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 2025-06-10.

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	18% 10% 3 8% 5% 4 4 4 5% 7% 31%	119701
	Night	18% 10% 3 7% 5% 4 4 3 5% 6% 33%	118636
Transparency	Day	51% 6% 12% 19% 10% 1	123294
	Night	51% 6% 11% 20% 11% 1	123203
Seeing	Day	23% 18% 30% 21% 7% 1	17574
	Night	24% 17% 30% 22% 6% 1	25273

By Month

Percentage Cloud Cover NE (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	11% 16% 4 9% 6% 5% 5% 4 5% 5% 30%	3157
	11th to 20th	Day	13% 14% 5% 11% 5% 4 4 3 4 5% 31%	3552
	21st to end	Day	10% 9% 4 8% 5% 4 5% 4 6% 8% 38%	4466
February	1st to 10th	Day	15% 13% 4 9% 5% 4 4 3 5% 6% 31%	1800
	11th to 20th	Day	14% 13% 4 8% 6% 5% 4 5% 5% 5% 30%	1753
	21st to end	Day	10% 15% 4 9% 6% 3 4 4 5% 6% 34%	1520
March	1st to 10th	Day	20% 13% 5% 13% 5% 3 4 3 4 5% 27%	4880
	11th to 20th	Day	17% 16% 4 9% 5% 4 4 3 4 6% 28%	4980
	21st to end	Day	25% 13% 5% 8% 6% 4 4 4 6% 5% 21%	5466
April	1st to 10th	Day	21% 14% 3 8% 5% 5% 4 4 5% 5% 26%	2355
	11th to 20th	Day	17% 13% 3 8% 6% 4 4 3 5% 7% 31%	2522
	21st to end	Day	21% 10% 3 9% 5% 4 4 3 5% 7% 29%	2600
May	1st to 10th	Day	23% 13% 4 9% 6% 5% 5% 4 5% 5% 21%	5968
	11th to 20th	Day	18% 10% 4 8% 5% 5% 5% 3 5% 6% 32%	6318
	21st to end	Day	20% 11% 3 8% 5% 4 5% 4 6% 7% 27%	6948
June	1st to 10th	Day	23% 10% 3 8% 5% 4 4 4 5% 7% 26%	2915
	11th to 20th	Day	20% 8% 3 6% 5% 4 4 4 6% 8% 31%	2850
	21st to end	Day	28% 9% 2 7% 5% 4 4 3 4 7% 27%	2850
July	1st to 10th	Day	17% 7% 3 7% 5% 5% 6% 5% 8% 9% 28%	4760
	11th to 20th	Day	17% 8% 3 7% 5% 5% 4 4 6% 10% 31%	4646
	21st to end	Day	15% 7% 3 7% 6% 5% 6% 4 7% 8% 32%	4628
August	1st to 10th	Day	19% 8% 3 9% 5% 6% 5% 4 7% 8% 26%	4105
	11th to 20th	Day	15% 7% 3 6% 5% 5% 5% 4 6% 9% 35%	4045
	21st to end	Day	12% 6% 2 5% 4 4 4 4 7% 8% 43%	4441
September	1st to 10th	Day	14% 5% 2 5% 5% 4 5% 4 6% 8% 40%	2280
	11th to 20th	Day	18% 7% 2 7% 3 4 4 4 4 7% 41%	2152
	21st to end	Day	18% 7% 3 5% 3 4 4 3 5% 7% 42%	2066
October	1st to 10th	Day	16% 8% 3 6% 4 4 4 3 5% 8% 38%	1900
	11th to 20th	Day	20% 7% 2 7% 5% 3 4 3 5% 6% 37%	1900
	21st to end	Day	19% 8% 4 9% 5% 4 4 4 5% 6% 33%	2057
November	1st to 10th	Day	22% 10% 3 7% 5% 5% 4 3 4 6% 32%	1613
	11th to 20th	Day	19% 11% 3 7% 4 4 4 3 5% 6% 34%	1600
	21st to end	Day	19% 12% 3 8% 5% 4 5% 3 5% 5% 30%	1600
December	1st to 10th	Day	20% 11% 3 7% 3 4 4 3 5% 8% 32%	2610
	11th to 20th	Day	24% 12% 2 6% 4 4 3 2 4 4 34%	2922
	21st to end	Day	17% 12% 5% 7% 4 5% 4 4 6% 5% 31%	3476
January	1st to 10th	Night	11% 14% 4 9% 6% 5% 4 3 4 6% 34%	6808
	11th to 20th	Night	16% 12% 3 9% 5% 5% 5% 4 5% 6% 29%	6328
	21st to end	Night	10% 11% 5% 8% 5% 4 4 3 5% 7% 39%	6490
February	1st to 10th	Night	18% 11% 3 8% 5% 4 4 4 6% 7% 31%	2600
	11th to 20th	Night	15% 14% 4 8% 5% 4 5% 3 5% 6% 31%	2533
	21st to end	Night	12% 13% 5% 7% 6% 4 4 3 6% 6% 34%	2105
March	1st to 10th	Night	24% 15% 3 9% 5% 5% 4 2 4 4 25%	4959
	11th to 20th	Night	19% 11% 5% 10% 5% 4 4 3 5% 6% 28%	4979
	21st to end	Night	28% 12% 3 7% 4 4 4 3 4 4 26%	5466
April	1st to 10th	Night	22% 12% 4 7% 5% 4 5% 4 5% 4 28%	2045
	11th to 20th	Night	20% 10% 3 6% 5% 5% 4 3 6% 6% 32%	1823
	21st to end	Night	20% 11% 3 7% 6% 4 5% 3 5% 6% 30%	1800
May	1st to 10th	Night	24% 12% 3 8% 5% 5% 5% 4 5% 6% 24%	3976
	11th to 20th	Night	19% 9% 3 8% 5% 5% 4 4 5% 5% 34%	3596
	21st to end	Night	17% 10% 3 8% 5% 5% 5% 4 6% 6% 30%	3948
June	1st to 10th	Night	24% 9% 3 8% 4 5% 5% 3 5% 8% 26%	1486
	11th to 20th	Night	18% 9% 3 6% 5% 6% 4 3 5% 6% 35%	1330
	21st to end	Night	26% 9% 3 6% 4 6% 5% 4 5% 6% 26%	1330
July	1st to 10th	Night	17% 8% 3 9% 6% 5% 6% 4 7% 8% 27%	2210
	11th to 20th	Night	16% 9% 3 7% 6% 4 5% 5% 7% 8% 29%	2291
	21st to end	Night	16% 7% 3 7% 4 4 5% 4 6% 7% 36%	2928
August	1st to 10th	Night	18% 9% 3 8% 6% 5% 5% 4 7% 7% 28%	2825
	11th to 20th	Night	12% 6% 2 7% 5% 5% 6% 4 8% 8% 35%	2782
	21st to end	Night	12% 6% 2 5% 4 5% 4 4 5% 7% 47%	3215
September	1st to 10th	Night	13% 6% 3 6% 4 4 5% 3 7% 7% 42%	1900
	11th to 20th	Night	17% 8% 3 5% 3 3 3 3 4 7% 44%	2025
	21st to end	Night	20% 8% 2 4 3 3 4 3 4 7% 42%	2114
October	1st to 10th	Night	19% 8% 2 6% 4 3 4 3 5% 6% 39%	2280
	11th to 20th	Night	19% 7% 3 6% 4 4 4 4 6% 6% 38%	2280
	21st to end	Night	24% 9% 3 6% 5% 4 3 4 4 5% 34%	2536
November	1st to 10th	Night	22% 9% 3 6% 4 4 4 3 4 5% 36%	2684
	11th to 20th	Night	20% 9% 3 7% 4 5% 4 3 6% 7% 32%	2800
	21st to end	Night	21% 9% 3 7% 5% 4 4 4 5% 6% 31%	2800
December	1st to 10th	Night	16% 10% 4 7% 4 3 3 3 5% 7% 37%	4580
	11th to 20th	Night	24% 12% 3 6% 3 3 3 3 4 5% 35%	5282
	21st to end	Night	21% 13% 4 7% 4 4 3 3 4 5% 32%	7502

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	54% 6% 13% 18% 8% 1	3158
	11th to 20th	Day	54% 4 13% 20% 9% 1	3582
	21st to end	Day	44% 5% 12% 16% 17% 6%	4466
February	1st to 10th	Day	54% 4 10% 17% 12% 2	1800
	11th to 20th	Day	55% 5% 11% 18% 10% 1	1761
	21st to end	Day	49% 5% 11% 20% 11% 3	1520
March	1st to 10th	Day	61% 4 8% 13% 11% 3	4884
	11th to 20th	Day	55% 6% 16% 16% 7% 2	4980
	21st to end	Day	62% 5% 7% 16% 9% 1	5466
April	1st to 10th	Day	58% 5% 9% 15% 11% 3	2355
	11th to 20th	Day	52% 5% 10% 20% 12% 1	2522
	21st to end	Day	55% 5% 11% 17% 11% 2	2600
May	1st to 10th	Day	61% 8% 10% 14% 7%	5979
	11th to 20th	Day	51% 7% 15% 21% 5% 1	6322
	21st to end	Day	53% 10% 12% 17% 8%	6948
June	1st to 10th	Day	56% 7% 12% 17% 8%	2915
	11th to 20th	Day	49% 7% 14% 21% 8% 1	2850
	21st to end	Day	57% 7% 11% 16% 8% 1	2850
July	1st to 10th	Day	48% 8% 16% 18% 10%	4685
	11th to 20th	Day	47% 8% 14% 24% 7%	4565
	21st to end	Day	47% 10% 16% 22% 5%	4480
August	1st to 10th	Day	53% 9% 13% 20% 5%	4092
	11th to 20th	Day	44% 9% 13% 22% 12%	4084
	21st to end	Day	36% 7% 15% 25% 16% 1	4441
September	1st to 10th	Day	39% 9% 14% 25% 13%	2280
	11th to 20th	Day	43% 6% 11% 24% 14% 2	2152
	21st to end	Day	42% 6% 11% 27% 14% 1	2066
October	1st to 10th	Day	43% 5% 12% 28% 12%	3160
	11th to 20th	Day	41% 4 11% 30% 13% 1	3140
	21st to end	Day	43% 4 16% 20% 17% 1	3371
November	1st to 10th	Day	53% 5% 10% 21% 10% 1	1613
	11th to 20th	Day	50% 5% 14% 17% 13% 1	1600
	21st to end	Day	56% 6% 12% 19% 8%	1600
December	1st to 10th	Day	51% 3 14% 26% 6% 1	2610
	11th to 20th	Day	55% 4 9% 16% 14% 2	2921
	21st to end	Day	52% 4 14% 17% 12% 1	3476
January	1st to 10th	Night	51% 6% 11% 25% 6% 1	6808
	11th to 20th	Night	52% 7% 14% 20% 7% 1	6372
	21st to end	Night	45% 5% 9% 21% 16% 4	6490
February	1st to 10th	Night	53% 5% 12% 16% 13% 2	2600
	11th to 20th	Night	53% 5% 11% 17% 12% 2	2536
	21st to end	Night	51% 4 10% 19% 13% 3	2105
March	1st to 10th	Night	64% 3 7% 11% 11% 4	4968
	11th to 20th	Night	55% 7% 14% 13% 9% 1	4980
	21st to end	Night	61% 4 6% 17% 11% 1	5466
April	1st to 10th	Night	58% 5% 8% 15% 11% 4	2045
	11th to 20th	Night	51% 5% 12% 20% 11% 1	1823
	21st to end	Night	55% 5% 11% 15% 13% 2	1800
May	1st to 10th	Night	58% 9% 11% 14% 8% 1	3982
	11th to 20th	Night	50% 8% 12% 21% 7% 2	3597
	21st to end	Night	49% 11% 15% 17% 7%	3949
June	1st to 10th	Night	55% 8% 10% 20% 6%	1486
	11th to 20th	Night	50% 6% 11% 23% 8% 2	1330
	21st to end	Night	57% 9% 10% 13% 9% 1	1330
July	1st to 10th	Night	51% 10% 12% 16% 11% 1	2175
	11th to 20th	Night	48% 11% 14% 19% 8%	2249
	21st to end	Night	44% 11% 13% 20% 11% 1	2836
August	1st to 10th	Night	53% 9% 14% 18% 6%	2813
	11th to 20th	Night	41% 11% 16% 21% 10% 1	2809
	21st to end	Night	35% 7% 16% 25% 16% 1	3214
September	1st to 10th	Night	38% 8% 12% 28% 12% 1	1900
	11th to 20th	Night	41% 5% 12% 26% 14% 1	2025
	21st to end	Night	41% 7% 12% 25% 13% 2	2114
October	1st to 10th	Night	48% 5% 12% 24% 12%	3800
	11th to 20th	Night	36% 4 12% 27% 20% 1	3779
	21st to end	Night	45% 3 13% 22% 17%	4172
November	1st to 10th	Night	49% 6% 13% 20% 10% 1	2684
	11th to 20th	Night	51% 6% 14% 19% 9% 1	2800
	21st to end	Night	52% 8% 13% 18% 8% 1	2800
December	1st to 10th	Night	47% 4 10% 28% 10%	4580
	11th to 20th	Night	53% 3 8% 17% 16% 2	5284
	21st to end	Night	56% 3 10% 18% 11% 1	7502

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	25% 28% 33% 14% 1	200
	11th to 20th	Day	32% 21% 28% 15% 4	299
	21st to end	Day	19% 20% 39% 17% 4	440
February	1st to 10th	Day	27% 25% 26% 18% 4 1	400
	11th to 20th	Day	22% 28% 35% 12% 3	396
	21st to end	Day	17% 24% 37% 19% 3	349
March	1st to 10th	Day	23% 24% 36% 12% 4 1	500
	11th to 20th	Day	21% 25% 32% 18% 3	500
	21st to end	Day	26% 24% 32% 13% 4 1	550
April	1st to 10th	Day	32% 27% 26% 13% 2	500
	11th to 20th	Day	26% 23% 30% 15% 5%	492
	21st to end	Day	26% 22% 28% 16% 7% 1	500
May	1st to 10th	Day	26% 19% 32% 16% 5% 1	533
	11th to 20th	Day	23% 15% 30% 22% 9% 2	724
	21st to end	Day	22% 17% 31% 24% 5% 1	790
June	1st to 10th	Day	28% 16% 27% 21% 7% 1	725
	11th to 20th	Day	23% 14% 30% 25% 8% 1	680
	21st to end	Day	28% 11% 26% 24% 10% 2	680
July	1st to 10th	Day	22% 12% 31% 25% 9% 1	670
	11th to 20th	Day	18% 8% 32% 26% 15% 1	668
	21st to end	Day	19% 9% 30% 32% 9% 1	628
August	1st to 10th	Day	17% 8% 31% 31% 11% 1	486
	11th to 20th	Day	19% 11% 27% 27% 13% 1	479
	21st to end	Day	17% 14% 24% 31% 11% 3	539
September	1st to 10th	Day	14% 12% 33% 30% 9% 2	490
	11th to 20th	Day	19% 13% 29% 26% 11% 3	490
	21st to end	Day	18% 16% 32% 23% 10% 1	477
October	1st to 10th	Day	20% 14% 26% 26% 12% 2	390
	11th to 20th	Day	24% 18% 29% 20% 8% 1	390
	21st to end	Day	25% 19% 34% 15% 6%	429
November	1st to 10th	Day	27% 21% 30% 17% 5% 1	395
	11th to 20th	Day	28% 24% 28% 15% 5% 1	400
	21st to end	Day	28% 27% 32% 12% 2	400
December	1st to 10th	Day	31% 21% 30% 17% 2	400
	11th to 20th	Day	35% 28% 25% 11% 1	365
	21st to end	Day	25% 25% 31% 15% 2	220
January	1st to 10th	Night	26% 25% 29% 18% 2	900
	11th to 20th	Night	27% 22% 30% 17% 4	857
	21st to end	Night	20% 24% 35% 16% 4	880
February	1st to 10th	Night	27% 24% 30% 17% 2	800
	11th to 20th	Night	24% 26% 33% 14% 2	783
	21st to end	Night	20% 23% 34% 18% 4	660
March	1st to 10th	Night	24% 20% 36% 18% 2	800
	11th to 20th	Night	22% 19% 33% 21% 6%	800
	21st to end	Night	24% 19% 33% 17% 5%	880
April	1st to 10th	Night	29% 18% 29% 21% 3	645
	11th to 20th	Night	24% 18% 33% 20% 5%	594
	21st to end	Night	28% 14% 30% 21% 7% 1	600
May	1st to 10th	Night	27% 14% 31% 20% 8% 1	600
	11th to 20th	Night	24% 12% 29% 26% 8%	592
	21st to end	Night	18% 13% 36% 26% 6%	649
June	1st to 10th	Night	28% 10% 28% 28% 6% 1	487
	11th to 20th	Night	23% 10% 24% 35% 8%	380
	21st to end	Night	26% 8% 23% 32% 11% 1	380
July	1st to 10th	Night	21% 8% 28% 32% 11%	370
	11th to 20th	Night	18% 11% 25% 35% 10% 1	410
	21st to end	Night	18% 7% 26% 38% 10%	623
August	1st to 10th	Night	15% 6% 33% 32% 13% 1	568
	11th to 20th	Night	19% 7% 24% 32% 18% 1	562
	21st to end	Night	14% 9% 28% 31% 16% 2	627
September	1st to 10th	Night	16% 8% 32% 32% 11% 1	570
	11th to 20th	Night	18% 12% 25% 30% 14% 1	695
	21st to end	Night	21% 12% 29% 25% 10% 2	760
October	1st to 10th	Night	24% 11% 29% 24% 11% 1	760
	11th to 20th	Night	23% 16% 30% 22% 8% 1	760
	21st to end	Night	29% 20% 29% 18% 4	836
November	1st to 10th	Night	27% 21% 27% 18% 6% 1	855
	11th to 20th	Night	26% 23% 27% 17% 5% 1	900
	21st to end	Night	27% 23% 28% 18% 3	900
December	1st to 10th	Night	29% 22% 30% 16% 3	900
	11th to 20th	Night	29% 27% 28% 13% 3	900
	21st to end	Night	30% 24% 26% 16% 3 1	990

Page generated at 2025-06-07 11:28:18. Version ac07

Copyright © 2025 Attilla Danko.