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 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% 35%	108305
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 7% 33%	110468
Transparency	Day	26% 45% 10% 15% 5%	111771
	Night	29% 43% 8% 14% 5%	115151
Seeing	Day	14% 9% 30% 29% 14% 4	16541
	Night	14% 8% 32% 33% 12% 1	26661

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	4 13% 6% 15% 8% 6% 6% 5% 8% 9% 19%	3178
	11th to 20th	Day	4 9% 4 13% 11% 10% 9% 10% 8% 7% 16%	3272
	21st to end	Day	6% 14% 6% 14% 7% 7% 7% 3 6% 7% 24%	4004
February	1st to 10th	Day	8% 14% 5% 10% 8% 6% 8% 6% 6% 6% 22%	1721
	11th to 20th	Day	6% 11% 4 10% 9% 7% 8% 5% 8% 7% 25%	1754
	21st to end	Day	7% 9% 5% 12% 8% 8% 7% 5% 7% 6% 26%	1519
March	1st to 10th	Day	13% 10% 3 9% 10% 5% 5% 4 5% 8% 28%	3186
	11th to 20th	Day	9% 11% 4 13% 9% 4 5% 5% 6% 8% 26%	3480
	21st to end	Day	9% 13% 6% 13% 6% 5% 4 4 6% 6% 29%	3814
April	1st to 10th	Day	10% 12% 4 8% 5% 5% 5% 5% 6% 7% 33%	2304
	11th to 20th	Day	13% 12% 4 9% 6% 5% 5% 4 6% 6% 31%	2442
	21st to end	Day	19% 10% 3 7% 5% 4 4 3 5% 6% 33%	2470
May	1st to 10th	Day	18% 9% 3 7% 4 4 5% 4 5% 8% 33%	4163
	11th to 20th	Day	18% 8% 3 7% 5% 4 5% 4 6% 8% 33%	4597
	21st to end	Day	18% 9% 3 7% 5% 4 5% 4 5% 7% 33%	5173
June	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 7% 10% 37%	2850
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 5% 6% 8% 39%	2850
	21st to end	Day	11% 6% 2 6% 6% 5% 5% 5% 8% 9% 37%	2850
July	1st to 10th	Day	8% 3 1 5% 4 5% 5% 4 8% 11% 46%	4750
	11th to 20th	Day	7% 3 2 5% 3 4 4 4 6% 9% 51%	4750
	21st to end	Day	5% 4 2 4 4 4 5% 4 7% 9% 52%	4997
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 47%	4243
	11th to 20th	Day	11% 5% 2 5% 4 3 4 4 7% 9% 47%	4041
	21st to end	Day	10% 5% 2 6% 3 4 4 3 5% 8% 50%	4532
September	1st to 10th	Day	10% 5% 2 4 3 4 4 3 6% 7% 51%	2336
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 5% 8% 42%	2090
	21st to end	Day	15% 6% 2 7% 4 4 4 3 6% 8% 43%	2090
October	1st to 10th	Day	17% 9% 3 7% 4 4 3 3 5% 7% 39%	1954
	11th to 20th	Day	14% 10% 3 8% 5% 4 4 4 6% 7% 35%	1858
	21st to end	Day	15% 10% 4 10% 5% 5% 4 4 5% 8% 29%	1881
November	1st to 10th	Day	14% 14% 4 10% 5% 5% 4 3 5% 6% 30%	1634
	11th to 20th	Day	10% 15% 6% 13% 8% 5% 4 3 6% 5% 25%	1600
	21st to end	Day	12% 16% 5% 11% 7% 4 5% 4 6% 5% 26%	1495
December	1st to 10th	Day	9% 14% 6% 14% 11% 9% 6% 3 5% 5% 18%	2290
	11th to 20th	Day	9% 14% 5% 13% 6% 7% 6% 4 6% 7% 24%	2639
	21st to end	Day	8% 13% 6% 17% 8% 7% 5% 3 6% 5% 23%	3498
January	1st to 10th	Night	4 12% 7% 16% 8% 8% 6% 5% 7% 7% 20%	6787
	11th to 20th	Night	4 12% 5% 12% 10% 9% 9% 7% 7% 6% 19%	6615
	21st to end	Night	7% 16% 7% 15% 6% 5% 5% 4 5% 6% 24%	6974
February	1st to 10th	Night	8% 12% 5% 12% 7% 6% 6% 5% 5% 6% 27%	2678
	11th to 20th	Night	6% 10% 3 8% 7% 6% 7% 5% 8% 8% 32%	2533
	21st to end	Night	7% 10% 4 12% 7% 6% 6% 4 6% 5% 33%	1952
March	1st to 10th	Night	11% 11% 3 10% 7% 5% 5% 4 6% 7% 33%	3662
	11th to 20th	Night	6% 8% 5% 9% 7% 5% 5% 4 6% 8% 38%	3479
	21st to end	Night	10% 13% 5% 14% 6% 5% 4 3 5% 5% 31%	3817
April	1st to 10th	Night	8% 8% 2 7% 4 6% 5% 6% 5% 8% 41%	1871
	11th to 20th	Night	14% 11% 3 8% 5% 4 4 5% 5% 6% 35%	1689
	21st to end	Night	15% 9% 3 6% 5% 5% 5% 4 4 7% 37%	1710
May	1st to 10th	Night	20% 9% 3 6% 4 4 4 4 6% 6% 35%	2786
	11th to 20th	Night	15% 8% 3 6% 4 5% 4 3 6% 7% 40%	2306
	21st to end	Night	19% 7% 2 4 3 3 3 3 5% 5% 45%	2414
June	1st to 10th	Night	11% 6% 2 5% 3 5% 5% 5% 6% 8% 44%	1330
	11th to 20th	Night	13% 6% 2 5% 4 4 5% 3 5% 8% 45%	1330
	21st to end	Night	10% 4 2 5% 4 5% 4 5% 7% 11% 44%	1330
July	1st to 10th	Night	8% 3 1 3 3 3 4 4 7% 10% 54%	2220
	11th to 20th	Night	6% 2 1 3 3 3 3 4 5% 10% 59%	2220
	21st to end	Night	5% 3 2 5% 4 4 5% 4 8% 11% 49%	2565
August	1st to 10th	Night	4 3 2 7% 5% 4 4 4 7% 9% 51%	2695
	11th to 20th	Night	9% 4 2 5% 3 4 5% 3 7% 9% 49%	2798
	21st to end	Night	10% 5% 2 6% 4 4 4 3 5% 7% 51%	3135
September	1st to 10th	Night	8% 5% 2 5% 3 3 4 2 6% 8% 54%	1841
	11th to 20th	Night	10% 5% 2 6% 4 3 4 3 6% 8% 50%	2090
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 8% 45%	2090
October	1st to 10th	Night	15% 8% 3 7% 4 5% 4 3 5% 6% 40%	2206
	11th to 20th	Night	14% 11% 3 6% 4 4 3 3 5% 7% 39%	2318
	21st to end	Night	12% 11% 3 8% 5% 6% 4 4 6% 8% 34%	2717
November	1st to 10th	Night	13% 12% 5% 8% 6% 5% 4 4 6% 7% 31%	2661
	11th to 20th	Night	10% 15% 6% 13% 6% 5% 4 3 5% 5% 28%	2800
	21st to end	Night	11% 13% 5% 12% 7% 5% 4 3 4 6% 28%	2901
December	1st to 10th	Night	7% 14% 6% 18% 12% 7% 5% 4 5% 5% 17%	4900
	11th to 20th	Night	8% 15% 6% 16% 8% 6% 5% 4 6% 6% 22%	5568
	21st to end	Night	9% 12% 5% 17% 9% 8% 5% 4 5% 5% 22%	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	44% 27% 9% 16% 3	3179
	11th to 20th	Day	46% 29% 7% 12% 7%	3300
	21st to end	Day	42% 30% 12% 13% 1	4004
February	1st to 10th	Day	25% 63% 5% 4 2	1721
	11th to 20th	Day	20% 62% 6% 9% 3	1763
	21st to end	Day	18% 67% 4 7% 4	1519
March	1st to 10th	Day	26% 46% 12% 14% 3	3188
	11th to 20th	Day	34% 43% 10% 11% 2	3480
	21st to end	Day	33% 42% 6% 14% 5%	3814
April	1st to 10th	Day	17% 65% 5% 9% 3	2304
	11th to 20th	Day	18% 66% 5% 7% 4	2442
	21st to end	Day	15% 65% 6% 9% 3 2	2470
May	1st to 10th	Day	29% 41% 9% 14% 6% 1	4174
	11th to 20th	Day	31% 42% 8% 12% 7%	4601
	21st to end	Day	31% 42% 7% 15% 5%	5173
June	1st to 10th	Day	12% 66% 9% 10% 4	2850
	11th to 20th	Day	16% 66% 6% 8% 4	2850
	21st to end	Day	16% 65% 7% 9% 3	2850
July	1st to 10th	Day	20% 40% 20% 17% 3	4676
	11th to 20th	Day	17% 38% 17% 25% 4	4669
	21st to end	Day	17% 38% 17% 24% 5%	4838
August	1st to 10th	Day	21% 40% 12% 19% 8%	4227
	11th to 20th	Day	22% 39% 13% 24% 2	4080
	21st to end	Day	21% 38% 17% 20% 4	4531
September	1st to 10th	Day	13% 61% 7% 15% 4	2336
	11th to 20th	Day	17% 61% 9% 11% 2 1	2090
	21st to end	Day	19% 60% 6% 11% 4	2090
October	1st to 10th	Day	21% 38% 8% 22% 11%	3240
	11th to 20th	Day	21% 38% 6% 24% 11%	3064
	21st to end	Day	29% 37% 7% 19% 7%	3091
November	1st to 10th	Day	26% 57% 7% 8% 2	1634
	11th to 20th	Day	24% 58% 8% 7% 3	1600
	21st to end	Day	28% 58% 5% 6% 3	1495
December	1st to 10th	Day	46% 38% 7% 7% 2 1	2290
	11th to 20th	Day	38% 33% 10% 11% 7%	2640
	21st to end	Day	44% 31% 5% 16% 4	3498
January	1st to 10th	Night	47% 29% 8% 12% 4	6787
	11th to 20th	Night	46% 29% 6% 15% 4	6662
	21st to end	Night	44% 30% 10% 13% 3	6974
February	1st to 10th	Night	24% 62% 5% 6% 3	2678
	11th to 20th	Night	16% 62% 7% 11% 3	2534
	21st to end	Night	18% 65% 3 9% 5%	1952
March	1st to 10th	Night	26% 43% 11% 15% 5%	3673
	11th to 20th	Night	22% 45% 11% 16% 6%	3480
	21st to end	Night	34% 42% 5% 14% 5% 1	3817
April	1st to 10th	Night	14% 66% 5% 9% 6% 1	1871
	11th to 20th	Night	16% 66% 3 9% 5% 1	1689
	21st to end	Night	17% 65% 4 10% 3 2	1710
May	1st to 10th	Night	31% 42% 6% 16% 5%	2795
	11th to 20th	Night	27% 41% 7% 16% 7% 2	2307
	21st to end	Night	25% 41% 7% 19% 7%	2414
June	1st to 10th	Night	11% 66% 6% 11% 5% 1	1330
	11th to 20th	Night	15% 65% 4 9% 6%	1330
	21st to end	Night	13% 66% 6% 13% 2	1330
July	1st to 10th	Night	17% 41% 20% 18% 3	2185
	11th to 20th	Night	13% 37% 14% 30% 6% 1	2178
	21st to end	Night	18% 38% 19% 21% 4 1	2484
August	1st to 10th	Night	18% 39% 11% 21% 10% 1	2687
	11th to 20th	Night	21% 39% 16% 20% 4	2825
	21st to end	Night	23% 39% 12% 22% 4	3135
September	1st to 10th	Night	12% 60% 6% 16% 5% 1	1841
	11th to 20th	Night	15% 62% 9% 11% 4	2090
	21st to end	Night	17% 60% 6% 11% 6% 1	2090
October	1st to 10th	Night	22% 37% 6% 24% 10% 1	3687
	11th to 20th	Night	21% 37% 7% 23% 12%	3841
	21st to end	Night	25% 38% 9% 21% 7%	4467
November	1st to 10th	Night	25% 57% 6% 9% 2	2661
	11th to 20th	Night	25% 57% 7% 8% 3 1	2800
	21st to end	Night	27% 57% 4 7% 3	2901
December	1st to 10th	Night	48% 38% 6% 7% 1	4900
	11th to 20th	Night	41% 34% 8% 11% 6%	5566
	21st to end	Night	45% 29% 7% 14% 4	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	9% 9% 32% 32% 15% 3	300
	11th to 20th	Day	11% 8% 33% 34% 12% 2	300
	21st to end	Day	11% 11% 33% 30% 12% 2	330
February	1st to 10th	Day	16% 12% 33% 30% 8% 1	300
	11th to 20th	Day	9% 14% 32% 31% 13% 2	300
	21st to end	Day	9% 8% 44% 24% 13% 1	225
March	1st to 10th	Day	14% 8% 33% 31% 13% 1	270
	11th to 20th	Day	14% 10% 31% 28% 14% 4	270
	21st to end	Day	13% 10% 31% 35% 9% 2	297
April	1st to 10th	Day	14% 13% 30% 30% 10% 3	484
	11th to 20th	Day	19% 12% 32% 25% 13% 1	640
	21st to end	Day	21% 10% 30% 27% 10% 2	650
May	1st to 10th	Day	16% 11% 31% 27% 12% 2	653
	11th to 20th	Day	19% 11% 29% 25% 12% 4	675
	21st to end	Day	19% 9% 25% 29% 14% 4	734
June	1st to 10th	Day	14% 10% 28% 28% 15% 6%	680
	11th to 20th	Day	16% 9% 34% 25% 13% 2	680
	21st to end	Day	12% 7% 28% 33% 17% 4	680
July	1st to 10th	Day	9% 4 28% 28% 20% 10%	671
	11th to 20th	Day	11% 7% 28% 29% 19% 6%	668
	21st to end	Day	5% 4 28% 34% 21% 8%	739
August	1st to 10th	Day	6% 4 28% 31% 24% 7%	676
	11th to 20th	Day	10% 5% 22% 33% 20% 10%	665
	21st to end	Day	12% 8% 27% 30% 19% 5%	748
September	1st to 10th	Day	10% 8% 35% 30% 14% 3	546
	11th to 20th	Day	15% 13% 33% 26% 11% 2	300
	21st to end	Day	21% 10% 33% 26% 10% 1	300
October	1st to 10th	Day	20% 12% 28% 26% 12% 2	300
	11th to 20th	Day	14% 10% 40% 28% 7% 1	300
	21st to end	Day	22% 11% 33% 23% 9% 1	330
November	1st to 10th	Day	22% 11% 28% 27% 11% 1	300
	11th to 20th	Day	18% 14% 34% 25% 8% 1	300
	21st to end	Day	20% 10% 38% 23% 8%	300
December	1st to 10th	Day	15% 7% 34% 32% 9% 3	300
	11th to 20th	Day	17% 12% 32% 31% 8%	300
	21st to end	Day	15% 7% 31% 32% 10% 5%	330
January	1st to 10th	Night	11% 9% 34% 32% 12% 2	1000
	11th to 20th	Night	14% 9% 32% 35% 10%	1000
	21st to end	Night	14% 9% 36% 31% 9%	1100
February	1st to 10th	Night	12% 7% 34% 35% 11% 1	1000
	11th to 20th	Night	10% 6% 33% 35% 13% 2	979
	21st to end	Night	10% 8% 33% 34% 13% 1	696
March	1st to 10th	Night	15% 7% 32% 32% 12% 2	725
	11th to 20th	Night	9% 7% 37% 33% 12% 1	570
	21st to end	Night	19% 6% 30% 30% 14% 3	627
April	1st to 10th	Night	11% 9% 34% 35% 11% 1	570
	11th to 20th	Night	15% 9% 32% 32% 10%	563
	21st to end	Night	18% 7% 27% 38% 10% 1	570
May	1st to 10th	Night	18% 9% 33% 31% 9%	570
	11th to 20th	Night	16% 9% 28% 33% 14%	561
	21st to end	Night	18% 7% 30% 33% 10%	617
June	1st to 10th	Night	13% 7% 28% 36% 15% 1	570
	11th to 20th	Night	13% 6% 33% 34% 12% 1	570
	21st to end	Night	10% 5% 26% 39% 20% 1	570
July	1st to 10th	Night	7% 3 24% 43% 22% 2	555
	11th to 20th	Night	6% 5% 27% 42% 19% 1	552
	21st to end	Night	5% 4 24% 43% 21% 3	624
August	1st to 10th	Night	7% 2 24% 45% 21% 1	567
	11th to 20th	Night	8% 4 23% 41% 21% 3	564
	21st to end	Night	11% 4 26% 38% 20%	627
September	1st to 10th	Night	10% 4 31% 37% 16% 2	570
	11th to 20th	Night	12% 6% 28% 39% 14% 1	570
	21st to end	Night	17% 7% 31% 33% 11% 1	570
October	1st to 10th	Night	20% 10% 33% 29% 8%	686
	11th to 20th	Night	21% 9% 31% 30% 9%	798
	21st to end	Night	17% 9% 35% 30% 8% 1	1045
November	1st to 10th	Night	19% 9% 35% 27% 9%	975
	11th to 20th	Night	18% 10% 35% 28% 9% 2	1000
	21st to end	Night	17% 11% 39% 26% 6% 1	1000
December	1st to 10th	Night	14% 9% 31% 33% 12% 1	1000
	11th to 20th	Night	16% 10% 35% 31% 8% 1	1000
	21st to end	Night	19% 7% 34% 29% 11% 1	1100

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