6% 15% 30% 9%	2803
	11th to 20th	Night	42% 6% 17% 28% 6%	2812
	21st to end	Night	42% 5% 14% 27% 10% 2	3178
September	1st to 10th	Night	40% 8% 16% 25% 9% 1	1900
	11th to 20th	Night	42% 7% 15% 28% 9%	1983
	21st to end	Night	45% 7% 14% 26% 8%	2141
October	1st to 10th	Night	51% 5% 10% 24% 10%	3800
	11th to 20th	Night	57% 4 14% 22% 4	3779
	21st to end	Night	60% 6% 17% 11% 6%	4258
November	1st to 10th	Night	59% 7% 12% 17% 4	2641
	11th to 20th	Night	69% 7% 10% 9% 5%	2660
	21st to end	Night	73% 7% 10% 8% 2	2660
December	1st to 10th	Night	76% 5% 7% 9% 3	2660
	11th to 20th	Night	71% 5% 9% 12% 3	3491
	21st to end	Night	77% 4 6% 11% 3	5236

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% 16% 38% 13% 5% 1	180
	11th to 20th	Day	21% 14% 37% 19% 7% 2	180
	21st to end	Day	20% 11% 36% 21% 10% 1	285
February	1st to 10th	Day	21% 24% 34% 16% 5%	270
	11th to 20th	Day	21% 21% 39% 16% 3	270
	21st to end	Day	28% 25% 34% 12%	225
March	1st to 10th	Day	20% 17% 38% 20% 5%	438
	11th to 20th	Day	17% 18% 37% 22% 5% 1	460
	21st to end	Day	25% 17% 35% 16% 7% 1	506
April	1st to 10th	Day	25% 16% 31% 20% 7% 1	460
	11th to 20th	Day	28% 19% 23% 21% 7% 3	452
	21st to end	Day	28% 20% 27% 15% 8% 3	460
May	1st to 10th	Day	25% 12% 32% 20% 9% 2	463
	11th to 20th	Day	21% 15% 28% 22% 11% 3	487
	21st to end	Day	22% 12% 33% 22% 10% 1	544
June	1st to 10th	Day	20% 14% 32% 25% 8% 1	680
	11th to 20th	Day	19% 12% 30% 25% 11% 3	680
	21st to end	Day	21% 11% 27% 29% 11% 3	680
July	1st to 10th	Day	15% 11% 32% 30% 10% 2	671
	11th to 20th	Day	18% 11% 29% 25% 14% 3	668
	21st to end	Day	13% 9% 28% 31% 15% 4	569
August	1st to 10th	Day	18% 10% 27% 26% 15% 5%	486
	11th to 20th	Day	18% 5% 24% 25% 18% 10%	479
	21st to end	Day	21% 9% 30% 20% 14% 6%	539
September	1st to 10th	Day	22% 8% 30% 24% 12% 4	490
	11th to 20th	Day	20% 9% 23% 31% 11% 6%	490
	21st to end	Day	25% 11% 25% 21% 14% 3	490
October	1st to 10th	Day	27% 14% 27% 18% 12% 2	490
	11th to 20th	Day	31% 19% 29% 16% 3 1	490
	21st to end	Day	34% 16% 31% 16% 3 1	471
November	1st to 10th	Day	31% 18% 39% 10% 3	186
	11th to 20th	Day	26% 16% 37% 16% 4 1	180
	21st to end	Day	33% 18% 36% 10% 2 1	180
December	1st to 10th	Day	26% 16% 34% 20% 3 1	180
	11th to 20th	Day	27% 16% 37% 13% 6% 2	180
	21st to end	Day	24% 20% 29% 19% 6% 2	198
January	1st to 10th	Night	26% 14% 34% 21% 5%	950
	11th to 20th	Night	25% 14% 31% 24% 6%	950
	21st to end	Night	19% 11% 31% 27% 11% 1	836
February	1st to 10th	Night	19% 15% 34% 25% 7%	760
	11th to 20th	Night	21% 14% 37% 22% 5%	743
	21st to end	Night	22% 18% 33% 21% 5%	628
March	1st to 10th	Night	20% 18% 34% 23% 5% 1	760
	11th to 20th	Night	23% 16% 32% 22% 6% 1	760
	21st to end	Night	20% 13% 37% 24% 6% 1	836
April	1st to 10th	Night	26% 15% 29% 24% 6% 1	760
	11th to 20th	Night	26% 14% 27% 24% 9%	620
	21st to end	Night	24% 12% 28% 28% 7% 1	570
May	1st to 10th	Night	20% 9% 29% 33% 7% 1	449
	11th to 20th	Night	21% 9% 31% 28% 9% 1	374
	21st to end	Night	21% 9% 26% 38% 6%	411
June	1st to 10th	Night	18% 10% 28% 36% 8%	380
	11th to 20th	Night	21% 8% 27% 32% 12%	380
	21st to end	Night	16% 8% 27% 36% 13% 1	380
July	1st to 10th	Night	15% 6% 30% 40% 9% 1	370
	11th to 20th	Night	16% 6% 24% 40% 13%	368
	21st to end	Night	13% 5% 28% 37% 16% 1	416
August	1st to 10th	Night	15% 5% 25% 36% 17% 2	555
	11th to 20th	Night	18% 5% 23% 32% 20% 2	564
	21st to end	Night	20% 7% 27% 26% 17% 2	655
September	1st to 10th	Night	19% 8% 27% 29% 16% 1	760
	11th to 20th	Night	21% 8% 26% 26% 17% 2	760
	21st to end	Night	25% 6% 23% 31% 13% 2	760
October	1st to 10th	Night	27% 9% 29% 26% 9% 1	760
	11th to 20th	Night	33% 12% 28% 20% 7% 1	760
	21st to end	Night	33% 13% 29% 22% 3 1	836
November	1st to 10th	Night	27% 14% 32% 20% 6% 1	755
	11th to 20th	Night	25% 17% 35% 17% 6%	760
	21st to end	Night	33% 16% 28% 19% 4	760
December	1st to 10th	Night	31% 16% 29% 18% 5% 1	760
	11th to 20th	Night	27% 14% 32% 20% 5% 1	916
	21st to end	Night	29% 16% 25% 23% 6% 1	1045

Page generated at 2024-11-01 22:47:52. Version ac07

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