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 2025-04-26.

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% 12% 4 9% 5% 5% 4 4 5% 6% 30%	113675
	Night	16% 13% 4 10% 5% 4 4 3 5% 6% 28%	114903
Transparency	Day	54% 5% 11% 20% 9% 1	117188
	Night	57% 5% 11% 18% 9% 1	119554
Seeing	Day	25% 14% 30% 20% 9% 3	14920
	Night	24% 11% 27% 26% 10% 1	24161

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% 19% 9% 18% 8% 5% 4 2 3 3 15%	3597
	11th to 20th	Day	11% 17% 7% 16% 9% 5% 4 3 4 5% 18%	3570
	21st to end	Day	10% 18% 8% 15% 6% 5% 4 3 4 6% 21%	4097
February	1st to 10th	Day	11% 18% 6% 14% 8% 5% 4 4 5% 5% 21%	1800
	11th to 20th	Day	12% 18% 6% 12% 6% 5% 3 3 4 5% 26%	1753
	21st to end	Day	17% 17% 5% 9% 7% 4 4 3 3 4 26%	1636
March	1st to 10th	Day	18% 16% 5% 12% 7% 4 4 4 4 5% 23%	4476
	11th to 20th	Day	11% 15% 5% 10% 6% 4 5% 4 5% 7% 27%	4980
	21st to end	Day	16% 13% 4 8% 5% 4 4 3 4 5% 35%	5772
April	1st to 10th	Day	23% 14% 3 7% 5% 4 3 2 4 5% 31%	2400
	11th to 20th	Day	22% 11% 4 7% 4 3 3 3 4 5% 34%	2374
	21st to end	Day	22% 12% 3 7% 5% 4 4 3 4 4 31%	2512
May	1st to 10th	Day	20% 9% 3 7% 5% 5% 5% 3 6% 7% 30%	4426
	11th to 20th	Day	17% 8% 3 8% 5% 4 4 4 5% 7% 35%	4398
	21st to end	Day	17% 7% 2 7% 5% 5% 5% 4 6% 6% 36%	4837
June	1st to 10th	Day	22% 9% 3 7% 5% 5% 5% 4 6% 6% 29%	2660
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 4 6% 8% 38%	2660
	21st to end	Day	19% 7% 4 7% 6% 5% 5% 4 6% 7% 31%	2660
July	1st to 10th	Day	13% 7% 3 7% 6% 5% 5% 4 8% 9% 32%	4440
	11th to 20th	Day	14% 7% 3 7% 5% 4 5% 5% 6% 8% 37%	4440
	21st to end	Day	11% 7% 2 6% 4 4 4 3 6% 8% 45%	4814
August	1st to 10th	Day	14% 7% 2 8% 6% 6% 6% 4 7% 8% 32%	4376
	11th to 20th	Day	17% 7% 3 7% 4 6% 5% 4 5% 7% 36%	4022
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 7% 9% 38%	4180
September	1st to 10th	Day	16% 6% 2 6% 5% 4 4 3 6% 7% 42%	2280
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 6% 44%	2280
	21st to end	Day	21% 7% 2 6% 4 4 4 4 6% 7% 37%	2024
October	1st to 10th	Day	21% 10% 2 7% 4 4 4 4 6% 6% 34%	1900
	11th to 20th	Day	24% 12% 3 7% 5% 5% 4 3 4 6% 27%	1900
	21st to end	Day	26% 12% 2 8% 5% 4 5% 3 5% 6% 25%	1929
November	1st to 10th	Day	16% 12% 4 7% 5% 5% 3 4 5% 7% 32%	1736
	11th to 20th	Day	18% 16% 4 10% 5% 5% 4 4 5% 5% 23%	1600
	21st to end	Day	22% 17% 6% 13% 5% 5% 4 3 4 5% 17%	1600
December	1st to 10th	Day	21% 24% 7% 11% 6% 5% 4 3 4 5% 11%	2600
	11th to 20th	Day	17% 17% 7% 12% 6% 5% 4 3 4 4 20%	2986
	21st to end	Day	21% 19% 5% 12% 6% 5% 4 3 3 3 18%	3960
January	1st to 10th	Night	13% 22% 8% 15% 7% 4 3 2 3 4 17%	6349
	11th to 20th	Night	11% 15% 7% 17% 8% 6% 5% 3 4 5% 20%	6315
	21st to end	Night	10% 17% 8% 14% 7% 7% 5% 4 4 4 20%	6858
February	1st to 10th	Night	11% 15% 6% 13% 8% 6% 5% 4 4 5% 23%	2600
	11th to 20th	Night	12% 16% 5% 13% 7% 5% 5% 3 3 5% 25%	2533
	21st to end	Night	13% 16% 5% 12% 6% 5% 4 3 5% 6% 26%	1992
March	1st to 10th	Night	20% 18% 4 9% 5% 4 3 3 3 5% 26%	5347
	11th to 20th	Night	12% 15% 4 9% 6% 5% 5% 4 6% 7% 26%	4979
	21st to end	Night	16% 11% 4 11% 5% 4 4 3 5% 5% 32%	5159
April	1st to 10th	Night	21% 12% 3 8% 5% 4 4 3 5% 6% 29%	2000
	11th to 20th	Night	19% 11% 3 7% 4 3 4 3 5% 5% 36%	1975
	21st to end	Night	22% 10% 4 8% 3 4 4 2 4 4 34%	1801
May	1st to 10th	Night	20% 10% 4 8% 5% 4 5% 4 5% 6% 29%	2514
	11th to 20th	Night	17% 9% 3 7% 4 5% 4 4 5% 7% 36%	2495
	21st to end	Night	15% 8% 3 6% 5% 5% 5% 4 5% 7% 36%	2738
June	1st to 10th	Night	17% 8% 3 6% 5% 5% 5% 4 6% 9% 33%	1520
	11th to 20th	Night	16% 6% 3 6% 5% 4 4 4 5% 7% 41%	1520
	21st to end	Night	18% 7% 2 6% 5% 5% 4 4 6% 8% 34%	1520
July	1st to 10th	Night	14% 7% 3 8% 5% 5% 5% 5% 6% 8% 35%	2520
	11th to 20th	Night	14% 7% 2 7% 4 4 5% 3 6% 8% 39%	2520
	21st to end	Night	8% 6% 2 6% 4 4 5% 5% 7% 8% 45%	2739
August	1st to 10th	Night	12% 7% 3 8% 5% 5% 5% 4 7% 7% 37%	2537
	11th to 20th	Night	16% 7% 3 7% 4 5% 4 3 6% 7% 39%	2785
	21st to end	Night	16% 6% 3 7% 5% 4 5% 4 6% 7% 39%	3465
September	1st to 10th	Night	13% 5% 3 7% 4 4 4 3 5% 6% 45%	1900
	11th to 20th	Night	16% 5% 2 4 4 3 3 3 5% 8% 46%	1900
	21st to end	Night	18% 8% 3 6% 3 3 4 3 4 6% 41%	2152
October	1st to 10th	Night	20% 8% 2 6% 4 4 5% 3 5% 5% 38%	2280
	11th to 20th	Night	22% 11% 3 8% 5% 4 4 3 4 6% 30%	2280
	21st to end	Night	23% 11% 3 8% 5% 4 5% 3 5% 6% 26%	2668
November	1st to 10th	Night	18% 12% 4 9% 6% 4 4 3 5% 5% 30%	2558
	11th to 20th	Night	18% 13% 5% 10% 6% 5% 4 3 4 5% 26%	2800
	21st to end	Night	22% 17% 4 11% 6% 4 4 4 5% 5% 19%	2800
December	1st to 10th	Night	19% 20% 6% 14% 7% 5% 4 3 4 4 12%	4580
	11th to 20th	Night	21% 16% 5% 11% 5% 4 3 3 3 5% 24%	5208
	21st to end	Night	22% 21% 5% 11% 5% 4 3 3 4 5% 17%	6996

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	75% 3 7% 10% 4	3598
	11th to 20th	Day	69% 4 8% 12% 8%	3600
	21st to end	Day	65% 3 9% 13% 8% 1	4097
February	1st to 10th	Day	65% 4 8% 16% 6% 1	1800
	11th to 20th	Day	63% 2 11% 14% 9% 1	1762
	21st to end	Day	63% 2 9% 14% 11% 1	1636
March	1st to 10th	Day	63% 4 7% 12% 12% 2	4479
	11th to 20th	Day	56% 4 14% 14% 9% 1	4980
	21st to end	Day	52% 4 6% 20% 16% 2	5772
April	1st to 10th	Day	58% 3 6% 19% 12% 3	2400
	11th to 20th	Day	53% 3 6% 22% 14% 2	2374
	21st to end	Day	58% 3 7% 16% 12% 4	2512
May	1st to 10th	Day	52% 5% 9% 23% 10% 2	4440
	11th to 20th	Day	48% 5% 15% 21% 10% 1	4402
	21st to end	Day	48% 5% 11% 20% 15% 2	4837
June	1st to 10th	Day	55% 4 10% 20% 11% 1	2660
	11th to 20th	Day	43% 5% 11% 27% 11% 2	2660
	21st to end	Day	51% 5% 14% 20% 10%	2660
July	1st to 10th	Day	45% 7% 14% 26% 8%	4370
	11th to 20th	Day	42% 6% 16% 28% 7%	4365
	21st to end	Day	37% 5% 15% 33% 10%	4661
August	1st to 10th	Day	48% 6% 16% 23% 7%	4362
	11th to 20th	Day	45% 6% 15% 27% 6% 1	4061
	21st to end	Day	39% 7% 16% 24% 12% 1	4180
September	1st to 10th	Day	42% 4 14% 29% 12%	2280
	11th to 20th	Day	40% 4 14% 30% 12% 1	2280
	21st to end	Day	45% 6% 14% 25% 9%	2024
October	1st to 10th	Day	51% 5% 11% 27% 6%	3160
	11th to 20th	Day	54% 5% 13% 21% 7%	3140
	21st to end	Day	54% 6% 19% 15% 5%	3154
November	1st to 10th	Day	51% 5% 14% 23% 6% 1	1736
	11th to 20th	Day	63% 4 12% 14% 7%	1600
	21st to end	Day	70% 4 11% 9% 5%	1600
December	1st to 10th	Day	76% 5% 8% 9% 2	2600
	11th to 20th	Day	67% 4 7% 15% 6%	2986
	21st to end	Day	71% 5% 7% 15% 3	3960
January	1st to 10th	Night	74% 3 11% 9% 2	6349
	11th to 20th	Night	68% 4 11% 12% 5%	6360
	21st to end	Night	68% 4 7% 10% 11%	6858
February	1st to 10th	Night	64% 4 8% 16% 7% 1	2600
	11th to 20th	Night	63% 4 8% 18% 7% 1	2535
	21st to end	Night	59% 4 8% 19% 9% 1	1992
March	1st to 10th	Night	63% 3 8% 13% 11% 3	5357
	11th to 20th	Night	54% 7% 14% 16% 8% 2	4980
	21st to end	Night	54% 4 7% 21% 11% 3	5159
April	1st to 10th	Night	56% 3 8% 18% 13% 2	2000
	11th to 20th	Night	50% 3 7% 24% 14% 2	1975
	21st to end	Night	55% 3 7% 18% 15% 3	1801
May	1st to 10th	Night	54% 5% 9% 20% 9% 2	2520
	11th to 20th	Night	47% 5% 13% 22% 11% 2	2496
	21st to end	Night	47% 5% 11% 22% 14% 2	2739
June	1st to 10th	Night	47% 7% 13% 22% 10% 1	1520
	11th to 20th	Night	43% 5% 11% 27% 12% 2	1520
	21st to end	Night	47% 5% 13% 22% 11%	1520
July	1st to 10th	Night	45% 7% 16% 20% 11% 1	2480
	11th to 20th	Night	42% 7% 16% 27% 8%	2472
	21st to end	Night	34% 7% 16% 32% 11%	2651
August	1st to 10th	Night	44% 7% 14% 27% 8%	2527
	11th to 20th	Night	43% 6% 17% 28% 7%	2812
	21st to end	Night	43% 6% 15% 24% 11% 2	3465
September	1st to 10th	Night	40% 5% 13% 29% 13%	1900
	11th to 20th	Night	36% 5% 15% 34% 9%	1900
	21st to end	Night	43% 5% 13% 29% 10%	2152
October	1st to 10th	Night	43% 6% 14% 27% 10%	3800
	11th to 20th	Night	51% 4 14% 22% 8%	3779
	21st to end	Night	53% 5% 18% 15% 8%	4393
November	1st to 10th	Night	56% 4 14% 19% 8%	2558
	11th to 20th	Night	61% 4 11% 15% 8% 1	2800
	21st to end	Night	68% 4 11% 14% 3	2800
December	1st to 10th	Night	74% 5% 9% 10% 2	4580
	11th to 20th	Night	65% 3 6% 15% 10%	5208
	21st to end	Night	70% 3 5% 14% 8%	6996

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% 37% 17% 4 1	200
	11th to 20th	Day	29% 12% 39% 14% 7% 1	200
	21st to end	Day	22% 16% 36% 19% 5% 1	250
February	1st to 10th	Day	31% 23% 33% 10% 3 1	300
	11th to 20th	Day	25% 28% 37% 10%	300
	21st to end	Day	29% 20% 35% 14% 2 1	400
March	1st to 10th	Day	27% 17% 34% 16% 6%	500
	11th to 20th	Day	28% 17% 33% 18% 4 1	500
	21st to end	Day	32% 16% 29% 16% 5% 1	550
April	1st to 10th	Day	33% 18% 25% 17% 6% 1	500
	11th to 20th	Day	35% 17% 22% 14% 10% 3	492
	21st to end	Day	34% 15% 31% 14% 5% 1	480
May	1st to 10th	Day	30% 12% 31% 17% 8% 3	463
	11th to 20th	Day	22% 16% 31% 21% 7% 3	487
	21st to end	Day	24% 14% 32% 19% 10% 2	529
June	1st to 10th	Day	25% 12% 32% 22% 7% 2	490
	11th to 20th	Day	18% 15% 30% 21% 12% 3	490
	21st to end	Day	22% 12% 30% 22% 10% 2	490
July	1st to 10th	Day	15% 7% 31% 34% 10% 3	485
	11th to 20th	Day	15% 9% 34% 25% 14% 3	484
	21st to end	Day	15% 7% 32% 29% 14% 3	532
August	1st to 10th	Day	18% 6% 28% 31% 15% 3	486
	11th to 20th	Day	20% 5% 22% 28% 16% 8%	479
	21st to end	Day	17% 10% 30% 22% 14% 7%	539
September	1st to 10th	Day	18% 10% 30% 22% 12% 8%	490
	11th to 20th	Day	16% 10% 23% 22% 19% 10%	490
	21st to end	Day	23% 11% 26% 18% 14% 8%	490
October	1st to 10th	Day	27% 14% 23% 21% 12% 3	490
	11th to 20th	Day	31% 17% 30% 14% 7% 1	490
	21st to end	Day	33% 13% 30% 18% 6%	454
November	1st to 10th	Day	29% 14% 29% 21% 5% 1	370
	11th to 20th	Day	30% 18% 34% 11% 7% 2	200
	21st to end	Day	31% 20% 33% 13% 2 2	200
December	1st to 10th	Day	35% 8% 36% 18% 5%	200
	11th to 20th	Day	30% 24% 31% 12% 4 1	200
	21st to end	Day	27% 17% 35% 16% 5% 1	220
January	1st to 10th	Night	28% 13% 34% 21% 4	800
	11th to 20th	Night	29% 15% 26% 23% 8%	800
	21st to end	Night	19% 11% 34% 27% 9% 1	880
February	1st to 10th	Night	20% 15% 38% 22% 4 1	800
	11th to 20th	Night	24% 15% 36% 20% 4	783
	21st to end	Night	23% 18% 30% 25% 5%	660
March	1st to 10th	Night	27% 15% 29% 24% 6%	800
	11th to 20th	Night	26% 14% 31% 24% 6%	800
	21st to end	Night	23% 15% 28% 24% 9%	880
April	1st to 10th	Night	29% 14% 25% 26% 6%	800
	11th to 20th	Night	27% 14% 28% 22% 9% 1	791
	21st to end	Night	29% 11% 26% 25% 8%	627
May	1st to 10th	Night	24% 8% 29% 28% 9% 1	380
	11th to 20th	Night	24% 8% 27% 32% 9%	374
	21st to end	Night	16% 10% 34% 30% 10%	411
June	1st to 10th	Night	19% 8% 32% 32% 8% 1	380
	11th to 20th	Night	19% 8% 23% 36% 13% 1	380
	21st to end	Night	22% 4 23% 37% 14% 1	380
July	1st to 10th	Night	11% 4 27% 42% 16%	370
	11th to 20th	Night	15% 4 20% 42% 18% 1	368
	21st to end	Night	11% 4 24% 43% 17% 2	416
August	1st to 10th	Night	11% 4 21% 38% 25% 1	396
	11th to 20th	Night	15% 3 16% 37% 26% 3	564
	21st to end	Night	17% 5% 24% 32% 20% 2	825
September	1st to 10th	Night	18% 5% 25% 32% 18% 1	760
	11th to 20th	Night	19% 4 21% 30% 22% 4	760
	21st to end	Night	22% 8% 22% 27% 19% 2	760
October	1st to 10th	Night	26% 10% 25% 25% 14% 1	760
	11th to 20th	Night	30% 9% 27% 24% 8% 1	760
	21st to end	Night	31% 11% 27% 24% 7%	836
November	1st to 10th	Night	28% 14% 26% 24% 8% 1	780
	11th to 20th	Night	28% 15% 28% 19% 9% 1	800
	21st to end	Night	34% 15% 24% 22% 6%	800
December	1st to 10th	Night	32% 14% 28% 21% 5% 1	800
	11th to 20th	Night	29% 16% 28% 21% 6% 1	800
	21st to end	Night	34% 13% 27% 21% 5% 1	880

Page generated at 2025-04-23 09:25:01. Version ac07

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