ALPHA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ALPHA Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	131088
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	130090
Transparency	Day	40% 5% 11% 24% 17% 4	134681
	Night	39% 4 11% 24% 18% 4	134568
Seeing	Day	20% 14% 28% 23% 12% 4	16631
	Night	18% 11% 29% 29% 11% 1	26228

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	15% 5% 2 6% 4 3 4 3 5% 6% 48%	4703
	11th to 20th	Day	15% 8% 2 5% 3 3 3 2 4 4 50%	5280
	21st to end	Day	14% 6% 2 7% 5% 4 4 3 5% 6% 44%	5780
February	1st to 10th	Day	14% 7% 2 6% 4 5% 5% 3 6% 7% 41%	1890
	11th to 20th	Day	18% 8% 3 5% 4 3 4 4 5% 6% 40%	1923
	21st to end	Day	18% 8% 3 8% 4 4 4 3 4 6% 40%	1730
March	1st to 10th	Day	19% 5% 2 5% 3 4 4 3 5% 7% 44%	4812
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 5% 7% 50%	4980
	21st to end	Day	16% 6% 2 6% 4 4 4 4 5% 7% 41%	5597
April	1st to 10th	Day	15% 6% 2 6% 5% 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 5% 6% 47%	2374
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 8% 42%	2400
May	1st to 10th	Day	21% 7% 3 6% 4 4 4 4 7% 9% 31%	5713
	11th to 20th	Day	22% 7% 2 6% 4 4 5% 4 7% 9% 31%	6226
	21st to end	Day	15% 5% 2 7% 4 5% 5% 4 6% 9% 38%	6934
June	1st to 10th	Day	11% 5% 2 5% 4 4 5% 5% 7% 9% 43%	2800
	11th to 20th	Day	12% 5% 2 6% 4 5% 6% 5% 6% 8% 42%	2800
	21st to end	Day	10% 5% 1 5% 5% 4 5% 5% 7% 10% 43%	2800
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	6360
	11th to 20th	Day	9% 7% 3 8% 6% 6% 6% 6% 9% 12% 29%	6253
	21st to end	Day	8% 6% 2 6% 5% 6% 5% 5% 7% 10% 40%	6424
August	1st to 10th	Day	16% 6% 2 7% 5% 5% 6% 5% 7% 9% 33%	4234
	11th to 20th	Day	11% 5% 2 7% 5% 5% 6% 5% 7% 10% 37%	3959
	21st to end	Day	10% 4 2 6% 4 4 6% 4 7% 8% 45%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 6% 7% 46%	2400
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 6% 9% 46%	2400
	21st to end	Day	20% 9% 2 6% 4 3 5% 4 5% 7% 36%	2115
October	1st to 10th	Day	16% 5% 2 6% 3 4 3 3 5% 6% 47%	2000
	11th to 20th	Day	16% 6% 2 5% 2 3 3 3 5% 6% 49%	2000
	21st to end	Day	13% 5% 2 6% 4 3 4 3 5% 5% 50%	2200
November	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 5% 6% 52%	1921
	11th to 20th	Day	13% 6% 2 4 3 3 4 4 6% 6% 49%	1890
	21st to end	Day	14% 6% 2 5% 4 3 4 4 3 6% 48%	1680
December	1st to 10th	Day	13% 5% 2 6% 4 4 4 3 6% 6% 48%	2680
	11th to 20th	Day	18% 8% 3 4 3 3 3 3 5% 6% 43%	3070
	21st to end	Day	16% 8% 3 7% 4 4 4 4 5% 7% 38%	4048
January	1st to 10th	Night	12% 7% 2 6% 5% 5% 4 4 5% 7% 44%	8173
	11th to 20th	Night	14% 7% 2 6% 3 3 3 3 4 5% 51%	7680
	21st to end	Night	14% 7% 2 5% 3 3 3 3 5% 5% 48%	8401
February	1st to 10th	Night	15% 7% 2 5% 4 4 5% 4 6% 6% 42%	2730
	11th to 20th	Night	19% 7% 2 5% 4 4 3 3 4 5% 43%	2581
	21st to end	Night	16% 7% 3 6% 3 4 4 3 4 6% 43%	2076
March	1st to 10th	Night	16% 6% 2 5% 3 3 4 3 3 5% 49%	5082
	11th to 20th	Night	11% 6% 2 5% 3 4 4 3 5% 7% 50%	4980
	21st to end	Night	13% 7% 3 6% 4 4 4 3 5% 6% 45%	5335
April	1st to 10th	Night	15% 8% 2 6% 4 4 5% 4 6% 6% 39%	2000
	11th to 20th	Night	14% 6% 2 6% 4 3 4 4 6% 6% 45%	1975
	21st to end	Night	18% 5% 2 5% 3 4 4 3 7% 7% 43%	2000
May	1st to 10th	Night	20% 9% 3 7% 4 4 5% 4 6% 8% 29%	4224
	11th to 20th	Night	22% 8% 2 5% 3 4 5% 4 6% 8% 32%	3667
	21st to end	Night	17% 6% 2 6% 4 5% 5% 4 6% 9% 36%	3920
June	1st to 10th	Night	10% 5% 2 5% 5% 4 4 5% 7% 10% 44%	1600
	11th to 20th	Night	12% 6% 2 6% 5% 4 5% 4 6% 10% 40%	1600
	21st to end	Night	9% 5% 3 6% 3 6% 5% 5% 7% 10% 42%	1600
July	1st to 10th	Night	8% 5% 2 7% 6% 5% 6% 5% 7% 10% 39%	3598
	11th to 20th	Night	9% 8% 3 9% 6% 6% 6% 5% 8% 10% 29%	3687
	21st to end	Night	9% 6% 2 6% 4 5% 5% 5% 7% 9% 42%	4427
August	1st to 10th	Night	17% 6% 3 7% 4 5% 5% 4 7% 9% 33%	2915
	11th to 20th	Night	10% 8% 3 9% 6% 5% 6% 5% 6% 7% 35%	3057
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 47%	3586
September	1st to 10th	Night	13% 6% 2 5% 4 4 5% 3 4 7% 46%	2000
	11th to 20th	Night	11% 6% 2 6% 3 4 4 3 5% 7% 49%	2000
	21st to end	Night	19% 7% 2 6% 4 4 4 3 6% 7% 37%	2279
October	1st to 10th	Night	18% 6% 2 5% 4 3 3 3 4 6% 46%	2400
	11th to 20th	Night	16% 4 2 5% 3 3 3 3 4 6% 52%	2400
	21st to end	Night	15% 5% 1 5% 3 3 3 4 5% 6% 50%	2640
November	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 4 6% 53%	2597
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 6% 49%	2730
	21st to end	Night	13% 5% 2 5% 3 4 4 4 5% 7% 48%	2935
December	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 5% 7% 51%	4720
	11th to 20th	Night	21% 6% 2 5% 3 4 4 4 5% 6% 41%	5345
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 39%	7150

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	39% 3 7% 27% 20% 5%	4704
	11th to 20th	Day	39% 2 6% 23% 20% 9%	5280
	21st to end	Day	42% 2 10% 32% 12% 3	5780
February	1st to 10th	Day	42% 4 13% 25% 14% 2	1890
	11th to 20th	Day	44% 4 8% 25% 15% 3	1932
	21st to end	Day	47% 3 10% 22% 15% 3	1730
March	1st to 10th	Day	40% 4 8% 23% 18% 7%	4812
	11th to 20th	Day	35% 3 9% 25% 22% 6%	4980
	21st to end	Day	43% 3 10% 19% 18% 6%	5597
April	1st to 10th	Day	41% 5% 11% 21% 18% 5%	2400
	11th to 20th	Day	38% 4 10% 22% 21% 6%	2374
	21st to end	Day	41% 4 10% 23% 19% 4	2400
May	1st to 10th	Day	49% 5% 15% 18% 11% 2	5746
	11th to 20th	Day	49% 5% 12% 24% 10% 1	6230
	21st to end	Day	42% 5% 13% 22% 16% 2	6949
June	1st to 10th	Day	36% 6% 14% 25% 17% 2	2800
	11th to 20th	Day	38% 8% 12% 23% 16% 2	2800
	21st to end	Day	34% 8% 14% 24% 19% 2	2800
July	1st to 10th	Day	35% 9% 17% 20% 15% 4	6290
	11th to 20th	Day	42% 11% 16% 22% 8% 1	6178
	21st to end	Day	37% 7% 11% 22% 23% 1	6240
August	1st to 10th	Day	45% 7% 14% 25% 8% 1	4221
	11th to 20th	Day	40% 7% 12% 27% 13% 1	3998
	21st to end	Day	35% 6% 13% 27% 17% 2	4312
September	1st to 10th	Day	38% 5% 12% 21% 20% 5%	2400
	11th to 20th	Day	35% 5% 9% 26% 21% 4	2400
	21st to end	Day	47% 6% 7% 21% 17% 3	2115
October	1st to 10th	Day	38% 4 7% 17% 29% 5%	3260
	11th to 20th	Day	26% 3 7% 29% 26% 9%	3240
	21st to end	Day	24% 2 8% 30% 29% 7%	3538
November	1st to 10th	Day	32% 3 12% 27% 21% 5%	1921
	11th to 20th	Day	35% 3 10% 24% 23% 5%	1890
	21st to end	Day	39% 3 10% 28% 18% 2	1680
December	1st to 10th	Day	38% 3 14% 28% 14% 4	2680
	11th to 20th	Day	42% 3 12% 30% 11% 2	3066
	21st to end	Day	45% 4 14% 22% 15% 1	4048
January	1st to 10th	Night	40% 3 9% 26% 19% 4	8174
	11th to 20th	Night	37% 2 7% 23% 22% 8%	7680
	21st to end	Night	38% 2 6% 30% 19% 3	8401
February	1st to 10th	Night	42% 4 12% 25% 15% 2	2730
	11th to 20th	Night	44% 3 8% 22% 18% 5%	2583
	21st to end	Night	43% 3 10% 22% 17% 5%	2076
March	1st to 10th	Night	39% 3 7% 26% 14% 10%	5082
	11th to 20th	Night	33% 4 11% 27% 19% 7%	4980
	21st to end	Night	41% 3 9% 21% 21% 5%	5335
April	1st to 10th	Night	43% 6% 9% 19% 20% 4	2000
	11th to 20th	Night	38% 4 12% 21% 20% 5%	1975
	21st to end	Night	39% 5% 9% 25% 17% 5%	2000
May	1st to 10th	Night	53% 5% 12% 17% 9% 3	4254
	11th to 20th	Night	50% 5% 12% 21% 12% 1	3668
	21st to end	Night	45% 5% 14% 22% 12% 2	3927
June	1st to 10th	Night	34% 8% 11% 27% 18% 2	1600
	11th to 20th	Night	39% 6% 14% 25% 14% 3	1600
	21st to end	Night	36% 6% 14% 23% 19% 3	1600
July	1st to 10th	Night	40% 8% 18% 17% 14% 2	3560
	11th to 20th	Night	46% 9% 18% 18% 8% 1	3642
	21st to end	Night	37% 6% 13% 23% 19% 1	4297
August	1st to 10th	Night	45% 6% 18% 22% 9% 1	2904
	11th to 20th	Night	45% 7% 11% 24% 11% 1	3084
	21st to end	Night	34% 5% 12% 29% 17% 2	3586
September	1st to 10th	Night	38% 5% 10% 23% 18% 6%	2000
	11th to 20th	Night	35% 5% 8% 27% 22% 4	2000
	21st to end	Night	46% 6% 10% 18% 17% 3	2279
October	1st to 10th	Night	41% 4 9% 21% 21% 5%	3920
	11th to 20th	Night	25% 3 11% 23% 31% 8%	3899
	21st to end	Night	23% 3 9% 32% 25% 8%	4252
November	1st to 10th	Night	34% 3 9% 25% 23% 6%	2597
	11th to 20th	Night	36% 3 10% 24% 22% 6%	2730
	21st to end	Night	35% 3 12% 25% 21% 3	2935
December	1st to 10th	Night	34% 3 12% 28% 20% 3	4720
	11th to 20th	Night	44% 3 13% 19% 17% 4	5348
	21st to end	Night	43% 4 14% 21% 16% 1	7150

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	30% 21% 30% 14% 4	228
	11th to 20th	Day	25% 31% 30% 11% 3	330
	21st to end	Day	22% 28% 34% 12% 4 1	363
February	1st to 10th	Day	26% 24% 34% 12% 4	330
	11th to 20th	Day	36% 21% 34% 8% 1	410
	21st to end	Day	28% 19% 34% 17% 3	446
March	1st to 10th	Day	25% 22% 32% 17% 3 1	508
	11th to 20th	Day	26% 22% 30% 16% 4 1	500
	21st to end	Day	26% 20% 31% 16% 5% 1	550
April	1st to 10th	Day	25% 22% 33% 15% 4 1	500
	11th to 20th	Day	21% 18% 34% 18% 7% 2	492
	21st to end	Day	26% 15% 33% 17% 8% 2	500
May	1st to 10th	Day	24% 17% 31% 18% 7% 3	503
	11th to 20th	Day	22% 15% 28% 24% 10% 2	527
	21st to end	Day	19% 10% 23% 29% 16% 4	573
June	1st to 10th	Day	14% 11% 29% 24% 16% 6%	530
	11th to 20th	Day	17% 8% 26% 30% 16% 4	530
	21st to end	Day	12% 5% 27% 32% 18% 6%	530
July	1st to 10th	Day	12% 4 18% 36% 21% 9%	525
	11th to 20th	Day	6% 2 23% 34% 28% 7%	524
	21st to end	Day	12% 3 23% 34% 20% 7%	576
August	1st to 10th	Day	13% 5% 21% 31% 21% 8%	526
	11th to 20th	Day	15% 4 17% 32% 22% 9%	519
	21st to end	Day	10% 4 23% 33% 18% 11%	583
September	1st to 10th	Day	16% 7% 22% 28% 16% 11%	530
	11th to 20th	Day	13% 4 23% 34% 17% 9%	530
	21st to end	Day	26% 9% 24% 27% 9% 5%	530
October	1st to 10th	Day	20% 8% 23% 26% 16% 6%	530
	11th to 20th	Day	19% 17% 27% 22% 13% 3	530
	21st to end	Day	17% 16% 34% 22% 10% 2	583
November	1st to 10th	Day	16% 15% 34% 23% 9% 3	463
	11th to 20th	Day	20% 15% 33% 22% 8% 1	430
	21st to end	Day	27% 22% 26% 17% 7%	220
December	1st to 10th	Day	23% 22% 37% 15% 3 1	220
	11th to 20th	Day	24% 22% 32% 16% 6%	220
	21st to end	Day	25% 21% 31% 18% 5%	242
January	1st to 10th	Night	19% 18% 39% 21% 3	840
	11th to 20th	Night	22% 18% 31% 24% 4	840
	21st to end	Night	20% 17% 38% 24% 1	924
February	1st to 10th	Night	19% 16% 36% 27% 2	840
	11th to 20th	Night	26% 21% 32% 19% 1	823
	21st to end	Night	25% 17% 36% 20% 2	692
March	1st to 10th	Night	20% 16% 36% 24% 3	809
	11th to 20th	Night	22% 18% 34% 20% 5% 1	800
	21st to end	Night	20% 16% 40% 21% 4 1	880
April	1st to 10th	Night	21% 20% 34% 20% 4	800
	11th to 20th	Night	16% 17% 33% 27% 6%	791
	21st to end	Night	20% 13% 32% 27% 8%	800
May	1st to 10th	Night	21% 10% 30% 29% 10%	690
	11th to 20th	Night	20% 7% 29% 34% 9%	434
	21st to end	Night	16% 6% 24% 37% 16% 1	433
June	1st to 10th	Night	11% 4 29% 38% 18% 1	400
	11th to 20th	Night	13% 6% 25% 41% 14% 1	400
	21st to end	Night	12% 3 25% 46% 15% 1	400
July	1st to 10th	Night	8% 3 24% 41% 23% 2	390
	11th to 20th	Night	8% 1 16% 43% 31% 2	426
	21st to end	Night	8% 3 19% 40% 25% 4	657
August	1st to 10th	Night	11% 2 13% 42% 29% 2	597
	11th to 20th	Night	12% 3 14% 42% 26% 2	698
	21st to end	Night	12% 3 19% 36% 26% 4	880
September	1st to 10th	Night	16% 4 21% 31% 25% 3	800
	11th to 20th	Night	13% 5% 14% 38% 26% 4	800
	21st to end	Night	24% 5% 22% 32% 16% 2	800
October	1st to 10th	Night	21% 7% 21% 27% 20% 4	800
	11th to 20th	Night	18% 10% 27% 31% 14% 1	800
	21st to end	Night	18% 12% 31% 26% 13%	880
November	1st to 10th	Night	14% 10% 33% 31% 11% 1	820
	11th to 20th	Night	19% 15% 31% 28% 7% 1	840
	21st to end	Night	18% 15% 33% 26% 8% 1	840
December	1st to 10th	Night	21% 16% 34% 21% 7% 1	840
	11th to 20th	Night	19% 13% 36% 27% 5%	840
	21st to end	Night	20% 15% 31% 30% 4	924

Page generated at 2026-02-28 01:40:24. Version ac07

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