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 2024-03-06.

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 7% 6% 3 5% 5% 6% 7% 39%	78181
	Night	10% 8% 4 7% 6% 4 5% 5% 5% 7% 39%	81388
Transparency	Day	44% 5% 15% 25% 10% 1	77874
	Night	44% 5% 14% 24% 12% 1	81217
Seeing	Day	15% 8% 30% 31% 14% 3	15037
	Night	14% 7% 32% 36% 11% 1	25182

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	9% 12% 5% 11% 10% 6% 8% 7% 6% 8% 18%	2288
	11th to 20th	Day	5% 9% 4 10% 10% 6% 10% 10% 8% 9% 19%	2540
	21st to end	Day	6% 12% 8% 17% 8% 4 4 5% 5% 6% 24%	2829
February	1st to 10th	Day	10% 10% 6% 10% 8% 4 7% 6% 6% 6% 28%	1710
	11th to 20th	Day	7% 10% 5% 10% 8% 6% 7% 7% 6% 6% 28%	1690
	21st to end	Day	8% 11% 6% 12% 9% 5% 5% 5% 5% 9% 27%	1570
March	1st to 10th	Day	12% 12% 5% 11% 9% 3 6% 4 5% 5% 28%	1887
	11th to 20th	Day	10% 10% 5% 9% 6% 4 5% 5% 6% 7% 34%	1980
	21st to end	Day	14% 12% 5% 10% 7% 4 5% 6% 5% 8% 24%	2178
April	1st to 10th	Day	14% 8% 3 6% 6% 3 4 5% 5% 7% 38%	2122
	11th to 20th	Day	19% 10% 4 6% 5% 3 4 3 5% 7% 33%	2309
	21st to end	Day	19% 10% 3 6% 5% 3 4 5% 6% 5% 34%	2340
May	1st to 10th	Day	16% 9% 4 7% 6% 4 4 5% 6% 6% 35%	2340
	11th to 20th	Day	14% 6% 3 5% 5% 3 5% 5% 7% 7% 40%	2354
	21st to end	Day	15% 7% 3 5% 5% 4 4 4 6% 9% 37%	2900
June	1st to 10th	Day	12% 6% 2 4 5% 2 5% 6% 7% 8% 42%	2700
	11th to 20th	Day	11% 5% 2 5% 5% 3 5% 5% 7% 9% 43%	2700
	21st to end	Day	8% 4 2 5% 5% 3 6% 6% 7% 8% 46%	2700
July	1st to 10th	Day	6% 3 2 4 4 3 4 5% 8% 9% 52%	2700
	11th to 20th	Day	5% 4 2 3 4 2 3 5% 6% 10% 56%	2612
	21st to end	Day	5% 4 2 4 5% 3 5% 5% 7% 9% 52%	2734
August	1st to 10th	Day	7% 5% 3 6% 5% 3 5% 5% 6% 7% 47%	2328
	11th to 20th	Day	10% 4 2 4 6% 2 4 5% 6% 7% 49%	2301
	21st to end	Day	8% 4 2 3 3 2 3 4 6% 7% 58%	2574
September	1st to 10th	Day	10% 4 2 4 4 3 3 4 6% 7% 53%	2136
	11th to 20th	Day	11% 5% 2 5% 5% 3 4 4 5% 7% 49%	1980
	21st to end	Day	14% 6% 2 4 4 3 4 4 4 6% 49%	1980
October	1st to 10th	Day	16% 7% 2 5% 5% 3 3 4 5% 7% 43%	1885
	11th to 20th	Day	16% 8% 2 5% 4 3 4 4 5% 6% 44%	1800
	21st to end	Day	15% 9% 3 6% 5% 2 4 4 4 7% 40%	1784
November	1st to 10th	Day	13% 9% 4 7% 5% 2 4 4 5% 8% 39%	1673
	11th to 20th	Day	9% 10% 5% 9% 7% 4 5% 5% 4 7% 34%	1532
	21st to end	Day	9% 8% 4 8% 8% 3 5% 5% 6% 7% 36%	1520
December	1st to 10th	Day	7% 12% 5% 10% 9% 5% 7% 5% 5% 6% 28%	1380
	11th to 20th	Day	11% 11% 5% 8% 6% 5% 6% 5% 5% 7% 30%	1683
	21st to end	Day	20% 13% 4 11% 6% 3 4 3 4 4 29%	2442
January	1st to 10th	Night	7% 11% 5% 11% 10% 7% 7% 7% 8% 7% 21%	4664
	11th to 20th	Night	5% 8% 5% 10% 10% 6% 10% 9% 7% 8% 21%	4430
	21st to end	Night	6% 12% 7% 13% 6% 4 6% 8% 6% 7% 26%	4838
February	1st to 10th	Night	8% 11% 5% 9% 7% 4 6% 6% 5% 7% 31%	2470
	11th to 20th	Night	7% 9% 3 9% 7% 4 6% 7% 6% 7% 35%	2394
	21st to end	Night	9% 10% 3 8% 7% 4 6% 5% 5% 6% 36%	1884
March	1st to 10th	Night	11% 9% 5% 9% 7% 4 5% 5% 6% 8% 32%	2187
	11th to 20th	Night	13% 10% 4 8% 5% 3 4 5% 4 5% 40%	1980
	21st to end	Night	12% 9% 5% 8% 6% 3 6% 5% 5% 8% 34%	2178
April	1st to 10th	Night	12% 9% 3 7% 5% 3 4 5% 5% 7% 41%	1837
	11th to 20th	Night	18% 10% 3 7% 3 3 3 3 5% 7% 39%	1599
	21st to end	Night	17% 9% 4 6% 5% 3 4 4 4 6% 37%	1620
May	1st to 10th	Night	13% 6% 3 6% 6% 5% 6% 5% 4 6% 41%	1620
	11th to 20th	Night	11% 5% 2 6% 5% 3 5% 6% 6% 8% 43%	1540
	21st to end	Night	12% 4 3 5% 6% 4 4 6% 7% 9% 40%	1374
June	1st to 10th	Night	9% 5% 2 4 4 3 5% 3 6% 10% 50%	1260
	11th to 20th	Night	8% 4 2 5% 4 3 4 5% 7% 9% 50%	1260
	21st to end	Night	6% 3 1 4 3 2 4 4 5% 8% 59%	1260
July	1st to 10th	Night	5% 3 2 2 3 2 4 4 6% 8% 61%	1260
	11th to 20th	Night	3 2 1 3 2 2 3 4 6% 6% 68%	1344
	21st to end	Night	3 2 1 4 4 3 4 4 5% 8% 63%	1584
August	1st to 10th	Night	6% 3 2 5% 4 3 5% 4 6% 8% 53%	1611
	11th to 20th	Night	7% 4 2 3 4 3 4 4 4 8% 58%	1600
	21st to end	Night	6% 3 1 3 3 2 3 3 5% 7% 63%	1782
September	1st to 10th	Night	8% 4 2 4 4 2 4 4 4 6% 59%	1820
	11th to 20th	Night	9% 5% 2 3 4 2 4 3 5% 6% 57%	1980
	21st to end	Night	13% 4 2 3 3 2 3 4 4 6% 55%	1980
October	1st to 10th	Night	14% 7% 2 5% 4 3 4 4 4 6% 46%	2056
	11th to 20th	Night	15% 7% 3 5% 4 3 3 4 4 6% 47%	2160
	21st to end	Night	13% 8% 3 5% 5% 3 4 4 4 8% 41%	2569
November	1st to 10th	Night	9% 9% 4 7% 4 3 5% 4 6% 7% 42%	2405
	11th to 20th	Night	8% 9% 5% 9% 8% 3 5% 4 5% 6% 38%	2646
	21st to end	Night	9% 8% 3 7% 6% 3 5% 4 5% 8% 41%	2660
December	1st to 10th	Night	9% 10% 5% 10% 8% 4 6% 4 5% 7% 31%	2797
	11th to 20th	Night	9% 13% 4 9% 8% 4 5% 5% 5% 6% 32%	3514
	21st to end	Night	19% 13% 5% 9% 6% 3 4 4 4 5% 28%	5225

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	63% 5% 13% 14% 4 1	2289
	11th to 20th	Day	59% 6% 16% 15% 3	2540
	21st to end	Day	60% 5% 11% 19% 5%	2829
February	1st to 10th	Day	55% 6% 13% 18% 8% 1	1710
	11th to 20th	Day	53% 5% 14% 20% 7% 1	1699
	21st to end	Day	55% 4 14% 19% 8%	1570
March	1st to 10th	Day	58% 6% 14% 16% 6% 1	1887
	11th to 20th	Day	51% 4 13% 23% 9%	1980
	21st to end	Day	57% 6% 17% 16% 4	2178
April	1st to 10th	Day	45% 5% 14% 24% 11% 1	2122
	11th to 20th	Day	51% 5% 11% 19% 13% 1	2312
	21st to end	Day	50% 6% 13% 24% 7%	2340
May	1st to 10th	Day	49% 5% 12% 23% 11% 1	2340
	11th to 20th	Day	42% 5% 14% 26% 13% 1	2358
	21st to end	Day	42% 5% 17% 25% 10% 1	2900
June	1st to 10th	Day	37% 6% 17% 29% 10% 1	2700
	11th to 20th	Day	37% 6% 17% 27% 12% 1	2700
	21st to end	Day	32% 7% 19% 30% 11% 1	2700
July	1st to 10th	Day	25% 6% 18% 34% 15% 1	2626
	11th to 20th	Day	23% 5% 18% 32% 18% 3	2531
	21st to end	Day	27% 5% 17% 32% 17% 2	2582
August	1st to 10th	Day	33% 7% 16% 28% 13% 3	2314
	11th to 20th	Day	32% 6% 16% 32% 12% 1	2301
	21st to end	Day	25% 5% 15% 35% 18% 3	2574
September	1st to 10th	Day	30% 5% 14% 35% 15% 1	2136
	11th to 20th	Day	35% 5% 17% 31% 12% 1	1980
	21st to end	Day	37% 4 13% 31% 14% 1	1980
October	1st to 10th	Day	41% 4 15% 27% 13% 1	1885
	11th to 20th	Day	42% 4 12% 27% 13% 2	1800
	21st to end	Day	45% 4 14% 26% 10% 1	1784
November	1st to 10th	Day	45% 4 14% 24% 12% 2	1673
	11th to 20th	Day	49% 5% 16% 20% 8% 1	1532
	21st to end	Day	46% 6% 17% 23% 7% 2	1520
December	1st to 10th	Day	56% 6% 12% 20% 6% 1	1380
	11th to 20th	Day	54% 5% 17% 19% 4	1680
	21st to end	Day	60% 3 9% 23% 4	2442
January	1st to 10th	Night	59% 5% 14% 17% 6%	4664
	11th to 20th	Night	55% 7% 16% 16% 5%	4430
	21st to end	Night	56% 5% 13% 19% 6%	4838
February	1st to 10th	Night	51% 5% 14% 19% 10% 1	2470
	11th to 20th	Night	46% 5% 14% 23% 11% 1	2395
	21st to end	Night	48% 5% 14% 20% 12% 2	1884
March	1st to 10th	Night	50% 5% 16% 21% 8% 1	2187
	11th to 20th	Night	47% 5% 11% 24% 12% 1	1980
	21st to end	Night	47% 6% 16% 23% 7% 1	2178
April	1st to 10th	Night	42% 4 13% 26% 14% 1	1837
	11th to 20th	Night	46% 3 8% 24% 16% 2	1599
	21st to end	Night	48% 5% 12% 25% 9% 1	1620
May	1st to 10th	Night	43% 6% 12% 23% 16% 1	1620
	11th to 20th	Night	37% 6% 14% 25% 17% 2	1541
	21st to end	Night	38% 6% 17% 27% 12% 1	1374
June	1st to 10th	Night	31% 4 18% 32% 13% 2	1260
	11th to 20th	Night	29% 7% 18% 27% 15% 4	1260
	21st to end	Night	23% 6% 21% 32% 16% 2	1260
July	1st to 10th	Night	21% 5% 17% 35% 21% 2	1225
	11th to 20th	Night	16% 6% 15% 35% 23% 5%	1302
	21st to end	Night	20% 5% 16% 34% 20% 5%	1496
August	1st to 10th	Night	29% 4 17% 30% 17% 3	1601
	11th to 20th	Night	26% 5% 18% 33% 16% 2	1600
	21st to end	Night	21% 4 14% 34% 23% 3	1782
September	1st to 10th	Night	26% 5% 13% 34% 21% 1	1820
	11th to 20th	Night	29% 4 15% 33% 18% 1	1980
	21st to end	Night	31% 4 12% 32% 18% 2	1980
October	1st to 10th	Night	39% 5% 13% 26% 16% 2	2056
	11th to 20th	Night	39% 3 11% 28% 15% 3	2160
	21st to end	Night	42% 5% 14% 27% 12% 1	2569
November	1st to 10th	Night	40% 5% 12% 23% 17% 3	2405
	11th to 20th	Night	47% 5% 14% 22% 12% 2	2646
	21st to end	Night	41% 5% 19% 25% 9% 1	2660
December	1st to 10th	Night	53% 3 16% 20% 7%	2797
	11th to 20th	Night	52% 5% 17% 18% 6%	3516
	21st to end	Night	60% 4 10% 19% 7%	5225

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	12% 8% 29% 40% 9% 1	270
	11th to 20th	Day	13% 12% 33% 33% 8% 1	270
	21st to end	Day	19% 9% 29% 34% 9% 1	297
February	1st to 10th	Day	13% 10% 40% 31% 6%	270
	11th to 20th	Day	13% 9% 43% 25% 9% 1	270
	21st to end	Day	14% 7% 44% 31% 5%	225
March	1st to 10th	Day	20% 10% 38% 24% 7% 1	255
	11th to 20th	Day	15% 8% 31% 28% 14% 3	240
	21st to end	Day	17% 6% 29% 29% 14% 5%	264
April	1st to 10th	Day	21% 13% 35% 23% 7% 1	382
	11th to 20th	Day	27% 11% 28% 22% 10% 1	590
	21st to end	Day	23% 9% 28% 27% 11% 2	600
May	1st to 10th	Day	19% 7% 30% 30% 13%	603
	11th to 20th	Day	20% 7% 27% 29% 15% 3	625
	21st to end	Day	18% 8% 25% 30% 15% 4	679
June	1st to 10th	Day	10% 8% 25% 29% 21% 7%	630
	11th to 20th	Day	11% 8% 37% 28% 14% 3	630
	21st to end	Day	10% 7% 28% 36% 16% 3	630
July	1st to 10th	Day	8% 1 24% 38% 24% 5%	621
	11th to 20th	Day	8% 6% 30% 35% 18% 3	618
	21st to end	Day	6% 3 23% 40% 23% 5%	684
August	1st to 10th	Day	9% 3 20% 39% 24% 4	626
	11th to 20th	Day	12% 3 26% 33% 20% 6%	615
	21st to end	Day	10% 5% 24% 34% 25% 3	693
September	1st to 10th	Day	12% 8% 25% 36% 17% 2	426
	11th to 20th	Day	14% 5% 31% 34% 13% 2	270
	21st to end	Day	20% 11% 28% 26% 11% 3	270
October	1st to 10th	Day	19% 6% 30% 27% 16% 2	270
	11th to 20th	Day	16% 18% 26% 30% 9%	270
	21st to end	Day	21% 16% 36% 19% 6% 2	297
November	1st to 10th	Day	23% 7% 30% 33% 7%	270
	11th to 20th	Day	15% 10% 39% 27% 8% 1	270
	21st to end	Day	14% 15% 39% 24% 7% 1	270
December	1st to 10th	Day	14% 7% 37% 29% 11% 1	270
	11th to 20th	Day	17% 6% 44% 29% 3	270
	21st to end	Day	23% 10% 35% 23% 7% 2	297
January	1st to 10th	Night	12% 9% 39% 31% 8% 1	950
	11th to 20th	Night	14% 8% 37% 33% 8%	950
	21st to end	Night	15% 8% 37% 33% 7% 1	1045
February	1st to 10th	Night	13% 9% 34% 34% 9% 1	950
	11th to 20th	Night	12% 7% 40% 30% 9% 2	920
	21st to end	Night	14% 7% 32% 32% 12% 2	628
March	1st to 10th	Night	16% 5% 36% 32% 10% 1	703
	11th to 20th	Night	17% 6% 34% 32% 9%	540
	21st to end	Night	15% 7% 32% 31% 13% 1	594
April	1st to 10th	Night	16% 10% 37% 30% 8%	540
	11th to 20th	Night	23% 9% 26% 32% 10%	533
	21st to end	Night	21% 6% 29% 34% 11%	540
May	1st to 10th	Night	17% 5% 32% 36% 10% 1	540
	11th to 20th	Night	12% 5% 31% 40% 12%	531
	21st to end	Night	15% 6% 29% 37% 12% 1	584
June	1st to 10th	Night	11% 5% 25% 42% 14% 2	540
	11th to 20th	Night	11% 4 33% 43% 9%	540
	21st to end	Night	7% 3 26% 46% 16% 1	540
July	1st to 10th	Night	6% 2 23% 46% 22% 1	525
	11th to 20th	Night	4 2 26% 52% 16% 1	522
	21st to end	Night	5% 2 21% 52% 19% 1	591
August	1st to 10th	Night	7% 1 17% 56% 17% 1	537
	11th to 20th	Night	9% 3 24% 43% 19% 3	534
	21st to end	Night	6% 2 27% 46% 18% 1	594
September	1st to 10th	Night	9% 4 26% 42% 17% 2	540
	11th to 20th	Night	12% 3 24% 46% 14% 1	540
	21st to end	Night	15% 6% 32% 33% 14%	540
October	1st to 10th	Night	17% 9% 32% 32% 10%	616
	11th to 20th	Night	19% 8% 34% 30% 9%	720
	21st to end	Night	18% 10% 34% 31% 7%	985
November	1st to 10th	Night	14% 10% 32% 34% 9%	925
	11th to 20th	Night	13% 9% 37% 30% 10% 1	950
	21st to end	Night	14% 9% 37% 33% 6%	950
December	1st to 10th	Night	14% 7% 35% 34% 9%	950
	11th to 20th	Night	18% 9% 33% 33% 7% 1	950
	21st to end	Night	22% 8% 35% 28% 6% 1	1045

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