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 2026-07-01.

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	17% 11% 4 10% 6% 5% 5% 4 6% 7% 26%	138012
	Night	16% 14% 5% 12% 7% 5% 5% 4 5% 6% 22%	134450
Transparency	Day	58% 4 12% 18% 7% 1	141427
	Night	63% 4 10% 16% 6%	139125
Seeing	Day	25% 14% 30% 20% 9% 2	17418
	Night	26% 12% 28% 25% 8% 1	25747

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	12% 20% 7% 18% 8% 6% 7% 4 4 4 12%	4490
	11th to 20th	Day	9% 18% 7% 20% 9% 6% 8% 4 4 5% 11%	4650
	21st to end	Day	4 9% 7% 16% 9% 7% 6% 5% 7% 8% 22%	5124
February	1st to 10th	Day	7% 13% 4 11% 9% 9% 8% 7% 7% 6% 20%	1680
	11th to 20th	Day	12% 16% 5% 14% 8% 6% 5% 3 5% 5% 20%	1936
	21st to end	Day	10% 13% 4 11% 8% 8% 7% 4 5% 6% 25%	1730
March	1st to 10th	Day	15% 17% 5% 9% 4 4 5% 4 5% 6% 28%	4576
	11th to 20th	Day	16% 16% 5% 12% 6% 5% 6% 3 5% 5% 21%	4807
	21st to end	Day	13% 14% 4 9% 6% 5% 5% 4 5% 7% 27%	6003
April	1st to 10th	Day	18% 14% 5% 10% 6% 5% 4 3 5% 5% 26%	2520
	11th to 20th	Day	18% 10% 3 8% 5% 4 5% 4 5% 6% 32%	2610
	21st to end	Day	18% 10% 3 7% 5% 5% 5% 4 5% 6% 31%	2730
May	1st to 10th	Day	21% 10% 2 7% 5% 4 4 4 6% 7% 30%	8110
	11th to 20th	Day	21% 8% 3 8% 5% 5% 5% 4 5% 7% 30%	8221
	21st to end	Day	18% 9% 3 7% 5% 5% 5% 4 6% 7% 31%	9533
June	1st to 10th	Day	19% 7% 2 7% 5% 4 5% 4 7% 8% 32%	3150
	11th to 20th	Day	16% 8% 2 7% 5% 5% 5% 5% 6% 8% 33%	3150
	21st to end	Day	21% 8% 3 7% 6% 5% 4 5% 6% 7% 29%	3150
July	1st to 10th	Day	20% 7% 3 7% 5% 5% 5% 5% 7% 9% 28%	6784
	11th to 20th	Day	16% 7% 3 8% 5% 5% 5% 5% 7% 9% 30%	6360
	21st to end	Day	13% 6% 2 7% 5% 4 5% 5% 7% 9% 38%	6926
August	1st to 10th	Day	15% 6% 3 8% 6% 5% 6% 5% 8% 10% 27%	4563
	11th to 20th	Day	18% 7% 3 8% 5% 5% 6% 5% 7% 9% 28%	4254
	21st to end	Day	18% 7% 3 8% 5% 5% 4 3 5% 8% 33%	4511
September	1st to 10th	Day	18% 8% 3 7% 5% 6% 5% 3 6% 8% 31%	2400
	11th to 20th	Day	19% 6% 3 7% 4 5% 5% 4 6% 7% 35%	2247
	21st to end	Day	24% 8% 3 8% 4 4 4 3 5% 7% 31%	2200
October	1st to 10th	Day	18% 10% 3 9% 5% 4 5% 3 4 8% 31%	2151
	11th to 20th	Day	25% 15% 3 10% 5% 5% 5% 4 5% 5% 18%	1825
	21st to end	Day	27% 16% 4 8% 6% 4 4 4 4 5% 19%	1980
November	1st to 10th	Day	21% 18% 5% 11% 6% 5% 4 4 4 5% 19%	1851
	11th to 20th	Day	21% 20% 6% 12% 5% 5% 5% 3 3 5% 16%	1729
	21st to end	Day	18% 22% 6% 13% 8% 6% 5% 3 3 3 11%	1470
December	1st to 10th	Day	15% 17% 8% 16% 12% 7% 5% 5% 4 4 8%	2350
	11th to 20th	Day	17% 17% 6% 13% 9% 6% 4 4 7% 4 12%	2688
	21st to end	Day	16% 17% 5% 12% 8% 8% 6% 4 5% 6% 12%	3553
January	1st to 10th	Night	12% 23% 8% 16% 8% 6% 6% 4 4 4 9%	8397
	11th to 20th	Night	8% 22% 8% 23% 9% 5% 4 3 4 4 9%	8235
	21st to end	Night	4 9% 6% 15% 11% 9% 8% 6% 8% 7% 17%	9057
February	1st to 10th	Night	8% 13% 6% 13% 9% 7% 5% 5% 6% 7% 21%	2940
	11th to 20th	Night	10% 20% 6% 14% 9% 7% 5% 3 4 5% 18%	2567
	21st to end	Night	10% 18% 5% 12% 7% 5% 4 4 4 7% 24%	2076
March	1st to 10th	Night	16% 17% 6% 12% 7% 4 5% 4 4 6% 19%	5484
	11th to 20th	Night	15% 16% 5% 12% 6% 6% 6% 4 6% 6% 20%	5371
	21st to end	Night	14% 15% 5% 11% 5% 4 4 3 5% 5% 28%	5169
April	1st to 10th	Night	22% 16% 4 9% 5% 5% 3 2 3 5% 25%	2100
	11th to 20th	Night	21% 12% 3 8% 5% 4 4 3 4 6% 30%	1958
	21st to end	Night	20% 11% 3 8% 3 4 4 3 4 6% 33%	1890
May	1st to 10th	Night	21% 10% 3 8% 5% 4 5% 4 5% 6% 30%	4845
	11th to 20th	Night	23% 10% 3 9% 5% 5% 5% 3 5% 7% 27%	4639
	21st to end	Night	21% 8% 3 8% 5% 4 5% 3 6% 6% 31%	4663
June	1st to 10th	Night	19% 8% 3 7% 5% 5% 5% 5% 7% 8% 28%	1470
	11th to 20th	Night	19% 8% 3 7% 5% 5% 4 3 6% 7% 33%	1470
	21st to end	Night	20% 8% 2 7% 7% 5% 5% 4 6% 9% 27%	1470
July	1st to 10th	Night	17% 7% 3 8% 5% 6% 6% 5% 7% 7% 27%	3190
	11th to 20th	Night	16% 8% 3 8% 5% 6% 6% 5% 7% 8% 27%	3600
	21st to end	Night	16% 6% 2 6% 5% 4 4 4 6% 8% 38%	3926
August	1st to 10th	Night	15% 8% 3 8% 6% 5% 5% 6% 7% 8% 28%	2586
	11th to 20th	Night	17% 7% 4 7% 6% 5% 6% 5% 7% 8% 30%	2761
	21st to end	Night	15% 7% 3 8% 5% 6% 5% 4 6% 7% 34%	3383
September	1st to 10th	Night	19% 5% 2 8% 4 5% 5% 4 5% 7% 36%	2000
	11th to 20th	Night	20% 8% 3 8% 5% 5% 4 4 5% 7% 33%	2152
	21st to end	Night	21% 8% 3 8% 5% 4 5% 3 4 5% 34%	2200
October	1st to 10th	Night	21% 8% 2 7% 5% 5% 4 3 5% 6% 33%	2245
	11th to 20th	Night	29% 12% 4 9% 5% 5% 4 3 4 4 21%	2575
	21st to end	Night	30% 15% 4 9% 5% 5% 4 4 4 5% 16%	2860
November	1st to 10th	Night	24% 19% 5% 11% 4 4 4 3 3 5% 19%	2667
	11th to 20th	Night	20% 21% 6% 11% 5% 5% 4 4 4 4 16%	2886
	21st to end	Night	21% 22% 7% 14% 6% 5% 4 3 3 4 11%	3150
December	1st to 10th	Night	14% 18% 8% 20% 11% 6% 5% 3 4 4 8%	5060
	11th to 20th	Night	21% 18% 6% 14% 7% 5% 4 3 5% 4 12%	5741
	21st to end	Night	16% 19% 6% 13% 6% 5% 4 3 4 4 18%	7667

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	79% 3 8% 10% 1	4491
	11th to 20th	Day	76% 3 6% 10% 4	4680
	21st to end	Day	59% 3 10% 21% 7%	5124
February	1st to 10th	Day	61% 5% 9% 16% 8% 1	1680
	11th to 20th	Day	66% 5% 9% 14% 6%	1945
	21st to end	Day	59% 5% 8% 21% 6% 1	1730
March	1st to 10th	Day	58% 5% 11% 13% 11% 2	4579
	11th to 20th	Day	66% 4 10% 12% 8%	4807
	21st to end	Day	56% 5% 8% 17% 12% 1	6003
April	1st to 10th	Day	62% 3 8% 16% 9% 3	2520
	11th to 20th	Day	53% 4 10% 22% 11% 1	2610
	21st to end	Day	55% 4 9% 20% 11% 1	2730
May	1st to 10th	Day	55% 3 10% 21% 11% 1	8122
	11th to 20th	Day	54% 5% 9% 21% 10% 1	8225
	21st to end	Day	52% 4 10% 18% 14% 1	9534
June	1st to 10th	Day	51% 3 11% 22% 12% 1	3150
	11th to 20th	Day	48% 5% 13% 21% 11% 2	3150
	21st to end	Day	55% 5% 12% 19% 9% 1	3150
July	1st to 10th	Day	52% 4 14% 27% 3	6709
	11th to 20th	Day	51% 4 19% 23% 3	6285
	21st to end	Day	43% 4 19% 27% 7%	6728
August	1st to 10th	Day	51% 5% 18% 24% 2	4549
	11th to 20th	Day	51% 6% 22% 17% 4	4295
	21st to end	Day	51% 5% 15% 20% 9%	4511
September	1st to 10th	Day	52% 4 16% 22% 6%	2400
	11th to 20th	Day	47% 5% 18% 23% 7%	2247
	21st to end	Day	54% 4 16% 21% 6%	2200
October	1st to 10th	Day	55% 4 12% 24% 4 1	3502
	11th to 20th	Day	65% 4 16% 12% 2	2950
	21st to end	Day	65% 4 14% 15% 2	3180
November	1st to 10th	Day	67% 5% 12% 11% 4	1851
	11th to 20th	Day	73% 5% 8% 11% 3	1729
	21st to end	Day	77% 5% 9% 6% 2	1470
December	1st to 10th	Day	77% 4 11% 7% 1	2350
	11th to 20th	Day	73% 6% 8% 11% 3	2688
	21st to end	Day	72% 5% 6% 13% 5%	3553
January	1st to 10th	Night	79% 3 7% 10% 1	8397
	11th to 20th	Night	80% 3 5% 9% 3	8280
	21st to end	Night	62% 3 10% 19% 6%	9057
February	1st to 10th	Night	61% 4 10% 17% 8%	2940
	11th to 20th	Night	69% 4 8% 11% 7%	2569
	21st to end	Night	61% 3 8% 21% 7%	2076
March	1st to 10th	Night	68% 3 8% 14% 8%	5494
	11th to 20th	Night	64% 4 10% 13% 7% 1	5372
	21st to end	Night	58% 3 6% 21% 11% 1	5169
April	1st to 10th	Night	64% 3 7% 16% 8% 2	2100
	11th to 20th	Night	56% 4 9% 19% 13% 1	1958
	21st to end	Night	53% 4 9% 22% 11% 2	1890
May	1st to 10th	Night	55% 3 8% 22% 11% 1	4851
	11th to 20th	Night	58% 3 9% 21% 9% 1	4640
	21st to end	Night	53% 4 9% 16% 16% 2	4664
June	1st to 10th	Night	52% 5% 11% 21% 10% 1	1470
	11th to 20th	Night	50% 5% 11% 18% 13% 3	1470
	21st to end	Night	54% 6% 12% 18% 9%	1470
July	1st to 10th	Night	54% 4 13% 22% 7%	3155
	11th to 20th	Night	54% 4 18% 21% 3	3552
	21st to end	Night	45% 4 18% 26% 8%	3809
August	1st to 10th	Night	51% 6% 17% 21% 5%	2576
	11th to 20th	Night	51% 5% 22% 17% 4	2786
	21st to end	Night	49% 5% 12% 27% 6%	3383
September	1st to 10th	Night	48% 5% 15% 24% 9%	2000
	11th to 20th	Night	50% 4 17% 21% 6%	2152
	21st to end	Night	53% 5% 11% 23% 9%	2200
October	1st to 10th	Night	56% 4 13% 21% 5% 1	3674
	11th to 20th	Night	64% 4 11% 18% 4	4189
	21st to end	Night	69% 5% 11% 12% 3	4610
November	1st to 10th	Night	68% 4 12% 12% 4	2667
	11th to 20th	Night	69% 5% 10% 12% 3	2886
	21st to end	Night	78% 4 8% 8% 2	3150
December	1st to 10th	Night	80% 3 10% 6% 1	5060
	11th to 20th	Night	74% 4 5% 14% 3	5742
	21st to end	Night	71% 3 7% 15% 4	7667

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	21% 18% 41% 15% 5%	220
	11th to 20th	Day	22% 15% 35% 21% 6% 1	220
	21st to end	Day	18% 12% 33% 24% 10% 2	242
February	1st to 10th	Day	12% 20% 41% 17% 6% 3	220
	11th to 20th	Day	23% 19% 35% 20% 3	445
	21st to end	Day	22% 18% 35% 18% 5% 1	446
March	1st to 10th	Day	28% 16% 33% 16% 6% 1	540
	11th to 20th	Day	27% 16% 32% 17% 7% 1	540
	21st to end	Day	28% 16% 30% 19% 7% 1	594
April	1st to 10th	Day	29% 17% 31% 17% 6% 1	540
	11th to 20th	Day	27% 15% 31% 19% 7% 1	532
	21st to end	Day	26% 17% 34% 17% 5% 1	540
May	1st to 10th	Day	29% 17% 28% 19% 6% 1	543
	11th to 20th	Day	21% 16% 29% 25% 7% 1	567
	21st to end	Day	26% 16% 30% 20% 8% 1	778
June	1st to 10th	Day	24% 10% 33% 23% 9% 1	780
	11th to 20th	Day	22% 12% 31% 22% 11% 2	780
	21st to end	Day	26% 10% 27% 24% 11% 2	780
July	1st to 10th	Day	23% 10% 31% 23% 12% 1	710
	11th to 20th	Day	20% 10% 31% 26% 12% 2	524
	21st to end	Day	19% 9% 30% 27% 13% 2	576
August	1st to 10th	Day	21% 6% 25% 29% 15% 4	526
	11th to 20th	Day	20% 8% 25% 27% 16% 4	519
	21st to end	Day	24% 11% 26% 20% 14% 5%	583
September	1st to 10th	Day	23% 14% 27% 20% 11% 4	530
	11th to 20th	Day	20% 13% 25% 26% 11% 4	530
	21st to end	Day	27% 12% 26% 20% 13% 3	530
October	1st to 10th	Day	24% 15% 29% 17% 12% 2	530
	11th to 20th	Day	34% 17% 26% 16% 6% 1	434
	21st to end	Day	39% 15% 22% 18% 5% 1	462
November	1st to 10th	Day	38% 19% 26% 13% 4 1	425
	11th to 20th	Day	32% 18% 25% 19% 5% 1	330
	21st to end	Day	32% 20% 30% 15% 4	220
December	1st to 10th	Day	23% 11% 39% 16% 8% 2	220
	11th to 20th	Day	28% 19% 35% 12% 6% 1	220
	21st to end	Day	27% 20% 28% 18% 5% 1	242
January	1st to 10th	Night	26% 14% 31% 24% 5%	872
	11th to 20th	Night	25% 13% 27% 27% 8% 1	840
	21st to end	Night	15% 11% 31% 32% 10% 1	924
February	1st to 10th	Night	14% 13% 36% 27% 10% 2	840
	11th to 20th	Night	21% 15% 34% 22% 7%	823
	21st to end	Night	20% 15% 30% 27% 8% 1	692
March	1st to 10th	Night	22% 13% 35% 23% 6%	840
	11th to 20th	Night	24% 18% 30% 21% 8%	840
	21st to end	Night	22% 15% 29% 27% 6% 1	924
April	1st to 10th	Night	31% 13% 28% 22% 5% 1	840
	11th to 20th	Night	29% 12% 27% 25% 6% 1	714
	21st to end	Night	28% 14% 30% 22% 6%	630
May	1st to 10th	Night	31% 9% 31% 23% 6%	488
	11th to 20th	Night	24% 9% 33% 27% 7%	414
	21st to end	Night	27% 11% 34% 22% 5%	455
June	1st to 10th	Night	26% 5% 35% 28% 6%	420
	11th to 20th	Night	23% 8% 25% 32% 12%	420
	21st to end	Night	24% 8% 27% 30% 9% 1	420
July	1st to 10th	Night	24% 7% 28% 32% 9%	390
	11th to 20th	Night	18% 5% 25% 39% 12% 1	388
	21st to end	Night	21% 5% 23% 38% 12% 1	438
August	1st to 10th	Night	21% 4 23% 38% 13% 2	398
	11th to 20th	Night	20% 5% 23% 35% 17% 1	517
	21st to end	Night	22% 8% 23% 30% 15% 2	755
September	1st to 10th	Night	23% 9% 24% 30% 13% 1	800
	11th to 20th	Night	24% 8% 26% 26% 14% 2	800
	21st to end	Night	25% 10% 27% 28% 9% 2	800
October	1st to 10th	Night	26% 10% 27% 28% 9% 1	800
	11th to 20th	Night	36% 11% 23% 22% 8% 1	800
	21st to end	Night	40% 13% 21% 18% 7%	880
November	1st to 10th	Night	37% 15% 24% 18% 6% 1	820
	11th to 20th	Night	31% 17% 25% 19% 7% 1	870
	21st to end	Night	33% 15% 28% 19% 4 1	950
December	1st to 10th	Night	30% 13% 27% 21% 9% 1	950
	11th to 20th	Night	27% 16% 27% 21% 8% 1	950
	21st to end	Night	27% 14% 26% 25% 7% 1	1045

Page generated at 2026-06-28 02:10:48. Version ac07

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