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 2024-01-15.

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% 5% 2 5% 5% 3 5% 5% 6% 8% 42%	74696
	Night	15% 5% 2 5% 5% 3 4 5% 5% 7% 44%	75092
Transparency	Day	39% 7% 12% 23% 16% 3	74406
	Night	39% 6% 11% 23% 17% 3	74901
Seeing	Day	18% 10% 26% 25% 15% 6%	13862
	Night	17% 9% 29% 30% 13% 1	23495

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% 6% 3 5% 5% 3 4 5% 5% 6% 43%	1572
	11th to 20th	Day	16% 8% 3 5% 4 2 4 4 5% 7% 42%	1662
	21st to end	Day	15% 7% 3 5% 6% 3 5% 3 5% 6% 42%	1782
February	1st to 10th	Day	15% 6% 3 5% 7% 3 5% 5% 6% 7% 38%	1620
	11th to 20th	Day	18% 7% 3 4 5% 3 3 5% 5% 7% 41%	1622
	21st to end	Day	19% 7% 3 6% 5% 4 4 4 4 5% 40%	1480
March	1st to 10th	Day	13% 5% 2 4 5% 3 4 4 4 7% 49%	1948
	11th to 20th	Day	17% 6% 3 5% 5% 3 4 4 5% 5% 42%	1980
	21st to end	Day	17% 6% 3 6% 6% 4 5% 3 5% 6% 38%	2207
April	1st to 10th	Day	13% 6% 3 5% 5% 3 5% 5% 5% 8% 44%	2160
	11th to 20th	Day	14% 4 3 4 5% 3 4 4 4 7% 48%	2134
	21st to end	Day	19% 6% 2 5% 4 3 4 5% 5% 7% 41%	2160
May	1st to 10th	Day	15% 7% 2 5% 5% 3 5% 5% 6% 8% 38%	2299
	11th to 20th	Day	19% 6% 3 5% 5% 3 5% 5% 6% 8% 36%	2401
	21st to end	Day	14% 5% 2 4 5% 3 5% 5% 7% 8% 41%	2725
June	1st to 10th	Day	11% 5% 2 4 5% 3 5% 5% 6% 10% 43%	2520
	11th to 20th	Day	11% 5% 3 6% 4 3 7% 6% 6% 9% 40%	2520
	21st to end	Day	9% 4 2 5% 5% 3 6% 6% 7% 11% 43%	2520
July	1st to 10th	Day	10% 3 2 6% 6% 4 6% 6% 6% 10% 41%	2520
	11th to 20th	Day	6% 4 2 4 5% 4 5% 8% 8% 11% 43%	2434
	21st to end	Day	9% 4 3 6% 6% 5% 6% 7% 8% 10% 36%	2547
August	1st to 10th	Day	10% 6% 2 6% 6% 5% 6% 6% 8% 9% 36%	2327
	11th to 20th	Day	11% 5% 2 5% 6% 5% 6% 7% 7% 8% 38%	2210
	21st to end	Day	11% 4 2 5% 5% 4 5% 5% 7% 10% 42%	2376
September	1st to 10th	Day	14% 4 2 5% 5% 4 4 5% 5% 9% 43%	2160
	11th to 20th	Day	12% 5% 2 4 4 2 4 5% 6% 8% 48%	2160
	21st to end	Day	18% 6% 2 6% 4 3 4 5% 6% 6% 39%	1891
October	1st to 10th	Day	17% 5% 2 4 4 4 4 3 5% 6% 46%	1800
	11th to 20th	Day	18% 4 2 4 3 2 4 4 4 6% 49%	1800
	21st to end	Day	14% 5% 2 4 4 3 4 4 4 6% 48%	1980
November	1st to 10th	Day	13% 5% 2 4 4 2 4 3 4 8% 51%	1767
	11th to 20th	Day	16% 3 3 3 4 2 4 4 5% 7% 50%	1690
	21st to end	Day	16% 5% 2 4 4 2 5% 4 5% 6% 48%	1520
December	1st to 10th	Day	15% 7% 2 5% 5% 3 5% 5% 5% 8% 41%	1520
	11th to 20th	Day	16% 5% 2 5% 4 3 4 4 4 7% 45%	1910
	21st to end	Day	17% 8% 2 6% 5% 3 5% 4 5% 7% 36%	2772
January	1st to 10th	Night	17% 6% 2 5% 4 3 3 4 5% 7% 45%	2592
	11th to 20th	Night	17% 6% 2 6% 5% 3 4 4 5% 6% 41%	2404
	21st to end	Night	15% 7% 3 4 4 2 4 4 4 7% 46%	2574
February	1st to 10th	Night	16% 7% 2 5% 5% 3 5% 5% 6% 6% 40%	2340
	11th to 20th	Night	19% 6% 3 5% 4 2 4 3 5% 5% 43%	2223
	21st to end	Night	18% 8% 3 5% 4 3 3 4 5% 6% 42%	1776
March	1st to 10th	Night	14% 4 2 5% 4 3 4 5% 4 7% 48%	2008
	11th to 20th	Night	18% 6% 2 5% 4 3 4 4 4 8% 43%	1980
	21st to end	Night	15% 6% 4 6% 5% 2 5% 4 5% 7% 41%	2147
April	1st to 10th	Night	15% 6% 3 6% 6% 3 4 5% 6% 5% 42%	1800
	11th to 20th	Night	14% 5% 2 5% 5% 3 4 4 5% 7% 47%	1775
	21st to end	Night	18% 5% 2 3 4 2 5% 5% 6% 7% 42%	1800
May	1st to 10th	Night	16% 6% 3 5% 6% 3 5% 5% 5% 8% 39%	1674
	11th to 20th	Night	19% 6% 3 6% 4 2 5% 6% 5% 8% 37%	1490
	21st to end	Night	13% 5% 2 4 5% 3 4 5% 7% 10% 41%	1550
June	1st to 10th	Night	11% 4 2 4 4 3 5% 6% 5% 10% 45%	1440
	11th to 20th	Night	12% 5% 3 5% 4 3 6% 5% 6% 9% 41%	1440
	21st to end	Night	10% 4 3 6% 5% 3 4 5% 7% 10% 43%	1440
July	1st to 10th	Night	10% 4 2 6% 5% 4 6% 7% 7% 9% 40%	1440
	11th to 20th	Night	6% 4 2 5% 5% 3 5% 7% 7% 10% 45%	1521
	21st to end	Night	8% 4 2 5% 5% 4 5% 6% 8% 11% 41%	1771
August	1st to 10th	Night	9% 4 2 5% 6% 5% 7% 7% 6% 9% 40%	1611
	11th to 20th	Night	10% 6% 2 6% 7% 4 6% 6% 7% 9% 39%	1670
	21st to end	Night	10% 6% 3 5% 5% 3 4 5% 6% 9% 43%	1980
September	1st to 10th	Night	14% 7% 2 5% 4 3 4 5% 5% 7% 44%	1800
	11th to 20th	Night	10% 5% 2 6% 4 2 4 4 4 7% 51%	1800
	21st to end	Night	17% 7% 2 6% 5% 3 4 4 5% 7% 40%	2063
October	1st to 10th	Night	20% 5% 2 4 3 3 4 3 4 6% 46%	2160
	11th to 20th	Night	17% 4 2 4 4 2 3 3 4 5% 51%	2160
	21st to end	Night	15% 5% 3 4 4 2 4 4 4 6% 49%	2376
November	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 3 5% 54%	2309
	11th to 20th	Night	14% 6% 2 4 4 2 4 3 4 6% 51%	2489
	21st to end	Night	14% 3 2 4 5% 3 4 6% 5% 6% 49%	2660
December	1st to 10th	Night	17% 7% 3 5% 4 3 3 4 5% 6% 42%	2660
	11th to 20th	Night	17% 5% 2 5% 4 3 4 4 5% 7% 44%	3285
	21st to end	Night	17% 6% 3 5% 5% 3 5% 5% 6% 7% 37%	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	41% 5% 9% 25% 16% 4	1573
	11th to 20th	Day	43% 5% 13% 23% 14% 2	1662
	21st to end	Day	44% 4 9% 23% 15% 4	1782
February	1st to 10th	Day	45% 5% 13% 23% 12% 2	1620
	11th to 20th	Day	43% 4 9% 24% 16% 3	1631
	21st to end	Day	47% 3 12% 19% 15% 4	1480
March	1st to 10th	Day	36% 5% 11% 25% 20% 3	1948
	11th to 20th	Day	44% 5% 10% 18% 18% 6%	1980
	21st to end	Day	47% 5% 8% 19% 17% 4	2207
April	1st to 10th	Day	38% 5% 12% 20% 19% 5%	2160
	11th to 20th	Day	38% 4 10% 21% 21% 6%	2134
	21st to end	Day	42% 6% 10% 21% 17% 4	2160
May	1st to 10th	Day	42% 7% 13% 20% 16% 3	2299
	11th to 20th	Day	45% 7% 13% 19% 13% 2	2405
	21st to end	Day	39% 9% 13% 26% 11% 2	2725
June	1st to 10th	Day	36% 8% 14% 26% 16% 1	2520
	11th to 20th	Day	38% 9% 12% 23% 15% 2	2520
	21st to end	Day	33% 10% 15% 24% 17% 2	2520
July	1st to 10th	Day	36% 10% 14% 21% 17% 3	2450
	11th to 20th	Day	30% 10% 16% 26% 16% 2	2359
	21st to end	Day	39% 9% 13% 20% 17% 1	2405
August	1st to 10th	Day	41% 9% 14% 23% 12% 1	2314
	11th to 20th	Day	40% 10% 13% 20% 15% 1	2210
	21st to end	Day	36% 9% 14% 24% 13% 4	2376
September	1st to 10th	Day	38% 8% 13% 19% 18% 4	2160
	11th to 20th	Day	34% 5% 10% 26% 21% 4	2160
	21st to end	Day	43% 6% 11% 19% 19% 2	1891
October	1st to 10th	Day	39% 6% 10% 24% 18% 3	1800
	11th to 20th	Day	37% 5% 9% 25% 21% 3	1800
	21st to end	Day	36% 4 9% 25% 23% 3	1980
November	1st to 10th	Day	33% 5% 11% 26% 20% 5%	1767
	11th to 20th	Day	34% 5% 10% 24% 22% 5%	1690
	21st to end	Day	38% 4 9% 28% 18% 3	1520
December	1st to 10th	Day	41% 7% 12% 25% 12% 3	1520
	11th to 20th	Day	38% 6% 9% 32% 12% 3	1906
	21st to end	Day	45% 6% 13% 24% 11% 1	2772
January	1st to 10th	Night	40% 5% 9% 23% 19% 5%	2592
	11th to 20th	Night	44% 5% 12% 21% 17% 2	2404
	21st to end	Night	39% 4 8% 26% 20% 2	2574
February	1st to 10th	Night	43% 7% 11% 23% 14% 2	2340
	11th to 20th	Night	43% 4 9% 22% 17% 5%	2225
	21st to end	Night	43% 5% 10% 21% 16% 5%	1776
March	1st to 10th	Night	36% 5% 11% 26% 19% 3	2008
	11th to 20th	Night	41% 6% 10% 21% 18% 4	1980
	21st to end	Night	42% 6% 10% 21% 16% 6%	2147
April	1st to 10th	Night	41% 6% 10% 18% 20% 4	1800
	11th to 20th	Night	37% 5% 12% 20% 20% 6%	1775
	21st to end	Night	39% 7% 11% 24% 14% 6%	1800
May	1st to 10th	Night	42% 8% 11% 22% 13% 4	1674
	11th to 20th	Night	43% 8% 11% 19% 16% 3	1491
	21st to end	Night	37% 9% 17% 21% 13% 2	1551
June	1st to 10th	Night	35% 7% 12% 27% 16% 2	1440
	11th to 20th	Night	38% 8% 14% 23% 14% 3	1440
	21st to end	Night	34% 8% 15% 22% 18% 3	1440
July	1st to 10th	Night	38% 11% 15% 18% 13% 4	1400
	11th to 20th	Night	31% 10% 16% 24% 16% 2	1473
	21st to end	Night	32% 9% 17% 26% 16% 1	1672
August	1st to 10th	Night	38% 11% 13% 23% 14% 1	1600
	11th to 20th	Night	40% 9% 14% 21% 13% 2	1670
	21st to end	Night	36% 9% 14% 23% 14% 3	1980
September	1st to 10th	Night	39% 6% 10% 22% 17% 5%	1800
	11th to 20th	Night	33% 7% 8% 26% 22% 4	1800
	21st to end	Night	44% 6% 11% 18% 18% 4	2063
October	1st to 10th	Night	41% 5% 10% 23% 19% 3	2160
	11th to 20th	Night	36% 5% 8% 26% 22% 3	2160
	21st to end	Night	36% 5% 9% 24% 22% 4	2376
November	1st to 10th	Night	34% 3 9% 26% 22% 6%	2309
	11th to 20th	Night	35% 4 9% 23% 23% 6%	2489
	21st to end	Night	34% 6% 11% 25% 20% 3	2660
December	1st to 10th	Night	43% 5% 10% 24% 15% 2	2660
	11th to 20th	Night	40% 5% 10% 23% 18% 4	3288
	21st to end	Night	44% 7% 15% 21% 11% 1	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	25% 18% 36% 17% 3 1	207
	11th to 20th	Day	28% 22% 30% 17% 4	254
	21st to end	Day	23% 22% 35% 13% 6% 1	264
February	1st to 10th	Day	25% 23% 35% 12% 4	240
	11th to 20th	Day	31% 21% 37% 9% 1	289
	21st to end	Day	28% 18% 32% 20% 2	346
March	1st to 10th	Day	22% 19% 33% 22% 4	420
	11th to 20th	Day	26% 17% 32% 19% 5% 1	420
	21st to end	Day	22% 18% 31% 20% 7% 2	462
April	1st to 10th	Day	21% 16% 33% 21% 7% 1	420
	11th to 20th	Day	17% 15% 30% 21% 14% 4	412
	21st to end	Day	26% 13% 29% 22% 8% 2	420
May	1st to 10th	Day	23% 14% 27% 19% 14% 3	423
	11th to 20th	Day	21% 8% 31% 24% 13% 4	447
	21st to end	Day	20% 5% 22% 26% 20% 7%	485
June	1st to 10th	Day	14% 4 25% 28% 20% 8%	450
	11th to 20th	Day	15% 6% 26% 29% 17% 8%	450
	21st to end	Day	10% 3 27% 35% 18% 7%	450
July	1st to 10th	Day	13% 3 15% 31% 24% 14%	445
	11th to 20th	Day	4 2 17% 32% 33% 12%	444
	21st to end	Day	11% 2 20% 32% 27% 8%	488
August	1st to 10th	Day	11% 5% 15% 28% 31% 11%	446
	11th to 20th	Day	11% 4 16% 27% 30% 12%	439
	21st to end	Day	9% 2 19% 29% 24% 17%	495
September	1st to 10th	Day	18% 4 19% 22% 21% 16%	450
	11th to 20th	Day	14% 4 19% 30% 20% 14%	450
	21st to end	Day	24% 7% 24% 26% 13% 5%	450
October	1st to 10th	Day	20% 6% 20% 27% 20% 7%	450
	11th to 20th	Day	16% 13% 24% 28% 15% 3	450
	21st to end	Day	17% 11% 33% 26% 9% 3	495
November	1st to 10th	Day	15% 11% 31% 28% 12% 3	413
	11th to 20th	Day	19% 12% 33% 23% 11% 1	350
	21st to end	Day	22% 16% 29% 22% 9% 2	180
December	1st to 10th	Day	18% 14% 42% 18% 5% 2	180
	11th to 20th	Day	25% 15% 32% 20% 8%	180
	21st to end	Day	24% 13% 32% 23% 7% 1	198
January	1st to 10th	Night	20% 17% 40% 19% 4	760
	11th to 20th	Night	22% 12% 35% 26% 4	740
	21st to end	Night	20% 15% 39% 24% 2	792
February	1st to 10th	Night	20% 14% 37% 26% 3	720
	11th to 20th	Night	24% 18% 38% 18% 1	703
	21st to end	Night	26% 17% 35% 21% 2	592
March	1st to 10th	Night	19% 15% 40% 23% 3	720
	11th to 20th	Night	22% 15% 36% 21% 5% 1	720
	21st to end	Night	18% 15% 39% 24% 5% 1	792
April	1st to 10th	Night	18% 19% 37% 23% 4	720
	11th to 20th	Night	16% 12% 35% 28% 9% 1	711
	21st to end	Night	21% 9% 31% 32% 7%	720
May	1st to 10th	Night	20% 7% 30% 30% 11% 1	594
	11th to 20th	Night	21% 4 28% 35% 11%	429
	21st to end	Night	14% 4 22% 40% 20% 2	389
June	1st to 10th	Night	10% 3 25% 39% 23% 1	360
	11th to 20th	Night	13% 4 23% 39% 19% 1	360
	21st to end	Night	8% 2 23% 49% 17% 1	360
July	1st to 10th	Night	10% 1 18% 41% 26% 3	350
	11th to 20th	Night	6% 1 15% 39% 34% 5%	429
	21st to end	Night	9% 1 15% 42% 30% 3	591
August	1st to 10th	Night	9% 2 14% 42% 31% 2	537
	11th to 20th	Night	12% 2 13% 41% 28% 3	606
	21st to end	Night	11% 2 16% 37% 29% 4	792
September	1st to 10th	Night	17% 3 16% 29% 30% 5%	720
	11th to 20th	Night	12% 2 15% 38% 30% 3	720
	21st to end	Night	20% 4 23% 32% 20% 1	720
October	1st to 10th	Night	22% 5% 19% 29% 20% 5%	720
	11th to 20th	Night	17% 8% 28% 31% 16%	720
	21st to end	Night	18% 9% 33% 27% 13%	792
November	1st to 10th	Night	14% 8% 32% 32% 11% 2	740
	11th to 20th	Night	17% 12% 33% 29% 8% 1	760
	21st to end	Night	15% 13% 33% 29% 9% 1	760
December	1st to 10th	Night	21% 13% 34% 25% 7%	760
	11th to 20th	Night	18% 10% 39% 27% 5%	760
	21st to end	Night	20% 11% 34% 31% 3	836

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