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 2024-09-29.

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% 2 6% 4 4 4 3 5% 6% 51%	101198
	Night	8% 9% 3 7% 5% 4 4 4 5% 6% 45%	97158
Transparency	Day	33% 6% 11% 21% 18% 11%	100944
	Night	38% 7% 12% 20% 15% 8%	97001
Seeing	Day	10% 7% 20% 26% 24% 13%	14261
	Night	12% 6% 21% 36% 23% 3	23289

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	12% 21% 6% 12% 6% 5% 6% 4 5% 6% 17%	2539
	11th to 20th	Day	20% 18% 7% 10% 4 4 4 4 5% 5% 19%	2540
	21st to end	Day	20% 20% 8% 14% 8% 5% 5% 3 3 3 11%	2794
February	1st to 10th	Day	12% 14% 5% 13% 7% 6% 5% 4 4 5% 25%	1653
	11th to 20th	Day	11% 11% 4 10% 5% 5% 5% 4 5% 8% 32%	1727
	21st to end	Day	8% 9% 4 9% 7% 5% 7% 5% 7% 8% 32%	1570
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%	2069
October	1st to 10th	Day	7% 6% 1 5% 4 3 4 3 5% 8% 52%	1823
	11th to 20th	Day	7% 5% 1 6% 4 4 5% 4 7% 9% 49%	1786
	21st to end	Day	13% 7% 2 6% 5% 6% 5% 4 6% 7% 40%	1782
November	1st to 10th	Day	11% 9% 3 9% 7% 5% 5% 4 6% 7% 34%	1674
	11th to 20th	Day	10% 15% 4 10% 8% 6% 5% 4 6% 6% 26%	1577
	21st to end	Day	10% 14% 5% 13% 7% 8% 6% 5% 5% 5% 22%	1520
December	1st to 10th	Day	9% 13% 5% 14% 9% 7% 5% 4 6% 6% 22%	1520
	11th to 20th	Day	14% 19% 8% 16% 8% 5% 6% 3 4 5% 13%	1928
	21st to end	Day	12% 15% 7% 17% 9% 7% 6% 4 4 4 15%	2794
January	1st to 10th	Night	14% 20% 5% 11% 6% 6% 5% 4 5% 6% 17%	4406
	11th to 20th	Night	16% 16% 5% 9% 5% 5% 5% 5% 6% 7% 21%	4420
	21st to end	Night	16% 16% 8% 15% 7% 6% 5% 3 5% 5% 15%	4862
February	1st to 10th	Night	12% 13% 3 7% 6% 5% 6% 4 6% 6% 32%	2526
	11th to 20th	Night	9% 11% 4 8% 6% 6% 5% 4 6% 7% 36%	2339
	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 4 3 5% 9% 60%	2079
October	1st to 10th	Night	8% 6% 2 5% 5% 3 4 3 5% 6% 54%	2132
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 6% 6% 51%	2173
	21st to end	Night	9% 6% 2 7% 5% 4 5% 4 6% 8% 45%	2574
November	1st to 10th	Night	13% 8% 3 7% 6% 5% 5% 3 6% 6% 38%	2405
	11th to 20th	Night	12% 11% 3 8% 7% 5% 5% 5% 5% 6% 31%	2603
	21st to end	Night	10% 11% 4 8% 5% 5% 6% 4 7% 8% 31%	2660
December	1st to 10th	Night	10% 15% 6% 11% 7% 6% 5% 4 5% 6% 25%	2660
	11th to 20th	Night	14% 18% 7% 14% 7% 6% 5% 4 5% 6% 15%	3276
	21st to end	Night	12% 17% 6% 12% 7% 6% 5% 4 6% 6% 20%	4862

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% 11% 10% 9% 4	2539
	11th to 20th	Day	67% 7% 15% 9% 2	2540
	21st to end	Day	79% 6% 8% 5% 1	2794
February	1st to 10th	Day	60% 8% 17% 10% 4	1653
	11th to 20th	Day	50% 9% 15% 18% 7%	1736
	21st to end	Day	46% 10% 16% 19% 8% 1	1570
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% 15% 32% 17% 4	2069
October	1st to 10th	Day	31% 5% 13% 31% 18% 2	1823
	11th to 20th	Day	30% 8% 21% 29% 11% 2	1786
	21st to end	Day	43% 8% 17% 22% 9% 1	1782
November	1st to 10th	Day	50% 9% 15% 18% 7% 1	1674
	11th to 20th	Day	58% 8% 14% 14% 5% 1	1577
	21st to end	Day	63% 9% 13% 10% 5%	1520
December	1st to 10th	Day	63% 9% 12% 12% 4	1520
	11th to 20th	Day	74% 8% 10% 6% 2	1928
	21st to end	Day	72% 7% 10% 9% 2	2794
January	1st to 10th	Night	66% 10% 13% 9% 3	4406
	11th to 20th	Night	58% 10% 17% 14% 2	4420
	21st to end	Night	71% 9% 11% 7% 2	4862
February	1st to 10th	Night	51% 9% 19% 16% 4 1	2526
	11th to 20th	Night	45% 9% 16% 21% 8%	2341
	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%	2079
October	1st to 10th	Night	31% 5% 10% 26% 27% 2	2132
	11th to 20th	Night	34% 5% 12% 29% 18% 2	2173
	21st to end	Night	36% 8% 13% 26% 15% 2	2574
November	1st to 10th	Night	46% 8% 15% 21% 9% 1	2405
	11th to 20th	Night	51% 10% 14% 19% 5% 1	2603
	21st to end	Night	49% 9% 19% 16% 6% 1	2660
December	1st to 10th	Night	59% 11% 13% 14% 3	2660
	11th to 20th	Night	71% 9% 11% 6% 2	3276
	21st to end	Night	64% 9% 13% 9% 6%	4862

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	28% 9% 21% 22% 16% 4	180
	11th to 20th	Day	29% 7% 22% 24% 13% 4	180
	21st to end	Day	19% 9% 23% 25% 22% 3	198
February	1st to 10th	Day	21% 12% 21% 27% 14% 5%	313
	11th to 20th	Day	17% 10% 23% 26% 18% 5%	395
	21st to end	Day	14% 12% 26% 25% 19% 4	382
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%	483
October	1st to 10th	Day	10% 5% 22% 22% 25% 16%	450
	11th to 20th	Day	12% 8% 14% 25% 26% 14%	436
	21st to end	Day	18% 8% 24% 28% 13% 8%	297
November	1st to 10th	Day	20% 10% 21% 27% 19% 3	270
	11th to 20th	Day	25% 7% 24% 20% 19% 5%	207
	21st to end	Day	14% 7% 20% 27% 25% 7%	180
December	1st to 10th	Day	15% 9% 23% 29% 13% 11%	180
	11th to 20th	Day	19% 7% 24% 34% 14% 2	180
	21st to end	Day	18% 7% 27% 34% 11% 4	198
January	1st to 10th	Night	26% 9% 22% 27% 14% 2	760
	11th to 20th	Night	20% 7% 18% 35% 18% 2	760
	21st to end	Night	18% 5% 15% 37% 22% 2	836
February	1st to 10th	Night	17% 9% 22% 33% 18% 2	760
	11th to 20th	Night	12% 8% 24% 33% 20% 2	744
	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%	756
October	1st to 10th	Night	11% 4 15% 37% 30% 3	720
	11th to 20th	Night	11% 5% 19% 33% 25% 7%	720
	21st to end	Night	11% 7% 23% 33% 22% 5%	792
November	1st to 10th	Night	16% 6% 21% 39% 17% 1	740
	11th to 20th	Night	17% 9% 22% 33% 16% 3	760
	21st to end	Night	14% 6% 19% 37% 20% 3	760
December	1st to 10th	Night	15% 7% 20% 36% 20% 3	760
	11th to 20th	Night	22% 7% 18% 37% 16% 1	760
	21st to end	Night	23% 7% 22% 33% 13% 1	836

Page generated at 2024-09-26 06:52:23. Version ac07

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