Jackson College Astronomy Club Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jackson College Astronomy Club Astronomical Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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	13% 10% 4 9% 5% 5% 5% 4 6% 7% 33%	125826
	Night	14% 11% 4 9% 5% 4 4 4 5% 6% 33%	123047
Transparency	Day	48% 7% 12% 21% 12% 1	129394
	Night	49% 6% 11% 20% 12% 1	127549
Seeing	Day	17% 12% 29% 23% 13% 5%	16849
	Night	18% 11% 30% 28% 12% 1	26629

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	12% 16% 7% 19% 6% 5% 5% 3 4 5% 18%	3617
	11th to 20th	Day	14% 14% 7% 17% 11% 6% 8% 5% 5% 4 9%	3620
	21st to end	Day	21% 18% 6% 15% 5% 4 3 3 3 4 17%	4226
February	1st to 10th	Day	13% 15% 6% 11% 9% 5% 5% 4 5% 6% 20%	1813
	11th to 20th	Day	14% 11% 6% 14% 8% 6% 5% 4 4 5% 23%	1948
	21st to end	Day	16% 14% 6% 12% 5% 4 4 4 5% 6% 23%	1650
March	1st to 10th	Day	14% 13% 5% 12% 5% 5% 5% 4 4 5% 29%	4480
	11th to 20th	Day	11% 12% 3 8% 5% 4 4 4 5% 7% 36%	4979
	21st to end	Day	15% 15% 5% 10% 6% 5% 4 4 5% 6% 25%	5465
April	1st to 10th	Day	17% 14% 4 8% 5% 5% 4 3 4 7% 30%	2391
	11th to 20th	Day	16% 10% 3 8% 5% 4 4 4 6% 6% 35%	2482
	21st to end	Day	17% 9% 3 8% 5% 5% 4 4 5% 7% 35%	2600
May	1st to 10th	Day	20% 9% 3 7% 5% 4 4 4 4 6% 34%	5862
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 6% 9% 34%	5861
	21st to end	Day	14% 6% 3 7% 4 4 4 3 5% 8% 40%	6569
June	1st to 10th	Day	11% 7% 2 6% 5% 5% 4 4 6% 9% 41%	2800
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 8% 9% 41%	2800
	21st to end	Day	8% 5% 3 7% 5% 6% 6% 5% 8% 9% 39%	2800
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 11% 38%	6360
	11th to 20th	Day	7% 5% 3 6% 5% 5% 5% 4 7% 10% 45%	6360
	21st to end	Day	7% 6% 2 6% 5% 5% 5% 5% 7% 9% 43%	6680
August	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 44%	4234
	11th to 20th	Day	10% 5% 3 6% 4 5% 5% 4 7% 8% 42%	4174
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 4 7% 9% 43%	4487
September	1st to 10th	Day	11% 6% 2 5% 6% 5% 6% 4 6% 7% 42%	2225
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 7% 9% 47%	2090
	21st to end	Day	13% 8% 3 6% 4 5% 5% 4 6% 7% 39%	2090
October	1st to 10th	Day	12% 7% 2 7% 5% 4 5% 3 6% 7% 42%	2085
	11th to 20th	Day	17% 10% 3 7% 6% 5% 5% 3 5% 6% 34%	1900
	21st to end	Day	19% 11% 3 8% 6% 4 4 4 6% 7% 29%	1881
November	1st to 10th	Day	11% 9% 3 8% 5% 4 5% 4 5% 7% 40%	1762
	11th to 20th	Day	14% 15% 4 10% 6% 5% 4 4 5% 6% 28%	1800
	21st to end	Day	19% 14% 5% 9% 5% 5% 4 4 4 6% 26%	1800
December	1st to 10th	Day	11% 16% 9% 14% 6% 5% 4 4 5% 5% 21%	2930
	11th to 20th	Day	17% 20% 7% 12% 7% 5% 4 3 5% 4 15%	3023
	21st to end	Day	23% 21% 8% 12% 6% 4 3 2 4 5% 12%	3982
January	1st to 10th	Night	13% 15% 7% 16% 6% 5% 4 3 4 5% 19%	6348
	11th to 20th	Night	12% 16% 7% 16% 9% 6% 6% 5% 6% 5% 12%	6360
	21st to end	Night	18% 15% 7% 12% 6% 5% 5% 4 4 5% 20%	6738
February	1st to 10th	Night	15% 15% 5% 13% 7% 5% 4 3 5% 4 23%	2585
	11th to 20th	Night	13% 13% 5% 10% 8% 6% 5% 3 5% 5% 27%	2338
	21st to end	Night	18% 10% 5% 10% 6% 6% 3 3 4 7% 28%	1980
March	1st to 10th	Night	15% 11% 5% 9% 4 4 4 3 5% 7% 35%	5373
	11th to 20th	Night	10% 8% 2 6% 4 4 4 4 5% 8% 44%	4980
	21st to end	Night	16% 10% 3 9% 5% 4 4 4 4 6% 35%	5467
April	1st to 10th	Night	16% 11% 3 6% 5% 4 4 3 6% 6% 36%	2025
	11th to 20th	Night	18% 9% 3 6% 4 4 4 4 5% 6% 37%	1867
	21st to end	Night	16% 8% 3 8% 5% 5% 4 4 5% 7% 37%	1800
May	1st to 10th	Night	20% 12% 3 8% 5% 5% 4 3 5% 7% 30%	4033
	11th to 20th	Night	18% 8% 3 7% 4 3 3 3 5% 6% 39%	4032
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 8% 43%	4275
June	1st to 10th	Night	10% 4 2 5% 5% 4 4 4 7% 10% 47%	1600
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 7% 8% 43%	1600
	21st to end	Night	8% 5% 2 5% 5% 3 5% 5% 8% 10% 46%	1600
July	1st to 10th	Night	7% 4 2 5% 4 4 5% 4 6% 10% 48%	3618
	11th to 20th	Night	7% 5% 2 5% 4 3 4 4 6% 9% 51%	3617
	21st to end	Night	9% 5% 2 6% 4 4 4 4 5% 9% 48%	4181
August	1st to 10th	Night	10% 5% 2 6% 4 5% 4 3 6% 8% 48%	2915
	11th to 20th	Night	9% 6% 3 6% 3 4 4 4 6% 8% 48%	2875
	21st to end	Night	9% 7% 2 7% 4 4 5% 4 5% 7% 46%	3409
September	1st to 10th	Night	9% 6% 3 5% 4 4 4 4 5% 6% 50%	2001
	11th to 20th	Night	9% 5% 2 4 3 3 4 3 4 7% 56%	2090
	21st to end	Night	13% 8% 2 6% 4 5% 5% 3 6% 7% 43%	2090
October	1st to 10th	Night	12% 7% 3 6% 4 4 4 4 5% 6% 47%	2094
	11th to 20th	Night	16% 11% 3 8% 4 4 3 3 4 6% 37%	2280
	21st to end	Night	20% 9% 3 7% 5% 4 4 3 5% 6% 33%	2714
November	1st to 10th	Night	13% 10% 3 8% 4 4 5% 3 4 5% 41%	2536
	11th to 20th	Night	15% 12% 3 9% 5% 5% 5% 3 6% 7% 30%	2600
	21st to end	Night	16% 16% 5% 9% 5% 4 4 4 4 7% 26%	2600
December	1st to 10th	Night	12% 14% 6% 14% 7% 5% 5% 4 6% 6% 22%	4250
	11th to 20th	Night	15% 19% 6% 14% 7% 5% 4 3 4 5% 17%	5180
	21st to end	Night	24% 21% 6% 11% 5% 4 3 3 4 5% 13%	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	67% 6% 10% 13% 3	3618
	11th to 20th	Day	74% 6% 10% 6% 5%	3620
	21st to end	Day	72% 4 8% 9% 6%	4226
February	1st to 10th	Day	63% 7% 10% 13% 7% 1	1813
	11th to 20th	Day	62% 6% 8% 15% 8% 1	1958
	21st to end	Day	60% 7% 12% 14% 6% 2	1650
March	1st to 10th	Day	56% 6% 11% 17% 8% 1	4480
	11th to 20th	Day	45% 6% 15% 24% 8% 2	4979
	21st to end	Day	58% 7% 9% 15% 10% 1	5465
April	1st to 10th	Day	54% 6% 9% 19% 10%	2391
	11th to 20th	Day	48% 7% 11% 20% 12% 2	2482
	21st to end	Day	48% 7% 13% 18% 14% 1	2600
May	1st to 10th	Day	50% 7% 10% 18% 14% 1	5897
	11th to 20th	Day	44% 8% 14% 21% 12% 1	5865
	21st to end	Day	41% 7% 10% 22% 18% 3	6583
June	1st to 10th	Day	37% 8% 13% 25% 15% 2	2800
	11th to 20th	Day	37% 8% 14% 23% 17% 2	2800
	21st to end	Day	39% 9% 13% 22% 15% 2	2800
July	1st to 10th	Day	37% 7% 13% 26% 15% 2	6291
	11th to 20th	Day	33% 7% 13% 29% 17% 1	6285
	21st to end	Day	35% 8% 13% 23% 20% 1	6489
August	1st to 10th	Day	35% 8% 14% 25% 17% 1	4221
	11th to 20th	Day	37% 8% 12% 30% 13% 1	4213
	21st to end	Day	35% 6% 13% 32% 12% 2	4486
September	1st to 10th	Day	39% 8% 15% 21% 15% 2	2225
	11th to 20th	Day	31% 8% 12% 30% 18% 1	2090
	21st to end	Day	42% 8% 14% 20% 17%	2090
October	1st to 10th	Day	30% 5% 11% 34% 20% 1	3462
	11th to 20th	Day	41% 5% 10% 24% 20%	3140
	21st to end	Day	46% 4 12% 24% 11% 2	3081
November	1st to 10th	Day	43% 7% 11% 25% 14% 1	1762
	11th to 20th	Day	56% 6% 11% 17% 8% 1	1800
	21st to end	Day	60% 5% 14% 15% 6% 1	1800
December	1st to 10th	Day	64% 7% 15% 10% 3	2930
	11th to 20th	Day	71% 6% 10% 11% 2	3020
	21st to end	Day	75% 5% 7% 9% 3	3982
January	1st to 10th	Night	65% 6% 8% 14% 5% 1	6348
	11th to 20th	Night	70% 5% 10% 8% 5%	6360
	21st to end	Night	65% 6% 12% 10% 7%	6738
February	1st to 10th	Night	63% 7% 10% 12% 6% 1	2585
	11th to 20th	Night	58% 6% 10% 18% 7% 1	2339
	21st to end	Night	57% 5% 13% 16% 8% 2	1980
March	1st to 10th	Night	47% 5% 13% 24% 10% 1	5373
	11th to 20th	Night	37% 6% 14% 27% 15% 1	4980
	21st to end	Night	50% 6% 11% 18% 14% 1	5467
April	1st to 10th	Night	47% 6% 10% 21% 14% 1	2025
	11th to 20th	Night	46% 6% 11% 18% 17% 2	1867
	21st to end	Night	46% 6% 12% 20% 14% 3	1800
May	1st to 10th	Night	53% 7% 9% 15% 13% 2	4060
	11th to 20th	Night	43% 7% 11% 25% 12% 2	4033
	21st to end	Night	38% 7% 13% 21% 19% 2	4283
June	1st to 10th	Night	32% 6% 14% 27% 19% 2	1600
	11th to 20th	Night	37% 7% 13% 25% 16% 2	1600
	21st to end	Night	31% 9% 16% 26% 18% 2	1600
July	1st to 10th	Night	31% 6% 14% 27% 17% 5%	3580
	11th to 20th	Night	30% 6% 15% 28% 19% 2	3572
	21st to end	Night	32% 7% 13% 21% 24% 2	4059
August	1st to 10th	Night	33% 7% 14% 30% 15% 1	2904
	11th to 20th	Night	33% 7% 12% 33% 14% 2	2902
	21st to end	Night	35% 7% 10% 30% 16% 1	3409
September	1st to 10th	Night	32% 7% 10% 27% 19% 3	2001
	11th to 20th	Night	28% 6% 10% 30% 23% 3	2090
	21st to end	Night	39% 7% 12% 26% 15% 1	2090
October	1st to 10th	Night	25% 4 10% 34% 26% 1	3497
	11th to 20th	Night	42% 4 10% 22% 21%	3779
	21st to end	Night	39% 4 10% 32% 15% 1	4464
November	1st to 10th	Night	45% 6% 11% 23% 15% 1	2536
	11th to 20th	Night	53% 7% 13% 17% 9% 2	2600
	21st to end	Night	57% 6% 12% 18% 6%	2600
December	1st to 10th	Night	61% 6% 15% 13% 4	4250
	11th to 20th	Night	68% 6% 11% 12% 4	5182
	21st to end	Night	74% 6% 8% 8% 4	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	19% 17% 40% 19% 4 1	300
	11th to 20th	Day	22% 18% 44% 12% 2 1	300
	21st to end	Day	26% 19% 35% 16% 3	330
February	1st to 10th	Day	25% 20% 36% 15% 2 1	300
	11th to 20th	Day	27% 19% 36% 16% 2	300
	21st to end	Day	27% 25% 33% 12% 2	249
March	1st to 10th	Day	22% 21% 42% 12% 2	300
	11th to 20th	Day	23% 14% 35% 21% 6% 1	499
	21st to end	Day	25% 18% 34% 16% 5% 1	550
April	1st to 10th	Day	22% 16% 36% 18% 7%	500
	11th to 20th	Day	23% 20% 31% 15% 7% 3	492
	21st to end	Day	21% 15% 33% 19% 10% 2	500
May	1st to 10th	Day	24% 14% 31% 20% 10% 2	503
	11th to 20th	Day	20% 11% 29% 22% 12% 6%	527
	21st to end	Day	14% 10% 24% 26% 18% 7%	631
June	1st to 10th	Day	13% 9% 29% 30% 15% 4	730
	11th to 20th	Day	14% 7% 30% 28% 17% 5%	730
	21st to end	Day	9% 9% 31% 31% 15% 5%	730
July	1st to 10th	Day	8% 5% 21% 31% 23% 11%	721
	11th to 20th	Day	10% 6% 25% 27% 21% 10%	718
	21st to end	Day	8% 3 26% 31% 24% 9%	689
August	1st to 10th	Day	8% 4 18% 30% 22% 18%	526
	11th to 20th	Day	12% 4 20% 26% 28% 10%	519
	21st to end	Day	9% 3 21% 31% 24% 12%	583
September	1st to 10th	Day	13% 6% 23% 23% 22% 13%	502
	11th to 20th	Day	9% 3 21% 23% 28% 15%	490
	21st to end	Day	17% 11% 23% 25% 16% 8%	490
October	1st to 10th	Day	13% 9% 29% 25% 18% 6%	490
	11th to 20th	Day	19% 16% 29% 26% 7% 2	490
	21st to end	Day	28% 17% 34% 15% 4 1	330
November	1st to 10th	Day	13% 19% 35% 24% 7% 2	300
	11th to 20th	Day	21% 25% 30% 18% 5% 1	300
	21st to end	Day	27% 19% 37% 15% 2	300
December	1st to 10th	Day	21% 28% 31% 13% 7% 1	300
	11th to 20th	Day	23% 25% 33% 15% 4	300
	21st to end	Day	24% 26% 32% 17% 1	330
January	1st to 10th	Night	20% 17% 39% 19% 5%	1000
	11th to 20th	Night	22% 17% 37% 20% 4 1	1000
	21st to end	Night	19% 14% 38% 24% 5%	985
February	1st to 10th	Night	24% 15% 38% 21% 3	800
	11th to 20th	Night	21% 16% 37% 22% 3	783
	21st to end	Night	24% 16% 39% 19% 2	660
March	1st to 10th	Night	20% 13% 37% 24% 6%	800
	11th to 20th	Night	17% 16% 39% 23% 5% 1	800
	21st to end	Night	24% 12% 31% 25% 7%	880
April	1st to 10th	Night	19% 16% 36% 26% 4 1	625
	11th to 20th	Night	20% 13% 35% 22% 9% 1	593
	21st to end	Night	19% 10% 35% 28% 8% 1	600
May	1st to 10th	Night	20% 11% 29% 32% 7% 2	600
	11th to 20th	Night	23% 6% 30% 27% 14% 1	591
	21st to end	Night	12% 4 26% 38% 17% 2	650
June	1st to 10th	Night	11% 3 27% 43% 15%	600
	11th to 20th	Night	12% 4 24% 42% 17% 1	600
	21st to end	Night	9% 3 24% 42% 20% 3	600
July	1st to 10th	Night	8% 2 18% 42% 26% 4	585
	11th to 20th	Night	9% 2 18% 43% 25% 3	582
	21st to end	Night	7% 2 19% 44% 28% 1	657
August	1st to 10th	Night	8% 2 14% 44% 30% 3	597
	11th to 20th	Night	8% 2 17% 41% 28% 4	594
	21st to end	Night	11% 2 16% 38% 29% 4	660
September	1st to 10th	Night	11% 3 20% 35% 26% 5%	650
	11th to 20th	Night	11% 4 16% 34% 32% 4	760
	21st to end	Night	16% 7% 23% 30% 20% 4	760
October	1st to 10th	Night	15% 7% 26% 32% 18% 2	760
	11th to 20th	Night	20% 14% 30% 25% 9% 1	760
	21st to end	Night	24% 14% 33% 23% 6%	836
November	1st to 10th	Night	19% 13% 31% 27% 11% 1	780
	11th to 20th	Night	21% 17% 35% 22% 6% 1	800
	21st to end	Night	24% 13% 36% 23% 4	800
December	1st to 10th	Night	23% 16% 37% 20% 5%	800
	11th to 20th	Night	23% 15% 38% 20% 3	981
	21st to end	Night	30% 15% 34% 19% 2	1100

Page generated at 2025-09-01 01:51:05. Version ac07

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