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 2024-01-27.

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% 9% 4 7% 6% 3 5% 4 5% 7% 35%	74985
	Night	15% 9% 4 7% 6% 3 4 4 5% 6% 37%	75076
Transparency	Day	48% 5% 12% 21% 12% 2	74693
	Night	48% 5% 11% 21% 14% 2	74889
Seeing	Day	17% 9% 25% 24% 17% 7%	14451
	Night	19% 8% 28% 29% 14% 2	24418

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	17% 16% 7% 10% 8% 4 5% 4 4 4 22%	1517
	11th to 20th	Day	18% 15% 8% 9% 7% 5% 5% 5% 4 5% 20%	1520
	21st to end	Day	15% 18% 9% 10% 5% 4 5% 4 5% 5% 19%	1732
February	1st to 10th	Day	18% 14% 8% 9% 8% 3 6% 3 3 5% 22%	1764
	11th to 20th	Day	16% 12% 5% 10% 8% 4 4 4 5% 5% 27%	1748
	21st to end	Day	18% 14% 7% 8% 5% 3 3 4 4 6% 28%	1480
March	1st to 10th	Day	18% 13% 4 8% 6% 3 3 3 5% 6% 31%	1800
	11th to 20th	Day	17% 10% 4 7% 6% 3 4 5% 5% 7% 30%	1930
	21st to end	Day	22% 12% 5% 8% 6% 3 4 3 4 6% 27%	2238
April	1st to 10th	Day	15% 12% 4 6% 6% 4 4 4 5% 6% 35%	2160
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 4 5% 6% 37%	2138
	21st to end	Day	21% 8% 3 7% 6% 3 5% 3 4 7% 33%	2340
May	1st to 10th	Day	18% 9% 3 7% 6% 4 5% 5% 5% 6% 32%	2340
	11th to 20th	Day	21% 8% 3 6% 5% 3 5% 5% 5% 7% 34%	2301
	21st to end	Day	11% 6% 3 6% 6% 4 7% 5% 7% 8% 36%	2557
June	1st to 10th	Day	10% 6% 3 6% 7% 4 5% 5% 6% 8% 41%	2520
	11th to 20th	Day	10% 5% 2 5% 4 3 6% 6% 7% 8% 43%	2520
	21st to end	Day	10% 5% 3 7% 5% 4 5% 5% 6% 9% 41%	2520
July	1st to 10th	Day	11% 5% 3 6% 5% 3 6% 7% 6% 11% 38%	2520
	11th to 20th	Day	7% 5% 2 5% 5% 3 5% 5% 7% 11% 46%	2520
	21st to end	Day	9% 5% 2 4 5% 4 5% 7% 6% 10% 43%	2642
August	1st to 10th	Day	8% 5% 2 5% 5% 3 5% 5% 6% 10% 46%	2327
	11th to 20th	Day	10% 5% 2 6% 6% 4 6% 6% 7% 9% 39%	2253
	21st to end	Day	8% 4 2 5% 5% 3 5% 6% 7% 10% 46%	2376
September	1st to 10th	Day	12% 6% 3 5% 5% 3 5% 4 5% 8% 45%	2111
	11th to 20th	Day	10% 5% 2 4 4 4 4 5% 6% 8% 47%	1980
	21st to end	Day	14% 5% 2 4 5% 2 4 3 4 7% 49%	1980
October	1st to 10th	Day	12% 6% 3 6% 5% 3 3 4 4 7% 47%	1980
	11th to 20th	Day	14% 6% 2 5% 4 3 4 5% 6% 7% 43%	1837
	21st to end	Day	20% 10% 3 5% 5% 3 4 3 5% 7% 33%	1782
November	1st to 10th	Day	9% 7% 2 5% 4 3 4 4 5% 8% 50%	1672
	11th to 20th	Day	15% 13% 4 7% 6% 2 3 4 6% 7% 33%	1710
	21st to end	Day	20% 11% 5% 6% 5% 2 3 4 5% 6% 33%	1710
December	1st to 10th	Day	20% 15% 5% 10% 6% 3 4 3 4 5% 24%	1710
	11th to 20th	Day	15% 17% 7% 11% 5% 3 4 4 4 4 26%	1967
	21st to end	Day	22% 21% 9% 10% 7% 3 4 3 4 5% 13%	2783
January	1st to 10th	Night	18% 14% 7% 10% 7% 3 4 4 4 4 26%	2648
	11th to 20th	Night	18% 18% 7% 10% 7% 4 4 4 3 3 23%	2660
	21st to end	Night	17% 15% 7% 10% 8% 3 4 4 4 5% 25%	2765
February	1st to 10th	Night	20% 14% 5% 9% 7% 3 4 3 5% 4 25%	2196
	11th to 20th	Night	15% 12% 5% 9% 7% 4 4 4 5% 6% 31%	2098
	21st to end	Night	19% 11% 5% 10% 6% 4 4 4 4 5% 28%	1776
March	1st to 10th	Night	16% 10% 4 6% 6% 3 4 4 5% 6% 35%	2160
	11th to 20th	Night	15% 10% 4 5% 7% 3 4 4 5% 7% 37%	2029
	21st to end	Night	21% 9% 4 6% 5% 3 4 4 4 7% 32%	2134
April	1st to 10th	Night	14% 9% 3 5% 5% 3 5% 5% 5% 7% 39%	1800
	11th to 20th	Night	17% 8% 2 4 5% 3 4 4 5% 8% 39%	1771
	21st to end	Night	18% 8% 3 5% 4 3 5% 5% 7% 7% 34%	1620
May	1st to 10th	Night	19% 10% 4 7% 5% 4 5% 5% 5% 8% 29%	1620
	11th to 20th	Night	18% 8% 3 5% 5% 3 3 5% 5% 8% 38%	1592
	21st to end	Night	9% 6% 2 5% 5% 4 5% 5% 8% 11% 42%	1713
June	1st to 10th	Night	8% 5% 2 5% 5% 4 5% 4 4 8% 49%	1440
	11th to 20th	Night	8% 6% 3 5% 5% 3 4 4 5% 9% 49%	1440
	21st to end	Night	8% 3 2 5% 5% 3 5% 6% 8% 10% 44%	1440
July	1st to 10th	Night	7% 5% 2 5% 6% 3 6% 5% 7% 8% 47%	1440
	11th to 20th	Night	7% 4 2 4 5% 2 4 5% 7% 9% 52%	1440
	21st to end	Night	8% 5% 2 5% 5% 3 4 4 6% 8% 51%	1673
August	1st to 10th	Night	7% 3 2 4 5% 4 5% 6% 7% 8% 49%	1611
	11th to 20th	Night	6% 5% 2 5% 5% 2 5% 5% 5% 8% 51%	1644
	21st to end	Night	7% 4 2 4 4 4 4 5% 6% 8% 52%	1980
September	1st to 10th	Night	12% 5% 2 5% 5% 2 4 5% 5% 8% 47%	1829
	11th to 20th	Night	9% 4 2 4 4 2 3 4 5% 8% 54%	1980
	21st to end	Night	12% 4 2 4 4 2 4 4 5% 6% 52%	1980
October	1st to 10th	Night	9% 5% 2 5% 5% 3 3 4 4 6% 53%	1980
	11th to 20th	Night	19% 7% 2 4 4 3 4 4 4 5% 44%	2117
	21st to end	Night	18% 8% 2 6% 6% 3 4 5% 4 7% 37%	2574
November	1st to 10th	Night	12% 9% 3 4 4 3 3 4 4 6% 48%	2406
	11th to 20th	Night	16% 9% 4 7% 5% 3 3 3 5% 7% 39%	2470
	21st to end	Night	19% 13% 4 7% 6% 3 4 4 4 5% 31%	2470
December	1st to 10th	Night	18% 12% 6% 8% 7% 3 5% 4 5% 5% 27%	2470
	11th to 20th	Night	17% 14% 7% 9% 6% 3 3 4 4 6% 28%	3226
	21st to end	Night	23% 18% 8% 10% 6% 3 4 3 4 5% 15%	4884

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	64% 3 9% 16% 8%	1518
	11th to 20th	Day	68% 4 11% 13% 5%	1520
	21st to end	Day	67% 4 11% 12% 5% 1	1732
February	1st to 10th	Day	66% 3 9% 13% 8% 1	1764
	11th to 20th	Day	60% 3 10% 19% 7% 1	1758
	21st to end	Day	58% 4 11% 19% 7% 2	1480
March	1st to 10th	Day	54% 3 11% 21% 9% 1	1800
	11th to 20th	Day	55% 5% 12% 18% 9% 1	1930
	21st to end	Day	60% 4 12% 20% 4 1	2238
April	1st to 10th	Day	50% 4 12% 20% 13% 1	2160
	11th to 20th	Day	47% 5% 10% 23% 11% 4	2138
	21st to end	Day	52% 4 10% 21% 10% 3	2340
May	1st to 10th	Day	52% 6% 12% 20% 9% 1	2340
	11th to 20th	Day	50% 7% 12% 18% 13% 1	2305
	21st to end	Day	43% 7% 13% 21% 14% 1	2557
June	1st to 10th	Day	41% 5% 12% 23% 16% 3	2520
	11th to 20th	Day	37% 6% 12% 24% 19% 2	2520
	21st to end	Day	39% 6% 16% 20% 15% 4	2520
July	1st to 10th	Day	39% 7% 14% 21% 15% 3	2451
	11th to 20th	Day	32% 6% 15% 27% 18% 2	2445
	21st to end	Day	33% 8% 17% 24% 14% 3	2495
August	1st to 10th	Day	32% 7% 15% 30% 14% 2	2314
	11th to 20th	Day	38% 7% 14% 26% 15% 1	2253
	21st to end	Day	33% 7% 11% 26% 16% 7%	2376
September	1st to 10th	Day	37% 6% 12% 24% 18% 4	2111
	11th to 20th	Day	34% 6% 12% 25% 20% 3	1980
	21st to end	Day	36% 4 9% 26% 23% 1	1980
October	1st to 10th	Day	38% 4 11% 26% 18% 3	1980
	11th to 20th	Day	39% 4 11% 25% 19% 2	1837
	21st to end	Day	52% 3 10% 22% 12% 1	1782
November	1st to 10th	Day	34% 4 13% 28% 18% 3	1672
	11th to 20th	Day	52% 3 10% 21% 11% 3	1710
	21st to end	Day	53% 4 10% 23% 8% 2	1710
December	1st to 10th	Day	64% 3 10% 16% 7%	1710
	11th to 20th	Day	61% 3 9% 21% 6%	1964
	21st to end	Day	76% 3 7% 9% 5%	2783
January	1st to 10th	Night	62% 4 7% 17% 9%	2648
	11th to 20th	Night	67% 3 8% 15% 6%	2660
	21st to end	Night	63% 3 9% 15% 8% 1	2765
February	1st to 10th	Night	63% 3 11% 15% 7% 1	2196
	11th to 20th	Night	55% 3 11% 18% 11% 1	2099
	21st to end	Night	58% 4 11% 16% 9% 2	1776
March	1st to 10th	Night	50% 4 10% 22% 12% 1	2160
	11th to 20th	Night	47% 5% 12% 23% 10% 3	2029
	21st to end	Night	54% 4 11% 21% 8% 2	2134
April	1st to 10th	Night	44% 6% 11% 25% 13% 1	1800
	11th to 20th	Night	44% 5% 10% 20% 16% 5%	1771
	21st to end	Night	48% 6% 10% 20% 12% 4	1620
May	1st to 10th	Night	54% 5% 11% 19% 9% 2	1620
	11th to 20th	Night	45% 6% 12% 22% 12% 2	1593
	21st to end	Night	34% 7% 17% 24% 14% 4	1714
June	1st to 10th	Night	34% 5% 10% 26% 20% 3	1440
	11th to 20th	Night	33% 6% 12% 25% 21% 3	1440
	21st to end	Night	32% 8% 15% 25% 16% 4	1440
July	1st to 10th	Night	33% 9% 15% 21% 16% 6%	1400
	11th to 20th	Night	27% 7% 14% 30% 20% 2	1392
	21st to end	Night	31% 5% 14% 28% 18% 3	1580
August	1st to 10th	Night	29% 8% 16% 27% 18% 2	1600
	11th to 20th	Night	32% 7% 13% 27% 20% 2	1644
	21st to end	Night	28% 6% 14% 25% 22% 5%	1980
September	1st to 10th	Night	35% 5% 11% 24% 19% 6%	1829
	11th to 20th	Night	29% 5% 9% 28% 25% 3	1980
	21st to end	Night	33% 6% 9% 25% 24% 3	1980
October	1st to 10th	Night	32% 5% 11% 26% 23% 3	1980
	11th to 20th	Night	43% 3 8% 25% 19% 2	2117
	21st to end	Night	48% 3 9% 24% 14% 2	2574
November	1st to 10th	Night	38% 4 9% 25% 21% 3	2406
	11th to 20th	Night	46% 3 10% 22% 16% 3	2470
	21st to end	Night	56% 4 10% 21% 9%	2470
December	1st to 10th	Night	58% 4 10% 18% 9% 1	2470
	11th to 20th	Night	59% 2 9% 21% 8% 1	3228
	21st to end	Night	73% 4 9% 10% 5%	4884

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	24% 18% 33% 19% 4 1	270
	11th to 20th	Day	21% 19% 38% 15% 5% 1	270
	21st to end	Day	29% 18% 29% 18% 6%	282
February	1st to 10th	Day	31% 15% 35% 17% 3	240
	11th to 20th	Day	25% 19% 34% 17% 5%	240
	21st to end	Day	24% 15% 40% 19% 2	198
March	1st to 10th	Day	22% 20% 38% 18% 2	240
	11th to 20th	Day	26% 12% 35% 17% 8% 1	370
	21st to end	Day	31% 14% 30% 16% 8% 1	462
April	1st to 10th	Day	19% 15% 32% 22% 10% 3	420
	11th to 20th	Day	19% 16% 30% 20% 12% 4	412
	21st to end	Day	26% 14% 32% 18% 8% 2	420
May	1st to 10th	Day	22% 14% 28% 25% 9% 3	423
	11th to 20th	Day	24% 5% 28% 24% 13% 6%	447
	21st to end	Day	15% 4 24% 27% 20% 11%	511
June	1st to 10th	Day	11% 5% 25% 32% 21% 6%	630
	11th to 20th	Day	11% 5% 23% 31% 22% 8%	630
	21st to end	Day	13% 4 23% 29% 21% 9%	630
July	1st to 10th	Day	12% 2 15% 24% 28% 19%	621
	11th to 20th	Day	7% 4 18% 32% 25% 14%	618
	21st to end	Day	11% 3 13% 29% 30% 15%	583
August	1st to 10th	Day	11% 1 11% 28% 30% 20%	446
	11th to 20th	Day	13% 3 16% 25% 28% 14%	439
	21st to end	Day	9% 3 12% 29% 30% 17%	495
September	1st to 10th	Day	13% 4 15% 24% 31% 12%	450
	11th to 20th	Day	10% 2 13% 27% 29% 19%	450
	21st to end	Day	14% 8% 21% 23% 23% 10%	450
October	1st to 10th	Day	12% 5% 25% 32% 18% 8%	450
	11th to 20th	Day	14% 14% 33% 26% 12% 1	410
	21st to end	Day	30% 13% 34% 20% 3	297
November	1st to 10th	Day	13% 15% 30% 24% 14% 3	270
	11th to 20th	Day	21% 14% 33% 19% 10% 2	270
	21st to end	Day	28% 10% 37% 21% 4	270
December	1st to 10th	Day	20% 21% 36% 17% 6% 1	270
	11th to 20th	Day	21% 16% 37% 19% 6% 1	270
	21st to end	Day	29% 19% 31% 17% 4	297
January	1st to 10th	Night	23% 15% 34% 22% 5%	950
	11th to 20th	Night	27% 13% 36% 20% 4	950
	21st to end	Night	25% 14% 35% 23% 4	921
February	1st to 10th	Night	28% 10% 35% 24% 3	720
	11th to 20th	Night	22% 12% 37% 27% 3	703
	21st to end	Night	26% 10% 40% 21% 3	592
March	1st to 10th	Night	21% 15% 33% 26% 5%	720
	11th to 20th	Night	21% 12% 35% 26% 6% 1	720
	21st to end	Night	26% 9% 30% 29% 5% 1	748
April	1st to 10th	Night	17% 10% 37% 29% 7%	540
	11th to 20th	Night	20% 9% 32% 28% 10% 1	533
	21st to end	Night	22% 7% 39% 21% 9% 1	540
May	1st to 10th	Night	23% 5% 34% 27% 10%	540
	11th to 20th	Night	20% 5% 24% 33% 16% 2	531
	21st to end	Night	10% 3 22% 38% 23% 3	584
June	1st to 10th	Night	9% 5% 22% 46% 19% 1	540
	11th to 20th	Night	11% 4 21% 40% 23% 1	540
	21st to end	Night	9% 2 22% 39% 26% 2	540
July	1st to 10th	Night	9% 2 10% 38% 34% 7%	525
	11th to 20th	Night	8% 3 12% 40% 32% 5%	522
	21st to end	Night	8% 3 10% 42% 35% 3	591
August	1st to 10th	Night	9% 3 10% 39% 35% 4	537
	11th to 20th	Night	8% 13% 39% 34% 6%	534
	21st to end	Night	6% 3 12% 36% 38% 4	594
September	1st to 10th	Night	15% 2 12% 34% 32% 5%	569
	11th to 20th	Night	10% 1 15% 34% 34% 6%	720
	21st to end	Night	13% 4 21% 35% 24% 3	720
October	1st to 10th	Night	12% 6% 23% 36% 18% 5%	720
	11th to 20th	Night	23% 9% 29% 26% 12% 1	720
	21st to end	Night	21% 10% 35% 26% 7%	792
November	1st to 10th	Night	16% 9% 34% 27% 13% 1	740
	11th to 20th	Night	20% 15% 30% 26% 8% 1	760
	21st to end	Night	26% 11% 29% 27% 8%	760
December	1st to 10th	Night	24% 12% 35% 24% 4	760
	11th to 20th	Night	25% 9% 35% 24% 6%	897
	21st to end	Night	30% 13% 31% 22% 4	1045

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