Jackson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jackson 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	11% 8% 3 8% 5% 5% 5% 4 6% 7% 39%	117377
	Night	9% 8% 3 8% 5% 5% 5% 4 6% 7% 39%	120672
Transparency	Day	43% 5% 15% 26% 11% 1	117110
	Night	43% 5% 14% 24% 12% 1	120529
Seeing	Day	15% 8% 30% 31% 14% 3	16776
	Night	14% 7% 32% 36% 11% 1	26798

By Month

Percentage Cloud Cover SW (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	6% 11% 5% 14% 12% 10% 10% 6% 6% 6% 14%	3290
	11th to 20th	Day	5% 8% 3 11% 8% 8% 13% 8% 10% 9% 18%	3640
	21st to end	Day	4 9% 7% 18% 9% 6% 4 3 5% 6% 29%	4065
February	1st to 10th	Day	10% 12% 5% 11% 7% 6% 6% 5% 6% 6% 27%	1800
	11th to 20th	Day	6% 10% 4 11% 7% 7% 7% 6% 7% 7% 28%	1782
	21st to end	Day	8% 11% 5% 13% 8% 6% 5% 3 5% 9% 27%	1650
March	1st to 10th	Day	8% 9% 3 8% 5% 4 4 4 4 7% 43%	4602
	11th to 20th	Day	8% 10% 3 8% 5% 5% 5% 4 6% 8% 37%	4980
	21st to end	Day	18% 13% 5% 12% 7% 4 4 3 5% 7% 22%	5464
April	1st to 10th	Day	14% 9% 2 7% 6% 4 4 4 5% 7% 39%	2359
	11th to 20th	Day	17% 11% 4 7% 4 4 4 3 6% 7% 33%	2569
	21st to end	Day	20% 10% 3 7% 4 4 4 4 6% 5% 33%	2600
May	1st to 10th	Day	15% 8% 3 6% 4 4 4 3 5% 6% 41%	4250
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 4 6% 6% 43%	4309
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 9% 37%	5218
June	1st to 10th	Day	12% 6% 2 5% 5% 4 5% 4 7% 8% 43%	2850
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 7% 8% 42%	2850
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 7% 8% 46%	2850
July	1st to 10th	Day	9% 4 2 6% 4 4 5% 5% 8% 9% 45%	4750
	11th to 20th	Day	4 3 1 3 3 3 4 4 6% 9% 61%	4592
	21st to end	Day	5% 4 2 5% 5% 5% 5% 5% 7% 9% 48%	4782
August	1st to 10th	Day	10% 5% 2 6% 3 4 4 4 6% 8% 48%	4103
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 6% 8% 42%	4041
	21st to end	Day	10% 5% 2 4 3 3 4 3 6% 8% 52%	4532
September	1st to 10th	Day	9% 4 2 5% 3 4 3 4 6% 7% 53%	2253
	11th to 20th	Day	10% 5% 1 6% 4 4 4 3 5% 8% 49%	2090
	21st to end	Day	13% 6% 2 5% 3 4 3 3 4 6% 51%	2090
October	1st to 10th	Day	12% 4 2 4 3 3 3 3 5% 6% 56%	3301
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 6% 51%	3140
	21st to end	Day	13% 9% 2 6% 4 4 4 3 5% 8% 42%	3093
November	1st to 10th	Day	13% 10% 4 8% 4 3 4 3 5% 7% 38%	1763
	11th to 20th	Day	9% 11% 4 10% 6% 5% 4 4 5% 7% 35%	1612
	21st to end	Day	9% 9% 4 10% 7% 5% 5% 4 6% 6% 36%	1600
December	1st to 10th	Day	6% 12% 5% 12% 10% 7% 7% 4 6% 7% 23%	2366
	11th to 20th	Day	8% 14% 7% 13% 6% 7% 6% 4 5% 7% 24%	2643
	21st to end	Day	15% 13% 3 15% 6% 4 4 3 5% 5% 26%	3498
January	1st to 10th	Night	5% 10% 4 14% 11% 10% 8% 6% 7% 7% 17%	6672
	11th to 20th	Night	5% 9% 4 10% 8% 9% 10% 7% 9% 8% 22%	6340
	21st to end	Night	4 11% 7% 12% 5% 5% 5% 5% 6% 7% 32%	6913
February	1st to 10th	Night	8% 12% 4 10% 6% 6% 5% 5% 6% 7% 31%	2600
	11th to 20th	Night	6% 10% 3 10% 6% 5% 6% 5% 6% 7% 36%	2523
	21st to end	Night	9% 10% 3 8% 6% 6% 6% 4 5% 6% 37%	1980
March	1st to 10th	Night	8% 7% 3 7% 5% 5% 4 4 7% 8% 44%	5265
	11th to 20th	Night	9% 9% 4 9% 5% 5% 5% 5% 6% 7% 37%	4980
	21st to end	Night	17% 13% 4 9% 5% 4 4 4 5% 7% 29%	5467
April	1st to 10th	Night	12% 9% 2 7% 5% 4 5% 4 5% 6% 41%	2039
	11th to 20th	Night	17% 10% 3 7% 3 3 3 3 5% 7% 39%	1779
	21st to end	Night	18% 9% 3 7% 5% 4 4 3 5% 6% 37%	1800
May	1st to 10th	Night	14% 6% 2 6% 4 5% 4 3 4 7% 46%	2940
	11th to 20th	Night	9% 5% 2 7% 4 4 5% 5% 6% 7% 45%	2815
	21st to end	Night	13% 5% 2 6% 5% 4 4 4 6% 8% 42%	2470
June	1st to 10th	Night	9% 5% 2 5% 3 4 5% 3 6% 10% 50%	1330
	11th to 20th	Night	8% 5% 2 5% 3 4 4 4 7% 9% 49%	1330
	21st to end	Night	7% 3 1 4 3 3 4 3 5% 8% 58%	1330
July	1st to 10th	Night	6% 4 3 5% 4 5% 5% 5% 6% 10% 47%	2220
	11th to 20th	Night	3 1 1 3 2 2 3 3 5% 7% 69%	2374
	21st to end	Night	3 3 1 5% 4 4 6% 4 8% 10% 51%	2781
August	1st to 10th	Night	6% 3 2 5% 4 5% 4 4 6% 7% 53%	2836
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 5% 8% 49%	2798
	21st to end	Night	9% 5% 2 5% 3 3 4 3 6% 7% 53%	3135
September	1st to 10th	Night	8% 4 2 4 3 4 3 3 4 6% 60%	1922
	11th to 20th	Night	8% 6% 2 4 3 3 4 3 5% 6% 57%	2090
	21st to end	Night	13% 4 2 4 3 2 3 3 4 6% 56%	2090
October	1st to 10th	Night	8% 4 1 4 3 3 3 3 4 5% 62%	3626
	11th to 20th	Night	12% 6% 2 6% 3 3 3 4 6% 7% 48%	3779
	21st to end	Night	13% 7% 2 7% 4 4 4 3 4 7% 45%	4462
November	1st to 10th	Night	9% 10% 4 7% 4 5% 4 3 6% 7% 41%	2535
	11th to 20th	Night	8% 11% 4 9% 7% 4 4 4 5% 6% 39%	2786
	21st to end	Night	10% 8% 3 9% 5% 4 4 3 6% 7% 40%	2800
December	1st to 10th	Night	5% 9% 4 12% 8% 7% 5% 4 6% 8% 32%	4821
	11th to 20th	Night	7% 12% 5% 12% 8% 6% 6% 4 5% 6% 27%	5564
	21st to end	Night	14% 13% 5% 13% 7% 5% 4 4 5% 5% 25%	7480

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	68% 5% 11% 11% 4 1	3291
	11th to 20th	Day	57% 5% 16% 15% 7%	3640
	21st to end	Day	56% 5% 15% 20% 5%	4065
February	1st to 10th	Day	55% 6% 12% 18% 8% 1	1800
	11th to 20th	Day	52% 5% 13% 21% 8% 1	1791
	21st to end	Day	55% 4 15% 19% 7%	1650
March	1st to 10th	Day	41% 5% 14% 32% 8% 1	4602
	11th to 20th	Day	43% 5% 14% 28% 9% 1	4980
	21st to end	Day	62% 5% 14% 15% 4 1	5464
April	1st to 10th	Day	44% 5% 14% 25% 11% 2	2359
	11th to 20th	Day	50% 5% 12% 20% 13% 1	2572
	21st to end	Day	50% 6% 13% 23% 8%	2600
May	1st to 10th	Day	42% 5% 12% 26% 15% 1	4250
	11th to 20th	Day	39% 6% 10% 28% 14% 3	4313
	21st to end	Day	42% 6% 15% 27% 9%	5218
June	1st to 10th	Day	36% 6% 17% 28% 11% 1	2850
	11th to 20th	Day	37% 6% 16% 27% 12% 1	2850
	21st to end	Day	33% 7% 18% 30% 11% 1	2850
July	1st to 10th	Day	32% 8% 19% 29% 11% 1	4676
	11th to 20th	Day	19% 5% 16% 40% 19% 2	4511
	21st to end	Day	30% 7% 18% 31% 12% 1	4631
August	1st to 10th	Day	32% 6% 15% 31% 15% 2	4090
	11th to 20th	Day	38% 6% 16% 32% 7% 1	4080
	21st to end	Day	29% 7% 17% 33% 12% 2	4531
September	1st to 10th	Day	29% 5% 14% 37% 15% 1	2253
	11th to 20th	Day	34% 5% 18% 31% 12% 1	2090
	21st to end	Day	35% 4 13% 31% 15% 1	2090
October	1st to 10th	Day	30% 3 9% 32% 23% 3	3301
	11th to 20th	Day	35% 4 10% 32% 18% 2	3140
	21st to end	Day	41% 4 16% 24% 15% 1	3093
November	1st to 10th	Day	45% 4 14% 24% 11% 2	1763
	11th to 20th	Day	49% 5% 16% 21% 9% 1	1612
	21st to end	Day	47% 5% 17% 23% 7% 2	1600
December	1st to 10th	Day	59% 6% 14% 17% 4	2366
	11th to 20th	Day	60% 6% 15% 17% 3	2640
	21st to end	Day	60% 3 12% 19% 6%	3498
January	1st to 10th	Night	62% 5% 12% 16% 5%	6672
	11th to 20th	Night	53% 6% 16% 15% 9% 1	6340
	21st to end	Night	50% 5% 18% 20% 7%	6913
February	1st to 10th	Night	51% 5% 14% 19% 11% 1	2600
	11th to 20th	Night	45% 5% 13% 23% 13% 1	2524
	21st to end	Night	47% 5% 14% 21% 12% 1	1980
March	1st to 10th	Night	36% 4 16% 31% 11% 2	5265
	11th to 20th	Night	43% 7% 14% 26% 11% 1	4980
	21st to end	Night	54% 5% 14% 20% 7%	5467
April	1st to 10th	Night	43% 4 12% 26% 14% 1	2039
	11th to 20th	Night	45% 4 8% 24% 17% 2	1779
	21st to end	Night	48% 5% 12% 24% 10% 1	1800
May	1st to 10th	Night	37% 6% 11% 27% 16% 3	2940
	11th to 20th	Night	34% 7% 12% 29% 17% 2	2816
	21st to end	Night	37% 7% 14% 31% 11%	2470
June	1st to 10th	Night	31% 4 18% 31% 13% 3	1330
	11th to 20th	Night	30% 7% 18% 26% 16% 4	1330
	21st to end	Night	24% 6% 20% 32% 15% 2	1330
July	1st to 10th	Night	31% 9% 19% 26% 14% 1	2185
	11th to 20th	Night	15% 5% 14% 45% 19% 3	2332
	21st to end	Night	26% 8% 20% 31% 13% 3	2694
August	1st to 10th	Night	29% 5% 16% 30% 17% 2	2826
	11th to 20th	Night	32% 7% 17% 30% 12% 1	2825
	21st to end	Night	29% 6% 16% 31% 17% 2	3135
September	1st to 10th	Night	26% 4 12% 36% 20% 1	1922
	11th to 20th	Night	29% 4 15% 33% 18% 1	2090
	21st to end	Night	29% 4 12% 32% 20% 3	2090
October	1st to 10th	Night	25% 3 8% 30% 30% 4	3626
	11th to 20th	Night	34% 4 10% 31% 19% 2	3779
	21st to end	Night	40% 4 12% 28% 15% 1	4462
November	1st to 10th	Night	41% 5% 12% 23% 16% 3	2535
	11th to 20th	Night	46% 5% 14% 22% 12% 2	2786
	21st to end	Night	42% 5% 19% 24% 8% 1	2800
December	1st to 10th	Night	50% 5% 21% 17% 7%	4821
	11th to 20th	Night	56% 6% 15% 18% 4	5566
	21st to end	Night	60% 4 13% 16% 6%	7480

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	11% 8% 29% 40% 10% 2	300
	11th to 20th	Day	14% 12% 32% 33% 8% 1	300
	21st to end	Day	17% 10% 31% 32% 8% 1	330
February	1st to 10th	Day	14% 12% 39% 30% 5%	300
	11th to 20th	Day	12% 9% 42% 27% 9% 1	300
	21st to end	Day	14% 7% 42% 33% 5%	249
March	1st to 10th	Day	18% 10% 38% 26% 7% 1	300
	11th to 20th	Day	15% 11% 32% 26% 13% 3	300
	21st to end	Day	18% 6% 32% 27% 13% 4	330
April	1st to 10th	Day	20% 13% 34% 25% 7% 1	459
	11th to 20th	Day	25% 14% 29% 22% 9% 1	690
	21st to end	Day	24% 10% 29% 25% 11% 2	700
May	1st to 10th	Day	18% 8% 30% 30% 14%	703
	11th to 20th	Day	21% 7% 26% 30% 14% 2	725
	21st to end	Day	18% 8% 25% 31% 15% 3	760
June	1st to 10th	Day	10% 8% 27% 28% 20% 7%	680
	11th to 20th	Day	11% 9% 37% 28% 13% 2	680
	21st to end	Day	12% 7% 28% 36% 15% 3	680
July	1st to 10th	Day	8% 2 24% 36% 25% 6%	671
	11th to 20th	Day	7% 5% 30% 35% 19% 4	668
	21st to end	Day	5% 4 23% 39% 24% 6%	739
August	1st to 10th	Day	9% 4 20% 39% 23% 4	676
	11th to 20th	Day	13% 3 25% 34% 19% 6%	665
	21st to end	Day	9% 6% 25% 34% 23% 2	748
September	1st to 10th	Day	11% 7% 26% 35% 18% 3	463
	11th to 20th	Day	13% 6% 33% 33% 14% 2	300
	21st to end	Day	18% 12% 28% 26% 12% 4	300
October	1st to 10th	Day	18% 6% 31% 27% 16% 3	300
	11th to 20th	Day	16% 18% 26% 32% 9%	300
	21st to end	Day	19% 17% 35% 20% 7% 2	330
November	1st to 10th	Day	21% 8% 32% 31% 7% 1	300
	11th to 20th	Day	16% 10% 39% 26% 8% 1	300
	21st to end	Day	15% 14% 39% 24% 7% 1	300
December	1st to 10th	Day	15% 7% 37% 30% 10% 1	300
	11th to 20th	Day	17% 7% 44% 29% 3	300
	21st to end	Day	21% 9% 36% 24% 7% 2	330
January	1st to 10th	Night	11% 8% 40% 31% 9% 1	1000
	11th to 20th	Night	14% 8% 37% 33% 8%	1000
	21st to end	Night	14% 8% 38% 33% 7% 1	1100
February	1st to 10th	Night	13% 9% 35% 34% 8% 1	1000
	11th to 20th	Night	11% 7% 41% 30% 9% 2	969
	21st to end	Night	14% 8% 33% 31% 12% 2	660
March	1st to 10th	Night	15% 6% 37% 32% 10% 1	756
	11th to 20th	Night	17% 8% 35% 32% 9%	600
	21st to end	Night	17% 7% 33% 30% 12% 1	660
April	1st to 10th	Night	16% 10% 37% 30% 7%	600
	11th to 20th	Night	22% 10% 28% 31% 9%	593
	21st to end	Night	22% 6% 28% 34% 10%	600
May	1st to 10th	Night	16% 5% 33% 36% 10% 1	600
	11th to 20th	Night	13% 5% 31% 39% 12%	591
	21st to end	Night	15% 6% 29% 37% 11% 1	634
June	1st to 10th	Night	11% 5% 27% 41% 14% 2	570
	11th to 20th	Night	11% 5% 34% 41% 9%	570
	21st to end	Night	8% 4 26% 46% 16% 1	570
July	1st to 10th	Night	6% 2 23% 45% 22% 2	555
	11th to 20th	Night	4 2 25% 52% 16% 1	552
	21st to end	Night	5% 2 21% 51% 20% 1	624
August	1st to 10th	Night	7% 2 18% 56% 16% 1	567
	11th to 20th	Night	9% 3 23% 43% 19% 3	564
	21st to end	Night	6% 2 27% 46% 18% 1	627
September	1st to 10th	Night	9% 4 26% 42% 17% 2	570
	11th to 20th	Night	12% 4 26% 45% 14% 1	570
	21st to end	Night	15% 6% 32% 33% 14%	570
October	1st to 10th	Night	16% 10% 32% 32% 10%	651
	11th to 20th	Night	18% 8% 35% 30% 8%	760
	21st to end	Night	18% 10% 34% 31% 7%	1040
November	1st to 10th	Night	15% 10% 31% 35% 9%	975
	11th to 20th	Night	13% 9% 38% 29% 10% 1	1000
	21st to end	Night	15% 10% 37% 33% 6%	1000
December	1st to 10th	Night	14% 8% 36% 34% 9%	1000
	11th to 20th	Night	18% 9% 33% 33% 6% 1	1000
	21st to end	Night	21% 8% 35% 28% 7% 1	1100

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