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 2025-03-27.

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% 5% 1 3 2 2 3 2 4 5% 64%	112962
	Night	11% 6% 1 3 2 3 3 2 4 5% 60%	115164
Transparency	Day	25% 2 7% 18% 25% 24%	112712
	Night	29% 2 7% 17% 23% 22%	115003
Seeing	Day	13% 12% 30% 29% 11% 4	15582
	Night	14% 4 16% 35% 25% 6%	25053

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	18% 8% 2 4 3 2 3 2 4 6% 48%	3639
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 3 3 55%	3916
	21st to end	Day	19% 9% 2 5% 3 3 3 2 4 5% 45%	4510
February	1st to 10th	Day	22% 8% 2 4 2 3 3 3 4 4 46%	1800
	11th to 20th	Day	15% 10% 2 4 3 4 3 4 4 6% 45%	1916
	21st to end	Day	20% 11% 2 5% 3 2 2 3 4 5% 45%	1650
March	1st to 10th	Day	18% 17% 3 8% 4 4 4 4 5% 6% 29%	3287
	11th to 20th	Day	15% 7% 1 4 3 2 3 2 3 6% 54%	3600
	21st to end	Day	17% 12% 3 7% 5% 4 4 4 6% 6% 32%	4078
April	1st to 10th	Day	15% 10% 1 5% 3 3 3 4 5% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 4 6% 52%	2249
	21st to end	Day	12% 4 1 3 3 2 3 3 5% 6% 58%	2280
May	1st to 10th	Day	8% 3 1 3 2 2 2 2 4 4 69%	3879
	11th to 20th	Day	4 2 1 2 1 2 2 2 3 5% 77%	4367
	21st to end	Day	5% 3 1 3 2 2 2 2 3 5% 72%	4813
June	1st to 10th	Day	6% 4 1 3 3 3 3 2 5% 6% 63%	2660
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 4 79%	2660
	21st to end	Day	2 2 1 1 1 1 2 1 3 3 83%	2660
July	1st to 10th	Day	1 1 1 1 1 2 92%	4420
	11th to 20th	Day	2 1 1 1 1 2 2 3 4 84%	4420
	21st to end	Day	1 1 1 1 2 2 2 3 87%	4530
August	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 79%	4091
	11th to 20th	Day	1 1 1 1 1 1 2 3 89%	3729
	21st to end	Day	2 1 1 1 1 1 1 2 2 89%	4180
September	1st to 10th	Day	2 1 1 1 2 1 2 3 87%	2280
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 4 75%	2280
	21st to end	Day	3 2 1 2 2 1 2 2 3 4 78%	2068
October	1st to 10th	Day	3 2 1 3 2 2 3 3 4 4 72%	3180
	11th to 20th	Day	6% 2 1 2 2 2 2 2 3 4 75%	3161
	21st to end	Day	5% 3 1 4 3 3 3 3 4 6% 64%	3448
November	1st to 10th	Day	12% 5% 1 4 4 3 4 2 4 8% 53%	1774
	11th to 20th	Day	13% 5% 1 4 3 3 4 3 5% 7% 53%	1800
	21st to end	Day	13% 6% 2 4 3 2 3 3 5% 5% 54%	1705
December	1st to 10th	Day	9% 4 1 3 3 3 4 3 4 7% 58%	2620
	11th to 20th	Day	27% 9% 2 5% 3 2 3 3 5% 6% 35%	3028
	21st to end	Day	24% 7% 2 5% 3 3 4 4 7% 8% 31%	4004
January	1st to 10th	Night	12% 8% 2 5% 3 3 3 3 5% 9% 46%	6306
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 3 4 53%	6039
	21st to end	Night	17% 8% 2 6% 3 4 3 3 4 4 47%	6468
February	1st to 10th	Night	19% 8% 2 4 3 3 3 2 3 5% 47%	2600
	11th to 20th	Night	15% 10% 1 5% 3 3 4 3 5% 6% 44%	2370
	21st to end	Night	17% 10% 2 4 2 3 3 4 4 5% 47%	1980
March	1st to 10th	Night	22% 18% 2 5% 3 3 4 2 4 5% 33%	3804
	11th to 20th	Night	18% 7% 1 3 2 2 2 2 4 4 55%	3600
	21st to end	Night	22% 11% 2 5% 3 4 5% 3 5% 6% 34%	3712
April	1st to 10th	Night	14% 7% 2 5% 4 4 3 3 5% 5% 47%	1900
	11th to 20th	Night	12% 6% 2 4 4 3 3 2 4 6% 53%	1879
	21st to end	Night	10% 5% 1 3 2 2 2 3 5% 6% 59%	1900
May	1st to 10th	Night	5% 6% 1 2 2 2 2 2 4 5% 69%	3096
	11th to 20th	Night	5% 2 1 1 1 2 1 2 3 5% 78%	2521
	21st to end	Night	5% 2 1 2 1 1 2 2 4 4 76%	2762
June	1st to 10th	Night	6% 4 1 3 2 2 2 3 4 5% 66%	1520
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 4 81%	1520
	21st to end	Night	2 1 1 2 2 1 2 1 2 3 82%	1520
July	1st to 10th	Night	1 1 1 1 2 93%	2540
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 3 79%	2540
	21st to end	Night	1 1 1 1 1 1 2 1 2 3 85%	3014
August	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 76%	2848
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 88%	3120
	21st to end	Night	4 1 1 1 1 1 1 1 2 88%	3498
September	1st to 10th	Night	2 1 1 1 1 1 1 2 3 88%	1900
	11th to 20th	Night	3 2 1 2 1 2 2 1 3 3 80%	1900
	21st to end	Night	3 1 1 2 2 2 1 2 3 4 80%	2106
October	1st to 10th	Night	3 2 1 3 2 2 2 2 3 3 77%	3800
	11th to 20th	Night	4 1 1 2 1 2 2 2 3 5% 78%	3777
	21st to end	Night	9% 3 1 3 3 3 3 3 4 5% 65%	4121
November	1st to 10th	Night	13% 6% 1 3 2 2 2 2 5% 7% 57%	2525
	11th to 20th	Night	11% 5% 1 3 2 3 3 3 4 7% 56%	2600
	21st to end	Night	13% 6% 2 3 3 3 2 3 4 5% 55%	2690
December	1st to 10th	Night	11% 4 1 3 2 2 3 3 4 6% 61%	4560
	11th to 20th	Night	25% 9% 2 4 3 4 4 3 6% 9% 30%	5176
	21st to end	Night	23% 11% 3 7% 4 4 4 3 5% 6% 31%	6952

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	38% 2 12% 21% 17% 9%	3639
	11th to 20th	Day	36% 2 7% 14% 24% 17%	3916
	21st to end	Day	42% 2 6% 14% 19% 15%	4510
February	1st to 10th	Day	43% 2 7% 19% 17% 11%	1800
	11th to 20th	Day	41% 3 9% 18% 23% 7%	1926
	21st to end	Day	44% 2 8% 21% 17% 8%	1650
March	1st to 10th	Day	55% 4 13% 14% 11% 4	3287
	11th to 20th	Day	34% 2 12% 30% 13% 10%	3600
	21st to end	Day	50% 4 14% 20% 9% 2	4078
April	1st to 10th	Day	39% 4 15% 22% 16% 4	2280
	11th to 20th	Day	33% 3 13% 28% 17% 5%	2252
	21st to end	Day	27% 4 13% 30% 23% 3	2280
May	1st to 10th	Day	21% 2 10% 31% 21% 15%	3879
	11th to 20th	Day	13% 2 6% 28% 42% 9%	4370
	21st to end	Day	17% 2 11% 35% 31% 4	4814
June	1st to 10th	Day	23% 3 10% 25% 29% 11%	2660
	11th to 20th	Day	10% 2 8% 22% 37% 21%	2660
	21st to end	Day	10% 2 2 13% 34% 39%	2660
July	1st to 10th	Day	3 1 1 6% 15% 75%	4351
	11th to 20th	Day	7% 2 2 7% 17% 64%	4345
	21st to end	Day	6% 1 1 4 27% 60%	4387
August	1st to 10th	Day	11% 2 2 7% 22% 57%	4076
	11th to 20th	Day	4 1 1 6% 38% 50%	3765
	21st to end	Day	7% 1 2 8% 42% 41%	4179
September	1st to 10th	Day	5% 2 2 8% 32% 51%	2280
	11th to 20th	Day	14% 3 4 21% 38% 21%	2280
	21st to end	Day	13% 2 4 19% 35% 28%	2068
October	1st to 10th	Day	16% 1 3 12% 27% 40%	3180
	11th to 20th	Day	15% 2 4 19% 40% 20%	3161
	21st to end	Day	22% 2 6% 20% 35% 15%	3448
November	1st to 10th	Day	32% 2 8% 19% 24% 14%	1774
	11th to 20th	Day	32% 4 9% 18% 26% 12%	1800
	21st to end	Day	32% 3 7% 17% 27% 14%	1705
December	1st to 10th	Day	26% 2 6% 22% 27% 16%	2620
	11th to 20th	Day	52% 2 7% 14% 16% 9%	3028
	21st to end	Day	50% 3 12% 19% 11% 5%	4004
January	1st to 10th	Night	36% 2 16% 21% 15% 10%	6306
	11th to 20th	Night	37% 2 6% 14% 25% 17%	6039
	21st to end	Night	42% 2 6% 12% 17% 21%	6468
February	1st to 10th	Night	42% 3 5% 17% 20% 14%	2600
	11th to 20th	Night	40% 3 7% 21% 20% 9%	2371
	21st to end	Night	39% 3 8% 22% 20% 8%	1980
March	1st to 10th	Night	55% 2 10% 16% 11% 5%	3804
	11th to 20th	Night	34% 2 8% 25% 20% 11%	3600
	21st to end	Night	50% 5% 11% 22% 9% 3	3712
April	1st to 10th	Night	40% 3 13% 19% 19% 6%	1900
	11th to 20th	Night	33% 4 13% 26% 16% 8%	1879
	21st to end	Night	26% 5% 11% 32% 20% 6%	1900
May	1st to 10th	Night	19% 2 14% 28% 22% 15%	3096
	11th to 20th	Night	11% 3 7% 26% 37% 16%	2521
	21st to end	Night	13% 3 11% 40% 28% 5%	2762
June	1st to 10th	Night	20% 4 12% 25% 27% 13%	1520
	11th to 20th	Night	9% 2 6% 24% 35% 24%	1520
	21st to end	Night	11% 1 3 12% 33% 40%	1520
July	1st to 10th	Night	3 1 1 5% 15% 75%	2500
	11th to 20th	Night	12% 2 2 6% 15% 63%	2492
	21st to end	Night	8% 2 1 4 22% 63%	2919
August	1st to 10th	Night	13% 2 1 8% 22% 55%	2839
	11th to 20th	Night	5% 1 1 6% 34% 52%	3150
	21st to end	Night	8% 1 1 12% 35% 43%	3498
September	1st to 10th	Night	6% 1 2 8% 32% 51%	1900
	11th to 20th	Night	12% 2 5% 17% 42% 23%	1900
	21st to end	Night	12% 1 3 21% 33% 30%	2106
October	1st to 10th	Night	15% 2 3 10% 24% 45%	3800
	11th to 20th	Night	11% 2 4 24% 37% 21%	3777
	21st to end	Night	23% 2 3 19% 34% 19%	4121
November	1st to 10th	Night	29% 3 9% 20% 22% 17%	2525
	11th to 20th	Night	29% 3 8% 21% 25% 13%	2600
	21st to end	Night	33% 2 6% 18% 24% 17%	2690
December	1st to 10th	Night	26% 3 6% 21% 26% 19%	4560
	11th to 20th	Night	51% 3 7% 13% 19% 7%	5176
	21st to end	Night	54% 3 11% 16% 10% 6%	6952

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	38% 13% 21% 19% 8% 3	200
	11th to 20th	Day	23% 10% 23% 28% 12% 3	320
	21st to end	Day	25% 10% 23% 26% 13% 3	440
February	1st to 10th	Day	31% 12% 21% 23% 12% 2	400
	11th to 20th	Day	25% 16% 31% 20% 7% 1	479
	21st to end	Day	22% 18% 29% 21% 8% 1	414
March	1st to 10th	Day	33% 19% 26% 15% 5% 2	500
	11th to 20th	Day	28% 20% 26% 16% 8% 2	500
	21st to end	Day	26% 18% 29% 20% 6%	531
April	1st to 10th	Day	22% 19% 33% 18% 7% 1	460
	11th to 20th	Day	20% 23% 25% 25% 6% 1	452
	21st to end	Day	12% 18% 30% 26% 12% 2	460
May	1st to 10th	Day	12% 21% 31% 25% 8% 3	463
	11th to 20th	Day	12% 16% 28% 27% 11% 5%	488
	21st to end	Day	12% 12% 29% 27% 14% 6%	529
June	1st to 10th	Day	8% 13% 30% 32% 10% 8%	490
	11th to 20th	Day	6% 16% 36% 28% 10% 6%	490
	21st to end	Day	3 10% 36% 32% 11% 8%	490
July	1st to 10th	Day	1 4 41% 34% 12% 8%	486
	11th to 20th	Day	1 3 35% 41% 11% 9%	484
	21st to end	Day	2 3 36% 38% 14% 7%	531
August	1st to 10th	Day	4 6% 39% 34% 12% 6%	487
	11th to 20th	Day	2 7% 36% 37% 12% 6%	477
	21st to end	Day	2 5% 35% 41% 14% 4	539
September	1st to 10th	Day	3 6% 42% 34% 13% 2	490
	11th to 20th	Day	7% 12% 34% 34% 12% 1	490
	21st to end	Day	4 11% 35% 40% 9% 2	490
October	1st to 10th	Day	6% 12% 32% 35% 12% 3	490
	11th to 20th	Day	8% 10% 29% 32% 17% 4	490
	21st to end	Day	10% 11% 29% 31% 15% 5%	539
November	1st to 10th	Day	17% 10% 23% 25% 20% 5%	310
	11th to 20th	Day	16% 8% 24% 30% 17% 7%	300
	21st to end	Day	19% 7% 24% 28% 18% 3	253
December	1st to 10th	Day	20% 9% 20% 27% 16% 10%	200
	11th to 20th	Day	21% 15% 19% 28% 11% 7%	200
	21st to end	Day	26% 12% 23% 21% 16% 2	220
January	1st to 10th	Night	23% 6% 22% 26% 16% 7%	800
	11th to 20th	Night	18% 7% 23% 35% 14% 4	800
	21st to end	Night	22% 4 22% 29% 17% 5%	880
February	1st to 10th	Night	23% 6% 22% 30% 16% 4	800
	11th to 20th	Night	20% 8% 21% 29% 17% 4	784
	21st to end	Night	23% 7% 21% 34% 13% 2	660
March	1st to 10th	Night	27% 7% 18% 30% 15% 4	800
	11th to 20th	Night	26% 6% 16% 27% 20% 6%	800
	21st to end	Night	24% 6% 18% 31% 19% 2	862
April	1st to 10th	Night	19% 5% 18% 37% 19% 3	760
	11th to 20th	Night	16% 7% 17% 36% 19% 6%	751
	21st to end	Night	11% 5% 15% 37% 23% 8%	760
May	1st to 10th	Night	11% 4 21% 35% 23% 6%	715
	11th to 20th	Night	9% 2 11% 40% 32% 5%	379
	21st to end	Night	8% 4 10% 37% 35% 7%	411
June	1st to 10th	Night	8% 2 11% 35% 36% 8%	380
	11th to 20th	Night	2 1 13% 39% 38% 6%	380
	21st to end	Night	2 10% 38% 40% 9%	380
July	1st to 10th	Night	1 6% 39% 47% 7%	370
	11th to 20th	Night	2 1 5% 44% 40% 8%	368
	21st to end	Night	2 1 7% 32% 43% 15%	568
August	1st to 10th	Night	2 1 10% 39% 40% 8%	567
	11th to 20th	Night	1 1 11% 41% 36% 10%	746
	21st to end	Night	2 1 7% 42% 42% 6%	836
September	1st to 10th	Night	2 1 8% 41% 41% 7%	760
	11th to 20th	Night	4 1 12% 37% 35% 11%	760
	21st to end	Night	4 1 15% 40% 29% 11%	760
October	1st to 10th	Night	7% 3 14% 45% 24% 9%	760
	11th to 20th	Night	8% 4 14% 38% 30% 7%	760
	21st to end	Night	9% 5% 17% 33% 23% 13%	836
November	1st to 10th	Night	17% 2 15% 33% 25% 9%	780
	11th to 20th	Night	13% 5% 21% 36% 19% 7%	800
	21st to end	Night	16% 7% 23% 32% 17% 6%	800
December	1st to 10th	Night	20% 7% 19% 33% 15% 6%	800
	11th to 20th	Night	28% 4 20% 29% 16% 4	800
	21st to end	Night	29% 6% 21% 27% 15% 2	880

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