Fossil
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fossil clear sky chart from 2005-11-06 to 2026-08-30.

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	7% 8% 3 7% 4 4 4 3 5% 6% 49%	153179
	Night	8% 9% 3 7% 4 4 4 4 5% 6% 45%	146351
Transparency	Day	34% 6% 11% 21% 19% 9%	147943
	Night	37% 7% 12% 22% 16% 6%	142588
Seeing	Day	13% 13% 26% 27% 16% 5%	16815
	Night	14% 7% 20% 37% 20% 2	25806

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	14% 18% 5% 13% 7% 6% 5% 4 5% 6% 18%	4722
	11th to 20th	Day	10% 10% 3 7% 5% 5% 3 3 5% 6% 44%	4740
	21st to end	Day	7% 10% 4 9% 5% 5% 5% 5% 6% 8% 36%	5307
February	1st to 10th	Day	11% 17% 5% 9% 6% 5% 4 4 6% 7% 26%	1890
	11th to 20th	Day	12% 15% 5% 10% 5% 5% 4 5% 5% 7% 27%	1892
	21st to end	Day	11% 17% 4 10% 6% 5% 5% 3 5% 7% 26%	1730
March	1st to 10th	Day	8% 12% 4 11% 6% 5% 6% 5% 7% 8% 27%	4618
	11th to 20th	Day	6% 14% 6% 11% 6% 5% 5% 4 4 5% 34%	5006
	21st to end	Day	8% 14% 6% 13% 8% 7% 6% 5% 6% 7% 21%	5961
April	1st to 10th	Day	11% 13% 4 9% 7% 5% 5% 5% 5% 7% 28%	2520
	11th to 20th	Day	10% 14% 4 8% 5% 4 5% 4 5% 5% 37%	2489
	21st to end	Day	9% 11% 3 6% 4 4 4 5% 6% 7% 40%	2619
May	1st to 10th	Day	8% 7% 2 5% 3 4 4 4 5% 6% 52%	8195
	11th to 20th	Day	8% 7% 3 7% 4 4 4 4 5% 7% 48%	8160
	21st to end	Day	8% 7% 2 6% 4 4 4 3 6% 7% 48%	8992
June	1st to 10th	Day	12% 8% 3 6% 5% 4 4 4 6% 7% 43%	2940
	11th to 20th	Day	9% 7% 3 5% 5% 4 4 3 5% 6% 49%	2940
	21st to end	Day	5% 4 1 3 2 2 4 3 5% 6% 64%	2940
July	1st to 10th	Day	2 2 1 2 2 2 2 2 3 5% 78%	8219
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 3 77%	8220
	21st to end	Day	2 2 1 2 1 2 2 2 3 4 79%	8922
August	1st to 10th	Day	3 4 1 4 2 2 3 2 4 6% 68%	6308
	11th to 20th	Day	3 2 1 3 1 2 2 2 3 5% 76%	5802
	21st to end	Day	4 4 2 5% 3 3 3 3 5% 6% 62%	6093
September	1st to 10th	Day	3 2 1 3 3 3 2 3 4 6% 71%	2400
	11th to 20th	Day	6% 5% 2 5% 4 3 4 3 4 7% 57%	2394
	21st to end	Day	9% 6% 2 6% 3 3 3 3 4 5% 56%	2170
October	1st to 10th	Day	7% 5% 2 5% 4 3 3 3 4 5% 59%	3280
	11th to 20th	Day	6% 6% 3 7% 5% 5% 5% 4 7% 9% 44%	3261
	21st to end	Day	10% 11% 4 8% 6% 6% 5% 4 6% 7% 33%	3279
November	1st to 10th	Day	14% 12% 4 9% 6% 6% 5% 4 6% 7% 28%	1854
	11th to 20th	Day	14% 13% 3 11% 6% 5% 7% 5% 6% 6% 23%	1736
	21st to end	Day	12% 13% 4 9% 5% 6% 4 4 6% 8% 29%	1680
December	1st to 10th	Day	10% 9% 3 7% 4 4 4 3 5% 6% 45%	2700
	11th to 20th	Day	11% 14% 5% 13% 7% 6% 6% 5% 7% 7% 19%	3108
	21st to end	Day	11% 17% 5% 13% 6% 6% 6% 5% 6% 6% 19%	4092
January	1st to 10th	Night	13% 18% 5% 13% 7% 6% 6% 5% 6% 6% 16%	8172
	11th to 20th	Night	11% 12% 3 6% 3 3 4 3 4 7% 45%	8220
	21st to end	Night	8% 12% 4 9% 5% 5% 5% 4 5% 7% 36%	8865
February	1st to 10th	Night	13% 14% 5% 10% 6% 4 5% 5% 5% 6% 27%	2730
	11th to 20th	Night	10% 16% 5% 11% 6% 4 4 4 5% 5% 31%	2615
	21st to end	Night	11% 18% 5% 9% 5% 5% 5% 4 5% 6% 27%	2076
March	1st to 10th	Night	6% 11% 3 9% 6% 5% 6% 6% 7% 8% 33%	5496
	11th to 20th	Night	6% 11% 4 10% 6% 5% 5% 4 6% 7% 36%	5211
	21st to end	Night	8% 14% 5% 12% 7% 7% 6% 5% 7% 7% 22%	5255
April	1st to 10th	Night	10% 13% 4 8% 5% 5% 4 4 5% 7% 35%	2100
	11th to 20th	Night	10% 11% 4 7% 4 4 5% 3 5% 6% 40%	2079
	21st to end	Night	9% 10% 3 5% 4 5% 5% 4 6% 6% 42%	1997
May	1st to 10th	Night	6% 8% 2 6% 4 3 3 3 4 6% 56%	4723
	11th to 20th	Night	7% 8% 2 5% 4 3 3 3 4 5% 54%	4700
	21st to end	Night	9% 7% 1 4 3 3 4 3 5% 7% 54%	5181
June	1st to 10th	Night	11% 6% 2 6% 4 4 3 4 5% 8% 47%	1680
	11th to 20th	Night	9% 8% 2 5% 3 3 3 3 4 7% 52%	1680
	21st to end	Night	6% 4 1 4 3 2 4 2 4 4 65%	1680
July	1st to 10th	Night	1 1 2 2 2 2 3 4 5% 78%	4740
	11th to 20th	Night	2 2 1 4 3 3 2 2 3 4 75%	4740
	21st to end	Night	2 2 1 2 2 2 2 2 4 5% 77%	5153
August	1st to 10th	Night	2 3 2 4 3 3 3 3 6% 7% 64%	3703
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 5% 74%	4270
	21st to end	Night	3 4 2 5% 2 2 4 3 4 5% 67%	5105
September	1st to 10th	Night	2 2 1 3 2 2 3 3 4 6% 72%	2000
	11th to 20th	Night	5% 4 1 4 3 3 4 4 5% 6% 60%	2005
	21st to end	Night	8% 5% 2 5% 3 3 3 3 3 6% 59%	2226
October	1st to 10th	Night	6% 5% 2 4 3 3 3 2 4 5% 64%	3920
	11th to 20th	Night	8% 5% 2 7% 4 5% 5% 3 6% 8% 47%	3897
	21st to end	Night	9% 8% 3 9% 5% 6% 5% 4 6% 7% 37%	4523
November	1st to 10th	Night	14% 13% 4 9% 5% 5% 5% 3 4 6% 31%	2665
	11th to 20th	Night	14% 12% 4 10% 6% 5% 5% 4 5% 6% 28%	2882
	21st to end	Night	11% 12% 4 9% 5% 5% 5% 5% 6% 8% 32%	2940
December	1st to 10th	Night	8% 10% 3 6% 4 4 4 3 5% 7% 47%	4700
	11th to 20th	Night	11% 17% 6% 14% 7% 7% 6% 5% 6% 6% 16%	5316
	21st to end	Night	12% 15% 5% 11% 6% 6% 6% 4 6% 7% 23%	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	61% 11% 14% 11% 2	4723
	11th to 20th	Day	39% 6% 9% 19% 22% 4	4740
	21st to end	Day	43% 10% 12% 15% 16% 4	5307
February	1st to 10th	Day	54% 8% 16% 16% 6%	1890
	11th to 20th	Day	52% 9% 15% 16% 7% 1	1901
	21st to end	Day	54% 8% 15% 14% 8% 2	1730
March	1st to 10th	Day	45% 8% 13% 21% 10% 3	4618
	11th to 20th	Day	46% 8% 12% 24% 9%	5006
	21st to end	Day	54% 10% 14% 16% 5%	5961
April	1st to 10th	Day	51% 8% 14% 19% 8% 1	2520
	11th to 20th	Day	45% 6% 14% 19% 13% 2	2492
	21st to end	Day	39% 7% 14% 26% 13% 2	2619
May	1st to 10th	Day	27% 7% 9% 30% 25% 3	8195
	11th to 20th	Day	32% 5% 13% 27% 20% 2	8163
	21st to end	Day	30% 7% 16% 29% 16% 1	8994
June	1st to 10th	Day	39% 6% 11% 26% 17% 2	2940
	11th to 20th	Day	31% 5% 11% 28% 21% 4	2940
	21st to end	Day	21% 4 11% 29% 26% 9%	2940
July	1st to 10th	Day	11% 2 5% 19% 34% 30%	8151
	11th to 20th	Day	13% 2 4 9% 29% 43%	8145
	21st to end	Day	10% 2 5% 26% 37% 20%	8723
August	1st to 10th	Day	16% 3 4 15% 37% 25%	4634
	11th to 20th	Day	13% 3 6% 33% 33% 13%	4237
	21st to end	Day	18% 3 7% 28% 26% 18%	4420
September	1st to 10th	Day	15% 3 9% 25% 30% 18%	2400
	11th to 20th	Day	26% 6% 9% 25% 25% 11%	2394
	21st to end	Day	29% 5% 12% 25% 22% 8%	2170
October	1st to 10th	Day	28% 3 8% 23% 29% 9%	3280
	11th to 20th	Day	35% 6% 17% 25% 16% 2	3261
	21st to end	Day	46% 6% 10% 24% 12% 1	3279
November	1st to 10th	Day	52% 7% 14% 19% 7% 1	1854
	11th to 20th	Day	56% 9% 15% 15% 4	1736
	21st to end	Day	50% 7% 18% 17% 7% 1	1680
December	1st to 10th	Day	39% 8% 12% 23% 18% 1	2700
	11th to 20th	Day	61% 10% 16% 11% 2	3108
	21st to end	Day	62% 10% 15% 11% 2	4092
January	1st to 10th	Night	62% 12% 12% 12% 2	8172
	11th to 20th	Night	38% 7% 8% 20% 22% 4	8220
	21st to end	Night	47% 10% 8% 14% 17% 4	8865
February	1st to 10th	Night	54% 8% 16% 15% 6% 1	2730
	11th to 20th	Night	51% 7% 15% 19% 7% 1	2617
	21st to end	Night	53% 9% 12% 15% 9% 1	2076
March	1st to 10th	Night	40% 8% 12% 24% 13% 3	5496
	11th to 20th	Night	40% 9% 15% 28% 8%	5211
	21st to end	Night	52% 13% 16% 16% 3	5255
April	1st to 10th	Night	45% 8% 15% 24% 7%	2100
	11th to 20th	Night	42% 7% 17% 22% 11% 2	2079
	21st to end	Night	39% 8% 14% 23% 13% 2	1997
May	1st to 10th	Night	26% 5% 11% 30% 28% 1	4721
	11th to 20th	Night	29% 5% 17% 32% 17% 1	4700
	21st to end	Night	27% 7% 23% 28% 15% 1	5182
June	1st to 10th	Night	34% 7% 13% 28% 15% 2	1680
	11th to 20th	Night	31% 6% 12% 31% 15% 4	1680
	21st to end	Night	21% 4 13% 31% 25% 7%	1680
July	1st to 10th	Night	9% 2 7% 25% 29% 27%	4700
	11th to 20th	Night	15% 3 5% 10% 28% 39%	4692
	21st to end	Night	11% 2 5% 31% 35% 16%	5040
August	1st to 10th	Night	15% 3 6% 16% 36% 23%	2709
	11th to 20th	Night	12% 3 8% 39% 28% 10%	3108
	21st to end	Night	15% 3 11% 29% 26% 16%	3698
September	1st to 10th	Night	14% 3 10% 27% 29% 16%	2000
	11th to 20th	Night	23% 5% 11% 25% 23% 13%	2005
	21st to end	Night	26% 4 9% 28% 25% 8%	2226
October	1st to 10th	Night	23% 3 10% 24% 34% 4	3920
	11th to 20th	Night	33% 5% 18% 23% 19% 2	3897
	21st to end	Night	42% 8% 10% 27% 12% 1	4523
November	1st to 10th	Night	52% 7% 12% 20% 8% 1	2665
	11th to 20th	Night	53% 9% 16% 18% 5%	2882
	21st to end	Night	46% 9% 16% 20% 8%	2940
December	1st to 10th	Night	36% 8% 11% 25% 20% 1	4700
	11th to 20th	Night	65% 8% 13% 12% 2	5316
	21st to end	Night	57% 11% 17% 12% 3	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	29% 16% 21% 20% 12% 3	220
	11th to 20th	Day	15% 13% 29% 26% 14% 4	220
	21st to end	Day	15% 11% 31% 26% 14% 3	284
February	1st to 10th	Day	25% 15% 25% 24% 9% 2	430
	11th to 20th	Day	22% 14% 31% 21% 12%	450
	21st to end	Day	21% 17% 28% 25% 8% 1	446
March	1st to 10th	Day	20% 19% 28% 24% 8% 1	540
	11th to 20th	Day	21% 20% 24% 21% 11% 3	540
	21st to end	Day	20% 20% 28% 24% 8% 1	594
April	1st to 10th	Day	21% 15% 26% 23% 11% 4	540
	11th to 20th	Day	17% 17% 32% 21% 10% 4	532
	21st to end	Day	11% 19% 33% 21% 13% 3	540
May	1st to 10th	Day	13% 16% 29% 25% 13% 4	543
	11th to 20th	Day	21% 14% 24% 25% 12% 4	568
	21st to end	Day	13% 14% 24% 24% 18% 7%	617
June	1st to 10th	Day	14% 15% 23% 25% 16% 7%	570
	11th to 20th	Day	12% 18% 28% 26% 12% 3	570
	21st to end	Day	6% 10% 27% 32% 18% 5%	570
July	1st to 10th	Day	2 7% 29% 36% 22% 4	566
	11th to 20th	Day	4 10% 30% 32% 20% 5%	564
	21st to end	Day	2 8% 28% 32% 24% 6%	619
August	1st to 10th	Day	3 8% 25% 35% 18% 10%	567
	11th to 20th	Day	3 6% 22% 32% 25% 11%	557
	21st to end	Day	6% 9% 26% 28% 24% 7%	620
September	1st to 10th	Day	3 6% 23% 32% 25% 10%	530
	11th to 20th	Day	9% 7% 21% 33% 22% 6%	530
	21st to end	Day	12% 9% 22% 30% 23% 5%	530
October	1st to 10th	Day	15% 9% 26% 25% 19% 6%	530
	11th to 20th	Day	14% 12% 25% 23% 22% 5%	530
	21st to end	Day	17% 14% 24% 25% 15% 5%	418
November	1st to 10th	Day	16% 18% 22% 27% 15% 2	330
	11th to 20th	Day	19% 15% 19% 28% 16% 4	248
	21st to end	Day	18% 15% 22% 25% 19% 1	220
December	1st to 10th	Day	16% 16% 19% 23% 18% 9%	220
	11th to 20th	Day	27% 11% 32% 22% 8%	220
	21st to end	Day	21% 16% 24% 24% 13% 2	242
January	1st to 10th	Night	22% 12% 24% 28% 13% 1	840
	11th to 20th	Night	19% 10% 22% 30% 18% 3	840
	21st to end	Night	18% 7% 21% 35% 18% 1	924
February	1st to 10th	Night	20% 8% 23% 33% 14% 1	840
	11th to 20th	Night	16% 10% 25% 34% 15% 1	824
	21st to end	Night	21% 13% 25% 32% 8% 1	692
March	1st to 10th	Night	16% 10% 25% 35% 13% 1	840
	11th to 20th	Night	14% 10% 23% 37% 15% 1	840
	21st to end	Night	15% 12% 28% 33% 12% 1	924
April	1st to 10th	Night	15% 9% 23% 35% 16% 2	840
	11th to 20th	Night	15% 9% 26% 32% 16% 3	831
	21st to end	Night	13% 8% 26% 33% 18% 2	737
May	1st to 10th	Night	9% 7% 20% 40% 22% 2	420
	11th to 20th	Night	13% 5% 17% 37% 26% 1	414
	21st to end	Night	11% 5% 18% 39% 24% 3	455
June	1st to 10th	Night	12% 5% 19% 40% 23% 1	420
	11th to 20th	Night	11% 6% 20% 44% 16% 1	420
	21st to end	Night	8% 3 13% 47% 28% 1	420
July	1st to 10th	Night	2 2 10% 50% 34% 2	410
	11th to 20th	Night	2 13% 56% 26% 2	408
	21st to end	Night	1 1 10% 53% 34% 2	460
August	1st to 10th	Night	3 7% 50% 37% 2	433
	11th to 20th	Night	3 1 9% 38% 41% 8%	674
	21st to end	Night	3 4 16% 45% 28% 4	916
September	1st to 10th	Night	4 2 14% 41% 36% 4	800
	11th to 20th	Night	6% 3 13% 40% 33% 7%	800
	21st to end	Night	10% 3 14% 37% 31% 5%	800
October	1st to 10th	Night	12% 3 16% 40% 27% 2	800
	11th to 20th	Night	13% 6% 17% 37% 24% 4	800
	21st to end	Night	15% 7% 21% 36% 17% 4	880
November	1st to 10th	Night	18% 9% 21% 37% 15%	820
	11th to 20th	Night	20% 9% 23% 32% 14% 2	840
	21st to end	Night	16% 9% 20% 36% 16% 3	840
December	1st to 10th	Night	19% 8% 21% 34% 16% 3	840
	11th to 20th	Night	22% 9% 22% 33% 13% 1	840
	21st to end	Night	23% 12% 27% 27% 10%	924

Page generated at 2026-08-27 01:07:15. Version ac07

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