Lac Heney
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lac Heney clear sky chart from 2005-11-06 to 2026-09-14.

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% 11% 4 9% 5% 5% 5% 4 5% 7% 30%	149766
	Night	16% 13% 4 10% 6% 5% 4 3 5% 6% 29%	141279
Transparency	Day	53% 5% 12% 21% 9% 1	153233
	Night	56% 5% 11% 19% 9% 1	145901
Seeing	Day	25% 14% 30% 19% 9% 3	16743
	Night	24% 11% 27% 27% 10% 1	25741

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% 18% 8% 17% 7% 5% 5% 3 4 4 16%	4661
	11th to 20th	Day	10% 22% 8% 17% 8% 4 4 3 4 5% 16%	4650
	21st to end	Day	8% 15% 7% 16% 10% 7% 5% 4 5% 5% 19%	5303
February	1st to 10th	Day	11% 17% 6% 14% 8% 5% 4 4 5% 5% 21%	1890
	11th to 20th	Day	11% 18% 6% 12% 7% 5% 3 4 4 5% 26%	1843
	21st to end	Day	16% 17% 5% 10% 7% 4 4 3 3 4 25%	1716
March	1st to 10th	Day	17% 16% 5% 12% 7% 4 4 4 4 5% 23%	4576
	11th to 20th	Day	12% 15% 5% 10% 6% 4 5% 4 5% 7% 27%	5090
	21st to end	Day	16% 12% 4 8% 5% 4 4 3 4 5% 35%	5899
April	1st to 10th	Day	23% 14% 3 7% 5% 4 3 2 4 4 30%	2520
	11th to 20th	Day	23% 11% 4 7% 4 3 3 2 4 5% 34%	2494
	21st to end	Day	22% 12% 3 7% 5% 4 4 3 5% 4 31%	2709
May	1st to 10th	Day	19% 10% 3 8% 5% 5% 6% 4 6% 8% 25%	8237
	11th to 20th	Day	17% 8% 3 8% 5% 5% 4 4 6% 7% 33%	8221
	21st to end	Day	20% 9% 3 7% 4 5% 5% 3 5% 6% 33%	9060
June	1st to 10th	Day	22% 9% 3 7% 5% 4 4 4 6% 6% 30%	2940
	11th to 20th	Day	16% 6% 2 6% 5% 4 5% 4 6% 9% 37%	2940
	21st to end	Day	18% 7% 3 7% 6% 6% 5% 4 6% 7% 31%	2940
July	1st to 10th	Day	14% 5% 2 7% 5% 5% 5% 5% 7% 10% 35%	8280
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 7% 10% 43%	8280
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 7% 10% 40%	8993
August	1st to 10th	Day	16% 7% 2 8% 5% 5% 6% 4 6% 7% 32%	6421
	11th to 20th	Day	15% 7% 3 7% 4 6% 6% 4 6% 9% 34%	5928
	21st to end	Day	13% 7% 2 7% 5% 5% 5% 5% 7% 10% 34%	6080
September	1st to 10th	Day	15% 6% 2 6% 5% 4 4 4 6% 7% 41%	2520
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 45%	2436
	21st to end	Day	21% 6% 2 6% 4 4 4 3 6% 7% 37%	2130
October	1st to 10th	Day	20% 9% 2 7% 4 4 4 4 6% 6% 35%	2000
	11th to 20th	Day	23% 11% 3 7% 5% 5% 4 3 4 6% 28%	2000
	21st to end	Day	25% 12% 2 8% 5% 4 5% 3 5% 6% 25%	2030
November	1st to 10th	Day	16% 12% 4 8% 5% 5% 3 4 5% 7% 31%	1825
	11th to 20th	Day	17% 17% 5% 10% 5% 5% 4 4 5% 5% 22%	1680
	21st to end	Day	22% 18% 6% 13% 5% 5% 4 3 3 5% 16%	1680
December	1st to 10th	Day	20% 23% 7% 11% 6% 5% 4 3 4 4 11%	2680
	11th to 20th	Day	17% 17% 7% 12% 6% 5% 4 3 4 4 20%	3066
	21st to end	Day	21% 18% 5% 12% 7% 5% 4 3 3 3 18%	4048
January	1st to 10th	Night	12% 20% 8% 16% 8% 5% 4 3 4 4 18%	8234
	11th to 20th	Night	12% 19% 7% 17% 8% 5% 5% 3 4 4 17%	8235
	21st to end	Night	8% 16% 7% 15% 8% 8% 6% 4 5% 5% 19%	8877
February	1st to 10th	Night	11% 15% 6% 14% 8% 6% 5% 4 4 5% 23%	2730
	11th to 20th	Night	12% 15% 6% 13% 7% 5% 5% 3 3 5% 25%	2663
	21st to end	Night	12% 16% 5% 12% 7% 5% 4 2 5% 5% 26%	2088
March	1st to 10th	Night	20% 18% 4 9% 5% 4 4 3 3 5% 26%	5467
	11th to 20th	Night	12% 15% 4 9% 6% 5% 5% 4 6% 7% 26%	5089
	21st to end	Night	16% 11% 4 11% 5% 4 4 3 5% 5% 32%	5274
April	1st to 10th	Night	21% 12% 3 8% 5% 4 4 3 5% 5% 29%	2100
	11th to 20th	Night	19% 11% 3 7% 4 3 4 3 5% 5% 35%	2075
	21st to end	Night	22% 10% 3 8% 3 4 4 2 4 5% 35%	1929
May	1st to 10th	Night	19% 11% 4 10% 5% 5% 5% 4 6% 7% 25%	4660
	11th to 20th	Night	15% 9% 3 8% 4 6% 5% 4 5% 7% 35%	4639
	21st to end	Night	19% 8% 3 7% 5% 5% 5% 4 4 6% 33%	5114
June	1st to 10th	Night	17% 8% 3 6% 5% 4 4 4 6% 9% 34%	1680
	11th to 20th	Night	16% 6% 3 6% 5% 4 4 4 5% 7% 40%	1680
	21st to end	Night	18% 7% 2 6% 5% 5% 4 4 7% 8% 34%	1680
July	1st to 10th	Night	12% 6% 2 7% 4 5% 6% 5% 6% 10% 36%	4680
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 6% 8% 46%	4680
	21st to end	Night	12% 7% 3 7% 4 5% 4 4 6% 7% 41%	5084
August	1st to 10th	Night	15% 7% 3 8% 5% 5% 4 4 7% 7% 35%	3712
	11th to 20th	Night	15% 7% 3 7% 4 4 4 4 6% 8% 38%	4095
	21st to end	Night	15% 7% 2 7% 5% 4 5% 4 6% 7% 37%	5021
September	1st to 10th	Night	13% 6% 3 7% 5% 4 4 3 5% 6% 44%	2100
	11th to 20th	Night	15% 5% 2 4 4 3 3 3 5% 8% 48%	2031
	21st to end	Night	18% 8% 3 6% 3 3 4 3 4 6% 41%	2266
October	1st to 10th	Night	20% 8% 2 5% 4 4 4 3 5% 6% 39%	2400
	11th to 20th	Night	22% 11% 3 8% 5% 4 4 3 4 6% 30%	2400
	21st to end	Night	23% 11% 3 8% 5% 4 5% 3 5% 6% 26%	2809
November	1st to 10th	Night	18% 12% 4 9% 5% 4 4 3 5% 5% 30%	2689
	11th to 20th	Night	19% 14% 5% 10% 6% 5% 4 3 4 5% 26%	2940
	21st to end	Night	22% 18% 5% 11% 6% 4 4 4 4 5% 18%	2940
December	1st to 10th	Night	18% 20% 6% 14% 7% 5% 5% 3 4 4 13%	4720
	11th to 20th	Night	21% 16% 5% 11% 5% 4 4 3 4 5% 24%	5348
	21st to end	Night	22% 21% 5% 11% 5% 4 3 3 4 5% 17%	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	71% 4 8% 13% 4	4662
	11th to 20th	Day	71% 3 6% 10% 9%	4680
	21st to end	Day	67% 4 9% 11% 9% 1	5303
February	1st to 10th	Day	65% 4 8% 16% 6% 1	1890
	11th to 20th	Day	62% 2 11% 15% 9% 1	1852
	21st to end	Day	64% 2 9% 14% 10% 1	1716
March	1st to 10th	Day	63% 4 7% 12% 11% 2	4579
	11th to 20th	Day	57% 4 14% 14% 9% 1	5090
	21st to end	Day	53% 4 6% 19% 16% 2	5899
April	1st to 10th	Day	58% 3 6% 18% 12% 2	2520
	11th to 20th	Day	54% 3 6% 22% 13% 2	2494
	21st to end	Day	57% 3 7% 17% 13% 3	2709
May	1st to 10th	Day	53% 6% 8% 22% 9% 2	8249
	11th to 20th	Day	49% 6% 14% 22% 9% 1	8225
	21st to end	Day	51% 5% 10% 21% 13% 1	9061
June	1st to 10th	Day	54% 4 9% 21% 12% 1	2940
	11th to 20th	Day	44% 5% 11% 27% 11% 2	2940
	21st to end	Day	50% 5% 14% 21% 9%	2940
July	1st to 10th	Day	42% 7% 15% 25% 11%	8210
	11th to 20th	Day	35% 6% 15% 33% 10% 1	8205
	21st to end	Day	38% 7% 16% 31% 8%	8795
August	1st to 10th	Day	50% 6% 15% 20% 9% 1	6407
	11th to 20th	Day	44% 7% 16% 26% 6%	5967
	21st to end	Day	41% 8% 18% 24% 9% 1	6080
September	1st to 10th	Day	41% 5% 14% 29% 11%	2520
	11th to 20th	Day	39% 4 13% 31% 12% 1	2436
	21st to end	Day	45% 6% 14% 25% 11%	2130
October	1st to 10th	Day	50% 5% 11% 28% 6%	3260
	11th to 20th	Day	53% 5% 13% 21% 7%	3240
	21st to end	Day	55% 6% 18% 15% 5%	3255
November	1st to 10th	Day	52% 5% 14% 22% 6% 1	1825
	11th to 20th	Day	64% 4 11% 13% 7%	1680
	21st to end	Day	71% 5% 10% 9% 5%	1680
December	1st to 10th	Day	76% 4 8% 10% 2	2680
	11th to 20th	Day	68% 4 7% 15% 6%	3066
	21st to end	Day	71% 5% 7% 15% 3	4048
January	1st to 10th	Night	71% 4 11% 11% 3	8234
	11th to 20th	Night	71% 4 9% 11% 5%	8280
	21st to end	Night	67% 4 7% 11% 11%	8877
February	1st to 10th	Night	63% 4 8% 17% 7% 1	2730
	11th to 20th	Night	63% 4 8% 19% 7% 1	2665
	21st to end	Night	59% 4 8% 19% 9% 1	2088
March	1st to 10th	Night	62% 3 8% 13% 11% 3	5477
	11th to 20th	Night	55% 7% 13% 16% 8% 2	5090
	21st to end	Night	54% 4 7% 21% 11% 3	5274
April	1st to 10th	Night	56% 3 8% 18% 13% 2	2100
	11th to 20th	Night	50% 4 7% 24% 14% 2	2075
	21st to end	Night	53% 3 7% 19% 16% 3	1929
May	1st to 10th	Night	56% 6% 8% 21% 8% 2	4666
	11th to 20th	Night	48% 6% 12% 21% 11% 1	4640
	21st to end	Night	51% 5% 10% 21% 13% 1	5115
June	1st to 10th	Night	46% 6% 12% 23% 11% 1	1680
	11th to 20th	Night	44% 5% 11% 26% 12% 2	1680
	21st to end	Night	47% 5% 14% 23% 11%	1680
July	1st to 10th	Night	41% 8% 17% 22% 11% 1	4640
	11th to 20th	Night	36% 6% 16% 31% 11%	4632
	21st to end	Night	41% 7% 13% 30% 9%	4967
August	1st to 10th	Night	45% 7% 14% 23% 10% 1	3702
	11th to 20th	Night	43% 6% 17% 27% 6%	4122
	21st to end	Night	43% 7% 15% 24% 9% 1	5021
September	1st to 10th	Night	40% 5% 13% 29% 12%	2100
	11th to 20th	Night	35% 5% 15% 36% 9%	2031
	21st to end	Night	43% 5% 12% 28% 11%	2266
October	1st to 10th	Night	42% 6% 14% 27% 11%	3920
	11th to 20th	Night	50% 5% 14% 22% 8% 1	3899
	21st to end	Night	52% 5% 18% 15% 8%	4534
November	1st to 10th	Night	56% 4 13% 19% 7%	2689
	11th to 20th	Night	62% 4 10% 15% 8% 1	2940
	21st to end	Night	69% 4 11% 13% 3	2940
December	1st to 10th	Night	73% 5% 9% 10% 2	4720
	11th to 20th	Night	65% 3 7% 15% 10%	5348
	21st to end	Night	70% 3 5% 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	25% 16% 37% 17% 4	220
	11th to 20th	Day	30% 14% 37% 13% 6%	220
	21st to end	Day	21% 15% 37% 21% 5% 1	275
February	1st to 10th	Day	28% 21% 34% 12% 4 1	330
	11th to 20th	Day	24% 27% 38% 11%	330
	21st to end	Day	28% 19% 35% 15% 2	432
March	1st to 10th	Day	27% 18% 34% 15% 5%	540
	11th to 20th	Day	30% 17% 31% 17% 4 1	540
	21st to end	Day	32% 18% 29% 16% 5% 1	594
April	1st to 10th	Day	33% 19% 25% 16% 5% 1	540
	11th to 20th	Day	35% 17% 22% 14% 9% 2	532
	21st to end	Day	33% 16% 31% 13% 5% 1	540
May	1st to 10th	Day	29% 14% 31% 16% 7% 2	543
	11th to 20th	Day	23% 17% 32% 19% 7% 3	567
	21st to end	Day	25% 16% 30% 18% 9% 1	617
June	1st to 10th	Day	26% 12% 31% 21% 8% 2	570
	11th to 20th	Day	19% 16% 31% 20% 11% 3	570
	21st to end	Day	20% 14% 31% 21% 10% 2	570
July	1st to 10th	Day	15% 8% 32% 34% 9% 2	565
	11th to 20th	Day	15% 10% 34% 25% 13% 2	564
	21st to end	Day	15% 9% 32% 28% 13% 3	620
August	1st to 10th	Day	17% 6% 28% 32% 14% 3	566
	11th to 20th	Day	20% 5% 25% 28% 15% 7%	559
	21st to end	Day	17% 10% 30% 22% 13% 7%	627
September	1st to 10th	Day	19% 11% 30% 21% 12% 7%	570
	11th to 20th	Day	14% 9% 23% 23% 20% 11%	542
	21st to end	Day	23% 11% 27% 18% 13% 8%	530
October	1st to 10th	Day	25% 13% 25% 21% 12% 3	530
	11th to 20th	Day	30% 18% 30% 15% 6% 1	530
	21st to end	Day	32% 13% 30% 19% 6%	489
November	1st to 10th	Day	30% 15% 30% 20% 4 1	399
	11th to 20th	Day	29% 19% 33% 10% 7% 1	220
	21st to end	Day	33% 19% 34% 12% 2 1	220
December	1st to 10th	Day	34% 8% 37% 17% 4	220
	11th to 20th	Day	29% 24% 32% 11% 3	220
	21st to end	Day	28% 17% 34% 16% 5% 1	242
January	1st to 10th	Night	28% 12% 34% 22% 4	840
	11th to 20th	Night	29% 16% 25% 23% 7%	840
	21st to end	Night	19% 11% 34% 27% 9% 1	924
February	1st to 10th	Night	19% 15% 38% 23% 5%	840
	11th to 20th	Night	23% 15% 37% 21% 5%	823
	21st to end	Night	23% 18% 29% 26% 5%	692
March	1st to 10th	Night	27% 15% 29% 23% 5%	840
	11th to 20th	Night	26% 14% 31% 23% 5%	840
	21st to end	Night	24% 16% 28% 23% 9%	924
April	1st to 10th	Night	29% 14% 25% 26% 6%	840
	11th to 20th	Night	27% 14% 28% 22% 9% 1	831
	21st to end	Night	29% 12% 26% 24% 9%	669
May	1st to 10th	Night	24% 10% 28% 28% 9% 1	420
	11th to 20th	Night	23% 8% 29% 31% 8%	414
	21st to end	Night	17% 11% 32% 30% 10%	455
June	1st to 10th	Night	20% 8% 30% 34% 8% 1	420
	11th to 20th	Night	19% 9% 24% 35% 12% 1	420
	21st to end	Night	21% 5% 23% 38% 14%	420
July	1st to 10th	Night	10% 4 28% 43% 15%	410
	11th to 20th	Night	15% 5% 20% 43% 17%	408
	21st to end	Night	12% 4 25% 42% 16% 2	460
August	1st to 10th	Night	11% 4 21% 40% 24% 1	438
	11th to 20th	Night	14% 3 17% 38% 24% 3	624
	21st to end	Night	17% 5% 24% 32% 19% 2	912
September	1st to 10th	Night	17% 6% 25% 33% 18% 1	840
	11th to 20th	Night	19% 4 21% 31% 22% 4	813
	21st to end	Night	21% 8% 22% 28% 19% 2	800
October	1st to 10th	Night	25% 9% 26% 25% 14% 1	800
	11th to 20th	Night	29% 10% 28% 24% 8% 1	800
	21st to end	Night	31% 11% 26% 25% 7%	880
November	1st to 10th	Night	29% 13% 26% 24% 8% 1	820
	11th to 20th	Night	28% 15% 28% 20% 9% 1	840
	21st to end	Night	34% 15% 24% 21% 6%	840
December	1st to 10th	Night	31% 15% 28% 22% 5%	840
	11th to 20th	Night	28% 16% 29% 21% 5% 1	840
	21st to end	Night	34% 13% 27% 21% 5% 1	924

Page generated at 2026-09-11 04:11:55. Version ac07

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