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 2024-01-27.

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% 7% 3 6% 5% 3 5% 5% 5% 7% 42%	74906
	Night	14% 7% 3 6% 5% 3 4 4 5% 7% 43%	75153
Transparency	Day	41% 6% 12% 24% 15% 3	74607
	Night	41% 5% 11% 23% 17% 3	74973
Seeing	Day	16% 9% 24% 25% 18% 7%	14477
	Night	17% 9% 28% 30% 14% 2	24446

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	15% 10% 4 9% 5% 3 4 5% 4 6% 35%	1517
	11th to 20th	Day	17% 11% 4 6% 6% 3 4 4 4 6% 33%	1530
	21st to end	Day	14% 11% 5% 8% 7% 4 4 4 4 5% 35%	1836
February	1st to 10th	Day	18% 9% 5% 8% 5% 2 4 5% 5% 7% 35%	1701
	11th to 20th	Day	16% 9% 4 6% 6% 2 5% 5% 6% 7% 33%	1748
	21st to end	Day	15% 11% 4 7% 6% 3 5% 4 5% 6% 35%	1480
March	1st to 10th	Day	16% 8% 3 5% 4 3 4 5% 4 6% 42%	1800
	11th to 20th	Day	18% 10% 3 5% 5% 3 4 3 4 6% 36%	1893
	21st to end	Day	21% 10% 4 8% 6% 3 4 5% 4 6% 29%	2362
April	1st to 10th	Day	13% 6% 3 5% 4 3 4 4 6% 9% 43%	2160
	11th to 20th	Day	15% 9% 4 6% 5% 3 4 4 4 6% 41%	2134
	21st to end	Day	18% 7% 3 6% 6% 3 4 4 4 7% 40%	2160
May	1st to 10th	Day	16% 7% 3 6% 6% 4 4 5% 6% 7% 36%	2312
	11th to 20th	Day	16% 7% 3 5% 6% 3 5% 4 5% 8% 38%	2332
	21st to end	Day	9% 6% 2 6% 6% 4 7% 6% 8% 9% 38%	2725
June	1st to 10th	Day	8% 4 2 5% 5% 3 5% 6% 7% 9% 47%	2520
	11th to 20th	Day	7% 4 2 5% 5% 3 5% 6% 6% 9% 48%	2520
	21st to end	Day	6% 4 3 5% 6% 3 6% 7% 7% 10% 45%	2520
July	1st to 10th	Day	9% 5% 2 5% 5% 4 6% 6% 7% 11% 41%	2520
	11th to 20th	Day	4 4 2 4 5% 3 5% 6% 6% 12% 49%	2520
	21st to end	Day	7% 4 2 5% 6% 4 5% 5% 6% 9% 48%	2743
August	1st to 10th	Day	6% 4 3 5% 6% 3 6% 5% 7% 10% 45%	2226
	11th to 20th	Day	8% 5% 2 5% 6% 3 5% 7% 8% 9% 42%	2125
	21st to end	Day	6% 5% 2 5% 4 3 6% 5% 6% 8% 50%	2376
September	1st to 10th	Day	12% 6% 2 5% 5% 3 3 5% 5% 7% 47%	2160
	11th to 20th	Day	7% 4 3 5% 5% 4 4 5% 6% 8% 50%	2068
	21st to end	Day	9% 4 2 4 5% 2 4 4 5% 7% 53%	1980
October	1st to 10th	Day	10% 4 2 5% 5% 2 5% 3 5% 6% 54%	1913
	11th to 20th	Day	14% 6% 3 4 4 2 3 5% 5% 7% 48%	1800
	21st to end	Day	18% 7% 2 4 5% 2 4 3 5% 4 46%	1869
November	1st to 10th	Day	9% 5% 2 5% 3 2 5% 4 4 6% 55%	1672
	11th to 20th	Day	14% 7% 3 6% 5% 3 4 3 4 5% 46%	1710
	21st to end	Day	16% 9% 4 6% 6% 3 4 3 4 5% 40%	1710
December	1st to 10th	Day	14% 11% 4 6% 5% 3 5% 5% 5% 5% 37%	1563
	11th to 20th	Day	15% 11% 4 7% 6% 3 4 6% 4 6% 34%	1918
	21st to end	Day	21% 16% 5% 9% 5% 3 3 4 4 6% 24%	2783
January	1st to 10th	Night	14% 9% 4 8% 5% 4 4 4 5% 6% 37%	2648
	11th to 20th	Night	16% 10% 6% 9% 5% 3 4 4 4 5% 34%	2646
	21st to end	Night	15% 10% 4 8% 6% 3 4 4 5% 6% 34%	2665
February	1st to 10th	Night	18% 10% 5% 8% 6% 3 4 4 4 5% 33%	2257
	11th to 20th	Night	16% 8% 3 6% 7% 3 4 4 4 7% 39%	2098
	21st to end	Night	18% 9% 4 8% 5% 2 4 4 3 5% 37%	1776
March	1st to 10th	Night	16% 8% 4 6% 6% 2 4 4 4 4 41%	2160
	11th to 20th	Night	19% 7% 4 7% 5% 2 3 4 4 7% 37%	2083
	21st to end	Night	20% 9% 4 5% 5% 3 4 4 5% 7% 33%	1994
April	1st to 10th	Night	11% 5% 3 5% 6% 3 4 3 5% 7% 47%	1800
	11th to 20th	Night	15% 6% 2 5% 4 3 5% 4 5% 9% 42%	1775
	21st to end	Night	18% 9% 3 5% 5% 3 5% 4 5% 6% 39%	1800
May	1st to 10th	Night	14% 5% 4 6% 5% 4 4 4 5% 7% 40%	1648
	11th to 20th	Night	12% 7% 3 6% 6% 4 4 4 6% 7% 42%	1558
	21st to end	Night	7% 5% 3 6% 5% 4 6% 6% 7% 10% 40%	1551
June	1st to 10th	Night	9% 5% 2 4 4 2 5% 4 5% 7% 52%	1440
	11th to 20th	Night	5% 4 2 4 5% 3 6% 4 6% 8% 53%	1440
	21st to end	Night	5% 4 2 4 5% 3 6% 6% 7% 11% 50%	1440
July	1st to 10th	Night	8% 4 2 4 5% 3 4 5% 7% 10% 49%	1440
	11th to 20th	Night	4 4 2 5% 5% 4 4 5% 7% 8% 54%	1440
	21st to end	Night	6% 4 2 3 5% 4 5% 5% 5% 9% 52%	1575
August	1st to 10th	Night	7% 4 2 4 5% 3 5% 4 7% 9% 50%	1704
	11th to 20th	Night	5% 4 2 5% 5% 3 4 5% 7% 9% 49%	1776
	21st to end	Night	6% 3 1 4 4 2 3 4 5% 9% 57%	1980
September	1st to 10th	Night	11% 4 2 4 4 3 4 4 6% 6% 53%	1800
	11th to 20th	Night	6% 5% 2 5% 4 3 4 4 5% 7% 54%	1872
	21st to end	Night	9% 5% 1 4 4 2 5% 4 4 7% 52%	1980
October	1st to 10th	Night	9% 4 2 4 4 3 3 3 5% 7% 57%	2042
	11th to 20th	Night	14% 5% 2 4 4 3 3 3 4 6% 53%	2160
	21st to end	Night	18% 8% 2 5% 4 3 3 3 4 5% 45%	2484
November	1st to 10th	Night	10% 7% 2 4 4 2 3 4 4 6% 54%	2406
	11th to 20th	Night	14% 5% 3 5% 4 3 4 3 5% 6% 48%	2470
	21st to end	Night	20% 7% 4 5% 3 2 3 3 4 6% 43%	2470
December	1st to 10th	Night	18% 9% 4 6% 4 3 5% 4 5% 7% 35%	2612
	11th to 20th	Night	17% 7% 5% 5% 4 3 5% 4 4 5% 41%	3279
	21st to end	Night	22% 17% 5% 8% 7% 3 4 4 3 4 23%	4884

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	50% 5% 10% 20% 13% 2	1518
	11th to 20th	Day	52% 5% 10% 24% 9%	1530
	21st to end	Day	52% 4 10% 23% 9% 1	1836
February	1st to 10th	Day	50% 4 12% 22% 10% 1	1701
	11th to 20th	Day	49% 6% 9% 22% 11% 2	1758
	21st to end	Day	50% 5% 10% 21% 12% 3	1480
March	1st to 10th	Day	44% 5% 10% 25% 14% 3	1800
	11th to 20th	Day	50% 5% 13% 20% 10% 2	1893
	21st to end	Day	56% 6% 10% 19% 8% 1	2362
April	1st to 10th	Day	37% 6% 13% 27% 16% 1	2160
	11th to 20th	Day	45% 4 10% 23% 15% 3	2134
	21st to end	Day	45% 5% 11% 25% 11% 2	2160
May	1st to 10th	Day	46% 8% 14% 23% 8% 1	2312
	11th to 20th	Day	45% 8% 12% 21% 14% 1	2336
	21st to end	Day	39% 9% 16% 21% 12% 2	2725
June	1st to 10th	Day	31% 9% 14% 27% 17% 2	2520
	11th to 20th	Day	31% 8% 13% 25% 20% 3	2520
	21st to end	Day	32% 9% 18% 21% 16% 5%	2520
July	1st to 10th	Day	35% 8% 13% 26% 13% 4	2451
	11th to 20th	Day	27% 9% 14% 29% 18% 3	2445
	21st to end	Day	32% 7% 13% 28% 17% 2	2591
August	1st to 10th	Day	33% 8% 13% 30% 15% 1	2211
	11th to 20th	Day	34% 8% 16% 27% 13% 1	2125
	21st to end	Day	29% 7% 15% 24% 19% 6%	2376
September	1st to 10th	Day	36% 5% 12% 25% 19% 4	2160
	11th to 20th	Day	31% 7% 11% 25% 24% 3	2068
	21st to end	Day	31% 4 10% 25% 26% 4	1980
October	1st to 10th	Day	31% 5% 10% 25% 25% 4	1913
	11th to 20th	Day	36% 5% 8% 24% 23% 4	1800
	21st to end	Day	42% 4 8% 26% 17% 2	1869
November	1st to 10th	Day	31% 4 12% 27% 19% 6%	1672
	11th to 20th	Day	41% 4 8% 24% 18% 5%	1710
	21st to end	Day	48% 3 9% 24% 14% 2	1710
December	1st to 10th	Day	48% 6% 12% 22% 13%	1563
	11th to 20th	Day	49% 5% 11% 25% 10% 2	1915
	21st to end	Day	62% 5% 10% 14% 7% 1	2783
January	1st to 10th	Night	48% 5% 9% 23% 14% 1	2648
	11th to 20th	Night	54% 4 9% 22% 9% 1	2646
	21st to end	Night	50% 5% 12% 21% 11% 1	2665
February	1st to 10th	Night	54% 4 10% 20% 10% 2	2257
	11th to 20th	Night	46% 4 11% 21% 15% 2	2099
	21st to end	Night	50% 4 10% 21% 12% 3	1776
March	1st to 10th	Night	46% 4 8% 22% 18% 3	2160
	11th to 20th	Night	47% 6% 11% 20% 13% 3	2083
	21st to end	Night	51% 5% 12% 21% 9% 2	1994
April	1st to 10th	Night	37% 5% 11% 26% 18% 3	1800
	11th to 20th	Night	40% 4 10% 23% 20% 3	1775
	21st to end	Night	46% 6% 9% 21% 15% 3	1800
May	1st to 10th	Night	42% 7% 12% 25% 12% 2	1648
	11th to 20th	Night	41% 9% 11% 23% 15% 1	1559
	21st to end	Night	38% 9% 13% 21% 15% 5%	1552
June	1st to 10th	Night	31% 6% 13% 24% 23% 2	1440
	11th to 20th	Night	26% 8% 11% 26% 23% 5%	1440
	21st to end	Night	27% 8% 15% 23% 18% 7%	1440
July	1st to 10th	Night	30% 8% 13% 28% 15% 6%	1400
	11th to 20th	Night	26% 6% 14% 29% 19% 5%	1392
	21st to end	Night	28% 6% 13% 31% 19% 3	1487
August	1st to 10th	Night	29% 7% 16% 28% 19% 2	1695
	11th to 20th	Night	29% 9% 12% 26% 22% 2	1776
	21st to end	Night	24% 8% 14% 26% 23% 6%	1980
September	1st to 10th	Night	31% 6% 9% 29% 19% 7%	1800
	11th to 20th	Night	29% 5% 8% 30% 24% 4	1872
	21st to end	Night	31% 6% 8% 22% 27% 6%	1980
October	1st to 10th	Night	28% 6% 10% 24% 28% 5%	2042
	11th to 20th	Night	35% 3 8% 25% 23% 6%	2160
	21st to end	Night	43% 4 7% 24% 18% 4	2484
November	1st to 10th	Night	32% 4 11% 24% 24% 6%	2406
	11th to 20th	Night	37% 4 9% 24% 23% 4	2470
	21st to end	Night	44% 3 10% 24% 17% 1	2470
December	1st to 10th	Night	49% 4 12% 22% 12% 1	2612
	11th to 20th	Night	45% 4 11% 25% 14% 1	3281
	21st to end	Night	66% 4 8% 14% 7% 1	4884

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% 17% 31% 20% 6%	270
	11th to 20th	Day	31% 16% 31% 20% 1	270
	21st to end	Day	22% 21% 39% 15% 3	282
February	1st to 10th	Day	32% 15% 38% 13% 2	240
	11th to 20th	Day	28% 24% 25% 18% 5%	240
	21st to end	Day	30% 17% 33% 17% 3 1	198
March	1st to 10th	Day	25% 18% 39% 14% 4	240
	11th to 20th	Day	32% 15% 34% 14% 5%	240
	21st to end	Day	31% 14% 31% 17% 6%	448
April	1st to 10th	Day	17% 16% 32% 25% 8% 1	420
	11th to 20th	Day	20% 17% 27% 21% 12% 2	412
	21st to end	Day	24% 13% 33% 20% 8% 2	420
May	1st to 10th	Day	22% 12% 31% 23% 9% 3	423
	11th to 20th	Day	16% 8% 25% 29% 16% 5%	478
	21st to end	Day	9% 4 27% 31% 20% 9%	679
June	1st to 10th	Day	10% 4 18% 31% 27% 11%	630
	11th to 20th	Day	8% 3 22% 32% 27% 8%	630
	21st to end	Day	7% 5% 25% 32% 23% 9%	630
July	1st to 10th	Day	10% 2 13% 26% 29% 20%	621
	11th to 20th	Day	6% 3 14% 35% 29% 14%	618
	21st to end	Day	9% 2 12% 33% 30% 14%	684
August	1st to 10th	Day	8% 3 12% 32% 32% 14%	523
	11th to 20th	Day	10% 3 14% 29% 31% 14%	439
	21st to end	Day	7% 2 13% 28% 34% 16%	495
September	1st to 10th	Day	13% 3 15% 27% 30% 12%	450
	11th to 20th	Day	8% 2 12% 29% 33% 15%	450
	21st to end	Day	11% 7% 22% 24% 23% 13%	450
October	1st to 10th	Day	13% 6% 21% 36% 20% 5%	383
	11th to 20th	Day	16% 16% 33% 25% 9% 1	270
	21st to end	Day	31% 16% 28% 20% 6%	297
November	1st to 10th	Day	14% 12% 34% 21% 16% 3	270
	11th to 20th	Day	19% 19% 28% 23% 9% 2	270
	21st to end	Day	24% 12% 34% 22% 7% 1	270
December	1st to 10th	Day	15% 18% 41% 15% 6% 5%	270
	11th to 20th	Day	21% 17% 37% 20% 5%	270
	21st to end	Day	30% 18% 32% 15% 4	297
January	1st to 10th	Night	19% 17% 36% 24% 4	950
	11th to 20th	Night	24% 16% 34% 22% 4	950
	21st to end	Night	21% 14% 37% 25% 2	1025
February	1st to 10th	Night	25% 12% 38% 23% 2	817
	11th to 20th	Night	22% 16% 37% 23% 2	703
	21st to end	Night	24% 17% 36% 22% 2	592
March	1st to 10th	Night	21% 14% 40% 23% 3	720
	11th to 20th	Night	23% 10% 36% 26% 5%	643
	21st to end	Night	25% 9% 31% 27% 7%	594
April	1st to 10th	Night	15% 12% 36% 29% 7%	540
	11th to 20th	Night	18% 10% 32% 27% 13% 1	533
	21st to end	Night	23% 7% 32% 28% 8% 1	540
May	1st to 10th	Night	16% 7% 31% 35% 10% 1	540
	11th to 20th	Night	13% 5% 27% 36% 17% 2	531
	21st to end	Night	9% 3 22% 39% 24% 3	584
June	1st to 10th	Night	11% 4 17% 41% 26% 1	540
	11th to 20th	Night	6% 1 21% 46% 24% 3	540
	21st to end	Night	7% 2 21% 41% 26% 3	540
July	1st to 10th	Night	10% 1 10% 32% 37% 9%	525
	11th to 20th	Night	4 3 11% 40% 38% 4	522
	21st to end	Night	7% 1 12% 40% 34% 5%	591
August	1st to 10th	Night	8% 2 9% 40% 36% 5%	537
	11th to 20th	Night	7% 1 11% 39% 37% 5%	534
	21st to end	Night	7% 1 11% 32% 44% 5%	594
September	1st to 10th	Night	11% 1 13% 38% 30% 6%	540
	11th to 20th	Night	8% 2 15% 35% 33% 6%	612
	21st to end	Night	12% 4 21% 31% 26% 6%	720
October	1st to 10th	Night	10% 5% 24% 34% 23% 4	720
	11th to 20th	Night	17% 10% 29% 30% 13% 1	720
	21st to end	Night	22% 11% 32% 28% 7%	792
November	1st to 10th	Night	14% 8% 37% 30% 11%	740
	11th to 20th	Night	16% 13% 31% 31% 10% 1	760
	21st to end	Night	24% 11% 31% 26% 8%	760
December	1st to 10th	Night	22% 12% 35% 25% 5%	902
	11th to 20th	Night	23% 9% 31% 30% 6%	950
	21st to end	Night	28% 14% 33% 21% 3	1045

Page generated at 2024-01-24 08:37:29. Version ac07

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