Altavista
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Altavista clear sky chart from 2005-11-06 to 2025-06-10.

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	14% 5% 2 6% 4 4 4 4 6% 8% 43%	119004
	Night	14% 5% 2 5% 4 4 4 3 5% 7% 46%	119080
Transparency	Day	38% 6% 11% 22% 18% 5%	122651
	Night	37% 5% 10% 23% 20% 5%	123570
Seeing	Day	19% 11% 25% 24% 15% 6%	15886
	Night	17% 9% 26% 30% 16% 2	26196

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	15% 4 1 4 3 3 3 3 4 5% 54%	4056
	11th to 20th	Day	13% 6% 1 4 3 3 3 3 4 5% 55%	4060
	21st to end	Day	17% 6% 2 5% 4 4 4 2 4 5% 47%	4466
February	1st to 10th	Day	12% 7% 2 6% 5% 4 5% 4 6% 8% 41%	1800
	11th to 20th	Day	21% 6% 2 5% 3 4 3 4 5% 7% 41%	1753
	21st to end	Day	19% 6% 2 5% 4 4 4 3 5% 6% 40%	1767
March	1st to 10th	Day	18% 6% 2 7% 4 4 4 4 5% 7% 39%	4935
	11th to 20th	Day	11% 4 2 5% 3 3 4 3 5% 7% 52%	4980
	21st to end	Day	17% 6% 2 6% 5% 4 4 4 5% 5% 42%	5465
April	1st to 10th	Day	12% 4 2 5% 4 4 4 4 6% 7% 49%	2312
	11th to 20th	Day	13% 5% 2 6% 4 3 3 3 5% 8% 47%	2374
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 7% 43%	2461
May	1st to 10th	Day	12% 5% 2 7% 5% 6% 6% 5% 8% 10% 34%	5862
	11th to 20th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 8% 30%	5861
	21st to end	Day	14% 4 2 5% 4 4 5% 4 6% 9% 42%	6433
June	1st to 10th	Day	10% 4 2 5% 4 5% 4 4 7% 9% 44%	2587
	11th to 20th	Day	10% 4 1 6% 5% 5% 6% 4 8% 9% 40%	2498
	21st to end	Day	8% 5% 2 5% 5% 5% 5% 4 8% 10% 42%	2470
July	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 5% 9% 12% 38%	4120
	11th to 20th	Day	9% 5% 2 6% 6% 5% 6% 5% 9% 11% 36%	4120
	21st to end	Day	12% 7% 3 8% 6% 7% 6% 5% 8% 9% 28%	4466
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 10% 31%	4104
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 6% 8% 11% 38%	3997
	21st to end	Day	8% 5% 2 5% 3 4 4 4 6% 11% 49%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 48%	2280
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 4 7% 43%	2123
	21st to end	Day	24% 6% 2 5% 4 4 4 3 6% 7% 36%	2066
October	1st to 10th	Day	17% 5% 2 5% 2 2 3 2 4 5% 53%	1900
	11th to 20th	Day	15% 5% 2 4 3 3 2 2 4 5% 55%	1900
	21st to end	Day	13% 6% 2 4 3 4 3 3 4 5% 54%	2090
November	1st to 10th	Day	13% 5% 2 4 3 3 3 3 4 5% 55%	1957
	11th to 20th	Day	14% 6% 2 4 4 3 3 3 4 6% 50%	1905
	21st to end	Day	14% 5% 1 4 3 3 3 3 5% 7% 52%	1618
December	1st to 10th	Day	12% 6% 2 5% 3 4 4 4 5% 6% 50%	2600
	11th to 20th	Day	20% 6% 2 4 3 3 3 3 5% 6% 46%	2990
	21st to end	Day	19% 6% 2 6% 4 4 5% 4 5% 6% 40%	4448
January	1st to 10th	Night	13% 5% 1 4 3 2 3 3 4 5% 56%	5889
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 3 4 57%	5900
	21st to end	Night	18% 5% 2 5% 3 3 2 2 4 5% 51%	6490
February	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 5% 7% 44%	2600
	11th to 20th	Night	20% 6% 2 4 4 4 4 3 4 6% 43%	2533
	21st to end	Night	14% 7% 2 5% 4 4 4 3 5% 6% 44%	1856
March	1st to 10th	Night	17% 6% 2 6% 4 4 4 3 5% 6% 45%	4918
	11th to 20th	Night	10% 3 2 4 3 3 3 3 6% 7% 55%	4980
	21st to end	Night	14% 6% 2 5% 4 3 4 3 6% 7% 45%	5467
April	1st to 10th	Night	15% 6% 2 6% 4 5% 2 3 5% 7% 45%	2086
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 50%	1975
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 8% 45%	1955
May	1st to 10th	Night	15% 6% 2 7% 5% 5% 5% 4 6% 9% 36%	4033
	11th to 20th	Night	22% 8% 3 8% 4 4 4 3 5% 6% 33%	4032
	21st to end	Night	15% 5% 2 5% 4 4 4 3 6% 10% 42%	4421
June	1st to 10th	Night	9% 4 2 6% 5% 4 5% 4 8% 10% 43%	1781
	11th to 20th	Night	10% 5% 2 5% 5% 4 4 4 6% 10% 45%	1599
	21st to end	Night	9% 5% 2 5% 4 4 4 5% 8% 11% 44%	1710
July	1st to 10th	Night	6% 4 2 7% 5% 6% 6% 5% 7% 10% 41%	2840
	11th to 20th	Night	8% 6% 3 7% 5% 5% 5% 6% 8% 11% 37%	2840
	21st to end	Night	12% 8% 3 8% 6% 5% 5% 5% 6% 10% 30%	3087
August	1st to 10th	Night	17% 7% 3 7% 5% 5% 5% 5% 8% 9% 30%	2825
	11th to 20th	Night	7% 4 2 7% 5% 5% 6% 5% 8% 10% 41%	2785
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 6% 8% 47%	3476
September	1st to 10th	Night	12% 6% 2 6% 4 3 4 5% 7% 7% 45%	1900
	11th to 20th	Night	15% 6% 2 5% 4 5% 4 3 4 7% 43%	2052
	21st to end	Night	25% 7% 3 6% 4 3 3 3 5% 5% 36%	2114
October	1st to 10th	Night	18% 5% 2 4 4 3 4 2 4 6% 49%	2280
	11th to 20th	Night	14% 4 2 4 4 3 4 2 4 5% 53%	2280
	21st to end	Night	14% 4 1 4 3 3 3 3 4 6% 56%	2508
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 2 4 5% 53%	2341
	11th to 20th	Night	13% 5% 2 4 3 3 4 4 5% 5% 52%	2492
	21st to end	Night	13% 3 1 5% 3 4 4 3 6% 6% 52%	2777
December	1st to 10th	Night	11% 5% 2 4 3 4 4 3 4 5% 54%	4580
	11th to 20th	Night	19% 8% 2 6% 4 4 3 3 4 5% 41%	5188
	21st to end	Night	18% 5% 2 4 3 4 4 4 6% 7% 43%	6490

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	33% 3 5% 23% 29% 7%	4057
	11th to 20th	Day	32% 3 4 20% 29% 10%	4060
	21st to end	Day	41% 3 6% 26% 21% 3	4466
February	1st to 10th	Day	40% 5% 12% 23% 18% 3	1800
	11th to 20th	Day	43% 4 7% 23% 18% 5%	1762
	21st to end	Day	43% 6% 10% 17% 18% 6%	1767
March	1st to 10th	Day	45% 4 7% 20% 15% 9%	4935
	11th to 20th	Day	31% 5% 7% 22% 24% 11%	4980
	21st to end	Day	43% 5% 8% 19% 19% 6%	5465
April	1st to 10th	Day	33% 6% 10% 23% 19% 8%	2312
	11th to 20th	Day	36% 5% 10% 24% 19% 6%	2374
	21st to end	Day	41% 5% 10% 24% 16% 4	2461
May	1st to 10th	Day	41% 9% 15% 22% 11% 2	5897
	11th to 20th	Day	52% 6% 9% 20% 11% 2	5865
	21st to end	Day	37% 8% 14% 21% 17% 3	6447
June	1st to 10th	Day	34% 8% 15% 27% 15% 1	2587
	11th to 20th	Day	36% 9% 14% 22% 17% 2	2498
	21st to end	Day	34% 11% 15% 24% 15% 2	2470
July	1st to 10th	Day	35% 12% 20% 17% 11% 4	4055
	11th to 20th	Day	38% 9% 19% 24% 9% 1	4051
	21st to end	Day	49% 9% 12% 19% 11%	4324
August	1st to 10th	Day	45% 8% 16% 24% 6% 1	4091
	11th to 20th	Day	36% 10% 18% 26% 10% 1	4036
	21st to end	Day	29% 7% 20% 27% 16% 2	4180
September	1st to 10th	Day	36% 6% 12% 20% 21% 4	2280
	11th to 20th	Day	41% 5% 9% 23% 19% 3	2123
	21st to end	Day	48% 6% 8% 18% 18% 2	2066
October	1st to 10th	Day	34% 4 7% 19% 29% 7%	3160
	11th to 20th	Day	24% 2 5% 25% 28% 15%	3140
	21st to end	Day	24% 3 10% 25% 25% 12%	3428
November	1st to 10th	Day	32% 4 8% 24% 26% 6%	1957
	11th to 20th	Day	36% 3 8% 23% 21% 8%	1905
	21st to end	Day	32% 5% 9% 24% 26% 4	1618
December	1st to 10th	Day	34% 6% 7% 24% 22% 7%	2600
	11th to 20th	Day	40% 3 10% 21% 21% 5%	2986
	21st to end	Day	44% 5% 10% 24% 16% 1	4448
January	1st to 10th	Night	31% 3 5% 22% 29% 10%	5889
	11th to 20th	Night	32% 3 5% 19% 32% 8%	5900
	21st to end	Night	38% 3 5% 29% 20% 6%	6490
February	1st to 10th	Night	39% 5% 9% 25% 19% 3	2600
	11th to 20th	Night	43% 4 8% 19% 19% 6%	2535
	21st to end	Night	41% 5% 8% 23% 16% 7%	1856
March	1st to 10th	Night	41% 5% 5% 21% 21% 9%	4918
	11th to 20th	Night	28% 5% 7% 20% 28% 13%	4980
	21st to end	Night	39% 4 7% 23% 22% 5%	5467
April	1st to 10th	Night	40% 5% 10% 18% 22% 6%	2086
	11th to 20th	Night	34% 5% 9% 24% 21% 6%	1975
	21st to end	Night	37% 5% 11% 24% 18% 4	1955
May	1st to 10th	Night	44% 8% 15% 18% 12% 2	4060
	11th to 20th	Night	52% 5% 10% 19% 13% 1	4033
	21st to end	Night	37% 7% 15% 25% 13% 3	4429
June	1st to 10th	Night	33% 9% 15% 25% 17% 1	1781
	11th to 20th	Night	34% 8% 18% 21% 17% 2	1599
	21st to end	Night	32% 11% 15% 21% 17% 4	1710
July	1st to 10th	Night	36% 11% 19% 17% 13% 3	2795
	11th to 20th	Night	38% 11% 21% 21% 7% 2	2786
	21st to end	Night	49% 9% 13% 20% 9%	2988
August	1st to 10th	Night	48% 8% 16% 20% 7% 1	2814
	11th to 20th	Night	35% 8% 18% 26% 11% 1	2812
	21st to end	Night	34% 7% 12% 33% 12% 2	3476
September	1st to 10th	Night	36% 8% 13% 20% 17% 7%	1900
	11th to 20th	Night	41% 7% 9% 20% 21% 2	2052
	21st to end	Night	50% 5% 7% 18% 16% 3	2114
October	1st to 10th	Night	39% 4 8% 18% 24% 6%	3800
	11th to 20th	Night	25% 3 6% 22% 30% 14%	3779
	21st to end	Night	21% 3 6% 29% 30% 12%	4120
November	1st to 10th	Night	35% 4 8% 21% 25% 7%	2341
	11th to 20th	Night	34% 4 8% 20% 25% 9%	2492
	21st to end	Night	32% 4 8% 25% 24% 7%	2777
December	1st to 10th	Night	33% 4 6% 29% 25% 4	4580
	11th to 20th	Night	47% 4 8% 19% 15% 7%	5191
	21st to end	Night	39% 5% 11% 25% 18% 2	6490

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	31% 24% 26% 14% 5%	300
	11th to 20th	Day	31% 23% 34% 10% 1	300
	21st to end	Day	28% 20% 38% 12% 2	330
February	1st to 10th	Day	20% 25% 38% 13% 4	300
	11th to 20th	Day	39% 26% 25% 7% 2	300
	21st to end	Day	30% 18% 33% 15% 3	376
March	1st to 10th	Day	25% 17% 33% 19% 6% 1	500
	11th to 20th	Day	24% 20% 32% 18% 6%	500
	21st to end	Day	23% 18% 35% 20% 3 1	550
April	1st to 10th	Day	23% 20% 29% 20% 7% 1	500
	11th to 20th	Day	21% 15% 30% 20% 10% 4	492
	21st to end	Day	19% 12% 34% 22% 11% 1	500
May	1st to 10th	Day	17% 15% 34% 22% 9% 3	503
	11th to 20th	Day	22% 8% 25% 25% 14% 5%	527
	21st to end	Day	21% 5% 22% 26% 20% 7%	573
June	1st to 10th	Day	12% 5% 27% 30% 20% 7%	526
	11th to 20th	Day	12% 3 21% 31% 22% 11%	490
	21st to end	Day	10% 3 24% 33% 22% 7%	490
July	1st to 10th	Day	11% 3 14% 30% 30% 12%	485
	11th to 20th	Day	6% 1 14% 34% 33% 13%	484
	21st to end	Day	10% 2 18% 32% 26% 12%	532
August	1st to 10th	Day	15% 3 19% 25% 25% 13%	486
	11th to 20th	Day	11% 2 16% 29% 27% 16%	479
	21st to end	Day	12% 2 12% 30% 27% 18%	539
September	1st to 10th	Day	14% 4 15% 25% 27% 16%	490
	11th to 20th	Day	18% 4 14% 27% 25% 12%	490
	21st to end	Day	29% 6% 22% 24% 15% 4	490
October	1st to 10th	Day	23% 4 17% 30% 19% 7%	490
	11th to 20th	Day	15% 13% 21% 25% 20% 5%	490
	21st to end	Day	16% 11% 29% 28% 12% 4	539
November	1st to 10th	Day	19% 9% 30% 25% 13% 4	495
	11th to 20th	Day	19% 17% 28% 23% 11% 2	405
	21st to end	Day	20% 21% 29% 21% 7% 2	208
December	1st to 10th	Day	26% 17% 36% 15% 6% 1	200
	11th to 20th	Day	26% 15% 28% 26% 5% 2	200
	21st to end	Day	28% 15% 30% 18% 6% 2	327
January	1st to 10th	Night	21% 18% 33% 24% 3	800
	11th to 20th	Night	19% 18% 33% 28% 2	800
	21st to end	Night	19% 15% 40% 24% 2	880
February	1st to 10th	Night	19% 15% 36% 27% 3	800
	11th to 20th	Night	27% 17% 38% 17% 1	783
	21st to end	Night	19% 16% 40% 22% 3	660
March	1st to 10th	Night	19% 15% 36% 26% 4	800
	11th to 20th	Night	20% 15% 34% 23% 7% 1	800
	21st to end	Night	19% 13% 36% 26% 5%	880
April	1st to 10th	Night	19% 16% 34% 25% 6%	800
	11th to 20th	Night	14% 13% 35% 28% 10%	791
	21st to end	Night	16% 9% 33% 33% 9%	755
May	1st to 10th	Night	16% 9% 33% 28% 15%	600
	11th to 20th	Night	19% 6% 25% 33% 15% 2	591
	21st to end	Night	16% 2 16% 35% 28% 3	650
June	1st to 10th	Night	10% 2 19% 38% 29% 2	583
	11th to 20th	Night	12% 1 15% 38% 31% 3	459
	21st to end	Night	11% 2 13% 43% 27% 4	570
July	1st to 10th	Night	11% 1 13% 35% 35% 6%	555
	11th to 20th	Night	6% 1 9% 35% 45% 5%	552
	21st to end	Night	8% 1 12% 42% 33% 5%	624
August	1st to 10th	Night	16% 2 9% 36% 32% 5%	567
	11th to 20th	Night	12% 2 10% 39% 35% 2	564
	21st to end	Night	11% 2 10% 35% 36% 6%	836
September	1st to 10th	Night	15% 2 10% 31% 35% 7%	760
	11th to 20th	Night	17% 3 11% 33% 33% 4	760
	21st to end	Night	27% 4 18% 28% 22% 1	760
October	1st to 10th	Night	20% 5% 18% 29% 23% 6%	760
	11th to 20th	Night	17% 8% 23% 33% 17% 2	760
	21st to end	Night	17% 6% 27% 35% 14% 2	836
November	1st to 10th	Night	17% 7% 28% 33% 13% 1	780
	11th to 20th	Night	17% 11% 31% 30% 12%	800
	21st to end	Night	15% 10% 34% 30% 10% 1	800
December	1st to 10th	Night	22% 12% 35% 25% 6%	800
	11th to 20th	Night	19% 11% 35% 31% 5%	800
	21st to end	Night	22% 14% 32% 29% 5%	880

Page generated at 2025-06-07 10:28:13. Version ac07

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