Albert L. Ruppel Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Albert L. Ruppel Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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% 7% 3 6% 4 4 4 4 6% 7% 42%	125686
	Night	13% 8% 3 7% 4 4 4 3 5% 6% 43%	122583
Transparency	Day	39% 5% 12% 25% 17% 3	129267
	Night	41% 4 11% 24% 18% 3	127079
Seeing	Day	17% 12% 28% 25% 14% 4	16665
	Night	17% 11% 30% 28% 12% 1	26447

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	12% 12% 7% 13% 5% 4 4 3 5% 6% 29%	3617
	11th to 20th	Day	9% 9% 4 8% 5% 5% 5% 3 5% 6% 41%	3960
	21st to end	Day	17% 14% 5% 7% 3 4 4 3 4 4 36%	4488
February	1st to 10th	Day	15% 11% 5% 9% 5% 3 4 4 6% 6% 33%	1800
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 3 4 6% 37%	1919
	21st to end	Day	14% 10% 3 8% 4 4 3 3 5% 7% 40%	1650
March	1st to 10th	Day	15% 11% 3 5% 3 3 2 2 3 4 49%	4572
	11th to 20th	Day	9% 7% 2 4 4 3 4 3 5% 7% 51%	4980
	21st to end	Day	13% 8% 3 7% 5% 4 5% 4 6% 8% 36%	5735
April	1st to 10th	Day	12% 7% 2 7% 5% 4 5% 3 6% 7% 42%	2400
	11th to 20th	Day	17% 9% 2 6% 4 4 4 3 5% 6% 41%	2374
	21st to end	Day	17% 7% 2 8% 5% 5% 4 4 5% 7% 36%	2400
May	1st to 10th	Day	16% 8% 3 8% 5% 5% 5% 4 7% 9% 31%	5497
	11th to 20th	Day	16% 8% 3 6% 4 4 4 4 7% 8% 36%	6272
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 7% 40%	6935
June	1st to 10th	Day	9% 5% 2 5% 5% 5% 5% 4 6% 8% 47%	2800
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 10% 49%	2800
	21st to end	Day	5% 4 2 5% 4 5% 5% 4 8% 11% 47%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 6% 9% 46%	6360
	11th to 20th	Day	6% 5% 2 7% 5% 5% 5% 5% 8% 11% 42%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 6% 5% 7% 11% 42%	6525
August	1st to 10th	Day	5% 4 1 4 4 5% 5% 4 7% 10% 51%	4157
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 9% 42%	3729
	21st to end	Day	7% 4 2 4 4 4 4 4 6% 9% 53%	4180
September	1st to 10th	Day	13% 4 2 5% 3 3 3 3 5% 8% 50%	2280
	11th to 20th	Day	10% 5% 2 6% 3 3 4 3 5% 9% 49%	2262
	21st to end	Day	8% 5% 2 5% 4 3 4 3 5% 8% 53%	2072
October	1st to 10th	Day	9% 5% 2 4 4 3 3 2 5% 6% 55%	1900
	11th to 20th	Day	13% 6% 2 6% 3 3 3 2 4 5% 53%	1900
	21st to end	Day	17% 8% 2 5% 3 4 3 2 4 5% 47%	2083
November	1st to 10th	Day	12% 5% 1 4 4 3 4 3 4 6% 54%	1762
	11th to 20th	Day	13% 7% 3 5% 3 3 3 3 5% 6% 47%	1800
	21st to end	Day	16% 10% 3 6% 4 4 4 3 4 5% 42%	1717
December	1st to 10th	Day	9% 10% 3 6% 4 3 4 3 4 6% 50%	2610
	11th to 20th	Day	12% 9% 5% 8% 4 4 4 3 5% 5% 38%	3008
	21st to end	Day	22% 17% 5% 9% 5% 4 3 3 4 4 24%	3982
January	1st to 10th	Night	16% 10% 6% 13% 5% 4 4 2 4 4 32%	6348
	11th to 20th	Night	11% 10% 4 12% 6% 6% 5% 3 4 4 36%	6019
	21st to end	Night	18% 13% 4 8% 4 3 3 3 3 5% 36%	6490
February	1st to 10th	Night	16% 12% 4 8% 5% 4 4 3 5% 5% 34%	2600
	11th to 20th	Night	16% 9% 3 9% 5% 4 3 3 4 5% 40%	2365
	21st to end	Night	15% 9% 3 8% 4 3 4 2 5% 4 41%	1980
March	1st to 10th	Night	19% 12% 2 5% 4 3 3 2 4 4 43%	5309
	11th to 20th	Night	12% 8% 2 5% 3 3 3 3 4 6% 50%	4980
	21st to end	Night	17% 9% 3 8% 5% 5% 4 4 4 6% 35%	5192
April	1st to 10th	Night	12% 6% 2 7% 4 3 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 5% 7% 45%	1975
	21st to end	Night	18% 8% 3 6% 4 3 3 3 5% 7% 41%	2000
May	1st to 10th	Night	18% 9% 2 7% 4 4 4 4 6% 8% 34%	4382
	11th to 20th	Night	14% 7% 2 6% 3 4 4 4 6% 8% 42%	3625
	21st to end	Night	13% 7% 3 7% 4 4 4 4 6% 9% 40%	3942
June	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 7% 10% 47%	1600
	11th to 20th	Night	6% 4 1 4 3 4 5% 4 5% 9% 55%	1600
	21st to end	Night	4 4 1 4 4 4 5% 4 6% 9% 56%	1600
July	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 10% 48%	3618
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 7% 9% 48%	3617
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 6% 7% 10% 42%	4328
August	1st to 10th	Night	5% 3 1 5% 3 3 4 3 6% 8% 58%	2872
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 6% 8% 47%	3099
	21st to end	Night	5% 3 1 4 3 4 4 4 5% 9% 58%	3476
September	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 6% 53%	1900
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 5% 7% 52%	1900
	21st to end	Night	10% 4 2 5% 3 4 4 3 4 7% 53%	2102
October	1st to 10th	Night	8% 4 1 5% 3 3 3 3 4 6% 60%	2280
	11th to 20th	Night	13% 6% 2 4 2 3 3 3 4 6% 54%	2280
	21st to end	Night	17% 8% 2 5% 3 3 4 2 4 5% 48%	2513
November	1st to 10th	Night	11% 5% 2 4 3 3 3 3 4 5% 56%	2536
	11th to 20th	Night	14% 6% 3 5% 4 3 4 3 5% 6% 49%	2600
	21st to end	Night	18% 8% 3 5% 3 3 3 3 5% 6% 43%	2680
December	1st to 10th	Night	12% 10% 3 6% 4 3 3 3 4 5% 47%	4580
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 4 6% 41%	5199
	21st to end	Night	24% 17% 4 9% 5% 4 4 3 4 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	54% 5% 11% 18% 9% 2	3618
	11th to 20th	Day	42% 4 11% 23% 16% 4	3960
	21st to end	Day	52% 3 8% 16% 16% 4	4488
February	1st to 10th	Day	51% 4 11% 23% 10% 1	1800
	11th to 20th	Day	48% 5% 10% 19% 15% 3	1929
	21st to end	Day	45% 4 8% 24% 15% 4	1650
March	1st to 10th	Day	41% 2 6% 22% 24% 5%	4572
	11th to 20th	Day	32% 4 9% 24% 25% 6%	4980
	21st to end	Day	44% 6% 14% 27% 9% 1	5737
April	1st to 10th	Day	41% 5% 11% 25% 17% 1	2400
	11th to 20th	Day	44% 4 10% 23% 15% 3	2374
	21st to end	Day	47% 5% 11% 23% 13% 1	2400
May	1st to 10th	Day	48% 6% 12% 22% 11%	5532
	11th to 20th	Day	44% 6% 11% 24% 13% 1	6276
	21st to end	Day	42% 6% 10% 22% 19% 1	6949
June	1st to 10th	Day	34% 6% 14% 28% 17% 2	2800
	11th to 20th	Day	29% 6% 13% 27% 22% 3	2800
	21st to end	Day	28% 7% 19% 25% 16% 5%	2800
July	1st to 10th	Day	34% 5% 12% 30% 17% 2	6291
	11th to 20th	Day	34% 7% 17% 27% 13% 1	6285
	21st to end	Day	35% 7% 18% 30% 10% 1	6340
August	1st to 10th	Day	27% 6% 15% 37% 14%	4143
	11th to 20th	Day	37% 6% 14% 25% 17% 1	3765
	21st to end	Day	27% 6% 15% 34% 15% 3	4179
September	1st to 10th	Day	34% 4 10% 27% 20% 5%	2280
	11th to 20th	Day	32% 4 9% 27% 24% 3	2262
	21st to end	Day	30% 5% 8% 30% 24% 3	2072
October	1st to 10th	Day	18% 3 5% 20% 47% 7%	3160
	11th to 20th	Day	24% 2 6% 22% 37% 9%	3140
	21st to end	Day	33% 3 7% 28% 26% 3	3409
November	1st to 10th	Day	33% 4 12% 26% 20% 5%	1762
	11th to 20th	Day	38% 3 9% 24% 21% 5%	1800
	21st to end	Day	46% 4 8% 27% 15%	1717
December	1st to 10th	Day	37% 4 12% 31% 16%	2610
	11th to 20th	Day	47% 4 12% 22% 14% 1	3005
	21st to end	Day	65% 3 12% 13% 7% 1	3982
January	1st to 10th	Night	57% 3 9% 22% 9% 1	6348
	11th to 20th	Night	52% 5% 6% 22% 13% 2	6019
	21st to end	Night	53% 3 7% 19% 16% 2	6490
February	1st to 10th	Night	52% 4 10% 20% 13% 1	2600
	11th to 20th	Night	47% 4 9% 20% 17% 2	2366
	21st to end	Night	46% 3 10% 22% 15% 4	1980
March	1st to 10th	Night	46% 3 6% 22% 21% 2	5309
	11th to 20th	Night	36% 3 10% 23% 22% 7%	4980
	21st to end	Night	50% 5% 10% 23% 10% 1	5192
April	1st to 10th	Night	37% 5% 10% 26% 18% 3	2000
	11th to 20th	Night	39% 4 11% 24% 18% 4	1975
	21st to end	Night	44% 4 8% 26% 16% 2	2000
May	1st to 10th	Night	48% 7% 14% 19% 12% 1	4412
	11th to 20th	Night	40% 6% 12% 25% 15% 3	3626
	21st to end	Night	41% 6% 13% 23% 15% 2	3950
June	1st to 10th	Night	31% 6% 16% 27% 17% 4	1600
	11th to 20th	Night	26% 5% 11% 28% 26% 3	1600
	21st to end	Night	24% 6% 16% 29% 20% 7%	1600
July	1st to 10th	Night	34% 5% 11% 29% 19% 2	3580
	11th to 20th	Night	31% 6% 16% 31% 14% 2	3572
	21st to end	Night	33% 7% 20% 27% 12% 1	4202
August	1st to 10th	Night	23% 7% 17% 34% 18% 1	2862
	11th to 20th	Night	36% 5% 13% 26% 19% 2	3129
	21st to end	Night	23% 5% 15% 35% 18% 3	3476
September	1st to 10th	Night	33% 4 9% 25% 23% 6%	1900
	11th to 20th	Night	31% 5% 8% 26% 25% 4	1900
	21st to end	Night	31% 6% 8% 25% 27% 4	2102
October	1st to 10th	Night	16% 2 6% 18% 50% 8%	3800
	11th to 20th	Night	22% 3 7% 23% 36% 10%	3779
	21st to end	Night	31% 3 9% 26% 27% 5%	4137
November	1st to 10th	Night	31% 4 11% 25% 23% 6%	2536
	11th to 20th	Night	37% 3 8% 24% 23% 4	2600
	21st to end	Night	43% 3 11% 25% 17% 2	2680
December	1st to 10th	Night	40% 4 9% 32% 15% 1	4580
	11th to 20th	Night	45% 3 13% 22% 17% 1	5201
	21st to end	Night	65% 5% 11% 13% 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	23% 23% 36% 14% 4	300
	11th to 20th	Day	29% 23% 36% 11% 1	300
	21st to end	Day	22% 24% 38% 13% 2	330
February	1st to 10th	Day	25% 22% 40% 11% 2	300
	11th to 20th	Day	28% 29% 28% 12% 3	300
	21st to end	Day	29% 24% 36% 9% 1	249
March	1st to 10th	Day	22% 22% 45% 11% 1	300
	11th to 20th	Day	31% 22% 32% 11% 3 1	300
	21st to end	Day	33% 21% 31% 12% 4	448
April	1st to 10th	Day	23% 18% 35% 15% 7% 1	500
	11th to 20th	Day	27% 17% 30% 18% 7%	492
	21st to end	Day	25% 15% 38% 17% 6%	500
May	1st to 10th	Day	21% 15% 34% 22% 6% 1	503
	11th to 20th	Day	18% 14% 30% 24% 11% 2	704
	21st to end	Day	14% 9% 33% 27% 11% 5%	789
June	1st to 10th	Day	12% 6% 25% 32% 20% 5%	730
	11th to 20th	Day	9% 5% 23% 40% 18% 5%	730
	21st to end	Day	6% 7% 31% 36% 16% 5%	730
July	1st to 10th	Day	11% 4 15% 30% 27% 12%	721
	11th to 20th	Day	6% 5% 23% 38% 23% 6%	718
	21st to end	Day	9% 3 20% 34% 29% 5%	794
August	1st to 10th	Day	8% 3 19% 37% 26% 7%	697
	11th to 20th	Day	13% 5% 18% 33% 23% 8%	479
	21st to end	Day	10% 2 15% 32% 30% 11%	539
September	1st to 10th	Day	13% 4 18% 30% 27% 8%	490
	11th to 20th	Day	11% 2 16% 32% 26% 13%	490
	21st to end	Day	11% 9% 22% 31% 20% 7%	472
October	1st to 10th	Day	10% 11% 27% 34% 12% 6%	300
	11th to 20th	Day	21% 18% 30% 22% 8%	300
	21st to end	Day	31% 18% 29% 17% 5%	330
November	1st to 10th	Day	17% 10% 38% 22% 9% 3	300
	11th to 20th	Day	23% 20% 31% 18% 7% 2	300
	21st to end	Day	28% 17% 31% 18% 6% 1	300
December	1st to 10th	Day	18% 24% 36% 14% 5% 3	300
	11th to 20th	Day	21% 22% 33% 20% 3	300
	21st to end	Day	29% 22% 36% 10% 3	330
January	1st to 10th	Night	21% 17% 38% 20% 4	1000
	11th to 20th	Night	23% 22% 34% 18% 4	1000
	21st to end	Night	19% 19% 39% 21% 2	1100
February	1st to 10th	Night	23% 18% 37% 20% 2	1000
	11th to 20th	Night	24% 19% 37% 19% 2	810
	21st to end	Night	22% 18% 38% 19% 2	660
March	1st to 10th	Night	23% 17% 37% 19% 4	775
	11th to 20th	Night	24% 12% 37% 21% 5%	600
	21st to end	Night	27% 15% 33% 21% 4	660
April	1st to 10th	Night	17% 16% 37% 24% 6%	600
	11th to 20th	Night	19% 14% 35% 24% 7%	593
	21st to end	Night	22% 12% 38% 23% 6%	600
May	1st to 10th	Night	18% 7% 36% 30% 8% 1	600
	11th to 20th	Night	19% 5% 29% 31% 14% 2	591
	21st to end	Night	12% 6% 26% 35% 19% 2	650
June	1st to 10th	Night	10% 4 22% 39% 24% 1	600
	11th to 20th	Night	8% 2 24% 46% 19% 1	600
	21st to end	Night	5% 3 25% 45% 20% 2	600
July	1st to 10th	Night	11% 2 12% 37% 32% 5%	585
	11th to 20th	Night	6% 2 16% 43% 30% 3	582
	21st to end	Night	7% 2 15% 42% 31% 3	657
August	1st to 10th	Night	7% 2 13% 41% 34% 3	579
	11th to 20th	Night	7% 2 14% 41% 31% 4	564
	21st to end	Night	6% 1 14% 39% 36% 3	627
September	1st to 10th	Night	13% 3 13% 37% 28% 5%	570
	11th to 20th	Night	10% 4 16% 33% 32% 5%	570
	21st to end	Night	11% 7% 23% 31% 24% 4	758
October	1st to 10th	Night	9% 5% 29% 31% 22% 4	760
	11th to 20th	Night	16% 13% 31% 28% 11% 1	760
	21st to end	Night	24% 11% 35% 24% 6%	836
November	1st to 10th	Night	13% 10% 39% 26% 11% 1	780
	11th to 20th	Night	18% 16% 34% 26% 6%	800
	21st to end	Night	24% 14% 33% 22% 6% 1	880
December	1st to 10th	Night	23% 14% 36% 23% 4 1	1000
	11th to 20th	Night	24% 13% 32% 27% 4	1000
	21st to end	Night	28% 17% 33% 20% 2	1100

Page generated at 2025-08-02 00:32:26. Version ac07

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