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 2024-01-16.

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	14% 8% 4 6% 6% 3 5% 5% 6% 7% 36%	74841
	Night	14% 9% 4 6% 5% 3 4 4 5% 7% 39%	74991
Transparency	Day	46% 6% 12% 22% 13% 2	74542
	Night	46% 5% 11% 22% 14% 2	74811
Seeing	Day	16% 10% 28% 25% 15% 6%	14597
	Night	17% 9% 30% 30% 12% 1	24572

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	16% 13% 6% 8% 7% 4 4 6% 4 5% 28%	1517
	11th to 20th	Day	14% 18% 6% 11% 8% 3 5% 4 4 6% 21%	1485
	21st to end	Day	14% 16% 8% 11% 7% 3 4 4 5% 6% 23%	1637
February	1st to 10th	Day	19% 12% 5% 8% 6% 4 6% 5% 5% 6% 25%	1692
	11th to 20th	Day	15% 10% 5% 8% 7% 5% 5% 3 5% 7% 30%	1748
	21st to end	Day	17% 12% 6% 9% 7% 3 4 4 5% 6% 28%	1480
March	1st to 10th	Day	18% 14% 5% 7% 6% 3 4 4 5% 5% 29%	1800
	11th to 20th	Day	14% 10% 3 8% 6% 3 5% 5% 5% 9% 31%	1848
	21st to end	Day	22% 12% 5% 6% 6% 4 5% 4 4 7% 25%	2376
April	1st to 10th	Day	17% 10% 4 7% 5% 3 5% 4 5% 7% 33%	2160
	11th to 20th	Day	17% 9% 4 6% 5% 3 4 4 5% 7% 37%	2134
	21st to end	Day	20% 8% 3 7% 5% 3 5% 5% 5% 7% 33%	2254
May	1st to 10th	Day	18% 9% 3 6% 7% 4 5% 5% 6% 7% 30%	2340
	11th to 20th	Day	18% 8% 3 6% 5% 3 5% 5% 6% 7% 34%	2371
	21st to end	Day	12% 6% 2 6% 6% 4 5% 5% 7% 8% 38%	2725
June	1st to 10th	Day	10% 5% 3 5% 5% 3 6% 5% 6% 8% 44%	2520
	11th to 20th	Day	9% 5% 2 5% 6% 4 5% 5% 7% 8% 45%	2520
	21st to end	Day	10% 6% 2 6% 7% 3 6% 6% 7% 9% 38%	2520
July	1st to 10th	Day	8% 4 3 5% 5% 4 6% 5% 7% 10% 44%	2520
	11th to 20th	Day	10% 4 2 4 4 4 5% 5% 7% 10% 45%	2520
	21st to end	Day	6% 5% 2 4 6% 3 6% 6% 8% 9% 43%	2743
August	1st to 10th	Day	7% 5% 2 5% 6% 3 6% 6% 7% 9% 46%	2424
	11th to 20th	Day	9% 5% 2 5% 5% 3 6% 6% 7% 8% 44%	2133
	21st to end	Day	8% 5% 2 5% 4 3 5% 6% 6% 9% 48%	2376
September	1st to 10th	Day	11% 6% 2 5% 5% 4 5% 4 7% 7% 44%	2160
	11th to 20th	Day	12% 5% 2 4 3 3 4 5% 5% 7% 50%	2032
	21st to end	Day	9% 5% 2 5% 5% 4 4 5% 6% 8% 45%	1980
October	1st to 10th	Day	12% 6% 2 6% 4 3 4 5% 5% 6% 45%	1958
	11th to 20th	Day	19% 7% 3 5% 6% 3 5% 4 6% 7% 36%	1787
	21st to end	Day	20% 9% 3 7% 5% 3 5% 3 6% 6% 33%	1782
November	1st to 10th	Day	12% 7% 3 5% 4 3 3 4 5% 8% 47%	1672
	11th to 20th	Day	17% 8% 4 7% 6% 3 4 4 5% 6% 35%	1710
	21st to end	Day	18% 10% 4 8% 6% 2 3 5% 4 5% 35%	1696
December	1st to 10th	Day	16% 13% 5% 10% 6% 3 6% 3 4 5% 29%	1520
	11th to 20th	Day	14% 13% 4 9% 7% 4 6% 4 5% 6% 28%	1918
	21st to end	Day	23% 19% 8% 13% 7% 3 3 3 3 4 14%	2783
January	1st to 10th	Night	15% 13% 5% 8% 6% 3 4 4 4 5% 32%	2648
	11th to 20th	Night	16% 14% 6% 10% 7% 2 4 4 4 6% 27%	2603
	21st to end	Night	16% 12% 7% 10% 7% 3 4 4 5% 7% 26%	2718
February	1st to 10th	Night	17% 14% 6% 7% 7% 3 4 4 4 6% 28%	2268
	11th to 20th	Night	15% 10% 4 7% 6% 3 5% 4 4 7% 34%	2098
	21st to end	Night	20% 11% 6% 7% 6% 3 3 4 4 5% 33%	1776
March	1st to 10th	Night	16% 12% 4 7% 7% 3 4 4 5% 5% 34%	2160
	11th to 20th	Night	14% 8% 3 6% 5% 2 5% 4 6% 8% 39%	2125
	21st to end	Night	20% 10% 4 6% 5% 3 5% 4 5% 6% 32%	1980
April	1st to 10th	Night	14% 7% 3 7% 5% 3 3 5% 5% 7% 40%	1800
	11th to 20th	Night	18% 9% 3 5% 5% 3 4 4 5% 6% 39%	1775
	21st to end	Night	19% 7% 4 5% 5% 4 5% 4 6% 6% 36%	1704
May	1st to 10th	Night	16% 8% 3 5% 5% 3 5% 5% 5% 9% 36%	1620
	11th to 20th	Night	15% 6% 2 5% 5% 3 5% 4 6% 8% 41%	1519
	21st to end	Night	10% 6% 2 6% 6% 3 5% 6% 7% 9% 40%	1551
June	1st to 10th	Night	9% 5% 2 5% 5% 3 4 3 6% 7% 51%	1440
	11th to 20th	Night	7% 6% 3 5% 4 3 4 5% 6% 9% 49%	1440
	21st to end	Night	10% 5% 3 5% 6% 3 5% 6% 7% 8% 43%	1440
July	1st to 10th	Night	7% 4 2 5% 4 3 3 7% 5% 10% 49%	1440
	11th to 20th	Night	6% 3 2 4 4 2 5% 6% 4 7% 56%	1440
	21st to end	Night	4 4 2 4 5% 3 5% 5% 6% 9% 54%	1575
August	1st to 10th	Night	5% 4 2 5% 5% 3 6% 5% 6% 9% 51%	1511
	11th to 20th	Night	7% 4 2 4 4 3 4 5% 6% 9% 52%	1767
	21st to end	Night	8% 3 2 5% 4 3 4 3 5% 8% 54%	1980
September	1st to 10th	Night	10% 6% 2 4 5% 2 4 4 5% 7% 53%	1800
	11th to 20th	Night	9% 4 2 4 4 2 3 5% 4 8% 55%	1908
	21st to end	Night	8% 5% 2 6% 5% 3 5% 5% 6% 8% 46%	1980
October	1st to 10th	Night	11% 6% 3 4 5% 3 4 4 4 6% 51%	2000
	11th to 20th	Night	16% 7% 3 5% 4 2 3 3 5% 5% 46%	2173
	21st to end	Night	20% 9% 3 6% 5% 2 3 3 4 6% 38%	2574
November	1st to 10th	Night	12% 7% 3 5% 4 2 4 5% 5% 6% 48%	2406
	11th to 20th	Night	14% 8% 3 7% 5% 3 4 4 5% 6% 38%	2470
	21st to end	Night	18% 9% 4 5% 6% 3 4 3 4 6% 37%	2479
December	1st to 10th	Night	16% 11% 4 8% 6% 3 5% 4 5% 6% 33%	2660
	11th to 20th	Night	16% 14% 4 8% 6% 3 4 3 4 5% 32%	3279
	21st to end	Night	24% 17% 7% 10% 6% 4 4 4 3 5% 15%	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	58% 5% 10% 16% 10% 1	1518
	11th to 20th	Day	66% 3 11% 13% 6% 1	1485
	21st to end	Day	63% 3 11% 14% 8% 1	1637
February	1st to 10th	Day	61% 4 11% 15% 7% 2	1692
	11th to 20th	Day	55% 4 10% 21% 9% 2	1758
	21st to end	Day	58% 4 9% 20% 6% 2	1480
March	1st to 10th	Day	56% 5% 11% 21% 7% 1	1800
	11th to 20th	Day	49% 5% 12% 21% 11% 1	1848
	21st to end	Day	60% 6% 11% 18% 5% 1	2376
April	1st to 10th	Day	50% 6% 11% 20% 13%	2160
	11th to 20th	Day	48% 5% 10% 21% 12% 4	2134
	21st to end	Day	51% 6% 12% 17% 12% 2	2254
May	1st to 10th	Day	51% 6% 11% 23% 8%	2340
	11th to 20th	Day	48% 7% 11% 20% 12% 1	2375
	21st to end	Day	44% 6% 11% 21% 17% 2	2725
June	1st to 10th	Day	35% 7% 11% 25% 19% 2	2520
	11th to 20th	Day	36% 6% 11% 24% 21% 4	2520
	21st to end	Day	40% 8% 15% 21% 14% 2	2520
July	1st to 10th	Day	34% 8% 14% 26% 16% 3	2451
	11th to 20th	Day	34% 7% 15% 27% 15% 2	2445
	21st to end	Day	34% 8% 16% 24% 16% 3	2591
August	1st to 10th	Day	33% 7% 16% 29% 13% 2	2409
	11th to 20th	Day	34% 7% 14% 26% 17% 1	2133
	21st to end	Day	31% 7% 13% 26% 19% 4	2376
September	1st to 10th	Day	37% 6% 12% 25% 15% 4	2160
	11th to 20th	Day	33% 5% 12% 26% 20% 4	2032
	21st to end	Day	35% 6% 10% 26% 21% 2	1980
October	1st to 10th	Day	38% 6% 12% 25% 16% 3	1958
	11th to 20th	Day	46% 5% 11% 22% 15% 1	1787
	21st to end	Day	51% 5% 10% 22% 10% 2	1782
November	1st to 10th	Day	36% 5% 15% 25% 17% 3	1672
	11th to 20th	Day	49% 3 12% 23% 12% 1	1710
	21st to end	Day	52% 5% 10% 22% 11% 1	1696
December	1st to 10th	Day	58% 5% 11% 18% 8% 1	1520
	11th to 20th	Day	57% 4 11% 21% 7% 1	1915
	21st to end	Day	76% 4 7% 9% 4	2783
January	1st to 10th	Night	55% 4 9% 19% 12% 1	2648
	11th to 20th	Night	60% 4 14% 14% 8%	2603
	21st to end	Night	60% 3 11% 17% 9% 1	2718
February	1st to 10th	Night	59% 4 10% 17% 8% 2	2268
	11th to 20th	Night	51% 4 10% 22% 12% 2	2099
	21st to end	Night	54% 4 9% 18% 11% 3	1776
March	1st to 10th	Night	52% 5% 10% 22% 10% 1	2160
	11th to 20th	Night	42% 4 13% 21% 16% 3	2125
	21st to end	Night	53% 6% 11% 20% 10% 1	1980
April	1st to 10th	Night	43% 5% 13% 24% 13% 2	1800
	11th to 20th	Night	47% 4 9% 20% 17% 4	1775
	21st to end	Night	48% 6% 11% 19% 14% 3	1704
May	1st to 10th	Night	46% 6% 12% 24% 12%	1620
	11th to 20th	Night	42% 6% 13% 21% 16% 1	1520
	21st to end	Night	39% 7% 13% 19% 17% 4	1552
June	1st to 10th	Night	33% 5% 11% 26% 21% 4	1440
	11th to 20th	Night	32% 6% 11% 25% 23% 3	1440
	21st to end	Night	38% 7% 14% 22% 14% 4	1440
July	1st to 10th	Night	30% 7% 15% 26% 16% 5%	1400
	11th to 20th	Night	27% 6% 13% 28% 24% 3	1392
	21st to end	Night	26% 6% 16% 28% 19% 5%	1487
August	1st to 10th	Night	28% 7% 16% 30% 17% 2	1502
	11th to 20th	Night	27% 7% 16% 26% 21% 3	1767
	21st to end	Night	29% 5% 12% 28% 21% 5%	1980
September	1st to 10th	Night	32% 6% 11% 25% 21% 5%	1800
	11th to 20th	Night	28% 5% 9% 29% 26% 4	1908
	21st to end	Night	35% 6% 10% 27% 19% 3	1980
October	1st to 10th	Night	35% 6% 9% 26% 23% 3	2000
	11th to 20th	Night	41% 4 9% 25% 20% 2	2173
	21st to end	Night	49% 4 9% 22% 14% 2	2574
November	1st to 10th	Night	37% 4 10% 27% 18% 4	2406
	11th to 20th	Night	46% 4 11% 22% 14% 2	2470
	21st to end	Night	50% 3 11% 24% 12%	2479
December	1st to 10th	Night	53% 5% 12% 20% 9% 1	2660
	11th to 20th	Night	56% 3 10% 18% 12% 1	3281
	21st to end	Night	72% 5% 8% 10% 5%	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% 11% 33% 25% 6% 1	270
	11th to 20th	Day	27% 14% 36% 18% 5% 1	257
	21st to end	Day	21% 16% 33% 23% 7%	264
February	1st to 10th	Day	26% 20% 37% 15% 2	240
	11th to 20th	Day	25% 14% 36% 20% 4	240
	21st to end	Day	27% 16% 41% 13% 3	198
March	1st to 10th	Day	21% 22% 35% 21% 2	240
	11th to 20th	Day	23% 14% 34% 22% 7%	273
	21st to end	Day	32% 12% 30% 18% 7% 2	462
April	1st to 10th	Day	22% 14% 30% 25% 9% 1	420
	11th to 20th	Day	20% 14% 32% 19% 11% 3	412
	21st to end	Day	21% 16% 34% 19% 7% 3	420
May	1st to 10th	Day	21% 12% 32% 21% 10% 3	423
	11th to 20th	Day	21% 8% 30% 22% 15% 4	517
	21st to end	Day	14% 8% 31% 23% 16% 9%	679
June	1st to 10th	Day	9% 7% 30% 29% 19% 6%	630
	11th to 20th	Day	11% 5% 29% 32% 19% 5%	630
	21st to end	Day	12% 9% 29% 30% 15% 5%	630
July	1st to 10th	Day	8% 4 19% 29% 23% 17%	621
	11th to 20th	Day	10% 4 22% 32% 21% 11%	618
	21st to end	Day	8% 3 21% 36% 23% 8%	684
August	1st to 10th	Day	7% 3 14% 32% 29% 15%	543
	11th to 20th	Day	10% 4 16% 28% 26% 15%	439
	21st to end	Day	8% 2 18% 35% 24% 13%	495
September	1st to 10th	Day	11% 6% 20% 29% 25% 9%	450
	11th to 20th	Day	12% 4 16% 26% 30% 12%	450
	21st to end	Day	11% 8% 27% 26% 21% 8%	450
October	1st to 10th	Day	14% 6% 29% 28% 18% 6%	428
	11th to 20th	Day	19% 21% 30% 23% 7%	270
	21st to end	Day	30% 12% 35% 19% 4	297
November	1st to 10th	Day	14% 13% 36% 24% 9% 5%	270
	11th to 20th	Day	22% 15% 36% 17% 9% 2	270
	21st to end	Day	25% 10% 37% 22% 6%	270
December	1st to 10th	Day	17% 18% 37% 19% 9% 1	270
	11th to 20th	Day	19% 15% 37% 23% 6%	270
	21st to end	Day	25% 21% 37% 14% 3	297
January	1st to 10th	Night	19% 15% 38% 22% 5%	950
	11th to 20th	Night	23% 13% 36% 22% 5%	930
	21st to end	Night	21% 13% 36% 26% 4	990
February	1st to 10th	Night	26% 12% 38% 22% 3	828
	11th to 20th	Night	20% 14% 35% 26% 5%	703
	21st to end	Night	23% 16% 38% 20% 3	592
March	1st to 10th	Night	22% 14% 35% 23% 5%	720
	11th to 20th	Night	17% 12% 38% 27% 6% 1	720
	21st to end	Night	26% 9% 34% 24% 7% 1	594
April	1st to 10th	Night	17% 11% 41% 25% 6%	540
	11th to 20th	Night	21% 11% 32% 28% 7% 1	533
	21st to end	Night	21% 10% 32% 29% 8% 1	540
May	1st to 10th	Night	18% 7% 35% 31% 9% 1	540
	11th to 20th	Night	18% 6% 29% 35% 12% 1	531
	21st to end	Night	12% 4 27% 36% 20% 1	584
June	1st to 10th	Night	11% 4 24% 47% 14%	540
	11th to 20th	Night	11% 3 26% 43% 15% 1	540
	21st to end	Night	10% 4 26% 41% 18% 1	540
July	1st to 10th	Night	9% 2 14% 44% 28% 4	525
	11th to 20th	Night	7% 2 18% 41% 29% 3	522
	21st to end	Night	5% 2 18% 45% 29% 1	591
August	1st to 10th	Night	6% 2 9% 49% 29% 4	537
	11th to 20th	Night	7% 2 12% 41% 32% 5%	534
	21st to end	Night	8% 2 16% 36% 34% 3	594
September	1st to 10th	Night	11% 3 15% 40% 26% 4	540
	11th to 20th	Night	9% 5% 16% 36% 30% 4	648
	21st to end	Night	11% 6% 24% 33% 23% 3	720
October	1st to 10th	Night	13% 6% 24% 36% 20% 1	720
	11th to 20th	Night	19% 13% 32% 27% 9%	720
	21st to end	Night	24% 13% 31% 24% 7%	792
November	1st to 10th	Night	14% 10% 40% 25% 10% 1	740
	11th to 20th	Night	18% 16% 34% 26% 6%	760
	21st to end	Night	22% 12% 35% 24% 7%	769
December	1st to 10th	Night	22% 13% 36% 24% 5%	950
	11th to 20th	Night	23% 11% 34% 26% 5%	950
	21st to end	Night	28% 13% 34% 22% 2	1045

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