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 2024-11-01.

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% 3 8% 5% 4 4 4 6% 8% 36%	101166
	Night	15% 10% 4 8% 5% 4 4 4 5% 6% 36%	97838
Transparency	Day	46% 6% 12% 21% 13% 2	100957
	Night	48% 6% 12% 21% 13% 2	97719
Seeing	Day	16% 11% 29% 25% 14% 5%	15744
	Night	18% 10% 30% 30% 12% 1	25572

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	15% 15% 11% 12% 6% 5% 4 4 5% 6% 17%	2527
	11th to 20th	Day	14% 14% 7% 14% 8% 6% 5% 6% 8% 4 14%	2530
	21st to end	Day	22% 24% 6% 10% 4 3 4 3 3 5% 15%	2783
February	1st to 10th	Day	15% 15% 4 11% 6% 6% 4 3 4 6% 24%	1833
	11th to 20th	Day	14% 11% 5% 9% 6% 4 4 5% 6% 6% 31%	1848
	21st to end	Day	16% 14% 5% 10% 4 5% 5% 3 6% 5% 27%	1570
March	1st to 10th	Day	18% 15% 5% 8% 4 4 4 3 5% 6% 30%	3120
	11th to 20th	Day	11% 8% 4 7% 5% 4 5% 4 6% 8% 38%	3283
	21st to end	Day	19% 11% 5% 11% 6% 5% 5% 4 5% 7% 23%	4100
April	1st to 10th	Day	16% 12% 3 8% 5% 4 4 4 6% 7% 32%	2280
	11th to 20th	Day	17% 11% 3 7% 4 4 3 3 5% 7% 36%	2254
	21st to end	Day	19% 9% 3 7% 4 4 4 4 5% 6% 34%	2429
May	1st to 10th	Day	18% 7% 2 7% 4 5% 5% 4 6% 9% 34%	4082
	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% 3 4 5% 6% 8% 42%	1672
	11th to 20th	Day	18% 11% 3 7% 5% 5% 4 4 6% 7% 31%	1710
	21st to end	Day	18% 12% 3 9% 5% 4 4 2 5% 5% 32%	1710
December	1st to 10th	Day	16% 16% 4 10% 5% 5% 4 5% 6% 6% 24%	1554
	11th to 20th	Day	13% 16% 7% 11% 6% 5% 5% 3 5% 6% 24%	1918
	21st to end	Day	22% 22% 7% 14% 5% 4 3 3 4 4 12%	2783
January	1st to 10th	Night	17% 16% 8% 12% 5% 4 4 3 4 4 22%	4428
	11th to 20th	Night	15% 19% 7% 14% 5% 4 5% 5% 6% 4 15%	4440
	21st to end	Night	24% 17% 7% 11% 5% 4 3 3 4 4 19%	4883
February	1st to 10th	Night	16% 14% 5% 9% 6% 5% 3 3 5% 6% 28%	2346
	11th to 20th	Night	15% 11% 4 7% 6% 5% 4 4 5% 7% 34%	2218
	21st to end	Night	20% 13% 4 8% 4 4 4 3 5% 4 31%	1884
March	1st to 10th	Night	17% 11% 4 7% 4 4 4 3 5% 6% 34%	3733
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 5% 6% 46%	3676
	21st to end	Night	19% 10% 5% 9% 5% 4 5% 4 5% 6% 28%	3548
April	1st to 10th	Night	14% 9% 3 7% 5% 4 5% 4 5% 7% 37%	1900
	11th to 20th	Night	19% 9% 2 7% 5% 4 4 3 4 5% 39%	1875
	21st to end	Night	17% 8% 3 8% 5% 5% 4 4 4 5% 37%	1749
May	1st to 10th	Night	16% 8% 3 7% 5% 5% 4 3 5% 9% 35%	2813
	11th to 20th	Night	18% 7% 2 7% 3 3 3 4 6% 7% 40%	2761
	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 6% 4 4 3 4 5% 6% 36%	2717
November	1st to 10th	Night	12% 9% 3 7% 4 3 4 4 6% 6% 41%	2413
	11th to 20th	Night	15% 10% 3 9% 4 4 5% 4 5% 6% 34%	2470
	21st to end	Night	19% 10% 4 7% 5% 4 4 4 6% 6% 32%	2470
December	1st to 10th	Night	16% 14% 4 8% 5% 4 4 4 5% 7% 27%	2622
	11th to 20th	Night	16% 14% 4 10% 6% 5% 5% 4 5% 6% 25%	3279
	21st to end	Night	24% 19% 6% 10% 6% 5% 4 3 4 5% 15%	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	66% 6% 11% 12% 4	2528
	11th to 20th	Day	70% 4 9% 11% 6%	2530
	21st to end	Day	72% 5% 8% 12% 3	2783
February	1st to 10th	Day	61% 5% 12% 15% 8% 1	1833
	11th to 20th	Day	52% 6% 11% 22% 9% 1	1858
	21st to end	Day	57% 6% 11% 18% 6% 2	1570
March	1st to 10th	Day	55% 4 11% 19% 9% 1	3120
	11th to 20th	Day	43% 6% 12% 28% 10% 1	3283
	21st to end	Day	60% 9% 14% 14% 3	4100
April	1st to 10th	Day	50% 5% 12% 20% 13%	2280
	11th to 20th	Day	47% 6% 10% 22% 13% 3	2254
	21st to end	Day	48% 7% 13% 18% 12% 2	2429
May	1st to 10th	Day	45% 8% 12% 24% 9% 1	4120
	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	38% 6% 12% 24% 18% 2	2090
	11th to 20th	Day	47% 5% 11% 22% 14% 1	1870
	21st to end	Day	53% 6% 12% 19% 9% 1	1881
November	1st to 10th	Day	39% 7% 15% 22% 15% 2	1672
	11th to 20th	Day	51% 5% 11% 22% 9% 1	1710
	21st to end	Day	55% 4 11% 21% 8%	1710
December	1st to 10th	Day	59% 6% 13% 16% 6%	1554
	11th to 20th	Day	60% 4 12% 19% 4	1915
	21st to end	Day	76% 5% 9% 7% 2	2783
January	1st to 10th	Night	64% 6% 11% 14% 5%	4428
	11th to 20th	Night	71% 5% 11% 10% 4	4440
	21st to end	Night	69% 4 10% 13% 5%	4883
February	1st to 10th	Night	57% 4 12% 17% 8% 2	2346
	11th to 20th	Night	49% 5% 13% 21% 11% 1	2219
	21st to end	Night	55% 5% 10% 18% 10% 2	1884
March	1st to 10th	Night	49% 6% 12% 22% 10% 1	3733
	11th to 20th	Night	37% 6% 11% 27% 17% 2	3676
	21st to end	Night	54% 9% 12% 20% 5%	3548
April	1st to 10th	Night	45% 7% 13% 21% 13% 1	1900
	11th to 20th	Night	47% 5% 7% 20% 17% 3	1875
	21st to end	Night	48% 6% 10% 19% 14% 3	1749
May	1st to 10th	Night	47% 7% 11% 22% 12% 2	2840
	11th to 20th	Night	41% 8% 14% 26% 11%	2762
	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	34% 6% 10% 27% 22% 1	2090
	11th to 20th	Night	43% 5% 9% 24% 18% 2	2306
	21st to end	Night	48% 5% 10% 23% 12% 1	2717
November	1st to 10th	Night	41% 5% 11% 23% 17% 2	2413
	11th to 20th	Night	49% 6% 12% 22% 10% 1	2470
	21st to end	Night	52% 5% 12% 23% 8%	2470
December	1st to 10th	Night	55% 6% 14% 18% 7% 1	2622
	11th to 20th	Night	57% 5% 14% 18% 6% 1	3281
	21st to end	Night	72% 5% 10% 10% 3	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	23% 14% 33% 24% 6% 1	270
	11th to 20th	Day	23% 19% 36% 15% 5% 1	270
	21st to end	Day	24% 18% 33% 20% 5%	297
February	1st to 10th	Day	20% 20% 37% 18% 5%	270
	11th to 20th	Day	23% 18% 35% 20% 4	270
	21st to end	Day	24% 19% 40% 14% 4	225
March	1st to 10th	Day	23% 21% 36% 17% 3	270
	11th to 20th	Day	21% 18% 35% 19% 7%	341
	21st to end	Day	27% 16% 30% 20% 6% 1	506
April	1st to 10th	Day	24% 13% 32% 22% 7% 1	460
	11th to 20th	Day	21% 17% 32% 16% 11% 2	452
	21st to end	Day	24% 14% 32% 20% 8% 2	460
May	1st to 10th	Day	22% 13% 30% 22% 10% 3	463
	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% 14% 37% 16% 4 1	330
November	1st to 10th	Day	15% 12% 38% 23% 9% 3	270
	11th to 20th	Day	21% 19% 34% 17% 7% 1	270
	21st to end	Day	27% 11% 39% 20% 3	270
December	1st to 10th	Day	18% 20% 37% 18% 6%	270
	11th to 20th	Day	19% 19% 40% 19% 4 1	270
	21st to end	Day	26% 22% 36% 14% 2	297
January	1st to 10th	Night	22% 15% 36% 22% 5%	950
	11th to 20th	Night	23% 15% 37% 21% 4 1	950
	21st to end	Night	22% 14% 34% 25% 4	1045
February	1st to 10th	Night	23% 13% 38% 23% 3	826
	11th to 20th	Night	21% 14% 35% 25% 4	743
	21st to end	Night	26% 15% 36% 19% 2	628
March	1st to 10th	Night	21% 13% 37% 23% 6%	760
	11th to 20th	Night	17% 12% 40% 25% 6%	760
	21st to end	Night	23% 11% 33% 26% 6%	683
April	1st to 10th	Night	17% 13% 39% 24% 6% 1	570
	11th to 20th	Night	23% 8% 35% 26% 7% 1	563
	21st to end	Night	20% 11% 33% 28% 8% 1	570
May	1st to 10th	Night	20% 7% 34% 29% 9% 1	570
	11th to 20th	Night	19% 5% 27% 35% 14% 1	561
	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	18% 12% 37% 22% 10% 1	743
	11th to 20th	Night	20% 18% 33% 24% 6%	760
	21st to end	Night	23% 14% 34% 23% 6%	760
December	1st to 10th	Night	24% 15% 35% 22% 3	912
	11th to 20th	Night	23% 12% 35% 25% 4	950
	21st to end	Night	30% 14% 33% 20% 3	1045

Page generated at 2024-10-29 11:20:45. Version ac07

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