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 2024-11-04.

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	9% 6% 2 7% 5% 5% 6% 5% 7% 9% 39%	100951
	Night	9% 6% 2 6% 5% 4 5% 4 6% 9% 43%	98015
Transparency	Day	37% 9% 16% 19% 14% 5%	104744
	Night	36% 8% 16% 19% 15% 6%	102361
Seeing	Day	10% 6% 19% 26% 23% 16%	15354
	Night	11% 5% 20% 33% 24% 6%	25381

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% 7% 3 8% 4 3 3 3 5% 7% 45%	2836
	11th to 20th	Day	18% 10% 4 7% 5% 3 3 3 4 6% 37%	2840
	21st to end	Day	10% 7% 2 5% 4 4 4 3 6% 8% 47%	3315
February	1st to 10th	Day	15% 6% 3 6% 4 3 4 4 6% 6% 42%	1900
	11th to 20th	Day	11% 5% 2 4 4 3 4 3 4 6% 53%	1848
	21st to end	Day	10% 6% 3 6% 4 4 3 4 5% 7% 48%	1589
March	1st to 10th	Day	11% 7% 3 8% 5% 5% 5% 5% 7% 8% 37%	3435
	11th to 20th	Day	7% 5% 2 4 4 3 4 4 8% 9% 50%	3480
	21st to end	Day	12% 5% 2 5% 3 4 4 4 5% 8% 49%	3815
April	1st to 10th	Day	7% 4 2 4 4 4 4 5% 6% 10% 49%	2090
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 4 6% 7% 46%	2066
	21st to end	Day	5% 3 2 4 4 4 5% 5% 6% 8% 55%	2372
May	1st to 10th	Day	4 3 1 3 3 4 4 4 7% 10% 57%	4082
	11th to 20th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	4081
	21st to end	Day	4 3 2 4 4 4 5% 4 7% 11% 53%	4476
June	1st to 10th	Day	10% 6% 3 8% 6% 6% 6% 7% 8% 11% 28%	2470
	11th to 20th	Day	6% 5% 2 7% 7% 8% 8% 8% 10% 11% 29%	2470
	21st to end	Day	9% 6% 2 7% 7% 7% 9% 7% 9% 10% 27%	2470
July	1st to 10th	Day	5% 5% 3 8% 8% 8% 10% 8% 12% 13% 20%	4120
	11th to 20th	Day	4 4 3 8% 9% 9% 9% 7% 11% 13% 23%	4120
	21st to end	Day	4 5% 3 9% 7% 9% 9% 8% 11% 14% 23%	4466
August	1st to 10th	Day	10% 5% 3 7% 6% 6% 7% 6% 10% 13% 28%	4104
	11th to 20th	Day	6% 5% 2 7% 6% 7% 9% 7% 10% 12% 29%	3789
	21st to end	Day	9% 6% 3 8% 6% 6% 7% 6% 10% 12% 27%	3892
September	1st to 10th	Day	11% 8% 3 9% 7% 8% 8% 7% 9% 9% 22%	2090
	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% 52%	1887
	11th to 20th	Day	9% 8% 3 6% 4 4 4 3 6% 6% 46%	1858
	21st to end	Day	10% 5% 2 6% 4 4 5% 3 6% 7% 47%	1710
December	1st to 10th	Day	8% 6% 2 6% 4 4 5% 4 5% 9% 47%	1710
	11th to 20th	Day	13% 7% 2 6% 4 4 3 5% 6% 7% 44%	2153
	21st to end	Day	16% 7% 3 7% 4 3 3 3 4 6% 43%	3124
January	1st to 10th	Night	14% 7% 3 6% 3 4 5% 4 6% 8% 42%	4109
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 3 6% 8% 37%	4120
	21st to end	Night	14% 7% 2 7% 4 5% 4 3 6% 7% 40%	4339
February	1st to 10th	Night	12% 6% 2 7% 4 4 4 3 4 7% 48%	2280
	11th to 20th	Night	10% 8% 2 5% 4 3 4 4 6% 6% 48%	2218
	21st to end	Night	9% 7% 2 6% 4 4 4 3 4 7% 50%	1863
March	1st to 10th	Night	15% 10% 4 7% 5% 4 4 4 4 6% 38%	3418
	11th to 20th	Night	7% 6% 2 5% 4 4 4 5% 7% 10% 46%	3480
	21st to end	Night	12% 5% 2 5% 4 3 4 3 5% 7% 49%	3817
April	1st to 10th	Night	9% 5% 2 4 4 5% 4 4 5% 7% 51%	2090
	11th to 20th	Night	7% 4 3 6% 4 4 4 4 5% 9% 50%	2063
	21st to end	Night	6% 3 1 6% 3 3 3 3 7% 7% 58%	1819
May	1st to 10th	Night	5% 3 1 4 3 3 4 3 7% 9% 58%	2813
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 4 6% 9% 47%	2812
	21st to end	Night	3 3 1 4 3 3 3 3 5% 9% 64%	3079
June	1st to 10th	Night	9% 5% 3 7% 4 5% 6% 5% 7% 10% 38%	1710
	11th to 20th	Night	5% 5% 2 5% 6% 5% 7% 5% 9% 14% 37%	1710
	21st to end	Night	6% 5% 3 5% 6% 4 6% 5% 10% 15% 36%	1710
July	1st to 10th	Night	6% 7% 3 10% 8% 8% 8% 7% 10% 13% 22%	2840
	11th to 20th	Night	2 4 2 8% 6% 7% 8% 6% 10% 14% 33%	2840
	21st to end	Night	4 7% 4 10% 7% 7% 9% 7% 11% 12% 23%	3087
August	1st to 10th	Night	7% 5% 2 7% 6% 5% 7% 6% 9% 13% 32%	2825
	11th to 20th	Night	4 4 2 5% 4 4 6% 5% 9% 15% 44%	3034
	21st to end	Night	7% 7% 3 9% 7% 7% 6% 5% 8% 12% 30%	3731
September	1st to 10th	Night	7% 7% 4 8% 7% 6% 7% 7% 8% 10% 28%	2090
	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 3 3 4 5% 8% 54%	2264
	11th to 20th	Night	9% 6% 2 5% 4 3 4 3 5% 6% 51%	2318
	21st to end	Night	9% 7% 2 6% 4 4 5% 4 5% 7% 47%	2470
December	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 6% 8% 47%	2470
	11th to 20th	Night	13% 8% 3 6% 4 3 4 3 4 6% 47%	3042
	21st to end	Night	15% 8% 3 7% 4 4 4 4 5% 7% 39%	4532

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% 4 8% 17% 20% 12%	2837
	11th to 20th	Day	48% 4 5% 13% 17% 12%	2840
	21st to end	Day	33% 6% 11% 20% 17% 12%	3315
February	1st to 10th	Day	40% 6% 9% 13% 20% 12%	1900
	11th to 20th	Day	32% 5% 10% 18% 24% 12%	1858
	21st to end	Day	35% 6% 10% 16% 22% 11%	1589
March	1st to 10th	Day	39% 8% 13% 19% 15% 6%	3435
	11th to 20th	Day	28% 6% 16% 20% 22% 8%	3480
	21st to end	Day	32% 6% 10% 29% 16% 8%	3815
April	1st to 10th	Day	28% 7% 14% 20% 19% 11%	2090
	11th to 20th	Day	36% 7% 9% 19% 21% 8%	2066
	21st to end	Day	23% 8% 13% 24% 23% 9%	2372
May	1st to 10th	Day	20% 8% 19% 31% 17% 5%	4120
	11th to 20th	Day	38% 9% 16% 23% 11% 2	4085
	21st to end	Day	23% 9% 17% 33% 17% 2	4489
June	1st to 10th	Day	43% 12% 18% 19% 7%	2470
	11th to 20th	Day	40% 15% 21% 20% 5%	2470
	21st to end	Day	46% 14% 21% 14% 5%	2470
July	1st to 10th	Day	44% 15% 32% 8% 1	4055
	11th to 20th	Day	42% 16% 35% 6% 1	4051
	21st to end	Day	41% 17% 32% 9% 1	4324
August	1st to 10th	Day	41% 12% 29% 14% 3	4091
	11th to 20th	Day	39% 13% 24% 18% 7%	3827
	21st to end	Day	43% 12% 23% 20% 2	3892
September	1st to 10th	Day	51% 15% 18% 9% 6% 1	2090
	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	30% 5% 12% 21% 18% 13%	1887
	11th to 20th	Day	36% 5% 9% 19% 21% 10%	1858
	21st to end	Day	35% 5% 10% 20% 23% 7%	1710
December	1st to 10th	Day	33% 6% 13% 25% 17% 6%	1710
	11th to 20th	Day	37% 6% 10% 18% 18% 11%	2150
	21st to end	Day	43% 4 7% 21% 17% 7%	3124
January	1st to 10th	Night	40% 5% 8% 16% 19% 12%	4109
	11th to 20th	Night	44% 5% 7% 18% 18% 7%	4120
	21st to end	Night	41% 7% 10% 13% 18% 10%	4339
February	1st to 10th	Night	37% 4 9% 17% 19% 13%	2280
	11th to 20th	Night	36% 5% 8% 18% 21% 12%	2219
	21st to end	Night	34% 6% 10% 18% 21% 11%	1863
March	1st to 10th	Night	46% 7% 11% 15% 14% 6%	3418
	11th to 20th	Night	31% 7% 17% 20% 18% 7%	3480
	21st to end	Night	33% 6% 10% 23% 20% 8%	3817
April	1st to 10th	Night	31% 7% 15% 19% 19% 9%	2090
	11th to 20th	Night	31% 5% 11% 25% 20% 7%	2063
	21st to end	Night	24% 6% 13% 29% 19% 9%	1819
May	1st to 10th	Night	18% 12% 20% 29% 16% 5%	2840
	11th to 20th	Night	31% 11% 21% 24% 12% 2	2813
	21st to end	Night	17% 7% 15% 38% 22% 1	3087
June	1st to 10th	Night	38% 11% 22% 23% 5%	1710
	11th to 20th	Night	34% 13% 29% 21% 4	1710
	21st to end	Night	33% 15% 27% 19% 5% 1	1710
July	1st to 10th	Night	47% 16% 28% 7% 1	2795
	11th to 20th	Night	38% 17% 37% 7% 1	2786
	21st to end	Night	47% 15% 27% 9% 1	2988
August	1st to 10th	Night	39% 12% 34% 12% 3	2814
	11th to 20th	Night	26% 13% 31% 23% 6%	3062
	21st to end	Night	44% 13% 24% 16% 3	3731
September	1st to 10th	Night	45% 15% 23% 12% 4 1	2090
	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	28% 5% 12% 22% 19% 13%	2264
	11th to 20th	Night	33% 5% 10% 20% 20% 13%	2318
	21st to end	Night	35% 6% 10% 19% 21% 9%	2470
December	1st to 10th	Night	34% 5% 12% 28% 15% 7%	2470
	11th to 20th	Night	39% 5% 8% 16% 21% 10%	3044
	21st to end	Night	43% 6% 11% 19% 16% 5%	4532

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	11% 13% 46% 23% 6%	270
	11th to 20th	Day	12% 13% 40% 23% 11% 2	270
	21st to end	Day	18% 21% 42% 16% 3	297
February	1st to 10th	Day	13% 16% 37% 24% 11%	270
	11th to 20th	Day	13% 14% 34% 29% 7% 1	270
	21st to end	Day	9% 14% 38% 27% 11% 1	244
March	1st to 10th	Day	10% 15% 33% 29% 12% 1	460
	11th to 20th	Day	15% 7% 33% 27% 16% 1	460
	21st to end	Day	12% 12% 30% 29% 15% 2	506
April	1st to 10th	Day	12% 11% 29% 29% 17% 2	460
	11th to 20th	Day	10% 10% 31% 30% 16% 3	452
	21st to end	Day	5% 8% 24% 32% 24% 7%	460
May	1st to 10th	Day	5% 5% 25% 27% 26% 12%	463
	11th to 20th	Day	9% 4 16% 31% 23% 17%	487
	21st to end	Day	8% 2 11% 27% 30% 22%	529
June	1st to 10th	Day	11% 1 9% 29% 26% 23%	490
	11th to 20th	Day	8% 3 9% 22% 30% 28%	490
	21st to end	Day	7% 1 6% 21% 32% 34%	490
July	1st to 10th	Day	8% 2 5% 12% 29% 44%	485
	11th to 20th	Day	6% 2 2 12% 31% 48%	484
	21st to end	Day	6% 2 3 16% 32% 40%	532
August	1st to 10th	Day	10% 3 5% 20% 27% 35%	486
	11th to 20th	Day	9% 2 3 13% 30% 43%	479
	21st to end	Day	11% 2 4 20% 33% 30%	539
September	1st to 10th	Day	16% 2 5% 20% 30% 28%	490
	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% 4 27% 30% 26% 4	467
	11th to 20th	Day	15% 7% 26% 34% 14% 3	418
	21st to end	Day	11% 8% 35% 34% 11% 1	270
December	1st to 10th	Day	9% 13% 37% 24% 14% 3	270
	11th to 20th	Day	17% 10% 34% 28% 10%	270
	21st to end	Day	12% 11% 31% 33% 12% 1	297
January	1st to 10th	Night	17% 9% 31% 33% 9% 1	760
	11th to 20th	Night	11% 10% 26% 39% 13% 1	760
	21st to end	Night	18% 11% 35% 29% 6%	836
February	1st to 10th	Night	16% 8% 30% 36% 10%	760
	11th to 20th	Night	13% 9% 34% 33% 9% 1	743
	21st to end	Night	11% 10% 34% 39% 6%	628
March	1st to 10th	Night	10% 10% 32% 37% 10%	760
	11th to 20th	Night	11% 8% 31% 35% 14% 1	760
	21st to end	Night	9% 9% 32% 37% 13%	836
April	1st to 10th	Night	11% 7% 31% 36% 13% 1	760
	11th to 20th	Night	10% 8% 33% 35% 13% 1	751
	21st to end	Night	9% 3 26% 42% 19% 1	662
May	1st to 10th	Night	6% 3 21% 45% 24% 2	570
	11th to 20th	Night	7% 4 17% 39% 29% 4	561
	21st to end	Night	7% 1 9% 41% 35% 7%	617
June	1st to 10th	Night	11% 4 10% 33% 35% 7%	570
	11th to 20th	Night	6% 2 6% 27% 38% 21%	570
	21st to end	Night	8% 1 4 22% 43% 22%	570
July	1st to 10th	Night	6% 2 3 17% 45% 27%	555
	11th to 20th	Night	4 1 1 13% 51% 30%	552
	21st to end	Night	5% 1 2 15% 54% 23%	624
August	1st to 10th	Night	7% 2 4 14% 53% 20%	567
	11th to 20th	Night	6% 2 3 12% 50% 27%	564
	21st to end	Night	12% 3 6% 22% 40% 17%	778
September	1st to 10th	Night	12% 3 7% 24% 39% 16%	760
	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% 36% 22% 1	755
	11th to 20th	Night	11% 6% 26% 40% 16% 1	760
	21st to end	Night	11% 7% 28% 40% 13% 1	760
December	1st to 10th	Night	13% 6% 26% 34% 19% 1	760
	11th to 20th	Night	13% 9% 26% 38% 13% 1	760
	21st to end	Night	14% 9% 26% 37% 14% 1	836

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