Alcova
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alcova clear sky chart from 2005-11-06 to 2025-04-26.

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	4 6% 2 5% 4 4 4 4 6% 7% 55%	117791
	Night	4 7% 3 6% 4 4 4 4 5% 7% 52%	119716
Transparency	Day	28% 6% 13% 25% 20% 8%	117537
	Night	30% 5% 14% 27% 17% 6%	119561
Seeing	Day	9% 14% 30% 28% 16% 4	16320
	Night	8% 12% 33% 34% 12% 1	26515

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	1 7% 3 11% 6% 6% 5% 4 6% 8% 45%	3638
	11th to 20th	Day	2 7% 4 10% 8% 6% 5% 4 6% 7% 41%	3709
	21st to end	Day	2 5% 3 6% 3 4 3 3 3 6% 61%	4510
February	1st to 10th	Day	1 10% 4 9% 4 3 5% 5% 7% 8% 43%	1800
	11th to 20th	Day	3 10% 4 8% 5% 4 4 4 5% 8% 44%	1769
	21st to end	Day	2 8% 3 6% 3 4 5% 4 5% 8% 52%	1650
March	1st to 10th	Day	3 9% 2 5% 3 3 3 3 5% 7% 56%	4653
	11th to 20th	Day	3 10% 2 5% 3 4 4 4 6% 8% 52%	5000
	21st to end	Day	4 9% 3 7% 5% 5% 5% 4 7% 9% 41%	5661
April	1st to 10th	Day	6% 9% 2 5% 4 4 3 5% 7% 8% 47%	2400
	11th to 20th	Day	9% 11% 2 6% 4 4 5% 4 6% 7% 42%	2371
	21st to end	Day	9% 9% 3 6% 4 5% 5% 4 6% 6% 44%	2372
May	1st to 10th	Day	12% 12% 3 6% 4 5% 4 5% 6% 8% 34%	4297
	11th to 20th	Day	9% 5% 2 4 3 4 4 4 6% 9% 50%	4388
	21st to end	Day	8% 6% 2 5% 4 3 4 4 6% 8% 50%	4825
June	1st to 10th	Day	6% 3 1 4 3 4 4 5% 7% 10% 52%	2660
	11th to 20th	Day	5% 3 1 4 3 3 5% 4 6% 8% 59%	2660
	21st to end	Day	2 2 1 3 3 3 4 3 6% 7% 66%	2660
July	1st to 10th	Day	2 2 1 2 2 3 3 3 6% 7% 68%	4430
	11th to 20th	Day	2 2 1 4 3 4 4 4 7% 8% 61%	4430
	21st to end	Day	2 2 1 3 2 3 4 4 6% 8% 63%	4727
August	1st to 10th	Day	2 3 1 4 3 4 5% 4 6% 7% 59%	4092
	11th to 20th	Day	3 3 1 4 4 4 4 4 6% 7% 62%	3976
	21st to end	Day	2 2 1 2 3 3 3 3 5% 7% 69%	4180
September	1st to 10th	Day	5% 3 1 3 3 3 3 3 4 6% 65%	2280
	11th to 20th	Day	5% 3 1 4 3 3 3 3 5% 7% 64%	2280
	21st to end	Day	6% 3 1 3 2 2 3 3 5% 5% 66%	2024
October	1st to 10th	Day	4 3 1 3 2 2 2 2 3 4 74%	3170
	11th to 20th	Day	7% 5% 1 3 3 3 3 3 4 6% 61%	3150
	21st to end	Day	7% 8% 2 4 3 3 3 3 5% 8% 54%	3356
November	1st to 10th	Day	5% 6% 2 5% 3 4 5% 3 6% 7% 54%	1763
	11th to 20th	Day	3 7% 2 5% 3 3 4 4 5% 8% 56%	1660
	21st to end	Day	3 7% 3 7% 4 3 4 3 5% 7% 55%	1600
December	1st to 10th	Day	1 7% 3 6% 3 3 3 3 4 4 63%	2620
	11th to 20th	Day	2 7% 2 6% 3 4 5% 3 5% 8% 55%	3026
	21st to end	Day	3 10% 4 8% 5% 5% 5% 3 5% 8% 45%	4004
January	1st to 10th	Night	1 11% 5% 11% 5% 4 5% 3 6% 7% 41%	6327
	11th to 20th	Night	1 9% 6% 13% 5% 5% 5% 4 5% 6% 40%	6312
	21st to end	Night	2 6% 4 9% 4 3 3 3 4 7% 56%	6468
February	1st to 10th	Night	3 8% 3 9% 5% 5% 4 3 6% 8% 45%	2600
	11th to 20th	Night	4 9% 3 8% 6% 5% 5% 4 6% 8% 43%	2517
	21st to end	Night	2 8% 4 9% 6% 5% 5% 4 5% 7% 46%	1980
March	1st to 10th	Night	4 10% 2 5% 4 3 3 3 4 6% 54%	5225
	11th to 20th	Night	4 9% 2 5% 4 4 4 3 5% 8% 52%	5000
	21st to end	Night	5% 10% 4 9% 5% 5% 6% 5% 7% 8% 36%	5292
April	1st to 10th	Night	7% 10% 2 5% 4 3 5% 4 5% 8% 46%	2000
	11th to 20th	Night	9% 10% 3 9% 4 4 4 4 6% 7% 39%	1977
	21st to end	Night	9% 9% 2 6% 4 5% 4 4 7% 7% 43%	1921
May	1st to 10th	Night	11% 11% 3 7% 4 4 4 3 5% 6% 41%	2671
	11th to 20th	Night	10% 4 1 3 3 4 3 4 6% 8% 53%	2517
	21st to end	Night	9% 5% 1 5% 4 5% 4 5% 6% 8% 49%	2762
June	1st to 10th	Night	4 4 1 4 4 4 4 4 7% 8% 55%	1520
	11th to 20th	Night	6% 4 1 3 3 3 3 3 5% 8% 60%	1520
	21st to end	Night	3 2 1 3 3 2 4 4 4 7% 67%	1520
July	1st to 10th	Night	3 3 1 4 3 4 4 4 6% 9% 58%	2540
	11th to 20th	Night	2 1 1 4 3 4 4 4 6% 10% 60%	2540
	21st to end	Night	2 2 1 4 3 3 5% 4 6% 9% 61%	2820
August	1st to 10th	Night	4 3 1 4 3 4 4 4 7% 9% 56%	2847
	11th to 20th	Night	3 2 1 5% 5% 6% 5% 5% 6% 7% 55%	2861
	21st to end	Night	2 2 1 3 4 4 4 4 6% 9% 62%	3487
September	1st to 10th	Night	4 3 1 3 3 3 3 3 4 7% 65%	1900
	11th to 20th	Night	5% 4 1 3 3 3 3 3 4 7% 64%	1900
	21st to end	Night	6% 3 1 3 3 2 2 2 3 5% 69%	2149
October	1st to 10th	Night	4 4 1 2 1 2 2 2 3 4 74%	3800
	11th to 20th	Night	7% 6% 2 4 3 3 3 3 5% 7% 58%	3778
	21st to end	Night	5% 7% 2 4 3 3 3 4 6% 7% 55%	4169
November	1st to 10th	Night	5% 7% 2 4 4 3 4 4 6% 7% 54%	2535
	11th to 20th	Night	4 7% 3 6% 4 4 4 4 6% 7% 52%	2736
	21st to end	Night	3 9% 2 6% 4 3 3 3 5% 7% 54%	2800
December	1st to 10th	Night	2 6% 3 7% 4 3 3 2 4 5% 62%	4570
	11th to 20th	Night	2 6% 3 6% 4 4 4 3 5% 8% 54%	5181
	21st to end	Night	3 9% 3 7% 4 5% 6% 5% 7% 9% 43%	6974

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	38% 8% 15% 27% 11% 1	3639
	11th to 20th	Day	41% 8% 17% 20% 13% 1	3709
	21st to end	Day	27% 6% 22% 27% 17% 1	4510
February	1st to 10th	Day	37% 9% 17% 27% 9% 2	1800
	11th to 20th	Day	38% 8% 16% 24% 11% 3	1778
	21st to end	Day	29% 7% 21% 26% 14% 3	1650
March	1st to 10th	Day	27% 4 10% 37% 15% 7%	4653
	11th to 20th	Day	28% 7% 18% 30% 17%	5000
	21st to end	Day	38% 7% 18% 26% 10%	5661
April	1st to 10th	Day	35% 8% 19% 22% 13% 3	2400
	11th to 20th	Day	41% 6% 13% 24% 14% 2	2374
	21st to end	Day	39% 7% 13% 29% 11% 1	2372
May	1st to 10th	Day	46% 7% 12% 16% 15% 3	4297
	11th to 20th	Day	29% 8% 14% 32% 14% 4	4392
	21st to end	Day	30% 7% 13% 28% 20% 3	4825
June	1st to 10th	Day	26% 8% 15% 26% 20% 5%	2660
	11th to 20th	Day	22% 6% 13% 19% 25% 15%	2660
	21st to end	Day	16% 5% 10% 21% 31% 17%	2660
July	1st to 10th	Day	14% 5% 7% 18% 35% 22%	4361
	11th to 20th	Day	18% 4 6% 19% 31% 21%	4355
	21st to end	Day	17% 5% 7% 16% 30% 25%	4578
August	1st to 10th	Day	22% 4 8% 25% 21% 19%	4078
	11th to 20th	Day	21% 4 7% 21% 30% 17%	4015
	21st to end	Day	15% 4 8% 18% 33% 23%	4179
September	1st to 10th	Day	19% 4 8% 20% 29% 20%	2280
	11th to 20th	Day	20% 4 9% 22% 29% 15%	2280
	21st to end	Day	21% 4 10% 27% 29% 9%	2024
October	1st to 10th	Day	17% 3 9% 19% 29% 24%	3170
	11th to 20th	Day	26% 4 11% 32% 24% 5%	3150
	21st to end	Day	30% 4 11% 33% 20% 3	3356
November	1st to 10th	Day	29% 6% 16% 27% 19% 3	1763
	11th to 20th	Day	27% 5% 18% 30% 17% 4	1660
	21st to end	Day	30% 6% 16% 30% 16% 2	1600
December	1st to 10th	Day	25% 5% 17% 37% 14% 1	2620
	11th to 20th	Day	29% 4 26% 30% 11%	3024
	21st to end	Day	38% 6% 13% 29% 13% 1	4004
January	1st to 10th	Night	40% 6% 16% 26% 11% 1	6327
	11th to 20th	Night	43% 7% 15% 22% 11% 2	6312
	21st to end	Night	30% 6% 23% 26% 13% 2	6468
February	1st to 10th	Night	36% 8% 19% 25% 10% 2	2600
	11th to 20th	Night	39% 8% 16% 24% 11% 2	2518
	21st to end	Night	36% 6% 17% 22% 16% 3	1980
March	1st to 10th	Night	31% 4 12% 30% 17% 6%	5225
	11th to 20th	Night	29% 5% 12% 35% 17% 1	5000
	21st to end	Night	43% 8% 16% 24% 8% 1	5292
April	1st to 10th	Night	36% 7% 17% 24% 13% 3	2000
	11th to 20th	Night	43% 6% 12% 20% 17% 2	1977
	21st to end	Night	37% 10% 12% 27% 14% 1	1921
May	1st to 10th	Night	41% 6% 11% 22% 16% 3	2671
	11th to 20th	Night	28% 6% 15% 32% 13% 5%	2518
	21st to end	Night	29% 6% 15% 33% 16% 1	2762
June	1st to 10th	Night	23% 7% 19% 27% 18% 5%	1520
	11th to 20th	Night	22% 5% 12% 21% 25% 16%	1520
	21st to end	Night	16% 5% 9% 22% 30% 19%	1520
July	1st to 10th	Night	21% 5% 8% 23% 26% 16%	2500
	11th to 20th	Night	18% 5% 7% 25% 29% 18%	2492
	21st to end	Night	19% 4 8% 20% 30% 18%	2732
August	1st to 10th	Night	23% 6% 9% 29% 16% 18%	2838
	11th to 20th	Night	28% 4 9% 22% 21% 16%	2888
	21st to end	Night	20% 4 9% 17% 28% 22%	3487
September	1st to 10th	Night	19% 4 9% 21% 29% 18%	1900
	11th to 20th	Night	21% 4 10% 22% 28% 14%	1900
	21st to end	Night	20% 2 8% 26% 31% 12%	2149
October	1st to 10th	Night	16% 4 7% 18% 33% 22%	3800
	11th to 20th	Night	26% 3 10% 29% 25% 7%	3778
	21st to end	Night	27% 4 11% 33% 19% 6%	4169
November	1st to 10th	Night	28% 6% 15% 27% 21% 3	2535
	11th to 20th	Night	30% 6% 15% 31% 15% 3	2736
	21st to end	Night	30% 5% 15% 31% 17% 2	2800
December	1st to 10th	Night	27% 4 17% 35% 16% 1	4570
	11th to 20th	Night	29% 5% 20% 36% 9% 1	5182
	21st to end	Night	35% 6% 17% 28% 11% 2	6974

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	10% 21% 30% 25% 11% 2	300
	11th to 20th	Day	7% 16% 45% 24% 7% 2	300
	21st to end	Day	9% 14% 38% 27% 10% 1	330
February	1st to 10th	Day	10% 25% 42% 20% 3	300
	11th to 20th	Day	5% 24% 46% 17% 7%	300
	21st to end	Day	13% 21% 38% 20% 8%	249
March	1st to 10th	Day	6% 22% 36% 27% 9% 1	362
	11th to 20th	Day	10% 19% 34% 27% 9% 1	500
	21st to end	Day	13% 15% 38% 27% 6% 1	550
April	1st to 10th	Day	13% 18% 33% 30% 6%	500
	11th to 20th	Day	14% 18% 35% 25% 7%	492
	21st to end	Day	16% 11% 29% 27% 13% 4	481
May	1st to 10th	Day	15% 16% 26% 27% 14% 3	572
	11th to 20th	Day	16% 9% 24% 27% 20% 4	675
	21st to end	Day	13% 12% 24% 27% 19% 5%	734
June	1st to 10th	Day	5% 10% 24% 27% 26% 7%	680
	11th to 20th	Day	8% 13% 32% 30% 14% 2	680
	21st to end	Day	4 7% 29% 34% 23% 4	680
July	1st to 10th	Day	4 6% 20% 36% 29% 6%	671
	11th to 20th	Day	2 4 22% 36% 30% 6%	668
	21st to end	Day	4 4 18% 37% 32% 6%	700
August	1st to 10th	Day	4 7% 23% 37% 24% 5%	487
	11th to 20th	Day	4 5% 27% 31% 25% 8%	477
	21st to end	Day	4 9% 25% 32% 22% 8%	539
September	1st to 10th	Day	5% 10% 23% 28% 24% 9%	490
	11th to 20th	Day	6% 11% 33% 25% 18% 6%	490
	21st to end	Day	8% 12% 30% 27% 15% 8%	353
October	1st to 10th	Day	7% 12% 27% 28% 20% 7%	300
	11th to 20th	Day	11% 16% 28% 31% 11% 4	300
	21st to end	Day	11% 21% 41% 21% 6% 1	330
November	1st to 10th	Day	10% 22% 41% 21% 4 2	300
	11th to 20th	Day	14% 26% 38% 18% 3	300
	21st to end	Day	7% 25% 44% 20% 4	300
December	1st to 10th	Day	9% 20% 39% 21% 10% 1	300
	11th to 20th	Day	11% 32% 36% 18% 4	300
	21st to end	Day	18% 20% 33% 22% 7% 1	330
January	1st to 10th	Night	7% 19% 35% 30% 9% 1	1000
	11th to 20th	Night	7% 18% 42% 24% 8% 1	985
	21st to end	Night	9% 11% 41% 32% 6% 1	880
February	1st to 10th	Night	8% 14% 46% 24% 7% 1	800
	11th to 20th	Night	9% 15% 39% 29% 9%	784
	21st to end	Night	7% 22% 40% 25% 6%	660
March	1st to 10th	Night	7% 13% 40% 31% 9% 1	800
	11th to 20th	Night	8% 11% 33% 37% 10%	800
	21st to end	Night	10% 13% 34% 33% 10% 1	799
April	1st to 10th	Night	12% 11% 37% 35% 5%	600
	11th to 20th	Night	14% 12% 33% 35% 6%	593
	21st to end	Night	12% 10% 27% 39% 12% 1	587
May	1st to 10th	Night	14% 9% 27% 32% 15% 2	570
	11th to 20th	Night	14% 5% 24% 35% 20% 1	561
	21st to end	Night	11% 8% 25% 40% 16% 1	617
June	1st to 10th	Night	6% 7% 24% 39% 22% 1	570
	11th to 20th	Night	7% 8% 35% 41% 8% 1	570
	21st to end	Night	3 4 26% 47% 18% 1	570
July	1st to 10th	Night	5% 3 19% 45% 26% 1	555
	11th to 20th	Night	2 3 24% 49% 21% 1	552
	21st to end	Night	3 2 16% 54% 23% 1	624
August	1st to 10th	Night	3 2 19% 52% 23% 1	567
	11th to 20th	Night	4 2 18% 50% 25% 1	564
	21st to end	Night	3 3 23% 47% 23% 2	627
September	1st to 10th	Night	6% 3 21% 45% 23% 2	570
	11th to 20th	Night	5% 6% 26% 41% 19% 2	570
	21st to end	Night	7% 6% 24% 40% 20% 3	684
October	1st to 10th	Night	11% 8% 28% 35% 18% 1	760
	11th to 20th	Night	9% 9% 29% 39% 14% 1	760
	21st to end	Night	10% 11% 36% 33% 10% 2	861
November	1st to 10th	Night	9% 13% 41% 28% 8% 1	975
	11th to 20th	Night	8% 20% 44% 22% 5%	1000
	21st to end	Night	7% 17% 42% 27% 6% 1	1000
December	1st to 10th	Night	10% 18% 39% 26% 7%	1000
	11th to 20th	Night	9% 22% 39% 26% 4	1000
	21st to end	Night	12% 23% 37% 22% 6%	1100

Page generated at 2025-04-23 07:25:08. Version ac07

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