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 2024-11-02.

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%	101687
	Night	19% 10% 3 7% 5% 4 4 4 5% 6% 32%	97362
Transparency	Day	51% 7% 13% 20% 9% 1	101439
	Night	51% 7% 12% 19% 10% 1	97230
Seeing	Day	23% 18% 30% 21% 7% 1	16808
	Night	24% 17% 30% 22% 7% 1	24394

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	14% 13% 4 8% 6% 4 4 4 5% 4 35%	2207
	11th to 20th	Day	14% 13% 4 9% 4 4 4 2 4 5% 37%	2497
	21st to end	Day	14% 9% 3 7% 5% 5% 5% 4 7% 9% 32%	3124
February	1st to 10th	Day	15% 13% 4 9% 5% 4 4 3 5% 6% 31%	1710
	11th to 20th	Day	15% 14% 4 9% 6% 5% 4 4 6% 5% 29%	1663
	21st to end	Day	10% 16% 4 8% 5% 3 4 4 4 6% 34%	1446
March	1st to 10th	Day	21% 14% 5% 8% 5% 3 4 3 4 5% 28%	3394
	11th to 20th	Day	14% 20% 5% 11% 6% 5% 4 3 4 5% 25%	3480
	21st to end	Day	28% 12% 5% 8% 6% 4 4 3 5% 4 22%	3816
April	1st to 10th	Day	20% 15% 3 8% 5% 5% 4 4 5% 5% 26%	2237
	11th to 20th	Day	17% 13% 3 7% 6% 4 4 3 5% 7% 32%	2394
	21st to end	Day	21% 10% 3 9% 5% 4 4 3 4 7% 28%	2470
May	1st to 10th	Day	19% 13% 5% 10% 6% 5% 4 4 4 6% 24%	4160
	11th to 20th	Day	16% 9% 3 8% 4 5% 4 3 5% 6% 36%	4398
	21st to end	Day	18% 9% 3 8% 6% 5% 6% 4 7% 8% 27%	4836
June	1st to 10th	Day	24% 10% 3 8% 5% 4 4 4 5% 7% 25%	2783
	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 3 6% 32%	1541
	11th to 20th	Day	19% 11% 3 6% 4 4 4 3 5% 6% 35%	1520
	21st to end	Day	19% 12% 3 8% 5% 4 5% 3 5% 5% 31%	1520
December	1st to 10th	Day	26% 11% 3 7% 3 4 4 3 4 7% 28%	1520
	11th to 20th	Day	22% 16% 3 7% 5% 5% 3 2 4 5% 27%	1860
	21st to end	Day	18% 10% 5% 8% 5% 5% 5% 4 6% 5% 29%	2431
January	1st to 10th	Night	13% 11% 3 7% 5% 4 4 3 4 6% 40%	4748
	11th to 20th	Night	18% 12% 3 8% 5% 4 4 3 5% 6% 31%	4466
	21st to end	Night	14% 10% 4 8% 7% 5% 4 3 5% 7% 31%	4532
February	1st to 10th	Night	18% 11% 3 8% 5% 3 4 4 6% 7% 30%	2470
	11th to 20th	Night	15% 13% 4 8% 5% 4 5% 3 6% 6% 31%	2403
	21st to end	Night	12% 13% 5% 7% 6% 4 4 3 6% 6% 34%	2003
March	1st to 10th	Night	20% 15% 3 9% 6% 6% 5% 3 4 4 26%	3445
	11th to 20th	Night	16% 13% 6% 13% 5% 5% 5% 4 5% 6% 22%	3479
	21st to end	Night	29% 11% 3 8% 5% 4 5% 3 3 3 25%	3816
April	1st to 10th	Night	22% 12% 4 7% 5% 4 5% 4 5% 4 28%	1943
	11th to 20th	Night	19% 10% 3 6% 5% 5% 4 3 6% 6% 32%	1731
	21st to end	Night	21% 11% 3 7% 6% 4 5% 3 5% 5% 30%	1710
May	1st to 10th	Night	18% 10% 3 8% 6% 5% 5% 5% 5% 7% 28%	2782
	11th to 20th	Night	18% 8% 2 7% 5% 3 3 4 5% 6% 38%	2506
	21st to end	Night	17% 10% 4 8% 5% 6% 6% 5% 6% 7% 28%	2749
June	1st to 10th	Night	24% 9% 3 8% 4 5% 5% 3 5% 7% 26%	1413
	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	23% 9% 3 6% 4 4 4 3 5% 5% 35%	2570
	11th to 20th	Night	21% 9% 3 7% 4 4 4 3 6% 7% 33%	2660
	21st to end	Night	21% 9% 3 7% 5% 4 4 4 5% 6% 32%	2660
December	1st to 10th	Night	25% 11% 4 8% 5% 4 3 3 5% 6% 27%	2660
	11th to 20th	Night	28% 12% 4 6% 4 3 3 3 4 5% 30%	3334
	21st to end	Night	22% 11% 5% 8% 4 5% 4 3 4 5% 29%	5236

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	51% 5% 15% 17% 10% 1	2208
	11th to 20th	Day	52% 4 10% 22% 12% 1	2497
	21st to end	Day	46% 6% 17% 17% 14% 1	3124
February	1st to 10th	Day	54% 4 10% 18% 12% 2	1710
	11th to 20th	Day	56% 5% 11% 17% 10% 1	1671
	21st to end	Day	49% 6% 11% 21% 11% 4	1446
March	1st to 10th	Day	58% 5% 10% 17% 7% 3	3398
	11th to 20th	Day	58% 6% 14% 14% 7% 2	3480
	21st to end	Day	63% 6% 6% 15% 8% 2	3816
April	1st to 10th	Day	58% 5% 9% 16% 10% 3	2237
	11th to 20th	Day	51% 5% 10% 20% 12% 1	2394
	21st to end	Day	54% 5% 11% 17% 11% 2	2470
May	1st to 10th	Day	56% 10% 11% 16% 7% 1	4174
	11th to 20th	Day	47% 7% 16% 23% 7%	4402
	21st to end	Day	51% 13% 14% 16% 6%	4836
June	1st to 10th	Day	57% 7% 12% 17% 7%	2783
	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% 29% 11%	1900
	11th to 20th	Day	47% 6% 12% 25% 10% 1	1900
	21st to end	Day	52% 5% 12% 19% 13%	2057
November	1st to 10th	Day	53% 5% 10% 21% 10% 1	1541
	11th to 20th	Day	49% 5% 14% 18% 13% 1	1520
	21st to end	Day	55% 6% 12% 19% 8%	1520
December	1st to 10th	Day	58% 4 12% 19% 6% 1	1520
	11th to 20th	Day	60% 5% 9% 17% 8% 1	1859
	21st to end	Day	53% 5% 17% 15% 7% 2	2431
January	1st to 10th	Night	46% 5% 13% 27% 8% 1	4748
	11th to 20th	Night	52% 8% 12% 19% 9% 1	4466
	21st to end	Night	52% 6% 12% 20% 9% 1	4532
February	1st to 10th	Night	53% 5% 12% 16% 12% 2	2470
	11th to 20th	Night	54% 5% 11% 17% 12% 2	2406
	21st to end	Night	51% 4 10% 19% 13% 3	2003
March	1st to 10th	Night	62% 4 9% 15% 9% 2	3454
	11th to 20th	Night	59% 8% 12% 12% 8% 1	3480
	21st to end	Night	62% 5% 7% 15% 9% 2	3816
April	1st to 10th	Night	58% 5% 8% 15% 11% 4	1943
	11th to 20th	Night	50% 5% 12% 20% 12% 1	1731
	21st to end	Night	55% 5% 11% 14% 13% 2	1710
May	1st to 10th	Night	51% 13% 14% 15% 7% 1	2788
	11th to 20th	Night	43% 9% 17% 23% 7%	2507
	21st to end	Night	51% 13% 16% 15% 5% 1	2750
June	1st to 10th	Night	56% 8% 10% 19% 6%	1413
	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	44% 6% 11% 25% 13%	2280
	11th to 20th	Night	45% 6% 12% 27% 9%	2280
	21st to end	Night	53% 4 12% 20% 12% 1	2536
November	1st to 10th	Night	50% 6% 13% 20% 10% 2	2570
	11th to 20th	Night	50% 5% 14% 20% 9% 1	2660
	21st to end	Night	52% 7% 13% 18% 9% 1	2660
December	1st to 10th	Night	58% 5% 11% 19% 7% 1	2660
	11th to 20th	Night	59% 4 9% 18% 10% 1	3336
	21st to end	Night	59% 4 12% 17% 6% 1	5236

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	23% 27% 33% 16% 1	180
	11th to 20th	Day	32% 22% 28% 14% 3	274
	21st to end	Day	18% 19% 39% 19% 4	407
February	1st to 10th	Day	25% 24% 28% 19% 4 1	370
	11th to 20th	Day	20% 28% 36% 13% 3	366
	21st to end	Day	16% 24% 37% 20% 2	323
March	1st to 10th	Day	21% 25% 37% 13% 4 1	460
	11th to 20th	Day	20% 24% 32% 19% 3	460
	21st to end	Day	25% 23% 34% 13% 4 1	506
April	1st to 10th	Day	30% 26% 28% 14% 3	460
	11th to 20th	Day	23% 23% 32% 16% 6%	452
	21st to end	Day	24% 20% 29% 18% 8% 2	460
May	1st to 10th	Day	26% 19% 32% 17% 5% 1	491
	11th to 20th	Day	22% 15% 30% 23% 9% 1	674
	21st to end	Day	21% 17% 32% 24% 5% 1	735
June	1st to 10th	Day	28% 15% 27% 21% 7% 1	680
	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% 20% 33% 15% 6%	429
November	1st to 10th	Day	26% 20% 30% 18% 5% 1	369
	11th to 20th	Day	27% 24% 28% 16% 6% 1	370
	21st to end	Day	27% 26% 33% 12% 2	370
December	1st to 10th	Day	30% 21% 29% 17% 2	370
	11th to 20th	Day	34% 29% 25% 11% 1	337
	21st to end	Day	25% 28% 30% 16% 2 1	198
January	1st to 10th	Night	26% 24% 30% 18% 2	850
	11th to 20th	Night	27% 23% 29% 17% 4	812
	21st to end	Night	19% 23% 36% 17% 5%	836
February	1st to 10th	Night	26% 23% 31% 18% 2	760
	11th to 20th	Night	23% 26% 33% 15% 3	743
	21st to end	Night	20% 23% 34% 19% 4	628
March	1st to 10th	Night	23% 21% 37% 18% 3	760
	11th to 20th	Night	22% 18% 33% 21% 6%	760
	21st to end	Night	24% 19% 33% 18% 6%	836
April	1st to 10th	Night	28% 18% 30% 21% 3	613
	11th to 20th	Night	23% 17% 34% 21% 5%	564
	21st to end	Night	27% 14% 30% 21% 7% 1	570
May	1st to 10th	Night	25% 13% 32% 21% 8% 1	570
	11th to 20th	Night	24% 12% 29% 27% 9%	562
	21st to end	Night	18% 13% 36% 26% 6%	616
June	1st to 10th	Night	27% 10% 29% 28% 6% 1	463
	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% 20% 27% 19% 7% 1	815
	11th to 20th	Night	27% 22% 27% 18% 6% 1	850
	21st to end	Night	26% 23% 29% 19% 4	850
December	1st to 10th	Night	30% 21% 30% 15% 3	850
	11th to 20th	Night	28% 27% 29% 14% 3	850
	21st to end	Night	30% 24% 26% 16% 3 1	935

Page generated at 2024-10-30 14:19:24. Version ac07

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