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 2026-10-14.

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	17% 10% 3 8% 5% 5% 5% 4 6% 7% 31%	150135
	Night	18% 10% 3 7% 5% 4 4 3 5% 6% 33%	141768
Transparency	Day	50% 6% 13% 20% 10% 1	153688
	Night	50% 6% 11% 20% 11% 1	146303
Seeing	Day	24% 18% 30% 20% 6% 1	19468
	Night	24% 18% 30% 22% 6% 1	26967

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% 15% 5% 9% 6% 5% 5% 4 5% 6% 30%	4093
	11th to 20th	Day	16% 15% 5% 11% 5% 4 4 3 4 5% 28%	4624
	21st to end	Day	10% 9% 4 8% 5% 4 4 4 6% 9% 39%	5781
February	1st to 10th	Day	14% 13% 4 10% 6% 5% 5% 3 5% 6% 30%	1890
	11th to 20th	Day	14% 13% 4 9% 6% 5% 4 5% 5% 5% 30%	1843
	21st to end	Day	10% 15% 4 9% 6% 4 5% 4 5% 6% 34%	1594
March	1st to 10th	Day	20% 13% 5% 12% 5% 3 4 2 4 5% 27%	4989
	11th to 20th	Day	17% 16% 4 9% 5% 4 4 3 4 6% 28%	5090
	21st to end	Day	25% 13% 5% 8% 6% 4 4 4 6% 5% 21%	5587
April	1st to 10th	Day	21% 14% 3 8% 5% 4 4 4 6% 5% 26%	2473
	11th to 20th	Day	17% 13% 3 8% 6% 4 4 3 5% 7% 30%	2650
	21st to end	Day	21% 10% 4 9% 5% 4 4 3 4 7% 29%	2730
May	1st to 10th	Day	27% 12% 4 8% 5% 4 4 3 4 5% 23%	7734
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 5% 6% 33%	8220
	21st to end	Day	21% 11% 3 8% 5% 4 5% 3 5% 6% 28%	9059
June	1st to 10th	Day	23% 9% 3 8% 5% 4 5% 4 5% 7% 27%	3076
	11th to 20th	Day	19% 8% 3 7% 5% 4 4 4 6% 8% 31%	3150
	21st to end	Day	27% 9% 2 7% 5% 5% 4 3 5% 7% 27%	3150
July	1st to 10th	Day	14% 8% 3 7% 5% 6% 6% 5% 8% 10% 28%	8880
	11th to 20th	Day	13% 7% 3 6% 4 5% 5% 4 7% 10% 36%	8682
	21st to end	Day	13% 6% 2 6% 5% 5% 6% 4 6% 9% 37%	8640
August	1st to 10th	Day	17% 8% 3 9% 5% 6% 5% 4 7% 7% 30%	6015
	11th to 20th	Day	15% 7% 3 7% 5% 5% 6% 5% 7% 9% 31%	5953
	21st to end	Day	13% 5% 2 5% 4 4 5% 5% 7% 9% 40%	6458
September	1st to 10th	Day	15% 5% 2 5% 5% 4 5% 4 6% 8% 40%	2520
	11th to 20th	Day	16% 7% 2 7% 3 4 4 3 4 7% 42%	2378
	21st to end	Day	18% 7% 3 6% 3 4 4 3 5% 6% 42%	2283
October	1st to 10th	Day	15% 8% 3 6% 3 4 4 3 5% 8% 40%	2100
	11th to 20th	Day	20% 7% 2 8% 5% 4 4 3 5% 6% 36%	2030
	21st to end	Day	19% 8% 4 10% 5% 4 4 4 4 6% 32%	2165
November	1st to 10th	Day	21% 10% 3 7% 4 5% 4 3 4 6% 32%	1695
	11th to 20th	Day	19% 11% 3 7% 4 4 4 3 5% 6% 34%	1680
	21st to end	Day	19% 12% 3 8% 5% 4 5% 4 5% 5% 30%	1680
December	1st to 10th	Day	20% 11% 3 7% 4 4 4 3 5% 8% 32%	2690
	11th to 20th	Day	24% 12% 3 6% 4 4 3 2 4 4 34%	3000
	21st to end	Day	17% 12% 5% 7% 4 5% 4 3 6% 5% 32%	3553
January	1st to 10th	Night	11% 15% 4 8% 6% 4 4 3 4 6% 34%	8831
	11th to 20th	Night	21% 13% 3 9% 5% 5% 5% 4 5% 5% 26%	8248
	21st to end	Night	9% 11% 5% 7% 5% 4 4 3 6% 7% 39%	8400
February	1st to 10th	Night	17% 11% 3 8% 5% 4 5% 4 6% 7% 30%	2730
	11th to 20th	Night	15% 13% 4 8% 5% 4 5% 3 5% 6% 32%	2663
	21st to end	Night	12% 13% 5% 8% 6% 4 4 3 6% 6% 34%	2207
March	1st to 10th	Night	23% 15% 3 9% 5% 5% 4 2 4 4 26%	5070
	11th to 20th	Night	19% 11% 5% 10% 5% 4 4 3 5% 6% 27%	5089
	21st to end	Night	28% 12% 3 7% 4 4 4 3 4 4 26%	5587
April	1st to 10th	Night	21% 13% 4 7% 5% 4 5% 4 5% 4 28%	2147
	11th to 20th	Night	20% 10% 3 7% 5% 4 3 3 5% 6% 32%	1915
	21st to end	Night	20% 11% 3 7% 6% 4 4 3 5% 6% 30%	1890
May	1st to 10th	Night	25% 12% 3 8% 5% 5% 5% 4 5% 6% 23%	5157
	11th to 20th	Night	19% 9% 3 8% 5% 5% 4 4 5% 5% 33%	4670
	21st to end	Night	17% 10% 4 10% 5% 5% 5% 4 5% 6% 30%	5147
June	1st to 10th	Night	24% 9% 3 8% 4 5% 5% 3 5% 8% 27%	1561
	11th to 20th	Night	19% 9% 3 6% 5% 5% 4 3 5% 5% 35%	1470
	21st to end	Night	25% 8% 3 6% 4 6% 5% 4 6% 6% 26%	1470
July	1st to 10th	Night	14% 8% 3 9% 6% 5% 6% 4 7% 8% 29%	4110
	11th to 20th	Night	12% 7% 3 7% 5% 4 5% 4 6% 10% 37%	4247
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 6% 7% 40%	5450
August	1st to 10th	Night	17% 8% 3 8% 6% 5% 5% 4 6% 7% 31%	4135
	11th to 20th	Night	14% 7% 2 7% 5% 5% 6% 4 7% 9% 34%	4092
	21st to end	Night	16% 7% 2 6% 4 5% 4 4 5% 7% 42%	4667
September	1st to 10th	Night	14% 6% 3 6% 4 4 5% 3 7% 7% 42%	2100
	11th to 20th	Night	16% 7% 3 5% 3 3 3 3 3 7% 46%	2239
	21st to end	Night	19% 8% 2 4 3 3 4 3 4 7% 42%	2336
October	1st to 10th	Night	18% 8% 2 5% 4 4 4 4 5% 6% 41%	2520
	11th to 20th	Night	18% 8% 3 7% 4 4 4 4 6% 6% 37%	2437
	21st to end	Night	24% 9% 3 6% 5% 4 3 4 4 6% 33%	2670
November	1st to 10th	Night	23% 9% 2 6% 4 4 4 3 4 6% 35%	2822
	11th to 20th	Night	20% 10% 3 7% 4 5% 4 3 6% 6% 32%	2940
	21st to end	Night	21% 9% 3 8% 5% 4 4 4 5% 6% 31%	2940
December	1st to 10th	Night	16% 10% 4 7% 4 4 3 3 5% 7% 37%	4720
	11th to 20th	Night	24% 12% 3 6% 3 3 3 3 4 5% 34%	5424
	21st to end	Night	20% 13% 4 7% 4 4 3 3 4 5% 32%	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	53% 6% 12% 18% 11% 1	4094
	11th to 20th	Day	58% 3 11% 18% 9% 1	4654
	21st to end	Day	42% 4 11% 19% 19% 5%	5781
February	1st to 10th	Day	54% 4 10% 18% 11% 2	1890
	11th to 20th	Day	54% 4 11% 18% 10% 1	1851
	21st to end	Day	49% 5% 11% 20% 12% 3	1594
March	1st to 10th	Day	61% 4 8% 13% 11% 3	4993
	11th to 20th	Day	55% 6% 16% 15% 7% 2	5090
	21st to end	Day	62% 5% 7% 16% 9% 1	5587
April	1st to 10th	Day	57% 5% 9% 15% 11% 3	2473
	11th to 20th	Day	53% 5% 10% 20% 12% 1	2650
	21st to end	Day	54% 5% 10% 17% 11% 3	2730
May	1st to 10th	Day	61% 7% 10% 14% 8%	7746
	11th to 20th	Day	49% 7% 15% 21% 7% 1	8224
	21st to end	Day	55% 8% 10% 15% 11% 1	9060
June	1st to 10th	Day	56% 7% 12% 17% 8%	3076
	11th to 20th	Day	49% 6% 14% 21% 8% 1	3150
	21st to end	Day	57% 7% 11% 16% 8% 1	3150
July	1st to 10th	Day	47% 8% 15% 18% 11%	8805
	11th to 20th	Day	41% 6% 15% 28% 9% 1	8601
	21st to end	Day	42% 8% 15% 28% 8%	8450
August	1st to 10th	Day	50% 8% 14% 20% 7% 1	6002
	11th to 20th	Day	46% 9% 16% 20% 8%	5992
	21st to end	Day	38% 6% 17% 24% 14% 1	6458
September	1st to 10th	Day	39% 8% 14% 25% 13%	2520
	11th to 20th	Day	42% 5% 11% 25% 16% 2	2378
	21st to end	Day	41% 6% 10% 26% 15% 2	2283
October	1st to 10th	Day	42% 5% 12% 28% 12%	3360
	11th to 20th	Day	42% 4 11% 30% 13% 1	3270
	21st to end	Day	43% 4 15% 20% 16% 1	3479
November	1st to 10th	Day	53% 5% 11% 21% 10% 1	1695
	11th to 20th	Day	50% 5% 14% 18% 12% 1	1680
	21st to end	Day	55% 6% 12% 19% 8%	1680
December	1st to 10th	Day	52% 3 13% 25% 6% 1	2690
	11th to 20th	Day	55% 4 9% 16% 14% 2	2999
	21st to end	Day	52% 4 14% 17% 12% 1	3553
January	1st to 10th	Night	51% 5% 10% 25% 9% 1	8831
	11th to 20th	Night	57% 5% 12% 17% 8% 1	8292
	21st to end	Night	44% 4 10% 21% 18% 4	8400
February	1st to 10th	Night	52% 5% 12% 16% 12% 2	2730
	11th to 20th	Night	53% 4 11% 18% 12% 2	2666
	21st to end	Night	50% 4 10% 19% 13% 2	2207
March	1st to 10th	Night	63% 3 7% 11% 12% 4	5079
	11th to 20th	Night	55% 7% 14% 13% 9% 1	5090
	21st to end	Night	61% 4 6% 16% 11% 1	5587
April	1st to 10th	Night	58% 5% 8% 15% 12% 3	2147
	11th to 20th	Night	51% 5% 11% 20% 12% 1	1915
	21st to end	Night	54% 5% 12% 15% 13% 2	1890
May	1st to 10th	Night	59% 8% 11% 15% 7%	5163
	11th to 20th	Night	51% 8% 11% 20% 8% 2	4671
	21st to end	Night	52% 9% 13% 15% 10% 2	5148
June	1st to 10th	Night	55% 8% 11% 21% 6%	1561
	11th to 20th	Night	50% 6% 11% 23% 9% 1	1470
	21st to end	Night	56% 9% 11% 14% 9% 1	1470
July	1st to 10th	Night	49% 8% 12% 20% 11%	4075
	11th to 20th	Night	41% 7% 14% 27% 10%	4205
	21st to end	Night	41% 8% 12% 25% 13%	5326
August	1st to 10th	Night	50% 9% 15% 17% 7% 1	4123
	11th to 20th	Night	43% 11% 18% 20% 7% 1	4119
	21st to end	Night	41% 6% 15% 24% 13%	4666
September	1st to 10th	Night	38% 8% 12% 28% 12% 1	2100
	11th to 20th	Night	40% 5% 12% 28% 14% 1	2239
	21st to end	Night	41% 6% 12% 24% 14% 2	2336
October	1st to 10th	Night	47% 5% 11% 24% 13%	4040
	11th to 20th	Night	37% 4 12% 26% 20% 1	3936
	21st to end	Night	46% 3 13% 22% 16%	4306
November	1st to 10th	Night	49% 6% 13% 20% 10% 1	2822
	11th to 20th	Night	51% 5% 13% 20% 9% 1	2940
	21st to end	Night	52% 7% 13% 18% 9% 1	2940
December	1st to 10th	Night	47% 4 10% 28% 10% 1	4720
	11th to 20th	Night	54% 3 8% 17% 16% 2	5426
	21st to end	Night	56% 3 10% 18% 11% 1	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	26% 28% 31% 14% 1	220
	11th to 20th	Day	34% 21% 27% 14% 3	324
	21st to end	Day	21% 21% 38% 16% 4	473
February	1st to 10th	Day	25% 24% 27% 19% 4	430
	11th to 20th	Day	22% 27% 35% 14% 3	426
	21st to end	Day	19% 24% 36% 18% 2	375
March	1st to 10th	Day	24% 26% 35% 11% 3 1	540
	11th to 20th	Day	24% 25% 30% 17% 3	540
	21st to end	Day	29% 24% 32% 12% 3 1	594
April	1st to 10th	Day	33% 27% 26% 12% 2	540
	11th to 20th	Day	29% 23% 29% 14% 5%	532
	21st to end	Day	25% 21% 28% 17% 7% 1	540
May	1st to 10th	Day	28% 20% 32% 15% 5% 1	574
	11th to 20th	Day	24% 16% 30% 20% 8% 1	774
	21st to end	Day	23% 17% 31% 23% 5% 1	845
June	1st to 10th	Day	27% 16% 28% 21% 7% 1	780
	11th to 20th	Day	24% 15% 30% 23% 7% 1	780
	21st to end	Day	27% 12% 27% 23% 9% 2	780
July	1st to 10th	Day	21% 13% 33% 24% 8% 1	770
	11th to 20th	Day	18% 11% 32% 25% 13% 1	768
	21st to end	Day	19% 10% 31% 30% 9% 1	726
August	1st to 10th	Day	17% 9% 33% 30% 10% 1	566
	11th to 20th	Day	18% 12% 30% 27% 12% 1	559
	21st to end	Day	18% 15% 26% 27% 10% 3	627
September	1st to 10th	Day	15% 14% 33% 28% 9% 2	570
	11th to 20th	Day	20% 14% 30% 24% 9% 3	570
	21st to end	Day	18% 18% 32% 22% 9% 1	554
October	1st to 10th	Day	21% 16% 28% 23% 11% 1	450
	11th to 20th	Day	23% 18% 30% 19% 8% 1	429
	21st to end	Day	26% 20% 32% 15% 6%	462
November	1st to 10th	Day	29% 21% 29% 16% 5% 1	425
	11th to 20th	Day	28% 24% 28% 14% 5%	430
	21st to end	Day	28% 27% 31% 11% 3	430
December	1st to 10th	Day	31% 22% 29% 16% 2	430
	11th to 20th	Day	36% 28% 25% 10% 1	393
	21st to end	Day	27% 26% 30% 14% 2	242
January	1st to 10th	Night	25% 25% 30% 17% 2	950
	11th to 20th	Night	28% 22% 30% 17% 4	902
	21st to end	Night	21% 24% 35% 16% 4	924
February	1st to 10th	Night	25% 24% 32% 17% 3	840
	11th to 20th	Night	24% 25% 33% 15% 3	823
	21st to end	Night	21% 23% 34% 18% 4	692
March	1st to 10th	Night	24% 21% 36% 17% 2	840
	11th to 20th	Night	23% 19% 32% 20% 5%	840
	21st to end	Night	25% 20% 32% 17% 5%	924
April	1st to 10th	Night	29% 19% 29% 20% 3	677
	11th to 20th	Night	25% 18% 32% 20% 5%	624
	21st to end	Night	27% 14% 30% 21% 7% 1	630
May	1st to 10th	Night	27% 14% 30% 20% 8% 1	630
	11th to 20th	Night	25% 13% 29% 25% 8%	622
	21st to end	Night	19% 14% 35% 25% 6%	682
June	1st to 10th	Night	27% 10% 28% 28% 6% 1	511
	11th to 20th	Night	24% 10% 25% 33% 8%	420
	21st to end	Night	26% 8% 22% 32% 11% 1	420
July	1st to 10th	Night	20% 8% 28% 34% 10%	410
	11th to 20th	Night	17% 12% 26% 35% 9% 1	454
	21st to end	Night	19% 7% 27% 36% 10%	689
August	1st to 10th	Night	15% 6% 33% 32% 14% 1	628
	11th to 20th	Night	18% 7% 24% 33% 16% 1	622
	21st to end	Night	14% 9% 28% 31% 15% 2	693
September	1st to 10th	Night	16% 8% 32% 32% 10% 1	630
	11th to 20th	Night	18% 12% 25% 30% 13% 2	769
	21st to end	Night	21% 13% 30% 24% 10% 2	840
October	1st to 10th	Night	24% 13% 28% 24% 10% 1	840
	11th to 20th	Night	24% 16% 30% 21% 8% 1	813
	21st to end	Night	30% 20% 29% 18% 4	880
November	1st to 10th	Night	29% 21% 27% 17% 6% 1	903
	11th to 20th	Night	27% 23% 28% 17% 5% 1	950
	21st to end	Night	27% 23% 29% 17% 4	950
December	1st to 10th	Night	30% 22% 30% 16% 3	950
	11th to 20th	Night	30% 27% 28% 12% 2	950
	21st to end	Night	31% 24% 26% 15% 3 1	1045

Page generated at 2026-10-11 06:06:30. Version ac07

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