Robastro Home Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robastro Home Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	11% 5% 2 5% 4 3 4 3 5% 7% 50%	127224
	Night	11% 5% 2 4 3 3 3 3 4 6% 55%	124087
Transparency	Day	33% 5% 10% 24% 21% 8%	130915
	Night	30% 4 9% 22% 24% 11%	128459
Seeing	Day	12% 8% 22% 28% 23% 7%	16615
	Night	13% 7% 27% 35% 16% 2	26496

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	14% 6% 2 6% 3 3 3 2 4 5% 51%	4077
	11th to 20th	Day	9% 5% 1 3 2 2 2 3 4 6% 61%	4080
	21st to end	Day	24% 9% 3 6% 2 2 2 2 2 4 45%	4488
February	1st to 10th	Day	20% 8% 3 7% 3 2 3 2 3 5% 44%	1889
	11th to 20th	Day	19% 8% 3 6% 3 3 3 2 4 5% 45%	1949
	21st to end	Day	21% 6% 2 4 3 3 3 2 3 3 50%	1710
March	1st to 10th	Day	16% 7% 2 5% 3 2 3 3 5% 6% 48%	4936
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 4 5% 52%	4980
	21st to end	Day	14% 7% 2 5% 3 4 4 3 6% 6% 45%	5462
April	1st to 10th	Day	15% 6% 2 5% 4 4 4 4 5% 6% 45%	2200
	11th to 20th	Day	14% 6% 2 6% 4 4 3 3 5% 7% 44%	2304
	21st to end	Day	13% 5% 2 6% 4 4 5% 5% 5% 7% 44%	2486
May	1st to 10th	Day	18% 8% 3 7% 4 4 4 3 5% 7% 38%	5862
	11th to 20th	Day	14% 7% 3 8% 5% 4 4 4 4 5% 43%	5862
	21st to end	Day	13% 8% 3 7% 5% 4 5% 5% 7% 9% 35%	6434
June	1st to 10th	Day	6% 4 2 6% 5% 4 5% 4 6% 10% 48%	2600
	11th to 20th	Day	6% 4 2 4 4 5% 4 4 7% 9% 51%	2600
	21st to end	Day	5% 4 2 5% 4 4 4 3 6% 9% 55%	2600
July	1st to 10th	Day	6% 6% 2 7% 5% 6% 6% 5% 7% 9% 39%	5900
	11th to 20th	Day	3 2 1 3 3 3 4 3 6% 10% 61%	5900
	21st to end	Day	5% 5% 2 5% 4 4 4 5% 7% 10% 51%	6422
August	1st to 10th	Day	1 1 1 2 2 2 3 3 5% 8% 71%	4234
	11th to 20th	Day	4 2 1 3 3 3 4 4 7% 9% 61%	4171
	21st to end	Day	3 3 1 4 4 4 6% 5% 7% 12% 52%	4223
September	1st to 10th	Day	7% 5% 2 5% 4 4 5% 3 5% 7% 53%	2200
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 3 5% 7% 48%	2200
	21st to end	Day	8% 4 1 4 3 3 3 2 5% 8% 59%	2200
October	1st to 10th	Day	5% 4 1 5% 2 3 3 2 5% 6% 64%	2120
	11th to 20th	Day	12% 4 1 3 3 2 2 3 4 6% 61%	2000
	21st to end	Day	14% 4 2 5% 2 2 3 2 3 4 59%	2200
November	1st to 10th	Day	12% 5% 2 4 3 3 3 2 4 5% 56%	2058
	11th to 20th	Day	8% 4 2 6% 4 3 3 3 5% 6% 58%	2015
	21st to end	Day	16% 6% 1 5% 3 4 3 2 4 5% 52%	1890
December	1st to 10th	Day	18% 5% 2 4 3 3 3 3 4 6% 50%	3030
	11th to 20th	Day	18% 8% 2 6% 4 3 3 3 4 4 43%	3454
	21st to end	Day	18% 8% 3 6% 3 3 3 3 3 4 47%	4488
January	1st to 10th	Night	15% 5% 2 5% 3 2 3 2 3 5% 55%	5888
	11th to 20th	Night	9% 3 1 3 2 2 2 2 3 4 70%	5900
	21st to end	Night	24% 8% 2 5% 3 2 2 2 3 4 45%	6490
February	1st to 10th	Night	16% 7% 2 5% 3 3 3 2 4 5% 51%	2528
	11th to 20th	Night	18% 6% 2 5% 4 3 3 2 4 5% 47%	2338
	21st to end	Night	19% 5% 2 5% 3 3 2 2 3 5% 50%	1919
March	1st to 10th	Night	17% 7% 3 6% 4 3 3 3 4 5% 45%	4919
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 4 6% 53%	4980
	21st to end	Night	14% 6% 2 6% 3 3 3 3 4 6% 50%	5467
April	1st to 10th	Night	10% 5% 2 4 4 4 4 4 6% 7% 52%	2200
	11th to 20th	Night	13% 5% 2 5% 3 4 4 3 5% 6% 50%	2043
	21st to end	Night	11% 6% 1 4 3 3 3 3 5% 7% 54%	1910
May	1st to 10th	Night	17% 9% 3 7% 4 3 4 3 5% 7% 37%	4053
	11th to 20th	Night	10% 6% 2 6% 5% 5% 4 4 5% 7% 46%	4053
	21st to end	Night	10% 6% 2 5% 4 4 4 4 6% 8% 47%	4443
June	1st to 10th	Night	5% 3 1 3 4 4 4 4 6% 9% 58%	1800
	11th to 20th	Night	4 4 1 3 3 3 3 3 4 8% 65%	1800
	21st to end	Night	4 3 1 4 3 4 4 4 6% 10% 59%	1800
July	1st to 10th	Night	5% 5% 2 6% 4 5% 6% 5% 8% 11% 44%	4078
	11th to 20th	Night	3 2 1 3 3 2 3 2 5% 10% 66%	4077
	21st to end	Night	3 3 1 4 4 3 5% 4 6% 8% 59%	4449
August	1st to 10th	Night	1 1 1 1 1 2 2 2 4 9% 77%	2926
	11th to 20th	Night	3 2 1 3 2 3 2 3 5% 10% 67%	2888
	21st to end	Night	3 2 1 3 3 3 4 3 6% 10% 61%	3670
September	1st to 10th	Night	6% 5% 2 5% 4 3 3 3 5% 7% 56%	2200
	11th to 20th	Night	9% 4 2 4 3 4 3 3 5% 7% 55%	2200
	21st to end	Night	6% 3 1 3 3 4 3 3 5% 8% 62%	2200
October	1st to 10th	Night	6% 2 1 3 2 2 3 3 5% 7% 67%	2258
	11th to 20th	Night	11% 4 1 3 2 2 3 2 4 5% 63%	2400
	21st to end	Night	15% 5% 1 3 2 2 2 1 3 4 61%	2640
November	1st to 10th	Night	10% 5% 2 4 2 3 3 2 3 5% 61%	2460
	11th to 20th	Night	8% 4 2 5% 3 3 3 3 5% 6% 59%	2602
	21st to end	Night	15% 5% 2 5% 3 3 3 2 4 5% 52%	2730
December	1st to 10th	Night	16% 6% 2 5% 3 2 2 2 3 5% 53%	4380
	11th to 20th	Night	17% 8% 2 5% 4 2 3 2 3 4 50%	4908
	21st to end	Night	15% 7% 2 5% 3 2 3 2 3 5% 52%	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	36% 3 7% 26% 25% 3	4078
	11th to 20th	Day	25% 3 7% 22% 35% 7%	4080
	21st to end	Day	47% 2 4 15% 21% 12%	4488
February	1st to 10th	Day	45% 2 5% 17% 21% 10%	1889
	11th to 20th	Day	44% 3 6% 19% 20% 8%	1959
	21st to end	Day	41% 3 5% 15% 24% 13%	1710
March	1st to 10th	Day	36% 5% 5% 16% 23% 15%	4936
	11th to 20th	Day	34% 4 6% 17% 17% 21%	4980
	21st to end	Day	38% 5% 10% 22% 19% 6%	5464
April	1st to 10th	Day	39% 5% 9% 19% 18% 10%	2200
	11th to 20th	Day	39% 6% 8% 17% 22% 8%	2304
	21st to end	Day	39% 5% 8% 20% 23% 5%	2486
May	1st to 10th	Day	45% 5% 9% 25% 12% 3	5900
	11th to 20th	Day	43% 5% 7% 20% 21% 3	5866
	21st to end	Day	42% 8% 12% 19% 15% 4	6447
June	1st to 10th	Day	29% 8% 15% 29% 16% 3	2600
	11th to 20th	Day	28% 7% 13% 29% 19% 4	2600
	21st to end	Day	25% 7% 13% 31% 21% 3	2600
July	1st to 10th	Day	37% 9% 19% 27% 8%	5836
	11th to 20th	Day	18% 5% 16% 39% 21% 2	5831
	21st to end	Day	26% 7% 15% 33% 18% 2	6240
August	1st to 10th	Day	12% 4 12% 33% 35% 4	4220
	11th to 20th	Day	18% 5% 16% 41% 17% 2	4210
	21st to end	Day	23% 8% 19% 34% 13% 2	4222
September	1st to 10th	Day	31% 6% 9% 23% 21% 11%	2200
	11th to 20th	Day	34% 6% 11% 20% 22% 8%	2200
	21st to end	Day	25% 4 9% 24% 26% 13%	2200
October	1st to 10th	Day	13% 3 6% 14% 33% 31%	3458
	11th to 20th	Day	17% 2 5% 23% 31% 22%	3240
	21st to end	Day	25% 3 6% 28% 29% 9%	3538
November	1st to 10th	Day	31% 4 6% 23% 26% 9%	2058
	11th to 20th	Day	28% 4 9% 20% 28% 12%	2015
	21st to end	Day	36% 3 5% 22% 24% 9%	1890
December	1st to 10th	Day	36% 4 6% 25% 24% 5%	3030
	11th to 20th	Day	44% 3 5% 12% 25% 11%	3452
	21st to end	Day	42% 3 5% 22% 22% 7%	4488
January	1st to 10th	Night	34% 2 6% 25% 27% 6%	5888
	11th to 20th	Night	21% 3 5% 19% 37% 15%	5900
	21st to end	Night	46% 2 4 13% 21% 14%	6490
February	1st to 10th	Night	38% 3 7% 17% 22% 13%	2528
	11th to 20th	Night	40% 3 7% 16% 24% 10%	2338
	21st to end	Night	38% 4 6% 14% 21% 16%	1919
March	1st to 10th	Night	42% 3 5% 11% 22% 17%	4919
	11th to 20th	Night	32% 5% 7% 16% 19% 22%	4980
	21st to end	Night	36% 4 7% 19% 27% 7%	5467
April	1st to 10th	Night	31% 5% 10% 21% 20% 14%	2200
	11th to 20th	Night	35% 5% 8% 19% 19% 13%	2043
	21st to end	Night	30% 5% 10% 23% 27% 5%	1910
May	1st to 10th	Night	44% 6% 10% 21% 14% 4	4080
	11th to 20th	Night	37% 7% 13% 16% 24% 3	4054
	21st to end	Night	33% 8% 14% 26% 10% 9%	4452
June	1st to 10th	Night	21% 8% 16% 30% 21% 4	1800
	11th to 20th	Night	18% 6% 14% 35% 20% 7%	1800
	21st to end	Night	20% 7% 13% 30% 24% 5%	1800
July	1st to 10th	Night	31% 8% 18% 33% 9% 1	4035
	11th to 20th	Night	15% 4 16% 40% 21% 4	4026
	21st to end	Night	21% 7% 17% 34% 18% 3	4320
August	1st to 10th	Night	7% 3 11% 34% 40% 6%	2916
	11th to 20th	Night	15% 4 14% 42% 22% 4	2915
	21st to end	Night	18% 7% 21% 32% 21% 2	3670
September	1st to 10th	Night	27% 5% 11% 23% 23% 11%	2200
	11th to 20th	Night	29% 5% 12% 22% 23% 9%	2200
	21st to end	Night	22% 3 9% 21% 29% 16%	2200
October	1st to 10th	Night	11% 3 6% 15% 27% 39%	3687
	11th to 20th	Night	16% 2 4 20% 31% 27%	3899
	21st to end	Night	24% 2 4 22% 31% 17%	4252
November	1st to 10th	Night	28% 3 7% 21% 28% 14%	2460
	11th to 20th	Night	26% 4 7% 21% 27% 15%	2602
	21st to end	Night	35% 3 7% 17% 27% 11%	2730
December	1st to 10th	Night	36% 3 6% 19% 28% 9%	4380
	11th to 20th	Night	41% 2 5% 14% 25% 13%	4909
	21st to end	Night	37% 3 5% 19% 26% 10%	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	23% 13% 35% 23% 6%	300
	11th to 20th	Day	20% 18% 35% 21% 5% 1	300
	21st to end	Day	23% 14% 35% 19% 8% 1	330
February	1st to 10th	Day	24% 15% 38% 18% 6%	300
	11th to 20th	Day	27% 19% 39% 11% 4	300
	21st to end	Day	29% 22% 33% 14% 1	249
March	1st to 10th	Day	23% 18% 33% 22% 3	300
	11th to 20th	Day	23% 20% 38% 13% 4 1	300
	21st to end	Day	16% 20% 39% 19% 6% 1	330
April	1st to 10th	Day	24% 15% 36% 19% 6%	300
	11th to 20th	Day	19% 16% 34% 23% 7%	422
	21st to end	Day	16% 16% 34% 24% 9% 1	586
May	1st to 10th	Day	14% 14% 32% 32% 8% 1	703
	11th to 20th	Day	16% 8% 30% 29% 14% 3	725
	21st to end	Day	12% 6% 26% 36% 16% 3	789
June	1st to 10th	Day	7% 3 17% 36% 28% 9%	730
	11th to 20th	Day	6% 3 17% 40% 29% 5%	730
	21st to end	Day	4 1 11% 38% 35% 11%	730
July	1st to 10th	Day	6% 2 11% 30% 39% 11%	721
	11th to 20th	Day	2 1 6% 36% 42% 13%	718
	21st to end	Day	5% 1 7% 29% 45% 14%	794
August	1st to 10th	Day	3 1 7% 28% 48% 13%	726
	11th to 20th	Day	8% 1 4 32% 43% 12%	715
	21st to end	Day	5% 1 5% 26% 44% 19%	533
September	1st to 10th	Day	8% 1 8% 24% 39% 21%	330
	11th to 20th	Day	5% 8% 28% 36% 23%	330
	21st to end	Day	10% 4 10% 25% 31% 19%	330
October	1st to 10th	Day	3 2 15% 35% 33% 12%	330
	11th to 20th	Day	11% 6% 25% 30% 22% 6%	330
	21st to end	Day	22% 12% 19% 26% 17% 3	363
November	1st to 10th	Day	19% 7% 28% 30% 12% 4	330
	11th to 20th	Day	12% 9% 31% 32% 14% 2	330
	21st to end	Day	18% 14% 33% 25% 9% 2	330
December	1st to 10th	Day	19% 11% 34% 24% 9% 3	330
	11th to 20th	Day	22% 15% 34% 23% 6%	321
	21st to end	Day	22% 14% 37% 22% 5% 1	330
January	1st to 10th	Night	16% 11% 34% 32% 7%	1000
	11th to 20th	Night	19% 14% 32% 30% 5% 1	1000
	21st to end	Night	19% 10% 35% 31% 5%	1100
February	1st to 10th	Night	20% 12% 36% 29% 4	928
	11th to 20th	Night	22% 13% 37% 24% 3	784
	21st to end	Night	22% 15% 40% 20% 3	599
March	1st to 10th	Night	21% 11% 36% 28% 5%	600
	11th to 20th	Night	16% 13% 38% 28% 6%	600
	21st to end	Night	15% 13% 39% 28% 5%	660
April	1st to 10th	Night	13% 11% 44% 29% 4	600
	11th to 20th	Night	16% 11% 36% 28% 8% 1	593
	21st to end	Night	14% 10% 38% 29% 9% 1	600
May	1st to 10th	Night	10% 11% 38% 33% 7% 1	600
	11th to 20th	Night	11% 5% 34% 35% 15% 1	591
	21st to end	Night	9% 5% 29% 39% 16% 2	650
June	1st to 10th	Night	7% 2 19% 45% 24% 3	600
	11th to 20th	Night	4 2 21% 54% 18% 1	600
	21st to end	Night	4 2 15% 41% 34% 5%	600
July	1st to 10th	Night	4 2 11% 42% 36% 5%	585
	11th to 20th	Night	3 1 9% 44% 40% 3	582
	21st to end	Night	2 1 5% 47% 41% 4	657
August	1st to 10th	Night	3 1 5% 44% 44% 4	597
	11th to 20th	Night	6% 4 41% 42% 7%	594
	21st to end	Night	5% 5% 40% 42% 8%	660
September	1st to 10th	Night	6% 1 11% 40% 38% 4	600
	11th to 20th	Night	11% 2 7% 39% 37% 4	600
	21st to end	Night	8% 1 12% 43% 31% 6%	600
October	1st to 10th	Night	6% 2 16% 44% 29% 3	658
	11th to 20th	Night	13% 4 24% 39% 18% 2	800
	21st to end	Night	17% 8% 28% 31% 16% 1	880
November	1st to 10th	Night	11% 5% 28% 40% 14% 1	820
	11th to 20th	Night	10% 8% 33% 37% 11% 1	922
	21st to end	Night	19% 9% 32% 28% 10% 1	1050
December	1st to 10th	Night	15% 9% 35% 33% 7%	1050
	11th to 20th	Night	21% 8% 32% 33% 6%	1036
	21st to end	Night	20% 9% 34% 31% 6%	1100

Page generated at 2025-12-15 00:13:56. Version ac07

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