Cerro Alto Campground
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cerro Alto Campground 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	4 3 1 3 2 2 3 3 4 6% 68%	146002
	Night	5% 4 2 5% 3 3 3 3 5% 6% 60%	144192
Transparency	Day	18% 4 8% 19% 28% 24%	141025
	Night	23% 5% 9% 18% 25% 21%	140151
Seeing	Day	6% 8% 22% 35% 23% 6%	17803
	Night	6% 3 14% 36% 33% 7%	27355

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	9% 4 2 5% 3 3 4 3 6% 7% 53%	5064
	11th to 20th	Day	6% 4 1 3 2 3 3 3 5% 6% 64%	5340
	21st to end	Day	7% 6% 2 6% 4 4 5% 4 6% 8% 48%	5843
February	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 5% 52%	1953
	11th to 20th	Day	10% 6% 3 6% 4 3 4 3 5% 8% 50%	2050
	21st to end	Day	9% 6% 2 5% 3 4 4 3 6% 6% 51%	1730
March	1st to 10th	Day	11% 7% 2 7% 4 4 5% 4 6% 9% 41%	4787
	11th to 20th	Day	7% 5% 2 6% 4 3 3 3 4 6% 57%	5110
	21st to end	Day	7% 6% 3 7% 4 5% 6% 5% 7% 9% 41%	5831
April	1st to 10th	Day	7% 5% 2 4 3 3 4 4 6% 7% 55%	2520
	11th to 20th	Day	6% 6% 2 4 4 3 3 3 6% 7% 57%	2489
	21st to end	Day	6% 3 2 3 3 3 3 3 6% 6% 61%	2520
May	1st to 10th	Day	2 2 1 2 2 2 2 2 3 5% 76%	7058
	11th to 20th	Day	1 2 1 3 2 2 3 2 4 7% 75%	7307
	21st to end	Day	2 2 1 3 2 2 2 2 4 7% 74%	8992
June	1st to 10th	Day	2 3 1 2 3 2 2 3 5% 7% 69%	2940
	11th to 20th	Day	1 2 1 2 1 2 2 2 3 5% 81%	2940
	21st to end	Day	1 1 2 2 1 2 1 3 4 83%	2940
July	1st to 10th	Day	1 1 1 1 1 2 4 89%	6459
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 4 81%	5953
	21st to end	Day	1 1 2 2 1 2 2 4 6% 79%	6402
August	1st to 10th	Day	1 1 1 1 1 1 1 3 5% 84%	5550
	11th to 20th	Day	1 1 1 1 1 1 3 90%	5364
	21st to end	Day	1 1 1 1 2 2 2 3 5% 83%	5984
September	1st to 10th	Day	1 1 1 1 1 1 2 1 3 5% 83%	2400
	11th to 20th	Day	2 2 1 2 2 2 2 2 2 5% 78%	2400
	21st to end	Day	2 3 1 3 2 2 3 3 4 5% 74%	2191
October	1st to 10th	Day	2 1 1 2 2 2 2 3 4 5% 76%	3280
	11th to 20th	Day	2 2 1 3 2 2 3 3 3 4 75%	3261
	21st to end	Day	2 3 2 4 2 3 3 3 4 7% 67%	3558
November	1st to 10th	Day	5% 4 1 3 3 2 3 3 4 6% 66%	1985
	11th to 20th	Day	7% 4 1 3 3 3 4 3 5% 5% 61%	1890
	21st to end	Day	7% 5% 2 5% 4 2 3 3 4 5% 60%	1890
December	1st to 10th	Day	4 5% 2 6% 4 3 4 3 5% 8% 55%	2821
	11th to 20th	Day	15% 6% 2 5% 3 3 4 4 6% 7% 46%	3108
	21st to end	Day	11% 8% 2 7% 4 5% 5% 5% 7% 10% 36%	4092
January	1st to 10th	Night	10% 7% 2 6% 3 3 3 2 4 6% 53%	7830
	11th to 20th	Night	6% 5% 2 4 3 3 3 3 5% 6% 62%	7650
	21st to end	Night	10% 7% 2 7% 4 6% 5% 4 5% 7% 44%	8371
February	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 54%	2664
	11th to 20th	Night	9% 5% 1 4 3 4 4 3 6% 7% 55%	2458
	21st to end	Night	8% 6% 2 6% 4 4 3 2 4 6% 55%	2076
March	1st to 10th	Night	10% 10% 3 7% 4 5% 5% 4 5% 7% 41%	5323
	11th to 20th	Night	6% 6% 2 5% 4 3 4 3 5% 6% 56%	5110
	21st to end	Night	11% 9% 3 7% 3 3 3 3 5% 6% 48%	5372
April	1st to 10th	Night	6% 6% 1 5% 3 2 4 3 5% 7% 58%	2100
	11th to 20th	Night	5% 5% 2 6% 3 3 3 2 5% 5% 63%	2079
	21st to end	Night	4 6% 2 5% 3 3 3 2 4 7% 62%	2100
May	1st to 10th	Night	1 3 1 5% 3 3 2 3 5% 6% 68%	5919
	11th to 20th	Night	1 2 3 7% 4 3 3 3 4 5% 66%	5562
	21st to end	Night	1 3 2 7% 5% 3 3 3 6% 6% 61%	5181
June	1st to 10th	Night	2 2 2 4 4 3 4 4 5% 7% 63%	1680
	11th to 20th	Night	1 4 1 3 3 2 3 2 4 6% 71%	1680
	21st to end	Night	1 2 1 4 3 2 2 1 4 6% 75%	1680
July	1st to 10th	Night	2 2 2 2 2 5% 5% 79%	3720
	11th to 20th	Night	1 2 3 3 3 4 4 8% 6% 65%	4132
	21st to end	Night	1 1 2 12% 7% 3 3 4 6% 6% 54%	4472
August	1st to 10th	Night	1 1 2 1 2 2 2 5% 6% 79%	4259
	11th to 20th	Night	1 1 1 1 2 5% 5% 83%	4506
	21st to end	Night	2 2 2 2 2 5% 6% 78%	5015
September	1st to 10th	Night	1 2 1 3 2 2 2 2 3 4 79%	2000
	11th to 20th	Night	2 3 1 4 2 2 3 2 3 4 74%	2000
	21st to end	Night	1 3 1 2 2 2 2 2 4 4 77%	2203
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 78%	3920
	11th to 20th	Night	2 2 1 4 3 4 4 3 4 5% 69%	3897
	21st to end	Night	3 4 2 3 2 2 3 2 4 5% 71%	4253
November	1st to 10th	Night	4 4 1 4 2 2 3 3 4 6% 68%	2531
	11th to 20th	Night	5% 4 2 4 3 3 4 3 5% 6% 63%	2730
	21st to end	Night	4 5% 2 5% 3 3 4 4 6% 6% 58%	2730
December	1st to 10th	Night	6% 4 1 4 3 3 4 3 5% 6% 60%	4567
	11th to 20th	Night	13% 7% 2 5% 4 4 5% 5% 6% 8% 41%	5316
	21st to end	Night	10% 8% 3 9% 6% 6% 5% 5% 6% 8% 32%	7106

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% 6% 13% 28% 17% 9%	5065
	11th to 20th	Day	22% 4 7% 11% 21% 35%	5340
	21st to end	Day	30% 6% 10% 15% 28% 12%	5843
February	1st to 10th	Day	32% 4 7% 19% 21% 17%	1953
	11th to 20th	Day	32% 5% 10% 22% 22% 9%	2060
	21st to end	Day	32% 5% 12% 21% 22% 7%	1730
March	1st to 10th	Day	34% 8% 13% 16% 25% 5%	4787
	11th to 20th	Day	28% 5% 10% 32% 16% 9%	5110
	21st to end	Day	31% 9% 17% 23% 16% 3	5831
April	1st to 10th	Day	27% 5% 12% 26% 23% 8%	2520
	11th to 20th	Day	25% 5% 13% 26% 24% 7%	2492
	21st to end	Day	21% 7% 12% 26% 27% 8%	2520
May	1st to 10th	Day	12% 3 7% 27% 38% 13%	7058
	11th to 20th	Day	10% 4 8% 25% 40% 14%	7310
	21st to end	Day	11% 4 10% 26% 41% 9%	8994
June	1st to 10th	Day	15% 5% 10% 26% 30% 14%	2940
	11th to 20th	Day	9% 2 7% 17% 37% 28%	2940
	21st to end	Day	8% 2 4 16% 32% 38%	2940
July	1st to 10th	Day	3 1 2 7% 23% 64%	6391
	11th to 20th	Day	8% 2 3 8% 21% 58%	5883
	21st to end	Day	7% 2 4 16% 34% 37%	6217
August	1st to 10th	Day	8% 2 3 13% 34% 41%	4034
	11th to 20th	Day	4 1 3 9% 31% 52%	3880
	21st to end	Day	6% 1 3 13% 28% 49%	4311
September	1st to 10th	Day	7% 2 4 14% 31% 41%	2400
	11th to 20th	Day	11% 2 5% 20% 37% 24%	2400
	21st to end	Day	12% 2 8% 16% 35% 27%	2191
October	1st to 10th	Day	11% 3 6% 13% 24% 43%	3280
	11th to 20th	Day	13% 2 8% 19% 37% 21%	3261
	21st to end	Day	18% 2 6% 13% 33% 27%	3558
November	1st to 10th	Day	19% 4 8% 21% 29% 18%	1985
	11th to 20th	Day	23% 4 10% 19% 25% 18%	1890
	21st to end	Day	27% 4 7% 19% 24% 20%	1890
December	1st to 10th	Day	27% 3 7% 15% 27% 21%	2821
	11th to 20th	Day	35% 6% 11% 18% 20% 10%	3108
	21st to end	Day	37% 9% 16% 21% 12% 5%	4092
January	1st to 10th	Night	31% 6% 11% 23% 21% 8%	7830
	11th to 20th	Night	22% 4 7% 13% 22% 32%	7650
	21st to end	Night	36% 6% 10% 14% 21% 14%	8371
February	1st to 10th	Night	32% 4 8% 18% 22% 16%	2664
	11th to 20th	Night	28% 5% 10% 23% 23% 11%	2459
	21st to end	Night	30% 5% 9% 23% 24% 9%	2076
March	1st to 10th	Night	38% 7% 11% 17% 22% 6%	5323
	11th to 20th	Night	27% 6% 14% 26% 18% 9%	5110
	21st to end	Night	35% 6% 15% 24% 17% 4	5372
April	1st to 10th	Night	26% 5% 14% 25% 20% 10%	2100
	11th to 20th	Night	24% 3 12% 28% 24% 9%	2079
	21st to end	Night	22% 5% 10% 27% 28% 7%	2100
May	1st to 10th	Night	15% 5% 13% 21% 34% 12%	5917
	11th to 20th	Night	15% 7% 10% 19% 34% 16%	5564
	21st to end	Night	14% 8% 12% 21% 35% 9%	5182
June	1st to 10th	Night	18% 6% 13% 22% 29% 13%	1680
	11th to 20th	Night	14% 3 6% 19% 32% 26%	1680
	21st to end	Night	11% 3 4 16% 30% 36%	1680
July	1st to 10th	Night	5% 3 2 10% 22% 58%	3680
	11th to 20th	Night	11% 5% 4 16% 18% 47%	4079
	21st to end	Night	24% 4 6% 15% 25% 25%	4345
August	1st to 10th	Night	10% 1 4 15% 32% 38%	3093
	11th to 20th	Night	5% 1 3 11% 25% 56%	3256
	21st to end	Night	7% 1 4 15% 29% 42%	3608
September	1st to 10th	Night	10% 2 3 13% 28% 44%	2000
	11th to 20th	Night	13% 3 6% 17% 34% 28%	2000
	21st to end	Night	11% 3 6% 16% 35% 29%	2203
October	1st to 10th	Night	11% 2 4 14% 23% 46%	3920
	11th to 20th	Night	15% 3 10% 18% 32% 23%	3897
	21st to end	Night	16% 2 5% 14% 36% 26%	4253
November	1st to 10th	Night	18% 4 8% 21% 27% 22%	2531
	11th to 20th	Night	21% 4 12% 18% 26% 19%	2730
	21st to end	Night	25% 5% 10% 17% 23% 20%	2730
December	1st to 10th	Night	24% 4 6% 15% 31% 19%	4567
	11th to 20th	Night	37% 6% 12% 17% 18% 11%	5316
	21st to end	Night	43% 10% 13% 19% 10% 4	7106

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% 14% 28% 27% 11% 1	341
	11th to 20th	Day	10% 10% 29% 35% 14% 1	430
	21st to end	Day	10% 12% 27% 33% 17% 2	473
February	1st to 10th	Day	16% 10% 27% 29% 15% 3	463
	11th to 20th	Day	14% 13% 30% 26% 15% 2	535
	21st to end	Day	8% 14% 33% 33% 9% 2	446
March	1st to 10th	Day	14% 17% 31% 30% 8%	540
	11th to 20th	Day	14% 11% 30% 33% 11% 1	540
	21st to end	Day	10% 18% 29% 31% 12% 1	594
April	1st to 10th	Day	11% 10% 29% 31% 15% 3	540
	11th to 20th	Day	7% 14% 27% 33% 15% 3	532
	21st to end	Day	5% 12% 30% 32% 17% 4	540
May	1st to 10th	Day	7% 9% 30% 31% 19% 4	543
	11th to 20th	Day	3 11% 23% 39% 19% 5%	568
	21st to end	Day	5% 11% 22% 36% 19% 6%	617
June	1st to 10th	Day	4 4 21% 38% 28% 6%	570
	11th to 20th	Day	1 5% 24% 38% 24% 8%	570
	21st to end	Day	1 3 19% 37% 30% 10%	570
July	1st to 10th	Day	1 13% 44% 31% 10%	566
	11th to 20th	Day	2 1 6% 39% 37% 16%	545
	21st to end	Day	8% 39% 37% 15%	575
August	1st to 10th	Day	1 1 16% 41% 33% 9%	527
	11th to 20th	Day	1 1 9% 39% 35% 15%	517
	21st to end	Day	1 1 9% 40% 37% 11%	583
September	1st to 10th	Day	1 2 13% 36% 38% 10%	530
	11th to 20th	Day	3 4 17% 39% 31% 6%	530
	21st to end	Day	2 7% 16% 35% 35% 6%	530
October	1st to 10th	Day	1 6% 22% 41% 26% 4	530
	11th to 20th	Day	3 6% 23% 38% 26% 4	530
	21st to end	Day	3 5% 22% 38% 28% 4	583
November	1st to 10th	Day	5% 7% 22% 36% 26% 5%	461
	11th to 20th	Day	11% 8% 21% 36% 19% 4	330
	21st to end	Day	8% 5% 26% 35% 23% 2	330
December	1st to 10th	Day	7% 11% 31% 34% 16% 2	262
	11th to 20th	Day	8% 12% 22% 32% 23% 3	220
	21st to end	Day	18% 16% 27% 27% 10% 2	242
January	1st to 10th	Night	15% 6% 21% 37% 18% 4	840
	11th to 20th	Night	10% 2 21% 43% 20% 4	840
	21st to end	Night	13% 3 18% 43% 20% 3	924
February	1st to 10th	Night	14% 7% 14% 39% 23% 4	840
	11th to 20th	Night	11% 6% 20% 37% 21% 4	824
	21st to end	Night	9% 7% 20% 44% 17% 2	692
March	1st to 10th	Night	11% 7% 22% 40% 18% 3	840
	11th to 20th	Night	9% 5% 19% 39% 24% 5%	840
	21st to end	Night	9% 6% 21% 39% 22% 3	924
April	1st to 10th	Night	8% 5% 19% 41% 25% 2	840
	11th to 20th	Night	7% 6% 19% 38% 26% 5%	831
	21st to end	Night	5% 5% 23% 34% 25% 8%	840
May	1st to 10th	Night	4 3 16% 39% 32% 7%	840
	11th to 20th	Night	3 3 13% 38% 37% 6%	721
	21st to end	Night	3 2 15% 42% 31% 7%	455
June	1st to 10th	Night	2 1 8% 40% 43% 6%	420
	11th to 20th	Night	2 2 10% 33% 47% 7%	420
	21st to end	Night	1 3 32% 52% 11%	420
July	1st to 10th	Night	1 1 23% 63% 12%	410
	11th to 20th	Night	1 1 10% 62% 25%	585
	21st to end	Night	1 1 14% 60% 23%	657
August	1st to 10th	Night	1 4 27% 53% 15%	696
	11th to 20th	Night	1 2 25% 59% 14%	792
	21st to end	Night	1 2 24% 59% 15%	880
September	1st to 10th	Night	1 1 3 25% 58% 12%	800
	11th to 20th	Night	3 1 7% 35% 44% 11%	800
	21st to end	Night	2 1 8% 32% 47% 10%	800
October	1st to 10th	Night	4 1 8% 39% 43% 6%	800
	11th to 20th	Night	4 3 10% 38% 40% 6%	800
	21st to end	Night	5% 2 11% 42% 32% 8%	880
November	1st to 10th	Night	5% 4 13% 41% 33% 4	820
	11th to 20th	Night	6% 3 17% 44% 25% 5%	840
	21st to end	Night	6% 5% 19% 44% 24% 3	840
December	1st to 10th	Night	10% 5% 18% 42% 23% 3	840
	11th to 20th	Night	13% 4 18% 43% 20% 2	840
	21st to end	Night	12% 7% 23% 40% 15% 2	924

Page generated at 2026-07-13 00:14:50. Version ac07

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