Mount Jennings Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Jennings Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	13% 9% 3 8% 5% 5% 5% 4 6% 8% 36%	131147
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 37%	130951
Transparency	Day	45% 5% 11% 23% 14% 2	134682
	Night	46% 5% 11% 22% 14% 2	135491
Seeing	Day	16% 11% 29% 25% 14% 6%	17602
	Night	17% 10% 30% 30% 12% 1	27587

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% 13% 7% 13% 6% 5% 5% 4 5% 7% 22%	4691
	11th to 20th	Day	8% 12% 6% 14% 7% 7% 6% 5% 5% 4 26%	4710
	21st to end	Day	14% 16% 5% 11% 6% 4 4 3 5% 6% 26%	5335
February	1st to 10th	Day	17% 12% 4 10% 6% 5% 5% 4 5% 6% 25%	1974
	11th to 20th	Day	13% 12% 4 9% 6% 6% 3 3 5% 7% 32%	2048
	21st to end	Day	15% 13% 5% 9% 6% 4 4 3 5% 7% 29%	1730
March	1st to 10th	Day	19% 13% 4 7% 4 3 3 2 4 4 39%	4580
	11th to 20th	Day	8% 8% 3 7% 5% 4 5% 4 6% 9% 40%	4755
	21st to end	Day	18% 13% 5% 10% 6% 6% 5% 4 5% 7% 22%	6099
April	1st to 10th	Day	17% 12% 4 7% 5% 4 4 3 5% 7% 33%	2412
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 3 5% 6% 36%	2374
	21st to end	Day	19% 8% 3 7% 5% 4 4 4 5% 7% 34%	2506
May	1st to 10th	Day	19% 8% 2 7% 5% 5% 5% 4 7% 8% 31%	5862
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 6% 7% 40%	6057
	21st to end	Day	13% 7% 2 8% 5% 4 4 4 6% 9% 38%	6935
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 8% 44%	2800
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 9% 44%	2800
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 9% 39%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 8% 11% 36%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 39%	6360
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 9% 38%	6924
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 48%	4405
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 5% 7% 10% 43%	3857
	21st to end	Day	6% 5% 2 6% 4 5% 4 4 7% 9% 48%	4312
September	1st to 10th	Day	10% 6% 2 5% 4 4 5% 3 7% 7% 47%	2400
	11th to 20th	Day	11% 5% 2 4 3 3 4 4 5% 7% 52%	2257
	21st to end	Day	10% 6% 2 6% 5% 5% 4 4 6% 8% 45%	2200
October	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 6% 47%	2175
	11th to 20th	Day	18% 7% 3 6% 5% 4 4 3 6% 6% 38%	1985
	21st to end	Day	19% 9% 3 8% 4 4 4 3 6% 6% 35%	1980
November	1st to 10th	Day	13% 8% 2 5% 4 3 3 4 5% 8% 46%	1852
	11th to 20th	Day	17% 10% 4 8% 5% 4 4 3 5% 6% 35%	1890
	21st to end	Day	17% 12% 3 10% 5% 3 3 3 4 5% 34%	1874
December	1st to 10th	Day	9% 11% 4 9% 5% 4 4 2 4 5% 41%	2690
	11th to 20th	Day	13% 15% 5% 12% 6% 5% 5% 3 6% 6% 23%	3088
	21st to end	Day	26% 21% 7% 13% 6% 4 4 3 3 3 11%	4070
January	1st to 10th	Night	13% 13% 6% 12% 5% 5% 4 3 5% 5% 28%	8232
	11th to 20th	Night	10% 13% 6% 15% 8% 6% 5% 4 5% 5% 22%	8280
	21st to end	Night	16% 13% 5% 9% 5% 5% 5% 4 4 5% 30%	8864
February	1st to 10th	Night	15% 14% 5% 8% 6% 4 4 3 4 6% 30%	2645
	11th to 20th	Night	14% 11% 3 8% 6% 5% 5% 3 5% 6% 34%	2458
	21st to end	Night	18% 11% 4 8% 5% 4 3 3 4 5% 34%	2076
March	1st to 10th	Night	16% 11% 4 6% 4 4 3 3 4 5% 39%	5493
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 45%	5467
	21st to end	Night	18% 10% 5% 10% 6% 5% 5% 4 5% 6% 27%	5072
April	1st to 10th	Night	15% 9% 3 8% 4 3 3 3 5% 7% 39%	2010
	11th to 20th	Night	18% 9% 3 6% 4 4 4 3 5% 6% 39%	1975
	21st to end	Night	17% 8% 3 6% 5% 5% 4 4 5% 7% 36%	1892
May	1st to 10th	Night	18% 9% 3 8% 5% 4 4 3 5% 7% 34%	4033
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 7% 9% 42%	3842
	21st to end	Night	14% 8% 3 7% 4 4 4 4 5% 8% 38%	3942
June	1st to 10th	Night	9% 6% 2 5% 5% 4 3 3 5% 7% 50%	1600
	11th to 20th	Night	8% 7% 3 5% 4 5% 4 3 7% 8% 47%	1600
	21st to end	Night	9% 6% 2 6% 5% 4 5% 4 6% 8% 43%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 5% 7% 9% 40%	3618
	11th to 20th	Night	6% 5% 2 7% 4 4 6% 4 5% 8% 49%	3617
	21st to end	Night	7% 6% 3 7% 5% 5% 5% 4 7% 7% 43%	3947
August	1st to 10th	Night	5% 4 2 6% 4 5% 5% 4 7% 9% 48%	2746
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 7% 8% 51%	3191
	21st to end	Night	8% 4 2 5% 3 4 4 3 6% 9% 53%	3586
September	1st to 10th	Night	9% 5% 2 4 4 3 3 3 5% 6% 55%	2000
	11th to 20th	Night	8% 4 2 4 3 2 4 4 4 8% 56%	2121
	21st to end	Night	9% 6% 2 7% 4 5% 5% 4 5% 7% 46%	2200
October	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 4 6% 53%	2223
	11th to 20th	Night	15% 7% 2 6% 4 3 3 3 5% 5% 47%	2415
	21st to end	Night	19% 9% 3 7% 4 3 4 2 4 6% 39%	2860
November	1st to 10th	Night	12% 8% 2 5% 4 3 3 4 5% 6% 47%	2666
	11th to 20th	Night	14% 9% 3 8% 4 5% 4 4 5% 6% 38%	2730
	21st to end	Night	17% 11% 4 7% 5% 4 4 3 4 6% 36%	2741
December	1st to 10th	Night	12% 9% 3 8% 4 4 4 3 5% 5% 43%	4720
	11th to 20th	Night	17% 14% 4 9% 5% 4 4 3 4 5% 30%	5339
	21st to end	Night	25% 18% 6% 11% 5% 4 4 3 4 5% 15%	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	62% 4 13% 14% 6% 1	4692
	11th to 20th	Day	61% 3 8% 14% 12% 3	4710
	21st to end	Day	59% 3 12% 18% 7%	5335
February	1st to 10th	Day	58% 4 12% 16% 8% 2	1974
	11th to 20th	Day	53% 4 10% 23% 9% 2	2058
	21st to end	Day	55% 4 10% 21% 7% 2	1730
March	1st to 10th	Day	50% 3 8% 23% 13% 3	4580
	11th to 20th	Day	40% 5% 11% 33% 11% 1	4755
	21st to end	Day	61% 7% 11% 14% 6% 1	6101
April	1st to 10th	Day	51% 5% 10% 20% 13%	2412
	11th to 20th	Day	48% 5% 10% 22% 12% 3	2374
	21st to end	Day	50% 5% 13% 19% 12% 2	2506
May	1st to 10th	Day	48% 6% 10% 21% 12% 2	5897
	11th to 20th	Day	42% 7% 10% 26% 13% 2	6061
	21st to end	Day	41% 6% 10% 24% 16% 2	6949
June	1st to 10th	Day	36% 7% 11% 25% 18% 3	2800
	11th to 20th	Day	36% 5% 11% 25% 20% 3	2800
	21st to end	Day	38% 8% 14% 21% 15% 3	2800
July	1st to 10th	Day	39% 6% 13% 24% 16% 2	6291
	11th to 20th	Day	37% 7% 15% 27% 14% 1	6285
	21st to end	Day	39% 8% 16% 22% 14% 2	6728
August	1st to 10th	Day	31% 7% 16% 30% 14% 1	4390
	11th to 20th	Day	33% 7% 11% 32% 16% 1	3893
	21st to end	Day	31% 6% 13% 29% 19% 3	4311
September	1st to 10th	Day	35% 6% 11% 27% 17% 4	2400
	11th to 20th	Day	31% 5% 11% 26% 23% 4	2257
	21st to end	Day	36% 6% 9% 25% 21% 2	2200
October	1st to 10th	Day	26% 4 8% 29% 32% 2	3539
	11th to 20th	Day	34% 4 11% 24% 25% 1	3213
	21st to end	Day	42% 4 9% 25% 18% 2	3180
November	1st to 10th	Day	37% 4 14% 26% 16% 2	1852
	11th to 20th	Day	50% 3 12% 22% 12% 1	1890
	21st to end	Day	53% 4 9% 23% 10% 1	1874
December	1st to 10th	Day	46% 3 13% 28% 9%	2690
	11th to 20th	Day	61% 4 12% 17% 5% 1	3085
	21st to end	Day	79% 3 6% 8% 4	4070
January	1st to 10th	Night	58% 4 11% 18% 8%	8233
	11th to 20th	Night	63% 3 10% 12% 10% 2	8280
	21st to end	Night	57% 3 9% 20% 9% 1	8864
February	1st to 10th	Night	56% 4 10% 18% 9% 2	2645
	11th to 20th	Night	50% 4 9% 23% 12% 1	2459
	21st to end	Night	52% 4 11% 19% 11% 3	2076
March	1st to 10th	Night	48% 4 9% 26% 10% 4	5493
	11th to 20th	Night	38% 5% 12% 27% 16% 3	5467
	21st to end	Night	56% 6% 9% 21% 8% 1	5072
April	1st to 10th	Night	44% 5% 12% 24% 13% 2	2010
	11th to 20th	Night	46% 4 9% 20% 17% 4	1975
	21st to end	Night	47% 6% 10% 21% 14% 3	1892
May	1st to 10th	Night	50% 6% 10% 17% 14% 4	4060
	11th to 20th	Night	38% 7% 13% 26% 13% 3	3843
	21st to end	Night	43% 7% 9% 20% 17% 4	3950
June	1st to 10th	Night	33% 5% 11% 27% 21% 4	1600
	11th to 20th	Night	33% 6% 11% 24% 22% 3	1600
	21st to end	Night	37% 7% 14% 23% 15% 4	1600
July	1st to 10th	Night	38% 6% 12% 26% 13% 4	3580
	11th to 20th	Night	32% 7% 14% 25% 21% 1	3572
	21st to end	Night	37% 7% 15% 23% 15% 4	3832
August	1st to 10th	Night	29% 8% 17% 29% 16% 1	2737
	11th to 20th	Night	29% 6% 15% 29% 19% 1	3221
	21st to end	Night	29% 5% 11% 33% 18% 3	3586
September	1st to 10th	Night	30% 6% 10% 26% 23% 5%	2000
	11th to 20th	Night	27% 5% 8% 29% 28% 4	2121
	21st to end	Night	36% 5% 10% 27% 19% 3	2200
October	1st to 10th	Night	21% 4 6% 32% 33% 3	3639
	11th to 20th	Night	32% 4 8% 24% 30% 2	3926
	21st to end	Night	39% 3 8% 24% 22% 4	4610
November	1st to 10th	Night	38% 4 10% 27% 18% 3	2666
	11th to 20th	Night	46% 4 11% 23% 14% 2	2730
	21st to end	Night	52% 3 10% 23% 11%	2741
December	1st to 10th	Night	43% 4 14% 27% 11% 1	4720
	11th to 20th	Night	57% 3 12% 16% 11% 1	5341
	21st to end	Night	72% 4 8% 10% 5%	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	22% 12% 33% 25% 6% 1	330
	11th to 20th	Day	24% 17% 40% 15% 4 1	330
	21st to end	Day	20% 18% 33% 23% 6%	363
February	1st to 10th	Day	21% 19% 37% 18% 4 1	330
	11th to 20th	Day	22% 17% 37% 20% 3	330
	21st to end	Day	24% 19% 42% 12% 3	273
March	1st to 10th	Day	22% 21% 37% 18% 2	330
	11th to 20th	Day	23% 18% 34% 20% 5%	369
	21st to end	Day	29% 15% 34% 16% 5% 1	594
April	1st to 10th	Day	24% 15% 30% 23% 8% 1	504
	11th to 20th	Day	21% 16% 31% 19% 10% 3	492
	21st to end	Day	21% 15% 34% 19% 8% 2	500
May	1st to 10th	Day	21% 12% 32% 21% 11% 3	503
	11th to 20th	Day	20% 9% 29% 24% 15% 4	606
	21st to end	Day	14% 8% 33% 22% 15% 8%	789
June	1st to 10th	Day	10% 9% 30% 27% 19% 5%	730
	11th to 20th	Day	11% 5% 28% 33% 18% 5%	730
	21st to end	Day	11% 8% 30% 30% 15% 6%	730
July	1st to 10th	Day	8% 5% 19% 30% 22% 15%	721
	11th to 20th	Day	9% 4 23% 32% 20% 12%	718
	21st to end	Day	9% 4 21% 35% 24% 8%	794
August	1st to 10th	Day	7% 4 15% 32% 29% 14%	633
	11th to 20th	Day	9% 5% 17% 30% 25% 14%	519
	21st to end	Day	8% 3 17% 35% 25% 13%	583
September	1st to 10th	Day	10% 6% 20% 29% 25% 10%	530
	11th to 20th	Day	10% 4 15% 25% 33% 13%	530
	21st to end	Day	12% 8% 26% 24% 21% 9%	530
October	1st to 10th	Day	13% 7% 27% 28% 20% 6%	505
	11th to 20th	Day	18% 19% 27% 26% 9% 1	330
	21st to end	Day	27% 14% 34% 20% 5% 1	363
November	1st to 10th	Day	15% 15% 36% 23% 8% 4	330
	11th to 20th	Day	23% 17% 34% 16% 8% 2	330
	21st to end	Day	25% 12% 35% 22% 5%	330
December	1st to 10th	Day	15% 18% 38% 20% 8% 1	330
	11th to 20th	Day	21% 16% 37% 21% 5%	330
	21st to end	Day	29% 18% 36% 14% 2	363
January	1st to 10th	Night	20% 15% 39% 22% 5%	1050
	11th to 20th	Night	22% 14% 38% 21% 4	1050
	21st to end	Night	20% 15% 37% 25% 4	1155
February	1st to 10th	Night	24% 12% 38% 23% 3	965
	11th to 20th	Night	19% 14% 36% 26% 4	823
	21st to end	Night	22% 16% 39% 20% 3	692
March	1st to 10th	Night	21% 15% 36% 23% 5%	840
	11th to 20th	Night	17% 13% 38% 25% 5% 1	840
	21st to end	Night	25% 10% 35% 23% 6%	693
April	1st to 10th	Night	19% 11% 40% 24% 5%	603
	11th to 20th	Night	21% 12% 33% 27% 7% 1	593
	21st to end	Night	20% 11% 33% 28% 8% 1	600
May	1st to 10th	Night	17% 7% 35% 32% 9% 1	600
	11th to 20th	Night	16% 7% 29% 35% 12% 1	591
	21st to end	Night	12% 5% 28% 36% 18% 1	650
June	1st to 10th	Night	12% 4 25% 46% 14%	600
	11th to 20th	Night	11% 3 27% 43% 15% 1	600
	21st to end	Night	10% 4 26% 41% 18% 1	600
July	1st to 10th	Night	9% 2 15% 44% 26% 4	585
	11th to 20th	Night	7% 3 19% 41% 28% 3	582
	21st to end	Night	5% 2 18% 45% 30% 1	657
August	1st to 10th	Night	6% 2 10% 50% 29% 4	597
	11th to 20th	Night	8% 2 13% 42% 31% 5%	594
	21st to end	Night	8% 2 15% 39% 33% 3	660
September	1st to 10th	Night	11% 3 15% 41% 27% 4	600
	11th to 20th	Night	8% 5% 15% 38% 30% 4	721
	21st to end	Night	11% 6% 24% 33% 23% 3	800
October	1st to 10th	Night	13% 5% 24% 37% 20% 1	800
	11th to 20th	Night	18% 12% 31% 28% 10%	800
	21st to end	Night	23% 13% 31% 25% 7%	880
November	1st to 10th	Night	15% 11% 40% 25% 9% 1	820
	11th to 20th	Night	19% 16% 34% 25% 6%	840
	21st to end	Night	23% 14% 34% 23% 6%	851
December	1st to 10th	Night	22% 12% 36% 25% 4	1050
	11th to 20th	Night	24% 12% 34% 25% 5%	1050
	21st to end	Night	29% 13% 34% 22% 2	1155

Page generated at 2026-03-30 01:18:41. Version ac07

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