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 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	4 3 1 3 3 2 3 3 4 6% 66%	112640
	Night	5% 4 2 5% 3 3 3 3 5% 6% 61%	115186
Transparency	Day	18% 4 9% 19% 26% 24%	112398
	Night	22% 5% 9% 19% 24% 21%	115019
Seeing	Day	6% 8% 21% 35% 24% 6%	15963
	Night	6% 3 13% 36% 34% 7%	25640

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% 3 1 4 3 3 4 3 6% 7% 59%	3897
	11th to 20th	Day	7% 5% 2 4 3 3 3 3 5% 5% 61%	4100
	21st to end	Day	9% 6% 2 5% 4 4 5% 5% 6% 9% 45%	4510
February	1st to 10th	Day	13% 6% 2 4 3 3 3 3 5% 5% 53%	1860
	11th to 20th	Day	9% 5% 3 5% 4 3 4 3 5% 8% 51%	1950
	21st to end	Day	10% 7% 2 5% 3 4 4 3 6% 6% 50%	1650
March	1st to 10th	Day	11% 8% 3 8% 4 4 5% 5% 6% 9% 39%	3356
	11th to 20th	Day	6% 4 1 4 3 3 3 3 5% 6% 62%	3503
	21st to end	Day	8% 7% 3 6% 5% 5% 6% 5% 6% 8% 41%	3979
April	1st to 10th	Day	8% 6% 2 4 3 3 4 3 6% 7% 54%	2280
	11th to 20th	Day	6% 5% 2 4 4 3 3 3 5% 7% 58%	2249
	21st to end	Day	6% 3 1 3 3 2 3 3 5% 6% 64%	2280
May	1st to 10th	Day	4 3 1 2 2 2 1 1 3 4 77%	3800
	11th to 20th	Day	1 3 2 4 3 3 4 2 5% 8% 65%	3924
	21st to end	Day	2 2 1 3 2 2 2 2 4 7% 74%	4813
June	1st to 10th	Day	3 3 1 2 3 2 2 3 5% 8% 68%	2660
	11th to 20th	Day	1 2 1 2 1 2 2 2 3 5% 80%	2660
	21st to end	Day	1 1 2 2 1 2 2 3 4 83%	2660
July	1st to 10th	Day	1 1 1 1 1 1 3 4 87%	4420
	11th to 20th	Day	1 1 1 2 2 2 3 2 4 5% 79%	4117
	21st to end	Day	1 1 1 2 2 2 3 6% 81%	4455
August	1st to 10th	Day	1 1 1 1 2 1 2 2 3 6% 80%	3924
	11th to 20th	Day	1 1 1 1 1 1 2 3 88%	3724
	21st to end	Day	1 1 1 1 2 1 1 2 4 85%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 1 3 5% 83%	2280
	11th to 20th	Day	2 2 2 2 2 2 2 2 5% 79%	2280
	21st to end	Day	1 2 1 2 2 1 3 3 4 5% 76%	2082
October	1st to 10th	Day	2 1 1 2 2 2 2 3 4 5% 76%	3180
	11th to 20th	Day	2 2 1 3 2 2 3 2 3 4 75%	3161
	21st to end	Day	2 3 2 4 2 3 3 3 4 7% 67%	3448
November	1st to 10th	Day	5% 4 1 3 3 2 3 3 4 6% 66%	1889
	11th to 20th	Day	5% 4 1 3 3 3 4 3 5% 5% 64%	1800
	21st to end	Day	7% 5% 2 4 4 2 4 3 4 5% 59%	1800
December	1st to 10th	Day	5% 5% 2 6% 4 3 4 3 5% 8% 55%	2737
	11th to 20th	Day	15% 6% 2 5% 3 3 4 4 6% 7% 45%	3028
	21st to end	Day	11% 8% 2 7% 4 5% 5% 5% 7% 10% 37%	4004
January	1st to 10th	Night	6% 6% 2 5% 3 3 3 2 4 7% 59%	6042
	11th to 20th	Night	7% 6% 2 4 3 3 3 3 4 6% 58%	5880
	21st to end	Night	11% 7% 2 6% 4 5% 5% 4 4 7% 45%	6468
February	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 4 5% 54%	2537
	11th to 20th	Night	9% 4 1 4 3 3 4 3 6% 7% 56%	2338
	21st to end	Night	9% 6% 2 5% 4 4 2 2 4 6% 56%	1980
March	1st to 10th	Night	11% 11% 3 7% 4 5% 5% 4 5% 7% 38%	3735
	11th to 20th	Night	5% 4 2 4 3 4 3 3 6% 6% 61%	3505
	21st to end	Night	11% 9% 2 6% 3 3 3 3 5% 7% 47%	3660
April	1st to 10th	Night	6% 6% 1 5% 3 2 4 3 5% 7% 58%	1900
	11th to 20th	Night	5% 5% 1 5% 3 2 3 2 5% 4 65%	1879
	21st to end	Night	4 6% 2 4 2 2 3 2 3 7% 64%	1900
May	1st to 10th	Night	2 3 1 3 2 2 2 2 3 5% 76%	3180
	11th to 20th	Night	2 4 5% 11% 5% 4 4 4 5% 6% 51%	2976
	21st to end	Night	1 3 2 8% 6% 4 4 4 5% 6% 56%	2762
June	1st to 10th	Night	3 2 2 4 4 3 4 4 5% 7% 63%	1520
	11th to 20th	Night	1 4 1 3 3 2 2 2 4 6% 72%	1520
	21st to end	Night	1 2 1 3 2 2 1 1 4 6% 77%	1520
July	1st to 10th	Night	3 2 2 1 2 3 3 84%	2540
	11th to 20th	Night	1 1 1 2 2 3 4 4 6% 5% 71%	2848
	21st to end	Night	1 1 4 5% 3 3 4 6% 7% 66%	3109
August	1st to 10th	Night	1 1 2 2 2 2 2 4 7% 76%	3007
	11th to 20th	Night	1 1 1 1 2 2 3 5% 84%	3126
	21st to end	Night	1 3 2 2 2 3 4 6% 77%	3498
September	1st to 10th	Night	1 2 1 3 1 2 2 2 3 4 80%	1900
	11th to 20th	Night	2 3 1 3 2 2 3 2 3 4 75%	1900
	21st to end	Night	3 1 2 1 2 2 2 3 4 79%	2092
October	1st to 10th	Night	2 2 1 2 1 2 2 2 3 5% 78%	3800
	11th to 20th	Night	2 2 1 4 3 4 4 3 4 5% 68%	3777
	21st to end	Night	3 4 2 3 2 2 2 2 4 5% 71%	4121
November	1st to 10th	Night	4 4 1 4 2 2 3 3 4 6% 67%	2407
	11th to 20th	Night	3 3 1 4 3 3 4 3 5% 6% 65%	2600
	21st to end	Night	5% 5% 2 5% 3 3 4 4 6% 6% 58%	2600
December	1st to 10th	Night	6% 4 2 4 3 4 4 3 5% 6% 59%	4431
	11th to 20th	Night	13% 7% 2 5% 4 4 5% 5% 6% 8% 41%	5176
	21st to end	Night	10% 8% 4 9% 6% 7% 5% 5% 6% 8% 32%	6952

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	21% 5% 14% 31% 19% 10%	3897
	11th to 20th	Day	26% 4 9% 14% 19% 27%	4100
	21st to end	Day	32% 7% 11% 14% 22% 15%	4510
February	1st to 10th	Day	32% 4 7% 19% 21% 16%	1860
	11th to 20th	Day	31% 5% 11% 22% 22% 9%	1960
	21st to end	Day	32% 5% 12% 21% 22% 8%	1650
March	1st to 10th	Day	36% 9% 15% 18% 17% 5%	3356
	11th to 20th	Day	22% 6% 11% 32% 16% 12%	3503
	21st to end	Day	31% 11% 18% 23% 13% 4	3979
April	1st to 10th	Day	28% 5% 12% 26% 20% 8%	2280
	11th to 20th	Day	25% 5% 13% 26% 24% 8%	2252
	21st to end	Day	20% 6% 12% 26% 28% 8%	2280
May	1st to 10th	Day	14% 2 7% 27% 35% 14%	3800
	11th to 20th	Day	15% 5% 11% 30% 28% 11%	3927
	21st to end	Day	11% 4 13% 29% 36% 6%	4814
June	1st to 10th	Day	15% 5% 11% 27% 29% 13%	2660
	11th to 20th	Day	9% 2 8% 19% 37% 25%	2660
	21st to end	Day	8% 2 5% 17% 31% 37%	2660
July	1st to 10th	Day	4 1 2 9% 19% 66%	4351
	11th to 20th	Day	9% 3 4 9% 20% 55%	4047
	21st to end	Day	6% 2 4 12% 30% 45%	4315
August	1st to 10th	Day	8% 2 3 13% 34% 40%	3909
	11th to 20th	Day	4 1 3 9% 30% 52%	3760
	21st to end	Day	6% 1 3 13% 27% 49%	4179
September	1st to 10th	Day	7% 2 4 14% 30% 43%	2280
	11th to 20th	Day	10% 2 5% 21% 37% 25%	2280
	21st to end	Day	11% 2 8% 16% 36% 28%	2082
October	1st to 10th	Day	11% 3 5% 13% 24% 44%	3180
	11th to 20th	Day	13% 2 8% 20% 37% 21%	3161
	21st to end	Day	19% 3 6% 12% 33% 27%	3448
November	1st to 10th	Day	20% 3 8% 21% 29% 19%	1889
	11th to 20th	Day	21% 4 11% 20% 26% 19%	1800
	21st to end	Day	27% 4 7% 18% 24% 20%	1800
December	1st to 10th	Day	28% 3 7% 14% 27% 20%	2737
	11th to 20th	Day	36% 6% 11% 17% 20% 10%	3028
	21st to end	Day	37% 9% 16% 22% 12% 5%	4004
January	1st to 10th	Night	24% 5% 12% 26% 24% 9%	6042
	11th to 20th	Night	26% 5% 9% 16% 21% 24%	5880
	21st to end	Night	36% 6% 11% 13% 18% 17%	6468
February	1st to 10th	Night	32% 4 8% 18% 22% 15%	2537
	11th to 20th	Night	27% 4 10% 23% 23% 12%	2339
	21st to end	Night	29% 5% 9% 22% 25% 10%	1980
March	1st to 10th	Night	40% 8% 12% 19% 16% 5%	3735
	11th to 20th	Night	22% 6% 13% 26% 22% 10%	3505
	21st to end	Night	33% 8% 17% 20% 17% 4	3660
April	1st to 10th	Night	26% 5% 14% 26% 18% 10%	1900
	11th to 20th	Night	22% 3 12% 29% 24% 9%	1879
	21st to end	Night	21% 5% 10% 27% 29% 8%	1900
May	1st to 10th	Night	11% 4 12% 26% 27% 20%	3180
	11th to 20th	Night	22% 10% 13% 22% 21% 11%	2978
	21st to end	Night	14% 13% 12% 25% 30% 6%	2762
June	1st to 10th	Night	17% 6% 13% 23% 28% 13%	1520
	11th to 20th	Night	14% 3 7% 20% 33% 24%	1520
	21st to end	Night	10% 3 3 17% 31% 36%	1520
July	1st to 10th	Night	6% 2 2 10% 16% 65%	2500
	11th to 20th	Night	9% 4 4 17% 18% 49%	2795
	21st to end	Night	14% 4 4 16% 30% 33%	3011
August	1st to 10th	Night	10% 1 4 15% 32% 37%	2998
	11th to 20th	Night	5% 1 3 11% 24% 56%	3156
	21st to end	Night	8% 1 4 15% 29% 42%	3498
September	1st to 10th	Night	10% 2 3 13% 27% 46%	1900
	11th to 20th	Night	13% 3 6% 17% 33% 28%	1900
	21st to end	Night	10% 2 5% 17% 36% 29%	2092
October	1st to 10th	Night	10% 2 4 14% 23% 47%	3800
	11th to 20th	Night	15% 3 10% 18% 31% 23%	3777
	21st to end	Night	17% 2 5% 14% 37% 26%	4121
November	1st to 10th	Night	18% 4 8% 21% 27% 22%	2407
	11th to 20th	Night	19% 4 11% 19% 27% 20%	2600
	21st to end	Night	26% 5% 10% 17% 22% 21%	2600
December	1st to 10th	Night	25% 4 7% 14% 31% 19%	4431
	11th to 20th	Night	38% 7% 11% 17% 17% 10%	5176
	21st to end	Night	43% 10% 13% 20% 10% 4	6952

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% 13% 28% 27% 12% 1	315
	11th to 20th	Day	11% 11% 28% 35% 14% 1	400
	21st to end	Day	10% 11% 27% 33% 17% 2	440
February	1st to 10th	Day	17% 11% 24% 29% 16% 3	430
	11th to 20th	Day	12% 13% 30% 27% 15% 2	495
	21st to end	Day	9% 15% 30% 33% 10% 2	414
March	1st to 10th	Day	15% 18% 28% 32% 8%	500
	11th to 20th	Day	12% 11% 31% 32% 12% 1	465
	21st to end	Day	10% 19% 30% 29% 12% 1	506
April	1st to 10th	Day	12% 10% 29% 31% 15% 3	460
	11th to 20th	Day	8% 13% 27% 34% 15% 3	452
	21st to end	Day	4 10% 30% 32% 18% 5%	460
May	1st to 10th	Day	7% 8% 30% 32% 19% 4	463
	11th to 20th	Day	3 10% 21% 40% 20% 6%	488
	21st to end	Day	5% 11% 22% 35% 20% 6%	529
June	1st to 10th	Day	4 4 19% 37% 29% 7%	490
	11th to 20th	Day	1 4 23% 37% 26% 8%	490
	21st to end	Day	1 2 17% 38% 33% 10%	490
July	1st to 10th	Day	9% 45% 34% 11%	486
	11th to 20th	Day	2 1 5% 38% 39% 16%	484
	21st to end	Day	7% 38% 38% 16%	531
August	1st to 10th	Day	1 1 14% 41% 34% 9%	487
	11th to 20th	Day	1 1 9% 38% 36% 15%	477
	21st to end	Day	1 1 9% 40% 37% 11%	539
September	1st to 10th	Day	1 2 12% 37% 39% 10%	490
	11th to 20th	Day	3 3 17% 39% 32% 6%	490
	21st to end	Day	1 5% 15% 36% 37% 6%	490
October	1st to 10th	Day	1 5% 21% 41% 27% 4	490
	11th to 20th	Day	3 5% 22% 38% 27% 4	490
	21st to end	Day	3 6% 20% 38% 29% 5%	539
November	1st to 10th	Day	4 8% 23% 35% 25% 4	425
	11th to 20th	Day	8% 8% 21% 39% 20% 4	300
	21st to end	Day	9% 6% 24% 35% 24% 2	300
December	1st to 10th	Day	8% 11% 28% 33% 18% 2	238
	11th to 20th	Day	9% 13% 22% 32% 22% 3	200
	21st to end	Day	15% 15% 28% 29% 11% 2	220
January	1st to 10th	Night	14% 6% 21% 37% 19% 4	800
	11th to 20th	Night	10% 3 21% 43% 20% 4	800
	21st to end	Night	13% 3 17% 43% 20% 3	880
February	1st to 10th	Night	15% 7% 12% 38% 24% 4	800
	11th to 20th	Night	10% 6% 20% 38% 22% 4	784
	21st to end	Night	10% 7% 19% 44% 17% 3	660
March	1st to 10th	Night	12% 7% 21% 40% 18% 3	800
	11th to 20th	Night	9% 5% 18% 39% 25% 5%	766
	21st to end	Night	9% 6% 22% 38% 22% 3	836
April	1st to 10th	Night	9% 4 19% 40% 26% 2	760
	11th to 20th	Night	6% 6% 19% 38% 26% 5%	751
	21st to end	Night	4 5% 23% 34% 26% 8%	760
May	1st to 10th	Night	4 3 17% 38% 31% 6%	760
	11th to 20th	Night	4 3 12% 37% 38% 6%	652
	21st to end	Night	3 2 16% 40% 32% 7%	411
June	1st to 10th	Night	3 2 8% 39% 42% 6%	380
	11th to 20th	Night	2 2 9% 32% 48% 7%	380
	21st to end	Night	1 3 32% 53% 11%	380
July	1st to 10th	Night	1 1 21% 64% 13%	370
	11th to 20th	Night	1 1 10% 63% 24%	541
	21st to end	Night	1 1 13% 61% 24%	624
August	1st to 10th	Night	1 3 26% 55% 14%	661
	11th to 20th	Night	1 2 24% 59% 14%	752
	21st to end	Night	1 2 22% 60% 15%	836
September	1st to 10th	Night	1 1 3 24% 59% 13%	760
	11th to 20th	Night	2 1 6% 35% 43% 11%	760
	21st to end	Night	1 1 8% 31% 49% 11%	760
October	1st to 10th	Night	4 1 8% 39% 43% 6%	760
	11th to 20th	Night	4 3 9% 38% 40% 6%	760
	21st to end	Night	5% 2 10% 41% 33% 9%	836
November	1st to 10th	Night	4 4 13% 41% 33% 5%	780
	11th to 20th	Night	5% 3 17% 45% 26% 5%	800
	21st to end	Night	6% 5% 17% 44% 25% 4	800
December	1st to 10th	Night	11% 5% 16% 41% 24% 3	800
	11th to 20th	Night	14% 4 18% 43% 20% 2	800
	21st to end	Night	11% 7% 23% 41% 16% 2	880

Page generated at 2025-03-09 05:08:46. Version ac07

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