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 2024-12-09.

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% 4 4 4 6% 7% 36%	101554
	Night	14% 10% 4 7% 5% 4 4 3 5% 6% 38%	98305
Transparency	Day	44% 6% 12% 23% 14% 2	105134
	Night	45% 5% 11% 22% 15% 2	102866
Seeing	Day	16% 10% 28% 25% 15% 6%	15865
	Night	17% 9% 30% 30% 12% 1	25736

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% 14% 8% 11% 5% 5% 4 4 5% 5% 24%	2527
	11th to 20th	Day	13% 16% 6% 15% 6% 5% 4 6% 7% 4 17%	2530
	21st to end	Day	20% 24% 7% 12% 5% 3 3 2 4 5% 17%	2878
February	1st to 10th	Day	18% 12% 4 9% 5% 5% 5% 4 5% 6% 26%	1786
	11th to 20th	Day	14% 11% 4 9% 6% 6% 4 3 5% 7% 32%	1848
	21st to end	Day	16% 13% 5% 9% 6% 4 4 3 5% 7% 28%	1570
March	1st to 10th	Day	18% 15% 4 7% 4 3 3 3 5% 5% 33%	3120
	11th to 20th	Day	10% 7% 3 8% 5% 5% 5% 4 7% 10% 38%	3250
	21st to end	Day	18% 12% 5% 11% 6% 6% 5% 4 5% 7% 22%	4165
April	1st to 10th	Day	16% 12% 4 7% 5% 4 4 3 5% 7% 33%	2280
	11th to 20th	Day	16% 9% 3 7% 4 4 4 3 5% 6% 37%	2254
	21st to end	Day	20% 8% 3 7% 5% 4 4 4 5% 7% 33%	2380
May	1st to 10th	Day	18% 8% 2 7% 5% 4 5% 4 7% 8% 31%	4082
	11th to 20th	Day	15% 7% 2 6% 4 5% 5% 4 6% 8% 40%	4221
	21st to end	Day	9% 6% 2 6% 4 4 4 4 7% 9% 45%	4824
June	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 7% 8% 44%	2660
	11th to 20th	Day	8% 5% 2 5% 5% 5% 5% 4 7% 8% 45%	2660
	21st to end	Day	10% 7% 2 8% 5% 5% 6% 5% 7% 8% 38%	2660
July	1st to 10th	Day	11% 6% 3 8% 5% 5% 6% 5% 8% 10% 33%	4440
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 7% 11% 46%	4440
	21st to end	Day	8% 6% 2 6% 5% 6% 6% 5% 7% 8% 41%	4813
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 47%	4270
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 43%	3737
	21st to end	Day	6% 5% 2 6% 4 5% 4 4 7% 9% 48%	4180
September	1st to 10th	Day	10% 6% 2 5% 4 4 5% 3 7% 7% 46%	2280
	11th to 20th	Day	11% 5% 2 4 3 3 4 4 5% 7% 51%	2144
	21st to end	Day	10% 6% 2 6% 5% 5% 4 4 6% 8% 44%	2090
October	1st to 10th	Day	11% 7% 2 6% 4 4 4 4 5% 6% 47%	2066
	11th to 20th	Day	18% 7% 2 6% 5% 4 4 3 6% 6% 38%	1886
	21st to end	Day	19% 9% 3 8% 4 4 5% 3 6% 6% 34%	1881
November	1st to 10th	Day	13% 8% 3 5% 4 3 3 4 5% 7% 46%	1762
	11th to 20th	Day	17% 10% 4 8% 5% 4 4 3 5% 6% 35%	1800
	21st to end	Day	17% 11% 3 10% 5% 3 4 3 4 5% 34%	1785
December	1st to 10th	Day	15% 13% 4 11% 5% 5% 5% 2 4 5% 31%	1584
	11th to 20th	Day	14% 14% 4 10% 6% 6% 5% 3 5% 6% 28%	1918
	21st to end	Day	23% 19% 8% 14% 5% 4 3 3 3 4 13%	2783
January	1st to 10th	Night	16% 15% 8% 10% 5% 5% 4 3 4 4 27%	4428
	11th to 20th	Night	14% 18% 8% 13% 6% 4 5% 4 6% 4 18%	4440
	21st to end	Night	24% 17% 6% 10% 5% 4 3 3 4 5% 20%	4788
February	1st to 10th	Night	16% 15% 5% 8% 6% 4 4 3 4 6% 30%	2394
	11th to 20th	Night	14% 10% 3 8% 6% 5% 5% 3 5% 7% 35%	2218
	21st to end	Night	19% 11% 4 8% 5% 4 3 3 5% 5% 33%	1884
March	1st to 10th	Night	18% 12% 3 6% 4 3 3 2 4 6% 38%	3733
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 47%	3737
	21st to end	Night	20% 9% 6% 9% 6% 5% 5% 4 5% 5% 27%	3466
April	1st to 10th	Night	14% 9% 3 8% 4 4 4 3 5% 7% 40%	1900
	11th to 20th	Night	18% 9% 3 6% 4 4 4 3 4 6% 40%	1875
	21st to end	Night	18% 8% 3 6% 4 5% 4 4 6% 6% 36%	1798
May	1st to 10th	Night	16% 9% 3 7% 4 4 4 4 6% 8% 35%	2813
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 7% 9% 39%	2674
	21st to end	Night	10% 7% 2 7% 5% 4 4 4 5% 8% 44%	2743
June	1st to 10th	Night	9% 6% 2 5% 5% 4 3 3 6% 7% 50%	1520
	11th to 20th	Night	7% 6% 2 5% 4 5% 4 3 7% 8% 48%	1520
	21st to end	Night	10% 6% 2 6% 5% 4 5% 4 6% 8% 43%	1520
July	1st to 10th	Night	11% 5% 2 7% 5% 5% 5% 5% 6% 9% 40%	2530
	11th to 20th	Night	6% 4 2 6% 4 3 5% 4 5% 7% 55%	2530
	21st to end	Night	4 5% 2 6% 6% 6% 5% 5% 7% 8% 46%	2750
August	1st to 10th	Night	5% 4 2 6% 4 5% 5% 4 7% 9% 48%	2661
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 7% 8% 52%	3091
	21st to end	Night	8% 4 2 5% 3 4 4 3 6% 9% 52%	3476
September	1st to 10th	Night	9% 5% 2 4 4 3 3 3 5% 7% 54%	1900
	11th to 20th	Night	8% 4 2 5% 3 2 4 4 5% 8% 55%	2015
	21st to end	Night	9% 6% 2 7% 4 5% 6% 4 6% 8% 45%	2090
October	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 4 6% 53%	2112
	11th to 20th	Night	15% 7% 3 6% 4 3 3 3 5% 5% 47%	2294
	21st to end	Night	20% 9% 3 6% 4 3 4 2 4 6% 39%	2717
November	1st to 10th	Night	12% 8% 2 5% 3 3 3 4 5% 6% 47%	2536
	11th to 20th	Night	14% 9% 3 8% 4 5% 4 4 5% 6% 38%	2600
	21st to end	Night	17% 11% 3 7% 6% 4 4 2 4 6% 36%	2610
December	1st to 10th	Night	15% 11% 3 9% 5% 4 4 3 5% 5% 34%	2779
	11th to 20th	Night	16% 14% 4 9% 5% 4 4 3 4 5% 32%	3279
	21st to end	Night	24% 18% 7% 11% 6% 5% 4 3 4 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	60% 7% 11% 16% 6%	2528
	11th to 20th	Day	67% 3 9% 12% 9% 1	2530
	21st to end	Day	72% 3 8% 13% 5%	2878
February	1st to 10th	Day	58% 5% 11% 16% 8% 2	1786
	11th to 20th	Day	53% 4 10% 23% 9% 2	1858
	21st to end	Day	56% 5% 10% 21% 6% 3	1570
March	1st to 10th	Day	54% 5% 10% 19% 11% 1	3120
	11th to 20th	Day	40% 6% 11% 29% 13% 1	3250
	21st to end	Day	60% 9% 13% 15% 3	4165
April	1st to 10th	Day	51% 6% 11% 20% 13%	2280
	11th to 20th	Day	47% 5% 10% 22% 12% 4	2254
	21st to end	Day	50% 5% 12% 18% 12% 2	2380
May	1st to 10th	Day	47% 7% 11% 25% 10%	4120
	11th to 20th	Day	41% 7% 11% 28% 11% 1	4225
	21st to end	Day	35% 6% 11% 23% 22% 4	4837
June	1st to 10th	Day	35% 7% 11% 25% 19% 3	2660
	11th to 20th	Day	35% 5% 11% 24% 21% 4	2660
	21st to end	Day	39% 8% 14% 21% 15% 3	2660
July	1st to 10th	Day	43% 7% 13% 21% 14% 2	4371
	11th to 20th	Day	31% 6% 17% 29% 17% 1	4365
	21st to end	Day	38% 8% 16% 17% 19% 2	4662
August	1st to 10th	Day	32% 7% 16% 30% 14% 1	4255
	11th to 20th	Day	33% 6% 11% 32% 16% 1	3773
	21st to end	Day	31% 6% 13% 28% 19% 3	4179
September	1st to 10th	Day	36% 6% 12% 26% 17% 5%	2280
	11th to 20th	Day	32% 5% 11% 26% 22% 4	2144
	21st to end	Day	37% 6% 9% 26% 21% 2	2090
October	1st to 10th	Day	26% 4 8% 29% 32% 2	3430
	11th to 20th	Day	34% 4 11% 24% 26% 1	3114
	21st to end	Day	43% 4 9% 25% 18% 2	3081
November	1st to 10th	Day	37% 5% 14% 26% 16% 2	1762
	11th to 20th	Day	50% 3 12% 22% 12% 1	1800
	21st to end	Day	53% 5% 9% 22% 10% 1	1785
December	1st to 10th	Day	57% 4 11% 19% 8% 1	1584
	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	60% 5% 10% 16% 8% 1	4428
	11th to 20th	Night	69% 3 10% 12% 6%	4440
	21st to end	Night	68% 3 8% 14% 6% 1	4788
February	1st to 10th	Night	56% 4 10% 18% 10% 2	2394
	11th to 20th	Night	50% 4 10% 23% 13% 1	2219
	21st to end	Night	53% 5% 10% 19% 11% 3	1884
March	1st to 10th	Night	48% 5% 11% 25% 9% 3	3733
	11th to 20th	Night	37% 5% 10% 28% 18% 2	3737
	21st to end	Night	57% 7% 10% 19% 6%	3466
April	1st to 10th	Night	43% 5% 12% 24% 13% 2	1900
	11th to 20th	Night	46% 4 8% 20% 17% 4	1875
	21st to end	Night	47% 6% 11% 19% 14% 3	1798
May	1st to 10th	Night	46% 6% 11% 22% 12% 2	2840
	11th to 20th	Night	40% 7% 15% 26% 11%	2675
	21st to end	Night	37% 8% 10% 19% 21% 6%	2751
June	1st to 10th	Night	33% 5% 11% 27% 21% 4	1520
	11th to 20th	Night	32% 6% 11% 24% 23% 3	1520
	21st to end	Night	38% 7% 14% 22% 15% 4	1520
July	1st to 10th	Night	39% 6% 14% 23% 12% 5%	2490
	11th to 20th	Night	27% 6% 12% 28% 25% 2	2482
	21st to end	Night	33% 6% 16% 20% 19% 5%	2662
August	1st to 10th	Night	30% 8% 17% 29% 16% 1	2652
	11th to 20th	Night	28% 6% 15% 30% 20% 2	3121
	21st to end	Night	29% 5% 11% 32% 18% 3	3476
September	1st to 10th	Night	30% 6% 10% 25% 23% 5%	1900
	11th to 20th	Night	27% 5% 8% 28% 27% 4	2015
	21st to end	Night	36% 6% 10% 27% 18% 3	2090
October	1st to 10th	Night	21% 4 7% 33% 33% 3	3528
	11th to 20th	Night	32% 4 8% 25% 30% 2	3805
	21st to end	Night	39% 3 8% 24% 21% 4	4467
November	1st to 10th	Night	37% 4 9% 27% 18% 3	2536
	11th to 20th	Night	47% 4 11% 22% 14% 2	2600
	21st to end	Night	51% 3 10% 23% 11%	2610
December	1st to 10th	Night	51% 5% 12% 21% 10% 1	2779
	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	26% 16% 36% 16% 5% 1	270
	21st to end	Day	23% 15% 31% 24% 7%	297
February	1st to 10th	Day	24% 19% 37% 16% 4	270
	11th to 20th	Day	23% 14% 36% 22% 4	270
	21st to end	Day	25% 17% 41% 13% 3	225
March	1st to 10th	Day	21% 22% 35% 19% 2	270
	11th to 20th	Day	22% 16% 34% 21% 7%	305
	21st to end	Day	30% 13% 32% 17% 6% 1	506
April	1st to 10th	Day	22% 14% 31% 24% 8% 1	460
	11th to 20th	Day	20% 15% 32% 19% 10% 3	452
	21st to end	Day	21% 16% 34% 19% 7% 2	460
May	1st to 10th	Day	21% 12% 32% 22% 10% 3	463
	11th to 20th	Day	20% 8% 29% 24% 15% 4	562
	21st to end	Day	13% 8% 32% 22% 16% 8%	734
June	1st to 10th	Day	9% 9% 30% 28% 19% 5%	680
	11th to 20th	Day	10% 5% 29% 33% 18% 5%	680
	21st to end	Day	11% 9% 31% 29% 15% 5%	680
July	1st to 10th	Day	8% 5% 20% 29% 22% 16%	671
	11th to 20th	Day	9% 4 23% 32% 21% 12%	668
	21st to end	Day	8% 3 21% 36% 24% 8%	739
August	1st to 10th	Day	7% 4 15% 32% 28% 14%	588
	11th to 20th	Day	10% 5% 17% 30% 25% 14%	479
	21st to end	Day	8% 3 16% 35% 25% 13%	539
September	1st to 10th	Day	10% 6% 19% 30% 25% 9%	490
	11th to 20th	Day	11% 4 15% 26% 33% 12%	490
	21st to end	Day	13% 8% 26% 25% 20% 8%	490
October	1st to 10th	Day	13% 7% 28% 27% 19% 6%	466
	11th to 20th	Day	19% 20% 28% 23% 10% 1	300
	21st to end	Day	29% 13% 33% 19% 5% 1	330
November	1st to 10th	Day	15% 14% 35% 24% 8% 4	300
	11th to 20th	Day	23% 15% 36% 16% 8% 2	300
	21st to end	Day	24% 12% 35% 23% 6%	300
December	1st to 10th	Day	16% 18% 37% 19% 8% 1	294
	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% 14% 36% 22% 5%	950
	21st to end	Night	21% 13% 35% 26% 4	1045
February	1st to 10th	Night	24% 11% 38% 23% 3	874
	11th to 20th	Night	20% 14% 36% 26% 5%	743
	21st to end	Night	23% 16% 38% 20% 3	628
March	1st to 10th	Night	21% 14% 36% 23% 5%	760
	11th to 20th	Night	16% 13% 39% 26% 6% 1	760
	21st to end	Night	26% 9% 34% 23% 6%	627
April	1st to 10th	Night	18% 11% 41% 24% 6%	570
	11th to 20th	Night	21% 12% 33% 27% 7% 1	563
	21st to end	Night	21% 11% 33% 28% 8% 1	570
May	1st to 10th	Night	17% 7% 35% 31% 9% 1	570
	11th to 20th	Night	17% 6% 29% 35% 12% 1	561
	21st to end	Night	12% 5% 28% 36% 19% 1	617
June	1st to 10th	Night	11% 4 25% 46% 14%	570
	11th to 20th	Night	10% 3 27% 42% 15% 1	570
	21st to end	Night	11% 4 26% 41% 17% 1	570
July	1st to 10th	Night	9% 2 14% 44% 26% 4	555
	11th to 20th	Night	7% 2 18% 41% 28% 3	552
	21st to end	Night	4 2 17% 46% 29% 1	624
August	1st to 10th	Night	6% 2 10% 49% 29% 4	567
	11th to 20th	Night	8% 2 12% 42% 31% 5%	564
	21st to end	Night	8% 2 16% 37% 34% 3	627
September	1st to 10th	Night	11% 3 15% 41% 27% 4	570
	11th to 20th	Night	8% 5% 15% 38% 30% 4	685
	21st to end	Night	11% 6% 25% 33% 22% 3	760
October	1st to 10th	Night	13% 6% 24% 37% 20% 1	760
	11th to 20th	Night	18% 13% 31% 27% 10%	760
	21st to end	Night	23% 13% 31% 25% 7%	836
November	1st to 10th	Night	15% 11% 39% 25% 9% 1	780
	11th to 20th	Night	19% 16% 34% 25% 6%	800
	21st to end	Night	22% 13% 34% 24% 7%	810
December	1st to 10th	Night	22% 12% 37% 24% 5%	993
	11th to 20th	Night	23% 11% 34% 26% 5%	950
	21st to end	Night	28% 13% 34% 22% 2	1045

Page generated at 2024-12-06 02:54:53. Version ac07

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