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 2026-09-14.

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% 5% 5% 4 6% 8% 36%	148754
	Night	14% 10% 4 8% 5% 4 4 4 5% 7% 36%	142612
Transparency	Day	46% 5% 12% 23% 13% 1	152293
	Night	47% 4 11% 22% 14% 2	147150
Seeing	Day	17% 11% 29% 25% 14% 5%	18333
	Night	18% 10% 30% 29% 12% 1	28078

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% 14% 8% 14% 7% 5% 5% 4 6% 7% 17%	4691
	11th to 20th	Day	10% 11% 7% 15% 7% 7% 6% 5% 6% 4 22%	4710
	21st to end	Day	15% 16% 5% 11% 6% 4 4 4 5% 6% 25%	5156
February	1st to 10th	Day	15% 14% 5% 12% 6% 6% 4 4 4 7% 24%	2027
	11th to 20th	Day	13% 12% 5% 9% 6% 4 4 4 5% 6% 30%	2048
	21st to end	Day	15% 14% 5% 10% 5% 5% 5% 3 6% 5% 28%	1730
March	1st to 10th	Day	18% 12% 4 9% 4 3 3 2 4 5% 35%	4580
	11th to 20th	Day	9% 9% 5% 7% 4 4 5% 4 5% 8% 41%	4803
	21st to end	Day	19% 12% 6% 11% 6% 6% 5% 3 5% 6% 22%	6004
April	1st to 10th	Day	18% 12% 3 8% 5% 4 4 4 6% 7% 30%	2520
	11th to 20th	Day	16% 10% 3 7% 4 4 3 4 5% 7% 36%	2494
	21st to end	Day	18% 9% 3 7% 4 4 4 4 5% 7% 34%	2685
May	1st to 10th	Day	18% 7% 2 7% 4 5% 5% 4 6% 8% 34%	7616
	11th to 20th	Day	13% 7% 3 7% 4 5% 5% 4 6% 8% 39%	7718
	21st to end	Day	13% 7% 3 7% 4 4 4 4 5% 9% 40%	9047
June	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 44%	2940
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 8% 45%	2940
	21st to end	Day	10% 5% 2 6% 6% 4 6% 5% 7% 10% 40%	2940
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 5% 5% 8% 10% 39%	8280
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 6% 11% 45%	8280
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 8% 40%	8990
August	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 6% 8% 43%	6154
	11th to 20th	Day	13% 7% 3 8% 5% 4 5% 4 6% 7% 39%	5670
	21st to end	Day	5% 4 2 6% 4 4 4 4 6% 9% 50%	6080
September	1st to 10th	Day	9% 5% 2 6% 4 5% 4 4 6% 8% 47%	2520
	11th to 20th	Day	9% 6% 2 5% 3 4 4 4 6% 8% 48%	2254
	21st to end	Day	10% 7% 2 7% 5% 4 4 4 6% 9% 42%	2200
October	1st to 10th	Day	11% 6% 3 6% 4 3 5% 4 5% 7% 46%	2200
	11th to 20th	Day	18% 7% 3 7% 5% 5% 4 4 6% 6% 36%	1968
	21st to end	Day	19% 9% 3 8% 5% 5% 5% 3 6% 7% 31%	1980
November	1st to 10th	Day	12% 8% 3 6% 5% 4 4 5% 6% 8% 41%	1852
	11th to 20th	Day	18% 11% 3 8% 5% 5% 4 3 5% 7% 31%	1890
	21st to end	Day	18% 13% 4 10% 5% 4 4 2 4 5% 31%	1890
December	1st to 10th	Day	10% 13% 3 11% 6% 4 4 4 6% 7% 32%	2739
	11th to 20th	Day	13% 16% 7% 11% 6% 5% 5% 4 6% 7% 21%	3088
	21st to end	Day	26% 21% 6% 13% 5% 4 4 3 4 4 10%	4070
January	1st to 10th	Night	13% 15% 8% 14% 6% 4 5% 4 5% 6% 22%	8232
	11th to 20th	Night	11% 15% 7% 15% 7% 6% 5% 4 6% 5% 19%	8280
	21st to end	Night	16% 14% 5% 10% 5% 5% 4 4 5% 5% 28%	9043
February	1st to 10th	Night	15% 14% 5% 10% 6% 5% 3 4 5% 6% 28%	2592
	11th to 20th	Night	15% 10% 4 8% 6% 5% 4 3 5% 7% 34%	2458
	21st to end	Night	19% 13% 4 9% 4 4 4 3 5% 5% 32%	2076
March	1st to 10th	Night	16% 10% 5% 8% 4 4 4 3 5% 6% 35%	5493
	11th to 20th	Night	12% 7% 2 6% 4 4 4 4 5% 7% 44%	5376
	21st to end	Night	18% 10% 5% 9% 5% 5% 5% 4 5% 6% 30%	5199
April	1st to 10th	Night	15% 9% 3 7% 5% 4 5% 4 6% 7% 35%	2100
	11th to 20th	Night	18% 9% 2 7% 5% 4 4 3 4 6% 38%	2075
	21st to end	Night	16% 8% 3 8% 5% 5% 5% 4 4 6% 38%	1933
May	1st to 10th	Night	17% 9% 3 8% 5% 5% 4 3 4 7% 37%	5237
	11th to 20th	Night	13% 6% 2 6% 4 3 4 3 6% 9% 43%	5144
	21st to end	Night	15% 7% 2 7% 4 4 4 4 5% 8% 41%	5139
June	1st to 10th	Night	10% 5% 2 6% 3 3 4 3 6% 9% 48%	1680
	11th to 20th	Night	9% 6% 2 6% 5% 5% 5% 4 6% 9% 44%	1680
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 5% 6% 9% 42%	1680
July	1st to 10th	Night	8% 6% 2 7% 4 4 4 4 7% 9% 46%	4708
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 5% 9% 51%	4707
	21st to end	Night	7% 5% 3 6% 5% 5% 6% 5% 7% 8% 44%	5117
August	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 6% 9% 43%	3999
	11th to 20th	Night	12% 7% 3 6% 4 5% 5% 4 6% 7% 42%	4372
	21st to end	Night	8% 4 3 5% 4 4 4 4 5% 8% 52%	5046
September	1st to 10th	Night	10% 5% 3 5% 3 4 3 4 5% 7% 52%	2100
	11th to 20th	Night	7% 4 1 5% 3 4 3 3 6% 8% 55%	2191
	21st to end	Night	10% 7% 2 6% 4 5% 5% 3 5% 8% 45%	2200
October	1st to 10th	Night	11% 5% 3 6% 3 4 4 3 5% 6% 51%	2200
	11th to 20th	Night	17% 8% 3 6% 3 3 4 3 4 6% 42%	2428
	21st to end	Night	20% 8% 2 7% 4 4 3 3 5% 6% 37%	2860
November	1st to 10th	Night	13% 9% 3 7% 4 4 4 4 6% 6% 41%	2666
	11th to 20th	Night	15% 10% 3 9% 5% 4 5% 4 5% 6% 34%	2730
	21st to end	Night	19% 10% 4 8% 5% 4 4 4 5% 5% 30%	2730
December	1st to 10th	Night	13% 10% 3 7% 6% 4 4 4 5% 7% 36%	4652
	11th to 20th	Night	17% 13% 5% 11% 6% 5% 5% 4 5% 6% 23%	5339
	21st to end	Night	25% 19% 6% 11% 6% 5% 4 3 4 5% 13%	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	65% 4 13% 14% 4	4692
	11th to 20th	Day	64% 2 7% 14% 10% 2	4710
	21st to end	Day	60% 3 13% 16% 7%	5156
February	1st to 10th	Day	61% 4 12% 15% 7% 1	2027
	11th to 20th	Day	53% 5% 11% 22% 8% 1	2058
	21st to end	Day	57% 6% 11% 18% 6% 2	1730
March	1st to 10th	Day	53% 3 8% 23% 10% 2	4580
	11th to 20th	Day	41% 5% 11% 32% 9% 1	4803
	21st to end	Day	63% 6% 12% 13% 5%	6006
April	1st to 10th	Day	52% 5% 11% 20% 12%	2520
	11th to 20th	Day	48% 5% 11% 22% 12% 3	2494
	21st to end	Day	48% 6% 13% 19% 11% 2	2685
May	1st to 10th	Day	47% 5% 11% 23% 13% 1	7651
	11th to 20th	Day	44% 5% 12% 23% 15% 1	7722
	21st to end	Day	41% 4 11% 22% 17% 4	9061
June	1st to 10th	Day	37% 7% 11% 24% 18% 2	2940
	11th to 20th	Day	35% 8% 10% 24% 20% 3	2940
	21st to end	Day	39% 9% 14% 20% 16% 2	2940
July	1st to 10th	Day	39% 4 14% 25% 16% 2	8211
	11th to 20th	Day	35% 3 14% 31% 16% 1	8205
	21st to end	Day	41% 4 14% 24% 16% 2	8794
August	1st to 10th	Day	40% 4 13% 25% 17% 1	6139
	11th to 20th	Day	44% 5% 12% 26% 12% 1	5709
	21st to end	Day	31% 4 14% 33% 16% 3	6079
September	1st to 10th	Day	34% 7% 13% 26% 16% 4	2520
	11th to 20th	Day	31% 6% 12% 27% 21% 3	2254
	21st to end	Day	38% 6% 11% 24% 19% 2	2200
October	1st to 10th	Day	28% 4 8% 32% 27% 1	3590
	11th to 20th	Day	38% 3 11% 23% 23%	3171
	21st to end	Day	46% 4 9% 26% 13% 2	3180
November	1st to 10th	Day	40% 7% 14% 23% 15% 2	1852
	11th to 20th	Day	52% 5% 11% 22% 9% 1	1890
	21st to end	Day	56% 4 10% 21% 8%	1890
December	1st to 10th	Day	51% 3 16% 24% 5%	2739
	11th to 20th	Day	63% 3 14% 16% 4	3085
	21st to end	Day	79% 4 7% 8% 2	4070
January	1st to 10th	Night	63% 3 10% 18% 5%	8233
	11th to 20th	Night	67% 3 9% 11% 9% 1	8280
	21st to end	Night	58% 3 12% 19% 8%	9043
February	1st to 10th	Night	58% 4 11% 17% 8% 2	2592
	11th to 20th	Night	50% 5% 12% 21% 11% 1	2459
	21st to end	Night	54% 5% 11% 18% 10% 2	2076
March	1st to 10th	Night	49% 4 10% 26% 10% 1	5493
	11th to 20th	Night	39% 4 12% 28% 14% 2	5376
	21st to end	Night	54% 6% 10% 21% 9% 1	5199
April	1st to 10th	Night	46% 6% 12% 21% 13% 1	2100
	11th to 20th	Night	48% 5% 8% 20% 16% 3	2075
	21st to end	Night	47% 5% 10% 22% 13% 2	1933
May	1st to 10th	Night	49% 4 9% 21% 14% 3	5264
	11th to 20th	Night	38% 5% 13% 24% 18% 2	5145
	21st to end	Night	42% 4 9% 18% 21% 5%	5149
June	1st to 10th	Night	33% 6% 13% 26% 20% 2	1680
	11th to 20th	Night	35% 6% 12% 22% 22% 2	1680
	21st to end	Night	38% 7% 15% 22% 15% 3	1680
July	1st to 10th	Night	36% 4 11% 29% 16% 4	4670
	11th to 20th	Night	32% 3 13% 25% 25% 2	4662
	21st to end	Night	37% 3 14% 24% 17% 4	5002
August	1st to 10th	Night	38% 5% 13% 27% 17% 1	3990
	11th to 20th	Night	42% 5% 14% 23% 15% 1	4399
	21st to end	Night	32% 3 11% 34% 18% 2	5046
September	1st to 10th	Night	32% 6% 11% 26% 20% 5%	2100
	11th to 20th	Night	27% 5% 7% 29% 27% 3	2191
	21st to end	Night	37% 6% 10% 26% 19% 3	2200
October	1st to 10th	Night	23% 4 8% 34% 29% 2	3590
	11th to 20th	Night	40% 3 7% 22% 26% 1	3964
	21st to end	Night	40% 4 8% 27% 19% 2	4610
November	1st to 10th	Night	42% 5% 11% 24% 16% 2	2666
	11th to 20th	Night	49% 5% 12% 22% 10% 2	2730
	21st to end	Night	54% 5% 11% 22% 8%	2730
December	1st to 10th	Night	47% 3 15% 26% 8%	4652
	11th to 20th	Night	61% 3 14% 14% 7% 1	5341
	21st to end	Night	74% 3 9% 10% 4	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	21% 14% 36% 23% 5% 1	330
	11th to 20th	Day	22% 22% 37% 14% 4 1	330
	21st to end	Day	22% 21% 34% 19% 4	363
February	1st to 10th	Day	18% 21% 38% 18% 4 1	330
	11th to 20th	Day	22% 19% 37% 19% 3	330
	21st to end	Day	23% 19% 42% 12% 4	273
March	1st to 10th	Day	23% 20% 38% 16% 3	330
	11th to 20th	Day	23% 18% 35% 18% 6%	409
	21st to end	Day	27% 17% 32% 18% 5% 1	594
April	1st to 10th	Day	26% 13% 33% 21% 6% 1	540
	11th to 20th	Day	22% 19% 31% 16% 10% 2	532
	21st to end	Day	24% 13% 33% 20% 9% 2	540
May	1st to 10th	Day	23% 14% 31% 21% 10% 2	543
	11th to 20th	Day	20% 11% 31% 21% 14% 3	594
	21st to end	Day	14% 8% 31% 25% 16% 7%	844
June	1st to 10th	Day	13% 9% 28% 28% 18% 5%	780
	11th to 20th	Day	12% 7% 33% 28% 17% 3	780
	21st to end	Day	12% 8% 32% 31% 13% 5%	780
July	1st to 10th	Day	9% 4 20% 34% 21% 13%	771
	11th to 20th	Day	10% 5% 23% 31% 22% 10%	768
	21st to end	Day	10% 4 23% 34% 22% 7%	849
August	1st to 10th	Day	10% 3 18% 30% 27% 13%	629
	11th to 20th	Day	12% 3 20% 29% 25% 10%	559
	21st to end	Day	10% 3 17% 33% 27% 9%	627
September	1st to 10th	Day	11% 6% 21% 32% 24% 7%	570
	11th to 20th	Day	9% 4 16% 27% 28% 15%	542
	21st to end	Day	14% 10% 23% 25% 19% 9%	530
October	1st to 10th	Day	13% 7% 28% 31% 17% 5%	530
	11th to 20th	Day	18% 20% 25% 27% 9% 1	360
	21st to end	Day	28% 14% 37% 17% 4 1	363
November	1st to 10th	Day	17% 13% 37% 23% 8% 2	330
	11th to 20th	Day	22% 19% 33% 18% 6% 1	330
	21st to end	Day	28% 13% 40% 16% 3	330
December	1st to 10th	Day	16% 22% 38% 18% 5%	330
	11th to 20th	Day	20% 20% 38% 18% 3 1	330
	21st to end	Day	28% 19% 38% 13% 2	363
January	1st to 10th	Night	21% 15% 37% 22% 4	1050
	11th to 20th	Night	22% 16% 38% 20% 4 1	1050
	21st to end	Night	22% 15% 36% 24% 3	1155
February	1st to 10th	Night	22% 13% 38% 23% 3	912
	11th to 20th	Night	21% 15% 35% 26% 4	823
	21st to end	Night	25% 15% 38% 20% 2	692
March	1st to 10th	Night	21% 14% 37% 22% 6%	840
	11th to 20th	Night	18% 12% 40% 24% 6%	840
	21st to end	Night	23% 12% 35% 25% 5%	755
April	1st to 10th	Night	18% 14% 38% 24% 5% 1	630
	11th to 20th	Night	23% 9% 35% 25% 7% 1	623
	21st to end	Night	19% 11% 32% 29% 8% 1	630
May	1st to 10th	Night	20% 7% 34% 30% 9% 1	630
	11th to 20th	Night	18% 6% 29% 34% 13% 1	621
	21st to end	Night	13% 4 23% 40% 19% 2	683
June	1st to 10th	Night	11% 4 22% 45% 17%	630
	11th to 20th	Night	11% 3 29% 40% 16% 1	630
	21st to end	Night	11% 3 24% 43% 17% 1	630
July	1st to 10th	Night	9% 2 14% 45% 25% 6%	615
	11th to 20th	Night	8% 2 19% 38% 29% 4	612
	21st to end	Night	6% 2 16% 45% 30% 1	690
August	1st to 10th	Night	8% 1 9% 49% 30% 3	627
	11th to 20th	Night	10% 1 13% 41% 31% 4	624
	21st to end	Night	8% 2 16% 40% 32% 3	693
September	1st to 10th	Night	11% 3 19% 37% 27% 3	630
	11th to 20th	Night	8% 3 18% 37% 29% 4	770
	21st to end	Night	13% 6% 24% 32% 23% 3	800
October	1st to 10th	Night	13% 5% 25% 37% 19% 1	800
	11th to 20th	Night	22% 13% 28% 28% 9%	800
	21st to end	Night	24% 12% 32% 26% 6%	880
November	1st to 10th	Night	19% 12% 37% 22% 10% 1	820
	11th to 20th	Night	20% 18% 33% 23% 5%	840
	21st to end	Night	24% 15% 33% 23% 5%	840
December	1st to 10th	Night	25% 14% 36% 22% 3	1008
	11th to 20th	Night	24% 13% 36% 23% 4	1050
	21st to end	Night	31% 14% 33% 20% 3	1155

Page generated at 2026-09-11 04:11:53. Version ac07

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