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 2024-11-02.

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	3 6% 2 6% 4 4 5% 4 6% 7% 53%	100928
	Night	4 6% 2 5% 4 4 4 4 5% 7% 55%	98287
Transparency	Day	26% 6% 11% 20% 24% 13%	100690
	Night	26% 6% 10% 20% 25% 14%	98115
Seeing	Day	12% 18% 29% 26% 13% 3	15091
	Night	9% 11% 26% 34% 18% 2	25221

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% 6% 4 4 4 4 4 6% 56%	2857
	11th to 20th	Day	2 7% 4 6% 3 3 3 3 5% 5% 59%	2860
	21st to end	Day	3 11% 3 8% 5% 4 4 3 4 6% 49%	3146
February	1st to 10th	Day	1 9% 5% 8% 4 3 3 3 5% 6% 53%	1710
	11th to 20th	Day	3 8% 4 7% 4 3 5% 4 4 7% 52%	1820
	21st to end	Day	2 11% 4 5% 4 3 3 3 5% 6% 53%	1570
March	1st to 10th	Day	2 8% 3 6% 4 3 3 3 4 6% 57%	3370
	11th to 20th	Day	3 16% 4 8% 4 4 4 3 5% 6% 43%	3490
	21st to end	Day	3 10% 4 7% 4 4 4 3 4 6% 51%	3907
April	1st to 10th	Day	3 6% 2 5% 4 3 3 4 4 7% 58%	2280
	11th to 20th	Day	3 6% 2 6% 4 4 4 4 5% 7% 56%	2251
	21st to end	Day	4 6% 2 5% 5% 4 4 4 5% 7% 55%	2280
May	1st to 10th	Day	2 4 1 3 3 3 3 3 5% 6% 66%	3800
	11th to 20th	Day	5% 5% 2 6% 5% 6% 5% 4 7% 7% 48%	3994
	21st to end	Day	3 2 1 3 3 2 2 3 4 5% 72%	4816
June	1st to 10th	Day	3 3 2 3 4 3 5% 4 6% 8% 60%	2660
	11th to 20th	Day	3 3 1 3 3 3 3 3 6% 5% 68%	2660
	21st to end	Day	4 4 2 5% 5% 4 5% 5% 7% 8% 52%	2660
July	1st to 10th	Day	3 3 2 5% 4 5% 6% 5% 7% 8% 53%	4287
	11th to 20th	Day	2 3 1 6% 6% 7% 11% 8% 11% 8% 37%	4110
	21st to end	Day	4 4 2 7% 8% 7% 7% 7% 10% 11% 32%	4454
August	1st to 10th	Day	4 4 2 8% 8% 8% 9% 8% 9% 9% 30%	4070
	11th to 20th	Day	3 4 2 5% 5% 4 5% 5% 8% 11% 49%	3726
	21st to end	Day	5% 5% 2 7% 7% 6% 7% 6% 9% 10% 35%	4180
September	1st to 10th	Day	7% 5% 2 6% 4 5% 6% 5% 8% 7% 46%	2280
	11th to 20th	Day	6% 4 1 4 5% 4 4 3 6% 7% 54%	2280
	21st to end	Day	4 4 1 4 2 4 4 3 5% 5% 64%	2038
October	1st to 10th	Day	9% 4 1 5% 5% 3 4 4 4 5% 56%	1900
	11th to 20th	Day	4 3 1 3 3 3 2 2 4 4 71%	1900
	21st to end	Day	4 5% 1 4 2 2 2 2 3 4 69%	2090
November	1st to 10th	Day	3 3 1 4 2 3 2 2 5% 7% 68%	1850
	11th to 20th	Day	3 4 1 4 3 3 3 2 5% 7% 67%	1710
	21st to end	Day	5% 8% 2 5% 4 3 2 3 4 5% 60%	1682
December	1st to 10th	Day	4 7% 2 5% 3 2 3 4 3 5% 61%	1520
	11th to 20th	Day	3 9% 2 8% 4 4 4 3 4 6% 51%	1926
	21st to end	Day	3 9% 3 5% 3 3 3 3 4 7% 59%	2794
January	1st to 10th	Night	1 7% 5% 10% 4 4 4 3 5% 5% 53%	4127
	11th to 20th	Night	2 8% 4 6% 5% 4 3 3 5% 5% 56%	4110
	21st to end	Night	3 9% 4 7% 4 4 4 4 5% 7% 50%	4521
February	1st to 10th	Night	2 8% 4 5% 3 3 3 2 4 6% 59%	2470
	11th to 20th	Night	5% 10% 2 4 4 3 3 4 5% 6% 54%	2246
	21st to end	Night	4 8% 2 5% 3 3 3 3 5% 6% 58%	1884
March	1st to 10th	Night	2 7% 2 5% 3 3 3 2 3 5% 65%	3498
	11th to 20th	Night	5% 13% 3 6% 5% 4 4 3 5% 6% 45%	3490
	21st to end	Night	3 9% 3 6% 3 3 4 3 5% 5% 55%	3743
April	1st to 10th	Night	2 6% 2 4 5% 3 3 4 5% 7% 58%	1900
	11th to 20th	Night	6% 6% 2 5% 4 3 3 2 4 7% 59%	1877
	21st to end	Night	4 6% 2 5% 3 3 4 3 4 7% 60%	1900
May	1st to 10th	Night	6% 6% 1 3 4 3 3 3 4 5% 62%	3170
	11th to 20th	Night	7% 7% 3 6% 4 5% 5% 4 5% 5% 50%	2907
	21st to end	Night	3 2 1 3 3 3 4 3 4 4 69%	2768
June	1st to 10th	Night	2 2 1 4 5% 4 4 5% 7% 8% 58%	1520
	11th to 20th	Night	3 2 1 3 3 3 4 3 4 6% 69%	1520
	21st to end	Night	4 4 2 6% 5% 5% 5% 5% 6% 8% 51%	1520
July	1st to 10th	Night	2 4 2 5% 5% 5% 5% 4 6% 7% 55%	2664
	11th to 20th	Night	3 3 1 6% 6% 6% 8% 7% 10% 13% 37%	2860
	21st to end	Night	5% 4 2 7% 6% 6% 7% 6% 10% 11% 35%	3109
August	1st to 10th	Night	3 4 3 9% 7% 7% 9% 7% 12% 10% 29%	2868
	11th to 20th	Night	3 4 2 6% 5% 5% 6% 6% 9% 13% 40%	3113
	21st to end	Night	3 5% 2 8% 6% 7% 9% 7% 10% 11% 32%	3487
September	1st to 10th	Night	5% 5% 2 5% 5% 5% 5% 4 6% 8% 50%	1900
	11th to 20th	Night	5% 3 2 4 4 4 4 4 5% 8% 58%	1900
	21st to end	Night	5% 2 1 4 3 4 3 3 5% 6% 64%	2138
October	1st to 10th	Night	6% 5% 2 4 4 3 3 4 4 5% 59%	2280
	11th to 20th	Night	4 2 1 3 3 3 2 2 3 4 74%	2280
	21st to end	Night	4 3 1 3 3 3 3 2 4 5% 70%	2508
November	1st to 10th	Night	2 2 1 2 2 3 3 3 4 5% 72%	2244
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 5% 72%	2468
	21st to end	Night	5% 8% 2 4 3 3 3 3 4 5% 60%	2493
December	1st to 10th	Night	4 7% 2 5% 3 3 3 2 4 6% 60%	2660
	11th to 20th	Night	4 7% 2 6% 5% 4 5% 5% 6% 7% 50%	3271
	21st to end	Night	4 9% 3 6% 3 3 3 2 4 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	22% 3 11% 20% 37% 7%	3370
	11th to 20th	Day	37% 6% 9% 23% 20% 5%	3490
	21st to end	Day	27% 5% 10% 25% 26% 6%	3907
April	1st to 10th	Day	23% 4 10% 20% 28% 15%	2280
	11th to 20th	Day	26% 4 7% 17% 31% 15%	2254
	21st to end	Day	25% 4 9% 21% 29% 11%	2280
May	1st to 10th	Day	17% 4 10% 15% 36% 18%	3800
	11th to 20th	Day	29% 7% 12% 16% 25% 11%	3997
	21st to end	Day	14% 3 8% 13% 29% 33%	4816
June	1st to 10th	Day	20% 5% 11% 18% 27% 20%	2660
	11th to 20th	Day	16% 4 7% 14% 25% 35%	2660
	21st to end	Day	27% 5% 10% 18% 22% 18%	2660
July	1st to 10th	Day	25% 7% 11% 12% 17% 27%	4222
	11th to 20th	Day	33% 13% 17% 26% 8% 4	4041
	21st to end	Day	37% 13% 20% 20% 6% 4	4314
August	1st to 10th	Day	41% 13% 20% 14% 8% 4	4056
	11th to 20th	Day	27% 7% 17% 22% 18% 10%	3762
	21st to end	Day	36% 11% 18% 18% 15% 3	4179
September	1st to 10th	Day	31% 8% 12% 21% 20% 8%	2280
	11th to 20th	Day	27% 5% 11% 17% 27% 13%	2280
	21st to end	Day	20% 3 8% 22% 33% 13%	2038
October	1st to 10th	Day	28% 5% 8% 20% 22% 16%	1900
	11th to 20th	Day	17% 2 4 14% 40% 22%	1900
	21st to end	Day	20% 2 8% 20% 36% 14%	2090
November	1st to 10th	Day	16% 3 8% 23% 36% 14%	1850
	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	20% 3 6% 19% 41% 11%	3498
	11th to 20th	Night	34% 6% 10% 21% 22% 6%	3490
	21st to end	Night	26% 5% 10% 26% 26% 7%	3743
April	1st to 10th	Night	23% 4 7% 21% 29% 15%	1900
	11th to 20th	Night	26% 3 7% 18% 28% 18%	1877
	21st to end	Night	22% 5% 9% 20% 33% 12%	1900
May	1st to 10th	Night	22% 5% 10% 12% 31% 20%	3170
	11th to 20th	Night	33% 6% 11% 19% 19% 13%	2908
	21st to end	Night	15% 3 10% 12% 26% 34%	2768
June	1st to 10th	Night	21% 7% 12% 18% 22% 21%	1520
	11th to 20th	Night	16% 3 8% 12% 24% 37%	1520
	21st to end	Night	30% 5% 12% 13% 17% 23%	1520
July	1st to 10th	Night	27% 9% 10% 12% 13% 30%	2621
	11th to 20th	Night	32% 11% 20% 27% 6% 6%	2806
	21st to end	Night	35% 11% 20% 25% 8% 1	3011
August	1st to 10th	Night	40% 13% 19% 14% 8% 6%	2859
	11th to 20th	Night	30% 9% 19% 15% 15% 11%	3143
	21st to end	Night	38% 12% 18% 16% 11% 4	3487
September	1st to 10th	Night	30% 7% 12% 20% 20% 11%	1900
	11th to 20th	Night	25% 5% 12% 15% 24% 19%	1900
	21st to end	Night	20% 4 8% 16% 34% 17%	2138
October	1st to 10th	Night	26% 5% 7% 16% 26% 20%	2280
	11th to 20th	Night	15% 3 6% 16% 34% 27%	2280
	21st to end	Night	17% 3 6% 21% 37% 16%	2508
November	1st to 10th	Night	12% 3 6% 24% 39% 16%	2244
	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% 15% 5%	408
	11th to 20th	Day	21% 28% 29% 16% 5% 1	460
	21st to end	Day	25% 31% 30% 9% 4	506
April	1st to 10th	Day	17% 33% 33% 13% 4	460
	11th to 20th	Day	19% 37% 30% 12% 3	452
	21st to end	Day	16% 33% 37% 11% 2	460
May	1st to 10th	Day	12% 33% 39% 12% 3	463
	11th to 20th	Day	10% 26% 35% 21% 6% 2	488
	21st to end	Day	11% 22% 31% 27% 8% 1	725
June	1st to 10th	Day	5% 12% 35% 31% 16% 1	680
	11th to 20th	Day	6% 13% 33% 29% 17% 2	680
	21st to end	Day	7% 9% 31% 31% 18% 4	680
July	1st to 10th	Day	7% 4 26% 32% 25% 5%	597
	11th to 20th	Day	6% 1 17% 37% 29% 11%	484
	21st to end	Day	6% 2 13% 37% 35% 8%	531
August	1st to 10th	Day	7% 3 18% 38% 25% 9%	487
	11th to 20th	Day	6% 2 26% 39% 21% 6%	477
	21st to end	Day	6% 4 19% 42% 23% 6%	539
September	1st to 10th	Day	7% 7% 21% 37% 22% 7%	490
	11th to 20th	Day	7% 8% 34% 34% 13% 4	490
	21st to end	Day	8% 14% 28% 30% 15% 5%	352
October	1st to 10th	Day	10% 16% 28% 24% 17% 5%	300
	11th to 20th	Day	12% 17% 30% 25% 12% 3	300
	21st to end	Day	15% 13% 33% 25% 11% 2	330
November	1st to 10th	Day	14% 22% 32% 26% 6%	273
	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	760
	11th to 20th	Night	13% 13% 30% 31% 13% 1	760
	21st to end	Night	15% 16% 33% 28% 8%	789
April	1st to 10th	Night	12% 22% 35% 26% 5%	570
	11th to 20th	Night	15% 20% 29% 28% 8%	563
	21st to end	Night	11% 18% 35% 28% 7%	570
May	1st to 10th	Night	9% 16% 34% 33% 9%	570
	11th to 20th	Night	9% 14% 30% 33% 13% 1	561
	21st to end	Night	7% 14% 34% 32% 12% 1	617
June	1st to 10th	Night	3 6% 29% 45% 15% 1	570
	11th to 20th	Night	4 7% 31% 41% 16% 1	570
	21st to end	Night	4 8% 23% 38% 26% 1	570
July	1st to 10th	Night	5% 2 13% 46% 31% 3	555
	11th to 20th	Night	5% 1 9% 37% 43% 5%	552
	21st to end	Night	5% 2 6% 32% 47% 8%	624
August	1st to 10th	Night	4 2 11% 30% 45% 8%	567
	11th to 20th	Night	6% 2 10% 40% 39% 4	564
	21st to end	Night	3 3 10% 41% 37% 6%	627
September	1st to 10th	Night	8% 2 12% 41% 33% 5%	570
	11th to 20th	Night	6% 3 18% 43% 26% 3	570
	21st to end	Night	7% 4 20% 41% 25% 3	670
October	1st to 10th	Night	8% 7% 25% 41% 17% 1	760
	11th to 20th	Night	7% 6% 28% 40% 15% 4	760
	21st to end	Night	8% 9% 25% 36% 17% 5%	836
November	1st to 10th	Night	4 9% 24% 40% 21% 2	748
	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 2024-10-30 11:19:25. Version ac07

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