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 2024-03-01.

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	5% 5% 2 3 3 2 3 3 4 6% 64%	78741
	Night	6% 7% 2 4 3 2 3 3 4 6% 60%	80697
Transparency	Day	21% 3 7% 16% 27% 25%	78459
	Night	25% 4 7% 17% 26% 22%	80500
Seeing	Day	12% 14% 36% 27% 9% 3	14479
	Night	10% 7% 22% 35% 22% 5%	24306

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% 14% 4 6% 4 2 3 4 5% 6% 46%	2539
	11th to 20th	Day	9% 7% 3 5% 5% 3 4 5% 7% 9% 43%	2762
	21st to end	Day	11% 10% 2 7% 5% 3 4 5% 6% 9% 37%	3280
February	1st to 10th	Day	8% 11% 3 5% 4 2 4 3 4 5% 51%	1900
	11th to 20th	Day	9% 10% 3 3 3 2 3 4 5% 8% 49%	1850
	21st to end	Day	8% 11% 4 5% 4 3 4 5% 5% 5% 48%	1570
March	1st to 10th	Day	10% 12% 3 5% 4 3 6% 5% 5% 9% 40%	1801
	11th to 20th	Day	7% 8% 3 4 4 2 5% 5% 6% 9% 48%	1862
	21st to end	Day	6% 11% 3 4 4 3 4 4 5% 6% 49%	2376
April	1st to 10th	Day	8% 6% 2 4 3 3 4 4 4 8% 54%	2160
	11th to 20th	Day	6% 6% 2 4 3 2 3 3 4 7% 60%	2129
	21st to end	Day	5% 6% 2 3 3 2 3 3 5% 8% 61%	2160
May	1st to 10th	Day	4 4 2 2 3 1 3 3 5% 7% 65%	2219
	11th to 20th	Day	5% 4 1 2 3 2 3 3 4 6% 68%	2301
	21st to end	Day	6% 5% 1 2 2 2 2 3 4 6% 67%	2530
June	1st to 10th	Day	3 1 1 2 2 2 2 3 5% 7% 73%	2503
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 88%	2520
	21st to end	Day	1 1 1 1 1 2 2 2 3 86%	2520
July	1st to 10th	Day	1 1 1 1 2 2 3 5% 84%	2520
	11th to 20th	Day	1 2 1 1 2 1 2 2 2 4 80%	2520
	21st to end	Day	1 2 1 2 2 1 2 3 4 5% 77%	2485
August	1st to 10th	Day	1 1 1 1 1 2 1 2 3 3 83%	2149
	11th to 20th	Day	1 1 1 2 1 2 2 3 3 85%	2122
	21st to end	Day	1 1 2 2 2 3 3 86%	2376
September	1st to 10th	Day	1 1 1 1 1 2 1 3 4 85%	2160
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	2057
	21st to end	Day	1 1 1 2 2 1 2 2 2 4 83%	1980
October	1st to 10th	Day	4 3 1 2 3 2 3 2 4 5% 70%	1980
	11th to 20th	Day	3 3 1 3 2 2 2 3 3 4 74%	1805
	21st to end	Day	3 3 1 3 3 1 2 2 5% 7% 70%	1947
November	1st to 10th	Day	5% 8% 3 3 3 1 3 3 4 6% 62%	1674
	11th to 20th	Day	4 4 2 3 4 2 4 3 4 7% 62%	1710
	21st to end	Day	5% 8% 2 3 3 3 3 4 5% 6% 57%	1710
December	1st to 10th	Day	7% 5% 2 4 4 2 4 5% 5% 7% 54%	1710
	11th to 20th	Day	10% 9% 3 6% 6% 3 5% 5% 6% 8% 41%	2060
	21st to end	Day	10% 10% 3 6% 7% 3 6% 5% 5% 7% 38%	2794
January	1st to 10th	Night	7% 16% 4 6% 4 2 4 4 5% 6% 43%	4406
	11th to 20th	Night	7% 10% 3 6% 5% 3 4 5% 6% 9% 42%	4203
	21st to end	Night	16% 13% 3 5% 4 2 3 4 5% 8% 37%	4387
February	1st to 10th	Night	8% 13% 3 5% 4 2 3 2 3 5% 50%	2280
	11th to 20th	Night	9% 10% 2 3 4 2 3 3 5% 6% 52%	2218
	21st to end	Night	10% 12% 3 4 5% 2 3 4 4 5% 48%	1884
March	1st to 10th	Night	8% 11% 3 5% 4 3 4 5% 6% 9% 42%	2165
	11th to 20th	Night	10% 12% 3 3 3 2 4 4 5% 7% 46%	2092
	21st to end	Night	10% 12% 2 5% 4 3 4 4 4 6% 47%	1980
April	1st to 10th	Night	8% 8% 3 4 4 3 3 4 5% 6% 53%	1800
	11th to 20th	Night	6% 7% 2 4 2 2 3 4 4 6% 61%	1779
	21st to end	Night	8% 5% 1 2 2 1 2 3 4 7% 64%	1800
May	1st to 10th	Night	7% 4 1 3 3 1 2 2 4 7% 66%	1738
	11th to 20th	Night	5% 3 1 2 2 1 3 3 4 6% 72%	1594
	21st to end	Night	5% 3 1 2 2 1 2 2 4 5% 71%	1746
June	1st to 10th	Night	4 2 1 2 2 1 2 4 3 5% 74%	1452
	11th to 20th	Night	1 1 1 2 1 2 2 3 87%	1440
	21st to end	Night	2 1 1 2 2 1 4 88%	1440
July	1st to 10th	Night	1 1 2 1 2 3 3 4 84%	1440
	11th to 20th	Night	1 1 1 2 1 2 3 3 4 83%	1440
	21st to end	Night	2 1 1 2 2 1 2 3 2 5% 80%	1824
August	1st to 10th	Night	2 1 1 1 1 1 2 2 3 4 83%	1790
	11th to 20th	Night	1 1 1 1 2 2 2 3 88%	1780
	21st to end	Night	1 1 2 1 1 2 2 4 87%	1980
September	1st to 10th	Night	1 2 1 1 1 1 1 2 3 88%	1800
	11th to 20th	Night	1 1 1 1 1 1 2 2 2 4 85%	1903
	21st to end	Night	1 1 1 1 1 2 2 2 4 84%	1980
October	1st to 10th	Night	3 3 2 3 2 1 3 3 5% 4 72%	1980
	11th to 20th	Night	5% 3 1 3 3 1 2 3 3 4 71%	2150
	21st to end	Night	4 3 1 2 2 1 2 3 3 5% 73%	2407
November	1st to 10th	Night	6% 7% 2 4 3 1 3 3 4 6% 60%	2405
	11th to 20th	Night	5% 5% 2 3 4 2 3 4 4 7% 62%	2470
	21st to end	Night	8% 7% 3 4 3 2 4 4 5% 7% 53%	2470
December	1st to 10th	Night	7% 8% 2 4 3 3 4 4 5% 6% 54%	2470
	11th to 20th	Night	10% 8% 3 5% 4 3 5% 5% 6% 9% 42%	3142
	21st to end	Night	7% 13% 4 7% 7% 4 5% 6% 6% 7% 34%	4862

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% 4 14% 27% 20% 7%	2539
	11th to 20th	Day	31% 7% 14% 19% 20% 9%	2762
	21st to end	Day	38% 6% 11% 19% 16% 10%	3280
February	1st to 10th	Day	34% 4 5% 17% 21% 19%	1900
	11th to 20th	Day	31% 4 6% 18% 26% 15%	1860
	21st to end	Day	36% 4 8% 19% 26% 7%	1570
March	1st to 10th	Day	40% 4 9% 15% 19% 12%	1801
	11th to 20th	Day	31% 5% 11% 19% 22% 12%	1862
	21st to end	Day	32% 5% 11% 27% 17% 8%	2376
April	1st to 10th	Day	29% 5% 12% 22% 21% 11%	2160
	11th to 20th	Day	23% 3 10% 28% 24% 12%	2132
	21st to end	Day	19% 4 13% 26% 30% 8%	2160
May	1st to 10th	Day	18% 3 10% 22% 35% 12%	2219
	11th to 20th	Day	17% 3 7% 24% 35% 14%	2304
	21st to end	Day	20% 3 7% 26% 35% 9%	2530
June	1st to 10th	Day	14% 3 7% 19% 34% 22%	2503
	11th to 20th	Day	5% 2 5% 13% 34% 40%	2520
	21st to end	Day	7% 1 3 6% 30% 53%	2520
July	1st to 10th	Day	6% 2 3 8% 22% 59%	2451
	11th to 20th	Day	12% 2 2 12% 29% 43%	2445
	21st to end	Day	12% 3 4 15% 33% 32%	2346
August	1st to 10th	Day	9% 2 3 7% 25% 54%	2135
	11th to 20th	Day	7% 2 2 8% 32% 49%	2122
	21st to end	Day	7% 2 3 6% 36% 47%	2376
September	1st to 10th	Day	7% 3 3 6% 30% 50%	2160
	11th to 20th	Day	5% 2 3 12% 36% 43%	2057
	21st to end	Day	9% 1 3 10% 39% 37%	1980
October	1st to 10th	Day	17% 3 6% 16% 24% 34%	1980
	11th to 20th	Day	14% 2 5% 13% 30% 36%	1805
	21st to end	Day	13% 3 6% 13% 28% 36%	1947
November	1st to 10th	Day	22% 3 8% 12% 31% 23%	1674
	11th to 20th	Day	21% 3 8% 17% 24% 28%	1710
	21st to end	Day	28% 4 7% 12% 25% 24%	1710
December	1st to 10th	Day	27% 5% 7% 18% 22% 20%	1710
	11th to 20th	Day	40% 6% 9% 15% 18% 12%	2059
	21st to end	Day	43% 5% 7% 19% 18% 9%	2794
January	1st to 10th	Night	33% 4 9% 22% 22% 10%	4406
	11th to 20th	Night	33% 7% 11% 19% 19% 10%	4203
	21st to end	Night	41% 4 10% 17% 17% 11%	4387
February	1st to 10th	Night	38% 2 6% 13% 21% 20%	2280
	11th to 20th	Night	32% 3 7% 16% 26% 17%	2219
	21st to end	Night	37% 4 8% 20% 22% 9%	1884
March	1st to 10th	Night	38% 6% 8% 18% 18% 12%	2165
	11th to 20th	Night	38% 3 9% 16% 21% 12%	2092
	21st to end	Night	36% 5% 11% 24% 18% 6%	1980
April	1st to 10th	Night	32% 4 11% 23% 21% 9%	1800
	11th to 20th	Night	25% 3 12% 23% 27% 11%	1779
	21st to end	Night	22% 3 8% 34% 27% 6%	1800
May	1st to 10th	Night	20% 3 9% 29% 31% 8%	1738
	11th to 20th	Night	17% 4 5% 27% 38% 8%	1595
	21st to end	Night	17% 3 8% 28% 35% 8%	1746
June	1st to 10th	Night	15% 3 8% 21% 35% 19%	1452
	11th to 20th	Night	6% 2 6% 16% 36% 35%	1440
	21st to end	Night	5% 2 3 8% 33% 49%	1440
July	1st to 10th	Night	7% 1 3 8% 26% 55%	1400
	11th to 20th	Night	10% 2 4 13% 30% 42%	1392
	21st to end	Night	11% 1 4 19% 33% 31%	1723
August	1st to 10th	Night	9% 2 4 9% 29% 48%	1780
	11th to 20th	Night	5% 2 2 9% 37% 44%	1780
	21st to end	Night	7% 2 1 8% 39% 42%	1980
September	1st to 10th	Night	6% 2 3 9% 31% 49%	1800
	11th to 20th	Night	7% 2 3 12% 40% 37%	1903
	21st to end	Night	8% 1 2 15% 37% 36%	1980
October	1st to 10th	Night	16% 2 4 17% 27% 34%	1980
	11th to 20th	Night	17% 2 4 16% 28% 33%	2150
	21st to end	Night	16% 3 4 12% 30% 36%	2407
November	1st to 10th	Night	24% 2 5% 15% 30% 24%	2405
	11th to 20th	Night	22% 3 7% 14% 26% 28%	2470
	21st to end	Night	29% 4 6% 13% 21% 26%	2470
December	1st to 10th	Night	30% 4 7% 15% 24% 21%	2470
	11th to 20th	Night	39% 10% 7% 15% 17% 12%	3142
	21st to end	Night	43% 6% 7% 17% 18% 9%	4862

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% 17% 33% 21% 4 1	180
	11th to 20th	Day	21% 19% 36% 19% 5% 1	311
	21st to end	Day	19% 19% 39% 17% 5% 1	447
February	1st to 10th	Day	19% 17% 40% 19% 4 2	460
	11th to 20th	Day	22% 23% 30% 19% 5% 1	455
	21st to end	Day	23% 21% 36% 16% 4	382
March	1st to 10th	Day	25% 23% 31% 18% 4	421
	11th to 20th	Day	23% 20% 31% 19% 5% 1	420
	21st to end	Day	24% 23% 34% 14% 4	462
April	1st to 10th	Day	21% 19% 31% 21% 8% 1	420
	11th to 20th	Day	22% 21% 32% 19% 5% 1	412
	21st to end	Day	14% 21% 33% 20% 10% 3	420
May	1st to 10th	Day	16% 18% 37% 20% 8% 1	423
	11th to 20th	Day	13% 17% 38% 23% 7% 3	448
	21st to end	Day	16% 15% 31% 25% 9% 3	485
June	1st to 10th	Day	6% 14% 38% 29% 9% 4	450
	11th to 20th	Day	4 11% 40% 32% 10% 4	450
	21st to end	Day	3 11% 41% 29% 11% 5%	450
July	1st to 10th	Day	1 4 40% 37% 11% 6%	446
	11th to 20th	Day	2 3 29% 44% 14% 9%	444
	21st to end	Day	2 2 33% 37% 16% 9%	487
August	1st to 10th	Day	2 6% 42% 31% 12% 8%	447
	11th to 20th	Day	3 3 33% 39% 15% 7%	437
	21st to end	Day	1 3 40% 37% 13% 6%	495
September	1st to 10th	Day	1 5% 41% 36% 12% 4	450
	11th to 20th	Day	4 11% 41% 34% 10% 2	450
	21st to end	Day	6% 9% 40% 30% 13% 2	450
October	1st to 10th	Day	8% 14% 35% 31% 11%	450
	11th to 20th	Day	8% 12% 37% 35% 7% 1	450
	21st to end	Day	9% 14% 34% 33% 8% 3	462
November	1st to 10th	Day	15% 8% 37% 27% 12% 1	270
	11th to 20th	Day	10% 12% 37% 29% 10% 1	270
	21st to end	Day	14% 15% 39% 21% 10% 1	270
December	1st to 10th	Day	16% 16% 38% 24% 5%	270
	11th to 20th	Day	20% 11% 33% 24% 9% 1	237
	21st to end	Day	26% 22% 27% 18% 8%	198
January	1st to 10th	Night	16% 8% 31% 29% 11% 5%	760
	11th to 20th	Night	11% 9% 32% 33% 12% 3	760
	21st to end	Night	18% 9% 27% 31% 12% 2	836
February	1st to 10th	Night	16% 9% 28% 30% 13% 4	760
	11th to 20th	Night	15% 9% 29% 29% 15% 4	744
	21st to end	Night	17% 11% 25% 32% 13% 3	628
March	1st to 10th	Night	16% 10% 27% 33% 12% 2	722
	11th to 20th	Night	16% 11% 21% 29% 19% 5%	720
	21st to end	Night	18% 13% 21% 32% 15% 2	792
April	1st to 10th	Night	14% 13% 27% 29% 15% 3	720
	11th to 20th	Night	13% 12% 24% 34% 14% 3	711
	21st to end	Night	11% 15% 23% 29% 18% 4	720
May	1st to 10th	Night	10% 11% 29% 32% 16% 1	658
	11th to 20th	Night	8% 12% 24% 35% 20% 1	531
	21st to end	Night	11% 10% 27% 34% 17% 1	584
June	1st to 10th	Night	6% 6% 25% 39% 22% 1	372
	11th to 20th	Night	1 4 24% 43% 25% 2	360
	21st to end	Night	1 2 23% 44% 30% 1	360
July	1st to 10th	Night	1 8% 43% 45% 3	350
	11th to 20th	Night	2 4 39% 52% 2	348
	21st to end	Night	2 7% 38% 45% 7%	642
August	1st to 10th	Night	2 1 14% 41% 36% 6%	716
	11th to 20th	Night	1 10% 39% 41% 10%	712
	21st to end	Night	1 1 7% 40% 40% 10%	792
September	1st to 10th	Night	2 2 8% 34% 44% 10%	720
	11th to 20th	Night	1 2 13% 42% 33% 8%	720
	21st to end	Night	3 3 16% 36% 33% 9%	720
October	1st to 10th	Night	4 5% 18% 45% 24% 4	720
	11th to 20th	Night	8% 5% 18% 38% 24% 7%	720
	21st to end	Night	7% 5% 21% 34% 22% 10%	792
November	1st to 10th	Night	12% 4 19% 38% 20% 7%	740
	11th to 20th	Night	7% 6% 26% 36% 21% 4	760
	21st to end	Night	11% 8% 28% 33% 17% 4	760
December	1st to 10th	Night	13% 9% 30% 31% 14% 3	760
	11th to 20th	Night	18% 8% 29% 31% 13% 2	760
	21st to end	Night	18% 9% 29% 31% 11% 2	836

Page generated at 2024-02-27 03:30:16. Version ac07

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