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 2025-05-11.

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	4 5% 1 3 2 2 3 3 4 5% 67%	117863
	Night	5% 8% 2 3 2 2 3 2 4 5% 64%	119992
Transparency	Day	19% 3 6% 15% 27% 30%	117618
	Night	23% 3 6% 16% 27% 26%	119826
Seeing	Day	12% 14% 36% 26% 9% 3	16106
	Night	10% 7% 23% 35% 21% 4	25914

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	3 10% 3 5% 3 3 3 3 5% 6% 55%	3639
	11th to 20th	Day	7% 5% 2 4 3 3 2 3 5% 6% 60%	3960
	21st to end	Day	8% 9% 2 6% 3 4 4 4 5% 8% 47%	4714
February	1st to 10th	Day	8% 12% 3 5% 4 3 3 3 4 5% 51%	2000
	11th to 20th	Day	8% 12% 3 3 3 3 3 4 5% 8% 48%	1950
	21st to end	Day	7% 12% 3 6% 3 4 3 4 5% 5% 48%	1650
March	1st to 10th	Day	10% 19% 3 6% 3 3 4 3 4 7% 38%	4518
	11th to 20th	Day	7% 10% 2 5% 3 4 5% 4 6% 8% 45%	4726
	21st to end	Day	7% 12% 2 4 4 4 5% 5% 7% 8% 43%	5969
April	1st to 10th	Day	7% 8% 1 4 3 3 4 3 5% 7% 55%	2400
	11th to 20th	Day	6% 7% 2 4 3 2 3 3 4 7% 59%	2369
	21st to end	Day	5% 7% 1 4 3 2 3 3 5% 7% 61%	2400
May	1st to 10th	Day	3 4 1 2 2 2 2 2 3 5% 73%	4035
	11th to 20th	Day	3 2 1 2 2 2 2 4 6% 77%	4073
	21st to end	Day	3 3 1 1 1 1 1 2 2 4 80%	4477
June	1st to 10th	Day	2 2 1 2 2 2 2 3 5% 6% 73%	2643
	11th to 20th	Day	1 1 1 1 1 1 2 3 88%	2660
	21st to end	Day	1 1 1 2 1 1 1 2 3 87%	2660
July	1st to 10th	Day	1 1 1 1 1 2 3 91%	4420
	11th to 20th	Day	1 1 1 1 1 2 2 2 2 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 1 3 4 86%	4345
August	1st to 10th	Day	1 1 1 1 2 1 2 2 3 85%	3783
	11th to 20th	Day	1 1 1 1 1 1 2 2 92%	3724
	21st to end	Day	1 1 1 1 1 2 92%	4180
September	1st to 10th	Day	1 1 1 1 1 2 1 3 4 85%	2280
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 89%	2171
	21st to end	Day	1 1 1 2 1 1 2 1 2 4 84%	2090
October	1st to 10th	Day	2 2 2 1 2 2 1 3 4 81%	3490
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 4 77%	3166
	21st to end	Day	4 4 1 3 3 3 3 3 5% 7% 64%	3389
November	1st to 10th	Day	5% 8% 2 3 2 2 2 3 4 6% 63%	1764
	11th to 20th	Day	4 6% 1 4 3 3 4 2 4 7% 62%	1800
	21st to end	Day	6% 9% 2 3 3 4 3 3 5% 6% 56%	1800
December	1st to 10th	Day	4 5% 1 4 3 3 4 4 5% 7% 61%	2950
	11th to 20th	Day	7% 7% 2 5% 5% 4 4 4 6% 8% 49%	3244
	21st to end	Day	10% 8% 2 6% 5% 5% 6% 5% 6% 7% 39%	4004
January	1st to 10th	Night	6% 12% 3 5% 2 3 4 3 5% 6% 50%	6306
	11th to 20th	Night	5% 7% 2 5% 3 3 3 3 4 6% 59%	5999
	21st to end	Night	12% 11% 2 4 3 3 3 3 5% 7% 46%	6269
February	1st to 10th	Night	8% 14% 2 6% 3 3 3 2 4 6% 49%	2400
	11th to 20th	Night	8% 11% 2 4 4 3 4 2 6% 7% 50%	2338
	21st to end	Night	9% 12% 2 4 4 3 3 3 4 6% 50%	1980
March	1st to 10th	Night	9% 23% 3 4 3 3 3 3 5% 7% 38%	5376
	11th to 20th	Night	7% 16% 2 4 2 4 4 3 4 6% 49%	5255
	21st to end	Night	7% 16% 2 4 3 3 4 3 4 7% 46%	5006
April	1st to 10th	Night	7% 9% 3 4 3 3 3 3 5% 6% 54%	2000
	11th to 20th	Night	6% 7% 1 4 2 3 3 3 4 6% 61%	1979
	21st to end	Night	8% 7% 1 3 1 2 2 3 4 7% 64%	2000
May	1st to 10th	Night	7% 4 1 2 1 1 1 1 3 4 76%	3158
	11th to 20th	Night	3 2 1 1 1 2 1 3 5% 80%	2838
	21st to end	Night	3 2 1 2 1 1 1 1 3 3 83%	3110
June	1st to 10th	Night	3 2 1 2 2 1 3 3 4 5% 75%	1532
	11th to 20th	Night	1 1 1 2 1 1 2 3 87%	1520
	21st to end	Night	1 1 1 1 1 2 3 89%	1520
July	1st to 10th	Night	1 1 1 1 1 2 2 90%	2540
	11th to 20th	Night	1 1 1 1 1 1 2 2 2 88%	2540
	21st to end	Night	1 1 1 1 1 1 1 1 3 88%	3213
August	1st to 10th	Night	1 1 1 1 1 1 1 1 2 3 86%	3166
	11th to 20th	Night	1 1 1 2 2 92%	3126
	21st to end	Night	1 1 1 1 1 1 2 93%	3498
September	1st to 10th	Night	1 2 1 1 1 1 2 3 88%	1900
	11th to 20th	Night	1 1 1 1 1 2 1 2 3 85%	2009
	21st to end	Night	1 1 1 1 2 1 1 2 3 85%	2090
October	1st to 10th	Night	2 2 1 2 1 1 1 1 3 3 84%	3490
	11th to 20th	Night	4 2 1 2 1 1 1 2 3 5% 77%	3767
	21st to end	Night	4 4 1 2 2 2 3 2 3 4 74%	4176
November	1st to 10th	Night	6% 7% 2 4 3 2 3 3 4 7% 61%	2535
	11th to 20th	Night	5% 6% 2 4 3 2 4 3 5% 6% 61%	2600
	21st to end	Night	9% 8% 2 5% 3 3 3 3 5% 7% 52%	2600
December	1st to 10th	Night	4 5% 1 3 3 3 4 3 5% 6% 62%	4240
	11th to 20th	Night	7% 9% 2 4 3 3 5% 4 7% 10% 46%	4964
	21st to end	Night	6% 15% 4 8% 5% 6% 5% 5% 7% 9% 30%	6952

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	23% 3 10% 27% 24% 13%	3639
	11th to 20th	Day	21% 5% 10% 13% 20% 30%	3960
	21st to end	Day	32% 4 7% 15% 21% 21%	4714
February	1st to 10th	Day	33% 5% 6% 16% 22% 19%	2000
	11th to 20th	Day	31% 4 6% 19% 25% 14%	1960
	21st to end	Day	35% 4 8% 18% 27% 9%	1650
March	1st to 10th	Day	42% 4 7% 14% 19% 14%	4518
	11th to 20th	Day	31% 6% 10% 29% 17% 8%	4726
	21st to end	Day	30% 7% 14% 25% 17% 6%	5969
April	1st to 10th	Day	29% 4 11% 22% 24% 11%	2400
	11th to 20th	Day	23% 3 9% 28% 25% 12%	2372
	21st to end	Day	19% 4 12% 27% 31% 8%	2400
May	1st to 10th	Day	14% 3 7% 19% 38% 18%	4035
	11th to 20th	Day	10% 3 4 18% 44% 21%	4076
	21st to end	Day	12% 2 4 17% 47% 19%	4477
June	1st to 10th	Day	13% 3 7% 19% 35% 22%	2643
	11th to 20th	Day	5% 1 5% 13% 35% 40%	2660
	21st to end	Day	7% 1 3 6% 30% 53%	2660
July	1st to 10th	Day	4 1 2 5% 12% 77%	4351
	11th to 20th	Day	10% 1 1 8% 22% 58%	4345
	21st to end	Day	7% 2 2 16% 29% 44%	4207
August	1st to 10th	Day	8% 1 2 6% 34% 49%	3770
	11th to 20th	Day	4 1 1 4 25% 64%	3760
	21st to end	Day	4 1 2 6% 29% 58%	4179
September	1st to 10th	Day	7% 3 3 6% 31% 51%	2280
	11th to 20th	Day	5% 1 2 12% 37% 42%	2171
	21st to end	Day	8% 1 3 9% 38% 40%	2090
October	1st to 10th	Day	10% 2 3 9% 21% 55%	3490
	11th to 20th	Day	11% 1 3 16% 39% 30%	3166
	21st to end	Day	22% 2 3 9% 31% 33%	3389
November	1st to 10th	Day	22% 3 8% 13% 32% 23%	1764
	11th to 20th	Day	21% 3 7% 18% 24% 28%	1800
	21st to end	Day	29% 4 7% 12% 25% 24%	1800
December	1st to 10th	Day	25% 3 4 12% 29% 26%	2950
	11th to 20th	Day	33% 4 8% 17% 25% 14%	3243
	21st to end	Day	40% 3 9% 20% 21% 7%	4004
January	1st to 10th	Night	28% 3 7% 22% 24% 16%	6306
	11th to 20th	Night	23% 5% 8% 14% 24% 26%	5999
	21st to end	Night	33% 3 7% 12% 21% 23%	6269
February	1st to 10th	Night	37% 2 6% 13% 22% 20%	2400
	11th to 20th	Night	33% 3 7% 16% 25% 16%	2339
	21st to end	Night	35% 3 8% 19% 23% 11%	1980
March	1st to 10th	Night	41% 5% 6% 18% 19% 12%	5376
	11th to 20th	Night	37% 4 8% 30% 15% 6%	5255
	21st to end	Night	33% 6% 17% 22% 17% 5%	5006
April	1st to 10th	Night	31% 4 10% 24% 22% 8%	2000
	11th to 20th	Night	24% 2 10% 23% 29% 11%	1979
	21st to end	Night	22% 3 8% 33% 28% 6%	2000
May	1st to 10th	Night	16% 2 5% 22% 39% 15%	3158
	11th to 20th	Night	10% 4 3 20% 47% 16%	2839
	21st to end	Night	10% 2 5% 20% 49% 14%	3110
June	1st to 10th	Night	14% 3 8% 21% 36% 19%	1532
	11th to 20th	Night	6% 2 5% 15% 37% 35%	1520
	21st to end	Night	5% 1 3 7% 33% 50%	1520
July	1st to 10th	Night	4 1 2 4 15% 75%	2500
	11th to 20th	Night	7% 1 2 7% 19% 63%	2492
	21st to end	Night	6% 1 2 19% 26% 45%	3113
August	1st to 10th	Night	7% 1 2 9% 37% 44%	3156
	11th to 20th	Night	3 1 1 5% 30% 59%	3156
	21st to end	Night	4 1 1 8% 32% 54%	3498
September	1st to 10th	Night	6% 2 3 8% 31% 50%	1900
	11th to 20th	Night	7% 2 2 12% 40% 36%	2009
	21st to end	Night	8% 1 2 15% 37% 37%	2090
October	1st to 10th	Night	9% 1 2 10% 21% 57%	3490
	11th to 20th	Night	12% 2 2 13% 43% 27%	3767
	21st to end	Night	18% 2 2 9% 34% 34%	4176
November	1st to 10th	Night	24% 2 5% 16% 29% 24%	2535
	11th to 20th	Night	23% 3 7% 14% 26% 27%	2600
	21st to end	Night	30% 4 6% 13% 21% 25%	2600
December	1st to 10th	Night	23% 3 4 11% 33% 27%	4240
	11th to 20th	Night	34% 7% 7% 16% 20% 16%	4964
	21st to end	Night	43% 5% 9% 17% 19% 7%	6952

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% 18% 34% 21% 4 1	200
	11th to 20th	Day	22% 19% 36% 18% 5% 1	338
	21st to end	Day	18% 19% 39% 17% 6% 1	485
February	1st to 10th	Day	19% 18% 40% 18% 4 1	500
	11th to 20th	Day	23% 23% 30% 19% 5% 1	495
	21st to end	Day	22% 22% 35% 16% 4	414
March	1st to 10th	Day	27% 23% 31% 15% 4	500
	11th to 20th	Day	24% 21% 32% 17% 5% 1	500
	21st to end	Day	26% 22% 33% 13% 4	550
April	1st to 10th	Day	21% 18% 31% 21% 8% 1	500
	11th to 20th	Day	21% 20% 32% 20% 5% 1	492
	21st to end	Day	14% 20% 33% 20% 10% 3	500
May	1st to 10th	Day	15% 18% 36% 21% 8% 1	503
	11th to 20th	Day	12% 16% 38% 23% 8% 3	489
	21st to end	Day	15% 14% 33% 25% 9% 3	529
June	1st to 10th	Day	6% 13% 37% 31% 9% 4	490
	11th to 20th	Day	3 12% 41% 30% 10% 3	490
	21st to end	Day	3 11% 42% 29% 11% 4	490
July	1st to 10th	Day	1 4 40% 37% 11% 7%	486
	11th to 20th	Day	2 3 29% 43% 14% 10%	484
	21st to end	Day	2 3 33% 37% 16% 9%	531
August	1st to 10th	Day	2 6% 42% 31% 12% 7%	487
	11th to 20th	Day	3 4 35% 37% 15% 6%	477
	21st to end	Day	1 4 39% 37% 12% 6%	539
September	1st to 10th	Day	1 5% 42% 36% 12% 4	490
	11th to 20th	Day	4 12% 40% 32% 10% 1	490
	21st to end	Day	6% 9% 40% 30% 13% 2	490
October	1st to 10th	Day	7% 13% 35% 32% 12%	490
	11th to 20th	Day	9% 11% 37% 34% 7% 1	490
	21st to end	Day	10% 14% 34% 33% 8% 2	504
November	1st to 10th	Day	15% 9% 36% 26% 12% 1	300
	11th to 20th	Day	11% 14% 37% 28% 9% 1	300
	21st to end	Day	16% 15% 39% 20% 9% 1	300
December	1st to 10th	Day	14% 16% 40% 25% 5%	300
	11th to 20th	Day	19% 11% 35% 25% 8% 1	263
	21st to end	Day	25% 22% 29% 16% 7%	220
January	1st to 10th	Night	16% 9% 32% 29% 11% 5%	800
	11th to 20th	Night	11% 10% 33% 32% 11% 3	800
	21st to end	Night	17% 10% 28% 31% 12% 2	880
February	1st to 10th	Night	16% 9% 30% 29% 12% 4	800
	11th to 20th	Night	15% 9% 29% 28% 14% 4	784
	21st to end	Night	17% 11% 26% 32% 12% 2	660
March	1st to 10th	Night	18% 11% 28% 32% 11% 2	800
	11th to 20th	Night	17% 10% 24% 27% 17% 4	800
	21st to end	Night	19% 12% 22% 31% 14% 2	880
April	1st to 10th	Night	14% 13% 27% 29% 15% 2	800
	11th to 20th	Night	13% 12% 25% 35% 13% 3	791
	21st to end	Night	12% 14% 23% 30% 17% 4	800
May	1st to 10th	Night	10% 11% 29% 33% 16% 1	730
	11th to 20th	Night	8% 12% 24% 36% 20% 1	563
	21st to end	Night	11% 10% 28% 33% 17% 1	617
June	1st to 10th	Night	6% 6% 25% 41% 21% 1	392
	11th to 20th	Night	1 5% 25% 43% 24% 2	380
	21st to end	Night	1 2 24% 45% 29% 1	380
July	1st to 10th	Night	1 8% 44% 44% 3	370
	11th to 20th	Night	2 4 40% 51% 2	368
	21st to end	Night	2 8% 39% 44% 7%	679
August	1st to 10th	Night	2 1 14% 42% 35% 6%	756
	11th to 20th	Night	1 11% 40% 39% 9%	752
	21st to end	Night	1 1 8% 41% 39% 10%	836
September	1st to 10th	Night	2 1 8% 36% 43% 9%	760
	11th to 20th	Night	2 2 15% 42% 31% 8%	760
	21st to end	Night	3 3 18% 37% 31% 8%	760
October	1st to 10th	Night	4 5% 17% 47% 23% 3	760
	11th to 20th	Night	8% 5% 18% 39% 23% 7%	760
	21st to end	Night	8% 5% 23% 34% 21% 10%	836
November	1st to 10th	Night	13% 4 20% 38% 19% 6%	780
	11th to 20th	Night	8% 6% 27% 35% 20% 4	800
	21st to end	Night	12% 9% 29% 31% 16% 4	800
December	1st to 10th	Night	13% 9% 30% 32% 14% 3	800
	11th to 20th	Night	17% 8% 30% 31% 12% 2	800
	21st to end	Night	18% 9% 30% 31% 11% 2	880

Page generated at 2025-05-08 06:33:59. Version ac07

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