Voice Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Voice Observing Site clear sky chart from 2005-11-06 to 2026-06-16.

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	6% 6% 3 6% 4 4 4 4 5% 7% 50%	146728
	Night	7% 8% 3 7% 5% 5% 5% 4 6% 7% 44%	142575
Transparency	Day	34% 5% 11% 22% 20% 9%	141423
	Night	38% 7% 12% 21% 16% 7%	138887
Seeing	Day	10% 8% 19% 26% 24% 13%	16356
	Night	12% 6% 20% 36% 23% 3	25461

By Month

Percentage Cloud Cover SW (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% 18% 7% 13% 7% 5% 6% 4 5% 6% 16%	4722
	11th to 20th	Day	11% 11% 6% 9% 7% 6% 6% 5% 7% 6% 27%	4740
	21st to end	Day	12% 13% 5% 11% 6% 5% 6% 5% 6% 5% 24%	5187
February	1st to 10th	Day	12% 15% 5% 13% 7% 5% 5% 4 4 5% 24%	1827
	11th to 20th	Day	11% 12% 4 10% 5% 5% 5% 3 5% 7% 32%	1915
	21st to end	Day	8% 9% 3 8% 7% 5% 7% 5% 7% 8% 33%	1730
March	1st to 10th	Day	5% 7% 3 7% 5% 6% 6% 6% 9% 10% 37%	4618
	11th to 20th	Day	6% 8% 3 8% 5% 4 4 4 5% 7% 47%	4840
	21st to end	Day	8% 10% 3 9% 7% 6% 6% 5% 7% 9% 30%	6056
April	1st to 10th	Day	7% 7% 2 8% 5% 6% 5% 4 7% 9% 39%	2520
	11th to 20th	Day	6% 6% 2 6% 5% 5% 5% 4 6% 8% 47%	2489
	21st to end	Day	6% 5% 2 6% 5% 4 4 4 6% 8% 51%	2686
May	1st to 10th	Day	5% 4 1 4 3 3 4 3 5% 8% 60%	8195
	11th to 20th	Day	4 4 2 6% 4 4 4 4 6% 8% 53%	8160
	21st to end	Day	5% 4 1 5% 4 4 5% 4 7% 9% 54%	8992
June	1st to 10th	Day	8% 6% 2 6% 4 4 5% 4 5% 7% 50%	2940
	11th to 20th	Day	6% 4 2 5% 4 3 4 4 5% 8% 56%	2871
	21st to end	Day	3 3 2 3 3 3 3 3 4 6% 67%	2800
July	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 79%	6319
	11th to 20th	Day	1 1 1 2 2 2 1 1 2 3 84%	6320
	21st to end	Day	1 1 1 2 2 2 2 2 4 6% 78%	6880
August	1st to 10th	Day	3 3 1 4 3 2 3 2 4 6% 69%	6210
	11th to 20th	Day	2 1 1 2 2 2 3 3 4 6% 75%	5846
	21st to end	Day	3 4 1 4 3 3 3 3 5% 6% 65%	5984
September	1st to 10th	Day	2 2 1 2 2 3 3 2 4 7% 73%	2400
	11th to 20th	Day	4 3 2 4 4 4 4 3 6% 7% 59%	2355
	21st to end	Day	6% 4 2 5% 3 5% 3 2 5% 7% 58%	2200
October	1st to 10th	Day	5% 4 1 4 3 3 3 3 5% 8% 63%	3317
	11th to 20th	Day	4 4 1 5% 4 5% 6% 5% 7% 10% 48%	3233
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 7% 41%	3200
November	1st to 10th	Day	11% 9% 3 9% 7% 5% 5% 4 6% 7% 34%	1854
	11th to 20th	Day	11% 14% 4 11% 8% 6% 6% 4 6% 6% 25%	1742
	21st to end	Day	9% 14% 5% 14% 7% 7% 6% 5% 5% 5% 22%	1680
December	1st to 10th	Day	8% 10% 6% 15% 11% 8% 7% 4 5% 6% 20%	2700
	11th to 20th	Day	15% 19% 7% 15% 8% 6% 5% 3 4 5% 15%	3108
	21st to end	Day	13% 16% 7% 17% 8% 6% 5% 3 4 4 16%	4092
January	1st to 10th	Night	15% 20% 7% 13% 6% 6% 5% 4 5% 5% 14%	8172
	11th to 20th	Night	9% 10% 3 7% 5% 6% 6% 5% 8% 9% 33%	8220
	21st to end	Night	10% 12% 6% 11% 6% 5% 5% 4 7% 8% 25%	8994
February	1st to 10th	Night	12% 13% 4 8% 6% 5% 6% 4 6% 6% 31%	2791
	11th to 20th	Night	9% 11% 4 8% 6% 5% 5% 4 6% 7% 34%	2591
	21st to end	Night	8% 10% 3 8% 6% 5% 5% 5% 6% 7% 37%	2076
March	1st to 10th	Night	5% 6% 3 7% 5% 5% 6% 5% 9% 10% 41%	5496
	11th to 20th	Night	6% 7% 3 9% 5% 5% 4 4 6% 6% 45%	5363
	21st to end	Night	5% 8% 3 9% 8% 7% 6% 6% 8% 9% 31%	5161
April	1st to 10th	Night	10% 9% 2 7% 4 5% 5% 4 6% 8% 39%	2100
	11th to 20th	Night	7% 7% 2 7% 5% 4 3 4 5% 6% 50%	2079
	21st to end	Night	6% 5% 3 7% 5% 3 5% 5% 6% 8% 47%	1928
May	1st to 10th	Night	4 3 1 3 3 3 4 4 6% 8% 60%	4723
	11th to 20th	Night	4 4 2 4 4 4 4 3 6% 8% 58%	4700
	21st to end	Night	5% 4 2 5% 3 4 4 4 7% 9% 53%	5181
June	1st to 10th	Night	7% 4 1 4 5% 4 4 4 6% 7% 52%	1680
	11th to 20th	Night	6% 5% 2 5% 4 4 3 3 5% 6% 56%	1640
	21st to end	Night	3 3 2 4 4 4 4 3 4 5% 63%	1600
July	1st to 10th	Night	2 2 1 2 1 1 2 2 4 5% 79%	3640
	11th to 20th	Night	1 1 1 1 1 2 1 3 4 85%	3640
	21st to end	Night	2 2 1 1 1 1 2 2 3 6% 78%	3970
August	1st to 10th	Night	3 2 1 3 2 3 3 3 5% 7% 66%	3582
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 5% 73%	3983
	21st to end	Night	2 3 1 3 3 3 4 3 5% 7% 65%	5015
September	1st to 10th	Night	2 2 1 3 2 2 3 2 4 5% 73%	2000
	11th to 20th	Night	3 3 1 4 3 4 4 4 5% 7% 62%	2045
	21st to end	Night	5% 3 1 4 3 3 4 3 5% 8% 60%	2200
October	1st to 10th	Night	4 3 1 4 3 3 3 3 4 6% 66%	3878
	11th to 20th	Night	6% 4 1 5% 4 4 4 4 7% 9% 52%	3923
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 7% 9% 46%	4600
November	1st to 10th	Night	12% 8% 3 7% 6% 6% 5% 4 6% 6% 38%	2665
	11th to 20th	Night	13% 11% 3 9% 7% 5% 5% 5% 5% 6% 31%	2877
	21st to end	Night	11% 12% 4 8% 5% 5% 6% 4 7% 8% 30%	2940
December	1st to 10th	Night	6% 10% 5% 9% 7% 6% 8% 5% 6% 8% 30%	4700
	11th to 20th	Night	15% 18% 7% 14% 7% 6% 6% 4 5% 6% 12%	5316
	21st to end	Night	14% 15% 5% 11% 7% 6% 5% 4 6% 6% 20%	7106

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	67% 9% 10% 10% 4	4723
	11th to 20th	Day	55% 8% 16% 18% 4	4740
	21st to end	Day	60% 7% 13% 14% 6%	5187
February	1st to 10th	Day	61% 8% 16% 10% 4	1827
	11th to 20th	Day	51% 9% 15% 17% 8%	1924
	21st to end	Day	46% 10% 15% 20% 8% 1	1730
March	1st to 10th	Day	38% 7% 14% 24% 16% 1	4618
	11th to 20th	Day	37% 4 14% 29% 15% 1	4840
	21st to end	Day	47% 8% 17% 19% 8% 1	6056
April	1st to 10th	Day	39% 7% 16% 24% 14% 1	2520
	11th to 20th	Day	34% 6% 14% 26% 16% 5%	2492
	21st to end	Day	31% 5% 14% 27% 19% 4	2686
May	1st to 10th	Day	22% 4 6% 26% 33% 8%	8195
	11th to 20th	Day	27% 5% 10% 29% 24% 6%	8163
	21st to end	Day	25% 6% 16% 28% 22% 3	8994
June	1st to 10th	Day	32% 5% 9% 25% 22% 6%	2940
	11th to 20th	Day	26% 5% 11% 27% 25% 6%	2871
	21st to end	Day	19% 3 9% 29% 28% 12%	2800
July	1st to 10th	Day	11% 2 3 15% 30% 38%	6251
	11th to 20th	Day	8% 2 3 10% 32% 46%	6245
	21st to end	Day	10% 2 5% 23% 36% 24%	6681
August	1st to 10th	Day	14% 3 5% 13% 37% 29%	4524
	11th to 20th	Day	12% 3 5% 33% 35% 13%	4224
	21st to end	Day	20% 3 7% 27% 32% 11%	4311
September	1st to 10th	Day	13% 3 9% 28% 31% 15%	2400
	11th to 20th	Day	24% 5% 12% 25% 26% 8%	2355
	21st to end	Day	26% 5% 14% 32% 18% 5%	2200
October	1st to 10th	Day	22% 4 11% 37% 23% 3	3317
	11th to 20th	Day	29% 7% 22% 27% 14% 1	3233
	21st to end	Day	41% 7% 14% 26% 11% 1	3200
November	1st to 10th	Day	49% 9% 15% 19% 7% 1	1854
	11th to 20th	Day	58% 8% 15% 13% 4 1	1742
	21st to end	Day	63% 8% 14% 10% 5%	1680
December	1st to 10th	Day	63% 9% 13% 13% 3	2700
	11th to 20th	Day	73% 7% 10% 8% 2	3108
	21st to end	Day	72% 6% 9% 11% 2	4092
January	1st to 10th	Night	70% 8% 11% 8% 3	8172
	11th to 20th	Night	44% 9% 19% 24% 4	8220
	21st to end	Night	55% 8% 15% 16% 6%	8994
February	1st to 10th	Night	52% 9% 19% 16% 4 1	2791
	11th to 20th	Night	46% 9% 16% 21% 7% 1	2593
	21st to end	Night	45% 8% 15% 21% 9% 2	2076
March	1st to 10th	Night	34% 7% 11% 29% 18% 1	5496
	11th to 20th	Night	38% 6% 12% 29% 15%	5363
	21st to end	Night	44% 10% 14% 25% 7% 1	5161
April	1st to 10th	Night	41% 6% 15% 23% 14% 1	2100
	11th to 20th	Night	34% 6% 10% 25% 18% 6%	2079
	21st to end	Night	32% 7% 13% 24% 18% 5%	1928
May	1st to 10th	Night	20% 5% 6% 24% 37% 8%	4721
	11th to 20th	Night	23% 6% 9% 30% 28% 4	4700
	21st to end	Night	26% 7% 15% 28% 22% 2	5182
June	1st to 10th	Night	29% 6% 11% 24% 23% 7%	1680
	11th to 20th	Night	29% 5% 8% 26% 26% 6%	1640
	21st to end	Night	23% 3 8% 24% 28% 14%	1600
July	1st to 10th	Night	10% 2 3 17% 26% 41%	3600
	11th to 20th	Night	7% 1 3 9% 31% 49%	3592
	21st to end	Night	10% 2 4 18% 35% 31%	3857
August	1st to 10th	Night	16% 2 4 10% 32% 36%	2601
	11th to 20th	Night	13% 3 5% 29% 33% 17%	2883
	21st to end	Night	16% 3 6% 25% 31% 19%	3608
September	1st to 10th	Night	14% 3 8% 23% 31% 21%	2000
	11th to 20th	Night	21% 6% 10% 24% 26% 14%	2045
	21st to end	Night	22% 5% 11% 29% 25% 9%	2200
October	1st to 10th	Night	21% 3 9% 34% 31% 2	3878
	11th to 20th	Night	29% 5% 15% 27% 22% 2	3923
	21st to end	Night	34% 6% 13% 30% 15% 2	4600
November	1st to 10th	Night	46% 7% 16% 21% 9% 1	2665
	11th to 20th	Night	52% 10% 14% 18% 6% 1	2877
	21st to end	Night	51% 9% 18% 15% 6% 1	2940
December	1st to 10th	Night	49% 13% 17% 17% 4	4700
	11th to 20th	Night	73% 9% 11% 6% 2	5316
	21st to end	Night	63% 8% 13% 11% 5%	7106

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% 10% 23% 24% 14% 3	220
	11th to 20th	Day	24% 6% 21% 28% 16% 5%	220
	21st to end	Day	17% 8% 19% 29% 23% 4	242
February	1st to 10th	Day	22% 13% 20% 27% 14% 5%	367
	11th to 20th	Day	18% 9% 22% 27% 19% 5%	463
	21st to end	Day	14% 12% 27% 25% 20% 3	446
March	1st to 10th	Day	16% 12% 27% 25% 14% 5%	540
	11th to 20th	Day	14% 11% 21% 26% 18% 9%	540
	21st to end	Day	13% 12% 28% 28% 16% 2	594
April	1st to 10th	Day	10% 14% 25% 24% 17% 9%	540
	11th to 20th	Day	9% 14% 27% 26% 16% 8%	532
	21st to end	Day	6% 10% 25% 28% 21% 9%	540
May	1st to 10th	Day	6% 8% 25% 25% 25% 11%	543
	11th to 20th	Day	12% 8% 21% 26% 21% 13%	568
	21st to end	Day	10% 4 19% 26% 26% 16%	617
June	1st to 10th	Day	9% 10% 17% 23% 24% 17%	570
	11th to 20th	Day	6% 9% 25% 31% 18% 10%	551
	21st to end	Day	2 5% 18% 24% 31% 19%	530
July	1st to 10th	Day	3 2 11% 27% 42% 16%	526
	11th to 20th	Day	1 3 14% 28% 34% 20%	524
	21st to end	Day	1 2 10% 23% 37% 28%	575
August	1st to 10th	Day	3 4 9% 22% 36% 25%	527
	11th to 20th	Day	1 2 10% 22% 34% 31%	517
	21st to end	Day	4 5% 16% 23% 28% 23%	583
September	1st to 10th	Day	3 2 10% 26% 33% 25%	530
	11th to 20th	Day	5% 4 14% 28% 27% 22%	530
	21st to end	Day	7% 7% 16% 26% 29% 15%	530
October	1st to 10th	Day	10% 5% 21% 24% 26% 15%	530
	11th to 20th	Day	11% 8% 16% 26% 26% 12%	514
	21st to end	Day	16% 8% 21% 33% 14% 7%	363
November	1st to 10th	Day	19% 11% 21% 27% 19% 3	330
	11th to 20th	Day	23% 8% 24% 21% 19% 4	252
	21st to end	Day	13% 7% 19% 29% 25% 6%	220
December	1st to 10th	Day	19% 8% 20% 27% 13% 13%	220
	11th to 20th	Day	21% 9% 24% 31% 13% 2	220
	21st to end	Day	19% 8% 25% 32% 12% 4	242
January	1st to 10th	Night	27% 9% 21% 28% 14% 2	840
	11th to 20th	Night	19% 7% 17% 36% 19% 2	840
	21st to end	Night	17% 5% 15% 39% 23% 2	924
February	1st to 10th	Night	17% 9% 22% 32% 19% 2	840
	11th to 20th	Night	13% 8% 23% 34% 20% 2	824
	21st to end	Night	12% 14% 21% 37% 15% 1	692
March	1st to 10th	Night	12% 7% 27% 34% 17% 2	840
	11th to 20th	Night	12% 9% 24% 33% 21% 2	840
	21st to end	Night	11% 10% 31% 34% 14% 1	924
April	1st to 10th	Night	14% 8% 22% 37% 18% 2	840
	11th to 20th	Night	11% 9% 28% 31% 19% 3	831
	21st to end	Night	7% 8% 27% 35% 21% 2	668
May	1st to 10th	Night	6% 6% 24% 38% 23% 3	420
	11th to 20th	Night	9% 5% 19% 37% 26% 4	414
	21st to end	Night	6% 4 20% 41% 25% 4	455
June	1st to 10th	Night	9% 4 26% 36% 22% 3	420
	11th to 20th	Night	8% 7% 31% 38% 16% 1	410
	21st to end	Night	3 4 23% 40% 29% 2	400
July	1st to 10th	Night	4 2 17% 46% 30% 2	390
	11th to 20th	Night	3 3 19% 48% 26% 2	388
	21st to end	Night	1 2 14% 45% 35% 3	438
August	1st to 10th	Night	3 2 13% 43% 35% 4	398
	11th to 20th	Night	2 1 12% 38% 38% 9%	561
	21st to end	Night	3 3 14% 41% 35% 5%	880
September	1st to 10th	Night	3 2 14% 39% 35% 6%	800
	11th to 20th	Night	4 1 13% 36% 38% 7%	800
	21st to end	Night	6% 4 17% 34% 33% 6%	800
October	1st to 10th	Night	10% 4 15% 38% 29% 3	800
	11th to 20th	Night	11% 5% 20% 33% 25% 6%	800
	21st to end	Night	10% 7% 22% 35% 21% 5%	880
November	1st to 10th	Night	15% 7% 21% 39% 17% 1	820
	11th to 20th	Night	18% 9% 21% 33% 16% 3	840
	21st to end	Night	15% 7% 19% 37% 19% 3	840
December	1st to 10th	Night	15% 8% 19% 35% 20% 3	840
	11th to 20th	Night	22% 8% 18% 36% 15% 1	840
	21st to end	Night	22% 7% 23% 33% 13% 1	924

Page generated at 2026-06-12 23:31:43. Version ac07

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