Altona Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Altona Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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	15% 9% 4 7% 7% 4 5% 5% 5% 7% 31%	74870
	Night	15% 9% 4 7% 7% 4 5% 5% 6% 7% 32%	75166
Transparency	Day	51% 4 12% 21% 10% 1	74575
	Night	51% 4 13% 21% 10% 1	74995
Seeing	Day	23% 17% 32% 19% 7% 2	13547
	Night	22% 16% 31% 22% 8% 1	22685

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	11% 12% 5% 12% 10% 4 5% 7% 6% 7% 21%	1517
	11th to 20th	Day	13% 11% 7% 12% 10% 5% 5% 5% 5% 5% 23%	1520
	21st to end	Day	7% 12% 6% 12% 9% 5% 7% 6% 6% 6% 25%	1632
February	1st to 10th	Day	12% 16% 5% 13% 9% 5% 6% 5% 6% 4 20%	1600
	11th to 20th	Day	10% 12% 5% 11% 9% 4 6% 6% 5% 6% 27%	1680
	21st to end	Day	13% 14% 5% 10% 8% 5% 7% 5% 4 6% 24%	1480
March	1st to 10th	Day	13% 12% 5% 9% 8% 5% 5% 6% 5% 6% 26%	1797
	11th to 20th	Day	13% 13% 5% 8% 8% 3 5% 4 6% 6% 29%	1862
	21st to end	Day	16% 12% 5% 7% 6% 3 4 4 4 5% 33%	2374
April	1st to 10th	Day	18% 11% 5% 8% 6% 4 4 4 4 6% 30%	2160
	11th to 20th	Day	16% 10% 3 7% 6% 4 4 4 4 6% 36%	2134
	21st to end	Day	22% 9% 4 7% 5% 4 4 4 4 6% 31%	2291
May	1st to 10th	Day	17% 9% 4 6% 6% 4 4 4 5% 6% 34%	2457
	11th to 20th	Day	16% 8% 3 6% 6% 4 5% 4 5% 6% 37%	2478
	21st to end	Day	16% 6% 3 6% 6% 4 5% 6% 6% 9% 34%	2725
June	1st to 10th	Day	22% 8% 2 5% 6% 3 5% 6% 5% 8% 29%	2520
	11th to 20th	Day	15% 7% 2 6% 5% 3 5% 5% 6% 8% 36%	2520
	21st to end	Day	20% 7% 3 7% 7% 5% 6% 5% 5% 8% 29%	2520
July	1st to 10th	Day	12% 5% 3 6% 6% 4 6% 5% 6% 8% 38%	2520
	11th to 20th	Day	12% 5% 2 7% 6% 4 7% 7% 7% 9% 34%	2520
	21st to end	Day	13% 6% 3 6% 6% 4 6% 6% 7% 10% 35%	2744
August	1st to 10th	Day	13% 6% 3 6% 6% 5% 6% 6% 6% 9% 34%	2506
	11th to 20th	Day	11% 6% 2 5% 6% 4 6% 6% 7% 10% 36%	2246
	21st to end	Day	13% 6% 3 6% 6% 4 5% 6% 5% 7% 40%	2376
September	1st to 10th	Day	13% 5% 3 5% 5% 3 4 5% 5% 9% 42%	2160
	11th to 20th	Day	14% 6% 2 4 5% 3 4 5% 5% 7% 45%	2082
	21st to end	Day	12% 5% 4 6% 7% 3 5% 6% 7% 8% 38%	1980
October	1st to 10th	Day	19% 9% 3 6% 5% 3 6% 5% 6% 7% 31%	1865
	11th to 20th	Day	15% 10% 4 7% 7% 4 6% 5% 5% 6% 31%	1770
	21st to end	Day	21% 12% 4 8% 8% 3 4 5% 4 5% 27%	1782
November	1st to 10th	Day	13% 12% 4 8% 7% 4 5% 6% 5% 6% 32%	1671
	11th to 20th	Day	16% 9% 5% 7% 7% 4 5% 5% 5% 7% 30%	1663
	21st to end	Day	14% 12% 5% 11% 6% 4 6% 5% 5% 6% 24%	1520
December	1st to 10th	Day	16% 16% 8% 12% 8% 4 4 4 5% 6% 17%	1520
	11th to 20th	Day	17% 13% 7% 9% 6% 4 4 5% 5% 6% 24%	1906
	21st to end	Day	22% 14% 6% 10% 7% 3 5% 4 5% 4 21%	2772
January	1st to 10th	Night	13% 11% 5% 11% 8% 5% 8% 6% 5% 7% 21%	2649
	11th to 20th	Night	13% 14% 6% 9% 7% 4 7% 6% 6% 6% 22%	2660
	21st to end	Night	9% 10% 5% 11% 9% 4 6% 5% 6% 7% 28%	2868
February	1st to 10th	Night	11% 12% 5% 11% 10% 5% 8% 5% 5% 6% 21%	2358
	11th to 20th	Night	11% 9% 5% 8% 7% 4 7% 6% 6% 7% 29%	2165
	21st to end	Night	12% 11% 5% 9% 8% 5% 7% 5% 4 6% 29%	1776
March	1st to 10th	Night	13% 11% 5% 9% 8% 5% 5% 5% 5% 6% 27%	2150
	11th to 20th	Night	14% 11% 4 7% 6% 4 6% 5% 5% 6% 33%	2092
	21st to end	Night	15% 9% 4 8% 7% 4 5% 4 4 5% 36%	1981
April	1st to 10th	Night	17% 10% 4 8% 7% 4 5% 4 6% 5% 29%	1800
	11th to 20th	Night	19% 8% 3 7% 6% 3 6% 4 4 7% 32%	1775
	21st to end	Night	19% 7% 4 6% 6% 3 3 5% 5% 6% 35%	1668
May	1st to 10th	Night	20% 8% 3 7% 5% 3 5% 5% 4 7% 33%	1517
	11th to 20th	Night	15% 9% 3 6% 5% 2 4 5% 6% 7% 38%	1415
	21st to end	Night	13% 5% 3 6% 5% 3 5% 5% 5% 9% 40%	1550
June	1st to 10th	Night	20% 6% 3 5% 5% 3 4 5% 8% 9% 33%	1440
	11th to 20th	Night	15% 5% 3 6% 5% 4 5% 5% 7% 8% 39%	1440
	21st to end	Night	15% 6% 3 6% 5% 2 5% 5% 6% 7% 39%	1440
July	1st to 10th	Night	9% 5% 3 5% 5% 3 5% 5% 7% 9% 45%	1440
	11th to 20th	Night	9% 4 2 4 6% 4 5% 6% 8% 10% 43%	1440
	21st to end	Night	12% 5% 2 4 5% 3 5% 6% 7% 9% 43%	1574
August	1st to 10th	Night	11% 4 3 3 4 2 3 5% 8% 9% 47%	1432
	11th to 20th	Night	11% 5% 3 4 6% 3 4 4 5% 9% 45%	1633
	21st to end	Night	10% 6% 2 6% 5% 3 4 6% 6% 8% 44%	1980
September	1st to 10th	Night	11% 5% 2 5% 5% 4 4 5% 6% 7% 46%	1800
	11th to 20th	Night	12% 6% 2 4 4 3 4 3 5% 7% 49%	1877
	21st to end	Night	15% 5% 3 4 5% 3 5% 4 6% 8% 43%	1980
October	1st to 10th	Night	18% 8% 3 6% 5% 3 4 4 6% 7% 36%	2092
	11th to 20th	Night	17% 7% 3 6% 6% 4 5% 5% 6% 7% 34%	2188
	21st to end	Night	22% 10% 4 7% 6% 3 5% 5% 5% 6% 28%	2574
November	1st to 10th	Night	16% 8% 4 6% 7% 4 6% 5% 5% 6% 33%	2407
	11th to 20th	Night	15% 10% 4 8% 7% 3 4 4 6% 6% 33%	2513
	21st to end	Night	16% 10% 4 8% 7% 4 6% 6% 5% 7% 26%	2660
December	1st to 10th	Night	16% 14% 6% 10% 9% 4 6% 5% 6% 6% 18%	2660
	11th to 20th	Night	17% 13% 6% 10% 7% 4 6% 6% 5% 5% 23%	3288
	21st to end	Night	24% 12% 4 11% 7% 4 4 4 5% 5% 20%	4884

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	60% 4 12% 19% 4	1518
	11th to 20th	Day	63% 4 10% 13% 9% 1	1520
	21st to end	Day	59% 4 12% 15% 9% 1	1632
February	1st to 10th	Day	66% 4 7% 16% 5% 2	1600
	11th to 20th	Day	58% 3 11% 19% 8% 1	1689
	21st to end	Day	63% 2 10% 13% 9% 2	1480
March	1st to 10th	Day	58% 3 11% 16% 9% 3	1800
	11th to 20th	Day	54% 2 13% 14% 13% 3	1862
	21st to end	Day	53% 3 7% 20% 13% 4	2374
April	1st to 10th	Day	56% 3 7% 20% 12% 3	2160
	11th to 20th	Day	51% 2 8% 21% 15% 2	2134
	21st to end	Day	55% 3 10% 18% 11% 3	2291
May	1st to 10th	Day	51% 3 8% 21% 16% 2	2457
	11th to 20th	Day	48% 4 8% 22% 16% 2	2482
	21st to end	Day	45% 5% 14% 21% 12% 2	2725
June	1st to 10th	Day	53% 5% 11% 19% 11% 1	2520
	11th to 20th	Day	44% 4 11% 26% 13% 2	2520
	21st to end	Day	52% 4 12% 21% 9% 1	2520
July	1st to 10th	Day	41% 5% 12% 27% 14% 1	2450
	11th to 20th	Day	44% 5% 12% 30% 9%	2445
	21st to end	Day	43% 5% 15% 26% 10% 1	2591
August	1st to 10th	Day	44% 5% 16% 26% 9%	2492
	11th to 20th	Day	40% 5% 18% 26% 10%	2246
	21st to end	Day	43% 5% 12% 27% 13% 1	2376
September	1st to 10th	Day	38% 5% 13% 29% 14% 1	2160
	11th to 20th	Day	38% 3 10% 33% 14% 1	2082
	21st to end	Day	41% 5% 16% 28% 10% 1	1980
October	1st to 10th	Day	51% 5% 10% 23% 11%	1865
	11th to 20th	Day	52% 4 16% 20% 8%	1770
	21st to end	Day	59% 3 13% 18% 7% 1	1782
November	1st to 10th	Day	52% 3 13% 21% 8% 1	1671
	11th to 20th	Day	53% 3 13% 19% 9% 2	1663
	21st to end	Day	60% 3 14% 19% 4 1	1520
December	1st to 10th	Day	69% 3 11% 13% 2	1520
	11th to 20th	Day	61% 4 12% 18% 6%	1906
	21st to end	Day	65% 2 10% 17% 5%	2772
January	1st to 10th	Night	62% 4 11% 17% 5% 1	2649
	11th to 20th	Night	63% 4 12% 14% 6% 1	2660
	21st to end	Night	54% 3 11% 20% 11% 1	2868
February	1st to 10th	Night	63% 4 12% 14% 7% 1	2358
	11th to 20th	Night	53% 3 13% 21% 9% 1	2167
	21st to end	Night	56% 3 10% 18% 12% 1	1776
March	1st to 10th	Night	58% 3 10% 18% 9% 2	2160
	11th to 20th	Night	54% 2 9% 21% 11% 3	2093
	21st to end	Night	51% 3 8% 22% 14% 3	1981
April	1st to 10th	Night	56% 2 7% 21% 12% 2	1800
	11th to 20th	Night	53% 2 10% 22% 12% 1	1775
	21st to end	Night	50% 3 9% 20% 14% 4	1668
May	1st to 10th	Night	51% 3 7% 19% 16% 4	1517
	11th to 20th	Night	45% 4 9% 21% 19% 3	1416
	21st to end	Night	40% 5% 16% 22% 15% 2	1551
June	1st to 10th	Night	47% 5% 13% 23% 11% 2	1440
	11th to 20th	Night	42% 5% 13% 24% 14% 2	1440
	21st to end	Night	43% 5% 16% 24% 12%	1440
July	1st to 10th	Night	36% 5% 15% 28% 16% 1	1400
	11th to 20th	Night	34% 6% 17% 32% 11%	1392
	21st to end	Night	37% 6% 16% 30% 11% 1	1486
August	1st to 10th	Night	33% 5% 21% 31% 11%	1422
	11th to 20th	Night	38% 4 17% 28% 13% 1	1633
	21st to end	Night	37% 4 16% 30% 12% 1	1980
September	1st to 10th	Night	35% 4 16% 31% 13% 1	1800
	11th to 20th	Night	36% 3 14% 34% 13% 1	1877
	21st to end	Night	38% 4 15% 29% 13% 1	1980
October	1st to 10th	Night	48% 4 12% 24% 12% 1	2092
	11th to 20th	Night	49% 4 13% 24% 9%	2188
	21st to end	Night	55% 3 14% 17% 9% 1	2574
November	1st to 10th	Night	51% 3 13% 21% 10% 2	2407
	11th to 20th	Night	52% 3 13% 21% 9% 2	2513
	21st to end	Night	55% 4 16% 19% 6%	2660
December	1st to 10th	Night	65% 4 11% 16% 3	2660
	11th to 20th	Night	63% 3 11% 15% 7% 1	3288
	21st to end	Night	67% 2 12% 16% 3	4884

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	24% 18% 37% 13% 8%	180
	11th to 20th	Day	23% 29% 35% 9% 3 1	180
	21st to end	Day	26% 21% 37% 12% 3 1	188
February	1st to 10th	Day	36% 27% 31% 6%	232
	11th to 20th	Day	27% 30% 35% 7% 1	348
	21st to end	Day	31% 24% 31% 12% 3	346
March	1st to 10th	Day	29% 22% 33% 13% 3	420
	11th to 20th	Day	26% 26% 32% 13% 3 1	420
	21st to end	Day	31% 24% 27% 13% 5%	462
April	1st to 10th	Day	33% 20% 27% 16% 3 1	420
	11th to 20th	Day	29% 22% 28% 14% 7% 1	412
	21st to end	Day	32% 17% 31% 15% 5% 1	420
May	1st to 10th	Day	30% 19% 30% 12% 6% 2	423
	11th to 20th	Day	21% 19% 35% 18% 6% 1	447
	21st to end	Day	19% 17% 36% 21% 6% 1	485
June	1st to 10th	Day	27% 13% 36% 18% 5% 1	450
	11th to 20th	Day	19% 18% 32% 21% 9% 2	450
	21st to end	Day	21% 16% 34% 20% 8% 2	450
July	1st to 10th	Day	16% 8% 35% 29% 9% 2	445
	11th to 20th	Day	15% 8% 38% 29% 9% 2	444
	21st to end	Day	15% 6% 40% 29% 8% 2	488
August	1st to 10th	Day	14% 8% 34% 28% 13% 3	446
	11th to 20th	Day	16% 7% 32% 29% 12% 5%	439
	21st to end	Day	16% 14% 25% 30% 12% 3	495
September	1st to 10th	Day	15% 10% 33% 23% 14% 5%	450
	11th to 20th	Day	16% 14% 26% 26% 13% 6%	450
	21st to end	Day	15% 11% 36% 23% 12% 3	450
October	1st to 10th	Day	26% 15% 27% 15% 14% 2	450
	11th to 20th	Day	22% 22% 33% 15% 8% 1	435
	21st to end	Day	32% 16% 31% 17% 4	396
November	1st to 10th	Day	27% 18% 29% 18% 7% 1	365
	11th to 20th	Day	28% 26% 26% 14% 5% 1	323
	21st to end	Day	33% 19% 29% 17% 2	180
December	1st to 10th	Day	22% 25% 33% 17% 3 1	180
	11th to 20th	Day	34% 20% 34% 11% 2	180
	21st to end	Day	30% 23% 32% 11% 4 1	198
January	1st to 10th	Night	23% 22% 36% 17% 3	760
	11th to 20th	Night	28% 19% 32% 17% 4	760
	21st to end	Night	18% 20% 39% 19% 4 1	820
February	1st to 10th	Night	21% 21% 36% 19% 3	720
	11th to 20th	Night	21% 22% 38% 17% 2	703
	21st to end	Night	22% 22% 35% 19% 2	592
March	1st to 10th	Night	28% 17% 34% 18% 3	720
	11th to 20th	Night	24% 22% 33% 18% 3 1	720
	21st to end	Night	26% 17% 31% 21% 5% 1	792
April	1st to 10th	Night	27% 18% 33% 20% 3	720
	11th to 20th	Night	27% 21% 28% 18% 6%	711
	21st to end	Night	26% 18% 31% 18% 6%	588
May	1st to 10th	Night	27% 14% 31% 21% 6% 1	437
	11th to 20th	Night	18% 15% 35% 24% 8%	354
	21st to end	Night	16% 12% 35% 27% 10% 1	389
June	1st to 10th	Night	24% 9% 32% 24% 9% 1	360
	11th to 20th	Night	17% 11% 29% 30% 12%	360
	21st to end	Night	18% 8% 34% 27% 12% 1	360
July	1st to 10th	Night	8% 8% 34% 34% 15%	350
	11th to 20th	Night	10% 4 28% 42% 13% 2	348
	21st to end	Night	12% 5% 33% 36% 13% 1	394
August	1st to 10th	Night	13% 4 26% 40% 16% 1	358
	11th to 20th	Night	14% 4 25% 35% 18% 3	569
	21st to end	Night	13% 8% 28% 33% 16% 1	792
September	1st to 10th	Night	14% 8% 32% 30% 15% 1	720
	11th to 20th	Night	15% 7% 28% 30% 17% 4	720
	21st to end	Night	17% 12% 30% 24% 15% 2	720
October	1st to 10th	Night	23% 14% 27% 22% 13% 1	720
	11th to 20th	Night	23% 16% 29% 24% 8% 1	720
	21st to end	Night	27% 17% 30% 20% 5%	792
November	1st to 10th	Night	23% 17% 32% 21% 6% 1	740
	11th to 20th	Night	24% 19% 32% 16% 8% 1	760
	21st to end	Night	25% 22% 27% 18% 6% 1	760
December	1st to 10th	Night	27% 21% 30% 18% 4	760
	11th to 20th	Night	26% 23% 31% 15% 4 1	760
	21st to end	Night	30% 21% 30% 17% 3	836

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