Arnes
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arnes clear sky chart from 2005-11-06 to 2025-02-21.

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	11% 9% 3 9% 6% 5% 5% 4 6% 8% 34%	108384
	Night	10% 10% 4 10% 6% 5% 5% 4 6% 7% 32%	110413
Transparency	Day	46% 8% 16% 23% 7%	111820
	Night	49% 7% 14% 21% 9% 1	115138
Seeing	Day	14% 6% 26% 32% 19% 4	16615
	Night	15% 5% 28% 35% 16% 2	26745

By Month

Percentage Cloud Cover NE (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% 16% 7% 12% 7% 8% 8% 6% 7% 6% 18%	3178
	11th to 20th	Day	5% 11% 4 16% 10% 7% 7% 8% 11% 8% 13%	3152
	21st to end	Day	5% 14% 7% 14% 8% 8% 8% 5% 7% 7% 19%	3885
February	1st to 10th	Day	9% 12% 5% 11% 8% 7% 7% 5% 8% 7% 20%	1688
	11th to 20th	Day	7% 10% 4 9% 5% 7% 7% 5% 8% 8% 28%	1754
	21st to end	Day	6% 13% 5% 10% 7% 6% 7% 6% 7% 7% 27%	1465
March	1st to 10th	Day	10% 11% 5% 11% 7% 6% 6% 5% 7% 8% 24%	3207
	11th to 20th	Day	7% 11% 4 13% 9% 6% 6% 5% 6% 6% 27%	3480
	21st to end	Day	6% 11% 4 10% 7% 6% 5% 5% 6% 6% 33%	3814
April	1st to 10th	Day	9% 9% 3 8% 5% 5% 5% 4 6% 7% 39%	2334
	11th to 20th	Day	11% 9% 3 6% 5% 4 6% 4 6% 7% 38%	2442
	21st to end	Day	13% 7% 2 8% 6% 5% 5% 5% 6% 7% 37%	2470
May	1st to 10th	Day	13% 8% 3 7% 4 4 4 3 6% 8% 38%	4319
	11th to 20th	Day	18% 8% 3 7% 4 4 4 3 5% 6% 35%	4670
	21st to end	Day	19% 7% 2 7% 4 4 4 3 5% 6% 37%	5173
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 4 4 6% 8% 38%	2850
	11th to 20th	Day	14% 6% 2 6% 6% 4 5% 5% 6% 8% 37%	2850
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 4 7% 9% 38%	2850
July	1st to 10th	Day	11% 5% 3 6% 5% 5% 6% 5% 7% 9% 39%	4750
	11th to 20th	Day	8% 4 1 5% 3 4 4 4 6% 10% 51%	4750
	21st to end	Day	5% 4 1 6% 4 4 4 5% 7% 9% 51%	5151
August	1st to 10th	Day	8% 3 2 6% 4 5% 5% 5% 8% 10% 44%	4340
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 5% 8% 47%	4041
	21st to end	Day	13% 6% 2 5% 4 4 4 3 6% 9% 43%	4532
September	1st to 10th	Day	11% 5% 2 5% 4 4 5% 5% 5% 8% 47%	2380
	11th to 20th	Day	14% 8% 3 8% 5% 5% 5% 4 5% 9% 36%	2090
	21st to end	Day	15% 7% 3 6% 4 4 5% 4 6% 9% 39%	2090
October	1st to 10th	Day	18% 11% 3 7% 5% 4 5% 3 5% 9% 30%	1925
	11th to 20th	Day	13% 12% 4 9% 6% 5% 5% 4 6% 6% 29%	1853
	21st to end	Day	14% 15% 4 11% 6% 7% 6% 4 4 6% 23%	1872
November	1st to 10th	Day	17% 14% 5% 13% 5% 5% 3 3 4 6% 25%	1567
	11th to 20th	Day	10% 17% 5% 13% 9% 5% 4 3 4 6% 24%	1600
	21st to end	Day	14% 15% 5% 11% 8% 6% 4 4 6% 6% 21%	1435
December	1st to 10th	Day	11% 13% 6% 15% 9% 10% 6% 5% 7% 6% 12%	2290
	11th to 20th	Day	9% 12% 5% 13% 8% 5% 5% 5% 6% 8% 24%	2639
	21st to end	Day	6% 14% 7% 17% 8% 6% 5% 4 5% 6% 22%	3498
January	1st to 10th	Night	5% 12% 6% 14% 8% 8% 7% 5% 6% 7% 20%	6787
	11th to 20th	Night	5% 12% 7% 14% 10% 9% 8% 7% 7% 5% 15%	6753
	21st to end	Night	7% 15% 7% 14% 7% 6% 6% 4 6% 7% 20%	7089
February	1st to 10th	Night	7% 13% 5% 11% 7% 7% 7% 6% 6% 6% 25%	2711
	11th to 20th	Night	6% 9% 4 10% 6% 5% 6% 5% 7% 8% 34%	2533
	21st to end	Night	5% 12% 4 10% 6% 5% 7% 5% 5% 6% 34%	2007
March	1st to 10th	Night	11% 12% 3 9% 6% 6% 6% 5% 7% 7% 31%	3641
	11th to 20th	Night	5% 9% 3 7% 5% 5% 5% 4 6% 8% 43%	3479
	21st to end	Night	9% 11% 4 9% 5% 5% 4 3 5% 6% 38%	3817
April	1st to 10th	Night	10% 8% 2 5% 5% 4 5% 4 7% 8% 41%	1843
	11th to 20th	Night	12% 10% 3 8% 6% 4 5% 4 6% 6% 36%	1689
	21st to end	Night	10% 8% 2 8% 6% 5% 6% 5% 7% 7% 37%	1710
May	1st to 10th	Night	18% 9% 3 6% 4 4 4 4 5% 6% 36%	2632
	11th to 20th	Night	18% 7% 2 7% 4 4 4 3 6% 7% 38%	2231
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 7% 39%	2414
June	1st to 10th	Night	14% 8% 2 6% 4 4 5% 4 7% 6% 38%	1330
	11th to 20th	Night	15% 7% 2 4 6% 4 3 4 6% 8% 41%	1330
	21st to end	Night	11% 5% 2 5% 4 4 6% 5% 9% 10% 39%	1330
July	1st to 10th	Night	10% 4 2 4 3 3 3 4 6% 9% 53%	2220
	11th to 20th	Night	7% 3 2 4 3 4 5% 3 6% 7% 56%	2220
	21st to end	Night	5% 3 1 4 4 4 4 5% 7% 8% 55%	2414
August	1st to 10th	Night	9% 4 2 5% 3 4 5% 4 6% 9% 48%	2596
	11th to 20th	Night	8% 3 1 4 3 4 4 4 7% 11% 50%	2798
	21st to end	Night	10% 5% 1 6% 4 4 4 4 5% 8% 48%	3135
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 6% 8% 49%	1796
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 6% 9% 42%	2090
	21st to end	Night	14% 7% 2 7% 4 4 5% 4 5% 7% 41%	2090
October	1st to 10th	Night	17% 10% 3 6% 4 5% 5% 4 5% 6% 36%	2235
	11th to 20th	Night	15% 12% 3 8% 6% 4 4 4 5% 7% 32%	2326
	21st to end	Night	15% 13% 4 9% 6% 5% 5% 4 5% 6% 28%	2725
November	1st to 10th	Night	14% 13% 4 10% 6% 5% 5% 4 5% 7% 26%	2731
	11th to 20th	Night	12% 15% 5% 14% 7% 5% 4 3 6% 6% 23%	2800
	21st to end	Night	13% 15% 6% 14% 7% 5% 5% 4 5% 6% 20%	2963
December	1st to 10th	Night	10% 17% 7% 15% 12% 8% 5% 4 4 4 14%	4900
	11th to 20th	Night	8% 13% 6% 13% 7% 5% 5% 5% 6% 7% 26%	5568
	21st to end	Night	7% 12% 5% 18% 9% 6% 5% 3 5% 6% 23%	7480

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	63% 7% 9% 15% 5%	3179
	11th to 20th	Day	62% 5% 8% 20% 5%	3180
	21st to end	Day	62% 6% 13% 14% 4	3885
February	1st to 10th	Day	58% 8% 12% 13% 8% 1	1688
	11th to 20th	Day	49% 7% 15% 22% 8%	1763
	21st to end	Day	53% 8% 12% 18% 8%	1465
March	1st to 10th	Day	52% 10% 16% 17% 5%	3209
	11th to 20th	Day	48% 14% 14% 17% 7%	3480
	21st to end	Day	46% 13% 13% 21% 7% 1	3814
April	1st to 10th	Day	43% 7% 14% 24% 11% 1	2334
	11th to 20th	Day	44% 8% 11% 21% 14% 2	2442
	21st to end	Day	44% 8% 14% 25% 8% 1	2470
May	1st to 10th	Day	41% 8% 14% 25% 11%	4330
	11th to 20th	Day	47% 10% 9% 21% 12% 2	4674
	21st to end	Day	46% 9% 12% 22% 11% 1	5173
June	1st to 10th	Day	44% 9% 14% 23% 11% 1	2850
	11th to 20th	Day	43% 10% 14% 21% 12% 1	2850
	21st to end	Day	41% 10% 18% 23% 8%	2850
July	1st to 10th	Day	40% 7% 23% 23% 7%	4676
	11th to 20th	Day	28% 6% 24% 36% 6%	4669
	21st to end	Day	28% 6% 24% 34% 7%	4988
August	1st to 10th	Day	33% 7% 21% 31% 8%	4323
	11th to 20th	Day	35% 6% 23% 31% 4	4080
	21st to end	Day	38% 6% 25% 26% 5%	4531
September	1st to 10th	Day	33% 9% 20% 30% 8%	2380
	11th to 20th	Day	46% 8% 18% 25% 3	2090
	21st to end	Day	42% 8% 17% 26% 7%	2090
October	1st to 10th	Day	44% 5% 16% 27% 6%	3198
	11th to 20th	Day	44% 7% 13% 27% 9%	3059
	21st to end	Day	53% 6% 20% 19% 3	3070
November	1st to 10th	Day	60% 7% 13% 16% 4 1	1567
	11th to 20th	Day	60% 9% 15% 13% 3	1600
	21st to end	Day	62% 9% 16% 12% 2	1435
December	1st to 10th	Day	68% 8% 13% 7% 4	2290
	11th to 20th	Day	56% 6% 15% 15% 8%	2640
	21st to end	Day	61% 7% 11% 14% 7%	3498
January	1st to 10th	Night	61% 7% 10% 14% 9%	6787
	11th to 20th	Night	67% 5% 7% 16% 5%	6800
	21st to end	Night	62% 6% 12% 14% 6%	7089
February	1st to 10th	Night	56% 7% 12% 16% 8% 1	2711
	11th to 20th	Night	43% 7% 18% 22% 9%	2534
	21st to end	Night	47% 8% 11% 23% 11%	2007
March	1st to 10th	Night	49% 8% 13% 22% 9%	3652
	11th to 20th	Night	34% 9% 18% 25% 13% 1	3480
	21st to end	Night	45% 7% 11% 26% 10% 1	3817
April	1st to 10th	Night	39% 6% 11% 24% 17% 3	1843
	11th to 20th	Night	47% 5% 9% 20% 16% 3	1689
	21st to end	Night	42% 8% 11% 24% 13% 2	1710
May	1st to 10th	Night	45% 9% 10% 25% 11%	2640
	11th to 20th	Night	42% 8% 12% 26% 10% 2	2232
	21st to end	Night	42% 8% 11% 26% 11% 1	2414
June	1st to 10th	Night	42% 9% 15% 23% 10% 1	1330
	11th to 20th	Night	39% 7% 17% 22% 13% 2	1330
	21st to end	Night	34% 11% 21% 24% 8% 1	1330
July	1st to 10th	Night	29% 4 23% 35% 9%	2185
	11th to 20th	Night	26% 6% 20% 39% 8% 1	2178
	21st to end	Night	26% 5% 23% 35% 11% 1	2337
August	1st to 10th	Night	32% 5% 17% 34% 12%	2589
	11th to 20th	Night	28% 5% 28% 32% 7%	2825
	21st to end	Night	35% 4 22% 31% 8%	3135
September	1st to 10th	Night	32% 7% 19% 31% 11%	1796
	11th to 20th	Night	38% 9% 19% 25% 9% 1	2090
	21st to end	Night	41% 7% 16% 27% 9% 1	2090
October	1st to 10th	Night	40% 4 14% 29% 13%	3729
	11th to 20th	Night	43% 5% 14% 28% 9% 1	3862
	21st to end	Night	47% 6% 18% 25% 4	4487
November	1st to 10th	Night	57% 9% 15% 14% 5% 1	2731
	11th to 20th	Night	61% 10% 14% 11% 4	2800
	21st to end	Night	64% 8% 15% 10% 3	2963
December	1st to 10th	Night	72% 5% 12% 7% 3	4900
	11th to 20th	Night	56% 6% 13% 12% 13%	5566
	21st to end	Night	60% 7% 9% 16% 8% 1	7480

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	11% 4 31% 33% 17% 4	300
	11th to 20th	Day	9% 4 30% 37% 15% 4	300
	21st to end	Day	15% 6% 26% 34% 14% 4	330
February	1st to 10th	Day	13% 5% 30% 34% 16% 2	300
	11th to 20th	Day	10% 6% 27% 35% 17% 5%	300
	21st to end	Day	7% 5% 36% 28% 23% 1	225
March	1st to 10th	Day	12% 5% 30% 29% 23% 2	270
	11th to 20th	Day	11% 7% 30% 34% 14% 3	270
	21st to end	Day	12% 6% 26% 34% 18% 4	297
April	1st to 10th	Day	12% 4 25% 39% 16% 4	514
	11th to 20th	Day	15% 7% 25% 32% 18% 2	640
	21st to end	Day	13% 7% 26% 33% 18% 3	650
May	1st to 10th	Day	13% 6% 25% 36% 17% 2	653
	11th to 20th	Day	17% 6% 23% 31% 19% 5%	675
	21st to end	Day	19% 6% 20% 29% 22% 3	734
June	1st to 10th	Day	16% 6% 25% 31% 19% 4	680
	11th to 20th	Day	14% 5% 29% 33% 14% 4	680
	21st to end	Day	14% 5% 21% 30% 24% 6%	680
July	1st to 10th	Day	9% 4 22% 34% 25% 7%	671
	11th to 20th	Day	10% 5% 25% 30% 24% 7%	668
	21st to end	Day	7% 4 22% 33% 25% 9%	739
August	1st to 10th	Day	9% 3 22% 33% 25% 8%	676
	11th to 20th	Day	11% 5% 19% 31% 25% 10%	665
	21st to end	Day	16% 4 21% 31% 23% 5%	748
September	1st to 10th	Day	11% 4 26% 37% 18% 3	590
	11th to 20th	Day	17% 7% 29% 32% 12% 2	300
	21st to end	Day	18% 8% 29% 33% 11% 2	300
October	1st to 10th	Day	20% 9% 25% 29% 13% 4	300
	11th to 20th	Day	15% 10% 28% 36% 9% 2	300
	21st to end	Day	21% 8% 29% 27% 12% 2	330
November	1st to 10th	Day	21% 9% 24% 31% 13% 1	300
	11th to 20th	Day	19% 7% 37% 23% 12% 1	300
	21st to end	Day	21% 8% 32% 30% 8% 1	300
December	1st to 10th	Day	15% 5% 31% 32% 12% 5%	300
	11th to 20th	Day	17% 8% 36% 27% 11% 2	300
	21st to end	Day	12% 5% 28% 32% 19% 5%	330
January	1st to 10th	Night	13% 4 33% 32% 17% 2	1000
	11th to 20th	Night	14% 5% 28% 39% 12% 2	1000
	21st to end	Night	15% 5% 29% 35% 14% 2	1100
February	1st to 10th	Night	14% 4 26% 40% 15% 2	1000
	11th to 20th	Night	9% 4 28% 36% 19% 4	979
	21st to end	Night	9% 5% 30% 36% 18% 1	751
March	1st to 10th	Night	14% 4 27% 35% 18% 3	717
	11th to 20th	Night	12% 6% 35% 34% 11% 2	570
	21st to end	Night	16% 6% 27% 32% 15% 5%	627
April	1st to 10th	Night	13% 4 29% 39% 14% 2	570
	11th to 20th	Night	16% 6% 26% 35% 17% 1	563
	21st to end	Night	12% 4 27% 42% 14% 1	570
May	1st to 10th	Night	19% 5% 28% 35% 12% 1	570
	11th to 20th	Night	15% 5% 26% 32% 19% 2	561
	21st to end	Night	19% 4 26% 35% 15% 1	617
June	1st to 10th	Night	17% 5% 22% 33% 21% 1	570
	11th to 20th	Night	16% 6% 25% 35% 17% 2	570
	21st to end	Night	13% 5% 16% 39% 25% 2	570
July	1st to 10th	Night	8% 3 19% 43% 25% 2	555
	11th to 20th	Night	9% 3 23% 40% 22% 3	552
	21st to end	Night	8% 2 20% 38% 28% 4	624
August	1st to 10th	Night	8% 3 19% 43% 24% 4	567
	11th to 20th	Night	8% 2 19% 39% 27% 5%	564
	21st to end	Night	13% 4 22% 37% 22% 2	627
September	1st to 10th	Night	13% 4 25% 39% 16% 4	570
	11th to 20th	Night	16% 5% 26% 36% 15% 1	570
	21st to end	Night	16% 5% 32% 33% 14% 1	570
October	1st to 10th	Night	22% 6% 31% 32% 8%	715
	11th to 20th	Night	20% 8% 29% 31% 11% 1	806
	21st to end	Night	20% 9% 30% 30% 9% 2	1045
November	1st to 10th	Night	20% 7% 34% 26% 13% 1	975
	11th to 20th	Night	18% 9% 34% 28% 9% 3	1000
	21st to end	Night	19% 8% 38% 27% 8% 1	1000
December	1st to 10th	Night	12% 6% 29% 36% 15% 3	1000
	11th to 20th	Night	14% 7% 35% 33% 10% 2	1000
	21st to end	Night	16% 5% 31% 29% 17% 2	1100

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