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 2025-02-25.

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% 10% 4 10% 6% 5% 5% 4 5% 6% 29%	108286
	Night	15% 13% 5% 13% 8% 6% 5% 3 5% 5% 23%	110522
Transparency	Day	54% 8% 13% 18% 7% 1	111755
	Night	61% 7% 11% 15% 6%	115208
Seeing	Day	22% 15% 28% 24% 10% 2	16785
	Night	24% 15% 26% 24% 10% 1	24516

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% 24% 8% 16% 18% 6% 3 2 2 2 3	3157
	11th to 20th	Day	8% 18% 10% 23% 12% 6% 4 4 4 4 7%	3134
	21st to end	Day	4 8% 3 11% 13% 15% 15% 5% 6% 6% 15%	3917
February	1st to 10th	Day	9% 10% 3 11% 11% 12% 8% 4 6% 7% 20%	1800
	11th to 20th	Day	11% 15% 5% 12% 9% 7% 6% 6% 6% 5% 19%	1753
	21st to end	Day	7% 11% 4 9% 7% 9% 6% 5% 7% 7% 27%	1449
March	1st to 10th	Day	11% 12% 4 10% 7% 7% 6% 4 5% 7% 29%	3318
	11th to 20th	Day	16% 20% 5% 10% 6% 5% 4 3 4 5% 19%	3480
	21st to end	Day	21% 14% 3 8% 7% 5% 4 4 4 5% 26%	3816
April	1st to 10th	Day	17% 13% 4 9% 6% 5% 5% 4 6% 6% 26%	2288
	11th to 20th	Day	15% 12% 3 7% 4 5% 5% 3 5% 7% 34%	2442
	21st to end	Day	21% 11% 4 7% 3 3 4 4 5% 5% 33%	2470
May	1st to 10th	Day	20% 12% 3 6% 5% 4 5% 4 5% 7% 29%	4356
	11th to 20th	Day	13% 9% 3 8% 5% 5% 5% 5% 6% 8% 33%	4398
	21st to end	Day	24% 9% 3 6% 5% 4 4 4 6% 6% 30%	5185
June	1st to 10th	Day	18% 8% 2 7% 5% 5% 5% 5% 5% 8% 32%	2850
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 6% 8% 38%	2850
	21st to end	Day	19% 7% 2 6% 4 5% 5% 4 7% 7% 34%	2850
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 7% 8% 40%	4760
	11th to 20th	Day	10% 6% 3 7% 6% 5% 6% 5% 7% 9% 38%	4760
	21st to end	Day	11% 5% 2 5% 3 4 5% 4 6% 8% 47%	5083
August	1st to 10th	Day	12% 5% 2 6% 4 4 5% 5% 6% 9% 41%	4185
	11th to 20th	Day	16% 6% 3 7% 5% 6% 5% 4 7% 8% 33%	4045
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 7% 49%	4532
September	1st to 10th	Day	18% 6% 2 6% 4 6% 5% 3 5% 6% 39%	2285
	11th to 20th	Day	21% 7% 2 5% 3 4 4 3 4 5% 43%	2168
	21st to end	Day	18% 6% 2 6% 4 4 4 4 5% 7% 40%	2059
October	1st to 10th	Day	16% 7% 2 6% 4 5% 4 4 6% 7% 39%	1900
	11th to 20th	Day	19% 11% 3 9% 5% 4 5% 3 5% 6% 29%	1900
	21st to end	Day	23% 14% 3 8% 4 4 3 3 5% 6% 27%	1911
November	1st to 10th	Day	19% 15% 3 10% 6% 5% 5% 3 6% 6% 20%	1565
	11th to 20th	Day	20% 15% 8% 13% 6% 4 4 3 4 5% 18%	1600
	21st to end	Day	22% 18% 6% 17% 6% 4 3 3 3 5% 14%	1600
December	1st to 10th	Day	11% 15% 9% 32% 7% 4 4 2 4 4 7%	2326
	11th to 20th	Day	17% 16% 7% 19% 9% 6% 3 3 3 3 14%	2618
	21st to end	Day	11% 15% 6% 20% 11% 5% 4 3 4 6% 15%	3476
January	1st to 10th	Night	17% 24% 6% 19% 14% 6% 3 2 3 2 4	6808
	11th to 20th	Night	10% 18% 9% 24% 13% 6% 5% 3 3 3 8%	6771
	21st to end	Night	3 8% 5% 17% 15% 15% 11% 4 4 5% 14%	7042
February	1st to 10th	Night	8% 13% 4 12% 11% 11% 8% 3 6% 5% 19%	2600
	11th to 20th	Night	13% 17% 5% 11% 8% 8% 6% 4 4 5% 18%	2533
	21st to end	Night	9% 13% 4 10% 8% 7% 5% 5% 6% 5% 27%	2100
March	1st to 10th	Night	10% 12% 3 10% 7% 6% 7% 4 6% 6% 29%	3521
	11th to 20th	Night	22% 20% 5% 10% 7% 5% 3 2 3 4 18%	3479
	21st to end	Night	25% 17% 4 7% 6% 4 4 3 4 6% 22%	3816
April	1st to 10th	Night	19% 13% 4 11% 8% 5% 4 3 5% 5% 22%	1888
	11th to 20th	Night	18% 11% 3 8% 4 4 5% 4 4 8% 31%	1688
	21st to end	Night	21% 11% 4 8% 5% 4 4 3 3 5% 32%	1710
May	1st to 10th	Night	25% 12% 3 7% 5% 4 4 4 4 6% 26%	2591
	11th to 20th	Night	16% 8% 2 7% 4 5% 5% 4 6% 7% 35%	2506
	21st to end	Night	20% 10% 3 8% 5% 5% 5% 4 7% 8% 27%	2415
June	1st to 10th	Night	23% 8% 2 7% 4 5% 4 4 5% 7% 30%	1330
	11th to 20th	Night	17% 8% 4 7% 6% 4 4 3 6% 6% 35%	1330
	21st to end	Night	18% 7% 3 8% 6% 5% 5% 4 6% 8% 30%	1330
July	1st to 10th	Night	15% 7% 2 6% 3 5% 5% 3 7% 7% 38%	2210
	11th to 20th	Night	10% 7% 3 7% 5% 5% 5% 5% 7% 9% 38%	2210
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 6% 7% 46%	2450
August	1st to 10th	Night	13% 7% 3 7% 6% 5% 5% 4 7% 9% 33%	2732
	11th to 20th	Night	14% 7% 3 7% 5% 5% 5% 4 6% 8% 36%	2782
	21st to end	Night	18% 5% 2 5% 3 4 3 3 5% 7% 45%	3124
September	1st to 10th	Night	19% 6% 3 6% 4 4 4 4 5% 6% 40%	1895
	11th to 20th	Night	21% 7% 2 6% 4 4 3 3 4 5% 42%	2009
	21st to end	Night	19% 7% 2 6% 4 3 3 3 5% 8% 41%	2120
October	1st to 10th	Night	19% 9% 3 5% 3 4 4 3 5% 7% 38%	2280
	11th to 20th	Night	23% 11% 4 8% 4 5% 4 3 4 6% 27%	2280
	21st to end	Night	22% 12% 3 7% 5% 5% 6% 4 5% 7% 25%	2684
November	1st to 10th	Night	20% 13% 4 9% 5% 5% 5% 4 5% 6% 23%	2733
	11th to 20th	Night	19% 20% 7% 11% 5% 4 4 3 4 5% 18%	2800
	21st to end	Night	22% 17% 7% 16% 6% 5% 4 3 3 4 13%	2800
December	1st to 10th	Night	13% 17% 9% 30% 6% 4 4 2 3 4 8%	4862
	11th to 20th	Night	13% 14% 7% 22% 8% 5% 3 2 4 5% 15%	5591
	21st to end	Night	14% 16% 7% 18% 11% 5% 4 3 4 5% 12%	7502

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	87% 4 6% 2 1	3158
	11th to 20th	Day	76% 6% 10% 6% 2	3160
	21st to end	Day	61% 9% 17% 9% 4	3917
February	1st to 10th	Day	61% 8% 13% 12% 6% 1	1800
	11th to 20th	Day	62% 8% 13% 12% 4	1761
	21st to end	Day	54% 9% 10% 22% 5%	1449
March	1st to 10th	Day	55% 8% 13% 18% 6% 1	3322
	11th to 20th	Day	68% 9% 7% 11% 5% 1	3480
	21st to end	Day	58% 9% 9% 15% 7% 1	3816
April	1st to 10th	Day	60% 8% 9% 13% 9% 1	2288
	11th to 20th	Day	52% 8% 10% 18% 12% 1	2442
	21st to end	Day	54% 8% 10% 16% 11% 1	2470
May	1st to 10th	Day	55% 10% 10% 17% 7% 1	4371
	11th to 20th	Day	46% 10% 14% 21% 8% 1	4402
	21st to end	Day	53% 9% 10% 18% 8% 1	5185
June	1st to 10th	Day	49% 10% 12% 17% 10% 1	2850
	11th to 20th	Day	45% 9% 12% 20% 12% 1	2850
	21st to end	Day	49% 12% 11% 18% 9%	2850
July	1st to 10th	Day	42% 9% 13% 25% 11%	4685
	11th to 20th	Day	43% 9% 17% 23% 8%	4679
	21st to end	Day	35% 9% 19% 27% 10%	4921
August	1st to 10th	Day	41% 8% 18% 28% 5%	4171
	11th to 20th	Day	47% 10% 18% 17% 7%	4084
	21st to end	Day	35% 8% 18% 30% 9% 1	4532
September	1st to 10th	Day	45% 8% 15% 23% 9%	2285
	11th to 20th	Day	43% 7% 16% 26% 8%	2168
	21st to end	Day	46% 9% 13% 23% 9% 1	2059
October	1st to 10th	Day	46% 7% 15% 24% 7% 1	3160
	11th to 20th	Day	56% 6% 14% 19% 4	3140
	21st to end	Day	54% 6% 14% 17% 7% 1	3115
November	1st to 10th	Day	64% 9% 12% 10% 5%	1565
	11th to 20th	Day	69% 8% 11% 9% 4	1600
	21st to end	Day	75% 5% 11% 6% 3	1600
December	1st to 10th	Day	78% 8% 5% 8%	2326
	11th to 20th	Day	73% 6% 8% 8% 4	2618
	21st to end	Day	67% 9% 10% 11% 2	3476
January	1st to 10th	Night	85% 6% 6% 3 1	6808
	11th to 20th	Night	79% 5% 10% 5% 1	6820
	21st to end	Night	67% 7% 11% 8% 4 2	7042
February	1st to 10th	Night	63% 7% 11% 11% 7% 1	2600
	11th to 20th	Night	66% 7% 9% 12% 5%	2536
	21st to end	Night	56% 7% 9% 20% 8% 1	2100
March	1st to 10th	Night	53% 10% 10% 20% 7%	3530
	11th to 20th	Night	70% 5% 7% 12% 5% 1	3480
	21st to end	Night	62% 5% 11% 14% 7% 1	3816
April	1st to 10th	Night	61% 8% 9% 12% 9% 1	1888
	11th to 20th	Night	51% 6% 9% 21% 12% 1	1688
	21st to end	Night	55% 5% 9% 15% 13% 2	1710
May	1st to 10th	Night	57% 6% 14% 16% 6% 1	2596
	11th to 20th	Night	43% 8% 13% 24% 11%	2507
	21st to end	Night	51% 10% 12% 19% 7%	2416
June	1st to 10th	Night	52% 6% 11% 20% 10%	1330
	11th to 20th	Night	50% 6% 13% 18% 13% 1	1330
	21st to end	Night	49% 10% 12% 19% 9% 1	1330
July	1st to 10th	Night	42% 8% 14% 22% 14%	2175
	11th to 20th	Night	39% 7% 19% 29% 5%	2168
	21st to end	Night	36% 7% 21% 23% 12%	2372
August	1st to 10th	Night	46% 8% 19% 21% 7%	2721
	11th to 20th	Night	45% 9% 21% 20% 6%	2809
	21st to end	Night	39% 6% 15% 29% 10% 1	3124
September	1st to 10th	Night	44% 8% 12% 25% 10%	1895
	11th to 20th	Night	45% 5% 14% 26% 10%	2009
	21st to end	Night	43% 7% 13% 28% 10% 1	2120
October	1st to 10th	Night	48% 6% 12% 25% 8% 1	3800
	11th to 20th	Night	56% 5% 10% 22% 7%	3779
	21st to end	Night	57% 8% 11% 20% 4	4420
November	1st to 10th	Night	61% 7% 15% 13% 5%	2733
	11th to 20th	Night	70% 6% 10% 10% 4	2800
	21st to end	Night	75% 6% 11% 6% 2	2800
December	1st to 10th	Night	77% 7% 6% 8% 1	4862
	11th to 20th	Night	70% 7% 10% 10% 3 1	5592
	21st to end	Night	72% 7% 9% 11% 1	7502

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% 26% 26% 21% 3 1	200
	11th to 20th	Day	23% 23% 26% 21% 7% 2	200
	21st to end	Day	18% 21% 32% 21% 7% 1	271
February	1st to 10th	Day	17% 20% 38% 15% 10% 1	400
	11th to 20th	Day	18% 29% 32% 16% 4 1	396
	21st to end	Day	13% 21% 38% 18% 8% 1	319
March	1st to 10th	Day	19% 22% 33% 20% 5% 2	456
	11th to 20th	Day	24% 20% 25% 20% 8% 2	460
	21st to end	Day	27% 20% 30% 17% 4 2	506
April	1st to 10th	Day	27% 21% 30% 17% 4 1	460
	11th to 20th	Day	22% 19% 33% 18% 7% 1	452
	21st to end	Day	25% 17% 27% 23% 5% 2	460
May	1st to 10th	Day	26% 13% 29% 20% 10% 3	610
	11th to 20th	Day	23% 13% 24% 24% 13% 2	674
	21st to end	Day	28% 15% 27% 23% 6% 1	735
June	1st to 10th	Day	24% 10% 26% 27% 11% 1	680
	11th to 20th	Day	21% 11% 25% 28% 13% 3	680
	21st to end	Day	22% 6% 24% 36% 11% 2	680
July	1st to 10th	Day	17% 6% 32% 29% 15% 1	670
	11th to 20th	Day	14% 9% 24% 38% 15% 1	668
	21st to end	Day	15% 8% 21% 37% 16% 2	740
August	1st to 10th	Day	17% 6% 25% 34% 16% 2	540
	11th to 20th	Day	15% 6% 25% 33% 19% 2	479
	21st to end	Day	18% 7% 26% 33% 13% 3	539
September	1st to 10th	Day	22% 11% 30% 22% 12% 3	490
	11th to 20th	Day	26% 11% 26% 25% 10% 2	490
	21st to end	Day	19% 19% 29% 22% 9% 1	473
October	1st to 10th	Day	19% 16% 32% 20% 11% 1	390
	11th to 20th	Day	27% 21% 25% 17% 9% 1	390
	21st to end	Day	28% 23% 23% 16% 9% 1	429
November	1st to 10th	Day	26% 25% 25% 21% 3 1	395
	11th to 20th	Day	29% 20% 30% 15% 6% 1	400
	21st to end	Day	33% 19% 27% 17% 4 1	400
December	1st to 10th	Day	21% 20% 31% 21% 6% 1	233
	11th to 20th	Day	22% 34% 27% 15% 3	200
	21st to end	Day	17% 31% 31% 17% 4	220
January	1st to 10th	Night	26% 22% 28% 18% 5% 1	900
	11th to 20th	Night	23% 22% 30% 19% 6%	900
	21st to end	Night	16% 24% 30% 21% 8% 1	915
February	1st to 10th	Night	15% 23% 32% 23% 7% 1	800
	11th to 20th	Night	25% 18% 32% 19% 5%	783
	21st to end	Night	15% 18% 35% 21% 9% 2	647
March	1st to 10th	Night	19% 17% 33% 23% 9% 1	760
	11th to 20th	Night	22% 20% 28% 20% 8% 2	760
	21st to end	Night	25% 16% 28% 23% 7% 1	836
April	1st to 10th	Night	26% 17% 26% 21% 9% 1	591
	11th to 20th	Night	24% 13% 27% 24% 10% 2	564
	21st to end	Night	26% 12% 25% 24% 11% 2	570
May	1st to 10th	Night	29% 7% 23% 25% 14% 2	570
	11th to 20th	Night	23% 7% 23% 28% 15% 2	562
	21st to end	Night	24% 11% 27% 30% 8% 1	425
June	1st to 10th	Night	26% 6% 17% 36% 14% 1	380
	11th to 20th	Night	22% 6% 22% 33% 17% 1	380
	21st to end	Night	23% 6% 22% 30% 18% 2	380
July	1st to 10th	Night	17% 4 22% 36% 18% 2	370
	11th to 20th	Night	16% 4 22% 40% 18% 1	368
	21st to end	Night	16% 6% 17% 37% 23% 2	440
August	1st to 10th	Night	17% 6% 17% 38% 21% 2	568
	11th to 20th	Night	17% 4 19% 37% 22% 1	562
	21st to end	Night	18% 5% 20% 33% 21% 3	627
September	1st to 10th	Night	21% 8% 24% 33% 14% 1	570
	11th to 20th	Night	24% 8% 24% 30% 12% 1	679
	21st to end	Night	23% 10% 28% 28% 10% 1	760
October	1st to 10th	Night	25% 13% 26% 26% 9% 1	760
	11th to 20th	Night	32% 14% 24% 17% 11% 1	760
	21st to end	Night	30% 16% 25% 22% 8% 1	859
November	1st to 10th	Night	30% 19% 27% 19% 4	880
	11th to 20th	Night	28% 21% 28% 17% 5% 1	900
	21st to end	Night	31% 19% 27% 18% 5%	900
December	1st to 10th	Night	25% 21% 31% 18% 4 1	900
	11th to 20th	Night	25% 28% 28% 14% 4	900
	21st to end	Night	27% 21% 27% 19% 4 2	990

Page generated at 2025-02-22 06:51:36. Version ac07

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