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 2024-02-10.

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% 11% 4 8% 7% 4 5% 5% 5% 7% 26%	77831
	Night	18% 14% 5% 10% 8% 4 5% 5% 4 6% 22%	81141
Transparency	Day	58% 5% 12% 18% 7% 1	77531
	Night	63% 5% 10% 15% 6% 1	80976
Seeing	Day	25% 14% 29% 21% 9% 2	14234
	Night	26% 12% 28% 25% 8% 1	22777

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	12% 16% 9% 17% 11% 5% 5% 5% 4 4 12%	2422
	11th to 20th	Day	11% 12% 7% 17% 11% 6% 8% 6% 5% 6% 11%	2520
	21st to end	Day	8% 14% 7% 12% 8% 4 5% 5% 5% 7% 24%	2772
February	1st to 10th	Day	8% 11% 5% 9% 11% 6% 9% 8% 7% 6% 20%	1512
	11th to 20th	Day	13% 15% 6% 11% 10% 5% 5% 5% 5% 5% 19%	1652
	21st to end	Day	10% 12% 5% 10% 9% 6% 7% 5% 5% 6% 26%	1480
March	1st to 10th	Day	14% 13% 5% 10% 6% 3 6% 5% 6% 6% 25%	1797
	11th to 20th	Day	15% 14% 5% 8% 7% 4 7% 4 4 6% 26%	1871
	21st to end	Day	16% 13% 5% 7% 5% 3 6% 5% 5% 6% 30%	2335
April	1st to 10th	Day	19% 14% 5% 8% 8% 4 5% 3 5% 5% 25%	2160
	11th to 20th	Day	18% 10% 3 6% 6% 3 5% 4 5% 6% 35%	2232
	21st to end	Day	19% 9% 4 7% 6% 3 5% 4 5% 6% 32%	2340
May	1st to 10th	Day	21% 9% 3 5% 5% 2 5% 4 5% 7% 34%	2477
	11th to 20th	Day	17% 7% 3 6% 6% 3 5% 5% 5% 7% 36%	2478
	21st to end	Day	19% 6% 3 6% 5% 3 6% 5% 6% 8% 33%	2862
June	1st to 10th	Day	20% 7% 2 6% 5% 3 5% 5% 6% 8% 32%	2700
	11th to 20th	Day	17% 7% 2 6% 6% 3 5% 5% 6% 9% 34%	2700
	21st to end	Day	21% 8% 3 6% 6% 3 5% 6% 5% 8% 30%	2700
July	1st to 10th	Day	19% 6% 3 6% 6% 3 5% 6% 7% 10% 30%	2692
	11th to 20th	Day	16% 6% 3 7% 6% 3 5% 6% 7% 9% 30%	2520
	21st to end	Day	15% 6% 3 6% 7% 4 6% 6% 7% 9% 31%	2744
August	1st to 10th	Day	18% 7% 4 7% 7% 4 6% 7% 7% 10% 25%	2506
	11th to 20th	Day	18% 6% 3 6% 6% 4 6% 5% 7% 8% 31%	2354
	21st to end	Day	20% 8% 3 7% 6% 3 5% 5% 6% 8% 30%	2485
September	1st to 10th	Day	19% 9% 4 5% 6% 4 5% 4 6% 8% 31%	2160
	11th to 20th	Day	20% 7% 3 6% 5% 4 4 5% 5% 7% 34%	2023
	21st to end	Day	24% 7% 3 7% 5% 3 4 4 4 7% 31%	1980
October	1st to 10th	Day	19% 10% 3 9% 6% 3 5% 4 4 7% 31%	1937
	11th to 20th	Day	27% 15% 4 8% 7% 3 6% 5% 4 5% 16%	1643
	21st to end	Day	28% 15% 4 7% 7% 3 4 5% 4 5% 19%	1782
November	1st to 10th	Day	20% 17% 6% 9% 8% 3 5% 4 3 5% 20%	1671
	11th to 20th	Day	22% 18% 6% 12% 6% 4 6% 3 3 5% 15%	1565
	21st to end	Day	20% 19% 6% 12% 9% 4 6% 4 3 4 12%	1330
December	1st to 10th	Day	20% 18% 7% 13% 10% 5% 6% 6% 4 3 9%	1330
	11th to 20th	Day	19% 18% 7% 10% 6% 4 4 4 6% 4 17%	1668
	21st to end	Day	18% 18% 6% 10% 9% 6% 7% 6% 4 5% 11%	2431
January	1st to 10th	Night	13% 20% 7% 14% 11% 6% 6% 4 4 4 11%	4524
	11th to 20th	Night	13% 16% 6% 15% 12% 4 6% 5% 5% 5% 13%	4440
	21st to end	Night	7% 15% 8% 12% 11% 5% 6% 5% 5% 5% 20%	4884
February	1st to 10th	Night	9% 12% 7% 11% 11% 5% 6% 6% 6% 7% 21%	2653
	11th to 20th	Night	11% 17% 8% 13% 9% 5% 6% 4 4 5% 18%	2191
	21st to end	Night	11% 17% 6% 11% 8% 4 5% 3 4 6% 24%	1776
March	1st to 10th	Night	13% 14% 7% 11% 8% 4 4 4 4 6% 24%	2150
	11th to 20th	Night	16% 14% 6% 10% 8% 4 6% 5% 5% 6% 22%	2087
	21st to end	Night	15% 16% 5% 8% 6% 3 4 4 4 5% 30%	2019
April	1st to 10th	Night	23% 15% 5% 9% 6% 4 4 3 3 6% 22%	1800
	11th to 20th	Night	21% 10% 3 7% 6% 3 4 4 4 6% 31%	1676
	21st to end	Night	20% 10% 3 6% 4 2 5% 5% 4 7% 34%	1620
May	1st to 10th	Night	23% 10% 3 7% 6% 3 4 5% 4 5% 30%	1499
	11th to 20th	Night	18% 10% 3 6% 5% 4 4 3 5% 7% 34%	1415
	21st to end	Night	23% 8% 3 5% 5% 4 5% 4 5% 7% 31%	1411
June	1st to 10th	Night	20% 7% 4 6% 6% 5% 6% 6% 7% 9% 26%	1260
	11th to 20th	Night	17% 8% 3 6% 5% 4 5% 4 5% 8% 35%	1260
	21st to end	Night	20% 7% 3 6% 8% 4 5% 6% 6% 9% 28%	1260
July	1st to 10th	Night	18% 7% 4 6% 7% 3 5% 6% 6% 8% 29%	1264
	11th to 20th	Night	15% 6% 3 8% 6% 4 7% 7% 6% 8% 30%	1440
	21st to end	Night	18% 7% 4 7% 7% 4 5% 5% 6% 7% 30%	1574
August	1st to 10th	Night	16% 7% 3 6% 9% 4 6% 8% 7% 8% 25%	1432
	11th to 20th	Night	18% 7% 4 6% 8% 4 5% 5% 6% 8% 30%	1528
	21st to end	Night	18% 7% 3 7% 6% 3 6% 6% 6% 7% 31%	1867
September	1st to 10th	Night	20% 5% 3 7% 5% 4 5% 5% 5% 8% 35%	1800
	11th to 20th	Night	21% 8% 3 7% 6% 3 5% 4 5% 7% 32%	1936
	21st to end	Night	22% 7% 3 7% 7% 4 5% 4 4 5% 33%	1980
October	1st to 10th	Night	21% 9% 2 6% 6% 4 5% 4 5% 6% 32%	2020
	11th to 20th	Night	31% 12% 4 7% 6% 4 4 4 4 4 19%	2317
	21st to end	Night	31% 14% 4 8% 5% 4 4 5% 4 5% 16%	2574
November	1st to 10th	Night	24% 18% 5% 9% 6% 3 4 4 3 5% 19%	2407
	11th to 20th	Night	21% 19% 6% 10% 7% 4 4 5% 4 5% 16%	2610
	21st to end	Night	21% 19% 8% 13% 8% 4 5% 4 4 4 12%	2850
December	1st to 10th	Night	21% 19% 8% 12% 9% 4 5% 4 4 4 10%	2850
	11th to 20th	Night	21% 16% 7% 11% 8% 5% 5% 6% 4 5% 13%	3531
	21st to end	Night	17% 23% 7% 11% 7% 3 4 3 3 4 17%	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	76% 4 10% 8% 2	2423
	11th to 20th	Day	73% 6% 8% 10% 3	2520
	21st to end	Day	60% 4 11% 18% 7%	2772
February	1st to 10th	Day	62% 6% 9% 16% 7% 1	1512
	11th to 20th	Day	67% 5% 9% 13% 7%	1661
	21st to end	Day	60% 5% 8% 20% 6% 1	1480
March	1st to 10th	Day	59% 6% 8% 16% 9% 1	1800
	11th to 20th	Day	61% 5% 8% 17% 9%	1871
	21st to end	Day	56% 4 10% 19% 10% 1	2335
April	1st to 10th	Day	64% 3 9% 13% 9% 2	2160
	11th to 20th	Day	51% 4 10% 23% 11% 1	2232
	21st to end	Day	54% 4 10% 19% 12% 1	2340
May	1st to 10th	Day	51% 3 10% 19% 14% 2	2477
	11th to 20th	Day	48% 5% 10% 23% 13% 1	2482
	21st to end	Day	49% 6% 9% 20% 14% 2	2862
June	1st to 10th	Day	51% 4 10% 21% 13% 1	2700
	11th to 20th	Day	47% 5% 12% 22% 12% 2	2700
	21st to end	Day	53% 5% 13% 20% 9% 1	2700
July	1st to 10th	Day	48% 5% 18% 23% 5%	2617
	11th to 20th	Day	49% 6% 17% 22% 6%	2445
	21st to end	Day	51% 6% 17% 22% 4	2591
August	1st to 10th	Day	54% 7% 18% 19% 2	2492
	11th to 20th	Day	50% 7% 19% 19% 5%	2354
	21st to end	Day	52% 6% 16% 21% 5%	2485
September	1st to 10th	Day	52% 4 15% 22% 7%	2160
	11th to 20th	Day	50% 5% 17% 21% 7%	2023
	21st to end	Day	54% 4 16% 20% 6%	1980
October	1st to 10th	Day	54% 4 15% 19% 7% 1	1937
	11th to 20th	Day	70% 5% 11% 12% 2	1643
	21st to end	Day	69% 5% 9% 14% 3 1	1782
November	1st to 10th	Day	66% 5% 12% 12% 4	1671
	11th to 20th	Day	74% 5% 9% 11% 2	1565
	21st to end	Day	76% 5% 10% 7% 2	1330
December	1st to 10th	Day	77% 6% 7% 9% 1	1330
	11th to 20th	Day	68% 5% 6% 16% 4	1668
	21st to end	Day	74% 6% 6% 8% 6%	2431
January	1st to 10th	Night	77% 4 8% 9% 2	4524
	11th to 20th	Night	72% 4 8% 11% 4	4440
	21st to end	Night	65% 4 10% 16% 5%	4884
February	1st to 10th	Night	62% 4 9% 17% 8%	2653
	11th to 20th	Night	69% 4 8% 12% 7%	2193
	21st to end	Night	62% 3 8% 20% 7%	1776
March	1st to 10th	Night	63% 3 8% 19% 7% 1	2160
	11th to 20th	Night	64% 3 7% 16% 8% 1	2088
	21st to end	Night	57% 3 8% 20% 10% 2	2019
April	1st to 10th	Night	66% 3 7% 14% 8% 2	1800
	11th to 20th	Night	55% 3 9% 19% 13%	1676
	21st to end	Night	51% 4 9% 22% 12% 2	1620
May	1st to 10th	Night	57% 3 8% 15% 14% 3	1499
	11th to 20th	Night	50% 4 10% 20% 15% 2	1416
	21st to end	Night	53% 5% 8% 19% 13% 1	1412
June	1st to 10th	Night	52% 5% 11% 20% 10% 2	1260
	11th to 20th	Night	48% 5% 11% 19% 13% 3	1260
	21st to end	Night	53% 6% 12% 19% 10%	1260
July	1st to 10th	Night	52% 5% 17% 20% 6%	1229
	11th to 20th	Night	51% 6% 15% 22% 7%	1392
	21st to end	Night	55% 6% 14% 19% 6%	1486
August	1st to 10th	Night	52% 9% 18% 19% 2	1422
	11th to 20th	Night	52% 6% 18% 17% 6%	1528
	21st to end	Night	51% 6% 14% 25% 4	1867
September	1st to 10th	Night	48% 5% 14% 23% 9%	1800
	11th to 20th	Night	52% 4 18% 19% 6%	1936
	21st to end	Night	54% 5% 11% 22% 8%	1980
October	1st to 10th	Night	52% 5% 14% 20% 8% 2	2020
	11th to 20th	Night	69% 5% 9% 13% 4	2317
	21st to end	Night	69% 5% 9% 12% 4 1	2574
November	1st to 10th	Night	68% 4 11% 12% 4	2407
	11th to 20th	Night	69% 6% 11% 12% 3	2610
	21st to end	Night	75% 5% 9% 9% 2	2850
December	1st to 10th	Night	77% 5% 8% 7% 2	2850
	11th to 20th	Night	71% 6% 6% 12% 4	3532
	21st to end	Night	73% 4 8% 10% 5%	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	19% 17% 44% 14% 6%	180
	11th to 20th	Day	22% 16% 35% 21% 6% 1	180
	21st to end	Day	18% 13% 33% 23% 11% 2	198
February	1st to 10th	Day	13% 21% 46% 11% 7% 2	178
	11th to 20th	Day	22% 22% 33% 20% 3	341
	21st to end	Day	23% 18% 33% 19% 6% 1	346
March	1st to 10th	Day	26% 15% 34% 18% 6% 1	420
	11th to 20th	Day	25% 17% 30% 18% 8% 1	420
	21st to end	Day	27% 15% 29% 20% 8% 1	462
April	1st to 10th	Day	28% 16% 31% 17% 7% 1	420
	11th to 20th	Day	26% 15% 31% 19% 8%	412
	21st to end	Day	25% 18% 33% 18% 5% 1	420
May	1st to 10th	Day	27% 17% 29% 19% 5% 1	423
	11th to 20th	Day	20% 17% 30% 24% 8% 1	447
	21st to end	Day	25% 16% 31% 20% 7% 1	622
June	1st to 10th	Day	24% 11% 33% 22% 9% 1	630
	11th to 20th	Day	22% 12% 30% 23% 11% 2	630
	21st to end	Day	25% 9% 27% 24% 12% 2	630
July	1st to 10th	Day	22% 10% 31% 23% 12% 1	612
	11th to 20th	Day	20% 10% 28% 26% 13% 3	444
	21st to end	Day	20% 7% 30% 27% 15% 2	488
August	1st to 10th	Day	23% 5% 24% 30% 14% 3	446
	11th to 20th	Day	20% 8% 25% 27% 16% 4	439
	21st to end	Day	25% 12% 26% 18% 14% 5%	495
September	1st to 10th	Day	23% 15% 25% 22% 12% 4	450
	11th to 20th	Day	22% 14% 26% 25% 9% 3	450
	21st to end	Day	27% 11% 26% 20% 12% 3	450
October	1st to 10th	Day	25% 16% 28% 18% 12% 2	450
	11th to 20th	Day	36% 17% 24% 14% 7% 2	372
	21st to end	Day	40% 15% 23% 16% 5% 1	396
November	1st to 10th	Day	37% 19% 26% 13% 4 1	365
	11th to 20th	Day	30% 20% 25% 20% 5% 1	280
	21st to end	Day	32% 20% 29% 14% 4 1	180
December	1st to 10th	Day	26% 11% 35% 16% 9% 3	180
	11th to 20th	Day	26% 21% 35% 12% 6% 1	180
	21st to end	Day	30% 23% 26% 17% 4 1	198
January	1st to 10th	Night	26% 14% 31% 24% 5% 1	787
	11th to 20th	Night	24% 13% 28% 27% 7% 1	760
	21st to end	Night	16% 11% 31% 32% 10% 1	836
February	1st to 10th	Night	15% 13% 35% 27% 9% 1	759
	11th to 20th	Night	23% 14% 33% 22% 7% 1	703
	21st to end	Night	19% 16% 30% 26% 8% 1	592
March	1st to 10th	Night	21% 13% 36% 24% 7%	720
	11th to 20th	Night	23% 16% 31% 21% 9%	720
	21st to end	Night	23% 16% 28% 27% 7% 1	792
April	1st to 10th	Night	31% 12% 28% 23% 5% 1	720
	11th to 20th	Night	29% 11% 27% 25% 6% 1	612
	21st to end	Night	25% 15% 31% 23% 6%	540
May	1st to 10th	Night	31% 9% 30% 24% 6%	419
	11th to 20th	Night	23% 9% 34% 27% 7% 1	354
	21st to end	Night	28% 11% 33% 24% 5%	389
June	1st to 10th	Night	25% 5% 37% 27% 6%	360
	11th to 20th	Night	20% 8% 26% 33% 13%	360
	21st to end	Night	24% 8% 28% 31% 9% 1	360
July	1st to 10th	Night	24% 8% 29% 30% 9%	350
	11th to 20th	Night	18% 5% 24% 39% 13% 1	348
	21st to end	Night	22% 5% 23% 37% 13% 1	394
August	1st to 10th	Night	23% 4 22% 38% 13% 1	358
	11th to 20th	Night	20% 5% 24% 34% 16% 1	464
	21st to end	Night	22% 8% 25% 28% 15% 2	679
September	1st to 10th	Night	23% 9% 24% 31% 13% 1	720
	11th to 20th	Night	26% 8% 27% 25% 12% 2	720
	21st to end	Night	25% 10% 26% 28% 9% 2	720
October	1st to 10th	Night	26% 10% 26% 28% 9% 1	720
	11th to 20th	Night	38% 10% 22% 21% 8% 1	720
	21st to end	Night	40% 13% 22% 17% 7%	792
November	1st to 10th	Night	36% 15% 25% 17% 6% 1	740
	11th to 20th	Night	30% 18% 26% 18% 7% 1	784
	21st to end	Night	31% 16% 30% 18% 5% 1	850
December	1st to 10th	Night	32% 13% 26% 20% 9% 1	850
	11th to 20th	Night	26% 16% 27% 21% 8% 1	850
	21st to end	Night	27% 15% 26% 24% 7% 1	935

Page generated at 2024-02-07 06:11:57. Version ac07

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