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 2024-10-25.

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	8% 9% 3 6% 4 4 4 3 5% 6% 48%	101456
	Night	9% 10% 3 7% 4 4 4 4 5% 6% 43%	97511
Transparency	Day	35% 7% 12% 20% 17% 10%	101199
	Night	39% 8% 13% 20% 13% 6%	97357
Seeing	Day	13% 13% 25% 27% 17% 5%	14510
	Night	14% 7% 19% 37% 21% 2	23545

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	16% 19% 5% 12% 7% 6% 5% 4 4 5% 17%	2539
	11th to 20th	Day	18% 18% 5% 10% 7% 5% 4 4 5% 5% 18%	2540
	21st to end	Day	13% 15% 5% 11% 6% 5% 5% 4 6% 8% 23%	2858
February	1st to 10th	Day	11% 18% 4 8% 6% 5% 4 4 5% 6% 27%	1710
	11th to 20th	Day	13% 15% 5% 10% 5% 5% 3 5% 5% 7% 28%	1707
	21st to end	Day	11% 18% 5% 11% 6% 5% 5% 4 5% 7% 24%	1570
March	1st to 10th	Day	8% 13% 4 10% 6% 5% 6% 6% 6% 8% 28%	3138
	11th to 20th	Day	6% 12% 3 7% 5% 3 4 3 4 5% 47%	3415
	21st to end	Day	10% 15% 5% 11% 7% 6% 5% 5% 6% 7% 22%	4067
April	1st to 10th	Day	12% 14% 4 9% 6% 5% 5% 5% 5% 7% 27%	2280
	11th to 20th	Day	10% 14% 4 8% 5% 4 5% 4 5% 5% 36%	2249
	21st to end	Day	9% 12% 3 6% 5% 4 4 5% 6% 7% 38%	2369
May	1st to 10th	Day	11% 11% 3 6% 4 4 4 3 5% 6% 42%	4420
	11th to 20th	Day	7% 5% 2 4 3 3 2 3 4 6% 60%	4377
	21st to end	Day	11% 9% 3 7% 4 4 5% 4 6% 7% 40%	4813
June	1st to 10th	Day	13% 8% 3 6% 5% 4 4 4 6% 7% 41%	2660
	11th to 20th	Day	10% 7% 3 5% 4 4 4 3 5% 6% 48%	2660
	21st to end	Day	5% 4 1 3 2 2 4 3 5% 6% 65%	2660
July	1st to 10th	Day	1 1 1 2 1 1 2 2 3 4 83%	4420
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 81%	4420
	21st to end	Day	2 2 1 2 2 2 2 2 3 4 79%	4790
August	1st to 10th	Day	3 4 1 3 2 2 3 2 4 5% 71%	4372
	11th to 20th	Day	2 2 1 3 2 2 3 2 4 5% 74%	3943
	21st to end	Day	3 3 2 5% 2 2 3 2 4 5% 70%	4180
September	1st to 10th	Day	3 2 1 3 2 2 2 3 4 5% 72%	2280
	11th to 20th	Day	6% 5% 2 5% 4 3 4 3 4 7% 56%	2275
	21st to end	Day	9% 6% 2 6% 3 4 3 3 4 5% 55%	2062
October	1st to 10th	Day	11% 7% 3 7% 5% 4 3 3 4 5% 48%	1900
	11th to 20th	Day	9% 7% 3 6% 5% 4 5% 3 7% 8% 44%	1900
	21st to end	Day	14% 8% 4 7% 5% 4 4 4 4 7% 39%	1874
November	1st to 10th	Day	15% 12% 3 8% 6% 5% 5% 5% 6% 7% 28%	1674
	11th to 20th	Day	15% 13% 3 11% 6% 5% 7% 4 6% 6% 24%	1572
	21st to end	Day	12% 14% 4 8% 5% 6% 4 4 6% 7% 29%	1520
December	1st to 10th	Day	11% 12% 3 9% 6% 6% 5% 4 6% 6% 31%	1520
	11th to 20th	Day	12% 14% 5% 10% 6% 6% 5% 5% 7% 8% 22%	1928
	21st to end	Day	13% 18% 4 11% 6% 6% 5% 4 6% 6% 21%	2794
January	1st to 10th	Night	14% 19% 5% 12% 7% 6% 6% 5% 6% 6% 15%	4406
	11th to 20th	Night	20% 21% 5% 9% 5% 4 5% 3 5% 7% 16%	4420
	21st to end	Night	12% 15% 4 11% 6% 6% 6% 5% 6% 8% 21%	4798
February	1st to 10th	Night	14% 14% 5% 10% 5% 4 5% 5% 5% 6% 27%	2470
	11th to 20th	Night	11% 16% 4 10% 6% 4 4 4 5% 5% 31%	2360
	21st to end	Night	12% 19% 5% 9% 5% 5% 5% 4 5% 6% 26%	1884
March	1st to 10th	Night	7% 12% 3 10% 6% 5% 6% 5% 6% 8% 32%	3736
	11th to 20th	Night	6% 9% 3 7% 4 4 4 3 6% 6% 47%	3562
	21st to end	Night	9% 16% 4 11% 7% 7% 7% 6% 7% 6% 23%	3585
April	1st to 10th	Night	11% 14% 4 8% 5% 5% 4 4 5% 7% 34%	1900
	11th to 20th	Night	11% 12% 4 7% 4 4 5% 3 5% 6% 39%	1879
	21st to end	Night	9% 11% 3 6% 5% 5% 5% 4 6% 7% 40%	1807
May	1st to 10th	Night	10% 13% 3 7% 5% 3 3 3 3 5% 44%	2540
	11th to 20th	Night	7% 4 1 4 3 3 2 3 4 5% 64%	2516
	21st to end	Night	11% 9% 2 4 3 4 4 3 5% 7% 49%	2762
June	1st to 10th	Night	12% 7% 2 5% 4 4 3 4 5% 8% 45%	1520
	11th to 20th	Night	10% 8% 2 5% 3 3 3 3 4 7% 51%	1520
	21st to end	Night	5% 4 1 3 3 2 4 2 4 5% 66%	1520
July	1st to 10th	Night	2 1 1 1 1 1 2 3 3 84%	2540
	11th to 20th	Night	1 1 1 3 2 2 2 1 3 4 80%	2540
	21st to end	Night	1 1 1 2 3 2 2 2 3 4 81%	2761
August	1st to 10th	Night	2 3 1 3 3 3 3 3 5% 6% 68%	2555
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 6% 74%	2896
	21st to end	Night	2 3 1 4 2 2 3 3 3 4 74%	3498
September	1st to 10th	Night	3 2 1 3 2 2 2 3 3 5% 75%	1900
	11th to 20th	Night	5% 4 1 4 4 3 4 4 5% 6% 60%	1905
	21st to end	Night	8% 5% 2 5% 3 3 3 3 4 7% 59%	2114
October	1st to 10th	Night	10% 6% 3 5% 4 3 3 3 5% 6% 53%	2280
	11th to 20th	Night	10% 7% 2 6% 4 4 4 3 5% 7% 47%	2280
	21st to end	Night	12% 8% 3 8% 5% 5% 4 3 5% 5% 41%	2587
November	1st to 10th	Night	15% 12% 4 9% 5% 5% 5% 3 4 6% 32%	2405
	11th to 20th	Night	15% 12% 4 9% 5% 5% 5% 4 5% 6% 29%	2607
	21st to end	Night	11% 12% 4 8% 5% 5% 5% 5% 6% 8% 32%	2660
December	1st to 10th	Night	12% 14% 4 8% 5% 5% 5% 4 6% 7% 29%	2660
	11th to 20th	Night	12% 14% 5% 11% 7% 6% 6% 5% 7% 6% 20%	3276
	21st to end	Night	12% 15% 4 10% 6% 5% 6% 4 6% 6% 25%	4862

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	62% 15% 15% 7% 1	2539
	11th to 20th	Day	65% 11% 10% 10% 3 1	2540
	21st to end	Day	57% 13% 17% 9% 3 1	2858
February	1st to 10th	Day	54% 9% 17% 15% 5%	1710
	11th to 20th	Day	51% 9% 16% 16% 7%	1716
	21st to end	Day	55% 8% 16% 14% 6% 1	1570
March	1st to 10th	Day	44% 9% 16% 20% 11%	3138
	11th to 20th	Day	35% 7% 14% 32% 11%	3415
	21st to end	Day	54% 12% 16% 15% 4	4067
April	1st to 10th	Day	52% 8% 14% 16% 9% 1	2280
	11th to 20th	Day	47% 7% 14% 18% 13% 2	2252
	21st to end	Day	41% 7% 14% 25% 11% 2	2369
May	1st to 10th	Day	36% 9% 12% 28% 14% 1	4420
	11th to 20th	Day	24% 4 10% 31% 27% 4	4380
	21st to end	Day	36% 10% 19% 25% 9% 1	4814
June	1st to 10th	Day	41% 7% 11% 26% 14% 2	2660
	11th to 20th	Day	33% 6% 12% 28% 19% 3	2660
	21st to end	Day	20% 4 11% 30% 27% 9%	2660
July	1st to 10th	Day	8% 2 5% 12% 24% 48%	4351
	11th to 20th	Day	9% 2 5% 7% 24% 53%	4345
	21st to end	Day	10% 2 5% 17% 36% 29%	4639
August	1st to 10th	Day	16% 3 5% 14% 38% 25%	4356
	11th to 20th	Day	13% 3 5% 34% 33% 12%	3982
	21st to end	Day	18% 3 7% 28% 26% 18%	4179
September	1st to 10th	Day	14% 2 9% 25% 31% 19%	2280
	11th to 20th	Day	26% 6% 9% 24% 24% 11%	2275
	21st to end	Day	29% 5% 12% 25% 21% 7%	2062
October	1st to 10th	Day	38% 5% 9% 22% 20% 6%	1900
	11th to 20th	Day	36% 7% 16% 23% 16% 3	1900
	21st to end	Day	44% 7% 14% 22% 13% 1	1874
November	1st to 10th	Day	52% 7% 14% 19% 7% 1	1674
	11th to 20th	Day	56% 9% 15% 15% 4	1572
	21st to end	Day	51% 7% 19% 15% 8% 1	1520
December	1st to 10th	Day	51% 12% 13% 16% 7%	1520
	11th to 20th	Day	57% 14% 15% 13% 1	1928
	21st to end	Day	62% 11% 15% 10% 2	2794
January	1st to 10th	Night	64% 15% 13% 7% 2	4406
	11th to 20th	Night	64% 12% 10% 9% 4 1	4420
	21st to end	Night	59% 16% 12% 9% 4	4798
February	1st to 10th	Night	54% 9% 17% 14% 6%	2470
	11th to 20th	Night	51% 8% 15% 19% 7% 1	2362
	21st to end	Night	55% 10% 12% 15% 7% 1	1884
March	1st to 10th	Night	42% 9% 13% 24% 12% 1	3736
	11th to 20th	Night	32% 7% 15% 36% 9%	3562
	21st to end	Night	51% 14% 18% 14% 3	3585
April	1st to 10th	Night	47% 8% 16% 21% 7%	1900
	11th to 20th	Night	43% 7% 17% 20% 11% 2	1879
	21st to end	Night	41% 8% 15% 22% 12% 2	1807
May	1st to 10th	Night	36% 7% 15% 25% 16%	2540
	11th to 20th	Night	21% 5% 13% 38% 22% 1	2516
	21st to end	Night	30% 10% 29% 23% 7% 1	2762
June	1st to 10th	Night	36% 7% 14% 27% 14% 2	1520
	11th to 20th	Night	31% 6% 13% 33% 13% 4	1520
	21st to end	Night	20% 4 12% 32% 25% 7%	1520
July	1st to 10th	Night	7% 2 7% 14% 27% 42%	2500
	11th to 20th	Night	10% 2 6% 12% 19% 50%	2492
	21st to end	Night	9% 2 5% 20% 35% 29%	2674
August	1st to 10th	Night	15% 4 6% 15% 37% 23%	2547
	11th to 20th	Night	12% 3 7% 40% 27% 10%	2923
	21st to end	Night	14% 3 12% 29% 26% 16%	3498
September	1st to 10th	Night	13% 3 9% 28% 30% 17%	1900
	11th to 20th	Night	23% 5% 11% 25% 22% 14%	1905
	21st to end	Night	27% 4 10% 27% 25% 7%	2114
October	1st to 10th	Night	31% 5% 11% 25% 24% 4	2280
	11th to 20th	Night	35% 6% 14% 22% 19% 4	2280
	21st to end	Night	42% 7% 13% 19% 16% 2	2587
November	1st to 10th	Night	52% 7% 10% 21% 9% 1	2405
	11th to 20th	Night	52% 9% 16% 18% 5%	2607
	21st to end	Night	46% 10% 16% 19% 9% 1	2660
December	1st to 10th	Night	53% 12% 15% 15% 5%	2660
	11th to 20th	Night	60% 11% 14% 12% 3	3276
	21st to end	Night	57% 12% 16% 11% 4	4862

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	26% 14% 20% 22% 14% 3	180
	11th to 20th	Day	17% 14% 25% 28% 12% 4	180
	21st to end	Day	16% 11% 31% 25% 15% 3	236
February	1st to 10th	Day	24% 15% 24% 26% 9% 2	370
	11th to 20th	Day	22% 14% 30% 22% 12% 1	385
	21st to end	Day	18% 18% 26% 27% 9% 1	382
March	1st to 10th	Day	20% 18% 27% 26% 9% 1	460
	11th to 20th	Day	17% 17% 25% 25% 13% 3	460
	21st to end	Day	20% 19% 26% 25% 8% 1	506
April	1st to 10th	Day	23% 16% 25% 23% 9% 4	460
	11th to 20th	Day	17% 17% 31% 21% 10% 4	452
	21st to end	Day	12% 19% 31% 22% 13% 3	460
May	1st to 10th	Day	14% 16% 29% 25% 13% 4	463
	11th to 20th	Day	19% 13% 23% 28% 13% 5%	488
	21st to end	Day	14% 13% 23% 25% 18% 7%	529
June	1st to 10th	Day	13% 14% 23% 25% 18% 7%	490
	11th to 20th	Day	13% 19% 27% 25% 13% 3	490
	21st to end	Day	6% 10% 27% 32% 19% 6%	490
July	1st to 10th	Day	1 7% 31% 35% 22% 3	486
	11th to 20th	Day	4 9% 29% 31% 22% 5%	484
	21st to end	Day	2 8% 27% 32% 25% 7%	531
August	1st to 10th	Day	3 8% 27% 33% 19% 10%	487
	11th to 20th	Day	4 6% 21% 32% 26% 12%	477
	21st to end	Day	6% 8% 25% 29% 24% 7%	539
September	1st to 10th	Day	3 7% 23% 32% 25% 10%	490
	11th to 20th	Day	9% 8% 21% 34% 22% 6%	490
	21st to end	Day	12% 9% 22% 30% 23% 4	490
October	1st to 10th	Day	14% 9% 25% 24% 20% 7%	490
	11th to 20th	Day	13% 12% 23% 23% 23% 6%	490
	21st to end	Day	17% 13% 24% 23% 16% 6%	363
November	1st to 10th	Day	13% 17% 22% 30% 15% 2	270
	11th to 20th	Day	16% 14% 19% 29% 18% 4	204
	21st to end	Day	18% 13% 22% 23% 22% 2	180
December	1st to 10th	Day	12% 16% 21% 27% 16% 9%	180
	11th to 20th	Day	24% 10% 30% 26% 9% 1	180
	21st to end	Day	17% 15% 23% 29% 13% 3	198
January	1st to 10th	Night	21% 11% 23% 29% 14% 1	760
	11th to 20th	Night	21% 11% 19% 30% 18% 3	760
	21st to end	Night	19% 8% 19% 35% 17% 1	836
February	1st to 10th	Night	21% 8% 23% 33% 14% 1	760
	11th to 20th	Night	17% 9% 22% 35% 15% 1	744
	21st to end	Night	21% 12% 24% 33% 9% 1	628
March	1st to 10th	Night	16% 10% 23% 36% 14% 1	760
	11th to 20th	Night	13% 9% 21% 40% 17% 1	760
	21st to end	Night	14% 11% 27% 34% 13% 1	836
April	1st to 10th	Night	16% 9% 23% 34% 16% 2	760
	11th to 20th	Night	15% 9% 25% 31% 16% 3	751
	21st to end	Night	14% 9% 25% 33% 18% 2	667
May	1st to 10th	Night	10% 8% 20% 39% 21% 2	380
	11th to 20th	Night	13% 5% 15% 38% 28% 2	374
	21st to end	Night	12% 4 18% 38% 24% 3	411
June	1st to 10th	Night	13% 4 19% 40% 23% 1	380
	11th to 20th	Night	12% 7% 21% 41% 17% 1	380
	21st to end	Night	8% 3 12% 48% 28% 1	380
July	1st to 10th	Night	2 2 11% 50% 33% 2	370
	11th to 20th	Night	2 1 13% 56% 27% 2	368
	21st to end	Night	1 10% 52% 34% 2	416
August	1st to 10th	Night	3 7% 49% 38% 3	391
	11th to 20th	Night	3 1 10% 36% 43% 8%	609
	21st to end	Night	3 3 16% 44% 29% 4	836
September	1st to 10th	Night	4 2 14% 41% 36% 4	760
	11th to 20th	Night	6% 3 12% 39% 33% 7%	760
	21st to end	Night	11% 3 13% 37% 31% 5%	760
October	1st to 10th	Night	13% 2 16% 39% 28% 2	760
	11th to 20th	Night	12% 6% 17% 37% 25% 4	760
	21st to end	Night	16% 7% 20% 35% 18% 5%	812
November	1st to 10th	Night	18% 8% 20% 38% 15% 1	740
	11th to 20th	Night	20% 8% 22% 33% 15% 2	760
	21st to end	Night	15% 9% 20% 37% 16% 4	760
December	1st to 10th	Night	18% 8% 20% 35% 16% 3	760
	11th to 20th	Night	21% 8% 21% 34% 14% 1	760
	21st to end	Night	23% 12% 26% 29% 10%	836

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