Anderson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anderson Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	12% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	114422
	Night	12% 7% 3 7% 5% 4 5% 4 6% 7% 41%	114540
Transparency	Day	42% 6% 11% 22% 15% 4	118085
	Night	40% 6% 11% 24% 16% 3	119016
Seeing	Day	21% 12% 22% 22% 16% 7%	15892
	Night	18% 11% 22% 31% 17% 2	26055

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% 7% 3 7% 4 5% 4 3 5% 7% 42%	4056
	11th to 20th	Day	11% 7% 4 8% 6% 4 4 4 6% 6% 42%	4060
	21st to end	Day	15% 8% 3 7% 4 4 4 3 4 5% 42%	4466
February	1st to 10th	Day	12% 9% 2 8% 6% 6% 6% 5% 6% 6% 36%	1800
	11th to 20th	Day	18% 6% 3 9% 7% 4 4 4 6% 6% 34%	1832
	21st to end	Day	18% 7% 3 8% 5% 4 4 4 5% 6% 36%	1703
March	1st to 10th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 7% 35%	4935
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 5% 7% 49%	4980
	21st to end	Day	13% 8% 3 8% 5% 5% 5% 4 5% 6% 40%	5465
April	1st to 10th	Day	10% 7% 3 7% 4 5% 4 4 6% 8% 43%	2250
	11th to 20th	Day	13% 6% 3 8% 4 4 4 4 6% 7% 42%	2374
	21st to end	Day	15% 6% 3 7% 5% 4 5% 3 6% 6% 39%	2570
May	1st to 10th	Day	15% 9% 3 8% 6% 6% 6% 5% 6% 8% 28%	4212
	11th to 20th	Day	19% 8% 3 7% 6% 5% 5% 4 6% 7% 29%	4081
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 10% 38%	4476
June	1st to 10th	Day	10% 5% 2 6% 6% 5% 6% 5% 7% 10% 38%	2470
	11th to 20th	Day	8% 5% 2 7% 6% 5% 6% 5% 7% 10% 37%	2470
	21st to end	Day	8% 6% 2 7% 5% 7% 6% 5% 7% 10% 37%	2470
July	1st to 10th	Day	6% 4 2 7% 5% 7% 7% 6% 9% 10% 37%	4120
	11th to 20th	Day	8% 5% 3 7% 6% 6% 7% 5% 8% 11% 35%	4120
	21st to end	Day	13% 9% 4 11% 8% 8% 6% 6% 7% 8% 22%	4466
August	1st to 10th	Day	13% 6% 3 8% 6% 7% 7% 5% 8% 10% 27%	4104
	11th to 20th	Day	8% 4 2 7% 6% 7% 8% 6% 9% 11% 32%	4044
	21st to end	Day	6% 5% 2 6% 4 5% 5% 5% 7% 11% 43%	4254
September	1st to 10th	Day	10% 7% 2 7% 5% 5% 4 4 6% 8% 42%	2205
	11th to 20th	Day	15% 6% 2 7% 6% 5% 5% 4 6% 8% 37%	2090
	21st to end	Day	19% 8% 3 8% 5% 4 5% 3 5% 5% 35%	2090
October	1st to 10th	Day	14% 6% 2 6% 3 4 3 3 5% 6% 48%	1933
	11th to 20th	Day	13% 6% 2 7% 4 4 4 3 6% 7% 45%	1900
	21st to end	Day	13% 6% 2 6% 5% 3 4 3 5% 7% 47%	2090
November	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 6% 7% 49%	1957
	11th to 20th	Day	10% 7% 2 5% 5% 4 5% 4 5% 8% 45%	1810
	21st to end	Day	13% 7% 2 7% 5% 4 3 3 4 7% 45%	1800
December	1st to 10th	Day	10% 9% 2 7% 4 4 5% 4 5% 5% 45%	2930
	11th to 20th	Day	17% 9% 3 6% 4 4 5% 4 7% 7% 36%	3373
	21st to end	Day	17% 9% 3 7% 5% 5% 6% 4 7% 7% 30%	4466
January	1st to 10th	Night	11% 7% 3 8% 5% 5% 4 4 6% 6% 39%	5889
	11th to 20th	Night	11% 6% 3 9% 5% 4 5% 4 5% 5% 43%	5900
	21st to end	Night	17% 8% 2 5% 4 3 3 3 5% 6% 43%	6490
February	1st to 10th	Night	14% 8% 3 5% 4 4 5% 5% 6% 8% 39%	2600
	11th to 20th	Night	19% 7% 2 7% 5% 4 4 5% 6% 6% 35%	2452
	21st to end	Night	13% 7% 3 6% 4 5% 5% 5% 6% 6% 39%	1925
March	1st to 10th	Night	17% 7% 2 6% 5% 4 5% 4 5% 5% 40%	4918
	11th to 20th	Night	11% 5% 2 5% 3 4 4 4 6% 8% 49%	4980
	21st to end	Night	14% 7% 2 6% 5% 4 4 4 5% 7% 42%	5467
April	1st to 10th	Night	12% 7% 2 6% 5% 4 4 4 6% 8% 40%	2150
	11th to 20th	Night	12% 5% 2 5% 4 4 4 4 6% 7% 46%	1975
	21st to end	Night	12% 7% 2 6% 4 4 4 4 5% 6% 44%	1845
May	1st to 10th	Night	14% 9% 3 8% 5% 5% 5% 3 6% 7% 35%	2903
	11th to 20th	Night	21% 9% 2 6% 5% 5% 3 3 4 7% 34%	2818
	21st to end	Night	8% 4 2 7% 5% 4 5% 4 7% 11% 42%	3079
June	1st to 10th	Night	8% 5% 2 6% 5% 4 6% 5% 7% 10% 42%	1710
	11th to 20th	Night	8% 6% 2 6% 5% 4 4 4 6% 9% 46%	1710
	21st to end	Night	6% 5% 2 5% 4 5% 6% 4 6% 11% 45%	1710
July	1st to 10th	Night	5% 4 2 6% 6% 6% 7% 5% 8% 10% 41%	2840
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 7% 11% 44%	2840
	21st to end	Night	9% 8% 4 10% 7% 8% 6% 5% 7% 10% 28%	3087
August	1st to 10th	Night	13% 7% 3 8% 6% 5% 5% 4 7% 11% 31%	2825
	11th to 20th	Night	7% 6% 2 6% 5% 6% 7% 5% 7% 9% 40%	2785
	21st to end	Night	6% 4 2 6% 4 4 5% 4 6% 9% 51%	3369
September	1st to 10th	Night	8% 6% 3 7% 4 5% 5% 4 5% 6% 46%	1971
	11th to 20th	Night	13% 6% 2 8% 4 5% 4 3 5% 7% 42%	2090
	21st to end	Night	17% 7% 2 7% 5% 5% 5% 3 6% 5% 39%	2090
October	1st to 10th	Night	13% 6% 3 6% 4 4 4 3 3 7% 45%	2246
	11th to 20th	Night	12% 7% 2 6% 5% 4 4 3 5% 5% 47%	2280
	21st to end	Night	11% 6% 2 5% 4 3 4 4 6% 6% 49%	2508
November	1st to 10th	Night	12% 6% 3 6% 4 3 4 4 5% 5% 48%	2341
	11th to 20th	Night	11% 5% 2 6% 5% 4 4 4 6% 6% 47%	2585
	21st to end	Night	12% 6% 2 6% 5% 4 4 4 5% 7% 46%	2600
December	1st to 10th	Night	10% 6% 2 7% 5% 4 4 4 5% 7% 45%	4250
	11th to 20th	Night	16% 9% 4 8% 6% 5% 5% 4 6% 7% 32%	4822
	21st to end	Night	17% 7% 4 7% 5% 5% 6% 5% 6% 7% 30%	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	42% 4 6% 24% 22% 3	4057
	11th to 20th	Day	42% 5% 6% 19% 24% 4	4060
	21st to end	Day	45% 3 7% 24% 18% 2	4466
February	1st to 10th	Day	48% 4 11% 21% 15% 2	1800
	11th to 20th	Day	50% 4 9% 20% 14% 3	1841
	21st to end	Day	49% 4 9% 17% 17% 4	1703
March	1st to 10th	Day	46% 5% 7% 20% 15% 7%	4935
	11th to 20th	Day	36% 4 7% 23% 23% 8%	4980
	21st to end	Day	45% 5% 8% 22% 17% 3	5465
April	1st to 10th	Day	39% 4 12% 24% 16% 5%	2250
	11th to 20th	Day	41% 3 11% 25% 16% 4	2374
	21st to end	Day	45% 5% 11% 25% 11% 3	2570
May	1st to 10th	Day	53% 6% 11% 19% 9% 1	4250
	11th to 20th	Day	53% 5% 10% 24% 7% 1	4085
	21st to end	Day	38% 8% 16% 22% 13% 3	4489
June	1st to 10th	Day	40% 8% 16% 23% 12%	2470
	11th to 20th	Day	39% 9% 17% 20% 14% 2	2470
	21st to end	Day	40% 8% 15% 22% 14% 2	2470
July	1st to 10th	Day	38% 12% 19% 19% 8% 5%	4055
	11th to 20th	Day	39% 9% 19% 24% 8% 1	4051
	21st to end	Day	57% 8% 12% 15% 8%	4324
August	1st to 10th	Day	47% 9% 18% 20% 6%	4091
	11th to 20th	Day	39% 10% 17% 23% 10% 1	4083
	21st to end	Day	31% 9% 17% 27% 14% 2	4254
September	1st to 10th	Day	39% 7% 13% 20% 18% 4	2205
	11th to 20th	Day	44% 5% 10% 24% 15% 2	2090
	21st to end	Day	50% 5% 10% 20% 15% 1	2090
October	1st to 10th	Day	31% 6% 8% 21% 26% 8%	3206
	11th to 20th	Day	28% 4 8% 26% 19% 15%	3140
	21st to end	Day	27% 4 9% 28% 19% 12%	3428
November	1st to 10th	Day	34% 5% 9% 24% 24% 4	1957
	11th to 20th	Day	38% 4 8% 23% 23% 4	1810
	21st to end	Day	40% 3 10% 23% 21% 2	1800
December	1st to 10th	Day	40% 5% 9% 20% 22% 4	2930
	11th to 20th	Day	46% 5% 11% 19% 17% 2	3370
	21st to end	Day	51% 6% 10% 22% 11% 1	4466
January	1st to 10th	Night	43% 4 8% 21% 19% 4	5889
	11th to 20th	Night	41% 5% 6% 20% 21% 6%	5900
	21st to end	Night	42% 3 7% 27% 17% 3	6490
February	1st to 10th	Night	43% 4 14% 23% 15% 1	2600
	11th to 20th	Night	50% 3 9% 19% 15% 4	2454
	21st to end	Night	44% 5% 10% 24% 12% 5%	1925
March	1st to 10th	Night	45% 5% 6% 24% 16% 5%	4918
	11th to 20th	Night	33% 5% 8% 24% 24% 6%	4980
	21st to end	Night	43% 4 10% 23% 18% 3	5467
April	1st to 10th	Night	41% 5% 13% 22% 16% 3	2150
	11th to 20th	Night	37% 4 12% 22% 20% 5%	1975
	21st to end	Night	40% 4 14% 27% 14% 2	1845
May	1st to 10th	Night	48% 6% 14% 21% 9% 1	2930
	11th to 20th	Night	50% 6% 12% 25% 7% 1	2819
	21st to end	Night	35% 8% 20% 26% 8% 3	3087
June	1st to 10th	Night	34% 10% 18% 25% 12% 1	1710
	11th to 20th	Night	35% 7% 19% 26% 12% 1	1710
	21st to end	Night	33% 9% 17% 25% 14% 2	1710
July	1st to 10th	Night	36% 12% 18% 20% 11% 2	2795
	11th to 20th	Night	31% 9% 24% 27% 7% 2	2786
	21st to end	Night	47% 12% 15% 20% 6%	2988
August	1st to 10th	Night	44% 10% 20% 19% 6%	2814
	11th to 20th	Night	36% 10% 18% 25% 10%	2812
	21st to end	Night	28% 8% 15% 35% 12% 1	3369
September	1st to 10th	Night	37% 8% 14% 20% 17% 4	1971
	11th to 20th	Night	41% 5% 10% 25% 18% 1	2090
	21st to end	Night	45% 6% 10% 23% 15% 2	2090
October	1st to 10th	Night	36% 9% 8% 19% 24% 5%	3753
	11th to 20th	Night	27% 5% 9% 25% 24% 10%	3779
	21st to end	Night	24% 3 9% 32% 22% 11%	4120
November	1st to 10th	Night	37% 5% 9% 22% 22% 5%	2341
	11th to 20th	Night	37% 4 7% 23% 22% 6%	2585
	21st to end	Night	38% 4 9% 26% 19% 4	2600
December	1st to 10th	Night	38% 4 9% 28% 18% 1	4250
	11th to 20th	Night	51% 5% 11% 19% 12% 2	4824
	21st to end	Night	50% 7% 10% 21% 12%	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	40% 23% 19% 10% 7%	300
	11th to 20th	Day	39% 20% 26% 12% 3	300
	21st to end	Day	33% 24% 26% 10% 7%	330
February	1st to 10th	Day	27% 22% 33% 12% 7%	300
	11th to 20th	Day	44% 26% 19% 9% 2	300
	21st to end	Day	34% 19% 31% 12% 3	302
March	1st to 10th	Day	29% 19% 25% 20% 6% 1	500
	11th to 20th	Day	28% 19% 27% 18% 7% 1	500
	21st to end	Day	28% 17% 29% 19% 6%	550
April	1st to 10th	Day	24% 22% 23% 20% 10% 1	500
	11th to 20th	Day	22% 16% 28% 20% 11% 4	492
	21st to end	Day	23% 13% 28% 19% 13% 4	500
May	1st to 10th	Day	20% 17% 30% 17% 10% 5%	503
	11th to 20th	Day	20% 9% 25% 23% 16% 8%	487
	21st to end	Day	14% 6% 21% 27% 20% 12%	529
June	1st to 10th	Day	13% 5% 25% 27% 19% 11%	490
	11th to 20th	Day	11% 6% 21% 29% 21% 12%	490
	21st to end	Day	11% 8% 23% 27% 20% 11%	490
July	1st to 10th	Day	12% 3 15% 26% 26% 18%	485
	11th to 20th	Day	7% 2 16% 33% 27% 15%	484
	21st to end	Day	11% 3 15% 29% 27% 15%	532
August	1st to 10th	Day	17% 4 18% 21% 25% 15%	486
	11th to 20th	Day	13% 2 16% 25% 27% 18%	479
	21st to end	Day	11% 2 9% 28% 27% 23%	539
September	1st to 10th	Day	12% 3 14% 26% 29% 16%	490
	11th to 20th	Day	16% 4 11% 25% 29% 15%	490
	21st to end	Day	24% 8% 20% 25% 19% 4	490
October	1st to 10th	Day	18% 6% 19% 27% 23% 7%	490
	11th to 20th	Day	17% 14% 19% 24% 22% 4	490
	21st to end	Day	19% 11% 27% 23% 17% 4	539
November	1st to 10th	Day	19% 10% 25% 27% 14% 4	495
	11th to 20th	Day	25% 16% 23% 22% 10% 3	310
	21st to end	Day	25% 20% 22% 21% 11% 1	300
December	1st to 10th	Day	28% 24% 26% 17% 5%	300
	11th to 20th	Day	30% 17% 26% 18% 8% 1	300
	21st to end	Day	29% 16% 26% 15% 11% 2	330
January	1st to 10th	Night	25% 21% 30% 20% 3	800
	11th to 20th	Night	25% 18% 29% 25% 3	800
	21st to end	Night	21% 21% 35% 21% 2	880
February	1st to 10th	Night	22% 17% 31% 28% 2	800
	11th to 20th	Night	30% 20% 30% 19% 1	783
	21st to end	Night	24% 16% 36% 21% 2	660
March	1st to 10th	Night	20% 16% 33% 28% 3	800
	11th to 20th	Night	24% 16% 29% 24% 7%	800
	21st to end	Night	20% 13% 36% 26% 5%	880
April	1st to 10th	Night	21% 18% 26% 29% 6%	800
	11th to 20th	Night	18% 14% 27% 30% 11%	791
	21st to end	Night	16% 10% 27% 34% 12% 1	645
May	1st to 10th	Night	17% 9% 26% 31% 16% 2	600
	11th to 20th	Night	16% 7% 20% 36% 19% 2	563
	21st to end	Night	14% 2 12% 33% 36% 4	617
June	1st to 10th	Night	9% 2 15% 42% 28% 3	570
	11th to 20th	Night	11% 2 13% 34% 36% 4	570
	21st to end	Night	7% 2 12% 41% 32% 5%	570
July	1st to 10th	Night	10% 2 6% 36% 39% 6%	555
	11th to 20th	Night	7% 1 7% 31% 47% 6%	552
	21st to end	Night	6% 2 7% 39% 41% 5%	624
August	1st to 10th	Night	12% 3 6% 37% 37% 5%	567
	11th to 20th	Night	12% 2 6% 33% 41% 6%	564
	21st to end	Night	11% 2 7% 32% 40% 9%	768
September	1st to 10th	Night	11% 2 9% 32% 39% 7%	760
	11th to 20th	Night	16% 3 10% 31% 34% 6%	760
	21st to end	Night	21% 3 18% 32% 24% 2	760
October	1st to 10th	Night	18% 7% 17% 30% 25% 3	760
	11th to 20th	Night	15% 10% 22% 34% 18% 1	760
	21st to end	Night	17% 9% 21% 37% 15% 1	836
November	1st to 10th	Night	17% 8% 21% 37% 16% 2	780
	11th to 20th	Night	18% 13% 24% 34% 10% 1	800
	21st to end	Night	19% 14% 26% 31% 10% 1	800
December	1st to 10th	Night	23% 16% 30% 25% 6%	800
	11th to 20th	Night	19% 15% 30% 30% 6%	800
	21st to end	Night	23% 16% 25% 31% 5%	880

Page generated at 2025-05-08 09:33:57. Version ac07

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