Cottontail Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cottontail Observatory clear sky chart from 2005-11-06 to 2025-06-10.

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	2 4 2 5% 3 3 3 3 5% 7% 62%	123325
	Night	1 4 2 5% 3 3 3 3 5% 7% 65%	123754
Transparency	Day	21% 5% 7% 19% 28% 20%	123071
	Night	19% 5% 8% 22% 29% 17%	123596
Seeing	Day	9% 22% 30% 25% 12% 3	15828
	Night	4 12% 27% 33% 18% 5%	25306

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	1 4 2 4 3 3 4 4 7% 10% 58%	3639
	11th to 20th	Day	1 4 2 4 3 4 4 4 5% 7% 63%	3732
	21st to end	Day	2 7% 3 10% 4 5% 4 3 6% 7% 47%	4510
February	1st to 10th	Day	1 5% 2 6% 3 3 3 3 6% 10% 57%	1858
	11th to 20th	Day	1 5% 3 7% 4 3 4 4 7% 11% 51%	1950
	21st to end	Day	1 4 2 8% 4 4 5% 3 6% 7% 57%	1650
March	1st to 10th	Day	1 9% 4 11% 5% 4 5% 4 7% 9% 41%	4518
	11th to 20th	Day	4 3 7% 3 3 4 4 6% 10% 57%	4858
	21st to end	Day	1 9% 3 7% 3 3 4 4 6% 7% 54%	5881
April	1st to 10th	Day	3 5% 2 7% 4 4 4 3 6% 8% 54%	2400
	11th to 20th	Day	2 6% 2 4 4 4 3 4 6% 7% 58%	2369
	21st to end	Day	3 7% 2 5% 4 3 3 4 5% 7% 58%	2400
May	1st to 10th	Day	3 7% 1 4 2 3 3 3 4 5% 65%	5735
	11th to 20th	Day	3 4 2 4 3 3 3 3 4 5% 67%	6010
	21st to end	Day	4 5% 2 4 3 3 3 2 4 6% 63%	6902
June	1st to 10th	Day	4 4 2 5% 4 4 4 4 6% 7% 57%	2789
	11th to 20th	Day	2 2 1 3 3 2 3 3 5% 5% 73%	2660
	21st to end	Day	2 3 1 4 3 3 3 3 4 4 70%	2660
July	1st to 10th	Day	1 1 1 2 1 2 2 2 3 4 79%	4420
	11th to 20th	Day	4 5% 2 7% 5% 4 5% 3 5% 5% 54%	4420
	21st to end	Day	3 3 1 4 4 4 4 3 4 6% 64%	4687
August	1st to 10th	Day	2 3 2 5% 3 5% 4 3 5% 5% 62%	3951
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 3 78%	3724
	21st to end	Day	1 3 2 3 2 2 2 2 2 3 79%	4180
September	1st to 10th	Day	2 2 1 3 3 3 3 3 3 4 73%	2280
	11th to 20th	Day	3 3 1 4 3 3 3 3 4 5% 69%	2224
	21st to end	Day	2 3 1 3 2 2 2 2 3 5% 76%	2090
October	1st to 10th	Day	3 5% 2 4 2 2 2 2 3 5% 69%	3304
	11th to 20th	Day	3 3 1 2 2 1 2 2 2 3 79%	3161
	21st to end	Day	1 3 1 4 2 2 3 3 5% 7% 69%	3337
November	1st to 10th	Day	1 5% 2 4 3 3 4 4 5% 7% 63%	1764
	11th to 20th	Day	1 2 1 3 2 4 4 3 7% 7% 66%	1800
	21st to end	Day	1 4 1 4 3 3 4 3 6% 11% 61%	1800
December	1st to 10th	Day	1 3 1 2 2 3 3 3 5% 8% 68%	2630
	11th to 20th	Day	2 5% 3 6% 5% 5% 6% 5% 7% 9% 48%	3028
	21st to end	Day	1 6% 3 8% 4 3 5% 5% 8% 13% 45%	4004
January	1st to 10th	Night	1 4 3 6% 4 3 4 3 6% 9% 56%	6306
	11th to 20th	Night	4 2 6% 4 3 4 3 6% 7% 61%	6215
	21st to end	Night	2 7% 3 9% 4 3 4 4 5% 6% 53%	6468
February	1st to 10th	Night	1 4 3 7% 4 4 4 5% 6% 7% 56%	2540
	11th to 20th	Night	6% 2 6% 4 4 4 3 7% 10% 54%	2338
	21st to end	Night	1 4 2 6% 3 3 4 3 7% 8% 59%	1980
March	1st to 10th	Night	1 6% 5% 8% 4 4 5% 4 6% 7% 50%	5376
	11th to 20th	Night	5% 3 7% 3 4 4 4 5% 8% 56%	5139
	21st to end	Night	5% 2 7% 5% 4 4 4 7% 9% 52%	5094
April	1st to 10th	Night	3 5% 2 5% 3 2 4 4 6% 8% 57%	2000
	11th to 20th	Night	1 6% 2 4 3 3 3 3 5% 6% 63%	1979
	21st to end	Night	3 6% 2 3 3 3 3 3 4 6% 64%	2000
May	1st to 10th	Night	3 5% 1 3 2 2 2 2 3 6% 74%	4248
	11th to 20th	Night	2 3 1 4 2 2 2 2 4 5% 71%	3892
	21st to end	Night	2 2 1 3 2 2 2 2 3 5% 75%	3972
June	1st to 10th	Night	4 4 1 2 3 2 3 3 5% 6% 66%	1595
	11th to 20th	Night	1 1 1 2 1 2 2 1 4 5% 80%	1520
	21st to end	Night	3 1 1 2 2 1 2 3 3 3 80%	1520
July	1st to 10th	Night	1 1 1 1 1 2 3 89%	2540
	11th to 20th	Night	2 2 1 4 3 3 4 3 5% 6% 67%	2540
	21st to end	Night	2 2 2 2 3 3 3 4 4 76%	2850
August	1st to 10th	Night	2 1 1 3 2 3 2 2 5% 5% 76%	2990
	11th to 20th	Night	1 1 1 1 2 2 2 3 3 85%	3126
	21st to end	Night	1 1 1 1 1 1 1 1 2 3 87%	3498
September	1st to 10th	Night	1 1 1 2 1 1 2 2 2 4 85%	1900
	11th to 20th	Night	2 3 1 2 2 2 2 2 3 4 78%	1954
	21st to end	Night	2 2 1 3 2 2 2 2 4 4 76%	2090
October	1st to 10th	Night	2 3 1 2 2 1 3 2 4 6% 75%	3671
	11th to 20th	Night	1 2 1 3 1 2 2 2 3 5% 79%	3777
	21st to end	Night	1 4 2 4 2 2 2 2 4 6% 72%	4228
November	1st to 10th	Night	1 3 1 4 2 3 3 4 4 9% 66%	2535
	11th to 20th	Night	1 3 2 3 3 2 4 3 5% 9% 65%	2600
	21st to end	Night	1 3 2 5% 3 3 3 3 6% 8% 64%	2600
December	1st to 10th	Night	1 2 1 3 2 2 3 3 6% 7% 69%	4545
	11th to 20th	Night	1 4 2 5% 4 4 5% 5% 9% 13% 48%	5176
	21st to end	Night	2 7% 4 9% 5% 5% 6% 5% 9% 11% 40%	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	19% 6% 18% 29% 24% 3	3639
	11th to 20th	Day	21% 3 11% 21% 29% 16%	3732
	21st to end	Day	33% 6% 8% 18% 24% 11%	4510
February	1st to 10th	Day	23% 6% 11% 23% 25% 12%	1858
	11th to 20th	Day	26% 5% 10% 24% 26% 9%	1960
	21st to end	Day	26% 6% 10% 25% 24% 8%	1650
March	1st to 10th	Day	36% 7% 6% 22% 20% 9%	4518
	11th to 20th	Day	21% 8% 11% 34% 23% 4	4858
	21st to end	Day	27% 7% 12% 24% 25% 4	5881
April	1st to 10th	Day	28% 6% 12% 27% 20% 8%	2400
	11th to 20th	Day	25% 7% 10% 27% 23% 9%	2372
	21st to end	Day	25% 6% 12% 29% 23% 5%	2400
May	1st to 10th	Day	20% 4 7% 25% 35% 9%	5735
	11th to 20th	Day	17% 7% 4 25% 38% 9%	6013
	21st to end	Day	20% 7% 4 17% 41% 9%	6904
June	1st to 10th	Day	25% 5% 8% 23% 29% 10%	2789
	11th to 20th	Day	14% 3 5% 21% 36% 21%	2660
	21st to end	Day	19% 3 3 10% 32% 33%	2660
July	1st to 10th	Day	9% 2 2 5% 14% 67%	4351
	11th to 20th	Day	27% 7% 3 8% 13% 42%	4345
	21st to end	Day	19% 5% 4 11% 19% 43%	4539
August	1st to 10th	Day	20% 6% 3 8% 21% 42%	3936
	11th to 20th	Day	12% 2 1 5% 16% 63%	3760
	21st to end	Day	12% 3 1 6% 34% 44%	4179
September	1st to 10th	Day	15% 3 3 9% 31% 38%	2280
	11th to 20th	Day	18% 3 3 18% 33% 24%	2224
	21st to end	Day	14% 3 2 18% 36% 27%	2090
October	1st to 10th	Day	16% 6% 3 11% 35% 29%	3304
	11th to 20th	Day	12% 3 3 13% 52% 17%	3161
	21st to end	Day	14% 5% 4 25% 37% 15%	3337
November	1st to 10th	Day	19% 6% 9% 23% 28% 14%	1764
	11th to 20th	Day	15% 5% 12% 24% 32% 11%	1800
	21st to end	Day	18% 5% 10% 26% 27% 15%	1800
December	1st to 10th	Day	13% 5% 9% 28% 31% 14%	2630
	11th to 20th	Day	28% 7% 16% 21% 19% 9%	3028
	21st to end	Day	27% 7% 18% 22% 20% 5%	4004
January	1st to 10th	Night	23% 4 16% 26% 25% 5%	6306
	11th to 20th	Night	22% 3 12% 21% 31% 12%	6215
	21st to end	Night	30% 4 8% 19% 25% 13%	6468
February	1st to 10th	Night	25% 6% 11% 24% 20% 14%	2540
	11th to 20th	Night	24% 5% 9% 27% 26% 10%	2339
	21st to end	Night	21% 6% 12% 28% 24% 9%	1980
March	1st to 10th	Night	30% 7% 7% 20% 26% 10%	5376
	11th to 20th	Night	22% 7% 9% 40% 19% 3	5139
	21st to end	Night	24% 9% 16% 27% 19% 4	5094
April	1st to 10th	Night	23% 7% 14% 30% 20% 7%	2000
	11th to 20th	Night	22% 5% 13% 27% 23% 10%	1979
	21st to end	Night	22% 4 15% 33% 23% 3	2000
May	1st to 10th	Night	15% 3 6% 36% 35% 6%	4245
	11th to 20th	Night	15% 4 7% 27% 37% 10%	3892
	21st to end	Night	13% 3 5% 25% 48% 7%	3972
June	1st to 10th	Night	18% 4 11% 28% 31% 8%	1595
	11th to 20th	Night	9% 2 8% 21% 39% 21%	1520
	21st to end	Night	11% 3 3 16% 31% 35%	1520
July	1st to 10th	Night	4 1 2 7% 19% 68%	2500
	11th to 20th	Night	15% 5% 2 7% 20% 50%	2492
	21st to end	Night	12% 3 2 14% 23% 46%	2761
August	1st to 10th	Night	11% 2 3 11% 32% 40%	2981
	11th to 20th	Night	7% 2 1 9% 20% 62%	3156
	21st to end	Night	6% 2 2 12% 36% 43%	3498
September	1st to 10th	Night	6% 2 1 14% 38% 39%	1900
	11th to 20th	Night	12% 2 5% 20% 36% 25%	1954
	21st to end	Night	13% 3 2 20% 37% 25%	2090
October	1st to 10th	Night	12% 3 4 13% 44% 25%	3671
	11th to 20th	Night	10% 3 3 16% 48% 19%	3777
	21st to end	Night	15% 2 5% 20% 38% 20%	4228
November	1st to 10th	Night	16% 5% 8% 29% 28% 14%	2535
	11th to 20th	Night	16% 5% 9% 27% 32% 11%	2600
	21st to end	Night	19% 5% 11% 24% 29% 13%	2600
December	1st to 10th	Night	14% 5% 10% 27% 26% 18%	4545
	11th to 20th	Night	24% 7% 14% 24% 24% 7%	5176
	21st to end	Night	33% 8% 12% 19% 20% 8%	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	13% 29% 30% 20% 10% 1	200
	11th to 20th	Day	9% 25% 30% 23% 11% 3	240
	21st to end	Day	7% 22% 32% 28% 10% 3	440
February	1st to 10th	Day	13% 27% 30% 19% 9% 2	430
	11th to 20th	Day	12% 28% 32% 20% 8% 1	495
	21st to end	Day	13% 25% 34% 22% 5%	414
March	1st to 10th	Day	15% 29% 32% 18% 5% 1	500
	11th to 20th	Day	12% 32% 29% 17% 10%	500
	21st to end	Day	14% 30% 32% 19% 5%	550
April	1st to 10th	Day	11% 30% 30% 20% 8% 1	500
	11th to 20th	Day	17% 28% 28% 18% 7% 1	492
	21st to end	Day	12% 28% 30% 19% 10% 2	500
May	1st to 10th	Day	15% 27% 27% 20% 8% 3	503
	11th to 20th	Day	14% 26% 27% 20% 10% 3	528
	21st to end	Day	14% 19% 25% 24% 13% 5%	573
June	1st to 10th	Day	9% 21% 30% 22% 14% 5%	527
	11th to 20th	Day	10% 24% 31% 21% 11% 4	490
	21st to end	Day	5% 18% 33% 26% 12% 5%	490
July	1st to 10th	Day	3 17% 32% 28% 14% 5%	486
	11th to 20th	Day	5% 13% 25% 33% 19% 5%	484
	21st to end	Day	6% 12% 30% 30% 17% 6%	531
August	1st to 10th	Day	4 17% 29% 27% 18% 4	487
	11th to 20th	Day	7% 17% 28% 27% 17% 4	477
	21st to end	Day	5% 14% 33% 29% 16% 3	539
September	1st to 10th	Day	4 14% 30% 31% 15% 4	490
	11th to 20th	Day	8% 21% 28% 28% 13% 3	490
	21st to end	Day	6% 15% 36% 30% 11% 2	490
October	1st to 10th	Day	7% 20% 29% 29% 14% 1	490
	11th to 20th	Day	7% 17% 28% 34% 11% 3	490
	21st to end	Day	6% 18% 33% 26% 14% 4	477
November	1st to 10th	Day	8% 19% 28% 23% 17% 5%	300
	11th to 20th	Day	5% 20% 28% 29% 13% 6%	300
	21st to end	Day	6% 20% 27% 29% 15% 3	300
December	1st to 10th	Day	9% 21% 21% 25% 17% 7%	205
	11th to 20th	Day	7% 24% 32% 24% 11% 4	200
	21st to end	Day	16% 20% 28% 25% 9% 2	220
January	1st to 10th	Night	6% 16% 33% 23% 14% 7%	800
	11th to 20th	Night	4 14% 31% 32% 14% 5%	800
	21st to end	Night	5% 12% 34% 30% 15% 5%	880
February	1st to 10th	Night	6% 15% 31% 30% 15% 3	800
	11th to 20th	Night	5% 16% 32% 28% 14% 6%	784
	21st to end	Night	5% 19% 29% 33% 10% 3	660
March	1st to 10th	Night	6% 18% 36% 27% 10% 3	800
	11th to 20th	Night	4 22% 28% 27% 15% 4	800
	21st to end	Night	5% 21% 34% 27% 11% 1	880
April	1st to 10th	Night	6% 15% 32% 33% 12% 2	800
	11th to 20th	Night	6% 18% 29% 29% 14% 4	791
	21st to end	Night	6% 14% 28% 32% 16% 4	800
May	1st to 10th	Night	8% 11% 31% 31% 15% 5%	670
	11th to 20th	Night	8% 10% 24% 40% 16% 2	514
	21st to end	Night	7% 6% 22% 40% 23% 2	433
June	1st to 10th	Night	6% 8% 23% 42% 18% 3	399
	11th to 20th	Night	2 7% 28% 40% 20% 3	380
	21st to end	Night	2 6% 22% 45% 22% 3	380
July	1st to 10th	Night	1 3 20% 49% 24% 3	370
	11th to 20th	Night	2 1 24% 47% 24% 2	368
	21st to end	Night	3 3 17% 42% 30% 4	469
August	1st to 10th	Night	2 2 21% 40% 29% 5%	644
	11th to 20th	Night	2 5% 22% 40% 26% 5%	752
	21st to end	Night	3 4 19% 45% 25% 4	836
September	1st to 10th	Night	1 4 22% 37% 30% 5%	760
	11th to 20th	Night	3 8% 21% 36% 26% 6%	760
	21st to end	Night	2 8% 24% 31% 28% 7%	760
October	1st to 10th	Night	5% 8% 19% 39% 23% 6%	760
	11th to 20th	Night	3 10% 19% 39% 23% 6%	760
	21st to end	Night	3 10% 25% 30% 21% 11%	836
November	1st to 10th	Night	3 13% 25% 31% 21% 7%	780
	11th to 20th	Night	5% 12% 31% 32% 14% 6%	800
	21st to end	Night	6% 14% 28% 31% 17% 6%	800
December	1st to 10th	Night	5% 13% 30% 33% 14% 5%	800
	11th to 20th	Night	5% 19% 32% 27% 13% 4	800
	21st to end	Night	7% 17% 35% 28% 11% 2	880

Page generated at 2025-06-07 07:28:14. Version ac07

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