Mais Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mais Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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% 3 1 3 2 2 3 3 4 6% 67%	113517
	Night	6% 6% 1 3 3 3 3 3 4 6% 63%	114475
Transparency	Day	19% 3 6% 15% 27% 31%	113287
	Night	23% 3 6% 14% 23% 30%	114295
Seeing	Day	8% 8% 27% 37% 15% 5%	16168
	Night	8% 3 11% 35% 34% 9%	26589

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	6% 4 1 4 2 2 2 2 3 4 70%	4098
	11th to 20th	Day	6% 4 1 3 2 2 3 2 3 6% 68%	4327
	21st to end	Day	11% 5% 1 4 3 4 4 4 6% 9% 49%	5016
February	1st to 10th	Day	11% 5% 1 5% 4 2 4 3 4 5% 57%	2000
	11th to 20th	Day	11% 6% 2 3 3 3 4 3 5% 7% 53%	1950
	21st to end	Day	12% 9% 2 5% 3 4 4 3 4 6% 49%	1650
March	1st to 10th	Day	13% 7% 2 4 4 3 5% 5% 8% 9% 39%	3238
	11th to 20th	Day	6% 5% 2 5% 4 4 4 3 5% 7% 56%	3341
	21st to end	Day	13% 7% 1 4 3 3 3 3 5% 6% 51%	4026
April	1st to 10th	Day	9% 6% 1 4 3 3 3 3 5% 7% 55%	2280
	11th to 20th	Day	5% 5% 1 3 3 2 4 4 4 6% 63%	2249
	21st to end	Day	7% 5% 1 3 2 2 2 3 4 6% 64%	2280
May	1st to 10th	Day	5% 4 1 3 3 2 2 3 3 4 70%	4007
	11th to 20th	Day	2 3 1 3 3 3 3 2 4 7% 68%	4071
	21st to end	Day	3 5% 2 4 3 3 3 3 5% 7% 63%	4477
June	1st to 10th	Day	4 3 1 3 3 2 3 3 4 5% 69%	2470
	11th to 20th	Day	2 3 1 2 2 2 2 1 3 5% 79%	2470
	21st to end	Day	1 2 1 2 2 2 2 2 3 3 81%	2470
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 85%	4110
	11th to 20th	Day	1 1 1 2 2 2 3 2 4 6% 77%	4110
	21st to end	Day	1 1 2 1 2 2 2 4 5% 80%	4283
August	1st to 10th	Day	1 1 1 1 1 2 2 3 3 6% 79%	3783
	11th to 20th	Day	1 1 2 1 1 1 1 2 5% 85%	3724
	21st to end	Day	1 1 1 2 2 2 2 3 85%	4180
September	1st to 10th	Day	2 1 2 2 2 2 2 4 5% 76%	2261
	11th to 20th	Day	2 2 2 1 1 2 1 3 4 82%	2090
	21st to end	Day	3 1 1 2 2 2 2 2 3 4 78%	2090
October	1st to 10th	Day	3 3 1 2 1 2 2 2 3 4 79%	3490
	11th to 20th	Day	3 3 1 3 3 2 2 1 3 5% 74%	3469
	21st to end	Day	3 2 1 3 2 3 3 3 5% 7% 69%	3274
November	1st to 10th	Day	5% 3 1 2 2 2 2 2 5% 6% 69%	1764
	11th to 20th	Day	5% 3 1 3 3 3 3 2 4 5% 69%	1800
	21st to end	Day	7% 5% 2 4 3 3 4 3 5% 8% 57%	1800
December	1st to 10th	Day	5% 3 1 2 2 2 3 3 5% 7% 67%	2950
	11th to 20th	Day	9% 4 1 4 4 4 6% 4 5% 7% 51%	3409
	21st to end	Day	9% 6% 2 5% 5% 5% 5% 4 6% 7% 45%	4510
January	1st to 10th	Night	5% 4 1 3 2 2 3 3 4 6% 67%	5867
	11th to 20th	Night	6% 5% 2 4 4 3 2 2 4 6% 63%	5660
	21st to end	Night	14% 8% 2 4 3 4 3 3 5% 5% 48%	5984
February	1st to 10th	Night	10% 7% 2 4 2 2 3 3 4 5% 57%	2400
	11th to 20th	Night	11% 8% 2 5% 4 3 3 3 3 5% 54%	2338
	21st to end	Night	12% 8% 3 4 3 3 4 3 3 5% 51%	1980
March	1st to 10th	Night	14% 16% 3 6% 3 3 3 2 5% 7% 37%	3856
	11th to 20th	Night	7% 8% 1 5% 3 3 4 3 6% 7% 53%	3665
	21st to end	Night	10% 13% 3 5% 3 3 3 4 6% 8% 42%	3623
April	1st to 10th	Night	8% 10% 1 4 3 2 4 3 5% 8% 51%	1900
	11th to 20th	Night	7% 9% 2 6% 4 2 2 2 4 5% 57%	1879
	21st to end	Night	7% 8% 2 4 3 4 4 2 4 6% 57%	1900
May	1st to 10th	Night	4 8% 2 3 3 2 2 2 3 5% 66%	2963
	11th to 20th	Night	3 9% 2 5% 6% 4 3 3 4 5% 57%	2834
	21st to end	Night	3 13% 3 5% 4 2 3 2 3 5% 57%	3110
June	1st to 10th	Night	2 6% 1 3 4 4 4 3 5% 7% 60%	1710
	11th to 20th	Night	1 4 1 3 3 3 2 2 4 5% 71%	1710
	21st to end	Night	2 2 1 2 2 3 2 4 4 76%	1710
July	1st to 10th	Night	1 1 2 2 2 1 2 4 86%	2860
	11th to 20th	Night	1 1 2 3 3 2 4 6% 78%	2860
	21st to end	Night	1 1 2 2 2 2 2 3 5% 80%	3283
August	1st to 10th	Night	1 1 1 1 1 2 2 5% 7% 79%	3166
	11th to 20th	Night	1 1 1 1 1 2 2 4 6% 83%	3126
	21st to end	Night	1 1 1 1 2 2 2 3 3 85%	3498
September	1st to 10th	Night	2 2 1 3 2 3 3 2 5% 5% 72%	1918
	11th to 20th	Night	2 2 1 4 2 2 2 2 3 3 77%	2090
	21st to end	Night	2 2 1 2 2 2 2 2 3 4 78%	2090
October	1st to 10th	Night	3 2 1 2 2 2 2 2 2 4 79%	3490
	11th to 20th	Night	5% 5% 1 4 6% 3 2 2 4 5% 64%	3469
	21st to end	Night	4 4 1 3 2 3 3 2 4 5% 69%	4282
November	1st to 10th	Night	5% 4 1 3 2 3 3 3 4 5% 67%	2535
	11th to 20th	Night	4 5% 2 4 3 3 2 3 3 5% 67%	2600
	21st to end	Night	7% 6% 2 4 4 3 4 4 6% 7% 53%	2600
December	1st to 10th	Night	5% 3 1 3 2 2 3 3 5% 5% 67%	4240
	11th to 20th	Night	9% 7% 2 4 3 3 5% 4 6% 7% 49%	4811
	21st to end	Night	8% 7% 2 6% 4 3 4 4 6% 10% 46%	6468

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	19% 2 4 17% 28% 30%	4099
	11th to 20th	Day	19% 3 7% 18% 22% 31%	4327
	21st to end	Day	30% 4 6% 11% 27% 21%	5016
February	1st to 10th	Day	31% 3 4 13% 21% 28%	2000
	11th to 20th	Day	31% 4 6% 14% 26% 20%	1960
	21st to end	Day	38% 3 6% 13% 19% 22%	1650
March	1st to 10th	Day	36% 4 8% 21% 18% 13%	3238
	11th to 20th	Day	24% 6% 11% 26% 20% 13%	3341
	21st to end	Day	33% 4 9% 31% 16% 8%	4026
April	1st to 10th	Day	29% 4 8% 23% 24% 13%	2280
	11th to 20th	Day	22% 4 7% 19% 29% 20%	2252
	21st to end	Day	21% 3 7% 22% 34% 13%	2280
May	1st to 10th	Day	18% 3 7% 17% 43% 12%	4007
	11th to 20th	Day	17% 2 8% 17% 40% 17%	4074
	21st to end	Day	21% 2 7% 18% 45% 8%	4477
June	1st to 10th	Day	18% 3 5% 23% 32% 20%	2470
	11th to 20th	Day	12% 2 4 11% 29% 43%	2470
	21st to end	Day	10% 2 3 7% 23% 56%	2470
July	1st to 10th	Day	6% 2 2 5% 13% 72%	4046
	11th to 20th	Day	8% 3 5% 13% 32% 38%	4041
	21st to end	Day	8% 3 6% 22% 24% 36%	4148
August	1st to 10th	Day	8% 2 5% 17% 39% 29%	3770
	11th to 20th	Day	6% 1 5% 12% 24% 51%	3760
	21st to end	Day	6% 2 2 7% 24% 59%	4179
September	1st to 10th	Day	12% 3 4 8% 33% 40%	2261
	11th to 20th	Day	9% 2 2 8% 35% 45%	2090
	21st to end	Day	12% 2 3 11% 31% 42%	2090
October	1st to 10th	Day	12% 2 3 10% 16% 58%	3490
	11th to 20th	Day	12% 3 5% 11% 29% 40%	3469
	21st to end	Day	15% 3 3 10% 30% 40%	3274
November	1st to 10th	Day	18% 3 4 14% 23% 37%	1764
	11th to 20th	Day	19% 3 5% 11% 23% 40%	1800
	21st to end	Day	26% 3 7% 11% 20% 33%	1800
December	1st to 10th	Day	17% 2 5% 11% 25% 39%	2950
	11th to 20th	Day	31% 4 7% 13% 23% 22%	3408
	21st to end	Day	36% 5% 7% 22% 22% 9%	4510
January	1st to 10th	Night	20% 2 4 14% 29% 30%	5867
	11th to 20th	Night	22% 4 6% 19% 17% 31%	5660
	21st to end	Night	37% 3 6% 9% 23% 23%	5984
February	1st to 10th	Night	30% 3 6% 12% 20% 28%	2400
	11th to 20th	Night	34% 3 5% 12% 22% 24%	2339
	21st to end	Night	36% 4 4 14% 20% 22%	1980
March	1st to 10th	Night	46% 5% 7% 13% 17% 12%	3856
	11th to 20th	Night	28% 6% 9% 25% 19% 13%	3665
	21st to end	Night	40% 5% 9% 20% 18% 8%	3623
April	1st to 10th	Night	32% 3 9% 21% 20% 15%	1900
	11th to 20th	Night	31% 2 8% 16% 23% 21%	1879
	21st to end	Night	30% 4 9% 17% 25% 15%	1900
May	1st to 10th	Night	24% 3 8% 18% 35% 12%	2963
	11th to 20th	Night	31% 3 14% 13% 28% 12%	2835
	21st to end	Night	32% 2 10% 14% 35% 7%	3110
June	1st to 10th	Night	23% 5% 9% 16% 28% 20%	1710
	11th to 20th	Night	16% 2 6% 12% 24% 40%	1710
	21st to end	Night	11% 2 5% 8% 21% 53%	1710
July	1st to 10th	Night	6% 1 2 6% 13% 72%	2815
	11th to 20th	Night	6% 4 5% 14% 36% 34%	2806
	21st to end	Night	9% 2 7% 21% 27% 35%	3180
August	1st to 10th	Night	6% 2 4 19% 43% 27%	3156
	11th to 20th	Night	5% 2 5% 16% 23% 49%	3156
	21st to end	Night	6% 1 4 6% 20% 62%	3498
September	1st to 10th	Night	14% 2 5% 11% 25% 42%	1918
	11th to 20th	Night	14% 2 4 9% 28% 44%	2090
	21st to end	Night	13% 1 4 10% 33% 39%	2090
October	1st to 10th	Night	13% 2 4 10% 16% 56%	3490
	11th to 20th	Night	18% 6% 6% 8% 20% 42%	3469
	21st to end	Night	19% 2 3 9% 25% 43%	4282
November	1st to 10th	Night	21% 3 4 13% 21% 39%	2535
	11th to 20th	Night	21% 2 4 12% 21% 40%	2600
	21st to end	Night	29% 4 7% 13% 16% 32%	2600
December	1st to 10th	Night	19% 2 4 13% 23% 39%	4240
	11th to 20th	Night	32% 4 7% 12% 19% 25%	4811
	21st to end	Night	32% 4 9% 21% 23% 10%	6468

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	18% 5% 31% 32% 13% 1	300
	11th to 20th	Day	13% 9% 27% 36% 13% 2	397
	21st to end	Day	13% 12% 32% 31% 11% 1	550
February	1st to 10th	Day	15% 11% 33% 28% 11% 3	500
	11th to 20th	Day	15% 15% 32% 30% 8%	495
	21st to end	Day	17% 16% 37% 23% 6%	414
March	1st to 10th	Day	17% 16% 35% 22% 8% 1	500
	11th to 20th	Day	15% 13% 34% 27% 10% 2	465
	21st to end	Day	13% 17% 37% 24% 8% 1	506
April	1st to 10th	Day	13% 12% 31% 30% 12% 3	460
	11th to 20th	Day	8% 19% 33% 30% 8% 2	452
	21st to end	Day	10% 12% 32% 28% 14% 4	460
May	1st to 10th	Day	13% 11% 32% 28% 13% 3	463
	11th to 20th	Day	8% 9% 33% 34% 14% 3	488
	21st to end	Day	9% 10% 32% 33% 13% 3	529
June	1st to 10th	Day	3 5% 25% 40% 20% 7%	490
	11th to 20th	Day	3 3 27% 43% 17% 6%	490
	21st to end	Day	2 2 24% 43% 20% 8%	490
July	1st to 10th	Day	2 1 12% 53% 21% 11%	486
	11th to 20th	Day	2 10% 50% 23% 15%	484
	21st to end	Day	2 1 10% 51% 24% 13%	531
August	1st to 10th	Day	2 17% 49% 20% 12%	487
	11th to 20th	Day	1 1 13% 52% 18% 15%	477
	21st to end	Day	1 1 16% 51% 21% 10%	539
September	1st to 10th	Day	3 1 26% 46% 18% 7%	490
	11th to 20th	Day	3 6% 28% 43% 17% 3	490
	21st to end	Day	4 6% 30% 37% 17% 5%	490
October	1st to 10th	Day	6% 7% 30% 37% 18% 3	490
	11th to 20th	Day	6% 7% 32% 38% 16% 2	490
	21st to end	Day	6% 6% 27% 38% 20% 3	435
November	1st to 10th	Day	8% 7% 23% 36% 22% 4	300
	11th to 20th	Day	6% 7% 23% 42% 18% 4	300
	21st to end	Day	11% 9% 32% 32% 12% 4	300
December	1st to 10th	Day	11% 5% 29% 36% 17% 3	300
	11th to 20th	Day	6% 6% 29% 37% 18% 3	300
	21st to end	Day	20% 9% 27% 30% 12% 2	330
January	1st to 10th	Night	11% 4 23% 37% 21% 6%	800
	11th to 20th	Night	10% 4 19% 41% 20% 6%	800
	21st to end	Night	14% 6% 18% 40% 19% 3	880
February	1st to 10th	Night	14% 4 16% 38% 22% 6%	800
	11th to 20th	Night	13% 4 20% 37% 21% 6%	784
	21st to end	Night	15% 6% 17% 38% 19% 4	660
March	1st to 10th	Night	16% 6% 16% 41% 19% 2	800
	11th to 20th	Night	13% 3 12% 40% 24% 8%	766
	21st to end	Night	11% 5% 16% 38% 26% 5%	836
April	1st to 10th	Night	13% 3 13% 43% 24% 4	760
	11th to 20th	Night	11% 5% 13% 40% 26% 5%	751
	21st to end	Night	10% 3 12% 37% 30% 7%	760
May	1st to 10th	Night	10% 4 11% 39% 32% 5%	634
	11th to 20th	Night	8% 2 13% 33% 37% 7%	561
	21st to end	Night	11% 4 12% 36% 34% 4	617
June	1st to 10th	Night	3 2 11% 38% 39% 7%	570
	11th to 20th	Night	3 1 8% 42% 42% 5%	570
	21st to end	Night	2 1 5% 37% 48% 7%	570
July	1st to 10th	Night	1 27% 59% 12%	555
	11th to 20th	Night	1 1 20% 66% 12%	552
	21st to end	Night	1 2 1 17% 61% 18%	723
August	1st to 10th	Night	1 1 25% 55% 16%	756
	11th to 20th	Night	1 1 16% 61% 20%	752
	21st to end	Night	1 1 17% 58% 22%	836
September	1st to 10th	Night	2 1 3 25% 54% 15%	760
	11th to 20th	Night	4 2 3 33% 48% 11%	760
	21st to end	Night	4 1 7% 35% 42% 12%	760
October	1st to 10th	Night	6% 2 6% 43% 35% 7%	760
	11th to 20th	Night	7% 3 10% 39% 31% 10%	760
	21st to end	Night	8% 1 12% 35% 31% 13%	836
November	1st to 10th	Night	7% 2 11% 41% 28% 10%	780
	11th to 20th	Night	6% 2 13% 42% 27% 10%	800
	21st to end	Night	10% 5% 17% 39% 23% 7%	800
December	1st to 10th	Night	11% 3 17% 41% 22% 6%	800
	11th to 20th	Night	15% 3 17% 42% 19% 4	800
	21st to end	Night	14% 4 19% 38% 20% 5%	880

Page generated at 2025-03-09 05:08:42. Version ac07

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