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 2024-11-04.

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	8% 6% 2 5% 3 3 4 3 5% 7% 53%	105177
	Night	8% 6% 2 5% 3 3 4 3 5% 7% 53%	102472
Transparency	Day	30% 6% 14% 27% 20% 4	104933
	Night	29% 6% 13% 26% 21% 5%	102306
Seeing	Day	11% 8% 31% 33% 15% 3	15712
	Night	11% 8% 34% 37% 10%	25595

By Month

Percentage Cloud Cover SW (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	6% 10% 4 7% 4 4 4 3 5% 7% 48%	2538
	11th to 20th	Day	6% 11% 5% 8% 4 4 3 3 7% 7% 41%	2631
	21st to end	Day	5% 8% 3 7% 4 4 4 2 5% 6% 51%	3135
February	1st to 10th	Day	8% 9% 4 7% 5% 5% 4 4 6% 6% 41%	1779
	11th to 20th	Day	5% 7% 2 6% 5% 4 5% 4 5% 8% 49%	1849
	21st to end	Day	7% 6% 2 5% 4 4 3 3 5% 7% 54%	1570
March	1st to 10th	Day	10% 6% 2 4 3 2 3 3 5% 5% 58%	3153
	11th to 20th	Day	10% 7% 2 4 3 3 2 3 4 6% 57%	3480
	21st to end	Day	12% 8% 3 6% 3 3 4 3 5% 7% 47%	3814
April	1st to 10th	Day	10% 6% 2 5% 3 3 4 3 5% 7% 53%	2228
	11th to 20th	Day	12% 7% 3 6% 4 4 3 3 5% 8% 46%	2322
	21st to end	Day	16% 5% 2 4 3 4 4 3 6% 8% 45%	2470
May	1st to 10th	Day	12% 6% 2 5% 3 3 4 3 6% 8% 48%	4120
	11th to 20th	Day	12% 6% 2 5% 4 3 3 3 6% 8% 48%	4082
	21st to end	Day	11% 6% 3 6% 5% 4 4 3 6% 8% 44%	4489
June	1st to 10th	Day	5% 3 2 3 3 3 4 3 6% 11% 59%	2545
	11th to 20th	Day	7% 4 1 4 3 4 4 4 6% 8% 54%	2660
	21st to end	Day	4 4 2 4 4 3 4 4 6% 7% 59%	2660
July	1st to 10th	Day	7% 5% 2 5% 4 4 4 4 5% 7% 52%	4430
	11th to 20th	Day	4 3 1 5% 3 4 5% 4 7% 9% 54%	4378
	21st to end	Day	3 3 1 4 4 4 4 4 7% 9% 57%	4466
August	1st to 10th	Day	9% 6% 2 5% 3 4 4 3 6% 9% 49%	4103
	11th to 20th	Day	10% 5% 2 5% 3 4 4 4 7% 9% 48%	4041
	21st to end	Day	5% 4 2 4 4 3 3 3 5% 7% 61%	4257
September	1st to 10th	Day	10% 4 1 4 2 3 4 3 4 6% 59%	2185
	11th to 20th	Day	9% 4 1 4 3 3 4 2 3 5% 60%	2090
	21st to end	Day	8% 3 1 4 2 3 3 3 3 6% 64%	2090
October	1st to 10th	Day	9% 2 1 2 1 2 2 2 3 4 72%	3480
	11th to 20th	Day	5% 3 1 3 2 3 3 2 4 6% 67%	3154
	21st to end	Day	9% 4 2 4 2 2 3 3 4 6% 62%	3162
November	1st to 10th	Day	7% 4 1 4 3 3 4 2 5% 6% 61%	1700
	11th to 20th	Day	7% 5% 2 3 3 2 3 3 5% 5% 61%	1710
	21st to end	Day	9% 7% 3 5% 3 3 4 3 3 6% 54%	1710
December	1st to 10th	Day	6% 6% 2 6% 3 3 3 3 5% 7% 54%	1710
	11th to 20th	Day	8% 7% 2 6% 4 3 3 2 5% 7% 52%	2161
	21st to end	Day	11% 12% 4 12% 4 3 4 3 3 5% 38%	2825
January	1st to 10th	Night	6% 11% 4 12% 4 3 3 3 5% 6% 43%	4417
	11th to 20th	Night	4 8% 3 6% 5% 4 3 3 4 7% 52%	4334
	21st to end	Night	8% 9% 4 8% 5% 5% 4 3 5% 5% 45%	4532
February	1st to 10th	Night	8% 9% 3 6% 4 4 5% 4 5% 6% 46%	2418
	11th to 20th	Night	4 9% 2 6% 5% 4 4 4 6% 8% 49%	2218
	21st to end	Night	5% 6% 2 6% 4 5% 4 2 4 6% 56%	1884
March	1st to 10th	Night	10% 5% 2 4 2 3 3 2 4 5% 60%	3707
	11th to 20th	Night	9% 5% 2 3 3 3 3 3 5% 7% 58%	3480
	21st to end	Night	13% 7% 2 6% 4 4 4 4 6% 8% 43%	3817
April	1st to 10th	Night	8% 6% 2 5% 5% 4 4 4 5% 7% 50%	1949
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 8% 46%	1804
	21st to end	Night	13% 5% 2 5% 3 3 3 3 5% 8% 49%	1710
May	1st to 10th	Night	19% 6% 2 5% 3 4 4 4 5% 6% 42%	2850
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 6% 8% 47%	2823
	21st to end	Night	14% 5% 2 5% 4 4 4 4 7% 10% 41%	3098
June	1st to 10th	Night	4 3 1 4 3 4 4 4 7% 10% 54%	1629
	11th to 20th	Night	8% 4 1 4 4 3 3 4 7% 9% 51%	1520
	21st to end	Night	4 3 2 4 4 3 5% 5% 7% 10% 54%	1520
July	1st to 10th	Night	8% 5% 1 5% 5% 3 4 4 6% 9% 50%	2540
	11th to 20th	Night	4 4 2 5% 4 4 5% 5% 8% 11% 49%	2589
	21st to end	Night	2 3 1 3 3 3 3 4 6% 8% 64%	3097
August	1st to 10th	Night	5% 4 2 4 4 4 4 4 6% 12% 51%	2836
	11th to 20th	Night	9% 6% 2 5% 3 4 5% 3 6% 8% 49%	2798
	21st to end	Night	4 3 2 5% 3 5% 5% 4 7% 9% 54%	3407
September	1st to 10th	Night	7% 3 1 4 2 3 3 3 5% 7% 63%	1995
	11th to 20th	Night	7% 5% 1 4 2 3 3 3 5% 6% 61%	2090
	21st to end	Night	7% 4 1 4 3 3 3 3 5% 6% 62%	2090
October	1st to 10th	Night	8% 2 1 3 2 1 2 2 4 5% 71%	3480
	11th to 20th	Night	6% 3 1 3 3 2 2 2 4 6% 68%	3732
	21st to end	Night	7% 3 1 4 3 3 3 3 4 6% 64%	4391
November	1st to 10th	Night	4 4 2 3 3 2 3 3 5% 7% 65%	2451
	11th to 20th	Night	7% 4 2 4 3 3 3 3 5% 6% 60%	2470
	21st to end	Night	8% 6% 1 4 3 3 3 2 5% 6% 59%	2470
December	1st to 10th	Night	5% 6% 3 5% 3 3 4 3 5% 6% 57%	2470
	11th to 20th	Night	7% 5% 2 4 4 3 4 3 6% 7% 57%	3036
	21st to end	Night	12% 13% 3 7% 4 3 3 3 4 6% 43%	4820

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	36% 5% 18% 27% 13% 1	2539
	11th to 20th	Day	41% 8% 12% 23% 12% 3	2631
	21st to end	Day	34% 5% 16% 29% 14% 2	3135
February	1st to 10th	Day	42% 6% 16% 21% 12% 3	1779
	11th to 20th	Day	33% 6% 16% 25% 16% 4	1859
	21st to end	Day	30% 5% 14% 26% 19% 7%	1570
March	1st to 10th	Day	28% 4 11% 22% 31% 5%	3153
	11th to 20th	Day	29% 4 13% 27% 24% 3	3480
	21st to end	Day	37% 7% 23% 26% 6% 1	3814
April	1st to 10th	Day	30% 6% 15% 26% 20% 3	2228
	11th to 20th	Day	37% 6% 15% 23% 16% 3	2325
	21st to end	Day	36% 7% 14% 28% 15% 1	2470
May	1st to 10th	Day	32% 8% 14% 26% 13% 7%	4120
	11th to 20th	Day	33% 8% 15% 30% 14%	4086
	21st to end	Day	35% 9% 15% 26% 13% 2	4489
June	1st to 10th	Day	21% 7% 20% 36% 14% 2	2545
	11th to 20th	Day	27% 7% 14% 28% 18% 5%	2660
	21st to end	Day	23% 7% 12% 29% 23% 5%	2660
July	1st to 10th	Day	30% 7% 10% 25% 23% 4	4361
	11th to 20th	Day	22% 8% 16% 27% 23% 4	4303
	21st to end	Day	20% 8% 15% 26% 23% 9%	4325
August	1st to 10th	Day	31% 7% 15% 24% 19% 4	4090
	11th to 20th	Day	33% 7% 17% 23% 18% 2	4080
	21st to end	Day	23% 6% 13% 27% 22% 9%	4256
September	1st to 10th	Day	28% 5% 11% 24% 25% 8%	2185
	11th to 20th	Day	26% 5% 11% 28% 25% 4	2090
	21st to end	Day	23% 4 9% 33% 28% 3	2090
October	1st to 10th	Day	19% 3 6% 23% 39% 11%	3480
	11th to 20th	Day	19% 5% 11% 37% 19% 8%	3154
	21st to end	Day	25% 3 9% 30% 29% 4	3162
November	1st to 10th	Day	24% 4 12% 31% 24% 4	1700
	11th to 20th	Day	25% 4 11% 28% 28% 4	1710
	21st to end	Day	33% 4 11% 25% 23% 5%	1710
December	1st to 10th	Day	30% 5% 14% 29% 19% 2	1710
	11th to 20th	Day	32% 4 14% 35% 14%	2159
	21st to end	Day	49% 6% 9% 21% 14% 2	2825
January	1st to 10th	Night	41% 5% 14% 25% 14% 1	4417
	11th to 20th	Night	31% 6% 15% 27% 21% 1	4334
	21st to end	Night	40% 6% 13% 24% 16% 2	4532
February	1st to 10th	Night	39% 6% 13% 22% 15% 5%	2418
	11th to 20th	Night	33% 6% 12% 31% 16% 2	2218
	21st to end	Night	29% 5% 12% 21% 24% 9%	1884
March	1st to 10th	Night	27% 4 8% 24% 28% 9%	3707
	11th to 20th	Night	25% 5% 10% 26% 28% 5%	3480
	21st to end	Night	36% 8% 22% 24% 8% 1	3817
April	1st to 10th	Night	32% 7% 13% 23% 21% 4	1949
	11th to 20th	Night	36% 7% 14% 23% 18% 2	1804
	21st to end	Night	33% 6% 14% 27% 17% 2	1710
May	1st to 10th	Night	39% 7% 13% 21% 14% 5%	2850
	11th to 20th	Night	33% 8% 17% 25% 16% 1	2824
	21st to end	Night	34% 11% 19% 24% 11% 1	3099
June	1st to 10th	Night	23% 9% 20% 28% 17% 3	1629
	11th to 20th	Night	28% 7% 15% 26% 18% 6%	1520
	21st to end	Night	24% 7% 16% 27% 22% 5%	1520
July	1st to 10th	Night	29% 7% 15% 25% 20% 4	2500
	11th to 20th	Night	25% 7% 16% 28% 19% 5%	2541
	21st to end	Night	16% 7% 16% 27% 24% 11%	2999
August	1st to 10th	Night	25% 7% 18% 26% 21% 3	2826
	11th to 20th	Night	33% 7% 17% 25% 16% 2	2825
	21st to end	Night	24% 8% 15% 27% 18% 9%	3407
September	1st to 10th	Night	22% 5% 11% 25% 27% 10%	1995
	11th to 20th	Night	24% 6% 10% 27% 26% 7%	2090
	21st to end	Night	23% 5% 9% 26% 30% 7%	2090
October	1st to 10th	Night	17% 3 6% 18% 45% 10%	3480
	11th to 20th	Night	19% 4 10% 28% 26% 12%	3732
	21st to end	Night	22% 4 11% 26% 31% 6%	4391
November	1st to 10th	Night	20% 4 11% 30% 27% 8%	2451
	11th to 20th	Night	26% 4 9% 28% 28% 6%	2470
	21st to end	Night	27% 4 11% 26% 26% 5%	2470
December	1st to 10th	Night	28% 5% 14% 29% 20% 3	2470
	11th to 20th	Night	27% 5% 12% 36% 19% 2	3037
	21st to end	Night	42% 6% 11% 27% 13% 2	4820

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	11% 10% 39% 30% 8% 3	270
	11th to 20th	Day	16% 15% 32% 28% 9%	270
	21st to end	Day	10% 8% 45% 26% 9%	297
February	1st to 10th	Day	15% 13% 46% 21% 5%	270
	11th to 20th	Day	10% 9% 44% 29% 6% 1	270
	21st to end	Day	10% 16% 50% 20% 4	225
March	1st to 10th	Day	13% 11% 44% 24% 7%	270
	11th to 20th	Day	20% 11% 33% 24% 12% 1	270
	21st to end	Day	20% 12% 39% 23% 5% 1	297
April	1st to 10th	Day	16% 18% 37% 24% 5%	408
	11th to 20th	Day	14% 17% 38% 25% 5%	523
	21st to end	Day	19% 13% 36% 22% 8% 2	650
May	1st to 10th	Day	15% 11% 33% 30% 11% 1	653
	11th to 20th	Day	19% 6% 31% 28% 13% 3	675
	21st to end	Day	16% 8% 33% 28% 13% 2	734
June	1st to 10th	Day	4 6% 25% 38% 21% 6%	680
	11th to 20th	Day	8% 5% 34% 36% 15% 2	680
	21st to end	Day	6% 8% 30% 39% 17% 1	680
July	1st to 10th	Day	7% 3 22% 39% 24% 5%	671
	11th to 20th	Day	4 3 21% 44% 24% 5%	668
	21st to end	Day	5% 2 18% 43% 26% 6%	739
August	1st to 10th	Day	6% 3 18% 45% 24% 4	676
	11th to 20th	Day	7% 4 22% 44% 21% 3	665
	21st to end	Day	9% 2 21% 42% 22% 5%	590
September	1st to 10th	Day	11% 4 26% 40% 17% 2	395
	11th to 20th	Day	5% 7% 29% 34% 21% 5%	300
	21st to end	Day	13% 10% 25% 28% 19% 5%	300
October	1st to 10th	Day	18% 7% 23% 32% 16% 4	300
	11th to 20th	Day	4 14% 40% 27% 12% 4	300
	21st to end	Day	17% 13% 31% 27% 10% 1	330
November	1st to 10th	Day	12% 12% 34% 28% 11% 4	279
	11th to 20th	Day	12% 10% 39% 27% 10% 1	270
	21st to end	Day	19% 13% 32% 25% 9% 3	270
December	1st to 10th	Day	9% 7% 41% 34% 8% 1	270
	11th to 20th	Day	14% 9% 40% 30% 6%	270
	21st to end	Day	17% 11% 40% 25% 6%	297
January	1st to 10th	Night	10% 9% 37% 36% 8%	950
	11th to 20th	Night	10% 12% 35% 34% 8%	950
	21st to end	Night	10% 9% 43% 31% 7%	1045
February	1st to 10th	Night	12% 10% 41% 31% 5%	898
	11th to 20th	Night	9% 10% 42% 34% 6%	744
	21st to end	Night	9% 13% 40% 33% 5%	628
March	1st to 10th	Night	13% 12% 37% 33% 5%	746
	11th to 20th	Night	16% 10% 35% 30% 7% 1	570
	21st to end	Night	17% 13% 39% 25% 6%	627
April	1st to 10th	Night	12% 14% 44% 26% 4	570
	11th to 20th	Night	15% 13% 40% 28% 4	563
	21st to end	Night	15% 13% 32% 32% 8%	570
May	1st to 10th	Night	17% 9% 36% 31% 7%	570
	11th to 20th	Night	15% 6% 37% 32% 9% 1	561
	21st to end	Night	16% 8% 34% 31% 10%	617
June	1st to 10th	Night	5% 5% 31% 42% 14% 2	570
	11th to 20th	Night	10% 6% 36% 38% 10%	570
	21st to end	Night	5% 5% 36% 45% 10%	570
July	1st to 10th	Night	7% 4 24% 44% 20% 1	555
	11th to 20th	Night	3 2 21% 56% 18% 1	552
	21st to end	Night	5% 1 19% 51% 23% 1	624
August	1st to 10th	Night	6% 2 17% 52% 22% 1	567
	11th to 20th	Night	7% 4 25% 46% 17% 1	564
	21st to end	Night	6% 2 25% 47% 19% 2	627
September	1st to 10th	Night	7% 3 28% 50% 10% 2	570
	11th to 20th	Night	10% 3 32% 38% 16%	570
	21st to end	Night	8% 7% 32% 37% 15% 1	570
October	1st to 10th	Night	14% 8% 29% 37% 12%	570
	11th to 20th	Night	11% 9% 36% 34% 8%	726
	21st to end	Night	13% 10% 33% 32% 12% 1	993
November	1st to 10th	Night	7% 9% 33% 42% 9%	943
	11th to 20th	Night	10% 9% 35% 37% 8%	950
	21st to end	Night	11% 9% 32% 37% 11% 1	950
December	1st to 10th	Night	9% 9% 37% 39% 6%	950
	11th to 20th	Night	12% 8% 36% 35% 8%	950
	21st to end	Night	15% 9% 35% 35% 5%	1045

Page generated at 2024-11-01 21:47:52. Version ac07

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