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 2026-07-16.

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 2 4 5% 68%	146762
	Night	6% 6% 2 4 3 3 3 2 4 5% 62%	143626
Transparency	Day	18% 3 5% 15% 28% 30%	141859
	Night	24% 4 6% 13% 23% 30%	139507
Seeing	Day	8% 8% 27% 36% 15% 5%	18050
	Night	8% 3 11% 36% 33% 8%	28394

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	10% 6% 2 4 2 2 2 2 3 5% 62%	5318
	11th to 20th	Day	5% 3 1 2 2 2 2 2 3 6% 73%	5637
	21st to end	Day	10% 6% 2 5% 3 4 5% 4 6% 8% 48%	6500
February	1st to 10th	Day	11% 5% 1 5% 4 3 4 3 4 5% 57%	2100
	11th to 20th	Day	12% 7% 2 3 3 3 4 3 5% 7% 51%	2050
	21st to end	Day	11% 9% 2 5% 3 4 4 3 4 6% 49%	1730
March	1st to 10th	Day	13% 8% 2 5% 4 3 5% 4 7% 7% 42%	4618
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 5% 6% 54%	4880
	21st to end	Day	10% 6% 1 4 3 3 3 3 5% 7% 54%	5892
April	1st to 10th	Day	8% 6% 1 4 3 3 3 3 5% 7% 56%	2520
	11th to 20th	Day	5% 4 2 4 3 3 4 4 5% 5% 62%	2489
	21st to end	Day	7% 5% 1 3 2 2 3 3 4 7% 63%	2520
May	1st to 10th	Day	6% 4 2 4 2 2 3 2 3 4 68%	7432
	11th to 20th	Day	1 3 1 4 3 3 3 2 4 7% 69%	7596
	21st to end	Day	2 4 1 3 3 3 3 3 5% 7% 67%	8370
June	1st to 10th	Day	3 3 1 3 3 2 3 3 4 5% 69%	2730
	11th to 20th	Day	1 2 1 2 2 2 2 1 3 5% 79%	2730
	21st to end	Day	1 2 1 2 2 2 2 2 2 3 81%	2730
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 88%	6009
	11th to 20th	Day	1 1 1 2 2 2 2 3 5% 80%	5946
	21st to end	Day	1 1 2 1 2 2 2 3 4 83%	6152
August	1st to 10th	Day	1 1 1 1 1 1 2 3 5% 84%	5346
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 89%	5364
	21st to end	Day	2 1 1 2 1 2 2 2 3 4 81%	5984
September	1st to 10th	Day	2 1 2 2 2 2 2 3 5% 77%	2380
	11th to 20th	Day	2 2 2 2 1 2 1 3 4 82%	2200
	21st to end	Day	3 1 1 3 2 2 2 2 3 4 77%	2200
October	1st to 10th	Day	3 3 1 2 1 2 2 2 3 4 79%	3600
	11th to 20th	Day	3 3 1 3 2 2 2 1 3 5% 74%	3579
	21st to end	Day	3 2 1 3 2 3 3 3 5% 7% 70%	3378
November	1st to 10th	Day	5% 3 1 2 2 2 2 2 5% 6% 70%	1854
	11th to 20th	Day	6% 3 1 3 3 3 3 2 4 5% 66%	1890
	21st to end	Day	7% 5% 2 4 3 3 4 3 5% 7% 57%	1890
December	1st to 10th	Day	5% 3 1 2 2 2 3 3 5% 7% 68%	3040
	11th to 20th	Day	9% 4 1 4 4 4 6% 4 6% 7% 52%	3499
	21st to end	Day	10% 6% 2 5% 5% 5% 5% 4 6% 7% 44%	4609
January	1st to 10th	Night	12% 6% 2 4 2 3 3 2 4 5% 57%	7605
	11th to 20th	Night	4 4 1 3 3 2 2 2 4 6% 69%	7365
	21st to end	Night	13% 8% 2 5% 3 4 3 3 5% 5% 47%	7747
February	1st to 10th	Night	10% 7% 2 4 2 2 3 3 4 5% 58%	2520
	11th to 20th	Night	12% 9% 2 5% 4 3 3 2 4 5% 52%	2458
	21st to end	Night	12% 8% 3 4 3 3 4 3 3 6% 51%	2076
March	1st to 10th	Night	16% 17% 2 5% 3 3 2 2 5% 6% 39%	5496
	11th to 20th	Night	11% 13% 1 5% 3 3 4 2 4 6% 47%	5338
	21st to end	Night	9% 14% 3 6% 4 3 3 3 5% 6% 44%	5308
April	1st to 10th	Night	8% 10% 2 4 3 2 3 3 5% 7% 52%	2100
	11th to 20th	Night	7% 9% 2 6% 4 2 3 3 4 5% 56%	2079
	21st to end	Night	7% 8% 2 4 3 4 4 2 4 6% 55%	2100
May	1st to 10th	Night	7% 8% 2 4 5% 2 2 2 3 4 61%	5508
	11th to 20th	Night	4 10% 3 6% 4 3 3 2 4 5% 56%	5296
	21st to end	Night	3 12% 2 7% 6% 3 3 2 3 5% 54%	5837
June	1st to 10th	Night	2 6% 1 4 4 4 4 3 4 7% 60%	1890
	11th to 20th	Night	1 4 1 3 3 3 2 2 4 5% 72%	1890
	21st to end	Night	2 2 1 2 2 2 3 2 4 5% 76%	1890
July	1st to 10th	Night	1 1 1 1 1 1 1 3 90%	4190
	11th to 20th	Night	1 3 3 3 2 4 5% 78%	4145
	21st to end	Night	1 1 3 3 2 2 2 3 4 79%	4716
August	1st to 10th	Night	1 1 1 1 1 2 1 3 5% 85%	4486
	11th to 20th	Night	1 1 1 1 3 4 86%	4506
	21st to end	Night	2 2 1 1 2 2 2 3 4 81%	5015
September	1st to 10th	Night	2 2 3 2 3 3 2 4 5% 73%	2019
	11th to 20th	Night	3 3 1 4 2 2 2 2 3 3 76%	2200
	21st to end	Night	3 3 1 2 2 2 1 2 3 3 77%	2200
October	1st to 10th	Night	3 2 1 2 2 2 2 2 2 4 79%	3600
	11th to 20th	Night	5% 5% 1 4 6% 3 2 1 4 5% 64%	3579
	21st to end	Night	4 4 1 3 2 3 3 2 4 5% 70%	4420
November	1st to 10th	Night	5% 4 1 3 2 3 3 3 4 5% 68%	2665
	11th to 20th	Night	5% 5% 2 4 3 3 2 3 3 5% 65%	2730
	21st to end	Night	7% 5% 2 4 4 3 4 4 6% 7% 54%	2730
December	1st to 10th	Night	5% 3 1 3 2 2 3 3 5% 5% 68%	4370
	11th to 20th	Night	9% 6% 2 4 3 3 5% 4 6% 7% 50%	4941
	21st to end	Night	9% 7% 2 6% 4 3 4 4 6% 10% 45%	6611

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	27% 3 5% 14% 24% 27%	5321
	11th to 20th	Day	15% 2 6% 14% 20% 43%	5637
	21st to end	Day	31% 4 6% 10% 23% 26%	6500
February	1st to 10th	Day	31% 3 4 13% 21% 28%	2100
	11th to 20th	Day	32% 4 6% 14% 25% 19%	2060
	21st to end	Day	36% 3 5% 13% 21% 21%	1730
March	1st to 10th	Day	36% 4 7% 18% 20% 15%	4618
	11th to 20th	Day	29% 5% 9% 22% 22% 14%	4880
	21st to end	Day	29% 5% 9% 30% 18% 10%	5892
April	1st to 10th	Day	28% 4 8% 22% 24% 14%	2520
	11th to 20th	Day	23% 4 6% 19% 29% 19%	2492
	21st to end	Day	22% 3 7% 23% 33% 11%	2520
May	1st to 10th	Day	20% 4 5% 19% 37% 15%	7432
	11th to 20th	Day	14% 4 6% 19% 39% 18%	7599
	21st to end	Day	15% 4 7% 21% 40% 13%	8371
June	1st to 10th	Day	17% 3 5% 21% 33% 20%	2730
	11th to 20th	Day	12% 2 4 10% 29% 44%	2730
	21st to end	Day	10% 2 3 6% 24% 55%	2730
July	1st to 10th	Day	5% 1 1 5% 17% 71%	5946
	11th to 20th	Day	7% 3 5% 11% 38% 36%	5877
	21st to end	Day	6% 3 5% 17% 30% 39%	5975
August	1st to 10th	Day	8% 2 5% 16% 40% 30%	3890
	11th to 20th	Day	6% 1 5% 12% 25% 52%	3880
	21st to end	Day	7% 2 2 7% 24% 58%	4311
September	1st to 10th	Day	11% 3 4 8% 34% 40%	2380
	11th to 20th	Day	9% 2 2 7% 35% 44%	2200
	21st to end	Day	13% 2 3 11% 30% 41%	2200
October	1st to 10th	Day	12% 2 3 9% 18% 57%	3600
	11th to 20th	Day	12% 3 5% 11% 29% 41%	3579
	21st to end	Day	15% 3 2 10% 30% 40%	3378
November	1st to 10th	Day	17% 3 4 14% 24% 38%	1854
	11th to 20th	Day	21% 3 5% 11% 22% 38%	1890
	21st to end	Day	26% 3 7% 12% 20% 33%	1890
December	1st to 10th	Day	17% 2 5% 11% 26% 39%	3040
	11th to 20th	Day	30% 4 7% 13% 24% 22%	3498
	21st to end	Day	36% 5% 7% 21% 22% 9%	4609
January	1st to 10th	Night	31% 3 4 12% 24% 26%	7605
	11th to 20th	Night	17% 3 5% 15% 16% 44%	7365
	21st to end	Night	38% 3 5% 8% 20% 27%	7747
February	1st to 10th	Night	29% 3 6% 12% 21% 29%	2520
	11th to 20th	Night	36% 3 5% 12% 21% 23%	2459
	21st to end	Night	35% 4 3 14% 21% 22%	2076
March	1st to 10th	Night	46% 5% 7% 12% 17% 14%	5496
	11th to 20th	Night	37% 5% 7% 20% 20% 11%	5338
	21st to end	Night	39% 6% 8% 16% 20% 12%	5308
April	1st to 10th	Night	31% 4 8% 19% 21% 16%	2100
	11th to 20th	Night	31% 3 8% 16% 23% 20%	2079
	21st to end	Night	30% 4 9% 18% 24% 14%	2100
May	1st to 10th	Night	25% 7% 7% 20% 30% 12%	5506
	11th to 20th	Night	29% 5% 9% 15% 28% 14%	5297
	21st to end	Night	28% 7% 9% 16% 30% 11%	5838
June	1st to 10th	Night	22% 5% 9% 16% 28% 20%	1890
	11th to 20th	Night	14% 2 6% 11% 25% 41%	1890
	21st to end	Night	11% 3 5% 7% 23% 51%	1890
July	1st to 10th	Night	4 1 1 7% 15% 72%	4145
	11th to 20th	Night	5% 4 7% 13% 41% 30%	4091
	21st to end	Night	7% 4 8% 16% 30% 35%	4582
August	1st to 10th	Night	6% 2 4 18% 43% 28%	3256
	11th to 20th	Night	5% 2 5% 15% 24% 49%	3256
	21st to end	Night	6% 1 4 7% 21% 61%	3608
September	1st to 10th	Night	14% 2 5% 11% 26% 43%	2019
	11th to 20th	Night	14% 2 4 8% 29% 43%	2200
	21st to end	Night	14% 2 4 10% 32% 38%	2200
October	1st to 10th	Night	13% 2 4 10% 17% 56%	3600
	11th to 20th	Night	18% 6% 6% 8% 21% 42%	3579
	21st to end	Night	18% 3 3 9% 25% 43%	4420
November	1st to 10th	Night	20% 3 4 13% 21% 40%	2665
	11th to 20th	Night	23% 2 4 11% 20% 39%	2730
	21st to end	Night	28% 4 7% 13% 16% 32%	2730
December	1st to 10th	Night	19% 2 4 12% 23% 39%	4370
	11th to 20th	Night	31% 4 7% 12% 20% 26%	4941
	21st to end	Night	33% 4 9% 21% 23% 10%	6611

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	19% 6% 32% 30% 12% 1	330
	11th to 20th	Day	12% 9% 27% 38% 13% 2	432
	21st to end	Day	13% 12% 32% 31% 11% 1	594
February	1st to 10th	Day	14% 12% 33% 29% 11% 3	540
	11th to 20th	Day	18% 15% 31% 28% 8%	535
	21st to end	Day	16% 15% 38% 24% 7%	446
March	1st to 10th	Day	17% 18% 35% 22% 7% 1	540
	11th to 20th	Day	15% 12% 34% 26% 11% 2	540
	21st to end	Day	12% 16% 36% 26% 9% 2	594
April	1st to 10th	Day	13% 13% 30% 29% 12% 3	540
	11th to 20th	Day	8% 19% 34% 28% 9% 2	532
	21st to end	Day	10% 14% 33% 26% 13% 4	540
May	1st to 10th	Day	13% 13% 32% 27% 12% 4	543
	11th to 20th	Day	8% 11% 32% 33% 13% 3	568
	21st to end	Day	8% 10% 34% 32% 13% 3	617
June	1st to 10th	Day	3 5% 28% 39% 19% 6%	570
	11th to 20th	Day	3 3 29% 42% 18% 5%	570
	21st to end	Day	2 3 26% 42% 19% 7%	570
July	1st to 10th	Day	2 1 15% 50% 20% 11%	566
	11th to 20th	Day	2 1 10% 50% 23% 14%	545
	21st to end	Day	2 1 11% 50% 24% 12%	575
August	1st to 10th	Day	2 17% 49% 20% 12%	527
	11th to 20th	Day	1 1 13% 50% 19% 16%	517
	21st to end	Day	2 2 17% 49% 20% 11%	583
September	1st to 10th	Day	3 2 25% 45% 17% 8%	530
	11th to 20th	Day	3 6% 28% 44% 16% 3	530
	21st to end	Day	5% 7% 30% 36% 17% 5%	530
October	1st to 10th	Day	5% 7% 32% 35% 17% 3	530
	11th to 20th	Day	7% 8% 32% 36% 15% 2	530
	21st to end	Day	5% 7% 26% 39% 19% 3	473
November	1st to 10th	Day	8% 7% 22% 37% 24% 3	330
	11th to 20th	Day	8% 7% 24% 40% 17% 3	330
	21st to end	Day	10% 9% 32% 33% 12% 4	330
December	1st to 10th	Day	10% 5% 29% 35% 17% 3	330
	11th to 20th	Day	6% 6% 28% 38% 19% 4	330
	21st to end	Day	21% 11% 27% 28% 12% 2	363
January	1st to 10th	Night	12% 4 24% 35% 20% 5%	840
	11th to 20th	Night	9% 4 19% 43% 20% 6%	840
	21st to end	Night	14% 6% 19% 41% 18% 3	924
February	1st to 10th	Night	13% 4 17% 39% 22% 6%	840
	11th to 20th	Night	14% 4 20% 36% 20% 5%	824
	21st to end	Night	15% 6% 16% 41% 18% 4	692
March	1st to 10th	Night	16% 6% 17% 41% 18% 2	840
	11th to 20th	Night	14% 3 11% 41% 25% 7%	840
	21st to end	Night	11% 5% 15% 39% 26% 5%	924
April	1st to 10th	Night	13% 3 13% 43% 24% 4	840
	11th to 20th	Night	11% 5% 13% 40% 25% 5%	831
	21st to end	Night	11% 4 12% 38% 29% 6%	840
May	1st to 10th	Night	10% 4 11% 38% 31% 5%	700
	11th to 20th	Night	9% 2 13% 32% 36% 7%	621
	21st to end	Night	10% 5% 12% 36% 33% 4	683
June	1st to 10th	Night	3 2 11% 39% 38% 7%	630
	11th to 20th	Night	2 1 7% 43% 42% 5%	630
	21st to end	Night	2 1 6% 38% 46% 7%	630
July	1st to 10th	Night	1 30% 57% 11%	615
	11th to 20th	Night	1 1 20% 66% 11%	597
	21st to end	Night	1 2 1 19% 60% 18%	761
August	1st to 10th	Night	1 1 26% 55% 16%	796
	11th to 20th	Night	1 1 17% 61% 20%	792
	21st to end	Night	2 1 17% 59% 22%	880
September	1st to 10th	Night	2 1 2 26% 55% 15%	800
	11th to 20th	Night	4 2 4 33% 47% 11%	800
	21st to end	Night	4 1 8% 35% 40% 12%	800
October	1st to 10th	Night	6% 2 6% 44% 35% 7%	800
	11th to 20th	Night	7% 3 11% 40% 30% 10%	800
	21st to end	Night	7% 1 11% 36% 31% 13%	880
November	1st to 10th	Night	7% 2 11% 43% 28% 9%	820
	11th to 20th	Night	8% 2 13% 42% 26% 10%	840
	21st to end	Night	10% 4 17% 40% 22% 7%	840
December	1st to 10th	Night	11% 2 17% 42% 21% 6%	840
	11th to 20th	Night	15% 3 17% 42% 20% 4	840
	21st to end	Night	14% 5% 20% 37% 19% 5%	924

Page generated at 2026-07-13 00:14:54. Version ac07

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