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 2026-05-17.

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	14% 10% 4 9% 6% 6% 5% 4 6% 7% 30%	131918
	Night	14% 10% 4 10% 6% 5% 5% 4 6% 7% 30%	130920
Transparency	Day	52% 4 11% 21% 10% 1	135375
	Night	52% 4 12% 21% 10% 1	135550
Seeing	Day	24% 18% 32% 18% 7% 2	16497
	Night	22% 16% 31% 22% 7% 1	25579

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	10% 11% 5% 15% 10% 7% 6% 5% 6% 7% 19%	4661
	11th to 20th	Day	7% 15% 7% 16% 10% 8% 6% 4 5% 5% 19%	4650
	21st to end	Day	8% 12% 5% 13% 9% 9% 7% 5% 6% 6% 22%	5124
February	1st to 10th	Day	10% 15% 4 15% 9% 6% 6% 4 5% 4 21%	1867
	11th to 20th	Day	9% 12% 4 13% 9% 6% 6% 4 5% 6% 26%	1969
	21st to end	Day	11% 15% 5% 11% 7% 7% 5% 4 5% 6% 24%	1730
March	1st to 10th	Day	16% 12% 5% 12% 6% 5% 5% 4 5% 6% 25%	4576
	11th to 20th	Day	10% 12% 4 11% 7% 6% 6% 4 6% 6% 29%	4798
	21st to end	Day	15% 11% 4 8% 6% 5% 5% 3 4 5% 34%	6100
April	1st to 10th	Day	17% 13% 4 9% 5% 4 4 3 4 6% 29%	2520
	11th to 20th	Day	16% 12% 3 9% 5% 4 5% 3 4 5% 34%	2494
	21st to end	Day	20% 9% 3 9% 4 5% 4 3 4 6% 32%	2674
May	1st to 10th	Day	23% 14% 4 9% 6% 6% 4 4 4 5% 22%	6336
	11th to 20th	Day	18% 10% 3 8% 5% 6% 5% 4 6% 7% 29%	6402
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	6949
June	1st to 10th	Day	20% 9% 2 7% 5% 5% 5% 4 6% 8% 29%	2800
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 8% 36%	2800
	21st to end	Day	19% 7% 3 8% 5% 6% 5% 4 6% 8% 29%	2800
July	1st to 10th	Day	11% 6% 3 8% 6% 6% 6% 5% 7% 8% 34%	6360
	11th to 20th	Day	10% 5% 2 6% 5% 6% 6% 5% 7% 9% 38%	6360
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 6% 10% 42%	6926
August	1st to 10th	Day	15% 7% 2 7% 5% 6% 6% 5% 7% 9% 32%	4563
	11th to 20th	Day	12% 5% 3 7% 6% 6% 5% 5% 7% 8% 35%	4056
	21st to end	Day	15% 5% 2 6% 4 5% 5% 4 6% 8% 40%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 5% 9% 44%	2400
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 3 5% 8% 46%	2312
	21st to end	Day	13% 5% 3 7% 5% 4 5% 4 6% 8% 39%	2200
October	1st to 10th	Day	17% 9% 3 7% 4 5% 5% 4 6% 6% 34%	2071
	11th to 20th	Day	14% 10% 3 8% 6% 5% 6% 4 6% 6% 33%	1966
	21st to end	Day	20% 12% 4 9% 7% 4 4 3 4 5% 28%	1980
November	1st to 10th	Day	13% 13% 4 9% 6% 4 5% 4 5% 6% 31%	1851
	11th to 20th	Day	15% 11% 4 8% 6% 5% 5% 4 6% 7% 30%	1837
	21st to end	Day	14% 14% 5% 12% 5% 6% 5% 4 5% 6% 24%	1680
December	1st to 10th	Day	10% 19% 6% 18% 8% 6% 4 3 4 6% 14%	2680
	11th to 20th	Day	15% 11% 5% 11% 5% 4 4 4 5% 6% 30%	3066
	21st to end	Day	17% 14% 5% 11% 6% 4 5% 3 5% 5% 25%	4048
January	1st to 10th	Night	10% 13% 5% 14% 8% 7% 6% 4 6% 6% 20%	8234
	11th to 20th	Night	8% 13% 6% 16% 8% 7% 7% 5% 5% 6% 19%	8235
	21st to end	Night	9% 10% 4 12% 8% 7% 8% 5% 7% 7% 22%	9057
February	1st to 10th	Night	10% 13% 4 13% 9% 7% 6% 4 5% 6% 22%	2751
	11th to 20th	Night	10% 10% 4 10% 7% 6% 7% 4 6% 6% 29%	2536
	21st to end	Night	11% 12% 4 10% 6% 7% 5% 4 5% 6% 30%	2076
March	1st to 10th	Night	17% 10% 3 9% 7% 7% 5% 3 5% 6% 28%	5484
	11th to 20th	Night	11% 11% 4 9% 5% 5% 5% 4 6% 7% 31%	5376
	21st to end	Night	12% 11% 5% 12% 7% 6% 5% 3 5% 5% 30%	5073
April	1st to 10th	Night	17% 12% 3 9% 6% 5% 5% 3 5% 5% 29%	2100
	11th to 20th	Night	19% 11% 3 8% 5% 5% 5% 4 4 6% 30%	2075
	21st to end	Night	18% 8% 3 6% 5% 4 4 4 5% 6% 36%	1945
May	1st to 10th	Night	23% 14% 4 8% 4 4 5% 4 5% 6% 22%	3867
	11th to 20th	Night	18% 10% 3 8% 5% 4 4 4 6% 7% 31%	3623
	21st to end	Night	16% 9% 3 8% 5% 4 5% 4 6% 8% 33%	3926
June	1st to 10th	Night	19% 8% 2 6% 5% 4 4 5% 8% 9% 32%	1600
	11th to 20th	Night	14% 5% 3 7% 4 5% 5% 4 7% 8% 38%	1600
	21st to end	Night	16% 6% 3 7% 4 4 5% 4 6% 7% 38%	1600
July	1st to 10th	Night	10% 5% 2 7% 5% 6% 6% 4 7% 9% 39%	3600
	11th to 20th	Night	11% 6% 2 5% 5% 4 4 4 6% 7% 46%	3600
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 6% 7% 44%	3926
August	1st to 10th	Night	13% 6% 3 6% 4 3 4 4 7% 8% 41%	2586
	11th to 20th	Night	12% 6% 3 6% 5% 5% 5% 3 5% 8% 42%	2956
	21st to end	Night	13% 5% 2 6% 4 3 4 4 6% 8% 46%	3586
September	1st to 10th	Night	11% 5% 2 5% 4 5% 4 3 6% 7% 48%	2000
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 5% 7% 50%	2087
	21st to end	Night	15% 6% 2 5% 4 4 4 3 6% 8% 43%	2200
October	1st to 10th	Night	17% 8% 3 6% 4 4 3 4 6% 7% 38%	2325
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 4 6% 7% 35%	2432
	21st to end	Night	21% 10% 3 8% 5% 4 4 4 5% 7% 30%	2860
November	1st to 10th	Night	15% 9% 4 7% 6% 6% 5% 4 6% 6% 33%	2667
	11th to 20th	Night	14% 11% 3 9% 6% 4 4 3 6% 6% 33%	2779
	21st to end	Night	16% 11% 4 10% 6% 5% 6% 4 6% 7% 26%	2940
December	1st to 10th	Night	13% 15% 6% 14% 8% 7% 6% 5% 6% 6% 15%	4720
	11th to 20th	Night	17% 13% 5% 12% 5% 5% 5% 3 4 5% 25%	5348
	21st to end	Night	19% 13% 4 11% 5% 5% 5% 3 5% 5% 24%	7150

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% 4 13% 17% 3	4662
	11th to 20th	Day	68% 4 8% 9% 9% 2	4680
	21st to end	Day	61% 5% 9% 14% 10% 1	5124
February	1st to 10th	Day	64% 3 8% 18% 6% 2	1867
	11th to 20th	Day	58% 3 11% 19% 8% 1	1978
	21st to end	Day	62% 2 10% 14% 9% 2	1730
March	1st to 10th	Day	60% 4 9% 14% 9% 5%	4579
	11th to 20th	Day	54% 3 15% 15% 10% 2	4798
	21st to end	Day	54% 3 6% 17% 18% 2	6100
April	1st to 10th	Day	56% 4 6% 19% 12% 2	2520
	11th to 20th	Day	53% 3 7% 20% 15% 2	2494
	21st to end	Day	54% 3 10% 19% 12% 3	2674
May	1st to 10th	Day	65% 3 7% 17% 8% 1	6347
	11th to 20th	Day	53% 5% 14% 18% 9% 1	6406
	21st to end	Day	51% 4 12% 23% 9% 1	6949
June	1st to 10th	Day	53% 5% 11% 20% 12% 1	2800
	11th to 20th	Day	44% 5% 11% 26% 13% 2	2800
	21st to end	Day	52% 4 12% 22% 9% 1	2800
July	1st to 10th	Day	45% 7% 13% 23% 12%	6290
	11th to 20th	Day	37% 9% 12% 28% 13%	6285
	21st to end	Day	37% 5% 12% 37% 8%	6728
August	1st to 10th	Day	46% 6% 15% 24% 9% 1	4549
	11th to 20th	Day	42% 6% 16% 29% 6%	4097
	21st to end	Day	41% 7% 14% 25% 14%	4312
September	1st to 10th	Day	36% 5% 12% 32% 14% 1	2400
	11th to 20th	Day	37% 3 11% 33% 15% 1	2312
	21st to end	Day	41% 5% 15% 27% 11% 1	2200
October	1st to 10th	Day	41% 6% 11% 31% 11%	3370
	11th to 20th	Day	44% 3 16% 25% 11%	3182
	21st to end	Day	45% 3 17% 26% 8% 1	3180
November	1st to 10th	Day	53% 4 13% 21% 9% 1	1851
	11th to 20th	Day	52% 3 13% 20% 10% 2	1837
	21st to end	Day	60% 3 14% 18% 4 1	1680
December	1st to 10th	Day	71% 4 10% 13% 2	2680
	11th to 20th	Day	54% 4 11% 20% 9% 1	3066
	21st to end	Day	61% 3 9% 17% 10%	4048
January	1st to 10th	Night	63% 3 14% 14% 5% 1	8234
	11th to 20th	Night	64% 5% 10% 13% 8%	8280
	21st to end	Night	58% 5% 10% 15% 12% 1	9057
February	1st to 10th	Night	61% 4 11% 16% 7% 1	2751
	11th to 20th	Night	53% 3 13% 22% 9%	2538
	21st to end	Night	55% 3 10% 19% 12% 1	2076
March	1st to 10th	Night	58% 3 8% 17% 12% 2	5494
	11th to 20th	Night	52% 3 16% 18% 10% 2	5377
	21st to end	Night	56% 3 6% 21% 13% 2	5073
April	1st to 10th	Night	56% 2 7% 20% 12% 1	2100
	11th to 20th	Night	55% 3 9% 20% 12% 1	2075
	21st to end	Night	49% 3 9% 20% 15% 3	1945
May	1st to 10th	Night	63% 4 9% 16% 7% 2	3873
	11th to 20th	Night	52% 4 10% 22% 10% 1	3624
	21st to end	Night	49% 5% 12% 21% 13% 1	3927
June	1st to 10th	Night	48% 5% 12% 21% 12% 2	1600
	11th to 20th	Night	42% 5% 14% 23% 15% 2	1600
	21st to end	Night	43% 5% 15% 24% 12% 1	1600
July	1st to 10th	Night	40% 5% 18% 26% 10%	3560
	11th to 20th	Night	36% 6% 15% 29% 13%	3552
	21st to end	Night	38% 6% 14% 31% 11%	3809
August	1st to 10th	Night	40% 6% 16% 28% 10%	2576
	11th to 20th	Night	41% 5% 19% 25% 9% 1	2981
	21st to end	Night	36% 5% 16% 29% 14% 1	3586
September	1st to 10th	Night	33% 4 16% 33% 13% 1	2000
	11th to 20th	Night	34% 3 14% 34% 14% 1	2087
	21st to end	Night	38% 4 14% 27% 15% 1	2200
October	1st to 10th	Night	42% 4 13% 29% 11% 1	3806
	11th to 20th	Night	42% 3 12% 28% 14% 1	3955
	21st to end	Night	46% 4 15% 24% 10% 1	4610
November	1st to 10th	Night	51% 3 13% 22% 10% 1	2667
	11th to 20th	Night	51% 3 12% 22% 10% 2	2779
	21st to end	Night	55% 4 15% 19% 6%	2940
December	1st to 10th	Night	67% 5% 11% 14% 3	4720
	11th to 20th	Night	61% 3 9% 14% 12% 1	5348
	21st to end	Night	63% 3 10% 16% 8% 1	7150

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% 21% 36% 13% 6%	220
	11th to 20th	Day	24% 30% 35% 9% 3	220
	21st to end	Day	26% 24% 33% 14% 2 1	242
February	1st to 10th	Day	33% 25% 34% 7% 1	319
	11th to 20th	Day	27% 30% 34% 7% 1	457
	21st to end	Day	30% 23% 33% 11% 2	446
March	1st to 10th	Day	31% 23% 31% 12% 3	540
	11th to 20th	Day	30% 24% 31% 12% 2 1	540
	21st to end	Day	32% 25% 27% 11% 4	594
April	1st to 10th	Day	35% 20% 27% 14% 3 1	540
	11th to 20th	Day	33% 23% 26% 12% 5% 1	532
	21st to end	Day	31% 20% 29% 14% 5% 1	540
May	1st to 10th	Day	33% 20% 29% 12% 5% 1	543
	11th to 20th	Day	22% 18% 33% 19% 7% 1	551
	21st to end	Day	21% 16% 35% 21% 6% 1	573
June	1st to 10th	Day	24% 13% 38% 18% 6% 1	530
	11th to 20th	Day	18% 19% 32% 21% 8% 2	530
	21st to end	Day	23% 16% 34% 19% 7% 1	530
July	1st to 10th	Day	17% 8% 35% 30% 8% 2	525
	11th to 20th	Day	15% 8% 37% 28% 10% 2	524
	21st to end	Day	14% 8% 39% 29% 8% 2	576
August	1st to 10th	Day	13% 9% 32% 31% 13% 3	526
	11th to 20th	Day	15% 8% 33% 29% 12% 4	519
	21st to end	Day	16% 13% 27% 29% 12% 3	583
September	1st to 10th	Day	15% 11% 32% 23% 13% 5%	530
	11th to 20th	Day	14% 12% 24% 28% 16% 7%	530
	21st to end	Day	15% 13% 35% 22% 12% 3	530
October	1st to 10th	Day	24% 15% 28% 17% 14% 2	530
	11th to 20th	Day	22% 22% 32% 15% 8% 1	511
	21st to end	Day	31% 16% 33% 16% 4	462
November	1st to 10th	Day	29% 19% 28% 16% 6% 1	425
	11th to 20th	Day	28% 25% 27% 15% 5% 1	377
	21st to end	Day	34% 24% 27% 14% 2	220
December	1st to 10th	Day	24% 25% 33% 16% 2	220
	11th to 20th	Day	34% 21% 34% 10% 1	220
	21st to end	Day	29% 24% 33% 10% 4 1	242
January	1st to 10th	Night	22% 24% 36% 16% 2	840
	11th to 20th	Night	29% 20% 31% 17% 4	840
	21st to end	Night	17% 21% 39% 19% 4 1	924
February	1st to 10th	Night	20% 20% 36% 21% 3	840
	11th to 20th	Night	22% 22% 37% 17% 2	823
	21st to end	Night	21% 22% 36% 19% 2	692
March	1st to 10th	Night	28% 19% 33% 17% 3	840
	11th to 20th	Night	26% 21% 32% 18% 3	840
	21st to end	Night	28% 19% 30% 19% 4 1	924
April	1st to 10th	Night	28% 18% 32% 19% 3	840
	11th to 20th	Night	29% 21% 27% 17% 5%	831
	21st to end	Night	26% 19% 31% 19% 5%	685
May	1st to 10th	Night	27% 14% 32% 20% 6% 1	509
	11th to 20th	Night	19% 15% 34% 24% 8%	406
	21st to end	Night	15% 12% 35% 28% 9%	433
June	1st to 10th	Night	24% 9% 32% 26% 9% 1	400
	11th to 20th	Night	17% 12% 30% 31% 11%	400
	21st to end	Night	19% 8% 35% 27% 11% 1	400
July	1st to 10th	Night	9% 8% 34% 35% 14%	390
	11th to 20th	Night	10% 6% 27% 44% 12% 2	388
	21st to end	Night	12% 5% 33% 36% 13% 1	438
August	1st to 10th	Night	13% 5% 25% 40% 17% 1	398
	11th to 20th	Night	13% 5% 25% 35% 19% 3	634
	21st to end	Night	14% 8% 28% 34% 15% 1	880
September	1st to 10th	Night	13% 9% 32% 30% 15% 1	800
	11th to 20th	Night	13% 7% 26% 30% 21% 4	800
	21st to end	Night	17% 11% 31% 23% 16% 2	800
October	1st to 10th	Night	23% 14% 26% 23% 14% 1	800
	11th to 20th	Night	22% 17% 29% 24% 8% 1	800
	21st to end	Night	27% 16% 31% 21% 5%	880
November	1st to 10th	Night	24% 19% 31% 20% 5% 1	820
	11th to 20th	Night	24% 20% 31% 16% 8% 1	840
	21st to end	Night	26% 22% 28% 18% 6%	840
December	1st to 10th	Night	26% 22% 30% 18% 4	840
	11th to 20th	Night	27% 23% 30% 15% 4 1	840
	21st to end	Night	30% 21% 29% 17% 3	924

Page generated at 2026-05-14 01:58:59. Version ac07

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