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 2024-11-03.

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% 8% 3 6% 5% 4 5% 4 6% 7% 36%	101375
	Night	18% 7% 2 6% 4 4 4 4 5% 7% 39%	97599
Transparency	Day	46% 6% 11% 22% 13% 2	101134
	Night	44% 6% 12% 23% 14% 2	97460
Seeing	Day	26% 15% 28% 19% 9% 3	14879
	Night	23% 15% 27% 24% 10% 1	24124

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	14% 11% 4 7% 4 4 4 3 5% 5% 40%	2517
	11th to 20th	Day	14% 12% 7% 13% 5% 6% 5% 3 4 5% 25%	2566
	21st to end	Day	21% 13% 5% 7% 5% 5% 4 3 5% 5% 29%	3124
February	1st to 10th	Day	16% 9% 4 9% 5% 4 5% 4 6% 6% 33%	1710
	11th to 20th	Day	14% 9% 2 7% 6% 5% 5% 4 6% 6% 36%	1761
	21st to end	Day	20% 11% 4 8% 5% 4 4 3 4 6% 33%	1570
March	1st to 10th	Day	26% 8% 2 6% 4 4 4 3 4 5% 32%	3166
	11th to 20th	Day	9% 6% 3 7% 5% 5% 5% 5% 5% 7% 41%	3480
	21st to end	Day	20% 10% 2 5% 3 3 3 2 4 5% 41%	4006
April	1st to 10th	Day	16% 10% 3 7% 5% 5% 5% 4 5% 6% 34%	2280
	11th to 20th	Day	18% 9% 3 6% 5% 4 4 3 5% 5% 39%	2254
	21st to end	Day	17% 6% 3 6% 5% 5% 5% 4 6% 7% 38%	2280
May	1st to 10th	Day	28% 7% 2 6% 4 4 4 4 5% 7% 29%	4177
	11th to 20th	Day	24% 11% 3 7% 5% 5% 4 4 5% 6% 26%	4398
	21st to end	Day	14% 5% 2 5% 4 4 5% 5% 6% 9% 41%	4837
June	1st to 10th	Day	14% 6% 2 6% 6% 6% 5% 5% 7% 8% 35%	2660
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 6% 7% 9% 34%	2660
	21st to end	Day	13% 5% 2 6% 5% 5% 5% 5% 7% 9% 37%	2660
July	1st to 10th	Day	10% 5% 2 7% 6% 6% 7% 5% 7% 10% 35%	4440
	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% 4 5% 5% 3 5% 6% 38%	2028
November	1st to 10th	Day	15% 8% 2 4 3 3 3 2 3 7% 50%	1689
	11th to 20th	Day	17% 8% 3 5% 5% 5% 4 4 6% 5% 40%	1710
	21st to end	Day	19% 7% 2 5% 4 3 5% 4 6% 7% 39%	1520
December	1st to 10th	Day	19% 11% 3 7% 6% 5% 4 4 5% 6% 31%	1520
	11th to 20th	Day	21% 9% 3 6% 4 4 4 4 5% 6% 34%	1906
	21st to end	Day	19% 10% 4 10% 6% 5% 5% 4 6% 6% 24%	2772
January	1st to 10th	Night	15% 7% 2 7% 5% 4 4 3 6% 7% 40%	4429
	11th to 20th	Night	16% 9% 4 9% 6% 5% 4 4 5% 5% 31%	4376
	21st to end	Night	23% 9% 3 7% 5% 4 4 3 5% 5% 32%	4532
February	1st to 10th	Night	16% 9% 2 7% 4 4 3 4 5% 7% 38%	2470
	11th to 20th	Night	16% 8% 3 7% 5% 3 5% 4 6% 6% 37%	2304
	21st to end	Night	18% 9% 2 6% 4 4 4 4 5% 7% 37%	1884
March	1st to 10th	Night	23% 11% 3 6% 3 3 3 3 4 5% 36%	3671
	11th to 20th	Night	11% 6% 3 6% 5% 5% 4 3 5% 6% 47%	3479
	21st to end	Night	15% 10% 2 7% 4 4 3 3 4 5% 43%	3611
April	1st to 10th	Night	17% 8% 3 7% 5% 4 5% 4 5% 7% 34%	1900
	11th to 20th	Night	17% 6% 2 6% 6% 5% 4 4 5% 7% 39%	1875
	21st to end	Night	20% 5% 2 5% 3 3 4 3 5% 6% 43%	1900
May	1st to 10th	Night	27% 9% 2 5% 4 3 4 4 5% 6% 30%	2789
	11th to 20th	Night	25% 11% 3 6% 4 3 3 3 5% 7% 30%	2495
	21st to end	Night	15% 7% 2 5% 4 4 4 4 7% 9% 39%	2738
June	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 5% 8% 43%	1520
	11th to 20th	Night	14% 4 2 4 4 5% 4 5% 6% 10% 42%	1520
	21st to end	Night	13% 5% 2 4 4 4 4 4 6% 9% 46%	1520
July	1st to 10th	Night	10% 5% 2 7% 5% 5% 5% 4 6% 9% 40%	2520
	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 5% 6% 49%	2434
	11th to 20th	Night	18% 7% 2 5% 4 3 3 3 5% 7% 42%	2470
	21st to end	Night	15% 6% 2 5% 4 4 5% 4 6% 8% 41%	2657
December	1st to 10th	Night	21% 7% 2 7% 6% 4 5% 4 5% 6% 32%	2660
	11th to 20th	Night	19% 9% 2 5% 4 4 5% 4 6% 7% 35%	3288
	21st to end	Night	20% 9% 3 7% 4 5% 4 4 5% 6% 32%	4884

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	45% 4 10% 29% 10% 2	2518
	11th to 20th	Day	57% 7% 9% 16% 9% 2	2566
	21st to end	Day	56% 4 10% 20% 9% 1	3124
February	1st to 10th	Day	50% 5% 13% 20% 12% 1	1710
	11th to 20th	Day	47% 5% 8% 25% 12% 2	1770
	21st to end	Day	52% 4 9% 21% 11% 3	1570
March	1st to 10th	Day	53% 5% 10% 23% 8% 2	3169
	11th to 20th	Day	37% 7% 12% 28% 12% 3	3480
	21st to end	Day	46% 4 10% 16% 21% 4	4006
April	1st to 10th	Day	48% 6% 8% 20% 14% 4	2280
	11th to 20th	Day	47% 5% 9% 23% 13% 3	2254
	21st to end	Day	44% 6% 9% 21% 15% 4	2280
May	1st to 10th	Day	53% 6% 10% 19% 9% 1	4190
	11th to 20th	Day	58% 6% 11% 16% 8% 1	4402
	21st to end	Day	37% 7% 12% 26% 16% 2	4837
June	1st to 10th	Day	43% 8% 12% 23% 13% 1	2660
	11th to 20th	Day	43% 8% 14% 22% 11% 2	2660
	21st to end	Day	40% 7% 14% 25% 13%	2660
July	1st to 10th	Day	42% 9% 13% 23% 13% 1	4370
	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	43% 5% 13% 22% 15% 2	1900
	11th to 20th	Day	41% 4 11% 25% 16% 2	1900
	21st to end	Day	47% 4 11% 21% 15% 2	2028
November	1st to 10th	Day	36% 3 13% 25% 20% 3	1689
	11th to 20th	Day	44% 5% 9% 23% 17% 2	1710
	21st to end	Day	43% 5% 14% 27% 10% 2	1520
December	1st to 10th	Day	53% 5% 14% 17% 10% 1	1520
	11th to 20th	Day	49% 5% 15% 22% 8% 1	1906
	21st to end	Day	56% 8% 11% 18% 7%	2772
January	1st to 10th	Night	42% 5% 12% 29% 10% 2	4429
	11th to 20th	Night	48% 8% 12% 21% 9% 2	4376
	21st to end	Night	52% 5% 10% 22% 10% 1	4532
February	1st to 10th	Night	45% 5% 14% 24% 10% 2	2470
	11th to 20th	Night	45% 5% 11% 21% 15% 3	2306
	21st to end	Night	47% 5% 8% 24% 14% 3	1884
March	1st to 10th	Night	51% 4 9% 23% 11% 2	3681
	11th to 20th	Night	36% 6% 14% 27% 16% 1	3480
	21st to end	Night	44% 4 7% 17% 22% 6%	3611
April	1st to 10th	Night	48% 6% 9% 22% 13% 3	1900
	11th to 20th	Night	43% 7% 9% 22% 18% 1	1875
	21st to end	Night	42% 6% 10% 21% 18% 4	1900
May	1st to 10th	Night	53% 6% 8% 23% 8% 1	2795
	11th to 20th	Night	54% 6% 13% 15% 10% 1	2496
	21st to end	Night	40% 7% 15% 22% 13% 2	2739
June	1st to 10th	Night	39% 6% 10% 25% 18% 1	1520
	11th to 20th	Night	37% 7% 16% 27% 12% 1	1520
	21st to end	Night	33% 8% 14% 31% 13% 1	1520
July	1st to 10th	Night	40% 8% 16% 18% 16% 2	2480
	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	43% 5% 11% 25% 15% 2	2280
	11th to 20th	Night	38% 5% 11% 30% 14% 2	2280
	21st to end	Night	41% 5% 11% 26% 16% 1	2570
November	1st to 10th	Night	36% 4 13% 24% 20% 3	2434
	11th to 20th	Night	41% 4 10% 25% 16% 3	2470
	21st to end	Night	40% 4 14% 28% 12% 1	2657
December	1st to 10th	Night	50% 5% 12% 23% 10% 1	2660
	11th to 20th	Night	46% 6% 14% 20% 14% 1	3288
	21st to end	Night	50% 7% 12% 22% 8%	4884

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% 21% 33% 9% 2	180
	11th to 20th	Day	40% 25% 24% 10% 2	195
	21st to end	Day	29% 25% 33% 10% 3	297
February	1st to 10th	Day	30% 30% 30% 7% 3	270
	11th to 20th	Day	35% 25% 33% 6% 1	368
	21st to end	Day	36% 21% 28% 13% 2	382
March	1st to 10th	Day	34% 21% 27% 15% 2	460
	11th to 20th	Day	31% 27% 26% 12% 2 1	460
	21st to end	Day	32% 24% 26% 13% 4 1	506
April	1st to 10th	Day	34% 22% 26% 13% 5% 1	460
	11th to 20th	Day	31% 19% 29% 12% 8% 1	452
	21st to end	Day	27% 20% 33% 13% 5% 2	460
May	1st to 10th	Day	35% 18% 25% 13% 7% 2	463
	11th to 20th	Day	26% 14% 32% 19% 7% 1	487
	21st to end	Day	21% 9% 29% 24% 14% 3	529
June	1st to 10th	Day	22% 15% 28% 22% 11% 2	490
	11th to 20th	Day	23% 13% 25% 25% 11% 2	490
	21st to end	Day	18% 12% 33% 22% 12% 2	490
July	1st to 10th	Day	15% 5% 24% 32% 18% 6%	485
	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	25% 15% 29% 20% 9% 2	371
	11th to 20th	Day	28% 23% 28% 15% 5% 1	370
	21st to end	Day	31% 21% 36% 9% 3	180
December	1st to 10th	Day	37% 23% 26% 9% 4 2	180
	11th to 20th	Day	42% 20% 25% 10% 3	180
	21st to end	Day	37% 18% 31% 9% 5%	198
January	1st to 10th	Night	25% 24% 33% 14% 3	760
	11th to 20th	Night	28% 20% 31% 19% 3	760
	21st to end	Night	20% 25% 35% 19% 1	836
February	1st to 10th	Night	27% 20% 33% 19% 2	760
	11th to 20th	Night	28% 26% 34% 11% 1	743
	21st to end	Night	31% 21% 31% 14% 3	628
March	1st to 10th	Night	28% 22% 30% 17% 3	760
	11th to 20th	Night	26% 22% 28% 19% 4 1	760
	21st to end	Night	25% 21% 32% 17% 5% 1	836
April	1st to 10th	Night	27% 18% 32% 19% 4	760
	11th to 20th	Night	23% 18% 32% 20% 7%	751
	21st to end	Night	26% 15% 30% 23% 7%	760
May	1st to 10th	Night	29% 11% 28% 25% 7% 1	551
	11th to 20th	Night	22% 12% 30% 28% 8% 1	374
	21st to end	Night	20% 8% 20% 35% 16% 1	411
June	1st to 10th	Night	19% 6% 23% 34% 17% 1	380
	11th to 20th	Night	17% 6% 26% 36% 15% 1	380
	21st to end	Night	14% 5% 24% 41% 15% 1	380
July	1st to 10th	Night	15% 6% 18% 39% 20% 2	370
	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% 17% 30% 23% 10% 1	749
	11th to 20th	Night	25% 18% 26% 24% 5% 1	760
	21st to end	Night	21% 20% 30% 23% 5% 1	760
December	1st to 10th	Night	28% 21% 30% 16% 5%	760
	11th to 20th	Night	27% 20% 31% 18% 4	760
	21st to end	Night	26% 22% 30% 19% 3	836

Page generated at 2024-10-31 03:39:20. Version ac07

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