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 2024-02-15.

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% 1 3 3 2 3 3 4 5% 60%	78433
	Night	13% 5% 2 3 3 2 3 3 4 5% 56%	80675
Transparency	Day	28% 3 8% 19% 24% 19%	78146
	Night	31% 3 8% 18% 22% 18%	80483
Seeing	Day	13% 12% 30% 29% 11% 4	14111
	Night	14% 4 15% 34% 26% 7%	23598

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	22% 11% 2 3 4 2 3 3 4 5% 42%	2539
	11th to 20th	Day	23% 8% 3 6% 5% 3 4 3 5% 5% 36%	2732
	21st to end	Day	23% 10% 2 5% 5% 3 4 3 5% 6% 33%	3146
February	1st to 10th	Day	21% 8% 2 4 2 2 3 3 4 5% 46%	1710
	11th to 20th	Day	15% 10% 2 4 3 2 3 4 4 6% 47%	1758
	21st to end	Day	21% 11% 2 4 3 2 2 3 3 5% 44%	1480
March	1st to 10th	Day	20% 11% 2 6% 4 3 4 4 5% 5% 36%	1822
	11th to 20th	Day	21% 8% 2 4 3 2 3 4 4 8% 39%	1980
	21st to end	Day	18% 8% 2 5% 5% 3 4 4 5% 6% 40%	2285
April	1st to 10th	Day	16% 9% 2 4 4 2 3 5% 5% 7% 44%	2160
	11th to 20th	Day	14% 5% 2 3 4 2 4 4 4 6% 53%	2129
	21st to end	Day	12% 4 2 2 3 2 3 4 4 7% 58%	2160
May	1st to 10th	Day	8% 4 2 4 3 2 3 3 5% 6% 61%	2200
	11th to 20th	Day	7% 4 1 3 3 2 3 4 5% 7% 62%	2467
	21st to end	Day	9% 4 1 4 3 2 3 4 4 7% 58%	2723
June	1st to 10th	Day	6% 4 2 3 3 2 3 3 4 7% 64%	2520
	11th to 20th	Day	2 2 1 2 1 1 2 3 3 4 79%	2520
	21st to end	Day	3 2 1 1 2 1 2 1 2 3 83%	2520
July	1st to 10th	Day	1 1 1 1 2 2 2 3 87%	2520
	11th to 20th	Day	2 1 1 1 1 1 2 2 2 3 85%	2520
	21st to end	Day	1 1 1 2 2 1 2 3 2 4 81%	2595
August	1st to 10th	Day	3 1 1 1 2 1 2 2 3 4 81%	2328
	11th to 20th	Day	1 1 1 1 1 2 2 4 85%	2127
	21st to end	Day	1 1 1 1 1 1 2 2 2 88%	2376
September	1st to 10th	Day	3 1 1 1 2 3 87%	2160
	11th to 20th	Day	3 2 1 2 2 2 2 3 3 4 76%	2160
	21st to end	Day	3 2 1 2 2 1 2 2 3 4 77%	1960
October	1st to 10th	Day	5% 3 1 3 3 2 4 4 4 5% 68%	1800
	11th to 20th	Day	7% 2 1 3 3 2 4 3 3 5% 69%	1800
	21st to end	Day	8% 2 1 3 3 2 4 3 3 5% 65%	1980
November	1st to 10th	Day	12% 5% 2 4 4 2 4 3 4 7% 53%	1684
	11th to 20th	Day	12% 4 2 3 4 3 3 4 4 7% 54%	1710
	21st to end	Day	12% 5% 2 4 3 2 3 4 5% 6% 55%	1620
December	1st to 10th	Day	16% 5% 2 3 4 2 4 4 3 8% 48%	1520
	11th to 20th	Day	27% 10% 3 5% 4 2 3 4 4 5% 33%	1928
	21st to end	Day	15% 7% 2 5% 4 3 5% 5% 7% 8% 38%	2794
January	1st to 10th	Night	17% 9% 3 5% 4 2 3 4 5% 7% 42%	4406
	11th to 20th	Night	22% 8% 2 5% 5% 3 5% 5% 4 6% 35%	4230
	21st to end	Night	23% 10% 2 5% 5% 3 4 4 5% 5% 33%	4521
February	1st to 10th	Night	17% 8% 2 4 4 3 3 3 3 5% 48%	2470
	11th to 20th	Night	14% 8% 1 4 3 2 4 4 5% 7% 47%	2182
	21st to end	Night	18% 9% 2 3 3 2 3 4 4 5% 47%	1776
March	1st to 10th	Night	22% 11% 3 4 3 2 4 3 3 4 40%	2136
	11th to 20th	Night	20% 8% 2 4 3 2 3 3 5% 7% 42%	1980
	21st to end	Night	20% 7% 2 4 3 3 4 5% 3 5% 43%	2070
April	1st to 10th	Night	15% 7% 3 5% 5% 3 4 3 4 6% 47%	1800
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 4 7% 53%	1779
	21st to end	Night	10% 4 1 3 2 2 3 4 5% 7% 59%	1800
May	1st to 10th	Night	8% 4 2 3 3 2 3 4 5% 6% 61%	1758
	11th to 20th	Night	8% 3 1 2 2 2 2 3 5% 7% 64%	1421
	21st to end	Night	8% 3 1 3 3 1 4 3 5% 6% 62%	1552
June	1st to 10th	Night	6% 4 1 3 3 1 2 4 4 6% 66%	1440
	11th to 20th	Night	1 2 1 1 2 1 3 2 3 4 80%	1440
	21st to end	Night	2 1 1 2 2 1 2 2 2 3 82%	1440
July	1st to 10th	Night	1 1 1 2 1 2 3 88%	1440
	11th to 20th	Night	1 1 1 1 2 1 2 2 3 3 84%	1440
	21st to end	Night	1 2 1 2 3 1 3 2 3 4 78%	1720
August	1st to 10th	Night	2 3 1 2 2 1 2 3 3 4 78%	1611
	11th to 20th	Night	1 1 1 1 2 1 2 3 4 4 80%	1774
	21st to end	Night	2 1 1 1 1 2 1 2 2 87%	1980
September	1st to 10th	Night	2 1 1 2 1 1 1 2 3 88%	1800
	11th to 20th	Night	3 2 1 1 1 1 1 2 3 3 81%	1800
	21st to end	Night	3 2 1 2 2 1 1 2 3 4 79%	1995
October	1st to 10th	Night	6% 2 1 3 3 2 2 3 3 4 71%	2160
	11th to 20th	Night	7% 2 1 1 2 1 2 3 4 6% 71%	2160
	21st to end	Night	8% 2 1 2 2 1 2 3 3 5% 70%	2376
November	1st to 10th	Night	14% 6% 1 3 3 1 2 3 5% 7% 56%	2395
	11th to 20th	Night	11% 4 1 3 3 2 3 4 4 7% 57%	2470
	21st to end	Night	12% 6% 2 3 3 2 3 3 4 5% 56%	2555
December	1st to 10th	Night	19% 6% 2 3 3 2 3 4 3 6% 49%	2660
	11th to 20th	Night	28% 8% 3 4 4 2 4 3 5% 7% 33%	3276
	21st to end	Night	18% 9% 3 6% 5% 3 4 5% 5% 7% 35%	4862

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	46% 3 7% 23% 17% 4	2539
	11th to 20th	Day	51% 3 11% 15% 12% 8%	2732
	21st to end	Day	52% 3 9% 19% 12% 6%	3146
February	1st to 10th	Day	42% 2 7% 20% 18% 11%	1710
	11th to 20th	Day	38% 3 9% 18% 24% 7%	1768
	21st to end	Day	46% 2 8% 20% 17% 7%	1480
March	1st to 10th	Day	50% 5% 10% 16% 15% 4	1822
	11th to 20th	Day	45% 3 10% 22% 16% 5%	1980
	21st to end	Day	45% 5% 14% 19% 14% 4	2285
April	1st to 10th	Day	40% 4 14% 21% 16% 5%	2160
	11th to 20th	Day	33% 4 12% 27% 18% 6%	2132
	21st to end	Day	27% 4 13% 29% 23% 4	2160
May	1st to 10th	Day	25% 3 11% 30% 25% 7%	2200
	11th to 20th	Day	23% 4 9% 30% 26% 7%	2470
	21st to end	Day	27% 4 13% 31% 20% 5%	2724
June	1st to 10th	Day	23% 3 10% 25% 28% 11%	2520
	11th to 20th	Day	10% 2 8% 23% 35% 22%	2520
	21st to end	Day	10% 2 2 14% 35% 37%	2520
July	1st to 10th	Day	5% 1 2 10% 26% 55%	2451
	11th to 20th	Day	8% 2 4 12% 27% 48%	2445
	21st to end	Day	10% 2 3 6% 29% 50%	2451
August	1st to 10th	Day	11% 1 3 11% 27% 47%	2312
	11th to 20th	Day	5% 2 2 10% 36% 45%	2127
	21st to end	Day	7% 1 1 10% 39% 43%	2376
September	1st to 10th	Day	6% 2 2 8% 32% 51%	2160
	11th to 20th	Day	13% 3 3 21% 38% 22%	2160
	21st to end	Day	13% 2 4 19% 35% 26%	1960
October	1st to 10th	Day	21% 2 6% 19% 29% 24%	1800
	11th to 20th	Day	20% 3 6% 19% 28% 25%	1800
	21st to end	Day	23% 2 6% 19% 27% 22%	1980
November	1st to 10th	Day	33% 3 8% 18% 23% 15%	1684
	11th to 20th	Day	30% 4 9% 17% 27% 12%	1710
	21st to end	Day	31% 3 8% 17% 28% 13%	1620
December	1st to 10th	Day	38% 3 8% 18% 21% 12%	1520
	11th to 20th	Day	55% 3 5% 14% 14% 9%	1928
	21st to end	Day	43% 3 13% 21% 14% 6%	2794
January	1st to 10th	Night	43% 2 13% 23% 16% 4	4406
	11th to 20th	Night	52% 3 8% 15% 12% 10%	4230
	21st to end	Night	54% 3 8% 15% 13% 7%	4521
February	1st to 10th	Night	41% 3 5% 18% 20% 14%	2470
	11th to 20th	Night	38% 3 7% 21% 21% 10%	2183
	21st to end	Night	40% 3 8% 22% 20% 7%	1776
March	1st to 10th	Night	49% 3 9% 18% 16% 5%	2136
	11th to 20th	Night	42% 4 7% 21% 18% 8%	1980
	21st to end	Night	43% 6% 10% 20% 15% 6%	2070
April	1st to 10th	Night	41% 3 13% 18% 18% 6%	1800
	11th to 20th	Night	34% 4 12% 25% 16% 9%	1779
	21st to end	Night	25% 5% 12% 32% 20% 6%	1800
May	1st to 10th	Night	24% 3 13% 26% 25% 9%	1758
	11th to 20th	Night	20% 4 13% 29% 25% 10%	1421
	21st to end	Night	22% 5% 15% 30% 23% 6%	1552
June	1st to 10th	Night	20% 4 12% 25% 27% 13%	1440
	11th to 20th	Night	10% 3 6% 24% 34% 24%	1440
	21st to end	Night	11% 1 3 13% 34% 38%	1440
July	1st to 10th	Night	5% 2 2 9% 28% 55%	1400
	11th to 20th	Night	8% 2 3 9% 25% 52%	1392
	21st to end	Night	12% 3 2 7% 24% 53%	1624
August	1st to 10th	Night	12% 2 2 13% 25% 46%	1602
	11th to 20th	Night	9% 3 2 10% 29% 48%	1774
	21st to end	Night	8% 1 1 11% 30% 49%	1980
September	1st to 10th	Night	6% 1 2 8% 31% 51%	1800
	11th to 20th	Night	11% 2 5% 17% 42% 24%	1800
	21st to end	Night	12% 2 4 21% 34% 28%	1995
October	1st to 10th	Night	19% 3 6% 17% 29% 26%	2160
	11th to 20th	Night	16% 3 6% 17% 32% 26%	2160
	21st to end	Night	18% 2 5% 21% 29% 25%	2376
November	1st to 10th	Night	29% 3 9% 19% 22% 18%	2395
	11th to 20th	Night	28% 3 8% 21% 26% 14%	2470
	21st to end	Night	32% 2 6% 18% 24% 17%	2555
December	1st to 10th	Night	37% 3 7% 18% 19% 15%	2660
	11th to 20th	Night	52% 3 7% 13% 18% 7%	3276
	21st to end	Night	49% 3 12% 17% 13% 6%	4862

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% 12% 21% 17% 8% 3	180
	11th to 20th	Day	25% 9% 22% 29% 12% 3	294
	21st to end	Day	26% 10% 21% 26% 14% 3	407
February	1st to 10th	Day	29% 13% 21% 24% 12% 2	370
	11th to 20th	Day	23% 16% 30% 22% 8% 1	417
	21st to end	Day	24% 20% 26% 22% 7% 1	346
March	1st to 10th	Day	34% 19% 26% 15% 5% 2	420
	11th to 20th	Day	27% 20% 25% 16% 9% 3	420
	21st to end	Day	27% 18% 29% 20% 6%	462
April	1st to 10th	Day	23% 18% 34% 18% 6% 1	420
	11th to 20th	Day	21% 25% 25% 23% 5% 1	412
	21st to end	Day	12% 18% 31% 24% 13% 2	420
May	1st to 10th	Day	13% 21% 29% 26% 8% 3	423
	11th to 20th	Day	13% 17% 28% 27% 9% 5%	448
	21st to end	Day	13% 13% 28% 26% 14% 6%	485
June	1st to 10th	Day	9% 14% 29% 31% 10% 8%	450
	11th to 20th	Day	6% 16% 35% 28% 10% 6%	450
	21st to end	Day	3 10% 34% 33% 11% 8%	450
July	1st to 10th	Day	1 4 42% 33% 12% 9%	446
	11th to 20th	Day	1 3 36% 40% 11% 10%	444
	21st to end	Day	2 3 36% 39% 14% 7%	487
August	1st to 10th	Day	4 6% 39% 33% 11% 6%	447
	11th to 20th	Day	2 5% 35% 39% 12% 7%	437
	21st to end	Day	1 4 36% 41% 13% 4	495
September	1st to 10th	Day	3 5% 40% 35% 14% 2	450
	11th to 20th	Day	6% 11% 35% 34% 13% 2	450
	21st to end	Day	4 11% 34% 40% 9% 2	450
October	1st to 10th	Day	6% 12% 33% 34% 12% 3	450
	11th to 20th	Day	8% 10% 29% 32% 17% 4	450
	21st to end	Day	11% 10% 27% 31% 16% 5%	495
November	1st to 10th	Day	18% 8% 21% 26% 20% 6%	280
	11th to 20th	Day	14% 6% 22% 32% 18% 7%	270
	21st to end	Day	16% 7% 25% 29% 19% 4	228
December	1st to 10th	Day	22% 9% 19% 27% 14% 9%	180
	11th to 20th	Day	19% 16% 21% 26% 11% 7%	180
	21st to end	Day	24% 12% 22% 22% 18% 2	198
January	1st to 10th	Night	24% 6% 20% 26% 17% 7%	760
	11th to 20th	Night	19% 6% 21% 35% 15% 4	760
	21st to end	Night	23% 4 21% 28% 18% 6%	836
February	1st to 10th	Night	22% 5% 21% 31% 17% 4	760
	11th to 20th	Night	18% 8% 20% 30% 19% 5%	722
	21st to end	Night	24% 7% 17% 35% 14% 3	592
March	1st to 10th	Night	27% 7% 16% 30% 16% 4	720
	11th to 20th	Night	25% 6% 15% 27% 22% 6%	720
	21st to end	Night	25% 6% 17% 31% 20% 2	792
April	1st to 10th	Night	19% 4 17% 37% 20% 3	720
	11th to 20th	Night	16% 7% 17% 35% 19% 6%	711
	21st to end	Night	11% 6% 14% 38% 23% 8%	720
May	1st to 10th	Night	11% 4 20% 35% 25% 6%	678
	11th to 20th	Night	10% 2 11% 39% 33% 6%	359
	21st to end	Night	8% 4 9% 35% 36% 7%	389
June	1st to 10th	Night	8% 2 9% 34% 37% 9%	360
	11th to 20th	Night	2 1 13% 39% 39% 7%	360
	21st to end	Night	3 9% 37% 42% 9%	360
July	1st to 10th	Night	1 5% 38% 49% 7%	350
	11th to 20th	Night	2 5% 42% 42% 9%	348
	21st to end	Night	2 1 6% 30% 45% 15%	538
August	1st to 10th	Night	3 1 9% 37% 42% 8%	537
	11th to 20th	Night	1 1 9% 41% 38% 10%	706
	21st to end	Night	2 6% 41% 44% 6%	792
September	1st to 10th	Night	2 1 7% 40% 43% 8%	720
	11th to 20th	Night	4 1 11% 36% 37% 12%	720
	21st to end	Night	4 1 14% 39% 30% 12%	720
October	1st to 10th	Night	7% 3 13% 43% 25% 9%	720
	11th to 20th	Night	8% 4 13% 38% 30% 7%	720
	21st to end	Night	8% 6% 14% 34% 24% 14%	792
November	1st to 10th	Night	17% 2 13% 33% 26% 9%	740
	11th to 20th	Night	12% 4 20% 37% 20% 7%	760
	21st to end	Night	15% 6% 22% 33% 18% 6%	760
December	1st to 10th	Night	21% 7% 18% 33% 15% 6%	760
	11th to 20th	Night	27% 4 19% 29% 17% 4	760
	21st to end	Night	28% 6% 21% 28% 15% 3	836

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