Astro-chalet
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astro-chalet clear sky chart from 2005-11-06 to 2026-10-29.

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	17% 7% 3 6% 4 4 5% 4 6% 7% 36%	149828
	Night	17% 7% 2 6% 4 4 4 3 5% 7% 40%	142228
Transparency	Day	44% 6% 12% 22% 14% 2	153360
	Night	42% 5% 12% 23% 15% 2	146784
Seeing	Day	26% 16% 28% 19% 8% 3	17447
	Night	24% 15% 27% 24% 9% 1	26650

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% 9% 3 7% 5% 5% 5% 4 5% 6% 38%	4661
	11th to 20th	Day	14% 10% 5% 12% 6% 5% 4 3 4 5% 32%	4752
	21st to end	Day	15% 10% 4 7% 5% 5% 5% 3 4 5% 37%	5780
February	1st to 10th	Day	15% 9% 4 9% 5% 4 5% 4 6% 5% 34%	1890
	11th to 20th	Day	15% 10% 3 7% 6% 5% 4 4 6% 6% 34%	1952
	21st to end	Day	18% 11% 4 8% 5% 4 3 3 4 6% 34%	1730
March	1st to 10th	Day	22% 7% 2 6% 4 4 4 3 4 5% 38%	4641
	11th to 20th	Day	12% 6% 3 8% 5% 5% 5% 4 5% 7% 41%	5090
	21st to end	Day	18% 8% 2 5% 4 4 4 3 5% 7% 40%	5867
April	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 4 5% 6% 34%	2520
	11th to 20th	Day	17% 8% 3 6% 5% 4 4 3 6% 5% 38%	2494
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 4 5% 7% 38%	2520
May	1st to 10th	Day	26% 6% 2 6% 4 4 5% 4 6% 7% 30%	7765
	11th to 20th	Day	20% 9% 3 6% 4 4 4 4 6% 7% 34%	8221
	21st to end	Day	23% 6% 2 5% 4 4 5% 4 6% 8% 31%	9060
June	1st to 10th	Day	14% 6% 2 6% 6% 5% 5% 5% 7% 8% 36%	2940
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 6% 7% 9% 34%	2940
	21st to end	Day	14% 5% 2 6% 5% 4 5% 5% 7% 9% 38%	2940
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 9% 36%	8280
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 10% 35%	8280
	21st to end	Day	16% 6% 2 5% 4 4 5% 4 7% 8% 39%	8875
August	1st to 10th	Day	20% 6% 3 8% 5% 6% 6% 5% 7% 9% 27%	6014
	11th to 20th	Day	15% 5% 2 6% 4 4 5% 4 7% 9% 39%	5607
	21st to end	Day	16% 5% 2 5% 3 3 5% 4 7% 10% 41%	6080
September	1st to 10th	Day	18% 4 2 6% 4 3 4 4 6% 7% 42%	2520
	11th to 20th	Day	14% 5% 2 4 3 3 4 3 6% 8% 48%	2510
	21st to end	Day	22% 7% 2 6% 4 4 4 4 5% 6% 35%	2269
October	1st to 10th	Day	19% 6% 2 4 3 4 4 4 4 6% 43%	2100
	11th to 20th	Day	18% 6% 2 5% 4 4 4 4 5% 6% 43%	2100
	21st to end	Day	19% 7% 3 6% 3 5% 5% 3 5% 6% 40%	2215
November	1st to 10th	Day	14% 7% 2 4 3 3 3 3 4 8% 50%	1851
	11th to 20th	Day	15% 8% 3 6% 4 5% 4 4 6% 5% 39%	1890
	21st to end	Day	19% 8% 2 5% 4 3 5% 4 6% 7% 37%	1680
December	1st to 10th	Day	16% 9% 3 6% 5% 5% 4 4 5% 6% 38%	2680
	11th to 20th	Day	22% 11% 3 6% 3 4 3 3 4 5% 36%	3066
	21st to end	Day	18% 9% 3 8% 5% 5% 5% 4 6% 7% 30%	4048
January	1st to 10th	Night	12% 8% 3 8% 5% 5% 5% 3 6% 7% 39%	8234
	11th to 20th	Night	13% 9% 3 10% 5% 5% 4 4 6% 6% 35%	8115
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 5% 6% 42%	8401
February	1st to 10th	Night	15% 9% 2 7% 4 4 3 4 5% 7% 38%	2730
	11th to 20th	Night	16% 8% 3 8% 5% 3 5% 4 6% 6% 35%	2552
	21st to end	Night	17% 9% 3 7% 5% 5% 4 4 5% 7% 37%	2076
March	1st to 10th	Night	19% 9% 2 6% 4 3 4 3 4 5% 39%	5402
	11th to 20th	Night	13% 5% 3 6% 4 4 4 3 4 6% 48%	5089
	21st to end	Night	16% 9% 2 7% 4 4 4 3 5% 6% 41%	5292
April	1st to 10th	Night	18% 8% 2 7% 6% 4 5% 4 5% 6% 34%	2100
	11th to 20th	Night	17% 6% 2 6% 6% 5% 4 4 5% 7% 38%	2075
	21st to end	Night	20% 5% 2 5% 3 3 4 3 5% 6% 43%	2100
May	1st to 10th	Night	26% 7% 2 5% 4 4 4 4 6% 7% 31%	5187
	11th to 20th	Night	21% 8% 3 5% 4 3 4 3 5% 8% 36%	4639
	21st to end	Night	25% 8% 2 5% 4 3 3 3 5% 7% 34%	5114
June	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 5% 8% 44%	1680
	11th to 20th	Night	15% 5% 2 5% 4 5% 4 5% 6% 9% 40%	1680
	21st to end	Night	13% 5% 2 5% 4 4 4 4 6% 10% 45%	1680
July	1st to 10th	Night	15% 6% 2 6% 4 4 5% 4 6% 9% 39%	4680
	11th to 20th	Night	15% 6% 2 7% 4 5% 5% 5% 7% 9% 35%	4680
	21st to end	Night	15% 6% 2 5% 4 4 4 4 5% 7% 44%	5198
August	1st to 10th	Night	20% 6% 3 7% 4 4 5% 4 6% 8% 32%	4135
	11th to 20th	Night	16% 6% 2 6% 4 4 5% 3 5% 7% 42%	4393
	21st to end	Night	14% 5% 2 5% 4 4 4 4 5% 7% 47%	5045
September	1st to 10th	Night	17% 6% 2 6% 3 3 4 3 5% 6% 46%	2100
	11th to 20th	Night	12% 5% 2 6% 3 3 4 3 5% 6% 50%	2110
	21st to end	Night	21% 7% 2 5% 4 3 4 4 5% 6% 40%	2347
October	1st to 10th	Night	21% 5% 2 4 3 3 3 3 5% 7% 44%	2520
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 6% 47%	2520
	21st to end	Night	20% 5% 1 4 3 3 4 3 5% 7% 44%	2802
November	1st to 10th	Night	13% 6% 2 5% 3 3 4 4 6% 6% 49%	2667
	11th to 20th	Night	18% 7% 2 5% 4 3 3 3 5% 6% 42%	2730
	21st to end	Night	15% 7% 2 6% 4 5% 5% 4 6% 8% 39%	2937
December	1st to 10th	Night	15% 9% 2 7% 4 4 4 3 4 6% 41%	4720
	11th to 20th	Night	24% 9% 2 5% 3 4 4 3 5% 6% 36%	5348
	21st to end	Night	20% 11% 3 7% 4 4 4 3 4 5% 34%	7150

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	44% 4 12% 25% 15% 1	4662
	11th to 20th	Day	52% 4 10% 17% 13% 5%	4782
	21st to end	Day	48% 4 11% 22% 15% 1	5780
February	1st to 10th	Day	49% 5% 12% 20% 12% 2	1890
	11th to 20th	Day	49% 5% 8% 24% 12% 2	1961
	21st to end	Day	51% 4 10% 22% 11% 2	1730
March	1st to 10th	Day	46% 4 11% 21% 11% 6%	4644
	11th to 20th	Day	40% 6% 13% 27% 9% 5%	5090
	21st to end	Day	44% 5% 11% 13% 22% 5%	5867
April	1st to 10th	Day	49% 6% 8% 19% 14% 4	2520
	11th to 20th	Day	46% 5% 11% 22% 14% 3	2494
	21st to end	Day	43% 6% 9% 22% 15% 4	2520
May	1st to 10th	Day	51% 7% 13% 20% 10% 1	7775
	11th to 20th	Day	47% 6% 12% 23% 12% 1	8225
	21st to end	Day	49% 6% 12% 20% 12% 1	9061
June	1st to 10th	Day	42% 8% 12% 23% 14% 1	2940
	11th to 20th	Day	43% 8% 14% 21% 12% 2	2940
	21st to end	Day	40% 7% 14% 25% 13%	2940
July	1st to 10th	Day	43% 8% 12% 22% 14% 1	8210
	11th to 20th	Day	40% 8% 16% 25% 10%	8205
	21st to end	Day	41% 6% 11% 22% 19% 1	8679
August	1st to 10th	Day	52% 8% 13% 16% 10%	6001
	11th to 20th	Day	39% 6% 16% 29% 9%	5645
	21st to end	Day	36% 6% 13% 29% 13% 2	6080
September	1st to 10th	Day	40% 6% 12% 21% 19% 2	2520
	11th to 20th	Day	33% 6% 10% 25% 23% 3	2510
	21st to end	Day	49% 4 9% 17% 18% 2	2269
October	1st to 10th	Day	39% 4 10% 21% 22% 5%	3360
	11th to 20th	Day	33% 4 10% 27% 20% 6%	3340
	21st to end	Day	30% 3 15% 25% 23% 4	3505
November	1st to 10th	Day	35% 4 13% 25% 20% 2	1851
	11th to 20th	Day	44% 5% 9% 23% 16% 2	1890
	21st to end	Day	44% 5% 13% 26% 10% 1	1680
December	1st to 10th	Day	46% 4 15% 24% 9% 1	2680
	11th to 20th	Day	50% 4 14% 21% 11% 1	3066
	21st to end	Day	49% 6% 13% 18% 13% 1	4048
January	1st to 10th	Night	42% 5% 14% 23% 14% 2	8234
	11th to 20th	Night	45% 6% 13% 17% 14% 5%	8160
	21st to end	Night	41% 4 10% 22% 21% 2	8401
February	1st to 10th	Night	44% 5% 13% 24% 11% 2	2730
	11th to 20th	Night	47% 4 10% 21% 14% 3	2554
	21st to end	Night	47% 4 8% 24% 15% 3	2076
March	1st to 10th	Night	46% 4 12% 21% 12% 6%	5412
	11th to 20th	Night	37% 5% 16% 25% 13% 5%	5090
	21st to end	Night	44% 5% 7% 16% 23% 5%	5292
April	1st to 10th	Night	49% 6% 9% 21% 13% 3	2100
	11th to 20th	Night	43% 7% 10% 21% 18% 1	2075
	21st to end	Night	42% 5% 10% 22% 17% 4	2100
May	1st to 10th	Night	51% 6% 12% 19% 11% 2	5193
	11th to 20th	Night	46% 6% 14% 21% 11% 1	4640
	21st to end	Night	50% 5% 11% 22% 11% 1	5115
June	1st to 10th	Night	39% 6% 10% 26% 19% 1	1680
	11th to 20th	Night	38% 7% 16% 25% 12% 2	1680
	21st to end	Night	34% 8% 15% 30% 13% 1	1680
July	1st to 10th	Night	41% 8% 14% 20% 16% 1	4640
	11th to 20th	Night	44% 7% 14% 22% 12% 1	4632
	21st to end	Night	38% 6% 13% 24% 18% 1	5080
August	1st to 10th	Night	49% 8% 14% 19% 9%	4124
	11th to 20th	Night	41% 6% 16% 25% 11%	4421
	21st to end	Night	36% 6% 13% 29% 15% 1	5045
September	1st to 10th	Night	39% 5% 11% 25% 18% 2	2100
	11th to 20th	Night	34% 5% 9% 25% 24% 2	2110
	21st to end	Night	45% 5% 9% 20% 19% 2	2347
October	1st to 10th	Night	40% 4 10% 23% 21% 2	4040
	11th to 20th	Night	30% 4 9% 29% 21% 6%	4019
	21st to end	Night	29% 4 15% 28% 21% 3	4462
November	1st to 10th	Night	35% 4 13% 25% 20% 3	2667
	11th to 20th	Night	41% 4 10% 24% 17% 3	2730
	21st to end	Night	42% 4 14% 27% 12% 1	2937
December	1st to 10th	Night	43% 3 14% 30% 8% 1	4720
	11th to 20th	Night	47% 4 12% 21% 14% 1	5348
	21st to end	Night	50% 5% 10% 19% 14% 1	7150

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	35% 24% 30% 8% 2	220
	11th to 20th	Day	45% 23% 22% 9% 1	239
	21st to end	Day	31% 28% 29% 8% 3	363
February	1st to 10th	Day	31% 30% 29% 7% 2	330
	11th to 20th	Day	38% 24% 32% 5% 1	439
	21st to end	Day	34% 24% 28% 12% 2	446
March	1st to 10th	Day	36% 20% 29% 13% 2	540
	11th to 20th	Day	32% 25% 27% 13% 2 1	540
	21st to end	Day	32% 25% 27% 11% 3 1	594
April	1st to 10th	Day	37% 21% 25% 12% 4 1	540
	11th to 20th	Day	30% 20% 29% 12% 7% 1	532
	21st to end	Day	26% 20% 34% 13% 4 2	540
May	1st to 10th	Day	35% 20% 26% 12% 6% 2	543
	11th to 20th	Day	26% 16% 32% 18% 6% 2	567
	21st to end	Day	25% 10% 29% 22% 12% 3	617
June	1st to 10th	Day	21% 16% 27% 24% 10% 2	570
	11th to 20th	Day	26% 14% 26% 23% 10% 2	570
	21st to end	Day	19% 13% 32% 24% 11% 2	570
July	1st to 10th	Day	16% 5% 24% 33% 17% 5%	565
	11th to 20th	Day	14% 7% 28% 30% 17% 4	564
	21st to end	Day	17% 8% 31% 29% 12% 3	620
August	1st to 10th	Day	17% 7% 23% 30% 17% 6%	566
	11th to 20th	Day	15% 7% 25% 32% 17% 4	559
	21st to end	Day	16% 8% 30% 27% 13% 7%	627
September	1st to 10th	Day	21% 11% 23% 24% 14% 6%	570
	11th to 20th	Day	15% 8% 28% 25% 15% 9%	570
	21st to end	Day	29% 12% 26% 19% 10% 3	570
October	1st to 10th	Day	26% 9% 27% 20% 12% 6%	570
	11th to 20th	Day	26% 17% 27% 21% 7% 2	570
	21st to end	Day	27% 15% 31% 20% 7% 1	579
November	1st to 10th	Day	24% 18% 29% 20% 8% 1	425
	11th to 20th	Day	28% 24% 29% 14% 4	430
	21st to end	Day	35% 23% 32% 8% 2	220
December	1st to 10th	Day	36% 25% 26% 8% 4 1	220
	11th to 20th	Day	43% 20% 24% 10% 3	220
	21st to end	Day	37% 20% 30% 9% 5%	242
January	1st to 10th	Night	25% 26% 32% 13% 3	840
	11th to 20th	Night	30% 20% 30% 17% 3	840
	21st to end	Night	21% 25% 34% 18% 1	924
February	1st to 10th	Night	27% 20% 33% 18% 1	840
	11th to 20th	Night	30% 24% 33% 11% 1	823
	21st to end	Night	30% 21% 32% 14% 3	692
March	1st to 10th	Night	29% 21% 29% 18% 3	840
	11th to 20th	Night	27% 21% 28% 20% 4	840
	21st to end	Night	25% 22% 33% 15% 4 1	924
April	1st to 10th	Night	28% 18% 30% 19% 4	840
	11th to 20th	Night	23% 18% 33% 20% 6%	831
	21st to end	Night	26% 15% 30% 23% 6%	840
May	1st to 10th	Night	28% 11% 29% 25% 7%	609
	11th to 20th	Night	21% 13% 29% 28% 8%	414
	21st to end	Night	23% 8% 20% 33% 15% 1	455
June	1st to 10th	Night	19% 6% 25% 33% 17% 1	420
	11th to 20th	Night	19% 6% 26% 35% 13% 1	420
	21st to end	Night	15% 5% 24% 40% 15% 1	420
July	1st to 10th	Night	15% 6% 18% 39% 20% 2	410
	11th to 20th	Night	15% 4 16% 45% 20% 1	408
	21st to end	Night	16% 4 18% 41% 20% 2	498
August	1st to 10th	Night	19% 3 14% 38% 24% 3	627
	11th to 20th	Night	18% 5% 13% 38% 23% 2	754
	21st to end	Night	18% 5% 19% 33% 24% 2	924
September	1st to 10th	Night	20% 6% 23% 30% 19% 2	840
	11th to 20th	Night	16% 6% 20% 32% 22% 4	840
	21st to end	Night	24% 8% 21% 29% 17% 1	840
October	1st to 10th	Night	26% 8% 23% 25% 17% 3	840
	11th to 20th	Night	21% 13% 28% 29% 9%	840
	21st to end	Night	25% 15% 27% 25% 8%	913
November	1st to 10th	Night	20% 16% 30% 24% 9%	820
	11th to 20th	Night	26% 19% 26% 23% 5% 1	840
	21st to end	Night	23% 20% 29% 22% 5%	840
December	1st to 10th	Night	27% 21% 31% 16% 4	840
	11th to 20th	Night	28% 20% 31% 18% 3	840
	21st to end	Night	27% 22% 29% 19% 3	924

Page generated at 2026-10-26 04:37:51. Version ac07

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