Valier
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Valier clear sky chart from 2005-11-06 to 2026-03-18.

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	7% 7% 3 7% 5% 4 5% 4 6% 8% 46%	139839
	Night	6% 8% 3 7% 5% 4 5% 4 6% 8% 45%	138962
Transparency	Day	35% 6% 14% 25% 16% 5%	134495
	Night	36% 6% 15% 25% 14% 4	135314
Seeing	Day	12% 12% 27% 28% 16% 5%	17095
	Night	10% 12% 30% 33% 13% 1	26542

By Month

Percentage Cloud Cover SW (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	4 10% 4 12% 7% 6% 4 4 5% 8% 36%	4124
	11th to 20th	Day	3 9% 4 10% 7% 6% 6% 5% 5% 8% 37%	4380
	21st to end	Day	2 7% 4 11% 5% 4 4 4 5% 8% 45%	5187
February	1st to 10th	Day	5% 12% 5% 10% 6% 6% 5% 5% 7% 8% 30%	1859
	11th to 20th	Day	6% 14% 6% 8% 5% 4 4 4 6% 8% 36%	1845
	21st to end	Day	3 8% 5% 10% 7% 5% 6% 4 6% 7% 39%	1655
March	1st to 10th	Day	4 9% 3 7% 5% 5% 4 3 5% 10% 46%	4748
	11th to 20th	Day	3 5% 3 8% 6% 6% 6% 6% 7% 9% 42%	5078
	21st to end	Day	8% 15% 6% 15% 7% 5% 5% 4 6% 7% 21%	5486
April	1st to 10th	Day	8% 12% 4 9% 6% 5% 5% 4 6% 7% 34%	2405
	11th to 20th	Day	11% 12% 4 8% 6% 5% 4 3 6% 7% 35%	2567
	21st to end	Day	10% 12% 3 7% 5% 4 6% 4 6% 7% 37%	2600
May	1st to 10th	Day	12% 12% 3 7% 4 4 4 3 5% 7% 41%	5920
	11th to 20th	Day	11% 7% 3 8% 6% 5% 6% 5% 7% 8% 34%	6444
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 8% 46%	7404
June	1st to 10th	Day	14% 5% 3 7% 4 5% 5% 4 7% 8% 39%	3000
	11th to 20th	Day	14% 6% 3 6% 5% 5% 6% 4 6% 8% 37%	3000
	21st to end	Day	6% 4 1 5% 4 4 5% 4 6% 9% 54%	3000
July	1st to 10th	Day	3 3 1 5% 4 4 4 3 6% 7% 59%	6779
	11th to 20th	Day	6% 2 1 3 3 3 3 3 5% 8% 63%	6780
	21st to end	Day	4 3 1 3 2 3 4 3 5% 8% 64%	7092
August	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 5% 7% 9% 45%	5874
	11th to 20th	Day	4 3 1 4 3 4 5% 5% 7% 9% 55%	5798
	21st to end	Day	6% 3 1 4 3 3 3 3 5% 7% 62%	6468
September	1st to 10th	Day	8% 4 2 5% 3 3 4 3 5% 6% 57%	2443
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 3 5% 7% 49%	2200
	21st to end	Day	12% 5% 2 3 3 3 4 3 5% 6% 54%	2200
October	1st to 10th	Day	7% 5% 2 5% 3 3 3 3 6% 9% 53%	3379
	11th to 20th	Day	5% 6% 2 5% 4 5% 5% 4 6% 7% 51%	3229
	21st to end	Day	6% 7% 3 9% 5% 4 4 4 6% 7% 46%	3200
November	1st to 10th	Day	8% 12% 4 8% 4 4 4 4 4 7% 40%	1730
	11th to 20th	Day	3 9% 3 8% 5% 5% 5% 5% 8% 8% 40%	1680
	21st to end	Day	5% 10% 4 7% 4 4 4 4 6% 10% 42%	1635
December	1st to 10th	Day	2 6% 3 6% 5% 4 4 4 8% 10% 48%	2360
	11th to 20th	Day	3 9% 4 9% 6% 5% 7% 5% 7% 7% 38%	2715
	21st to end	Day	3 11% 4 9% 5% 5% 5% 4 5% 7% 40%	3575
January	1st to 10th	Night	4 9% 4 9% 6% 5% 5% 4 7% 9% 39%	8768
	11th to 20th	Night	3 10% 4 9% 6% 6% 6% 4 6% 7% 39%	8622
	21st to end	Night	2 8% 4 10% 5% 5% 4 4 6% 8% 44%	8994
February	1st to 10th	Night	6% 12% 4 9% 6% 4 4 4 6% 7% 35%	2760
	11th to 20th	Night	4 13% 5% 10% 6% 6% 6% 4 6% 7% 33%	2662
	21st to end	Night	3 9% 4 11% 7% 5% 5% 4 5% 7% 38%	2149
March	1st to 10th	Night	2 7% 4 8% 5% 6% 5% 4 7% 11% 42%	5366
	11th to 20th	Night	3 6% 3 8% 5% 6% 6% 4 6% 8% 47%	5077
	21st to end	Night	10% 16% 5% 12% 6% 5% 4 3 6% 8% 24%	5489
April	1st to 10th	Night	8% 11% 3 8% 4 5% 4 4 7% 8% 39%	1992
	11th to 20th	Night	10% 13% 3 8% 5% 5% 6% 4 5% 6% 35%	1781
	21st to end	Night	10% 10% 4 6% 4 4 4 4 6% 8% 40%	1800
May	1st to 10th	Night	13% 11% 3 6% 4 4 4 3 5% 6% 41%	4059
	11th to 20th	Night	7% 7% 3 7% 6% 6% 6% 4 8% 9% 37%	3446
	21st to end	Night	12% 6% 2 5% 3 3 3 4 5% 7% 50%	3470
June	1st to 10th	Night	12% 5% 2 7% 4 4 4 4 6% 9% 43%	1400
	11th to 20th	Night	12% 6% 2 6% 6% 4 4 4 6% 8% 42%	1400
	21st to end	Night	6% 3 1 4 3 4 3 4 7% 9% 56%	1400
July	1st to 10th	Night	5% 2 1 3 3 3 3 3 4 7% 66%	3180
	11th to 20th	Night	6% 2 1 3 2 3 3 2 3 5% 70%	3180
	21st to end	Night	4 4 1 4 3 3 3 3 4 6% 65%	3741
August	1st to 10th	Night	9% 6% 2 6% 5% 4 5% 4 6% 8% 46%	3932
	11th to 20th	Night	3 3 1 4 3 4 6% 4 7% 9% 55%	4051
	21st to end	Night	7% 5% 2 4 3 3 3 3 4 6% 61%	4509
September	1st to 10th	Night	9% 4 1 4 3 3 4 3 5% 6% 58%	1954
	11th to 20th	Night	11% 6% 2 5% 3 4 4 3 4 7% 53%	2200
	21st to end	Night	11% 5% 2 4 3 3 3 3 4 6% 56%	2200
October	1st to 10th	Night	8% 6% 2 5% 3 3 3 3 5% 6% 56%	3814
	11th to 20th	Night	6% 8% 2 6% 3 4 5% 3 6% 7% 51%	3919
	21st to end	Night	3 6% 3 6% 4 4 4 4 6% 8% 51%	4600
November	1st to 10th	Night	9% 10% 3 7% 5% 4 5% 4 6% 8% 40%	2766
	11th to 20th	Night	4 7% 3 7% 6% 4 5% 5% 7% 8% 43%	2940
	21st to end	Night	4 10% 4 8% 5% 4 5% 4 5% 8% 42%	2982
December	1st to 10th	Night	3 8% 2 6% 5% 4 5% 5% 8% 9% 45%	5040
	11th to 20th	Night	3 10% 4 10% 6% 5% 5% 4 7% 8% 37%	5696
	21st to end	Night	3 12% 4 8% 4 4 4 4 6% 8% 42%	7623

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	45% 5% 13% 26% 10% 1	4125
	11th to 20th	Day	44% 5% 18% 19% 9% 5%	4380
	21st to end	Day	36% 6% 18% 26% 14% 1	5187
February	1st to 10th	Day	48% 7% 17% 18% 8% 2	1859
	11th to 20th	Day	46% 6% 16% 21% 9% 2	1854
	21st to end	Day	42% 6% 16% 22% 12% 2	1655
March	1st to 10th	Day	32% 5% 13% 25% 21% 4	4748
	11th to 20th	Day	33% 6% 22% 24% 12% 2	5078
	21st to end	Day	58% 6% 16% 15% 5%	5486
April	1st to 10th	Day	47% 6% 15% 21% 10% 1	2405
	11th to 20th	Day	46% 6% 13% 23% 11% 2	2570
	21st to end	Day	45% 6% 15% 24% 11% 1	2600
May	1st to 10th	Day	43% 4 10% 23% 16% 3	5920
	11th to 20th	Day	42% 7% 14% 25% 10% 2	6448
	21st to end	Day	33% 7% 13% 29% 18% 1	7406
June	1st to 10th	Day	40% 7% 17% 22% 13% 1	3000
	11th to 20th	Day	43% 7% 12% 17% 17% 4	3000
	21st to end	Day	25% 6% 13% 34% 18% 3	3000
July	1st to 10th	Day	23% 4 12% 25% 24% 13%	6706
	11th to 20th	Day	19% 4 12% 21% 25% 19%	6699
	21st to end	Day	19% 5% 9% 23% 28% 17%	6887
August	1st to 10th	Day	29% 5% 12% 21% 24% 8%	4277
	11th to 20th	Day	24% 6% 13% 34% 20% 4	4197
	21st to end	Day	22% 5% 10% 30% 27% 6%	4663
September	1st to 10th	Day	27% 5% 11% 28% 20% 11%	2443
	11th to 20th	Day	33% 6% 12% 28% 17% 4	2200
	21st to end	Day	30% 5% 12% 30% 17% 6%	2200
October	1st to 10th	Day	28% 4 10% 33% 22% 3	3379
	11th to 20th	Day	32% 4 13% 30% 18% 3	3229
	21st to end	Day	37% 6% 11% 27% 18% 2	3200
November	1st to 10th	Day	43% 5% 16% 22% 10% 4	1730
	11th to 20th	Day	36% 8% 18% 25% 12% 1	1680
	21st to end	Day	36% 6% 20% 26% 10% 2	1635
December	1st to 10th	Day	29% 6% 23% 29% 10% 2	2360
	11th to 20th	Day	41% 8% 20% 22% 7% 1	2714
	21st to end	Day	41% 5% 12% 22% 15% 3	3575
January	1st to 10th	Night	39% 6% 17% 26% 12% 1	8769
	11th to 20th	Night	42% 5% 17% 21% 11% 3	8622
	21st to end	Night	37% 6% 17% 26% 12% 2	8994
February	1st to 10th	Night	46% 6% 17% 18% 11% 2	2760
	11th to 20th	Night	47% 6% 16% 19% 10% 1	2663
	21st to end	Night	43% 5% 16% 23% 10% 2	2149
March	1st to 10th	Night	32% 7% 13% 30% 14% 4	5366
	11th to 20th	Night	34% 6% 19% 27% 14% 1	5077
	21st to end	Night	56% 7% 14% 16% 6% 1	5489
April	1st to 10th	Night	41% 6% 16% 26% 11% 1	1992
	11th to 20th	Night	46% 7% 12% 24% 10% 1	1781
	21st to end	Night	40% 7% 17% 26% 9% 1	1800
May	1st to 10th	Night	43% 4 11% 28% 10% 5%	4056
	11th to 20th	Night	37% 8% 15% 27% 12% 1	3447
	21st to end	Night	32% 6% 14% 30% 16% 1	3470
June	1st to 10th	Night	36% 5% 19% 26% 13% 1	1400
	11th to 20th	Night	38% 7% 14% 24% 14% 3	1400
	21st to end	Night	22% 6% 16% 35% 18% 2	1400
July	1st to 10th	Night	18% 4 12% 28% 27% 12%	3145
	11th to 20th	Night	18% 3 9% 28% 25% 16%	3138
	21st to end	Night	20% 3 12% 22% 25% 17%	3634
August	1st to 10th	Night	29% 3 13% 22% 23% 10%	2854
	11th to 20th	Night	25% 6% 13% 34% 18% 4	2928
	21st to end	Night	25% 3 9% 30% 24% 8%	3245
September	1st to 10th	Night	26% 5% 11% 28% 19% 11%	1954
	11th to 20th	Night	31% 4 13% 30% 16% 5%	2200
	21st to end	Night	30% 4 12% 29% 20% 5%	2200
October	1st to 10th	Night	29% 3 12% 31% 21% 4	3814
	11th to 20th	Night	32% 4 11% 31% 19% 2	3919
	21st to end	Night	29% 6% 12% 29% 21% 3	4600
November	1st to 10th	Night	39% 7% 17% 22% 12% 3	2766
	11th to 20th	Night	35% 7% 20% 25% 13% 1	2940
	21st to end	Night	38% 6% 21% 21% 11% 2	2982
December	1st to 10th	Night	31% 7% 23% 23% 13% 3	5040
	11th to 20th	Night	42% 7% 25% 19% 7% 1	5697
	21st to end	Night	38% 6% 14% 24% 14% 4	7623

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	9% 16% 30% 22% 15% 8%	220
	11th to 20th	Day	9% 18% 31% 25% 14% 4	220
	21st to end	Day	12% 17% 36% 23% 11% 2	242
February	1st to 10th	Day	18% 19% 31% 24% 7% 1	314
	11th to 20th	Day	13% 17% 29% 28% 9% 4	330
	21st to end	Day	8% 23% 40% 21% 7% 1	371
March	1st to 10th	Day	15% 15% 30% 27% 10% 3	540
	11th to 20th	Day	11% 16% 31% 26% 12% 3	529
	21st to end	Day	13% 17% 29% 29% 8% 3	550
April	1st to 10th	Day	16% 17% 30% 25% 10% 2	500
	11th to 20th	Day	18% 16% 33% 21% 9% 3	492
	21st to end	Day	17% 18% 25% 21% 14% 4	500
May	1st to 10th	Day	16% 11% 22% 26% 17% 8%	517
	11th to 20th	Day	20% 9% 20% 25% 19% 6%	725
	21st to end	Day	19% 8% 17% 26% 23% 6%	789
June	1st to 10th	Day	13% 13% 24% 25% 18% 6%	730
	11th to 20th	Day	17% 16% 28% 25% 12% 3	730
	21st to end	Day	8% 8% 26% 34% 19% 6%	730
July	1st to 10th	Day	4 5% 23% 36% 25% 7%	721
	11th to 20th	Day	7% 8% 24% 38% 21% 3	718
	21st to end	Day	4 4 25% 37% 23% 7%	670
August	1st to 10th	Day	9% 5% 21% 35% 21% 7%	527
	11th to 20th	Day	6% 5% 22% 33% 24% 11%	517
	21st to end	Day	7% 7% 25% 33% 21% 8%	583
September	1st to 10th	Day	9% 11% 25% 26% 21% 7%	530
	11th to 20th	Day	9% 10% 25% 30% 18% 8%	530
	21st to end	Day	16% 10% 27% 24% 17% 6%	530
October	1st to 10th	Day	17% 9% 22% 29% 17% 7%	400
	11th to 20th	Day	14% 14% 25% 22% 17% 8%	330
	21st to end	Day	15% 17% 32% 24% 9% 3	363
November	1st to 10th	Day	21% 14% 34% 21% 9% 1	330
	11th to 20th	Day	10% 18% 37% 24% 9% 2	330
	21st to end	Day	12% 18% 32% 24% 10% 3	305
December	1st to 10th	Day	9% 18% 34% 24% 11% 3	220
	11th to 20th	Day	20% 18% 29% 22% 9% 3	220
	21st to end	Day	10% 16% 27% 26% 16% 5%	242
January	1st to 10th	Night	9% 18% 36% 28% 9% 1	1050
	11th to 20th	Night	10% 19% 37% 26% 8% 1	982
	21st to end	Night	9% 17% 39% 26% 8% 2	924
February	1st to 10th	Night	13% 17% 32% 26% 10% 3	840
	11th to 20th	Night	9% 14% 34% 32% 10% 1	824
	21st to end	Night	8% 16% 33% 30% 12% 2	692
March	1st to 10th	Night	6% 11% 36% 34% 12% 1	840
	11th to 20th	Night	8% 12% 33% 34% 11% 2	828
	21st to end	Night	13% 11% 31% 32% 12% 1	880
April	1st to 10th	Night	14% 11% 33% 30% 12% 1	710
	11th to 20th	Night	13% 10% 35% 32% 9% 1	593
	21st to end	Night	15% 9% 30% 32% 15%	600
May	1st to 10th	Night	14% 8% 28% 32% 18% 1	600
	11th to 20th	Night	14% 8% 20% 37% 19% 2	525
	21st to end	Night	20% 7% 20% 30% 21% 3	433
June	1st to 10th	Night	15% 6% 24% 39% 15% 2	400
	11th to 20th	Night	14% 11% 29% 33% 13%	400
	21st to end	Night	8% 5% 21% 44% 21% 1	400
July	1st to 10th	Night	7% 3 19% 45% 26%	390
	11th to 20th	Night	5% 3 22% 55% 14% 1	388
	21st to end	Night	5% 3 17% 53% 20% 1	438
August	1st to 10th	Night	7% 4 20% 46% 22% 1	547
	11th to 20th	Night	6% 3 22% 44% 23% 2	594
	21st to end	Night	8% 6% 21% 45% 19% 1	660
September	1st to 10th	Night	11% 3 20% 44% 20% 2	648
	11th to 20th	Night	13% 4 18% 39% 23% 3	800
	21st to end	Night	13% 6% 27% 32% 18% 3	800
October	1st to 10th	Night	16% 6% 23% 35% 19% 1	800
	11th to 20th	Night	11% 12% 29% 32% 13% 3	800
	21st to end	Night	10% 14% 35% 31% 9% 1	880
November	1st to 10th	Night	13% 16% 34% 31% 6%	921
	11th to 20th	Night	9% 18% 39% 26% 8% 1	1050
	21st to end	Night	8% 16% 37% 30% 8% 1	1050
December	1st to 10th	Night	10% 17% 33% 27% 11% 2	1050
	11th to 20th	Night	10% 17% 36% 28% 9% 1	1050
	21st to end	Night	10% 17% 35% 28% 9% 1	1155

Page generated at 2026-03-15 00:23:53. Version ac07

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