Astro-chalet West
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astro-chalet West clear sky chart from 2005-11-06 to 2026-09-14.

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	3 5% 1 3 2 2 2 2 4 5% 71%	156744
	Night	4 7% 1 3 2 2 2 2 4 5% 67%	151963
Transparency	Day	17% 2 5% 16% 30% 30%	151825
	Night	21% 2 5% 17% 29% 26%	147857
Seeing	Day	12% 14% 36% 26% 9% 3	17953
	Night	10% 7% 23% 35% 20% 4	27616

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	5% 11% 3 5% 4 4 3 3 5% 7% 50%	4722
	11th to 20th	Day	5% 4 1 3 2 2 2 3 5% 7% 65%	5158
	21st to end	Day	6% 7% 2 6% 3 4 4 3 5% 8% 52%	6116
February	1st to 10th	Day	7% 11% 3 5% 4 3 3 3 4 5% 52%	2100
	11th to 20th	Day	9% 12% 3 3 3 3 3 4 5% 8% 47%	2050
	21st to end	Day	7% 11% 3 5% 3 4 3 4 5% 5% 50%	1730
March	1st to 10th	Day	10% 18% 3 6% 3 3 4 3 4 7% 39%	4618
	11th to 20th	Day	7% 10% 2 5% 3 4 4 4 6% 8% 46%	4829
	21st to end	Day	7% 12% 2 4 4 4 5% 5% 7% 8% 43%	6101
April	1st to 10th	Day	7% 7% 1 4 3 3 4 3 5% 7% 55%	2520
	11th to 20th	Day	6% 7% 2 4 2 2 3 3 4 7% 60%	2489
	21st to end	Day	5% 7% 1 3 3 2 3 3 5% 7% 61%	2520
May	1st to 10th	Day	3 5% 1 2 2 2 2 2 3 5% 74%	7258
	11th to 20th	Day	2 2 1 1 2 2 2 3 5% 80%	7596
	21st to end	Day	2 3 1 1 1 1 1 2 2 4 81%	8370
June	1st to 10th	Day	2 2 1 2 2 2 2 3 5% 6% 74%	2921
	11th to 20th	Day	1 1 1 1 1 1 2 3 89%	2940
	21st to end	Day	1 1 1 2 1 1 1 2 3 88%	2940
July	1st to 10th	Day	1 1 1 1 2 2 93%	8219
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 85%	8220
	21st to end	Day	1 1 1 1 1 1 2 3 90%	8089
August	1st to 10th	Day	1 1 1 1 2 2 1 2 4 87%	6986
	11th to 20th	Day	1 1 1 1 2 3 90%	7001
	21st to end	Day	1 1 1 1 2 2 2 3 3 85%	7750
September	1st to 10th	Day	1 1 1 1 1 1 2 1 3 4 85%	2520
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	2322
	21st to end	Day	1 2 1 2 1 1 2 1 2 4 83%	2200
October	1st to 10th	Day	2 2 2 1 2 2 1 3 4 81%	3600
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 4 77%	3266
	21st to end	Day	4 4 1 3 3 3 3 3 5% 7% 64%	3497
November	1st to 10th	Day	5% 8% 2 3 2 2 2 3 4 6% 64%	1854
	11th to 20th	Day	5% 6% 1 4 3 3 4 2 4 7% 61%	1890
	21st to end	Day	6% 9% 2 3 3 4 3 4 6% 6% 56%	1890
December	1st to 10th	Day	4 4 1 4 3 3 3 4 5% 6% 62%	3040
	11th to 20th	Day	7% 7% 1 5% 5% 4 4 4 6% 8% 49%	3330
	21st to end	Day	10% 8% 2 6% 5% 5% 5% 4 6% 8% 39%	4092
January	1st to 10th	Night	7% 13% 2 5% 3 4 4 3 5% 7% 48%	8172
	11th to 20th	Night	4 6% 2 4 2 2 2 3 4 6% 65%	7808
	21st to end	Night	9% 10% 2 5% 3 3 3 3 5% 7% 49%	8108
February	1st to 10th	Night	8% 14% 2 6% 3 3 3 2 4 5% 50%	2520
	11th to 20th	Night	8% 11% 2 4 3 3 4 3 6% 7% 49%	2458
	21st to end	Night	9% 11% 2 4 4 3 3 3 4 6% 50%	2076
March	1st to 10th	Night	8% 23% 3 4 2 3 3 3 5% 7% 39%	5496
	11th to 20th	Night	7% 16% 2 4 2 3 4 3 4 6% 50%	5371
	21st to end	Night	7% 16% 2 4 3 3 4 3 4 7% 47%	5116
April	1st to 10th	Night	7% 9% 3 4 3 3 3 3 5% 6% 54%	2100
	11th to 20th	Night	7% 8% 1 4 2 3 3 3 4 6% 61%	2079
	21st to end	Night	8% 7% 1 2 1 2 2 2 4 6% 63%	2100
May	1st to 10th	Night	8% 4 1 2 1 1 1 1 3 4 74%	5705
	11th to 20th	Night	3 1 1 1 1 1 1 3 4 83%	5296
	21st to end	Night	4 2 1 1 1 1 1 1 2 3 83%	5837
June	1st to 10th	Night	3 2 1 2 1 1 2 3 3 5% 77%	1693
	11th to 20th	Night	1 1 1 2 1 1 2 2 89%	1680
	21st to end	Night	1 1 1 1 1 2 3 89%	1680
July	1st to 10th	Night	1 1 1 1 1 1 2 93%	4740
	11th to 20th	Night	1 1 1 1 1 2 2 2 3 87%	4740
	21st to end	Night	1 1 1 1 1 1 1 1 3 91%	5996
August	1st to 10th	Night	1 1 1 1 1 1 2 3 89%	5866
	11th to 20th	Night	1 1 1 2 4 91%	5886
	21st to end	Night	1 1 1 1 1 1 1 2 4 87%	6499
September	1st to 10th	Night	1 2 1 1 1 1 2 3 89%	2100
	11th to 20th	Night	1 2 1 2 1 1 2 1 2 3 84%	2145
	21st to end	Night	1 1 1 1 2 1 1 2 4 84%	2200
October	1st to 10th	Night	2 2 1 2 1 1 1 1 3 3 84%	3600
	11th to 20th	Night	4 2 1 2 1 1 1 2 3 5% 77%	3887
	21st to end	Night	4 4 1 2 2 2 3 2 3 4 73%	4310
November	1st to 10th	Night	6% 7% 2 4 2 2 2 3 4 6% 62%	2665
	11th to 20th	Night	5% 7% 2 4 3 3 3 3 5% 7% 59%	2730
	21st to end	Night	8% 8% 2 4 3 3 3 3 5% 7% 53%	2730
December	1st to 10th	Night	4 5% 1 3 3 3 3 3 5% 6% 63%	4370
	11th to 20th	Night	6% 9% 2 4 3 3 4 4 7% 10% 47%	5098
	21st to end	Night	6% 15% 4 8% 5% 6% 5% 5% 7% 9% 30%	7106

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	28% 2 10% 26% 22% 13%	4723
	11th to 20th	Day	17% 4 7% 10% 23% 37%	5158
	21st to end	Day	29% 3 6% 13% 27% 22%	6116
February	1st to 10th	Day	33% 4 5% 16% 22% 19%	2100
	11th to 20th	Day	33% 4 6% 19% 24% 14%	2060
	21st to end	Day	34% 4 9% 19% 26% 8%	1730
March	1st to 10th	Day	41% 4 7% 14% 20% 14%	4618
	11th to 20th	Day	30% 6% 10% 29% 18% 8%	4829
	21st to end	Day	30% 7% 14% 25% 18% 6%	6101
April	1st to 10th	Day	28% 4 11% 21% 24% 11%	2520
	11th to 20th	Day	22% 3 9% 28% 25% 13%	2492
	21st to end	Day	19% 4 12% 27% 31% 7%	2520
May	1st to 10th	Day	15% 2 6% 21% 38% 17%	7258
	11th to 20th	Day	10% 1 3 20% 45% 21%	7599
	21st to end	Day	11% 1 2 18% 51% 16%	8371
June	1st to 10th	Day	13% 3 6% 20% 36% 22%	2921
	11th to 20th	Day	5% 1 5% 12% 34% 43%	2940
	21st to end	Day	6% 1 3 6% 30% 54%	2940
July	1st to 10th	Day	3 1 1 4 16% 75%	8151
	11th to 20th	Day	9% 1 3 11% 27% 50%	8145
	21st to end	Day	5% 1 2 14% 35% 44%	7909
August	1st to 10th	Day	7% 1 1 11% 34% 46%	5530
	11th to 20th	Day	6% 1 2 13% 31% 47%	5517
	21st to end	Day	8% 1 2 17% 29% 44%	6077
September	1st to 10th	Day	7% 2 3 8% 31% 48%	2520
	11th to 20th	Day	6% 1 2 11% 39% 40%	2322
	21st to end	Day	9% 1 3 11% 37% 39%	2200
October	1st to 10th	Day	10% 2 3 10% 21% 55%	3600
	11th to 20th	Day	11% 1 3 16% 39% 30%	3266
	21st to end	Day	21% 2 3 9% 31% 33%	3497
November	1st to 10th	Day	21% 3 7% 14% 32% 24%	1854
	11th to 20th	Day	23% 3 7% 17% 24% 26%	1890
	21st to end	Day	29% 4 7% 12% 25% 24%	1890
December	1st to 10th	Day	24% 3 4 13% 30% 26%	3040
	11th to 20th	Day	32% 4 8% 17% 25% 14%	3329
	21st to end	Day	40% 3 9% 20% 21% 8%	4092
January	1st to 10th	Night	32% 2 8% 21% 21% 14%	8172
	11th to 20th	Night	19% 4 6% 11% 25% 36%	7808
	21st to end	Night	33% 3 6% 10% 25% 24%	8108
February	1st to 10th	Night	37% 2 5% 13% 22% 21%	2520
	11th to 20th	Night	34% 3 6% 16% 25% 16%	2459
	21st to end	Night	34% 3 9% 20% 24% 11%	2076
March	1st to 10th	Night	40% 5% 6% 18% 19% 12%	5496
	11th to 20th	Night	36% 4 8% 30% 16% 7%	5371
	21st to end	Night	33% 6% 17% 22% 18% 5%	5116
April	1st to 10th	Night	31% 4 10% 24% 22% 9%	2100
	11th to 20th	Night	25% 2 10% 23% 29% 11%	2079
	21st to end	Night	23% 3 8% 33% 28% 6%	2100
May	1st to 10th	Night	18% 1 6% 24% 42% 9%	5703
	11th to 20th	Night	9% 2 2 27% 47% 13%	5297
	21st to end	Night	11% 1 3 25% 51% 10%	5838
June	1st to 10th	Night	13% 2 7% 22% 38% 18%	1693
	11th to 20th	Night	5% 1 5% 14% 35% 39%	1680
	21st to end	Night	5% 1 3 7% 33% 51%	1680
July	1st to 10th	Night	4 1 6% 20% 69%	4700
	11th to 20th	Night	8% 1 3 11% 24% 53%	4692
	21st to end	Night	4 2 18% 38% 37%	5864
August	1st to 10th	Night	5% 1 2 13% 39% 40%	4636
	11th to 20th	Night	4 1 2 17% 34% 41%	4636
	21st to end	Night	7% 1 1 19% 34% 39%	5092
September	1st to 10th	Night	6% 2 3 10% 32% 47%	2100
	11th to 20th	Night	8% 2 2 12% 40% 35%	2145
	21st to end	Night	8% 1 2 15% 36% 37%	2200
October	1st to 10th	Night	9% 1 2 10% 21% 56%	3600
	11th to 20th	Night	12% 2 2 13% 43% 28%	3887
	21st to end	Night	18% 2 2 9% 34% 35%	4310
November	1st to 10th	Night	23% 2 5% 15% 29% 25%	2665
	11th to 20th	Night	24% 3 7% 14% 25% 26%	2730
	21st to end	Night	29% 4 6% 13% 21% 26%	2730
December	1st to 10th	Night	22% 2 4 11% 34% 27%	4370
	11th to 20th	Night	34% 6% 7% 16% 20% 17%	5098
	21st to end	Night	44% 5% 9% 17% 18% 7%	7106

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% 20% 33% 20% 4	220
	11th to 20th	Day	21% 18% 38% 18% 5% 1	365
	21st to end	Day	17% 19% 40% 18% 5% 1	523
February	1st to 10th	Day	18% 18% 39% 19% 4 1	540
	11th to 20th	Day	24% 23% 30% 18% 4 1	535
	21st to end	Day	21% 23% 36% 17% 3	446
March	1st to 10th	Day	26% 24% 31% 15% 4	540
	11th to 20th	Day	22% 20% 32% 19% 6% 1	540
	21st to end	Day	24% 22% 34% 14% 5%	594
April	1st to 10th	Day	19% 19% 33% 20% 8% 1	540
	11th to 20th	Day	21% 19% 33% 20% 5% 1	532
	21st to end	Day	15% 21% 33% 19% 9% 3	540
May	1st to 10th	Day	16% 17% 37% 21% 8% 1	543
	11th to 20th	Day	14% 16% 37% 22% 8% 3	568
	21st to end	Day	15% 14% 34% 25% 9% 3	617
June	1st to 10th	Day	5% 13% 40% 28% 9% 5%	570
	11th to 20th	Day	3 13% 41% 29% 11% 3	570
	21st to end	Day	5% 12% 41% 27% 11% 5%	570
July	1st to 10th	Day	1 7% 40% 35% 11% 6%	566
	11th to 20th	Day	2 3 30% 41% 13% 10%	564
	21st to end	Day	2 3 34% 36% 16% 9%	619
August	1st to 10th	Day	2 6% 41% 31% 12% 8%	567
	11th to 20th	Day	2 5% 35% 35% 15% 7%	557
	21st to end	Day	1 5% 40% 35% 13% 6%	627
September	1st to 10th	Day	1 7% 42% 35% 12% 4	570
	11th to 20th	Day	5% 12% 39% 31% 11% 1	543
	21st to end	Day	6% 10% 40% 30% 13% 2	530
October	1st to 10th	Day	8% 13% 36% 32% 11%	530
	11th to 20th	Day	9% 12% 37% 33% 8% 1	530
	21st to end	Day	9% 14% 35% 32% 8% 2	546
November	1st to 10th	Day	14% 9% 38% 26% 12% 1	330
	11th to 20th	Day	14% 15% 36% 26% 8% 1	330
	21st to end	Day	15% 15% 40% 21% 8% 1	330
December	1st to 10th	Day	13% 16% 40% 25% 6%	330
	11th to 20th	Day	18% 11% 35% 27% 8% 1	289
	21st to end	Day	28% 21% 29% 16% 7%	242
January	1st to 10th	Night	16% 9% 32% 28% 10% 4	840
	11th to 20th	Night	11% 10% 35% 31% 11% 3	840
	21st to end	Night	16% 10% 30% 31% 11% 2	924
February	1st to 10th	Night	15% 9% 31% 29% 12% 4	840
	11th to 20th	Night	16% 10% 30% 27% 13% 4	824
	21st to end	Night	16% 11% 28% 32% 11% 2	692
March	1st to 10th	Night	17% 11% 29% 31% 11% 1	840
	11th to 20th	Night	16% 10% 24% 29% 17% 4	840
	21st to end	Night	18% 12% 22% 33% 14% 2	924
April	1st to 10th	Night	14% 12% 28% 29% 14% 2	840
	11th to 20th	Night	14% 11% 25% 35% 13% 2	831
	21st to end	Night	13% 14% 24% 30% 17% 3	840
May	1st to 10th	Night	11% 11% 29% 33% 16% 1	767
	11th to 20th	Night	8% 12% 25% 35% 19% 1	621
	21st to end	Night	10% 10% 28% 35% 16% 1	683
June	1st to 10th	Night	6% 6% 25% 42% 21% 1	433
	11th to 20th	Night	1 5% 25% 45% 22% 1	420
	21st to end	Night	1 3 24% 44% 27% 1	420
July	1st to 10th	Night	1 11% 46% 40% 2	410
	11th to 20th	Night	2 4 42% 50% 2	408
	21st to end	Night	2 9% 41% 42% 6%	751
August	1st to 10th	Night	1 1 14% 44% 35% 6%	836
	11th to 20th	Night	1 11% 43% 37% 8%	832
	21st to end	Night	1 1 8% 44% 37% 9%	924
September	1st to 10th	Night	2 1 10% 38% 40% 9%	840
	11th to 20th	Night	2 2 15% 44% 30% 7%	812
	21st to end	Night	3 3 19% 37% 30% 8%	800
October	1st to 10th	Night	4 5% 19% 46% 22% 3	800
	11th to 20th	Night	8% 6% 20% 39% 22% 7%	800
	21st to end	Night	7% 5% 23% 35% 20% 9%	880
November	1st to 10th	Night	12% 4 21% 39% 18% 6%	820
	11th to 20th	Night	8% 7% 28% 34% 19% 4	840
	21st to end	Night	12% 8% 29% 32% 15% 3	840
December	1st to 10th	Night	12% 9% 32% 31% 13% 3	840
	11th to 20th	Night	16% 8% 32% 31% 12% 2	840
	21st to end	Night	18% 10% 30% 30% 10% 2	924

Page generated at 2026-09-11 01:11:51. Version ac07

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