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 2026-08-30.

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% 6% 2 6% 4 4 5% 4 6% 8% 43%	143589
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	138639
Transparency	Day	39% 5% 12% 20% 18% 6%	147313
	Night	39% 5% 11% 19% 20% 7%	142987
Seeing	Day	17% 6% 20% 24% 21% 12%	17510
	Night	17% 5% 20% 33% 21% 4	27507

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% 6% 2 6% 4 3 3 3 4 5% 48%	5258
	11th to 20th	Day	11% 5% 2 4 2 2 3 2 3 4 63%	5280
	21st to end	Day	22% 8% 2 6% 4 3 3 3 4 5% 40%	5780
February	1st to 10th	Day	15% 6% 2 6% 4 4 4 3 5% 5% 45%	2027
	11th to 20th	Day	22% 5% 2 5% 3 3 3 3 5% 6% 43%	2048
	21st to end	Day	19% 7% 2 5% 4 3 4 3 5% 5% 43%	1730
March	1st to 10th	Day	21% 7% 2 7% 4 3 4 3 4 6% 39%	4948
	11th to 20th	Day	10% 4 2 5% 4 4 3 3 4 6% 54%	5090
	21st to end	Day	18% 6% 2 6% 3 4 4 3 4 5% 46%	5586
April	1st to 10th	Day	11% 5% 2 5% 5% 4 5% 4 7% 7% 47%	2310
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 5% 6% 50%	2470
	21st to end	Day	10% 5% 2 4 3 4 4 4 5% 8% 50%	2730
May	1st to 10th	Day	14% 7% 3 7% 4 4 5% 4 7% 9% 38%	7616
	11th to 20th	Day	15% 6% 2 7% 4 5% 5% 4 7% 8% 38%	7626
	21st to end	Day	16% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	8391
June	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 6% 8% 11% 35%	2730
	11th to 20th	Day	5% 4 2 5% 4 6% 6% 6% 8% 12% 42%	2730
	21st to end	Day	5% 3 1 6% 5% 6% 6% 6% 8% 12% 42%	2730
July	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 5% 9% 11% 41%	7680
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 6% 10% 13% 35%	7680
	21st to end	Day	8% 6% 3 8% 6% 7% 7% 6% 9% 12% 28%	8340
August	1st to 10th	Day	15% 6% 2 8% 5% 5% 5% 5% 8% 10% 30%	4364
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 5% 6% 11% 41%	4304
	21st to end	Day	7% 3 2 5% 4 5% 6% 5% 6% 8% 49%	4339
September	1st to 10th	Day	8% 5% 2 7% 5% 6% 5% 5% 7% 9% 42%	2200
	11th to 20th	Day	13% 6% 2 7% 4 5% 5% 4 6% 8% 41%	2200
	21st to end	Day	15% 6% 2 5% 3 4 5% 5% 6% 7% 44%	2200
October	1st to 10th	Day	12% 6% 2 4 2 3 3 3 4 5% 55%	2181
	11th to 20th	Day	11% 4 2 4 3 3 3 2 4 5% 59%	2000
	21st to end	Day	12% 5% 2 5% 3 3 3 2 4 4 58%	2200
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 3 5% 55%	1993
	11th to 20th	Day	14% 4 1 4 2 3 3 3 4 7% 53%	1890
	21st to end	Day	14% 5% 2 4 4 3 3 3 5% 7% 50%	1890
December	1st to 10th	Day	19% 6% 2 4 3 3 2 2 5% 5% 49%	3020
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 5% 5% 43%	3463
	21st to end	Day	23% 5% 2 4 2 2 2 2 4 7% 45%	4565
January	1st to 10th	Night	15% 5% 1 5% 3 3 3 2 4 5% 54%	7635
	11th to 20th	Night	11% 6% 2 5% 4 3 3 3 4 5% 55%	7680
	21st to end	Night	20% 7% 2 5% 3 3 3 2 3 5% 47%	8401
February	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 4 6% 48%	2588
	11th to 20th	Night	21% 6% 2 6% 4 3 3 2 4 4 44%	2458
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 6% 44%	2076
March	1st to 10th	Night	19% 6% 2 6% 4 3 4 3 5% 7% 41%	5123
	11th to 20th	Night	9% 4 1 4 3 3 4 4 6% 8% 55%	5090
	21st to end	Night	15% 6% 2 5% 3 3 4 3 5% 6% 48%	5588
April	1st to 10th	Night	12% 5% 2 5% 5% 4 4 4 5% 6% 49%	2310
	11th to 20th	Night	11% 5% 2 6% 3 3 4 3 5% 7% 53%	2115
	21st to end	Night	11% 5% 2 5% 3 4 4 3 5% 7% 52%	1890
May	1st to 10th	Night	14% 7% 3 8% 5% 4 5% 4 6% 9% 35%	5237
	11th to 20th	Night	18% 6% 2 6% 4 4 5% 4 6% 8% 38%	5234
	21st to end	Night	18% 7% 3 7% 5% 4 5% 4 6% 8% 33%	5761
June	1st to 10th	Night	10% 6% 3 8% 5% 5% 6% 6% 8% 9% 35%	1890
	11th to 20th	Night	8% 6% 3 5% 5% 5% 5% 5% 7% 12% 40%	1890
	21st to end	Night	8% 4 2 6% 5% 6% 6% 5% 9% 11% 39%	1890
July	1st to 10th	Night	8% 5% 2 8% 6% 6% 7% 6% 9% 12% 31%	5278
	11th to 20th	Night	9% 5% 3 8% 5% 6% 7% 6% 10% 13% 28%	5277
	21st to end	Night	12% 9% 4 10% 6% 6% 6% 5% 7% 12% 24%	5736
August	1st to 10th	Night	13% 8% 3 8% 5% 6% 7% 5% 8% 11% 27%	3005
	11th to 20th	Night	13% 7% 3 9% 6% 6% 5% 5% 7% 9% 29%	2965
	21st to end	Night	7% 5% 2 6% 5% 6% 5% 4 7% 8% 44%	3718
September	1st to 10th	Night	9% 7% 2 6% 5% 5% 5% 5% 7% 9% 40%	2200
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 5% 6% 7% 38%	2200
	21st to end	Night	14% 6% 2 7% 4 4 4 3 6% 6% 45%	2200
October	1st to 10th	Night	11% 5% 2 5% 4 3 3 2 6% 5% 53%	2217
	11th to 20th	Night	12% 4 2 5% 4 3 3 3 4 5% 56%	2400
	21st to end	Night	13% 4 1 3 3 2 3 2 3 5% 59%	2640
November	1st to 10th	Night	13% 5% 2 5% 3 3 2 2 4 5% 55%	2522
	11th to 20th	Night	15% 6% 2 4 3 2 3 2 4 6% 54%	2730
	21st to end	Night	15% 5% 2 4 3 3 4 3 5% 6% 49%	2730
December	1st to 10th	Night	16% 5% 2 5% 4 3 3 2 4 4 51%	4380
	11th to 20th	Night	19% 6% 2 6% 3 3 3 3 4 5% 46%	4952
	21st to end	Night	21% 5% 2 5% 4 3 3 3 5% 5% 44%	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	39% 3 7% 23% 23% 5%	5259
	11th to 20th	Day	28% 2 4 13% 38% 14%	5280
	21st to end	Day	48% 2 4 15% 19% 11%	5780
February	1st to 10th	Day	41% 4 8% 22% 20% 6%	2027
	11th to 20th	Day	43% 5% 6% 19% 22% 6%	2058
	21st to end	Day	43% 5% 7% 15% 21% 9%	1730
March	1st to 10th	Day	47% 4 6% 14% 18% 11%	4948
	11th to 20th	Day	32% 4 5% 15% 25% 19%	5090
	21st to end	Day	42% 3 6% 21% 22% 6%	5586
April	1st to 10th	Day	35% 4 11% 20% 24% 6%	2310
	11th to 20th	Day	33% 5% 9% 21% 22% 9%	2470
	21st to end	Day	32% 5% 11% 27% 21% 4	2730
May	1st to 10th	Day	43% 5% 14% 23% 13% 2	7651
	11th to 20th	Day	43% 4 16% 21% 13% 2	7630
	21st to end	Day	47% 8% 13% 16% 15% 2	8405
June	1st to 10th	Day	38% 11% 17% 23% 9% 2	2730
	11th to 20th	Day	31% 10% 18% 27% 12% 1	2730
	21st to end	Day	32% 9% 20% 22% 13% 3	2730
July	1st to 10th	Day	36% 7% 22% 28% 6% 1	7615
	11th to 20th	Day	38% 9% 30% 20% 3 1	7611
	21st to end	Day	46% 8% 19% 18% 8% 1	8156
August	1st to 10th	Day	47% 7% 18% 23% 5%	4351
	11th to 20th	Day	37% 6% 20% 19% 15% 2	4343
	21st to end	Day	32% 7% 14% 24% 19% 4	4339
September	1st to 10th	Day	36% 8% 15% 19% 18% 3	2200
	11th to 20th	Day	41% 7% 12% 19% 20% 2	2200
	21st to end	Day	39% 6% 12% 19% 18% 6%	2200
October	1st to 10th	Day	29% 3 6% 18% 35% 11%	3558
	11th to 20th	Day	21% 2 6% 15% 30% 27%	3240
	21st to end	Day	27% 2 4 23% 30% 14%	3538
November	1st to 10th	Day	33% 3 7% 20% 26% 10%	1993
	11th to 20th	Day	31% 4 8% 22% 26% 9%	1890
	21st to end	Day	34% 4 8% 22% 25% 6%	1890
December	1st to 10th	Day	38% 3 6% 17% 27% 10%	3020
	11th to 20th	Day	43% 4 6% 19% 20% 9%	3460
	21st to end	Day	40% 3 7% 25% 21% 2	4565
January	1st to 10th	Night	34% 3 9% 22% 25% 7%	7636
	11th to 20th	Night	33% 3 4 16% 33% 12%	7680
	21st to end	Night	43% 2 4 16% 25% 10%	8401
February	1st to 10th	Night	38% 4 7% 22% 22% 7%	2588
	11th to 20th	Night	44% 3 5% 18% 23% 7%	2459
	21st to end	Night	40% 4 7% 18% 20% 10%	2076
March	1st to 10th	Night	42% 4 8% 12% 27% 7%	5123
	11th to 20th	Night	27% 4 8% 13% 30% 18%	5090
	21st to end	Night	38% 4 6% 20% 22% 10%	5588
April	1st to 10th	Night	36% 4 9% 18% 25% 7%	2310
	11th to 20th	Night	32% 3 7% 24% 24% 10%	2115
	21st to end	Night	32% 4 11% 27% 21% 4	1890
May	1st to 10th	Night	45% 6% 14% 20% 13% 1	5264
	11th to 20th	Night	44% 5% 16% 19% 14% 1	5235
	21st to end	Night	47% 8% 12% 18% 14% 2	5771
June	1st to 10th	Night	39% 11% 16% 19% 12% 2	1890
	11th to 20th	Night	34% 9% 18% 24% 13% 2	1890
	21st to end	Night	34% 11% 18% 19% 13% 4	1890
July	1st to 10th	Night	45% 7% 23% 20% 5% 1	5235
	11th to 20th	Night	44% 11% 26% 16% 3 1	5226
	21st to end	Night	53% 7% 19% 14% 5% 1	5606
August	1st to 10th	Night	49% 8% 20% 18% 4	2994
	11th to 20th	Night	49% 8% 18% 16% 8%	2992
	21st to end	Night	35% 7% 13% 22% 20% 2	3718
September	1st to 10th	Night	37% 9% 16% 17% 16% 4	2200
	11th to 20th	Night	42% 9% 11% 19% 18% 2	2200
	21st to end	Night	39% 5% 13% 19% 19% 6%	2200
October	1st to 10th	Night	28% 3 8% 17% 35% 8%	3620
	11th to 20th	Night	20% 3 5% 17% 28% 27%	3899
	21st to end	Night	27% 2 4 22% 28% 18%	4252
November	1st to 10th	Night	33% 3 7% 18% 27% 11%	2522
	11th to 20th	Night	33% 3 5% 18% 28% 12%	2730
	21st to end	Night	36% 4 6% 21% 25% 8%	2730
December	1st to 10th	Night	38% 3 4 18% 30% 8%	4380
	11th to 20th	Night	42% 3 10% 16% 21% 8%	4954
	21st to end	Night	43% 3 7% 25% 19% 3	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	24% 14% 35% 22% 5%	330
	11th to 20th	Day	25% 18% 25% 23% 8%	330
	21st to end	Day	26% 16% 36% 18% 3 1	363
February	1st to 10th	Day	20% 12% 38% 26% 5%	330
	11th to 20th	Day	35% 12% 32% 17% 4	330
	21st to end	Day	31% 18% 27% 18% 5%	273
March	1st to 10th	Day	22% 9% 30% 30% 8% 1	491
	11th to 20th	Day	17% 12% 38% 25% 7% 1	540
	21st to end	Day	21% 10% 35% 24% 9% 2	594
April	1st to 10th	Day	19% 12% 26% 26% 14% 2	540
	11th to 20th	Day	12% 8% 32% 26% 17% 4	532
	21st to end	Day	15% 7% 24% 29% 17% 8%	540
May	1st to 10th	Day	13% 8% 29% 24% 17% 8%	543
	11th to 20th	Day	19% 4 15% 26% 23% 13%	567
	21st to end	Day	19% 3 17% 20% 23% 18%	617
June	1st to 10th	Day	14% 2 14% 27% 27% 16%	570
	11th to 20th	Day	8% 3 15% 24% 29% 21%	570
	21st to end	Day	9% 1 10% 26% 30% 23%	570
July	1st to 10th	Day	10% 1 7% 20% 33% 30%	565
	11th to 20th	Day	9% 1 4 22% 36% 29%	564
	21st to end	Day	7% 2 7% 21% 38% 25%	620
August	1st to 10th	Day	17% 2 9% 20% 28% 23%	566
	11th to 20th	Day	13% 1 4 21% 36% 24%	559
	21st to end	Day	13% 1 7% 25% 31% 23%	619
September	1st to 10th	Day	11% 1 10% 28% 30% 21%	530
	11th to 20th	Day	17% 2 10% 18% 36% 18%	530
	21st to end	Day	17% 3 14% 27% 28% 12%	530
October	1st to 10th	Day	19% 2 16% 29% 23% 11%	530
	11th to 20th	Day	12% 5% 20% 30% 27% 6%	530
	21st to end	Day	16% 6% 22% 28% 23% 5%	583
November	1st to 10th	Day	21% 4 22% 29% 17% 7%	471
	11th to 20th	Day	18% 9% 34% 23% 14% 2	330
	21st to end	Day	14% 11% 29% 28% 14% 3	330
December	1st to 10th	Day	31% 7% 30% 26% 6%	330
	11th to 20th	Day	25% 6% 32% 28% 7% 2	330
	21st to end	Day	31% 9% 29% 19% 10% 2	363
January	1st to 10th	Night	22% 11% 30% 30% 7%	840
	11th to 20th	Night	20% 11% 31% 31% 6%	840
	21st to end	Night	21% 12% 35% 29% 3	924
February	1st to 10th	Night	18% 8% 31% 37% 5%	840
	11th to 20th	Night	27% 7% 36% 26% 4	823
	21st to end	Night	20% 11% 33% 33% 3	692
March	1st to 10th	Night	18% 8% 30% 36% 7%	840
	11th to 20th	Night	17% 8% 35% 30% 9% 1	840
	21st to end	Night	17% 9% 31% 31% 12% 1	924
April	1st to 10th	Night	15% 10% 27% 37% 10% 1	840
	11th to 20th	Night	13% 8% 26% 36% 16% 1	745
	21st to end	Night	14% 4 23% 37% 21% 2	630
May	1st to 10th	Night	13% 4 24% 40% 17% 2	630
	11th to 20th	Night	16% 2 18% 37% 24% 4	621
	21st to end	Night	16% 2 11% 32% 33% 5%	683
June	1st to 10th	Night	14% 1 10% 35% 34% 6%	630
	11th to 20th	Night	10% 1 11% 29% 36% 12%	630
	21st to end	Night	10% 8% 34% 38% 10%	630
July	1st to 10th	Night	12% 2 3 26% 44% 13%	615
	11th to 20th	Night	10% 2 3 23% 48% 14%	612
	21st to end	Night	11% 2 6% 27% 45% 9%	690
August	1st to 10th	Night	15% 2 4 28% 40% 11%	627
	11th to 20th	Night	15% 1 6% 25% 42% 10%	624
	21st to end	Night	13% 1 6% 27% 42% 10%	753
September	1st to 10th	Night	11% 1 9% 30% 39% 10%	800
	11th to 20th	Night	19% 2 7% 29% 36% 8%	800
	21st to end	Night	16% 3 12% 33% 33% 4	800
October	1st to 10th	Night	14% 2 15% 33% 31% 6%	800
	11th to 20th	Night	14% 3 20% 39% 21% 4	800
	21st to end	Night	16% 4 20% 36% 21% 2	880
November	1st to 10th	Night	16% 3 21% 37% 20% 3	820
	11th to 20th	Night	18% 7% 26% 35% 13% 2	840
	21st to end	Night	17% 6% 24% 37% 15% 1	840
December	1st to 10th	Night	23% 8% 22% 37% 9%	840
	11th to 20th	Night	21% 5% 27% 36% 10% 1	840
	21st to end	Night	26% 8% 24% 34% 8%	924

Page generated at 2026-08-27 04:07:15. Version ac07

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