Anderson University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anderson University Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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 8% 5% 5% 4 4 6% 7% 34%	131665
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 35%	131673
Transparency	Day	48% 4 11% 22% 13% 1	135264
	Night	48% 4 10% 22% 14% 2	136134
Seeing	Day	17% 10% 26% 24% 16% 7%	17653
	Night	19% 9% 28% 29% 14% 2	27533

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% 16% 8% 14% 6% 4 4 3 5% 5% 21%	4691
	11th to 20th	Day	10% 14% 8% 14% 8% 9% 6% 5% 3 4 19%	4710
	21st to end	Day	17% 16% 7% 13% 6% 5% 5% 3 4 4 19%	5475
February	1st to 10th	Day	17% 14% 8% 10% 7% 5% 4 3 4 5% 23%	2058
	11th to 20th	Day	15% 14% 4 11% 7% 5% 4 3 5% 5% 28%	2048
	21st to end	Day	15% 15% 5% 9% 5% 4 3 3 5% 6% 30%	1730
March	1st to 10th	Day	18% 16% 7% 9% 4 4 2 2 4 4 30%	4580
	11th to 20th	Day	11% 11% 3 7% 4 4 3 4 5% 7% 41%	4967
	21st to end	Day	16% 12% 4 9% 5% 5% 4 4 5% 6% 29%	5757
April	1st to 10th	Day	16% 12% 3 8% 5% 5% 3 4 5% 6% 33%	2520
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 5% 6% 36%	2499
	21st to end	Day	19% 8% 3 7% 5% 5% 4 3 5% 7% 34%	2730
May	1st to 10th	Day	19% 10% 3 8% 6% 5% 4 4 6% 7% 29%	5992
	11th to 20th	Day	18% 8% 2 7% 4 4 4 4 6% 7% 36%	5991
	21st to end	Day	13% 8% 2 7% 5% 5% 5% 4 7% 9% 33%	6660
June	1st to 10th	Day	10% 7% 2 7% 6% 5% 5% 4 6% 8% 41%	2801
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 5% 7% 8% 42%	2800
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	2800
July	1st to 10th	Day	10% 6% 3 7% 4 4 5% 5% 6% 10% 41%	6360
	11th to 20th	Day	8% 6% 2 7% 4 5% 6% 4 8% 11% 38%	6360
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 9% 35%	6659
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 46%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 4 7% 11% 40%	4079
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 10% 50%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 5% 8% 48%	2345
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 8% 49%	2200
	21st to end	Day	15% 6% 2 5% 4 3 4 3 4 7% 47%	2200
October	1st to 10th	Day	11% 6% 2 7% 4 4 3 3 4 7% 49%	2200
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 7% 44%	2040
	21st to end	Day	19% 10% 3 7% 4 4 4 3 6% 7% 34%	1980
November	1st to 10th	Day	9% 8% 2 6% 4 4 4 3 5% 8% 48%	1852
	11th to 20th	Day	15% 13% 4 8% 5% 4 4 3 6% 6% 32%	1890
	21st to end	Day	20% 13% 5% 8% 4 3 3 3 5% 5% 32%	1890
December	1st to 10th	Day	14% 13% 4 10% 5% 3 3 2 4 5% 38%	3020
	11th to 20th	Day	15% 17% 5% 12% 5% 5% 4 4 5% 5% 23%	3165
	21st to end	Day	24% 20% 8% 11% 5% 5% 4 3 4 5% 12%	4070
January	1st to 10th	Night	14% 16% 8% 14% 6% 4 4 3 5% 5% 21%	8232
	11th to 20th	Night	10% 17% 8% 17% 7% 6% 5% 4 4 4 19%	8280
	21st to end	Night	17% 13% 5% 8% 6% 5% 5% 4 5% 6% 26%	8719
February	1st to 10th	Night	17% 15% 4 11% 5% 4 4 2 5% 5% 26%	2561
	11th to 20th	Night	15% 12% 5% 10% 6% 5% 3 3 5% 6% 31%	2458
	21st to end	Night	18% 11% 5% 11% 5% 5% 4 3 4 5% 30%	2076
March	1st to 10th	Night	19% 13% 3 6% 4 3 3 3 4 5% 37%	5493
	11th to 20th	Night	12% 7% 2 6% 5% 4 4 3 5% 7% 43%	5198
	21st to end	Night	17% 10% 4 8% 5% 5% 4 3 5% 6% 34%	5458
April	1st to 10th	Night	15% 11% 2 6% 4 4 5% 4 5% 7% 38%	2100
	11th to 20th	Night	16% 8% 2 6% 5% 5% 4 3 6% 8% 38%	2070
	21st to end	Night	17% 8% 2 6% 4 4 4 4 7% 8% 35%	1890
May	1st to 10th	Night	20% 11% 4 9% 5% 5% 4 3 5% 7% 28%	4123
	11th to 20th	Night	16% 7% 2 7% 5% 4 4 4 5% 9% 39%	4122
	21st to end	Night	13% 8% 3 7% 4 4 5% 4 7% 8% 38%	4435
June	1st to 10th	Night	9% 6% 2 6% 4 5% 5% 3 5% 8% 48%	1600
	11th to 20th	Night	9% 6% 3 6% 4 4 4 3 5% 9% 47%	1600
	21st to end	Night	8% 4 2 6% 5% 4 4 6% 8% 10% 45%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 7% 9% 47%	3618
	11th to 20th	Night	8% 5% 2 5% 4 4 4 5% 7% 10% 46%	3617
	21st to end	Night	9% 7% 3 7% 5% 4 5% 4 7% 9% 39%	4199
August	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 5% 8% 49%	2915
	11th to 20th	Night	7% 7% 3 7% 3 4 4 5% 6% 8% 47%	2966
	21st to end	Night	5% 4 2 6% 4 5% 4 4 6% 8% 53%	3586
September	1st to 10th	Night	11% 5% 2 6% 4 3 3 4 5% 7% 49%	2033
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 5% 8% 56%	2200
	21st to end	Night	13% 6% 2 5% 4 3 4 3 5% 6% 50%	2200
October	1st to 10th	Night	9% 5% 2 6% 4 4 3 4 4 6% 55%	2200
	11th to 20th	Night	17% 7% 2 5% 3 4 3 3 4 5% 46%	2353
	21st to end	Night	17% 8% 2 7% 5% 4 4 4 5% 7% 38%	2860
November	1st to 10th	Night	12% 9% 2 5% 3 3 3 3 4 6% 48%	2666
	11th to 20th	Night	15% 10% 3 7% 4 3 3 3 5% 7% 38%	2730
	21st to end	Night	19% 14% 4 8% 5% 4 4 3 4 5% 30%	2730
December	1st to 10th	Night	14% 10% 4 8% 6% 4 4 3 5% 5% 36%	4380
	11th to 20th	Night	17% 16% 7% 11% 6% 4 4 3 4 5% 24%	5255
	21st to end	Night	25% 19% 7% 11% 5% 4 4 2 4 5% 14%	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	65% 2 11% 16% 5%	4692
	11th to 20th	Day	71% 2 8% 9% 8% 2	4710
	21st to end	Day	70% 2 8% 13% 6% 2	5475
February	1st to 10th	Day	64% 3 10% 14% 9% 1	2058
	11th to 20th	Day	59% 3 9% 20% 8% 1	2058
	21st to end	Day	56% 4 12% 20% 8% 2	1730
March	1st to 10th	Day	60% 3 6% 19% 11% 1	4580
	11th to 20th	Day	42% 3 9% 33% 11% 1	4967
	21st to end	Day	55% 4 13% 18% 8% 2	5759
April	1st to 10th	Day	52% 4 12% 20% 12%	2520
	11th to 20th	Day	47% 5% 10% 24% 10% 3	2499
	21st to end	Day	50% 4 11% 23% 10% 3	2730
May	1st to 10th	Day	54% 5% 12% 18% 10%	6027
	11th to 20th	Day	47% 5% 12% 24% 11% 1	5995
	21st to end	Day	44% 6% 12% 25% 12% 1	6674
June	1st to 10th	Day	40% 5% 12% 23% 16% 3	2801
	11th to 20th	Day	38% 6% 11% 24% 19% 2	2800
	21st to end	Day	38% 6% 16% 20% 16% 4	2800
July	1st to 10th	Day	39% 5% 10% 27% 18% 1	6291
	11th to 20th	Day	37% 6% 17% 25% 14% 1	6285
	21st to end	Day	43% 6% 14% 20% 14% 2	6469
August	1st to 10th	Day	32% 6% 18% 29% 14% 1	4221
	11th to 20th	Day	36% 6% 13% 32% 14% 1	4118
	21st to end	Day	30% 5% 13% 31% 17% 4	4311
September	1st to 10th	Day	35% 5% 12% 25% 19% 4	2345
	11th to 20th	Day	32% 6% 11% 25% 23% 3	2200
	21st to end	Day	38% 4 9% 24% 24% 2	2200
October	1st to 10th	Day	25% 3 9% 32% 30% 2	3590
	11th to 20th	Day	34% 3 10% 27% 24% 2	3295
	21st to end	Day	42% 3 9% 24% 20% 2	3180
November	1st to 10th	Day	35% 4 13% 29% 17% 3	1852
	11th to 20th	Day	52% 3 10% 20% 11% 3	1890
	21st to end	Day	55% 4 10% 22% 8% 1	1890
December	1st to 10th	Day	51% 2 11% 26% 11%	3020
	11th to 20th	Day	63% 3 11% 17% 5%	3162
	21st to end	Day	75% 3 8% 10% 4	4070
January	1st to 10th	Night	66% 3 9% 16% 7%	8233
	11th to 20th	Night	70% 3 6% 11% 9% 1	8280
	21st to end	Night	59% 2 11% 20% 7% 1	8719
February	1st to 10th	Night	60% 3 10% 17% 8% 1	2561
	11th to 20th	Night	55% 3 11% 19% 11% 1	2459
	21st to end	Night	56% 3 12% 17% 10% 2	2076
March	1st to 10th	Night	51% 3 6% 26% 12% 2	5493
	11th to 20th	Night	41% 4 11% 28% 15% 2	5198
	21st to end	Night	51% 3 10% 26% 9% 1	5458
April	1st to 10th	Night	46% 5% 11% 24% 13% 1	2100
	11th to 20th	Night	44% 5% 11% 21% 16% 4	2070
	21st to end	Night	46% 5% 10% 23% 12% 4	1890
May	1st to 10th	Night	57% 4 8% 17% 11% 2	4150
	11th to 20th	Night	44% 5% 12% 27% 12% 1	4123
	21st to end	Night	43% 6% 14% 23% 12% 2	4443
June	1st to 10th	Night	35% 5% 10% 26% 21% 3	1600
	11th to 20th	Night	34% 6% 12% 25% 21% 3	1600
	21st to end	Night	31% 7% 16% 25% 16% 4	1600
July	1st to 10th	Night	33% 6% 12% 27% 17% 5%	3580
	11th to 20th	Night	32% 6% 16% 29% 16% 1	3572
	21st to end	Night	40% 4 15% 23% 15% 3	4076
August	1st to 10th	Night	34% 6% 14% 31% 14% 1	2904
	11th to 20th	Night	36% 5% 11% 29% 18% 1	2993
	21st to end	Night	29% 5% 12% 29% 23% 3	3586
September	1st to 10th	Night	33% 5% 11% 25% 21% 6%	2033
	11th to 20th	Night	28% 5% 9% 28% 28% 3	2200
	21st to end	Night	35% 5% 9% 24% 24% 3	2200
October	1st to 10th	Night	21% 3 6% 30% 37% 2	3590
	11th to 20th	Night	31% 2 10% 23% 32% 1	3824
	21st to end	Night	36% 3 7% 27% 24% 3	4610
November	1st to 10th	Night	38% 4 9% 26% 21% 3	2666
	11th to 20th	Night	46% 3 9% 23% 16% 3	2730
	21st to end	Night	58% 3 9% 20% 9%	2730
December	1st to 10th	Night	50% 3 12% 25% 10%	4380
	11th to 20th	Night	64% 2 10% 16% 7% 1	5257
	21st to end	Night	74% 3 9% 10% 4	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	25% 18% 32% 20% 4 1	330
	11th to 20th	Day	20% 24% 38% 14% 4 1	330
	21st to end	Day	28% 18% 30% 19% 5%	363
February	1st to 10th	Day	27% 14% 34% 20% 5%	330
	11th to 20th	Day	25% 18% 35% 16% 5%	330
	21st to end	Day	23% 16% 41% 18% 2	273
March	1st to 10th	Day	23% 18% 39% 18% 2	330
	11th to 20th	Day	26% 16% 34% 16% 7% 1	482
	21st to end	Day	29% 14% 32% 17% 7% 1	594
April	1st to 10th	Day	20% 16% 33% 20% 8% 2	540
	11th to 20th	Day	19% 18% 30% 20% 11% 3	532
	21st to end	Day	24% 13% 33% 21% 9% 2	540
May	1st to 10th	Day	22% 13% 30% 24% 8% 3	543
	11th to 20th	Day	22% 7% 28% 23% 12% 7%	567
	21st to end	Day	16% 5% 25% 26% 18% 9%	649
June	1st to 10th	Day	11% 6% 26% 31% 20% 6%	731
	11th to 20th	Day	11% 5% 22% 32% 22% 7%	730
	21st to end	Day	12% 4 23% 30% 21% 9%	730
July	1st to 10th	Day	12% 2 16% 25% 28% 17%	721
	11th to 20th	Day	8% 4 18% 32% 25% 13%	718
	21st to end	Day	11% 3 12% 30% 30% 14%	681
August	1st to 10th	Day	11% 1 13% 27% 30% 18%	526
	11th to 20th	Day	12% 3 17% 26% 29% 13%	519
	21st to end	Day	9% 3 11% 30% 31% 16%	583
September	1st to 10th	Day	12% 4 16% 24% 32% 12%	530
	11th to 20th	Day	9% 2 12% 26% 31% 20%	530
	21st to end	Day	17% 7% 20% 23% 23% 10%	530
October	1st to 10th	Day	11% 5% 22% 31% 22% 8%	530
	11th to 20th	Day	13% 13% 31% 25% 15% 3	485
	21st to end	Day	28% 13% 35% 20% 5%	363
November	1st to 10th	Day	12% 17% 32% 24% 12% 3	330
	11th to 20th	Day	24% 14% 33% 19% 8% 2	330
	21st to end	Day	29% 10% 38% 19% 3	330
December	1st to 10th	Day	18% 19% 40% 17% 5% 1	330
	11th to 20th	Day	22% 17% 37% 19% 5% 1	330
	21st to end	Day	29% 18% 32% 17% 4	363
January	1st to 10th	Night	23% 15% 35% 22% 5%	1050
	11th to 20th	Night	26% 14% 38% 19% 3	1050
	21st to end	Night	23% 14% 37% 22% 4	1035
February	1st to 10th	Night	25% 10% 36% 25% 3	840
	11th to 20th	Night	21% 11% 39% 26% 3	823
	21st to end	Night	24% 10% 41% 21% 3	692
March	1st to 10th	Night	23% 15% 33% 24% 5%	840
	11th to 20th	Night	20% 14% 36% 25% 5% 1	840
	21st to end	Night	25% 11% 31% 29% 5%	871
April	1st to 10th	Night	19% 11% 37% 27% 6%	630
	11th to 20th	Night	19% 11% 33% 27% 9% 1	623
	21st to end	Night	20% 7% 40% 23% 9% 1	630
May	1st to 10th	Night	25% 5% 34% 27% 9%	630
	11th to 20th	Night	19% 6% 26% 32% 16% 2	621
	21st to end	Night	12% 4 24% 37% 21% 3	683
June	1st to 10th	Night	9% 5% 23% 46% 17% 1	600
	11th to 20th	Night	12% 4 20% 42% 22% 1	600
	21st to end	Night	8% 2 22% 39% 25% 3	600
July	1st to 10th	Night	9% 2 10% 38% 34% 6%	585
	11th to 20th	Night	8% 3 13% 41% 31% 4	582
	21st to end	Night	8% 3 9% 42% 35% 3	657
August	1st to 10th	Night	9% 3 10% 40% 35% 4	597
	11th to 20th	Night	8% 1 13% 39% 34% 6%	594
	21st to end	Night	6% 3 11% 38% 38% 4	660
September	1st to 10th	Night	14% 3 12% 35% 32% 5%	633
	11th to 20th	Night	10% 1 13% 36% 34% 6%	800
	21st to end	Night	15% 4 20% 34% 23% 4	800
October	1st to 10th	Night	12% 5% 22% 37% 20% 4	800
	11th to 20th	Night	22% 9% 28% 27% 13% 1	800
	21st to end	Night	20% 10% 36% 26% 7%	880
November	1st to 10th	Night	16% 10% 34% 27% 12%	820
	11th to 20th	Night	20% 15% 32% 25% 7% 1	840
	21st to end	Night	27% 12% 28% 26% 7%	840
December	1st to 10th	Night	24% 12% 36% 24% 4	840
	11th to 20th	Night	26% 10% 36% 23% 6%	992
	21st to end	Night	31% 13% 32% 21% 3	1155

Page generated at 2026-05-29 01:53:24. Version ac07

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