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 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	7% 5% 2 5% 3 3 4 3 5% 7% 57%	117918
	Night	6% 5% 2 5% 3 3 4 3 5% 7% 57%	119422
Transparency	Day	27% 5% 11% 26% 23% 7%	117674
	Night	27% 5% 11% 25% 24% 8%	119256
Seeing	Day	10% 7% 30% 33% 16% 3	16154
	Night	10% 8% 35% 36% 10% 1	26229

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 5% 6% 50%	3638
	11th to 20th	Day	5% 7% 3 7% 4 4 5% 3 4 6% 51%	3884
	21st to end	Day	5% 6% 2 7% 4 4 3 2 4 5% 57%	4488
February	1st to 10th	Day	7% 9% 4 7% 4 3 3 2 5% 7% 50%	1965
	11th to 20th	Day	5% 5% 2 7% 5% 5% 4 4 5% 6% 51%	1949
	21st to end	Day	5% 6% 1 6% 4 3 3 2 5% 5% 60%	1650
March	1st to 10th	Day	5% 7% 2 4 3 2 3 2 4 6% 61%	4500
	11th to 20th	Day	6% 4 1 5% 3 3 3 3 4 6% 61%	4953
	21st to end	Day	6% 5% 1 4 3 4 4 3 5% 7% 57%	5464
April	1st to 10th	Day	9% 5% 2 5% 3 3 3 3 5% 6% 57%	2383
	11th to 20th	Day	10% 7% 2 6% 3 4 4 3 5% 6% 51%	2381
	21st to end	Day	12% 7% 2 5% 4 4 4 3 5% 6% 49%	2536
May	1st to 10th	Day	11% 6% 2 5% 4 4 3 4 5% 7% 49%	4120
	11th to 20th	Day	9% 5% 2 6% 3 4 4 4 6% 9% 47%	4082
	21st to end	Day	9% 4 2 5% 4 4 4 3 6% 8% 52%	4489
June	1st to 10th	Day	5% 3 1 4 3 3 4 4 6% 9% 60%	2488
	11th to 20th	Day	6% 4 1 4 3 4 3 3 5% 7% 59%	2660
	21st to end	Day	4 4 2 4 3 3 3 3 6% 8% 60%	2660
July	1st to 10th	Day	7% 4 2 6% 4 4 4 4 6% 8% 52%	4430
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 10% 60%	4316
	21st to end	Day	3 3 1 4 3 3 4 4 8% 10% 57%	4466
August	1st to 10th	Day	13% 5% 2 4 3 3 4 4 5% 7% 50%	4103
	11th to 20th	Day	8% 4 1 4 3 3 4 4 6% 8% 53%	3952
	21st to end	Day	4 3 1 4 3 3 4 3 5% 7% 62%	4180
September	1st to 10th	Day	9% 4 1 4 3 3 3 3 5% 6% 59%	2204
	11th to 20th	Day	7% 4 2 4 3 3 3 3 4 7% 60%	2090
	21st to end	Day	7% 3 1 3 3 2 4 3 4 7% 62%	2090
October	1st to 10th	Day	8% 3 1 2 2 2 2 1 4 4 71%	3480
	11th to 20th	Day	5% 3 1 3 2 3 3 3 5% 6% 66%	3246
	21st to end	Day	6% 4 2 3 3 2 2 2 4 6% 66%	3136
November	1st to 10th	Day	8% 3 1 3 3 3 3 2 4 6% 63%	1763
	11th to 20th	Day	5% 5% 1 3 3 3 3 2 5% 5% 65%	1800
	21st to end	Day	9% 5% 1 3 3 2 3 2 4 5% 62%	1800
December	1st to 10th	Day	4 4 1 2 2 2 3 2 4 6% 70%	2940
	11th to 20th	Day	5% 5% 2 4 2 2 3 2 4 6% 66%	3391
	21st to end	Day	8% 12% 5% 8% 4 3 3 3 4 6% 44%	4241
January	1st to 10th	Night	7% 8% 3 9% 5% 4 3 2 4 6% 49%	6327
	11th to 20th	Night	3 4 2 6% 4 4 4 3 4 6% 59%	6063
	21st to end	Night	6% 7% 3 7% 3 3 3 3 4 5% 57%	6490
February	1st to 10th	Night	7% 9% 3 7% 4 3 3 4 4 6% 49%	2450
	11th to 20th	Night	7% 6% 2 7% 6% 4 4 4 5% 7% 49%	2338
	21st to end	Night	5% 5% 3 6% 4 3 4 3 3 5% 59%	1980
March	1st to 10th	Night	6% 5% 1 4 2 3 3 2 4 5% 65%	5374
	11th to 20th	Night	6% 4 1 4 3 2 4 3 6% 7% 60%	4993
	21st to end	Night	7% 6% 3 6% 4 4 4 3 5% 8% 49%	5467
April	1st to 10th	Night	8% 5% 1 4 3 3 4 3 6% 8% 53%	2014
	11th to 20th	Night	11% 5% 2 6% 3 3 4 3 4 7% 51%	1965
	21st to end	Night	12% 5% 2 4 4 3 5% 3 5% 6% 53%	1760
May	1st to 10th	Night	11% 7% 2 4 4 4 4 3 5% 6% 50%	2850
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 5% 8% 50%	2823
	21st to end	Night	9% 4 2 4 4 4 5% 5% 7% 8% 49%	3098
June	1st to 10th	Night	5% 2 1 4 3 2 4 4 7% 13% 55%	1686
	11th to 20th	Night	7% 4 1 4 3 4 3 3 7% 8% 55%	1520
	21st to end	Night	5% 3 2 4 3 3 4 4 6% 11% 54%	1520
July	1st to 10th	Night	8% 4 2 5% 5% 4 5% 4 6% 9% 48%	2540
	11th to 20th	Night	3 3 1 4 5% 5% 5% 5% 7% 12% 51%	2652
	21st to end	Night	4 3 1 4 3 4 4 4 6% 10% 57%	3097
August	1st to 10th	Night	8% 4 2 6% 4 4 4 4 7% 10% 47%	2836
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 7% 8% 50%	2884
	21st to end	Night	3 3 1 4 4 4 5% 4 8% 11% 53%	3487
September	1st to 10th	Night	6% 3 1 4 3 3 3 3 4 6% 64%	1975
	11th to 20th	Night	7% 4 1 4 2 3 3 3 5% 6% 62%	2090
	21st to end	Night	7% 3 1 4 3 3 3 2 5% 5% 63%	2090
October	1st to 10th	Night	6% 3 1 2 2 1 2 2 3 5% 73%	3480
	11th to 20th	Night	4 3 1 2 2 2 3 3 4 7% 70%	3640
	21st to end	Night	5% 3 1 3 2 3 3 2 3 5% 69%	4405
November	1st to 10th	Night	6% 3 1 3 2 2 3 2 4 6% 66%	2535
	11th to 20th	Night	6% 4 2 3 3 3 3 3 4 5% 67%	2600
	21st to end	Night	8% 5% 2 4 3 3 3 3 4 6% 60%	2600
December	1st to 10th	Night	4 4 1 4 3 3 3 2 4 5% 67%	4250
	11th to 20th	Night	5% 4 1 3 3 3 3 3 4 6% 65%	4816
	21st to end	Night	8% 12% 4 7% 4 3 3 3 5% 6% 46%	6727

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	36% 4 12% 26% 18% 4	3639
	11th to 20th	Day	34% 5% 10% 22% 23% 5%	3884
	21st to end	Day	30% 4 11% 23% 25% 7%	4488
February	1st to 10th	Day	36% 4 12% 26% 16% 6%	1965
	11th to 20th	Day	33% 5% 14% 23% 18% 7%	1959
	21st to end	Day	27% 3 10% 24% 26% 11%	1650
March	1st to 10th	Day	26% 3 7% 16% 30% 17%	4500
	11th to 20th	Day	24% 4 8% 19% 30% 16%	4953
	21st to end	Day	26% 6% 14% 39% 13% 2	5464
April	1st to 10th	Day	28% 5% 11% 24% 24% 8%	2383
	11th to 20th	Day	34% 5% 11% 25% 19% 6%	2384
	21st to end	Day	36% 5% 11% 27% 19% 3	2536
May	1st to 10th	Day	33% 7% 12% 25% 12% 11%	4120
	11th to 20th	Day	30% 9% 16% 28% 16% 1	4086
	21st to end	Day	29% 8% 15% 22% 22% 5%	4489
June	1st to 10th	Day	22% 6% 19% 34% 16% 4	2488
	11th to 20th	Day	24% 6% 12% 28% 23% 7%	2660
	21st to end	Day	22% 6% 10% 29% 26% 8%	2660
July	1st to 10th	Day	29% 5% 11% 23% 24% 7%	4361
	11th to 20th	Day	18% 6% 16% 26% 28% 6%	4241
	21st to end	Day	20% 7% 13% 28% 24% 8%	4325
August	1st to 10th	Day	32% 6% 14% 25% 21% 2	4090
	11th to 20th	Day	28% 7% 16% 25% 20% 5%	3991
	21st to end	Day	21% 5% 14% 28% 24% 8%	4179
September	1st to 10th	Day	27% 4 9% 26% 22% 11%	2204
	11th to 20th	Day	25% 4 8% 27% 28% 7%	2090
	21st to end	Day	23% 5% 9% 28% 26% 9%	2090
October	1st to 10th	Day	19% 2 6% 20% 41% 13%	3480
	11th to 20th	Day	19% 4 11% 31% 24% 11%	3246
	21st to end	Day	21% 2 7% 26% 33% 10%	3136
November	1st to 10th	Day	23% 3 9% 26% 29% 9%	1763
	11th to 20th	Day	22% 3 8% 28% 32% 7%	1800
	21st to end	Day	27% 2 8% 25% 29% 9%	1800
December	1st to 10th	Day	19% 2 10% 35% 25% 9%	2940
	11th to 20th	Day	21% 2 7% 41% 26% 2	3389
	21st to end	Day	43% 3 8% 23% 18% 4	4241
January	1st to 10th	Night	37% 3 12% 26% 18% 3	6327
	11th to 20th	Night	27% 4 11% 22% 28% 9%	6063
	21st to end	Night	31% 4 10% 22% 23% 9%	6490
February	1st to 10th	Night	36% 4 13% 22% 17% 8%	2450
	11th to 20th	Night	34% 6% 11% 24% 20% 5%	2338
	21st to end	Night	28% 4 7% 21% 26% 14%	1980
March	1st to 10th	Night	22% 3 7% 18% 31% 18%	5374
	11th to 20th	Night	24% 3 9% 17% 28% 18%	4993
	21st to end	Night	33% 6% 13% 35% 11% 2	5467
April	1st to 10th	Night	28% 5% 11% 23% 26% 7%	2014
	11th to 20th	Night	33% 5% 13% 25% 19% 7%	1965
	21st to end	Night	31% 7% 12% 25% 22% 3	1760
May	1st to 10th	Night	33% 7% 15% 20% 17% 9%	2850
	11th to 20th	Night	31% 7% 15% 25% 20% 2	2824
	21st to end	Night	30% 9% 17% 21% 16% 7%	3099
June	1st to 10th	Night	20% 7% 19% 33% 17% 4	1686
	11th to 20th	Night	27% 6% 13% 26% 23% 5%	1520
	21st to end	Night	24% 7% 11% 27% 22% 8%	1520
July	1st to 10th	Night	31% 7% 13% 18% 28% 3	2500
	11th to 20th	Night	23% 6% 16% 24% 24% 5%	2604
	21st to end	Night	21% 6% 15% 25% 24% 10%	2999
August	1st to 10th	Night	31% 6% 12% 26% 22% 3	2826
	11th to 20th	Night	32% 6% 15% 26% 17% 3	2911
	21st to end	Night	23% 6% 15% 27% 21% 8%	3487
September	1st to 10th	Night	23% 5% 9% 24% 27% 11%	1975
	11th to 20th	Night	24% 5% 8% 24% 30% 9%	2090
	21st to end	Night	24% 4 7% 23% 30% 11%	2090
October	1st to 10th	Night	17% 2 6% 18% 44% 13%	3480
	11th to 20th	Night	17% 3 12% 23% 32% 14%	3640
	21st to end	Night	19% 3 7% 25% 36% 10%	4405
November	1st to 10th	Night	21% 3 9% 27% 27% 13%	2535
	11th to 20th	Night	21% 3 7% 28% 31% 11%	2600
	21st to end	Night	27% 3 9% 25% 25% 11%	2600
December	1st to 10th	Night	21% 3 9% 37% 24% 6%	4250
	11th to 20th	Night	22% 2 7% 35% 28% 6%	4817
	21st to end	Night	40% 4 9% 28% 16% 3	6727

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% 33% 9% 2	300
	11th to 20th	Day	15% 9% 36% 33% 7% 1	300
	21st to end	Day	9% 8% 45% 29% 9%	330
February	1st to 10th	Day	13% 12% 43% 23% 9%	300
	11th to 20th	Day	10% 12% 43% 23% 9% 3	300
	21st to end	Day	6% 18% 49% 24% 4	249
March	1st to 10th	Day	12% 13% 39% 29% 7% 1	300
	11th to 20th	Day	16% 13% 31% 31% 7% 2	300
	21st to end	Day	17% 12% 44% 21% 6% 1	330
April	1st to 10th	Day	12% 18% 41% 22% 6% 1	483
	11th to 20th	Day	15% 16% 41% 22% 5% 1	502
	21st to end	Day	17% 12% 37% 24% 8% 1	676
May	1st to 10th	Day	14% 9% 35% 29% 11% 1	653
	11th to 20th	Day	16% 8% 29% 30% 15% 3	675
	21st to end	Day	12% 9% 34% 33% 12% 1	734
June	1st to 10th	Day	5% 5% 28% 37% 18% 8%	680
	11th to 20th	Day	7% 6% 32% 36% 17% 2	680
	21st to end	Day	7% 4 30% 39% 19% 2	680
July	1st to 10th	Day	7% 3 20% 40% 24% 7%	671
	11th to 20th	Day	4 2 18% 45% 27% 5%	668
	21st to end	Day	6% 1 12% 42% 32% 8%	739
August	1st to 10th	Day	8% 1 13% 41% 32% 5%	676
	11th to 20th	Day	7% 1 18% 45% 26% 2	615
	21st to end	Day	6% 2 19% 43% 24% 6%	539
September	1st to 10th	Day	9% 2 18% 43% 23% 5%	414
	11th to 20th	Day	4 4 27% 38% 20% 7%	300
	21st to end	Day	11% 7% 23% 33% 21% 5%	300
October	1st to 10th	Day	18% 4 23% 33% 15% 6%	300
	11th to 20th	Day	4 7% 44% 26% 14% 3	300
	21st to end	Day	15% 12% 30% 30% 11% 2	330
November	1st to 10th	Day	12% 10% 33% 31% 11% 3	300
	11th to 20th	Day	11% 10% 40% 24% 13% 2	300
	21st to end	Day	16% 10% 31% 28% 12% 1	300
December	1st to 10th	Day	9% 4 39% 31% 15% 3	300
	11th to 20th	Day	14% 11% 38% 27% 8% 1	300
	21st to end	Day	17% 13% 39% 25% 6% 1	330
January	1st to 10th	Night	9% 8% 38% 38% 7% 1	1000
	11th to 20th	Night	9% 9% 39% 33% 9% 1	1000
	21st to end	Night	8% 8% 42% 35% 6%	1100
February	1st to 10th	Night	11% 9% 43% 31% 6%	850
	11th to 20th	Night	9% 11% 44% 30% 5%	784
	21st to end	Night	8% 13% 45% 28% 6%	660
March	1st to 10th	Night	10% 15% 37% 33% 6%	800
	11th to 20th	Night	13% 10% 35% 33% 7% 1	613
	21st to end	Night	17% 13% 40% 26% 4	660
April	1st to 10th	Night	12% 14% 45% 27% 3	600
	11th to 20th	Night	15% 15% 40% 26% 3	593
	21st to end	Night	16% 13% 34% 30% 6% 1	587
May	1st to 10th	Night	14% 11% 39% 31% 6%	570
	11th to 20th	Night	13% 10% 35% 32% 9% 1	561
	21st to end	Night	11% 11% 39% 30% 9%	617
June	1st to 10th	Night	6% 7% 34% 38% 15% 1	570
	11th to 20th	Night	8% 5% 44% 34% 8%	570
	21st to end	Night	6% 4 35% 44% 11% 1	570
July	1st to 10th	Night	7% 2 29% 46% 16% 1	555
	11th to 20th	Night	4 1 25% 51% 18% 1	552
	21st to end	Night	7% 1 18% 50% 23% 1	624
August	1st to 10th	Night	7% 2 19% 50% 22% 1	567
	11th to 20th	Night	9% 1 23% 50% 17%	564
	21st to end	Night	4 1 26% 46% 21% 2	627
September	1st to 10th	Night	8% 2 26% 48% 14% 1	570
	11th to 20th	Night	10% 3 35% 37% 13% 2	570
	21st to end	Night	9% 5% 29% 42% 14% 1	570
October	1st to 10th	Night	13% 6% 32% 36% 13%	570
	11th to 20th	Night	9% 7% 38% 36% 10% 1	673
	21st to end	Night	12% 9% 33% 33% 13% 1	1007
November	1st to 10th	Night	8% 9% 33% 42% 8% 1	975
	11th to 20th	Night	8% 11% 38% 35% 8% 1	1000
	21st to end	Night	12% 8% 31% 37% 10% 2	1000
December	1st to 10th	Night	9% 8% 37% 39% 7%	1000
	11th to 20th	Night	11% 11% 38% 32% 8%	1000
	21st to end	Night	13% 10% 36% 33% 7% 1	1100

Page generated at 2025-04-23 08:25:03. Version ac07

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