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 2025-01-22.

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% 3 9% 6% 5% 5% 4 6% 7% 31%	105226
	Night	14% 10% 4 9% 6% 5% 5% 4 5% 6% 31%	104152
Transparency	Day	51% 4 12% 22% 10% 1	108701
	Night	51% 4 13% 21% 10% 1	108766
Seeing	Day	24% 17% 32% 19% 7% 2	14806
	Night	22% 16% 31% 22% 8% 1	23911

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	12% 13% 4 13% 8% 5% 5% 5% 6% 7% 21%	2597
	11th to 20th	Day	10% 14% 7% 16% 9% 6% 5% 5% 5% 4 19%	2600
	21st to end	Day	12% 16% 6% 15% 6% 7% 5% 4 5% 5% 18%	2780
February	1st to 10th	Day	11% 16% 4 15% 8% 6% 5% 4 5% 4 21%	1689
	11th to 20th	Day	9% 13% 4 13% 8% 6% 6% 4 5% 5% 26%	1776
	21st to end	Day	12% 14% 4 10% 7% 7% 5% 4 5% 6% 25%	1570
March	1st to 10th	Day	19% 13% 4 10% 5% 5% 4 4 5% 6% 24%	3116
	11th to 20th	Day	10% 15% 5% 13% 8% 6% 5% 3 5% 6% 25%	3278
	21st to end	Day	13% 10% 4 7% 6% 5% 4 3 5% 5% 39%	4164
April	1st to 10th	Day	18% 13% 4 9% 5% 4 4 3 4 6% 30%	2280
	11th to 20th	Day	16% 11% 3 8% 6% 4 5% 3 4 5% 35%	2254
	21st to end	Day	21% 9% 3 9% 4 5% 4 3 4 6% 31%	2419
May	1st to 10th	Day	19% 12% 4 7% 5% 6% 4 4 4 6% 28%	4321
	11th to 20th	Day	17% 8% 3 7% 5% 6% 5% 4 6% 7% 31%	4398
	21st to end	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	4837
June	1st to 10th	Day	21% 9% 2 7% 5% 5% 5% 4 6% 7% 29%	2660
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 8% 36%	2660
	21st to end	Day	20% 7% 3 8% 5% 6% 5% 3 6% 8% 28%	2660
July	1st to 10th	Day	11% 5% 2 7% 6% 6% 6% 5% 8% 8% 35%	4440
	11th to 20th	Day	12% 5% 1 6% 5% 6% 7% 5% 7% 10% 35%	4440
	21st to end	Day	12% 6% 2 7% 4 5% 5% 4 7% 9% 40%	4814
August	1st to 10th	Day	15% 7% 3 7% 5% 6% 6% 5% 7% 9% 31%	4423
	11th to 20th	Day	12% 5% 3 7% 6% 6% 5% 5% 7% 8% 35%	3929
	21st to end	Day	15% 6% 2 5% 4 5% 5% 4 6% 8% 40%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 5% 9% 43%	2280
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 3 5% 7% 45%	2197
	21st to end	Day	13% 5% 3 7% 5% 4 5% 4 7% 8% 39%	2090
October	1st to 10th	Day	18% 10% 3 7% 4 5% 6% 4 6% 6% 32%	1968
	11th to 20th	Day	15% 10% 3 8% 6% 5% 6% 4 6% 6% 32%	1868
	21st to end	Day	20% 12% 3 9% 6% 4 4 3 4 5% 28%	1881
November	1st to 10th	Day	13% 12% 4 9% 6% 5% 5% 4 5% 6% 32%	1761
	11th to 20th	Day	15% 10% 4 8% 6% 5% 5% 4 6% 7% 31%	1750
	21st to end	Day	14% 14% 5% 12% 5% 6% 6% 4 5% 6% 24%	1600
December	1st to 10th	Day	10% 20% 7% 18% 8% 6% 4 3 5% 6% 14%	2600
	11th to 20th	Day	15% 11% 5% 11% 4 4 4 4 5% 6% 30%	2986
	21st to end	Day	17% 14% 5% 11% 6% 4 5% 3 5% 5% 26%	3960
January	1st to 10th	Night	12% 12% 4 13% 7% 7% 7% 4 5% 6% 22%	4569
	11th to 20th	Night	9% 13% 6% 13% 7% 7% 7% 5% 6% 6% 21%	4580
	21st to end	Night	14% 14% 6% 13% 7% 5% 5% 4 5% 5% 21%	4899
February	1st to 10th	Night	10% 13% 4 12% 9% 7% 6% 4 5% 6% 21%	2489
	11th to 20th	Night	10% 10% 4 9% 7% 6% 7% 5% 6% 7% 29%	2289
	21st to end	Night	11% 12% 4 11% 6% 7% 6% 4 5% 6% 30%	1884
March	1st to 10th	Night	18% 9% 3 8% 6% 5% 5% 4 5% 6% 31%	3724
	11th to 20th	Night	12% 14% 5% 11% 5% 5% 5% 3 4 6% 29%	3676
	21st to end	Night	11% 10% 5% 11% 6% 5% 5% 3 4 4 36%	3467
April	1st to 10th	Night	17% 12% 3 9% 7% 5% 5% 3 5% 5% 29%	1900
	11th to 20th	Night	19% 10% 3 8% 5% 5% 5% 4 4 6% 31%	1875
	21st to end	Night	19% 8% 3 6% 5% 4 4 4 5% 6% 36%	1760
May	1st to 10th	Night	21% 13% 4 8% 4 4 5% 4 5% 6% 26%	2647
	11th to 20th	Night	17% 11% 3 8% 5% 3 3 3 6% 7% 35%	2495
	21st to end	Night	13% 6% 2 7% 4 4 5% 4 6% 9% 39%	2738
June	1st to 10th	Night	19% 7% 2 6% 5% 4 4 5% 8% 9% 32%	1520
	11th to 20th	Night	14% 5% 3 7% 4 5% 5% 3 7% 8% 38%	1520
	21st to end	Night	16% 6% 3 7% 4 4 5% 4 6% 7% 38%	1520
July	1st to 10th	Night	10% 6% 3 8% 5% 6% 5% 4 7% 8% 38%	2520
	11th to 20th	Night	11% 5% 1 4 5% 5% 4 4 7% 8% 46%	2520
	21st to end	Night	10% 6% 2 7% 6% 5% 5% 4 6% 7% 42%	2739
August	1st to 10th	Night	13% 6% 3 7% 4 3 4 4 7% 8% 40%	2506
	11th to 20th	Night	12% 6% 3 7% 5% 5% 5% 3 6% 8% 41%	2864
	21st to end	Night	13% 5% 2 6% 4 3 4 4 6% 8% 46%	3476
September	1st to 10th	Night	11% 5% 2 5% 4 5% 4 3 6% 7% 48%	1900
	11th to 20th	Night	12% 6% 2 5% 3 4 3 3 5% 7% 50%	1982
	21st to end	Night	15% 5% 2 4 4 4 4 4 6% 8% 43%	2090
October	1st to 10th	Night	18% 8% 3 6% 5% 4 3 4 6% 7% 37%	2209
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 4 6% 7% 35%	2310
	21st to end	Night	21% 10% 3 8% 5% 4 4 4 5% 6% 29%	2717
November	1st to 10th	Night	15% 8% 4 7% 6% 6% 5% 4 6% 6% 33%	2537
	11th to 20th	Night	14% 10% 3 9% 6% 4 4 3 6% 6% 33%	2646
	21st to end	Night	16% 11% 4 9% 6% 5% 6% 5% 6% 7% 26%	2800
December	1st to 10th	Night	13% 15% 6% 14% 8% 6% 6% 5% 6% 6% 15%	4580
	11th to 20th	Night	17% 13% 5% 12% 5% 5% 5% 3 4 5% 25%	5208
	21st to end	Night	19% 13% 4 11% 5% 5% 5% 3 5% 5% 24%	6996

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% 2 14% 20% 3	2598
	11th to 20th	Day	68% 3 12% 10% 6% 1	2600
	21st to end	Day	67% 3 13% 12% 5% 1	2780
February	1st to 10th	Day	65% 3 8% 17% 5% 2	1689
	11th to 20th	Day	58% 3 10% 19% 8% 1	1785
	21st to end	Day	62% 2 11% 14% 9% 2	1570
March	1st to 10th	Day	61% 3 9% 18% 7% 1	3119
	11th to 20th	Day	60% 2 13% 14% 10% 2	3278
	21st to end	Day	48% 2 8% 21% 19% 2	4164
April	1st to 10th	Day	56% 3 7% 20% 12% 3	2280
	11th to 20th	Day	52% 2 8% 21% 15% 2	2254
	21st to end	Day	54% 3 10% 19% 12% 3	2419
May	1st to 10th	Day	57% 2 9% 20% 11% 1	4335
	11th to 20th	Day	51% 3 16% 19% 9% 1	4402
	21st to end	Day	43% 4 12% 28% 11% 2	4837
June	1st to 10th	Day	53% 4 10% 20% 11% 1	2660
	11th to 20th	Day	44% 5% 11% 25% 13% 2	2660
	21st to end	Day	53% 4 12% 21% 9% 1	2660
July	1st to 10th	Day	42% 7% 13% 26% 12% 1	4370
	11th to 20th	Day	40% 8% 13% 32% 7%	4365
	21st to end	Day	40% 6% 12% 33% 9%	4661
August	1st to 10th	Day	47% 6% 15% 23% 9% 1	4409
	11th to 20th	Day	42% 6% 17% 28% 6%	3970
	21st to end	Day	41% 7% 14% 24% 14%	4180
September	1st to 10th	Day	37% 5% 12% 31% 14% 1	2280
	11th to 20th	Day	38% 3 11% 33% 14% 1	2197
	21st to end	Day	41% 5% 15% 27% 10% 1	2090
October	1st to 10th	Day	42% 6% 11% 31% 10%	3267
	11th to 20th	Day	44% 3 17% 25% 11%	3084
	21st to end	Day	45% 3 18% 26% 8% 1	3081
November	1st to 10th	Day	52% 4 13% 22% 9% 1	1761
	11th to 20th	Day	51% 4 13% 20% 10% 2	1750
	21st to end	Day	60% 3 14% 18% 4 1	1600
December	1st to 10th	Day	72% 4 10% 13% 2	2600
	11th to 20th	Day	54% 4 11% 20% 9% 1	2986
	21st to end	Day	60% 3 9% 17% 10%	3960
January	1st to 10th	Night	62% 2 17% 14% 5%	4569
	11th to 20th	Night	62% 3 14% 15% 5% 1	4580
	21st to end	Night	63% 2 12% 15% 7% 1	4899
February	1st to 10th	Night	62% 4 12% 15% 7% 1	2489
	11th to 20th	Night	53% 3 13% 22% 9%	2291
	21st to end	Night	55% 3 10% 18% 12% 1	1884
March	1st to 10th	Night	55% 3 10% 21% 10% 2	3734
	11th to 20th	Night	58% 2 10% 19% 10% 2	3677
	21st to end	Night	51% 2 6% 23% 15% 2	3467
April	1st to 10th	Night	56% 2 7% 21% 12% 1	1900
	11th to 20th	Night	54% 2 10% 21% 12% 1	1875
	21st to end	Night	49% 3 9% 20% 14% 3	1760
May	1st to 10th	Night	58% 2 10% 19% 9% 2	2653
	11th to 20th	Night	50% 3 12% 23% 11% 2	2496
	21st to end	Night	41% 4 13% 23% 17% 1	2739
June	1st to 10th	Night	47% 5% 13% 22% 11% 2	1520
	11th to 20th	Night	42% 5% 13% 24% 14% 2	1520
	21st to end	Night	44% 5% 15% 23% 12% 1	1520
July	1st to 10th	Night	43% 5% 14% 25% 13% 1	2480
	11th to 20th	Night	35% 6% 16% 33% 10%	2472
	21st to end	Night	41% 6% 14% 25% 12% 1	2651
August	1st to 10th	Night	41% 6% 16% 27% 10%	2496
	11th to 20th	Night	42% 5% 19% 24% 9%	2889
	21st to end	Night	36% 5% 16% 29% 14% 1	3476
September	1st to 10th	Night	34% 4 16% 31% 13% 1	1900
	11th to 20th	Night	35% 3 14% 34% 14% 1	1982
	21st to end	Night	38% 4 15% 28% 14% 1	2090
October	1st to 10th	Night	43% 4 13% 29% 10% 1	3690
	11th to 20th	Night	42% 3 12% 28% 14%	3833
	21st to end	Night	46% 4 15% 24% 10% 1	4467
November	1st to 10th	Night	50% 3 13% 22% 10% 2	2537
	11th to 20th	Night	51% 3 12% 21% 11% 2	2646
	21st to end	Night	55% 4 16% 19% 6%	2800
December	1st to 10th	Night	67% 5% 11% 14% 2	4580
	11th to 20th	Night	61% 3 9% 14% 12% 1	5208
	21st to end	Night	62% 3 10% 15% 8% 1	6996

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	23% 21% 38% 12% 7%	200
	11th to 20th	Day	23% 27% 37% 10% 3 1	200
	21st to end	Day	25% 21% 37% 14% 3 1	200
February	1st to 10th	Day	34% 25% 34% 6%	261
	11th to 20th	Day	27% 31% 35% 7% 1	384
	21st to end	Day	32% 23% 32% 12% 2	382
March	1st to 10th	Day	30% 21% 33% 13% 3	460
	11th to 20th	Day	27% 25% 32% 13% 2 1	460
	21st to end	Day	32% 25% 27% 12% 5%	506
April	1st to 10th	Day	33% 20% 28% 15% 3 1	460
	11th to 20th	Day	31% 22% 27% 13% 6% 1	452
	21st to end	Day	32% 18% 30% 14% 5% 1	460
May	1st to 10th	Day	30% 19% 31% 13% 6% 2	463
	11th to 20th	Day	20% 18% 34% 20% 7% 1	487
	21st to end	Day	19% 17% 36% 22% 6% 1	529
June	1st to 10th	Day	26% 13% 36% 18% 6% 1	490
	11th to 20th	Day	19% 17% 32% 21% 8% 2	490
	21st to end	Day	24% 16% 33% 19% 7% 1	490
July	1st to 10th	Day	16% 8% 35% 30% 8% 2	485
	11th to 20th	Day	15% 8% 38% 28% 9% 1	484
	21st to end	Day	14% 7% 39% 30% 8% 2	532
August	1st to 10th	Day	14% 9% 33% 28% 12% 3	486
	11th to 20th	Day	16% 7% 33% 28% 12% 4	479
	21st to end	Day	17% 13% 25% 29% 12% 4	539
September	1st to 10th	Day	16% 11% 32% 22% 13% 4	490
	11th to 20th	Day	14% 13% 24% 27% 16% 7%	490
	21st to end	Day	16% 12% 36% 22% 12% 3	490
October	1st to 10th	Day	25% 15% 28% 16% 13% 2	490
	11th to 20th	Day	23% 21% 32% 15% 9% 1	473
	21st to end	Day	32% 15% 32% 17% 4	429
November	1st to 10th	Day	28% 19% 29% 17% 6% 1	395
	11th to 20th	Day	28% 26% 25% 16% 5% 1	350
	21st to end	Day	34% 22% 28% 15% 2	200
December	1st to 10th	Day	24% 24% 34% 17% 3 1	200
	11th to 20th	Day	34% 21% 34% 11% 2	200
	21st to end	Day	28% 23% 33% 11% 4 1	220
January	1st to 10th	Night	22% 23% 36% 16% 2	800
	11th to 20th	Night	28% 19% 32% 17% 4	800
	21st to end	Night	17% 20% 39% 19% 4 1	841
February	1st to 10th	Night	20% 20% 37% 20% 3	760
	11th to 20th	Night	21% 23% 38% 17% 2	743
	21st to end	Night	22% 22% 35% 19% 2	628
March	1st to 10th	Night	28% 16% 34% 18% 3	760
	11th to 20th	Night	24% 21% 32% 19% 3 1	760
	21st to end	Night	27% 18% 30% 20% 5% 1	836
April	1st to 10th	Night	27% 18% 33% 20% 3	760
	11th to 20th	Night	28% 21% 27% 18% 5%	751
	21st to end	Night	26% 19% 31% 19% 5%	620
May	1st to 10th	Night	27% 13% 32% 21% 6% 1	461
	11th to 20th	Night	18% 14% 34% 24% 9%	374
	21st to end	Night	15% 11% 36% 27% 9%	411
June	1st to 10th	Night	24% 9% 32% 26% 9% 1	380
	11th to 20th	Night	16% 11% 30% 31% 11%	380
	21st to end	Night	19% 8% 34% 26% 12% 1	380
July	1st to 10th	Night	9% 8% 33% 35% 15%	370
	11th to 20th	Night	11% 5% 28% 42% 13% 2	368
	21st to end	Night	12% 5% 32% 37% 13% 1	416
August	1st to 10th	Night	14% 5% 25% 39% 16% 1	378
	11th to 20th	Night	13% 5% 26% 35% 18% 3	602
	21st to end	Night	14% 8% 28% 33% 16% 1	836
September	1st to 10th	Night	14% 8% 32% 30% 15% 1	760
	11th to 20th	Night	14% 7% 26% 29% 20% 4	760
	21st to end	Night	17% 11% 30% 24% 16% 2	760
October	1st to 10th	Night	23% 14% 27% 22% 13% 1	760
	11th to 20th	Night	23% 16% 29% 24% 9% 1	760
	21st to end	Night	27% 17% 31% 20% 5%	836
November	1st to 10th	Night	24% 18% 31% 21% 6% 1	780
	11th to 20th	Night	24% 19% 31% 17% 8% 1	800
	21st to end	Night	25% 23% 28% 18% 6% 1	800
December	1st to 10th	Night	27% 21% 30% 18% 4	800
	11th to 20th	Night	27% 23% 31% 15% 4 1	800
	21st to end	Night	30% 20% 30% 17% 3	880

Page generated at 2025-01-19 03:06:48. Version ac07

Copyright © 2025 Attilla Danko.