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 2025-07-06.

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	18% 7% 3 6% 4 4 5% 4 6% 7% 36%	120029
	Night	18% 7% 2 6% 4 4 4 3 5% 7% 40%	118731
Transparency	Day	45% 6% 11% 22% 14% 2	123605
	Night	42% 5% 12% 23% 15% 3	123316
Seeing	Day	27% 16% 28% 19% 9% 3	15696
	Night	24% 15% 27% 24% 9% 1	24980

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% 4 3 5% 6% 42%	3597
	11th to 20th	Day	13% 11% 6% 13% 6% 6% 4 3 4 5% 29%	3644
	21st to end	Day	17% 11% 4 7% 5% 5% 4 3 4 4 36%	4466
February	1st to 10th	Day	16% 9% 4 9% 5% 4 6% 4 6% 6% 32%	1800
	11th to 20th	Day	15% 10% 3 7% 6% 5% 5% 4 6% 6% 35%	1857
	21st to end	Day	19% 10% 4 8% 5% 4 4 3 4 6% 34%	1650
March	1st to 10th	Day	22% 7% 2 6% 4 4 4 3 4 5% 38%	4540
	11th to 20th	Day	12% 6% 3 8% 5% 5% 5% 4 5% 7% 41%	4980
	21st to end	Day	18% 8% 2 5% 4 4 4 3 5% 7% 40%	5740
April	1st to 10th	Day	17% 10% 3 7% 5% 5% 5% 4 5% 6% 34%	2400
	11th to 20th	Day	18% 9% 3 6% 5% 4 4 3 5% 5% 38%	2374
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 4 6% 7% 38%	2400
May	1st to 10th	Day	29% 6% 2 6% 4 4 5% 4 6% 7% 27%	5985
	11th to 20th	Day	25% 10% 3 6% 4 4 4 3 5% 6% 30%	6318
	21st to end	Day	21% 6% 2 5% 4 4 5% 5% 7% 9% 33%	6949
June	1st to 10th	Day	14% 6% 2 6% 6% 6% 5% 5% 7% 8% 35%	2800
	11th to 20th	Day	17% 7% 2 6% 5% 5% 5% 6% 7% 9% 34%	2800
	21st to end	Day	13% 5% 2 6% 5% 5% 5% 5% 7% 9% 38%	2800
July	1st to 10th	Day	10% 5% 2 7% 6% 6% 7% 5% 7% 10% 35%	4510
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 10% 38%	4440
	21st to end	Day	16% 9% 3 7% 6% 5% 5% 4 7% 6% 33%	4752
August	1st to 10th	Day	23% 5% 2 7% 5% 5% 5% 4 5% 8% 30%	4104
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 7% 40%	3804
	21st to end	Day	17% 5% 2 4 2 3 4 4 6% 8% 45%	4180
September	1st to 10th	Day	19% 4 2 6% 4 4 3 4 6% 7% 42%	2280
	11th to 20th	Day	14% 5% 2 4 3 3 4 3 6% 8% 48%	2271
	21st to end	Day	22% 7% 2 6% 4 4 4 3 5% 6% 37%	2053
October	1st to 10th	Day	20% 7% 2 4 3 4 4 4 4 7% 40%	1900
	11th to 20th	Day	17% 6% 2 5% 4 4 4 4 5% 6% 43%	1900
	21st to end	Day	20% 7% 3 6% 3 5% 5% 3 5% 6% 38%	2028
November	1st to 10th	Day	14% 7% 2 4 3 3 3 3 4 7% 50%	1761
	11th to 20th	Day	16% 8% 3 5% 4 5% 4 4 6% 5% 40%	1800
	21st to end	Day	19% 7% 2 5% 4 3 5% 4 6% 7% 38%	1600
December	1st to 10th	Day	17% 9% 3 6% 5% 5% 4 4 5% 6% 38%	2600
	11th to 20th	Day	22% 11% 3 6% 3 4 3 3 4 5% 35%	2986
	21st to end	Day	18% 9% 3 8% 5% 5% 5% 4 6% 7% 30%	3960
January	1st to 10th	Night	12% 6% 2 7% 5% 4 5% 3 6% 7% 43%	6349
	11th to 20th	Night	13% 9% 4 9% 5% 5% 4 4 6% 7% 36%	6223
	21st to end	Night	17% 8% 3 7% 5% 4 4 4 5% 6% 37%	6490
February	1st to 10th	Night	16% 9% 3 7% 4 4 4 4 5% 7% 37%	2600
	11th to 20th	Night	16% 8% 2 7% 5% 3 5% 4 6% 6% 36%	2428
	21st to end	Night	18% 9% 2 6% 5% 4 4 4 5% 7% 37%	1980
March	1st to 10th	Night	19% 9% 2 6% 4 3 4 3 4 5% 39%	5283
	11th to 20th	Night	12% 5% 3 6% 4 4 4 3 4 6% 49%	4979
	21st to end	Night	16% 9% 2 7% 4 4 4 3 4 6% 41%	5177
April	1st to 10th	Night	18% 8% 3 8% 6% 4 5% 4 5% 6% 34%	2000
	11th to 20th	Night	17% 6% 2 6% 6% 5% 4 4 5% 7% 38%	1975
	21st to end	Night	20% 5% 2 5% 3 3 4 3 5% 6% 43%	2000
May	1st to 10th	Night	31% 8% 2 5% 4 4 4 3 5% 7% 28%	3995
	11th to 20th	Night	24% 9% 3 6% 4 3 4 3 5% 7% 33%	3575
	21st to end	Night	23% 7% 2 5% 4 4 4 3 6% 8% 36%	3926
June	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 5% 8% 42%	1600
	11th to 20th	Night	16% 5% 2 4 4 5% 4 5% 6% 9% 41%	1600
	21st to end	Night	13% 5% 2 4 4 4 4 4 6% 9% 46%	1600
July	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 6% 9% 40%	2560
	11th to 20th	Night	14% 5% 2 6% 4 4 5% 5% 8% 9% 39%	2520
	21st to end	Night	18% 6% 2 5% 4 4 4 4 6% 7% 40%	2797
August	1st to 10th	Night	22% 5% 3 6% 4 4 5% 4 6% 8% 34%	2825
	11th to 20th	Night	16% 7% 2 7% 4 4 4 3 4 6% 43%	2993
	21st to end	Night	14% 4 1 4 3 3 3 4 6% 7% 50%	3476
September	1st to 10th	Night	17% 6% 1 6% 3 3 4 3 5% 6% 47%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 6% 51%	1909
	21st to end	Night	21% 6% 2 5% 4 3 4 3 4 5% 43%	2123
October	1st to 10th	Night	22% 6% 2 5% 3 3 3 3 5% 7% 42%	2280
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 6% 47%	2280
	21st to end	Night	21% 5% 1 4 3 3 4 3 5% 7% 42%	2570
November	1st to 10th	Night	13% 6% 2 5% 3 3 4 4 6% 6% 50%	2537
	11th to 20th	Night	18% 7% 2 5% 4 3 4 3 5% 6% 42%	2600
	21st to end	Night	15% 6% 2 6% 4 5% 5% 4 6% 8% 40%	2797
December	1st to 10th	Night	16% 9% 2 7% 4 4 4 3 4 6% 41%	4580
	11th to 20th	Night	24% 9% 2 5% 3 4 4 3 5% 6% 36%	5208
	21st to end	Night	20% 11% 3 7% 4 4 4 3 4 5% 35%	6996

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	42% 4 9% 27% 16% 2	3598
	11th to 20th	Day	54% 5% 9% 17% 11% 4	3674
	21st to end	Day	51% 4 9% 22% 13% 1	4466
February	1st to 10th	Day	50% 5% 12% 19% 12% 1	1800
	11th to 20th	Day	48% 5% 8% 24% 12% 2	1866
	21st to end	Day	51% 4 10% 22% 11% 3	1650
March	1st to 10th	Day	46% 4 11% 21% 11% 6%	4543
	11th to 20th	Day	40% 6% 13% 27% 9% 5%	4980
	21st to end	Day	44% 5% 11% 13% 22% 5%	5740
April	1st to 10th	Day	49% 6% 8% 19% 14% 4	2400
	11th to 20th	Day	47% 5% 10% 22% 14% 3	2374
	21st to end	Day	43% 6% 9% 22% 15% 5%	2400
May	1st to 10th	Day	54% 7% 12% 17% 9% 1	5995
	11th to 20th	Day	54% 6% 9% 20% 11% 1	6322
	21st to end	Day	45% 6% 12% 23% 12% 2	6949
June	1st to 10th	Day	43% 8% 12% 23% 13% 1	2800
	11th to 20th	Day	44% 8% 14% 21% 11% 2	2800
	21st to end	Day	40% 7% 14% 25% 14%	2800
July	1st to 10th	Day	41% 9% 13% 23% 13% 1	4440
	11th to 20th	Day	38% 8% 16% 24% 13% 1	4365
	21st to end	Day	49% 7% 10% 19% 15% 1	4601
August	1st to 10th	Day	51% 7% 12% 20% 10%	4091
	11th to 20th	Day	40% 6% 14% 29% 11%	3842
	21st to end	Day	36% 5% 11% 32% 14% 2	4180
September	1st to 10th	Day	42% 6% 11% 21% 18% 2	2280
	11th to 20th	Day	33% 6% 10% 25% 24% 2	2271
	21st to end	Day	48% 4 9% 19% 19% 2	2053
October	1st to 10th	Day	40% 4 10% 20% 21% 5%	3160
	11th to 20th	Day	33% 4 10% 28% 20% 6%	3140
	21st to end	Day	31% 3 15% 25% 23% 3	3318
November	1st to 10th	Day	35% 3 13% 25% 20% 3	1761
	11th to 20th	Day	44% 5% 9% 24% 17% 2	1800
	21st to end	Day	43% 5% 14% 27% 10% 2	1600
December	1st to 10th	Day	46% 4 15% 24% 9% 1	2600
	11th to 20th	Day	50% 4 14% 22% 10% 1	2986
	21st to end	Day	49% 6% 13% 18% 13% 1	3960
January	1st to 10th	Night	38% 4 12% 25% 18% 3	6349
	11th to 20th	Night	43% 7% 15% 19% 12% 5%	6268
	21st to end	Night	46% 5% 10% 19% 17% 3	6490
February	1st to 10th	Night	46% 5% 14% 24% 10% 2	2600
	11th to 20th	Night	46% 4 11% 21% 15% 3	2430
	21st to end	Night	47% 4 8% 23% 15% 3	1980
March	1st to 10th	Night	46% 4 12% 21% 11% 6%	5293
	11th to 20th	Night	36% 5% 16% 25% 13% 5%	4980
	21st to end	Night	45% 5% 7% 16% 23% 5%	5177
April	1st to 10th	Night	49% 6% 8% 21% 13% 3	2000
	11th to 20th	Night	43% 7% 10% 21% 18% 1	1975
	21st to end	Night	41% 6% 10% 22% 18% 4	2000
May	1st to 10th	Night	55% 5% 11% 18% 9% 2	4001
	11th to 20th	Night	51% 7% 12% 16% 13% 1	3576
	21st to end	Night	46% 6% 13% 23% 11% 2	3927
June	1st to 10th	Night	40% 6% 10% 25% 19% 1	1600
	11th to 20th	Night	38% 7% 16% 26% 12% 1	1600
	21st to end	Night	33% 8% 15% 31% 13% 1	1600
July	1st to 10th	Night	40% 8% 16% 18% 16% 2	2520
	11th to 20th	Night	39% 8% 16% 24% 13% 1	2472
	21st to end	Night	43% 6% 14% 18% 18% 1	2708
August	1st to 10th	Night	47% 8% 16% 20% 9%	2814
	11th to 20th	Night	42% 5% 14% 24% 15% 1	3021
	21st to end	Night	33% 6% 11% 32% 17% 1	3476
September	1st to 10th	Night	39% 5% 10% 26% 18% 2	1900
	11th to 20th	Night	34% 5% 9% 26% 24% 2	1909
	21st to end	Night	44% 4 8% 22% 20% 2	2123
October	1st to 10th	Night	42% 4 10% 23% 20% 2	3800
	11th to 20th	Night	30% 4 9% 30% 22% 6%	3779
	21st to end	Night	30% 4 15% 29% 21% 2	4230
November	1st to 10th	Night	35% 4 13% 25% 20% 3	2537
	11th to 20th	Night	42% 4 10% 25% 17% 3	2600
	21st to end	Night	41% 4 14% 27% 12% 1	2797
December	1st to 10th	Night	43% 3 14% 31% 7% 1	4580
	11th to 20th	Night	48% 4 12% 21% 14% 1	5208
	21st to end	Night	50% 5% 10% 19% 15% 1	6996

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	37% 23% 31% 9% 2	200
	11th to 20th	Day	41% 24% 24% 9% 1	217
	21st to end	Day	28% 28% 32% 9% 3	330
February	1st to 10th	Day	31% 30% 29% 7% 2	300
	11th to 20th	Day	38% 24% 31% 5% 1	404
	21st to end	Day	35% 23% 28% 12% 2	414
March	1st to 10th	Day	35% 21% 27% 14% 2	500
	11th to 20th	Day	31% 27% 27% 13% 2 1	500
	21st to end	Day	32% 24% 27% 12% 3 1	550
April	1st to 10th	Day	36% 22% 25% 12% 4 1	500
	11th to 20th	Day	32% 20% 28% 12% 8% 1	492
	21st to end	Day	27% 21% 33% 13% 5% 2	500
May	1st to 10th	Day	35% 18% 26% 13% 6% 2	503
	11th to 20th	Day	28% 15% 31% 19% 6% 1	527
	21st to end	Day	23% 9% 28% 24% 13% 3	573
June	1st to 10th	Day	22% 15% 28% 23% 10% 2	530
	11th to 20th	Day	25% 13% 25% 24% 11% 2	530
	21st to end	Day	19% 12% 32% 24% 11% 2	530
July	1st to 10th	Day	15% 5% 25% 31% 18% 6%	505
	11th to 20th	Day	13% 6% 29% 31% 18% 4	484
	21st to end	Day	18% 8% 32% 27% 12% 3	532
August	1st to 10th	Day	18% 7% 22% 29% 17% 6%	486
	11th to 20th	Day	15% 7% 23% 32% 17% 5%	479
	21st to end	Day	17% 7% 28% 27% 13% 8%	539
September	1st to 10th	Day	23% 11% 20% 24% 15% 7%	490
	11th to 20th	Day	15% 8% 28% 23% 16% 10%	490
	21st to end	Day	29% 13% 24% 19% 11% 4	490
October	1st to 10th	Day	28% 9% 26% 20% 13% 5%	490
	11th to 20th	Day	25% 17% 28% 22% 7% 2	490
	21st to end	Day	29% 15% 29% 20% 7% 1	506
November	1st to 10th	Day	23% 17% 30% 20% 9% 2	395
	11th to 20th	Day	28% 23% 30% 14% 5% 1	400
	21st to end	Day	33% 23% 34% 9% 3	200
December	1st to 10th	Day	37% 23% 27% 8% 4 2	200
	11th to 20th	Day	43% 20% 24% 11% 3	200
	21st to end	Day	35% 19% 31% 10% 5%	220
January	1st to 10th	Night	26% 25% 32% 13% 3	800
	11th to 20th	Night	29% 20% 30% 18% 3	800
	21st to end	Night	20% 25% 35% 19% 1	880
February	1st to 10th	Night	27% 20% 33% 18% 2	800
	11th to 20th	Night	30% 25% 33% 11% 1	783
	21st to end	Night	30% 21% 32% 14% 3	660
March	1st to 10th	Night	29% 22% 30% 17% 3	800
	11th to 20th	Night	27% 21% 28% 20% 4 1	800
	21st to end	Night	25% 21% 32% 16% 5% 1	880
April	1st to 10th	Night	28% 18% 31% 19% 4	800
	11th to 20th	Night	23% 18% 32% 20% 6%	791
	21st to end	Night	26% 15% 29% 23% 6%	800
May	1st to 10th	Night	29% 10% 28% 25% 7% 1	580
	11th to 20th	Night	22% 12% 29% 28% 8% 1	394
	21st to end	Night	21% 8% 20% 34% 15% 1	433
June	1st to 10th	Night	20% 7% 24% 32% 17% 1	400
	11th to 20th	Night	19% 6% 26% 36% 14% 1	400
	21st to end	Night	14% 5% 24% 41% 15% 1	400
July	1st to 10th	Night	15% 6% 18% 39% 20% 2	380
	11th to 20th	Night	14% 3 16% 45% 21% 1	368
	21st to end	Night	16% 5% 17% 41% 19% 2	450
August	1st to 10th	Night	19% 2 14% 39% 23% 3	567
	11th to 20th	Night	18% 5% 13% 38% 24% 3	682
	21st to end	Night	18% 5% 17% 33% 26% 2	836
September	1st to 10th	Night	20% 6% 23% 29% 20% 2	760
	11th to 20th	Night	16% 6% 20% 31% 22% 5%	760
	21st to end	Night	24% 7% 21% 30% 17% 1	760
October	1st to 10th	Night	28% 8% 22% 23% 16% 3	760
	11th to 20th	Night	21% 12% 28% 30% 9%	760
	21st to end	Night	26% 15% 26% 24% 7%	836
November	1st to 10th	Night	19% 16% 31% 24% 10% 1	780
	11th to 20th	Night	26% 18% 27% 23% 5% 1	800
	21st to end	Night	22% 21% 30% 22% 5% 1	800
December	1st to 10th	Night	27% 21% 30% 16% 5%	800
	11th to 20th	Night	28% 20% 31% 18% 4	800
	21st to end	Night	26% 22% 29% 20% 3	880

Page generated at 2025-07-03 01:30:15. Version ac07

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