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 2026-10-14.

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 5% 4 6% 8% 42%	148199
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 45%	143384
Transparency	Day	38% 6% 12% 22% 17% 4	151804
	Night	38% 5% 10% 22% 19% 5%	147851
Seeing	Day	19% 11% 25% 24% 15% 6%	17722
	Night	17% 9% 26% 30% 16% 2	27961

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	17% 5% 1 4 3 3 3 3 4 6% 50%	5258
	11th to 20th	Day	12% 6% 1 4 3 3 3 2 4 5% 57%	5280
	21st to end	Day	16% 7% 2 6% 4 4 4 2 4 5% 47%	5780
February	1st to 10th	Day	12% 7% 2 6% 4 4 5% 4 6% 8% 42%	1890
	11th to 20th	Day	21% 6% 1 5% 3 4 3 4 5% 7% 41%	1843
	21st to end	Day	19% 6% 3 6% 4 4 4 3 5% 6% 39%	1853
March	1st to 10th	Day	18% 6% 2 7% 4 4 4 4 5% 7% 39%	5045
	11th to 20th	Day	12% 4 2 5% 3 3 4 3 5% 7% 52%	5090
	21st to end	Day	16% 5% 2 6% 5% 4 4 4 5% 5% 42%	5586
April	1st to 10th	Day	11% 4 2 4 3 4 4 4 6% 7% 50%	2428
	11th to 20th	Day	13% 5% 2 6% 4 3 3 3 5% 8% 48%	2494
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 6% 43%	2584
May	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 7% 9% 39%	7616
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 5% 7% 39%	7626
	21st to end	Day	19% 5% 2 6% 4 4 5% 4 6% 8% 38%	8391
June	1st to 10th	Day	10% 4 2 5% 5% 5% 4 4 7% 10% 44%	2730
	11th to 20th	Day	10% 4 1 6% 5% 4 6% 5% 8% 10% 40%	2758
	21st to end	Day	8% 5% 2 5% 5% 5% 5% 4 8% 11% 43%	2730
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 5% 5% 8% 11% 41%	7680
	11th to 20th	Day	11% 5% 2 7% 5% 5% 6% 5% 9% 13% 31%	7680
	21st to end	Day	12% 6% 3 7% 5% 6% 6% 5% 8% 9% 34%	8340
August	1st to 10th	Day	15% 7% 3 8% 5% 6% 6% 5% 8% 11% 27%	6014
	11th to 20th	Day	8% 5% 2 6% 5% 5% 7% 6% 9% 12% 36%	5890
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 11% 48%	6080
September	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 5% 8% 50%	2520
	11th to 20th	Day	13% 7% 2 5% 4 4 4 4 5% 7% 43%	2347
	21st to end	Day	24% 6% 2 6% 4 4 4 3 6% 7% 35%	2283
October	1st to 10th	Day	17% 5% 2 5% 2 2 3 2 4 5% 53%	2100
	11th to 20th	Day	15% 5% 1 4 3 3 3 2 3 5% 55%	2030
	21st to end	Day	14% 6% 2 4 3 3 3 3 4 5% 54%	2200
November	1st to 10th	Day	12% 5% 2 4 3 3 3 3 4 6% 56%	2057
	11th to 20th	Day	14% 6% 2 4 4 3 3 3 4 6% 51%	2000
	21st to end	Day	14% 5% 1 4 3 3 3 3 5% 7% 52%	1699
December	1st to 10th	Day	12% 6% 2 5% 3 4 4 4 5% 6% 49%	2680
	11th to 20th	Day	19% 6% 2 4 3 3 3 3 5% 6% 47%	3070
	21st to end	Day	19% 6% 2 6% 4 4 5% 4 5% 6% 40%	4547
January	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 4 6% 54%	7635
	11th to 20th	Night	13% 5% 2 4 3 3 3 2 3 4 58%	7680
	21st to end	Night	17% 5% 2 5% 3 3 3 2 4 5% 50%	8401
February	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 5% 7% 45%	2730
	11th to 20th	Night	20% 7% 3 5% 4 4 3 3 4 6% 43%	2663
	21st to end	Night	15% 7% 3 5% 4 4 4 3 5% 6% 44%	1946
March	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 5% 6% 45%	5028
	11th to 20th	Night	10% 3 2 4 3 3 3 3 6% 7% 55%	5090
	21st to end	Night	14% 6% 2 5% 4 4 4 4 6% 7% 45%	5588
April	1st to 10th	Night	15% 6% 2 5% 4 4 3 4 5% 7% 46%	2190
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 50%	2075
	21st to end	Night	14% 5% 2 4 4 4 5% 3 5% 8% 45%	2053
May	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	5237
	11th to 20th	Night	17% 6% 2 7% 3 4 4 3 5% 7% 41%	5234
	21st to end	Night	21% 5% 2 6% 4 4 4 3 6% 9% 35%	5761
June	1st to 10th	Night	9% 4 2 6% 5% 4 5% 4 8% 9% 43%	1873
	11th to 20th	Night	10% 5% 2 5% 5% 4 4 4 6% 10% 45%	1768
	21st to end	Night	8% 5% 2 5% 4 4 4 5% 8% 11% 44%	1890
July	1st to 10th	Night	9% 6% 2 7% 5% 6% 6% 5% 7% 10% 37%	5278
	11th to 20th	Night	12% 7% 3 8% 5% 5% 6% 6% 8% 11% 29%	5277
	21st to end	Night	13% 7% 3 8% 6% 6% 6% 5% 6% 8% 32%	5736
August	1st to 10th	Night	18% 8% 3 7% 5% 5% 5% 5% 9% 9% 26%	4135
	11th to 20th	Night	8% 4 2 7% 5% 5% 6% 5% 8% 11% 37%	4095
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 6% 8% 46%	5045
September	1st to 10th	Night	11% 6% 2 6% 3 3 4 4 6% 7% 48%	2100
	11th to 20th	Night	15% 6% 2 5% 5% 5% 4 3 5% 7% 43%	2268
	21st to end	Night	25% 7% 3 6% 4 3 3 2 5% 5% 35%	2336
October	1st to 10th	Night	17% 5% 2 4 4 3 4 2 4 5% 49%	2520
	11th to 20th	Night	15% 4 2 4 4 3 4 2 4 5% 52%	2437
	21st to end	Night	14% 4 1 4 3 3 3 3 4 6% 55%	2640
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 2 3 5% 53%	2461
	11th to 20th	Night	13% 5% 2 4 3 3 4 4 5% 5% 53%	2617
	21st to end	Night	13% 3 1 4 3 4 4 3 6% 6% 51%	2916
December	1st to 10th	Night	11% 5% 2 4 3 4 4 3 5% 5% 53%	4720
	11th to 20th	Night	19% 8% 2 6% 4 4 3 3 4 5% 41%	5328
	21st to end	Night	18% 5% 2 5% 3 4 4 4 6% 7% 43%	6633

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	36% 3 9% 21% 26% 6%	5259
	11th to 20th	Day	31% 3 4 18% 31% 13%	5280
	21st to end	Day	41% 3 5% 27% 19% 5%	5780
February	1st to 10th	Day	39% 5% 12% 22% 19% 4	1890
	11th to 20th	Day	43% 4 7% 23% 18% 5%	1852
	21st to end	Day	44% 6% 10% 17% 18% 5%	1853
March	1st to 10th	Day	45% 4 7% 20% 15% 9%	5045
	11th to 20th	Day	31% 5% 7% 22% 24% 11%	5090
	21st to end	Day	43% 5% 9% 19% 19% 6%	5586
April	1st to 10th	Day	32% 6% 10% 24% 20% 8%	2428
	11th to 20th	Day	35% 4 11% 24% 19% 6%	2494
	21st to end	Day	41% 5% 10% 24% 16% 4	2584
May	1st to 10th	Day	40% 7% 13% 24% 15% 1	7651
	11th to 20th	Day	44% 6% 10% 24% 16% 1	7630
	21st to end	Day	43% 7% 12% 18% 15% 4	8405
June	1st to 10th	Day	34% 8% 15% 26% 16% 1	2730
	11th to 20th	Day	36% 9% 15% 22% 16% 2	2758
	21st to end	Day	33% 10% 15% 25% 15% 2	2730
July	1st to 10th	Day	35% 10% 18% 23% 11% 2	7615
	11th to 20th	Day	41% 10% 20% 21% 8%	7611
	21st to end	Day	44% 8% 14% 20% 15% 1	8156
August	1st to 10th	Day	49% 10% 18% 19% 4	6001
	11th to 20th	Day	36% 10% 22% 24% 8%	5929
	21st to end	Day	30% 7% 20% 26% 16% 2	6080
September	1st to 10th	Day	34% 6% 13% 21% 21% 4	2520
	11th to 20th	Day	40% 6% 10% 23% 19% 3	2347
	21st to end	Day	49% 6% 9% 17% 16% 3	2283
October	1st to 10th	Day	34% 4 6% 19% 30% 7%	3360
	11th to 20th	Day	24% 2 5% 25% 28% 15%	3270
	21st to end	Day	25% 3 10% 25% 25% 12%	3538
November	1st to 10th	Day	31% 4 8% 24% 26% 6%	2057
	11th to 20th	Day	35% 3 9% 25% 21% 8%	2000
	21st to end	Day	32% 5% 9% 24% 26% 4	1699
December	1st to 10th	Day	35% 6% 7% 24% 22% 7%	2680
	11th to 20th	Day	40% 3 10% 21% 21% 5%	3066
	21st to end	Day	44% 5% 10% 24% 16% 1	4547
January	1st to 10th	Night	33% 3 10% 22% 24% 8%	7636
	11th to 20th	Night	33% 3 4 19% 33% 9%	7680
	21st to end	Night	38% 2 5% 28% 20% 6%	8401
February	1st to 10th	Night	38% 5% 9% 24% 20% 3	2730
	11th to 20th	Night	44% 4 8% 19% 20% 6%	2665
	21st to end	Night	41% 5% 8% 23% 17% 6%	1946
March	1st to 10th	Night	41% 5% 5% 21% 20% 8%	5028
	11th to 20th	Night	28% 5% 7% 20% 28% 13%	5090
	21st to end	Night	38% 4 7% 24% 22% 5%	5588
April	1st to 10th	Night	38% 5% 11% 18% 23% 5%	2190
	11th to 20th	Night	33% 5% 10% 24% 21% 6%	2075
	21st to end	Night	37% 5% 11% 25% 18% 4	2053
May	1st to 10th	Night	42% 7% 13% 21% 13% 4	5264
	11th to 20th	Night	44% 5% 11% 23% 16% 1	5235
	21st to end	Night	46% 6% 13% 20% 13% 2	5771
June	1st to 10th	Night	34% 9% 14% 25% 17% 1	1873
	11th to 20th	Night	35% 8% 18% 21% 16% 2	1768
	21st to end	Night	32% 10% 15% 22% 17% 4	1890
July	1st to 10th	Night	41% 10% 16% 20% 10% 2	5235
	11th to 20th	Night	47% 11% 17% 17% 7% 1	5226
	21st to end	Night	48% 7% 13% 18% 13%	5606
August	1st to 10th	Night	52% 11% 15% 17% 6%	4124
	11th to 20th	Night	38% 8% 20% 25% 8%	4122
	21st to end	Night	35% 6% 13% 30% 14% 1	5045
September	1st to 10th	Night	35% 7% 13% 22% 17% 6%	2100
	11th to 20th	Night	40% 7% 9% 21% 21% 2	2268
	21st to end	Night	51% 5% 8% 17% 15% 3	2336
October	1st to 10th	Night	39% 4 8% 18% 25% 6%	4040
	11th to 20th	Night	26% 3 5% 22% 30% 13%	3936
	21st to end	Night	22% 2 6% 29% 30% 12%	4252
November	1st to 10th	Night	35% 4 8% 21% 25% 7%	2461
	11th to 20th	Night	33% 4 8% 21% 26% 8%	2617
	21st to end	Night	33% 4 8% 25% 24% 7%	2916
December	1st to 10th	Night	33% 4 6% 29% 25% 3	4720
	11th to 20th	Night	46% 4 8% 19% 16% 7%	5331
	21st to end	Night	39% 5% 11% 25% 18% 2	6633

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% 25% 26% 14% 5%	330
	11th to 20th	Day	32% 25% 33% 9% 1	330
	21st to end	Day	28% 22% 37% 11% 2	363
February	1st to 10th	Day	20% 24% 40% 12% 5%	330
	11th to 20th	Day	40% 25% 26% 7% 2	330
	21st to end	Day	31% 18% 33% 16% 3	406
March	1st to 10th	Day	24% 16% 33% 20% 6% 1	540
	11th to 20th	Day	25% 20% 32% 18% 6%	540
	21st to end	Day	22% 18% 35% 20% 4 1	594
April	1st to 10th	Day	21% 20% 29% 21% 7% 1	540
	11th to 20th	Day	20% 15% 31% 21% 10% 4	532
	21st to end	Day	19% 13% 33% 22% 12% 1	540
May	1st to 10th	Day	18% 16% 34% 21% 8% 2	543
	11th to 20th	Day	21% 8% 26% 26% 14% 5%	567
	21st to end	Day	22% 5% 23% 25% 19% 6%	617
June	1st to 10th	Day	11% 5% 28% 31% 19% 7%	570
	11th to 20th	Day	12% 4 22% 32% 21% 9%	570
	21st to end	Day	10% 3 23% 32% 23% 8%	570
July	1st to 10th	Day	10% 2 14% 29% 31% 13%	565
	11th to 20th	Day	6% 1 14% 33% 33% 13%	564
	21st to end	Day	11% 2 17% 31% 25% 13%	620
August	1st to 10th	Day	17% 3 18% 26% 24% 12%	566
	11th to 20th	Day	10% 2 15% 32% 27% 14%	559
	21st to end	Day	11% 3 15% 30% 25% 16%	627
September	1st to 10th	Day	12% 4 16% 26% 27% 15%	570
	11th to 20th	Day	17% 4 13% 28% 26% 12%	570
	21st to end	Day	29% 6% 22% 24% 15% 4	570
October	1st to 10th	Day	22% 4 17% 30% 19% 8%	570
	11th to 20th	Day	15% 13% 23% 25% 19% 4	542
	21st to end	Day	18% 11% 28% 27% 11% 4	583
November	1st to 10th	Day	18% 10% 32% 25% 12% 4	535
	11th to 20th	Day	19% 18% 29% 23% 10% 2	440
	21st to end	Day	21% 21% 28% 21% 7% 2	229
December	1st to 10th	Day	27% 16% 37% 15% 5% 1	220
	11th to 20th	Day	26% 14% 27% 27% 4 1	220
	21st to end	Day	28% 16% 31% 17% 6% 2	360
January	1st to 10th	Night	21% 18% 33% 25% 3	840
	11th to 20th	Night	19% 18% 34% 27% 2	840
	21st to end	Night	19% 15% 40% 23% 2	924
February	1st to 10th	Night	19% 15% 37% 27% 3	840
	11th to 20th	Night	26% 17% 38% 18% 1	823
	21st to end	Night	20% 17% 39% 22% 3	692
March	1st to 10th	Night	19% 15% 35% 26% 4	840
	11th to 20th	Night	20% 15% 34% 23% 7% 1	840
	21st to end	Night	19% 13% 37% 26% 5%	924
April	1st to 10th	Night	18% 16% 35% 25% 7%	840
	11th to 20th	Night	13% 13% 36% 28% 10%	831
	21st to end	Night	16% 8% 33% 33% 9%	793
May	1st to 10th	Night	15% 9% 34% 27% 14%	630
	11th to 20th	Night	19% 5% 26% 33% 15% 1	621
	21st to end	Night	18% 2 16% 34% 26% 3	683
June	1st to 10th	Night	10% 2 19% 38% 29% 2	613
	11th to 20th	Night	12% 1 16% 38% 29% 3	508
	21st to end	Night	10% 2 13% 43% 29% 4	630
July	1st to 10th	Night	11% 1 13% 33% 35% 7%	615
	11th to 20th	Night	6% 1 8% 34% 45% 5%	612
	21st to end	Night	9% 1 12% 41% 32% 5%	690
August	1st to 10th	Night	17% 2 8% 36% 32% 5%	627
	11th to 20th	Night	11% 2 9% 39% 35% 3	624
	21st to end	Night	11% 2 11% 36% 34% 6%	924
September	1st to 10th	Night	14% 2 10% 33% 35% 6%	840
	11th to 20th	Night	17% 3 10% 34% 32% 4	840
	21st to end	Night	27% 4 17% 29% 22% 1	840
October	1st to 10th	Night	19% 4 18% 29% 24% 5%	840
	11th to 20th	Night	18% 8% 23% 33% 17% 1	813
	21st to end	Night	17% 7% 27% 34% 13% 1	880
November	1st to 10th	Night	17% 8% 28% 33% 13% 1	820
	11th to 20th	Night	16% 11% 31% 30% 11%	840
	21st to end	Night	16% 10% 34% 30% 10% 1	840
December	1st to 10th	Night	21% 11% 36% 26% 5%	840
	11th to 20th	Night	19% 11% 35% 30% 4	840
	21st to end	Night	21% 13% 33% 28% 4	924

Page generated at 2026-10-11 05:06:29. Version ac07

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