Peralta Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Peralta Skies Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	4 2 1 3 2 2 3 2 4 6% 71%	141016
	Night	5% 2 1 3 2 3 3 3 4 6% 68%	139045
Transparency	Day	17% 2 3 12% 26% 40%	135886
	Night	19% 2 3 11% 25% 40%	135177
Seeing	Day	5% 4 15% 32% 31% 13%	16886
	Night	5% 2 13% 43% 33% 4	27093

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	8% 5% 2 5% 3 3 3 3 4 5% 60%	5318
	11th to 20th	Day	4 2 1 2 2 2 2 2 2 4 79%	5340
	21st to end	Day	8% 5% 2 4 3 4 4 3 5% 6% 56%	5844
February	1st to 10th	Day	5% 4 1 3 2 2 3 3 4 6% 64%	1943
	11th to 20th	Day	9% 3 1 4 4 3 3 3 4 6% 59%	2050
	21st to end	Day	7% 4 1 3 3 3 4 3 4 6% 63%	1781
March	1st to 10th	Day	7% 3 1 3 3 3 3 2 4 5% 66%	5064
	11th to 20th	Day	7% 4 1 4 3 3 3 3 4 6% 62%	5110
	21st to end	Day	7% 4 2 4 3 3 3 2 3 4 65%	5607
April	1st to 10th	Day	3 2 1 2 2 3 3 2 4 8% 70%	2310
	11th to 20th	Day	2 3 1 2 2 2 2 2 3 5% 76%	2428
	21st to end	Day	2 2 2 1 2 2 2 3 5% 79%	2641
May	1st to 10th	Day	2 1 1 2 2 2 2 2 2 3 81%	5894
	11th to 20th	Day	1 1 2 1 2 2 2 3 4 82%	5841
	21st to end	Day	1 1 1 2 1 2 2 2 2 4 82%	6423
June	1st to 10th	Day	1 2 2 1 2 3 2 3 6% 79%	2600
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 88%	2600
	21st to end	Day	2 2 1 2 2 2 2 2 4 5% 76%	2600
July	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 77%	5879
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 9% 63%	5880
	21st to end	Day	3 2 1 3 2 3 4 4 7% 10% 61%	6401
August	1st to 10th	Day	2 2 1 2 2 3 3 4 7% 10% 64%	5779
	11th to 20th	Day	2 2 1 3 2 3 4 4 6% 9% 65%	5798
	21st to end	Day	2 2 1 3 2 3 3 3 6% 8% 67%	5927
September	1st to 10th	Day	4 3 1 3 2 3 3 3 5% 7% 65%	2200
	11th to 20th	Day	3 1 2 2 2 2 2 4 5% 77%	2200
	21st to end	Day	3 1 1 2 2 3 3 2 3 4 76%	2200
October	1st to 10th	Day	2 1 1 1 2 1 2 2 3 4 81%	3484
	11th to 20th	Day	4 1 2 1 1 1 1 2 3 84%	3250
	21st to end	Day	1 1 1 1 1 1 1 2 4 5% 81%	3548
November	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 75%	2059
	11th to 20th	Day	2 3 1 3 2 3 2 3 4 5% 72%	1979
	21st to end	Day	6% 3 1 5% 3 2 3 3 4 5% 64%	1890
December	1st to 10th	Day	4 2 1 2 2 2 2 2 3 4 77%	3040
	11th to 20th	Day	5% 3 2 4 3 3 2 4 7% 8% 60%	3499
	21st to end	Day	8% 4 1 4 2 3 3 2 4 6% 62%	4609
January	1st to 10th	Night	9% 5% 2 4 3 3 3 3 4 6% 58%	7605
	11th to 20th	Night	3 1 2 1 2 2 1 2 4 82%	7650
	21st to end	Night	11% 5% 2 5% 3 4 4 3 4 5% 55%	8370
February	1st to 10th	Night	7% 3 1 4 3 2 3 3 5% 5% 65%	2672
	11th to 20th	Night	9% 5% 1 4 4 3 3 3 5% 5% 59%	2458
	21st to end	Night	8% 4 1 5% 3 3 2 3 4 6% 62%	2024
March	1st to 10th	Night	7% 3 1 3 2 2 2 2 5% 6% 67%	5050
	11th to 20th	Night	10% 3 1 3 2 3 3 3 4 5% 63%	5110
	21st to end	Night	5% 2 1 3 2 3 2 3 5% 8% 66%	5610
April	1st to 10th	Night	4 3 1 3 2 3 4 3 5% 6% 68%	2310
	11th to 20th	Night	3 2 1 2 2 2 3 2 3 4 75%	2140
	21st to end	Night	2 1 1 1 2 2 2 3 4 80%	1974
May	1st to 10th	Night	3 3 1 3 2 2 2 2 3 4 76%	4109
	11th to 20th	Night	1 1 1 3 1 2 2 2 3 3 82%	4074
	21st to end	Night	1 1 1 1 1 2 1 2 3 85%	4474
June	1st to 10th	Night	2 2 1 2 2 2 2 2 4 6% 77%	1800
	11th to 20th	Night	1 1 1 1 2 2 2 4 87%	1800
	21st to end	Night	2 2 1 3 3 3 3 3 3 5% 74%	1800
July	1st to 10th	Night	2 2 1 4 3 3 3 3 4 6% 70%	4100
	11th to 20th	Night	1 2 1 4 4 4 5% 5% 8% 10% 57%	4100
	21st to end	Night	2 2 1 4 4 4 6% 6% 9% 13% 49%	4472
August	1st to 10th	Night	1 2 1 4 3 5% 5% 6% 9% 11% 52%	4033
	11th to 20th	Night	1 2 1 4 3 4 5% 4 8% 10% 58%	4051
	21st to end	Night	2 2 1 5% 3 3 5% 4 6% 10% 58%	5051
September	1st to 10th	Night	4 3 1 5% 3 3 4 4 6% 7% 60%	2200
	11th to 20th	Night	3 2 1 3 2 2 3 2 3 4 74%	2200
	21st to end	Night	2 2 1 3 2 3 3 2 4 5% 73%	2200
October	1st to 10th	Night	2 1 2 2 1 2 2 3 5% 79%	3710
	11th to 20th	Night	2 1 1 1 1 1 1 2 3 86%	3898
	21st to end	Night	3 1 1 1 1 2 2 3 4 80%	4252
November	1st to 10th	Night	3 2 1 3 2 2 3 2 4 5% 75%	2460
	11th to 20th	Night	3 2 1 3 2 2 3 2 3 4 74%	2636
	21st to end	Night	6% 3 2 3 3 2 3 3 4 6% 66%	2730
December	1st to 10th	Night	5% 1 1 2 2 2 2 2 4 5% 74%	4370
	11th to 20th	Night	6% 3 1 5% 3 3 3 2 4 5% 64%	4941
	21st to end	Night	8% 3 1 3 3 3 4 3 3 4 64%	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	28% 1 5% 12% 34% 20%	5321
	11th to 20th	Day	14% 1 9% 28% 48%	5340
	21st to end	Day	30% 2 3 9% 24% 32%	5844
February	1st to 10th	Day	22% 2 3 15% 23% 36%	1943
	11th to 20th	Day	28% 2 3 14% 27% 26%	2060
	21st to end	Day	25% 2 1 14% 27% 32%	1781
March	1st to 10th	Day	22% 2 1 11% 36% 28%	5064
	11th to 20th	Day	25% 2 2 17% 27% 27%	5110
	21st to end	Day	26% 1 1 12% 34% 27%	5607
April	1st to 10th	Day	16% 2 1 8% 35% 39%	2310
	11th to 20th	Day	14% 2 1 7% 27% 50%	2431
	21st to end	Day	10% 1 1 8% 27% 52%	2641
May	1st to 10th	Day	11% 1 2 8% 22% 55%	5894
	11th to 20th	Day	9% 1 1 5% 23% 62%	5844
	21st to end	Day	9% 1 1 3 12% 74%	6424
June	1st to 10th	Day	10% 2 4 21% 63%	2600
	11th to 20th	Day	6% 1 1 11% 80%	2600
	21st to end	Day	13% 2 2 7% 16% 61%	2600
July	1st to 10th	Day	12% 2 3 7% 20% 56%	5816
	11th to 20th	Day	17% 3 7% 27% 30% 15%	5811
	21st to end	Day	17% 3 10% 26% 18% 27%	6217
August	1st to 10th	Day	19% 4 13% 29% 17% 18%	4208
	11th to 20th	Day	17% 3 7% 23% 29% 20%	4197
	21st to end	Day	16% 3 8% 17% 26% 30%	4266
September	1st to 10th	Day	20% 2 5% 15% 25% 32%	2200
	11th to 20th	Day	11% 2 2 9% 31% 45%	2200
	21st to end	Day	14% 2 2 8% 23% 51%	2200
October	1st to 10th	Day	10% 1 2 7% 15% 64%	3484
	11th to 20th	Day	9% 1 1 5% 28% 56%	3250
	21st to end	Day	8% 1 1 6% 25% 59%	3548
November	1st to 10th	Day	12% 2 11% 36% 39%	2059
	11th to 20th	Day	16% 2 1 11% 25% 44%	1979
	21st to end	Day	24% 2 2 12% 25% 34%	1890
December	1st to 10th	Day	15% 1 1 13% 40% 30%	3040
	11th to 20th	Day	22% 1 4 12% 29% 32%	3498
	21st to end	Day	26% 1 3 20% 34% 17%	4609
January	1st to 10th	Night	29% 2 4 11% 35% 20%	7605
	11th to 20th	Night	11% 1 9% 31% 47%	7650
	21st to end	Night	33% 1 3 8% 25% 30%	8370
February	1st to 10th	Night	22% 1 3 11% 24% 38%	2672
	11th to 20th	Night	28% 2 3 16% 26% 26%	2459
	21st to end	Night	25% 2 1 10% 28% 34%	2024
March	1st to 10th	Night	20% 2 1 12% 35% 30%	5050
	11th to 20th	Night	25% 2 2 13% 31% 27%	5110
	21st to end	Night	19% 1 1 11% 38% 29%	5610
April	1st to 10th	Night	18% 2 1 8% 30% 41%	2310
	11th to 20th	Night	15% 2 1 6% 25% 52%	2140
	21st to end	Night	10% 1 1 6% 26% 56%	1974
May	1st to 10th	Night	14% 2 2 7% 25% 51%	4106
	11th to 20th	Night	10% 1 1 4 21% 63%	4075
	21st to end	Night	7% 1 1 2 10% 78%	4474
June	1st to 10th	Night	12% 1 3 19% 65%	1800
	11th to 20th	Night	5% 1 3 12% 79%	1800
	21st to end	Night	15% 2 3 6% 12% 64%	1800
July	1st to 10th	Night	17% 2 3 7% 17% 54%	4055
	11th to 20th	Night	20% 4 7% 24% 29% 16%	4046
	21st to end	Night	23% 4 10% 22% 12% 28%	4344
August	1st to 10th	Night	26% 6% 15% 23% 12% 18%	2928
	11th to 20th	Night	20% 4 9% 21% 23% 22%	2928
	21st to end	Night	21% 4 7% 14% 20% 34%	3639
September	1st to 10th	Night	22% 3 6% 18% 21% 30%	2200
	11th to 20th	Night	16% 2 2 9% 25% 46%	2200
	21st to end	Night	16% 3 2 8% 23% 49%	2200
October	1st to 10th	Night	10% 1 2 8% 15% 64%	3710
	11th to 20th	Night	7% 1 1 8% 21% 63%	3898
	21st to end	Night	10% 1 1 6% 18% 63%	4252
November	1st to 10th	Night	14% 1 1 8% 29% 46%	2460
	11th to 20th	Night	17% 1 1 11% 24% 45%	2636
	21st to end	Night	22% 1 2 11% 29% 34%	2730
December	1st to 10th	Night	15% 2 2 15% 32% 35%	4370
	11th to 20th	Night	25% 1 2 10% 26% 37%	4941
	21st to end	Night	25% 1 2 19% 31% 22%	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	13% 4 16% 39% 23% 5%	330
	11th to 20th	Day	11% 2 14% 40% 27% 6%	330
	21st to end	Day	5% 5% 20% 43% 20% 7%	363
February	1st to 10th	Day	7% 5% 20% 38% 25% 5%	383
	11th to 20th	Day	10% 6% 25% 39% 19% 2	535
	21st to end	Day	8% 7% 25% 40% 18% 2	446
March	1st to 10th	Day	6% 10% 26% 38% 19% 2	540
	11th to 20th	Day	8% 6% 22% 33% 25% 6%	540
	21st to end	Day	6% 8% 23% 37% 21% 6%	594
April	1st to 10th	Day	5% 5% 23% 34% 22% 9%	540
	11th to 20th	Day	3 8% 26% 36% 21% 7%	532
	21st to end	Day	1 8% 26% 33% 24% 9%	540
May	1st to 10th	Day	2 6% 23% 34% 25% 9%	508
	11th to 20th	Day	2 4 21% 39% 25% 10%	528
	21st to end	Day	1 5% 18% 38% 30% 8%	573
June	1st to 10th	Day	2 2 14% 32% 36% 13%	530
	11th to 20th	Day	2 2 15% 33% 35% 14%	530
	21st to end	Day	2 2 8% 34% 41% 13%	530
July	1st to 10th	Day	2 4 30% 41% 22%	526
	11th to 20th	Day	4 1 3 22% 40% 30%	524
	21st to end	Day	4 1 2 18% 43% 32%	575
August	1st to 10th	Day	3 1 2 21% 42% 31%	527
	11th to 20th	Day	3 2 1 20% 43% 31%	517
	21st to end	Day	1 1 3 18% 44% 33%	583
September	1st to 10th	Day	5% 1 6% 26% 42% 20%	530
	11th to 20th	Day	4 1 9% 27% 43% 16%	530
	21st to end	Day	3 3 11% 32% 34% 17%	530
October	1st to 10th	Day	3 2 14% 31% 34% 15%	466
	11th to 20th	Day	5% 2 15% 27% 37% 14%	330
	21st to end	Day	2 1 13% 34% 36% 13%	363
November	1st to 10th	Day	5% 3 13% 33% 35% 12%	330
	11th to 20th	Day	4 3 12% 36% 34% 10%	330
	21st to end	Day	6% 2 18% 37% 30% 6%	330
December	1st to 10th	Day	8% 2 13% 38% 30% 9%	330
	11th to 20th	Day	4 3 15% 42% 28% 8%	330
	21st to end	Day	15% 5% 20% 32% 23% 6%	363
January	1st to 10th	Night	8% 2 17% 48% 22% 3	840
	11th to 20th	Night	8% 2 17% 46% 24% 3	840
	21st to end	Night	11% 4 19% 45% 18% 3	924
February	1st to 10th	Night	8% 3 17% 47% 23% 2	840
	11th to 20th	Night	10% 2 21% 48% 18% 1	824
	21st to end	Night	9% 3 19% 49% 18% 1	692
March	1st to 10th	Night	7% 3 24% 49% 16% 2	840
	11th to 20th	Night	8% 3 16% 41% 26% 6%	840
	21st to end	Night	4 4 17% 46% 28% 2	924
April	1st to 10th	Night	4 2 17% 51% 24% 2	840
	11th to 20th	Night	4 2 23% 49% 21% 1	686
	21st to end	Night	3 2 18% 52% 22% 3	630
May	1st to 10th	Night	2 3 14% 51% 26% 3	603
	11th to 20th	Night	1 2 14% 53% 27% 3	591
	21st to end	Night	3 1 12% 52% 29% 3	650
June	1st to 10th	Night	2 1 6% 50% 39% 2	600
	11th to 20th	Night	1 7% 54% 36% 2	600
	21st to end	Night	2 1 6% 38% 49% 4	600
July	1st to 10th	Night	2 3 4 34% 53% 5%	585
	11th to 20th	Night	3 1 2 22% 65% 8%	582
	21st to end	Night	4 1 3 17% 63% 11%	657
August	1st to 10th	Night	2 1 2 20% 65% 10%	597
	11th to 20th	Night	3 2 3 21% 63% 8%	594
	21st to end	Night	3 1 2 24% 61% 9%	730
September	1st to 10th	Night	5% 1 4 30% 51% 8%	800
	11th to 20th	Night	3 1 8% 41% 42% 5%	800
	21st to end	Night	2 2 8% 41% 40% 6%	800
October	1st to 10th	Night	3 1 11% 47% 35% 3	800
	11th to 20th	Night	3 2 10% 48% 34% 3	800
	21st to end	Night	2 2 13% 43% 34% 7%	880
November	1st to 10th	Night	4 1 11% 48% 31% 5%	820
	11th to 20th	Night	3 2 15% 47% 30% 3	840
	21st to end	Night	6% 2 17% 46% 26% 3	840
December	1st to 10th	Night	7% 3 14% 46% 28% 2	840
	11th to 20th	Night	11% 2 12% 46% 27% 2	840
	21st to end	Night	10% 2 16% 47% 22% 3	924

Page generated at 2026-04-28 23:21:42. Version ac07

Copyright © 2026 Attilla Danko.