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 2024-11-02.

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% 42%	100546
	Night	15% 6% 2 5% 4 4 4 4 5% 7% 44%	98247
Transparency	Day	39% 7% 12% 23% 16% 4	100357
	Night	39% 6% 10% 22% 18% 4	98106
Seeing	Day	19% 10% 25% 24% 16% 6%	15135
	Night	17% 9% 26% 30% 16% 2	25360

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	18% 4 1 4 3 3 3 3 4 6% 51%	2836
	11th to 20th	Day	15% 7% 1 6% 4 4 3 3 5% 6% 47%	2840
	21st to end	Day	22% 7% 3 6% 4 4 3 2 4 5% 41%	3124
February	1st to 10th	Day	12% 7% 2 6% 4 4 5% 4 5% 8% 42%	1710
	11th to 20th	Day	19% 6% 2 5% 3 4 3 4 5% 7% 42%	1663
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 7% 39%	1680
March	1st to 10th	Day	23% 7% 2 6% 3 3 3 4 5% 6% 37%	3435
	11th to 20th	Day	11% 4 2 4 4 4 4 4 5% 6% 53%	3480
	21st to end	Day	21% 6% 2 6% 5% 4 4 4 5% 5% 39%	3815
April	1st to 10th	Day	11% 4 2 5% 3 4 4 4 6% 7% 50%	2196
	11th to 20th	Day	14% 5% 2 6% 4 3 4 3 5% 7% 47%	2254
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 6% 43%	2338
May	1st to 10th	Day	16% 6% 2 7% 5% 5% 6% 5% 7% 8% 33%	4082
	11th to 20th	Day	21% 6% 2 6% 4 4 4 4 5% 9% 34%	4081
	21st to end	Day	9% 4 1 5% 4 4 5% 5% 7% 10% 46%	4476
June	1st to 10th	Day	10% 4 2 5% 4 6% 4 5% 7% 9% 44%	2470
	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 3 3 3 4 6% 54%	2090
November	1st to 10th	Day	13% 5% 2 4 3 3 3 3 4 5% 55%	1868
	11th to 20th	Day	14% 6% 2 4 3 3 3 3 4 6% 51%	1810
	21st to end	Day	15% 5% 1 4 3 3 3 3 5% 8% 52%	1538
December	1st to 10th	Day	16% 7% 2 5% 4 5% 5% 6% 6% 6% 39%	1520
	11th to 20th	Day	16% 5% 1 3 3 3 3 2 3 6% 54%	1910
	21st to end	Day	19% 5% 2 5% 4 4 4 4 5% 7% 41%	3106
January	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 5% 5% 50%	4109
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 4 5% 49%	4120
	21st to end	Night	23% 6% 2 6% 4 3 3 3 4 4 43%	4532
February	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 5% 7% 45%	2470
	11th to 20th	Night	19% 6% 2 4 4 4 3 3 4 6% 44%	2403
	21st to end	Night	15% 7% 2 5% 4 4 4 3 5% 6% 44%	1767
March	1st to 10th	Night	22% 8% 2 6% 4 4 4 3 5% 6% 36%	3418
	11th to 20th	Night	11% 4 2 4 4 3 3 3 5% 7% 54%	3480
	21st to end	Night	17% 6% 2 4 3 3 3 3 5% 6% 47%	3817
April	1st to 10th	Night	15% 6% 2 5% 4 5% 3 4 5% 7% 46%	1982
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 50%	1875
	21st to end	Night	15% 5% 2 4 4 4 4 3 5% 7% 45%	1857
May	1st to 10th	Night	17% 7% 2 7% 5% 5% 4 4 5% 7% 38%	2813
	11th to 20th	Night	24% 7% 2 6% 3 4 4 2 4 6% 37%	2812
	21st to end	Night	11% 3 2 5% 4 4 4 3 7% 10% 47%	3079
June	1st to 10th	Night	10% 4 2 6% 5% 4 5% 4 8% 10% 43%	1694
	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 3 5% 54%	2244
	11th to 20th	Night	13% 5% 1 4 3 3 4 4 5% 5% 53%	2367
	21st to end	Night	14% 3 1 4 3 4 4 3 6% 6% 51%	2638
December	1st to 10th	Night	18% 6% 2 6% 4 4 4 3 4 5% 42%	2660
	11th to 20th	Night	16% 6% 2 6% 4 3 3 3 5% 5% 49%	3282
	21st to end	Night	18% 6% 2 5% 4 4 4 4 6% 7% 40%	4532

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	34% 4 6% 30% 20% 6%	2837
	11th to 20th	Day	38% 4 6% 21% 25% 5%	2840
	21st to end	Day	48% 4 5% 23% 18% 3	3124
February	1st to 10th	Day	39% 5% 12% 23% 18% 3	1710
	11th to 20th	Day	41% 4 8% 24% 19% 5%	1672
	21st to end	Day	45% 6% 11% 16% 18% 6%	1680
March	1st to 10th	Day	47% 5% 8% 21% 13% 6%	3435
	11th to 20th	Day	30% 6% 8% 24% 21% 10%	3480
	21st to end	Day	46% 6% 9% 19% 17% 3	3815
April	1st to 10th	Day	31% 7% 11% 23% 20% 8%	2196
	11th to 20th	Day	36% 5% 10% 23% 19% 7%	2254
	21st to end	Day	41% 5% 10% 23% 16% 4	2338
May	1st to 10th	Day	45% 10% 12% 20% 12% 2	4120
	11th to 20th	Day	47% 8% 9% 24% 11% 1	4085
	21st to end	Day	30% 9% 16% 22% 19% 3	4489
June	1st to 10th	Day	34% 8% 15% 27% 14% 1	2470
	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	35% 4 8% 25% 23% 5%	1900
	11th to 20th	Day	33% 3 8% 26% 22% 7%	1900
	21st to end	Day	33% 4 7% 26% 22% 7%	2090
November	1st to 10th	Day	32% 4 8% 23% 27% 6%	1868
	11th to 20th	Day	36% 3 8% 23% 21% 9%	1810
	21st to end	Day	32% 5% 9% 24% 25% 4	1538
December	1st to 10th	Day	43% 8% 8% 20% 17% 4	1520
	11th to 20th	Day	34% 3 7% 26% 23% 7%	1906
	21st to end	Day	42% 5% 10% 26% 15% 2	3106
January	1st to 10th	Night	36% 4 7% 25% 24% 5%	4109
	11th to 20th	Night	39% 4 7% 19% 26% 5%	4120
	21st to end	Night	46% 3 6% 23% 16% 6%	4532
February	1st to 10th	Night	38% 6% 9% 25% 20% 3	2470
	11th to 20th	Night	42% 4 8% 20% 20% 7%	2405
	21st to end	Night	42% 5% 8% 23% 16% 6%	1767
March	1st to 10th	Night	48% 6% 6% 19% 16% 4	3418
	11th to 20th	Night	30% 5% 7% 20% 26% 12%	3480
	21st to end	Night	39% 5% 8% 24% 20% 5%	3817
April	1st to 10th	Night	39% 5% 9% 18% 23% 5%	1982
	11th to 20th	Night	34% 5% 9% 24% 22% 6%	1875
	21st to end	Night	38% 5% 11% 24% 18% 4	1857
May	1st to 10th	Night	44% 8% 13% 20% 12% 2	2840
	11th to 20th	Night	48% 6% 8% 22% 13% 2	2813
	21st to end	Night	30% 9% 17% 25% 15% 4	3087
June	1st to 10th	Night	33% 10% 15% 26% 16% 1	1694
	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	37% 5% 8% 23% 22% 4	2280
	11th to 20th	Night	34% 4 6% 25% 24% 6%	2280
	21st to end	Night	30% 3 7% 24% 28% 7%	2508
November	1st to 10th	Night	35% 4 8% 21% 25% 8%	2244
	11th to 20th	Night	33% 4 8% 21% 25% 9%	2367
	21st to end	Night	33% 4 8% 25% 23% 7%	2638
December	1st to 10th	Night	44% 5% 7% 24% 17% 4	2660
	11th to 20th	Night	38% 4 7% 22% 22% 7%	3285
	21st to end	Night	41% 6% 9% 22% 19% 2	4532

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	29% 23% 28% 15% 6%	270
	11th to 20th	Day	31% 21% 35% 11% 1	270
	21st to end	Day	30% 20% 37% 12% 2	297
February	1st to 10th	Day	21% 24% 36% 14% 5%	270
	11th to 20th	Day	37% 27% 26% 8% 3	270
	21st to end	Day	32% 19% 32% 14% 3	345
March	1st to 10th	Day	25% 17% 33% 19% 6% 1	460
	11th to 20th	Day	23% 21% 32% 18% 6%	460
	21st to end	Day	24% 17% 34% 22% 4 1	506
April	1st to 10th	Day	22% 21% 29% 20% 8% 1	460
	11th to 20th	Day	21% 15% 29% 20% 11% 4	452
	21st to end	Day	20% 13% 34% 21% 11% 1	460
May	1st to 10th	Day	19% 14% 33% 22% 10% 3	463
	11th to 20th	Day	20% 8% 25% 27% 15% 5%	487
	21st to end	Day	18% 3 22% 28% 21% 8%	529
June	1st to 10th	Day	12% 5% 26% 30% 20% 7%	490
	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% 24% 13% 4	460
	11th to 20th	Day	19% 17% 27% 23% 11% 2	370
	21st to end	Day	19% 19% 30% 22% 7% 3	188
December	1st to 10th	Day	26% 16% 36% 15% 6% 1	180
	11th to 20th	Day	24% 16% 28% 26% 5% 2	180
	21st to end	Day	29% 14% 32% 16% 6% 3	294
January	1st to 10th	Night	21% 16% 33% 25% 3	760
	11th to 20th	Night	19% 18% 33% 29% 3	760
	21st to end	Night	20% 15% 39% 24% 3	836
February	1st to 10th	Night	19% 15% 36% 28% 3	760
	11th to 20th	Night	26% 17% 38% 18% 1	743
	21st to end	Night	20% 15% 39% 22% 3	628
March	1st to 10th	Night	19% 14% 35% 27% 4	760
	11th to 20th	Night	21% 15% 34% 22% 8% 1	760
	21st to end	Night	20% 12% 35% 27% 5%	836
April	1st to 10th	Night	18% 16% 34% 25% 6%	760
	11th to 20th	Night	14% 12% 34% 28% 11%	751
	21st to end	Night	17% 8% 33% 33% 9%	717
May	1st to 10th	Night	16% 9% 33% 28% 15%	570
	11th to 20th	Night	20% 5% 25% 32% 16% 2	561
	21st to end	Night	16% 2 15% 35% 29% 3	617
June	1st to 10th	Night	10% 2 18% 38% 29% 2	554
	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% 29% 32% 14% 1	748
	11th to 20th	Night	16% 11% 31% 29% 12%	760
	21st to end	Night	16% 10% 32% 31% 11% 1	760
December	1st to 10th	Night	22% 12% 34% 26% 6%	760
	11th to 20th	Night	18% 10% 36% 31% 5%	760
	21st to end	Night	22% 13% 33% 28% 4	836

Page generated at 2024-10-30 13:19:23. Version ac07

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