Ashton Observatory at Johnson Farms
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashton Observatory at Johnson Farms clear sky chart from 2005-11-06 to 2026-09-14.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	149031
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	142018
Transparency	Day	39% 6% 12% 23% 16% 3	152626
	Night	40% 5% 11% 23% 17% 4	146495
Seeing	Day	19% 13% 28% 23% 12% 4	17392
	Night	17% 12% 30% 29% 11% 1	26822

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	15% 4 2 6% 4 4 4 3 5% 6% 47%	4737
	11th to 20th	Day	14% 9% 2 5% 4 3 3 2 3 4 50%	5280
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 7% 43%	5780
February	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	1890
	11th to 20th	Day	18% 8% 2 5% 4 5% 4 4 5% 6% 39%	1911
	21st to end	Day	17% 9% 2 7% 4 5% 5% 2 4 5% 40%	1730
March	1st to 10th	Day	19% 5% 2 5% 3 4 4 4 5% 6% 44%	4923
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 5% 6% 49%	5090
	21st to end	Day	16% 6% 2 6% 5% 4 4 3 6% 7% 41%	5713
April	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 6% 8% 42%	2520
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 4 7% 47%	2494
	21st to end	Day	18% 6% 3 4 4 4 3 4 5% 7% 41%	2520
May	1st to 10th	Day	18% 7% 2 7% 4 4 4 4 6% 8% 35%	7454
	11th to 20th	Day	19% 6% 2 6% 4 4 4 4 6% 9% 36%	8079
	21st to end	Day	23% 5% 2 6% 4 4 4 3 5% 7% 37%	9045
June	1st to 10th	Day	11% 5% 2 5% 5% 4 5% 4 7% 8% 44%	2940
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 41%	2940
	21st to end	Day	10% 5% 2 5% 4 4 5% 5% 8% 10% 44%	2940
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	8280
	11th to 20th	Day	9% 6% 3 7% 5% 6% 6% 6% 9% 12% 32%	8109
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 5% 7% 11% 37%	8340
August	1st to 10th	Day	15% 6% 3 8% 5% 6% 7% 5% 8% 9% 28%	6014
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 10% 35%	5678
	21st to end	Day	9% 4 2 5% 4 4 6% 5% 7% 9% 45%	6080
September	1st to 10th	Day	13% 5% 2 5% 3 5% 4 4 5% 6% 47%	2520
	11th to 20th	Day	13% 6% 2 5% 3 4 5% 3 5% 7% 46%	2436
	21st to end	Day	20% 8% 3 6% 4 4 4 3 5% 6% 36%	2110
October	1st to 10th	Day	16% 5% 2 5% 3 3 3 3 5% 7% 47%	2000
	11th to 20th	Day	17% 4 2 5% 3 3 3 3 6% 6% 49%	2000
	21st to end	Day	13% 5% 2 5% 4 3 4 3 5% 6% 49%	2200
November	1st to 10th	Day	12% 5% 2 4 3 3 4 3 6% 6% 52%	1926
	11th to 20th	Day	14% 6% 2 4 3 3 4 4 6% 5% 49%	1874
	21st to end	Day	14% 6% 2 5% 5% 3 4 3 5% 5% 47%	1680
December	1st to 10th	Day	12% 5% 2 6% 3 4 4 3 6% 7% 47%	2680
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 5% 7% 42%	3070
	21st to end	Day	17% 8% 3 7% 4 4 4 3 5% 7% 37%	4048
January	1st to 10th	Night	11% 7% 2 6% 5% 5% 4 4 6% 7% 44%	8141
	11th to 20th	Night	13% 7% 3 6% 3 3 3 3 4 5% 51%	7680
	21st to end	Night	14% 7% 2 5% 3 3 4 3 5% 6% 49%	8401
February	1st to 10th	Night	15% 7% 2 6% 4 5% 4 4 6% 6% 41%	2730
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 5% 5% 43%	2594
	21st to end	Night	16% 7% 2 6% 4 4 4 3 4 6% 43%	2076
March	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 5% 48%	5134
	11th to 20th	Night	11% 5% 3 5% 4 3 4 4 5% 7% 51%	5090
	21st to end	Night	13% 8% 3 6% 4 4 4 4 5% 6% 44%	5459
April	1st to 10th	Night	16% 8% 2 6% 4 3 5% 4 6% 7% 40%	2100
	11th to 20th	Night	14% 5% 2 7% 4 3 5% 3 5% 7% 44%	2075
	21st to end	Night	16% 5% 2 5% 4 3 4 4 6% 8% 42%	2100
May	1st to 10th	Night	20% 8% 2 7% 4 4 5% 4 6% 7% 33%	5441
	11th to 20th	Night	20% 7% 2 5% 4 4 4 4 6% 9% 35%	4764
	21st to end	Night	25% 7% 2 6% 4 4 4 3 5% 6% 34%	5108
June	1st to 10th	Night	10% 4 2 6% 4 4 4 4 7% 10% 46%	1680
	11th to 20th	Night	12% 6% 2 6% 5% 4 5% 4 6% 9% 40%	1680
	21st to end	Night	9% 5% 2 6% 6% 5% 6% 4 6% 10% 42%	1680
July	1st to 10th	Night	11% 7% 3 7% 6% 5% 6% 4 6% 8% 39%	4678
	11th to 20th	Night	9% 7% 3 9% 6% 6% 7% 5% 7% 9% 32%	4830
	21st to end	Night	10% 6% 3 6% 4 5% 5% 5% 7% 9% 40%	5736
August	1st to 10th	Night	15% 7% 3 8% 5% 5% 6% 5% 7% 9% 29%	4135
	11th to 20th	Night	11% 8% 3 9% 6% 5% 6% 4 7% 8% 33%	4320
	21st to end	Night	10% 6% 2 7% 4 4 5% 4 5% 8% 45%	5045
September	1st to 10th	Night	14% 5% 2 5% 3 3 4 3 6% 7% 46%	2100
	11th to 20th	Night	12% 6% 3 6% 4 4 3 3 4 7% 49%	2031
	21st to end	Night	20% 7% 2 6% 4 4 5% 4 4 7% 37%	2285
October	1st to 10th	Night	19% 6% 2 4 3 3 3 3 4 5% 47%	2400
	11th to 20th	Night	16% 5% 1 4 3 3 4 2 4 5% 53%	2400
	21st to end	Night	15% 5% 2 4 3 3 4 3 5% 7% 50%	2640
November	1st to 10th	Night	12% 5% 1 6% 3 3 4 2 5% 6% 54%	2589
	11th to 20th	Night	15% 6% 2 5% 4 3 3 2 5% 6% 50%	2741
	21st to end	Night	13% 5% 2 5% 4 5% 4 3 6% 7% 47%	2940
December	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 5% 7% 50%	4720
	11th to 20th	Night	21% 6% 2 5% 4 4 4 4 5% 6% 41%	5345
	21st to end	Night	17% 7% 2 6% 4 5% 5% 4 6% 7% 38%	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	38% 4 8% 27% 18% 5%	4738
	11th to 20th	Day	39% 3 7% 22% 20% 9%	5280
	21st to end	Day	41% 4 10% 30% 12% 3	5780
February	1st to 10th	Day	42% 6% 13% 24% 13% 2	1890
	11th to 20th	Day	45% 5% 10% 23% 15% 3	1920
	21st to end	Day	46% 5% 10% 21% 14% 3	1730
March	1st to 10th	Day	39% 5% 10% 21% 17% 7%	4923
	11th to 20th	Day	34% 5% 10% 26% 19% 6%	5090
	21st to end	Day	43% 5% 10% 19% 19% 5%	5713
April	1st to 10th	Day	39% 6% 12% 20% 17% 4	2520
	11th to 20th	Day	37% 5% 11% 22% 19% 5%	2494
	21st to end	Day	41% 5% 12% 21% 18% 3	2520
May	1st to 10th	Day	45% 6% 13% 20% 14% 1	7488
	11th to 20th	Day	42% 7% 12% 26% 12% 1	8083
	21st to end	Day	46% 6% 11% 19% 16% 3	9061
June	1st to 10th	Day	35% 8% 14% 24% 17% 2	2940
	11th to 20th	Day	38% 10% 12% 24% 14% 2	2940
	21st to end	Day	33% 9% 15% 24% 17% 2	2940
July	1st to 10th	Day	37% 9% 15% 20% 15% 3	8210
	11th to 20th	Day	40% 9% 15% 25% 10% 1	8034
	21st to end	Day	39% 7% 12% 22% 20% 1	8156
August	1st to 10th	Day	48% 10% 16% 20% 7%	6001
	11th to 20th	Day	41% 8% 16% 24% 11% 1	5717
	21st to end	Day	34% 7% 15% 27% 16% 1	6080
September	1st to 10th	Day	36% 7% 13% 20% 19% 4	2520
	11th to 20th	Day	35% 6% 11% 24% 21% 3	2436
	21st to end	Day	47% 6% 9% 18% 17% 2	2110
October	1st to 10th	Day	37% 5% 7% 18% 28% 5%	3260
	11th to 20th	Day	26% 4 9% 27% 25% 9%	3240
	21st to end	Day	24% 3 9% 30% 29% 6%	3538
November	1st to 10th	Day	31% 6% 12% 26% 20% 5%	1926
	11th to 20th	Day	35% 5% 10% 23% 22% 4	1874
	21st to end	Day	39% 5% 10% 28% 17% 2	1680
December	1st to 10th	Day	36% 5% 14% 28% 13% 4	2680
	11th to 20th	Day	42% 4 13% 28% 10% 2	3066
	21st to end	Day	45% 5% 15% 19% 15% 1	4048
January	1st to 10th	Night	39% 4 10% 25% 18% 3	8142
	11th to 20th	Night	37% 3 8% 22% 22% 8%	7680
	21st to end	Night	37% 3 7% 30% 19% 3	8401
February	1st to 10th	Night	42% 6% 12% 24% 15% 2	2730
	11th to 20th	Night	44% 4 9% 22% 17% 4	2596
	21st to end	Night	43% 4 11% 22% 15% 5%	2076
March	1st to 10th	Night	40% 4 8% 25% 14% 9%	5134
	11th to 20th	Night	32% 5% 12% 27% 17% 7%	5090
	21st to end	Night	40% 4 10% 20% 21% 5%	5459
April	1st to 10th	Night	42% 7% 10% 19% 18% 4	2100
	11th to 20th	Night	38% 5% 13% 20% 18% 5%	2075
	21st to end	Night	38% 7% 11% 24% 16% 4	2100
May	1st to 10th	Night	50% 6% 11% 19% 12% 3	5470
	11th to 20th	Night	44% 7% 12% 25% 11% 1	4765
	21st to end	Night	50% 6% 11% 18% 13% 1	5115
June	1st to 10th	Night	34% 7% 12% 26% 19% 2	1680
	11th to 20th	Night	39% 8% 13% 24% 13% 2	1680
	21st to end	Night	37% 7% 13% 23% 18% 3	1680
July	1st to 10th	Night	43% 8% 15% 18% 14% 2	4640
	11th to 20th	Night	46% 9% 14% 20% 10% 1	4785
	21st to end	Night	38% 7% 14% 22% 18% 1	5606
August	1st to 10th	Night	49% 9% 17% 17% 8%	4124
	11th to 20th	Night	45% 8% 15% 22% 10% 1	4347
	21st to end	Night	37% 6% 12% 29% 15% 1	5045
September	1st to 10th	Night	37% 5% 12% 22% 19% 5%	2100
	11th to 20th	Night	36% 5% 9% 26% 21% 3	2031
	21st to end	Night	47% 7% 10% 17% 16% 3	2285
October	1st to 10th	Night	40% 5% 10% 20% 20% 5%	3920
	11th to 20th	Night	25% 3 11% 23% 30% 8%	3899
	21st to end	Night	24% 4 10% 32% 24% 7%	4252
November	1st to 10th	Night	32% 4 10% 25% 23% 5%	2589
	11th to 20th	Night	36% 5% 9% 23% 21% 6%	2741
	21st to end	Night	36% 5% 12% 25% 20% 3	2940
December	1st to 10th	Night	34% 4 13% 28% 18% 2	4720
	11th to 20th	Night	45% 4 12% 18% 17% 4	5348
	21st to end	Night	43% 6% 15% 20% 15% 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	33% 21% 29% 12% 4	234
	11th to 20th	Day	32% 26% 30% 11% 2	330
	21st to end	Day	23% 27% 35% 12% 3	363
February	1st to 10th	Day	27% 22% 37% 10% 4	330
	11th to 20th	Day	35% 21% 36% 6% 2	398
	21st to end	Day	27% 21% 35% 16% 2	446
March	1st to 10th	Day	27% 21% 30% 17% 4	540
	11th to 20th	Day	27% 21% 31% 17% 3 1	540
	21st to end	Day	23% 22% 33% 17% 4 1	594
April	1st to 10th	Day	23% 24% 31% 16% 5% 1	540
	11th to 20th	Day	19% 20% 33% 17% 8% 2	532
	21st to end	Day	25% 15% 32% 20% 7% 1	540
May	1st to 10th	Day	22% 21% 30% 17% 7% 2	543
	11th to 20th	Day	22% 14% 27% 25% 10% 2	567
	21st to end	Day	21% 8% 25% 25% 17% 3	617
June	1st to 10th	Day	12% 11% 30% 24% 17% 5%	570
	11th to 20th	Day	17% 7% 28% 28% 16% 4	570
	21st to end	Day	10% 7% 29% 32% 18% 5%	570
July	1st to 10th	Day	11% 4 19% 35% 21% 10%	565
	11th to 20th	Day	5% 3 23% 35% 26% 8%	564
	21st to end	Day	12% 4 23% 34% 21% 6%	620
August	1st to 10th	Day	13% 5% 22% 29% 22% 10%	566
	11th to 20th	Day	13% 5% 21% 32% 22% 8%	559
	21st to end	Day	9% 5% 24% 31% 21% 10%	627
September	1st to 10th	Day	14% 6% 23% 28% 18% 10%	570
	11th to 20th	Day	14% 3 23% 32% 18% 10%	542
	21st to end	Day	26% 10% 24% 26% 11% 4	530
October	1st to 10th	Day	21% 8% 23% 24% 18% 6%	530
	11th to 20th	Day	18% 16% 27% 25% 12% 3	530
	21st to end	Day	16% 16% 32% 25% 9% 2	583
November	1st to 10th	Day	16% 14% 34% 23% 11% 2	466
	11th to 20th	Day	20% 14% 36% 21% 8% 1	414
	21st to end	Day	28% 17% 32% 16% 7%	220
December	1st to 10th	Day	20% 24% 38% 14% 3 1	220
	11th to 20th	Day	24% 21% 31% 18% 6%	220
	21st to end	Day	28% 18% 33% 17% 5%	242
January	1st to 10th	Night	19% 18% 43% 17% 3	840
	11th to 20th	Night	22% 18% 34% 22% 3	840
	21st to end	Night	20% 18% 39% 22% 1	924
February	1st to 10th	Night	18% 17% 37% 26% 2	840
	11th to 20th	Night	26% 21% 37% 15% 1	823
	21st to end	Night	24% 18% 37% 19% 2	692
March	1st to 10th	Night	20% 17% 37% 23% 2	840
	11th to 20th	Night	22% 18% 35% 19% 4 1	840
	21st to end	Night	18% 18% 41% 19% 3 1	924
April	1st to 10th	Night	21% 20% 35% 20% 4	840
	11th to 20th	Night	16% 16% 35% 27% 6%	831
	21st to end	Night	20% 11% 34% 28% 7%	840
May	1st to 10th	Night	21% 10% 31% 28% 10%	706
	11th to 20th	Night	22% 7% 29% 34% 8%	463
	21st to end	Night	17% 7% 23% 36% 16% 2	455
June	1st to 10th	Night	11% 4 29% 36% 20%	420
	11th to 20th	Night	12% 5% 27% 40% 15%	420
	21st to end	Night	11% 2 25% 45% 16%	420
July	1st to 10th	Night	9% 3 20% 43% 23% 2	410
	11th to 20th	Night	6% 3 16% 42% 32% 2	457
	21st to end	Night	9% 3 19% 41% 26% 3	690
August	1st to 10th	Night	12% 2 13% 42% 30% 2	627
	11th to 20th	Night	11% 4 16% 43% 23% 3	719
	21st to end	Night	11% 3 19% 38% 25% 4	924
September	1st to 10th	Night	15% 3 22% 33% 24% 4	840
	11th to 20th	Night	12% 4 17% 37% 27% 4	813
	21st to end	Night	23% 4 23% 32% 18% 1	800
October	1st to 10th	Night	20% 6% 22% 28% 20% 4	800
	11th to 20th	Night	18% 10% 28% 30% 14% 1	800
	21st to end	Night	17% 12% 33% 27% 11%	880
November	1st to 10th	Night	15% 9% 34% 31% 9% 1	820
	11th to 20th	Night	18% 17% 32% 26% 7% 1	840
	21st to end	Night	17% 16% 32% 27% 7%	840
December	1st to 10th	Night	20% 16% 36% 21% 6% 1	840
	11th to 20th	Night	20% 13% 39% 24% 5%	840
	21st to end	Night	19% 15% 34% 28% 3	924

Page generated at 2026-09-11 04:11:55. Version ac07

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