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 2025-05-11.

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 4 4 6% 7% 42%	113896
	Night	15% 6% 2 5% 4 4 4 3 5% 7% 44%	115053
Transparency	Day	39% 6% 12% 23% 16% 4	117534
	Night	38% 5% 12% 23% 18% 4	119555
Seeing	Day	19% 13% 28% 23% 12% 4	15584
	Night	17% 12% 30% 29% 11% 1	25192

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	14% 4 2 4 3 4 4 3 5% 7% 50%	3659
	11th to 20th	Day	14% 9% 3 6% 4 3 3 2 4 4 47%	4060
	21st to end	Day	16% 8% 3 6% 4 4 4 3 4 6% 43%	4466
February	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 7% 38%	1800
	11th to 20th	Day	18% 8% 2 5% 4 5% 4 4 6% 6% 39%	1818
	21st to end	Day	17% 8% 2 6% 4 5% 5% 2 4 5% 41%	1650
March	1st to 10th	Day	19% 5% 1 5% 3 4 4 4 5% 6% 44%	4816
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 5% 6% 50%	4980
	21st to end	Day	16% 6% 2 6% 5% 4 4 3 6% 7% 41%	5590
April	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 42%	2400
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 4 6% 47%	2374
	21st to end	Day	18% 6% 3 4 5% 4 3 4 5% 7% 41%	2400
May	1st to 10th	Day	24% 7% 2 7% 4 4 5% 4 6% 8% 30%	4131
	11th to 20th	Day	23% 7% 3 7% 4 4 5% 4 6% 8% 30%	4324
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 9% 45%	4823
June	1st to 10th	Day	12% 5% 2 5% 5% 4 5% 4 7% 8% 43%	2660
	11th to 20th	Day	12% 5% 2 6% 5% 5% 4 4 7% 9% 41%	2660
	21st to end	Day	10% 4 1 4 4 4 5% 5% 8% 10% 43%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 6% 6% 5% 8% 11% 42%	4440
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 5% 8% 11% 39%	4353
	21st to end	Day	11% 6% 3 7% 5% 5% 6% 5% 6% 9% 35%	4466
August	1st to 10th	Day	17% 6% 2 7% 4 6% 6% 4 7% 9% 32%	4104
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 8% 8% 38%	3860
	21st to end	Day	11% 5% 2 6% 4 4 5% 5% 6% 8% 46%	4180
September	1st to 10th	Day	14% 6% 2 5% 3 5% 4 4 5% 6% 46%	2280
	11th to 20th	Day	12% 5% 1 5% 3 4 5% 3 5% 8% 47%	2280
	21st to end	Day	20% 8% 2 6% 4 4 4 3 5% 6% 37%	2005
October	1st to 10th	Day	17% 5% 3 5% 3 3 4 3 5% 7% 46%	1900
	11th to 20th	Day	18% 3 2 4 3 3 3 3 5% 6% 49%	1900
	21st to end	Day	14% 5% 2 5% 4 3 4 3 5% 6% 49%	2090
November	1st to 10th	Day	12% 5% 2 4 3 3 4 3 5% 6% 51%	1832
	11th to 20th	Day	14% 6% 2 4 3 3 4 4 6% 5% 49%	1785
	21st to end	Day	15% 6% 2 5% 5% 3 4 3 5% 5% 48%	1600
December	1st to 10th	Day	12% 5% 2 6% 3 4 4 3 6% 7% 48%	2600
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 5% 7% 42%	2990
	21st to end	Day	17% 8% 3 7% 4 4 4 3 5% 8% 38%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 3 3 5% 6% 49%	6285
	11th to 20th	Night	14% 7% 2 4 3 3 3 3 4 5% 51%	5900
	21st to end	Night	16% 8% 2 5% 3 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	15% 7% 2 6% 4 5% 4 4 6% 6% 41%	2600
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 5% 5% 44%	2468
	21st to end	Night	16% 7% 2 6% 4 4 4 3 4 6% 42%	1980
March	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 5% 48%	5021
	11th to 20th	Night	10% 5% 3 5% 4 3 4 4 5% 7% 51%	4980
	21st to end	Night	13% 8% 3 5% 4 4 4 4 5% 6% 45%	5340
April	1st to 10th	Night	16% 8% 2 6% 4 4 5% 4 6% 7% 40%	2000
	11th to 20th	Night	14% 5% 2 7% 4 3 5% 3 5% 7% 44%	1975
	21st to end	Night	16% 5% 2 5% 4 3 4 4 7% 7% 42%	2000
May	1st to 10th	Night	25% 8% 2 6% 4 4 4 4 6% 6% 32%	3012
	11th to 20th	Night	24% 9% 2 6% 5% 4 4 4 5% 7% 31%	2572
	21st to end	Night	11% 4 1 6% 3 5% 4 4 8% 10% 43%	2732
June	1st to 10th	Night	11% 4 2 6% 4 4 4 4 7% 10% 45%	1520
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 4 7% 9% 42%	1520
	21st to end	Night	9% 4 2 5% 6% 5% 6% 4 6% 11% 42%	1520
July	1st to 10th	Night	7% 5% 2 6% 6% 6% 7% 4 7% 10% 40%	2520
	11th to 20th	Night	8% 6% 2 7% 5% 5% 7% 5% 7% 10% 37%	2603
	21st to end	Night	12% 7% 3 6% 4 5% 5% 4 7% 8% 38%	3087
August	1st to 10th	Night	18% 6% 3 7% 4 5% 5% 5% 6% 9% 33%	2825
	11th to 20th	Night	10% 8% 3 9% 6% 5% 6% 4 7% 8% 35%	2935
	21st to end	Night	11% 5% 1 6% 4 3 5% 4 5% 8% 48%	3476
September	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 6% 7% 45%	1900
	11th to 20th	Night	11% 5% 3 5% 4 4 3 4 4 7% 50%	1900
	21st to end	Night	20% 6% 3 6% 4 4 4 3 4 7% 38%	2170
October	1st to 10th	Night	20% 6% 2 5% 3 3 4 3 4 5% 46%	2280
	11th to 20th	Night	15% 4 1 4 3 3 4 2 4 5% 53%	2280
	21st to end	Night	15% 5% 2 4 3 3 4 3 5% 7% 50%	2508
November	1st to 10th	Night	12% 5% 1 6% 3 3 4 2 5% 6% 54%	2463
	11th to 20th	Night	15% 6% 2 5% 4 3 3 2 5% 6% 49%	2610
	21st to end	Night	13% 5% 2 4 4 5% 4 3 6% 7% 47%	2800
December	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 5% 7% 51%	4580
	11th to 20th	Night	21% 6% 2 5% 4 4 4 4 5% 6% 40%	5205
	21st to end	Night	17% 7% 2 6% 4 5% 5% 4 6% 7% 38%	6996

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	34% 4 7% 27% 22% 6%	3660
	11th to 20th	Day	41% 3 9% 21% 18% 8%	4060
	21st to end	Day	42% 5% 10% 26% 13% 3	4466
February	1st to 10th	Day	43% 7% 13% 24% 12% 2	1800
	11th to 20th	Day	44% 5% 10% 23% 15% 3	1827
	21st to end	Day	46% 5% 10% 21% 15% 4	1650
March	1st to 10th	Day	38% 5% 11% 22% 17% 7%	4816
	11th to 20th	Day	34% 5% 10% 26% 19% 6%	4980
	21st to end	Day	43% 5% 10% 18% 19% 5%	5590
April	1st to 10th	Day	40% 7% 12% 20% 17% 5%	2400
	11th to 20th	Day	38% 5% 11% 22% 19% 5%	2374
	21st to end	Day	41% 5% 12% 21% 19% 3	2400
May	1st to 10th	Day	51% 7% 11% 19% 11% 1	4168
	11th to 20th	Day	48% 10% 14% 18% 8% 1	4328
	21st to end	Day	34% 8% 15% 24% 17% 3	4837
June	1st to 10th	Day	36% 9% 14% 25% 15% 1	2660
	11th to 20th	Day	38% 10% 12% 24% 14% 2	2660
	21st to end	Day	33% 9% 15% 24% 17% 2	2660
July	1st to 10th	Day	33% 11% 19% 20% 12% 5%	4370
	11th to 20th	Day	36% 8% 16% 27% 11% 2	4278
	21st to end	Day	43% 8% 12% 19% 17% 1	4324
August	1st to 10th	Day	45% 9% 15% 23% 8% 1	4091
	11th to 20th	Day	39% 8% 14% 25% 14% 1	3899
	21st to end	Day	35% 7% 12% 27% 16% 2	4180
September	1st to 10th	Day	38% 8% 12% 19% 19% 4	2280
	11th to 20th	Day	34% 6% 11% 24% 21% 3	2280
	21st to end	Day	46% 6% 9% 19% 18% 2	2005
October	1st to 10th	Day	38% 5% 7% 17% 28% 5%	3160
	11th to 20th	Day	25% 4 9% 27% 26% 10%	3140
	21st to end	Day	23% 3 10% 30% 28% 6%	3428
November	1st to 10th	Day	32% 6% 12% 26% 20% 5%	1832
	11th to 20th	Day	34% 6% 11% 23% 22% 4	1785
	21st to end	Day	38% 5% 11% 28% 17% 2	1600
December	1st to 10th	Day	36% 5% 14% 28% 13% 4	2600
	11th to 20th	Day	43% 4 13% 28% 10% 1	2986
	21st to end	Day	45% 5% 15% 19% 15% 1	3960
January	1st to 10th	Night	36% 4 8% 25% 23% 4	6285
	11th to 20th	Night	35% 4 9% 24% 20% 7%	5900
	21st to end	Night	39% 3 8% 28% 19% 2	6490
February	1st to 10th	Night	42% 6% 13% 24% 14% 1	2600
	11th to 20th	Night	43% 4 9% 22% 17% 4	2470
	21st to end	Night	43% 4 10% 22% 16% 5%	1980
March	1st to 10th	Night	39% 4 8% 26% 14% 9%	5021
	11th to 20th	Night	32% 5% 12% 27% 17% 7%	4980
	21st to end	Night	40% 4 9% 20% 21% 5%	5340
April	1st to 10th	Night	42% 7% 10% 18% 19% 4	2000
	11th to 20th	Night	39% 5% 13% 20% 18% 5%	1975
	21st to end	Night	38% 7% 11% 23% 16% 5%	2000
May	1st to 10th	Night	52% 6% 11% 19% 10% 3	3041
	11th to 20th	Night	50% 9% 11% 18% 10% 1	2573
	21st to end	Night	33% 9% 18% 25% 12% 3	2739
June	1st to 10th	Night	33% 8% 13% 27% 17% 2	1520
	11th to 20th	Night	37% 9% 13% 25% 14% 3	1520
	21st to end	Night	36% 8% 13% 23% 18% 3	1520
July	1st to 10th	Night	38% 9% 20% 17% 12% 4	2480
	11th to 20th	Night	39% 8% 18% 24% 10% 1	2555
	21st to end	Night	41% 8% 14% 20% 16% 1	2988
August	1st to 10th	Night	45% 9% 17% 20% 8% 1	2814
	11th to 20th	Night	43% 8% 12% 25% 11% 1	2962
	21st to end	Night	34% 6% 12% 30% 16% 2	3476
September	1st to 10th	Night	39% 5% 11% 21% 18% 5%	1900
	11th to 20th	Night	34% 5% 9% 26% 22% 4	1900
	21st to end	Night	46% 6% 10% 17% 17% 4	2170
October	1st to 10th	Night	41% 5% 10% 20% 19% 5%	3800
	11th to 20th	Night	24% 4 11% 23% 30% 8%	3779
	21st to end	Night	24% 4 10% 32% 24% 7%	4120
November	1st to 10th	Night	32% 5% 10% 25% 22% 5%	2463
	11th to 20th	Night	37% 5% 9% 23% 21% 6%	2610
	21st to end	Night	35% 5% 12% 24% 20% 3	2800
December	1st to 10th	Night	34% 4 13% 28% 18% 2	4580
	11th to 20th	Night	45% 4 12% 18% 17% 4	5208
	21st to end	Night	43% 6% 15% 20% 16% 1	6996

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% 22% 29% 12% 4	213
	11th to 20th	Day	31% 24% 31% 12% 2	300
	21st to end	Day	23% 25% 36% 13% 3	330
February	1st to 10th	Day	26% 23% 37% 9% 4	300
	11th to 20th	Day	35% 22% 36% 5% 2	365
	21st to end	Day	28% 20% 34% 16% 2	414
March	1st to 10th	Day	26% 22% 31% 18% 3	500
	11th to 20th	Day	25% 22% 31% 17% 3 1	500
	21st to end	Day	23% 21% 33% 17% 5% 1	550
April	1st to 10th	Day	24% 24% 31% 15% 4 1	500
	11th to 20th	Day	19% 19% 33% 18% 9% 2	492
	21st to end	Day	25% 15% 32% 19% 7% 1	500
May	1st to 10th	Day	23% 20% 29% 18% 8% 3	503
	11th to 20th	Day	21% 13% 28% 25% 11% 3	487
	21st to end	Day	17% 7% 26% 27% 20% 4	529
June	1st to 10th	Day	13% 10% 29% 25% 18% 6%	490
	11th to 20th	Day	16% 6% 28% 28% 18% 4	490
	21st to end	Day	9% 7% 30% 33% 17% 4	490
July	1st to 10th	Day	10% 5% 19% 36% 20% 10%	485
	11th to 20th	Day	5% 3 23% 35% 27% 8%	484
	21st to end	Day	13% 3 23% 34% 21% 6%	532
August	1st to 10th	Day	12% 5% 21% 28% 24% 10%	486
	11th to 20th	Day	12% 4 20% 31% 24% 9%	479
	21st to end	Day	9% 3 22% 32% 22% 12%	539
September	1st to 10th	Day	16% 7% 21% 25% 20% 12%	490
	11th to 20th	Day	14% 4 22% 32% 18% 10%	490
	21st to end	Day	26% 10% 23% 26% 11% 4	490
October	1st to 10th	Day	22% 8% 23% 23% 18% 5%	490
	11th to 20th	Day	17% 15% 26% 27% 12% 3	490
	21st to end	Day	16% 13% 33% 26% 9% 2	539
November	1st to 10th	Day	16% 13% 34% 23% 11% 3	432
	11th to 20th	Day	18% 14% 37% 22% 9% 1	385
	21st to end	Day	27% 18% 32% 16% 8% 1	200
December	1st to 10th	Day	20% 23% 39% 15% 3 2	200
	11th to 20th	Day	25% 21% 30% 18% 7%	200
	21st to end	Day	25% 17% 34% 18% 5%	220
January	1st to 10th	Night	19% 19% 42% 17% 3	800
	11th to 20th	Night	22% 18% 34% 23% 3	800
	21st to end	Night	19% 18% 39% 22% 1	880
February	1st to 10th	Night	18% 17% 37% 26% 2	800
	11th to 20th	Night	26% 21% 37% 15% 1	783
	21st to end	Night	24% 18% 37% 18% 2	660
March	1st to 10th	Night	20% 17% 38% 23% 2	800
	11th to 20th	Night	22% 19% 35% 19% 4 1	800
	21st to end	Night	18% 16% 41% 20% 4 1	880
April	1st to 10th	Night	21% 20% 35% 20% 4	800
	11th to 20th	Night	16% 16% 35% 27% 6%	791
	21st to end	Night	20% 11% 34% 28% 7%	800
May	1st to 10th	Night	21% 9% 31% 29% 10%	672
	11th to 20th	Night	21% 7% 28% 35% 9%	421
	21st to end	Night	15% 6% 21% 39% 17% 2	411
June	1st to 10th	Night	11% 3 28% 36% 20% 1	380
	11th to 20th	Night	11% 4 26% 42% 16% 1	380
	21st to end	Night	11% 2 26% 46% 15%	380
July	1st to 10th	Night	8% 2 20% 43% 24% 3	370
	11th to 20th	Night	6% 2 16% 42% 32% 2	412
	21st to end	Night	8% 3 18% 42% 26% 3	624
August	1st to 10th	Night	12% 2 12% 42% 30% 2	567
	11th to 20th	Night	11% 4 15% 43% 24% 3	649
	21st to end	Night	11% 3 17% 39% 26% 4	836
September	1st to 10th	Night	16% 3 21% 31% 24% 4	760
	11th to 20th	Night	11% 4 17% 37% 28% 4	760
	21st to end	Night	23% 4 23% 30% 18% 1	760
October	1st to 10th	Night	20% 6% 23% 28% 19% 4	760
	11th to 20th	Night	17% 9% 28% 31% 14% 1	760
	21st to end	Night	17% 11% 32% 27% 12%	836
November	1st to 10th	Night	15% 9% 34% 32% 9% 2	780
	11th to 20th	Night	18% 16% 32% 26% 7% 1	800
	21st to end	Night	17% 16% 32% 27% 7% 1	800
December	1st to 10th	Night	21% 16% 36% 21% 7% 1	800
	11th to 20th	Night	19% 13% 39% 24% 5%	800
	21st to end	Night	19% 15% 35% 29% 3	880

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