Fox Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fox Observatory clear sky chart from 2005-11-06 to 2025-07-06.

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	8% 5% 2 7% 6% 6% 7% 6% 8% 10% 36%	119555
	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	119221
Transparency	Day	40% 8% 17% 21% 11% 3	123263
	Night	35% 8% 19% 23% 12% 3	123650
Seeing	Day	8% 3 16% 28% 25% 20%	16895
	Night	8% 3 13% 33% 32% 12%	27032

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	7% 6% 2 5% 3 4 4 4 6% 9% 49%	4056
	11th to 20th	Day	17% 6% 3 9% 6% 5% 5% 5% 6% 9% 29%	4060
	21st to end	Day	11% 7% 3 8% 5% 5% 5% 4 5% 7% 40%	4540
February	1st to 10th	Day	10% 7% 2 6% 6% 4 5% 4 6% 9% 41%	2000
	11th to 20th	Day	7% 4 1 5% 4 4 5% 4 6% 10% 51%	2064
	21st to end	Day	7% 5% 1 5% 6% 5% 6% 5% 6% 8% 45%	1815
March	1st to 10th	Day	6% 4 2 6% 6% 6% 6% 6% 7% 10% 42%	4935
	11th to 20th	Day	4 3 1 4 3 4 4 4 7% 8% 60%	4980
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 9% 43%	5465
April	1st to 10th	Day	5% 3 1 5% 6% 5% 6% 5% 7% 11% 46%	2200
	11th to 20th	Day	7% 4 2 7% 6% 6% 5% 5% 8% 9% 42%	2268
	21st to end	Day	7% 4 2 6% 5% 5% 6% 4 8% 10% 44%	2400
May	1st to 10th	Day	3 2 1 4 4 4 5% 4 8% 10% 55%	5405
	11th to 20th	Day	7% 5% 2 6% 4 5% 6% 5% 7% 9% 45%	5485
	21st to end	Day	5% 4 2 6% 6% 6% 7% 6% 9% 11% 38%	6433
June	1st to 10th	Day	15% 10% 4 12% 9% 7% 8% 5% 8% 7% 17%	2600
	11th to 20th	Day	9% 8% 4 10% 8% 7% 8% 6% 8% 11% 20%	2600
	21st to end	Day	9% 7% 3 10% 9% 8% 9% 7% 9% 9% 20%	2600
July	1st to 10th	Day	7% 7% 3 10% 7% 7% 7% 6% 10% 13% 24%	4185
	11th to 20th	Day	6% 6% 3 9% 8% 8% 9% 7% 10% 13% 23%	4120
	21st to end	Day	5% 5% 2 8% 6% 7% 8% 7% 9% 14% 28%	4466
August	1st to 10th	Day	6% 5% 2 10% 8% 8% 9% 8% 11% 13% 19%	4024
	11th to 20th	Day	5% 4 2 8% 7% 8% 9% 8% 11% 14% 23%	3730
	21st to end	Day	6% 5% 2 9% 7% 8% 10% 8% 11% 15% 19%	3874
September	1st to 10th	Day	8% 6% 4 11% 8% 9% 9% 7% 8% 10% 19%	2090
	11th to 20th	Day	10% 7% 3 11% 9% 8% 9% 5% 9% 10% 20%	2090
	21st to end	Day	12% 9% 4 11% 9% 7% 8% 7% 7% 8% 20%	2090
October	1st to 10th	Day	9% 8% 3 10% 7% 8% 8% 7% 8% 10% 23%	2000
	11th to 20th	Day	10% 5% 3 8% 7% 7% 9% 7% 8% 8% 27%	1900
	21st to end	Day	13% 6% 2 8% 6% 5% 5% 5% 6% 9% 36%	2090
November	1st to 10th	Day	7% 6% 2 6% 6% 5% 7% 5% 7% 9% 40%	1957
	11th to 20th	Day	10% 6% 3 7% 8% 6% 5% 5% 6% 9% 34%	2000
	21st to end	Day	8% 6% 2 6% 5% 5% 7% 5% 7% 8% 41%	2000
December	1st to 10th	Day	6% 4 2 4 4 4 5% 4 6% 7% 55%	3194
	11th to 20th	Day	9% 6% 2 6% 5% 5% 6% 4 7% 10% 40%	3373
	21st to end	Day	11% 6% 3 8% 6% 6% 7% 5% 8% 10% 30%	4466
January	1st to 10th	Night	6% 5% 2 7% 5% 4 5% 4 6% 10% 45%	5889
	11th to 20th	Night	15% 9% 4 9% 6% 5% 5% 4 5% 7% 31%	5900
	21st to end	Night	12% 6% 3 7% 4 4 4 4 5% 7% 43%	6398
February	1st to 10th	Night	8% 6% 2 5% 4 4 4 4 6% 9% 48%	2400
	11th to 20th	Night	6% 3 1 3 4 4 5% 5% 5% 7% 56%	2220
	21st to end	Night	6% 2 2 4 4 4 4 3 5% 8% 59%	1815
March	1st to 10th	Night	5% 4 1 6% 4 4 4 4 7% 8% 52%	4918
	11th to 20th	Night	5% 3 1 5% 3 3 3 3 6% 8% 60%	4980
	21st to end	Night	6% 4 2 5% 5% 5% 5% 4 6% 8% 50%	5467
April	1st to 10th	Night	4 2 1 4 3 3 4 4 7% 10% 58%	2200
	11th to 20th	Night	6% 3 1 4 3 4 5% 4 7% 9% 54%	2080
	21st to end	Night	6% 3 2 4 4 4 4 5% 6% 9% 53%	2000
May	1st to 10th	Night	3 2 1 3 3 3 4 3 7% 11% 61%	4490
	11th to 20th	Night	7% 4 1 5% 4 4 4 4 6% 9% 52%	4449
	21st to end	Night	5% 4 2 6% 4 5% 6% 5% 9% 11% 44%	4421
June	1st to 10th	Night	11% 7% 3 9% 7% 7% 7% 7% 7% 10% 27%	1800
	11th to 20th	Night	6% 7% 2 6% 5% 5% 6% 5% 9% 14% 34%	1800
	21st to end	Night	6% 5% 1 7% 6% 5% 7% 6% 10% 14% 32%	1800
July	1st to 10th	Night	5% 5% 2 8% 6% 5% 6% 6% 8% 12% 37%	2885
	11th to 20th	Night	4 5% 2 7% 5% 5% 7% 7% 9% 12% 37%	2840
	21st to end	Night	3 4 2 6% 5% 4 6% 5% 10% 14% 41%	3087
August	1st to 10th	Night	4 4 2 7% 6% 6% 7% 6% 9% 14% 35%	2857
	11th to 20th	Night	4 4 1 6% 5% 6% 7% 6% 10% 15% 35%	3097
	21st to end	Night	5% 4 2 6% 5% 6% 8% 8% 12% 15% 28%	3761
September	1st to 10th	Night	6% 5% 2 7% 6% 7% 8% 7% 10% 11% 30%	2090
	11th to 20th	Night	8% 6% 3 9% 7% 6% 6% 5% 8% 12% 30%	2090
	21st to end	Night	10% 6% 3 9% 7% 8% 7% 6% 9% 11% 26%	2090
October	1st to 10th	Night	9% 6% 3 9% 6% 7% 7% 5% 9% 10% 30%	2176
	11th to 20th	Night	8% 6% 3 7% 5% 5% 5% 5% 7% 11% 39%	2280
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 6% 8% 43%	2508
November	1st to 10th	Night	9% 5% 2 5% 4 4 5% 4 6% 9% 50%	2341
	11th to 20th	Night	8% 6% 2 6% 6% 6% 5% 4 6% 8% 41%	2400
	21st to end	Night	7% 5% 2 6% 5% 5% 4 5% 6% 8% 48%	2400
December	1st to 10th	Night	6% 4 2 4 4 3 4 3 5% 8% 57%	3980
	11th to 20th	Night	12% 7% 3 7% 5% 4 4 4 6% 8% 40%	4822
	21st to end	Night	9% 7% 3 9% 7% 6% 6% 5% 7% 9% 31%	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	31% 4 9% 30% 20% 7%	4057
	11th to 20th	Day	51% 5% 14% 19% 8% 2	4060
	21st to end	Day	42% 4 7% 28% 15% 4	4540
February	1st to 10th	Day	39% 5% 10% 24% 16% 5%	2000
	11th to 20th	Day	29% 5% 10% 28% 22% 6%	2074
	21st to end	Day	33% 6% 10% 19% 24% 8%	1815
March	1st to 10th	Day	35% 8% 12% 19% 22% 4	4935
	11th to 20th	Day	21% 4 13% 29% 20% 12%	4980
	21st to end	Day	35% 6% 9% 28% 17% 6%	5465
April	1st to 10th	Day	29% 5% 16% 28% 18% 4	2200
	11th to 20th	Day	36% 6% 15% 21% 17% 5%	2268
	21st to end	Day	33% 6% 10% 25% 22% 4	2400
May	1st to 10th	Day	21% 6% 17% 34% 19% 3	5437
	11th to 20th	Day	32% 6% 18% 35% 7% 2	5489
	21st to end	Day	34% 9% 18% 29% 9%	6447
June	1st to 10th	Day	62% 9% 16% 11% 3	2600
	11th to 20th	Day	54% 12% 19% 13% 2	2600
	21st to end	Day	54% 13% 23% 10% 1	2600
July	1st to 10th	Day	44% 13% 28% 14%	4120
	11th to 20th	Day	46% 12% 31% 10%	4051
	21st to end	Day	39% 14% 32% 15%	4324
August	1st to 10th	Day	49% 16% 25% 10% 1	4011
	11th to 20th	Day	42% 15% 29% 14%	3766
	21st to end	Day	46% 16% 32% 6% 1	3874
September	1st to 10th	Day	54% 15% 19% 11% 1	2090
	11th to 20th	Day	54% 12% 21% 9% 3	2090
	21st to end	Day	60% 10% 14% 11% 4 1	2090
October	1st to 10th	Day	52% 11% 17% 15% 5% 1	3325
	11th to 20th	Day	49% 8% 14% 17% 11% 1	3140
	21st to end	Day	39% 5% 10% 26% 17% 3	3428
November	1st to 10th	Day	38% 5% 12% 20% 19% 7%	1957
	11th to 20th	Day	46% 6% 9% 18% 15% 6%	2000
	21st to end	Day	38% 6% 11% 19% 22% 4	2000
December	1st to 10th	Day	27% 5% 12% 29% 22% 5%	3194
	11th to 20th	Day	37% 6% 12% 21% 20% 4	3370
	21st to end	Day	46% 6% 15% 17% 14% 2	4466
January	1st to 10th	Night	34% 4 13% 28% 18% 3	5889
	11th to 20th	Night	52% 5% 17% 16% 9% 2	5900
	21st to end	Night	40% 5% 8% 29% 14% 4	6398
February	1st to 10th	Night	32% 5% 13% 28% 18% 4	2400
	11th to 20th	Night	25% 6% 11% 28% 23% 7%	2221
	21st to end	Night	24% 5% 11% 26% 27% 7%	1815
March	1st to 10th	Night	29% 6% 14% 22% 25% 5%	4918
	11th to 20th	Night	22% 5% 19% 27% 18% 10%	4980
	21st to end	Night	30% 6% 12% 27% 21% 3	5467
April	1st to 10th	Night	20% 5% 18% 36% 18% 4	2200
	11th to 20th	Night	25% 6% 18% 24% 21% 5%	2080
	21st to end	Night	25% 6% 15% 30% 21% 4	2000
May	1st to 10th	Night	16% 6% 25% 38% 13% 2	4520
	11th to 20th	Night	27% 7% 19% 35% 12% 1	4450
	21st to end	Night	29% 10% 26% 29% 6%	4429
June	1st to 10th	Night	50% 11% 22% 15% 2	1800
	11th to 20th	Night	38% 12% 29% 18% 3	1800
	21st to end	Night	38% 15% 35% 11% 1	1800
July	1st to 10th	Night	37% 13% 34% 16%	2840
	11th to 20th	Night	35% 13% 41% 11%	2786
	21st to end	Night	29% 14% 39% 16% 1	2988
August	1st to 10th	Night	37% 14% 30% 16% 2	2846
	11th to 20th	Night	31% 16% 34% 18%	3127
	21st to end	Night	37% 17% 38% 6% 1	3761
September	1st to 10th	Night	41% 17% 28% 13% 1	2090
	11th to 20th	Night	45% 15% 26% 12% 2	2090
	21st to end	Night	48% 14% 18% 15% 5%	2090
October	1st to 10th	Night	49% 11% 18% 17% 4 1	3631
	11th to 20th	Night	45% 7% 18% 18% 12% 1	3779
	21st to end	Night	37% 5% 12% 29% 15% 3	4120
November	1st to 10th	Night	32% 5% 14% 23% 19% 7%	2341
	11th to 20th	Night	39% 5% 12% 24% 14% 5%	2400
	21st to end	Night	33% 6% 14% 22% 20% 5%	2400
December	1st to 10th	Night	26% 5% 14% 34% 18% 4	3980
	11th to 20th	Night	40% 6% 11% 21% 16% 5%	4824
	21st to end	Night	46% 8% 15% 17% 13% 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	10% 7% 35% 34% 12% 2	300
	11th to 20th	Day	14% 7% 35% 29% 12% 2	300
	21st to end	Day	13% 6% 43% 29% 8% 1	330
February	1st to 10th	Day	9% 6% 26% 39% 15% 5%	300
	11th to 20th	Day	7% 3 27% 38% 18% 5%	417
	21st to end	Day	6% 5% 24% 37% 26% 2	414
March	1st to 10th	Day	5% 7% 31% 35% 20% 2	500
	11th to 20th	Day	6% 6% 28% 37% 20% 3	500
	21st to end	Day	6% 3 29% 35% 22% 5%	550
April	1st to 10th	Day	3 5% 31% 32% 22% 6%	500
	11th to 20th	Day	8% 4 28% 32% 21% 8%	492
	21st to end	Day	7% 3 21% 34% 26% 9%	500
May	1st to 10th	Day	4 1 19% 34% 29% 14%	503
	11th to 20th	Day	8% 2 17% 31% 27% 14%	527
	21st to end	Day	10% 3 7% 28% 28% 24%	573
June	1st to 10th	Day	15% 3 8% 24% 28% 22%	530
	11th to 20th	Day	10% 3 4 22% 29% 33%	530
	21st to end	Day	6% 2 2 18% 25% 47%	530
July	1st to 10th	Day	5% 2 2 13% 29% 49%	505
	11th to 20th	Day	6% 2 1 14% 31% 46%	484
	21st to end	Day	4 3 2 15% 28% 47%	532
August	1st to 10th	Day	8% 2 3 15% 29% 44%	486
	11th to 20th	Day	6% 2 3 15% 28% 46%	479
	21st to end	Day	9% 3 3 22% 29% 35%	539
September	1st to 10th	Day	8% 3 3 14% 29% 43%	490
	11th to 20th	Day	7% 2 5% 15% 27% 45%	490
	21st to end	Day	12% 3 6% 21% 27% 32%	490
October	1st to 10th	Day	12% 3 8% 34% 30% 13%	490
	11th to 20th	Day	12% 1 15% 30% 28% 14%	490
	21st to end	Day	11% 3 17% 35% 26% 9%	539
November	1st to 10th	Day	9% 4 20% 35% 24% 8%	495
	11th to 20th	Day	12% 2 21% 32% 25% 7%	500
	21st to end	Day	6% 3 24% 35% 28% 4	500
December	1st to 10th	Day	10% 4 20% 38% 23% 5%	460
	11th to 20th	Day	8% 5% 25% 39% 18% 5%	300
	21st to end	Day	11% 6% 29% 34% 15% 6%	330
January	1st to 10th	Night	9% 5% 21% 41% 23% 2	800
	11th to 20th	Night	10% 5% 25% 40% 19% 1	800
	21st to end	Night	9% 5% 27% 43% 15% 1	880
February	1st to 10th	Night	10% 5% 21% 41% 22% 2	800
	11th to 20th	Night	7% 4 23% 42% 22% 2	783
	21st to end	Night	7% 3 21% 48% 20% 1	660
March	1st to 10th	Night	5% 4 28% 44% 18% 1	800
	11th to 20th	Night	7% 4 21% 45% 21% 1	800
	21st to end	Night	7% 3 24% 43% 21% 2	880
April	1st to 10th	Night	4 2 18% 51% 24% 2	800
	11th to 20th	Night	6% 3 21% 46% 22% 2	791
	21st to end	Night	7% 2 17% 46% 27% 2	800
May	1st to 10th	Night	3 2 14% 46% 32% 3	800
	11th to 20th	Night	9% 2 11% 42% 32% 3	774
	21st to end	Night	8% 2 4 30% 46% 10%	650
June	1st to 10th	Night	11% 2 3 22% 46% 15%	600
	11th to 20th	Night	7% 2 2 13% 46% 30%	600
	21st to end	Night	5% 2 2 10% 45% 37%	600
July	1st to 10th	Night	6% 1 1 8% 42% 42%	570
	11th to 20th	Night	5% 2 8% 44% 41%	552
	21st to end	Night	2 1 6% 44% 46%	624
August	1st to 10th	Night	6% 2 1 9% 46% 37%	586
	11th to 20th	Night	3 2 2 10% 44% 39%	750
	21st to end	Night	9% 2 2 18% 41% 28%	836
September	1st to 10th	Night	6% 2 4 12% 41% 35%	760
	11th to 20th	Night	10% 3 2 14% 43% 29%	760
	21st to end	Night	12% 2 4 18% 41% 24%	760
October	1st to 10th	Night	11% 2 6% 33% 37% 11%	760
	11th to 20th	Night	11% 3 6% 32% 39% 9%	760
	21st to end	Night	13% 2 13% 35% 29% 7%	836
November	1st to 10th	Night	10% 3 18% 36% 30% 4	780
	11th to 20th	Night	9% 2 16% 37% 31% 5%	800
	21st to end	Night	7% 3 17% 44% 26% 3	800
December	1st to 10th	Night	7% 3 14% 40% 32% 3	800
	11th to 20th	Night	10% 4 17% 38% 28% 3	800
	21st to end	Night	8% 2 21% 39% 27% 3	880

Page generated at 2025-07-03 01:30:12. Version ac07

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