Lachute
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lachute clear sky chart from 2005-11-06 to 2026-03-18.

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% 4 4 5% 6% 32%	131287
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	130213
Transparency	Day	52% 3 12% 21% 10% 1	134731
	Night	55% 3 11% 20% 10% 1	134856
Seeing	Day	23% 14% 30% 21% 9% 2	16144
	Night	23% 12% 28% 26% 9% 1	25280

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	15% 18% 6% 14% 5% 5% 4 3 4 5% 22%	4661
	11th to 20th	Day	13% 18% 7% 13% 7% 5% 4 3 4 5% 22%	4650
	21st to end	Day	8% 16% 7% 13% 9% 6% 6% 4 5% 4 22%	5124
February	1st to 10th	Day	13% 16% 5% 12% 6% 5% 4 3 5% 5% 26%	1874
	11th to 20th	Day	14% 14% 5% 11% 6% 5% 4 3 6% 6% 25%	1918
	21st to end	Day	18% 15% 5% 9% 6% 5% 4 3 5% 5% 24%	1730
March	1st to 10th	Day	19% 15% 5% 11% 6% 4 4 2 4 4 25%	4576
	11th to 20th	Day	12% 14% 4 9% 5% 5% 4 4 6% 7% 31%	4856
	21st to end	Day	19% 9% 4 9% 5% 5% 3 3 4 4 35%	5915
April	1st to 10th	Day	21% 12% 3 7% 5% 4 4 3 4 5% 33%	2400
	11th to 20th	Day	21% 9% 3 7% 4 5% 4 3 5% 5% 35%	2374
	21st to end	Day	24% 9% 3 7% 4 4 4 3 4 5% 32%	2539
May	1st to 10th	Day	26% 11% 3 8% 5% 5% 5% 4 5% 6% 22%	6346
	11th to 20th	Day	20% 8% 3 8% 5% 4 4 3 5% 6% 31%	6318
	21st to end	Day	22% 9% 2 6% 4 4 4 4 6% 7% 31%	6949
June	1st to 10th	Day	21% 9% 3 7% 6% 4 4 4 6% 7% 30%	2800
	11th to 20th	Day	15% 6% 2 7% 5% 6% 5% 4 6% 9% 37%	2800
	21st to end	Day	21% 8% 3 7% 6% 5% 5% 5% 6% 7% 29%	2800
July	1st to 10th	Day	14% 6% 3 8% 6% 6% 6% 5% 7% 8% 33%	6360
	11th to 20th	Day	13% 6% 2 5% 5% 5% 5% 4 7% 8% 41%	6360
	21st to end	Day	10% 6% 2 5% 4 4 5% 4 6% 9% 47%	6926
August	1st to 10th	Day	14% 6% 2 8% 5% 5% 6% 4 7% 8% 34%	4563
	11th to 20th	Day	15% 7% 2 6% 4 4 4 5% 6% 8% 38%	4119
	21st to end	Day	16% 5% 2 6% 4 5% 5% 3 6% 9% 39%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 5% 5% 6% 8% 42%	2400
	11th to 20th	Day	14% 4 2 5% 4 4 4 3 4 7% 50%	2340
	21st to end	Day	18% 5% 2 5% 4 4 3 3 6% 7% 42%	2200
October	1st to 10th	Day	21% 7% 2 7% 4 3 4 3 5% 6% 38%	2025
	11th to 20th	Day	22% 9% 3 7% 6% 5% 4 4 4 7% 30%	1983
	21st to end	Day	24% 11% 3 7% 5% 5% 4 4 4 6% 28%	1980
November	1st to 10th	Day	15% 11% 3 8% 6% 5% 4 4 6% 6% 32%	1851
	11th to 20th	Day	19% 12% 4 7% 5% 6% 4 4 5% 6% 28%	1764
	21st to end	Day	23% 15% 3 7% 5% 5% 5% 3 5% 6% 23%	1680
December	1st to 10th	Day	20% 21% 5% 10% 5% 4 4 4 5% 5% 18%	2680
	11th to 20th	Day	18% 17% 5% 10% 5% 4 3 3 4 4 27%	3066
	21st to end	Day	23% 15% 4 9% 5% 4 4 3 4 4 25%	4048
January	1st to 10th	Night	14% 19% 7% 14% 6% 4 4 3 4 4 21%	8234
	11th to 20th	Night	11% 18% 7% 18% 7% 5% 4 3 4 5% 17%	8235
	21st to end	Night	10% 15% 6% 13% 7% 5% 5% 4 5% 6% 25%	9057
February	1st to 10th	Night	13% 14% 5% 11% 6% 5% 5% 3 5% 6% 26%	2742
	11th to 20th	Night	13% 12% 5% 10% 6% 6% 5% 3 5% 5% 29%	2587
	21st to end	Night	14% 15% 5% 9% 5% 5% 4 3 4 5% 30%	2076
March	1st to 10th	Night	21% 14% 4 8% 5% 4 4 3 4 4 29%	5484
	11th to 20th	Night	14% 15% 3 7% 4 4 5% 3 6% 7% 31%	5256
	21st to end	Night	17% 11% 4 8% 5% 5% 4 4 4 5% 33%	5016
April	1st to 10th	Night	23% 9% 3 7% 4 4 4 3 5% 6% 33%	2000
	11th to 20th	Night	20% 9% 3 6% 5% 5% 4 3 4 6% 36%	1975
	21st to end	Night	22% 9% 2 6% 5% 4 4 3 5% 6% 35%	1865
May	1st to 10th	Night	24% 8% 3 9% 5% 5% 5% 4 6% 6% 26%	3608
	11th to 20th	Night	22% 8% 3 6% 4 3 4 3 5% 6% 35%	3575
	21st to end	Night	21% 10% 3 8% 5% 5% 4 3 5% 6% 30%	3926
June	1st to 10th	Night	20% 7% 2 7% 4 6% 4 5% 6% 8% 32%	1600
	11th to 20th	Night	16% 7% 3 6% 4 4 5% 4 5% 7% 40%	1600
	21st to end	Night	20% 6% 2 6% 5% 5% 4 4 5% 6% 37%	1600
July	1st to 10th	Night	14% 6% 3 8% 5% 5% 5% 5% 7% 9% 33%	3600
	11th to 20th	Night	14% 7% 3 6% 4 5% 4 3 5% 8% 42%	3600
	21st to end	Night	11% 5% 2 6% 4 4 5% 4 5% 8% 47%	3926
August	1st to 10th	Night	14% 7% 2 7% 4 4 5% 4 5% 8% 39%	2586
	11th to 20th	Night	16% 6% 2 6% 4 4 4 4 6% 6% 42%	2894
	21st to end	Night	17% 6% 2 7% 4 4 4 3 5% 8% 41%	3586
September	1st to 10th	Night	12% 5% 2 6% 4 5% 4 3 5% 7% 46%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 7% 51%	2059
	21st to end	Night	17% 6% 2 5% 4 4 3 3 5% 7% 45%	2200
October	1st to 10th	Night	18% 7% 3 6% 3 3 4 3 5% 6% 41%	2371
	11th to 20th	Night	21% 8% 3 6% 5% 4 5% 3 5% 5% 36%	2416
	21st to end	Night	22% 9% 3 8% 5% 4 4 3 5% 5% 31%	2860
November	1st to 10th	Night	20% 11% 3 7% 4 4 4 3 5% 6% 33%	2667
	11th to 20th	Night	17% 11% 3 8% 5% 5% 4 4 4 5% 33%	2854
	21st to end	Night	20% 15% 4 8% 5% 4 4 3 4 5% 27%	2940
December	1st to 10th	Night	17% 17% 5% 10% 6% 5% 5% 3 6% 5% 20%	4720
	11th to 20th	Night	23% 15% 4 10% 5% 4 3 3 3 4 26%	5348
	21st to end	Night	25% 17% 4 8% 4 3 3 3 3 5% 26%	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	65% 3 10% 17% 4 1	4662
	11th to 20th	Day	65% 3 8% 14% 10% 1	4680
	21st to end	Day	63% 3 10% 12% 11% 1	5124
February	1st to 10th	Day	60% 2 8% 21% 7% 2	1874
	11th to 20th	Day	59% 2 10% 18% 10% 1	1927
	21st to end	Day	63% 2 9% 14% 11% 2	1730
March	1st to 10th	Day	63% 2 8% 13% 11% 3	4579
	11th to 20th	Day	54% 2 16% 15% 11% 2	4856
	21st to end	Day	54% 2 6% 17% 20% 2	5915
April	1st to 10th	Day	55% 2 7% 20% 14% 3	2400
	11th to 20th	Day	52% 2 6% 22% 16% 2	2374
	21st to end	Day	55% 3 8% 17% 13% 3	2539
May	1st to 10th	Day	63% 2 6% 19% 8% 1	6357
	11th to 20th	Day	55% 3 13% 19% 9% 2	6322
	21st to end	Day	53% 3 11% 20% 13% 1	6949
June	1st to 10th	Day	53% 4 9% 20% 13% 1	2800
	11th to 20th	Day	44% 3 12% 24% 14% 3	2800
	21st to end	Day	53% 3 13% 19% 10% 1	2800
July	1st to 10th	Day	46% 5% 14% 26% 8%	6290
	11th to 20th	Day	39% 5% 14% 30% 12%	6285
	21st to end	Day	33% 5% 16% 36% 10%	6728
August	1st to 10th	Day	45% 5% 18% 23% 9%	4549
	11th to 20th	Day	42% 5% 17% 29% 7%	4161
	21st to end	Day	41% 5% 17% 23% 13% 1	4312
September	1st to 10th	Day	40% 4 14% 30% 13%	2400
	11th to 20th	Day	35% 3 14% 32% 15% 1	2340
	21st to end	Day	42% 4 14% 27% 12% 1	2200
October	1st to 10th	Day	46% 4 13% 27% 10%	3298
	11th to 20th	Day	46% 4 16% 27% 7% 1	3211
	21st to end	Day	46% 4 21% 21% 8%	3180
November	1st to 10th	Day	52% 3 13% 22% 9% 1	1851
	11th to 20th	Day	57% 3 11% 18% 11% 1	1764
	21st to end	Day	63% 5% 13% 14% 5% 1	1680
December	1st to 10th	Day	68% 4 11% 14% 3	2680
	11th to 20th	Day	63% 3 9% 17% 8%	3066
	21st to end	Day	64% 2 10% 17% 7%	4048
January	1st to 10th	Night	68% 3 11% 14% 5% 1	8234
	11th to 20th	Night	71% 2 8% 12% 6%	8280
	21st to end	Night	61% 3 9% 15% 12% 1	9057
February	1st to 10th	Night	58% 4 9% 19% 9% 1	2742
	11th to 20th	Night	57% 3 10% 21% 9% 1	2589
	21st to end	Night	57% 3 9% 18% 13% 1	2076
March	1st to 10th	Night	59% 2 7% 16% 12% 4	5494
	11th to 20th	Night	55% 3 13% 16% 11% 3	5257
	21st to end	Night	56% 2 6% 21% 14% 2	5016
April	1st to 10th	Night	55% 2 7% 19% 15% 2	2000
	11th to 20th	Night	52% 3 6% 22% 15% 2	1975
	21st to end	Night	52% 3 7% 19% 15% 4	1865
May	1st to 10th	Night	60% 3 9% 19% 8% 2	3614
	11th to 20th	Night	52% 3 12% 21% 11% 2	3576
	21st to end	Night	55% 3 9% 17% 14% 1	3927
June	1st to 10th	Night	50% 3 11% 22% 13% 1	1600
	11th to 20th	Night	45% 3 10% 24% 16% 2	1600
	21st to end	Night	47% 4 13% 20% 15% 1	1600
July	1st to 10th	Night	46% 5% 18% 21% 9% 1	3560
	11th to 20th	Night	40% 4 16% 30% 9%	3552
	21st to end	Night	36% 4 16% 30% 14%	3809
August	1st to 10th	Night	43% 4 15% 27% 9%	2576
	11th to 20th	Night	42% 5% 17% 28% 8%	2918
	21st to end	Night	42% 5% 16% 22% 13% 1	3586
September	1st to 10th	Night	38% 4 12% 29% 16% 1	2000
	11th to 20th	Night	35% 3 14% 35% 13% 1	2059
	21st to end	Night	40% 3 12% 30% 15%	2200
October	1st to 10th	Night	42% 4 15% 26% 11% 1	3878
	11th to 20th	Night	45% 4 14% 28% 10%	3927
	21st to end	Night	47% 4 18% 19% 11% 1	4610
November	1st to 10th	Night	53% 3 13% 22% 9% 1	2667
	11th to 20th	Night	54% 3 12% 19% 10% 2	2854
	21st to end	Night	60% 4 11% 18% 6%	2940
December	1st to 10th	Night	66% 3 13% 14% 4	4720
	11th to 20th	Night	64% 3 6% 16% 10%	5348
	21st to end	Night	65% 2 7% 15% 11%	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	26% 17% 31% 21% 5%	220
	11th to 20th	Day	29% 21% 32% 13% 5%	220
	21st to end	Day	21% 18% 36% 18% 7%	242
February	1st to 10th	Day	30% 20% 34% 12% 3	322
	11th to 20th	Day	23% 24% 39% 14%	405
	21st to end	Day	30% 19% 30% 18% 3	446
March	1st to 10th	Day	27% 20% 31% 18% 4	540
	11th to 20th	Day	27% 20% 34% 14% 4 1	528
	21st to end	Day	28% 18% 29% 15% 8% 1	550
April	1st to 10th	Day	32% 15% 29% 17% 6% 2	500
	11th to 20th	Day	30% 18% 27% 16% 6% 2	492
	21st to end	Day	30% 18% 29% 15% 6% 1	500
May	1st to 10th	Day	27% 14% 32% 20% 6% 1	503
	11th to 20th	Day	23% 13% 29% 22% 10% 2	527
	21st to end	Day	23% 13% 31% 23% 8% 2	573
June	1st to 10th	Day	24% 14% 30% 23% 8% 1	530
	11th to 20th	Day	18% 14% 32% 24% 10% 2	530
	21st to end	Day	25% 10% 32% 23% 7% 3	530
July	1st to 10th	Day	14% 9% 34% 27% 14% 3	525
	11th to 20th	Day	15% 9% 33% 29% 11% 2	524
	21st to end	Day	15% 8% 33% 27% 14% 2	576
August	1st to 10th	Day	16% 7% 26% 33% 16% 3	526
	11th to 20th	Day	13% 7% 26% 30% 16% 7%	519
	21st to end	Day	16% 8% 29% 25% 16% 6%	583
September	1st to 10th	Day	15% 8% 31% 25% 15% 6%	530
	11th to 20th	Day	12% 7% 24% 25% 21% 11%	530
	21st to end	Day	20% 12% 28% 21% 16% 4	530
October	1st to 10th	Day	23% 12% 28% 22% 13% 3	530
	11th to 20th	Day	23% 18% 29% 20% 9% 2	520
	21st to end	Day	32% 11% 29% 22% 6%	462
November	1st to 10th	Day	30% 15% 27% 21% 7% 1	425
	11th to 20th	Day	28% 16% 35% 16% 5%	304
	21st to end	Day	30% 19% 30% 17% 4	220
December	1st to 10th	Day	30% 17% 26% 18% 7% 2	220
	11th to 20th	Day	30% 21% 34% 12% 3	220
	21st to end	Day	33% 15% 31% 15% 3 2	242
January	1st to 10th	Night	27% 15% 31% 23% 3	840
	11th to 20th	Night	29% 17% 27% 21% 6%	840
	21st to end	Night	19% 15% 33% 26% 6% 1	924
February	1st to 10th	Night	20% 16% 34% 25% 4	840
	11th to 20th	Night	21% 17% 37% 21% 4	823
	21st to end	Night	23% 16% 31% 27% 3	692
March	1st to 10th	Night	24% 16% 33% 24% 3	840
	11th to 20th	Night	28% 15% 31% 21% 5%	829
	21st to end	Night	25% 13% 29% 26% 6%	880
April	1st to 10th	Night	29% 15% 27% 25% 5%	800
	11th to 20th	Night	27% 13% 32% 23% 5% 1	791
	21st to end	Night	28% 10% 33% 20% 8%	665
May	1st to 10th	Night	24% 11% 31% 25% 7% 3	414
	11th to 20th	Night	22% 9% 29% 29% 10% 1	394
	21st to end	Night	18% 9% 34% 30% 9%	433
June	1st to 10th	Night	24% 9% 28% 29% 11% 1	400
	11th to 20th	Night	19% 7% 27% 35% 11% 1	400
	21st to end	Night	22% 6% 28% 30% 15% 1	400
July	1st to 10th	Night	11% 7% 29% 36% 16% 1	390
	11th to 20th	Night	15% 5% 20% 43% 17%	388
	21st to end	Night	14% 5% 23% 42% 15% 1	438
August	1st to 10th	Night	14% 4 17% 44% 20% 1	398
	11th to 20th	Night	13% 4 22% 34% 24% 3	597
	21st to end	Night	16% 5% 24% 34% 19% 2	880
September	1st to 10th	Night	13% 5% 27% 38% 16% 2	800
	11th to 20th	Night	16% 6% 18% 33% 24% 3	800
	21st to end	Night	20% 6% 26% 28% 18% 2	800
October	1st to 10th	Night	23% 9% 25% 30% 13% 1	800
	11th to 20th	Night	24% 12% 27% 28% 9% 1	800
	21st to end	Night	28% 12% 29% 26% 5%	880
November	1st to 10th	Night	27% 15% 24% 26% 8%	820
	11th to 20th	Night	24% 16% 30% 21% 9%	840
	21st to end	Night	30% 16% 28% 18% 7%	840
December	1st to 10th	Night	29% 14% 31% 21% 4	840
	11th to 20th	Night	30% 18% 30% 18% 4 1	840
	21st to end	Night	34% 16% 28% 16% 6%	924

Page generated at 2026-03-15 02:23:51. Version ac07

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