Jefferson College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jefferson College Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	12% 7% 3 6% 4 4 4 4 5% 7% 43%	131184
	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	130553
Transparency	Day	39% 4 11% 25% 17% 3	134778
	Night	40% 4 11% 24% 18% 3	135038
Seeing	Day	17% 11% 26% 26% 16% 5%	17378
	Night	17% 11% 28% 29% 13% 1	27338

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	12% 10% 6% 11% 5% 4 4 3 5% 6% 35%	4691
	11th to 20th	Day	7% 9% 3 7% 5% 4 4 3 5% 6% 47%	5172
	21st to end	Day	16% 11% 4 7% 4 4 3 3 4 5% 39%	5812
February	1st to 10th	Day	16% 11% 3 7% 4 4 4 4 5% 6% 36%	1890
	11th to 20th	Day	16% 8% 3 9% 4 4 4 3 5% 6% 38%	2031
	21st to end	Day	14% 9% 3 6% 4 4 4 4 6% 6% 40%	1730
March	1st to 10th	Day	15% 12% 2 5% 3 3 3 2 3 4 48%	4674
	11th to 20th	Day	10% 7% 2 4 3 3 4 3 5% 7% 53%	5057
	21st to end	Day	13% 8% 3 7% 4 5% 5% 4 6% 8% 38%	5737
April	1st to 10th	Day	13% 5% 2 7% 4 4 4 4 6% 8% 43%	2400
	11th to 20th	Day	17% 8% 3 6% 4 3 4 3 5% 7% 41%	2374
	21st to end	Day	18% 7% 3 6% 5% 4 4 4 5% 6% 37%	2400
May	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 4 7% 9% 31%	5487
	11th to 20th	Day	18% 7% 3 7% 4 4 5% 4 6% 8% 36%	6227
	21st to end	Day	14% 6% 2 7% 5% 5% 4 4 5% 7% 41%	6935
June	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 5% 8% 46%	2800
	11th to 20th	Day	7% 4 1 6% 4 4 4 4 7% 10% 49%	2800
	21st to end	Day	5% 3 1 5% 5% 5% 5% 5% 7% 10% 48%	2800
July	1st to 10th	Day	11% 4 2 6% 4 4 5% 4 6% 9% 47%	6360
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 12% 42%	6360
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 5% 7% 11% 41%	6497
August	1st to 10th	Day	6% 3 1 4 3 4 4 4 7% 9% 53%	4208
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 8% 45%	3849
	21st to end	Day	7% 4 2 4 3 4 4 4 5% 9% 53%	4312
September	1st to 10th	Day	13% 5% 2 4 3 4 5% 3 5% 7% 49%	2400
	11th to 20th	Day	10% 5% 2 6% 3 4 4 3 5% 7% 51%	2380
	21st to end	Day	9% 5% 2 4 3 5% 3 3 5% 7% 53%	2188
October	1st to 10th	Day	9% 4 1 5% 3 3 4 3 4 6% 57%	2000
	11th to 20th	Day	13% 5% 2 5% 3 3 3 2 4 6% 54%	2000
	21st to end	Day	17% 7% 2 5% 3 4 3 3 4 5% 47%	2200
November	1st to 10th	Day	12% 5% 1 5% 4 4 3 3 4 5% 54%	1852
	11th to 20th	Day	14% 7% 1 6% 4 3 3 2 5% 7% 47%	1890
	21st to end	Day	16% 10% 3 7% 3 3 3 3 4 5% 42%	1823
December	1st to 10th	Day	10% 10% 2 6% 4 3 3 2 4 6% 50%	2690
	11th to 20th	Day	13% 10% 5% 7% 4 4 4 3 4 5% 41%	3088
	21st to end	Day	22% 16% 5% 8% 5% 4 4 3 3 4 26%	4070
January	1st to 10th	Night	14% 10% 5% 10% 5% 4 4 3 4 5% 37%	8232
	11th to 20th	Night	9% 8% 4 10% 5% 5% 5% 3 4 4 42%	7800
	21st to end	Night	16% 11% 3 8% 5% 4 3 3 4 5% 37%	8402
February	1st to 10th	Night	17% 11% 4 7% 5% 4 3 3 5% 5% 37%	2730
	11th to 20th	Night	15% 9% 3 8% 4 4 3 3 4 6% 41%	2474
	21st to end	Night	15% 8% 3 8% 5% 4 3 3 5% 5% 42%	2076
March	1st to 10th	Night	19% 11% 2 5% 4 4 4 2 4 5% 42%	5428
	11th to 20th	Night	11% 8% 2 5% 3 3 4 3 5% 5% 50%	5057
	21st to end	Night	17% 8% 3 7% 5% 5% 5% 4 5% 6% 35%	5192
April	1st to 10th	Night	12% 6% 2 6% 5% 3 4 3 4 7% 48%	2000
	11th to 20th	Night	14% 8% 2 6% 4 3 4 2 5% 6% 45%	1975
	21st to end	Night	20% 7% 2 6% 3 4 4 2 5% 6% 40%	2000
May	1st to 10th	Night	18% 7% 3 7% 5% 5% 5% 3 6% 8% 34%	4405
	11th to 20th	Night	14% 7% 2 6% 4 4 5% 4 6% 7% 43%	3668
	21st to end	Night	15% 7% 3 6% 5% 4 4 3 6% 8% 39%	3942
June	1st to 10th	Night	9% 4 2 6% 4 4 4 4 7% 9% 47%	1600
	11th to 20th	Night	5% 5% 2 4 4 3 4 4 7% 9% 55%	1600
	21st to end	Night	4 3 1 4 4 5% 5% 4 7% 10% 55%	1600
July	1st to 10th	Night	11% 4 2 6% 4 3 4 3 6% 9% 49%	3618
	11th to 20th	Night	7% 4 2 6% 4 4 4 4 7% 8% 50%	3617
	21st to end	Night	6% 4 2 6% 5% 4 5% 4 8% 11% 44%	4356
August	1st to 10th	Night	5% 3 1 4 3 3 4 4 5% 9% 59%	2948
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 4 5% 8% 49%	3200
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 9% 58%	3586
September	1st to 10th	Night	12% 5% 2 4 4 4 4 3 5% 6% 52%	2000
	11th to 20th	Night	9% 5% 2 4 4 4 4 4 5% 7% 54%	2000
	21st to end	Night	10% 4 1 4 4 3 4 3 5% 6% 55%	2209
October	1st to 10th	Night	7% 4 2 4 3 3 3 2 4 7% 62%	2400
	11th to 20th	Night	13% 5% 2 5% 3 2 4 2 4 6% 55%	2400
	21st to end	Night	19% 8% 2 4 2 3 3 2 4 5% 48%	2640
November	1st to 10th	Night	11% 5% 2 4 3 3 3 3 4 5% 57%	2666
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 4 5% 50%	2730
	21st to end	Night	17% 10% 3 5% 4 2 3 2 4 5% 44%	2793
December	1st to 10th	Night	13% 10% 3 7% 3 3 3 3 4 5% 46%	4720
	11th to 20th	Night	14% 9% 3 7% 4 3 4 3 4 6% 43%	5339
	21st to end	Night	23% 15% 4 8% 5% 4 4 3 4 5% 25%	7150

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	51% 3 12% 24% 9% 2	4692
	11th to 20th	Day	39% 2 8% 27% 19% 4	5172
	21st to end	Day	49% 3 10% 18% 15% 5%	5812
February	1st to 10th	Day	49% 4 12% 23% 12% 1	1890
	11th to 20th	Day	48% 4 10% 20% 15% 4	2041
	21st to end	Day	44% 4 9% 24% 15% 4	1730
March	1st to 10th	Day	42% 2 5% 22% 22% 7%	4674
	11th to 20th	Day	33% 3 8% 26% 24% 7%	5057
	21st to end	Day	44% 4 14% 27% 10% 1	5739
April	1st to 10th	Day	39% 4 11% 26% 18% 1	2400
	11th to 20th	Day	43% 3 10% 24% 16% 4	2374
	21st to end	Day	47% 5% 11% 24% 13% 1	2400
May	1st to 10th	Day	48% 6% 11% 22% 12%	5522
	11th to 20th	Day	46% 5% 10% 24% 13% 1	6231
	21st to end	Day	42% 6% 9% 22% 20% 2	6949
June	1st to 10th	Day	35% 6% 12% 27% 18% 2	2800
	11th to 20th	Day	30% 6% 13% 27% 22% 3	2800
	21st to end	Day	28% 7% 18% 26% 16% 5%	2800
July	1st to 10th	Day	34% 5% 10% 30% 18% 2	6291
	11th to 20th	Day	34% 8% 15% 29% 13% 2	6285
	21st to end	Day	35% 7% 16% 29% 11% 1	6313
August	1st to 10th	Day	26% 6% 16% 36% 16%	4194
	11th to 20th	Day	37% 5% 13% 25% 18% 2	3885
	21st to end	Day	29% 5% 13% 35% 15% 3	4311
September	1st to 10th	Day	34% 5% 10% 27% 20% 5%	2400
	11th to 20th	Day	33% 4 9% 28% 24% 2	2380
	21st to end	Day	31% 6% 7% 28% 26% 3	2188
October	1st to 10th	Day	18% 3 6% 20% 45% 9%	3260
	11th to 20th	Day	23% 2 6% 22% 36% 10%	3240
	21st to end	Day	32% 3 7% 28% 27% 3	3538
November	1st to 10th	Day	33% 3 12% 27% 20% 6%	1852
	11th to 20th	Day	39% 3 8% 24% 21% 5%	1890
	21st to end	Day	45% 3 9% 26% 16%	1823
December	1st to 10th	Day	37% 2 11% 34% 16% 1	2690
	11th to 20th	Day	46% 3 11% 24% 16% 1	3085
	21st to end	Day	63% 3 12% 13% 7% 1	4070
January	1st to 10th	Night	51% 3 13% 24% 9%	8233
	11th to 20th	Night	46% 2 6% 26% 18% 3	7800
	21st to end	Night	51% 3 10% 19% 15% 2	8402
February	1st to 10th	Night	50% 4 10% 22% 13% 1	2730
	11th to 20th	Night	46% 3 9% 22% 17% 3	2475
	21st to end	Night	45% 3 10% 23% 15% 4	2076
March	1st to 10th	Night	47% 2 6% 22% 20% 3	5428
	11th to 20th	Night	37% 3 9% 23% 21% 7%	5057
	21st to end	Night	50% 4 11% 24% 11% 1	5192
April	1st to 10th	Night	37% 3 11% 28% 18% 3	2000
	11th to 20th	Night	40% 3 10% 26% 17% 5%	1975
	21st to end	Night	45% 4 9% 24% 16% 2	2000
May	1st to 10th	Night	48% 7% 13% 19% 12% 1	4435
	11th to 20th	Night	40% 5% 14% 25% 15% 2	3669
	21st to end	Night	42% 6% 11% 23% 15% 2	3950
June	1st to 10th	Night	32% 5% 15% 27% 17% 4	1600
	11th to 20th	Night	25% 6% 12% 29% 25% 4	1600
	21st to end	Night	24% 5% 15% 29% 20% 7%	1600
July	1st to 10th	Night	34% 5% 11% 28% 20% 3	3580
	11th to 20th	Night	30% 7% 15% 32% 14% 2	3572
	21st to end	Night	32% 7% 20% 27% 13% 1	4230
August	1st to 10th	Night	22% 6% 18% 33% 19% 1	2938
	11th to 20th	Night	34% 6% 14% 26% 18% 3	3230
	21st to end	Night	24% 5% 14% 35% 19% 3	3586
September	1st to 10th	Night	33% 4 9% 26% 22% 6%	2000
	11th to 20th	Night	30% 6% 9% 26% 26% 3	2000
	21st to end	Night	31% 5% 9% 23% 28% 4	2209
October	1st to 10th	Night	15% 2 7% 16% 50% 10%	3920
	11th to 20th	Night	22% 2 7% 22% 36% 10%	3899
	21st to end	Night	31% 3 8% 26% 27% 5%	4252
November	1st to 10th	Night	31% 3 12% 26% 23% 6%	2666
	11th to 20th	Night	37% 3 8% 25% 23% 4	2730
	21st to end	Night	44% 3 10% 25% 18% 2	2793
December	1st to 10th	Night	41% 3 9% 31% 15% 1	4720
	11th to 20th	Night	44% 3 12% 22% 18% 1	5341
	21st to end	Night	62% 4 12% 15% 6% 1	7150

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% 19% 34% 16% 4 1	330
	11th to 20th	Day	27% 24% 34% 14% 1	330
	21st to end	Day	21% 26% 38% 13% 3	363
February	1st to 10th	Day	26% 20% 35% 17% 2	330
	11th to 20th	Day	28% 26% 29% 12% 4 1	330
	21st to end	Day	26% 24% 36% 13% 1	273
March	1st to 10th	Day	24% 22% 39% 12% 3	330
	11th to 20th	Day	32% 19% 33% 12% 4 1	321
	21st to end	Day	31% 18% 33% 14% 4	450
April	1st to 10th	Day	22% 17% 34% 20% 7%	500
	11th to 20th	Day	26% 16% 29% 21% 7% 1	492
	21st to end	Day	25% 12% 35% 20% 7% 1	500
May	1st to 10th	Day	21% 14% 32% 24% 8% 1	503
	11th to 20th	Day	21% 11% 27% 24% 13% 4	685
	21st to end	Day	14% 8% 31% 29% 13% 5%	789
June	1st to 10th	Day	12% 6% 23% 29% 23% 6%	730
	11th to 20th	Day	9% 3 23% 39% 19% 6%	730
	21st to end	Day	5% 6% 27% 38% 17% 6%	730
July	1st to 10th	Day	11% 4 14% 28% 30% 13%	721
	11th to 20th	Day	5% 4 19% 38% 26% 8%	718
	21st to end	Day	9% 2 16% 34% 31% 8%	794
August	1st to 10th	Day	9% 3 14% 37% 29% 8%	713
	11th to 20th	Day	11% 4 16% 29% 30% 10%	519
	21st to end	Day	9% 3 12% 31% 31% 14%	583
September	1st to 10th	Day	12% 3 16% 28% 32% 8%	530
	11th to 20th	Day	9% 3 12% 31% 32% 14%	530
	21st to end	Day	11% 8% 20% 31% 21% 9%	518
October	1st to 10th	Day	11% 8% 22% 40% 15% 4	330
	11th to 20th	Day	17% 13% 35% 24% 10% 1	330
	21st to end	Day	30% 17% 29% 19% 5%	363
November	1st to 10th	Day	17% 11% 41% 19% 8% 4	330
	11th to 20th	Day	19% 19% 35% 16% 9% 2	330
	21st to end	Day	29% 16% 29% 18% 6% 1	330
December	1st to 10th	Day	18% 20% 37% 17% 6% 2	330
	11th to 20th	Day	24% 18% 35% 19% 4 1	330
	21st to end	Day	28% 21% 32% 14% 4	363
January	1st to 10th	Night	20% 19% 33% 23% 4	1050
	11th to 20th	Night	22% 21% 35% 18% 4	1050
	21st to end	Night	19% 19% 38% 22% 2	1155
February	1st to 10th	Night	22% 18% 35% 23% 2	1050
	11th to 20th	Night	22% 20% 35% 21% 1	839
	21st to end	Night	20% 20% 36% 22% 2	692
March	1st to 10th	Night	25% 16% 35% 22% 3	814
	11th to 20th	Night	23% 14% 33% 26% 4	621
	21st to end	Night	28% 10% 32% 25% 5%	660
April	1st to 10th	Night	17% 14% 36% 27% 7%	600
	11th to 20th	Night	19% 12% 32% 27% 9% 1	593
	21st to end	Night	24% 8% 37% 25% 6%	600
May	1st to 10th	Night	17% 9% 30% 35% 9% 1	600
	11th to 20th	Night	20% 6% 24% 31% 17% 1	591
	21st to end	Night	11% 6% 26% 36% 18% 4	650
June	1st to 10th	Night	11% 4 19% 39% 26% 1	600
	11th to 20th	Night	6% 2 21% 48% 21% 2	600
	21st to end	Night	4 2 23% 44% 26% 2	600
July	1st to 10th	Night	10% 2 9% 38% 35% 6%	585
	11th to 20th	Night	4 3 13% 43% 33% 4	582
	21st to end	Night	7% 2 14% 40% 34% 4	657
August	1st to 10th	Night	8% 2 11% 40% 36% 3	597
	11th to 20th	Night	8% 1 11% 40% 36% 4	594
	21st to end	Night	6% 3 11% 34% 41% 5%	660
September	1st to 10th	Night	12% 3 13% 36% 30% 6%	600
	11th to 20th	Night	11% 4 11% 35% 34% 6%	600
	21st to end	Night	11% 5% 21% 33% 26% 5%	800
October	1st to 10th	Night	8% 5% 24% 36% 23% 4	800
	11th to 20th	Night	16% 11% 28% 29% 15% 1	800
	21st to end	Night	24% 13% 30% 27% 6%	880
November	1st to 10th	Night	13% 10% 37% 30% 10% 1	820
	11th to 20th	Night	18% 15% 32% 27% 8%	840
	21st to end	Night	24% 13% 31% 26% 5%	903
December	1st to 10th	Night	23% 13% 34% 25% 4 1	1050
	11th to 20th	Night	24% 13% 32% 27% 5%	1050
	21st to end	Night	24% 16% 34% 23% 3	1155

Page generated at 2026-03-15 01:23:52. Version ac07

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