Fenton Hill Observtory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fenton Hill Observtory clear sky chart from 2005-11-06 to 2024-03-08.

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	4 5% 3 5% 5% 3 4 5% 5% 7% 54%	78761
	Night	4 6% 3 4 4 3 4 4 5% 7% 56%	80861
Transparency	Day	27% 6% 10% 20% 25% 12%	78486
	Night	25% 6% 10% 20% 26% 14%	80658
Seeing	Day	12% 18% 29% 26% 13% 3	14130
	Night	9% 11% 26% 34% 18% 2	24443

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	2 6% 5% 5% 4 3 4 5% 4 6% 56%	2857
	11th to 20th	Day	2 6% 5% 6% 3 2 3 4 5% 6% 59%	2860
	21st to end	Day	3 10% 3 7% 6% 3 4 4 4 6% 49%	3146
February	1st to 10th	Day	1 9% 5% 7% 4 2 4 3 5% 6% 53%	1710
	11th to 20th	Day	3 7% 4 6% 4 2 4 5% 4 7% 52%	1820
	21st to end	Day	3 11% 4 5% 4 2 3 4 5% 6% 54%	1570
March	1st to 10th	Day	4 8% 3 6% 5% 3 4 5% 5% 6% 50%	2006
	11th to 20th	Day	3 11% 4 7% 5% 3 3 3 4 6% 51%	1980
	21st to end	Day	5% 7% 3 5% 6% 3 4 4 4 6% 53%	2222
April	1st to 10th	Day	3 5% 2 4 4 3 4 4 4 7% 59%	2160
	11th to 20th	Day	3 6% 3 6% 5% 3 4 5% 5% 7% 54%	2131
	21st to end	Day	5% 5% 3 4 5% 2 4 5% 5% 8% 55%	2160
May	1st to 10th	Day	3 6% 2 3 4 3 4 4 5% 7% 59%	2160
	11th to 20th	Day	5% 5% 3 5% 6% 3 5% 4 5% 7% 52%	2250
	21st to end	Day	4 3 2 3 4 3 3 5% 5% 6% 62%	2715
June	1st to 10th	Day	3 2 2 3 4 2 5% 6% 6% 8% 60%	2520
	11th to 20th	Day	3 3 1 2 3 2 3 4 5% 6% 68%	2520
	21st to end	Day	4 3 2 4 5% 3 5% 6% 6% 8% 54%	2520
July	1st to 10th	Day	4 5% 3 6% 6% 5% 6% 7% 8% 9% 42%	2447
	11th to 20th	Day	3 4 2 6% 6% 4 7% 9% 9% 10% 40%	2340
	21st to end	Day	5% 4 3 6% 10% 5% 8% 8% 10% 11% 29%	2545
August	1st to 10th	Day	6% 4 3 8% 8% 5% 7% 8% 8% 9% 34%	2319
	11th to 20th	Day	5% 6% 3 7% 6% 4 6% 6% 7% 10% 40%	2123
	21st to end	Day	5% 4 2 5% 6% 4 6% 7% 8% 10% 42%	2376
September	1st to 10th	Day	7% 5% 2 5% 5% 4 6% 7% 8% 7% 46%	2160
	11th to 20th	Day	6% 4 2 3 6% 3 4 4 6% 7% 54%	2160
	21st to end	Day	4 4 2 3 3 3 4 4 5% 5% 65%	1932
October	1st to 10th	Day	9% 4 2 4 5% 2 4 5% 4 6% 55%	1800
	11th to 20th	Day	4 3 1 3 3 2 2 2 4 4 73%	1800
	21st to end	Day	4 5% 2 4 3 2 3 2 3 4 69%	1980
November	1st to 10th	Day	3 3 1 3 3 2 3 3 5% 7% 68%	1840
	11th to 20th	Day	3 3 1 3 3 2 3 3 5% 7% 67%	1710
	21st to end	Day	5% 7% 2 4 4 2 3 3 4 5% 60%	1682
December	1st to 10th	Day	4 6% 3 5% 4 2 3 4 3 5% 61%	1520
	11th to 20th	Day	3 9% 2 8% 5% 3 4 4 4 6% 51%	1926
	21st to end	Day	3 9% 3 4 3 2 3 3 3 7% 60%	2794
January	1st to 10th	Night	1 7% 5% 9% 5% 3 4 4 5% 5% 53%	4127
	11th to 20th	Night	2 7% 4 5% 6% 3 3 3 5% 5% 56%	4110
	21st to end	Night	3 9% 4 6% 5% 3 4 5% 5% 7% 50%	4521
February	1st to 10th	Night	2 8% 4 5% 4 2 3 3 4 6% 60%	2470
	11th to 20th	Night	5% 9% 2 4 5% 2 4 4 5% 6% 54%	2246
	21st to end	Night	4 8% 2 4 4 2 3 3 5% 6% 58%	1884
March	1st to 10th	Night	4 7% 2 4 4 3 4 4 4 6% 59%	2110
	11th to 20th	Night	5% 8% 3 4 4 3 3 4 4 6% 55%	1980
	21st to end	Night	5% 7% 3 4 4 2 3 4 4 5% 57%	2134
April	1st to 10th	Night	3 5% 3 4 5% 3 4 4 5% 7% 59%	1800
	11th to 20th	Night	6% 5% 2 5% 4 2 3 3 4 7% 59%	1777
	21st to end	Night	4 5% 2 4 4 3 4 3 4 7% 61%	1800
May	1st to 10th	Night	5% 4 2 4 4 3 4 4 5% 6% 60%	1800
	11th to 20th	Night	6% 6% 3 5% 5% 3 6% 5% 6% 6% 51%	1641
	21st to end	Night	3 2 1 3 4 2 5% 4 5% 5% 64%	1558
June	1st to 10th	Night	2 2 1 4 5% 2 4 5% 7% 8% 58%	1440
	11th to 20th	Night	3 2 1 3 2 2 3 4 3 6% 70%	1440
	21st to end	Night	3 3 2 5% 6% 3 5% 6% 5% 8% 53%	1440
July	1st to 10th	Night	2 5% 3 6% 7% 4 7% 6% 9% 9% 41%	1508
	11th to 20th	Night	3 4 2 5% 6% 5% 7% 7% 8% 13% 40%	1620
	21st to end	Night	4 4 2 6% 9% 5% 8% 8% 10% 12% 32%	1773
August	1st to 10th	Night	3 3 2 5% 6% 3 8% 7% 10% 10% 43%	1619
	11th to 20th	Night	6% 5% 3 6% 6% 5% 6% 7% 8% 10% 37%	1778
	21st to end	Night	3 4 2 4 4 3 7% 7% 9% 11% 45%	1980
September	1st to 10th	Night	5% 4 3 5% 6% 4 5% 6% 7% 8% 49%	1800
	11th to 20th	Night	5% 3 2 3 5% 3 4 5% 5% 8% 57%	1800
	21st to end	Night	5% 2 1 3 3 3 3 4 5% 6% 63%	2024
October	1st to 10th	Night	6% 5% 2 4 4 3 3 4 4 6% 58%	2160
	11th to 20th	Night	3 2 1 2 3 2 3 2 3 4 75%	2160
	21st to end	Night	4 3 1 3 3 2 3 3 4 6% 69%	2376
November	1st to 10th	Night	2 2 1 2 3 2 3 4 4 6% 72%	2220
	11th to 20th	Night	2 2 1 2 3 2 3 3 4 6% 72%	2468
	21st to end	Night	5% 7% 3 4 3 2 4 4 4 5% 60%	2493
December	1st to 10th	Night	4 7% 2 4 4 3 3 3 4 6% 60%	2660
	11th to 20th	Night	4 6% 2 5% 5% 3 5% 6% 6% 7% 51%	3271
	21st to end	Night	4 9% 3 5% 4 2 3 3 3 5% 58%	4873

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	25% 4 11% 28% 27% 5%	2858
	11th to 20th	Day	18% 8% 13% 35% 22% 4	2860
	21st to end	Day	33% 6% 10% 21% 24% 5%	3146
February	1st to 10th	Day	31% 3 11% 21% 24% 11%	1710
	11th to 20th	Day	30% 4 9% 22% 25% 11%	1830
	21st to end	Day	30% 3 7% 16% 29% 15%	1570
March	1st to 10th	Day	29% 5% 8% 20% 28% 11%	2006
	11th to 20th	Day	33% 3 8% 17% 29% 10%	1980
	21st to end	Day	29% 4 9% 20% 29% 9%	2222
April	1st to 10th	Day	23% 4 10% 21% 28% 15%	2160
	11th to 20th	Day	27% 5% 7% 18% 31% 13%	2134
	21st to end	Day	26% 4 9% 22% 29% 11%	2160
May	1st to 10th	Day	24% 4 8% 18% 34% 12%	2160
	11th to 20th	Day	31% 6% 11% 17% 26% 9%	2253
	21st to end	Day	22% 5% 10% 17% 26% 20%	2715
June	1st to 10th	Day	20% 5% 11% 18% 27% 19%	2520
	11th to 20th	Day	16% 4 6% 14% 26% 34%	2520
	21st to end	Day	25% 5% 10% 17% 23% 19%	2520
July	1st to 10th	Day	34% 9% 13% 14% 15% 15%	2382
	11th to 20th	Day	32% 11% 18% 20% 11% 8%	2271
	21st to end	Day	39% 15% 23% 19% 4 1	2404
August	1st to 10th	Day	40% 11% 16% 13% 11% 8%	2304
	11th to 20th	Day	35% 9% 14% 19% 17% 6%	2123
	21st to end	Day	30% 10% 15% 20% 21% 4	2376
September	1st to 10th	Day	32% 8% 12% 21% 19% 8%	2160
	11th to 20th	Day	27% 5% 11% 17% 27% 13%	2160
	21st to end	Day	20% 4 8% 22% 33% 13%	1932
October	1st to 10th	Day	29% 6% 8% 21% 22% 15%	1800
	11th to 20th	Day	16% 2 4 14% 41% 23%	1800
	21st to end	Day	20% 2 8% 20% 36% 13%	1980
November	1st to 10th	Day	16% 3 8% 23% 36% 14%	1840
	11th to 20th	Day	16% 3 8% 24% 37% 11%	1710
	21st to end	Day	26% 3 5% 21% 31% 14%	1682
December	1st to 10th	Day	23% 5% 9% 30% 22% 10%	1520
	11th to 20th	Day	32% 5% 9% 18% 27% 9%	1924
	21st to end	Day	24% 4 9% 18% 29% 16%	2794
January	1st to 10th	Night	29% 5% 10% 23% 24% 10%	4127
	11th to 20th	Night	19% 8% 12% 31% 23% 7%	4110
	21st to end	Night	30% 6% 8% 21% 28% 6%	4521
February	1st to 10th	Night	26% 3 10% 23% 26% 12%	2470
	11th to 20th	Night	29% 4 8% 22% 24% 13%	2246
	21st to end	Night	23% 4 7% 17% 33% 17%	1884
March	1st to 10th	Night	24% 5% 7% 22% 30% 13%	2110
	11th to 20th	Night	27% 4 9% 20% 30% 10%	1980
	21st to end	Night	27% 5% 8% 19% 31% 10%	2134
April	1st to 10th	Night	23% 4 7% 22% 29% 15%	1800
	11th to 20th	Night	26% 3 7% 19% 28% 16%	1777
	21st to end	Night	22% 4 8% 20% 33% 12%	1800
May	1st to 10th	Night	24% 4 8% 15% 33% 15%	1800
	11th to 20th	Night	30% 7% 11% 17% 24% 11%	1642
	21st to end	Night	19% 5% 10% 18% 27% 21%	1558
June	1st to 10th	Night	21% 7% 12% 18% 22% 20%	1440
	11th to 20th	Night	16% 3 8% 13% 24% 36%	1440
	21st to end	Night	28% 5% 12% 13% 17% 24%	1440
July	1st to 10th	Night	35% 12% 14% 13% 9% 17%	1465
	11th to 20th	Night	31% 12% 19% 19% 10% 10%	1566
	21st to end	Night	36% 14% 24% 20% 5% 1	1674
August	1st to 10th	Night	29% 14% 20% 16% 11% 10%	1610
	11th to 20th	Night	35% 11% 18% 15% 14% 8%	1778
	21st to end	Night	25% 11% 19% 20% 17% 7%	1980
September	1st to 10th	Night	31% 8% 13% 19% 19% 10%	1800
	11th to 20th	Night	25% 5% 12% 15% 24% 19%	1800
	21st to end	Night	21% 4 9% 17% 32% 17%	2024
October	1st to 10th	Night	28% 6% 7% 17% 24% 19%	2160
	11th to 20th	Night	14% 3 6% 15% 35% 28%	2160
	21st to end	Night	18% 3 7% 21% 36% 15%	2376
November	1st to 10th	Night	12% 3 6% 24% 39% 15%	2220
	11th to 20th	Night	14% 2 7% 24% 39% 13%	2468
	21st to end	Night	26% 3 6% 17% 32% 16%	2493
December	1st to 10th	Night	24% 4 9% 28% 26% 10%	2660
	11th to 20th	Night	28% 6% 10% 17% 26% 13%	3272
	21st to end	Night	26% 5% 8% 18% 28% 15%	4873

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% 27% 29% 24% 7%	270
	11th to 20th	Day	25% 21% 24% 22% 7%	270
	21st to end	Day	8% 21% 36% 23% 11% 1	297
February	1st to 10th	Day	14% 25% 34% 20% 6%	270
	11th to 20th	Day	19% 29% 35% 16% 2	270
	21st to end	Day	27% 32% 27% 12% 3	225
March	1st to 10th	Day	17% 32% 31% 13% 5%	397
	11th to 20th	Day	21% 28% 30% 16% 5% 1	420
	21st to end	Day	25% 32% 30% 8% 4	462
April	1st to 10th	Day	17% 33% 33% 12% 4	420
	11th to 20th	Day	19% 37% 29% 11% 3	412
	21st to end	Day	17% 34% 36% 10% 2	420
May	1st to 10th	Day	11% 33% 40% 12% 4	423
	11th to 20th	Day	10% 27% 35% 21% 6% 2	448
	21st to end	Day	10% 21% 31% 27% 9% 1	670
June	1st to 10th	Day	5% 12% 35% 31% 16% 1	630
	11th to 20th	Day	6% 12% 32% 30% 18% 2	630
	21st to end	Day	6% 8% 32% 31% 19% 4	630
July	1st to 10th	Day	7% 3 25% 33% 27% 6%	552
	11th to 20th	Day	6% 1 18% 38% 27% 10%	444
	21st to end	Day	6% 1 12% 37% 36% 8%	487
August	1st to 10th	Day	7% 4 18% 39% 24% 8%	447
	11th to 20th	Day	6% 2 26% 39% 21% 6%	437
	21st to end	Day	6% 3 18% 43% 23% 6%	495
September	1st to 10th	Day	8% 8% 20% 37% 22% 6%	450
	11th to 20th	Day	6% 6% 34% 36% 14% 4	450
	21st to end	Day	8% 14% 28% 29% 15% 5%	320
October	1st to 10th	Day	11% 17% 29% 24% 16% 4	270
	11th to 20th	Day	11% 17% 31% 26% 12% 3	270
	21st to end	Day	15% 13% 32% 26% 11% 3	297
November	1st to 10th	Day	14% 22% 31% 26% 6%	270
	11th to 20th	Day	9% 19% 31% 29% 11% 1	270
	21st to end	Day	16% 24% 29% 21% 10%	270
December	1st to 10th	Day	17% 21% 27% 27% 8%	270
	11th to 20th	Day	16% 25% 29% 24% 5% 1	270
	21st to end	Day	26% 25% 28% 17% 4	297
January	1st to 10th	Night	8% 13% 29% 34% 13% 3	777
	11th to 20th	Night	12% 12% 29% 31% 13% 3	760
	21st to end	Night	10% 14% 28% 31% 14% 3	836
February	1st to 10th	Night	11% 14% 35% 29% 10% 1	760
	11th to 20th	Night	13% 15% 35% 26% 9%	744
	21st to end	Night	14% 18% 35% 29% 4	628
March	1st to 10th	Night	10% 17% 34% 30% 8% 1	750
	11th to 20th	Night	12% 13% 30% 31% 14% 1	720
	21st to end	Night	14% 16% 33% 29% 8%	748
April	1st to 10th	Night	11% 22% 35% 27% 5%	540
	11th to 20th	Night	15% 20% 28% 28% 8%	533
	21st to end	Night	11% 18% 35% 27% 8%	540
May	1st to 10th	Night	8% 15% 34% 34% 9%	540
	11th to 20th	Night	9% 14% 30% 32% 14% 1	531
	21st to end	Night	7% 13% 34% 33% 12% 1	584
June	1st to 10th	Night	3 6% 29% 44% 16% 1	540
	11th to 20th	Night	4 6% 31% 41% 17% 1	540
	21st to end	Night	3 7% 24% 38% 27% 1	540
July	1st to 10th	Night	5% 2 12% 45% 32% 3	525
	11th to 20th	Night	5% 1 8% 36% 44% 6%	522
	21st to end	Night	5% 2 6% 30% 49% 8%	591
August	1st to 10th	Night	4 2 10% 29% 46% 8%	537
	11th to 20th	Night	6% 2 10% 41% 38% 4	534
	21st to end	Night	3 3 10% 41% 38% 6%	594
September	1st to 10th	Night	8% 2 11% 40% 33% 5%	540
	11th to 20th	Night	6% 3 17% 44% 28% 3	540
	21st to end	Night	7% 4 20% 40% 26% 3	634
October	1st to 10th	Night	8% 8% 26% 40% 17% 2	720
	11th to 20th	Night	6% 7% 28% 41% 15% 4	720
	21st to end	Night	9% 10% 23% 35% 18% 5%	792
November	1st to 10th	Night	4 9% 23% 41% 21% 2	740
	11th to 20th	Night	7% 11% 29% 34% 17% 3	948
	21st to end	Night	13% 14% 23% 28% 18% 3	950
December	1st to 10th	Night	11% 14% 29% 33% 11% 1	950
	11th to 20th	Night	14% 16% 27% 30% 12% 1	950
	21st to end	Night	15% 16% 30% 29% 8% 1	1045

Page generated at 2024-03-05 05:24:02. Version ac07

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