Vienna
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vienna clear sky chart from 2005-11-06 to 2026-03-18.

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% 6% 2 6% 4 4 5% 4 6% 8% 42%	131257
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	130244
Transparency	Day	39% 6% 12% 24% 16% 4	134851
	Night	39% 6% 11% 23% 18% 4	134721
Seeing	Day	19% 11% 27% 24% 14% 5%	16690
	Night	18% 10% 30% 30% 12% 1	26331

By Month

Percentage Cloud Cover SE (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	14% 4 2 6% 4 4 5% 3 4 6% 47%	4839
	11th to 20th	Day	14% 8% 3 6% 3 3 3 3 3 4 51%	5280
	21st to end	Day	14% 5% 2 7% 4 4 5% 4 6% 6% 43%	5780
February	1st to 10th	Day	14% 6% 3 6% 5% 5% 4 4 6% 7% 41%	1890
	11th to 20th	Day	18% 8% 3 5% 4 3 3 4 5% 7% 40%	1900
	21st to end	Day	18% 8% 2 7% 5% 4 4 2 4 5% 41%	1730
March	1st to 10th	Day	19% 5% 2 5% 4 4 4 4 5% 7% 42%	4971
	11th to 20th	Day	11% 6% 2 5% 4 4 3 4 5% 6% 50%	5057
	21st to end	Day	16% 6% 2 6% 4 5% 4 3 5% 7% 40%	5549
April	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 42%	2400
	11th to 20th	Day	14% 5% 2 5% 5% 4 4 3 4 7% 47%	2374
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 7% 41%	2400
May	1st to 10th	Day	20% 7% 2 6% 5% 5% 5% 4 7% 9% 31%	5766
	11th to 20th	Day	22% 6% 2 6% 4 4 4 4 7% 10% 30%	6129
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 9% 39%	6934
June	1st to 10th	Day	11% 5% 2 5% 4 5% 4 4 7% 9% 43%	2800
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 5% 6% 9% 41%	2800
	21st to end	Day	9% 4 2 5% 4 4 6% 5% 7% 10% 44%	2800
July	1st to 10th	Day	8% 4 2 6% 5% 5% 6% 5% 7% 10% 43%	6360
	11th to 20th	Day	9% 6% 2 8% 6% 6% 6% 6% 10% 11% 30%	6134
	21st to end	Day	9% 5% 2 6% 5% 6% 5% 5% 7% 10% 39%	6424
August	1st to 10th	Day	16% 6% 2 6% 5% 5% 6% 4 7% 9% 33%	4234
	11th to 20th	Day	10% 5% 2 6% 5% 6% 6% 5% 7% 10% 39%	3992
	21st to end	Day	10% 4 2 5% 4 4 4 3 6% 9% 48%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 3 4 5% 8% 45%	2400
	11th to 20th	Day	12% 6% 2 5% 4 3 4 4 6% 8% 48%	2400
	21st to end	Day	21% 7% 2 7% 4 4 4 4 6% 6% 36%	2100
October	1st to 10th	Day	16% 5% 2 4 3 4 3 3 5% 6% 47%	2000
	11th to 20th	Day	17% 4 2 4 3 3 3 3 5% 6% 50%	2000
	21st to end	Day	14% 5% 2 5% 3 4 3 4 4 6% 50%	2200
November	1st to 10th	Day	12% 5% 2 4 3 4 4 3 4 7% 52%	1957
	11th to 20th	Day	15% 4 2 4 3 3 4 4 5% 6% 50%	1867
	21st to end	Day	15% 6% 2 5% 4 4 5% 3 5% 5% 47%	1680
December	1st to 10th	Day	12% 5% 2 7% 4 3 4 4 5% 7% 48%	2680
	11th to 20th	Day	18% 8% 2 5% 3 3 4 3 4 7% 43%	3070
	21st to end	Day	17% 8% 3 6% 4 4 4 3 5% 7% 38%	4048
January	1st to 10th	Night	11% 6% 2 6% 4 4 5% 4 6% 6% 45%	8039
	11th to 20th	Night	13% 7% 2 6% 3 3 3 2 4 5% 52%	7680
	21st to end	Night	14% 6% 2 6% 3 3 4 3 4 6% 49%	8401
February	1st to 10th	Night	15% 7% 2 6% 5% 5% 4 4 6% 6% 42%	2730
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 5% 5% 43%	2605
	21st to end	Night	16% 8% 3 5% 3 4 3 3 5% 6% 43%	2076
March	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 4 5% 47%	5084
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 6% 7% 50%	5058
	21st to end	Night	12% 7% 3 6% 4 4 5% 4 5% 6% 44%	5381
April	1st to 10th	Night	16% 7% 2 6% 6% 4 4 4 6% 6% 41%	2000
	11th to 20th	Night	14% 5% 2 6% 4 4 4 3 5% 7% 46%	1975
	21st to end	Night	16% 5% 2 4 4 3 5% 4 6% 8% 43%	2000
May	1st to 10th	Night	21% 9% 3 6% 5% 5% 5% 4 6% 7% 30%	4169
	11th to 20th	Night	24% 8% 2 6% 4 4 4 4 5% 8% 31%	3762
	21st to end	Night	17% 6% 2 6% 4 4 5% 4 6% 8% 36%	3920
June	1st to 10th	Night	11% 4 2 5% 4 5% 5% 5% 6% 9% 45%	1600
	11th to 20th	Night	13% 5% 2 6% 4 5% 5% 5% 5% 9% 41%	1600
	21st to end	Night	9% 5% 2 6% 5% 5% 4 4 7% 10% 43%	1600
July	1st to 10th	Night	9% 5% 2 7% 5% 5% 6% 5% 7% 9% 40%	3598
	11th to 20th	Night	9% 8% 3 9% 6% 6% 7% 6% 9% 10% 29%	3818
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 41%	4427
August	1st to 10th	Night	17% 6% 2 7% 4 5% 5% 5% 6% 9% 33%	2915
	11th to 20th	Night	11% 7% 3 8% 5% 6% 5% 5% 6% 9% 36%	3021
	21st to end	Night	11% 5% 2 4 3 3 4 4 5% 8% 50%	3586
September	1st to 10th	Night	13% 7% 2 5% 4 3 5% 4 5% 6% 46%	2000
	11th to 20th	Night	11% 6% 2 6% 3 4 4 4 5% 7% 49%	2000
	21st to end	Night	20% 8% 2 6% 4 4 3 3 5% 6% 37%	2294
October	1st to 10th	Night	19% 5% 2 5% 3 3 3 3 4 5% 48%	2400
	11th to 20th	Night	16% 4 2 5% 3 3 3 2 4 5% 52%	2400
	21st to end	Night	14% 5% 2 4 3 3 4 3 5% 6% 51%	2640
November	1st to 10th	Night	12% 5% 2 5% 3 4 3 3 4 5% 54%	2559
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 50%	2751
	21st to end	Night	13% 4 1 5% 5% 4 4 5% 5% 6% 48%	2940
December	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 5% 7% 52%	4720
	11th to 20th	Night	20% 6% 2 5% 3 4 3 3 5% 6% 41%	5345
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 38%	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	39% 3 8% 26% 19% 5%	4840
	11th to 20th	Day	38% 3 7% 22% 21% 9%	5280
	21st to end	Day	40% 4 9% 31% 13% 3	5780
February	1st to 10th	Day	42% 5% 13% 24% 14% 2	1890
	11th to 20th	Day	44% 4 10% 23% 17% 3	1909
	21st to end	Day	46% 3 11% 21% 15% 4	1730
March	1st to 10th	Day	41% 5% 9% 21% 17% 8%	4971
	11th to 20th	Day	34% 5% 10% 25% 21% 6%	5057
	21st to end	Day	44% 4 10% 18% 17% 6%	5549
April	1st to 10th	Day	40% 5% 12% 20% 18% 5%	2400
	11th to 20th	Day	38% 4 11% 21% 20% 6%	2374
	21st to end	Day	42% 6% 10% 23% 17% 4	2400
May	1st to 10th	Day	47% 8% 15% 19% 10% 2	5800
	11th to 20th	Day	47% 8% 11% 24% 9% 1	6133
	21st to end	Day	40% 8% 13% 22% 15% 2	6949
June	1st to 10th	Day	35% 8% 14% 26% 16% 1	2800
	11th to 20th	Day	37% 9% 12% 24% 15% 2	2800
	21st to end	Day	32% 9% 15% 25% 17% 2	2800
July	1st to 10th	Day	33% 10% 18% 20% 14% 5%	6290
	11th to 20th	Day	42% 11% 17% 22% 7% 1	6059
	21st to end	Day	38% 7% 10% 24% 20% 1	6240
August	1st to 10th	Day	44% 8% 15% 25% 8%	4221
	11th to 20th	Day	38% 8% 13% 26% 14% 1	4031
	21st to end	Day	33% 7% 13% 28% 18% 2	4312
September	1st to 10th	Day	36% 7% 13% 19% 20% 5%	2400
	11th to 20th	Day	34% 5% 10% 26% 21% 3	2400
	21st to end	Day	46% 6% 11% 18% 17% 2	2100
October	1st to 10th	Day	37% 4 7% 17% 29% 5%	3260
	11th to 20th	Day	25% 3 7% 29% 26% 8%	3240
	21st to end	Day	22% 3 9% 30% 28% 8%	3538
November	1st to 10th	Day	32% 5% 12% 26% 20% 5%	1957
	11th to 20th	Day	34% 5% 10% 24% 22% 5%	1867
	21st to end	Day	39% 4 9% 28% 18% 3	1680
December	1st to 10th	Day	36% 5% 12% 27% 16% 4	2680
	11th to 20th	Day	41% 4 13% 28% 10% 3	3066
	21st to end	Day	44% 5% 14% 21% 15% 1	4048
January	1st to 10th	Night	39% 4 9% 25% 20% 4	8040
	11th to 20th	Night	36% 3 7% 22% 24% 8%	7680
	21st to end	Night	37% 3 7% 30% 19% 3	8401
February	1st to 10th	Night	42% 7% 11% 23% 16% 2	2730
	11th to 20th	Night	43% 4 10% 22% 17% 4	2607
	21st to end	Night	41% 5% 11% 21% 16% 5%	2076
March	1st to 10th	Night	39% 4 8% 24% 16% 9%	5084
	11th to 20th	Night	32% 5% 11% 25% 20% 7%	5058
	21st to end	Night	39% 5% 9% 22% 20% 6%	5381
April	1st to 10th	Night	42% 6% 10% 18% 20% 4	2000
	11th to 20th	Night	37% 5% 12% 20% 20% 5%	1975
	21st to end	Night	37% 7% 11% 24% 15% 5%	2000
May	1st to 10th	Night	52% 7% 12% 17% 8% 4	4198
	11th to 20th	Night	49% 7% 11% 21% 11% 1	3763
	21st to end	Night	44% 8% 13% 22% 10% 2	3927
June	1st to 10th	Night	36% 7% 12% 26% 17% 2	1600
	11th to 20th	Night	38% 8% 14% 24% 14% 2	1600
	21st to end	Night	34% 8% 15% 21% 19% 3	1600
July	1st to 10th	Night	38% 10% 17% 18% 14% 3	3560
	11th to 20th	Night	46% 11% 18% 16% 7% 1	3773
	21st to end	Night	36% 8% 15% 22% 18%	4297
August	1st to 10th	Night	46% 8% 15% 22% 8% 1	2904
	11th to 20th	Night	42% 8% 13% 24% 12% 1	3048
	21st to end	Night	31% 7% 12% 30% 18% 2	3586
September	1st to 10th	Night	38% 6% 10% 22% 18% 6%	2000
	11th to 20th	Night	34% 7% 8% 27% 21% 3	2000
	21st to end	Night	47% 6% 11% 17% 16% 3	2294
October	1st to 10th	Night	40% 4 9% 21% 20% 6%	3920
	11th to 20th	Night	24% 4 11% 22% 31% 8%	3899
	21st to end	Night	23% 4 9% 31% 26% 8%	4252
November	1st to 10th	Night	34% 3 9% 25% 23% 6%	2559
	11th to 20th	Night	36% 4 9% 23% 22% 6%	2751
	21st to end	Night	35% 6% 11% 25% 21% 3	2940
December	1st to 10th	Night	33% 4 12% 27% 21% 2	4720
	11th to 20th	Night	43% 4 12% 20% 16% 4	5348
	21st to end	Night	44% 6% 15% 20% 14% 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	28% 17% 34% 17% 2 1	253
	11th to 20th	Day	29% 23% 30% 15% 3	330
	21st to end	Day	22% 23% 36% 14% 5% 1	363
February	1st to 10th	Day	25% 22% 35% 14% 4	330
	11th to 20th	Day	34% 22% 34% 9% 2	387
	21st to end	Day	27% 18% 33% 20% 2	446
March	1st to 10th	Day	24% 18% 34% 19% 5%	540
	11th to 20th	Day	27% 18% 32% 18% 5% 1	528
	21st to end	Day	23% 17% 33% 19% 7% 1	550
April	1st to 10th	Day	23% 19% 32% 19% 6% 1	500
	11th to 20th	Day	18% 17% 30% 20% 12% 3	492
	21st to end	Day	24% 13% 29% 23% 9% 2	500
May	1st to 10th	Day	23% 14% 28% 19% 12% 3	503
	11th to 20th	Day	21% 9% 30% 23% 13% 4	527
	21st to end	Day	20% 7% 24% 24% 19% 6%	573
June	1st to 10th	Day	13% 5% 28% 27% 19% 8%	530
	11th to 20th	Day	14% 6% 24% 31% 18% 6%	530
	21st to end	Day	10% 4 25% 34% 20% 7%	530
July	1st to 10th	Day	11% 3 14% 34% 24% 13%	525
	11th to 20th	Day	5% 3 18% 32% 31% 11%	524
	21st to end	Day	11% 2 18% 33% 27% 8%	576
August	1st to 10th	Day	13% 4 17% 28% 28% 10%	526
	11th to 20th	Day	11% 4 18% 29% 27% 11%	519
	21st to end	Day	9% 2 22% 28% 23% 15%	583
September	1st to 10th	Day	16% 4 22% 24% 19% 15%	530
	11th to 20th	Day	14% 3 21% 30% 20% 12%	530
	21st to end	Day	26% 8% 24% 26% 12% 5%	530
October	1st to 10th	Day	19% 7% 20% 28% 18% 8%	530
	11th to 20th	Day	16% 14% 26% 27% 14% 3	530
	21st to end	Day	17% 12% 32% 26% 10% 3	583
November	1st to 10th	Day	15% 13% 33% 26% 11% 3	483
	11th to 20th	Day	21% 12% 33% 22% 10% 1	407
	21st to end	Day	24% 18% 29% 20% 8% 1	220
December	1st to 10th	Day	20% 15% 42% 17% 4 2	220
	11th to 20th	Day	24% 15% 34% 20% 7%	220
	21st to end	Day	25% 16% 32% 21% 6%	242
January	1st to 10th	Night	20% 18% 40% 19% 3	840
	11th to 20th	Night	22% 15% 35% 24% 4	840
	21st to end	Night	20% 15% 40% 23% 2	924
February	1st to 10th	Night	20% 14% 38% 26% 3	840
	11th to 20th	Night	25% 19% 37% 18% 1	823
	21st to end	Night	25% 16% 35% 21% 2	692
March	1st to 10th	Night	20% 15% 39% 23% 3	840
	11th to 20th	Night	22% 17% 36% 20% 5% 1	829
	21st to end	Night	18% 16% 38% 23% 4 1	880
April	1st to 10th	Night	19% 18% 37% 22% 4	800
	11th to 20th	Night	17% 13% 35% 27% 8% 1	791
	21st to end	Night	20% 9% 33% 31% 7%	800
May	1st to 10th	Night	21% 8% 29% 31% 11% 1	660
	11th to 20th	Night	21% 5% 29% 34% 11%	477
	21st to end	Night	14% 3 23% 39% 19% 1	433
June	1st to 10th	Night	10% 4 25% 40% 21% 1	400
	11th to 20th	Night	14% 4 23% 40% 18% 1	400
	21st to end	Night	9% 2 24% 48% 17% 1	400
July	1st to 10th	Night	10% 2 19% 42% 25% 3	390
	11th to 20th	Night	6% 1 16% 39% 33% 5%	479
	21st to end	Night	9% 1 16% 42% 29% 3	657
August	1st to 10th	Night	11% 3 13% 42% 30% 2	597
	11th to 20th	Night	13% 3 15% 41% 26% 3	675
	21st to end	Night	11% 2 17% 38% 28% 4	880
September	1st to 10th	Night	16% 3 18% 31% 28% 4	800
	11th to 20th	Night	13% 2 15% 39% 28% 3	800
	21st to end	Night	23% 4 22% 31% 19% 1	800
October	1st to 10th	Night	22% 5% 20% 28% 22% 4	800
	11th to 20th	Night	17% 10% 28% 30% 16%	800
	21st to end	Night	17% 9% 33% 29% 13%	880
November	1st to 10th	Night	14% 9% 33% 32% 11% 2	820
	11th to 20th	Night	18% 13% 33% 28% 8%	840
	21st to end	Night	16% 14% 33% 28% 8%	840
December	1st to 10th	Night	20% 13% 35% 25% 6%	840
	11th to 20th	Night	20% 10% 37% 27% 5%	840
	21st to end	Night	20% 12% 34% 31% 3	924

Page generated at 2026-03-15 02:23:56. Version ac07

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