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 2024-11-04.

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 5% 7% 44%	100814
	Night	12% 6% 2 5% 4 4 4 3 5% 7% 48%	98227
Transparency	Day	37% 6% 10% 23% 19% 4	104516
	Night	34% 6% 10% 23% 21% 6%	102653
Seeing	Day	14% 9% 21% 25% 21% 10%	15313
	Night	15% 8% 25% 31% 19% 3	25065

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	13% 14% 3 7% 4 4 3 2 3 5% 43%	2568
	11th to 20th	Day	18% 9% 2 7% 4 3 3 3 3 4 44%	2850
	21st to end	Day	21% 10% 3 6% 4 3 3 2 4 5% 39%	3135
February	1st to 10th	Day	20% 10% 3 8% 4 3 3 2 3 5% 40%	1881
	11th to 20th	Day	20% 9% 3 6% 4 4 3 3 4 5% 39%	1848
	21st to end	Day	22% 9% 4 7% 4 4 4 3 4 4 36%	1570
March	1st to 10th	Day	17% 12% 3 9% 5% 4 4 3 4 5% 34%	3120
	11th to 20th	Day	14% 6% 3 7% 4 4 5% 3 5% 5% 44%	3443
	21st to end	Day	12% 7% 3 8% 6% 5% 4 3 4 6% 41%	4089
April	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	16% 6% 2 6% 4 4 3 3 5% 6% 44%	2254
	21st to end	Day	13% 6% 2 7% 5% 4 4 3 6% 8% 43%	2280
May	1st to 10th	Day	13% 6% 3 7% 4 5% 5% 4 6% 8% 39%	3765
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 7% 8% 39%	4067
	21st to end	Day	11% 6% 3 7% 5% 6% 5% 5% 7% 10% 36%	4632
June	1st to 10th	Day	8% 5% 2 6% 5% 6% 4 5% 7% 9% 42%	2660
	11th to 20th	Day	5% 4 2 5% 5% 5% 5% 5% 8% 10% 48%	2660
	21st to end	Day	4 4 2 5% 5% 5% 6% 5% 8% 10% 46%	2660
July	1st to 10th	Day	6% 6% 2 7% 6% 6% 6% 5% 8% 10% 39%	4440
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 11% 39%	4353
	21st to end	Day	14% 8% 3 8% 6% 6% 5% 4 7% 8% 31%	4439
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	10% 6% 2 3 2 2 3 3 3 6% 59%	1712
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 5% 7% 52%	1710
	21st to end	Day	17% 8% 2 5% 4 3 4 3 5% 6% 43%	1710
December	1st to 10th	Day	19% 9% 3 6% 4 4 4 3 4 5% 39%	1710
	11th to 20th	Day	14% 8% 4 6% 5% 5% 4 3 4 5% 44%	1933
	21st to end	Day	13% 11% 4 9% 5% 5% 5% 3 5% 6% 35%	2783
January	1st to 10th	Night	19% 9% 3 5% 4 3 3 3 3 5% 43%	4382
	11th to 20th	Night	17% 8% 3 6% 4 3 3 2 4 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 6% 4 3 2 3 4 5% 40%	2297
	11th to 20th	Night	19% 9% 3 7% 4 3 2 2 3 5% 43%	2218
	21st to end	Night	19% 8% 2 6% 4 4 4 3 4 5% 40%	1884
March	1st to 10th	Night	19% 10% 3 6% 4 4 3 3 4 5% 39%	3733
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 5% 50%	3544
	21st to end	Night	13% 6% 3 7% 5% 4 5% 4 6% 6% 43%	3540
April	1st to 10th	Night	10% 6% 2 6% 4 3 4 4 6% 7% 48%	1900
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 5% 7% 49%	1875
	21st to end	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	1900
May	1st to 10th	Night	18% 7% 2 6% 3 4 4 3 5% 8% 41%	3130
	11th to 20th	Night	12% 7% 2 7% 4 4 4 3 5% 7% 46%	2822
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 8% 10% 40%	2911
June	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	1520
	11th to 20th	Night	3 3 1 3 3 3 3 3 6% 11% 60%	1520
	21st to end	Night	3 2 1 3 3 3 4 5% 9% 11% 55%	1520
July	1st to 10th	Night	5% 4 2 6% 4 5% 6% 4 6% 11% 47%	2530
	11th to 20th	Night	5% 4 2 5% 4 3 4 4 6% 13% 50%	2612
	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	10% 5% 1 4 3 3 3 3 5% 6% 57%	2438
	11th to 20th	Night	10% 5% 2 4 3 4 4 4 5% 6% 52%	2470
	21st to end	Night	14% 6% 2 5% 4 3 4 3 4 6% 49%	2470
December	1st to 10th	Night	17% 9% 3 6% 3 3 3 3 4 5% 44%	2470
	11th to 20th	Night	14% 8% 3 5% 4 3 4 3 4 6% 47%	3260
	21st to end	Night	15% 9% 3 6% 5% 5% 5% 4 5% 6% 35%	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	53% 5% 7% 12% 14% 9%	3120
	11th to 20th	Day	41% 6% 8% 22% 18% 5%	3443
	21st to end	Day	44% 5% 9% 21% 20% 1	4089
April	1st to 10th	Day	40% 6% 7% 24% 20% 3	2280
	11th to 20th	Day	41% 4 8% 20% 21% 5%	2254
	21st to end	Day	40% 6% 10% 24% 18% 1	2280
May	1st to 10th	Day	41% 8% 12% 23% 15% 1	3800
	11th to 20th	Day	39% 8% 17% 22% 13% 1	4071
	21st to end	Day	40% 10% 14% 25% 11% 1	4645
June	1st to 10th	Day	35% 10% 13% 23% 18% 2	2660
	11th to 20th	Day	28% 8% 14% 28% 18% 3	2660
	21st to end	Day	31% 8% 15% 25% 18% 4	2660
July	1st to 10th	Day	36% 9% 15% 27% 11% 2	4371
	11th to 20th	Day	36% 8% 15% 30% 9% 2	4278
	21st to end	Day	50% 8% 12% 22% 7% 1	4299
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	28% 4 9% 24% 26% 8%	1712
	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	47% 5% 9% 13% 17% 9%	3733
	11th to 20th	Night	36% 5% 9% 22% 19% 8%	3544
	21st to end	Night	40% 6% 11% 19% 23% 2	3540
April	1st to 10th	Night	35% 5% 11% 23% 21% 5%	1900
	11th to 20th	Night	34% 5% 8% 21% 24% 8%	1875
	21st to end	Night	35% 6% 10% 25% 20% 4	1900
May	1st to 10th	Night	42% 8% 11% 22% 15% 3	3160
	11th to 20th	Night	37% 7% 15% 26% 14% 1	2823
	21st to end	Night	35% 10% 13% 26% 13% 2	2919
June	1st to 10th	Night	33% 9% 12% 21% 23% 2	1520
	11th to 20th	Night	19% 7% 14% 31% 24% 5%	1520
	21st to end	Night	20% 8% 19% 28% 20% 5%	1520
July	1st to 10th	Night	29% 8% 21% 27% 13% 2	2490
	11th to 20th	Night	25% 7% 15% 37% 13% 3	2564
	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	29% 4 7% 23% 29% 8%	2438
	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	24% 17% 32% 19% 8%	456
April	1st to 10th	Day	20% 16% 32% 22% 8% 1	460
	11th to 20th	Day	19% 16% 32% 22% 9% 2	452
	21st to end	Day	17% 14% 31% 23% 12% 2	460
May	1st to 10th	Day	16% 15% 25% 29% 11% 3	463
	11th to 20th	Day	13% 6% 24% 28% 21% 8%	487
	21st to end	Day	9% 6% 22% 29% 23% 11%	620
June	1st to 10th	Day	10% 4 16% 27% 28% 14%	680
	11th to 20th	Day	5% 3 13% 32% 31% 16%	680
	21st to end	Day	6% 1 17% 32% 29% 15%	680
July	1st to 10th	Day	6% 2 9% 28% 34% 21%	671
	11th to 20th	Day	7% 1 10% 23% 38% 20%	668
	21st to end	Day	8% 2 9% 26% 35% 19%	725
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	16% 11% 23% 33% 12% 5%	279
	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	17% 12% 33% 26% 12%	627
April	1st to 10th	Night	14% 13% 35% 30% 8% 1	570
	11th to 20th	Night	18% 8% 33% 30% 11% 1	563
	21st to end	Night	13% 9% 34% 31% 11% 2	570
May	1st to 10th	Night	15% 7% 25% 38% 13% 2	570
	11th to 20th	Night	12% 4 25% 37% 18% 3	561
	21st to end	Night	7% 2 18% 40% 28% 5%	617
June	1st to 10th	Night	8% 3 15% 36% 32% 7%	570
	11th to 20th	Night	2 1 12% 35% 45% 4	570
	21st to end	Night	4 1 10% 40% 40% 5%	570
July	1st to 10th	Night	5% 1 9% 31% 45% 10%	555
	11th to 20th	Night	6% 2 9% 28% 43% 13%	552
	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	12% 6% 24% 38% 18% 2	755
	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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