AST@RHO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AST@RHO clear sky chart from 2005-11-06 to 2025-09-04.

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% 5% 2 6% 5% 5% 6% 5% 7% 10% 41%	125965
	Night	9% 6% 2 6% 4 4 5% 4 6% 9% 44%	122750
Transparency	Day	35% 8% 16% 19% 14% 6%	129714
	Night	35% 8% 15% 19% 16% 7%	127070
Seeing	Day	10% 6% 20% 26% 23% 16%	16468
	Night	10% 5% 20% 33% 24% 6%	26475

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	10% 5% 2 6% 4 2 3 2 4 7% 55%	4056
	11th to 20th	Day	19% 10% 4 7% 4 3 3 3 4 6% 36%	4060
	21st to end	Day	10% 6% 2 5% 3 3 4 3 6% 9% 49%	4741
February	1st to 10th	Day	14% 6% 3 6% 4 4 5% 4 6% 7% 43%	2000
	11th to 20th	Day	11% 6% 2 5% 4 3 4 3 4 6% 53%	1948
	21st to end	Day	10% 6% 2 6% 4 4 3 4 5% 7% 48%	1670
March	1st to 10th	Day	12% 7% 3 7% 4 4 4 4 6% 9% 41%	4935
	11th to 20th	Day	7% 4 2 4 3 3 3 3 6% 8% 58%	4980
	21st to end	Day	10% 5% 2 5% 3 4 4 4 6% 8% 50%	5465
April	1st to 10th	Day	7% 4 2 4 4 4 4 5% 6% 10% 50%	2200
	11th to 20th	Day	9% 6% 2 6% 5% 4 4 4 6% 6% 48%	2176
	21st to end	Day	4 2 2 4 4 4 5% 5% 6% 8% 54%	2497
May	1st to 10th	Day	4 4 1 5% 4 5% 5% 4 7% 11% 50%	5862
	11th to 20th	Day	9% 6% 2 6% 4 4 4 4 6% 9% 46%	5861
	21st to end	Day	4 3 2 4 4 4 5% 5% 7% 12% 50%	6433
June	1st to 10th	Day	10% 6% 3 8% 6% 6% 7% 7% 9% 11% 29%	2600
	11th to 20th	Day	5% 4 2 8% 7% 8% 8% 8% 10% 11% 28%	2600
	21st to end	Day	9% 6% 2 7% 7% 7% 8% 7% 9% 10% 27%	2600
July	1st to 10th	Day	5% 6% 3 9% 8% 9% 10% 8% 12% 13% 17%	5900
	11th to 20th	Day	3 4 2 7% 7% 8% 8% 7% 12% 15% 27%	5900
	21st to end	Day	4 4 2 8% 6% 7% 8% 7% 11% 14% 28%	6424
August	1st to 10th	Day	10% 5% 3 6% 6% 6% 7% 6% 10% 13% 27%	4234
	11th to 20th	Day	6% 5% 2 7% 6% 7% 9% 7% 10% 12% 29%	3911
	21st to end	Day	9% 6% 3 9% 7% 7% 7% 6% 10% 12% 26%	4015
September	1st to 10th	Day	11% 8% 3 9% 7% 8% 8% 7% 9% 9% 22%	2123
	11th to 20th	Day	10% 9% 3 9% 6% 6% 7% 6% 7% 10% 26%	2090
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 5% 7% 8% 36%	2090
October	1st to 10th	Day	11% 7% 3 8% 6% 5% 5% 4 5% 8% 37%	2090
	11th to 20th	Day	5% 4 2 5% 5% 4 5% 3 5% 7% 56%	1933
	21st to end	Day	6% 4 2 6% 4 5% 4 4 6% 8% 51%	2090
November	1st to 10th	Day	8% 5% 2 6% 3 4 5% 4 5% 7% 51%	1957
	11th to 20th	Day	9% 8% 3 6% 4 5% 4 3 6% 6% 47%	1955
	21st to end	Day	10% 5% 2 6% 4 4 4 3 6% 7% 49%	1800
December	1st to 10th	Day	5% 4 1 4 4 4 4 4 5% 8% 57%	2930
	11th to 20th	Day	11% 6% 2 6% 4 4 4 5% 6% 7% 46%	3373
	21st to end	Day	14% 7% 3 7% 4 4 4 3 5% 6% 44%	4466
January	1st to 10th	Night	10% 5% 2 4 2 3 3 3 4 7% 56%	5889
	11th to 20th	Night	20% 8% 3 7% 4 4 5% 4 6% 7% 32%	5900
	21st to end	Night	15% 7% 2 6% 4 4 4 3 6% 8% 41%	6199
February	1st to 10th	Night	12% 6% 2 7% 4 4 4 3 4 7% 47%	2400
	11th to 20th	Night	10% 8% 2 5% 4 3 4 4 6% 6% 47%	2338
	21st to end	Night	9% 7% 2 6% 4 4 4 3 4 7% 50%	1958
March	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 4 6% 42%	4918
	11th to 20th	Night	6% 5% 2 4 3 3 3 4 6% 9% 55%	4980
	21st to end	Night	10% 6% 2 5% 4 3 4 3 6% 7% 50%	5467
April	1st to 10th	Night	9% 5% 2 4 4 5% 4 4 5% 7% 52%	2200
	11th to 20th	Night	7% 4 3 6% 4 4 4 4 5% 9% 51%	2173
	21st to end	Night	6% 3 1 6% 3 4 3 3 7% 7% 57%	1915
May	1st to 10th	Night	5% 4 1 5% 4 3 4 4 7% 10% 53%	4033
	11th to 20th	Night	9% 5% 2 5% 4 3 4 3 5% 9% 50%	4032
	21st to end	Night	3 3 1 4 3 3 4 4 6% 11% 59%	4421
June	1st to 10th	Night	9% 6% 3 8% 4 5% 5% 5% 7% 10% 38%	1800
	11th to 20th	Night	5% 4 2 5% 6% 6% 8% 6% 9% 14% 35%	1800
	21st to end	Night	5% 5% 3 5% 6% 4 6% 5% 10% 14% 37%	1800
July	1st to 10th	Night	5% 8% 3 10% 8% 8% 9% 7% 10% 13% 19%	4058
	11th to 20th	Night	3 4 2 7% 5% 7% 7% 6% 9% 13% 36%	4057
	21st to end	Night	4 6% 3 9% 6% 7% 8% 7% 11% 11% 27%	4427
August	1st to 10th	Night	7% 5% 2 7% 6% 5% 7% 6% 9% 13% 32%	2915
	11th to 20th	Night	4 4 2 5% 4 4 6% 5% 9% 15% 43%	3132
	21st to end	Night	6% 7% 3 9% 7% 7% 6% 6% 8% 12% 30%	3849
September	1st to 10th	Night	7% 7% 4 9% 7% 6% 7% 7% 8% 10% 28%	2124
	11th to 20th	Night	12% 8% 3 8% 6% 6% 6% 5% 7% 10% 28%	2090
	21st to end	Night	9% 8% 3 7% 5% 5% 4 4 6% 9% 40%	2090
October	1st to 10th	Night	10% 6% 3 7% 4 5% 4 4 6% 8% 43%	2090
	11th to 20th	Night	6% 4 2 4 3 4 4 4 5% 8% 58%	2244
	21st to end	Night	7% 5% 2 6% 4 4 4 4 5% 7% 52%	2508
November	1st to 10th	Night	8% 4 2 5% 4 4 4 4 5% 8% 53%	2341
	11th to 20th	Night	9% 6% 2 6% 4 3 5% 3 5% 6% 51%	2440
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 5% 7% 48%	2600
December	1st to 10th	Night	8% 5% 2 4 3 3 4 3 5% 7% 56%	4250
	11th to 20th	Night	10% 7% 2 7% 4 4 4 3 4 6% 48%	4822
	21st to end	Night	13% 8% 3 7% 4 5% 5% 4 5% 6% 40%	6490

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	30% 3 8% 15% 27% 16%	4057
	11th to 20th	Day	51% 3 4 13% 18% 11%	4060
	21st to end	Day	32% 5% 11% 25% 17% 11%	4741
February	1st to 10th	Day	40% 5% 9% 13% 20% 12%	2000
	11th to 20th	Day	33% 5% 10% 18% 23% 12%	1958
	21st to end	Day	35% 6% 10% 16% 22% 12%	1670
March	1st to 10th	Day	37% 6% 11% 16% 20% 9%	4935
	11th to 20th	Day	24% 5% 12% 19% 23% 18%	4980
	21st to end	Day	31% 5% 10% 26% 20% 9%	5465
April	1st to 10th	Day	27% 7% 14% 21% 20% 11%	2200
	11th to 20th	Day	34% 6% 9% 19% 23% 10%	2176
	21st to end	Day	23% 7% 13% 25% 23% 9%	2497
May	1st to 10th	Day	26% 8% 22% 28% 13% 4	5897
	11th to 20th	Day	33% 7% 14% 32% 12% 2	5865
	21st to end	Day	24% 8% 19% 32% 15% 2	6447
June	1st to 10th	Day	43% 12% 19% 19% 8%	2600
	11th to 20th	Day	40% 15% 22% 19% 4	2600
	21st to end	Day	45% 14% 21% 15% 4	2600
July	1st to 10th	Day	48% 16% 29% 6%	5835
	11th to 20th	Day	36% 14% 38% 11% 1	5831
	21st to end	Day	37% 14% 28% 19% 2	6240
August	1st to 10th	Day	41% 13% 29% 14% 3	4221
	11th to 20th	Day	39% 12% 25% 18% 6%	3949
	21st to end	Day	44% 12% 23% 20% 2	4015
September	1st to 10th	Day	51% 15% 19% 9% 6% 1	2123
	11th to 20th	Day	48% 13% 19% 14% 6% 1	2090
	21st to end	Day	43% 9% 13% 18% 13% 4	2090
October	1st to 10th	Day	53% 7% 10% 13% 11% 6%	3480
	11th to 20th	Day	22% 4 6% 20% 36% 13%	3186
	21st to end	Day	22% 5% 7% 25% 33% 9%	3428
November	1st to 10th	Day	31% 5% 12% 21% 18% 13%	1957
	11th to 20th	Day	36% 5% 9% 19% 21% 11%	1955
	21st to end	Day	34% 5% 10% 20% 23% 8%	1800
December	1st to 10th	Day	26% 4 14% 23% 22% 11%	2930
	11th to 20th	Day	35% 5% 10% 22% 18% 9%	3370
	21st to end	Day	41% 4 9% 19% 22% 5%	4466
January	1st to 10th	Night	28% 3 7% 15% 31% 15%	5889
	11th to 20th	Night	49% 4 7% 16% 18% 6%	5900
	21st to end	Night	41% 5% 9% 19% 17% 10%	6199
February	1st to 10th	Night	38% 4 9% 17% 20% 12%	2400
	11th to 20th	Night	36% 5% 8% 19% 20% 11%	2339
	21st to end	Night	34% 5% 10% 18% 22% 11%	1958
March	1st to 10th	Night	43% 5% 9% 15% 18% 11%	4918
	11th to 20th	Night	25% 6% 12% 18% 20% 18%	4980
	21st to end	Night	32% 5% 10% 24% 19% 10%	5467
April	1st to 10th	Night	31% 6% 14% 20% 20% 9%	2200
	11th to 20th	Night	30% 5% 10% 24% 21% 9%	2173
	21st to end	Night	25% 5% 14% 29% 19% 8%	1915
May	1st to 10th	Night	23% 11% 22% 28% 12% 3	4060
	11th to 20th	Night	30% 8% 16% 28% 16% 2	4033
	21st to end	Night	20% 7% 18% 36% 18% 1	4429
June	1st to 10th	Night	38% 11% 23% 22% 6% 1	1800
	11th to 20th	Night	35% 13% 29% 20% 3	1800
	21st to end	Night	33% 15% 26% 20% 5% 1	1800
July	1st to 10th	Night	50% 17% 27% 5% 1	4015
	11th to 20th	Night	36% 14% 37% 12% 1	4006
	21st to end	Night	43% 13% 25% 16% 2	4297
August	1st to 10th	Night	39% 12% 34% 12% 3	2904
	11th to 20th	Night	27% 13% 32% 23% 6%	3160
	21st to end	Night	44% 13% 24% 16% 3	3849
September	1st to 10th	Night	45% 15% 23% 12% 4 1	2124
	11th to 20th	Night	46% 13% 19% 16% 5% 1	2090
	21st to end	Night	38% 9% 17% 19% 13% 3	2090
October	1st to 10th	Night	51% 7% 12% 13% 10% 6%	3480
	11th to 20th	Night	22% 5% 7% 21% 36% 10%	3730
	21st to end	Night	24% 4 7% 22% 35% 8%	4120
November	1st to 10th	Night	29% 5% 12% 22% 19% 12%	2341
	11th to 20th	Night	34% 4 10% 20% 20% 13%	2440
	21st to end	Night	34% 5% 10% 19% 21% 10%	2600
December	1st to 10th	Night	29% 3 11% 23% 19% 15%	4250
	11th to 20th	Night	36% 5% 11% 20% 18% 10%	4824
	21st to end	Night	42% 5% 10% 19% 18% 6%	6490

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	10% 13% 49% 21% 6%	300
	11th to 20th	Day	13% 12% 40% 23% 10% 2	300
	21st to end	Day	17% 20% 43% 17% 3	330
February	1st to 10th	Day	11% 15% 34% 26% 13% 1	300
	11th to 20th	Day	14% 14% 36% 28% 7% 1	300
	21st to end	Day	10% 13% 38% 28% 11% 1	269
March	1st to 10th	Day	12% 15% 33% 28% 11% 1	500
	11th to 20th	Day	15% 8% 32% 28% 16% 1	500
	21st to end	Day	12% 12% 30% 30% 15% 1	550
April	1st to 10th	Day	11% 11% 28% 30% 18% 2	500
	11th to 20th	Day	10% 9% 30% 31% 17% 3	492
	21st to end	Day	4 7% 22% 31% 27% 9%	500
May	1st to 10th	Day	5% 5% 25% 29% 25% 11%	503
	11th to 20th	Day	9% 3 16% 32% 23% 17%	527
	21st to end	Day	8% 2 12% 27% 30% 21%	573
June	1st to 10th	Day	11% 1 10% 30% 26% 22%	530
	11th to 20th	Day	8% 3 9% 21% 31% 29%	530
	21st to end	Day	7% 1 5% 21% 33% 33%	530
July	1st to 10th	Day	8% 2 4 13% 30% 43%	525
	11th to 20th	Day	5% 2 2 12% 32% 47%	524
	21st to end	Day	6% 3 3 16% 31% 40%	576
August	1st to 10th	Day	10% 3 5% 19% 27% 37%	526
	11th to 20th	Day	8% 2 3 13% 30% 44%	519
	21st to end	Day	11% 2 4 20% 33% 30%	583
September	1st to 10th	Day	15% 2 5% 21% 30% 27%	502
	11th to 20th	Day	12% 3 7% 25% 34% 19%	490
	21st to end	Day	9% 3 11% 24% 36% 18%	490
October	1st to 10th	Day	15% 4 21% 31% 21% 8%	490
	11th to 20th	Day	4 6% 19% 35% 29% 7%	490
	21st to end	Day	5% 4 22% 40% 25% 4	539
November	1st to 10th	Day	9% 5% 26% 31% 26% 4	495
	11th to 20th	Day	14% 7% 27% 34% 15% 4	455
	21st to end	Day	10% 9% 36% 32% 11% 2	300
December	1st to 10th	Day	8% 12% 39% 25% 13% 3	300
	11th to 20th	Day	15% 11% 34% 29% 10%	300
	21st to end	Day	12% 11% 31% 34% 11% 1	330
January	1st to 10th	Night	16% 10% 32% 33% 9% 1	800
	11th to 20th	Night	13% 10% 26% 38% 12% 1	800
	21st to end	Night	18% 11% 35% 30% 6%	880
February	1st to 10th	Night	15% 8% 30% 36% 11%	800
	11th to 20th	Night	14% 9% 34% 34% 9% 1	783
	21st to end	Night	12% 9% 35% 39% 6%	660
March	1st to 10th	Night	11% 10% 32% 37% 10%	800
	11th to 20th	Night	11% 9% 31% 35% 14% 1	800
	21st to end	Night	9% 9% 32% 38% 13%	880
April	1st to 10th	Night	11% 7% 31% 36% 14% 1	800
	11th to 20th	Night	9% 8% 32% 37% 13% 1	791
	21st to end	Night	9% 3 26% 42% 20% 1	697
May	1st to 10th	Night	5% 3 21% 46% 23% 2	600
	11th to 20th	Night	7% 4 17% 40% 29% 4	591
	21st to end	Night	7% 1 9% 41% 35% 7%	650
June	1st to 10th	Night	11% 3 10% 35% 35% 7%	600
	11th to 20th	Night	6% 2 6% 26% 39% 21%	600
	21st to end	Night	7% 2 4 22% 44% 22%	600
July	1st to 10th	Night	6% 2 3 18% 45% 26%	585
	11th to 20th	Night	4 1 2 12% 51% 29%	582
	21st to end	Night	5% 1 2 15% 54% 23%	657
August	1st to 10th	Night	7% 2 4 15% 53% 20%	597
	11th to 20th	Night	6% 2 3 12% 50% 28%	594
	21st to end	Night	12% 3 6% 23% 39% 17%	819
September	1st to 10th	Night	12% 3 7% 25% 39% 16%	773
	11th to 20th	Night	12% 4 9% 31% 36% 8%	760
	21st to end	Night	12% 3 9% 34% 37% 5%	760
October	1st to 10th	Night	15% 2 18% 36% 25% 3	760
	11th to 20th	Night	6% 4 19% 41% 28% 2	760
	21st to end	Night	9% 4 19% 43% 24% 2	836
November	1st to 10th	Night	11% 4 25% 37% 22% 1	780
	11th to 20th	Night	11% 6% 26% 40% 16% 1	800
	21st to end	Night	11% 7% 29% 40% 13% 1	800
December	1st to 10th	Night	12% 6% 26% 36% 19% 1	800
	11th to 20th	Night	13% 9% 26% 39% 13% 1	800
	21st to end	Night	14% 9% 26% 37% 14% 1	880

Page generated at 2025-09-01 01:51:08. Version ac07

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