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 2026-09-14.

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 7% 8% 37%	148879
	Night	12% 7% 3 7% 5% 5% 5% 4 6% 8% 40%	142211
Transparency	Day	42% 7% 12% 22% 14% 3	152498
	Night	41% 7% 12% 23% 15% 3	146664
Seeing	Day	20% 12% 23% 22% 16% 7%	17749
	Night	18% 11% 22% 30% 17% 2	27789

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	14% 7% 3 6% 4 4 4 3 5% 7% 42%	5258
	11th to 20th	Day	8% 7% 4 8% 5% 4 5% 4 5% 6% 44%	5280
	21st to end	Day	15% 9% 3 7% 4 4 4 3 3 5% 43%	5780
February	1st to 10th	Day	11% 9% 2 8% 6% 6% 5% 5% 6% 6% 37%	1890
	11th to 20th	Day	18% 6% 3 8% 6% 4 4 4 6% 6% 35%	1926
	21st to end	Day	17% 7% 3 8% 5% 4 4 4 5% 6% 35%	1786
March	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	5045
	11th to 20th	Day	10% 6% 2 5% 4 4 4 4 5% 7% 48%	5090
	21st to end	Day	13% 8% 3 8% 4 5% 5% 4 5% 6% 40%	5586
April	1st to 10th	Day	10% 7% 3 7% 4 5% 4 4 5% 8% 44%	2362
	11th to 20th	Day	12% 6% 3 7% 4 4 4 4 6% 7% 42%	2494
	21st to end	Day	15% 6% 3 7% 5% 4 5% 4 6% 6% 40%	2699
May	1st to 10th	Day	11% 7% 3 7% 6% 5% 6% 5% 7% 10% 33%	7616
	11th to 20th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	7626
	21st to end	Day	17% 6% 3 8% 5% 6% 5% 5% 7% 9% 30%	8391
June	1st to 10th	Day	10% 5% 2 6% 6% 5% 6% 5% 7% 10% 39%	2730
	11th to 20th	Day	8% 5% 2 7% 6% 5% 6% 5% 7% 11% 38%	2730
	21st to end	Day	7% 6% 2 7% 5% 7% 6% 5% 7% 11% 37%	2730
July	1st to 10th	Day	6% 4 2 7% 5% 6% 7% 6% 9% 11% 35%	7680
	11th to 20th	Day	9% 5% 3 8% 6% 6% 7% 5% 9% 11% 29%	7680
	21st to end	Day	12% 7% 3 9% 7% 7% 7% 6% 8% 10% 25%	8340
August	1st to 10th	Day	13% 7% 3 9% 7% 7% 7% 6% 8% 11% 22%	6014
	11th to 20th	Day	7% 4 2 7% 6% 7% 8% 6% 9% 12% 31%	5951
	21st to end	Day	6% 5% 2 6% 4 5% 5% 5% 7% 11% 43%	6195
September	1st to 10th	Day	10% 6% 2 7% 5% 4 5% 4 6% 8% 43%	2437
	11th to 20th	Day	15% 6% 2 7% 6% 5% 5% 3 6% 8% 37%	2233
	21st to end	Day	19% 8% 3 8% 5% 5% 5% 4 5% 5% 33%	2200
October	1st to 10th	Day	14% 6% 2 6% 3 3 3 3 5% 6% 48%	2035
	11th to 20th	Day	12% 6% 2 7% 4 4 4 3 6% 7% 46%	2000
	21st to end	Day	14% 6% 3 6% 5% 3 4 3 5% 6% 46%	2200
November	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 6% 7% 49%	2057
	11th to 20th	Day	10% 7% 2 5% 5% 4 5% 4 5% 8% 46%	1900
	21st to end	Day	13% 7% 2 7% 5% 4 3 3 4 7% 44%	1890
December	1st to 10th	Day	10% 9% 2 7% 4 5% 5% 4 5% 5% 45%	3020
	11th to 20th	Day	17% 9% 3 6% 4 4 5% 4 7% 7% 36%	3463
	21st to end	Day	17% 9% 3 7% 5% 5% 6% 4 7% 7% 30%	4565
January	1st to 10th	Night	12% 7% 3 8% 5% 5% 5% 4 6% 7% 39%	7635
	11th to 20th	Night	10% 6% 3 9% 5% 4 4 4 5% 5% 46%	7680
	21st to end	Night	16% 8% 2 6% 4 3 4 3 5% 6% 43%	8401
February	1st to 10th	Night	13% 8% 3 5% 4 4 5% 5% 6% 7% 39%	2730
	11th to 20th	Night	19% 7% 2 7% 5% 4 4 5% 5% 6% 35%	2578
	21st to end	Night	13% 7% 3 7% 4 6% 5% 5% 6% 6% 38%	2018
March	1st to 10th	Night	17% 7% 2 6% 5% 4 5% 4 5% 6% 40%	5028
	11th to 20th	Night	11% 5% 2 5% 3 4 4 4 6% 8% 49%	5090
	21st to end	Night	14% 7% 2 6% 5% 4 4 4 6% 7% 42%	5588
April	1st to 10th	Night	12% 7% 2 6% 5% 4 4 4 6% 9% 41%	2258
	11th to 20th	Night	12% 5% 2 5% 4 4 4 4 6% 7% 46%	2075
	21st to end	Night	12% 7% 2 6% 4 4 4 4 5% 7% 45%	1937
May	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 4 7% 8% 37%	5237
	11th to 20th	Night	15% 7% 3 6% 5% 5% 5% 4 6% 8% 36%	5234
	21st to end	Night	21% 6% 3 8% 5% 5% 5% 4 6% 8% 29%	5761
June	1st to 10th	Night	8% 5% 2 6% 5% 4 6% 5% 7% 9% 42%	1890
	11th to 20th	Night	8% 6% 2 6% 5% 4 4 4 6% 10% 45%	1890
	21st to end	Night	6% 4 2 5% 4 5% 6% 5% 7% 11% 44%	1890
July	1st to 10th	Night	6% 5% 2 7% 6% 6% 7% 5% 8% 10% 38%	5278
	11th to 20th	Night	8% 6% 3 8% 5% 6% 7% 5% 8% 10% 33%	5277
	21st to end	Night	11% 7% 3 9% 7% 7% 6% 5% 7% 9% 29%	5736
August	1st to 10th	Night	14% 9% 3 9% 6% 5% 6% 4 8% 11% 25%	4135
	11th to 20th	Night	7% 6% 2 6% 5% 7% 6% 5% 8% 10% 36%	4095
	21st to end	Night	7% 4 2 6% 4 4 5% 4 6% 9% 49%	4883
September	1st to 10th	Night	8% 6% 3 6% 4 5% 5% 4 5% 7% 47%	2179
	11th to 20th	Night	12% 6% 3 8% 4 5% 4 3 5% 7% 42%	2234
	21st to end	Night	17% 7% 2 7% 4 5% 5% 4 6% 5% 38%	2200
October	1st to 10th	Night	13% 6% 3 6% 4 4 4 3 3 6% 46%	2364
	11th to 20th	Night	12% 7% 2 6% 5% 4 4 3 5% 5% 47%	2400
	21st to end	Night	12% 6% 2 5% 4 3 4 3 6% 6% 49%	2640
November	1st to 10th	Night	12% 6% 3 6% 4 4 4 4 5% 5% 48%	2461
	11th to 20th	Night	11% 5% 2 6% 5% 4 4 4 5% 6% 47%	2714
	21st to end	Night	11% 6% 2 6% 5% 4 4 4 5% 7% 45%	2730
December	1st to 10th	Night	10% 6% 2 7% 5% 4 4 4 6% 7% 44%	4380
	11th to 20th	Night	16% 9% 4 8% 6% 5% 5% 4 6% 7% 32%	4952
	21st to end	Night	17% 7% 4 8% 5% 5% 6% 5% 6% 7% 30%	6633

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 11% 21% 20% 2	5259
	11th to 20th	Day	39% 5% 6% 20% 25% 6%	5280
	21st to end	Day	45% 3 6% 26% 17% 4	5780
February	1st to 10th	Day	47% 4 10% 21% 16% 2	1890
	11th to 20th	Day	49% 4 9% 21% 14% 3	1935
	21st to end	Day	49% 4 9% 17% 16% 4	1786
March	1st to 10th	Day	46% 5% 7% 20% 15% 7%	5045
	11th to 20th	Day	36% 4 7% 23% 22% 8%	5090
	21st to end	Day	45% 5% 9% 22% 16% 3	5586
April	1st to 10th	Day	38% 5% 12% 25% 16% 4	2362
	11th to 20th	Day	40% 3 12% 24% 16% 4	2494
	21st to end	Day	44% 5% 11% 25% 12% 2	2699
May	1st to 10th	Day	45% 7% 12% 22% 13% 1	7651
	11th to 20th	Day	45% 6% 13% 25% 11% 1	7630
	21st to end	Day	48% 8% 12% 17% 12% 3	8405
June	1st to 10th	Day	39% 8% 16% 23% 14%	2730
	11th to 20th	Day	38% 9% 17% 21% 14% 2	2730
	21st to end	Day	39% 9% 15% 23% 13% 1	2730
July	1st to 10th	Day	37% 13% 17% 21% 10% 3	7615
	11th to 20th	Day	43% 12% 17% 21% 7%	7611
	21st to end	Day	48% 11% 13% 16% 10%	8156
August	1st to 10th	Day	51% 13% 17% 15% 4	6001
	11th to 20th	Day	38% 12% 20% 22% 7% 1	5990
	21st to end	Day	31% 10% 18% 26% 14% 1	6195
September	1st to 10th	Day	37% 7% 13% 21% 18% 4	2437
	11th to 20th	Day	43% 6% 10% 24% 15% 2	2233
	21st to end	Day	51% 5% 10% 19% 14% 1	2200
October	1st to 10th	Day	30% 6% 8% 21% 26% 8%	3308
	11th to 20th	Day	27% 4 8% 26% 20% 14%	3240
	21st to end	Day	28% 4 9% 28% 19% 12%	3538
November	1st to 10th	Day	33% 5% 10% 24% 24% 4	2057
	11th to 20th	Day	37% 4 8% 24% 22% 4	1900
	21st to end	Day	40% 3 10% 23% 20% 2	1890
December	1st to 10th	Day	41% 5% 9% 20% 22% 4	3020
	11th to 20th	Day	46% 5% 11% 19% 17% 3	3460
	21st to end	Day	50% 6% 10% 22% 10% 1	4565
January	1st to 10th	Night	43% 5% 12% 22% 16% 3	7636
	11th to 20th	Night	38% 5% 6% 22% 23% 6%	7680
	21st to end	Night	41% 4 8% 27% 17% 3	8401
February	1st to 10th	Night	43% 4 14% 23% 15% 1	2730
	11th to 20th	Night	49% 3 9% 20% 14% 4	2580
	21st to end	Night	46% 5% 10% 23% 11% 5%	2018
March	1st to 10th	Night	44% 5% 7% 23% 16% 5%	5028
	11th to 20th	Night	33% 5% 8% 25% 24% 6%	5090
	21st to end	Night	42% 4 10% 23% 17% 2	5588
April	1st to 10th	Night	39% 5% 14% 22% 16% 3	2258
	11th to 20th	Night	37% 4 13% 23% 19% 4	2075
	21st to end	Night	39% 4 14% 28% 14% 2	1937
May	1st to 10th	Night	43% 8% 14% 22% 12% 1	5264
	11th to 20th	Night	44% 7% 15% 24% 10%	5235
	21st to end	Night	51% 8% 14% 19% 7% 2	5771
June	1st to 10th	Night	35% 10% 17% 24% 13% 1	1890
	11th to 20th	Night	35% 8% 18% 26% 12% 1	1890
	21st to end	Night	32% 10% 17% 25% 14% 2	1890
July	1st to 10th	Night	37% 14% 16% 22% 10% 1	5235
	11th to 20th	Night	42% 13% 18% 21% 5% 1	5226
	21st to end	Night	46% 13% 14% 16% 10%	5606
August	1st to 10th	Night	49% 13% 17% 15% 4	4124
	11th to 20th	Night	38% 12% 19% 22% 8%	4122
	21st to end	Night	29% 9% 16% 33% 12% 1	4883
September	1st to 10th	Night	35% 8% 14% 21% 17% 4	2179
	11th to 20th	Night	41% 6% 10% 24% 18% 1	2234
	21st to end	Night	46% 6% 10% 22% 14% 2	2200
October	1st to 10th	Night	35% 9% 8% 19% 24% 4	3871
	11th to 20th	Night	27% 5% 9% 25% 24% 10%	3899
	21st to end	Night	24% 3 9% 31% 22% 10%	4252
November	1st to 10th	Night	37% 5% 9% 23% 23% 4	2461
	11th to 20th	Night	37% 4 8% 24% 21% 6%	2714
	21st to end	Night	38% 4 9% 26% 19% 4	2730
December	1st to 10th	Night	39% 5% 9% 28% 18% 1	4380
	11th to 20th	Night	51% 5% 11% 19% 12% 2	4954
	21st to end	Night	50% 7% 10% 21% 12% 1	6633

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% 21% 10% 7%	330
	11th to 20th	Day	40% 21% 26% 11% 2	330
	21st to end	Day	35% 24% 26% 9% 6%	363
February	1st to 10th	Day	26% 22% 34% 12% 6%	330
	11th to 20th	Day	44% 26% 19% 9% 2	330
	21st to end	Day	34% 20% 31% 13% 2	329
March	1st to 10th	Day	27% 21% 25% 19% 7% 1	540
	11th to 20th	Day	29% 19% 26% 18% 7% 1	540
	21st to end	Day	27% 18% 30% 19% 6%	594
April	1st to 10th	Day	23% 22% 24% 20% 10% 1	540
	11th to 20th	Day	22% 15% 29% 20% 11% 4	532
	21st to end	Day	23% 13% 28% 19% 13% 4	540
May	1st to 10th	Day	20% 18% 31% 16% 10% 5%	543
	11th to 20th	Day	20% 10% 26% 23% 15% 7%	567
	21st to end	Day	17% 7% 21% 25% 18% 11%	617
June	1st to 10th	Day	12% 6% 26% 28% 18% 11%	570
	11th to 20th	Day	10% 8% 22% 29% 20% 11%	570
	21st to end	Day	10% 7% 23% 27% 20% 12%	570
July	1st to 10th	Day	11% 3 15% 27% 26% 18%	565
	11th to 20th	Day	7% 2 16% 32% 27% 15%	564
	21st to end	Day	12% 4 15% 29% 26% 14%	620
August	1st to 10th	Day	18% 5% 17% 23% 23% 14%	566
	11th to 20th	Day	12% 3 18% 25% 26% 16%	559
	21st to end	Day	10% 3 11% 29% 26% 20%	627
September	1st to 10th	Day	11% 4 15% 26% 29% 15%	570
	11th to 20th	Day	15% 4 12% 25% 30% 15%	542
	21st to end	Day	24% 8% 20% 26% 19% 4	530
October	1st to 10th	Day	17% 6% 19% 27% 23% 8%	530
	11th to 20th	Day	17% 14% 20% 25% 21% 3	530
	21st to end	Day	22% 11% 26% 21% 16% 4	583
November	1st to 10th	Day	19% 11% 26% 26% 13% 4	535
	11th to 20th	Day	25% 16% 26% 21% 9% 3	340
	21st to end	Day	26% 21% 21% 20% 11% 1	330
December	1st to 10th	Day	27% 24% 28% 16% 5%	330
	11th to 20th	Day	30% 18% 26% 18% 7% 1	330
	21st to end	Day	30% 17% 26% 15% 10% 2	363
January	1st to 10th	Night	25% 21% 30% 20% 3	840
	11th to 20th	Night	25% 18% 29% 24% 3	840
	21st to end	Night	21% 21% 35% 21% 2	924
February	1st to 10th	Night	22% 17% 31% 27% 2	840
	11th to 20th	Night	29% 21% 30% 19% 1	823
	21st to end	Night	25% 17% 35% 21% 2	692
March	1st to 10th	Night	20% 16% 33% 28% 3	840
	11th to 20th	Night	24% 16% 28% 25% 7%	840
	21st to end	Night	19% 13% 36% 26% 5%	924
April	1st to 10th	Night	20% 18% 27% 29% 6%	840
	11th to 20th	Night	17% 13% 28% 31% 10%	831
	21st to end	Night	16% 10% 27% 34% 12% 1	677
May	1st to 10th	Night	16% 9% 28% 30% 15% 1	630
	11th to 20th	Night	15% 7% 20% 38% 18% 2	621
	21st to end	Night	17% 3 13% 31% 33% 4	683
June	1st to 10th	Night	9% 2 16% 42% 27% 3	630
	11th to 20th	Night	10% 3 14% 35% 35% 4	630
	21st to end	Night	7% 2 12% 40% 34% 5%	630
July	1st to 10th	Night	10% 2 6% 35% 41% 6%	615
	11th to 20th	Night	7% 2 7% 31% 47% 6%	612
	21st to end	Night	8% 3 7% 37% 40% 5%	690
August	1st to 10th	Night	12% 4 6% 36% 37% 5%	627
	11th to 20th	Night	11% 2 7% 34% 41% 5%	624
	21st to end	Night	11% 2 8% 34% 37% 8%	849
September	1st to 10th	Night	10% 2 9% 34% 39% 6%	840
	11th to 20th	Night	15% 3 10% 32% 34% 6%	813
	21st to end	Night	22% 3 18% 32% 24% 2	800
October	1st to 10th	Night	17% 6% 16% 30% 26% 4	800
	11th to 20th	Night	15% 10% 22% 34% 18% 1	800
	21st to end	Night	18% 10% 21% 36% 14% 1	880
November	1st to 10th	Night	16% 9% 21% 37% 15% 1	820
	11th to 20th	Night	18% 14% 24% 34% 10%	840
	21st to end	Night	19% 14% 25% 31% 10% 1	840
December	1st to 10th	Night	23% 16% 31% 24% 6%	840
	11th to 20th	Night	19% 16% 30% 29% 6%	840
	21st to end	Night	23% 16% 26% 30% 5%	924

Page generated at 2026-09-11 04:11:54. Version ac07

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