VU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the VU Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	13% 9% 4 8% 5% 4 4 4 6% 7% 35%	113532
	Night	14% 10% 4 8% 5% 4 4 4 5% 6% 35%	115572
Transparency	Day	46% 5% 12% 22% 13% 2	117118
	Night	48% 5% 11% 21% 14% 2	120129
Seeing	Day	17% 11% 29% 25% 14% 5%	16372
	Night	18% 10% 30% 29% 12% 1	26350

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	13% 12% 9% 15% 7% 5% 4 3 6% 7% 19%	3617
	11th to 20th	Day	12% 12% 6% 14% 8% 7% 5% 5% 7% 4 20%	3620
	21st to end	Day	18% 18% 5% 10% 4 3 4 3 4 6% 25%	3982
February	1st to 10th	Day	15% 15% 5% 12% 6% 6% 4 3 4 6% 24%	1930
	11th to 20th	Day	14% 12% 5% 9% 6% 5% 4 4 5% 6% 30%	1948
	21st to end	Day	15% 14% 5% 10% 5% 4 5% 3 6% 5% 28%	1650
March	1st to 10th	Day	18% 12% 4 9% 4 3 3 2 4 5% 35%	4480
	11th to 20th	Day	8% 9% 5% 7% 4 4 5% 4 6% 8% 41%	4699
	21st to end	Day	19% 13% 6% 11% 6% 6% 5% 3 5% 6% 21%	5874
April	1st to 10th	Day	17% 12% 3 8% 5% 4 4 4 6% 7% 31%	2400
	11th to 20th	Day	16% 11% 3 7% 4 4 3 3 5% 7% 36%	2374
	21st to end	Day	19% 9% 3 7% 4 4 4 4 5% 7% 34%	2557
May	1st to 10th	Day	18% 7% 2 7% 4 5% 5% 4 6% 9% 33%	4212
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 6% 8% 41%	4131
	21st to end	Day	9% 6% 2 6% 4 4 4 4 6% 9% 45%	4824
June	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 6% 8% 44%	2660
	11th to 20th	Day	9% 4 2 6% 5% 4 5% 4 7% 8% 45%	2660
	21st to end	Day	10% 5% 2 7% 6% 4 5% 5% 6% 10% 39%	2660
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 10% 35%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 11% 47%	4440
	21st to end	Day	8% 6% 3 7% 5% 5% 5% 5% 7% 8% 42%	4813
August	1st to 10th	Day	7% 5% 2 6% 5% 4 4 4 6% 8% 48%	4199
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 48%	3853
	21st to end	Day	7% 4 2 6% 4 4 4 4 6% 9% 49%	4180
September	1st to 10th	Day	10% 5% 2 6% 4 5% 4 4 6% 8% 46%	2280
	11th to 20th	Day	10% 6% 2 6% 3 4 4 4 7% 8% 47%	2109
	21st to end	Day	10% 7% 2 7% 4 4 4 4 6% 9% 41%	2090
October	1st to 10th	Day	12% 6% 3 6% 4 4 4 4 5% 7% 46%	2090
	11th to 20th	Day	18% 7% 3 7% 5% 4 4 4 6% 6% 36%	1870
	21st to end	Day	20% 9% 3 8% 5% 5% 5% 3 6% 7% 29%	1881
November	1st to 10th	Day	12% 8% 3 6% 5% 4 4 5% 6% 8% 41%	1762
	11th to 20th	Day	18% 11% 3 8% 5% 5% 4 4 6% 7% 30%	1800
	21st to end	Day	18% 13% 4 10% 5% 4 4 2 5% 5% 31%	1800
December	1st to 10th	Day	10% 13% 3 10% 5% 4 4 4 5% 7% 33%	2657
	11th to 20th	Day	13% 16% 7% 11% 6% 5% 5% 4 5% 7% 21%	3008
	21st to end	Day	26% 22% 6% 12% 5% 4 4 3 4 4 10%	3982
January	1st to 10th	Night	14% 15% 8% 14% 5% 4 4 4 5% 5% 21%	6348
	11th to 20th	Night	14% 17% 7% 14% 6% 5% 4 4 5% 4 19%	6360
	21st to end	Night	19% 14% 6% 10% 4 4 3 3 4 4 29%	6981
February	1st to 10th	Night	16% 14% 5% 9% 6% 5% 3 4 5% 6% 28%	2469
	11th to 20th	Night	15% 11% 4 8% 6% 5% 4 3 5% 7% 33%	2338
	21st to end	Night	19% 13% 3 8% 4 4 4 3 5% 4 32%	1980
March	1st to 10th	Night	16% 10% 5% 8% 4 4 4 3 5% 6% 36%	5373
	11th to 20th	Night	12% 7% 2 6% 4 3 4 4 5% 7% 45%	5260
	21st to end	Night	18% 10% 4 9% 5% 5% 5% 4 5% 6% 30%	5086
April	1st to 10th	Night	15% 9% 3 7% 5% 4 5% 4 6% 7% 36%	2000
	11th to 20th	Night	18% 9% 2 7% 5% 4 4 4 4 6% 38%	1975
	21st to end	Night	17% 8% 3 8% 5% 5% 4 4 4 5% 38%	1841
May	1st to 10th	Night	16% 8% 3 8% 5% 5% 4 3 5% 9% 34%	2903
	11th to 20th	Night	18% 7% 2 7% 3 3 3 4 6% 7% 40%	2767
	21st to end	Night	12% 6% 2 7% 5% 5% 4 3 5% 6% 44%	2743
June	1st to 10th	Night	10% 5% 1 6% 3 3 4 3 7% 10% 48%	1520
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	1520
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 5% 6% 8% 41%	1520
July	1st to 10th	Night	10% 4 2 6% 5% 4 5% 4 7% 9% 44%	2530
	11th to 20th	Night	7% 4 1 5% 4 4 4 4 5% 7% 55%	2530
	21st to end	Night	5% 5% 2 6% 5% 5% 6% 5% 7% 9% 45%	2750
August	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 4 7% 10% 45%	2731
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 5% 7% 52%	2972
	21st to end	Night	9% 4 3 5% 4 4 4 4 5% 8% 51%	3476
September	1st to 10th	Night	10% 5% 3 5% 3 4 3 4 4 6% 52%	1900
	11th to 20th	Night	8% 4 1 5% 3 4 4 3 6% 8% 53%	2051
	21st to end	Night	10% 7% 2 6% 4 4 5% 3 5% 9% 45%	2090
October	1st to 10th	Night	11% 5% 3 6% 3 4 4 3 6% 6% 51%	2090
	11th to 20th	Night	17% 9% 3 6% 3 3 4 3 4 6% 43%	2306
	21st to end	Night	21% 9% 2 7% 4 4 3 3 5% 6% 36%	2717
November	1st to 10th	Night	13% 9% 3 7% 4 4 4 4 6% 6% 41%	2536
	11th to 20th	Night	15% 10% 3 9% 5% 4 5% 4 5% 6% 33%	2600
	21st to end	Night	19% 10% 4 8% 5% 4 4 4 6% 5% 31%	2600
December	1st to 10th	Night	12% 10% 3 7% 6% 4 4 4 5% 7% 37%	4514
	11th to 20th	Night	17% 13% 5% 11% 6% 5% 5% 4 5% 6% 23%	5199
	21st to end	Night	25% 19% 6% 11% 6% 5% 4 3 4 5% 13%	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	63% 5% 11% 17% 5%	3618
	11th to 20th	Day	65% 3 9% 9% 11% 2	3620
	21st to end	Day	61% 4 12% 16% 7%	3982
February	1st to 10th	Day	61% 4 12% 14% 7% 1	1930
	11th to 20th	Day	53% 5% 11% 22% 8% 1	1958
	21st to end	Day	56% 6% 11% 18% 7% 2	1650
March	1st to 10th	Day	53% 3 8% 23% 10% 2	4480
	11th to 20th	Day	41% 5% 12% 33% 9% 1	4699
	21st to end	Day	63% 6% 12% 13% 5%	5876
April	1st to 10th	Day	52% 5% 12% 19% 12%	2400
	11th to 20th	Day	48% 6% 10% 22% 12% 3	2374
	21st to end	Day	48% 7% 14% 18% 11% 2	2557
May	1st to 10th	Day	46% 8% 12% 24% 9% 1	4250
	11th to 20th	Day	40% 8% 13% 27% 11% 1	4135
	21st to end	Day	34% 7% 12% 22% 23% 2	4837
June	1st to 10th	Day	36% 8% 12% 25% 18% 3	2660
	11th to 20th	Day	34% 8% 11% 24% 20% 3	2660
	21st to end	Day	39% 9% 14% 19% 16% 2	2660
July	1st to 10th	Day	44% 5% 13% 21% 14% 2	4371
	11th to 20th	Day	31% 5% 16% 29% 17% 1	4365
	21st to end	Day	39% 6% 16% 17% 19% 3	4662
August	1st to 10th	Day	34% 5% 15% 28% 15% 1	4184
	11th to 20th	Day	34% 6% 12% 31% 16% 1	3892
	21st to end	Day	32% 5% 14% 26% 20% 4	4179
September	1st to 10th	Day	35% 7% 14% 24% 17% 4	2280
	11th to 20th	Day	32% 7% 12% 26% 20% 3	2109
	21st to end	Day	38% 6% 12% 24% 19% 2	2090
October	1st to 10th	Day	28% 4 9% 32% 27% 1	3480
	11th to 20th	Day	38% 4 11% 23% 23%	3073
	21st to end	Day	47% 4 10% 26% 12% 2	3081
November	1st to 10th	Day	40% 7% 14% 23% 14% 2	1762
	11th to 20th	Day	52% 5% 11% 22% 9% 1	1800
	21st to end	Day	56% 4 11% 21% 8%	1800
December	1st to 10th	Day	51% 3 17% 24% 5%	2657
	11th to 20th	Day	63% 3 14% 16% 4	3005
	21st to end	Day	79% 4 7% 8% 2	3982
January	1st to 10th	Night	64% 4 9% 17% 6%	6348
	11th to 20th	Night	68% 4 9% 8% 10% 1	6360
	21st to end	Night	60% 3 12% 16% 9% 1	6981
February	1st to 10th	Night	57% 4 12% 17% 9% 2	2469
	11th to 20th	Night	51% 5% 12% 20% 11% 1	2339
	21st to end	Night	54% 5% 11% 18% 10% 2	1980
March	1st to 10th	Night	49% 4 10% 26% 10% 1	5373
	11th to 20th	Night	38% 4 12% 29% 14% 2	5260
	21st to end	Night	54% 6% 9% 21% 9% 1	5086
April	1st to 10th	Night	45% 6% 13% 22% 13% 1	2000
	11th to 20th	Night	48% 5% 8% 20% 16% 3	1975
	21st to end	Night	48% 6% 10% 20% 14% 3	1841
May	1st to 10th	Night	47% 7% 11% 21% 13% 2	2930
	11th to 20th	Night	41% 8% 14% 26% 11%	2768
	21st to end	Night	39% 7% 10% 18% 22% 4	2751
June	1st to 10th	Night	31% 7% 14% 26% 20% 2	1520
	11th to 20th	Night	34% 6% 12% 23% 22% 2	1520
	21st to end	Night	39% 8% 15% 22% 14% 3	1520
July	1st to 10th	Night	38% 5% 15% 24% 13% 5%	2490
	11th to 20th	Night	29% 4 13% 24% 27% 2	2482
	21st to end	Night	34% 6% 17% 19% 19% 5%	2662
August	1st to 10th	Night	34% 6% 15% 29% 15% 1	2722
	11th to 20th	Night	31% 5% 13% 28% 21% 1	2999
	21st to end	Night	33% 5% 11% 29% 19% 3	3476
September	1st to 10th	Night	32% 6% 11% 24% 21% 6%	1900
	11th to 20th	Night	28% 6% 8% 28% 27% 3	2051
	21st to end	Night	37% 6% 10% 26% 18% 3	2090
October	1st to 10th	Night	23% 4 9% 34% 29% 2	3480
	11th to 20th	Night	40% 3 7% 22% 26% 1	3842
	21st to end	Night	40% 4 9% 27% 19% 2	4467
November	1st to 10th	Night	42% 5% 11% 24% 16% 2	2536
	11th to 20th	Night	49% 5% 12% 22% 10% 2	2600
	21st to end	Night	53% 5% 11% 23% 8%	2600
December	1st to 10th	Night	46% 3 15% 27% 8% 1	4514
	11th to 20th	Night	61% 3 14% 14% 7% 1	5201
	21st to end	Night	74% 3 9% 10% 4	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	21% 14% 36% 23% 5% 1	300
	11th to 20th	Day	22% 20% 38% 14% 5% 1	300
	21st to end	Day	23% 20% 34% 19% 4	330
February	1st to 10th	Day	19% 22% 37% 18% 4	300
	11th to 20th	Day	22% 19% 36% 18% 3	300
	21st to end	Day	24% 19% 41% 12% 4	249
March	1st to 10th	Day	23% 21% 37% 16% 3	300
	11th to 20th	Day	21% 19% 35% 19% 6%	375
	21st to end	Day	28% 16% 31% 19% 5% 1	550
April	1st to 10th	Day	26% 13% 32% 22% 6% 1	500
	11th to 20th	Day	22% 18% 31% 16% 11% 2	492
	21st to end	Day	24% 14% 32% 20% 8% 2	500
May	1st to 10th	Day	23% 14% 29% 22% 11% 2	503
	11th to 20th	Day	21% 8% 30% 23% 15% 3	511
	21st to end	Day	13% 9% 29% 26% 16% 7%	734
June	1st to 10th	Day	12% 8% 28% 29% 19% 5%	680
	11th to 20th	Day	11% 6% 32% 29% 19% 4	680
	21st to end	Day	12% 8% 31% 31% 13% 5%	680
July	1st to 10th	Day	9% 5% 21% 31% 20% 13%	671
	11th to 20th	Day	10% 5% 24% 30% 21% 10%	668
	21st to end	Day	9% 3 24% 34% 22% 8%	739
August	1st to 10th	Day	9% 3 18% 30% 26% 13%	543
	11th to 20th	Day	11% 4 21% 29% 25% 11%	479
	21st to end	Day	11% 3 15% 34% 28% 9%	539
September	1st to 10th	Day	11% 6% 20% 33% 23% 7%	490
	11th to 20th	Day	10% 4 16% 28% 27% 14%	490
	21st to end	Day	14% 10% 24% 24% 19% 9%	490
October	1st to 10th	Day	13% 8% 29% 30% 16% 5%	490
	11th to 20th	Day	18% 21% 26% 25% 10% 1	329
	21st to end	Day	29% 15% 36% 16% 4 1	330
November	1st to 10th	Day	16% 12% 38% 23% 8% 2	300
	11th to 20th	Day	22% 19% 35% 17% 6% 1	300
	21st to end	Day	27% 13% 39% 18% 3	300
December	1st to 10th	Day	16% 22% 37% 19% 6%	300
	11th to 20th	Day	19% 20% 39% 18% 4 1	300
	21st to end	Day	28% 20% 35% 14% 2	330
January	1st to 10th	Night	22% 15% 37% 22% 5%	1000
	11th to 20th	Night	22% 15% 37% 21% 4 1	1000
	21st to end	Night	22% 14% 35% 25% 4	1100
February	1st to 10th	Night	23% 13% 38% 22% 3	869
	11th to 20th	Night	21% 15% 35% 25% 4	783
	21st to end	Night	25% 15% 38% 20% 2	660
March	1st to 10th	Night	21% 13% 37% 23% 6%	800
	11th to 20th	Night	17% 12% 40% 25% 6%	800
	21st to end	Night	23% 11% 34% 26% 5%	719
April	1st to 10th	Night	18% 13% 38% 25% 6% 1	600
	11th to 20th	Night	23% 9% 35% 26% 7% 1	593
	21st to end	Night	19% 11% 32% 29% 8% 1	600
May	1st to 10th	Night	20% 7% 33% 30% 10% 1	600
	11th to 20th	Night	19% 5% 27% 35% 13% 1	563
	21st to end	Night	12% 4 23% 40% 20% 2	617
June	1st to 10th	Night	10% 4 22% 45% 18%	570
	11th to 20th	Night	11% 3 28% 40% 17% 1	570
	21st to end	Night	11% 3 25% 41% 18% 1	570
July	1st to 10th	Night	8% 2 14% 45% 25% 5%	555
	11th to 20th	Night	8% 3 19% 38% 28% 4	552
	21st to end	Night	5% 2 16% 46% 30% 1	624
August	1st to 10th	Night	7% 2 9% 49% 30% 4	567
	11th to 20th	Night	9% 1 13% 41% 31% 5%	564
	21st to end	Night	8% 2 17% 38% 32% 3	627
September	1st to 10th	Night	12% 4 17% 37% 27% 4	570
	11th to 20th	Night	9% 3 19% 37% 29% 4	721
	21st to end	Night	13% 6% 24% 32% 22% 3	760
October	1st to 10th	Night	14% 5% 25% 36% 19% 1	760
	11th to 20th	Night	22% 13% 28% 27% 9%	760
	21st to end	Night	25% 11% 31% 25% 7%	836
November	1st to 10th	Night	19% 12% 36% 22% 10% 1	780
	11th to 20th	Night	20% 18% 33% 24% 5%	800
	21st to end	Night	24% 14% 34% 23% 6%	800
December	1st to 10th	Night	24% 15% 35% 22% 3	960
	11th to 20th	Night	24% 13% 35% 24% 4	1000
	21st to end	Night	30% 14% 33% 20% 3	1100

Page generated at 2025-05-08 09:33:59. Version ac07

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