Alma
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alma clear sky chart from 2005-11-06 to 2026-06-01.

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	12% 8% 3 7% 4 4 4 4 6% 8% 41%	132070
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	131317
Transparency	Day	41% 5% 11% 25% 16% 2	135643
	Night	41% 4 11% 24% 17% 3	135820
Seeing	Day	16% 9% 25% 25% 18% 7%	17685
	Night	17% 9% 28% 30% 14% 2	27561

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	12% 10% 5% 12% 6% 4 4 3 5% 6% 33%	4691
	11th to 20th	Day	8% 8% 3 8% 5% 6% 4 4 5% 8% 40%	4748
	21st to end	Day	17% 14% 4 8% 5% 4 4 3 4 5% 32%	5812
February	1st to 10th	Day	16% 9% 4 8% 4 4 5% 3 5% 7% 35%	1985
	11th to 20th	Day	16% 10% 3 8% 5% 3 5% 4 5% 6% 35%	2048
	21st to end	Day	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	1730
March	1st to 10th	Day	17% 13% 3 5% 3 3 3 2 3 4 45%	4580
	11th to 20th	Day	10% 9% 2 5% 3 3 4 3 5% 8% 48%	4872
	21st to end	Day	15% 9% 3 7% 5% 5% 5% 4 6% 8% 35%	6070
April	1st to 10th	Day	14% 7% 3 7% 4 4 4 4 5% 9% 40%	2520
	11th to 20th	Day	15% 9% 3 7% 4 4 3 4 4 6% 41%	2494
	21st to end	Day	16% 7% 2 7% 4 4 4 4 5% 7% 40%	2520
May	1st to 10th	Day	16% 9% 3 7% 5% 5% 5% 4 7% 10% 29%	5934
	11th to 20th	Day	16% 8% 3 7% 4 4 5% 4 5% 8% 37%	6080
	21st to end	Day	11% 7% 2 7% 5% 5% 6% 4 7% 8% 38%	7089
June	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 46%	2801
	11th to 20th	Day	7% 4 2 6% 5% 4 5% 5% 6% 8% 48%	2800
	21st to end	Day	6% 5% 2 5% 5% 4 6% 5% 7% 10% 45%	2800
July	1st to 10th	Day	9% 4 2 6% 4 4 4 4 6% 10% 46%	6360
	11th to 20th	Day	6% 6% 2 7% 5% 6% 6% 5% 8% 12% 38%	6360
	21st to end	Day	7% 6% 2 8% 6% 5% 6% 5% 8% 11% 37%	6924
August	1st to 10th	Day	6% 4 2 5% 4 3 4 3 6% 9% 54%	4042
	11th to 20th	Day	9% 5% 2 6% 4 4 6% 5% 7% 9% 42%	3849
	21st to end	Day	5% 4 2 5% 3 4 5% 3 7% 9% 53%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 3 4 4 5% 7% 49%	2400
	11th to 20th	Day	6% 5% 2 5% 4 5% 5% 4 6% 8% 51%	2297
	21st to end	Day	10% 5% 2 5% 4 4 4 3 5% 7% 51%	2200
October	1st to 10th	Day	9% 4 2 5% 4 3 4 2 5% 6% 56%	2125
	11th to 20th	Day	12% 6% 3 5% 3 3 4 3 5% 7% 50%	2000
	21st to end	Day	18% 7% 2 5% 3 3 4 2 5% 4 47%	2076
November	1st to 10th	Day	10% 6% 1 6% 3 4 4 3 5% 6% 54%	1852
	11th to 20th	Day	14% 8% 2 7% 4 4 3 2 4 6% 45%	1890
	21st to end	Day	17% 10% 4 8% 5% 4 3 3 4 5% 39%	1890
December	1st to 10th	Day	9% 10% 3 6% 4 3 3 3 5% 5% 48%	2761
	11th to 20th	Day	13% 10% 4 9% 5% 5% 5% 4 4 6% 34%	3088
	21st to end	Day	26% 17% 4 10% 4 4 3 2 3 4 21%	4070
January	1st to 10th	Night	14% 10% 5% 10% 4 4 4 3 5% 5% 37%	8232
	11th to 20th	Night	10% 10% 4 11% 4 5% 6% 4 4 5% 37%	8238
	21st to end	Night	17% 10% 5% 8% 4 4 4 3 4 6% 35%	8402
February	1st to 10th	Night	17% 10% 4 9% 5% 4 3 4 4 6% 33%	2633
	11th to 20th	Night	16% 9% 3 7% 5% 4 3 3 5% 6% 38%	2458
	21st to end	Night	16% 9% 3 9% 5% 3 4 3 4 5% 39%	2076
March	1st to 10th	Night	17% 12% 4 7% 4 4 4 3 4 5% 37%	5493
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 4 7% 50%	5339
	21st to end	Night	16% 8% 3 8% 5% 5% 5% 3 5% 7% 34%	5087
April	1st to 10th	Night	12% 6% 3 6% 5% 5% 4 3 5% 7% 45%	2100
	11th to 20th	Night	15% 7% 2 5% 3 4 4 3 5% 9% 42%	2075
	21st to end	Night	17% 8% 3 6% 5% 4 4 3 5% 6% 39%	2100
May	1st to 10th	Night	16% 8% 3 8% 5% 5% 4 4 6% 9% 32%	4180
	11th to 20th	Night	13% 7% 3 6% 5% 4 4 4 7% 8% 39%	4034
	21st to end	Night	13% 7% 3 7% 4 5% 5% 4 6% 9% 39%	4030
June	1st to 10th	Night	9% 6% 2 5% 4 3 4 4 5% 7% 51%	1600
	11th to 20th	Night	5% 5% 2 5% 5% 4 5% 4 6% 9% 52%	1600
	21st to end	Night	4 5% 1 4 5% 4 6% 5% 7% 10% 49%	1600
July	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 9% 49%	3618
	11th to 20th	Night	5% 5% 2 7% 5% 5% 5% 4 7% 9% 46%	3617
	21st to end	Night	6% 5% 2 6% 5% 6% 6% 5% 7% 11% 41%	3947
August	1st to 10th	Night	5% 3 1 4 4 4 4 3 7% 9% 55%	3103
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 8% 51%	3200
	21st to end	Night	4 3 1 4 3 3 4 3 5% 10% 59%	3586
September	1st to 10th	Night	10% 4 3 5% 3 4 4 3 6% 6% 53%	2000
	11th to 20th	Night	6% 5% 2 6% 4 4 4 3 6% 7% 54%	2081
	21st to end	Night	11% 6% 2 5% 4 4 5% 4 5% 7% 50%	2200
October	1st to 10th	Night	8% 4 2 4 3 4 3 3 5% 6% 59%	2270
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 5% 55%	2400
	21st to end	Night	18% 8% 2 6% 3 3 3 3 4 5% 45%	2761
November	1st to 10th	Night	10% 7% 2 6% 3 3 3 3 5% 6% 53%	2666
	11th to 20th	Night	13% 6% 2 6% 4 4 3 3 5% 6% 48%	2730
	21st to end	Night	20% 8% 3 6% 3 3 3 2 4 5% 42%	2730
December	1st to 10th	Night	15% 9% 3 7% 4 3 3 3 4 6% 44%	4642
	11th to 20th	Night	17% 8% 4 5% 3 4 4 3 5% 7% 41%	5339
	21st to end	Night	27% 16% 5% 8% 5% 4 3 3 3 4 23%	7150

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	52% 3 11% 24% 8% 1	4692
	11th to 20th	Day	43% 3 11% 23% 17% 3	4748
	21st to end	Day	56% 2 8% 18% 11% 5%	5812
February	1st to 10th	Day	50% 4 12% 23% 10% 1	1985
	11th to 20th	Day	49% 5% 9% 21% 14% 2	2058
	21st to end	Day	47% 4 10% 22% 14% 3	1730
March	1st to 10th	Day	45% 4 5% 21% 22% 4	4580
	11th to 20th	Day	35% 4 10% 29% 18% 3	4872
	21st to end	Day	47% 5% 13% 25% 10% 1	6072
April	1st to 10th	Day	41% 6% 13% 25% 15% 1	2520
	11th to 20th	Day	44% 4 11% 24% 15% 2	2494
	21st to end	Day	44% 5% 11% 26% 11% 2	2520
May	1st to 10th	Day	49% 7% 13% 21% 10% 1	5970
	11th to 20th	Day	46% 6% 9% 26% 12% 1	6084
	21st to end	Day	42% 6% 11% 22% 18% 1	7103
June	1st to 10th	Day	32% 9% 13% 27% 17% 3	2801
	11th to 20th	Day	32% 7% 12% 24% 21% 3	2800
	21st to end	Day	32% 8% 18% 22% 16% 5%	2800
July	1st to 10th	Day	34% 4 10% 32% 18% 3	6291
	11th to 20th	Day	38% 5% 16% 27% 12% 1	6285
	21st to end	Day	41% 4 19% 26% 9% 1	6728
August	1st to 10th	Day	27% 4 12% 40% 15% 1	4027
	11th to 20th	Day	36% 5% 15% 27% 16% 1	3885
	21st to end	Day	27% 4 15% 35% 15% 3	4311
September	1st to 10th	Day	33% 5% 11% 25% 21% 4	2400
	11th to 20th	Day	30% 6% 11% 24% 25% 3	2297
	21st to end	Day	33% 4 10% 23% 26% 4	2200
October	1st to 10th	Day	19% 3 6% 24% 42% 6%	3463
	11th to 20th	Day	25% 3 8% 22% 37% 6%	3240
	21st to end	Day	34% 3 6% 25% 30% 2	3327
November	1st to 10th	Day	32% 4 12% 28% 19% 6%	1852
	11th to 20th	Day	41% 3 8% 24% 17% 6%	1890
	21st to end	Day	49% 3 9% 24% 14% 1	1890
December	1st to 10th	Day	38% 4 14% 28% 16%	2761
	11th to 20th	Day	49% 3 11% 24% 11% 1	3085
	21st to end	Day	68% 3 10% 12% 5% 1	4070
January	1st to 10th	Night	51% 3 13% 25% 8%	8233
	11th to 20th	Night	50% 2 6% 20% 17% 4	8238
	21st to end	Night	52% 2 10% 19% 15% 1	8402
February	1st to 10th	Night	52% 4 11% 21% 11% 1	2633
	11th to 20th	Night	47% 4 10% 22% 16% 2	2459
	21st to end	Night	47% 4 9% 23% 13% 3	2076
March	1st to 10th	Night	49% 4 6% 20% 19% 2	5493
	11th to 20th	Night	36% 4 10% 25% 22% 4	5339
	21st to end	Night	49% 5% 10% 27% 9% 1	5087
April	1st to 10th	Night	39% 5% 11% 26% 17% 3	2100
	11th to 20th	Night	40% 4 11% 23% 19% 3	2075
	21st to end	Night	45% 5% 10% 22% 15% 3	2100
May	1st to 10th	Night	47% 7% 15% 21% 9% 1	4207
	11th to 20th	Night	42% 7% 12% 25% 14% 1	4035
	21st to end	Night	41% 6% 12% 24% 15% 2	4038
June	1st to 10th	Night	32% 6% 13% 25% 22% 3	1600
	11th to 20th	Night	27% 8% 11% 25% 24% 5%	1600
	21st to end	Night	28% 7% 15% 24% 19% 7%	1600
July	1st to 10th	Night	34% 5% 8% 27% 23% 3	3580
	11th to 20th	Night	34% 5% 15% 30% 15% 2	3572
	21st to end	Night	38% 4 20% 26% 11% 1	3832
August	1st to 10th	Night	25% 5% 17% 37% 16% 1	3094
	11th to 20th	Night	31% 6% 13% 28% 22% 1	3230
	21st to end	Night	22% 5% 14% 34% 22% 3	3586
September	1st to 10th	Night	31% 6% 8% 28% 21% 6%	2000
	11th to 20th	Night	28% 5% 8% 29% 25% 4	2081
	21st to end	Night	33% 6% 8% 20% 26% 6%	2200
October	1st to 10th	Night	17% 3 5% 19% 48% 7%	3712
	11th to 20th	Night	24% 2 8% 23% 37% 6%	3899
	21st to end	Night	34% 3 7% 25% 25% 7%	4460
November	1st to 10th	Night	33% 3 10% 25% 24% 5%	2666
	11th to 20th	Night	37% 3 9% 24% 22% 4	2730
	21st to end	Night	46% 3 10% 24% 16% 1	2730
December	1st to 10th	Night	43% 3 11% 26% 17%	4642
	11th to 20th	Night	44% 3 13% 24% 15%	5341
	21st to end	Night	67% 3 10% 14% 5% 1	7150

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	24% 16% 33% 20% 7%	330
	11th to 20th	Day	27% 18% 35% 19% 1	330
	21st to end	Day	21% 19% 42% 15% 3	363
February	1st to 10th	Day	28% 14% 39% 16% 3	330
	11th to 20th	Day	26% 24% 30% 15% 4 1	330
	21st to end	Day	26% 18% 37% 16% 3	273
March	1st to 10th	Day	24% 18% 39% 15% 4	330
	11th to 20th	Day	28% 18% 37% 13% 4	330
	21st to end	Day	28% 14% 34% 17% 6%	578
April	1st to 10th	Day	21% 15% 33% 23% 7% 1	540
	11th to 20th	Day	20% 20% 27% 19% 12% 2	532
	21st to end	Day	21% 13% 33% 22% 9% 2	540
May	1st to 10th	Day	21% 11% 32% 24% 9% 2	543
	11th to 20th	Day	16% 9% 26% 28% 16% 5%	604
	21st to end	Day	11% 5% 28% 31% 18% 7%	844
June	1st to 10th	Day	11% 4 20% 30% 24% 10%	731
	11th to 20th	Day	9% 3 20% 33% 27% 8%	730
	21st to end	Day	7% 4 24% 32% 24% 9%	730
July	1st to 10th	Day	10% 2 14% 26% 28% 19%	721
	11th to 20th	Day	6% 3 14% 36% 28% 13%	718
	21st to end	Day	9% 2 12% 32% 31% 14%	794
August	1st to 10th	Day	7% 3 13% 32% 32% 13%	611
	11th to 20th	Day	9% 3 14% 30% 30% 15%	519
	21st to end	Day	7% 2 12% 30% 34% 16%	583
September	1st to 10th	Day	11% 3 15% 28% 30% 12%	530
	11th to 20th	Day	8% 2 11% 28% 35% 16%	530
	21st to end	Day	13% 7% 23% 22% 23% 13%	530
October	1st to 10th	Day	11% 5% 20% 33% 24% 7%	455
	11th to 20th	Day	15% 15% 30% 26% 11% 4	330
	21st to end	Day	29% 16% 29% 18% 7%	363
November	1st to 10th	Day	14% 12% 35% 22% 14% 3	330
	11th to 20th	Day	19% 18% 31% 22% 8% 2	330
	21st to end	Day	26% 12% 30% 24% 7% 1	330
December	1st to 10th	Day	15% 18% 41% 16% 5% 4	330
	11th to 20th	Day	20% 18% 39% 18% 5%	330
	21st to end	Day	32% 16% 32% 15% 4	363
January	1st to 10th	Night	18% 16% 37% 25% 4	1050
	11th to 20th	Night	23% 17% 35% 22% 3	1050
	21st to end	Night	20% 15% 39% 25% 2	1155
February	1st to 10th	Night	23% 11% 40% 25% 1	953
	11th to 20th	Night	23% 16% 36% 23% 2	823
	21st to end	Night	23% 16% 38% 22% 1	692
March	1st to 10th	Night	22% 14% 39% 23% 3	840
	11th to 20th	Night	22% 12% 35% 26% 5%	749
	21st to end	Night	24% 11% 32% 26% 6%	693
April	1st to 10th	Night	17% 13% 36% 28% 7%	630
	11th to 20th	Night	18% 11% 33% 27% 11%	623
	21st to end	Night	21% 7% 34% 29% 8% 1	630
May	1st to 10th	Night	16% 7% 31% 36% 10%	630
	11th to 20th	Night	13% 5% 27% 36% 16% 2	621
	21st to end	Night	10% 3 22% 41% 21% 3	683
June	1st to 10th	Night	11% 4 18% 42% 24% 1	600
	11th to 20th	Night	7% 1 20% 46% 24% 3	600
	21st to end	Night	7% 2 21% 41% 27% 3	600
July	1st to 10th	Night	11% 1 11% 31% 37% 8%	585
	11th to 20th	Night	5% 3 11% 40% 37% 4	582
	21st to end	Night	8% 1 11% 41% 35% 5%	657
August	1st to 10th	Night	8% 2 10% 40% 36% 5%	597
	11th to 20th	Night	7% 1 11% 39% 37% 4	594
	21st to end	Night	7% 1 11% 34% 43% 5%	660
September	1st to 10th	Night	11% 2 13% 39% 30% 6%	600
	11th to 20th	Night	8% 2 14% 36% 35% 6%	681
	21st to end	Night	14% 4 20% 31% 26% 5%	800
October	1st to 10th	Night	9% 4 23% 36% 24% 4	800
	11th to 20th	Night	16% 9% 29% 30% 15% 1	800
	21st to end	Night	22% 10% 33% 28% 7%	880
November	1st to 10th	Night	14% 8% 37% 31% 10%	820
	11th to 20th	Night	16% 12% 32% 30% 9%	840
	21st to end	Night	25% 11% 31% 25% 8%	840
December	1st to 10th	Night	22% 12% 36% 26% 5%	998
	11th to 20th	Night	23% 10% 32% 30% 5%	1050
	21st to end	Night	29% 13% 34% 21% 3	1155

Page generated at 2026-05-29 00:53:32. Version ac07

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