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 2024-03-28.

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% 43%	78922
	Night	13% 6% 2 4 4 3 4 4 5% 7% 48%	81109
Transparency	Day	39% 7% 11% 22% 18% 4	78637
	Night	36% 6% 10% 22% 21% 5%	80915
Seeing	Day	14% 9% 21% 25% 21% 10%	14393
	Night	15% 8% 25% 31% 19% 3	24351

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	14% 13% 3 6% 5% 3 3 3 3 5% 43%	2568
	11th to 20th	Day	18% 8% 3 6% 5% 2 4 4 3 4 44%	2850
	21st to end	Day	21% 10% 3 5% 4 2 4 3 4 5% 39%	3135
February	1st to 10th	Day	20% 9% 3 7% 5% 2 3 3 3 5% 40%	1881
	11th to 20th	Day	20% 8% 3 5% 5% 2 4 4 4 5% 39%	1848
	21st to end	Day	23% 8% 4 6% 5% 3 4 4 4 5% 36%	1570
March	1st to 10th	Day	16% 8% 3 6% 5% 3 4 4 5% 5% 41%	1900
	11th to 20th	Day	19% 7% 3 7% 6% 4 5% 4 4 5% 36%	2066
	21st to end	Day	16% 8% 4 7% 6% 3 5% 4 5% 7% 37%	2410
April	1st to 10th	Day	12% 6% 3 5% 5% 3 4 5% 4 7% 46%	2160
	11th to 20th	Day	17% 5% 2 5% 5% 3 3 4 5% 7% 45%	2134
	21st to end	Day	13% 5% 2 7% 6% 3 4 4 6% 8% 42%	2160
May	1st to 10th	Day	15% 5% 3 5% 5% 3 5% 5% 5% 7% 43%	2160
	11th to 20th	Day	11% 5% 3 5% 5% 4 5% 5% 6% 8% 43%	2287
	21st to end	Day	8% 6% 3 6% 5% 4 6% 6% 8% 11% 37%	2617
June	1st to 10th	Day	8% 5% 3 5% 6% 5% 4 6% 7% 9% 43%	2520
	11th to 20th	Day	4 3 2 4 6% 4 5% 6% 8% 10% 49%	2520
	21st to end	Day	4 3 2 4 5% 3 6% 7% 8% 11% 46%	2520
July	1st to 10th	Day	6% 5% 3 5% 7% 4 7% 6% 8% 11% 36%	2520
	11th to 20th	Day	8% 3 3 6% 5% 4 6% 7% 8% 11% 39%	2475
	21st to end	Day	7% 4 2 4 6% 4 7% 7% 8% 10% 41%	2533
August	1st to 10th	Day	6% 5% 2 6% 5% 3 6% 6% 8% 12% 40%	2148
	11th to 20th	Day	8% 5% 2 5% 6% 4 5% 6% 7% 10% 42%	2125
	21st to end	Day	9% 5% 2 4 5% 4 6% 6% 7% 9% 42%	2376
September	1st to 10th	Day	9% 5% 3 5% 6% 3 5% 5% 6% 9% 44%	2160
	11th to 20th	Day	11% 5% 2 5% 5% 3 4 5% 5% 8% 46%	2084
	21st to end	Day	10% 4 2 4 4 3 3 5% 4 7% 54%	1980
October	1st to 10th	Day	7% 4 2 4 4 3 5% 4 4 6% 58%	1904
	11th to 20th	Day	11% 4 2 4 4 2 3 4 4 6% 56%	1800
	21st to end	Day	19% 6% 1 4 3 2 3 3 4 4 52%	1980
November	1st to 10th	Day	11% 6% 2 3 2 2 3 3 3 5% 60%	1685
	11th to 20th	Day	14% 4 2 4 4 2 3 3 5% 7% 52%	1710
	21st to end	Day	17% 7% 3 5% 4 2 4 4 4 7% 43%	1710
December	1st to 10th	Day	19% 8% 3 6% 4 3 5% 4 4 5% 39%	1710
	11th to 20th	Day	14% 7% 4 6% 5% 4 4 3 4 5% 44%	1933
	21st to end	Day	13% 11% 4 7% 6% 4 4 5% 5% 6% 35%	2783
January	1st to 10th	Night	19% 9% 3 5% 5% 2 3 3 3 5% 43%	4382
	11th to 20th	Night	17% 7% 3 5% 5% 3 3 3 3 5% 44%	4120
	21st to end	Night	24% 9% 2 3 3 2 3 3 3 5% 44%	4532
February	1st to 10th	Night	20% 10% 3 5% 5% 2 3 4 4 5% 41%	2297
	11th to 20th	Night	19% 8% 3 6% 5% 2 3 3 3 5% 43%	2218
	21st to end	Night	20% 7% 3 6% 4 3 5% 4 4 5% 40%	1884
March	1st to 10th	Night	16% 7% 2 4 5% 4 4 4 4 6% 45%	2280
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 4 4 5% 42%	2128
	21st to end	Night	15% 6% 3 6% 5% 3 5% 5% 6% 6% 42%	2098
April	1st to 10th	Night	10% 5% 2 5% 5% 3 4 5% 6% 7% 49%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 3 4 5% 8% 49%	1775
	21st to end	Night	11% 5% 2 5% 5% 3 3 4 5% 7% 49%	1800
May	1st to 10th	Night	14% 5% 2 4 4 2 3 3 4 9% 50%	1800
	11th to 20th	Night	10% 5% 2 5% 5% 3 5% 5% 5% 7% 46%	1602
	21st to end	Night	5% 4 2 6% 6% 4 6% 5% 7% 13% 42%	1654
June	1st to 10th	Night	7% 5% 2 5% 6% 4 5% 5% 6% 10% 46%	1440
	11th to 20th	Night	3 3 1 3 4 2 4 3 6% 11% 61%	1440
	21st to end	Night	3 2 2 3 3 3 4 6% 9% 11% 55%	1440
July	1st to 10th	Night	3 3 2 6% 5% 5% 6% 6% 8% 10% 47%	1440
	11th to 20th	Night	6% 4 2 4 4 3 4 5% 7% 13% 49%	1480
	21st to end	Night	5% 3 2 3 3 2 5% 5% 6% 12% 54%	1783
August	1st to 10th	Night	3 3 1 4 5% 3 6% 6% 6% 10% 53%	1790
	11th to 20th	Night	5% 3 2 4 5% 3 5% 5% 8% 11% 48%	1776
	21st to end	Night	6% 4 2 5% 5% 3 5% 6% 6% 10% 49%	1980
September	1st to 10th	Night	7% 4 3 5% 4 3 4 5% 5% 8% 53%	1800
	11th to 20th	Night	8% 6% 2 5% 4 2 4 4 5% 8% 52%	1856
	21st to end	Night	9% 4 1 3 3 3 3 4 4 6% 59%	1980
October	1st to 10th	Night	8% 3 2 4 3 2 3 4 4 7% 61%	2052
	11th to 20th	Night	12% 5% 2 3 3 2 3 4 4 5% 57%	2160
	21st to end	Night	17% 6% 2 3 4 3 3 3 4 5% 51%	2376
November	1st to 10th	Night	10% 5% 2 3 3 2 3 3 5% 6% 58%	2392
	11th to 20th	Night	11% 5% 2 4 4 3 4 4 5% 6% 53%	2470
	21st to end	Night	15% 6% 2 5% 4 3 4 3 4 6% 49%	2470
December	1st to 10th	Night	17% 9% 3 6% 4 2 4 3 4 5% 44%	2470
	11th to 20th	Night	14% 8% 3 4 5% 2 3 4 4 6% 47%	3260
	21st to end	Night	15% 9% 3 5% 6% 4 6% 5% 5% 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	46% 3 6% 21% 21% 3	2569
	11th to 20th	Day	45% 6% 7% 21% 20% 1	2850
	21st to end	Day	49% 4 7% 18% 20% 2	3135
February	1st to 10th	Day	49% 3 8% 20% 17% 3	1881
	11th to 20th	Day	49% 4 6% 19% 17% 5%	1858
	21st to end	Day	50% 4 9% 14% 16% 6%	1570
March	1st to 10th	Day	46% 5% 7% 17% 17% 8%	1900
	11th to 20th	Day	51% 6% 6% 18% 16% 3	2066
	21st to end	Day	48% 5% 10% 22% 14% 1	2410
April	1st to 10th	Day	39% 6% 8% 24% 20% 3	2160
	11th to 20th	Day	40% 4 9% 21% 21% 5%	2134
	21st to end	Day	40% 6% 10% 23% 19% 1	2160
May	1st to 10th	Day	41% 6% 12% 21% 18% 2	2160
	11th to 20th	Day	37% 8% 15% 22% 16% 2	2291
	21st to end	Day	39% 9% 16% 25% 10% 1	2617
June	1st to 10th	Day	35% 10% 12% 23% 19% 2	2520
	11th to 20th	Day	28% 8% 15% 28% 17% 3	2520
	21st to end	Day	30% 8% 15% 24% 18% 4	2520
July	1st to 10th	Day	37% 11% 17% 19% 13% 3	2451
	11th to 20th	Day	34% 10% 21% 20% 11% 4	2400
	21st to end	Day	34% 11% 16% 27% 11% 1	2392
August	1st to 10th	Day	33% 10% 18% 25% 12% 1	2136
	11th to 20th	Day	35% 9% 14% 23% 15% 3	2125
	21st to end	Day	35% 9% 13% 24% 15% 4	2376
September	1st to 10th	Day	35% 9% 11% 24% 17% 4	2160
	11th to 20th	Day	36% 7% 11% 18% 24% 5%	2084
	21st to end	Day	30% 5% 9% 23% 26% 7%	1980
October	1st to 10th	Day	28% 6% 10% 24% 25% 8%	1904
	11th to 20th	Day	30% 5% 8% 21% 29% 8%	1800
	21st to end	Day	37% 4 5% 22% 24% 8%	1980
November	1st to 10th	Day	28% 4 8% 24% 27% 9%	1685
	11th to 20th	Day	33% 4 9% 21% 26% 8%	1710
	21st to end	Day	43% 4 7% 19% 23% 5%	1710
December	1st to 10th	Day	48% 5% 7% 19% 17% 4	1710
	11th to 20th	Day	44% 4 7% 20% 22% 3	1930
	21st to end	Day	47% 7% 10% 20% 14% 2	2783
January	1st to 10th	Night	45% 4 6% 20% 22% 3	4382
	11th to 20th	Night	43% 5% 7% 21% 20% 4	4120
	21st to end	Night	44% 4 7% 23% 19% 3	4532
February	1st to 10th	Night	48% 4 8% 17% 20% 3	2297
	11th to 20th	Night	46% 4 6% 20% 18% 6%	2219
	21st to end	Night	47% 5% 8% 16% 18% 6%	1884
March	1st to 10th	Night	41% 5% 8% 15% 21% 10%	2280
	11th to 20th	Night	45% 5% 10% 19% 18% 4	2128
	21st to end	Night	41% 6% 11% 21% 18% 3	2098
April	1st to 10th	Night	34% 6% 12% 23% 20% 5%	1800
	11th to 20th	Night	34% 5% 9% 22% 23% 7%	1775
	21st to end	Night	35% 6% 9% 26% 21% 3	1800
May	1st to 10th	Night	34% 6% 12% 24% 20% 4	1800
	11th to 20th	Night	36% 7% 14% 23% 18% 2	1603
	21st to end	Night	32% 10% 15% 27% 14% 2	1655
June	1st to 10th	Night	33% 9% 12% 22% 23% 2	1440
	11th to 20th	Night	18% 7% 15% 32% 23% 5%	1440
	21st to end	Night	19% 9% 18% 28% 20% 5%	1440
July	1st to 10th	Night	28% 10% 18% 25% 15% 3	1400
	11th to 20th	Night	25% 9% 22% 25% 13% 6%	1432
	21st to end	Night	22% 9% 21% 31% 15% 1	1684
August	1st to 10th	Night	23% 9% 19% 33% 16% 1	1778
	11th to 20th	Night	28% 12% 16% 25% 17% 3	1776
	21st to end	Night	28% 9% 16% 25% 16% 6%	1980
September	1st to 10th	Night	29% 8% 10% 23% 23% 7%	1800
	11th to 20th	Night	30% 7% 11% 19% 27% 5%	1856
	21st to end	Night	27% 4 9% 26% 24% 9%	1980
October	1st to 10th	Night	24% 5% 10% 25% 25% 12%	2052
	11th to 20th	Night	29% 5% 7% 20% 30% 10%	2160
	21st to end	Night	37% 4 7% 19% 25% 8%	2376
November	1st to 10th	Night	28% 4 7% 23% 29% 8%	2392
	11th to 20th	Night	31% 4 8% 22% 25% 11%	2470
	21st to end	Night	38% 4 7% 20% 24% 6%	2470
December	1st to 10th	Night	43% 5% 8% 21% 18% 5%	2470
	11th to 20th	Night	39% 4 6% 23% 23% 5%	3262
	21st to end	Night	44% 7% 11% 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	23% 20% 37% 13% 6% 1	270
	11th to 20th	Day	27% 20% 36% 12% 3 1	270
	21st to end	Day	24% 21% 37% 14% 3	297
February	1st to 10th	Day	25% 18% 37% 14% 5% 1	270
	11th to 20th	Day	36% 20% 29% 14% 2 1	270
	21st to end	Day	37% 23% 28% 8% 4	225
March	1st to 10th	Day	21% 21% 35% 16% 6% 1	270
	11th to 20th	Day	23% 21% 35% 19% 1	270
	21st to end	Day	25% 18% 31% 18% 8%	440
April	1st to 10th	Day	18% 15% 33% 24% 9% 1	420
	11th to 20th	Day	20% 15% 31% 23% 10% 2	412
	21st to end	Day	18% 14% 30% 24% 12% 2	420
May	1st to 10th	Day	17% 13% 26% 29% 12% 3	423
	11th to 20th	Day	13% 6% 24% 27% 22% 9%	447
	21st to end	Day	9% 4 21% 30% 24% 12%	571
June	1st to 10th	Day	10% 4 16% 27% 28% 15%	630
	11th to 20th	Day	4 3 14% 33% 31% 15%	630
	21st to end	Day	5% 2 18% 32% 29% 15%	630
July	1st to 10th	Day	6% 2 9% 28% 33% 22%	621
	11th to 20th	Day	7% 1 10% 22% 39% 21%	618
	21st to end	Day	7% 2 9% 26% 37% 20%	671
August	1st to 10th	Day	7% 3 8% 29% 36% 17%	446
	11th to 20th	Day	10% 1 8% 32% 36% 13%	439
	21st to end	Day	12% 1 5% 23% 38% 21%	495
September	1st to 10th	Day	9% 2 7% 30% 36% 16%	450
	11th to 20th	Day	8% 3 11% 24% 36% 19%	450
	21st to end	Day	14% 3 15% 31% 23% 14%	450
October	1st to 10th	Day	9% 4 20% 28% 32% 6%	374
	11th to 20th	Day	12% 10% 36% 29% 10% 4	270
	21st to end	Day	24% 12% 28% 25% 8% 2	297
November	1st to 10th	Day	17% 11% 23% 33% 12% 4	270
	11th to 20th	Day	10% 16% 35% 29% 10% 1	270
	21st to end	Day	21% 16% 28% 19% 12% 3	270
December	1st to 10th	Day	24% 20% 29% 19% 7% 1	270
	11th to 20th	Day	22% 19% 31% 19% 9% 1	270
	21st to end	Day	23% 20% 34% 18% 5% 1	297
January	1st to 10th	Night	23% 13% 36% 24% 5%	950
	11th to 20th	Night	23% 16% 34% 22% 4	950
	21st to end	Night	22% 11% 39% 25% 2	1045
February	1st to 10th	Night	24% 12% 35% 25% 4	777
	11th to 20th	Night	24% 15% 35% 23% 2	743
	21st to end	Night	25% 17% 34% 22% 3	628
March	1st to 10th	Night	18% 15% 32% 29% 5%	760
	11th to 20th	Night	23% 10% 35% 27% 4 1	608
	21st to end	Night	18% 12% 32% 26% 12%	615
April	1st to 10th	Night	13% 12% 34% 32% 9% 1	540
	11th to 20th	Night	17% 8% 32% 31% 12% 1	533
	21st to end	Night	13% 9% 33% 32% 11% 2	540
May	1st to 10th	Night	15% 7% 25% 39% 12% 2	540
	11th to 20th	Night	12% 4 25% 37% 19% 4	531
	21st to end	Night	7% 1 17% 41% 30% 5%	584
June	1st to 10th	Night	7% 2 14% 36% 33% 7%	540
	11th to 20th	Night	2 1 12% 35% 45% 4	540
	21st to end	Night	4 1 10% 41% 38% 6%	540
July	1st to 10th	Night	4 1 9% 31% 45% 11%	525
	11th to 20th	Night	7% 2 9% 27% 43% 13%	522
	21st to end	Night	5% 1 8% 28% 46% 12%	591
August	1st to 10th	Night	4 1 6% 29% 51% 9%	537
	11th to 20th	Night	5% 2 7% 36% 43% 6%	534
	21st to end	Night	7% 1 8% 26% 48% 9%	594
September	1st to 10th	Night	6% 1 7% 41% 39% 7%	540
	11th to 20th	Night	10% 2 12% 31% 37% 7%	596
	21st to end	Night	11% 4 15% 36% 30% 5%	720
October	1st to 10th	Night	9% 3 17% 36% 31% 4	720
	11th to 20th	Night	14% 6% 26% 35% 17% 2	720
	21st to end	Night	20% 8% 23% 33% 16% 1	792
November	1st to 10th	Night	12% 6% 24% 38% 19% 2	740
	11th to 20th	Night	11% 9% 34% 33% 11%	760
	21st to end	Night	18% 9% 31% 29% 11% 1	760
December	1st to 10th	Night	22% 11% 31% 28% 7%	760
	11th to 20th	Night	22% 11% 29% 31% 7%	931
	21st to end	Night	23% 15% 31% 26% 5%	1045

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