Arnold
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arnold 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	7% 5% 2 4 3 3 3 3 5% 7% 57%	152569
	Night	7% 5% 2 5% 3 3 4 3 5% 7% 56%	147025
Transparency	Day	27% 5% 11% 27% 24% 7%	147435
	Night	28% 4 11% 26% 24% 8%	143171
Seeing	Day	10% 8% 31% 33% 15% 3	18145
	Night	10% 8% 36% 36% 9% 1	27951

By Month

Percentage Cloud Cover SW (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	7% 8% 3 6% 3 4 3 3 4 7% 51%	4720
	11th to 20th	Day	4 6% 3 6% 3 4 5% 3 4 6% 56%	5062
	21st to end	Day	6% 7% 2 7% 4 4 3 2 4 5% 56%	5812
February	1st to 10th	Day	6% 9% 3 7% 4 3 3 2 4 7% 51%	2064
	11th to 20th	Day	5% 5% 2 7% 6% 5% 4 5% 5% 6% 51%	2049
	21st to end	Day	5% 5% 1 5% 3 3 3 2 5% 5% 61%	1730
March	1st to 10th	Day	5% 7% 2 4 3 3 3 2 4 6% 61%	4600
	11th to 20th	Day	6% 4 1 5% 3 3 3 3 4 6% 61%	5062
	21st to end	Day	6% 5% 1 4 3 4 4 3 5% 7% 57%	5585
April	1st to 10th	Day	8% 5% 2 5% 3 3 3 3 5% 6% 57%	2502
	11th to 20th	Day	9% 7% 2 6% 3 4 4 3 5% 7% 52%	2502
	21st to end	Day	13% 7% 2 5% 4 4 4 3 5% 6% 48%	2730
May	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 6% 51%	7655
	11th to 20th	Day	6% 3 1 4 3 3 4 3 5% 7% 61%	7627
	21st to end	Day	18% 6% 2 5% 3 3 4 3 5% 6% 46%	8404
June	1st to 10th	Day	6% 3 1 4 3 3 4 3 6% 8% 59%	2750
	11th to 20th	Day	6% 4 1 4 3 4 4 3 5% 7% 59%	2940
	21st to end	Day	5% 4 2 4 3 3 4 3 6% 8% 58%	2940
July	1st to 10th	Day	4 3 1 5% 4 4 4 4 6% 9% 56%	8249
	11th to 20th	Day	4 2 1 3 2 2 3 3 5% 7% 68%	8032
	21st to end	Day	4 3 1 4 3 3 4 4 6% 9% 57%	8339
August	1st to 10th	Day	12% 5% 2 4 3 3 4 4 5% 7% 51%	5931
	11th to 20th	Day	6% 3 1 4 3 3 4 4 7% 9% 56%	5817
	21st to end	Day	14% 5% 2 5% 3 3 4 3 4 6% 51%	6093
September	1st to 10th	Day	9% 4 1 4 3 3 3 3 5% 6% 59%	2320
	11th to 20th	Day	7% 4 2 4 3 3 3 3 4 7% 60%	2200
	21st to end	Day	7% 3 1 3 3 2 4 3 4 7% 62%	2200
October	1st to 10th	Day	8% 3 1 2 2 2 2 1 3 4 71%	3590
	11th to 20th	Day	5% 3 1 3 2 3 3 3 5% 6% 65%	3349
	21st to end	Day	6% 4 2 3 3 2 2 2 4 6% 66%	3237
November	1st to 10th	Day	7% 3 1 3 3 3 3 2 4 6% 64%	1853
	11th to 20th	Day	5% 5% 1 3 3 3 3 2 5% 5% 65%	1890
	21st to end	Day	9% 5% 1 3 3 2 3 2 4 5% 61%	1890
December	1st to 10th	Day	4 5% 1 3 2 2 3 2 3 6% 69%	3030
	11th to 20th	Day	4 5% 2 4 2 2 2 2 4 6% 67%	3481
	21st to end	Day	8% 12% 5% 7% 4 3 3 3 4 6% 44%	4334
January	1st to 10th	Night	7% 8% 3 8% 4 4 3 3 4 6% 50%	8201
	11th to 20th	Night	2 5% 2 5% 4 4 4 3 4 5% 63%	7895
	21st to end	Night	6% 8% 3 8% 4 3 3 3 4 5% 53%	8402
February	1st to 10th	Night	7% 9% 3 7% 4 3 3 4 4 6% 50%	2572
	11th to 20th	Night	7% 6% 2 7% 6% 4 4 3 5% 7% 49%	2458
	21st to end	Night	5% 5% 3 6% 4 3 4 2 3 5% 59%	2076
March	1st to 10th	Night	6% 5% 1 4 2 3 3 2 4 5% 65%	5494
	11th to 20th	Night	6% 4 1 4 3 2 4 3 5% 7% 60%	5104
	21st to end	Night	7% 6% 3 6% 4 4 4 3 5% 8% 50%	5588
April	1st to 10th	Night	8% 5% 2 5% 3 3 4 3 6% 8% 53%	2115
	11th to 20th	Night	11% 5% 2 6% 3 3 4 3 4 7% 52%	2064
	21st to end	Night	13% 5% 2 4 4 3 4 3 5% 6% 52%	1890
May	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 6% 50%	5292
	11th to 20th	Night	6% 3 1 4 2 3 3 3 4 7% 63%	5265
	21st to end	Night	17% 6% 2 4 3 3 4 4 5% 7% 45%	5803
June	1st to 10th	Night	5% 3 1 4 4 2 4 3 7% 12% 54%	1864
	11th to 20th	Night	6% 4 1 4 3 4 3 3 6% 9% 56%	1680
	21st to end	Night	6% 4 2 5% 3 3 4 4 6% 11% 52%	1680
July	1st to 10th	Night	6% 4 2 5% 5% 5% 5% 5% 8% 11% 46%	4740
	11th to 20th	Night	5% 3 1 4 4 4 4 4 6% 9% 55%	4943
	21st to end	Night	5% 3 2 5% 4 4 4 4 7% 12% 50%	5770
August	1st to 10th	Night	7% 4 2 5% 4 4 5% 4 7% 11% 46%	4101
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 7% 9% 50%	4248
	21st to end	Night	11% 6% 2 6% 4 4 4 4 7% 9% 43%	5083
September	1st to 10th	Night	6% 3 1 4 3 3 3 3 4 6% 63%	2079
	11th to 20th	Night	7% 4 1 4 2 3 4 3 5% 6% 61%	2200
	21st to end	Night	8% 4 1 4 3 3 3 2 5% 5% 62%	2200
October	1st to 10th	Night	7% 3 1 2 2 1 2 2 3 5% 73%	3590
	11th to 20th	Night	4 3 1 2 2 2 3 3 4 7% 70%	3756
	21st to end	Night	6% 3 1 3 2 3 3 2 3 5% 68%	4546
November	1st to 10th	Night	6% 3 1 3 2 3 3 2 4 6% 66%	2665
	11th to 20th	Night	5% 3 2 3 2 2 2 3 4 5% 68%	2730
	21st to end	Night	8% 6% 2 4 3 3 2 3 4 6% 59%	2730
December	1st to 10th	Night	4 4 1 4 3 3 3 2 3 6% 67%	4380
	11th to 20th	Night	4 4 1 3 3 3 3 3 4 6% 65%	4946
	21st to end	Night	8% 12% 4 7% 3 3 3 3 5% 6% 46%	6875

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	34% 3 10% 26% 22% 5%	4723
	11th to 20th	Day	29% 5% 10% 26% 23% 7%	5062
	21st to end	Day	32% 4 10% 23% 25% 6%	5812
February	1st to 10th	Day	35% 4 12% 27% 17% 6%	2064
	11th to 20th	Day	33% 5% 14% 24% 18% 6%	2059
	21st to end	Day	26% 3 9% 24% 26% 11%	1730
March	1st to 10th	Day	26% 3 7% 16% 30% 17%	4600
	11th to 20th	Day	24% 3 8% 19% 30% 16%	5062
	21st to end	Day	26% 6% 14% 39% 13% 3	5585
April	1st to 10th	Day	28% 4 11% 24% 24% 8%	2502
	11th to 20th	Day	33% 5% 11% 24% 20% 8%	2505
	21st to end	Day	37% 5% 10% 26% 19% 3	2730
May	1st to 10th	Day	33% 5% 11% 26% 19% 6%	7655
	11th to 20th	Day	22% 6% 10% 23% 27% 12%	7631
	21st to end	Day	39% 6% 12% 20% 19% 4	8405
June	1st to 10th	Day	23% 6% 18% 33% 16% 4	2750
	11th to 20th	Day	24% 6% 12% 28% 24% 7%	2940
	21st to end	Day	23% 5% 9% 28% 25% 8%	2940
July	1st to 10th	Day	24% 5% 10% 27% 28% 5%	8181
	11th to 20th	Day	17% 4 15% 28% 27% 10%	7957
	21st to end	Day	23% 6% 12% 34% 19% 5%	8155
August	1st to 10th	Day	31% 6% 14% 25% 21% 3	4350
	11th to 20th	Day	27% 7% 16% 25% 20% 5%	4245
	21st to end	Day	22% 5% 14% 28% 23% 8%	4420
September	1st to 10th	Day	27% 4 9% 27% 21% 11%	2320
	11th to 20th	Day	25% 4 9% 28% 27% 7%	2200
	21st to end	Day	23% 5% 9% 28% 26% 9%	2200
October	1st to 10th	Day	19% 2 6% 20% 41% 12%	3590
	11th to 20th	Day	20% 4 11% 30% 25% 11%	3349
	21st to end	Day	22% 2 7% 26% 34% 10%	3237
November	1st to 10th	Day	22% 3 9% 28% 29% 9%	1853
	11th to 20th	Day	22% 3 8% 29% 31% 8%	1890
	21st to end	Day	27% 2 9% 25% 27% 9%	1890
December	1st to 10th	Day	19% 2 10% 35% 25% 8%	3030
	11th to 20th	Day	21% 2 8% 41% 26% 2	3479
	21st to end	Day	42% 3 8% 23% 19% 4	4334
January	1st to 10th	Night	36% 3 11% 25% 21% 4	8202
	11th to 20th	Night	25% 3 10% 26% 27% 9%	7895
	21st to end	Night	34% 3 10% 22% 22% 8%	8402
February	1st to 10th	Night	36% 4 12% 23% 19% 8%	2572
	11th to 20th	Night	34% 6% 11% 24% 20% 5%	2458
	21st to end	Night	28% 4 7% 21% 27% 14%	2076
March	1st to 10th	Night	22% 3 7% 18% 31% 18%	5494
	11th to 20th	Night	24% 3 9% 17% 28% 18%	5104
	21st to end	Night	33% 6% 12% 35% 11% 2	5588
April	1st to 10th	Night	29% 5% 11% 23% 26% 7%	2115
	11th to 20th	Night	32% 4 12% 24% 19% 8%	2064
	21st to end	Night	32% 6% 12% 25% 21% 3	1890
May	1st to 10th	Night	33% 6% 12% 26% 18% 5%	5293
	11th to 20th	Night	21% 5% 11% 28% 25% 10%	5266
	21st to end	Night	38% 7% 16% 20% 13% 6%	5805
June	1st to 10th	Night	22% 6% 19% 32% 17% 4	1864
	11th to 20th	Night	26% 6% 14% 26% 23% 5%	1680
	21st to end	Night	26% 6% 11% 26% 22% 8%	1680
July	1st to 10th	Night	30% 7% 17% 22% 23% 2	4700
	11th to 20th	Night	25% 5% 13% 26% 22% 9%	4895
	21st to end	Night	26% 5% 13% 30% 20% 5%	5641
August	1st to 10th	Night	31% 6% 12% 26% 22% 3	3006
	11th to 20th	Night	32% 6% 15% 26% 18% 3	3096
	21st to end	Night	25% 6% 16% 27% 20% 7%	3687
September	1st to 10th	Night	23% 5% 10% 24% 27% 11%	2079
	11th to 20th	Night	24% 5% 8% 25% 29% 9%	2200
	21st to end	Night	25% 4 7% 24% 29% 11%	2200
October	1st to 10th	Night	17% 2 6% 18% 44% 13%	3590
	11th to 20th	Night	17% 3 12% 23% 32% 13%	3756
	21st to end	Night	20% 3 6% 25% 36% 10%	4546
November	1st to 10th	Night	21% 3 9% 28% 27% 13%	2665
	11th to 20th	Night	20% 3 8% 29% 31% 10%	2730
	21st to end	Night	28% 3 9% 25% 24% 11%	2730
December	1st to 10th	Night	21% 3 9% 36% 24% 6%	4380
	11th to 20th	Night	22% 2 7% 35% 29% 6%	4947
	21st to end	Night	40% 4 9% 28% 17% 3	6875

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	10% 8% 38% 32% 10% 2	330
	11th to 20th	Day	15% 11% 36% 32% 7% 1	330
	21st to end	Day	9% 8% 44% 29% 9%	363
February	1st to 10th	Day	12% 11% 45% 24% 8%	330
	11th to 20th	Day	11% 12% 42% 24% 8% 2	330
	21st to end	Day	5% 17% 49% 25% 4	273
March	1st to 10th	Day	11% 12% 40% 29% 7% 1	330
	11th to 20th	Day	15% 14% 32% 31% 7% 2	330
	21st to end	Day	16% 12% 43% 22% 7% 1	363
April	1st to 10th	Day	11% 18% 41% 22% 6% 1	522
	11th to 20th	Day	15% 17% 42% 22% 5% 1	543
	21st to end	Day	18% 12% 38% 23% 7% 1	750
May	1st to 10th	Day	14% 9% 34% 30% 12% 1	753
	11th to 20th	Day	14% 9% 30% 29% 14% 3	775
	21st to end	Day	14% 9% 33% 32% 11% 1	844
June	1st to 10th	Day	6% 6% 29% 36% 16% 7%	780
	11th to 20th	Day	7% 7% 34% 35% 16% 2	780
	21st to end	Day	8% 5% 32% 36% 17% 2	780
July	1st to 10th	Day	6% 3 21% 41% 23% 6%	771
	11th to 20th	Day	4 2 16% 44% 29% 6%	768
	21st to end	Day	6% 2 13% 42% 31% 7%	849
August	1st to 10th	Day	7% 2 16% 41% 29% 4	776
	11th to 20th	Day	6% 2 18% 46% 25% 3	709
	21st to end	Day	8% 3 18% 42% 23% 6%	620
September	1st to 10th	Day	9% 2 19% 43% 22% 4	450
	11th to 20th	Day	4 4 27% 39% 18% 7%	330
	21st to end	Day	11% 7% 24% 33% 20% 5%	330
October	1st to 10th	Day	18% 4 25% 32% 14% 6%	330
	11th to 20th	Day	5% 8% 45% 25% 13% 3	330
	21st to end	Day	15% 12% 31% 29% 10% 2	363
November	1st to 10th	Day	11% 10% 33% 32% 11% 3	330
	11th to 20th	Day	11% 9% 41% 24% 13% 2	330
	21st to end	Day	18% 12% 30% 28% 12% 1	330
December	1st to 10th	Day	9% 5% 41% 29% 14% 3	330
	11th to 20th	Day	14% 11% 39% 26% 8% 1	330
	21st to end	Day	16% 12% 39% 26% 7% 1	363
January	1st to 10th	Night	9% 7% 38% 38% 6% 1	1050
	11th to 20th	Night	9% 10% 39% 32% 8% 1	1050
	21st to end	Night	8% 9% 42% 35% 6%	1155
February	1st to 10th	Night	11% 9% 43% 31% 6%	892
	11th to 20th	Night	9% 12% 44% 30% 5%	824
	21st to end	Night	8% 12% 45% 29% 5%	692
March	1st to 10th	Night	9% 15% 37% 33% 6%	840
	11th to 20th	Night	13% 11% 36% 32% 7% 1	644
	21st to end	Night	16% 13% 41% 26% 4	693
April	1st to 10th	Night	11% 14% 45% 27% 3	630
	11th to 20th	Night	15% 16% 40% 26% 3	623
	21st to end	Night	17% 15% 33% 29% 6%	630
May	1st to 10th	Night	13% 11% 38% 32% 6%	630
	11th to 20th	Night	12% 10% 36% 32% 9% 1	621
	21st to end	Night	13% 10% 39% 30% 8%	683
June	1st to 10th	Night	6% 6% 35% 38% 13% 1	630
	11th to 20th	Night	8% 6% 44% 33% 8%	630
	21st to end	Night	7% 6% 36% 41% 10% 1	630
July	1st to 10th	Night	6% 3 32% 44% 14%	615
	11th to 20th	Night	4 1 24% 50% 20% 1	612
	21st to end	Night	6% 1 18% 51% 22% 1	690
August	1st to 10th	Night	6% 3 21% 49% 20% 1	627
	11th to 20th	Night	8% 1 25% 48% 17%	624
	21st to end	Night	5% 1 27% 46% 20% 2	687
September	1st to 10th	Night	8% 2 26% 48% 14% 1	600
	11th to 20th	Night	10% 4 35% 37% 13% 2	600
	21st to end	Night	10% 6% 28% 42% 14% 1	600
October	1st to 10th	Night	13% 7% 32% 36% 12%	600
	11th to 20th	Night	9% 8% 39% 35% 9% 1	709
	21st to end	Night	13% 10% 33% 32% 12% 1	1060
November	1st to 10th	Night	8% 9% 33% 41% 8% 1	1025
	11th to 20th	Night	8% 10% 38% 36% 8%	1050
	21st to end	Night	12% 9% 31% 36% 10% 2	1050
December	1st to 10th	Night	9% 8% 37% 38% 7%	1050
	11th to 20th	Night	10% 11% 39% 32% 8%	1050
	21st to end	Night	12% 10% 37% 34% 7% 1	1155

Page generated at 2026-08-27 03:07:14. Version ac07

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