Rancho Del Sol Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rancho Del Sol Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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% 4 1 3 2 2 2 2 3 5% 66%	146862
	Night	11% 6% 1 3 2 2 3 2 4 5% 61%	143574
Transparency	Day	24% 2 6% 18% 26% 23%	141822
	Night	28% 2 7% 18% 24% 21%	139599
Seeing	Day	13% 12% 31% 29% 11% 4	17393
	Night	14% 4 17% 35% 24% 6%	26689

By Month

Percentage Cloud Cover SW (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	21% 9% 2 4 2 2 3 3 5% 6% 45%	4722
	11th to 20th	Day	12% 5% 2 4 3 2 2 2 4 4 60%	5100
	21st to end	Day	14% 8% 2 5% 4 4 4 3 5% 7% 45%	5843
February	1st to 10th	Day	21% 8% 2 4 2 3 3 2 4 5% 46%	1890
	11th to 20th	Day	15% 11% 2 4 3 4 3 3 4 6% 44%	2015
	21st to end	Day	20% 10% 2 5% 3 3 2 3 4 5% 44%	1730
March	1st to 10th	Day	16% 16% 3 7% 4 4 4 3 5% 6% 33%	4680
	11th to 20th	Day	17% 10% 2 4 3 2 3 2 3 6% 49%	5110
	21st to end	Day	19% 11% 2 6% 4 3 4 3 5% 6% 35%	5875
April	1st to 10th	Day	14% 10% 1 5% 3 3 3 4 5% 6% 46%	2520
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 4 6% 53%	2489
	21st to end	Day	12% 5% 2 3 3 2 3 3 5% 6% 56%	2520
May	1st to 10th	Day	7% 4 1 4 2 2 2 2 3 4 68%	7187
	11th to 20th	Day	4 2 1 1 1 1 2 2 3 5% 78%	8150
	21st to end	Day	4 2 1 3 1 2 2 2 3 5% 75%	8992
June	1st to 10th	Day	5% 4 1 3 3 3 2 2 5% 6% 65%	2940
	11th to 20th	Day	2 2 1 2 1 1 2 2 4 4 80%	2940
	21st to end	Day	2 2 1 1 1 1 1 1 3 3 84%	2940
July	1st to 10th	Day	1 1 1 1 1 1 2 93%	6459
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	6460
	21st to end	Day	1 1 1 1 1 2 2 2 3 84%	6650
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 3 85%	5777
	11th to 20th	Day	1 1 1 1 1 1 2 92%	5369
	21st to end	Day	2 1 1 2 1 1 2 2 2 3 83%	5984
September	1st to 10th	Day	2 1 1 1 1 2 1 2 3 86%	2400
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 4 75%	2400
	21st to end	Day	3 2 1 3 2 1 2 2 3 4 78%	2177
October	1st to 10th	Day	3 2 1 3 2 2 3 3 4 4 72%	3280
	11th to 20th	Day	6% 2 1 2 2 2 2 2 3 4 75%	3261
	21st to end	Day	6% 3 1 4 3 3 3 3 4 6% 64%	3558
November	1st to 10th	Day	12% 5% 1 4 4 3 4 2 5% 7% 54%	1864
	11th to 20th	Day	14% 5% 1 4 4 4 4 3 5% 6% 51%	1890
	21st to end	Day	13% 6% 2 3 3 2 3 3 5% 5% 56%	1790
December	1st to 10th	Day	9% 3 1 3 3 3 3 3 4 8% 59%	2700
	11th to 20th	Day	26% 9% 2 5% 3 2 3 3 5% 6% 35%	3108
	21st to end	Day	25% 7% 2 5% 3 3 4 4 7% 7% 32%	4092
January	1st to 10th	Night	18% 9% 2 5% 3 3 3 3 5% 8% 41%	8172
	11th to 20th	Night	12% 4 1 4 2 3 3 3 4 5% 59%	7861
	21st to end	Night	14% 8% 2 6% 4 4 3 3 4 6% 46%	8371
February	1st to 10th	Night	18% 8% 2 4 3 3 3 2 3 5% 48%	2730
	11th to 20th	Night	15% 11% 2 5% 3 3 4 3 5% 6% 43%	2491
	21st to end	Night	17% 10% 2 4 2 3 3 4 4 5% 47%	2076
March	1st to 10th	Night	19% 16% 2 5% 3 3 4 2 4 6% 37%	5419
	11th to 20th	Night	20% 12% 2 3 2 2 2 2 3 5% 48%	5110
	21st to end	Night	22% 11% 2 5% 3 4 4 3 5% 6% 36%	5341
April	1st to 10th	Night	14% 8% 2 5% 4 4 3 3 5% 5% 48%	2100
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 5% 7% 52%	2079
	21st to end	Night	10% 5% 1 3 2 2 2 3 5% 6% 58%	2100
May	1st to 10th	Night	5% 5% 1 3 2 2 3 2 4 4 67%	5767
	11th to 20th	Night	3 2 1 2 2 2 1 2 3 4 79%	4705
	21st to end	Night	3 2 1 2 2 2 2 2 3 4 77%	5181
June	1st to 10th	Night	6% 4 1 3 2 2 3 3 4 5% 67%	1680
	11th to 20th	Night	1 1 1 1 2 2 1 3 4 82%	1680
	21st to end	Night	2 1 1 2 2 1 2 1 2 3 83%	1680
July	1st to 10th	Night	1 1 1 1 2 94%	3720
	11th to 20th	Night	1 2 1 2 1 1 1 1 2 2 84%	3720
	21st to end	Night	1 1 1 2 2 2 3 2 3 5% 78%	4422
August	1st to 10th	Night	1 2 1 2 1 1 2 2 3 4 83%	4035
	11th to 20th	Night	1 1 1 1 1 2 2 91%	4500
	21st to end	Night	3 1 2 1 2 2 2 2 3 82%	5015
September	1st to 10th	Night	2 1 1 1 1 1 1 2 3 88%	2000
	11th to 20th	Night	3 2 1 2 1 2 2 1 3 3 79%	2000
	21st to end	Night	3 2 1 2 2 2 1 2 3 4 79%	2217
October	1st to 10th	Night	3 2 1 3 2 2 2 2 3 3 77%	3920
	11th to 20th	Night	4 2 1 2 1 2 2 2 3 5% 78%	3897
	21st to end	Night	9% 3 1 3 3 3 3 3 4 5% 65%	4253
November	1st to 10th	Night	13% 6% 1 3 2 2 2 3 5% 7% 57%	2655
	11th to 20th	Night	12% 6% 1 4 3 3 3 3 4 7% 54%	2730
	21st to end	Night	12% 6% 2 3 3 3 2 3 5% 5% 56%	2825
December	1st to 10th	Night	11% 4 1 3 2 2 3 3 4 6% 62%	4700
	11th to 20th	Night	25% 9% 2 4 3 4 4 3 6% 9% 31%	5316
	21st to end	Night	23% 11% 3 7% 4 4 4 3 4 6% 31%	7106

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	41% 2 12% 21% 16% 7%	4723
	11th to 20th	Day	30% 2 6% 13% 24% 27%	5100
	21st to end	Day	40% 2 6% 18% 20% 14%	5843
February	1st to 10th	Day	43% 2 7% 20% 17% 12%	1890
	11th to 20th	Day	42% 3 8% 18% 22% 6%	2025
	21st to end	Day	44% 2 9% 20% 17% 8%	1730
March	1st to 10th	Day	51% 3 10% 15% 16% 5%	4680
	11th to 20th	Day	40% 2 13% 27% 11% 7%	5110
	21st to end	Day	49% 4 14% 21% 11% 2	5875
April	1st to 10th	Day	38% 4 14% 23% 17% 4	2520
	11th to 20th	Day	33% 3 13% 28% 17% 6%	2492
	21st to end	Day	28% 4 13% 30% 23% 3	2520
May	1st to 10th	Day	22% 2 7% 27% 29% 13%	7187
	11th to 20th	Day	11% 2 5% 28% 42% 12%	8153
	21st to end	Day	15% 2 7% 27% 42% 7%	8994
June	1st to 10th	Day	21% 3 9% 25% 31% 11%	2940
	11th to 20th	Day	9% 2 7% 21% 37% 24%	2940
	21st to end	Day	9% 1 2 12% 37% 38%	2940
July	1st to 10th	Day	3 1 1 5% 21% 70%	6391
	11th to 20th	Day	5% 1 2 5% 13% 74%	6385
	21st to end	Day	8% 2 2 13% 30% 46%	6462
August	1st to 10th	Day	11% 2 2 7% 23% 56%	4206
	11th to 20th	Day	4 1 1 6% 38% 50%	3885
	21st to end	Day	7% 1 2 8% 42% 40%	4311
September	1st to 10th	Day	6% 2 2 10% 32% 49%	2400
	11th to 20th	Day	14% 3 4 21% 39% 20%	2400
	21st to end	Day	13% 2 4 18% 34% 28%	2177
October	1st to 10th	Day	17% 2 3 13% 27% 39%	3280
	11th to 20th	Day	15% 2 4 19% 40% 19%	3261
	21st to end	Day	22% 2 5% 20% 35% 15%	3558
November	1st to 10th	Day	31% 2 9% 19% 25% 14%	1864
	11th to 20th	Day	34% 4 9% 17% 25% 11%	1890
	21st to end	Day	31% 3 7% 17% 28% 14%	1790
December	1st to 10th	Day	26% 2 6% 23% 27% 16%	2700
	11th to 20th	Day	52% 2 7% 14% 16% 9%	3108
	21st to end	Day	51% 3 12% 19% 11% 5%	4092
January	1st to 10th	Night	42% 2 15% 18% 16% 8%	8172
	11th to 20th	Night	30% 2 5% 12% 25% 27%	7861
	21st to end	Night	41% 2 5% 15% 18% 19%	8371
February	1st to 10th	Night	41% 3 5% 18% 20% 15%	2730
	11th to 20th	Night	42% 3 7% 20% 20% 8%	2492
	21st to end	Night	40% 3 9% 22% 19% 8%	2076
March	1st to 10th	Night	50% 2 8% 18% 14% 8%	5419
	11th to 20th	Night	41% 2 9% 23% 16% 8%	5110
	21st to end	Night	49% 4 12% 22% 11% 3	5341
April	1st to 10th	Night	39% 4 12% 20% 20% 6%	2100
	11th to 20th	Night	33% 4 13% 26% 15% 9%	2079
	21st to end	Night	27% 5% 11% 32% 20% 5%	2100
May	1st to 10th	Night	22% 2 11% 27% 24% 15%	5765
	11th to 20th	Night	12% 2 6% 27% 39% 15%	4705
	21st to end	Night	13% 2 7% 31% 41% 6%	5182
June	1st to 10th	Night	20% 4 11% 25% 28% 13%	1680
	11th to 20th	Night	9% 2 5% 22% 35% 27%	1680
	21st to end	Night	10% 1 3 12% 36% 38%	1680
July	1st to 10th	Night	2 1 1 4 24% 69%	3680
	11th to 20th	Night	9% 2 1 4 14% 70%	3672
	21st to end	Night	10% 3 2 12% 23% 49%	4298
August	1st to 10th	Night	12% 2 1 8% 23% 54%	2929
	11th to 20th	Night	5% 1 1 7% 34% 52%	3250
	21st to end	Night	9% 1 1 12% 36% 42%	3608
September	1st to 10th	Night	6% 1 2 9% 32% 49%	2000
	11th to 20th	Night	13% 2 5% 18% 41% 22%	2000
	21st to end	Night	12% 1 4 20% 33% 29%	2217
October	1st to 10th	Night	15% 2 3 11% 25% 44%	3920
	11th to 20th	Night	12% 2 4 25% 37% 20%	3897
	21st to end	Night	23% 2 3 19% 34% 18%	4253
November	1st to 10th	Night	28% 3 9% 21% 22% 17%	2655
	11th to 20th	Night	31% 3 8% 21% 24% 12%	2730
	21st to end	Night	31% 2 6% 19% 24% 17%	2825
December	1st to 10th	Night	25% 3 6% 21% 26% 19%	4700
	11th to 20th	Night	50% 3 7% 13% 19% 8%	5316
	21st to end	Night	55% 3 11% 16% 10% 6%	7106

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	39% 13% 20% 18% 8% 2	220
	11th to 20th	Day	21% 10% 23% 31% 13% 3	346
	21st to end	Day	23% 10% 24% 27% 13% 3	473
February	1st to 10th	Day	29% 11% 23% 23% 12% 2	430
	11th to 20th	Day	27% 17% 30% 19% 6% 1	518
	21st to end	Day	23% 17% 30% 21% 8% 1	446
March	1st to 10th	Day	32% 19% 27% 16% 4 2	540
	11th to 20th	Day	26% 19% 27% 17% 8% 2	540
	21st to end	Day	26% 18% 31% 20% 6%	594
April	1st to 10th	Day	22% 18% 31% 20% 7% 1	540
	11th to 20th	Day	20% 23% 26% 24% 6% 2	532
	21st to end	Day	14% 17% 31% 24% 12% 2	540
May	1st to 10th	Day	12% 20% 33% 24% 8% 3	543
	11th to 20th	Day	13% 17% 29% 26% 10% 4	568
	21st to end	Day	11% 12% 30% 28% 13% 7%	617
June	1st to 10th	Day	7% 13% 31% 31% 9% 8%	570
	11th to 20th	Day	5% 14% 37% 27% 10% 6%	570
	21st to end	Day	5% 10% 36% 30% 11% 8%	570
July	1st to 10th	Day	1 7% 41% 32% 12% 7%	566
	11th to 20th	Day	2 3 37% 38% 12% 9%	564
	21st to end	Day	2 3 38% 37% 13% 7%	616
August	1st to 10th	Day	4 6% 39% 34% 12% 5%	527
	11th to 20th	Day	2 6% 35% 38% 11% 7%	517
	21st to end	Day	2 5% 35% 41% 14% 4	583
September	1st to 10th	Day	3 7% 41% 34% 13% 2	530
	11th to 20th	Day	7% 12% 33% 34% 13% 1	530
	21st to end	Day	4 11% 34% 39% 9% 2	530
October	1st to 10th	Day	7% 12% 31% 34% 12% 3	530
	11th to 20th	Day	8% 11% 28% 32% 16% 4	530
	21st to end	Day	11% 10% 29% 31% 14% 5%	583
November	1st to 10th	Day	16% 9% 22% 28% 19% 6%	340
	11th to 20th	Day	20% 7% 23% 28% 15% 6%	330
	21st to end	Day	18% 7% 25% 28% 18% 4	278
December	1st to 10th	Day	18% 8% 20% 28% 17% 9%	220
	11th to 20th	Day	21% 14% 20% 27% 11% 7%	220
	21st to end	Day	28% 12% 23% 21% 15% 2	242
January	1st to 10th	Night	24% 6% 23% 25% 15% 7%	840
	11th to 20th	Night	17% 7% 23% 36% 14% 3	840
	21st to end	Night	21% 5% 24% 29% 16% 5%	924
February	1st to 10th	Night	22% 5% 23% 30% 16% 3	840
	11th to 20th	Night	21% 8% 21% 29% 17% 4	824
	21st to end	Night	23% 7% 22% 33% 12% 2	692
March	1st to 10th	Night	26% 7% 20% 30% 14% 4	840
	11th to 20th	Night	24% 6% 17% 28% 19% 5%	840
	21st to end	Night	24% 6% 19% 32% 18% 2	924
April	1st to 10th	Night	19% 5% 19% 37% 18% 3	840
	11th to 20th	Night	15% 6% 19% 36% 19% 5%	831
	21st to end	Night	12% 5% 16% 38% 22% 7%	840
May	1st to 10th	Night	11% 4 22% 36% 22% 5%	790
	11th to 20th	Night	9% 3 15% 39% 29% 5%	419
	21st to end	Night	7% 4 11% 38% 33% 6%	455
June	1st to 10th	Night	7% 2 11% 38% 34% 8%	420
	11th to 20th	Night	2 1 13% 43% 35% 6%	420
	21st to end	Night	2 11% 41% 37% 8%	420
July	1st to 10th	Night	10% 40% 43% 6%	410
	11th to 20th	Night	2 1 8% 45% 37% 7%	408
	21st to end	Night	2 1 8% 34% 42% 13%	625
August	1st to 10th	Night	2 1 11% 40% 39% 8%	597
	11th to 20th	Night	1 1 11% 41% 37% 9%	786
	21st to end	Night	2 1 7% 43% 41% 6%	880
September	1st to 10th	Night	2 1 10% 42% 39% 7%	800
	11th to 20th	Night	4 1 12% 38% 34% 11%	800
	21st to end	Night	4 1 15% 41% 28% 11%	800
October	1st to 10th	Night	7% 3 15% 44% 23% 8%	800
	11th to 20th	Night	8% 4 15% 39% 28% 7%	800
	21st to end	Night	9% 5% 18% 34% 23% 12%	880
November	1st to 10th	Night	17% 2 15% 34% 24% 8%	820
	11th to 20th	Night	14% 5% 21% 35% 18% 7%	840
	21st to end	Night	15% 6% 25% 32% 16% 6%	840
December	1st to 10th	Night	20% 7% 20% 34% 15% 5%	840
	11th to 20th	Night	27% 4 21% 29% 15% 3	840
	21st to end	Night	30% 6% 22% 26% 14% 2	924

Page generated at 2026-07-28 00:25:26. Version ac07

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