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 2025-02-06.

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% 8% 3 7% 4 4 4 4 6% 7% 41%	108407
	Night	14% 8% 3 7% 4 4 4 3 5% 6% 42%	110026
Transparency	Day	40% 5% 11% 25% 16% 2	112024
	Night	41% 4 11% 24% 18% 3	114550
Seeing	Day	16% 9% 25% 25% 18% 7%	15883
	Night	17% 9% 28% 30% 14% 2	25838

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	13% 11% 6% 13% 5% 4 3 4 5% 6% 31%	3617
	11th to 20th	Day	11% 9% 4 8% 6% 7% 4 4 5% 7% 36%	3644
	21st to end	Day	19% 14% 4 8% 4 3 4 3 4 5% 33%	4488
February	1st to 10th	Day	17% 9% 4 8% 4 3 4 3 5% 7% 36%	1840
	11th to 20th	Day	15% 10% 3 7% 4 3 5% 4 6% 7% 35%	1848
	21st to end	Day	14% 11% 4 7% 5% 4 4 4 5% 6% 36%	1570
March	1st to 10th	Day	20% 14% 3 6% 3 3 3 3 4 5% 36%	3120
	11th to 20th	Day	12% 8% 2 5% 4 3 4 3 5% 8% 47%	3337
	21st to end	Day	14% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	4151
April	1st to 10th	Day	13% 7% 3 6% 4 4 4 3 5% 9% 41%	2280
	11th to 20th	Day	15% 9% 3 7% 4 4 3 4 4 6% 42%	2254
	21st to end	Day	17% 7% 2 7% 4 4 4 3 4 7% 39%	2280
May	1st to 10th	Day	14% 8% 3 7% 6% 5% 5% 4 8% 10% 31%	4040
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 5% 8% 38%	4140
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 7% 8% 43%	4824
June	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 7% 9% 46%	2660
	11th to 20th	Day	7% 4 2 6% 5% 4 5% 5% 6% 8% 49%	2660
	21st to end	Day	6% 5% 2 5% 5% 4 6% 5% 7% 10% 45%	2660
July	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 6% 10% 42%	4440
	11th to 20th	Day	4 4 2 6% 4 5% 5% 5% 7% 12% 47%	4440
	21st to end	Day	7% 6% 2 8% 6% 6% 6% 5% 8% 10% 37%	4813
August	1st to 10th	Day	7% 4 2 5% 4 3 5% 3 6% 9% 54%	3918
	11th to 20th	Day	9% 6% 2 6% 4 4 6% 5% 7% 9% 42%	3729
	21st to end	Day	5% 4 2 5% 3 4 5% 3 7% 9% 53%	4180
September	1st to 10th	Day	11% 6% 2 6% 4 3 4 4 5% 7% 48%	2280
	11th to 20th	Day	6% 5% 2 5% 4 5% 5% 3 6% 8% 49%	2182
	21st to end	Day	10% 5% 2 5% 4 3 4 3 5% 7% 51%	2090
October	1st to 10th	Day	10% 4 2 5% 4 3 4 2 5% 6% 56%	2019
	11th to 20th	Day	13% 6% 3 5% 3 3 4 3 5% 7% 49%	1900
	21st to end	Day	17% 7% 2 5% 3 3 4 3 5% 4 47%	1972
November	1st to 10th	Day	10% 5% 1 6% 3 4 4 3 5% 5% 54%	1762
	11th to 20th	Day	13% 8% 3 7% 4 4 3 3 4 6% 46%	1800
	21st to end	Day	16% 10% 4 7% 5% 4 3 3 4 5% 40%	1800
December	1st to 10th	Day	9% 10% 3 6% 4 3 3 3 5% 5% 49%	2679
	11th to 20th	Day	13% 10% 4 9% 5% 5% 5% 4 4 6% 34%	3008
	21st to end	Day	27% 17% 4 10% 4 4 3 2 3 4 21%	3982
January	1st to 10th	Night	15% 11% 5% 11% 4 4 4 3 5% 5% 33%	6348
	11th to 20th	Night	12% 11% 5% 11% 4 4 7% 4 4 5% 33%	6332
	21st to end	Night	19% 10% 5% 8% 4 4 4 3 4 5% 34%	6490
February	1st to 10th	Night	17% 10% 4 9% 5% 4 3 4 4 6% 34%	2449
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 3 5% 6% 40%	2218
	21st to end	Night	17% 9% 3 9% 5% 3 4 4 4 5% 37%	1884
March	1st to 10th	Night	19% 12% 3 7% 4 4 4 3 4 5% 36%	3733
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 7% 51%	3653
	21st to end	Night	17% 8% 4 7% 5% 4 5% 3 6% 7% 35%	3480
April	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 6% 47%	1900
	11th to 20th	Night	15% 7% 1 5% 3 4 4 3 5% 9% 42%	1875
	21st to end	Night	18% 9% 2 6% 4 4 4 3 5% 6% 39%	1900
May	1st to 10th	Night	15% 8% 3 7% 4 5% 4 4 5% 8% 35%	2855
	11th to 20th	Night	15% 7% 3 6% 4 4 4 4 6% 6% 42%	2752
	21st to end	Night	8% 6% 3 7% 4 5% 6% 4 7% 10% 41%	2743
June	1st to 10th	Night	9% 6% 2 5% 4 3 4 4 5% 7% 51%	1520
	11th to 20th	Night	4 4 2 4 4 4 4 4 6% 9% 53%	1520
	21st to end	Night	4 5% 1 4 5% 4 6% 5% 7% 10% 49%	1520
July	1st to 10th	Night	13% 5% 2 5% 3 3 4 3 5% 9% 48%	2530
	11th to 20th	Night	4 4 1 6% 4 4 4 3 6% 8% 56%	2530
	21st to end	Night	7% 6% 3 7% 6% 6% 6% 5% 7% 9% 39%	2750
August	1st to 10th	Night	5% 3 1 4 4 4 4 3 7% 9% 56%	3007
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 8% 50%	3100
	21st to end	Night	4 3 1 4 3 3 4 3 6% 10% 58%	3476
September	1st to 10th	Night	10% 4 2 5% 3 4 4 3 6% 5% 54%	1900
	11th to 20th	Night	6% 5% 2 6% 4 4 4 3 5% 8% 53%	1977
	21st to end	Night	10% 6% 1 5% 4 4 5% 4 5% 7% 50%	2090
October	1st to 10th	Night	8% 4 2 4 3 4 3 3 5% 6% 59%	2156
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 5% 54%	2280
	21st to end	Night	17% 8% 2 6% 3 3 3 3 4 5% 45%	2623
November	1st to 10th	Night	10% 7% 2 6% 3 3 3 3 4 6% 54%	2536
	11th to 20th	Night	14% 6% 2 6% 4 4 3 3 5% 6% 48%	2600
	21st to end	Night	19% 8% 3 6% 3 3 3 2 4 5% 42%	2600
December	1st to 10th	Night	15% 9% 3 6% 3 3 3 3 5% 5% 45%	4504
	11th to 20th	Night	17% 8% 3 5% 3 4 4 3 5% 7% 41%	5199
	21st to end	Night	27% 16% 5% 8% 5% 4 3 3 3 4 22%	6996

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	55% 4 10% 21% 9% 1	3618
	11th to 20th	Day	47% 3 11% 20% 15% 3	3644
	21st to end	Day	55% 2 7% 18% 13% 5%	4488
February	1st to 10th	Day	49% 4 13% 22% 11% 1	1840
	11th to 20th	Day	48% 6% 9% 22% 13% 2	1858
	21st to end	Day	48% 5% 10% 21% 13% 3	1570
March	1st to 10th	Day	51% 5% 6% 20% 13% 4	3120
	11th to 20th	Day	37% 5% 13% 25% 16% 4	3337
	21st to end	Day	47% 7% 15% 22% 9%	4151
April	1st to 10th	Day	39% 6% 13% 26% 16% 1	2280
	11th to 20th	Day	44% 4 10% 24% 15% 3	2254
	21st to end	Day	45% 5% 11% 25% 11% 2	2280
May	1st to 10th	Day	45% 10% 14% 24% 7%	4078
	11th to 20th	Day	44% 8% 9% 28% 10% 1	4144
	21st to end	Day	36% 8% 12% 22% 21% 1	4837
June	1st to 10th	Day	32% 9% 13% 26% 17% 3	2660
	11th to 20th	Day	31% 7% 13% 25% 21% 3	2660
	21st to end	Day	32% 8% 17% 22% 16% 5%	2660
July	1st to 10th	Day	38% 5% 11% 30% 14% 3	4371
	11th to 20th	Day	29% 6% 13% 33% 17% 2	4365
	21st to end	Day	41% 5% 17% 25% 11% 1	4662
August	1st to 10th	Day	28% 5% 12% 40% 15% 1	3903
	11th to 20th	Day	37% 5% 14% 27% 16% 1	3765
	21st to end	Day	27% 4 16% 35% 15% 3	4179
September	1st to 10th	Day	35% 5% 11% 24% 21% 4	2280
	11th to 20th	Day	31% 6% 11% 24% 24% 3	2182
	21st to end	Day	33% 4 10% 24% 25% 4	2090
October	1st to 10th	Day	19% 3 6% 24% 42% 6%	3357
	11th to 20th	Day	25% 3 8% 22% 37% 6%	3140
	21st to end	Day	34% 3 6% 25% 30% 2	3223
November	1st to 10th	Day	32% 4 12% 27% 19% 6%	1762
	11th to 20th	Day	41% 4 8% 23% 18% 6%	1800
	21st to end	Day	49% 3 9% 24% 14% 2	1800
December	1st to 10th	Day	38% 4 14% 29% 16%	2679
	11th to 20th	Day	50% 3 11% 24% 11% 1	3005
	21st to end	Day	68% 3 10% 12% 5% 1	3982
January	1st to 10th	Night	54% 3 11% 23% 8% 1	6348
	11th to 20th	Night	55% 3 8% 19% 12% 3	6332
	21st to end	Night	54% 3 8% 19% 16% 1	6490
February	1st to 10th	Night	52% 4 11% 20% 11% 2	2449
	11th to 20th	Night	45% 4 10% 22% 17% 2	2219
	21st to end	Night	49% 4 10% 21% 13% 3	1884
March	1st to 10th	Night	49% 6% 8% 21% 15% 2	3733
	11th to 20th	Night	34% 5% 12% 24% 21% 3	3653
	21st to end	Night	46% 6% 11% 28% 8% 1	3480
April	1st to 10th	Night	38% 5% 11% 26% 17% 3	1900
	11th to 20th	Night	40% 4 10% 24% 20% 3	1875
	21st to end	Night	46% 6% 9% 21% 15% 3	1900
May	1st to 10th	Night	44% 9% 17% 21% 9% 1	2882
	11th to 20th	Night	41% 8% 12% 26% 13% 1	2753
	21st to end	Night	36% 9% 12% 23% 17% 3	2751
June	1st to 10th	Night	31% 6% 14% 23% 23% 3	1520
	11th to 20th	Night	26% 8% 11% 26% 24% 5%	1520
	21st to end	Night	28% 8% 15% 23% 19% 7%	1520
July	1st to 10th	Night	34% 5% 9% 28% 19% 4	2490
	11th to 20th	Night	26% 4 11% 35% 21% 3	2482
	21st to end	Night	41% 5% 17% 24% 12% 2	2662
August	1st to 10th	Night	24% 5% 16% 37% 17% 1	2998
	11th to 20th	Night	32% 6% 12% 27% 22% 1	3130
	21st to end	Night	23% 5% 14% 34% 21% 3	3476
September	1st to 10th	Night	30% 6% 8% 29% 21% 7%	1900
	11th to 20th	Night	29% 5% 8% 29% 24% 4	1977
	21st to end	Night	33% 6% 8% 21% 26% 6%	2090
October	1st to 10th	Night	17% 3 6% 19% 48% 7%	3598
	11th to 20th	Night	24% 2 7% 23% 37% 6%	3779
	21st to end	Night	33% 3 7% 25% 25% 7%	4322
November	1st to 10th	Night	33% 3 10% 24% 24% 5%	2536
	11th to 20th	Night	37% 4 9% 23% 23% 4	2600
	21st to end	Night	45% 3 10% 24% 17% 1	2600
December	1st to 10th	Night	42% 3 11% 27% 17%	4504
	11th to 20th	Night	44% 3 14% 24% 15%	5201
	21st to end	Night	67% 3 10% 14% 5% 1	6996

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% 32% 20% 6%	300
	11th to 20th	Day	28% 17% 34% 20% 1	300
	21st to end	Day	21% 19% 42% 15% 3	330
February	1st to 10th	Day	29% 14% 38% 16% 2	285
	11th to 20th	Day	26% 23% 27% 18% 5% 1	270
	21st to end	Day	29% 18% 34% 16% 2	225
March	1st to 10th	Day	24% 18% 39% 14% 4	270
	11th to 20th	Day	29% 18% 36% 13% 4	270
	21st to end	Day	30% 14% 33% 16% 6%	492
April	1st to 10th	Day	19% 16% 32% 25% 8% 1	460
	11th to 20th	Day	20% 19% 27% 20% 12% 2	452
	21st to end	Day	23% 15% 33% 19% 8% 2	460
May	1st to 10th	Day	21% 12% 30% 24% 10% 2	463
	11th to 20th	Day	16% 8% 25% 29% 17% 6%	520
	21st to end	Day	9% 5% 28% 30% 19% 8%	734
June	1st to 10th	Day	10% 4 20% 30% 25% 11%	680
	11th to 20th	Day	8% 3 21% 33% 27% 8%	680
	21st to end	Day	7% 5% 26% 32% 22% 9%	680
July	1st to 10th	Day	11% 2 14% 27% 27% 19%	671
	11th to 20th	Day	6% 3 14% 35% 28% 14%	668
	21st to end	Day	9% 2 12% 33% 30% 13%	739
August	1st to 10th	Day	8% 3 13% 32% 32% 13%	567
	11th to 20th	Day	10% 3 15% 31% 29% 13%	479
	21st to end	Day	7% 2 12% 28% 35% 17%	539
September	1st to 10th	Day	12% 3 15% 28% 30% 13%	490
	11th to 20th	Day	8% 2 12% 28% 34% 16%	490
	21st to end	Day	13% 7% 23% 23% 22% 12%	490
October	1st to 10th	Day	12% 5% 21% 34% 21% 7%	419
	11th to 20th	Day	15% 15% 32% 26% 9% 4	300
	21st to end	Day	28% 16% 28% 20% 7%	330
November	1st to 10th	Day	15% 12% 34% 22% 15% 3	300
	11th to 20th	Day	19% 19% 30% 21% 9% 2	300
	21st to end	Day	24% 12% 33% 23% 7% 1	300
December	1st to 10th	Day	15% 19% 42% 15% 5% 4	300
	11th to 20th	Day	20% 18% 37% 19% 5%	300
	21st to end	Day	32% 17% 31% 16% 4	330
January	1st to 10th	Night	19% 16% 37% 24% 4	1000
	11th to 20th	Night	24% 16% 34% 22% 4	1000
	21st to end	Night	20% 14% 39% 25% 2	1100
February	1st to 10th	Night	24% 11% 39% 24% 1	889
	11th to 20th	Night	21% 15% 37% 24% 2	743
	21st to end	Night	24% 17% 37% 21% 1	628
March	1st to 10th	Night	21% 14% 39% 23% 3	760
	11th to 20th	Night	22% 10% 37% 26% 5%	678
	21st to end	Night	24% 10% 32% 26% 7%	627
April	1st to 10th	Night	16% 13% 36% 28% 7%	570
	11th to 20th	Night	18% 11% 32% 27% 12% 1	563
	21st to end	Night	22% 8% 34% 27% 8% 1	570
May	1st to 10th	Night	16% 8% 30% 35% 10% 1	570
	11th to 20th	Night	14% 5% 26% 36% 17% 2	561
	21st to end	Night	9% 3 22% 40% 23% 3	617
June	1st to 10th	Night	11% 4 17% 42% 25% 1	570
	11th to 20th	Night	6% 1 21% 46% 24% 3	570
	21st to end	Night	7% 2 21% 41% 26% 3	570
July	1st to 10th	Night	10% 1 12% 32% 36% 8%	555
	11th to 20th	Night	5% 3 11% 40% 37% 4	552
	21st to end	Night	8% 1 12% 41% 33% 5%	624
August	1st to 10th	Night	8% 2 10% 39% 37% 5%	567
	11th to 20th	Night	7% 1 12% 40% 35% 4	564
	21st to end	Night	7% 1 11% 32% 44% 5%	627
September	1st to 10th	Night	11% 2 13% 38% 30% 6%	570
	11th to 20th	Night	8% 2 14% 35% 34% 6%	647
	21st to end	Night	13% 4 21% 31% 25% 5%	760
October	1st to 10th	Night	9% 4 24% 35% 23% 5%	760
	11th to 20th	Night	16% 10% 29% 29% 15% 1	760
	21st to end	Night	21% 11% 32% 29% 7%	836
November	1st to 10th	Night	14% 7% 37% 30% 11%	780
	11th to 20th	Night	16% 12% 31% 30% 10% 1	800
	21st to end	Night	24% 12% 31% 25% 8%	800
December	1st to 10th	Night	22% 12% 35% 25% 5%	950
	11th to 20th	Night	23% 9% 32% 30% 6%	1000
	21st to end	Night	29% 14% 33% 21% 3	1100

Page generated at 2025-02-03 02:05:52. Version ac07

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