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 2025-05-26.

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% 7% 34%	115085
	Night	14% 10% 4 8% 5% 4 4 3 5% 7% 35%	114226
Transparency	Day	47% 6% 11% 22% 13% 2	118771
	Night	47% 5% 10% 22% 14% 2	118679
Seeing	Day	17% 9% 26% 24% 16% 7%	15955
	Night	17% 8% 27% 30% 15% 2	25561

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% 16% 9% 15% 6% 5% 4 3 4 4 20%	4056
	11th to 20th	Day	12% 14% 6% 16% 7% 8% 5% 3 3 4 22%	4060
	21st to end	Day	21% 12% 8% 10% 6% 6% 5% 4 4 5% 19%	4466
February	1st to 10th	Day	19% 14% 6% 11% 6% 5% 5% 3 5% 5% 23%	1849
	11th to 20th	Day	15% 13% 5% 11% 8% 5% 4 4 4 6% 25%	1948
	21st to end	Day	17% 14% 5% 10% 5% 4 4 3 4 4 29%	1650
March	1st to 10th	Day	16% 12% 7% 11% 6% 5% 4 3 4 5% 28%	4604
	11th to 20th	Day	12% 8% 3 7% 5% 4 4 3 6% 7% 39%	4980
	21st to end	Day	16% 10% 4 8% 4 5% 4 4 5% 7% 32%	5465
April	1st to 10th	Day	14% 11% 4 8% 4 4 4 4 6% 7% 33%	2310
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 4 5% 6% 37%	2472
	21st to end	Day	17% 8% 3 7% 6% 5% 5% 4 5% 7% 33%	2600
May	1st to 10th	Day	20% 10% 3 7% 4 5% 4 3 5% 8% 29%	4212
	11th to 20th	Day	20% 8% 3 7% 4 4 4 3 6% 8% 32%	4211
	21st to end	Day	9% 7% 3 8% 5% 5% 6% 5% 8% 10% 34%	4541
June	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 7% 10% 37%	2470
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 4 6% 10% 41%	2586
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 10% 39%	2660
July	1st to 10th	Day	10% 7% 3 7% 6% 5% 5% 4 7% 10% 37%	4440
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 7% 11% 44%	4162
	21st to end	Day	10% 7% 2 7% 6% 5% 6% 5% 8% 10% 34%	4466
August	1st to 10th	Day	8% 6% 2 6% 4 4 6% 5% 7% 10% 41%	4104
	11th to 20th	Day	8% 6% 2 7% 5% 5% 6% 5% 8% 11% 39%	4044
	21st to end	Day	6% 4 2 5% 4 5% 4 4 6% 9% 52%	4318
September	1st to 10th	Day	11% 5% 1 5% 4 4 5% 4 6% 6% 49%	2133
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 5% 8% 49%	2090
	21st to end	Day	15% 7% 2 6% 5% 4 3 3 5% 7% 45%	2090
October	1st to 10th	Day	10% 5% 3 6% 5% 5% 4 3 6% 7% 47%	2081
	11th to 20th	Day	13% 8% 2 6% 4 3 4 3 5% 6% 45%	1900
	21st to end	Day	18% 10% 3 7% 3 4 3 3 5% 7% 36%	1986
November	1st to 10th	Day	9% 7% 2 6% 4 3 4 2 5% 7% 49%	1762
	11th to 20th	Day	14% 11% 5% 8% 5% 4 4 4 4 6% 34%	1800
	21st to end	Day	18% 11% 5% 9% 6% 4 4 3 4 6% 32%	1800
December	1st to 10th	Day	16% 10% 4 9% 6% 4 4 4 4 6% 33%	2930
	11th to 20th	Day	20% 15% 5% 11% 7% 4 4 3 4 4 23%	3373
	21st to end	Day	20% 18% 7% 12% 6% 5% 4 3 4 5% 16%	4466
January	1st to 10th	Night	13% 15% 7% 15% 9% 5% 5% 3 4 5% 18%	5889
	11th to 20th	Night	12% 15% 8% 14% 7% 7% 5% 3 5% 5% 20%	5900
	21st to end	Night	19% 13% 5% 9% 5% 6% 5% 3 3 5% 28%	6490
February	1st to 10th	Night	19% 13% 4 10% 5% 4 5% 3 4 5% 29%	2550
	11th to 20th	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 30%	2338
	21st to end	Night	19% 13% 5% 9% 4 4 4 3 4 4 30%	1980
March	1st to 10th	Night	18% 12% 4 7% 5% 4 4 3 4 5% 36%	5247
	11th to 20th	Night	11% 6% 3 6% 4 5% 4 4 6% 8% 42%	4980
	21st to end	Night	17% 10% 4 8% 5% 4 4 3 5% 6% 34%	5467
April	1st to 10th	Night	15% 8% 3 7% 4 4 5% 4 6% 7% 37%	2108
	11th to 20th	Night	13% 8% 3 6% 4 4 5% 3 6% 8% 40%	1877
	21st to end	Night	15% 8% 3 7% 4 4 4 4 6% 8% 37%	1800
May	1st to 10th	Night	22% 11% 3 8% 6% 4 4 4 5% 6% 27%	2903
	11th to 20th	Night	18% 7% 2 6% 3 4 4 4 6% 9% 37%	2902
	21st to end	Night	8% 5% 2 6% 4 5% 5% 5% 7% 9% 44%	3130
June	1st to 10th	Night	8% 6% 3 5% 4 4 5% 4 5% 9% 48%	1710
	11th to 20th	Night	7% 4 2 5% 5% 3 4 3 8% 11% 47%	1587
	21st to end	Night	5% 4 2 5% 5% 5% 5% 5% 8% 11% 45%	1520
July	1st to 10th	Night	10% 5% 3 6% 4 4 4 3 6% 7% 48%	2530
	11th to 20th	Night	6% 5% 2 5% 4 4 5% 4 6% 10% 49%	2795
	21st to end	Night	10% 6% 2 7% 6% 5% 5% 5% 7% 9% 39%	3087
August	1st to 10th	Night	6% 7% 3 8% 5% 4 5% 5% 6% 9% 41%	2825
	11th to 20th	Night	7% 8% 3 8% 5% 4 4 4 6% 9% 44%	2785
	21st to end	Night	4 2 2 4 4 5% 5% 4 6% 10% 54%	3334
September	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 4 7% 51%	2027
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 5% 7% 54%	2090
	21st to end	Night	13% 6% 2 6% 4 3 3 3 5% 7% 48%	2090
October	1st to 10th	Night	8% 6% 2 6% 4 3 4 3 6% 7% 52%	2097
	11th to 20th	Night	14% 8% 3 5% 4 4 4 3 5% 6% 45%	2280
	21st to end	Night	17% 9% 3 7% 4 4 3 3 5% 6% 39%	2610
November	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 4 7% 46%	2536
	11th to 20th	Night	15% 8% 3 7% 5% 3 4 4 6% 7% 37%	2600
	21st to end	Night	17% 13% 5% 7% 6% 4 5% 3 5% 6% 29%	2600
December	1st to 10th	Night	15% 11% 4 9% 5% 5% 4 3 4 5% 35%	4250
	11th to 20th	Night	18% 14% 6% 11% 5% 4 4 3 4 5% 24%	4822
	21st to end	Night	20% 19% 7% 11% 6% 5% 4 3 4 5% 16%	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	67% 4 8% 16% 6%	4057
	11th to 20th	Day	67% 2 10% 10% 9% 1	4060
	21st to end	Day	67% 3 11% 10% 7% 1	4466
February	1st to 10th	Day	62% 4 10% 14% 9% 1	1849
	11th to 20th	Day	61% 4 10% 15% 9% 1	1958
	21st to end	Day	58% 5% 11% 18% 8% 1	1650
March	1st to 10th	Day	58% 5% 8% 19% 9% 1	4604
	11th to 20th	Day	42% 5% 10% 28% 14% 1	4980
	21st to end	Day	50% 5% 12% 19% 11% 2	5465
April	1st to 10th	Day	49% 5% 12% 22% 11% 1	2310
	11th to 20th	Day	47% 4 11% 22% 13% 3	2472
	21st to end	Day	48% 7% 10% 21% 12% 2	2600
May	1st to 10th	Day	52% 6% 12% 22% 7% 1	4250
	11th to 20th	Day	48% 7% 13% 23% 9% 1	4215
	21st to end	Day	39% 10% 15% 20% 14% 2	4554
June	1st to 10th	Day	41% 7% 11% 24% 15% 2	2470
	11th to 20th	Day	37% 8% 15% 23% 16% 1	2586
	21st to end	Day	37% 9% 15% 21% 14% 3	2660
July	1st to 10th	Day	42% 6% 12% 22% 17% 2	4371
	11th to 20th	Day	32% 7% 12% 31% 18% 1	4091
	21st to end	Day	42% 9% 14% 18% 14% 4	4324
August	1st to 10th	Day	35% 8% 17% 29% 11% 1	4091
	11th to 20th	Day	35% 7% 12% 29% 16% 1	4083
	21st to end	Day	29% 6% 14% 29% 18% 3	4317
September	1st to 10th	Day	34% 6% 10% 24% 21% 5%	2133
	11th to 20th	Day	32% 6% 11% 25% 22% 3	2090
	21st to end	Day	40% 5% 10% 22% 22% 1	2090
October	1st to 10th	Day	24% 5% 10% 30% 30% 1	3458
	11th to 20th	Day	31% 4 10% 29% 24% 2	3140
	21st to end	Day	38% 3 12% 24% 21% 3	3249
November	1st to 10th	Day	36% 3 11% 28% 19% 3	1762
	11th to 20th	Day	51% 4 10% 19% 14% 2	1800
	21st to end	Day	54% 4 10% 20% 11% 1	1800
December	1st to 10th	Day	52% 4 12% 24% 8%	2930
	11th to 20th	Day	65% 4 9% 16% 6%	3370
	21st to end	Day	71% 4 10% 10% 5%	4466
January	1st to 10th	Night	67% 4 9% 13% 7%	5889
	11th to 20th	Night	66% 3 9% 11% 8% 2	5900
	21st to end	Night	59% 3 11% 18% 8% 2	6490
February	1st to 10th	Night	59% 4 9% 18% 8% 1	2550
	11th to 20th	Night	55% 4 10% 17% 12% 1	2339
	21st to end	Night	58% 4 10% 16% 10% 2	1980
March	1st to 10th	Night	50% 4 7% 26% 11% 2	5247
	11th to 20th	Night	38% 6% 12% 30% 13% 1	4980
	21st to end	Night	51% 4 8% 24% 12% 1	5467
April	1st to 10th	Night	44% 6% 12% 22% 13% 2	2108
	11th to 20th	Night	42% 5% 13% 18% 18% 4	1877
	21st to end	Night	44% 5% 11% 24% 14% 2	1800
May	1st to 10th	Night	56% 6% 9% 20% 7% 2	2930
	11th to 20th	Night	44% 7% 13% 26% 9% 1	2903
	21st to end	Night	31% 10% 14% 28% 13% 5%	3138
June	1st to 10th	Night	33% 8% 11% 26% 21% 2	1710
	11th to 20th	Night	30% 9% 16% 23% 19% 4	1587
	21st to end	Night	29% 10% 15% 24% 17% 4	1520
July	1st to 10th	Night	35% 6% 12% 24% 18% 6%	2490
	11th to 20th	Night	29% 7% 12% 32% 18% 2	2743
	21st to end	Night	39% 6% 13% 23% 15% 4	2988
August	1st to 10th	Night	38% 7% 14% 27% 12% 1	2814
	11th to 20th	Night	36% 7% 12% 26% 19%	2812
	21st to end	Night	24% 7% 16% 29% 21% 3	3334
September	1st to 10th	Night	33% 6% 12% 24% 19% 6%	2027
	11th to 20th	Night	28% 6% 9% 26% 27% 3	2090
	21st to end	Night	36% 5% 10% 21% 26% 2	2090
October	1st to 10th	Night	24% 4 7% 28% 35% 3	3500
	11th to 20th	Night	30% 4 9% 27% 28% 3	3779
	21st to end	Night	34% 3 7% 24% 26% 5%	4297
November	1st to 10th	Night	38% 4 11% 22% 21% 3	2536
	11th to 20th	Night	45% 5% 9% 21% 17% 4	2600
	21st to end	Night	55% 5% 10% 19% 10% 1	2600
December	1st to 10th	Night	51% 4 11% 24% 9%	4250
	11th to 20th	Night	61% 4 11% 17% 7%	4824
	21st to end	Night	71% 4 8% 14% 3	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	23% 16% 37% 18% 5%	300
	11th to 20th	Day	21% 23% 39% 13% 4 1	300
	21st to end	Day	28% 15% 35% 17% 5%	330
February	1st to 10th	Day	27% 15% 36% 18% 4	300
	11th to 20th	Day	28% 20% 32% 16% 4	300
	21st to end	Day	25% 13% 41% 18% 3	249
March	1st to 10th	Day	21% 19% 36% 20% 4	346
	11th to 20th	Day	22% 16% 35% 19% 7% 1	500
	21st to end	Day	28% 15% 34% 19% 5%	550
April	1st to 10th	Day	21% 14% 33% 23% 8% 2	500
	11th to 20th	Day	17% 19% 28% 23% 10% 3	492
	21st to end	Day	21% 11% 35% 20% 10% 3	500
May	1st to 10th	Day	23% 12% 30% 21% 9% 4	503
	11th to 20th	Day	18% 9% 26% 27% 13% 8%	527
	21st to end	Day	11% 6% 25% 29% 18% 12%	549
June	1st to 10th	Day	13% 4 22% 28% 24% 9%	490
	11th to 20th	Day	12% 4 21% 31% 24% 9%	606
	21st to end	Day	11% 4 26% 33% 19% 9%	680
July	1st to 10th	Day	12% 3 17% 26% 23% 20%	671
	11th to 20th	Day	9% 3 21% 24% 27% 16%	511
	21st to end	Day	10% 2 15% 29% 28% 15%	532
August	1st to 10th	Day	10% 3 11% 28% 27% 21%	486
	11th to 20th	Day	12% 3 15% 30% 27% 14%	479
	21st to end	Day	8% 3 12% 26% 33% 18%	539
September	1st to 10th	Day	13% 3 13% 24% 32% 15%	490
	11th to 20th	Day	9% 2 12% 27% 32% 17%	490
	21st to end	Day	18% 9% 18% 24% 23% 8%	490
October	1st to 10th	Day	9% 4 23% 34% 20% 9%	490
	11th to 20th	Day	14% 12% 31% 23% 15% 5%	490
	21st to end	Day	22% 10% 35% 25% 8% 1	435
November	1st to 10th	Day	14% 10% 35% 25% 12% 5%	300
	11th to 20th	Day	19% 18% 30% 20% 10% 2	300
	21st to end	Day	24% 10% 39% 22% 5%	300
December	1st to 10th	Day	19% 19% 38% 16% 8% 1	300
	11th to 20th	Day	21% 14% 40% 20% 5%	300
	21st to end	Day	29% 17% 32% 16% 6%	330
January	1st to 10th	Night	20% 18% 35% 21% 6%	800
	11th to 20th	Night	23% 16% 36% 21% 3	800
	21st to end	Night	20% 14% 37% 25% 4	880
February	1st to 10th	Night	26% 11% 34% 27% 2	800
	11th to 20th	Night	24% 14% 36% 23% 3	783
	21st to end	Night	25% 11% 40% 22% 2	660
March	1st to 10th	Night	20% 13% 35% 27% 6% 1	800
	11th to 20th	Night	19% 13% 35% 26% 7%	800
	21st to end	Night	23% 11% 33% 25% 8%	880
April	1st to 10th	Night	18% 13% 34% 29% 6% 1	708
	11th to 20th	Night	17% 10% 34% 29% 10% 1	593
	21st to end	Night	17% 7% 38% 29% 8% 1	600
May	1st to 10th	Night	21% 8% 31% 28% 11% 1	600
	11th to 20th	Night	18% 5% 25% 34% 16% 2	591
	21st to end	Night	8% 3 22% 36% 26% 5%	634
June	1st to 10th	Night	8% 3 21% 43% 23% 2	570
	11th to 20th	Night	8% 3 17% 45% 25% 2	570
	21st to end	Night	6% 3 22% 40% 26% 3	570
July	1st to 10th	Night	10% 2 11% 36% 32% 8%	555
	11th to 20th	Night	6% 3 14% 38% 34% 6%	552
	21st to end	Night	11% 2 11% 40% 33% 3	624
August	1st to 10th	Night	7% 1 11% 40% 36% 5%	567
	11th to 20th	Night	9% 2 9% 45% 31% 4	564
	21st to end	Night	8% 1 12% 34% 37% 8%	627
September	1st to 10th	Night	13% 3 14% 31% 33% 5%	697
	11th to 20th	Night	8% 3 11% 37% 36% 5%	760
	21st to end	Night	16% 4 19% 36% 23% 2	760
October	1st to 10th	Night	11% 4 23% 34% 23% 4	760
	11th to 20th	Night	18% 9% 27% 32% 14% 1	760
	21st to end	Night	21% 9% 31% 27% 11%	836
November	1st to 10th	Night	17% 7% 30% 29% 16% 1	780
	11th to 20th	Night	19% 14% 32% 26% 9% 1	800
	21st to end	Night	22% 12% 30% 30% 6% 1	800
December	1st to 10th	Night	25% 11% 36% 24% 5% 1	800
	11th to 20th	Night	24% 11% 34% 25% 6% 1	800
	21st to end	Night	29% 13% 29% 27% 3	880

Page generated at 2025-05-23 09:21:02. Version ac07

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