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 2025-02-25.

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	16% 11% 4 10% 6% 5% 5% 4 6% 7% 26%	108296
	Night	16% 14% 5% 12% 7% 6% 5% 4 5% 6% 22%	110431
Transparency	Day	57% 5% 13% 18% 7% 1	111757
	Night	63% 4 10% 16% 6%	115135
Seeing	Day	25% 14% 30% 21% 9% 2	15583
	Night	26% 12% 28% 25% 8% 1	24111

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	13% 24% 8% 20% 8% 5% 4 3 3 3 9%	3463
	11th to 20th	Day	10% 15% 6% 21% 9% 7% 7% 4 5% 5% 12%	3570
	21st to end	Day	6% 11% 6% 13% 8% 6% 5% 4 7% 8% 26%	3960
February	1st to 10th	Day	7% 13% 4 12% 9% 9% 7% 6% 7% 6% 19%	1600
	11th to 20th	Day	13% 16% 6% 14% 9% 6% 5% 4 5% 5% 19%	1841
	21st to end	Day	10% 13% 4 11% 8% 7% 6% 4 5% 6% 25%	1610
March	1st to 10th	Day	13% 12% 4 9% 4 4 5% 3 5% 7% 33%	3116
	11th to 20th	Day	15% 18% 5% 11% 6% 5% 6% 3 5% 6% 21%	3287
	21st to end	Day	11% 13% 4 9% 5% 5% 6% 5% 5% 8% 29%	4097
April	1st to 10th	Day	18% 15% 5% 10% 7% 5% 4 3 5% 5% 26%	2280
	11th to 20th	Day	17% 10% 3 7% 5% 4 5% 4 5% 7% 34%	2358
	21st to end	Day	19% 10% 3 7% 6% 5% 5% 3 5% 6% 32%	2470
May	1st to 10th	Day	17% 10% 2 7% 4 4 4 4 6% 8% 34%	4355
	11th to 20th	Day	16% 7% 3 8% 6% 5% 5% 4 6% 7% 32%	4398
	21st to end	Day	16% 7% 3 7% 5% 5% 6% 5% 7% 8% 31%	5086
June	1st to 10th	Day	20% 8% 2 7% 5% 4 5% 4 6% 7% 32%	2850
	11th to 20th	Day	16% 8% 2 7% 5% 4 5% 5% 6% 8% 33%	2850
	21st to end	Day	21% 8% 3 7% 6% 4 4 4 6% 8% 30%	2850
July	1st to 10th	Day	18% 7% 3 7% 5% 4 5% 4 7% 9% 31%	4738
	11th to 20th	Day	16% 7% 3 9% 5% 5% 5% 5% 7% 9% 29%	4440
	21st to end	Day	14% 6% 3 7% 5% 4 5% 5% 6% 8% 36%	4814
August	1st to 10th	Day	15% 7% 3 8% 6% 5% 6% 5% 8% 10% 27%	4423
	11th to 20th	Day	18% 7% 3 8% 5% 5% 6% 5% 7% 9% 27%	4121
	21st to end	Day	18% 7% 3 8% 5% 5% 4 3 5% 8% 33%	4373
September	1st to 10th	Day	19% 9% 3 6% 5% 6% 5% 3 6% 8% 31%	2280
	11th to 20th	Day	19% 7% 3 7% 4 5% 4 4 6% 7% 35%	2135
	21st to end	Day	24% 8% 3 8% 4 4 4 3 5% 7% 31%	2090
October	1st to 10th	Day	19% 10% 3 10% 5% 4 4 3 4 7% 30%	2044
	11th to 20th	Day	26% 16% 3 10% 6% 5% 5% 4 5% 5% 17%	1734
	21st to end	Day	27% 16% 4 8% 6% 4 3 4 4 5% 19%	1881
November	1st to 10th	Day	20% 18% 5% 10% 6% 5% 4 4 4 5% 20%	1761
	11th to 20th	Day	21% 19% 6% 12% 5% 5% 5% 3 3 5% 16%	1647
	21st to end	Day	19% 21% 6% 14% 8% 6% 5% 3 3 3 12%	1400
December	1st to 10th	Day	15% 17% 8% 16% 12% 6% 5% 5% 4 4 9%	2280
	11th to 20th	Day	17% 17% 6% 13% 9% 6% 4 4 7% 4 12%	2618
	21st to end	Day	16% 17% 5% 12% 8% 8% 6% 4 5% 6% 12%	3476
January	1st to 10th	Night	14% 27% 8% 16% 7% 6% 4 3 3 3 9%	6472
	11th to 20th	Night	9% 18% 8% 22% 10% 6% 5% 4 4 4 10%	6315
	21st to end	Night	5% 11% 6% 13% 10% 8% 7% 5% 7% 7% 21%	6996
February	1st to 10th	Night	8% 13% 6% 13% 9% 7% 5% 5% 6% 7% 20%	2800
	11th to 20th	Night	10% 19% 6% 14% 9% 7% 5% 3 4 5% 17%	2442
	21st to end	Night	10% 18% 5% 12% 7% 6% 4 3 4 6% 24%	1939
March	1st to 10th	Night	16% 15% 5% 10% 6% 5% 6% 5% 4 6% 22%	3724
	11th to 20th	Night	15% 19% 5% 13% 7% 6% 6% 3 5% 5% 17%	3671
	21st to end	Night	13% 16% 5% 11% 5% 5% 4 4 5% 6% 27%	3533
April	1st to 10th	Night	22% 16% 4 9% 5% 5% 3 2 3 5% 24%	1900
	11th to 20th	Night	21% 11% 3 8% 5% 4 4 3 4 6% 29%	1770
	21st to end	Night	19% 11% 3 8% 3 4 5% 3 4 7% 33%	1710
May	1st to 10th	Night	19% 10% 3 8% 5% 5% 5% 3 5% 5% 32%	2615
	11th to 20th	Night	18% 10% 3 8% 5% 5% 5% 3 5% 7% 32%	2495
	21st to end	Night	21% 8% 2 6% 6% 5% 5% 3 6% 6% 30%	2495
June	1st to 10th	Night	19% 8% 3 7% 6% 5% 5% 5% 7% 8% 27%	1330
	11th to 20th	Night	17% 8% 3 7% 5% 5% 4 3 6% 7% 35%	1330
	21st to end	Night	20% 8% 2 7% 7% 5% 5% 4 6% 9% 27%	1330
July	1st to 10th	Night	17% 7% 4 8% 6% 6% 5% 5% 7% 7% 28%	2227
	11th to 20th	Night	14% 8% 3 8% 6% 6% 6% 5% 6% 8% 30%	2520
	21st to end	Night	14% 6% 2 6% 5% 5% 4 4 6% 9% 39%	2739
August	1st to 10th	Night	15% 8% 3 8% 6% 6% 6% 6% 7% 8% 27%	2506
	11th to 20th	Night	18% 7% 4 7% 6% 5% 6% 5% 7% 8% 29%	2675
	21st to end	Night	15% 7% 3 8% 5% 6% 5% 4 6% 7% 35%	3279
September	1st to 10th	Night	20% 5% 2 8% 4 5% 5% 4 5% 7% 35%	1900
	11th to 20th	Night	20% 8% 3 8% 5% 5% 4 4 5% 7% 32%	2044
	21st to end	Night	22% 8% 3 8% 5% 5% 5% 3 4 5% 33%	2090
October	1st to 10th	Night	21% 9% 2 7% 5% 5% 5% 3 5% 6% 32%	2133
	11th to 20th	Night	31% 13% 4 9% 5% 5% 4 3 4 5% 20%	2446
	21st to end	Night	30% 15% 4 9% 4 5% 4 4 4 5% 16%	2717
November	1st to 10th	Night	24% 19% 5% 10% 4 4 4 3 3 5% 19%	2537
	11th to 20th	Night	20% 20% 6% 11% 5% 5% 4 4 4 4 17%	2748
	21st to end	Night	21% 22% 7% 14% 6% 5% 4 3 3 4 11%	3000
December	1st to 10th	Night	14% 18% 9% 20% 11% 6% 5% 3 3 4 8%	4910
	11th to 20th	Night	21% 18% 6% 14% 7% 5% 5% 3 5% 4 13%	5591
	21st to end	Night	16% 20% 6% 13% 6% 5% 4 3 4 4 18%	7502

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	83% 3 7% 6% 1	3464
	11th to 20th	Day	74% 4 7% 11% 3	3600
	21st to end	Day	56% 4 11% 21% 9%	3960
February	1st to 10th	Day	62% 5% 9% 16% 7% 1	1600
	11th to 20th	Day	68% 5% 9% 13% 6%	1850
	21st to end	Day	60% 5% 8% 20% 6% 1	1610
March	1st to 10th	Day	50% 6% 16% 17% 9% 2	3119
	11th to 20th	Day	65% 4 10% 15% 5%	3287
	21st to end	Day	52% 5% 11% 18% 12% 2	4097
April	1st to 10th	Day	63% 3 9% 14% 9% 3	2280
	11th to 20th	Day	51% 4 10% 23% 11% 1	2358
	21st to end	Day	54% 4 9% 20% 12% 1	2470
May	1st to 10th	Day	49% 4 14% 20% 13% 1	4369
	11th to 20th	Day	49% 6% 11% 23% 10% 1	4402
	21st to end	Day	48% 6% 10% 22% 12% 1	5086
June	1st to 10th	Day	51% 4 10% 22% 12% 1	2850
	11th to 20th	Day	47% 5% 12% 22% 11% 2	2850
	21st to end	Day	53% 5% 12% 20% 9% 1	2850
July	1st to 10th	Day	49% 4 15% 28% 4	4663
	11th to 20th	Day	51% 4 19% 21% 4	4365
	21st to end	Day	46% 4 20% 24% 6%	4661
August	1st to 10th	Day	51% 5% 17% 24% 2	4409
	11th to 20th	Day	52% 6% 23% 16% 3	4162
	21st to end	Day	51% 5% 15% 20% 9%	4373
September	1st to 10th	Day	52% 4 15% 22% 6%	2280
	11th to 20th	Day	48% 5% 18% 23% 6%	2135
	21st to end	Day	54% 4 16% 20% 6%	2090
October	1st to 10th	Day	56% 4 12% 23% 4 1	3395
	11th to 20th	Day	66% 4 16% 12% 1	2859
	21st to end	Day	65% 5% 14% 15% 2	3081
November	1st to 10th	Day	66% 5% 12% 12% 4	1761
	11th to 20th	Day	73% 5% 8% 11% 3	1647
	21st to end	Day	77% 5% 10% 6% 2	1400
December	1st to 10th	Day	77% 4 11% 7% 1	2280
	11th to 20th	Day	72% 6% 8% 11% 3	2618
	21st to end	Day	72% 5% 6% 12% 5%	3476
January	1st to 10th	Night	81% 3 6% 8% 2	6472
	11th to 20th	Night	78% 3 6% 9% 3	6360
	21st to end	Night	60% 4 10% 20% 7%	6996
February	1st to 10th	Night	62% 4 9% 17% 8%	2800
	11th to 20th	Night	69% 4 8% 11% 7%	2444
	21st to end	Night	62% 3 8% 20% 7%	1939
March	1st to 10th	Night	64% 3 11% 15% 6%	3734
	11th to 20th	Night	68% 4 9% 13% 5% 1	3672
	21st to end	Night	57% 4 8% 20% 10% 1	3533
April	1st to 10th	Night	64% 3 7% 15% 8% 2	1900
	11th to 20th	Night	56% 4 9% 19% 12%	1770
	21st to end	Night	52% 4 9% 22% 12% 2	1710
May	1st to 10th	Night	53% 5% 10% 18% 13% 2	2621
	11th to 20th	Night	51% 4 12% 21% 12% 1	2496
	21st to end	Night	52% 6% 9% 19% 14% 1	2496
June	1st to 10th	Night	52% 5% 11% 21% 9% 2	1330
	11th to 20th	Night	48% 5% 11% 19% 13% 3	1330
	21st to end	Night	54% 6% 12% 19% 10%	1330
July	1st to 10th	Night	53% 4 14% 23% 5%	2192
	11th to 20th	Night	51% 5% 17% 22% 4	2472
	21st to end	Night	44% 5% 21% 20% 11%	2651
August	1st to 10th	Night	52% 6% 17% 20% 5%	2496
	11th to 20th	Night	52% 6% 23% 16% 4	2700
	21st to end	Night	49% 5% 12% 28% 6%	3279
September	1st to 10th	Night	48% 5% 14% 24% 9%	1900
	11th to 20th	Night	51% 4 17% 21% 6%	2044
	21st to end	Night	54% 5% 11% 22% 8%	2090
October	1st to 10th	Night	57% 4 13% 20% 5% 1	3562
	11th to 20th	Night	65% 4 11% 17% 4	4060
	21st to end	Night	69% 5% 12% 12% 3	4467
November	1st to 10th	Night	67% 4 12% 13% 4	2537
	11th to 20th	Night	69% 6% 10% 12% 3	2748
	21st to end	Night	77% 4 8% 8% 2	3000
December	1st to 10th	Night	80% 3 10% 6% 1	4910
	11th to 20th	Night	74% 4 5% 15% 3	5592
	21st to end	Night	71% 3 7% 15% 4	7502

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% 17% 42% 14% 5%	200
	11th to 20th	Day	22% 15% 34% 22% 7% 2	200
	21st to end	Day	17% 13% 34% 23% 11% 2	220
February	1st to 10th	Day	14% 21% 45% 14% 6% 2	200
	11th to 20th	Day	23% 20% 35% 19% 3	410
	21st to end	Day	23% 18% 34% 19% 6% 1	398
March	1st to 10th	Day	26% 16% 34% 18% 6% 1	460
	11th to 20th	Day	25% 17% 32% 18% 8% 1	460
	21st to end	Day	27% 17% 30% 19% 7% 1	506
April	1st to 10th	Day	27% 16% 32% 18% 7% 1	460
	11th to 20th	Day	25% 16% 32% 19% 7% 1	452
	21st to end	Day	25% 19% 33% 17% 5% 1	460
May	1st to 10th	Day	27% 18% 29% 19% 6% 2	463
	11th to 20th	Day	19% 16% 30% 26% 8% 1	487
	21st to end	Day	25% 17% 31% 19% 7% 1	674
June	1st to 10th	Day	24% 11% 33% 22% 9% 1	680
	11th to 20th	Day	21% 12% 30% 23% 11% 2	680
	21st to end	Day	26% 9% 28% 24% 12% 2	680
July	1st to 10th	Day	22% 10% 31% 24% 12% 1	661
	11th to 20th	Day	20% 10% 29% 27% 12% 3	484
	21st to end	Day	20% 7% 30% 27% 14% 2	532
August	1st to 10th	Day	22% 7% 26% 29% 14% 3	486
	11th to 20th	Day	21% 8% 24% 26% 17% 4	479
	21st to end	Day	24% 12% 25% 19% 15% 5%	539
September	1st to 10th	Day	24% 14% 27% 21% 11% 4	490
	11th to 20th	Day	21% 13% 26% 26% 11% 4	490
	21st to end	Day	27% 11% 26% 20% 12% 3	490
October	1st to 10th	Day	25% 16% 28% 18% 12% 2	490
	11th to 20th	Day	35% 17% 25% 14% 7% 1	403
	21st to end	Day	39% 15% 23% 17% 5% 1	429
November	1st to 10th	Day	38% 18% 26% 13% 4 1	395
	11th to 20th	Day	31% 18% 25% 20% 5% 1	305
	21st to end	Day	32% 20% 30% 15% 4 1	200
December	1st to 10th	Day	26% 11% 36% 17% 9% 3	200
	11th to 20th	Day	26% 20% 35% 12% 6% 1	200
	21st to end	Day	29% 21% 26% 18% 5% 1	220
January	1st to 10th	Night	27% 14% 31% 24% 5%	829
	11th to 20th	Night	24% 13% 27% 27% 8% 1	800
	21st to end	Night	15% 10% 31% 32% 9% 1	880
February	1st to 10th	Night	14% 13% 36% 26% 9% 1	800
	11th to 20th	Night	22% 15% 34% 22% 7% 1	783
	21st to end	Night	20% 15% 30% 27% 7% 1	647
March	1st to 10th	Night	20% 13% 37% 23% 7%	760
	11th to 20th	Night	23% 16% 31% 21% 9%	760
	21st to end	Night	23% 16% 29% 26% 6% 1	836
April	1st to 10th	Night	30% 12% 29% 23% 5% 1	760
	11th to 20th	Night	29% 11% 28% 25% 6% 1	646
	21st to end	Night	26% 15% 31% 22% 6%	570
May	1st to 10th	Night	31% 9% 31% 23% 6%	442
	11th to 20th	Night	23% 9% 33% 28% 7% 1	374
	21st to end	Night	27% 11% 34% 23% 4	411
June	1st to 10th	Night	26% 5% 36% 27% 6%	380
	11th to 20th	Night	21% 8% 26% 32% 13%	380
	21st to end	Night	25% 8% 28% 30% 8% 1	380
July	1st to 10th	Night	24% 7% 28% 32% 9%	370
	11th to 20th	Night	18% 5% 25% 39% 13% 1	368
	21st to end	Night	21% 5% 22% 38% 13% 1	416
August	1st to 10th	Night	22% 4 23% 38% 13% 1	378
	11th to 20th	Night	20% 5% 23% 34% 17% 1	491
	21st to end	Night	21% 8% 24% 29% 15% 2	717
September	1st to 10th	Night	23% 9% 25% 30% 13% 1	760
	11th to 20th	Night	25% 8% 27% 26% 13% 2	760
	21st to end	Night	25% 10% 27% 28% 9% 2	760
October	1st to 10th	Night	26% 10% 26% 28% 9% 1	760
	11th to 20th	Night	38% 11% 22% 21% 8% 1	760
	21st to end	Night	40% 14% 22% 17% 7%	836
November	1st to 10th	Night	36% 14% 24% 18% 6% 1	780
	11th to 20th	Night	31% 18% 25% 19% 7% 1	827
	21st to end	Night	33% 16% 29% 18% 4 1	900
December	1st to 10th	Night	31% 13% 26% 20% 9% 1	900
	11th to 20th	Night	27% 16% 26% 21% 8% 1	900
	21st to end	Night	27% 14% 26% 24% 7% 1	990

Page generated at 2025-02-22 06:51:35. Version ac07

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