Fennell Auxiliary Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fennell Auxiliary Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	12% 7% 2 6% 4 4 4 4 6% 7% 43%	126623
	Night	13% 6% 2 5% 4 4 4 3 5% 7% 48%	124891
Transparency	Day	38% 6% 11% 23% 18% 4	130261
	Night	35% 5% 10% 23% 20% 6%	129330
Seeing	Day	15% 9% 22% 24% 20% 9%	16845
	Night	16% 7% 24% 31% 19% 3	26640

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% 11% 4 11% 3 2 2 2 3 4 44%	3855
	11th to 20th	Day	16% 7% 2 7% 4 3 3 2 3 4 48%	4080
	21st to end	Day	18% 9% 2 6% 3 3 3 2 3 4 46%	4488
February	1st to 10th	Day	21% 9% 3 7% 4 3 3 2 4 4 39%	1936
	11th to 20th	Day	21% 8% 3 7% 3 4 4 3 4 5% 38%	1948
	21st to end	Day	22% 8% 3 7% 4 3 4 3 4 4 39%	1650
March	1st to 10th	Day	17% 13% 4 9% 5% 4 4 3 3 5% 35%	4564
	11th to 20th	Day	12% 6% 3 6% 4 4 4 3 4 6% 48%	4980
	21st to end	Day	11% 8% 3 8% 5% 5% 4 3 5% 6% 41%	5778
April	1st to 10th	Day	13% 6% 3 6% 4 5% 4 4 6% 6% 44%	2400
	11th to 20th	Day	15% 6% 2 6% 4 4 3 3 5% 7% 46%	2374
	21st to end	Day	13% 7% 3 7% 5% 4 4 3 6% 7% 41%	2400
May	1st to 10th	Day	16% 8% 3 8% 5% 4 5% 5% 6% 8% 33%	5405
	11th to 20th	Day	13% 7% 3 8% 5% 6% 6% 4 7% 9% 33%	5668
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 6% 8% 34%	6768
June	1st to 10th	Day	7% 5% 2 7% 4 6% 5% 5% 8% 10% 42%	2800
	11th to 20th	Day	5% 4 2 6% 5% 5% 5% 5% 7% 10% 47%	2800
	21st to end	Day	4 4 2 4 4 5% 5% 6% 7% 10% 48%	2800
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 6% 6% 8% 10% 38%	6360
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 6% 9% 12% 39%	6005
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 11% 35%	6422
August	1st to 10th	Day	3 3 1 4 3 4 4 4 6% 10% 59%	3942
	11th to 20th	Day	5% 4 2 6% 4 5% 6% 4 7% 10% 47%	3849
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 8% 52%	4312
September	1st to 10th	Day	9% 6% 2 6% 4 5% 5% 4 7% 9% 44%	2400
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 3 6% 8% 45%	2347
	21st to end	Day	11% 5% 2 4 3 3 4 3 6% 6% 55%	2200
October	1st to 10th	Day	7% 5% 1 4 4 4 3 3 4 6% 59%	2061
	11th to 20th	Day	11% 4 1 4 3 3 3 3 5% 6% 57%	2000
	21st to end	Day	19% 6% 1 4 2 2 3 3 4 4 52%	2200
November	1st to 10th	Day	12% 6% 2 5% 2 2 2 3 3 4 58%	1923
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 4 7% 52%	1890
	21st to end	Day	18% 8% 3 6% 3 3 3 3 4 6% 44%	1890
December	1st to 10th	Day	17% 9% 3 6% 3 3 3 2 4 4 45%	2970
	11th to 20th	Day	19% 9% 3 7% 4 3 3 2 3 4 43%	3088
	21st to end	Day	14% 10% 3 7% 4 5% 4 3 5% 6% 37%	4070
January	1st to 10th	Night	17% 8% 3 7% 4 4 3 2 4 4 45%	6115
	11th to 20th	Night	15% 7% 3 6% 3 3 3 2 3 4 51%	5900
	21st to end	Night	21% 9% 3 6% 3 2 2 2 3 5% 44%	6490
February	1st to 10th	Night	21% 8% 3 7% 3 3 3 2 3 5% 41%	2461
	11th to 20th	Night	20% 8% 3 6% 4 3 3 2 4 4 43%	2338
	21st to end	Night	19% 8% 3 6% 4 3 3 3 4 4 43%	1980
March	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 4 5% 41%	5331
	11th to 20th	Night	12% 7% 2 4 4 3 3 3 4 7% 50%	4980
	21st to end	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	5151
April	1st to 10th	Night	11% 6% 2 6% 4 4 4 5% 5% 7% 47%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 4 4 4 6% 51%	1975
	21st to end	Night	12% 6% 2 6% 4 3 5% 3 4 6% 49%	2000
May	1st to 10th	Night	18% 9% 3 7% 4 3 4 3 5% 8% 37%	4490
	11th to 20th	Night	17% 9% 3 8% 4 4 4 4 5% 6% 37%	4221
	21st to end	Night	13% 7% 3 6% 5% 5% 5% 4 6% 9% 38%	4085
June	1st to 10th	Night	7% 5% 1 5% 4 3 5% 5% 8% 11% 46%	1600
	11th to 20th	Night	3 3 1 4 4 3 5% 3 8% 9% 58%	1600
	21st to end	Night	3 3 1 4 4 3 4 4 7% 13% 54%	1600
July	1st to 10th	Night	5% 3 2 5% 5% 5% 5% 5% 7% 12% 48%	3618
	11th to 20th	Night	4 3 1 4 3 4 5% 4 8% 14% 49%	3953
	21st to end	Night	8% 6% 2 6% 5% 4 5% 4 6% 12% 41%	4449
August	1st to 10th	Night	2 2 1 2 2 3 3 3 5% 8% 70%	3208
	11th to 20th	Night	3 3 1 5% 3 5% 5% 4 6% 12% 53%	3200
	21st to end	Night	4 3 1 4 3 4 4 3 6% 9% 57%	3586
September	1st to 10th	Night	7% 5% 2 6% 4 4 4 4 6% 6% 52%	2000
	11th to 20th	Night	10% 5% 2 5% 2 4 5% 4 5% 7% 51%	2031
	21st to end	Night	9% 5% 2 4 3 3 3 3 4 6% 57%	2200
October	1st to 10th	Night	6% 3 1 4 2 3 3 2 5% 6% 63%	2335
	11th to 20th	Night	10% 4 2 3 2 2 3 2 4 6% 61%	2400
	21st to end	Night	16% 5% 2 5% 3 3 3 2 3 4 53%	2640
November	1st to 10th	Night	11% 5% 1 4 3 3 4 2 4 5% 56%	2591
	11th to 20th	Night	11% 5% 2 4 3 4 4 3 4 7% 55%	2730
	21st to end	Night	14% 6% 2 4 4 4 4 3 4 6% 49%	2730
December	1st to 10th	Night	18% 7% 2 5% 3 3 2 3 4 5% 48%	4414
	11th to 20th	Night	18% 8% 3 6% 3 3 3 3 4 4 45%	5339
	21st to end	Night	17% 7% 2 6% 5% 5% 5% 4 5% 6% 38%	7150

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	46% 2 9% 24% 16% 2	3856
	11th to 20th	Day	39% 5% 6% 26% 22% 2	4080
	21st to end	Day	43% 3 5% 18% 24% 6%	4488
February	1st to 10th	Day	50% 4 7% 21% 15% 3	1936
	11th to 20th	Day	49% 4 8% 17% 17% 5%	1958
	21st to end	Day	48% 4 9% 16% 17% 7%	1650
March	1st to 10th	Day	54% 4 6% 15% 12% 10%	4564
	11th to 20th	Day	37% 5% 7% 19% 23% 9%	4980
	21st to end	Day	44% 4 8% 24% 19% 1	5780
April	1st to 10th	Day	39% 6% 9% 22% 20% 3	2400
	11th to 20th	Day	39% 5% 8% 22% 20% 6%	2374
	21st to end	Day	41% 7% 9% 23% 18% 2	2400
May	1st to 10th	Day	46% 7% 12% 23% 11% 1	5440
	11th to 20th	Day	44% 9% 17% 19% 10% 1	5672
	21st to end	Day	45% 8% 12% 21% 12% 2	6782
June	1st to 10th	Day	34% 9% 14% 22% 19% 2	2800
	11th to 20th	Day	30% 8% 13% 28% 17% 3	2800
	21st to end	Day	28% 9% 15% 26% 18% 4	2800
July	1st to 10th	Day	37% 6% 16% 26% 14% 1	6291
	11th to 20th	Day	34% 7% 21% 30% 7% 2	5933
	21st to end	Day	41% 6% 18% 30% 5%	6240
August	1st to 10th	Day	20% 6% 17% 31% 23% 3	3928
	11th to 20th	Day	30% 7% 18% 28% 16% 2	3885
	21st to end	Day	29% 6% 12% 25% 25% 3	4311
September	1st to 10th	Day	34% 10% 11% 22% 17% 6%	2400
	11th to 20th	Day	36% 7% 11% 18% 23% 6%	2347
	21st to end	Day	30% 5% 10% 23% 25% 8%	2200
October	1st to 10th	Day	18% 3 5% 14% 31% 29%	3360
	11th to 20th	Day	17% 3 4 22% 38% 16%	3240
	21st to end	Day	28% 3 4 26% 30% 10%	3538
November	1st to 10th	Day	31% 3 8% 24% 26% 9%	1923
	11th to 20th	Day	32% 4 9% 21% 26% 8%	1890
	21st to end	Day	41% 4 8% 19% 24% 4	1890
December	1st to 10th	Day	43% 3 6% 23% 21% 3	2970
	11th to 20th	Day	46% 3 5% 22% 19% 4	3085
	21st to end	Day	46% 6% 10% 25% 12% 2	4070
January	1st to 10th	Night	44% 4 9% 21% 20% 3	6115
	11th to 20th	Night	38% 4 6% 20% 27% 6%	5900
	21st to end	Night	45% 3 5% 20% 22% 5%	6490
February	1st to 10th	Night	48% 3 8% 18% 20% 4	2461
	11th to 20th	Night	45% 4 7% 21% 17% 6%	2339
	21st to end	Night	44% 4 7% 16% 22% 7%	1980
March	1st to 10th	Night	45% 4 7% 14% 19% 10%	5331
	11th to 20th	Night	34% 4 7% 21% 18% 16%	4980
	21st to end	Night	38% 5% 9% 23% 21% 4	5151
April	1st to 10th	Night	36% 6% 11% 22% 20% 6%	2000
	11th to 20th	Night	35% 4 8% 22% 23% 8%	1975
	21st to end	Night	36% 5% 11% 24% 22% 3	2000
May	1st to 10th	Night	46% 5% 14% 19% 13% 2	4520
	11th to 20th	Night	48% 6% 14% 22% 9% 1	4222
	21st to end	Night	41% 8% 12% 21% 14% 3	4093
June	1st to 10th	Night	29% 10% 13% 23% 24% 2	1600
	11th to 20th	Night	20% 8% 15% 28% 25% 5%	1600
	21st to end	Night	21% 7% 17% 28% 22% 5%	1600
July	1st to 10th	Night	29% 7% 20% 25% 17% 2	3580
	11th to 20th	Night	25% 5% 23% 35% 10% 2	3905
	21st to end	Night	36% 6% 19% 30% 9%	4320
August	1st to 10th	Night	13% 5% 12% 41% 25% 3	3198
	11th to 20th	Night	24% 6% 21% 29% 18% 2	3230
	21st to end	Night	24% 6% 13% 24% 29% 4	3586
September	1st to 10th	Night	29% 7% 11% 23% 22% 8%	2000
	11th to 20th	Night	32% 6% 10% 21% 25% 6%	2031
	21st to end	Night	28% 3 11% 23% 25% 10%	2200
October	1st to 10th	Night	15% 3 6% 19% 28% 29%	3816
	11th to 20th	Night	17% 3 4 16% 35% 26%	3899
	21st to end	Night	26% 2 5% 26% 26% 15%	4252
November	1st to 10th	Night	30% 4 7% 22% 28% 8%	2591
	11th to 20th	Night	29% 5% 7% 22% 26% 10%	2730
	21st to end	Night	37% 4 8% 21% 25% 6%	2730
December	1st to 10th	Night	39% 3 6% 27% 20% 3	4414
	11th to 20th	Night	43% 3 7% 21% 21% 4	5341
	21st to end	Night	44% 5% 10% 23% 14% 3	7150

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	25% 19% 37% 14% 5% 1	303
	11th to 20th	Day	27% 19% 35% 13% 4 1	300
	21st to end	Day	21% 22% 40% 14% 3	330
February	1st to 10th	Day	27% 18% 34% 14% 6% 1	300
	11th to 20th	Day	35% 21% 30% 12% 2	300
	21st to end	Day	36% 22% 29% 10% 4	249
March	1st to 10th	Day	26% 19% 32% 16% 6% 1	300
	11th to 20th	Day	25% 24% 28% 21% 2	300
	21st to end	Day	22% 18% 34% 18% 8%	465
April	1st to 10th	Day	19% 18% 33% 21% 9%	500
	11th to 20th	Day	20% 16% 32% 21% 8% 2	492
	21st to end	Day	16% 15% 30% 25% 11% 3	500
May	1st to 10th	Day	16% 14% 30% 24% 14% 2	503
	11th to 20th	Day	12% 8% 25% 28% 20% 7%	527
	21st to end	Day	12% 5% 24% 30% 21% 9%	713
June	1st to 10th	Day	9% 5% 17% 28% 27% 15%	730
	11th to 20th	Day	6% 3 15% 33% 32% 12%	730
	21st to end	Day	5% 1 16% 32% 29% 16%	730
July	1st to 10th	Day	7% 2 10% 27% 34% 20%	721
	11th to 20th	Day	6% 2 11% 26% 36% 19%	718
	21st to end	Day	8% 2 7% 30% 33% 21%	794
August	1st to 10th	Day	7% 2 9% 28% 36% 18%	551
	11th to 20th	Day	9% 3 11% 29% 34% 14%	519
	21st to end	Day	11% 2 9% 21% 32% 25%	583
September	1st to 10th	Day	9% 2 13% 28% 33% 15%	530
	11th to 20th	Day	10% 2 13% 20% 37% 19%	530
	21st to end	Day	15% 3 15% 32% 24% 11%	530
October	1st to 10th	Day	7% 5% 21% 34% 26% 8%	391
	11th to 20th	Day	8% 7% 39% 28% 14% 4	330
	21st to end	Day	25% 10% 29% 25% 9% 2	363
November	1st to 10th	Day	19% 11% 27% 28% 14% 2	330
	11th to 20th	Day	11% 14% 39% 28% 8% 1	330
	21st to end	Day	25% 15% 32% 15% 12% 2	330
December	1st to 10th	Day	26% 19% 29% 19% 6% 1	330
	11th to 20th	Day	25% 15% 33% 18% 8% 1	330
	21st to end	Day	23% 22% 32% 18% 5%	363
January	1st to 10th	Night	23% 11% 34% 26% 5%	1006
	11th to 20th	Night	24% 14% 33% 25% 3	1000
	21st to end	Night	21% 11% 39% 26% 2	1100
February	1st to 10th	Night	25% 13% 33% 25% 4	861
	11th to 20th	Night	25% 14% 32% 25% 3	783
	21st to end	Night	27% 13% 33% 24% 3	660
March	1st to 10th	Night	20% 13% 31% 30% 6%	784
	11th to 20th	Night	22% 10% 35% 27% 6% 1	600
	21st to end	Night	18% 11% 33% 28% 10%	660
April	1st to 10th	Night	15% 11% 37% 28% 9% 1	600
	11th to 20th	Night	17% 10% 32% 27% 12% 1	593
	21st to end	Night	14% 9% 32% 32% 11% 2	600
May	1st to 10th	Night	16% 5% 27% 35% 15% 2	600
	11th to 20th	Night	14% 7% 23% 36% 18% 3	591
	21st to end	Night	8% 2 20% 38% 26% 6%	650
June	1st to 10th	Night	9% 2 12% 38% 34% 6%	600
	11th to 20th	Night	4 1 10% 37% 45% 3	600
	21st to end	Night	4 2 10% 37% 41% 7%	600
July	1st to 10th	Night	4 1 7% 33% 45% 10%	585
	11th to 20th	Night	7% 1 8% 26% 43% 13%	582
	21st to end	Night	6% 1 6% 31% 46% 11%	657
August	1st to 10th	Night	4 5% 30% 49% 11%	597
	11th to 20th	Night	6% 1 7% 37% 42% 7%	594
	21st to end	Night	8% 2 6% 27% 48% 9%	660
September	1st to 10th	Night	8% 2 9% 34% 43% 5%	600
	11th to 20th	Night	12% 2 10% 30% 39% 7%	631
	21st to end	Night	11% 3 15% 37% 30% 5%	800
October	1st to 10th	Night	8% 3 18% 36% 31% 5%	800
	11th to 20th	Night	13% 5% 22% 40% 18% 2	800
	21st to end	Night	20% 6% 24% 34% 15% 1	880
November	1st to 10th	Night	14% 5% 25% 37% 17% 2	820
	11th to 20th	Night	11% 8% 33% 36% 12%	840
	21st to end	Night	17% 10% 30% 29% 12% 1	840
December	1st to 10th	Night	22% 11% 31% 29% 6%	861
	11th to 20th	Night	23% 9% 31% 30% 7%	1050
	21st to end	Night	25% 12% 30% 29% 4	1155

Page generated at 2025-12-30 00:14:08. Version ac07

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