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 2024-01-14.

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% 6% 3 5% 5% 3 5% 5% 5% 7% 44%	74728
	Night	12% 5% 2 4 4 3 4 4 5% 7% 49%	75036
Transparency	Day	38% 7% 11% 22% 18% 4	74444
	Night	35% 6% 10% 22% 21% 6%	74841
Seeing	Day	14% 8% 21% 24% 22% 10%	14163
	Night	15% 7% 24% 31% 20% 3	24052

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	18% 9% 3 6% 4 2 3 2 3 5% 44%	1612
	11th to 20th	Day	21% 8% 4 5% 5% 3 3 4 4 5% 38%	1647
	21st to end	Day	15% 8% 3 5% 4 2 3 4 3 5% 48%	1782
February	1st to 10th	Day	20% 8% 4 6% 4 2 3 4 4 4 40%	1742
	11th to 20th	Day	21% 8% 3 6% 4 3 4 3 4 5% 38%	1748
	21st to end	Day	24% 8% 4 5% 5% 2 3 3 4 4 37%	1480
March	1st to 10th	Day	16% 7% 3 6% 5% 3 4 4 4 6% 42%	1828
	11th to 20th	Day	20% 7% 4 6% 6% 3 5% 5% 4 5% 34%	1980
	21st to end	Day	16% 7% 3 7% 5% 4 4 5% 5% 7% 37%	2299
April	1st to 10th	Day	12% 6% 3 6% 5% 4 5% 4 6% 6% 45%	2160
	11th to 20th	Day	16% 6% 3 4 4 3 3 4 5% 7% 46%	2134
	21st to end	Day	13% 7% 4 6% 6% 3 4 4 6% 6% 42%	2160
May	1st to 10th	Day	14% 5% 3 6% 5% 3 4 5% 4 7% 43%	2160
	11th to 20th	Day	11% 5% 2 5% 5% 4 5% 5% 7% 8% 43%	2220
	21st to end	Day	8% 5% 3 6% 7% 4 6% 7% 7% 9% 38%	2656
June	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 6% 7% 10% 42%	2520
	11th to 20th	Day	4 3 2 5% 5% 3 5% 6% 7% 10% 49%	2520
	21st to end	Day	5% 3 2 4 4 4 5% 7% 7% 10% 49%	2520
July	1st to 10th	Day	6% 5% 3 6% 7% 4 6% 7% 8% 11% 36%	2520
	11th to 20th	Day	7% 4 2 5% 7% 4 6% 6% 8% 10% 41%	2389
	21st to end	Day	7% 3 2 5% 6% 3 6% 7% 7% 12% 42%	2546
August	1st to 10th	Day	6% 5% 3 5% 6% 4 6% 7% 8% 11% 41%	2171
	11th to 20th	Day	8% 5% 2 6% 6% 4 6% 5% 6% 10% 43%	2125
	21st to end	Day	9% 5% 2 5% 5% 4 6% 5% 6% 9% 43%	2376
September	1st to 10th	Day	9% 5% 2 5% 5% 4 5% 5% 6% 9% 44%	2160
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 4 5% 8% 46%	2113
	21st to end	Day	10% 4 2 3 3 2 4 4 5% 6% 57%	1980
October	1st to 10th	Day	7% 5% 2 4 4 4 4 5% 4 6% 56%	1855
	11th to 20th	Day	12% 4 1 3 4 2 3 4 5% 6% 55%	1800
	21st to end	Day	19% 6% 1 3 3 2 3 3 4 4 52%	1980
November	1st to 10th	Day	11% 6% 2 4 2 2 2 3 3 5% 61%	1737
	11th to 20th	Day	13% 5% 3 4 4 2 3 4 4 6% 52%	1710
	21st to end	Day	19% 7% 3 5% 4 2 4 4 4 7% 43%	1710
December	1st to 10th	Day	20% 8% 3 6% 4 3 5% 3 5% 5% 39%	1687
	11th to 20th	Day	14% 7% 3 6% 5% 3 4 3 4 4 45%	1918
	21st to end	Day	14% 10% 4 7% 5% 4 5% 4 5% 6% 36%	2783
January	1st to 10th	Night	19% 7% 2 4 4 2 3 3 4 5% 47%	2549
	11th to 20th	Night	19% 9% 4 6% 5% 2 4 3 3 5% 40%	2386
	21st to end	Night	17% 7% 2 5% 4 3 2 3 4 6% 47%	2574
February	1st to 10th	Night	20% 8% 4 7% 4 2 3 3 3 5% 41%	2215
	11th to 20th	Night	19% 8% 4 5% 5% 3 3 3 3 4 44%	2098
	21st to end	Night	21% 8% 3 5% 4 3 3 3 4 5% 41%	1776
March	1st to 10th	Night	16% 6% 2 4 4 2 4 3 5% 6% 46%	2146
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 3 4 5% 41%	1980
	21st to end	Night	15% 6% 3 5% 5% 3 4 5% 5% 7% 42%	2057
April	1st to 10th	Night	10% 5% 2 5% 5% 3 4 5% 5% 7% 48%	1800
	11th to 20th	Night	14% 5% 2 4 3 2 4 5% 4 6% 51%	1775
	21st to end	Night	12% 5% 2 5% 5% 2 5% 3 4 6% 50%	1800
May	1st to 10th	Night	13% 6% 2 4 3 2 4 4 5% 9% 49%	1800
	11th to 20th	Night	11% 5% 2 5% 5% 3 4 5% 4 9% 46%	1669
	21st to end	Night	5% 4 3 5% 5% 4 6% 5% 7% 12% 43%	1616
June	1st to 10th	Night	6% 5% 2 5% 5% 2 5% 5% 8% 11% 47%	1440
	11th to 20th	Night	2 2 1 3 3 2 4 4 7% 10% 61%	1440
	21st to end	Night	3 3 2 3 4 2 4 5% 6% 13% 55%	1440
July	1st to 10th	Night	3 2 2 5% 6% 3 5% 6% 8% 11% 48%	1440
	11th to 20th	Night	6% 3 2 4 5% 3 4 6% 6% 11% 50%	1566
	21st to end	Night	4 2 2 3 4 2 5% 5% 7% 11% 54%	1772
August	1st to 10th	Night	3 3 1 3 4 4 4 5% 7% 10% 56%	1767
	11th to 20th	Night	6% 4 2 4 5% 4 6% 6% 7% 11% 46%	1776
	21st to end	Night	6% 5% 2 4 4 3 5% 5% 6% 11% 49%	1980
September	1st to 10th	Night	7% 4 2 5% 4 3 5% 4 6% 7% 53%	1800
	11th to 20th	Night	9% 5% 2 4 3 3 5% 5% 5% 6% 52%	1827
	21st to end	Night	9% 5% 2 3 3 3 3 3 4 6% 59%	1980
October	1st to 10th	Night	7% 3 2 3 3 2 3 4 4 7% 62%	2101
	11th to 20th	Night	11% 4 2 3 3 2 3 3 4 6% 59%	2160
	21st to end	Night	17% 5% 2 4 3 2 3 3 3 4 52%	2376
November	1st to 10th	Night	10% 4 2 3 3 2 3 3 4 6% 58%	2338
	11th to 20th	Night	11% 4 2 4 3 3 4 4 4 7% 54%	2470
	21st to end	Night	14% 5% 3 4 4 3 4 4 4 6% 49%	2470
December	1st to 10th	Night	18% 8% 3 4 5% 2 2 4 4 6% 45%	2489
	11th to 20th	Night	16% 6% 2 5% 4 2 3 4 4 5% 48%	3279
	21st to end	Night	15% 7% 2 6% 6% 4 5% 5% 6% 6% 36%	4884

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	45% 4 8% 22% 17% 5%	1613
	11th to 20th	Day	49% 5% 8% 20% 17% 2	1647
	21st to end	Day	39% 3 9% 24% 22% 4	1782
February	1st to 10th	Day	48% 5% 8% 21% 16% 3	1742
	11th to 20th	Day	49% 4 7% 18% 17% 4	1758
	21st to end	Day	51% 4 8% 16% 15% 6%	1480
March	1st to 10th	Day	45% 4 8% 17% 18% 8%	1828
	11th to 20th	Day	52% 6% 8% 17% 16% 2	1980
	21st to end	Day	46% 6% 11% 23% 14% 1	2299
April	1st to 10th	Day	39% 6% 9% 23% 20% 3	2160
	11th to 20th	Day	39% 5% 8% 22% 21% 5%	2134
	21st to end	Day	41% 7% 9% 23% 19% 1	2160
May	1st to 10th	Day	41% 8% 11% 20% 18% 3	2160
	11th to 20th	Day	36% 9% 16% 21% 16% 2	2224
	21st to end	Day	38% 10% 16% 24% 10% 2	2656
June	1st to 10th	Day	34% 9% 14% 22% 19% 1	2520
	11th to 20th	Day	27% 8% 14% 29% 18% 4	2520
	21st to end	Day	28% 9% 14% 26% 19% 5%	2520
July	1st to 10th	Day	35% 12% 18% 19% 12% 3	2451
	11th to 20th	Day	33% 12% 19% 21% 11% 4	2317
	21st to end	Day	33% 10% 17% 28% 10% 1	2405
August	1st to 10th	Day	33% 10% 19% 25% 12% 1	2157
	11th to 20th	Day	33% 11% 14% 24% 15% 2	2125
	21st to end	Day	34% 9% 16% 22% 15% 4	2376
September	1st to 10th	Day	34% 10% 12% 22% 17% 5%	2160
	11th to 20th	Day	36% 7% 11% 15% 24% 6%	2113
	21st to end	Day	28% 5% 9% 24% 25% 8%	1980
October	1st to 10th	Day	29% 6% 9% 23% 23% 10%	1855
	11th to 20th	Day	30% 5% 7% 22% 28% 9%	1800
	21st to end	Day	37% 4 5% 21% 25% 8%	1980
November	1st to 10th	Day	28% 3 8% 25% 26% 9%	1737
	11th to 20th	Day	33% 4 9% 20% 26% 8%	1710
	21st to end	Day	42% 4 8% 18% 23% 5%	1710
December	1st to 10th	Day	47% 5% 8% 18% 17% 5%	1687
	11th to 20th	Day	43% 5% 7% 19% 23% 4	1915
	21st to end	Day	45% 7% 11% 20% 14% 2	2783
January	1st to 10th	Night	42% 4 7% 20% 23% 4	2549
	11th to 20th	Night	48% 4 7% 18% 18% 4	2386
	21st to end	Night	40% 3 6% 22% 23% 5%	2574
February	1st to 10th	Night	47% 3 8% 18% 20% 3	2215
	11th to 20th	Night	45% 5% 7% 21% 17% 6%	2099
	21st to end	Night	46% 4 8% 15% 20% 7%	1776
March	1st to 10th	Night	39% 5% 9% 16% 20% 12%	2146
	11th to 20th	Night	46% 6% 8% 17% 18% 5%	1980
	21st to end	Night	40% 6% 10% 22% 18% 3	2057
April	1st to 10th	Night	35% 6% 11% 23% 20% 6%	1800
	11th to 20th	Night	34% 5% 8% 22% 24% 7%	1775
	21st to end	Night	35% 5% 10% 24% 23% 3	1800
May	1st to 10th	Night	34% 5% 12% 23% 21% 5%	1800
	11th to 20th	Night	36% 7% 14% 23% 18% 2	1670
	21st to end	Night	30% 10% 15% 27% 16% 2	1617
June	1st to 10th	Night	29% 11% 12% 23% 23% 2	1440
	11th to 20th	Night	17% 8% 16% 30% 25% 5%	1440
	21st to end	Night	21% 8% 15% 28% 23% 5%	1440
July	1st to 10th	Night	25% 12% 19% 26% 15% 3	1400
	11th to 20th	Night	27% 9% 19% 27% 13% 5%	1515
	21st to end	Night	22% 10% 19% 32% 16% 1	1673
August	1st to 10th	Night	20% 9% 20% 35% 15%	1757
	11th to 20th	Night	30% 10% 17% 25% 16% 3	1776
	21st to end	Night	29% 8% 16% 24% 18% 5%	1980
September	1st to 10th	Night	28% 7% 11% 24% 23% 8%	1800
	11th to 20th	Night	31% 7% 10% 20% 26% 6%	1827
	21st to end	Night	27% 4 10% 24% 25% 10%	1980
October	1st to 10th	Night	22% 5% 9% 28% 25% 12%	2101
	11th to 20th	Night	28% 5% 7% 20% 29% 11%	2160
	21st to end	Night	37% 4 6% 19% 26% 9%	2376
November	1st to 10th	Night	28% 4 7% 22% 29% 9%	2338
	11th to 20th	Night	30% 5% 7% 21% 27% 10%	2470
	21st to end	Night	36% 5% 8% 20% 25% 6%	2470
December	1st to 10th	Night	42% 5% 8% 22% 18% 6%	2489
	11th to 20th	Night	39% 4 8% 21% 24% 5%	3281
	21st to end	Night	42% 7% 12% 19% 15% 4	4884

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	26% 17% 37% 14% 5% 1	270
	11th to 20th	Day	28% 17% 34% 14% 5% 1	249
	21st to end	Day	18% 23% 42% 13% 4	264
February	1st to 10th	Day	25% 19% 35% 15% 6% 1	240
	11th to 20th	Day	36% 19% 31% 11% 3	240
	21st to end	Day	38% 22% 26% 10% 5%	198
March	1st to 10th	Day	25% 18% 33% 18% 6% 1	240
	11th to 20th	Day	26% 23% 25% 24% 1	240
	21st to end	Day	22% 16% 32% 20% 9% 1	385
April	1st to 10th	Day	16% 16% 34% 23% 11%	420
	11th to 20th	Day	21% 13% 31% 23% 10% 3	412
	21st to end	Day	16% 16% 29% 25% 12% 2	420
May	1st to 10th	Day	16% 12% 29% 25% 15% 3	423
	11th to 20th	Day	11% 6% 24% 28% 23% 8%	447
	21st to end	Day	10% 4 22% 30% 24% 10%	610
June	1st to 10th	Day	10% 4 14% 27% 29% 16%	630
	11th to 20th	Day	4 3 13% 34% 33% 13%	630
	21st to end	Day	6% 1 17% 32% 27% 16%	630
July	1st to 10th	Day	7% 2 9% 26% 35% 21%	621
	11th to 20th	Day	7% 2 10% 23% 37% 21%	618
	21st to end	Day	6% 2 7% 28% 34% 23%	684
August	1st to 10th	Day	8% 2 9% 26% 36% 19%	469
	11th to 20th	Day	10% 3 10% 29% 34% 14%	439
	21st to end	Day	11% 2 7% 18% 34% 27%	495
September	1st to 10th	Day	8% 1 9% 27% 36% 18%	450
	11th to 20th	Day	9% 2 12% 19% 37% 20%	450
	21st to end	Day	14% 2 13% 33% 25% 12%	450
October	1st to 10th	Day	7% 5% 22% 31% 26% 8%	325
	11th to 20th	Day	9% 7% 41% 27% 12% 4	270
	21st to end	Day	25% 11% 29% 23% 10% 3	297
November	1st to 10th	Day	16% 10% 26% 30% 17% 2	270
	11th to 20th	Day	11% 14% 38% 28% 9% 1	270
	21st to end	Day	26% 13% 31% 16% 13% 1	270
December	1st to 10th	Day	26% 19% 28% 19% 7% 1	270
	11th to 20th	Day	23% 16% 31% 20% 9% 1	270
	21st to end	Day	22% 23% 32% 18% 5%	297
January	1st to 10th	Night	23% 11% 35% 26% 5%	950
	11th to 20th	Night	25% 13% 32% 26% 4	918
	21st to end	Night	20% 11% 39% 27% 3	990
February	1st to 10th	Night	24% 13% 34% 25% 4	775
	11th to 20th	Night	24% 14% 33% 26% 3	703
	21st to end	Night	28% 13% 32% 24% 3	592
March	1st to 10th	Night	20% 11% 31% 31% 7%	706
	11th to 20th	Night	22% 9% 33% 28% 6% 1	540
	21st to end	Night	19% 10% 31% 28% 11%	594
April	1st to 10th	Night	13% 9% 37% 31% 9% 1	540
	11th to 20th	Night	17% 8% 31% 29% 13% 1	533
	21st to end	Night	14% 8% 32% 33% 11% 2	540
May	1st to 10th	Night	16% 4 26% 37% 15% 2	540
	11th to 20th	Night	13% 5% 23% 36% 20% 3	531
	21st to end	Night	7% 2 18% 40% 28% 6%	584
June	1st to 10th	Night	8% 2 10% 38% 35% 6%	540
	11th to 20th	Night	2 1 9% 38% 47% 3	540
	21st to end	Night	4 2 11% 38% 39% 8%	540
July	1st to 10th	Night	3 7% 33% 46% 10%	525
	11th to 20th	Night	7% 1 9% 25% 43% 15%	522
	21st to end	Night	5% 1 6% 30% 46% 12%	591
August	1st to 10th	Night	5% 5% 29% 50% 11%	537
	11th to 20th	Night	7% 1 7% 37% 42% 7%	534
	21st to end	Night	8% 2 4 26% 51% 9%	594
September	1st to 10th	Night	8% 2 6% 33% 45% 5%	540
	11th to 20th	Night	11% 2 11% 30% 38% 8%	567
	21st to end	Night	11% 3 14% 37% 31% 5%	720
October	1st to 10th	Night	8% 3 18% 34% 32% 5%	720
	11th to 20th	Night	14% 5% 24% 38% 18% 2	720
	21st to end	Night	20% 6% 24% 33% 16% 1	792
November	1st to 10th	Night	13% 4 25% 38% 18% 2	740
	11th to 20th	Night	12% 8% 32% 36% 13%	760
	21st to end	Night	17% 10% 30% 29% 13% 1	760
December	1st to 10th	Night	21% 10% 31% 31% 7%	779
	11th to 20th	Night	23% 9% 29% 32% 8%	950
	21st to end	Night	25% 13% 29% 29% 4	1045

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