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 2024-02-10.

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% 4 8% 8% 4 5% 5% 5% 7% 29%	77729
	Night	16% 12% 5% 11% 9% 4 5% 4 5% 6% 23%	81273
Transparency	Day	55% 9% 12% 16% 7% 1	77414
	Night	61% 7% 11% 14% 6% 1	81123
Seeing	Day	22% 15% 28% 24% 10% 2	15396
	Night	23% 15% 26% 24% 10% 1	23141

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	12% 15% 8% 14% 29% 6% 4 3 2 2 4	2207
	11th to 20th	Day	10% 14% 7% 14% 19% 6% 5% 5% 6% 5% 8%	2210
	21st to end	Day	5% 10% 4 12% 13% 13% 8% 6% 5% 6% 18%	2726
February	1st to 10th	Day	10% 9% 3 9% 11% 9% 8% 6% 6% 7% 20%	1701
	11th to 20th	Day	12% 15% 5% 10% 9% 5% 6% 7% 6% 5% 20%	1573
	21st to end	Day	8% 10% 4 9% 8% 7% 7% 6% 7% 8% 26%	1332
March	1st to 10th	Day	12% 13% 6% 9% 7% 4 6% 4 5% 5% 30%	1901
	11th to 20th	Day	13% 14% 5% 9% 7% 4 6% 4 5% 6% 27%	1980
	21st to end	Day	15% 15% 3 7% 8% 4 5% 5% 5% 4 29%	2178
April	1st to 10th	Day	17% 12% 5% 8% 7% 4 5% 5% 6% 6% 25%	2167
	11th to 20th	Day	15% 11% 3 6% 5% 3 5% 4 5% 7% 35%	2312
	21st to end	Day	21% 9% 5% 7% 4 2 4 5% 5% 5% 33%	2340
May	1st to 10th	Day	21% 9% 3 5% 5% 3 4 5% 5% 6% 33%	2479
	11th to 20th	Day	18% 9% 3 6% 5% 3 4 5% 5% 8% 34%	2478
	21st to end	Day	22% 7% 3 4 5% 3 4 5% 5% 7% 35%	2919
June	1st to 10th	Day	19% 7% 3 6% 5% 4 5% 5% 5% 8% 32%	2700
	11th to 20th	Day	17% 7% 2 5% 5% 2 5% 5% 6% 8% 38%	2700
	21st to end	Day	19% 7% 2 5% 5% 4 5% 5% 6% 7% 34%	2700
July	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 5% 7% 8% 37%	2700
	11th to 20th	Day	12% 5% 3 4 6% 3 4 6% 5% 9% 42%	2700
	21st to end	Day	14% 7% 2 6% 5% 4 6% 6% 6% 7% 38%	2901
August	1st to 10th	Day	13% 5% 3 5% 6% 3 5% 7% 7% 10% 37%	2379
	11th to 20th	Day	15% 6% 3 6% 5% 4 4 4 6% 7% 39%	2304
	21st to end	Day	16% 6% 2 5% 5% 4 5% 5% 6% 8% 38%	2574
September	1st to 10th	Day	19% 6% 3 5% 5% 5% 5% 5% 4 6% 39%	2165
	11th to 20th	Day	22% 7% 3 4 4 2 4 3 4 5% 42%	2054
	21st to end	Day	18% 6% 2 5% 5% 3 4 5% 5% 7% 40%	1951
October	1st to 10th	Day	16% 7% 2 6% 5% 3 5% 4 6% 7% 39%	1800
	11th to 20th	Day	20% 11% 3 8% 6% 3 5% 4 5% 7% 29%	1800
	21st to end	Day	24% 14% 3 7% 5% 3 4 3 5% 6% 26%	1812
November	1st to 10th	Day	19% 14% 4 8% 7% 4 6% 5% 5% 7% 21%	1485
	11th to 20th	Day	18% 14% 8% 12% 8% 3 5% 4 4 6% 18%	1520
	21st to end	Day	21% 17% 8% 16% 7% 3 4 3 3 5% 14%	1520
December	1st to 10th	Day	16% 15% 7% 20% 11% 4 6% 3 5% 5% 8%	1362
	11th to 20th	Day	17% 15% 8% 17% 13% 5% 4 4 3 3 10%	1668
	21st to end	Day	12% 14% 6% 17% 12% 4 4 3 5% 7% 15%	2431
January	1st to 10th	Night	11% 16% 6% 19% 23% 6% 5% 3 3 3 6%	4748
	11th to 20th	Night	11% 15% 7% 17% 20% 5% 6% 5% 3 3 8%	4760
	21st to end	Night	4 11% 6% 15% 14% 11% 7% 4 4 5% 19%	4930
February	1st to 10th	Night	9% 12% 5% 11% 11% 7% 10% 5% 6% 5% 19%	2464
	11th to 20th	Night	13% 17% 5% 9% 10% 5% 8% 5% 4 5% 18%	2273
	21st to end	Night	10% 13% 5% 10% 9% 5% 6% 6% 6% 6% 26%	1924
March	1st to 10th	Night	14% 16% 5% 8% 8% 4 5% 5% 5% 5% 26%	2045
	11th to 20th	Night	13% 16% 6% 8% 9% 5% 4 3 4 5% 27%	1979
	21st to end	Night	16% 16% 6% 7% 7% 3 5% 4 4 7% 25%	2178
April	1st to 10th	Night	20% 11% 5% 9% 9% 4 4 4 5% 5% 22%	1789
	11th to 20th	Night	19% 10% 3 7% 5% 3 5% 5% 4 8% 32%	1598
	21st to end	Night	22% 10% 4 7% 5% 2 4 5% 3 5% 33%	1620
May	1st to 10th	Night	24% 9% 3 5% 6% 3 4 4 4 6% 32%	1480
	11th to 20th	Night	22% 8% 3 6% 5% 4 4 4 4 6% 34%	1416
	21st to end	Night	23% 7% 3 7% 5% 3 5% 5% 6% 7% 30%	1370
June	1st to 10th	Night	24% 7% 3 7% 5% 3 5% 4 5% 8% 29%	1260
	11th to 20th	Night	17% 7% 4 6% 7% 3 4 4 6% 6% 35%	1260
	21st to end	Night	19% 6% 3 7% 7% 4 5% 5% 6% 8% 30%	1260
July	1st to 10th	Night	14% 8% 3 6% 4 4 5% 5% 7% 9% 36%	1260
	11th to 20th	Night	12% 5% 3 6% 6% 3 6% 5% 7% 8% 38%	1260
	21st to end	Night	13% 7% 2 7% 7% 4 5% 6% 5% 7% 37%	1399
August	1st to 10th	Night	15% 5% 2 5% 7% 3 5% 5% 6% 9% 38%	1558
	11th to 20th	Night	15% 7% 3 6% 7% 3 5% 5% 5% 8% 37%	1596
	21st to end	Night	16% 6% 3 5% 5% 4 5% 5% 6% 8% 37%	1782
September	1st to 10th	Night	18% 6% 3 5% 5% 3 4 5% 5% 6% 41%	1795
	11th to 20th	Night	21% 7% 2 5% 5% 3 3 4 4 5% 41%	1903
	21st to end	Night	19% 6% 2 5% 5% 2 3 4 5% 8% 42%	2008
October	1st to 10th	Night	19% 9% 3 5% 4 3 4 3 5% 7% 39%	2160
	11th to 20th	Night	23% 11% 4 7% 6% 4 5% 4 4 6% 26%	2160
	21st to end	Night	23% 12% 3 6% 5% 3 5% 5% 5% 7% 26%	2542
November	1st to 10th	Night	20% 12% 4 8% 6% 4 5% 5% 5% 6% 24%	2593
	11th to 20th	Night	18% 19% 9% 10% 6% 3 4 4 4 5% 18%	2660
	21st to end	Night	21% 15% 9% 16% 7% 3 4 4 3 4 13%	2660
December	1st to 10th	Night	18% 16% 10% 20% 9% 4 4 4 3 4 9%	2816
	11th to 20th	Night	16% 17% 7% 17% 13% 5% 4 2 3 4 11%	3531
	21st to end	Night	16% 16% 9% 16% 12% 4 4 4 5% 4 10%	5236

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% 5% 6% 3 1	2208
	11th to 20th	Day	69% 7% 14% 7% 3	2210
	21st to end	Day	60% 9% 15% 11% 5%	2726
February	1st to 10th	Day	60% 8% 13% 12% 6% 1	1701
	11th to 20th	Day	62% 8% 13% 13% 5%	1581
	21st to end	Day	55% 10% 11% 20% 5%	1332
March	1st to 10th	Day	59% 6% 10% 16% 9% 1	1905
	11th to 20th	Day	60% 7% 10% 13% 9% 1	1980
	21st to end	Day	58% 7% 10% 14% 9% 2	2178
April	1st to 10th	Day	61% 8% 9% 12% 10% 1	2167
	11th to 20th	Day	52% 8% 9% 18% 12% 1	2312
	21st to end	Day	53% 8% 10% 16% 12% 1	2340
May	1st to 10th	Day	53% 8% 9% 18% 11% 2	2479
	11th to 20th	Day	51% 8% 10% 21% 10% 1	2482
	21st to end	Day	50% 8% 12% 18% 11% 1	2919
June	1st to 10th	Day	51% 10% 12% 17% 10% 1	2700
	11th to 20th	Day	46% 9% 11% 20% 12% 1	2700
	21st to end	Day	49% 13% 11% 18% 9%	2700
July	1st to 10th	Day	43% 13% 17% 19% 7%	2625
	11th to 20th	Day	40% 10% 14% 24% 11%	2619
	21st to end	Day	45% 12% 14% 20% 8%	2739
August	1st to 10th	Day	43% 12% 18% 21% 5% 1	2365
	11th to 20th	Day	43% 12% 18% 21% 6%	2304
	21st to end	Day	44% 11% 16% 23% 5%	2574
September	1st to 10th	Day	45% 8% 15% 23% 9%	2165
	11th to 20th	Day	44% 7% 15% 25% 8%	2054
	21st to end	Day	46% 9% 13% 22% 9% 1	1951
October	1st to 10th	Day	45% 9% 16% 20% 9% 1	1800
	11th to 20th	Day	57% 8% 15% 15% 5%	1800
	21st to end	Day	62% 8% 10% 13% 6% 1	1812
November	1st to 10th	Day	63% 9% 12% 10% 5%	1485
	11th to 20th	Day	68% 8% 12% 9% 3	1520
	21st to end	Day	75% 5% 11% 5% 3	1520
December	1st to 10th	Day	76% 11% 8% 5% 1	1362
	11th to 20th	Day	77% 6% 9% 6% 2	1668
	21st to end	Day	66% 11% 13% 9% 1 1	2431
January	1st to 10th	Night	81% 7% 8% 4 1	4748
	11th to 20th	Night	77% 6% 10% 5% 2	4760
	21st to end	Night	62% 8% 13% 9% 6% 2	4930
February	1st to 10th	Night	61% 7% 12% 11% 7% 1	2464
	11th to 20th	Night	66% 7% 10% 13% 4	2276
	21st to end	Night	58% 7% 8% 18% 8% 1	1924
March	1st to 10th	Night	59% 7% 8% 17% 8% 1	2054
	11th to 20th	Night	62% 4 8% 16% 10% 1	1980
	21st to end	Night	60% 5% 10% 16% 8% 2	2178
April	1st to 10th	Night	63% 7% 9% 12% 10%	1789
	11th to 20th	Night	51% 5% 9% 22% 12% 1	1598
	21st to end	Night	55% 5% 10% 15% 13% 2	1620
May	1st to 10th	Night	53% 6% 9% 21% 10% 1	1480
	11th to 20th	Night	50% 7% 9% 21% 12%	1417
	21st to end	Night	53% 7% 11% 17% 11% 1	1371
June	1st to 10th	Night	54% 6% 11% 19% 10%	1260
	11th to 20th	Night	50% 6% 12% 18% 13% 1	1260
	21st to end	Night	50% 10% 12% 18% 9% 1	1260
July	1st to 10th	Night	41% 11% 17% 25% 5% 1	1225
	11th to 20th	Night	40% 7% 18% 27% 9%	1218
	21st to end	Night	45% 9% 16% 19% 12%	1321
August	1st to 10th	Night	41% 10% 21% 22% 6%	1547
	11th to 20th	Night	44% 11% 17% 22% 7%	1596
	21st to end	Night	43% 9% 15% 26% 7%	1782
September	1st to 10th	Night	44% 8% 12% 25% 10%	1795
	11th to 20th	Night	46% 5% 14% 25% 10%	1903
	21st to end	Night	42% 7% 13% 27% 11% 1	2008
October	1st to 10th	Night	47% 6% 13% 23% 10% 1	2160
	11th to 20th	Night	59% 7% 12% 17% 5%	2160
	21st to end	Night	58% 9% 11% 16% 5% 1	2542
November	1st to 10th	Night	60% 7% 15% 13% 5%	2593
	11th to 20th	Night	69% 7% 11% 9% 4	2660
	21st to end	Night	74% 6% 11% 6% 2	2660
December	1st to 10th	Night	78% 7% 8% 6% 1	2816
	11th to 20th	Night	76% 7% 8% 7% 2	3532
	21st to end	Night	74% 8% 9% 8% 1	5236

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	23% 25% 26% 22% 3 1	180
	11th to 20th	Day	23% 23% 28% 18% 7% 1	180
	21st to end	Day	18% 20% 33% 20% 7% 2	246
February	1st to 10th	Day	17% 18% 39% 16% 9% 1	367
	11th to 20th	Day	18% 28% 32% 17% 5% 1	336
	21st to end	Day	11% 22% 38% 18% 9% 1	280
March	1st to 10th	Day	21% 22% 33% 19% 5% 2	416
	11th to 20th	Day	23% 21% 26% 20% 8% 2	420
	21st to end	Day	25% 20% 30% 18% 5% 2	462
April	1st to 10th	Day	28% 21% 31% 16% 4 1	420
	11th to 20th	Day	23% 18% 33% 17% 8% 1	412
	21st to end	Day	25% 17% 28% 24% 5% 2	420
May	1st to 10th	Day	27% 13% 29% 20% 8% 3	562
	11th to 20th	Day	24% 13% 25% 23% 13% 2	624
	21st to end	Day	28% 14% 26% 24% 7% 1	680
June	1st to 10th	Day	25% 10% 26% 27% 11% 1	630
	11th to 20th	Day	22% 10% 24% 28% 13% 3	630
	21st to end	Day	21% 6% 23% 37% 11% 2	630
July	1st to 10th	Day	17% 6% 31% 29% 15% 1	620
	11th to 20th	Day	15% 8% 22% 38% 15% 1	618
	21st to end	Day	16% 8% 21% 37% 16% 2	685
August	1st to 10th	Day	17% 5% 23% 35% 17% 2	498
	11th to 20th	Day	15% 5% 26% 33% 19% 2	439
	21st to end	Day	19% 7% 26% 32% 13% 3	495
September	1st to 10th	Day	23% 11% 28% 22% 13% 3	450
	11th to 20th	Day	28% 11% 26% 24% 9% 2	450
	21st to end	Day	20% 20% 30% 21% 8% 1	435
October	1st to 10th	Day	19% 18% 32% 19% 11% 1	360
	11th to 20th	Day	27% 20% 25% 18% 10% 1	360
	21st to end	Day	30% 22% 20% 17% 10% 1	396
November	1st to 10th	Day	24% 25% 25% 22% 4 1	365
	11th to 20th	Day	25% 20% 31% 16% 7% 1	370
	21st to end	Day	31% 20% 27% 17% 4 1	370
December	1st to 10th	Day	19% 19% 32% 22% 7% 1	212
	11th to 20th	Day	21% 34% 27% 16% 3	180
	21st to end	Day	18% 32% 31% 17% 3	198
January	1st to 10th	Night	24% 23% 29% 19% 5% 1	850
	11th to 20th	Night	24% 22% 32% 18% 5%	850
	21st to end	Night	14% 24% 29% 23% 9% 1	868
February	1st to 10th	Night	14% 22% 31% 24% 8% 1	759
	11th to 20th	Night	25% 18% 32% 19% 6%	703
	21st to end	Night	15% 19% 35% 20% 9% 2	592
March	1st to 10th	Night	19% 17% 32% 22% 9% 1	720
	11th to 20th	Night	21% 20% 29% 20% 8% 2	720
	21st to end	Night	24% 16% 28% 24% 8% 1	792
April	1st to 10th	Night	27% 16% 27% 21% 9% 1	560
	11th to 20th	Night	24% 13% 27% 24% 10% 2	534
	21st to end	Night	26% 12% 25% 24% 11% 2	540
May	1st to 10th	Night	29% 6% 23% 25% 14% 2	540
	11th to 20th	Night	24% 7% 24% 27% 14% 3	532
	21st to end	Night	24% 11% 26% 30% 8% 1	403
June	1st to 10th	Night	26% 6% 18% 35% 14% 1	360
	11th to 20th	Night	22% 6% 21% 34% 17% 1	360
	21st to end	Night	23% 5% 21% 31% 19% 2	360
July	1st to 10th	Night	17% 4 23% 37% 18% 2	350
	11th to 20th	Night	17% 3 21% 39% 19% 1	348
	21st to end	Night	17% 6% 15% 37% 24% 2	416
August	1st to 10th	Night	17% 6% 16% 38% 22% 2	538
	11th to 20th	Night	17% 4 19% 36% 23% 1	532
	21st to end	Night	18% 5% 21% 33% 21% 2	594
September	1st to 10th	Night	20% 8% 24% 33% 14% 1	540
	11th to 20th	Night	26% 9% 23% 29% 12% 1	643
	21st to end	Night	22% 10% 29% 28% 10% 1	720
October	1st to 10th	Night	25% 13% 26% 26% 9% 1	720
	11th to 20th	Night	32% 14% 24% 17% 11% 2	720
	21st to end	Night	30% 15% 24% 23% 8% 1	812
November	1st to 10th	Night	29% 18% 28% 20% 4	830
	11th to 20th	Night	26% 21% 29% 17% 6% 1	850
	21st to end	Night	29% 20% 27% 19% 5%	850
December	1st to 10th	Night	25% 21% 30% 18% 4 1	850
	11th to 20th	Night	25% 28% 28% 14% 4	850
	21st to end	Night	27% 21% 27% 19% 4 2	935

Page generated at 2024-02-07 06:11:59. Version ac07

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