Anderson
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anderson clear sky chart from 2005-11-06 to 2025-04-26.

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% 5% 2 6% 4 4 4 4 6% 7% 44%	114082
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	114547
Transparency	Day	37% 6% 11% 20% 20% 7%	117850
	Night	37% 5% 10% 18% 22% 8%	118918
Seeing	Day	17% 6% 20% 24% 21% 12%	15661
	Night	17% 5% 20% 33% 21% 4	25803

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	16% 4 1 4 3 2 3 2 4 5% 54%	4056
	11th to 20th	Day	12% 5% 2 4 3 2 3 2 3 3 60%	4060
	21st to end	Day	20% 7% 2 6% 3 3 3 2 4 5% 45%	4466
February	1st to 10th	Day	15% 6% 2 6% 4 4 4 3 5% 5% 44%	1930
	11th to 20th	Day	23% 5% 2 5% 3 3 3 3 5% 6% 43%	1948
	21st to end	Day	19% 7% 2 4 4 3 4 3 4 5% 43%	1650
March	1st to 10th	Day	21% 7% 2 7% 4 3 4 3 4 6% 39%	4840
	11th to 20th	Day	9% 4 2 5% 4 4 3 3 4 6% 54%	4980
	21st to end	Day	18% 6% 2 6% 3 4 4 3 4 5% 45%	5465
April	1st to 10th	Day	12% 4 2 5% 5% 4 5% 4 6% 7% 47%	2200
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 5% 6% 49%	2351
	21st to end	Day	11% 5% 2 4 3 4 4 3 5% 8% 50%	2535
May	1st to 10th	Day	11% 7% 2 7% 4 4 4 4 7% 9% 39%	4082
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 6% 8% 38%	4081
	21st to end	Day	7% 4 2 6% 4 4 5% 4 8% 10% 47%	4476
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 6% 8% 11% 34%	2470
	11th to 20th	Day	5% 4 2 5% 4 6% 6% 7% 8% 12% 42%	2470
	21st to end	Day	6% 4 1 6% 5% 6% 6% 7% 8% 11% 40%	2470
July	1st to 10th	Day	8% 6% 3 8% 6% 6% 6% 5% 10% 10% 33%	4120
	11th to 20th	Day	7% 4 2 7% 5% 5% 6% 6% 9% 11% 38%	4120
	21st to end	Day	9% 8% 4 9% 7% 8% 7% 6% 10% 10% 23%	4466
August	1st to 10th	Day	14% 6% 2 8% 5% 5% 5% 5% 8% 10% 31%	4104
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 5% 6% 11% 41%	4044
	21st to end	Day	7% 4 2 5% 4 5% 6% 4 6% 8% 50%	4101
September	1st to 10th	Day	8% 5% 2 7% 5% 6% 5% 5% 7% 9% 41%	2090
	11th to 20th	Day	14% 6% 2 7% 4 5% 5% 4 6% 8% 40%	2090
	21st to end	Day	15% 5% 2 5% 3 4 4 4 6% 7% 44%	2090
October	1st to 10th	Day	13% 6% 2 4 2 3 3 2 4 5% 56%	2072
	11th to 20th	Day	12% 4 2 4 3 3 2 2 4 5% 58%	1900
	21st to end	Day	11% 5% 2 5% 3 3 3 2 4 4 58%	2090
November	1st to 10th	Day	14% 6% 2 4 3 3 3 3 3 5% 55%	1896
	11th to 20th	Day	15% 5% 1 5% 3 4 3 3 4 7% 51%	1800
	21st to end	Day	15% 5% 2 4 4 3 3 3 4 7% 50%	1800
December	1st to 10th	Day	19% 6% 2 4 3 2 2 2 5% 5% 51%	2930
	11th to 20th	Day	18% 7% 2 5% 4 4 5% 4 5% 5% 42%	3373
	21st to end	Day	24% 5% 2 4 2 2 3 2 4 7% 45%	4466
January	1st to 10th	Night	15% 4 1 4 3 3 3 2 3 4 58%	5889
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 5% 54%	5900
	21st to end	Night	19% 7% 2 4 3 3 3 2 4 6% 49%	6490
February	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 4 6% 47%	2465
	11th to 20th	Night	22% 6% 2 5% 4 3 3 2 4 4 44%	2338
	21st to end	Night	16% 6% 2 5% 4 3 4 3 5% 6% 45%	1980
March	1st to 10th	Night	19% 6% 2 6% 4 3 3 3 5% 7% 41%	5011
	11th to 20th	Night	9% 4 1 4 3 3 4 4 6% 8% 54%	4980
	21st to end	Night	16% 6% 2 5% 3 3 4 3 5% 5% 48%	5467
April	1st to 10th	Night	13% 5% 1 5% 5% 4 4 4 5% 6% 50%	2200
	11th to 20th	Night	11% 5% 2 6% 3 3 4 3 5% 6% 52%	2013
	21st to end	Night	11% 5% 2 5% 3 4 4 3 5% 7% 51%	1761
May	1st to 10th	Night	12% 7% 3 8% 5% 4 4 4 5% 9% 40%	2813
	11th to 20th	Night	17% 6% 2 6% 4 4 4 4 5% 8% 39%	2812
	21st to end	Night	7% 4 2 6% 5% 5% 5% 4 7% 11% 44%	3079
June	1st to 10th	Night	10% 5% 2 7% 5% 5% 6% 6% 8% 10% 35%	1710
	11th to 20th	Night	7% 6% 3 5% 5% 5% 5% 5% 7% 12% 40%	1710
	21st to end	Night	8% 4 2 6% 5% 6% 6% 5% 9% 11% 38%	1710
July	1st to 10th	Night	11% 7% 2 9% 7% 6% 7% 6% 9% 10% 28%	2840
	11th to 20th	Night	8% 5% 3 7% 4 6% 7% 5% 11% 14% 31%	2840
	21st to end	Night	12% 10% 4 10% 6% 6% 7% 5% 8% 10% 21%	3087
August	1st to 10th	Night	11% 7% 3 9% 5% 5% 6% 5% 8% 11% 28%	2825
	11th to 20th	Night	13% 7% 3 9% 6% 6% 5% 6% 7% 9% 29%	2785
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 7% 8% 45%	3517
September	1st to 10th	Night	9% 7% 2 6% 5% 4 5% 4 7% 9% 41%	2090
	11th to 20th	Night	15% 7% 2 7% 4 4 5% 4 7% 7% 38%	2090
	21st to end	Night	13% 6% 2 7% 4 4 4 3 5% 6% 46%	2090
October	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 5% 5% 53%	2106
	11th to 20th	Night	12% 4 2 5% 4 3 3 3 5% 5% 55%	2280
	21st to end	Night	12% 4 1 3 3 2 4 2 3 5% 60%	2508
November	1st to 10th	Night	14% 5% 2 5% 3 3 2 3 4 5% 55%	2399
	11th to 20th	Night	16% 6% 2 4 3 2 3 2 4 6% 53%	2600
	21st to end	Night	15% 5% 2 4 3 3 4 3 5% 7% 49%	2600
December	1st to 10th	Night	16% 5% 2 5% 3 3 3 3 4 4 52%	4250
	11th to 20th	Night	19% 6% 2 6% 4 3 3 3 4 5% 45%	4822
	21st to end	Night	21% 5% 2 5% 4 3 3 3 5% 5% 43%	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	33% 3 5% 22% 30% 7%	4057
	11th to 20th	Day	30% 3 5% 17% 36% 9%	4060
	21st to end	Day	44% 2 4 17% 23% 11%	4466
February	1st to 10th	Day	41% 4 8% 22% 20% 6%	1930
	11th to 20th	Day	43% 5% 6% 18% 22% 6%	1958
	21st to end	Day	43% 5% 6% 15% 22% 9%	1650
March	1st to 10th	Day	48% 4 6% 14% 18% 12%	4840
	11th to 20th	Day	32% 4 5% 14% 25% 20%	4980
	21st to end	Day	43% 3 6% 21% 22% 6%	5465
April	1st to 10th	Day	35% 5% 11% 20% 24% 6%	2200
	11th to 20th	Day	34% 6% 9% 21% 21% 10%	2351
	21st to end	Day	31% 6% 11% 27% 21% 4	2535
May	1st to 10th	Day	38% 8% 15% 21% 14% 3	4120
	11th to 20th	Day	43% 7% 12% 23% 14% 1	4085
	21st to end	Day	29% 10% 16% 22% 21% 2	4489
June	1st to 10th	Day	38% 11% 18% 22% 9% 2	2470
	11th to 20th	Day	31% 11% 16% 28% 13% 1	2470
	21st to end	Day	34% 10% 18% 21% 13% 4	2470
July	1st to 10th	Day	43% 9% 18% 21% 7% 1	4055
	11th to 20th	Day	36% 11% 26% 20% 5% 2	4051
	21st to end	Day	51% 12% 19% 12% 6%	4324
August	1st to 10th	Day	46% 7% 18% 24% 6%	4091
	11th to 20th	Day	38% 7% 20% 19% 15% 2	4083
	21st to end	Day	32% 7% 14% 24% 19% 4	4101
September	1st to 10th	Day	37% 9% 14% 18% 18% 3	2090
	11th to 20th	Day	41% 7% 11% 18% 20% 2	2090
	21st to end	Day	38% 6% 11% 20% 18% 6%	2090
October	1st to 10th	Day	28% 3 6% 17% 35% 11%	3449
	11th to 20th	Day	21% 2 6% 15% 30% 27%	3140
	21st to end	Day	27% 2 4 23% 29% 14%	3428
November	1st to 10th	Day	34% 3 7% 19% 27% 10%	1896
	11th to 20th	Day	33% 4 7% 21% 26% 9%	1800
	21st to end	Day	35% 4 8% 21% 25% 7%	1800
December	1st to 10th	Day	37% 3 6% 18% 27% 10%	2930
	11th to 20th	Day	44% 4 6% 19% 19% 8%	3370
	21st to end	Day	41% 3 7% 25% 21% 2	4466
January	1st to 10th	Night	31% 3 6% 18% 32% 9%	5889
	11th to 20th	Night	33% 3 5% 16% 33% 9%	5900
	21st to end	Night	39% 3 4 16% 30% 9%	6490
February	1st to 10th	Night	39% 4 8% 21% 21% 7%	2465
	11th to 20th	Night	44% 3 5% 18% 23% 7%	2339
	21st to end	Night	39% 5% 7% 18% 21% 10%	1980
March	1st to 10th	Night	42% 4 7% 12% 27% 7%	5011
	11th to 20th	Night	28% 4 8% 13% 30% 18%	4980
	21st to end	Night	38% 4 6% 20% 22% 10%	5467
April	1st to 10th	Night	35% 5% 9% 18% 26% 7%	2200
	11th to 20th	Night	33% 3 7% 23% 24% 10%	2013
	21st to end	Night	33% 5% 11% 26% 21% 5%	1761
May	1st to 10th	Night	38% 10% 13% 22% 15% 2	2840
	11th to 20th	Night	41% 7% 15% 19% 17% 1	2813
	21st to end	Night	28% 11% 14% 26% 19% 2	3087
June	1st to 10th	Night	38% 12% 17% 19% 12% 2	1710
	11th to 20th	Night	33% 9% 18% 24% 14% 2	1710
	21st to end	Night	35% 12% 18% 17% 13% 4	1710
July	1st to 10th	Night	49% 10% 19% 16% 6% 1	2795
	11th to 20th	Night	38% 12% 24% 20% 4 1	2786
	21st to end	Night	54% 11% 18% 12% 4 1	2988
August	1st to 10th	Night	48% 8% 20% 19% 4	2814
	11th to 20th	Night	50% 8% 17% 16% 9%	2812
	21st to end	Night	34% 7% 14% 22% 21% 3	3517
September	1st to 10th	Night	37% 9% 16% 17% 16% 4	2090
	11th to 20th	Night	42% 9% 11% 19% 17% 2	2090
	21st to end	Night	38% 6% 12% 19% 19% 6%	2090
October	1st to 10th	Night	28% 3 8% 17% 35% 8%	3509
	11th to 20th	Night	20% 3 5% 17% 27% 28%	3779
	21st to end	Night	26% 2 4 22% 27% 18%	4120
November	1st to 10th	Night	34% 3 7% 18% 26% 11%	2399
	11th to 20th	Night	34% 4 5% 18% 27% 12%	2600
	21st to end	Night	36% 4 6% 21% 26% 8%	2600
December	1st to 10th	Night	37% 3 4 18% 30% 8%	4250
	11th to 20th	Night	43% 3 10% 16% 21% 8%	4824
	21st to end	Night	43% 3 7% 25% 19% 3	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	23% 14% 34% 23% 5%	300
	11th to 20th	Day	26% 16% 25% 25% 8%	300
	21st to end	Day	24% 16% 36% 19% 3 1	330
February	1st to 10th	Day	20% 12% 36% 27% 5%	300
	11th to 20th	Day	37% 11% 32% 16% 4	300
	21st to end	Day	30% 18% 28% 19% 5%	249
March	1st to 10th	Day	22% 9% 29% 30% 8% 1	453
	11th to 20th	Day	17% 11% 38% 25% 8% 1	500
	21st to end	Day	22% 11% 33% 23% 9% 2	550
April	1st to 10th	Day	19% 13% 26% 27% 13% 2	500
	11th to 20th	Day	13% 8% 33% 24% 18% 4	492
	21st to end	Day	16% 8% 24% 26% 18% 8%	480
May	1st to 10th	Day	14% 8% 27% 24% 18% 9%	463
	11th to 20th	Day	20% 3 13% 26% 23% 14%	487
	21st to end	Day	16% 3 16% 20% 24% 20%	529
June	1st to 10th	Day	13% 1 13% 26% 29% 18%	490
	11th to 20th	Day	8% 3 13% 23% 28% 24%	490
	21st to end	Day	11% 1 10% 27% 29% 22%	490
July	1st to 10th	Day	11% 1 6% 20% 31% 31%	485
	11th to 20th	Day	10% 1 4 21% 36% 28%	484
	21st to end	Day	7% 2 6% 20% 39% 26%	532
August	1st to 10th	Day	15% 2 9% 19% 29% 26%	486
	11th to 20th	Day	14% 1 5% 22% 36% 23%	479
	21st to end	Day	13% 1 7% 22% 32% 25%	539
September	1st to 10th	Day	12% 1 9% 26% 30% 23%	490
	11th to 20th	Day	18% 2 10% 17% 34% 18%	490
	21st to end	Day	17% 2 13% 26% 29% 13%	490
October	1st to 10th	Day	20% 2 16% 28% 23% 12%	490
	11th to 20th	Day	13% 4 20% 30% 27% 6%	490
	21st to end	Day	15% 5% 21% 29% 25% 5%	539
November	1st to 10th	Day	23% 3 21% 28% 18% 7%	434
	11th to 20th	Day	20% 8% 34% 24% 13% 2	300
	21st to end	Day	13% 10% 29% 29% 15% 4	300
December	1st to 10th	Day	30% 6% 31% 26% 7%	300
	11th to 20th	Day	24% 7% 31% 28% 8% 2	300
	21st to end	Day	32% 9% 27% 19% 10% 2	330
January	1st to 10th	Night	22% 11% 29% 30% 7%	800
	11th to 20th	Night	21% 10% 31% 31% 7%	800
	21st to end	Night	21% 11% 35% 30% 3	880
February	1st to 10th	Night	19% 8% 30% 38% 6%	800
	11th to 20th	Night	28% 7% 35% 27% 4	783
	21st to end	Night	19% 10% 33% 34% 3	660
March	1st to 10th	Night	19% 9% 31% 36% 6%	800
	11th to 20th	Night	18% 8% 35% 30% 10% 1	800
	21st to end	Night	18% 9% 30% 31% 12% 1	880
April	1st to 10th	Night	16% 10% 27% 36% 9% 1	800
	11th to 20th	Night	14% 8% 26% 35% 16% 1	709
	21st to end	Night	14% 4 23% 36% 21% 2	587
May	1st to 10th	Night	13% 4 22% 39% 19% 2	570
	11th to 20th	Night	15% 1 17% 37% 25% 4	561
	21st to end	Night	14% 1 10% 33% 36% 6%	617
June	1st to 10th	Night	15% 1 8% 36% 34% 6%	570
	11th to 20th	Night	9% 1 11% 29% 37% 13%	570
	21st to end	Night	10% 1 8% 34% 36% 11%	570
July	1st to 10th	Night	13% 2 4 25% 44% 13%	555
	11th to 20th	Night	11% 1 3 23% 49% 13%	552
	21st to end	Night	12% 1 5% 27% 46% 10%	624
August	1st to 10th	Night	13% 2 4 28% 42% 11%	567
	11th to 20th	Night	16% 1 6% 25% 42% 10%	564
	21st to end	Night	14% 1 6% 25% 43% 11%	688
September	1st to 10th	Night	11% 1 8% 29% 39% 11%	760
	11th to 20th	Night	19% 2 7% 28% 36% 8%	760
	21st to end	Night	16% 2 12% 33% 33% 4	760
October	1st to 10th	Night	14% 2 15% 32% 31% 6%	760
	11th to 20th	Night	14% 3 20% 39% 21% 4	760
	21st to end	Night	15% 4 19% 37% 22% 2	836
November	1st to 10th	Night	17% 2 21% 36% 20% 3	780
	11th to 20th	Night	19% 7% 25% 34% 14% 2	800
	21st to end	Night	17% 6% 23% 38% 15% 2	800
December	1st to 10th	Night	23% 8% 23% 37% 9%	800
	11th to 20th	Night	21% 5% 27% 36% 10% 1	800
	21st to end	Night	27% 8% 23% 34% 8%	880

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