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 2025-08-05.

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	8% 6% 2 5% 3 3 4 3 5% 7% 54%	129608
	Night	8% 6% 2 5% 4 4 4 3 5% 7% 53%	127270
Transparency	Day	30% 6% 13% 27% 20% 5%	129323
	Night	30% 6% 13% 26% 21% 5%	127073
Seeing	Day	11% 8% 31% 32% 14% 3	16768
	Night	11% 8% 34% 37% 10%	26636

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	8% 10% 4 6% 4 4 4 3 5% 7% 46%	3638
	11th to 20th	Day	5% 9% 4 8% 5% 5% 3 3 5% 6% 46%	3770
	21st to end	Day	4 7% 3 7% 4 4 4 2 5% 5% 53%	4488
February	1st to 10th	Day	8% 10% 4 7% 5% 5% 4 4 6% 6% 41%	1873
	11th to 20th	Day	5% 7% 3 6% 6% 4 5% 4 5% 8% 48%	1949
	21st to end	Day	6% 6% 2 5% 4 4 3 3 5% 8% 55%	1650
March	1st to 10th	Day	10% 7% 2 4 2 2 2 2 4 5% 59%	4536
	11th to 20th	Day	9% 6% 2 4 3 3 3 3 4 7% 58%	4980
	21st to end	Day	10% 6% 2 5% 3 3 4 3 6% 8% 49%	5464
April	1st to 10th	Day	10% 6% 2 5% 3 3 4 3 5% 8% 52%	2345
	11th to 20th	Day	11% 6% 3 5% 4 4 3 3 5% 8% 47%	2446
	21st to end	Day	16% 6% 2 5% 3 4 4 3 5% 7% 45%	2600
May	1st to 10th	Day	10% 5% 2 4 3 3 4 3 6% 8% 52%	5900
	11th to 20th	Day	10% 5% 2 4 3 3 3 3 5% 7% 57%	5862
	21st to end	Day	17% 7% 3 5% 4 3 4 3 5% 7% 40%	6446
June	1st to 10th	Day	5% 3 2 3 3 3 3 3 6% 10% 59%	2679
	11th to 20th	Day	7% 4 1 4 3 4 4 4 6% 8% 55%	2800
	21st to end	Day	5% 4 1 4 4 3 4 4 6% 7% 59%	2800
July	1st to 10th	Day	6% 4 2 5% 4 4 4 4 5% 8% 55%	6339
	11th to 20th	Day	6% 3 2 4 3 4 4 4 7% 9% 54%	6262
	21st to end	Day	5% 4 2 5% 4 4 5% 4 6% 8% 52%	6424
August	1st to 10th	Day	9% 6% 2 6% 3 4 4 3 6% 9% 49%	4156
	11th to 20th	Day	10% 5% 2 5% 3 4 4 4 7% 9% 48%	4041
	21st to end	Day	5% 4 2 4 4 3 3 3 5% 7% 61%	4257
September	1st to 10th	Day	10% 4 1 4 2 3 4 3 4 6% 59%	2185
	11th to 20th	Day	9% 4 1 4 3 3 4 2 3 5% 60%	2090
	21st to end	Day	8% 3 1 4 2 3 3 3 3 6% 64%	2090
October	1st to 10th	Day	9% 2 1 2 1 2 2 2 3 4 72%	3480
	11th to 20th	Day	5% 3 1 3 2 3 3 2 4 6% 67%	3154
	21st to end	Day	9% 4 2 4 2 2 3 3 4 6% 62%	3162
November	1st to 10th	Day	8% 4 1 4 4 3 4 2 5% 6% 60%	1763
	11th to 20th	Day	7% 5% 2 3 3 2 3 3 4 5% 61%	1800
	21st to end	Day	9% 7% 3 5% 3 3 4 3 3 6% 55%	1800
December	1st to 10th	Day	4 5% 2 5% 3 3 3 3 4 6% 62%	2940
	11th to 20th	Day	6% 7% 2 5% 3 3 3 3 5% 6% 58%	3391
	21st to end	Day	10% 12% 4 11% 4 3 4 3 4 5% 40%	4048
January	1st to 10th	Night	7% 10% 4 11% 5% 3 4 3 5% 7% 40%	6327
	11th to 20th	Night	3 7% 3 7% 4 4 4 3 4 7% 54%	6192
	21st to end	Night	6% 8% 3 8% 5% 5% 4 3 4 5% 48%	6490
February	1st to 10th	Night	8% 9% 3 6% 4 4 5% 4 5% 6% 46%	2545
	11th to 20th	Night	5% 10% 2 6% 5% 4 4 4 6% 8% 47%	2338
	21st to end	Night	5% 6% 2 6% 4 4 4 3 4 6% 56%	1980
March	1st to 10th	Night	8% 5% 2 4 3 3 3 2 4 5% 61%	5333
	11th to 20th	Night	8% 5% 2 3 3 3 3 3 5% 6% 59%	4980
	21st to end	Night	10% 6% 2 6% 4 5% 5% 5% 6% 9% 41%	5467
April	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 5% 7% 50%	2051
	11th to 20th	Night	12% 5% 2 6% 4 4 4 4 5% 8% 46%	1900
	21st to end	Night	13% 5% 2 5% 3 4 3 3 5% 8% 48%	1800
May	1st to 10th	Night	16% 5% 2 5% 3 4 4 3 5% 7% 46%	4080
	11th to 20th	Night	8% 5% 2 5% 3 3 4 3 6% 7% 53%	4053
	21st to end	Night	18% 7% 2 5% 3 3 4 3 6% 8% 40%	4451
June	1st to 10th	Night	5% 3 2 4 3 4 4 4 7% 10% 54%	1715
	11th to 20th	Night	8% 4 1 4 4 3 4 4 7% 9% 52%	1600
	21st to end	Night	4 3 2 5% 4 3 5% 5% 7% 10% 54%	1600
July	1st to 10th	Night	6% 4 1 5% 4 4 4 4 7% 10% 50%	3640
	11th to 20th	Night	7% 5% 2 6% 4 5% 6% 4 7% 10% 43%	3715
	21st to end	Night	4 4 2 5% 3 4 4 4 6% 8% 57%	4449
August	1st to 10th	Night	5% 4 2 4 4 4 4 4 6% 12% 51%	2872
	11th to 20th	Night	9% 6% 2 5% 3 4 5% 3 6% 8% 49%	2798
	21st to end	Night	4 3 2 5% 3 5% 5% 4 7% 9% 54%	3407
September	1st to 10th	Night	7% 3 1 4 2 3 3 3 5% 7% 63%	1995
	11th to 20th	Night	7% 5% 1 4 2 3 3 3 5% 6% 61%	2090
	21st to end	Night	7% 4 1 4 3 3 3 3 5% 6% 62%	2090
October	1st to 10th	Night	8% 2 1 3 2 1 2 2 4 5% 71%	3480
	11th to 20th	Night	6% 3 1 3 3 2 2 2 4 6% 68%	3732
	21st to end	Night	7% 3 1 4 3 3 3 3 4 6% 64%	4391
November	1st to 10th	Night	5% 4 2 3 3 3 3 3 5% 7% 63%	2535
	11th to 20th	Night	7% 4 2 4 3 3 3 2 5% 6% 61%	2600
	21st to end	Night	8% 6% 2 4 3 3 3 2 5% 6% 59%	2600
December	1st to 10th	Night	3 4 2 3 3 2 3 3 5% 5% 67%	4250
	11th to 20th	Night	5% 4 1 4 3 3 4 3 6% 7% 59%	4816
	21st to end	Night	10% 13% 4 8% 4 4 3 3 4 6% 42%	6908

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% 5% 13% 26% 16% 1	3639
	11th to 20th	Day	38% 7% 9% 25% 18% 3	3770
	21st to end	Day	33% 5% 12% 27% 19% 4	4488
February	1st to 10th	Day	42% 6% 16% 21% 12% 3	1873
	11th to 20th	Day	35% 6% 16% 24% 16% 4	1959
	21st to end	Day	28% 5% 13% 26% 21% 7%	1650
March	1st to 10th	Day	29% 3 9% 20% 29% 9%	4536
	11th to 20th	Day	28% 4 9% 23% 27% 10%	4980
	21st to end	Day	32% 6% 21% 30% 9% 1	5464
April	1st to 10th	Day	31% 6% 15% 26% 20% 3	2345
	11th to 20th	Day	36% 5% 15% 24% 17% 3	2449
	21st to end	Day	38% 7% 13% 27% 15% 1	2600
May	1st to 10th	Day	29% 7% 13% 29% 17% 5%	5900
	11th to 20th	Day	27% 6% 11% 26% 21% 8%	5866
	21st to end	Day	40% 8% 13% 25% 13% 1	6447
June	1st to 10th	Day	21% 7% 19% 35% 15% 3	2679
	11th to 20th	Day	26% 8% 14% 28% 19% 5%	2800
	21st to end	Day	24% 7% 12% 29% 24% 5%	2800
July	1st to 10th	Day	27% 7% 10% 27% 25% 4	6271
	11th to 20th	Day	24% 8% 19% 27% 20% 3	6187
	21st to end	Day	25% 8% 14% 25% 22% 6%	6240
August	1st to 10th	Day	31% 7% 15% 24% 19% 4	4143
	11th to 20th	Day	33% 7% 17% 23% 18% 2	4080
	21st to end	Day	23% 6% 13% 27% 22% 9%	4256
September	1st to 10th	Day	28% 5% 11% 24% 25% 8%	2185
	11th to 20th	Day	26% 5% 11% 28% 25% 4	2090
	21st to end	Day	23% 4 9% 33% 28% 3	2090
October	1st to 10th	Day	19% 3 6% 23% 39% 11%	3480
	11th to 20th	Day	19% 5% 11% 37% 19% 8%	3154
	21st to end	Day	25% 3 9% 30% 29% 4	3162
November	1st to 10th	Day	26% 4 12% 31% 24% 4	1763
	11th to 20th	Day	25% 4 11% 28% 29% 4	1800
	21st to end	Day	32% 4 12% 26% 22% 4	1800
December	1st to 10th	Day	24% 4 11% 34% 23% 3	2940
	11th to 20th	Day	28% 4 17% 36% 15% 1	3389
	21st to end	Day	47% 5% 8% 25% 14% 1	4048
January	1st to 10th	Night	43% 5% 12% 23% 15% 2	6327
	11th to 20th	Night	30% 5% 12% 26% 25% 2	6192
	21st to end	Night	37% 6% 10% 21% 19% 7%	6490
February	1st to 10th	Night	39% 6% 13% 23% 15% 5%	2545
	11th to 20th	Night	35% 6% 11% 30% 16% 2	2338
	21st to end	Night	28% 5% 12% 22% 24% 9%	1980
March	1st to 10th	Night	25% 4 7% 24% 30% 10%	5333
	11th to 20th	Night	26% 4 8% 19% 27% 17%	4980
	21st to end	Night	36% 7% 21% 25% 9% 1	5467
April	1st to 10th	Night	32% 7% 12% 23% 21% 4	2051
	11th to 20th	Night	35% 6% 14% 23% 19% 2	1900
	21st to end	Night	34% 7% 13% 27% 17% 2	1800
May	1st to 10th	Night	36% 7% 13% 23% 18% 3	4080
	11th to 20th	Night	28% 7% 13% 25% 21% 6%	4054
	21st to end	Night	39% 9% 15% 23% 13% 1	4452
June	1st to 10th	Night	23% 9% 19% 28% 17% 3	1715
	11th to 20th	Night	28% 7% 15% 26% 19% 5%	1600
	21st to end	Night	25% 7% 16% 27% 22% 5%	1600
July	1st to 10th	Night	27% 7% 15% 29% 19% 3	3600
	11th to 20th	Night	33% 7% 16% 24% 16% 4	3667
	21st to end	Night	24% 7% 15% 25% 23% 8%	4320
August	1st to 10th	Night	25% 7% 18% 26% 21% 3	2862
	11th to 20th	Night	33% 7% 17% 25% 16% 2	2825
	21st to end	Night	24% 8% 15% 27% 18% 9%	3407
September	1st to 10th	Night	22% 5% 11% 25% 27% 10%	1995
	11th to 20th	Night	24% 6% 10% 27% 26% 7%	2090
	21st to end	Night	23% 5% 9% 26% 30% 7%	2090
October	1st to 10th	Night	17% 3 6% 18% 45% 10%	3480
	11th to 20th	Night	19% 4 10% 28% 26% 12%	3732
	21st to end	Night	22% 4 11% 26% 31% 6%	4391
November	1st to 10th	Night	21% 4 10% 29% 27% 8%	2535
	11th to 20th	Night	25% 4 9% 28% 27% 7%	2600
	21st to end	Night	27% 4 11% 27% 26% 5%	2600
December	1st to 10th	Night	19% 4 14% 36% 22% 4	4250
	11th to 20th	Night	23% 4 15% 36% 19% 3	4817
	21st to end	Night	43% 6% 9% 28% 13% 1	6908

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	11% 11% 38% 30% 9% 2	300
	11th to 20th	Day	15% 15% 34% 27% 9%	300
	21st to end	Day	10% 9% 45% 27% 9% 1	330
February	1st to 10th	Day	14% 15% 45% 22% 4	300
	11th to 20th	Day	12% 9% 45% 27% 6% 1	300
	21st to end	Day	9% 16% 51% 20% 4	249
March	1st to 10th	Day	15% 12% 42% 24% 7%	300
	11th to 20th	Day	19% 10% 33% 24% 11% 2	300
	21st to end	Day	19% 12% 40% 22% 5% 1	330
April	1st to 10th	Day	16% 19% 36% 22% 6%	445
	11th to 20th	Day	13% 18% 38% 26% 5%	567
	21st to end	Day	20% 13% 36% 22% 8% 2	700
May	1st to 10th	Day	14% 10% 32% 30% 13% 1	703
	11th to 20th	Day	18% 7% 31% 28% 13% 3	725
	21st to end	Day	17% 9% 32% 28% 12% 2	789
June	1st to 10th	Day	5% 6% 25% 37% 20% 7%	730
	11th to 20th	Day	7% 5% 34% 36% 16% 2	730
	21st to end	Day	6% 8% 31% 37% 17% 1	730
July	1st to 10th	Day	7% 3 22% 39% 24% 5%	721
	11th to 20th	Day	4 3 20% 44% 24% 5%	718
	21st to end	Day	5% 2 19% 43% 26% 5%	794
August	1st to 10th	Day	6% 2 19% 45% 24% 4	697
	11th to 20th	Day	7% 4 22% 44% 21% 3	665
	21st to end	Day	9% 2 21% 42% 22% 5%	590
September	1st to 10th	Day	11% 4 26% 40% 17% 2	395
	11th to 20th	Day	5% 7% 29% 34% 21% 5%	300
	21st to end	Day	13% 10% 25% 28% 19% 5%	300
October	1st to 10th	Day	18% 7% 23% 32% 16% 4	300
	11th to 20th	Day	4 14% 40% 27% 12% 4	300
	21st to end	Day	17% 13% 31% 27% 10% 1	330
November	1st to 10th	Day	14% 12% 34% 27% 10% 3	300
	11th to 20th	Day	13% 12% 39% 26% 10% 1	300
	21st to end	Day	17% 13% 33% 26% 9% 3	300
December	1st to 10th	Day	8% 7% 40% 36% 7% 2	300
	11th to 20th	Day	14% 9% 40% 30% 6%	300
	21st to end	Day	16% 11% 40% 25% 7%	330
January	1st to 10th	Night	11% 8% 37% 36% 8%	1000
	11th to 20th	Night	10% 12% 36% 34% 7%	1000
	21st to end	Night	10% 8% 43% 31% 7%	1100
February	1st to 10th	Night	12% 10% 42% 31% 5%	945
	11th to 20th	Night	10% 11% 41% 33% 5%	784
	21st to end	Night	9% 12% 40% 33% 5%	660
March	1st to 10th	Night	13% 12% 37% 33% 5%	785
	11th to 20th	Night	16% 10% 36% 30% 7% 1	600
	21st to end	Night	17% 13% 40% 25% 6%	660
April	1st to 10th	Night	12% 15% 43% 26% 4	600
	11th to 20th	Night	15% 13% 41% 27% 4	593
	21st to end	Night	15% 13% 32% 32% 8%	600
May	1st to 10th	Night	17% 9% 34% 32% 8%	600
	11th to 20th	Night	15% 7% 37% 32% 9% 1	591
	21st to end	Night	16% 8% 34% 31% 10%	650
June	1st to 10th	Night	6% 6% 31% 42% 14% 2	600
	11th to 20th	Night	9% 6% 36% 39% 11%	600
	21st to end	Night	5% 5% 36% 44% 10%	600
July	1st to 10th	Night	6% 4 24% 45% 19% 1	585
	11th to 20th	Night	3 2 21% 54% 18% 1	582
	21st to end	Night	5% 1 18% 51% 23% 1	657
August	1st to 10th	Night	6% 2 17% 53% 21% 1	579
	11th to 20th	Night	7% 4 25% 46% 17% 1	564
	21st to end	Night	6% 2 25% 47% 19% 2	627
September	1st to 10th	Night	7% 3 28% 50% 10% 2	570
	11th to 20th	Night	10% 3 32% 38% 16%	570
	21st to end	Night	8% 7% 32% 37% 15% 1	570
October	1st to 10th	Night	14% 8% 29% 37% 12%	570
	11th to 20th	Night	11% 9% 36% 34% 8%	726
	21st to end	Night	13% 10% 33% 32% 12% 1	993
November	1st to 10th	Night	8% 9% 33% 41% 8%	975
	11th to 20th	Night	10% 10% 35% 37% 8%	1000
	21st to end	Night	11% 9% 32% 37% 11% 1	1000
December	1st to 10th	Night	9% 9% 38% 39% 6%	1000
	11th to 20th	Night	12% 8% 38% 34% 8%	1000
	21st to end	Night	15% 9% 35% 35% 5%	1100

Page generated at 2025-08-02 00:32:26. Version ac07

Copyright © 2025 Attilla Danko.