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 2024-10-20.

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% 8% 42%	100855
	Night	14% 6% 2 6% 4 4 4 4 6% 7% 43%	97790
Transparency	Day	39% 8% 11% 19% 17% 5%	100668
	Night	38% 7% 11% 19% 19% 6%	97647
Seeing	Day	18% 9% 22% 23% 18% 9%	14908
	Night	17% 8% 22% 29% 20% 4	24924

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	17% 9% 3 6% 3 3 4 4 5% 6% 40%	2836
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 4 5% 52%	2840
	21st to end	Day	28% 6% 2 4 3 2 2 2 4 4 43%	3156
February	1st to 10th	Day	17% 8% 2 6% 4 4 4 4 5% 6% 41%	1900
	11th to 20th	Day	22% 7% 1 5% 3 2 3 4 3 5% 44%	1848
	21st to end	Day	21% 7% 2 7% 4 3 4 2 4 6% 40%	1570
March	1st to 10th	Day	25% 7% 3 6% 3 3 3 2 4 5% 38%	3277
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 4 5% 51%	3480
	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%	1933
	21st to end	Day	14% 4 2 5% 3 3 3 3 3 5% 56%	1980
November	1st to 10th	Day	12% 5% 2 4 3 3 3 2 4 4 58%	1736
	11th to 20th	Day	14% 5% 1 4 3 3 4 3 4 6% 53%	1710
	21st to end	Day	15% 5% 2 4 4 3 4 3 5% 7% 48%	1710
December	1st to 10th	Day	22% 7% 3 6% 4 4 3 3 6% 5% 38%	1710
	11th to 20th	Day	18% 5% 2 5% 4 3 3 3 4 5% 47%	2153
	21st to end	Day	23% 6% 2 5% 3 3 3 3 5% 8% 39%	3124
January	1st to 10th	Night	19% 7% 2 5% 3 3 3 3 4 6% 45%	4109
	11th to 20th	Night	13% 7% 3 6% 4 4 4 3 5% 5% 46%	4120
	21st to end	Night	25% 9% 2 5% 3 2 3 2 3 4 42%	4499
February	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 4 6% 45%	2280
	11th to 20th	Night	21% 6% 3 6% 3 3 4 3 4 4 44%	2218
	21st to end	Night	20% 7% 2 6% 3 4 3 3 5% 6% 41%	1884
March	1st to 10th	Night	25% 7% 2 6% 3 3 4 3 4 5% 38%	3574
	11th to 20th	Night	12% 4 1 4 3 3 4 4 6% 8% 51%	3480
	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%	2232
	21st to end	Night	13% 3 2 4 3 3 3 3 4 5% 57%	2376
November	1st to 10th	Night	13% 5% 1 3 3 3 3 2 4 4 58%	2338
	11th to 20th	Night	15% 5% 1 4 3 3 3 3 4 6% 53%	2470
	21st to end	Night	16% 5% 1 4 4 3 4 3 5% 6% 47%	2470
December	1st to 10th	Night	19% 7% 3 6% 4 4 4 3 5% 6% 42%	2470
	11th to 20th	Night	18% 6% 2 4 3 2 3 2 3 5% 52%	3042
	21st to end	Night	21% 7% 2 7% 4 4 5% 3 5% 6% 36%	4532

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	43% 6% 7% 16% 20% 8%	2837
	11th to 20th	Day	33% 5% 8% 21% 26% 8%	2840
	21st to end	Day	46% 3 4 17% 21% 9%	3156
February	1st to 10th	Day	44% 4 8% 19% 20% 6%	1900
	11th to 20th	Day	43% 4 6% 18% 22% 7%	1858
	21st to end	Day	46% 5% 7% 14% 19% 10%	1570
March	1st to 10th	Day	48% 6% 8% 14% 13% 11%	3277
	11th to 20th	Day	35% 5% 7% 17% 24% 12%	3480
	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	31% 4 9% 19% 29% 8%	2090
	11th to 20th	Day	30% 4 8% 22% 26% 11%	1933
	21st to end	Day	31% 4 5% 19% 29% 12%	1980
November	1st to 10th	Day	30% 4 7% 15% 31% 12%	1736
	11th to 20th	Day	32% 4 8% 20% 26% 10%	1710
	21st to end	Day	35% 6% 7% 19% 26% 7%	1710
December	1st to 10th	Day	48% 4 7% 18% 16% 7%	1710
	11th to 20th	Day	40% 4 7% 17% 24% 9%	2150
	21st to end	Day	43% 5% 9% 19% 21% 3	3124
January	1st to 10th	Night	40% 5% 6% 17% 25% 7%	4109
	11th to 20th	Night	38% 5% 6% 21% 21% 9%	4120
	21st to end	Night	46% 4 5% 13% 25% 7%	4499
February	1st to 10th	Night	40% 4 8% 18% 22% 8%	2280
	11th to 20th	Night	44% 4 5% 17% 21% 8%	2219
	21st to end	Night	45% 4 8% 16% 18% 9%	1884
March	1st to 10th	Night	47% 6% 9% 16% 15% 8%	3574
	11th to 20th	Night	30% 6% 9% 16% 28% 12%	3480
	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	31% 5% 10% 18% 28% 7%	2090
	11th to 20th	Night	31% 5% 7% 20% 25% 12%	2232
	21st to end	Night	29% 4 6% 20% 29% 12%	2376
November	1st to 10th	Night	30% 4 7% 19% 26% 13%	2338
	11th to 20th	Night	32% 4 7% 17% 27% 12%	2470
	21st to end	Night	37% 5% 7% 19% 24% 8%	2470
December	1st to 10th	Night	44% 4 7% 20% 18% 6%	2470
	11th to 20th	Night	36% 4 7% 18% 28% 7%	3044
	21st to end	Night	46% 6% 7% 21% 17% 3	4532

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	26% 20% 37% 14% 1 1	270
	11th to 20th	Day	31% 17% 29% 19% 4	270
	21st to end	Day	26% 22% 37% 12% 3 1	297
February	1st to 10th	Day	24% 20% 31% 21% 2	270
	11th to 20th	Day	41% 15% 31% 12% 1	270
	21st to end	Day	37% 20% 31% 9% 2	225
March	1st to 10th	Day	27% 13% 28% 22% 9% 1	362
	11th to 20th	Day	24% 18% 33% 18% 6% 1	460
	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% 5%	486
	21st to end	Day	17% 9% 24% 25% 20% 4	495
November	1st to 10th	Day	22% 8% 23% 28% 18% 2	334
	11th to 20th	Day	18% 20% 30% 24% 6% 2	270
	21st to end	Day	16% 12% 35% 24% 11% 2	270
December	1st to 10th	Day	32% 14% 29% 21% 3	270
	11th to 20th	Day	26% 13% 31% 24% 5% 1	270
	21st to end	Day	35% 16% 25% 15% 6% 2	297
January	1st to 10th	Night	26% 16% 29% 23% 6%	760
	11th to 20th	Night	22% 14% 31% 26% 7%	760
	21st to end	Night	22% 16% 33% 25% 3	836
February	1st to 10th	Night	20% 14% 31% 30% 6%	760
	11th to 20th	Night	24% 16% 33% 24% 4	743
	21st to end	Night	24% 14% 33% 26% 3	628
March	1st to 10th	Night	20% 14% 31% 30% 5%	760
	11th to 20th	Night	19% 11% 34% 27% 9% 1	760
	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	759
	21st to end	Night	14% 8% 22% 33% 22% 2	792
November	1st to 10th	Night	15% 7% 22% 33% 21% 2	740
	11th to 20th	Night	18% 10% 26% 31% 14% 1	760
	21st to end	Night	19% 8% 27% 31% 14% 1	760
December	1st to 10th	Night	22% 13% 27% 30% 8% 1	760
	11th to 20th	Night	20% 11% 29% 29% 10%	760
	21st to end	Night	29% 11% 25% 29% 6%	836

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