Cox Field
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cox Field clear sky chart from 2005-11-06 to 2025-04-11.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	113809
	Night	12% 6% 2 6% 4 4 4 4 6% 7% 45%	114504
Transparency	Day	38% 7% 10% 19% 19% 7%	117651
	Night	36% 6% 10% 19% 21% 8%	118800
Seeing	Day	16% 10% 24% 24% 18% 8%	15398
	Night	15% 9% 23% 29% 20% 4	25515

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	16% 8% 2 5% 3 3 3 3 4 5% 50%	4056
	11th to 20th	Day	10% 8% 3 4 2 2 2 3 5% 7% 52%	4060
	21st to end	Day	21% 9% 3 7% 3 3 3 2 4 6% 39%	4760
February	1st to 10th	Day	15% 9% 3 7% 4 4 4 3 6% 6% 39%	2000
	11th to 20th	Day	20% 8% 3 5% 3 3 3 3 4 5% 42%	1948
	21st to end	Day	16% 8% 3 7% 4 4 4 4 4 7% 39%	1650
March	1st to 10th	Day	20% 9% 3 8% 4 3 4 3 5% 6% 36%	4612
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 7% 51%	4980
	21st to end	Day	16% 7% 2 7% 4 4 3 3 5% 6% 44%	5465
April	1st to 10th	Day	11% 7% 2 6% 5% 3 4 4 6% 6% 47%	2255
	11th to 20th	Day	11% 6% 2 6% 4 3 4 4 5% 7% 48%	2254
	21st to end	Day	8% 4 1 5% 6% 4 4 3 6% 6% 53%	2403
May	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 6% 8% 45%	4082
	11th to 20th	Day	16% 7% 2 7% 4 4 5% 4 6% 7% 37%	4081
	21st to end	Day	7% 4 2 6% 5% 5% 5% 5% 6% 10% 46%	4476
June	1st to 10th	Day	9% 6% 3 9% 6% 7% 6% 6% 7% 10% 31%	2470
	11th to 20th	Day	5% 3 2 5% 5% 6% 7% 5% 8% 11% 41%	2470
	21st to end	Day	5% 5% 2 7% 7% 8% 8% 5% 8% 10% 36%	2470
July	1st to 10th	Day	10% 6% 3 11% 8% 7% 8% 6% 9% 11% 21%	4120
	11th to 20th	Day	6% 6% 2 8% 7% 7% 7% 6% 8% 11% 32%	4120
	21st to end	Day	8% 7% 4 10% 8% 8% 9% 7% 9% 10% 20%	4466
August	1st to 10th	Day	7% 6% 2 6% 5% 6% 7% 6% 9% 12% 34%	4080
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 7% 9% 48%	3729
	21st to end	Day	8% 4 2 5% 4 5% 6% 5% 7% 9% 46%	4180
September	1st to 10th	Day	11% 7% 2 6% 6% 6% 6% 5% 7% 7% 36%	2095
	11th to 20th	Day	14% 6% 3 6% 4 5% 5% 5% 6% 8% 39%	2090
	21st to end	Day	14% 5% 2 6% 4 4 5% 4 4 6% 46%	2090
October	1st to 10th	Day	12% 5% 2 4 3 3 4 3 4 6% 54%	2090
	11th to 20th	Day	11% 4 1 4 2 3 3 3 5% 7% 57%	2006
	21st to end	Day	11% 4 2 5% 3 4 3 3 4 5% 55%	2090
November	1st to 10th	Day	11% 7% 2 4 4 3 4 3 4 5% 52%	1792
	11th to 20th	Day	14% 4 2 4 3 3 3 3 4 7% 54%	1800
	21st to end	Day	12% 6% 2 5% 4 4 4 4 5% 8% 47%	1800
December	1st to 10th	Day	15% 7% 2 5% 3 3 3 3 4 5% 48%	2930
	11th to 20th	Day	15% 8% 2 6% 4 4 4 3 5% 5% 44%	3373
	21st to end	Day	20% 8% 2 6% 3 3 3 3 5% 6% 42%	4466
January	1st to 10th	Night	14% 8% 3 5% 3 3 3 3 4 5% 49%	5889
	11th to 20th	Night	13% 7% 3 6% 4 4 4 3 5% 6% 46%	5900
	21st to end	Night	18% 10% 2 6% 3 3 4 3 5% 5% 42%	6191
February	1st to 10th	Night	14% 8% 2 7% 5% 5% 2 3 5% 6% 44%	2400
	11th to 20th	Night	20% 7% 3 5% 3 3 3 3 4 5% 43%	2338
	21st to end	Night	15% 7% 2 5% 5% 4 4 3 5% 6% 43%	1980
March	1st to 10th	Night	20% 9% 3 7% 4 4 4 3 5% 6% 36%	5240
	11th to 20th	Night	10% 5% 2 5% 3 4 4 3 5% 7% 51%	4980
	21st to end	Night	14% 6% 2 6% 4 4 4 3 5% 6% 46%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 6% 48%	2160
	11th to 20th	Night	12% 5% 2 5% 4 4 3 3 6% 7% 49%	1881
	21st to end	Night	6% 5% 2 4 4 4 4 4 6% 6% 55%	1777
May	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 4 8% 12% 43%	2813
	11th to 20th	Night	15% 8% 2 6% 4 4 4 3 6% 8% 40%	2812
	21st to end	Night	6% 4 1 5% 3 4 4 4 7% 12% 50%	3079
June	1st to 10th	Night	5% 6% 3 7% 5% 6% 6% 6% 8% 12% 36%	1710
	11th to 20th	Night	3 5% 2 5% 5% 4 5% 5% 7% 12% 47%	1710
	21st to end	Night	5% 5% 2 7% 5% 6% 5% 5% 9% 12% 39%	1710
July	1st to 10th	Night	9% 6% 3 9% 7% 6% 6% 6% 8% 12% 28%	2840
	11th to 20th	Night	6% 5% 3 8% 6% 6% 6% 6% 10% 12% 33%	2840
	21st to end	Night	8% 8% 3 10% 7% 7% 8% 7% 9% 10% 23%	3087
August	1st to 10th	Night	6% 4 2 8% 6% 6% 8% 6% 9% 12% 33%	2846
	11th to 20th	Night	6% 4 2 6% 4 4 5% 3 7% 11% 48%	3100
	21st to end	Night	6% 5% 2 5% 4 4 4 3 6% 8% 53%	3476
September	1st to 10th	Night	8% 5% 2 6% 5% 6% 6% 5% 7% 10% 40%	2064
	11th to 20th	Night	12% 6% 2 7% 4 5% 5% 3 6% 7% 44%	2090
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 6% 8% 46%	2090
October	1st to 10th	Night	10% 5% 2 5% 3 3 3 2 5% 6% 56%	2090
	11th to 20th	Night	11% 4 2 4 2 3 3 3 4 5% 59%	2171
	21st to end	Night	10% 5% 2 4 4 3 3 3 4 5% 58%	2508
November	1st to 10th	Night	11% 6% 2 4 3 3 3 3 5% 5% 56%	2503
	11th to 20th	Night	13% 5% 2 5% 3 4 3 3 3 5% 54%	2600
	21st to end	Night	13% 6% 2 5% 4 3 4 4 5% 6% 49%	2600
December	1st to 10th	Night	14% 7% 2 5% 3 3 3 2 4 6% 52%	4250
	11th to 20th	Night	15% 5% 2 6% 3 3 3 3 5% 7% 47%	4822
	21st to end	Night	19% 8% 3 6% 4 4 4 3 4 5% 41%	6490

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	36% 5% 5% 20% 27% 8%	4057
	11th to 20th	Day	32% 3 4 20% 30% 11%	4060
	21st to end	Day	47% 3 6% 14% 21% 9%	4760
February	1st to 10th	Day	44% 4 7% 18% 20% 7%	2000
	11th to 20th	Day	45% 4 5% 18% 22% 7%	1958
	21st to end	Day	45% 5% 7% 15% 20% 9%	1650
March	1st to 10th	Day	48% 5% 7% 11% 22% 6%	4612
	11th to 20th	Day	32% 5% 7% 14% 22% 20%	4980
	21st to end	Day	42% 4 6% 20% 19% 9%	5465
April	1st to 10th	Day	37% 5% 9% 21% 19% 8%	2255
	11th to 20th	Day	34% 5% 8% 19% 23% 11%	2254
	21st to end	Day	30% 6% 10% 23% 27% 4	2403
May	1st to 10th	Day	34% 8% 12% 22% 21% 3	4120
	11th to 20th	Day	43% 8% 13% 22% 13% 1	4085
	21st to end	Day	30% 9% 15% 25% 19% 2	4489
June	1st to 10th	Day	45% 10% 14% 18% 12% 2	2470
	11th to 20th	Day	33% 9% 18% 24% 15% 1	2470
	21st to end	Day	39% 12% 16% 19% 11% 3	2470
July	1st to 10th	Day	50% 14% 19% 13% 4 1	4055
	11th to 20th	Day	41% 16% 18% 20% 5% 1	4051
	21st to end	Day	52% 15% 16% 13% 4	4324
August	1st to 10th	Day	37% 10% 16% 29% 7%	4067
	11th to 20th	Day	30% 9% 14% 31% 15% 1	3765
	21st to end	Day	31% 10% 16% 24% 16% 3	4179
September	1st to 10th	Day	43% 9% 14% 14% 15% 4	2095
	11th to 20th	Day	41% 8% 11% 18% 19% 3	2090
	21st to end	Day	38% 5% 9% 21% 20% 6%	2090
October	1st to 10th	Day	27% 4 7% 14% 34% 15%	3480
	11th to 20th	Day	17% 3 4 17% 28% 30%	3311
	21st to end	Day	24% 3 5% 17% 38% 13%	3428
November	1st to 10th	Day	37% 4 6% 14% 27% 12%	1792
	11th to 20th	Day	31% 4 7% 18% 26% 14%	1800
	21st to end	Day	37% 5% 8% 16% 25% 9%	1800
December	1st to 10th	Day	39% 4 6% 18% 26% 8%	2930
	11th to 20th	Day	42% 3 9% 17% 22% 7%	3370
	21st to end	Day	42% 5% 7% 22% 22% 3	4466
January	1st to 10th	Night	36% 4 5% 20% 28% 8%	5889
	11th to 20th	Night	38% 5% 5% 18% 25% 9%	5900
	21st to end	Night	44% 3 5% 14% 27% 6%	6191
February	1st to 10th	Night	41% 3 6% 19% 20% 10%	2400
	11th to 20th	Night	44% 4 4 16% 22% 9%	2339
	21st to end	Night	41% 5% 8% 16% 18% 12%	1980
March	1st to 10th	Night	48% 5% 8% 14% 15% 10%	5240
	11th to 20th	Night	32% 5% 8% 13% 23% 19%	4980
	21st to end	Night	38% 5% 5% 19% 21% 11%	5467
April	1st to 10th	Night	35% 5% 10% 20% 19% 11%	2160
	11th to 20th	Night	34% 5% 6% 21% 23% 11%	1881
	21st to end	Night	28% 6% 11% 24% 24% 7%	1777
May	1st to 10th	Night	29% 10% 17% 23% 19% 3	2840
	11th to 20th	Night	40% 8% 13% 25% 12% 2	2813
	21st to end	Night	24% 7% 19% 27% 21% 3	3087
June	1st to 10th	Night	37% 12% 17% 20% 14% 1	1710
	11th to 20th	Night	27% 9% 21% 29% 12% 2	1710
	21st to end	Night	33% 9% 22% 20% 13% 3	1710
July	1st to 10th	Night	46% 14% 22% 13% 5% 1	2795
	11th to 20th	Night	37% 14% 26% 17% 6% 1	2786
	21st to end	Night	49% 15% 18% 13% 4 1	2988
August	1st to 10th	Night	38% 11% 18% 26% 6%	2835
	11th to 20th	Night	28% 9% 17% 30% 14% 2	3130
	21st to end	Night	28% 7% 15% 27% 19% 3	3476
September	1st to 10th	Night	36% 10% 16% 18% 16% 4	2064
	11th to 20th	Night	39% 8% 11% 22% 17% 5%	2090
	21st to end	Night	35% 8% 10% 19% 21% 8%	2090
October	1st to 10th	Night	28% 5% 8% 14% 28% 16%	3480
	11th to 20th	Night	17% 2 4 15% 30% 32%	3605
	21st to end	Night	21% 3 4 21% 33% 18%	4120
November	1st to 10th	Night	32% 4 8% 16% 26% 14%	2503
	11th to 20th	Night	34% 3 5% 17% 27% 15%	2600
	21st to end	Night	36% 4 7% 18% 26% 9%	2600
December	1st to 10th	Night	35% 3 6% 19% 27% 9%	4250
	11th to 20th	Night	37% 4 11% 17% 24% 7%	4824
	21st to end	Night	44% 4 6% 22% 20% 2	6490

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	28% 21% 39% 11%	300
	11th to 20th	Day	24% 23% 34% 11% 6% 2	300
	21st to end	Day	25% 28% 33% 10% 3 1	330
February	1st to 10th	Day	23% 24% 36% 15% 3	300
	11th to 20th	Day	36% 19% 33% 10% 2	300
	21st to end	Day	29% 21% 35% 13% 2	249
March	1st to 10th	Day	25% 20% 32% 17% 4 1	354
	11th to 20th	Day	22% 20% 33% 18% 7% 1	500
	21st to end	Day	21% 21% 31% 21% 6% 1	550
April	1st to 10th	Day	19% 18% 33% 20% 9% 1	500
	11th to 20th	Day	15% 17% 30% 24% 12% 2	452
	21st to end	Day	13% 13% 32% 24% 14% 3	460
May	1st to 10th	Day	13% 13% 28% 28% 13% 5%	463
	11th to 20th	Day	12% 7% 22% 33% 17% 9%	487
	21st to end	Day	12% 6% 18% 32% 21% 11%	529
June	1st to 10th	Day	12% 3 18% 29% 28% 10%	490
	11th to 20th	Day	6% 3 18% 28% 27% 17%	490
	21st to end	Day	8% 3 18% 28% 25% 18%	490
July	1st to 10th	Day	9% 1 11% 24% 30% 25%	485
	11th to 20th	Day	7% 3 7% 23% 36% 24%	484
	21st to end	Day	8% 1 9% 26% 34% 22%	532
August	1st to 10th	Day	10% 3 11% 28% 26% 23%	486
	11th to 20th	Day	9% 1 9% 28% 33% 19%	479
	21st to end	Day	13% 3 10% 31% 28% 15%	539
September	1st to 10th	Day	12% 3 13% 28% 29% 15%	490
	11th to 20th	Day	16% 3 12% 29% 29% 10%	490
	21st to end	Day	13% 3 20% 29% 26% 10%	490
October	1st to 10th	Day	15% 4 26% 28% 19% 8%	490
	11th to 20th	Day	13% 9% 24% 29% 19% 5%	490
	21st to end	Day	16% 9% 24% 28% 18% 4	539
November	1st to 10th	Day	22% 11% 28% 24% 13% 3	330
	11th to 20th	Day	18% 16% 38% 22% 5% 1	300
	21st to end	Day	13% 18% 39% 22% 7% 1	300
December	1st to 10th	Day	24% 21% 33% 18% 5%	300
	11th to 20th	Day	26% 17% 35% 16% 6%	300
	21st to end	Day	30% 14% 36% 15% 3 1	330
January	1st to 10th	Night	23% 20% 29% 22% 5%	800
	11th to 20th	Night	22% 17% 28% 26% 7%	800
	21st to end	Night	22% 17% 35% 22% 4	880
February	1st to 10th	Night	19% 14% 32% 29% 7%	800
	11th to 20th	Night	25% 17% 32% 24% 3	783
	21st to end	Night	20% 19% 35% 23% 3	660
March	1st to 10th	Night	18% 16% 33% 28% 5%	800
	11th to 20th	Night	17% 16% 33% 24% 10%	800
	21st to end	Night	18% 15% 29% 29% 9%	880
April	1st to 10th	Night	15% 15% 30% 28% 11%	760
	11th to 20th	Night	16% 13% 28% 27% 15% 1	565
	21st to end	Night	10% 9% 26% 37% 18% 1	570
May	1st to 10th	Night	9% 8% 26% 32% 23% 2	570
	11th to 20th	Night	12% 4 21% 34% 24% 4	561
	21st to end	Night	12% 2 13% 36% 32% 6%	617
June	1st to 10th	Night	7% 2 13% 39% 35% 5%	570
	11th to 20th	Night	4 1 15% 31% 36% 13%	570
	21st to end	Night	6% 1 11% 33% 38% 11%	570
July	1st to 10th	Night	8% 2 8% 25% 46% 12%	555
	11th to 20th	Night	8% 1 5% 22% 46% 17%	552
	21st to end	Night	7% 8% 27% 45% 13%	624
August	1st to 10th	Night	5% 1 8% 28% 45% 13%	567
	11th to 20th	Night	9% 1 7% 30% 43% 10%	564
	21st to end	Night	12% 2 11% 26% 35% 14%	627
September	1st to 10th	Night	10% 2 10% 25% 40% 13%	734
	11th to 20th	Night	14% 3 13% 29% 37% 5%	760
	21st to end	Night	13% 3 14% 34% 33% 4	760
October	1st to 10th	Night	12% 4 22% 32% 24% 7%	760
	11th to 20th	Night	14% 7% 21% 32% 23% 3	760
	21st to end	Night	14% 8% 21% 34% 20% 3	836
November	1st to 10th	Night	16% 7% 23% 34% 19% 2	780
	11th to 20th	Night	16% 12% 28% 28% 14% 2	800
	21st to end	Night	15% 13% 28% 31% 12% 1	800
December	1st to 10th	Night	19% 14% 31% 27% 10% 1	800
	11th to 20th	Night	21% 12% 31% 27% 8% 1	800
	21st to end	Night	25% 14% 27% 27% 7%	880

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