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 2026-07-16.

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% 6% 2 6% 4 4 5% 4 6% 7% 42%	137849
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 44%	134837
Transparency	Day	39% 7% 11% 19% 18% 6%	141612
	Night	38% 7% 10% 19% 20% 7%	139146
Seeing	Day	18% 10% 23% 23% 18% 8%	17200
	Night	17% 8% 23% 29% 20% 3	27221

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 7% 4 3 3 3 4 4 44%	5258
	11th to 20th	Day	11% 5% 1 4 2 2 3 2 4 4 61%	5280
	21st to end	Day	24% 9% 3 6% 3 2 3 2 4 5% 38%	5840
February	1st to 10th	Day	16% 8% 2 6% 4 4 4 4 5% 6% 42%	2100
	11th to 20th	Day	23% 7% 1 5% 3 2 3 4 4 5% 43%	2048
	21st to end	Day	20% 7% 2 7% 4 3 4 2 4 6% 42%	1730
March	1st to 10th	Day	22% 7% 3 7% 4 4 4 3 4 5% 37%	4812
	11th to 20th	Day	10% 5% 2 6% 4 3 3 3 5% 6% 53%	5090
	21st to end	Day	17% 6% 2 6% 4 4 4 3 4 5% 46%	5586
April	1st to 10th	Day	13% 5% 2 4 5% 4 5% 4 7% 7% 45%	2310
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 4 6% 50%	2493
	21st to end	Day	10% 5% 2 5% 4 4 4 4 5% 8% 50%	2730
May	1st to 10th	Day	14% 7% 3 7% 4 4 5% 4 6% 8% 37%	7616
	11th to 20th	Day	15% 7% 3 7% 5% 5% 5% 5% 7% 8% 35%	7626
	21st to end	Day	18% 7% 3 7% 5% 5% 6% 5% 5% 7% 31%	8391
June	1st to 10th	Day	10% 6% 2 7% 6% 7% 6% 5% 8% 10% 34%	2730
	11th to 20th	Day	6% 4 2 5% 5% 6% 6% 7% 9% 11% 38%	2730
	21st to end	Day	6% 4 2 7% 5% 6% 6% 5% 8% 12% 39%	2730
July	1st to 10th	Day	6% 5% 2 7% 6% 7% 7% 6% 8% 11% 35%	6030
	11th to 20th	Day	6% 4 2 6% 6% 7% 7% 6% 9% 12% 34%	5965
	21st to end	Day	9% 7% 3 8% 6% 7% 7% 6% 9% 11% 25%	6424
August	1st to 10th	Day	11% 6% 2 7% 5% 6% 6% 6% 8% 11% 32%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 7% 11% 39%	4046
	21st to end	Day	8% 5% 2 6% 4 4 4 4 6% 8% 49%	4255
September	1st to 10th	Day	10% 6% 3 7% 6% 5% 5% 5% 6% 8% 40%	2200
	11th to 20th	Day	15% 7% 2 5% 4 4 4 4 7% 8% 40%	2200
	21st to end	Day	15% 6% 2 5% 3 4 3 3 5% 7% 47%	2200
October	1st to 10th	Day	13% 5% 2 4 3 3 3 3 5% 5% 54%	2200
	11th to 20th	Day	12% 4 2 5% 3 3 2 2 4 5% 59%	2045
	21st to end	Day	14% 4 2 5% 3 3 3 3 3 5% 56%	2200
November	1st to 10th	Day	13% 5% 2 3 3 3 3 2 4 5% 56%	1922
	11th to 20th	Day	13% 5% 1 4 3 3 4 3 5% 6% 54%	1890
	21st to end	Day	14% 5% 1 4 4 3 4 3 5% 6% 50%	1890
December	1st to 10th	Day	19% 6% 2 5% 3 3 3 3 5% 5% 46%	3020
	11th to 20th	Day	17% 6% 2 6% 4 3 4 3 4 5% 44%	3463
	21st to end	Day	23% 6% 2 4 3 3 3 3 4 7% 42%	4565
January	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 4 6% 48%	7635
	11th to 20th	Night	13% 7% 2 5% 3 3 3 2 4 5% 52%	7680
	21st to end	Night	21% 8% 2 5% 3 3 3 2 4 4 44%	8326
February	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 4 6% 47%	2520
	11th to 20th	Night	22% 6% 3 6% 3 3 4 3 4 4 43%	2458
	21st to end	Night	20% 7% 2 6% 3 4 3 3 4 5% 42%	2076
March	1st to 10th	Night	22% 7% 2 6% 4 3 4 3 5% 6% 37%	5259
	11th to 20th	Night	10% 3 1 4 3 3 4 3 6% 7% 54%	5090
	21st to end	Night	15% 6% 2 5% 4 4 3 3 5% 6% 46%	5588
April	1st to 10th	Night	12% 4 2 4 4 4 4 4 5% 7% 49%	2310
	11th to 20th	Night	12% 4 2 4 3 3 3 4 5% 8% 52%	2092
	21st to end	Night	11% 4 2 5% 4 4 4 4 5% 7% 50%	1890
May	1st to 10th	Night	13% 7% 3 7% 5% 5% 5% 4 7% 9% 35%	5237
	11th to 20th	Night	16% 6% 2 6% 5% 4 5% 4 6% 9% 36%	5234
	21st to end	Night	18% 7% 3 7% 4 4 5% 4 6% 7% 33%	5761
June	1st to 10th	Night	8% 5% 2 6% 5% 6% 6% 5% 8% 11% 37%	1890
	11th to 20th	Night	6% 5% 3 5% 4 5% 5% 5% 8% 12% 41%	1890
	21st to end	Night	6% 5% 2 6% 5% 5% 4 5% 8% 11% 43%	1890
July	1st to 10th	Night	8% 7% 3 9% 7% 6% 6% 5% 8% 10% 31%	4148
	11th to 20th	Night	5% 5% 2 8% 6% 7% 7% 6% 10% 14% 30%	4102
	21st to end	Night	11% 8% 3 9% 7% 6% 6% 5% 7% 10% 28%	4427
August	1st to 10th	Night	12% 6% 2 5% 5% 5% 7% 5% 8% 13% 32%	2915
	11th to 20th	Night	10% 6% 3 7% 5% 5% 5% 5% 6% 9% 39%	2998
	21st to end	Night	8% 5% 2 6% 4 5% 4 4 6% 8% 48%	3609
September	1st to 10th	Night	10% 6% 2 7% 5% 6% 5% 4 6% 9% 40%	2200
	11th to 20th	Night	15% 6% 2 5% 4 4 5% 4 7% 7% 41%	2200
	21st to end	Night	15% 5% 2 6% 4 3 4 3 6% 7% 45%	2200
October	1st to 10th	Night	13% 4 2 5% 3 3 3 3 4 6% 53%	2200
	11th to 20th	Night	11% 5% 1 4 3 3 4 2 4 4 58%	2355
	21st to end	Night	13% 3 2 4 3 3 3 3 3 5% 58%	2640
November	1st to 10th	Night	14% 5% 2 3 3 3 4 2 4 4 56%	2592
	11th to 20th	Night	15% 4 1 3 3 3 3 2 4 6% 54%	2730
	21st to end	Night	16% 5% 1 4 4 3 4 3 4 6% 49%	2730
December	1st to 10th	Night	16% 6% 2 5% 3 3 3 2 3 5% 51%	4380
	11th to 20th	Night	19% 6% 2 5% 3 3 3 3 4 5% 45%	4952
	21st to end	Night	22% 6% 2 5% 3 4 4 3 4 5% 43%	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	44% 5% 7% 19% 19% 7%	5259
	11th to 20th	Day	27% 3 5% 15% 33% 15%	5280
	21st to end	Day	50% 2 4 15% 17% 11%	5840
February	1st to 10th	Day	42% 4 7% 21% 20% 6%	2100
	11th to 20th	Day	43% 4 6% 19% 21% 6%	2058
	21st to end	Day	44% 5% 7% 14% 20% 10%	1730
March	1st to 10th	Day	47% 5% 7% 13% 18% 9%	4812
	11th to 20th	Day	32% 4 6% 14% 23% 20%	5090
	21st to end	Day	41% 5% 6% 21% 21% 6%	5586
April	1st to 10th	Day	36% 6% 10% 20% 22% 6%	2310
	11th to 20th	Day	35% 5% 8% 19% 24% 9%	2493
	21st to end	Day	32% 6% 10% 27% 20% 4	2730
May	1st to 10th	Day	43% 8% 12% 21% 13% 2	7651
	11th to 20th	Day	43% 9% 15% 20% 11% 2	7630
	21st to end	Day	50% 9% 10% 16% 14% 1	8405
June	1st to 10th	Day	41% 10% 16% 20% 10% 3	2730
	11th to 20th	Day	33% 11% 20% 23% 12% 1	2730
	21st to end	Day	34% 12% 18% 20% 13% 3	2730
July	1st to 10th	Day	36% 15% 18% 24% 7% 1	5965
	11th to 20th	Day	35% 15% 28% 17% 3 1	5896
	21st to end	Day	46% 14% 17% 18% 4	6240
August	1st to 10th	Day	41% 11% 16% 27% 6%	4221
	11th to 20th	Day	35% 11% 19% 19% 13% 2	4085
	21st to end	Day	31% 9% 14% 21% 20% 4	4255
September	1st to 10th	Day	39% 9% 15% 16% 17% 4	2200
	11th to 20th	Day	40% 9% 11% 19% 20% 2	2200
	21st to end	Day	36% 6% 13% 20% 19% 6%	2200
October	1st to 10th	Day	26% 4 7% 15% 36% 12%	3590
	11th to 20th	Day	21% 2 5% 14% 29% 29%	3311
	21st to end	Day	26% 4 5% 20% 31% 14%	3538
November	1st to 10th	Day	32% 4 8% 16% 29% 11%	1922
	11th to 20th	Day	31% 4 8% 21% 27% 10%	1890
	21st to end	Day	34% 5% 7% 19% 27% 7%	1890
December	1st to 10th	Day	41% 3 5% 16% 25% 9%	3020
	11th to 20th	Day	43% 5% 7% 17% 20% 8%	3460
	21st to end	Day	41% 5% 8% 25% 19% 2	4565
January	1st to 10th	Night	36% 5% 9% 21% 23% 6%	7636
	11th to 20th	Night	34% 4 4 19% 27% 11%	7680
	21st to end	Night	44% 3 4 16% 24% 9%	8326
February	1st to 10th	Night	39% 4 8% 20% 22% 7%	2520
	11th to 20th	Night	45% 4 6% 18% 21% 7%	2459
	21st to end	Night	43% 4 7% 16% 19% 10%	2076
March	1st to 10th	Night	46% 5% 8% 13% 20% 7%	5259
	11th to 20th	Night	27% 5% 8% 14% 29% 17%	5090
	21st to end	Night	38% 5% 6% 21% 20% 9%	5588
April	1st to 10th	Night	33% 6% 11% 20% 23% 7%	2310
	11th to 20th	Night	30% 5% 7% 23% 24% 10%	2092
	21st to end	Night	33% 6% 11% 25% 20% 5%	1890
May	1st to 10th	Night	42% 9% 14% 18% 13% 3	5264
	11th to 20th	Night	42% 8% 16% 20% 13% 1	5235
	21st to end	Night	46% 10% 11% 19% 13% 1	5771
June	1st to 10th	Night	36% 11% 18% 19% 13% 2	1890
	11th to 20th	Night	32% 12% 18% 24% 13% 1	1890
	21st to end	Night	31% 11% 19% 21% 13% 4	1890
July	1st to 10th	Night	43% 13% 19% 20% 5% 1	4105
	11th to 20th	Night	38% 18% 24% 16% 4 1	4051
	21st to end	Night	48% 12% 18% 17% 4 1	4297
August	1st to 10th	Night	39% 12% 20% 23% 6%	2904
	11th to 20th	Night	39% 11% 19% 17% 12% 1	3025
	21st to end	Night	33% 10% 13% 21% 21% 2	3609
September	1st to 10th	Night	37% 9% 16% 17% 17% 4	2200
	11th to 20th	Night	39% 9% 11% 19% 18% 3	2200
	21st to end	Night	37% 6% 13% 18% 19% 7%	2200
October	1st to 10th	Night	26% 5% 9% 15% 35% 9%	3590
	11th to 20th	Night	19% 3 5% 17% 31% 26%	3828
	21st to end	Night	24% 3 5% 21% 28% 18%	4252
November	1st to 10th	Night	32% 5% 7% 18% 26% 12%	2592
	11th to 20th	Night	31% 4 7% 18% 29% 12%	2730
	21st to end	Night	36% 5% 6% 20% 25% 8%	2730
December	1st to 10th	Night	38% 3 5% 17% 30% 7%	4380
	11th to 20th	Night	41% 6% 9% 16% 22% 7%	4954
	21st to end	Night	42% 5% 6% 28% 17% 2	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	28% 21% 36% 13% 1 1	330
	11th to 20th	Day	31% 20% 27% 17% 5%	330
	21st to end	Day	28% 22% 36% 12% 2 1	363
February	1st to 10th	Day	22% 22% 33% 21% 2	330
	11th to 20th	Day	41% 17% 28% 12% 2	330
	21st to end	Day	34% 19% 32% 11% 4	273
March	1st to 10th	Day	26% 15% 27% 22% 9% 1	432
	11th to 20th	Day	24% 18% 32% 18% 6% 1	540
	21st to end	Day	22% 17% 31% 23% 8%	594
April	1st to 10th	Day	23% 13% 29% 23% 11% 1	540
	11th to 20th	Day	15% 15% 32% 22% 14% 2	532
	21st to end	Day	17% 11% 28% 26% 14% 5%	540
May	1st to 10th	Day	16% 14% 29% 22% 15% 5%	543
	11th to 20th	Day	18% 9% 22% 26% 16% 10%	567
	21st to end	Day	17% 8% 20% 25% 18% 12%	617
June	1st to 10th	Day	13% 4 19% 31% 24% 10%	570
	11th to 20th	Day	9% 4 18% 29% 23% 17%	570
	21st to end	Day	9% 3 18% 30% 24% 16%	570
July	1st to 10th	Day	10% 2 10% 26% 29% 23%	565
	11th to 20th	Day	10% 2 9% 23% 35% 20%	544
	21st to end	Day	7% 1 9% 26% 37% 19%	576
August	1st to 10th	Day	15% 5% 15% 22% 25% 18%	526
	11th to 20th	Day	13% 2 10% 25% 35% 16%	519
	21st to end	Day	16% 2 13% 21% 29% 20%	583
September	1st to 10th	Day	10% 3 16% 28% 29% 13%	530
	11th to 20th	Day	16% 5% 12% 23% 30% 13%	530
	21st to end	Day	16% 4 18% 25% 28% 9%	530
October	1st to 10th	Day	17% 5% 20% 30% 19% 9%	530
	11th to 20th	Day	13% 9% 23% 26% 25% 5%	530
	21st to end	Day	18% 10% 25% 25% 20% 3	583
November	1st to 10th	Day	24% 8% 24% 27% 16% 2	400
	11th to 20th	Day	17% 18% 33% 23% 7% 2	330
	21st to end	Day	17% 15% 34% 23% 10% 2	330
December	1st to 10th	Day	32% 16% 30% 20% 2	330
	11th to 20th	Day	27% 12% 31% 25% 5% 1	330
	21st to end	Day	33% 15% 28% 16% 6% 2	363
January	1st to 10th	Night	26% 16% 29% 23% 6%	840
	11th to 20th	Night	22% 15% 32% 25% 6%	840
	21st to end	Night	22% 16% 34% 25% 3	924
February	1st to 10th	Night	20% 14% 31% 30% 5%	840
	11th to 20th	Night	25% 16% 33% 23% 4	823
	21st to end	Night	24% 14% 33% 26% 3	692
March	1st to 10th	Night	20% 13% 30% 31% 5%	840
	11th to 20th	Night	18% 11% 34% 28% 8%	840
	21st to end	Night	18% 13% 31% 29% 9%	924
April	1st to 10th	Night	15% 13% 30% 31% 10%	840
	11th to 20th	Night	14% 10% 27% 34% 16%	650
	21st to end	Night	13% 7% 25% 34% 21% 1	630
May	1st to 10th	Night	13% 8% 25% 32% 21% 1	630
	11th to 20th	Night	14% 7% 21% 31% 23% 4	621
	21st to end	Night	16% 3 14% 32% 30% 5%	683
June	1st to 10th	Night	10% 2 15% 37% 31% 5%	630
	11th to 20th	Night	7% 2 16% 31% 33% 12%	630
	21st to end	Night	9% 1 11% 36% 35% 9%	630
July	1st to 10th	Night	9% 2 9% 26% 42% 13%	615
	11th to 20th	Night	10% 2 8% 20% 47% 13%	597
	21st to end	Night	7% 2 10% 27% 44% 10%	657
August	1st to 10th	Night	16% 1 8% 26% 37% 11%	597
	11th to 20th	Night	13% 3 8% 25% 40% 11%	594
	21st to end	Night	15% 1 11% 25% 37% 11%	670
September	1st to 10th	Night	11% 3 12% 29% 37% 9%	800
	11th to 20th	Night	18% 3 12% 27% 36% 5%	800
	21st to end	Night	16% 3 15% 31% 32% 3	800
October	1st to 10th	Night	14% 5% 18% 31% 27% 6%	800
	11th to 20th	Night	15% 6% 22% 33% 22% 3	800
	21st to end	Night	14% 8% 23% 32% 22% 2	880
November	1st to 10th	Night	16% 7% 22% 34% 19% 1	820
	11th to 20th	Night	18% 9% 27% 31% 14% 1	840
	21st to end	Night	18% 9% 27% 30% 14% 1	840
December	1st to 10th	Night	22% 13% 28% 30% 7% 1	840
	11th to 20th	Night	21% 12% 29% 29% 10%	840
	21st to end	Night	28% 12% 25% 29% 6%	924

Page generated at 2026-07-13 03:14:51. Version ac07

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