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 2024-11-02.

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% 5% 4 5% 4 6% 7% 39%	101509
	Night	10% 8% 3 8% 5% 5% 5% 4 6% 7% 40%	97635
Transparency	Day	42% 5% 15% 26% 10% 1	101242
	Night	42% 5% 14% 25% 12% 1	97492
Seeing	Day	15% 8% 30% 31% 14% 3	16062
	Night	14% 7% 32% 36% 11% 1	25945

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	8% 13% 5% 12% 9% 8% 8% 5% 7% 7% 17%	2288
	11th to 20th	Day	4 10% 4 12% 8% 8% 12% 7% 7% 9% 19%	2540
	21st to end	Day	6% 12% 7% 19% 6% 5% 4 4 5% 7% 24%	2829
February	1st to 10th	Day	9% 11% 5% 11% 7% 6% 6% 5% 6% 6% 27%	1710
	11th to 20th	Day	7% 11% 4 11% 7% 7% 7% 6% 6% 6% 28%	1690
	21st to end	Day	8% 12% 5% 13% 8% 6% 5% 3 5% 9% 26%	1570
March	1st to 10th	Day	9% 12% 4 10% 6% 5% 5% 4 4 5% 37%	3206
	11th to 20th	Day	8% 11% 3 8% 6% 5% 5% 4 6% 8% 36%	3480
	21st to end	Day	19% 14% 4 13% 6% 4 4 3 4 6% 21%	3814
April	1st to 10th	Day	14% 9% 2 7% 6% 4 4 4 5% 6% 39%	2241
	11th to 20th	Day	18% 11% 4 7% 5% 4 4 3 5% 7% 33%	2439
	21st to end	Day	19% 11% 3 7% 4 4 4 4 6% 5% 33%	2470
May	1st to 10th	Day	16% 8% 3 6% 4 4 4 3 5% 6% 40%	4120
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 7% 6% 43%	4176
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 6% 9% 37%	5142
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	16% 6% 2 5% 4 4 3 3 5% 7% 45%	1989
	11th to 20th	Day	16% 8% 2 5% 3 4 3 3 5% 6% 44%	1900
	21st to end	Day	15% 10% 3 7% 4 3 4 3 4 7% 40%	1883
November	1st to 10th	Day	13% 10% 4 8% 4 3 4 3 5% 7% 39%	1682
	11th to 20th	Day	9% 11% 4 10% 6% 5% 5% 4 5% 7% 34%	1532
	21st to end	Day	9% 9% 4 10% 7% 5% 5% 3 6% 6% 36%	1520
December	1st to 10th	Day	7% 14% 4 12% 8% 6% 6% 4 6% 6% 28%	1380
	11th to 20th	Day	10% 13% 5% 10% 5% 6% 6% 4 5% 7% 30%	1683
	21st to end	Day	20% 13% 4 12% 6% 4 4 2 4 4 29%	2442
January	1st to 10th	Night	7% 12% 5% 12% 9% 9% 7% 6% 8% 7% 21%	4664
	11th to 20th	Night	5% 9% 4 11% 8% 8% 11% 6% 8% 8% 21%	4430
	21st to end	Night	6% 12% 7% 14% 5% 5% 6% 6% 6% 7% 26%	4838
February	1st to 10th	Night	8% 12% 5% 11% 6% 6% 5% 5% 6% 6% 31%	2470
	11th to 20th	Night	7% 10% 3 10% 6% 5% 6% 5% 6% 7% 35%	2394
	21st to end	Night	9% 11% 3 8% 7% 6% 6% 4 5% 6% 35%	1884
March	1st to 10th	Night	8% 7% 4 8% 5% 5% 4 4 7% 8% 41%	3653
	11th to 20th	Night	10% 9% 4 9% 5% 5% 5% 4 5% 7% 37%	3480
	21st to end	Night	19% 15% 3 10% 5% 3 4 3 5% 7% 27%	3817
April	1st to 10th	Night	12% 10% 2 7% 5% 4 5% 4 5% 6% 41%	1938
	11th to 20th	Night	17% 10% 3 7% 3 3 3 3 5% 7% 39%	1689
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 4 6% 36%	1710
May	1st to 10th	Night	14% 6% 2 6% 4 5% 4 3 4 7% 45%	2850
	11th to 20th	Night	8% 5% 2 7% 4 4 5% 5% 6% 7% 45%	2728
	21st to end	Night	13% 5% 2 6% 5% 4 4 4 6% 8% 42%	2430
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	13% 7% 2 5% 3 4 3 3 4 6% 48%	2171
	11th to 20th	Night	14% 8% 2 6% 3 4 3 3 4 6% 47%	2280
	21st to end	Night	13% 9% 3 6% 4 4 5% 3 4 8% 41%	2712
November	1st to 10th	Night	9% 10% 4 7% 4 5% 4 3 6% 7% 42%	2419
	11th to 20th	Night	7% 11% 4 10% 7% 4 4 4 5% 6% 38%	2646
	21st to end	Night	9% 8% 3 9% 5% 4 4 3 6% 7% 41%	2660
December	1st to 10th	Night	9% 12% 5% 10% 8% 6% 5% 3 5% 7% 31%	2797
	11th to 20th	Night	9% 14% 4 11% 6% 5% 5% 4 5% 6% 32%	3514
	21st to end	Night	19% 14% 5% 10% 6% 4 3 3 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	49% 5% 12% 27% 6% 1	3206
	11th to 20th	Day	44% 5% 15% 27% 8% 1	3480
	21st to end	Day	63% 5% 14% 12% 4 1	3814
April	1st to 10th	Day	44% 5% 14% 24% 11% 2	2241
	11th to 20th	Day	50% 5% 11% 20% 13% 1	2442
	21st to end	Day	50% 6% 13% 23% 8%	2470
May	1st to 10th	Day	43% 5% 12% 26% 14% 1	4120
	11th to 20th	Day	38% 6% 11% 28% 14% 3	4180
	21st to end	Day	42% 6% 16% 27% 9%	5142
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	40% 4 14% 28% 14% 1	1989
	11th to 20th	Day	41% 4 12% 28% 13% 2	1900
	21st to end	Day	44% 4 14% 26% 11% 1	1883
November	1st to 10th	Day	45% 4 14% 24% 12% 2	1682
	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	40% 5% 15% 28% 10% 2	3653
	11th to 20th	Night	42% 7% 13% 28% 9% 1	3480
	21st to end	Night	57% 5% 14% 17% 7% 1	3817
April	1st to 10th	Night	43% 4 12% 26% 14% 1	1938
	11th to 20th	Night	45% 3 8% 24% 17% 2	1689
	21st to end	Night	48% 5% 12% 24% 10% 1	1710
May	1st to 10th	Night	38% 6% 12% 27% 15% 3	2850
	11th to 20th	Night	34% 7% 12% 30% 16% 2	2729
	21st to end	Night	36% 7% 14% 31% 11%	2430
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	37% 4 12% 27% 17% 2	2171
	11th to 20th	Night	39% 4 11% 28% 15% 3	2280
	21st to end	Night	42% 5% 14% 26% 12% 1	2712
November	1st to 10th	Night	40% 5% 12% 23% 17% 3	2419
	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% 37% 25% 7% 1	270
	11th to 20th	Day	16% 9% 31% 27% 14% 3	270
	21st to end	Day	19% 5% 30% 28% 14% 4	297
April	1st to 10th	Day	21% 13% 34% 24% 7% 1	421
	11th to 20th	Day	26% 13% 28% 22% 10% 1	640
	21st to end	Day	23% 9% 28% 26% 12% 2	650
May	1st to 10th	Day	19% 8% 31% 30% 12%	653
	11th to 20th	Day	19% 7% 26% 30% 15% 3	675
	21st to end	Day	18% 8% 25% 31% 15% 4	734
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% 36% 19% 7% 2	330
November	1st to 10th	Day	23% 8% 30% 32% 7%	273
	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	15% 5% 36% 32% 10% 1	718
	11th to 20th	Night	16% 8% 34% 32% 9%	570
	21st to end	Night	16% 7% 32% 31% 12% 1	627
April	1st to 10th	Night	16% 10% 36% 30% 8%	570
	11th to 20th	Night	22% 10% 27% 32% 9%	563
	21st to end	Night	22% 5% 28% 34% 10%	570
May	1st to 10th	Night	17% 5% 33% 35% 9% 1	570
	11th to 20th	Night	12% 4 31% 40% 13%	561
	21st to end	Night	15% 6% 29% 38% 12% 1	617
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	14% 10% 32% 34% 9%	931
	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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