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 2026-06-16.

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	10% 8% 3 9% 5% 5% 5% 4 6% 8% 35%	138214
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 7% 33%	134148
Transparency	Day	45% 7% 16% 24% 8% 1	141601
	Night	49% 6% 13% 22% 10% 1	138850
Seeing	Day	14% 6% 26% 32% 18% 4	18515
	Night	15% 5% 29% 34% 15% 2	28494

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% 15% 7% 11% 7% 8% 8% 5% 6% 6% 20%	4124
	11th to 20th	Day	5% 11% 4 14% 8% 7% 9% 8% 11% 8% 14%	4112
	21st to end	Day	4 11% 5% 12% 7% 8% 8% 5% 8% 9% 23%	5041
February	1st to 10th	Day	9% 12% 5% 12% 8% 7% 7% 5% 8% 7% 20%	1773
	11th to 20th	Day	7% 10% 5% 9% 5% 7% 7% 6% 8% 8% 28%	1844
	21st to end	Day	6% 12% 5% 10% 7% 6% 7% 6% 7% 7% 27%	1615
March	1st to 10th	Day	9% 9% 4 11% 6% 6% 6% 5% 7% 9% 28%	4705
	11th to 20th	Day	7% 10% 4 13% 9% 6% 6% 4 6% 6% 27%	5090
	21st to end	Day	8% 10% 4 9% 6% 5% 5% 5% 6% 6% 36%	5585
April	1st to 10th	Day	9% 9% 3 9% 5% 6% 5% 3 6% 7% 38%	2580
	11th to 20th	Day	11% 9% 3 7% 5% 4 6% 4 6% 7% 38%	2702
	21st to end	Day	13% 7% 2 8% 6% 5% 5% 5% 6% 7% 37%	2730
May	1st to 10th	Day	9% 8% 3 8% 4 4 4 3 6% 7% 44%	8020
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 7% 32%	8726
	21st to end	Day	13% 5% 2 6% 4 4 4 3 5% 7% 48%	9683
June	1st to 10th	Day	15% 7% 2 7% 5% 5% 4 4 6% 8% 38%	3150
	11th to 20th	Day	13% 6% 2 7% 6% 5% 5% 5% 7% 8% 36%	3076
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 4 6% 9% 38%	3000
July	1st to 10th	Day	9% 5% 2 6% 4 5% 6% 5% 7% 9% 43%	6799
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 45%	6800
	21st to end	Day	7% 4 2 6% 4 4 5% 5% 7% 10% 47%	7406
August	1st to 10th	Day	8% 3 2 6% 4 5% 5% 5% 8% 10% 45%	4478
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 6% 8% 46%	4171
	21st to end	Day	13% 6% 2 5% 4 4 4 3 6% 9% 43%	4675
September	1st to 10th	Day	11% 5% 2 5% 4 4 5% 5% 5% 8% 46%	2505
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 4 5% 9% 35%	2200
	21st to end	Day	14% 7% 3 6% 4 4 5% 4 6% 8% 40%	2200
October	1st to 10th	Day	18% 11% 3 6% 5% 4 4 3 5% 9% 31%	2026
	11th to 20th	Day	14% 12% 4 9% 6% 5% 5% 4 6% 6% 28%	1950
	21st to end	Day	15% 14% 4 10% 6% 7% 6% 5% 5% 6% 23%	1970
November	1st to 10th	Day	16% 14% 5% 14% 5% 5% 3 3 4 6% 26%	1647
	11th to 20th	Day	10% 17% 4 13% 8% 5% 4 3 4 6% 25%	1680
	21st to end	Day	14% 14% 5% 11% 8% 6% 4 4 6% 6% 22%	1507
December	1st to 10th	Day	12% 13% 6% 14% 9% 9% 6% 5% 7% 6% 12%	2360
	11th to 20th	Day	9% 12% 5% 13% 8% 5% 5% 5% 6% 8% 23%	2709
	21st to end	Day	6% 14% 7% 17% 8% 6% 5% 4 5% 6% 22%	3575
January	1st to 10th	Night	6% 15% 6% 14% 8% 7% 7% 5% 6% 7% 20%	8800
	11th to 20th	Night	5% 13% 7% 14% 9% 8% 8% 8% 7% 5% 15%	8803
	21st to end	Night	6% 12% 5% 12% 7% 6% 8% 6% 9% 8% 22%	9169
February	1st to 10th	Night	7% 13% 5% 11% 7% 7% 7% 6% 6% 6% 25%	2846
	11th to 20th	Night	6% 10% 4 10% 5% 5% 6% 5% 7% 8% 34%	2663
	21st to end	Night	5% 12% 4 11% 6% 5% 7% 4 6% 6% 33%	2210
March	1st to 10th	Night	9% 12% 4 9% 6% 6% 5% 4 7% 7% 32%	5357
	11th to 20th	Night	7% 8% 3 8% 6% 5% 5% 4 6% 8% 40%	5089
	21st to end	Night	10% 11% 4 9% 5% 5% 4 3 5% 6% 39%	5588
April	1st to 10th	Night	10% 8% 2 6% 5% 4 5% 4 6% 8% 40%	2037
	11th to 20th	Night	13% 10% 3 8% 6% 4 5% 4 6% 6% 37%	1869
	21st to end	Night	10% 8% 2 8% 6% 5% 6% 5% 7% 7% 37%	1890
May	1st to 10th	Night	11% 6% 3 6% 4 5% 4 4 6% 7% 43%	4908
	11th to 20th	Night	18% 6% 2 8% 4 4 4 3 5% 7% 36%	4162
	21st to end	Night	12% 7% 2 6% 4 3 3 3 5% 7% 49%	4526
June	1st to 10th	Night	14% 8% 2 7% 5% 4 5% 4 7% 7% 38%	1470
	11th to 20th	Night	14% 6% 2 4 6% 4 3 4 6% 9% 41%	1435
	21st to end	Night	11% 5% 2 5% 5% 4 6% 5% 8% 10% 40%	1400
July	1st to 10th	Night	8% 4 2 5% 4 4 4 4 6% 9% 52%	3180
	11th to 20th	Night	8% 3 2 6% 4 5% 6% 4 6% 8% 48%	3180
	21st to end	Night	6% 3 1 5% 4 4 4 5% 7% 8% 53%	3469
August	1st to 10th	Night	9% 4 2 5% 3 4 4 4 6% 9% 48%	2678
	11th to 20th	Night	8% 3 1 4 3 4 5% 4 7% 11% 49%	2888
	21st to end	Night	10% 5% 1 6% 4 4 4 4 5% 8% 48%	3234
September	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 6% 8% 48%	1891
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 6% 9% 41%	2200
	21st to end	Night	13% 6% 2 7% 4 4 4 4 5% 7% 42%	2200
October	1st to 10th	Night	17% 9% 2 6% 4 4 5% 4 5% 7% 37%	2353
	11th to 20th	Night	15% 12% 4 8% 5% 4 4 4 5% 7% 32%	2449
	21st to end	Night	16% 13% 4 9% 6% 5% 5% 4 5% 6% 28%	2869
November	1st to 10th	Night	13% 13% 4 10% 6% 5% 5% 4 6% 7% 26%	2871
	11th to 20th	Night	12% 15% 5% 14% 7% 5% 4 3 5% 6% 24%	2940
	21st to end	Night	13% 14% 6% 14% 7% 5% 5% 4 5% 6% 21%	3111
December	1st to 10th	Night	10% 17% 7% 15% 12% 7% 5% 4 4 4 15%	5050
	11th to 20th	Night	8% 13% 6% 13% 7% 5% 5% 5% 6% 7% 25%	5718
	21st to end	Night	7% 12% 5% 18% 9% 6% 5% 3 5% 6% 23%	7645

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% 6% 9% 16% 7%	4125
	11th to 20th	Day	61% 4 8% 20% 6%	4140
	21st to end	Day	55% 5% 11% 23% 6%	5041
February	1st to 10th	Day	59% 7% 12% 13% 8% 1	1773
	11th to 20th	Day	49% 7% 15% 22% 7%	1853
	21st to end	Day	53% 8% 12% 18% 9% 1	1615
March	1st to 10th	Day	50% 7% 17% 15% 9% 1	4707
	11th to 20th	Day	52% 10% 13% 17% 8%	5090
	21st to end	Day	45% 9% 13% 23% 10% 1	5585
April	1st to 10th	Day	44% 7% 13% 23% 11% 1	2580
	11th to 20th	Day	45% 8% 12% 21% 13% 1	2702
	21st to end	Day	45% 7% 14% 25% 8% 1	2730
May	1st to 10th	Day	39% 5% 12% 29% 14% 1	8031
	11th to 20th	Day	51% 6% 10% 23% 9% 1	8730
	21st to end	Day	35% 6% 10% 24% 20% 5%	9683
June	1st to 10th	Day	43% 8% 14% 23% 10% 1	3150
	11th to 20th	Day	43% 9% 15% 21% 12% 1	3076
	21st to end	Day	41% 10% 18% 23% 8%	3000
July	1st to 10th	Day	37% 5% 23% 27% 7%	6726
	11th to 20th	Day	34% 5% 22% 34% 4	6719
	21st to end	Day	32% 5% 25% 32% 5%	7193
August	1st to 10th	Day	33% 7% 20% 32% 8%	4461
	11th to 20th	Day	36% 6% 24% 31% 4	4210
	21st to end	Day	38% 6% 25% 27% 5%	4674
September	1st to 10th	Day	35% 8% 19% 30% 8%	2505
	11th to 20th	Day	47% 7% 18% 24% 3	2200
	21st to end	Day	41% 7% 17% 28% 7%	2200
October	1st to 10th	Day	44% 5% 17% 27% 7%	3299
	11th to 20th	Day	44% 7% 12% 26% 9%	3156
	21st to end	Day	53% 5% 20% 19% 3	3168
November	1st to 10th	Day	59% 6% 13% 16% 5% 1	1647
	11th to 20th	Day	59% 8% 15% 13% 4	1680
	21st to end	Day	61% 8% 15% 13% 3	1507
December	1st to 10th	Day	68% 8% 13% 7% 4	2360
	11th to 20th	Day	57% 6% 15% 15% 8%	2710
	21st to end	Day	61% 7% 11% 14% 6%	3575
January	1st to 10th	Night	62% 6% 9% 15% 8%	8800
	11th to 20th	Night	67% 4 7% 15% 5%	8850
	21st to end	Night	56% 5% 11% 21% 7%	9169
February	1st to 10th	Night	56% 6% 12% 17% 8% 1	2846
	11th to 20th	Night	44% 7% 17% 22% 9%	2664
	21st to end	Night	49% 7% 11% 22% 10%	2210
March	1st to 10th	Night	49% 6% 14% 16% 10% 5%	5368
	11th to 20th	Night	39% 7% 16% 23% 14% 1	5090
	21st to end	Night	45% 5% 9% 26% 13% 1	5588
April	1st to 10th	Night	40% 5% 11% 25% 16% 3	2037
	11th to 20th	Night	47% 5% 10% 20% 15% 2	1869
	21st to end	Night	43% 7% 11% 24% 13% 2	1890
May	1st to 10th	Night	38% 5% 10% 31% 15%	4917
	11th to 20th	Night	46% 5% 11% 27% 10% 1	4163
	21st to end	Night	36% 5% 8% 20% 27% 5%	4526
June	1st to 10th	Night	43% 9% 14% 23% 10% 1	1470
	11th to 20th	Night	38% 6% 17% 24% 13% 2	1435
	21st to end	Night	35% 11% 20% 25% 9% 1	1400
July	1st to 10th	Night	30% 3 22% 36% 9% 1	3145
	11th to 20th	Night	33% 5% 18% 37% 6% 1	3138
	21st to end	Night	28% 4 24% 33% 9% 1	3368
August	1st to 10th	Night	32% 5% 16% 34% 12%	2671
	11th to 20th	Night	28% 5% 28% 31% 7%	2915
	21st to end	Night	35% 4 22% 32% 8%	3234
September	1st to 10th	Night	32% 7% 19% 31% 10%	1891
	11th to 20th	Night	39% 9% 19% 24% 9% 1	2200
	21st to end	Night	40% 6% 16% 28% 9% 1	2200
October	1st to 10th	Night	40% 4 14% 29% 13%	3847
	11th to 20th	Night	44% 5% 14% 28% 9% 1	3985
	21st to end	Night	48% 5% 18% 25% 4	4631
November	1st to 10th	Night	56% 8% 15% 14% 5% 1	2871
	11th to 20th	Night	60% 9% 15% 11% 5%	2940
	21st to end	Night	63% 7% 15% 11% 3	3111
December	1st to 10th	Night	71% 5% 12% 8% 4	5050
	11th to 20th	Night	56% 6% 13% 12% 12%	5716
	21st to end	Night	60% 7% 9% 16% 8% 1	7645

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% 5% 32% 32% 17% 4	330
	11th to 20th	Day	9% 4 32% 37% 15% 4	330
	21st to end	Day	13% 6% 29% 34% 14% 4	363
February	1st to 10th	Day	14% 6% 30% 33% 15% 2	330
	11th to 20th	Day	11% 6% 28% 34% 17% 5%	330
	21st to end	Day	8% 5% 35% 30% 20% 1	273
March	1st to 10th	Day	13% 5% 33% 27% 20% 2	330
	11th to 20th	Day	11% 8% 30% 36% 13% 3	330
	21st to end	Day	12% 8% 29% 31% 17% 4	363
April	1st to 10th	Day	12% 5% 24% 41% 15% 4	600
	11th to 20th	Day	15% 7% 27% 32% 17% 2	740
	21st to end	Day	13% 7% 26% 33% 18% 3	750
May	1st to 10th	Day	12% 7% 26% 35% 19% 2	753
	11th to 20th	Day	19% 5% 25% 30% 17% 4	775
	21st to end	Day	17% 6% 19% 30% 24% 4	844
June	1st to 10th	Day	16% 5% 27% 30% 17% 4	780
	11th to 20th	Day	13% 5% 29% 34% 15% 4	756
	21st to end	Day	14% 5% 22% 31% 22% 6%	730
July	1st to 10th	Day	9% 4 22% 34% 25% 7%	721
	11th to 20th	Day	10% 5% 25% 31% 24% 7%	718
	21st to end	Day	7% 4 20% 33% 26% 9%	794
August	1st to 10th	Day	9% 3 22% 33% 25% 8%	726
	11th to 20th	Day	12% 4 20% 30% 23% 10%	715
	21st to end	Day	15% 4 21% 30% 24% 5%	803
September	1st to 10th	Day	12% 4 26% 37% 18% 3	635
	11th to 20th	Day	18% 7% 29% 33% 12% 2	330
	21st to end	Day	17% 7% 30% 34% 11% 2	330
October	1st to 10th	Day	19% 8% 26% 30% 12% 4	330
	11th to 20th	Day	18% 10% 27% 35% 8% 2	330
	21st to end	Day	21% 8% 29% 28% 12% 2	363
November	1st to 10th	Day	21% 11% 25% 29% 12% 1	330
	11th to 20th	Day	18% 8% 37% 24% 13% 2	330
	21st to end	Day	19% 8% 32% 32% 8% 1	330
December	1st to 10th	Day	16% 5% 31% 32% 12% 5%	330
	11th to 20th	Day	17% 9% 37% 25% 10% 2	330
	21st to end	Day	12% 6% 28% 31% 18% 5%	363
January	1st to 10th	Night	13% 4 32% 32% 16% 2	1050
	11th to 20th	Night	13% 6% 30% 38% 11% 2	1050
	21st to end	Night	14% 5% 30% 35% 14% 1	1155
February	1st to 10th	Night	14% 4 27% 39% 14% 2	1050
	11th to 20th	Night	10% 4 28% 35% 19% 4	1029
	21st to end	Night	10% 6% 31% 35% 17% 1	826
March	1st to 10th	Night	15% 5% 28% 34% 16% 2	792
	11th to 20th	Night	11% 6% 36% 34% 11% 2	630
	21st to end	Night	16% 6% 28% 31% 14% 5%	693
April	1st to 10th	Night	13% 3 28% 40% 14% 2	630
	11th to 20th	Night	16% 6% 26% 35% 16% 1	623
	21st to end	Night	14% 4 26% 41% 14% 1	630
May	1st to 10th	Night	17% 6% 28% 36% 12% 1	630
	11th to 20th	Night	16% 5% 28% 31% 18% 2	621
	21st to end	Night	19% 4 26% 36% 15% 1	683
June	1st to 10th	Night	16% 5% 24% 34% 20% 1	630
	11th to 20th	Night	16% 5% 25% 36% 16% 1	615
	21st to end	Night	13% 5% 18% 39% 24% 2	600
July	1st to 10th	Night	8% 2 20% 43% 25% 3	585
	11th to 20th	Night	9% 3 23% 40% 22% 3	582
	21st to end	Night	8% 2 20% 40% 27% 4	657
August	1st to 10th	Night	8% 3 19% 43% 25% 3	597
	11th to 20th	Night	8% 2 19% 39% 26% 5%	594
	21st to end	Night	13% 4 22% 37% 23% 2	660
September	1st to 10th	Night	13% 4 25% 38% 17% 4	600
	11th to 20th	Night	17% 5% 26% 36% 15% 1	600
	21st to end	Night	15% 5% 32% 32% 15% 2	600
October	1st to 10th	Night	21% 6% 32% 32% 8%	753
	11th to 20th	Night	20% 8% 29% 31% 11% 1	849
	21st to end	Night	21% 9% 30% 30% 9% 1	1100
November	1st to 10th	Night	19% 8% 34% 25% 12% 1	1025
	11th to 20th	Night	18% 9% 35% 27% 9% 3	1050
	21st to end	Night	18% 8% 39% 27% 8% 1	1050
December	1st to 10th	Night	12% 6% 30% 36% 14% 2	1050
	11th to 20th	Night	15% 7% 36% 32% 9% 2	1050
	21st to end	Night	17% 5% 31% 29% 16% 2	1155

Page generated at 2026-06-13 01:31:42. Version ac07

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