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 2026-09-29.

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% 4 8% 5% 5% 5% 4 6% 8% 34%	149886
	Night	13% 10% 4 8% 5% 5% 4 4 5% 7% 35%	141575
Transparency	Day	47% 5% 12% 22% 13% 1	153528
	Night	47% 5% 11% 22% 14% 2	146005
Seeing	Day	16% 10% 26% 24% 16% 7%	17840
	Night	17% 8% 27% 30% 15% 2	27272

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	15% 16% 8% 13% 6% 5% 4 3 5% 5% 20%	5258
	11th to 20th	Day	11% 13% 5% 17% 8% 7% 5% 3 3 4 23%	5280
	21st to end	Day	18% 12% 7% 11% 7% 7% 7% 4 5% 6% 16%	5780
February	1st to 10th	Day	19% 13% 6% 10% 6% 5% 5% 3 5% 5% 24%	1942
	11th to 20th	Day	15% 13% 5% 11% 8% 5% 4 4 4 6% 25%	2048
	21st to end	Day	16% 14% 5% 10% 5% 4 4 3 4 4 30%	1730
March	1st to 10th	Day	16% 12% 7% 11% 5% 5% 4 3 4 5% 28%	4706
	11th to 20th	Day	12% 9% 3 8% 5% 5% 4 3 6% 7% 39%	5090
	21st to end	Day	16% 10% 4 8% 4 5% 4 4 5% 7% 32%	5586
April	1st to 10th	Day	14% 11% 4 8% 4 4 4 4 6% 8% 34%	2425
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 5% 7% 37%	2597
	21st to end	Day	16% 8% 3 7% 6% 4 5% 4 5% 7% 34%	2730
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 4 4 6% 8% 30%	7616
	11th to 20th	Day	17% 8% 3 7% 4 4 5% 4 7% 9% 33%	7626
	21st to end	Day	16% 9% 3 8% 5% 5% 6% 4 7% 9% 30%	8391
June	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 4 7% 9% 38%	2730
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 5% 7% 10% 40%	2858
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 10% 39%	2940
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 4 6% 9% 42%	8280
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 4 7% 11% 39%	7764
	21st to end	Day	9% 6% 3 7% 6% 5% 6% 5% 8% 10% 35%	8340
August	1st to 10th	Day	10% 7% 3 7% 4 5% 6% 5% 7% 9% 38%	6014
	11th to 20th	Day	11% 8% 3 8% 5% 5% 6% 4 8% 10% 31%	5951
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 10% 52%	6289
September	1st to 10th	Day	11% 5% 1 4 4 4 4 4 5% 6% 51%	2357
	11th to 20th	Day	8% 6% 2 5% 4 4 4 4 5% 8% 49%	2310
	21st to end	Day	16% 7% 2 6% 5% 3 3 3 5% 7% 45%	2288
October	1st to 10th	Day	10% 5% 3 6% 4 5% 4 3 6% 7% 47%	2190
	11th to 20th	Day	13% 8% 2 5% 4 4 4 3 5% 7% 46%	2000
	21st to end	Day	18% 10% 3 7% 3 4 3 3 5% 7% 37%	2090
November	1st to 10th	Day	10% 7% 2 6% 4 3 4 3 5% 7% 48%	1852
	11th to 20th	Day	14% 11% 4 8% 5% 4 4 4 4 6% 35%	1890
	21st to end	Day	18% 11% 5% 9% 6% 3 4 3 4 5% 31%	1890
December	1st to 10th	Day	16% 10% 4 9% 6% 5% 4 4 4 6% 32%	3020
	11th to 20th	Day	19% 15% 4 11% 7% 5% 4 3 4 4 23%	3463
	21st to end	Day	20% 18% 7% 12% 6% 5% 4 3 4 5% 16%	4565
January	1st to 10th	Night	12% 14% 7% 15% 9% 6% 6% 4 5% 5% 18%	7635
	11th to 20th	Night	10% 14% 8% 16% 7% 7% 5% 3 4 5% 21%	7680
	21st to end	Night	16% 12% 5% 8% 5% 6% 5% 3 4 6% 29%	8401
February	1st to 10th	Night	18% 13% 4 10% 5% 4 5% 3 4 5% 29%	2677
	11th to 20th	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 30%	2458
	21st to end	Night	18% 13% 5% 10% 4 4 4 3 4 5% 30%	2076
March	1st to 10th	Night	18% 12% 4 7% 5% 4 3 3 4 5% 35%	5365
	11th to 20th	Night	11% 6% 3 7% 4 5% 5% 4 6% 8% 41%	5090
	21st to end	Night	16% 10% 4 8% 5% 4 4 3 5% 6% 34%	5588
April	1st to 10th	Night	14% 8% 3 7% 4 4 4 4 6% 7% 37%	2213
	11th to 20th	Night	13% 8% 3 6% 4 4 5% 4 6% 8% 40%	1972
	21st to end	Night	15% 8% 3 7% 4 4 4 4 5% 8% 38%	1890
May	1st to 10th	Night	20% 8% 3 7% 5% 5% 5% 4 6% 8% 29%	5237
	11th to 20th	Night	14% 7% 2 7% 4 4 4 4 7% 8% 37%	5234
	21st to end	Night	15% 8% 3 8% 4 5% 4 4 6% 9% 35%	5761
June	1st to 10th	Night	8% 6% 3 5% 4 4 4 4 5% 8% 48%	1890
	11th to 20th	Night	7% 5% 2 5% 5% 4 5% 3 8% 10% 46%	1754
	21st to end	Night	6% 5% 2 5% 5% 5% 5% 5% 8% 11% 45%	1680
July	1st to 10th	Night	8% 6% 3 7% 4 5% 4 4 7% 8% 44%	4708
	11th to 20th	Night	8% 7% 2 7% 5% 4 5% 4 6% 9% 42%	5180
	21st to end	Night	8% 6% 2 6% 5% 5% 5% 5% 8% 10% 41%	5736
August	1st to 10th	Night	9% 8% 3 9% 5% 5% 5% 5% 6% 8% 36%	4135
	11th to 20th	Night	11% 10% 3 8% 5% 5% 4 4 5% 8% 37%	4095
	21st to end	Night	3 2 1 4 4 4 5% 4 7% 10% 54%	4833
September	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 5% 7% 52%	2241
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 5% 7% 54%	2310
	21st to end	Night	14% 6% 2 6% 3 3 3 3 4 7% 48%	2289
October	1st to 10th	Night	8% 6% 2 5% 4 3 4 3 6% 7% 53%	2208
	11th to 20th	Night	13% 8% 2 6% 4 4 5% 3 5% 6% 45%	2400
	21st to end	Night	16% 9% 3 7% 4 4 3 3 5% 6% 39%	2748
November	1st to 10th	Night	13% 7% 2 6% 3 4 4 3 4 7% 47%	2666
	11th to 20th	Night	15% 8% 3 7% 4 3 4 4 5% 7% 38%	2730
	21st to end	Night	18% 13% 5% 7% 6% 4 4 3 5% 6% 29%	2730
December	1st to 10th	Night	15% 11% 4 9% 5% 5% 4 3 4 5% 35%	4380
	11th to 20th	Night	18% 14% 6% 11% 5% 5% 4 3 4 5% 24%	4952
	21st to end	Night	20% 18% 7% 12% 6% 5% 4 3 4 5% 16%	6633

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	66% 4 11% 14% 4	5259
	11th to 20th	Day	65% 2 10% 11% 11% 1	5280
	21st to end	Day	68% 3 10% 11% 6% 1	5780
February	1st to 10th	Day	62% 4 10% 15% 8% 1	1942
	11th to 20th	Day	60% 4 11% 16% 9% 1	2058
	21st to end	Day	57% 5% 11% 18% 7% 1	1730
March	1st to 10th	Day	58% 5% 8% 19% 9% 1	4706
	11th to 20th	Day	43% 5% 10% 27% 14% 1	5090
	21st to end	Day	50% 5% 12% 19% 11% 2	5586
April	1st to 10th	Day	49% 5% 12% 22% 11% 1	2425
	11th to 20th	Day	47% 4 12% 22% 13% 3	2597
	21st to end	Day	47% 7% 11% 22% 11% 2	2730
May	1st to 10th	Day	51% 6% 12% 20% 11% 1	7651
	11th to 20th	Day	46% 6% 13% 24% 11%	7630
	21st to end	Day	48% 7% 11% 20% 12% 3	8405
June	1st to 10th	Day	41% 7% 11% 23% 16% 2	2730
	11th to 20th	Day	38% 8% 14% 23% 16% 1	2858
	21st to end	Day	38% 9% 15% 22% 14% 3	2940
July	1st to 10th	Day	38% 6% 12% 27% 16% 1	8211
	11th to 20th	Day	38% 7% 13% 31% 11%	7693
	21st to end	Day	40% 7% 13% 22% 15% 3	8156
August	1st to 10th	Day	39% 8% 15% 23% 11% 3	6001
	11th to 20th	Day	45% 7% 11% 25% 11%	5990
	21st to end	Day	27% 6% 16% 32% 17% 2	6288
September	1st to 10th	Day	32% 6% 11% 25% 21% 4	2357
	11th to 20th	Day	32% 6% 11% 25% 23% 3	2310
	21st to end	Day	40% 5% 9% 21% 23% 2	2288
October	1st to 10th	Day	24% 4 10% 30% 31% 2	3567
	11th to 20th	Day	31% 4 10% 30% 24% 2	3240
	21st to end	Day	38% 3 11% 24% 21% 2	3353
November	1st to 10th	Day	37% 3 11% 27% 19% 3	1852
	11th to 20th	Day	50% 4 11% 20% 13% 2	1890
	21st to end	Day	56% 4 10% 20% 10% 1	1890
December	1st to 10th	Day	53% 4 12% 23% 8%	3020
	11th to 20th	Day	64% 4 9% 16% 6%	3460
	21st to end	Day	71% 4 10% 10% 5%	4565
January	1st to 10th	Night	66% 4 10% 13% 6%	7636
	11th to 20th	Night	65% 3 9% 12% 9% 2	7680
	21st to end	Night	56% 3 12% 19% 8% 1	8401
February	1st to 10th	Night	58% 4 9% 18% 9% 1	2677
	11th to 20th	Night	55% 4 11% 18% 11% 1	2459
	21st to end	Night	57% 4 10% 16% 10% 2	2076
March	1st to 10th	Night	51% 4 7% 26% 11% 2	5365
	11th to 20th	Night	39% 6% 12% 30% 13% 1	5090
	21st to end	Night	51% 4 8% 24% 12% 1	5588
April	1st to 10th	Night	44% 6% 13% 23% 13% 2	2213
	11th to 20th	Night	41% 5% 14% 18% 18% 4	1972
	21st to end	Night	44% 5% 11% 24% 14% 2	1890
May	1st to 10th	Night	52% 6% 10% 21% 8% 3	5264
	11th to 20th	Night	43% 6% 11% 25% 14% 1	5235
	21st to end	Night	44% 7% 12% 22% 11% 5%	5771
June	1st to 10th	Night	34% 7% 11% 25% 22% 2	1890
	11th to 20th	Night	31% 8% 15% 23% 18% 3	1754
	21st to end	Night	30% 10% 16% 23% 17% 4	1680
July	1st to 10th	Night	37% 6% 13% 24% 17% 3	4670
	11th to 20th	Night	37% 7% 11% 32% 12% 1	5131
	21st to end	Night	36% 6% 13% 24% 18% 2	5606
August	1st to 10th	Night	43% 7% 13% 22% 12% 3	4124
	11th to 20th	Night	44% 7% 13% 22% 14%	4122
	21st to end	Night	23% 7% 17% 32% 20% 2	4833
September	1st to 10th	Night	31% 6% 12% 24% 21% 6%	2241
	11th to 20th	Night	28% 6% 9% 25% 28% 3	2310
	21st to end	Night	37% 5% 9% 20% 27% 2	2289
October	1st to 10th	Night	24% 4 7% 27% 35% 3	3611
	11th to 20th	Night	30% 3 9% 26% 28% 3	3899
	21st to end	Night	35% 3 7% 24% 26% 5%	4435
November	1st to 10th	Night	38% 4 11% 23% 21% 3	2666
	11th to 20th	Night	44% 5% 9% 22% 16% 3	2730
	21st to end	Night	55% 5% 10% 19% 10% 1	2730
December	1st to 10th	Night	52% 4 11% 24% 9% 1	4380
	11th to 20th	Night	61% 4 11% 17% 7%	4954
	21st to end	Night	71% 4 8% 14% 3	6633

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% 16% 37% 19% 5%	330
	11th to 20th	Day	21% 25% 38% 13% 3 1	330
	21st to end	Day	27% 17% 34% 18% 4	363
February	1st to 10th	Day	26% 14% 36% 19% 5% 1	330
	11th to 20th	Day	28% 20% 31% 17% 4	330
	21st to end	Day	23% 15% 41% 19% 3	273
March	1st to 10th	Day	21% 19% 37% 19% 4	378
	11th to 20th	Day	23% 16% 35% 19% 7% 1	540
	21st to end	Day	26% 15% 35% 20% 5%	594
April	1st to 10th	Day	20% 15% 33% 22% 7% 2	540
	11th to 20th	Day	16% 20% 28% 23% 10% 3	532
	21st to end	Day	20% 11% 36% 20% 11% 3	540
May	1st to 10th	Day	23% 13% 33% 20% 9% 3	543
	11th to 20th	Day	17% 8% 27% 27% 13% 7%	567
	21st to end	Day	13% 6% 24% 28% 17% 11%	617
June	1st to 10th	Day	12% 4 22% 29% 24% 9%	570
	11th to 20th	Day	11% 5% 22% 30% 23% 8%	698
	21st to end	Day	11% 4 24% 32% 20% 9%	780
July	1st to 10th	Day	11% 2 16% 25% 26% 19%	771
	11th to 20th	Day	10% 3 20% 25% 27% 16%	594
	21st to end	Day	9% 2 16% 29% 28% 15%	620
August	1st to 10th	Day	10% 3 13% 26% 28% 20%	566
	11th to 20th	Day	12% 4 14% 29% 27% 14%	559
	21st to end	Day	7% 3 13% 26% 34% 16%	627
September	1st to 10th	Day	11% 4 13% 26% 31% 15%	570
	11th to 20th	Day	9% 2 11% 26% 34% 18%	570
	21st to end	Day	19% 8% 18% 24% 22% 9%	562
October	1st to 10th	Day	9% 4 23% 32% 22% 10%	530
	11th to 20th	Day	14% 11% 31% 24% 16% 4	530
	21st to end	Day	22% 11% 35% 24% 7% 1	473
November	1st to 10th	Day	15% 10% 34% 25% 12% 4	330
	11th to 20th	Day	18% 19% 31% 19% 9% 2	330
	21st to end	Day	25% 11% 39% 21% 5%	330
December	1st to 10th	Day	18% 19% 39% 16% 7% 1	330
	11th to 20th	Day	20% 15% 41% 20% 4	330
	21st to end	Day	29% 17% 33% 15% 5%	363
January	1st to 10th	Night	20% 18% 35% 21% 6%	840
	11th to 20th	Night	23% 17% 37% 20% 3	840
	21st to end	Night	19% 14% 39% 24% 4	924
February	1st to 10th	Night	25% 11% 34% 28% 2	840
	11th to 20th	Night	24% 14% 36% 24% 3	823
	21st to end	Night	24% 11% 40% 22% 2	692
March	1st to 10th	Night	21% 13% 34% 26% 5%	840
	11th to 20th	Night	19% 14% 34% 25% 6%	840
	21st to end	Night	23% 11% 33% 25% 7%	924
April	1st to 10th	Night	17% 13% 34% 29% 6% 1	743
	11th to 20th	Night	16% 10% 35% 28% 10% 1	623
	21st to end	Night	17% 7% 38% 29% 8% 1	630
May	1st to 10th	Night	20% 8% 32% 28% 11% 1	630
	11th to 20th	Night	18% 5% 26% 33% 15% 2	621
	21st to end	Night	10% 4 22% 36% 24% 5%	683
June	1st to 10th	Night	9% 3 21% 43% 22% 1	630
	11th to 20th	Night	9% 3 17% 45% 23% 2	630
	21st to end	Night	7% 2 22% 40% 26% 3	630
July	1st to 10th	Night	9% 2 11% 36% 35% 8%	615
	11th to 20th	Night	6% 2 14% 37% 35% 6%	612
	21st to end	Night	10% 2 12% 39% 34% 3	690
August	1st to 10th	Night	8% 1 12% 40% 35% 4	627
	11th to 20th	Night	9% 2 10% 43% 32% 4	624
	21st to end	Night	8% 1 13% 35% 36% 7%	693
September	1st to 10th	Night	12% 3 14% 33% 32% 5%	771
	11th to 20th	Night	8% 3 11% 37% 37% 5%	840
	21st to end	Night	17% 4 19% 35% 22% 2	833
October	1st to 10th	Night	11% 4 22% 34% 24% 4	800
	11th to 20th	Night	17% 9% 26% 33% 13% 1	800
	21st to end	Night	21% 9% 31% 27% 11%	880
November	1st to 10th	Night	17% 7% 30% 29% 15% 1	820
	11th to 20th	Night	18% 14% 32% 26% 9% 1	840
	21st to end	Night	22% 12% 29% 30% 6%	840
December	1st to 10th	Night	24% 11% 36% 24% 4 1	840
	11th to 20th	Night	24% 11% 34% 25% 5% 1	840
	21st to end	Night	29% 13% 29% 26% 3	924

Page generated at 2026-09-26 04:18:41. Version ac07

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