Altona
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Altona 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 8% 5% 5% 5% 4 6% 8% 36%	137941
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 7% 34%	134385
Transparency	Day	29% 37% 10% 17% 7% 1	141359
	Night	32% 36% 9% 16% 6% 1	139044
Seeing	Day	14% 9% 31% 29% 14% 4	18435
	Night	14% 8% 32% 33% 12% 1	28402

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	5% 13% 6% 15% 9% 7% 6% 5% 7% 8% 20%	4124
	11th to 20th	Day	4 9% 4 13% 10% 10% 11% 9% 8% 6% 15%	4274
	21st to end	Day	4 11% 5% 12% 6% 8% 8% 5% 7% 8% 25%	5188
February	1st to 10th	Day	8% 14% 6% 10% 8% 6% 8% 6% 6% 6% 22%	1807
	11th to 20th	Day	6% 11% 4 10% 9% 7% 8% 5% 8% 7% 25%	1844
	21st to end	Day	6% 9% 4 12% 9% 8% 8% 5% 8% 6% 25%	1675
March	1st to 10th	Day	10% 10% 3 8% 8% 5% 5% 4 6% 10% 31%	4684
	11th to 20th	Day	9% 10% 4 13% 8% 5% 6% 5% 7% 9% 25%	5090
	21st to end	Day	10% 13% 7% 12% 6% 5% 4 4 6% 6% 28%	5585
April	1st to 10th	Day	9% 12% 4 9% 5% 4 5% 5% 6% 7% 33%	2546
	11th to 20th	Day	13% 12% 4 9% 6% 5% 5% 4 6% 7% 31%	2702
	21st to end	Day	19% 9% 3 7% 5% 4 4 3 5% 6% 33%	2730
May	1st to 10th	Day	11% 6% 3 7% 4 4 4 3 5% 7% 46%	7739
	11th to 20th	Day	17% 8% 2 7% 5% 4 4 4 6% 8% 35%	8576
	21st to end	Day	12% 7% 2 6% 4 4 4 3 5% 8% 45%	9683
June	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 7% 10% 37%	3150
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 5% 6% 8% 39%	3076
	21st to end	Day	11% 6% 2 6% 6% 5% 5% 5% 8% 9% 37%	3000
July	1st to 10th	Day	7% 3 2 5% 4 5% 5% 4 8% 11% 46%	6799
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 9% 46%	6800
	21st to end	Day	7% 5% 2 5% 4 5% 5% 4 7% 10% 46%	7182
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 47%	4378
	11th to 20th	Day	11% 5% 2 5% 4 3 4 4 7% 9% 47%	4171
	21st to end	Day	9% 5% 2 6% 3 4 4 3 5% 8% 50%	4675
September	1st to 10th	Day	10% 5% 2 4 4 4 4 4 6% 7% 50%	2459
	11th to 20th	Day	14% 6% 2 7% 4 4 5% 4 5% 8% 40%	2200
	21st to end	Day	14% 5% 2 6% 3 4 4 3 6% 8% 44%	2200
October	1st to 10th	Day	17% 9% 3 7% 4 4 3 3 5% 7% 40%	2057
	11th to 20th	Day	15% 11% 3 8% 4 3 4 3 6% 7% 35%	1956
	21st to end	Day	15% 11% 4 10% 5% 5% 4 4 5% 7% 29%	1980
November	1st to 10th	Day	14% 14% 5% 10% 5% 5% 4 3 4 6% 30%	1717
	11th to 20th	Day	10% 14% 6% 12% 7% 5% 4 3 6% 5% 27%	1680
	21st to end	Day	11% 15% 5% 12% 7% 4 4 4 6% 5% 26%	1570
December	1st to 10th	Day	9% 14% 6% 14% 11% 9% 6% 3 5% 5% 18%	2360
	11th to 20th	Day	9% 15% 5% 13% 6% 6% 6% 4 6% 7% 23%	2709
	21st to end	Day	8% 13% 6% 17% 8% 7% 5% 3 6% 5% 23%	3575
January	1st to 10th	Night	4 12% 7% 16% 8% 8% 6% 5% 7% 7% 20%	8800
	11th to 20th	Night	4 12% 5% 13% 10% 9% 10% 7% 6% 6% 19%	8623
	21st to end	Night	5% 13% 6% 12% 5% 6% 7% 5% 7% 8% 27%	9026
February	1st to 10th	Night	8% 12% 5% 13% 7% 6% 6% 5% 5% 6% 27%	2812
	11th to 20th	Night	6% 10% 3 8% 7% 6% 7% 5% 8% 8% 33%	2663
	21st to end	Night	7% 9% 4 12% 7% 6% 6% 4 6% 5% 33%	2149
March	1st to 10th	Night	8% 10% 3 9% 6% 5% 5% 4 7% 8% 34%	5392
	11th to 20th	Night	7% 10% 4 9% 6% 5% 5% 4 7% 7% 35%	5089
	21st to end	Night	10% 15% 6% 14% 6% 5% 4 3 4 5% 31%	5588
April	1st to 10th	Night	10% 9% 2 7% 4 5% 5% 5% 5% 8% 40%	2069
	11th to 20th	Night	13% 11% 3 9% 5% 4 4 5% 4 6% 35%	1869
	21st to end	Night	15% 9% 3 6% 5% 5% 5% 4 4 7% 37%	1890
May	1st to 10th	Night	12% 7% 2 6% 4 4 5% 4 6% 7% 42%	5174
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 6% 8% 40%	4301
	21st to end	Night	13% 6% 2 4 3 3 3 3 6% 6% 51%	4526
June	1st to 10th	Night	11% 6% 2 5% 3 5% 5% 5% 7% 8% 44%	1470
	11th to 20th	Night	12% 6% 2 6% 4 4 5% 3 6% 8% 45%	1435
	21st to end	Night	10% 4 2 5% 4 5% 5% 5% 7% 11% 44%	1400
July	1st to 10th	Night	7% 3 2 4 3 4 4 4 7% 9% 52%	3180
	11th to 20th	Night	7% 3 1 4 3 4 4 4 6% 9% 54%	3180
	21st to end	Night	5% 3 2 5% 4 5% 6% 4 8% 11% 47%	3690
August	1st to 10th	Night	4 3 2 7% 5% 4 4 4 7% 9% 51%	2780
	11th to 20th	Night	9% 4 2 5% 3 4 4 3 7% 9% 50%	2888
	21st to end	Night	9% 5% 2 6% 4 4 4 3 5% 7% 51%	3234
September	1st to 10th	Night	8% 5% 2 5% 3 3 4 2 6% 9% 53%	1938
	11th to 20th	Night	12% 5% 2 6% 4 3 4 3 6% 8% 49%	2200
	21st to end	Night	13% 6% 2 5% 4 4 4 3 5% 8% 46%	2200
October	1st to 10th	Night	15% 8% 3 7% 4 5% 4 3 5% 6% 40%	2322
	11th to 20th	Night	15% 11% 3 7% 4 4 3 3 5% 7% 38%	2440
	21st to end	Night	13% 11% 3 8% 5% 6% 4 4 5% 8% 34%	2860
November	1st to 10th	Night	13% 12% 5% 9% 6% 5% 4 4 6% 7% 31%	2798
	11th to 20th	Night	10% 14% 6% 13% 5% 5% 4 3 5% 5% 29%	2940
	21st to end	Night	11% 13% 5% 13% 7% 5% 4 3 4 6% 29%	3046
December	1st to 10th	Night	6% 14% 6% 18% 12% 7% 5% 3 5% 5% 18%	5050
	11th to 20th	Night	8% 15% 6% 15% 8% 6% 5% 4 6% 6% 22%	5718
	21st to end	Night	9% 12% 5% 16% 9% 8% 5% 4 5% 5% 22%	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	49% 22% 9% 17% 4	4125
	11th to 20th	Day	50% 24% 8% 12% 6%	4302
	21st to end	Day	40% 25% 11% 19% 5%	5188
February	1st to 10th	Day	27% 61% 6% 4 2	1807
	11th to 20th	Day	22% 59% 7% 9% 2	1853
	21st to end	Day	22% 61% 5% 7% 4	1675
March	1st to 10th	Day	29% 33% 17% 13% 8%	4686
	11th to 20th	Day	40% 31% 13% 13% 3	5090
	21st to end	Day	41% 30% 8% 17% 5%	5585
April	1st to 10th	Day	21% 59% 6% 9% 4	2546
	11th to 20th	Day	21% 60% 6% 8% 4	2702
	21st to end	Day	19% 59% 6% 10% 4 2	2730
May	1st to 10th	Day	26% 24% 9% 26% 14% 1	7750
	11th to 20th	Day	38% 24% 8% 17% 11% 2	8580
	21st to end	Day	27% 24% 8% 21% 18% 3	9683
June	1st to 10th	Day	14% 60% 9% 12% 4	3150
	11th to 20th	Day	18% 61% 7% 10% 4	3076
	21st to end	Day	17% 62% 8% 10% 3	3000
July	1st to 10th	Day	24% 30% 23% 20% 3	6726
	11th to 20th	Day	27% 28% 16% 25% 3	6719
	21st to end	Day	26% 29% 19% 21% 5%	6974
August	1st to 10th	Day	21% 39% 12% 19% 8%	4362
	11th to 20th	Day	22% 38% 13% 24% 2	4210
	21st to end	Day	21% 37% 17% 21% 4	4674
September	1st to 10th	Day	14% 58% 8% 16% 4	2459
	11th to 20th	Day	20% 58% 9% 11% 2 1	2200
	21st to end	Day	19% 57% 7% 13% 4	2200
October	1st to 10th	Day	21% 37% 8% 23% 11%	3343
	11th to 20th	Day	22% 37% 6% 24% 11%	3162
	21st to end	Day	30% 36% 8% 19% 7%	3190
November	1st to 10th	Day	28% 54% 6% 9% 3	1717
	11th to 20th	Day	24% 55% 9% 8% 3 1	1680
	21st to end	Day	30% 55% 6% 6% 3	1570
December	1st to 10th	Day	47% 37% 7% 7% 2 1	2360
	11th to 20th	Day	39% 33% 10% 11% 8%	2710
	21st to end	Day	45% 30% 5% 16% 4	3575
January	1st to 10th	Night	50% 23% 11% 12% 4	8800
	11th to 20th	Night	50% 23% 7% 15% 4	8670
	21st to end	Night	41% 24% 10% 20% 5%	9026
February	1st to 10th	Night	26% 59% 6% 6% 3	2812
	11th to 20th	Night	18% 59% 7% 12% 3	2664
	21st to end	Night	22% 59% 4 10% 5%	2149
March	1st to 10th	Night	30% 32% 16% 13% 8% 2	5403
	11th to 20th	Night	32% 32% 11% 16% 8%	5090
	21st to end	Night	42% 30% 5% 17% 6%	5588
April	1st to 10th	Night	18% 60% 5% 11% 6% 1	2069
	11th to 20th	Night	19% 60% 4 11% 5% 1	1869
	21st to end	Night	20% 59% 4 11% 4 2	1890
May	1st to 10th	Night	29% 25% 8% 25% 13%	5184
	11th to 20th	Night	33% 24% 8% 19% 14% 1	4302
	21st to end	Night	25% 23% 7% 21% 21% 3	4526
June	1st to 10th	Night	13% 60% 7% 14% 6% 1	1470
	11th to 20th	Night	17% 61% 5% 11% 6%	1435
	21st to end	Night	15% 63% 6% 13% 3	1400
July	1st to 10th	Night	22% 31% 20% 21% 5%	3145
	11th to 20th	Night	20% 28% 16% 30% 5%	3138
	21st to end	Night	22% 29% 23% 21% 4	3584
August	1st to 10th	Night	18% 38% 11% 22% 10% 1	2772
	11th to 20th	Night	21% 38% 16% 21% 4	2915
	21st to end	Night	23% 37% 13% 22% 4	3234
September	1st to 10th	Night	13% 57% 7% 17% 5% 1	1938
	11th to 20th	Night	17% 59% 10% 11% 3	2200
	21st to end	Night	16% 58% 6% 12% 7% 1	2200
October	1st to 10th	Night	23% 36% 6% 24% 10% 1	3803
	11th to 20th	Night	22% 36% 7% 22% 12%	3963
	21st to end	Night	27% 37% 9% 21% 7%	4610
November	1st to 10th	Night	26% 54% 7% 10% 3 1	2798
	11th to 20th	Night	25% 54% 8% 8% 4 1	2940
	21st to end	Night	29% 55% 5% 8% 3	3046
December	1st to 10th	Night	48% 37% 6% 8% 2	5050
	11th to 20th	Night	41% 33% 8% 11% 7%	5716
	21st to end	Night	46% 28% 8% 14% 4	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	9% 9% 32% 33% 14% 3	330
	11th to 20th	Day	11% 8% 35% 33% 11% 2	330
	21st to end	Day	10% 11% 35% 29% 12% 2	363
February	1st to 10th	Day	15% 13% 35% 29% 7% 1	330
	11th to 20th	Day	10% 14% 30% 31% 13% 2	330
	21st to end	Day	9% 9% 44% 26% 12% 1	273
March	1st to 10th	Day	14% 8% 35% 28% 12% 1	330
	11th to 20th	Day	15% 11% 32% 25% 13% 3	330
	21st to end	Day	13% 13% 33% 31% 8% 2	363
April	1st to 10th	Day	14% 13% 31% 29% 10% 2	566
	11th to 20th	Day	19% 12% 32% 24% 11% 1	740
	21st to end	Day	21% 10% 30% 27% 10% 2	750
May	1st to 10th	Day	15% 11% 33% 28% 12% 2	753
	11th to 20th	Day	21% 11% 30% 24% 11% 3	775
	21st to end	Day	17% 9% 24% 29% 15% 5%	844
June	1st to 10th	Day	14% 10% 29% 27% 14% 6%	780
	11th to 20th	Day	15% 10% 35% 26% 13% 2	756
	21st to end	Day	12% 8% 29% 32% 16% 4	730
July	1st to 10th	Day	8% 4 27% 30% 20% 10%	721
	11th to 20th	Day	11% 8% 28% 29% 19% 6%	718
	21st to end	Day	5% 4 27% 36% 20% 8%	794
August	1st to 10th	Day	6% 5% 28% 31% 24% 7%	726
	11th to 20th	Day	10% 5% 23% 32% 20% 10%	715
	21st to end	Day	12% 8% 27% 30% 18% 5%	803
September	1st to 10th	Day	10% 8% 35% 29% 14% 4	589
	11th to 20th	Day	18% 12% 32% 25% 11% 2	330
	21st to end	Day	19% 9% 34% 26% 11% 1	330
October	1st to 10th	Day	19% 12% 28% 27% 12% 2	330
	11th to 20th	Day	17% 12% 38% 26% 6% 1	330
	21st to end	Day	23% 12% 32% 23% 10% 1	363
November	1st to 10th	Day	23% 12% 28% 26% 10% 1	330
	11th to 20th	Day	17% 14% 34% 25% 9% 1	330
	21st to end	Day	18% 12% 38% 23% 9%	330
December	1st to 10th	Day	16% 8% 34% 31% 9% 3	330
	11th to 20th	Day	18% 13% 33% 28% 8%	330
	21st to end	Day	16% 8% 30% 32% 10% 4	363
January	1st to 10th	Night	11% 9% 34% 32% 12% 2	1050
	11th to 20th	Night	14% 9% 33% 34% 9%	1050
	21st to end	Night	13% 9% 37% 32% 9%	1155
February	1st to 10th	Night	13% 8% 34% 34% 11% 1	1050
	11th to 20th	Night	11% 7% 32% 35% 14% 2	1029
	21st to end	Night	10% 9% 35% 33% 13% 1	765
March	1st to 10th	Night	14% 8% 33% 31% 11% 2	801
	11th to 20th	Night	10% 7% 37% 33% 11% 1	630
	21st to end	Night	18% 6% 31% 29% 13% 2	693
April	1st to 10th	Night	12% 9% 33% 36% 10% 1	630
	11th to 20th	Night	15% 10% 33% 32% 9%	623
	21st to end	Night	18% 8% 28% 36% 10% 1	630
May	1st to 10th	Night	16% 9% 34% 31% 9%	630
	11th to 20th	Night	17% 9% 30% 31% 12%	621
	21st to end	Night	18% 7% 29% 34% 11%	683
June	1st to 10th	Night	13% 7% 30% 36% 13% 1	630
	11th to 20th	Night	13% 7% 33% 35% 12% 1	615
	21st to end	Night	10% 5% 27% 38% 19% 1	600
July	1st to 10th	Night	7% 3 24% 42% 22% 2	585
	11th to 20th	Night	6% 5% 27% 43% 18% 1	582
	21st to end	Night	5% 4 24% 44% 21% 3	657
August	1st to 10th	Night	7% 2 24% 45% 21% 1	597
	11th to 20th	Night	8% 4 22% 42% 21% 3	594
	21st to end	Night	10% 4 26% 39% 20%	660
September	1st to 10th	Night	10% 4 31% 37% 16% 2	600
	11th to 20th	Night	14% 6% 29% 39% 13% 1	600
	21st to end	Night	16% 7% 30% 33% 13% 1	600
October	1st to 10th	Night	19% 10% 34% 28% 8%	722
	11th to 20th	Night	21% 10% 30% 30% 9%	840
	21st to end	Night	18% 9% 35% 30% 8% 1	1100
November	1st to 10th	Night	19% 9% 35% 27% 9%	1025
	11th to 20th	Night	17% 10% 35% 27% 8% 2	1050
	21st to end	Night	17% 11% 39% 26% 6% 1	1050
December	1st to 10th	Night	13% 10% 32% 33% 11% 1	1050
	11th to 20th	Night	16% 10% 35% 30% 7% 1	1050
	21st to end	Night	18% 8% 35% 28% 10% 1	1155

Page generated at 2026-06-13 01:31:41. Version ac07

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