Cottonwood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cottonwood clear sky chart from 2005-11-06 to 2026-02-16.

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	9% 3 1 3 2 2 2 2 4 5% 65%	139958
	Night	12% 4 1 4 3 3 3 3 4 6% 58%	138311
Transparency	Day	23% 3 7% 17% 24% 25%	134838
	Night	28% 4 8% 18% 22% 19%	134421
Seeing	Day	11% 8% 18% 29% 24% 10%	16650
	Night	13% 5% 17% 36% 26% 3	25945

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	23% 8% 3 6% 4 4 4 3 5% 8% 33%	4722
	11th to 20th	Day	16% 5% 2 4 2 2 2 2 2 5% 59%	5218
	21st to end	Day	17% 6% 2 5% 3 3 3 3 5% 7% 45%	5843
February	1st to 10th	Day	18% 5% 1 5% 4 3 4 4 5% 6% 46%	1890
	11th to 20th	Day	18% 5% 2 5% 3 3 3 3 4 6% 48%	1826
	21st to end	Day	16% 5% 2 4 3 3 4 3 4 8% 49%	1650
March	1st to 10th	Day	21% 9% 3 6% 4 3 4 3 3 5% 38%	4814
	11th to 20th	Day	18% 8% 2 5% 3 2 3 2 3 6% 46%	5000
	21st to end	Day	15% 8% 3 6% 5% 4 4 3 5% 7% 39%	5589
April	1st to 10th	Day	13% 7% 2 4 4 4 4 3 4 7% 48%	2400
	11th to 20th	Day	12% 4 1 4 2 3 3 3 5% 7% 57%	2369
	21st to end	Day	6% 3 1 3 3 3 3 3 5% 7% 63%	2400
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 3 6% 73%	5877
	11th to 20th	Day	3 2 1 2 1 2 2 3 4 5% 75%	6267
	21st to end	Day	2 1 1 1 2 2 2 4 5% 80%	6902
June	1st to 10th	Day	5% 3 1 3 2 3 4 3 4 8% 63%	2800
	11th to 20th	Day	2 1 1 1 2 2 2 2 4 5% 80%	2800
	21st to end	Day	3 1 1 2 1 1 2 2 2 3 82%	2800
July	1st to 10th	Day	1 1 1 1 1 2 3 91%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	6320
	21st to end	Day	1 1 1 1 2 2 2 4 5% 82%	6405
August	1st to 10th	Day	1 1 2 1 1 1 2 3 5% 82%	5777
	11th to 20th	Day	1 1 1 1 1 1 2 3 90%	5674
	21st to end	Day	1 1 1 1 1 1 1 2 4 85%	5984
September	1st to 10th	Day	1 1 1 1 2 1 1 3 3 87%	2400
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 6% 75%	2400
	21st to end	Day	3 1 1 3 2 1 2 1 3 4 80%	2114
October	1st to 10th	Day	3 1 1 2 1 1 2 2 3 4 80%	3280
	11th to 20th	Day	6% 2 1 3 2 2 3 3 5% 6% 69%	3261
	21st to end	Day	10% 4 1 4 3 3 4 3 5% 7% 56%	3558
November	1st to 10th	Day	14% 4 1 3 3 2 3 3 5% 8% 53%	1924
	11th to 20th	Day	17% 5% 1 4 3 4 3 4 6% 8% 46%	1795
	21st to end	Day	12% 5% 2 3 2 4 4 3 4 7% 55%	1680
December	1st to 10th	Day	13% 3 1 3 3 3 3 3 4 6% 57%	2700
	11th to 20th	Day	31% 7% 2 6% 4 4 5% 4 6% 7% 24%	3108
	21st to end	Day	33% 7% 2 5% 3 3 3 3 4 5% 31%	4092
January	1st to 10th	Night	24% 8% 3 6% 4 4 5% 4 5% 6% 31%	8172
	11th to 20th	Night	15% 4 1 4 2 3 4 3 5% 8% 52%	7751
	21st to end	Night	18% 7% 2 6% 5% 4 4 4 5% 4 40%	8371
February	1st to 10th	Night	18% 5% 2 4 3 4 3 3 5% 6% 47%	2730
	11th to 20th	Night	19% 5% 2 4 4 3 3 3 5% 6% 47%	2572
	21st to end	Night	15% 6% 2 4 4 3 3 3 4 6% 50%	1980
March	1st to 10th	Night	19% 8% 3 7% 4 3 4 3 4 6% 39%	5078
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 5% 6% 47%	5000
	21st to end	Night	12% 9% 3 7% 5% 4 4 4 6% 8% 38%	5385
April	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 7% 49%	2000
	11th to 20th	Night	10% 5% 2 4 3 3 3 2 4 8% 56%	1979
	21st to end	Night	8% 4 1 4 3 2 2 3 5% 8% 60%	1999
May	1st to 10th	Night	6% 4 1 3 2 1 2 2 3 5% 70%	4100
	11th to 20th	Night	2 2 1 3 2 3 3 2 4 5% 71%	3625
	21st to end	Night	3 2 1 2 2 2 2 2 3 6% 75%	3972
June	1st to 10th	Night	6% 4 2 3 3 3 3 3 7% 9% 59%	1600
	11th to 20th	Night	3 2 1 2 2 2 2 2 4 5% 76%	1600
	21st to end	Night	2 1 2 2 2 2 2 3 2 82%	1600
July	1st to 10th	Night	1 1 1 1 1 2 2 89%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 2 88%	3640
	21st to end	Night	1 1 1 2 2 4 5% 84%	4465
August	1st to 10th	Night	2 1 1 1 1 2 2 2 3 4 82%	4035
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 89%	4178
	21st to end	Night	3 2 1 1 1 2 1 2 4 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 3 86%	2000
	11th to 20th	Night	4 2 1 2 2 1 2 2 3 5% 76%	2000
	21st to end	Night	3 1 1 2 1 2 2 2 3 4 78%	2280
October	1st to 10th	Night	4 2 1 1 1 2 2 2 3 3 79%	3920
	11th to 20th	Night	5% 2 1 3 2 2 3 2 4 6% 72%	3897
	21st to end	Night	8% 2 1 3 2 2 4 3 7% 8% 61%	4253
November	1st to 10th	Night	13% 5% 2 4 3 3 4 4 6% 8% 50%	2592
	11th to 20th	Night	15% 5% 2 4 3 3 4 3 5% 8% 47%	2820
	21st to end	Night	15% 4 1 4 3 3 3 3 4 7% 53%	2940
December	1st to 10th	Night	13% 3 1 3 2 3 4 4 6% 8% 54%	4700
	11th to 20th	Night	34% 7% 2 6% 4 3 4 4 5% 6% 25%	5316
	21st to end	Night	34% 7% 2 5% 3 3 3 3 4 5% 31%	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	49% 7% 10% 21% 11% 2	4723
	11th to 20th	Day	32% 2 5% 15% 23% 23%	5218
	21st to end	Day	39% 4 10% 16% 18% 13%	5843
February	1st to 10th	Day	37% 6% 10% 18% 20% 9%	1890
	11th to 20th	Day	38% 5% 9% 19% 23% 6%	1835
	21st to end	Day	35% 5% 9% 24% 19% 8%	1650
March	1st to 10th	Day	47% 5% 8% 16% 20% 4	4814
	11th to 20th	Day	41% 4 7% 22% 19% 7%	5000
	21st to end	Day	42% 8% 12% 24% 13% 2	5589
April	1st to 10th	Day	35% 6% 11% 21% 21% 6%	2400
	11th to 20th	Day	28% 5% 10% 22% 22% 13%	2372
	21st to end	Day	21% 5% 12% 25% 25% 11%	2400
May	1st to 10th	Day	15% 3 7% 20% 29% 26%	5877
	11th to 20th	Day	12% 3 6% 19% 35% 25%	6270
	21st to end	Day	9% 3 8% 27% 37% 16%	6904
June	1st to 10th	Day	21% 5% 9% 21% 27% 17%	2800
	11th to 20th	Day	9% 3 7% 18% 33% 30%	2800
	21st to end	Day	9% 1 4 13% 29% 42%	2800
July	1st to 10th	Day	3 1 3 9% 21% 63%	6251
	11th to 20th	Day	6% 1 2 6% 18% 67%	6245
	21st to end	Day	6% 2 6% 17% 28% 41%	6219
August	1st to 10th	Day	10% 1 3 9% 25% 52%	4206
	11th to 20th	Day	4 1 3 13% 38% 42%	4109
	21st to end	Day	7% 1 3 16% 33% 39%	4311
September	1st to 10th	Day	6% 1 3 10% 24% 56%	2400
	11th to 20th	Day	13% 3 6% 19% 35% 24%	2400
	21st to end	Day	11% 2 6% 15% 30% 35%	2114
October	1st to 10th	Day	11% 2 4 12% 24% 47%	3280
	11th to 20th	Day	16% 3 8% 21% 32% 19%	3261
	21st to end	Day	27% 5% 8% 19% 28% 14%	3558
November	1st to 10th	Day	30% 5% 9% 26% 18% 12%	1924
	11th to 20th	Day	36% 6% 12% 22% 16% 9%	1795
	21st to end	Day	30% 5% 9% 20% 23% 12%	1680
December	1st to 10th	Day	29% 3 6% 18% 31% 12%	2700
	11th to 20th	Day	58% 5% 11% 13% 10% 3	3108
	21st to end	Day	55% 5% 9% 17% 11% 2	4092
January	1st to 10th	Night	50% 8% 11% 18% 11% 2	8172
	11th to 20th	Night	30% 5% 7% 15% 24% 19%	7751
	21st to end	Night	45% 6% 6% 10% 20% 13%	8371
February	1st to 10th	Night	37% 5% 8% 21% 20% 9%	2730
	11th to 20th	Night	38% 5% 10% 18% 22% 7%	2574
	21st to end	Night	36% 4 11% 21% 21% 7%	1980
March	1st to 10th	Night	43% 6% 13% 19% 15% 5%	5078
	11th to 20th	Night	36% 6% 11% 23% 17% 7%	5000
	21st to end	Night	39% 9% 15% 26% 9% 2	5385
April	1st to 10th	Night	34% 6% 14% 22% 17% 7%	2000
	11th to 20th	Night	28% 6% 12% 23% 20% 11%	1979
	21st to end	Night	22% 6% 13% 27% 25% 7%	1999
May	1st to 10th	Night	19% 3 6% 19% 34% 20%	4097
	11th to 20th	Night	16% 3 5% 22% 34% 19%	3625
	21st to end	Night	12% 3 9% 30% 37% 9%	3972
June	1st to 10th	Night	22% 5% 10% 24% 29% 10%	1600
	11th to 20th	Night	12% 3 7% 18% 33% 26%	1600
	21st to end	Night	10% 3 4 13% 34% 37%	1600
July	1st to 10th	Night	4 1 2 8% 24% 61%	3600
	11th to 20th	Night	6% 1 2 4 17% 70%	3592
	21st to end	Night	6% 2 6% 18% 30% 38%	4339
August	1st to 10th	Night	12% 1 2 10% 26% 49%	2929
	11th to 20th	Night	3 1 2 15% 40% 38%	3016
	21st to end	Night	8% 1 2 16% 36% 38%	3608
September	1st to 10th	Night	6% 2 3 9% 25% 56%	2000
	11th to 20th	Night	12% 3 5% 20% 35% 25%	2000
	21st to end	Night	11% 2 7% 15% 28% 37%	2280
October	1st to 10th	Night	12% 3 4 10% 22% 49%	3920
	11th to 20th	Night	16% 3 6% 30% 27% 17%	3897
	21st to end	Night	20% 4 8% 25% 28% 16%	4253
November	1st to 10th	Night	30% 6% 11% 24% 17% 12%	2592
	11th to 20th	Night	34% 7% 13% 21% 16% 9%	2820
	21st to end	Night	31% 5% 10% 20% 22% 12%	2940
December	1st to 10th	Night	26% 5% 8% 22% 25% 14%	4700
	11th to 20th	Night	58% 5% 8% 14% 12% 2	5316
	21st to end	Night	55% 6% 9% 16% 12% 2	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	37% 12% 16% 23% 10% 3	220
	11th to 20th	Day	24% 8% 20% 24% 18% 7%	386
	21st to end	Day	21% 9% 18% 26% 19% 8%	473
February	1st to 10th	Day	22% 11% 18% 27% 19% 4	430
	11th to 20th	Day	21% 14% 23% 25% 13% 3	424
	21st to end	Day	16% 11% 23% 32% 15% 3	414
March	1st to 10th	Day	29% 12% 25% 20% 9% 4	500
	11th to 20th	Day	24% 10% 21% 23% 17% 4	500
	21st to end	Day	21% 12% 27% 23% 15% 2	550
April	1st to 10th	Day	17% 14% 28% 21% 18% 2	500
	11th to 20th	Day	14% 15% 21% 27% 15% 7%	492
	21st to end	Day	7% 11% 27% 27% 20% 8%	500
May	1st to 10th	Day	7% 15% 29% 25% 18% 7%	503
	11th to 20th	Day	9% 8% 24% 28% 21% 10%	528
	21st to end	Day	5% 9% 20% 29% 27% 11%	573
June	1st to 10th	Day	4 10% 16% 30% 27% 12%	530
	11th to 20th	Day	4 10% 24% 31% 25% 7%	530
	21st to end	Day	2 4 18% 30% 33% 13%	530
July	1st to 10th	Day	1 1 15% 38% 30% 14%	526
	11th to 20th	Day	1 10% 31% 39% 19%	524
	21st to end	Day	1 1 6% 35% 35% 23%	575
August	1st to 10th	Day	2 1 9% 35% 32% 21%	527
	11th to 20th	Day	1 2 12% 30% 36% 19%	517
	21st to end	Day	1 3 10% 33% 34% 19%	583
September	1st to 10th	Day	1 2 14% 31% 38% 13%	530
	11th to 20th	Day	4 3 18% 30% 30% 15%	530
	21st to end	Day	3 5% 18% 32% 29% 11%	530
October	1st to 10th	Day	5% 10% 16% 28% 27% 13%	530
	11th to 20th	Day	8% 5% 17% 30% 26% 13%	530
	21st to end	Day	12% 9% 18% 28% 23% 9%	583
November	1st to 10th	Day	16% 7% 19% 24% 24% 10%	400
	11th to 20th	Day	19% 4 14% 32% 23% 8%	280
	21st to end	Day	15% 6% 14% 32% 25% 7%	220
December	1st to 10th	Day	21% 7% 17% 23% 21% 11%	220
	11th to 20th	Day	30% 3 11% 28% 18% 10%	220
	21st to end	Day	26% 7% 18% 29% 17% 3	242
January	1st to 10th	Night	26% 4 17% 32% 19% 3	840
	11th to 20th	Night	21% 6% 17% 34% 19% 3	840
	21st to end	Night	20% 5% 18% 34% 19% 4	924
February	1st to 10th	Night	20% 5% 19% 37% 17% 2	840
	11th to 20th	Night	20% 7% 21% 34% 16% 2	804
	21st to end	Night	16% 9% 23% 38% 13% 1	660
March	1st to 10th	Night	23% 9% 23% 28% 15% 2	800
	11th to 20th	Night	20% 8% 18% 32% 22% 1	800
	21st to end	Night	19% 10% 22% 34% 15% 1	880
April	1st to 10th	Night	15% 9% 25% 33% 16% 2	800
	11th to 20th	Night	11% 9% 25% 35% 17% 3	791
	21st to end	Night	9% 5% 27% 35% 21% 2	799
May	1st to 10th	Night	9% 6% 31% 30% 24% 1	600
	11th to 20th	Night	7% 4 24% 38% 26% 1	403
	21st to end	Night	7% 7% 23% 37% 25% 1	433
June	1st to 10th	Night	5% 4 22% 42% 26% 2	400
	11th to 20th	Night	3 6% 26% 46% 19% 1	400
	21st to end	Night	3 2 14% 52% 29% 1	400
July	1st to 10th	Night	1 1 11% 49% 38% 1	390
	11th to 20th	Night	1 1 10% 51% 36% 2	388
	21st to end	Night	1 3 41% 50% 5%	650
August	1st to 10th	Night	2 1 6% 34% 51% 6%	597
	11th to 20th	Night	1 1 6% 42% 44% 6%	642
	21st to end	Night	1 1 6% 40% 49% 4	880
September	1st to 10th	Night	2 1 10% 43% 42% 3	800
	11th to 20th	Night	4 2 11% 36% 41% 7%	800
	21st to end	Night	4 3 15% 36% 34% 8%	800
October	1st to 10th	Night	7% 5% 17% 35% 33% 5%	800
	11th to 20th	Night	7% 3 15% 37% 32% 6%	800
	21st to end	Night	10% 6% 15% 35% 28% 6%	880
November	1st to 10th	Night	13% 7% 15% 32% 29% 5%	820
	11th to 20th	Night	17% 3 17% 38% 22% 3	840
	21st to end	Night	18% 4 15% 37% 24% 3	840
December	1st to 10th	Night	21% 4 16% 33% 21% 4	840
	11th to 20th	Night	30% 5% 13% 31% 19% 2	840
	21st to end	Night	28% 4 19% 33% 14% 1	924

Page generated at 2026-02-12 22:42:14. Version ac07

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