Fennell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fennell Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	12% 7% 2 7% 4 4 4 4 5% 7% 43%	119657
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 47%	119601
Transparency	Day	39% 6% 10% 22% 19% 4	123362
	Night	36% 5% 9% 23% 20% 6%	124027
Seeing	Day	14% 9% 21% 25% 21% 9%	16266
	Night	15% 8% 25% 31% 19% 3	26020

By Month

Percentage Cloud Cover SE (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	15% 10% 4 10% 3 3 3 2 3 5% 44%	3686
	11th to 20th	Day	14% 7% 2 8% 3 3 3 3 3 4 49%	4080
	21st to end	Day	17% 9% 2 6% 4 3 3 2 4 5% 44%	4488
February	1st to 10th	Day	20% 10% 3 8% 4 3 3 2 3 5% 39%	1980
	11th to 20th	Day	20% 9% 3 6% 4 3 3 3 4 5% 38%	1948
	21st to end	Day	21% 8% 3 7% 4 4 3 3 4 4 38%	1650
March	1st to 10th	Day	16% 12% 3 9% 5% 5% 4 3 4 5% 35%	4480
	11th to 20th	Day	13% 7% 3 6% 4 4 4 3 4 5% 47%	4929
	21st to end	Day	11% 7% 3 8% 5% 5% 4 3 4 7% 42%	5863
April	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 7% 45%	2400
	11th to 20th	Day	15% 5% 2 6% 5% 3 3 3 6% 6% 45%	2374
	21st to end	Day	13% 6% 2 7% 5% 4 4 4 6% 8% 42%	2400
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 5% 7% 8% 32%	5405
	11th to 20th	Day	13% 8% 3 8% 5% 5% 5% 5% 7% 9% 33%	5833
	21st to end	Day	16% 7% 3 7% 4 5% 5% 4 6% 8% 35%	6659
June	1st to 10th	Day	8% 5% 2 6% 5% 6% 4 5% 7% 9% 42%	2800
	11th to 20th	Day	5% 4 2 5% 5% 5% 5% 5% 8% 10% 46%	2800
	21st to end	Day	4 4 2 5% 5% 5% 6% 5% 8% 11% 46%	2800
July	1st to 10th	Day	6% 6% 2 7% 6% 6% 6% 5% 8% 10% 39%	4580
	11th to 20th	Day	8% 5% 2 7% 5% 5% 5% 5% 8% 11% 39%	4490
	21st to end	Day	14% 8% 3 8% 6% 6% 5% 4 7% 8% 31%	4440
August	1st to 10th	Day	4 4 1 4 3 4 3 4 6% 10% 58%	3785
	11th to 20th	Day	6% 4 2 5% 4 4 5% 5% 7% 9% 48%	3729
	21st to end	Day	5% 4 1 5% 4 5% 5% 4 6% 8% 54%	4180
September	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 6% 9% 44%	2280
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 6% 7% 45%	2199
	21st to end	Day	11% 5% 2 5% 4 4 3 4 4 7% 52%	2090
October	1st to 10th	Day	6% 4 1 5% 3 4 4 3 4 6% 59%	2009
	11th to 20th	Day	11% 4 2 4 3 3 3 3 4 6% 57%	1900
	21st to end	Day	18% 6% 1 4 2 3 3 2 4 3 53%	2090
November	1st to 10th	Day	11% 7% 2 4 2 2 3 3 3 5% 57%	1775
	11th to 20th	Day	13% 5% 2 4 3 2 3 3 5% 7% 53%	1800
	21st to end	Day	17% 8% 3 5% 4 3 4 3 5% 6% 44%	1800
December	1st to 10th	Day	16% 9% 3 6% 3 4 3 2 3 4 47%	2930
	11th to 20th	Day	18% 9% 3 6% 5% 4 3 2 3 4 42%	3023
	21st to end	Day	13% 10% 4 8% 5% 5% 5% 4 5% 6% 35%	3982
January	1st to 10th	Night	17% 9% 4 8% 4 3 3 3 3 5% 42%	6274
	11th to 20th	Night	16% 8% 3 6% 4 3 3 2 3 4 49%	5900
	21st to end	Night	21% 8% 3 5% 3 3 3 2 3 5% 44%	6490
February	1st to 10th	Night	21% 10% 3 6% 4 3 2 3 4 5% 40%	2417
	11th to 20th	Night	20% 9% 3 7% 4 3 2 2 3 5% 42%	2338
	21st to end	Night	19% 7% 2 6% 4 4 4 3 4 5% 41%	1980
March	1st to 10th	Night	17% 9% 3 6% 5% 4 4 3 4 5% 40%	5373
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 4 6% 51%	5072
	21st to end	Night	12% 6% 3 7% 5% 4 4 3 5% 6% 45%	5064
April	1st to 10th	Night	11% 6% 2 5% 4 3 4 4 6% 7% 48%	2000
	11th to 20th	Night	14% 6% 2 5% 4 3 3 4 5% 7% 49%	1975
	21st to end	Night	11% 6% 2 6% 4 4 4 3 5% 7% 49%	2000
May	1st to 10th	Night	19% 9% 3 7% 4 4 4 3 5% 8% 36%	4490
	11th to 20th	Night	14% 10% 3 9% 5% 4 4 3 4 5% 37%	4042
	21st to end	Night	13% 7% 3 7% 5% 4 5% 4 7% 8% 36%	4188
June	1st to 10th	Night	7% 5% 2 6% 5% 4 5% 4 7% 9% 46%	1600
	11th to 20th	Night	3 3 1 4 4 3 3 4 6% 11% 59%	1600
	21st to end	Night	3 2 1 3 4 4 5% 5% 9% 11% 54%	1600
July	1st to 10th	Night	5% 4 2 6% 4 5% 5% 4 6% 11% 47%	2610
	11th to 20th	Night	4 4 2 5% 4 3 4 4 6% 13% 51%	2695
	21st to end	Night	11% 8% 3 8% 4 4 4 3 5% 8% 40%	3121
August	1st to 10th	Night	2 2 1 3 3 3 3 3 4 8% 68%	3144
	11th to 20th	Night	4 2 1 5% 4 4 5% 4 7% 11% 54%	3100
	21st to end	Night	4 3 1 5% 3 4 5% 4 6% 9% 57%	3476
September	1st to 10th	Night	7% 4 3 6% 4 4 4 5% 5% 7% 52%	1900
	11th to 20th	Night	9% 6% 2 5% 4 3 4 3 5% 8% 51%	1960
	21st to end	Night	10% 5% 1 4 3 3 3 3 5% 6% 57%	2090
October	1st to 10th	Night	7% 2 2 5% 3 3 3 2 5% 6% 62%	2166
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 4 5% 58%	2280
	21st to end	Night	16% 6% 2 4 3 3 3 2 4 5% 52%	2508
November	1st to 10th	Night	11% 5% 2 4 4 3 4 3 5% 6% 55%	2522
	11th to 20th	Night	10% 5% 2 4 3 4 4 4 5% 6% 53%	2600
	21st to end	Night	14% 7% 2 5% 4 3 4 3 4 6% 50%	2600
December	1st to 10th	Night	17% 7% 3 5% 3 3 3 2 4 5% 49%	4250
	11th to 20th	Night	17% 8% 3 6% 4 3 4 3 4 5% 44%	5180
	21st to end	Night	17% 8% 3 7% 5% 5% 5% 4 5% 6% 37%	6996

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	47% 3 10% 22% 17% 2	3687
	11th to 20th	Day	39% 5% 5% 26% 23% 2	4080
	21st to end	Day	44% 3 6% 17% 24% 7%	4488
February	1st to 10th	Day	51% 3 7% 20% 16% 3	1980
	11th to 20th	Day	49% 4 7% 18% 16% 5%	1958
	21st to end	Day	49% 4 8% 15% 18% 6%	1650
March	1st to 10th	Day	53% 4 5% 16% 13% 9%	4480
	11th to 20th	Day	39% 5% 6% 20% 22% 8%	4929
	21st to end	Day	43% 4 8% 23% 21% 1	5865
April	1st to 10th	Day	40% 6% 8% 24% 20% 3	2400
	11th to 20th	Day	40% 4 8% 21% 21% 5%	2374
	21st to end	Day	41% 6% 10% 24% 18% 1	2400
May	1st to 10th	Day	47% 8% 12% 21% 11% 1	5440
	11th to 20th	Day	45% 8% 17% 19% 10% 1	5837
	21st to end	Day	44% 8% 11% 22% 12% 2	6673
June	1st to 10th	Day	34% 10% 12% 23% 19% 2	2800
	11th to 20th	Day	30% 9% 14% 27% 18% 3	2800
	21st to end	Day	31% 8% 15% 24% 18% 4	2800
July	1st to 10th	Day	36% 9% 15% 27% 12% 2	4511
	11th to 20th	Day	36% 8% 15% 30% 9% 2	4415
	21st to end	Day	50% 8% 12% 22% 7% 1	4300
August	1st to 10th	Day	21% 7% 17% 29% 23% 3	3773
	11th to 20th	Day	31% 7% 17% 26% 17% 2	3765
	21st to end	Day	28% 7% 10% 27% 25% 3	4179
September	1st to 10th	Day	35% 8% 10% 24% 17% 5%	2280
	11th to 20th	Day	37% 6% 11% 18% 23% 5%	2199
	21st to end	Day	32% 5% 10% 22% 25% 7%	2090
October	1st to 10th	Day	17% 3 6% 15% 37% 22%	3334
	11th to 20th	Day	18% 3 5% 21% 37% 16%	3140
	21st to end	Day	27% 3 3 26% 31% 10%	3428
November	1st to 10th	Day	31% 4 8% 23% 25% 8%	1775
	11th to 20th	Day	32% 4 8% 21% 27% 8%	1800
	21st to end	Day	42% 4 7% 20% 23% 5%	1800
December	1st to 10th	Day	42% 3 5% 25% 22% 2	2930
	11th to 20th	Day	48% 3 6% 21% 18% 4	3020
	21st to end	Day	47% 6% 9% 24% 12% 1	3982
January	1st to 10th	Night	47% 3 9% 19% 18% 3	6274
	11th to 20th	Night	41% 4 6% 19% 25% 5%	5900
	21st to end	Night	45% 3 5% 21% 22% 4	6490
February	1st to 10th	Night	48% 4 8% 18% 19% 3	2417
	11th to 20th	Night	47% 4 6% 20% 17% 5%	2339
	21st to end	Night	46% 5% 7% 16% 19% 7%	1980
March	1st to 10th	Night	47% 4 6% 15% 19% 9%	5373
	11th to 20th	Night	34% 4 7% 21% 18% 16%	5072
	21st to end	Night	39% 5% 9% 21% 22% 3	5064
April	1st to 10th	Night	35% 5% 11% 22% 21% 5%	2000
	11th to 20th	Night	34% 5% 8% 22% 23% 7%	1975
	21st to end	Night	35% 6% 10% 25% 20% 3	2000
May	1st to 10th	Night	47% 7% 12% 20% 12% 2	4520
	11th to 20th	Night	48% 6% 13% 23% 10% 1	4043
	21st to end	Night	43% 9% 11% 22% 13% 3	4196
June	1st to 10th	Night	32% 9% 12% 22% 23% 2	1600
	11th to 20th	Night	21% 7% 14% 30% 23% 5%	1600
	21st to end	Night	20% 8% 19% 28% 20% 5%	1600
July	1st to 10th	Night	29% 8% 21% 27% 13% 2	2570
	11th to 20th	Night	25% 7% 16% 37% 12% 3	2647
	21st to end	Night	43% 7% 15% 23% 11% 1	3022
August	1st to 10th	Night	14% 6% 12% 40% 26% 2	3132
	11th to 20th	Night	23% 8% 20% 28% 19% 2	3130
	21st to end	Night	23% 6% 13% 27% 27% 4	3476
September	1st to 10th	Night	30% 8% 10% 22% 22% 8%	1900
	11th to 20th	Night	31% 7% 11% 19% 27% 5%	1960
	21st to end	Night	28% 5% 10% 25% 24% 8%	2090
October	1st to 10th	Night	15% 3 6% 16% 33% 26%	3608
	11th to 20th	Night	18% 3 4 16% 36% 23%	3779
	21st to end	Night	25% 3 5% 28% 24% 15%	4120
November	1st to 10th	Night	31% 4 7% 23% 28% 8%	2522
	11th to 20th	Night	31% 4 8% 21% 25% 11%	2600
	21st to end	Night	37% 4 8% 21% 24% 6%	2600
December	1st to 10th	Night	39% 3 7% 27% 21% 3	4250
	11th to 20th	Night	44% 4 5% 23% 21% 4	5182
	21st to end	Night	46% 6% 9% 23% 13% 3	6996

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	22% 19% 37% 15% 5% 1	300
	11th to 20th	Day	25% 20% 37% 14% 3 1	300
	21st to end	Day	24% 20% 38% 15% 3	330
February	1st to 10th	Day	25% 18% 37% 14% 5% 1	300
	11th to 20th	Day	35% 21% 29% 13% 2 1	300
	21st to end	Day	33% 21% 30% 11% 5%	249
March	1st to 10th	Day	22% 22% 35% 15% 6% 1	300
	11th to 20th	Day	22% 22% 36% 19% 1	300
	21st to end	Day	23% 18% 33% 19% 7%	498
April	1st to 10th	Day	21% 15% 32% 22% 8% 1	500
	11th to 20th	Day	18% 16% 32% 23% 9% 2	492
	21st to end	Day	17% 14% 30% 23% 13% 3	500
May	1st to 10th	Day	16% 15% 26% 29% 11% 3	503
	11th to 20th	Day	13% 7% 25% 28% 20% 7%	527
	21st to end	Day	10% 6% 22% 29% 22% 11%	669
June	1st to 10th	Day	10% 5% 16% 27% 28% 14%	730
	11th to 20th	Day	6% 3 13% 32% 30% 16%	730
	21st to end	Day	5% 2 16% 30% 31% 17%	730
July	1st to 10th	Day	6% 2 8% 27% 35% 21%	721
	11th to 20th	Day	7% 2 9% 23% 38% 20%	718
	21st to end	Day	8% 2 9% 26% 35% 19%	726
August	1st to 10th	Day	6% 3 8% 29% 35% 18%	486
	11th to 20th	Day	9% 1 8% 33% 36% 13%	479
	21st to end	Day	11% 1 5% 22% 37% 23%	539
September	1st to 10th	Day	10% 1 8% 30% 36% 14%	490
	11th to 20th	Day	10% 3 10% 23% 35% 18%	490
	21st to end	Day	16% 4 15% 30% 22% 13%	490
October	1st to 10th	Day	8% 4 19% 28% 34% 7%	409
	11th to 20th	Day	11% 9% 34% 28% 12% 5%	300
	21st to end	Day	22% 11% 27% 28% 9% 2	330
November	1st to 10th	Day	19% 10% 23% 31% 12% 4	300
	11th to 20th	Day	10% 15% 35% 29% 10% 1	300
	21st to end	Day	21% 16% 29% 20% 11% 3	300
December	1st to 10th	Day	23% 20% 30% 19% 7% 1	300
	11th to 20th	Day	24% 18% 31% 18% 8% 1	300
	21st to end	Day	22% 20% 34% 18% 5% 1	330
January	1st to 10th	Night	23% 13% 36% 24% 5%	1000
	11th to 20th	Night	23% 15% 35% 23% 4	1000
	21st to end	Night	22% 11% 40% 25% 2	1100
February	1st to 10th	Night	25% 12% 34% 25% 4	817
	11th to 20th	Night	25% 15% 36% 22% 2	783
	21st to end	Night	23% 16% 34% 24% 3	660
March	1st to 10th	Night	18% 15% 33% 29% 5%	800
	11th to 20th	Night	23% 10% 35% 27% 4 1	640
	21st to end	Night	17% 12% 33% 27% 11%	660
April	1st to 10th	Night	15% 13% 35% 29% 8% 1	600
	11th to 20th	Night	17% 8% 33% 30% 11% 1	593
	21st to end	Night	13% 9% 33% 32% 12% 2	600
May	1st to 10th	Night	16% 7% 25% 38% 13% 2	600
	11th to 20th	Night	13% 5% 25% 36% 18% 3	591
	21st to end	Night	8% 2 19% 40% 27% 4	650
June	1st to 10th	Night	7% 3 15% 37% 32% 7%	600
	11th to 20th	Night	3 1 12% 36% 44% 4	600
	21st to end	Night	4 1 9% 39% 41% 6%	600
July	1st to 10th	Night	5% 1 8% 31% 46% 10%	585
	11th to 20th	Night	6% 2 8% 27% 45% 12%	582
	21st to end	Night	6% 2 8% 29% 44% 11%	624
August	1st to 10th	Night	4 1 6% 30% 51% 9%	567
	11th to 20th	Night	5% 2 8% 36% 43% 6%	564
	21st to end	Night	7% 2 8% 26% 47% 10%	627
September	1st to 10th	Night	6% 1 7% 42% 38% 6%	570
	11th to 20th	Night	12% 2 12% 31% 37% 7%	630
	21st to end	Night	12% 4 15% 36% 30% 4	760
October	1st to 10th	Night	9% 3 17% 36% 32% 4	760
	11th to 20th	Night	13% 5% 26% 36% 18% 2	760
	21st to end	Night	19% 7% 23% 34% 16% 1	836
November	1st to 10th	Night	13% 6% 24% 37% 18% 2	780
	11th to 20th	Night	11% 9% 33% 35% 11%	800
	21st to end	Night	18% 9% 32% 30% 11% 1	800
December	1st to 10th	Night	22% 11% 32% 28% 7%	800
	11th to 20th	Night	22% 11% 30% 30% 6%	981
	21st to end	Night	23% 15% 31% 27% 5%	1100

Page generated at 2025-07-18 00:10:03. Version ac07

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