Grande-Vallee
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grande-Vallee clear sky chart from 2005-11-06 to 2026-07-01.

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	15% 11% 4 9% 6% 5% 5% 4 5% 6% 29%	138192
	Night	15% 13% 5% 12% 8% 6% 5% 3 5% 5% 22%	134407
Transparency	Day	55% 7% 12% 18% 7%	141612
	Night	61% 6% 11% 15% 6%	139067
Seeing	Day	22% 15% 28% 23% 9% 2	18685
	Night	24% 15% 27% 24% 10% 1	26197

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% 22% 7% 15% 16% 10% 4 3 3 2 4	4093
	11th to 20th	Day	9% 20% 10% 20% 11% 6% 4 4 4 5% 8%	4084
	21st to end	Day	3 10% 5% 13% 12% 14% 16% 6% 5% 5% 12%	5071
February	1st to 10th	Day	9% 10% 3 11% 12% 12% 8% 4 6% 6% 19%	1890
	11th to 20th	Day	11% 15% 5% 12% 9% 7% 6% 6% 5% 5% 19%	1843
	21st to end	Day	8% 12% 4 10% 7% 9% 6% 5% 7% 7% 26%	1557
March	1st to 10th	Day	14% 14% 5% 12% 8% 8% 6% 3 4 6% 21%	4881
	11th to 20th	Day	15% 17% 4 11% 9% 7% 5% 4 5% 6% 18%	5090
	21st to end	Day	19% 15% 4 10% 9% 5% 5% 4 5% 5% 21%	5587
April	1st to 10th	Day	17% 13% 4 9% 7% 5% 5% 4 6% 6% 25%	2530
	11th to 20th	Day	16% 12% 3 7% 5% 5% 5% 3 5% 7% 34%	2702
	21st to end	Day	21% 11% 4 7% 3 3 4 4 5% 5% 33%	2730
May	1st to 10th	Day	23% 11% 3 6% 5% 4 5% 4 5% 6% 28%	8111
	11th to 20th	Day	16% 9% 3 8% 5% 4 5% 4 5% 8% 34%	8220
	21st to end	Day	23% 9% 3 7% 4 4 3 3 4 5% 34%	9716
June	1st to 10th	Day	18% 7% 2 7% 5% 5% 5% 5% 5% 8% 33%	3150
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 6% 8% 37%	3150
	21st to end	Day	18% 7% 2 7% 5% 5% 5% 4 6% 7% 33%	3150
July	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 7% 8% 40%	6820
	11th to 20th	Day	11% 7% 3 7% 6% 5% 5% 5% 7% 8% 35%	6820
	21st to end	Day	11% 5% 2 5% 3 4 4 4 6% 8% 47%	7321
August	1st to 10th	Day	12% 5% 2 6% 4 4 5% 5% 6% 9% 42%	4318
	11th to 20th	Day	16% 6% 2 7% 5% 6% 5% 4 7% 8% 34%	4175
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 7% 48%	4675
September	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 3 5% 7% 40%	2405
	11th to 20th	Day	21% 7% 2 5% 3 4 4 3 4 5% 43%	2282
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 7% 40%	2167
October	1st to 10th	Day	16% 7% 2 6% 4 5% 4 3 6% 8% 39%	2000
	11th to 20th	Day	19% 12% 3 9% 5% 4 4 3 5% 7% 29%	2000
	21st to end	Day	23% 14% 3 8% 4 4 3 3 5% 7% 27%	2011
November	1st to 10th	Day	19% 16% 4 9% 6% 5% 5% 3 5% 6% 21%	1645
	11th to 20th	Day	20% 15% 8% 13% 6% 4 4 3 4 5% 18%	1680
	21st to end	Day	21% 18% 6% 17% 6% 4 4 3 3 4 14%	1680
December	1st to 10th	Day	10% 15% 9% 32% 8% 4 4 2 4 4 7%	2397
	11th to 20th	Day	17% 15% 7% 20% 9% 6% 3 3 3 3 14%	2688
	21st to end	Day	11% 15% 6% 20% 11% 5% 4 3 4 6% 15%	3553
January	1st to 10th	Night	16% 22% 5% 18% 15% 8% 4 2 3 3 5%	8831
	11th to 20th	Night	10% 17% 8% 22% 12% 6% 5% 3 4 3 10%	8831
	21st to end	Night	3 10% 6% 15% 13% 14% 14% 4 4 5% 13%	9103
February	1st to 10th	Night	8% 13% 4 12% 11% 10% 8% 4 6% 5% 18%	2730
	11th to 20th	Night	13% 17% 5% 12% 8% 8% 6% 4 4 5% 18%	2663
	21st to end	Night	9% 14% 4 10% 8% 7% 5% 4 5% 5% 27%	2249
March	1st to 10th	Night	15% 14% 4 10% 10% 8% 6% 3 5% 4 21%	5177
	11th to 20th	Night	19% 17% 5% 11% 10% 6% 4 3 4 5% 17%	5089
	21st to end	Night	22% 17% 5% 10% 7% 4 4 3 4 5% 18%	5587
April	1st to 10th	Night	19% 13% 5% 11% 7% 5% 4 3 5% 5% 22%	2086
	11th to 20th	Night	20% 11% 3 8% 4 4 5% 4 4 7% 30%	1868
	21st to end	Night	21% 11% 4 8% 4 4 4 3 3 5% 33%	1890
May	1st to 10th	Night	26% 10% 3 7% 5% 4 4 3 3 5% 29%	4810
	11th to 20th	Night	21% 8% 3 8% 4 4 5% 4 5% 5% 33%	4670
	21st to end	Night	20% 10% 3 7% 4 5% 5% 4 6% 7% 29%	4533
June	1st to 10th	Night	22% 7% 2 8% 5% 5% 5% 4 5% 7% 30%	1470
	11th to 20th	Night	18% 8% 3 7% 6% 4 4 3 6% 6% 33%	1470
	21st to end	Night	19% 7% 3 8% 6% 5% 5% 4 6% 8% 31%	1470
July	1st to 10th	Night	16% 7% 2 6% 4 5% 5% 4 7% 8% 35%	3160
	11th to 20th	Night	10% 6% 2 7% 4 6% 5% 5% 9% 10% 36%	3160
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 6% 7% 42%	3508
August	1st to 10th	Night	12% 7% 3 7% 6% 5% 5% 4 7% 9% 34%	2819
	11th to 20th	Night	14% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	2872
	21st to end	Night	17% 5% 2 6% 3 4 4 3 5% 7% 45%	3223
September	1st to 10th	Night	18% 6% 3 6% 3 4 4 4 5% 7% 41%	1995
	11th to 20th	Night	20% 7% 2 6% 4 4 3 3 5% 6% 41%	2115
	21st to end	Night	18% 6% 2 6% 4 3 4 3 5% 8% 42%	2232
October	1st to 10th	Night	19% 9% 3 5% 3 4 4 3 5% 7% 39%	2400
	11th to 20th	Night	23% 11% 4 8% 4 5% 4 3 4 6% 28%	2400
	21st to end	Night	21% 12% 3 7% 5% 5% 6% 4 5% 7% 26%	2825
November	1st to 10th	Night	20% 13% 4 9% 5% 5% 5% 4 5% 6% 23%	2873
	11th to 20th	Night	20% 21% 7% 11% 5% 4 4 3 4 5% 18%	2940
	21st to end	Night	21% 17% 7% 16% 6% 5% 4 3 4 4 13%	2940
December	1st to 10th	Night	12% 17% 9% 29% 7% 4 4 3 3 4 8%	5010
	11th to 20th	Night	13% 14% 7% 23% 9% 5% 3 2 4 5% 15%	5741
	21st to end	Night	13% 16% 7% 18% 12% 5% 4 3 4 5% 12%	7667

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	85% 4 7% 3 1	4094
	11th to 20th	Day	76% 5% 9% 7% 3	4110
	21st to end	Day	67% 8% 14% 8% 3	5071
February	1st to 10th	Day	62% 7% 13% 11% 6% 1	1890
	11th to 20th	Day	63% 8% 13% 13% 4	1851
	21st to end	Day	55% 9% 10% 22% 5%	1557
March	1st to 10th	Day	65% 7% 9% 14% 5%	4885
	11th to 20th	Day	67% 8% 8% 12% 5%	5090
	21st to end	Day	65% 7% 7% 14% 6% 1	5587
April	1st to 10th	Day	60% 8% 9% 13% 10% 1	2530
	11th to 20th	Day	53% 7% 10% 18% 11% 1	2702
	21st to end	Day	54% 7% 11% 16% 11% 1	2730
May	1st to 10th	Day	57% 8% 10% 16% 8% 1	8124
	11th to 20th	Day	48% 7% 12% 23% 9%	8224
	21st to end	Day	53% 6% 9% 20% 12% 1	9717
June	1st to 10th	Day	49% 9% 12% 18% 11% 1	3150
	11th to 20th	Day	46% 9% 12% 20% 12% 1	3150
	21st to end	Day	49% 11% 10% 19% 10% 1	3150
July	1st to 10th	Day	42% 8% 13% 26% 11%	6745
	11th to 20th	Day	46% 8% 18% 22% 6%	6739
	21st to end	Day	36% 7% 18% 28% 10%	7111
August	1st to 10th	Day	40% 8% 18% 29% 5%	4304
	11th to 20th	Day	46% 10% 18% 19% 7%	4214
	21st to end	Day	35% 8% 18% 29% 9% 1	4675
September	1st to 10th	Day	44% 8% 16% 23% 9%	2405
	11th to 20th	Day	43% 7% 16% 25% 8%	2282
	21st to end	Day	45% 9% 13% 24% 9% 1	2167
October	1st to 10th	Day	46% 7% 15% 24% 7% 1	3260
	11th to 20th	Day	56% 6% 14% 19% 5%	3240
	21st to end	Day	54% 6% 14% 18% 7% 1	3215
November	1st to 10th	Day	64% 9% 12% 10% 5%	1645
	11th to 20th	Day	69% 7% 11% 9% 4	1680
	21st to end	Day	75% 5% 10% 6% 4	1680
December	1st to 10th	Day	78% 8% 5% 8%	2397
	11th to 20th	Day	73% 6% 8% 8% 4	2688
	21st to end	Day	68% 9% 10% 11% 2	3553
January	1st to 10th	Night	84% 5% 7% 3 1	8831
	11th to 20th	Night	76% 5% 9% 6% 3	8880
	21st to end	Night	70% 7% 11% 7% 4 1	9103
February	1st to 10th	Night	63% 7% 12% 11% 6% 1	2730
	11th to 20th	Night	66% 7% 9% 13% 5%	2666
	21st to end	Night	57% 7% 9% 20% 8%	2249
March	1st to 10th	Night	64% 8% 7% 15% 6%	5186
	11th to 20th	Night	69% 5% 8% 12% 5%	5090
	21st to end	Night	67% 4 8% 13% 6% 1	5587
April	1st to 10th	Night	63% 7% 9% 12% 9% 1	2086
	11th to 20th	Night	53% 5% 9% 21% 11% 1	1868
	21st to end	Night	55% 5% 10% 16% 13% 2	1890
May	1st to 10th	Night	57% 5% 12% 17% 9% 1	4815
	11th to 20th	Night	50% 6% 9% 21% 13% 1	4671
	21st to end	Night	53% 7% 9% 20% 11%	4534
June	1st to 10th	Night	52% 6% 11% 20% 10%	1470
	11th to 20th	Night	51% 6% 12% 18% 13% 1	1470
	21st to end	Night	49% 9% 13% 19% 9% 1	1470
July	1st to 10th	Night	45% 7% 12% 25% 11%	3125
	11th to 20th	Night	39% 7% 19% 31% 4	3118
	21st to end	Night	40% 7% 19% 23% 11%	3404
August	1st to 10th	Night	45% 8% 19% 22% 7%	2808
	11th to 20th	Night	44% 9% 21% 20% 6%	2899
	21st to end	Night	39% 6% 15% 29% 10% 1	3223
September	1st to 10th	Night	43% 8% 13% 26% 10%	1995
	11th to 20th	Night	44% 5% 15% 25% 9%	2115
	21st to end	Night	42% 6% 12% 28% 11% 1	2232
October	1st to 10th	Night	48% 6% 12% 25% 8% 1	3920
	11th to 20th	Night	56% 5% 11% 22% 7%	3899
	21st to end	Night	57% 8% 11% 20% 4	4561
November	1st to 10th	Night	61% 7% 15% 12% 4	2873
	11th to 20th	Night	70% 6% 10% 10% 4	2940
	21st to end	Night	75% 6% 11% 6% 2	2940
December	1st to 10th	Night	77% 7% 7% 9% 1	5010
	11th to 20th	Night	70% 7% 10% 10% 3 1	5742
	21st to end	Night	72% 7% 9% 11% 1	7667

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	24% 24% 29% 20% 3 1	220
	11th to 20th	Day	24% 23% 27% 19% 6% 1	220
	21st to end	Day	18% 22% 32% 20% 6% 1	296
February	1st to 10th	Day	16% 19% 37% 16% 10% 1	430
	11th to 20th	Day	19% 28% 31% 17% 5% 1	426
	21st to end	Day	14% 21% 37% 19% 8% 1	355
March	1st to 10th	Day	22% 23% 33% 17% 4 1	535
	11th to 20th	Day	27% 20% 26% 18% 7% 1	540
	21st to end	Day	27% 20% 31% 16% 4 2	594
April	1st to 10th	Day	30% 21% 29% 16% 3 1	540
	11th to 20th	Day	26% 19% 31% 17% 6% 1	532
	21st to end	Day	27% 16% 28% 22% 6% 1	540
May	1st to 10th	Day	28% 13% 29% 18% 9% 3	706
	11th to 20th	Day	25% 14% 24% 24% 12% 2	774
	21st to end	Day	28% 13% 27% 24% 7% 1	845
June	1st to 10th	Day	23% 10% 28% 27% 11% 1	780
	11th to 20th	Day	22% 11% 25% 27% 12% 3	780
	21st to end	Day	22% 7% 24% 34% 11% 2	780
July	1st to 10th	Day	18% 7% 32% 28% 14% 1	720
	11th to 20th	Day	14% 10% 23% 37% 14% 1	718
	21st to end	Day	16% 9% 23% 35% 15% 2	795
August	1st to 10th	Day	15% 6% 26% 34% 17% 2	583
	11th to 20th	Day	15% 6% 25% 32% 20% 2	519
	21st to end	Day	18% 7% 26% 32% 14% 3	583
September	1st to 10th	Day	21% 11% 29% 23% 13% 3	530
	11th to 20th	Day	27% 10% 26% 25% 11% 2	530
	21st to end	Day	19% 20% 29% 22% 9% 2	511
October	1st to 10th	Day	19% 16% 32% 20% 11% 1	420
	11th to 20th	Day	28% 20% 24% 17% 9% 1	420
	21st to end	Day	27% 22% 23% 17% 10% 1	462
November	1st to 10th	Day	28% 24% 24% 20% 3 1	425
	11th to 20th	Day	30% 20% 29% 14% 6% 1	430
	21st to end	Day	33% 20% 27% 16% 4	430
December	1st to 10th	Day	21% 20% 31% 21% 6% 1	254
	11th to 20th	Day	21% 34% 28% 14% 3	220
	21st to end	Day	17% 30% 32% 16% 5%	242
January	1st to 10th	Night	26% 22% 28% 18% 5% 1	950
	11th to 20th	Night	23% 22% 30% 18% 6%	950
	21st to end	Night	15% 25% 30% 21% 8% 1	962
February	1st to 10th	Night	14% 22% 32% 23% 7% 2	840
	11th to 20th	Night	25% 18% 32% 19% 6%	823
	21st to end	Night	16% 18% 35% 21% 8% 2	692
March	1st to 10th	Night	21% 18% 32% 21% 8%	840
	11th to 20th	Night	22% 21% 28% 20% 8% 2	840
	21st to end	Night	25% 16% 28% 23% 7% 1	924
April	1st to 10th	Night	27% 18% 27% 20% 8%	653
	11th to 20th	Night	26% 13% 27% 23% 9% 2	624
	21st to end	Night	27% 12% 25% 23% 11% 2	630
May	1st to 10th	Night	30% 7% 23% 24% 13% 2	630
	11th to 20th	Night	24% 8% 23% 28% 14% 2	622
	21st to end	Night	25% 11% 27% 29% 8% 1	471
June	1st to 10th	Night	25% 6% 21% 35% 13% 1	420
	11th to 20th	Night	23% 7% 22% 31% 16%	420
	21st to end	Night	24% 6% 23% 29% 17% 2	420
July	1st to 10th	Night	17% 5% 24% 35% 17% 2	390
	11th to 20th	Night	15% 4 24% 40% 17% 1	388
	21st to end	Night	17% 6% 18% 36% 22% 2	463
August	1st to 10th	Night	16% 6% 18% 38% 21% 2	598
	11th to 20th	Night	17% 4 19% 38% 21% 1	592
	21st to end	Night	18% 5% 20% 33% 21% 3	660
September	1st to 10th	Night	20% 8% 24% 32% 14% 1	600
	11th to 20th	Night	24% 9% 24% 30% 12% 1	715
	21st to end	Night	23% 10% 28% 28% 10% 1	800
October	1st to 10th	Night	25% 13% 26% 26% 9% 1	800
	11th to 20th	Night	32% 14% 24% 18% 11% 1	800
	21st to end	Night	29% 16% 24% 22% 8% 1	905
November	1st to 10th	Night	31% 19% 28% 18% 4	930
	11th to 20th	Night	29% 21% 27% 16% 5% 1	950
	21st to end	Night	31% 19% 27% 18% 5%	950
December	1st to 10th	Night	24% 22% 32% 17% 4 1	950
	11th to 20th	Night	25% 28% 29% 14% 4	950
	21st to end	Night	27% 21% 27% 19% 4 2	1045

Page generated at 2026-06-28 02:10:44. Version ac07

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