Fossil Falls
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fossil Falls clear sky chart from 2005-11-06 to 2026-08-15.

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	1 1 1 2 1 2 2 2 3 5% 80%	151922
	Night	1 1 1 2 1 2 2 2 3 5% 79%	147464
Transparency	Day	9% 3 5% 14% 29% 40%	146997
	Night	9% 3 5% 17% 31% 34%	143364
Seeing	Day	5% 17% 37% 29% 9% 2	17695
	Night	3 10% 29% 34% 19% 5%	27334

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	3 3 1 2 2 2 2 2 5% 7% 71%	4722
	11th to 20th	Day	1 1 2 2 2 2 2 3 5% 79%	4878
	21st to end	Day	3 3 1 3 2 3 3 3 5% 8% 66%	6218
February	1st to 10th	Day	3 2 1 2 2 3 3 2 4 7% 72%	2100
	11th to 20th	Day	3 2 1 2 2 2 2 3 4 10% 68%	2050
	21st to end	Day	3 2 1 2 2 2 3 2 5% 7% 72%	1730
March	1st to 10th	Day	1 2 2 4 3 2 3 2 5% 8% 69%	4618
	11th to 20th	Day	2 3 1 3 2 2 3 3 5% 8% 68%	4765
	21st to end	Day	2 2 1 2 2 3 3 3 5% 8% 71%	6101
April	1st to 10th	Day	2 2 1 2 2 2 3 2 4 7% 74%	2520
	11th to 20th	Day	1 2 1 2 1 2 2 3 4 5% 78%	2489
	21st to end	Day	1 1 1 2 2 1 2 2 5% 7% 78%	2520
May	1st to 10th	Day	1 1 1 2 2 2 2 2 3 4 81%	7503
	11th to 20th	Day	1 1 1 1 1 2 3 5% 84%	7596
	21st to end	Day	1 1 1 1 1 1 1 3 4 86%	8370
June	1st to 10th	Day	1 1 2 1 2 2 2 4 6% 79%	2897
	11th to 20th	Day	1 1 1 1 2 3 4 86%	2940
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	2940
July	1st to 10th	Day	1 1 1 1 1 1 1 2 92%	8219
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 4 80%	8220
	21st to end	Day	1 1 1 1 1 2 2 2 3 87%	8355
August	1st to 10th	Day	1 1 1 1 2 2 1 2 4 84%	5466
	11th to 20th	Day	1 1 1 1 2 93%	5413
	21st to end	Day	1 1 2 1 2 2 2 3 3 84%	5984
September	1st to 10th	Day	1 1 1 2 1 1 2 2 3 5% 81%	2400
	11th to 20th	Day	1 1 1 1 1 1 2 3 88%	2250
	21st to end	Day	1 1 1 1 1 1 3 3 88%	2200
October	1st to 10th	Day	1 1 1 1 1 1 1 3 4 86%	3600
	11th to 20th	Day	1 1 1 1 1 2 1 3 4 84%	3340
	21st to end	Day	1 1 2 1 1 2 2 3 4 82%	3376
November	1st to 10th	Day	1 1 1 1 2 2 1 4 5% 81%	1854
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 5% 77%	1890
	21st to end	Day	3 1 2 2 1 2 3 5% 6% 73%	1890
December	1st to 10th	Day	1 1 1 2 2 2 3 3 6% 7% 73%	3040
	11th to 20th	Day	2 2 1 3 1 3 3 3 5% 8% 69%	3376
	21st to end	Day	4 3 1 4 2 2 3 3 6% 10% 61%	4092
January	1st to 10th	Night	4 2 1 2 2 5% 3 2 3 5% 71%	8172
	11th to 20th	Night	1 1 2 1 2 2 2 3 7% 79%	8074
	21st to end	Night	3 4 1 3 2 3 3 3 5% 6% 65%	8000
February	1st to 10th	Night	2 3 1 3 2 3 3 4 4 7% 69%	2520
	11th to 20th	Night	2 2 1 2 2 2 2 2 5% 6% 72%	2458
	21st to end	Night	3 2 1 2 2 2 3 2 4 7% 73%	2076
March	1st to 10th	Night	1 3 1 3 2 3 3 3 5% 7% 71%	5496
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 7% 68%	5439
	21st to end	Night	1 2 1 3 2 3 3 3 5% 7% 71%	5116
April	1st to 10th	Night	2 1 1 2 2 2 3 2 4 6% 75%	2100
	11th to 20th	Night	1 1 1 1 2 1 2 2 3 5% 81%	2079
	21st to end	Night	1 1 1 2 1 2 4 5% 81%	2100
May	1st to 10th	Night	1 1 2 1 1 2 1 3 5% 83%	5446
	11th to 20th	Night	1 1 1 1 1 1 2 4 90%	5296
	21st to end	Night	1 1 1 1 1 2 3 89%	5837
June	1st to 10th	Night	1 1 1 1 1 1 2 3 5% 84%	1717
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 89%	1680
	21st to end	Night	1 1 1 2 1 3 90%	1680
July	1st to 10th	Night	1 1 1 1 1 2 93%	4740
	11th to 20th	Night	1 1 2 1 2 2 2 2 3 83%	4740
	21st to end	Night	1 1 1 1 1 2 3 88%	5718
August	1st to 10th	Night	1 1 1 1 1 1 1 2 4 87%	4586
	11th to 20th	Night	1 1 1 1 2 94%	4546
	21st to end	Night	1 1 1 1 1 2 2 3 3 85%	5015
September	1st to 10th	Night	1 1 2 1 1 1 1 2 4 86%	2000
	11th to 20th	Night	1 1 1 1 1 2 2 89%	2150
	21st to end	Night	1 1 1 1 3 4 89%	2200
October	1st to 10th	Night	1 1 1 1 1 1 2 3 90%	3600
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 88%	3801
	21st to end	Night	1 1 1 1 1 1 1 3 4 84%	4427
November	1st to 10th	Night	1 1 2 1 1 1 2 2 5% 83%	2665
	11th to 20th	Night	2 1 2 2 2 2 2 4 6% 77%	2730
	21st to end	Night	2 2 1 2 2 2 2 2 5% 6% 75%	2730
December	1st to 10th	Night	1 1 1 2 2 2 2 2 4 7% 75%	4370
	11th to 20th	Night	1 2 1 2 2 2 3 2 5% 9% 70%	5054
	21st to end	Night	5% 3 1 3 3 3 3 4 6% 9% 60%	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	13% 4 9% 28% 33% 12%	4723
	11th to 20th	Day	10% 4 9% 14% 34% 30%	4878
	21st to end	Day	17% 6% 7% 21% 33% 17%	6218
February	1st to 10th	Day	15% 4 7% 21% 31% 22%	2100
	11th to 20th	Day	13% 5% 8% 23% 31% 20%	2060
	21st to end	Day	13% 5% 10% 26% 31% 15%	1730
March	1st to 10th	Day	15% 5% 6% 20% 27% 27%	4618
	11th to 20th	Day	13% 6% 8% 29% 28% 16%	4765
	21st to end	Day	10% 6% 10% 25% 31% 18%	6101
April	1st to 10th	Day	12% 4 8% 23% 32% 21%	2520
	11th to 20th	Day	10% 4 6% 20% 36% 24%	2492
	21st to end	Day	8% 4 7% 21% 40% 20%	2520
May	1st to 10th	Day	9% 4 4 15% 42% 26%	7503
	11th to 20th	Day	5% 3 4 10% 45% 34%	7599
	21st to end	Day	5% 2 3 10% 45% 35%	8371
June	1st to 10th	Day	8% 4 5% 12% 29% 43%	2897
	11th to 20th	Day	5% 2 3 8% 19% 63%	2940
	21st to end	Day	6% 1 2 4 17% 70%	2940
July	1st to 10th	Day	3 1 1 3 9% 83%	8151
	11th to 20th	Day	8% 3 3 9% 15% 62%	8145
	21st to end	Day	5% 2 2 10% 20% 61%	8169
August	1st to 10th	Day	8% 2 2 6% 23% 59%	4010
	11th to 20th	Day	3 2 2 3 15% 76%	3929
	21st to end	Day	4 1 1 5% 16% 73%	4311
September	1st to 10th	Day	8% 3 3 8% 18% 59%	2400
	11th to 20th	Day	4 2 3 7% 29% 55%	2250
	21st to end	Day	5% 2 2 8% 35% 48%	2200
October	1st to 10th	Day	5% 2 2 8% 24% 60%	3600
	11th to 20th	Day	6% 2 2 6% 33% 51%	3340
	21st to end	Day	8% 3 2 8% 38% 41%	3376
November	1st to 10th	Day	8% 3 4 15% 41% 29%	1854
	11th to 20th	Day	11% 4 6% 20% 34% 26%	1890
	21st to end	Day	12% 4 8% 19% 31% 25%	1890
December	1st to 10th	Day	11% 5% 7% 25% 29% 23%	3040
	11th to 20th	Day	14% 6% 8% 26% 35% 12%	3375
	21st to end	Day	19% 6% 12% 29% 23% 10%	4092
January	1st to 10th	Night	13% 5% 9% 25% 35% 12%	8172
	11th to 20th	Night	9% 3 7% 20% 39% 22%	8074
	21st to end	Night	18% 5% 8% 21% 32% 15%	8000
February	1st to 10th	Night	16% 4 8% 22% 28% 21%	2520
	11th to 20th	Night	13% 4 6% 23% 33% 22%	2459
	21st to end	Night	12% 4 9% 24% 34% 18%	2076
March	1st to 10th	Night	13% 5% 7% 21% 28% 26%	5496
	11th to 20th	Night	14% 8% 7% 27% 30% 14%	5439
	21st to end	Night	11% 5% 7% 30% 32% 14%	5116
April	1st to 10th	Night	12% 4 6% 25% 35% 18%	2100
	11th to 20th	Night	9% 3 5% 26% 33% 24%	2079
	21st to end	Night	7% 3 5% 24% 43% 17%	2100
May	1st to 10th	Night	7% 3 5% 17% 46% 22%	5444
	11th to 20th	Night	4 1 2 16% 49% 28%	5297
	21st to end	Night	4 2 3 14% 48% 30%	5838
June	1st to 10th	Night	6% 2 4 15% 34% 38%	1717
	11th to 20th	Night	5% 2 2 10% 24% 57%	1680
	21st to end	Night	4 1 1 5% 19% 69%	1680
July	1st to 10th	Night	3 1 1 2 10% 84%	4700
	11th to 20th	Night	8% 2 2 8% 14% 65%	4692
	21st to end	Night	5% 2 1 10% 21% 61%	5592
August	1st to 10th	Night	7% 2 2 7% 24% 59%	3356
	11th to 20th	Night	2 1 2 4 16% 75%	3296
	21st to end	Night	4 1 1 4 18% 72%	3608
September	1st to 10th	Night	7% 2 3 6% 20% 63%	2000
	11th to 20th	Night	5% 3 2 7% 28% 55%	2150
	21st to end	Night	4 2 3 8% 30% 53%	2200
October	1st to 10th	Night	4 2 2 8% 20% 64%	3600
	11th to 20th	Night	5% 1 2 7% 31% 54%	3801
	21st to end	Night	6% 3 3 7% 39% 43%	4427
November	1st to 10th	Night	7% 2 5% 16% 41% 29%	2665
	11th to 20th	Night	10% 4 8% 18% 31% 29%	2730
	21st to end	Night	12% 3 6% 21% 33% 25%	2730
December	1st to 10th	Night	11% 4 5% 25% 33% 21%	4370
	11th to 20th	Night	12% 6% 11% 27% 33% 11%	5054
	21st to end	Night	19% 6% 12% 28% 23% 11%	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	11% 15% 43% 22% 8%	220
	11th to 20th	Day	11% 24% 30% 24% 10% 2	267
	21st to end	Day	11% 19% 32% 29% 7% 1	542
February	1st to 10th	Day	7% 27% 37% 21% 6% 2	540
	11th to 20th	Day	10% 26% 39% 19% 5% 1	535
	21st to end	Day	8% 25% 45% 18% 4	446
March	1st to 10th	Day	11% 28% 36% 19% 4 1	540
	11th to 20th	Day	9% 28% 41% 16% 4 1	540
	21st to end	Day	10% 28% 43% 16% 3	594
April	1st to 10th	Day	11% 24% 40% 19% 6% 1	540
	11th to 20th	Day	8% 26% 39% 22% 5% 1	532
	21st to end	Day	5% 25% 40% 21% 6% 3	540
May	1st to 10th	Day	7% 24% 42% 21% 5% 2	543
	11th to 20th	Day	5% 24% 36% 26% 7% 3	568
	21st to end	Day	5% 17% 38% 29% 8% 3	617
June	1st to 10th	Day	3 11% 38% 35% 10% 4	570
	11th to 20th	Day	3 15% 39% 30% 10% 3	570
	21st to end	Day	3 14% 36% 34% 10% 4	570
July	1st to 10th	Day	2 8% 36% 37% 13% 5%	566
	11th to 20th	Day	1 4 29% 41% 17% 7%	564
	21st to end	Day	2 5% 31% 41% 16% 6%	619
August	1st to 10th	Day	2 8% 35% 34% 15% 6%	567
	11th to 20th	Day	2 8% 31% 37% 15% 7%	534
	21st to end	Day	1 5% 35% 40% 14% 4	583
September	1st to 10th	Day	2 9% 34% 38% 16% 2	530
	11th to 20th	Day	3 14% 36% 36% 10% 1	530
	21st to end	Day	3 12% 40% 32% 12% 1	530
October	1st to 10th	Day	4 14% 37% 35% 9%	530
	11th to 20th	Day	5% 13% 36% 34% 11% 1	530
	21st to end	Day	4 14% 37% 32% 13% 1	476
November	1st to 10th	Day	7% 14% 30% 35% 12% 2	330
	11th to 20th	Day	7% 17% 37% 23% 13% 3	330
	21st to end	Day	7% 14% 38% 28% 12% 2	330
December	1st to 10th	Day	6% 15% 42% 29% 7% 1	330
	11th to 20th	Day	3 18% 39% 27% 12% 2	300
	21st to end	Day	14% 22% 36% 21% 6% 1	242
January	1st to 10th	Night	5% 13% 32% 24% 19% 7%	840
	11th to 20th	Night	3 12% 33% 32% 15% 4	840
	21st to end	Night	7% 12% 33% 28% 14% 5%	924
February	1st to 10th	Night	4 13% 34% 27% 16% 5%	840
	11th to 20th	Night	6% 13% 33% 29% 15% 4	824
	21st to end	Night	5% 15% 38% 26% 12% 3	692
March	1st to 10th	Night	5% 19% 35% 27% 11% 3	840
	11th to 20th	Night	5% 17% 34% 23% 15% 6%	840
	21st to end	Night	4 15% 38% 30% 11% 2	924
April	1st to 10th	Night	3 16% 37% 28% 13% 3	840
	11th to 20th	Night	2 17% 37% 27% 15% 3	831
	21st to end	Night	1 15% 35% 29% 16% 4	840
May	1st to 10th	Night	2 14% 39% 32% 12% 2	677
	11th to 20th	Night	2 11% 31% 37% 17% 2	621
	21st to end	Night	2 12% 29% 41% 16% 1	683
June	1st to 10th	Night	1 5% 26% 48% 18% 2	457
	11th to 20th	Night	1 7% 30% 40% 19% 2	420
	21st to end	Night	1 3 27% 46% 20% 2	420
July	1st to 10th	Night	1 1 21% 47% 26% 4	410
	11th to 20th	Night	1 12% 51% 33% 3	408
	21st to end	Night	1 1 15% 48% 32% 3	655
August	1st to 10th	Night	2 2 17% 48% 27% 5%	836
	11th to 20th	Night	1 1 16% 44% 31% 7%	808
	21st to end	Night	1 1 17% 40% 31% 9%	880
September	1st to 10th	Night	1 3 16% 38% 35% 8%	800
	11th to 20th	Night	1 6% 22% 37% 28% 8%	800
	21st to end	Night	6% 24% 33% 27% 10%	800
October	1st to 10th	Night	1 6% 25% 45% 18% 5%	800
	11th to 20th	Night	4 7% 23% 37% 20% 10%	800
	21st to end	Night	3 9% 26% 29% 24% 10%	880
November	1st to 10th	Night	3 10% 24% 34% 22% 8%	820
	11th to 20th	Night	2 8% 26% 34% 22% 7%	840
	21st to end	Night	4 9% 33% 31% 18% 5%	840
December	1st to 10th	Night	5% 10% 33% 29% 18% 5%	840
	11th to 20th	Night	4 9% 34% 34% 15% 4	840
	21st to end	Night	5% 16% 34% 26% 15% 4	924

Page generated at 2026-08-12 00:37:18. Version ac07

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