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 2024-03-01.

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	10% 9% 3 6% 5% 3 4 4 5% 6% 43%	78399
	Night	11% 11% 4 7% 5% 3 5% 5% 5% 6% 39%	81031
Transparency	Day	40% 7% 12% 19% 16% 6%	78102
	Night	43% 9% 13% 18% 12% 4	80849
Seeing	Day	13% 13% 25% 27% 17% 5%	13559
	Night	14% 7% 19% 37% 21% 2	22807

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% 18% 6% 10% 8% 4 6% 5% 4 5% 17%	2539
	11th to 20th	Day	18% 17% 5% 9% 8% 4 5% 5% 5% 5% 19%	2540
	21st to end	Day	13% 14% 5% 10% 7% 4 5% 5% 6% 8% 23%	2858
February	1st to 10th	Day	11% 17% 5% 7% 7% 4 5% 5% 5% 7% 27%	1710
	11th to 20th	Day	13% 14% 5% 9% 6% 4 4 6% 5% 7% 28%	1707
	21st to end	Day	12% 16% 5% 10% 7% 4 5% 5% 5% 7% 24%	1570
March	1st to 10th	Day	11% 16% 5% 9% 6% 4 5% 6% 6% 8% 24%	1801
	11th to 20th	Day	11% 17% 6% 8% 8% 4 5% 5% 4 6% 27%	1931
	21st to end	Day	14% 16% 6% 10% 7% 4 5% 5% 6% 6% 21%	2317
April	1st to 10th	Day	13% 13% 5% 8% 7% 4 6% 5% 5% 7% 26%	2160
	11th to 20th	Day	11% 13% 5% 7% 5% 3 5% 4 5% 5% 35%	2129
	21st to end	Day	10% 12% 3 6% 5% 2 5% 6% 5% 8% 39%	2244
May	1st to 10th	Day	9% 8% 3 5% 5% 3 5% 5% 6% 7% 44%	2520
	11th to 20th	Day	12% 8% 3 6% 6% 4 4 4 5% 8% 39%	2477
	21st to end	Day	15% 7% 3 6% 5% 3 5% 5% 6% 6% 37%	2723
June	1st to 10th	Day	14% 7% 3 6% 5% 3 4 4 6% 7% 41%	2520
	11th to 20th	Day	10% 7% 3 4 5% 3 5% 4 5% 6% 47%	2520
	21st to end	Day	5% 3 1 3 3 2 3 4 4 6% 65%	2520
July	1st to 10th	Day	2 2 1 2 2 2 3 3 4 5% 74%	2520
	11th to 20th	Day	3 2 1 2 3 2 3 3 3 5% 74%	2520
	21st to end	Day	2 1 1 2 2 1 2 2 3 3 81%	2741
August	1st to 10th	Day	3 2 1 3 2 2 3 3 4 6% 72%	2492
	11th to 20th	Day	3 2 1 2 2 1 2 3 4 5% 75%	2254
	21st to end	Day	3 3 1 2 2 2 2 3 4 6% 72%	2376
September	1st to 10th	Day	3 2 1 2 3 2 2 3 4 5% 73%	2160
	11th to 20th	Day	6% 5% 2 4 4 2 4 4 4 7% 57%	2156
	21st to end	Day	10% 6% 2 5% 4 2 4 4 4 5% 54%	1954
October	1st to 10th	Day	12% 7% 3 6% 6% 3 4 4 4 5% 47%	1800
	11th to 20th	Day	9% 6% 3 5% 5% 3 5% 4 6% 7% 45%	1800
	21st to end	Day	14% 8% 4 6% 6% 3 4 4 4 7% 39%	1832
November	1st to 10th	Day	15% 11% 4 7% 6% 4 6% 5% 5% 7% 28%	1674
	11th to 20th	Day	15% 12% 4 10% 7% 3 6% 7% 6% 6% 24%	1572
	21st to end	Day	13% 13% 4 8% 6% 4 6% 5% 6% 7% 29%	1520
December	1st to 10th	Day	12% 11% 4 9% 7% 4 6% 5% 5% 7% 31%	1520
	11th to 20th	Day	12% 14% 5% 9% 8% 4 6% 6% 7% 8% 23%	1928
	21st to end	Day	13% 17% 5% 9% 8% 5% 5% 6% 6% 6% 21%	2794
January	1st to 10th	Night	14% 18% 6% 11% 8% 4 6% 6% 6% 6% 15%	4406
	11th to 20th	Night	20% 20% 6% 8% 5% 3 5% 5% 5% 7% 16%	4420
	21st to end	Night	13% 14% 5% 9% 7% 4 6% 6% 6% 7% 22%	4798
February	1st to 10th	Night	14% 13% 5% 9% 6% 3 5% 6% 5% 6% 27%	2470
	11th to 20th	Night	11% 16% 4 9% 7% 3 4 4 5% 5% 32%	2360
	21st to end	Night	12% 17% 5% 9% 6% 3 5% 6% 5% 6% 26%	1884
March	1st to 10th	Night	12% 13% 4 8% 7% 4 5% 6% 6% 7% 28%	2165
	11th to 20th	Night	10% 12% 5% 9% 7% 4 6% 5% 7% 8% 28%	2026
	21st to end	Night	9% 14% 5% 8% 6% 4 7% 7% 7% 7% 26%	2038
April	1st to 10th	Night	11% 13% 5% 8% 6% 4 4 5% 5% 7% 33%	1800
	11th to 20th	Night	11% 12% 5% 6% 5% 3 5% 4 5% 5% 39%	1779
	21st to end	Night	10% 11% 3 5% 5% 3 6% 5% 6% 7% 39%	1713
May	1st to 10th	Night	9% 7% 3 6% 5% 3 4 4 4 6% 49%	1440
	11th to 20th	Night	11% 6% 2 5% 6% 3 4 4 7% 8% 44%	1416
	21st to end	Night	10% 9% 3 5% 4 3 4 5% 6% 9% 42%	1552
June	1st to 10th	Night	12% 6% 3 5% 4 3 4 5% 5% 9% 45%	1440
	11th to 20th	Night	10% 8% 3 5% 4 2 3 4 3 8% 50%	1440
	21st to end	Night	6% 3 1 3 4 2 3 3 4 5% 65%	1440
July	1st to 10th	Night	3 2 2 2 2 3 4 5% 5% 73%	1440
	11th to 20th	Night	1 1 1 2 2 1 3 3 4 4 78%	1440
	21st to end	Night	1 1 1 2 2 1 2 2 3 4 80%	1576
August	1st to 10th	Night	2 3 1 2 2 1 2 4 5% 7% 70%	1444
	11th to 20th	Night	1 3 1 2 2 2 3 3 3 6% 75%	1644
	21st to end	Night	2 3 1 3 3 2 2 3 3 5% 74%	1980
September	1st to 10th	Night	3 2 1 2 2 1 2 3 3 5% 76%	1800
	11th to 20th	Night	6% 3 1 4 4 2 4 4 5% 6% 61%	1804
	21st to end	Night	9% 5% 2 4 3 2 4 3 3 7% 58%	2002
October	1st to 10th	Night	10% 6% 3 4 4 2 3 3 5% 7% 52%	2160
	11th to 20th	Night	10% 6% 3 5% 5% 3 4 4 5% 7% 48%	2160
	21st to end	Night	12% 8% 4 6% 6% 3 4 4 5% 5% 42%	2524
November	1st to 10th	Night	15% 12% 5% 8% 6% 4 5% 3 4 5% 32%	2405
	11th to 20th	Night	15% 11% 5% 8% 6% 3 6% 6% 5% 6% 29%	2607
	21st to end	Night	12% 11% 4 8% 6% 3 5% 6% 6% 8% 33%	2660
December	1st to 10th	Night	12% 13% 5% 7% 6% 4 6% 5% 6% 7% 29%	2660
	11th to 20th	Night	12% 14% 6% 10% 8% 4 6% 7% 7% 6% 20%	3276
	21st to end	Night	12% 14% 5% 8% 8% 4 6% 6% 6% 7% 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	54% 7% 17% 14% 8%	1801
	11th to 20th	Day	55% 7% 13% 20% 5%	1931
	21st to end	Day	60% 9% 14% 14% 3	2317
April	1st to 10th	Day	53% 8% 14% 16% 9% 1	2160
	11th to 20th	Day	48% 6% 14% 17% 12% 2	2132
	21st to end	Day	41% 7% 13% 25% 12% 2	2244
May	1st to 10th	Day	36% 7% 13% 28% 15% 1	2520
	11th to 20th	Day	41% 6% 12% 25% 14% 1	2480
	21st to end	Day	43% 8% 14% 23% 11% 1	2724
June	1st to 10th	Day	41% 7% 11% 26% 14% 2	2520
	11th to 20th	Day	33% 6% 12% 28% 17% 3	2520
	21st to end	Day	20% 4 11% 30% 26% 9%	2520
July	1st to 10th	Day	13% 3 10% 20% 32% 22%	2451
	11th to 20th	Day	14% 3 8% 13% 33% 30%	2445
	21st to end	Day	10% 2 3 15% 40% 30%	2589
August	1st to 10th	Day	15% 3 7% 22% 35% 18%	2475
	11th to 20th	Day	13% 3 6% 24% 35% 20%	2254
	21st to end	Day	14% 3 6% 29% 35% 13%	2376
September	1st to 10th	Day	14% 2 9% 26% 31% 18%	2160
	11th to 20th	Day	25% 6% 9% 23% 25% 12%	2156
	21st to end	Day	31% 5% 12% 24% 21% 7%	1954
October	1st to 10th	Day	39% 5% 10% 22% 19% 5%	1800
	11th to 20th	Day	35% 7% 15% 23% 16% 3	1800
	21st to end	Day	43% 6% 14% 22% 13% 1	1832
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	52% 9% 12% 18% 8% 2	2165
	11th to 20th	Night	49% 8% 16% 22% 5%	2026
	21st to end	Night	51% 11% 17% 17% 4	2038
April	1st to 10th	Night	48% 8% 14% 22% 7%	1800
	11th to 20th	Night	45% 7% 17% 20% 10% 2	1779
	21st to end	Night	42% 8% 14% 21% 12% 3	1713
May	1st to 10th	Night	34% 5% 16% 28% 16%	1440
	11th to 20th	Night	35% 7% 18% 25% 13% 2	1416
	21st to end	Night	37% 9% 18% 23% 12% 1	1552
June	1st to 10th	Night	36% 7% 14% 28% 14% 1	1440
	11th to 20th	Night	31% 6% 14% 33% 12% 3	1440
	21st to end	Night	21% 4 12% 32% 24% 7%	1440
July	1st to 10th	Night	13% 4 12% 24% 26% 21%	1400
	11th to 20th	Night	11% 4 10% 18% 28% 30%	1392
	21st to end	Night	8% 2 5% 18% 40% 27%	1488
August	1st to 10th	Night	12% 4 9% 23% 34% 17%	1436
	11th to 20th	Night	13% 3 8% 24% 35% 18%	1644
	21st to end	Night	13% 3 8% 28% 37% 11%	1980
September	1st to 10th	Night	13% 3 10% 29% 30% 17%	1800
	11th to 20th	Night	22% 5% 11% 25% 23% 14%	1804
	21st to end	Night	27% 4 10% 26% 24% 8%	2002
October	1st to 10th	Night	32% 5% 11% 25% 22% 4	2160
	11th to 20th	Night	35% 6% 14% 22% 19% 4	2160
	21st to end	Night	42% 7% 13% 19% 17% 2	2524
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% 27% 27% 9% 1	382
March	1st to 10th	Day	20% 18% 27% 26% 8% 1	421
	11th to 20th	Day	18% 17% 26% 25% 12% 2	420
	21st to end	Day	21% 19% 25% 25% 9% 1	462
April	1st to 10th	Day	24% 16% 23% 23% 9% 4	420
	11th to 20th	Day	18% 17% 30% 20% 10% 4	412
	21st to end	Day	13% 19% 30% 21% 14% 3	420
May	1st to 10th	Day	13% 15% 29% 26% 13% 4	423
	11th to 20th	Day	20% 13% 23% 28% 12% 5%	448
	21st to end	Day	14% 12% 23% 26% 18% 6%	485
June	1st to 10th	Day	13% 14% 24% 24% 18% 7%	450
	11th to 20th	Day	12% 19% 26% 26% 13% 3	450
	21st to end	Day	5% 10% 26% 33% 20% 6%	450
July	1st to 10th	Day	2 7% 30% 36% 23% 3	446
	11th to 20th	Day	4 9% 28% 32% 21% 5%	444
	21st to end	Day	2 7% 27% 32% 25% 7%	487
August	1st to 10th	Day	3 7% 26% 34% 19% 11%	447
	11th to 20th	Day	3 5% 20% 32% 27% 13%	437
	21st to end	Day	6% 8% 25% 29% 26% 7%	495
September	1st to 10th	Day	3 7% 22% 32% 26% 9%	450
	11th to 20th	Day	9% 8% 20% 34% 23% 6%	450
	21st to end	Day	13% 9% 22% 29% 23% 4	450
October	1st to 10th	Day	15% 10% 24% 24% 20% 7%	450
	11th to 20th	Day	13% 12% 22% 23% 24% 6%	450
	21st to end	Day	17% 13% 24% 24% 16% 6%	347
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	17% 10% 22% 35% 14% 1	722
	11th to 20th	Night	13% 9% 21% 40% 16% 1	720
	21st to end	Night	15% 11% 26% 34% 13% 1	792
April	1st to 10th	Night	17% 8% 22% 35% 16% 2	720
	11th to 20th	Night	16% 9% 24% 31% 17% 3	711
	21st to end	Night	15% 9% 25% 32% 18% 2	633
May	1st to 10th	Night	10% 7% 18% 40% 23% 2	360
	11th to 20th	Night	13% 5% 15% 38% 28% 2	354
	21st to end	Night	12% 4 19% 37% 25% 3	389
June	1st to 10th	Night	13% 4 19% 39% 23% 1	360
	11th to 20th	Night	13% 8% 21% 41% 17% 1	360
	21st to end	Night	8% 3 12% 48% 29% 1	360
July	1st to 10th	Night	3 2 11% 48% 34% 2	350
	11th to 20th	Night	2 1 13% 55% 28% 2	348
	21st to end	Night	1 1 10% 52% 35% 2	394
August	1st to 10th	Night	3 6% 48% 39% 3	370
	11th to 20th	Night	3 1 9% 35% 44% 9%	576
	21st to end	Night	3 3 15% 45% 29% 4	792
September	1st to 10th	Night	3 2 14% 40% 36% 4	720
	11th to 20th	Night	6% 3 11% 39% 34% 7%	720
	21st to end	Night	11% 3 13% 37% 31% 5%	720
October	1st to 10th	Night	13% 2 16% 38% 28% 2	720
	11th to 20th	Night	12% 6% 17% 36% 25% 4	720
	21st to end	Night	16% 7% 20% 35% 18% 5%	792
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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