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 2025-02-21.

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	7% 7% 3 7% 4 4 4 3 5% 6% 49%	112533
	Night	8% 8% 3 7% 5% 4 4 4 6% 7% 44%	114753
Transparency	Day	34% 6% 11% 21% 18% 10%	112279
	Night	39% 7% 12% 20% 15% 7%	114596
Seeing	Day	10% 7% 19% 26% 24% 13%	14638
	Night	12% 6% 20% 36% 23% 3	23865

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	14% 20% 8% 14% 6% 5% 6% 4 5% 5% 14%	3639
	11th to 20th	Day	14% 13% 6% 8% 5% 5% 5% 5% 6% 6% 28%	3640
	21st to end	Day	14% 14% 6% 11% 6% 5% 6% 5% 5% 4 23%	4004
February	1st to 10th	Day	12% 14% 5% 13% 8% 5% 5% 4 4 5% 25%	1740
	11th to 20th	Day	11% 12% 4 10% 5% 5% 5% 4 5% 7% 32%	1821
	21st to end	Day	8% 9% 4 9% 7% 5% 7% 5% 7% 8% 32%	1571
March	1st to 10th	Day	6% 8% 3 6% 5% 5% 6% 5% 8% 11% 38%	3138
	11th to 20th	Day	4 6% 2 6% 4 4 4 4 5% 7% 56%	3304
	21st to end	Day	8% 8% 3 9% 7% 7% 7% 5% 7% 9% 30%	4134
April	1st to 10th	Day	8% 7% 2 8% 6% 6% 5% 4 7% 9% 39%	2280
	11th to 20th	Day	6% 6% 2 6% 5% 5% 5% 4 6% 8% 47%	2249
	21st to end	Day	6% 6% 2 6% 6% 4 4 4 6% 8% 49%	2430
May	1st to 10th	Day	8% 5% 2 4 3 3 4 3 6% 8% 53%	4420
	11th to 20th	Day	4 3 1 4 3 3 3 3 5% 7% 63%	4377
	21st to end	Day	6% 6% 2 6% 5% 5% 6% 5% 7% 7% 46%	4813
June	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 6% 7% 49%	2660
	11th to 20th	Day	6% 4 2 5% 4 3 4 4 6% 8% 54%	2660
	21st to end	Day	3 3 1 4 3 3 3 3 4 6% 67%	2660
July	1st to 10th	Day	1 1 2 2 1 1 2 3 4 83%	4420
	11th to 20th	Day	1 1 1 2 2 2 1 1 3 4 83%	4420
	21st to end	Day	1 1 1 2 2 2 3 2 4 4 81%	4790
August	1st to 10th	Day	2 2 1 3 2 2 2 3 4 5% 74%	4400
	11th to 20th	Day	1 1 2 2 2 3 3 4 6% 75%	4050
	21st to end	Day	2 4 1 4 3 3 3 2 4 5% 69%	4180
September	1st to 10th	Day	1 2 1 2 2 3 2 2 4 7% 74%	2280
	11th to 20th	Day	4 4 2 4 4 4 4 4 6% 8% 58%	2237
	21st to end	Day	6% 4 2 5% 3 5% 3 3 5% 7% 57%	2090
October	1st to 10th	Day	5% 4 1 4 3 3 3 3 5% 8% 63%	3216
	11th to 20th	Day	4 4 1 5% 4 5% 6% 5% 8% 10% 48%	3134
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 7% 41%	3101
November	1st to 10th	Day	11% 9% 3 9% 7% 5% 5% 4 6% 7% 34%	1764
	11th to 20th	Day	11% 14% 4 10% 8% 5% 5% 4 6% 6% 26%	1659
	21st to end	Day	9% 14% 6% 14% 7% 8% 6% 5% 5% 5% 21%	1600
December	1st to 10th	Day	8% 9% 6% 15% 11% 8% 7% 4 6% 6% 20%	2620
	11th to 20th	Day	15% 19% 7% 15% 8% 6% 4 3 4 5% 14%	3028
	21st to end	Day	13% 16% 7% 18% 8% 7% 5% 3 4 4 16%	4004
January	1st to 10th	Night	16% 22% 8% 13% 6% 6% 4 3 4 5% 12%	6306
	11th to 20th	Night	11% 12% 4 7% 4 4 5% 5% 7% 9% 32%	6320
	21st to end	Night	12% 14% 7% 11% 5% 5% 5% 4 7% 7% 24%	6952
February	1st to 10th	Night	12% 12% 3 8% 6% 5% 6% 4 6% 6% 31%	2658
	11th to 20th	Night	9% 11% 4 8% 6% 5% 5% 4 6% 7% 35%	2465
	21st to end	Night	7% 10% 4 8% 6% 5% 6% 5% 7% 7% 36%	1884
March	1st to 10th	Night	6% 7% 3 6% 5% 5% 5% 5% 9% 10% 40%	3736
	11th to 20th	Night	5% 6% 2 5% 3 4 3 4 5% 6% 56%	3659
	21st to end	Night	6% 7% 3 8% 8% 6% 7% 7% 9% 9% 31%	3519
April	1st to 10th	Night	11% 10% 3 7% 5% 5% 4 4 6% 8% 38%	1900
	11th to 20th	Night	7% 7% 2 7% 5% 4 3 4 5% 6% 49%	1879
	21st to end	Night	6% 6% 3 7% 5% 4 5% 5% 6% 8% 45%	1744
May	1st to 10th	Night	7% 5% 2 5% 5% 4 4 4 5% 6% 52%	2540
	11th to 20th	Night	5% 3 1 3 3 3 4 3 6% 7% 61%	2516
	21st to end	Night	4 5% 2 5% 4 5% 5% 4 7% 8% 51%	2762
June	1st to 10th	Night	8% 4 1 4 4 4 4 4 6% 8% 51%	1520
	11th to 20th	Night	7% 6% 3 5% 4 4 3 3 5% 6% 55%	1520
	21st to end	Night	3 3 2 4 3 4 4 3 4 5% 64%	1520
July	1st to 10th	Night	1 1 2 1 1 1 2 4 4 83%	2540
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 83%	2540
	21st to end	Night	1 1 1 1 2 2 2 2 4 82%	2761
August	1st to 10th	Night	3 2 1 3 2 2 3 3 3 5% 72%	2529
	11th to 20th	Night	2 2 1 2 2 2 3 2 4 5% 75%	2771
	21st to end	Night	1 2 1 2 3 3 3 3 4 6% 71%	3498
September	1st to 10th	Night	2 2 1 2 2 2 3 2 4 5% 74%	1900
	11th to 20th	Night	4 3 1 4 3 4 4 4 5% 7% 61%	1943
	21st to end	Night	5% 4 1 4 3 3 3 3 5% 9% 60%	2090
October	1st to 10th	Night	5% 3 1 3 3 3 3 3 4 6% 66%	3759
	11th to 20th	Night	6% 4 1 5% 4 4 4 4 7% 9% 52%	3802
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 7% 8% 45%	4457
November	1st to 10th	Night	12% 8% 3 7% 6% 5% 5% 4 6% 6% 38%	2535
	11th to 20th	Night	13% 11% 3 8% 7% 5% 5% 5% 5% 6% 31%	2740
	21st to end	Night	10% 12% 4 8% 5% 5% 6% 4 7% 8% 30%	2800
December	1st to 10th	Night	6% 9% 5% 9% 7% 6% 8% 5% 6% 8% 30%	4560
	11th to 20th	Night	16% 18% 7% 15% 7% 6% 6% 4 5% 5% 12%	5176
	21st to end	Night	14% 16% 5% 11% 7% 6% 5% 4 6% 6% 20%	6952

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	69% 10% 9% 8% 3	3639
	11th to 20th	Day	55% 8% 16% 17% 3	3640
	21st to end	Day	62% 7% 10% 15% 6%	4004
February	1st to 10th	Day	61% 8% 16% 10% 5%	1740
	11th to 20th	Day	51% 9% 15% 18% 7%	1830
	21st to end	Day	46% 10% 16% 19% 8% 1	1571
March	1st to 10th	Day	36% 8% 15% 25% 16% 1	3138
	11th to 20th	Day	29% 5% 17% 38% 12%	3304
	21st to end	Day	44% 10% 19% 18% 9% 1	4134
April	1st to 10th	Day	40% 7% 16% 22% 13% 1	2280
	11th to 20th	Day	34% 6% 14% 25% 15% 5%	2252
	21st to end	Day	32% 6% 14% 27% 17% 3	2430
May	1st to 10th	Day	26% 6% 10% 30% 23% 5%	4420
	11th to 20th	Day	21% 5% 8% 28% 28% 10%	4380
	21st to end	Day	32% 8% 18% 28% 12% 2	4814
June	1st to 10th	Day	34% 5% 9% 26% 20% 5%	2660
	11th to 20th	Day	27% 5% 12% 28% 23% 5%	2660
	21st to end	Day	19% 3 8% 29% 29% 12%	2660
July	1st to 10th	Day	8% 2 4 13% 26% 48%	4351
	11th to 20th	Day	9% 2 4 8% 24% 54%	4345
	21st to end	Day	9% 2 5% 17% 34% 33%	4639
August	1st to 10th	Day	14% 3 5% 12% 37% 29%	4384
	11th to 20th	Day	11% 3 5% 33% 35% 12%	4092
	21st to end	Day	19% 3 7% 27% 33% 11%	4179
September	1st to 10th	Day	12% 3 9% 28% 32% 16%	2280
	11th to 20th	Day	24% 5% 12% 25% 26% 8%	2237
	21st to end	Day	27% 5% 14% 32% 17% 5%	2090
October	1st to 10th	Day	21% 4 11% 37% 24% 3	3216
	11th to 20th	Day	29% 7% 22% 27% 14% 1	3134
	21st to end	Day	41% 7% 14% 27% 11% 1	3101
November	1st to 10th	Day	49% 9% 15% 19% 7% 1	1764
	11th to 20th	Day	58% 8% 15% 14% 5% 1	1659
	21st to end	Day	63% 9% 13% 10% 5%	1600
December	1st to 10th	Day	63% 9% 13% 13% 3	2620
	11th to 20th	Day	73% 7% 10% 8% 2	3028
	21st to end	Day	72% 6% 9% 11% 2	4004
January	1st to 10th	Night	73% 8% 10% 7% 2	6306
	11th to 20th	Night	47% 9% 19% 21% 4	6320
	21st to end	Night	56% 8% 13% 16% 7%	6952
February	1st to 10th	Night	52% 9% 18% 15% 4 1	2658
	11th to 20th	Night	46% 9% 16% 21% 7% 1	2467
	21st to end	Night	45% 9% 16% 20% 8% 2	1884
March	1st to 10th	Night	36% 8% 13% 29% 14% 1	3736
	11th to 20th	Night	26% 6% 14% 36% 17%	3659
	21st to end	Night	42% 12% 16% 23% 6% 1	3519
April	1st to 10th	Night	43% 6% 15% 22% 13% 1	1900
	11th to 20th	Night	35% 6% 10% 25% 17% 6%	1879
	21st to end	Night	34% 8% 14% 23% 17% 5%	1744
May	1st to 10th	Night	29% 6% 8% 27% 26% 4	2540
	11th to 20th	Night	19% 7% 9% 29% 30% 6%	2516
	21st to end	Night	26% 10% 19% 33% 11% 2	2762
June	1st to 10th	Night	30% 7% 12% 25% 21% 6%	1520
	11th to 20th	Night	30% 5% 8% 28% 24% 5%	1520
	21st to end	Night	22% 3 7% 24% 28% 15%	1520
July	1st to 10th	Night	7% 2 4 14% 24% 50%	2500
	11th to 20th	Night	7% 1 4 10% 22% 56%	2492
	21st to end	Night	8% 2 3 13% 30% 44%	2674
August	1st to 10th	Night	16% 2 3 9% 33% 37%	2521
	11th to 20th	Night	13% 3 5% 29% 33% 17%	2795
	21st to end	Night	15% 3 6% 26% 31% 19%	3498
September	1st to 10th	Night	14% 3 8% 22% 31% 22%	1900
	11th to 20th	Night	21% 6% 10% 23% 26% 14%	1943
	21st to end	Night	22% 5% 11% 29% 24% 9%	2090
October	1st to 10th	Night	21% 3 9% 33% 32% 2	3759
	11th to 20th	Night	28% 5% 16% 27% 22% 2	3802
	21st to end	Night	35% 7% 12% 30% 15% 2	4457
November	1st to 10th	Night	46% 8% 16% 21% 9% 1	2535
	11th to 20th	Night	51% 10% 14% 19% 6% 1	2740
	21st to end	Night	50% 10% 19% 15% 6% 1	2800
December	1st to 10th	Night	48% 13% 17% 18% 4	4560
	11th to 20th	Night	74% 9% 11% 5% 1	5176
	21st to end	Night	64% 8% 13% 11% 5%	6952

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	26% 10% 22% 24% 15% 4	200
	11th to 20th	Day	26% 7% 22% 26% 15% 5%	200
	21st to end	Day	17% 8% 20% 26% 24% 5%	220
February	1st to 10th	Day	21% 13% 21% 27% 14% 5%	340
	11th to 20th	Day	18% 10% 22% 26% 19% 5%	429
	21st to end	Day	14% 12% 26% 26% 19% 4	383
March	1st to 10th	Day	17% 13% 27% 24% 15% 5%	460
	11th to 20th	Day	12% 10% 20% 28% 20% 10%	460
	21st to end	Day	13% 12% 28% 28% 17% 3	506
April	1st to 10th	Day	12% 15% 27% 23% 15% 8%	460
	11th to 20th	Day	9% 14% 29% 25% 15% 8%	452
	21st to end	Day	6% 12% 27% 28% 19% 8%	460
May	1st to 10th	Day	7% 8% 26% 26% 23% 10%	463
	11th to 20th	Day	11% 7% 20% 26% 23% 14%	488
	21st to end	Day	11% 4 19% 26% 26% 15%	529
June	1st to 10th	Day	10% 9% 17% 23% 25% 17%	490
	11th to 20th	Day	7% 10% 24% 31% 19% 10%	490
	21st to end	Day	2 5% 18% 24% 31% 19%	490
July	1st to 10th	Day	2 2 11% 27% 43% 15%	486
	11th to 20th	Day	1 3 14% 29% 35% 19%	484
	21st to end	Day	1 2 10% 24% 36% 28%	531
August	1st to 10th	Day	3 3 9% 22% 38% 25%	487
	11th to 20th	Day	1 2 9% 22% 33% 32%	477
	21st to end	Day	4 5% 17% 24% 27% 23%	539
September	1st to 10th	Day	3 2 11% 27% 33% 23%	490
	11th to 20th	Day	5% 4 14% 28% 27% 21%	490
	21st to end	Day	7% 7% 16% 27% 29% 14%	490
October	1st to 10th	Day	10% 5% 22% 22% 26% 15%	490
	11th to 20th	Day	11% 8% 16% 25% 27% 13%	475
	21st to end	Day	18% 8% 21% 32% 13% 8%	330
November	1st to 10th	Day	20% 10% 21% 27% 19% 3	300
	11th to 20th	Day	24% 8% 24% 21% 18% 4	229
	21st to end	Day	13% 8% 19% 29% 26% 7%	200
December	1st to 10th	Day	15% 9% 22% 28% 14% 14%	200
	11th to 20th	Day	20% 8% 24% 34% 13% 3	200
	21st to end	Day	19% 9% 25% 33% 11% 4	220
January	1st to 10th	Night	27% 9% 21% 27% 14% 2	800
	11th to 20th	Night	20% 7% 18% 36% 18% 2	800
	21st to end	Night	18% 5% 15% 38% 22% 2	880
February	1st to 10th	Night	17% 9% 22% 32% 18% 2	800
	11th to 20th	Night	13% 8% 23% 34% 20% 2	784
	21st to end	Night	11% 14% 22% 37% 15% 1	628
March	1st to 10th	Night	13% 6% 26% 35% 17% 2	760
	11th to 20th	Night	10% 9% 22% 35% 23% 2	760
	21st to end	Night	11% 9% 31% 34% 14% 1	836
April	1st to 10th	Night	15% 8% 23% 36% 16% 2	760
	11th to 20th	Night	11% 9% 29% 30% 18% 3	751
	21st to end	Night	7% 9% 28% 35% 19% 2	604
May	1st to 10th	Night	7% 7% 26% 37% 21% 3	380
	11th to 20th	Night	10% 4 17% 37% 28% 4	374
	21st to end	Night	6% 4 22% 40% 24% 4	411
June	1st to 10th	Night	9% 3 27% 35% 22% 3	380
	11th to 20th	Night	9% 7% 31% 37% 16% 1	380
	21st to end	Night	3 4 22% 41% 28% 2	380
July	1st to 10th	Night	4 2 16% 46% 29% 2	370
	11th to 20th	Night	3 3 19% 48% 26% 2	368
	21st to end	Night	1 2 14% 45% 35% 3	416
August	1st to 10th	Night	3 2 13% 42% 35% 4	378
	11th to 20th	Night	3 1 13% 37% 38% 10%	533
	21st to end	Night	3 2 15% 41% 33% 5%	836
September	1st to 10th	Night	3 2 15% 40% 34% 6%	760
	11th to 20th	Night	4 1 14% 35% 38% 8%	760
	21st to end	Night	6% 3 17% 34% 33% 6%	760
October	1st to 10th	Night	10% 4 15% 37% 30% 3	760
	11th to 20th	Night	11% 5% 19% 33% 26% 7%	760
	21st to end	Night	11% 7% 22% 35% 21% 5%	836
November	1st to 10th	Night	16% 6% 21% 38% 18% 1	780
	11th to 20th	Night	18% 9% 22% 33% 16% 3	800
	21st to end	Night	15% 7% 19% 37% 20% 3	800
December	1st to 10th	Night	14% 7% 19% 36% 21% 4	800
	11th to 20th	Night	22% 7% 18% 36% 16% 1	800
	21st to end	Night	23% 7% 23% 34% 13% 1	880

Page generated at 2025-02-18 00:26:26. Version ac07

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