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 2025-02-06.

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	14% 10% 4 8% 5% 4 4 4 5% 7% 35%	108344
	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 35%	110049
Transparency	Day	47% 4 12% 22% 13% 2	111986
	Night	48% 4 10% 22% 14% 2	114534
Seeing	Day	17% 9% 26% 24% 16% 7%	15855
	Night	19% 8% 28% 30% 14% 2	25799

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	13% 17% 9% 15% 7% 4 3 3 4 4 21%	3617
	11th to 20th	Day	13% 16% 9% 12% 7% 7% 6% 5% 3 3 19%	3620
	21st to end	Day	20% 18% 8% 12% 5% 3 3 2 3 4 22%	4225
February	1st to 10th	Day	18% 14% 7% 11% 7% 5% 4 3 4 5% 23%	1910
	11th to 20th	Day	15% 13% 4 10% 7% 5% 4 3 5% 5% 29%	1848
	21st to end	Day	17% 15% 6% 9% 5% 4 3 3 5% 6% 29%	1570
March	1st to 10th	Day	20% 20% 5% 9% 4 3 2 2 4 4 26%	3120
	11th to 20th	Day	13% 9% 3 7% 4 4 3 4 5% 7% 41%	3402
	21st to end	Day	16% 11% 4 10% 6% 5% 4 3 5% 7% 29%	3929
April	1st to 10th	Day	15% 13% 3 8% 5% 5% 3 3 5% 6% 33%	2280
	11th to 20th	Day	18% 9% 3 7% 4 4 4 3 5% 6% 37%	2259
	21st to end	Day	20% 8% 3 8% 5% 5% 4 3 5% 7% 33%	2470
May	1st to 10th	Day	19% 10% 3 7% 5% 5% 4 4 6% 7% 28%	4082
	11th to 20th	Day	19% 8% 2 5% 4 4 4 3 6% 8% 37%	4081
	21st to end	Day	10% 7% 2 7% 5% 5% 5% 4 7% 10% 37%	4530
June	1st to 10th	Day	10% 7% 2 7% 6% 5% 5% 4 6% 8% 40%	2660
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 8% 43%	2660
	21st to end	Day	10% 6% 3 7% 5% 5% 4 4 6% 9% 41%	2660
July	1st to 10th	Day	13% 7% 3 8% 5% 5% 5% 5% 6% 10% 34%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 8% 11% 46%	4440
	21st to end	Day	9% 7% 2 7% 5% 5% 6% 5% 7% 10% 37%	4631
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 45%	4104
	11th to 20th	Day	9% 6% 2 6% 5% 5% 6% 4 7% 11% 41%	3952
	21st to end	Day	7% 5% 2 5% 4 4 4 4 6% 9% 50%	4180
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 5% 8% 46%	2228
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 7% 8% 48%	2090
	21st to end	Day	15% 6% 2 5% 4 3 4 3 4 7% 46%	2090
October	1st to 10th	Day	11% 6% 2 7% 4 4 3 3 4 7% 49%	2090
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 7% 44%	1938
	21st to end	Day	19% 10% 3 7% 4 4 5% 2 6% 7% 34%	1881
November	1st to 10th	Day	9% 7% 2 6% 4 4 4 3 5% 8% 49%	1762
	11th to 20th	Day	15% 13% 4 8% 6% 4 4 3 6% 6% 32%	1800
	21st to end	Day	19% 13% 4 7% 4 3 3 3 5% 5% 32%	1800
December	1st to 10th	Day	15% 13% 4 9% 5% 3 3 2 4 5% 39%	2930
	11th to 20th	Day	15% 17% 5% 11% 5% 5% 4 3 5% 5% 23%	3083
	21st to end	Day	24% 20% 8% 11% 5% 5% 4 3 5% 5% 12%	3982
January	1st to 10th	Night	16% 17% 8% 14% 6% 4 3 3 4 4 21%	6348
	11th to 20th	Night	12% 20% 8% 15% 7% 6% 5% 4 4 3 17%	6360
	21st to end	Night	20% 15% 6% 8% 6% 5% 4 3 4 4 26%	6739
February	1st to 10th	Night	18% 15% 4 11% 6% 4 4 3 5% 4 26%	2380
	11th to 20th	Night	14% 12% 5% 10% 6% 5% 3 3 5% 6% 31%	2218
	21st to end	Night	19% 12% 4 11% 5% 5% 4 3 4 5% 29%	1884
March	1st to 10th	Night	19% 14% 3 6% 4 4 3 3 4 5% 34%	3733
	11th to 20th	Night	12% 8% 2 5% 5% 4 4 3 5% 7% 45%	3557
	21st to end	Night	18% 9% 3 7% 5% 4 3 3 5% 6% 36%	3732
April	1st to 10th	Night	14% 11% 2 6% 4 4 5% 4 5% 7% 38%	1900
	11th to 20th	Night	17% 8% 2 5% 5% 4 3 3 5% 8% 38%	1870
	21st to end	Night	17% 8% 3 6% 4 4 5% 4 7% 7% 35%	1710
May	1st to 10th	Night	18% 11% 4 8% 5% 5% 5% 4 5% 8% 27%	2813
	11th to 20th	Night	19% 7% 2 5% 4 3 3 4 5% 8% 39%	2812
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 8% 9% 42%	3022
June	1st to 10th	Night	8% 6% 2 6% 4 5% 5% 3 5% 7% 49%	1520
	11th to 20th	Night	8% 6% 2 6% 4 3 4 3 5% 9% 48%	1520
	21st to end	Night	8% 4 2 6% 5% 4 5% 6% 8% 10% 43%	1520
July	1st to 10th	Night	9% 5% 2 6% 5% 4 4 4 7% 9% 46%	2530
	11th to 20th	Night	6% 3 2 4 3 3 4 4 6% 9% 55%	2530
	21st to end	Night	10% 7% 2 7% 5% 4 5% 4 7% 10% 40%	2924
August	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 5% 8% 48%	2825
	11th to 20th	Night	7% 7% 3 7% 3 4 4 5% 6% 8% 46%	2873
	21st to end	Night	5% 4 2 6% 4 5% 4 4 6% 8% 53%	3476
September	1st to 10th	Night	11% 5% 2 6% 4 3 4 4 5% 8% 49%	1931
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 5% 8% 55%	2090
	21st to end	Night	13% 5% 2 5% 4 3 4 3 5% 6% 50%	2090
October	1st to 10th	Night	9% 5% 2 6% 4 4 3 3 4 6% 55%	2090
	11th to 20th	Night	18% 7% 2 5% 3 3 3 3 4 5% 45%	2235
	21st to end	Night	17% 8% 2 6% 5% 4 4 4 5% 7% 38%	2717
November	1st to 10th	Night	12% 9% 2 5% 3 3 3 3 4 6% 47%	2536
	11th to 20th	Night	16% 10% 3 7% 4 3 3 3 5% 6% 38%	2600
	21st to end	Night	19% 14% 4 8% 6% 4 4 3 4 5% 30%	2600
December	1st to 10th	Night	14% 10% 4 8% 5% 4 4 3 5% 5% 36%	4250
	11th to 20th	Night	17% 16% 7% 11% 6% 3 4 3 4 5% 24%	5118
	21st to end	Night	25% 19% 7% 10% 5% 4 4 2 4 5% 14%	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% 2 7% 18% 6%	3618
	11th to 20th	Day	71% 2 9% 8% 8% 2	3620
	21st to end	Day	69% 2 8% 12% 7% 2	4225
February	1st to 10th	Day	64% 3 9% 13% 10% 1	1910
	11th to 20th	Day	58% 3 10% 20% 8% 1	1858
	21st to end	Day	57% 4 11% 19% 7% 2	1570
March	1st to 10th	Day	63% 3 7% 17% 9% 1	3120
	11th to 20th	Day	43% 4 10% 31% 11% 1	3402
	21st to end	Day	55% 4 14% 22% 4	3929
April	1st to 10th	Day	51% 4 12% 20% 13% 1	2280
	11th to 20th	Day	47% 5% 10% 24% 11% 3	2259
	21st to end	Day	51% 4 10% 22% 10% 3	2470
May	1st to 10th	Day	53% 6% 13% 21% 7%	4120
	11th to 20th	Day	45% 6% 14% 24% 10%	4085
	21st to end	Day	39% 7% 13% 23% 16% 1	4543
June	1st to 10th	Day	41% 5% 12% 23% 16% 3	2660
	11th to 20th	Day	36% 6% 11% 25% 20% 2	2660
	21st to end	Day	39% 6% 16% 20% 16% 4	2660
July	1st to 10th	Day	44% 5% 12% 21% 16% 2	4371
	11th to 20th	Day	29% 5% 16% 29% 19% 1	4365
	21st to end	Day	41% 7% 16% 18% 15% 3	4484
August	1st to 10th	Day	32% 6% 18% 29% 14% 1	4091
	11th to 20th	Day	35% 6% 13% 32% 14% 1	3991
	21st to end	Day	30% 5% 13% 30% 17% 4	4179
September	1st to 10th	Day	36% 5% 11% 24% 19% 4	2228
	11th to 20th	Day	33% 6% 11% 25% 22% 4	2090
	21st to end	Day	38% 4 9% 25% 22% 1	2090
October	1st to 10th	Day	25% 3 9% 32% 29% 2	3480
	11th to 20th	Day	34% 3 10% 27% 25% 1	3193
	21st to end	Day	42% 3 10% 24% 19% 2	3081
November	1st to 10th	Day	35% 4 13% 28% 17% 3	1762
	11th to 20th	Day	52% 3 10% 20% 12% 3	1800
	21st to end	Day	54% 4 10% 22% 8% 2	1800
December	1st to 10th	Day	50% 2 11% 26% 11%	2930
	11th to 20th	Day	63% 3 11% 17% 5%	3080
	21st to end	Day	75% 3 8% 10% 4	3982
January	1st to 10th	Night	68% 3 7% 15% 7%	6348
	11th to 20th	Night	73% 2 6% 9% 8% 1	6360
	21st to end	Night	62% 2 9% 18% 8% 1	6739
February	1st to 10th	Night	61% 3 11% 16% 8% 1	2380
	11th to 20th	Night	54% 4 11% 18% 12% 1	2219
	21st to end	Night	57% 4 11% 16% 10% 2	1884
March	1st to 10th	Night	53% 4 8% 24% 10% 1	3733
	11th to 20th	Night	39% 5% 11% 27% 16% 2	3557
	21st to end	Night	49% 4 13% 26% 7% 1	3732
April	1st to 10th	Night	46% 5% 11% 24% 13% 1	1900
	11th to 20th	Night	44% 5% 10% 20% 16% 4	1870
	21st to end	Night	47% 6% 10% 20% 12% 4	1710
May	1st to 10th	Night	55% 5% 10% 21% 7% 1	2840
	11th to 20th	Night	44% 5% 13% 28% 9% 1	2813
	21st to end	Night	35% 7% 15% 25% 15% 3	3030
June	1st to 10th	Night	34% 5% 10% 27% 20% 3	1520
	11th to 20th	Night	33% 6% 12% 25% 21% 3	1520
	21st to end	Night	32% 8% 15% 25% 16% 5%	1520
July	1st to 10th	Night	34% 7% 11% 27% 16% 6%	2490
	11th to 20th	Night	24% 6% 14% 32% 22% 2	2482
	21st to end	Night	39% 5% 15% 20% 17% 4	2831
August	1st to 10th	Night	34% 6% 15% 30% 14% 1	2814
	11th to 20th	Night	36% 5% 11% 29% 18% 1	2900
	21st to end	Night	29% 5% 12% 28% 22% 3	3476
September	1st to 10th	Night	33% 5% 11% 24% 20% 6%	1931
	11th to 20th	Night	28% 5% 9% 28% 26% 3	2090
	21st to end	Night	35% 5% 9% 24% 23% 3	2090
October	1st to 10th	Night	21% 3 7% 30% 37% 2	3480
	11th to 20th	Night	31% 2 10% 23% 33% 1	3706
	21st to end	Night	35% 3 8% 28% 24% 3	4467
November	1st to 10th	Night	38% 4 9% 25% 21% 3	2536
	11th to 20th	Night	46% 3 9% 22% 16% 3	2600
	21st to end	Night	58% 3 9% 21% 9%	2600
December	1st to 10th	Night	49% 3 12% 25% 10%	4250
	11th to 20th	Night	64% 2 10% 16% 7% 1	5120
	21st to end	Night	74% 3 9% 10% 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	24% 18% 34% 19% 4 1	300
	11th to 20th	Day	20% 21% 39% 15% 5% 1	300
	21st to end	Day	28% 17% 31% 19% 5%	330
February	1st to 10th	Day	28% 15% 34% 19% 4	285
	11th to 20th	Day	23% 19% 34% 18% 6%	270
	21st to end	Day	24% 15% 40% 19% 3	225
March	1st to 10th	Day	23% 20% 36% 19% 2	270
	11th to 20th	Day	25% 14% 35% 17% 8% 1	408
	21st to end	Day	30% 14% 31% 17% 8% 1	506
April	1st to 10th	Day	18% 17% 32% 21% 9% 3	460
	11th to 20th	Day	19% 16% 30% 20% 12% 3	452
	21st to end	Day	24% 14% 33% 18% 8% 2	460
May	1st to 10th	Day	21% 13% 29% 25% 9% 4	463
	11th to 20th	Day	23% 6% 26% 24% 13% 8%	487
	21st to end	Day	14% 4 25% 27% 20% 10%	557
June	1st to 10th	Day	10% 6% 26% 31% 21% 6%	680
	11th to 20th	Day	11% 5% 23% 32% 23% 7%	680
	21st to end	Day	13% 4 24% 29% 20% 9%	680
July	1st to 10th	Day	13% 2 16% 24% 27% 18%	671
	11th to 20th	Day	7% 4 19% 32% 25% 13%	668
	21st to end	Day	11% 3 12% 30% 29% 14%	632
August	1st to 10th	Day	11% 1 12% 28% 29% 18%	486
	11th to 20th	Day	13% 3 18% 26% 27% 13%	479
	21st to end	Day	9% 2 11% 29% 31% 17%	539
September	1st to 10th	Day	13% 3 15% 25% 32% 12%	490
	11th to 20th	Day	10% 2 13% 27% 30% 19%	490
	21st to end	Day	18% 8% 20% 23% 22% 9%	490
October	1st to 10th	Day	12% 5% 24% 32% 19% 8%	490
	11th to 20th	Day	13% 14% 32% 24% 13% 3	447
	21st to end	Day	28% 12% 35% 19% 5%	330
November	1st to 10th	Day	13% 16% 31% 24% 13% 3	300
	11th to 20th	Day	22% 14% 34% 19% 9% 2	300
	21st to end	Day	29% 9% 38% 20% 3	300
December	1st to 10th	Day	19% 21% 39% 16% 5% 1	300
	11th to 20th	Day	22% 16% 37% 19% 6% 1	300
	21st to end	Day	30% 18% 30% 18% 4	330
January	1st to 10th	Night	23% 15% 34% 22% 5%	1000
	11th to 20th	Night	27% 13% 37% 20% 3	1000
	21st to end	Night	24% 14% 36% 22% 4	986
February	1st to 10th	Night	27% 10% 35% 25% 3	781
	11th to 20th	Night	21% 12% 38% 27% 3	743
	21st to end	Night	26% 10% 39% 22% 3	628
March	1st to 10th	Night	21% 14% 33% 26% 5%	760
	11th to 20th	Night	20% 12% 36% 26% 5% 1	760
	21st to end	Night	25% 10% 31% 29% 5% 1	789
April	1st to 10th	Night	18% 11% 37% 28% 7%	570
	11th to 20th	Night	20% 10% 33% 28% 9% 1	563
	21st to end	Night	21% 7% 41% 21% 9% 1	570
May	1st to 10th	Night	23% 5% 34% 28% 10%	570
	11th to 20th	Night	20% 5% 24% 33% 17% 2	561
	21st to end	Night	11% 3 23% 38% 22% 3	617
June	1st to 10th	Night	9% 5% 23% 46% 18% 1	570
	11th to 20th	Night	11% 4 20% 41% 22% 1	570
	21st to end	Night	9% 2 23% 39% 25% 2	570
July	1st to 10th	Night	9% 2 10% 39% 33% 6%	555
	11th to 20th	Night	7% 3 13% 41% 31% 5%	552
	21st to end	Night	8% 3 9% 43% 34% 2	624
August	1st to 10th	Night	9% 2 10% 40% 34% 4	567
	11th to 20th	Night	8% 1 13% 40% 33% 6%	564
	21st to end	Night	6% 3 11% 36% 39% 4	627
September	1st to 10th	Night	14% 3 12% 35% 31% 5%	601
	11th to 20th	Night	10% 1 14% 36% 34% 6%	760
	21st to end	Night	15% 4 20% 34% 23% 3	760
October	1st to 10th	Night	12% 6% 23% 37% 19% 4	760
	11th to 20th	Night	23% 9% 28% 26% 13% 1	760
	21st to end	Night	20% 10% 36% 26% 7%	836
November	1st to 10th	Night	17% 10% 33% 27% 13% 1	780
	11th to 20th	Night	20% 15% 31% 26% 8% 1	800
	21st to end	Night	26% 11% 29% 26% 7%	800
December	1st to 10th	Night	24% 12% 35% 25% 4	800
	11th to 20th	Night	26% 9% 35% 23% 6%	945
	21st to end	Night	30% 13% 31% 22% 4	1100

Page generated at 2025-02-03 03:05:49. Version ac07

Copyright © 2025 Attilla Danko.