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 2024-10-25.

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% 55%	101242
	Night	7% 5% 2 5% 4 3 4 3 5% 7% 55%	97731
Transparency	Day	28% 5% 12% 26% 22% 6%	100998
	Night	28% 5% 12% 25% 23% 7%	97565
Seeing	Day	10% 7% 30% 33% 16% 3	15576
	Night	10% 8% 35% 37% 10% 1	25445

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	6% 9% 4 5% 3 4 3 4 5% 7% 49%	2538
	11th to 20th	Day	6% 8% 3 6% 4 5% 6% 4 5% 6% 48%	2706
	21st to end	Day	7% 9% 3 6% 3 3 2 2 4 6% 55%	3135
February	1st to 10th	Day	7% 9% 3 7% 4 4 3 2 5% 7% 49%	1866
	11th to 20th	Day	5% 5% 2 7% 5% 5% 4 4 5% 6% 52%	1849
	21st to end	Day	6% 6% 1 6% 4 3 3 2 5% 5% 59%	1570
March	1st to 10th	Day	6% 8% 2 3 3 2 3 3 5% 6% 60%	3130
	11th to 20th	Day	8% 5% 2 5% 3 3 3 3 4 6% 60%	3466
	21st to end	Day	8% 7% 2 4 4 4 4 3 5% 7% 52%	3814
April	1st to 10th	Day	8% 5% 2 4 3 3 3 3 5% 6% 57%	2264
	11th to 20th	Day	10% 7% 2 6% 3 4 4 3 5% 7% 50%	2260
	21st to end	Day	12% 7% 2 5% 4 4 4 3 5% 6% 49%	2470
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	13% 4 2 3 2 3 2 2 5% 5% 58%	2090
	11th to 20th	Day	7% 4 1 3 2 2 2 2 4 5% 66%	1967
	21st to end	Day	8% 7% 2 3 3 2 2 2 3 5% 62%	1857
November	1st to 10th	Day	6% 3 1 3 3 3 3 3 4 6% 65%	1673
	11th to 20th	Day	5% 5% 1 3 3 3 4 2 5% 5% 65%	1710
	21st to end	Day	9% 5% 1 3 4 2 3 2 4 5% 61%	1710
December	1st to 10th	Day	5% 7% 2 4 3 3 4 3 5% 8% 56%	1710
	11th to 20th	Day	7% 7% 2 5% 3 2 2 3 4 6% 58%	2161
	21st to end	Day	7% 14% 6% 8% 4 3 4 3 4 5% 42%	2966
January	1st to 10th	Night	5% 9% 3 9% 6% 4 3 2 4 5% 50%	4417
	11th to 20th	Night	4 5% 2 6% 4 4 4 3 4 6% 57%	4244
	21st to end	Night	8% 9% 3 7% 3 3 3 3 4 5% 54%	4532
February	1st to 10th	Night	8% 9% 3 7% 4 3 3 4 4 6% 49%	2328
	11th to 20th	Night	6% 6% 2 7% 6% 4 4 3 5% 7% 51%	2218
	21st to end	Night	5% 5% 3 6% 4 3 4 2 3 5% 59%	1884
March	1st to 10th	Night	8% 4 1 4 3 2 3 3 4 6% 63%	3734
	11th to 20th	Night	6% 5% 2 4 3 2 4 3 6% 7% 58%	3493
	21st to end	Night	7% 6% 3 5% 3 4 4 3 5% 7% 53%	3817
April	1st to 10th	Night	8% 5% 1 4 3 3 4 3 6% 8% 55%	1914
	11th to 20th	Night	11% 5% 2 6% 3 3 4 3 4 7% 51%	1866
	21st to end	Night	12% 5% 2 4 4 3 5% 3 5% 6% 53%	1710
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	11% 4 2 3 3 2 3 2 4 7% 59%	2090
	11th to 20th	Night	7% 4 1 3 2 2 2 2 3 6% 69%	2193
	21st to end	Night	8% 4 1 3 2 2 2 2 3 4 65%	2601
November	1st to 10th	Night	4 3 1 3 2 3 3 2 4 6% 68%	2405
	11th to 20th	Night	6% 4 2 3 3 2 3 3 4 5% 67%	2470
	21st to end	Night	8% 6% 2 4 3 3 3 3 4 6% 59%	2470
December	1st to 10th	Night	6% 5% 2 5% 3 3 3 3 5% 7% 58%	2470
	11th to 20th	Night	7% 4 2 4 3 4 3 3 4 7% 60%	3036
	21st to end	Night	8% 12% 4 6% 3 3 3 3 5% 6% 47%	4691

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% 5% 13% 27% 20% 2	2539
	11th to 20th	Day	35% 7% 14% 21% 17% 5%	2706
	21st to end	Day	32% 5% 14% 30% 16% 3	3135
February	1st to 10th	Day	36% 4 13% 25% 16% 6%	1866
	11th to 20th	Day	32% 5% 13% 24% 19% 7%	1859
	21st to end	Day	28% 3 10% 24% 25% 10%	1570
March	1st to 10th	Day	25% 4 9% 17% 31% 14%	3130
	11th to 20th	Day	26% 4 11% 25% 28% 5%	3466
	21st to end	Day	31% 7% 15% 32% 11% 3	3814
April	1st to 10th	Day	27% 5% 12% 24% 23% 9%	2264
	11th to 20th	Day	34% 5% 11% 25% 18% 6%	2263
	21st to end	Day	36% 5% 10% 27% 19% 3	2470
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	29% 4 9% 24% 28% 7%	2090
	11th to 20th	Day	21% 3 8% 25% 29% 13%	1967
	21st to end	Day	26% 3 9% 26% 27% 10%	1857
November	1st to 10th	Day	22% 3 9% 27% 29% 10%	1673
	11th to 20th	Day	23% 3 8% 29% 30% 7%	1710
	21st to end	Day	28% 2 7% 25% 29% 9%	1710
December	1st to 10th	Day	28% 4 12% 28% 23% 4	1710
	11th to 20th	Day	28% 3 9% 36% 21% 3	2159
	21st to end	Day	46% 4 9% 21% 17% 3	2966
January	1st to 10th	Night	37% 4 11% 27% 18% 3	4417
	11th to 20th	Night	29% 4 15% 25% 21% 6%	4244
	21st to end	Night	34% 5% 12% 25% 21% 3	4532
February	1st to 10th	Night	37% 4 13% 22% 17% 7%	2328
	11th to 20th	Night	33% 6% 11% 25% 21% 5%	2218
	21st to end	Night	29% 4 7% 22% 26% 13%	1884
March	1st to 10th	Night	23% 4 8% 21% 30% 14%	3734
	11th to 20th	Night	25% 4 12% 22% 29% 8%	3493
	21st to end	Night	30% 7% 15% 34% 11% 3	3817
April	1st to 10th	Night	27% 5% 11% 23% 26% 8%	1914
	11th to 20th	Night	33% 5% 13% 24% 18% 7%	1866
	21st to end	Night	31% 7% 12% 25% 22% 3	1710
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	27% 3 9% 20% 31% 9%	2090
	11th to 20th	Night	19% 2 10% 23% 31% 15%	2193
	21st to end	Night	24% 3 8% 24% 28% 13%	2601
November	1st to 10th	Night	19% 3 9% 27% 28% 14%	2405
	11th to 20th	Night	21% 3 8% 29% 30% 10%	2470
	21st to end	Night	28% 3 8% 23% 25% 12%	2470
December	1st to 10th	Night	26% 4 12% 29% 22% 7%	2470
	11th to 20th	Night	26% 3 9% 35% 23% 4	3037
	21st to end	Night	39% 4 11% 27% 16% 3	4691

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	9% 8% 38% 33% 10% 3	270
	11th to 20th	Day	17% 8% 36% 32% 7% 1	270
	21st to end	Day	10% 8% 42% 30% 9%	297
February	1st to 10th	Day	15% 11% 42% 23% 9%	270
	11th to 20th	Day	10% 10% 43% 24% 10% 3	270
	21st to end	Day	7% 17% 49% 23% 4	225
March	1st to 10th	Day	11% 11% 41% 30% 7% 1	270
	11th to 20th	Day	16% 11% 32% 31% 7% 2	270
	21st to end	Day	18% 10% 44% 22% 6% 1	297
April	1st to 10th	Day	11% 16% 43% 23% 5% 1	444
	11th to 20th	Day	16% 15% 41% 23% 5% 1	461
	21st to end	Day	17% 12% 38% 24% 8% 1	650
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% 29% 31% 11% 2	311
November	1st to 10th	Day	9% 11% 33% 32% 12% 4	270
	11th to 20th	Day	11% 9% 39% 26% 13% 2	270
	21st to end	Day	17% 10% 31% 29% 12% 1	270
December	1st to 10th	Day	9% 4 39% 31% 15% 2	270
	11th to 20th	Day	16% 11% 35% 28% 9% 1	270
	21st to end	Day	17% 13% 40% 24% 6%	297
January	1st to 10th	Night	8% 8% 38% 38% 7% 1	950
	11th to 20th	Night	10% 9% 38% 33% 9% 1	950
	21st to end	Night	8% 8% 41% 36% 7%	1045
February	1st to 10th	Night	12% 9% 41% 31% 7%	808
	11th to 20th	Night	9% 10% 45% 31% 6%	744
	21st to end	Night	8% 13% 44% 28% 6%	628
March	1st to 10th	Night	10% 14% 37% 34% 6%	760
	11th to 20th	Night	13% 10% 35% 33% 7% 1	583
	21st to end	Night	17% 12% 41% 26% 4	627
April	1st to 10th	Night	11% 13% 46% 27% 3	570
	11th to 20th	Night	15% 15% 40% 27% 3	563
	21st to end	Night	15% 13% 34% 30% 6% 1	570
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	977
November	1st to 10th	Night	7% 8% 33% 43% 9% 1	925
	11th to 20th	Night	8% 9% 38% 36% 8% 1	950
	21st to end	Night	12% 8% 31% 37% 11% 2	950
December	1st to 10th	Night	10% 8% 37% 38% 7%	950
	11th to 20th	Night	11% 11% 37% 33% 8%	950
	21st to end	Night	13% 10% 36% 34% 7%	1045

Page generated at 2024-10-22 19:21:02. Version ac07

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