Varennes
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Varennes 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	16% 10% 3 8% 5% 5% 5% 4 6% 7% 32%	131231
	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 30%	130291
Transparency	Day	50% 6% 13% 21% 9% 1	134689
	Night	54% 5% 11% 18% 10% 1	134920
Seeing	Day	21% 12% 28% 24% 12% 3	16190
	Night	23% 9% 29% 29% 10% 1	25272

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	14% 17% 6% 13% 5% 5% 4 3 3 4 23%	4661
	11th to 20th	Day	12% 15% 6% 13% 6% 6% 4 3 4 6% 23%	4650
	21st to end	Day	10% 13% 5% 11% 8% 8% 7% 4 5% 5% 24%	5124
February	1st to 10th	Day	14% 15% 4 10% 5% 5% 5% 3 4 6% 28%	1822
	11th to 20th	Day	13% 12% 5% 11% 6% 4 5% 5% 6% 5% 27%	1962
	21st to end	Day	17% 16% 4 9% 7% 4 4 2 5% 6% 27%	1730
March	1st to 10th	Day	18% 12% 4 11% 7% 5% 5% 3 4 5% 26%	4576
	11th to 20th	Day	12% 13% 4 9% 5% 5% 4 3 6% 7% 31%	4758
	21st to end	Day	17% 11% 4 10% 5% 4 4 3 5% 5% 33%	5970
April	1st to 10th	Day	21% 11% 3 8% 4 4 5% 3 4 6% 31%	2400
	11th to 20th	Day	20% 9% 3 7% 4 4 3 3 4 5% 37%	2374
	21st to end	Day	21% 9% 3 8% 4 4 4 4 5% 5% 32%	2579
May	1st to 10th	Day	24% 11% 4 9% 5% 4 5% 4 6% 6% 23%	6241
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 6% 8% 30%	6318
	21st to end	Day	20% 8% 3 7% 4 4 4 4 6% 8% 32%	6949
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 8% 30%	2800
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 4 6% 9% 38%	2800
	21st to end	Day	19% 8% 3 8% 6% 5% 5% 4 6% 7% 31%	2800
July	1st to 10th	Day	12% 6% 3 8% 6% 5% 6% 4 7% 9% 34%	6360
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	6360
	21st to end	Day	10% 6% 2 5% 4 4 4 3 7% 9% 46%	6926
August	1st to 10th	Day	13% 6% 2 8% 5% 5% 6% 5% 7% 10% 33%	4563
	11th to 20th	Day	13% 6% 3 8% 5% 4 4 4 6% 9% 38%	4150
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 40%	4312
September	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 5% 6% 9% 43%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 6% 7% 49%	2305
	21st to end	Day	15% 6% 2 7% 4 4 5% 4 5% 8% 41%	2200
October	1st to 10th	Day	18% 6% 3 6% 5% 5% 4 4 5% 8% 36%	2080
	11th to 20th	Day	16% 9% 3 8% 5% 6% 5% 5% 6% 7% 30%	1940
	21st to end	Day	24% 9% 3 7% 4 4 4 3 5% 5% 31%	1980
November	1st to 10th	Day	17% 11% 3 7% 5% 4 5% 4 6% 6% 31%	1851
	11th to 20th	Day	17% 11% 3 9% 5% 5% 5% 5% 5% 5% 30%	1816
	21st to end	Day	22% 13% 3 8% 5% 4 6% 4 4 6% 25%	1680
December	1st to 10th	Day	20% 19% 5% 12% 5% 4 4 3 4 5% 18%	2680
	11th to 20th	Day	19% 13% 4 10% 4 4 5% 4 5% 6% 26%	3066
	21st to end	Day	23% 13% 4 10% 6% 5% 6% 5% 6% 5% 19%	4048
January	1st to 10th	Night	13% 18% 7% 15% 6% 5% 4 3 4 5% 22%	8234
	11th to 20th	Night	11% 16% 7% 16% 8% 6% 5% 3 4 5% 18%	8235
	21st to end	Night	11% 13% 4 12% 9% 6% 5% 3 5% 6% 25%	9057
February	1st to 10th	Night	14% 14% 6% 12% 7% 5% 4 3 5% 6% 23%	2794
	11th to 20th	Night	14% 13% 5% 9% 5% 6% 4 3 5% 5% 30%	2542
	21st to end	Night	15% 15% 3 9% 5% 5% 4 3 4 6% 31%	2076
March	1st to 10th	Night	22% 12% 4 8% 5% 4 4 3 4 5% 30%	5484
	11th to 20th	Night	14% 14% 4 8% 5% 5% 5% 4 5% 7% 30%	5354
	21st to end	Night	15% 12% 4 9% 6% 5% 4 3 4 5% 32%	4962
April	1st to 10th	Night	23% 10% 3 7% 5% 4 4 3 4 6% 31%	2000
	11th to 20th	Night	22% 9% 3 7% 5% 4 4 4 4 6% 34%	1975
	21st to end	Night	22% 10% 3 6% 4 4 4 3 5% 5% 35%	1820
May	1st to 10th	Night	25% 10% 4 9% 5% 4 4 3 5% 6% 25%	3742
	11th to 20th	Night	22% 9% 3 8% 5% 4 4 3 5% 6% 30%	3575
	21st to end	Night	18% 10% 4 8% 5% 5% 4 4 5% 7% 30%	3926
June	1st to 10th	Night	22% 7% 2 8% 4 4 5% 4 5% 7% 32%	1600
	11th to 20th	Night	15% 6% 2 8% 4 4 5% 3 8% 7% 38%	1600
	21st to end	Night	18% 7% 2 8% 5% 6% 5% 4 5% 8% 33%	1600
July	1st to 10th	Night	13% 7% 3 7% 5% 5% 6% 5% 7% 10% 33%	3600
	11th to 20th	Night	14% 6% 2 7% 4 5% 5% 4 6% 8% 40%	3600
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 7% 9% 43%	3926
August	1st to 10th	Night	14% 8% 3 8% 5% 5% 5% 3 6% 7% 36%	2586
	11th to 20th	Night	15% 7% 3 8% 5% 5% 5% 3 6% 7% 38%	2862
	21st to end	Night	16% 6% 3 7% 5% 5% 4 3 6% 7% 38%	3586
September	1st to 10th	Night	11% 5% 2 7% 5% 4 4 4 6% 7% 44%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 7% 50%	2095
	21st to end	Night	16% 7% 3 6% 5% 4 3 3 5% 6% 43%	2200
October	1st to 10th	Night	17% 6% 2 7% 4 4 4 3 5% 7% 40%	2315
	11th to 20th	Night	20% 8% 3 7% 5% 4 4 3 5% 6% 35%	2460
	21st to end	Night	22% 8% 3 8% 5% 4 4 3 5% 5% 31%	2860
November	1st to 10th	Night	20% 9% 3 7% 5% 5% 4 3 5% 5% 33%	2667
	11th to 20th	Night	19% 11% 3 8% 5% 5% 4 3 5% 6% 32%	2800
	21st to end	Night	22% 13% 4 9% 6% 4 4 3 4 5% 26%	2940
December	1st to 10th	Night	17% 17% 6% 13% 6% 5% 5% 3 5% 5% 18%	4720
	11th to 20th	Night	22% 13% 4 10% 5% 4 4 3 6% 5% 24%	5348
	21st to end	Night	24% 15% 5% 9% 4 3 4 4 6% 5% 21%	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	62% 6% 11% 17% 4	4662
	11th to 20th	Day	62% 5% 9% 14% 9% 1	4680
	21st to end	Day	59% 5% 10% 14% 11% 1	5124
February	1st to 10th	Day	59% 5% 9% 19% 7% 2	1822
	11th to 20th	Day	57% 7% 11% 16% 9% 1	1971
	21st to end	Day	59% 4 10% 16% 10% 2	1730
March	1st to 10th	Day	59% 6% 8% 14% 11% 3	4579
	11th to 20th	Day	51% 7% 16% 14% 11% 2	4758
	21st to end	Day	53% 5% 8% 16% 16% 2	5970
April	1st to 10th	Day	55% 5% 8% 19% 12% 2	2400
	11th to 20th	Day	49% 4 9% 22% 14% 2	2374
	21st to end	Day	53% 5% 10% 18% 12% 3	2579
May	1st to 10th	Day	59% 6% 9% 19% 6% 1	6252
	11th to 20th	Day	50% 9% 14% 17% 8% 2	6322
	21st to end	Day	50% 7% 12% 19% 12% 1	6949
June	1st to 10th	Day	50% 7% 11% 19% 11% 2	2800
	11th to 20th	Day	40% 7% 13% 25% 12% 2	2800
	21st to end	Day	51% 7% 12% 20% 10% 1	2800
July	1st to 10th	Day	45% 7% 15% 25% 8%	6290
	11th to 20th	Day	38% 7% 15% 30% 10%	6285
	21st to end	Day	34% 6% 16% 37% 8%	6728
August	1st to 10th	Day	45% 7% 18% 22% 7%	4549
	11th to 20th	Day	41% 7% 20% 25% 7%	4192
	21st to end	Day	43% 6% 15% 21% 14%	4312
September	1st to 10th	Day	36% 8% 15% 30% 11% 1	2400
	11th to 20th	Day	34% 6% 13% 33% 14% 1	2305
	21st to end	Day	42% 7% 14% 25% 12%	2200
October	1st to 10th	Day	43% 6% 15% 27% 8%	3392
	11th to 20th	Day	44% 6% 18% 25% 7%	3143
	21st to end	Day	44% 6% 19% 22% 9% 1	3180
November	1st to 10th	Day	50% 7% 13% 21% 9% 1	1851
	11th to 20th	Day	52% 7% 11% 19% 11%	1816
	21st to end	Day	59% 8% 13% 14% 6%	1680
December	1st to 10th	Day	68% 4 8% 16% 4	2680
	11th to 20th	Day	59% 5% 9% 17% 10%	3066
	21st to end	Day	65% 6% 12% 11% 6%	4048
January	1st to 10th	Night	65% 5% 11% 13% 5% 1	8234
	11th to 20th	Night	67% 7% 8% 12% 6%	8280
	21st to end	Night	60% 5% 9% 13% 13%	9057
February	1st to 10th	Night	62% 5% 9% 16% 8% 1	2794
	11th to 20th	Night	58% 5% 8% 18% 10%	2544
	21st to end	Night	55% 5% 8% 17% 12% 1	2076
March	1st to 10th	Night	58% 4 8% 14% 11% 5%	5494
	11th to 20th	Night	53% 6% 13% 15% 13% 1	5355
	21st to end	Night	55% 5% 6% 21% 11% 2	4962
April	1st to 10th	Night	55% 4 7% 18% 14% 2	2000
	11th to 20th	Night	52% 4 7% 20% 14% 2	1975
	21st to end	Night	51% 4 8% 20% 14% 3	1820
May	1st to 10th	Night	59% 7% 9% 16% 6% 2	3748
	11th to 20th	Night	52% 7% 11% 18% 10% 2	3576
	21st to end	Night	52% 5% 10% 17% 14% 1	3927
June	1st to 10th	Night	49% 6% 10% 21% 13% 1	1600
	11th to 20th	Night	44% 7% 11% 21% 14% 3	1600
	21st to end	Night	49% 7% 10% 19% 14% 1	1600
July	1st to 10th	Night	44% 7% 19% 21% 9% 1	3560
	11th to 20th	Night	41% 6% 16% 26% 10%	3552
	21st to end	Night	37% 6% 17% 28% 11%	3809
August	1st to 10th	Night	46% 7% 12% 26% 9%	2576
	11th to 20th	Night	45% 7% 17% 23% 8%	2886
	21st to end	Night	45% 6% 13% 22% 13% 1	3586
September	1st to 10th	Night	38% 7% 12% 27% 15% 1	2000
	11th to 20th	Night	35% 5% 13% 32% 13% 1	2095
	21st to end	Night	40% 8% 10% 27% 15%	2200
October	1st to 10th	Night	43% 6% 13% 26% 11% 1	3783
	11th to 20th	Night	45% 5% 11% 30% 9%	3996
	21st to end	Night	48% 4 15% 19% 12% 2	4610
November	1st to 10th	Night	53% 5% 12% 22% 8% 1	2667
	11th to 20th	Night	53% 6% 11% 19% 10% 1	2800
	21st to end	Night	61% 5% 11% 17% 7%	2940
December	1st to 10th	Night	69% 5% 11% 12% 4	4720
	11th to 20th	Night	61% 5% 8% 14% 11%	5348
	21st to end	Night	64% 4 8% 14% 9%	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	27% 13% 32% 18% 10%	220
	11th to 20th	Day	25% 17% 34% 18% 5%	220
	21st to end	Day	18% 13% 35% 23% 10% 2	242
February	1st to 10th	Day	26% 15% 36% 19% 4	294
	11th to 20th	Day	21% 16% 32% 26% 4	450
	21st to end	Day	25% 13% 34% 24% 4	446
March	1st to 10th	Day	22% 15% 31% 24% 9%	540
	11th to 20th	Day	23% 14% 35% 21% 6% 1	528
	21st to end	Day	27% 12% 32% 21% 7% 1	550
April	1st to 10th	Day	28% 15% 27% 20% 8% 1	500
	11th to 20th	Day	28% 18% 24% 20% 7% 3	492
	21st to end	Day	28% 13% 31% 18% 9% 1	500
May	1st to 10th	Day	24% 11% 33% 21% 10% 2	503
	11th to 20th	Day	18% 11% 28% 26% 14% 3	527
	21st to end	Day	21% 12% 29% 25% 10% 2	573
June	1st to 10th	Day	21% 12% 26% 30% 10% 2	530
	11th to 20th	Day	15% 11% 26% 31% 14% 2	530
	21st to end	Day	23% 11% 25% 26% 13% 3	530
July	1st to 10th	Day	13% 8% 25% 30% 18% 5%	525
	11th to 20th	Day	13% 8% 27% 33% 15% 4	524
	21st to end	Day	13% 7% 26% 33% 17% 3	576
August	1st to 10th	Day	13% 5% 23% 31% 21% 6%	526
	11th to 20th	Day	12% 6% 19% 31% 23% 9%	519
	21st to end	Day	15% 8% 23% 25% 18% 10%	583
September	1st to 10th	Day	12% 9% 27% 29% 16% 7%	530
	11th to 20th	Day	13% 7% 19% 28% 20% 13%	530
	21st to end	Day	18% 9% 26% 23% 18% 6%	530
October	1st to 10th	Day	19% 13% 26% 23% 15% 4	530
	11th to 20th	Day	22% 12% 30% 23% 11% 2	497
	21st to end	Day	30% 11% 26% 24% 8%	462
November	1st to 10th	Day	26% 15% 30% 19% 9%	425
	11th to 20th	Day	23% 15% 31% 21% 8% 1	356
	21st to end	Day	34% 13% 30% 15% 8%	220
December	1st to 10th	Day	29% 10% 28% 19% 11% 3	220
	11th to 20th	Day	25% 20% 34% 15% 4 1	220
	21st to end	Day	31% 17% 27% 19% 4 2	242
January	1st to 10th	Night	28% 10% 32% 26% 5%	840
	11th to 20th	Night	25% 11% 33% 24% 6%	840
	21st to end	Night	19% 10% 33% 30% 6% 1	924
February	1st to 10th	Night	21% 10% 35% 28% 5% 1	840
	11th to 20th	Night	20% 11% 38% 25% 5% 1	823
	21st to end	Night	21% 11% 33% 31% 4	692
March	1st to 10th	Night	25% 9% 32% 28% 5%	840
	11th to 20th	Night	26% 12% 31% 25% 7%	829
	21st to end	Night	23% 9% 31% 28% 8%	880
April	1st to 10th	Night	28% 12% 26% 28% 6%	800
	11th to 20th	Night	25% 13% 30% 26% 6% 1	791
	21st to end	Night	28% 11% 30% 24% 7%	620
May	1st to 10th	Night	26% 8% 31% 26% 7% 1	470
	11th to 20th	Night	23% 9% 28% 31% 9% 1	394
	21st to end	Night	18% 9% 35% 29% 9%	433
June	1st to 10th	Night	22% 6% 27% 35% 9% 1	400
	11th to 20th	Night	19% 4 29% 37% 12%	400
	21st to end	Night	19% 7% 27% 33% 14% 1	400
July	1st to 10th	Night	11% 6% 29% 38% 15% 1	390
	11th to 20th	Night	12% 3 24% 46% 14% 1	388
	21st to end	Night	13% 4 22% 45% 16%	438
August	1st to 10th	Night	13% 5% 17% 44% 19% 2	398
	11th to 20th	Night	13% 4 20% 38% 24% 2	578
	21st to end	Night	16% 5% 21% 37% 20% 2	880
September	1st to 10th	Night	15% 5% 24% 40% 16% 1	800
	11th to 20th	Night	16% 5% 20% 34% 23% 2	800
	21st to end	Night	18% 6% 26% 32% 15% 3	800
October	1st to 10th	Night	20% 7% 28% 31% 13% 1	800
	11th to 20th	Night	25% 8% 28% 29% 9% 1	800
	21st to end	Night	27% 10% 31% 26% 7%	880
November	1st to 10th	Night	25% 11% 31% 24% 8%	820
	11th to 20th	Night	25% 10% 33% 23% 9% 1	840
	21st to end	Night	29% 12% 31% 21% 7%	840
December	1st to 10th	Night	28% 11% 30% 25% 6%	840
	11th to 20th	Night	28% 13% 32% 21% 6% 1	840
	21st to end	Night	31% 11% 30% 20% 7%	924

Page generated at 2026-03-15 02:23:52. Version ac07

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