Chambly
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chambly clear sky chart from 2005-11-06 to 2026-05-17.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	131934
	Night	18% 12% 4 9% 5% 5% 4 3 5% 6% 29%	130896
Transparency	Day	51% 6% 12% 21% 9% 1	135378
	Night	55% 5% 11% 18% 10% 1	135539
Seeing	Day	22% 13% 29% 23% 11% 3	16462
	Night	23% 10% 30% 27% 9% 1	25507

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	16% 18% 6% 15% 6% 5% 3 3 3 4 21%	4661
	11th to 20th	Day	13% 16% 6% 15% 7% 6% 4 3 4 6% 20%	4650
	21st to end	Day	11% 14% 5% 11% 8% 8% 6% 4 5% 6% 23%	5124
February	1st to 10th	Day	15% 16% 4 11% 8% 5% 4 4 4 5% 25%	1822
	11th to 20th	Day	14% 13% 5% 11% 6% 5% 4 4 5% 6% 26%	1976
	21st to end	Day	17% 15% 4 9% 6% 6% 4 3 5% 6% 26%	1730
March	1st to 10th	Day	17% 13% 5% 11% 6% 5% 6% 4 5% 5% 24%	4576
	11th to 20th	Day	13% 14% 4 9% 6% 4 4 3 6% 7% 30%	4774
	21st to end	Day	19% 11% 4 9% 5% 4 4 3 4 5% 32%	6102
April	1st to 10th	Day	22% 11% 3 8% 5% 4 4 3 5% 5% 29%	2520
	11th to 20th	Day	21% 9% 3 7% 5% 4 3 2 4 6% 36%	2494
	21st to end	Day	21% 9% 4 7% 4 4 4 4 5% 7% 31%	2709
May	1st to 10th	Day	26% 11% 4 9% 5% 4 4 4 5% 6% 23%	6332
	11th to 20th	Day	20% 7% 3 8% 5% 5% 5% 4 5% 8% 30%	6402
	21st to end	Day	20% 8% 3 7% 5% 4 5% 3 6% 8% 31%	6949
June	1st to 10th	Day	21% 8% 2 7% 5% 5% 5% 4 6% 8% 30%	2800
	11th to 20th	Day	15% 7% 3 6% 4 4 5% 4 6% 8% 39%	2800
	21st to end	Day	21% 8% 3 7% 5% 4 4 4 7% 6% 32%	2800
July	1st to 10th	Day	12% 7% 3 8% 5% 5% 6% 5% 7% 9% 33%	6360
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 9% 41%	6360
	21st to end	Day	11% 6% 2 5% 3 4 5% 4 6% 10% 45%	6926
August	1st to 10th	Day	14% 6% 3 8% 5% 5% 5% 4 7% 9% 33%	4563
	11th to 20th	Day	13% 7% 2 6% 5% 4 5% 4 6% 9% 38%	4125
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 8% 40%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 5% 5% 6% 8% 44%	2400
	11th to 20th	Day	14% 6% 2 4 3 3 3 4 5% 7% 49%	2296
	21st to end	Day	15% 6% 2 7% 4 4 4 4 5% 8% 40%	2200
October	1st to 10th	Day	18% 7% 3 7% 5% 5% 5% 4 5% 7% 34%	2092
	11th to 20th	Day	17% 9% 3 7% 6% 5% 5% 4 5% 8% 30%	1937
	21st to end	Day	25% 11% 3 6% 4 5% 4 3 5% 5% 30%	1980
November	1st to 10th	Day	18% 12% 3 7% 6% 5% 4 4 5% 6% 31%	1851
	11th to 20th	Day	17% 13% 3 9% 5% 5% 5% 4 5% 7% 29%	1837
	21st to end	Day	23% 13% 4 9% 5% 4 4 3 6% 5% 23%	1680
December	1st to 10th	Day	20% 20% 6% 10% 6% 4 5% 3 4 4 17%	2680
	11th to 20th	Day	20% 14% 5% 9% 4 4 5% 4 5% 6% 24%	3066
	21st to end	Day	22% 15% 4 11% 5% 5% 6% 4 6% 4 18%	4048
January	1st to 10th	Night	14% 20% 7% 14% 5% 4 4 3 4 4 20%	8234
	11th to 20th	Night	12% 18% 6% 16% 7% 7% 5% 3 4 5% 16%	8235
	21st to end	Night	12% 14% 5% 11% 9% 6% 5% 4 4 5% 24%	9057
February	1st to 10th	Night	16% 16% 6% 11% 7% 6% 5% 3 5% 5% 21%	2794
	11th to 20th	Night	14% 13% 5% 10% 6% 5% 5% 3 5% 5% 28%	2530
	21st to end	Night	16% 15% 4 8% 6% 6% 4 3 4 4 31%	2076
March	1st to 10th	Night	21% 12% 4 8% 5% 4 4 3 4 4 30%	5484
	11th to 20th	Night	15% 14% 4 7% 5% 4 5% 4 5% 7% 29%	5388
	21st to end	Night	16% 13% 4 10% 5% 5% 5% 3 4 4 30%	5072
April	1st to 10th	Night	24% 10% 3 7% 4 4 5% 3 4 5% 30%	2100
	11th to 20th	Night	23% 9% 3 8% 5% 4 4 3 4 5% 33%	2075
	21st to end	Night	21% 9% 3 6% 5% 3 4 3 4 6% 35%	1911
May	1st to 10th	Night	25% 11% 4 9% 5% 5% 4 3 5% 6% 25%	3872
	11th to 20th	Night	24% 9% 3 8% 5% 4 3 3 5% 6% 30%	3623
	21st to end	Night	18% 10% 3 9% 5% 5% 4 3 5% 7% 32%	3926
June	1st to 10th	Night	22% 8% 3 7% 4 3 4 4 5% 7% 32%	1600
	11th to 20th	Night	17% 5% 2 6% 5% 4 5% 4 6% 7% 37%	1600
	21st to end	Night	18% 7% 3 9% 5% 4 4 4 5% 7% 35%	1600
July	1st to 10th	Night	14% 7% 2 8% 6% 5% 5% 5% 7% 8% 34%	3600
	11th to 20th	Night	14% 6% 2 6% 5% 4 5% 4 5% 7% 41%	3600
	21st to end	Night	10% 5% 2 7% 5% 5% 5% 4 7% 8% 42%	3926
August	1st to 10th	Night	17% 8% 3 7% 4 4 4 3 6% 6% 37%	2586
	11th to 20th	Night	16% 7% 2 8% 5% 5% 5% 4 5% 6% 38%	2887
	21st to end	Night	17% 6% 2 7% 4 4 4 4 6% 7% 39%	3586
September	1st to 10th	Night	12% 6% 3 7% 4 4 4 4 7% 7% 44%	2000
	11th to 20th	Night	14% 6% 2 5% 4 3 3 2 5% 7% 49%	2102
	21st to end	Night	17% 7% 3 7% 4 3 4 3 4 6% 42%	2200
October	1st to 10th	Night	17% 6% 3 6% 5% 3 4 3 5% 6% 40%	2305
	11th to 20th	Night	20% 9% 2 7% 4 4 5% 4 4 6% 34%	2462
	21st to end	Night	24% 9% 3 8% 5% 5% 4 3 5% 5% 30%	2860
November	1st to 10th	Night	20% 10% 3 8% 4 4 4 4 5% 6% 32%	2667
	11th to 20th	Night	20% 11% 3 9% 5% 4 4 3 5% 5% 32%	2780
	21st to end	Night	22% 14% 4 8% 5% 5% 4 3 4 6% 25%	2940
December	1st to 10th	Night	18% 20% 6% 13% 7% 5% 4 3 4 5% 16%	4720
	11th to 20th	Night	23% 14% 5% 10% 4 4 4 3 4 5% 24%	5348
	21st to end	Night	24% 16% 5% 9% 4 3 4 4 6% 5% 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	68% 5% 10% 15% 2	4662
	11th to 20th	Day	66% 5% 8% 12% 8% 1	4680
	21st to end	Day	60% 7% 9% 13% 11%	5124
February	1st to 10th	Day	62% 3 10% 18% 5% 1	1822
	11th to 20th	Day	59% 4 11% 19% 6% 1	1985
	21st to end	Day	59% 4 11% 16% 9% 1	1730
March	1st to 10th	Day	60% 6% 8% 15% 9% 2	4579
	11th to 20th	Day	52% 6% 16% 15% 10% 2	4774
	21st to end	Day	55% 4 7% 18% 15% 1	6102
April	1st to 10th	Day	56% 4 8% 19% 11% 1	2520
	11th to 20th	Day	50% 4 8% 21% 14% 1	2494
	21st to end	Day	52% 5% 10% 19% 11% 3	2709
May	1st to 10th	Day	62% 6% 8% 17% 7% 1	6343
	11th to 20th	Day	50% 7% 14% 18% 9% 1	6406
	21st to end	Day	50% 6% 11% 20% 11% 1	6949
June	1st to 10th	Day	51% 6% 11% 19% 11% 1	2800
	11th to 20th	Day	41% 6% 12% 24% 14% 2	2800
	21st to end	Day	50% 6% 12% 21% 10% 1	2800
July	1st to 10th	Day	45% 7% 13% 25% 9%	6290
	11th to 20th	Day	38% 7% 14% 30% 11%	6285
	21st to end	Day	35% 6% 14% 37% 8%	6728
August	1st to 10th	Day	44% 7% 17% 23% 8%	4549
	11th to 20th	Day	41% 7% 19% 26% 7%	4166
	21st to end	Day	40% 6% 18% 22% 14%	4312
September	1st to 10th	Day	36% 6% 16% 30% 11%	2400
	11th to 20th	Day	34% 5% 12% 34% 14% 1	2296
	21st to end	Day	41% 6% 15% 26% 11% 1	2200
October	1st to 10th	Day	43% 6% 15% 28% 8%	3404
	11th to 20th	Day	43% 7% 18% 26% 6%	3127
	21st to end	Day	47% 4 19% 22% 8% 1	3180
November	1st to 10th	Day	53% 5% 13% 20% 8% 1	1851
	11th to 20th	Day	55% 6% 11% 18% 10%	1837
	21st to end	Day	62% 7% 12% 13% 5%	1680
December	1st to 10th	Day	71% 5% 9% 13% 2	2680
	11th to 20th	Day	59% 6% 10% 16% 8%	3066
	21st to end	Day	66% 7% 12% 10% 5%	4048
January	1st to 10th	Night	67% 5% 11% 13% 4	8234
	11th to 20th	Night	70% 5% 9% 10% 5%	8280
	21st to end	Night	62% 5% 8% 14% 12%	9057
February	1st to 10th	Night	64% 4 9% 15% 7% 1	2794
	11th to 20th	Night	59% 3 11% 18% 8%	2532
	21st to end	Night	57% 4 9% 18% 11% 1	2076
March	1st to 10th	Night	58% 4 8% 15% 11% 4	5494
	11th to 20th	Night	54% 5% 12% 16% 12% 1	5389
	21st to end	Night	56% 3 6% 22% 11% 2	5072
April	1st to 10th	Night	56% 3 8% 18% 13% 1	2100
	11th to 20th	Night	54% 4 7% 20% 13% 2	2075
	21st to end	Night	51% 4 8% 20% 14% 4	1911
May	1st to 10th	Night	60% 6% 8% 17% 7% 2	3878
	11th to 20th	Night	54% 6% 9% 20% 10% 2	3624
	21st to end	Night	52% 6% 8% 18% 14% 1	3927
June	1st to 10th	Night	52% 5% 9% 21% 12% 1	1600
	11th to 20th	Night	43% 6% 12% 21% 14% 3	1600
	21st to end	Night	48% 7% 11% 20% 13% 1	1600
July	1st to 10th	Night	45% 7% 17% 21% 9%	3560
	11th to 20th	Night	39% 8% 14% 27% 12%	3552
	21st to end	Night	38% 7% 16% 27% 12%	3809
August	1st to 10th	Night	46% 6% 13% 26% 8%	2576
	11th to 20th	Night	44% 8% 17% 22% 9%	2912
	21st to end	Night	41% 8% 12% 23% 14% 1	3586
September	1st to 10th	Night	39% 6% 12% 27% 15% 1	2000
	11th to 20th	Night	36% 5% 14% 31% 14% 1	2102
	21st to end	Night	43% 5% 11% 25% 15% 1	2200
October	1st to 10th	Night	43% 6% 14% 27% 10%	3773
	11th to 20th	Night	45% 5% 11% 30% 9%	4011
	21st to end	Night	49% 5% 14% 19% 12% 1	4610
November	1st to 10th	Night	53% 5% 11% 22% 8% 1	2667
	11th to 20th	Night	55% 4 12% 18% 10% 1	2780
	21st to end	Night	62% 4 12% 16% 6%	2940
December	1st to 10th	Night	72% 5% 10% 10% 3	4720
	11th to 20th	Night	63% 4 9% 15% 10%	5348
	21st to end	Night	64% 4 10% 14% 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	30% 15% 29% 17% 9%	220
	11th to 20th	Day	28% 20% 31% 16% 4	220
	21st to end	Day	19% 18% 33% 21% 8% 1	242
February	1st to 10th	Day	27% 19% 33% 19% 3	294
	11th to 20th	Day	22% 16% 38% 21% 3	464
	21st to end	Day	25% 18% 30% 24% 3	446
March	1st to 10th	Day	25% 14% 36% 20% 5%	540
	11th to 20th	Day	25% 16% 32% 19% 6% 1	540
	21st to end	Day	28% 18% 28% 19% 6% 1	594
April	1st to 10th	Day	31% 15% 30% 16% 7% 1	540
	11th to 20th	Day	30% 20% 25% 16% 8% 2	532
	21st to end	Day	26% 17% 30% 15% 10% 2	540
May	1st to 10th	Day	25% 14% 30% 20% 9% 2	543
	11th to 20th	Day	18% 11% 30% 27% 11% 3	551
	21st to end	Day	20% 13% 30% 24% 10% 2	573
June	1st to 10th	Day	23% 12% 27% 27% 9% 2	530
	11th to 20th	Day	17% 10% 29% 28% 14% 2	530
	21st to end	Day	23% 11% 29% 23% 13% 2	530
July	1st to 10th	Day	13% 9% 26% 31% 17% 3	525
	11th to 20th	Day	13% 8% 31% 31% 13% 4	524
	21st to end	Day	14% 7% 27% 31% 18% 2	576
August	1st to 10th	Day	14% 5% 23% 33% 21% 5%	526
	11th to 20th	Day	11% 7% 25% 31% 20% 7%	519
	21st to end	Day	15% 9% 25% 24% 19% 8%	583
September	1st to 10th	Day	12% 9% 27% 28% 15% 8%	530
	11th to 20th	Day	13% 8% 19% 27% 20% 12%	530
	21st to end	Day	17% 9% 28% 23% 17% 6%	530
October	1st to 10th	Day	18% 13% 28% 21% 14% 5%	530
	11th to 20th	Day	23% 15% 29% 20% 11% 2	494
	21st to end	Day	32% 15% 25% 22% 6%	462
November	1st to 10th	Day	28% 16% 28% 21% 7%	425
	11th to 20th	Day	24% 19% 31% 19% 7% 1	377
	21st to end	Day	32% 16% 30% 17% 3 1	220
December	1st to 10th	Day	32% 12% 25% 21% 7% 2	220
	11th to 20th	Day	24% 23% 38% 12% 3	220
	21st to end	Day	29% 20% 26% 19% 3 3	242
January	1st to 10th	Night	27% 12% 32% 24% 5%	840
	11th to 20th	Night	26% 13% 34% 21% 6%	840
	21st to end	Night	20% 11% 34% 27% 7% 1	924
February	1st to 10th	Night	23% 10% 35% 25% 5% 1	840
	11th to 20th	Night	22% 11% 38% 24% 5%	823
	21st to end	Night	24% 12% 32% 28% 4	692
March	1st to 10th	Night	25% 12% 32% 26% 5%	840
	11th to 20th	Night	25% 14% 33% 23% 6%	840
	21st to end	Night	24% 11% 30% 26% 7% 1	924
April	1st to 10th	Night	28% 13% 29% 25% 5%	840
	11th to 20th	Night	26% 15% 29% 23% 6%	831
	21st to end	Night	26% 11% 30% 25% 7% 1	651
May	1st to 10th	Night	26% 7% 34% 25% 7% 1	514
	11th to 20th	Night	21% 10% 32% 29% 7% 1	406
	21st to end	Night	18% 7% 36% 30% 9%	433
June	1st to 10th	Night	24% 8% 28% 30% 10% 1	400
	11th to 20th	Night	19% 6% 29% 34% 12%	400
	21st to end	Night	19% 5% 32% 33% 12% 1	400
July	1st to 10th	Night	13% 6% 28% 39% 14% 1	390
	11th to 20th	Night	12% 3 25% 44% 14% 1	388
	21st to end	Night	13% 3 23% 47% 14% 1	438
August	1st to 10th	Night	13% 4 17% 47% 17% 2	398
	11th to 20th	Night	13% 4 21% 37% 23% 2	591
	21st to end	Night	16% 5% 19% 39% 19% 2	880
September	1st to 10th	Night	15% 5% 25% 38% 17% 1	800
	11th to 20th	Night	16% 5% 19% 34% 23% 3	800
	21st to end	Night	19% 6% 28% 30% 14% 3	800
October	1st to 10th	Night	20% 7% 29% 30% 13% 1	800
	11th to 20th	Night	25% 8% 30% 28% 8%	800
	21st to end	Night	27% 13% 30% 25% 5%	880
November	1st to 10th	Night	25% 13% 31% 23% 7%	820
	11th to 20th	Night	25% 14% 30% 22% 8% 1	840
	21st to end	Night	28% 16% 30% 18% 7%	840
December	1st to 10th	Night	30% 13% 29% 23% 5%	840
	11th to 20th	Night	30% 15% 33% 17% 5% 1	840
	21st to end	Night	33% 13% 31% 18% 5%	924

Page generated at 2026-05-14 01:58:59. Version ac07

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