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 2025-06-10.

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 5% 2 5% 4 4 4 4 6% 7% 55%	123223
	Night	3 5% 2 5% 4 4 4 3 5% 6% 58%	123917
Transparency	Day	26% 5% 10% 19% 25% 14%	122986
	Night	24% 5% 9% 19% 27% 16%	123742
Seeing	Day	12% 18% 29% 25% 12% 3	15870
	Night	9% 11% 27% 34% 17% 2	26077

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	1 5% 4 6% 4 4 4 3 4 6% 58%	4097
	11th to 20th	Day	1 5% 3 7% 3 2 3 3 5% 5% 63%	4100
	21st to end	Day	2 8% 3 7% 5% 4 3 3 4 5% 56%	4510
February	1st to 10th	Day	1 9% 5% 8% 4 3 3 3 5% 6% 54%	1800
	11th to 20th	Day	3 8% 4 7% 4 3 5% 3 4 7% 52%	1919
	21st to end	Day	2 11% 4 5% 4 3 3 3 5% 6% 55%	1650
March	1st to 10th	Day	2 9% 3 6% 4 3 3 3 4 6% 57%	4838
	11th to 20th	Day	2 13% 4 9% 4 5% 4 3 5% 6% 45%	5000
	21st to end	Day	2 8% 3 6% 4 4 4 4 5% 7% 54%	5607
April	1st to 10th	Day	3 6% 2 5% 4 4 3 4 4 7% 58%	2400
	11th to 20th	Day	3 6% 2 6% 4 4 4 4 5% 7% 56%	2371
	21st to end	Day	4 6% 2 5% 4 3 4 4 5% 7% 56%	2400
May	1st to 10th	Day	4 6% 2 5% 4 5% 5% 4 5% 6% 54%	5440
	11th to 20th	Day	4 4 2 5% 4 5% 4 4 6% 6% 57%	5725
	21st to end	Day	2 2 1 3 3 3 3 4 5% 6% 68%	6916
June	1st to 10th	Day	3 3 1 4 4 3 5% 4 6% 8% 60%	2788
	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	6% 3 1 4 4 2 3 3 3 5% 66%	3170
	11th to 20th	Day	8% 4 1 5% 4 3 3 3 5% 6% 59%	3150
	21st to end	Day	4 3 1 3 2 2 2 2 3 5% 72%	3438
November	1st to 10th	Day	4 3 1 4 2 3 2 2 5% 7% 67%	1939
	11th to 20th	Day	2 4 1 3 3 3 3 2 5% 7% 68%	1800
	21st to end	Day	4 7% 2 5% 4 3 2 3 4 5% 61%	1770
December	1st to 10th	Day	2 4 1 3 2 1 2 2 3 5% 75%	2620
	11th to 20th	Day	2 6% 1 6% 4 4 3 3 4 7% 61%	3026
	21st to end	Day	2 7% 2 5% 2 2 4 3 3 6% 64%	4004
January	1st to 10th	Night	1 6% 4 9% 4 4 4 3 5% 5% 55%	5897
	11th to 20th	Night	1 6% 4 6% 5% 3 3 3 4 4 62%	5880
	21st to end	Night	2 7% 3 8% 4 4 4 3 4 6% 55%	6468
February	1st to 10th	Night	2 8% 3 6% 3 3 3 2 4 6% 60%	2600
	11th to 20th	Night	5% 10% 2 5% 4 3 3 4 5% 6% 53%	2367
	21st to end	Night	4 8% 2 4 3 3 3 3 5% 6% 59%	1980
March	1st to 10th	Night	2 9% 4 6% 3 2 2 2 3 4 62%	5034
	11th to 20th	Night	4 12% 4 6% 4 3 3 3 4 6% 52%	5000
	21st to end	Night	2 7% 2 4 3 3 4 3 5% 6% 61%	5362
April	1st to 10th	Night	2 6% 2 5% 5% 4 3 4 5% 7% 57%	2000
	11th to 20th	Night	6% 6% 2 5% 4 3 3 3 4 7% 58%	1977
	21st to end	Night	4 6% 2 5% 3 3 4 3 4 6% 61%	2000
May	1st to 10th	Night	6% 7% 2 5% 4 4 4 4 5% 6% 54%	4540
	11th to 20th	Night	5% 5% 2 4 3 3 4 3 4 5% 60%	4173
	21st to end	Night	3 3 1 3 3 3 4 3 4 6% 65%	3978
June	1st to 10th	Night	2 2 1 4 5% 4 5% 5% 7% 7% 57%	1596
	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	4 3 1 3 3 2 2 3 3 4 72%	3800
	11th to 20th	Night	9% 3 1 4 3 4 4 3 5% 6% 60%	3778
	21st to end	Night	3 2 1 2 2 2 2 2 3 5% 77%	4120
November	1st to 10th	Night	2 3 1 3 2 3 3 3 4 5% 71%	2340
	11th to 20th	Night	2 2 1 3 3 3 3 2 4 5% 72%	2598
	21st to end	Night	5% 7% 2 4 3 3 3 3 4 5% 60%	2625
December	1st to 10th	Night	2 4 1 3 2 2 2 2 3 6% 73%	4570
	11th to 20th	Night	3 4 1 4 3 3 3 4 5% 8% 62%	5181
	21st to end	Night	3 7% 3 5% 3 3 3 2 4 5% 63%	6974

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% 3 9% 25% 32% 6%	4098
	11th to 20th	Day	18% 5% 10% 25% 20% 22%	4100
	21st to end	Day	29% 4 8% 15% 27% 16%	4510
February	1st to 10th	Day	31% 3 10% 21% 24% 12%	1800
	11th to 20th	Day	30% 4 9% 21% 25% 11%	1929
	21st to end	Day	29% 3 6% 15% 29% 18%	1650
March	1st to 10th	Day	25% 2 9% 14% 35% 15%	4838
	11th to 20th	Day	37% 4 7% 19% 24% 9%	5000
	21st to end	Day	24% 4 9% 30% 26% 6%	5607
April	1st to 10th	Day	24% 4 10% 19% 29% 14%	2400
	11th to 20th	Day	26% 4 7% 17% 31% 15%	2374
	21st to end	Day	24% 4 9% 21% 31% 12%	2400
May	1st to 10th	Day	27% 6% 11% 18% 26% 13%	5440
	11th to 20th	Day	24% 5% 10% 12% 31% 18%	5728
	21st to end	Day	15% 3 10% 15% 26% 31%	6917
June	1st to 10th	Day	20% 5% 11% 18% 27% 19%	2788
	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	22% 3 6% 17% 26% 25%	3170
	11th to 20th	Day	26% 3 8% 15% 31% 16%	3150
	21st to end	Day	16% 2 6% 17% 38% 21%	3438
November	1st to 10th	Day	17% 3 8% 22% 36% 14%	1939
	11th to 20th	Day	16% 3 7% 23% 38% 13%	1800
	21st to end	Day	26% 3 5% 20% 30% 16%	1770
December	1st to 10th	Day	13% 3 6% 28% 32% 18%	2620
	11th to 20th	Day	23% 3 6% 25% 35% 8%	3024
	21st to end	Day	22% 3 8% 25% 31% 12%	4004
January	1st to 10th	Night	28% 3 9% 22% 26% 12%	5897
	11th to 20th	Night	17% 5% 10% 23% 24% 21%	5880
	21st to end	Night	27% 4 8% 15% 32% 14%	6468
February	1st to 10th	Night	25% 2 10% 23% 27% 13%	2600
	11th to 20th	Night	30% 4 8% 22% 24% 13%	2367
	21st to end	Night	22% 4 6% 16% 33% 19%	1980
March	1st to 10th	Night	25% 2 6% 14% 38% 16%	5034
	11th to 20th	Night	31% 4 8% 16% 29% 11%	5000
	21st to end	Night	22% 3 9% 29% 30% 6%	5362
April	1st to 10th	Night	24% 4 7% 21% 29% 15%	2000
	11th to 20th	Night	26% 3 7% 17% 29% 18%	1977
	21st to end	Night	22% 5% 8% 20% 34% 12%	2000
May	1st to 10th	Night	28% 6% 11% 17% 23% 14%	4537
	11th to 20th	Night	25% 4 8% 17% 29% 17%	4174
	21st to end	Night	17% 3 11% 12% 23% 33%	3978
June	1st to 10th	Night	22% 7% 12% 17% 22% 20%	1596
	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	17% 3 5% 13% 36% 26%	3800
	11th to 20th	Night	24% 4 6% 18% 26% 22%	3778
	21st to end	Night	13% 2 5% 19% 39% 23%	4120
November	1st to 10th	Night	14% 3 6% 23% 39% 15%	2340
	11th to 20th	Night	14% 2 7% 23% 39% 15%	2598
	21st to end	Night	26% 3 6% 17% 31% 18%	2625
December	1st to 10th	Night	14% 3 6% 27% 31% 18%	4570
	11th to 20th	Night	19% 4 7% 19% 37% 15%	5182
	21st to end	Night	23% 3 9% 22% 32% 11%	6974

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	15% 26% 30% 24% 6%	300
	11th to 20th	Day	24% 21% 26% 21% 7%	300
	21st to end	Day	10% 22% 35% 22% 10% 1	330
February	1st to 10th	Day	16% 24% 35% 19% 5%	300
	11th to 20th	Day	21% 29% 33% 15% 2	300
	21st to end	Day	25% 33% 26% 13% 3	249
March	1st to 10th	Day	18% 31% 31% 16% 4	445
	11th to 20th	Day	23% 28% 28% 15% 5% 1	500
	21st to end	Day	24% 31% 31% 9% 4 1	550
April	1st to 10th	Day	18% 32% 33% 13% 5%	500
	11th to 20th	Day	19% 37% 30% 11% 3	492
	21st to end	Day	16% 34% 37% 10% 2 1	500
May	1st to 10th	Day	13% 32% 38% 12% 4	503
	11th to 20th	Day	12% 27% 34% 20% 6% 2	528
	21st to end	Day	10% 22% 31% 28% 8% 1	780
June	1st to 10th	Day	5% 13% 35% 31% 15% 1	726
	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% 23% 31% 25% 6%	300
	11th to 20th	Day	10% 20% 30% 28% 11% 1	300
	21st to end	Day	16% 24% 29% 22% 9%	300
December	1st to 10th	Day	16% 20% 27% 27% 9%	300
	11th to 20th	Day	15% 24% 30% 25% 5% 1	300
	21st to end	Day	26% 25% 28% 17% 4 1	330
January	1st to 10th	Night	9% 13% 30% 34% 12% 3	817
	11th to 20th	Night	12% 12% 31% 30% 12% 3	800
	21st to end	Night	10% 14% 31% 30% 13% 2	880
February	1st to 10th	Night	11% 14% 37% 28% 9% 1	800
	11th to 20th	Night	14% 16% 35% 25% 9%	784
	21st to end	Night	13% 18% 37% 28% 3	660
March	1st to 10th	Night	11% 16% 34% 29% 8% 1	800
	11th to 20th	Night	14% 14% 31% 29% 13% 1	800
	21st to end	Night	14% 16% 34% 28% 8%	830
April	1st to 10th	Night	12% 21% 36% 26% 5%	600
	11th to 20th	Night	15% 20% 29% 28% 8%	593
	21st to end	Night	11% 18% 35% 29% 7%	600
May	1st to 10th	Night	9% 15% 33% 34% 9%	600
	11th to 20th	Night	9% 15% 30% 31% 13% 1	591
	21st to end	Night	6% 14% 34% 32% 12% 1	650
June	1st to 10th	Night	3 7% 31% 45% 15% 1	599
	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	5% 9% 25% 39% 20% 2	780
	11th to 20th	Night	7% 11% 30% 34% 16% 3	998
	21st to end	Night	14% 14% 24% 28% 17% 3	1000
December	1st to 10th	Night	11% 14% 29% 34% 11% 1	1000
	11th to 20th	Night	13% 16% 29% 29% 12% 1	1000
	21st to end	Night	15% 17% 30% 29% 8% 1	1100

Page generated at 2025-06-07 08:28:19. Version ac07

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