Jefferson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jefferson Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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% 6% 2 6% 4 4 5% 4 6% 7% 43%	109222
	Night	14% 6% 2 5% 4 4 4 3 5% 7% 45%	109791
Transparency	Day	38% 7% 11% 19% 19% 6%	113029
	Night	36% 7% 10% 19% 21% 7%	114123
Seeing	Day	18% 10% 23% 23% 18% 8%	15358
	Night	17% 8% 22% 29% 20% 3	25496

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% 8% 2 5% 3 3 3 3 4 4 49%	4056
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 4 4 57%	4060
	21st to end	Day	23% 8% 3 5% 3 2 3 2 4 5% 42%	4512
February	1st to 10th	Day	16% 8% 2 6% 4 4 4 4 5% 6% 42%	2000
	11th to 20th	Day	23% 7% 1 5% 3 2 3 4 3 5% 43%	1948
	21st to end	Day	20% 6% 2 7% 4 3 4 2 4 6% 42%	1650
March	1st to 10th	Day	25% 7% 3 6% 3 3 3 2 4 6% 38%	3382
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 4 5% 51%	3491
	21st to end	Day	19% 6% 2 6% 5% 5% 4 3 4 6% 41%	3815
April	1st to 10th	Day	13% 5% 2 4 5% 4 5% 4 6% 6% 46%	2090
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 5% 7% 47%	2253
	21st to end	Day	10% 4 2 4 3 4 4 4 5% 7% 51%	2470
May	1st to 10th	Day	12% 7% 3 7% 4 4 4 4 6% 9% 42%	4082
	11th to 20th	Day	17% 7% 3 7% 5% 4 4 4 5% 7% 37%	4081
	21st to end	Day	8% 4 2 5% 5% 5% 6% 5% 6% 10% 43%	4476
June	1st to 10th	Day	10% 6% 3 7% 6% 7% 6% 5% 8% 9% 35%	2470
	11th to 20th	Day	6% 4 2 5% 5% 6% 6% 6% 9% 11% 40%	2470
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 5% 8% 11% 38%	2470
July	1st to 10th	Day	8% 7% 2 8% 6% 7% 8% 6% 8% 11% 29%	4120
	11th to 20th	Day	9% 5% 2 7% 6% 7% 7% 6% 8% 10% 33%	4120
	21st to end	Day	10% 8% 4 10% 7% 8% 8% 7% 9% 10% 19%	4466
August	1st to 10th	Day	10% 6% 2 7% 5% 6% 6% 5% 8% 11% 33%	4104
	11th to 20th	Day	9% 5% 2 6% 5% 4 6% 5% 7% 11% 40%	3920
	21st to end	Day	8% 5% 2 6% 4 4 4 4 6% 8% 49%	4125
September	1st to 10th	Day	10% 7% 3 7% 6% 5% 5% 4 6% 8% 40%	2090
	11th to 20th	Day	16% 7% 2 5% 4 4 5% 4 7% 8% 38%	2090
	21st to end	Day	15% 6% 2 5% 3 4 3 3 4 7% 48%	2090
October	1st to 10th	Day	13% 5% 2 4 3 3 3 2 4 5% 56%	2090
	11th to 20th	Day	13% 4 2 5% 3 3 2 2 4 5% 58%	1943
	21st to end	Day	13% 4 2 5% 3 3 3 3 3 5% 56%	2090
November	1st to 10th	Day	14% 5% 2 3 3 3 3 2 4 5% 55%	1829
	11th to 20th	Day	14% 5% 1 4 3 3 4 3 4 6% 53%	1800
	21st to end	Day	14% 5% 2 4 4 3 4 3 5% 6% 50%	1800
December	1st to 10th	Day	19% 6% 2 4 3 3 3 3 5% 5% 47%	2930
	11th to 20th	Day	17% 7% 2 6% 4 3 4 3 5% 5% 43%	3373
	21st to end	Day	23% 6% 2 4 3 3 3 3 4 7% 42%	4466
January	1st to 10th	Night	16% 5% 2 4 3 3 3 2 4 5% 53%	5889
	11th to 20th	Night	13% 7% 2 5% 4 3 3 3 4 5% 50%	5900
	21st to end	Night	19% 8% 2 5% 3 3 3 3 4 5% 46%	6429
February	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 4 6% 46%	2400
	11th to 20th	Night	22% 6% 3 6% 3 3 4 3 4 4 43%	2338
	21st to end	Night	19% 7% 2 5% 3 4 3 3 5% 6% 43%	1980
March	1st to 10th	Night	25% 7% 2 6% 3 3 4 3 4 5% 38%	3689
	11th to 20th	Night	12% 4 1 4 3 3 4 4 6% 7% 51%	3498
	21st to end	Night	19% 6% 3 6% 4 3 3 3 5% 6% 42%	3817
April	1st to 10th	Night	12% 4 2 4 4 4 4 4 5% 6% 50%	2090
	11th to 20th	Night	13% 4 2 4 4 3 3 4 5% 8% 50%	1891
	21st to end	Night	11% 4 2 5% 4 4 4 4 5% 7% 52%	1710
May	1st to 10th	Night	11% 6% 3 7% 4 5% 5% 4 7% 9% 40%	2813
	11th to 20th	Night	16% 6% 2 5% 5% 4 4 4 5% 8% 41%	2812
	21st to end	Night	8% 4 2 5% 4 4 5% 5% 7% 10% 46%	3079
June	1st to 10th	Night	8% 5% 2 6% 5% 6% 6% 6% 9% 11% 37%	1710
	11th to 20th	Night	5% 5% 3 5% 4 5% 5% 5% 8% 12% 42%	1710
	21st to end	Night	7% 6% 2 6% 5% 5% 5% 5% 8% 11% 42%	1710
July	1st to 10th	Night	10% 7% 3 8% 7% 6% 6% 5% 8% 10% 30%	2840
	11th to 20th	Night	6% 5% 2 8% 6% 7% 8% 6% 9% 13% 31%	2840
	21st to end	Night	11% 9% 4 10% 7% 7% 7% 6% 8% 11% 21%	3087
August	1st to 10th	Night	11% 6% 2 6% 5% 5% 7% 5% 8% 13% 32%	2825
	11th to 20th	Night	9% 6% 3 7% 5% 5% 5% 5% 6% 10% 39%	2904
	21st to end	Night	8% 5% 2 5% 4 5% 4 4 6% 8% 49%	3498
September	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	2090
	11th to 20th	Night	16% 6% 2 5% 4 3 5% 4 6% 7% 41%	2090
	21st to end	Night	14% 5% 1 6% 4 3 4 3 6% 7% 46%	2090
October	1st to 10th	Night	13% 4 2 5% 3 3 3 3 4 6% 54%	2090
	11th to 20th	Night	12% 5% 1 4 3 3 4 2 4 4 57%	2237
	21st to end	Night	12% 3 2 4 3 3 3 3 4 5% 58%	2508
November	1st to 10th	Night	14% 5% 1 3 3 3 3 2 4 4 55%	2465
	11th to 20th	Night	16% 5% 1 3 3 3 3 3 4 6% 53%	2600
	21st to end	Night	16% 5% 1 4 4 3 4 3 4 6% 49%	2600
December	1st to 10th	Night	16% 6% 2 5% 3 3 3 2 4 5% 51%	4250
	11th to 20th	Night	20% 6% 2 5% 3 3 3 3 4 5% 45%	4822
	21st to end	Night	22% 6% 2 5% 3 3 4 3 4 5% 43%	6490

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	38% 5% 5% 19% 24% 9%	4057
	11th to 20th	Day	30% 4 6% 18% 31% 11%	4060
	21st to end	Day	46% 3 3 17% 21% 10%	4512
February	1st to 10th	Day	43% 5% 7% 20% 19% 6%	2000
	11th to 20th	Day	44% 4 6% 18% 22% 7%	1958
	21st to end	Day	44% 5% 7% 14% 20% 10%	1650
March	1st to 10th	Day	47% 6% 8% 14% 14% 11%	3382
	11th to 20th	Day	35% 5% 7% 17% 24% 12%	3491
	21st to end	Day	43% 6% 7% 20% 21% 4	3815
April	1st to 10th	Day	36% 6% 9% 20% 22% 6%	2090
	11th to 20th	Day	37% 6% 8% 17% 23% 10%	2253
	21st to end	Day	30% 6% 10% 28% 21% 4	2470
May	1st to 10th	Day	38% 9% 13% 21% 17% 2	4120
	11th to 20th	Day	44% 8% 11% 23% 13% 1	4085
	21st to end	Day	33% 10% 14% 21% 21% 1	4489
June	1st to 10th	Day	41% 11% 17% 19% 9% 3	2470
	11th to 20th	Day	32% 11% 20% 24% 13% 1	2470
	21st to end	Day	36% 12% 17% 19% 13% 4	2470
July	1st to 10th	Day	41% 16% 18% 19% 5% 1	4055
	11th to 20th	Day	40% 13% 23% 18% 5% 2	4051
	21st to end	Day	54% 14% 14% 13% 5%	4324
August	1st to 10th	Day	40% 11% 15% 28% 6%	4091
	11th to 20th	Day	34% 11% 19% 19% 13% 3	3959
	21st to end	Day	31% 9% 14% 22% 20% 4	4125
September	1st to 10th	Day	40% 9% 14% 16% 17% 4	2090
	11th to 20th	Day	41% 9% 10% 17% 21% 2	2090
	21st to end	Day	36% 5% 12% 20% 20% 7%	2090
October	1st to 10th	Day	26% 4 7% 15% 37% 12%	3480
	11th to 20th	Day	21% 2 5% 14% 29% 29%	3209
	21st to end	Day	26% 4 5% 20% 31% 14%	3428
November	1st to 10th	Day	33% 4 8% 15% 29% 11%	1829
	11th to 20th	Day	32% 4 7% 20% 27% 10%	1800
	21st to end	Day	34% 6% 7% 19% 27% 7%	1800
December	1st to 10th	Day	40% 3 5% 17% 26% 9%	2930
	11th to 20th	Day	43% 5% 7% 17% 20% 8%	3370
	21st to end	Day	41% 5% 8% 24% 19% 2	4466
January	1st to 10th	Night	33% 4 5% 20% 29% 8%	5889
	11th to 20th	Night	35% 4 5% 19% 27% 9%	5900
	21st to end	Night	41% 4 5% 15% 28% 7%	6429
February	1st to 10th	Night	40% 4 8% 19% 21% 7%	2400
	11th to 20th	Night	45% 4 5% 17% 21% 8%	2339
	21st to end	Night	43% 4 8% 16% 19% 10%	1980
March	1st to 10th	Night	47% 6% 9% 15% 16% 8%	3689
	11th to 20th	Night	29% 6% 9% 16% 28% 12%	3498
	21st to end	Night	42% 5% 6% 19% 18% 9%	3817
April	1st to 10th	Night	33% 6% 10% 19% 24% 7%	2090
	11th to 20th	Night	33% 5% 7% 21% 23% 11%	1891
	21st to end	Night	32% 6% 11% 26% 21% 6%	1710
May	1st to 10th	Night	36% 11% 15% 20% 16% 2	2840
	11th to 20th	Night	41% 8% 14% 22% 15% 1	2813
	21st to end	Night	27% 12% 14% 26% 19% 2	3087
June	1st to 10th	Night	36% 12% 18% 20% 12% 2	1710
	11th to 20th	Night	30% 12% 18% 25% 14% 1	1710
	21st to end	Night	33% 12% 19% 20% 13% 4	1710
July	1st to 10th	Night	45% 12% 20% 16% 5% 1	2795
	11th to 20th	Night	38% 17% 22% 17% 6% 1	2786
	21st to end	Night	54% 14% 15% 13% 3 1	2988
August	1st to 10th	Night	38% 12% 21% 23% 6%	2814
	11th to 20th	Night	38% 11% 19% 18% 13% 1	2931
	21st to end	Night	32% 10% 13% 21% 22% 2	3498
September	1st to 10th	Night	38% 9% 16% 17% 17% 4	2090
	11th to 20th	Night	40% 9% 11% 19% 18% 3	2090
	21st to end	Night	36% 6% 12% 18% 20% 7%	2090
October	1st to 10th	Night	25% 5% 9% 15% 36% 10%	3480
	11th to 20th	Night	19% 3 5% 17% 30% 26%	3710
	21st to end	Night	23% 3 6% 21% 28% 18%	4120
November	1st to 10th	Night	32% 5% 7% 19% 25% 13%	2465
	11th to 20th	Night	33% 4 7% 17% 28% 12%	2600
	21st to end	Night	36% 5% 7% 20% 25% 8%	2600
December	1st to 10th	Night	37% 3 5% 17% 30% 7%	4250
	11th to 20th	Night	41% 6% 8% 16% 22% 7%	4824
	21st to end	Night	43% 5% 6% 27% 17% 2	6490

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	27% 21% 37% 13% 1 1	300
	11th to 20th	Day	31% 19% 28% 18% 4	300
	21st to end	Day	26% 22% 36% 12% 2 1	330
February	1st to 10th	Day	22% 20% 33% 22% 2	300
	11th to 20th	Day	43% 16% 29% 12% 1	300
	21st to end	Day	34% 20% 32% 11% 3	249
March	1st to 10th	Day	27% 15% 27% 21% 9% 1	397
	11th to 20th	Day	23% 18% 33% 18% 6% 1	464
	21st to end	Day	23% 18% 29% 22% 8%	506
April	1st to 10th	Day	24% 13% 30% 22% 10% 1	460
	11th to 20th	Day	16% 16% 33% 21% 14% 1	452
	21st to end	Day	16% 11% 28% 25% 16% 4	460
May	1st to 10th	Day	15% 13% 30% 21% 16% 6%	463
	11th to 20th	Day	18% 8% 21% 27% 16% 10%	487
	21st to end	Day	14% 7% 20% 26% 20% 13%	529
June	1st to 10th	Day	12% 4 18% 31% 25% 10%	490
	11th to 20th	Day	9% 4 17% 29% 23% 18%	490
	21st to end	Day	10% 3 19% 30% 23% 15%	490
July	1st to 10th	Day	12% 2 11% 25% 27% 23%	485
	11th to 20th	Day	11% 2 10% 23% 35% 20%	484
	21st to end	Day	7% 2 10% 26% 37% 19%	532
August	1st to 10th	Day	12% 5% 15% 22% 26% 19%	486
	11th to 20th	Day	13% 2 10% 26% 34% 15%	479
	21st to end	Day	16% 2 12% 21% 30% 19%	539
September	1st to 10th	Day	11% 3 16% 26% 30% 14%	490
	11th to 20th	Day	18% 6% 12% 23% 29% 12%	490
	21st to end	Day	17% 4 17% 23% 29% 10%	490
October	1st to 10th	Day	18% 4 20% 29% 20% 10%	490
	11th to 20th	Day	13% 9% 24% 26% 23% 4	490
	21st to end	Day	16% 9% 24% 26% 22% 4	539
November	1st to 10th	Day	24% 8% 23% 26% 17% 2	367
	11th to 20th	Day	19% 18% 31% 23% 7% 2	300
	21st to end	Day	16% 13% 35% 24% 10% 2	300
December	1st to 10th	Day	31% 16% 30% 21% 2	300
	11th to 20th	Day	27% 13% 32% 24% 5% 1	300
	21st to end	Day	35% 15% 26% 15% 6% 3	330
January	1st to 10th	Night	26% 17% 29% 22% 6%	800
	11th to 20th	Night	23% 14% 31% 25% 7%	800
	21st to end	Night	22% 16% 33% 26% 3	880
February	1st to 10th	Night	20% 14% 31% 31% 6%	800
	11th to 20th	Night	25% 16% 32% 23% 4	783
	21st to end	Night	23% 14% 33% 27% 3	660
March	1st to 10th	Night	21% 14% 31% 30% 5%	800
	11th to 20th	Night	18% 11% 34% 27% 9% 1	767
	21st to end	Night	19% 13% 30% 28% 9%	836
April	1st to 10th	Night	16% 14% 30% 30% 10% 1	760
	11th to 20th	Night	15% 10% 27% 32% 15%	588
	21st to end	Night	13% 7% 26% 33% 21% 1	570
May	1st to 10th	Night	12% 8% 25% 31% 23% 1	570
	11th to 20th	Night	13% 6% 22% 31% 24% 4	561
	21st to end	Night	14% 2 13% 34% 32% 5%	617
June	1st to 10th	Night	10% 2 14% 37% 31% 6%	570
	11th to 20th	Night	6% 2 16% 31% 33% 13%	570
	21st to end	Night	10% 1 11% 36% 33% 9%	570
July	1st to 10th	Night	10% 2 9% 26% 41% 12%	555
	11th to 20th	Night	10% 2 8% 19% 47% 13%	552
	21st to end	Night	7% 2 10% 27% 44% 10%	624
August	1st to 10th	Night	14% 1 8% 27% 38% 11%	567
	11th to 20th	Night	13% 2 8% 26% 40% 10%	564
	21st to end	Night	14% 1 11% 24% 38% 11%	636
September	1st to 10th	Night	11% 3 11% 27% 38% 9%	760
	11th to 20th	Night	18% 3 12% 27% 34% 5%	760
	21st to end	Night	16% 3 15% 30% 33% 3	760
October	1st to 10th	Night	14% 4 18% 30% 28% 6%	760
	11th to 20th	Night	15% 6% 22% 32% 22% 3	760
	21st to end	Night	14% 7% 22% 33% 22% 2	836
November	1st to 10th	Night	16% 7% 22% 33% 20% 2	780
	11th to 20th	Night	19% 9% 26% 31% 14% 1	800
	21st to end	Night	19% 9% 27% 31% 14% 1	800
December	1st to 10th	Night	22% 13% 28% 29% 7% 1	800
	11th to 20th	Night	20% 12% 29% 29% 10%	800
	21st to end	Night	29% 11% 25% 29% 6%	880

Page generated at 2025-03-09 08:08:46. Version ac07

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