Sky Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sky Ranch Observatory clear sky chart from 2005-11-06 to 2024-11-01.

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	2 3 1 3 3 3 4 4 6% 8% 62%	100871
	Night	3 4 2 4 4 4 4 4 6% 8% 59%	98311
Transparency	Day	19% 6% 10% 22% 27% 17%	100631
	Night	21% 6% 10% 21% 26% 15%	98140
Seeing	Day	5% 12% 26% 30% 21% 6%	15085
	Night	4 7% 28% 41% 18% 1	25181

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	1 4 3 5% 3 3 4 3 6% 9% 60%	2808
	11th to 20th	Day	2 4 1 3 2 2 3 2 5% 7% 70%	2860
	21st to end	Day	2 5% 2 5% 4 4 4 4 7% 8% 54%	3146
February	1st to 10th	Day	2 4 1 5% 4 3 3 3 6% 7% 62%	1734
	11th to 20th	Day	2 4 1 4 3 3 3 3 5% 8% 62%	1849
	21st to end	Day	1 3 2 3 3 3 3 3 4 7% 66%	1570
March	1st to 10th	Day	2 4 1 2 2 2 3 3 5% 7% 68%	3308
	11th to 20th	Day	3 5% 2 6% 5% 5% 4 4 6% 9% 51%	3490
	21st to end	Day	2 3 1 4 3 3 4 4 6% 7% 62%	3946
April	1st to 10th	Day	2 2 1 3 3 3 3 4 6% 8% 66%	2280
	11th to 20th	Day	2 3 2 5% 3 3 3 3 5% 7% 65%	2251
	21st to end	Day	2 3 1 3 2 3 3 3 5% 6% 69%	2280
May	1st to 10th	Day	1 1 1 2 2 2 3 3 4 7% 73%	3800
	11th to 20th	Day	3 3 1 3 3 3 4 4 7% 9% 60%	3921
	21st to end	Day	1 1 2 2 2 2 2 3 6% 78%	4825
June	1st to 10th	Day	1 2 1 3 2 3 4 4 7% 9% 65%	2660
	11th to 20th	Day	2 2 1 3 2 2 3 3 5% 6% 72%	2660
	21st to end	Day	4 4 1 3 3 3 4 4 6% 8% 58%	2660
July	1st to 10th	Day	2 2 1 3 4 4 5% 5% 8% 8% 58%	4346
	11th to 20th	Day	1 2 1 3 4 4 6% 6% 9% 12% 51%	4110
	21st to end	Day	3 3 1 4 5% 5% 7% 7% 10% 12% 43%	4454
August	1st to 10th	Day	2 3 1 4 4 5% 5% 5% 10% 12% 48%	3979
	11th to 20th	Day	1 2 1 3 3 3 5% 4 6% 10% 61%	3726
	21st to end	Day	2 3 1 5% 5% 6% 7% 6% 9% 11% 44%	4180
September	1st to 10th	Day	5% 4 1 3 3 4 5% 4 6% 9% 55%	2280
	11th to 20th	Day	4 3 1 3 3 3 5% 4 6% 7% 61%	2280
	21st to end	Day	2 2 1 2 2 2 4 2 4 7% 70%	2067
October	1st to 10th	Day	5% 3 1 4 3 3 4 3 5% 6% 62%	1900
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 4 76%	1900
	21st to end	Day	3 3 1 3 2 2 2 2 3 4 74%	2090
November	1st to 10th	Day	2 1 1 1 2 2 3 3 5% 7% 74%	1809
	11th to 20th	Day	1 3 1 2 2 2 3 3 4 7% 72%	1710
	21st to end	Day	2 3 1 4 3 3 3 2 6% 6% 65%	1710
December	1st to 10th	Day	2 4 3 3 3 4 2 5% 7% 66%	1562
	11th to 20th	Day	2 4 1 5% 4 4 4 3 6% 7% 60%	1926
	21st to end	Day	2 4 1 4 4 3 3 3 5% 6% 65%	2794
January	1st to 10th	Night	1 4 3 6% 5% 4 4 3 5% 7% 57%	4162
	11th to 20th	Night	2 3 1 3 3 3 4 3 4 6% 69%	4110
	21st to end	Night	3 7% 2 5% 3 4 4 4 6% 10% 52%	4521
February	1st to 10th	Night	1 5% 2 5% 2 3 3 3 5% 6% 64%	2443
	11th to 20th	Night	3 6% 2 4 3 4 3 3 5% 8% 60%	2218
	21st to end	Night	2 4 2 4 3 3 4 2 5% 5% 66%	1884
March	1st to 10th	Night	2 4 2 4 3 3 4 3 5% 6% 67%	3560
	11th to 20th	Night	5% 6% 2 6% 5% 4 5% 4 6% 6% 51%	3490
	21st to end	Night	2 3 2 5% 3 3 4 4 5% 6% 63%	3701
April	1st to 10th	Night	3 3 1 3 3 3 3 3 5% 7% 65%	1900
	11th to 20th	Night	3 4 2 4 3 4 4 3 5% 6% 63%	1877
	21st to end	Night	3 2 1 3 3 3 3 4 4 6% 67%	1900
May	1st to 10th	Night	2 2 1 4 3 3 3 4 6% 7% 66%	3170
	11th to 20th	Night	4 4 2 5% 4 4 5% 5% 6% 8% 52%	2978
	21st to end	Night	2 2 1 3 2 3 3 2 4 7% 71%	2762
June	1st to 10th	Night	1 3 1 4 4 5% 4 5% 8% 10% 55%	1520
	11th to 20th	Night	2 2 1 3 2 3 4 4 5% 7% 67%	1520
	21st to end	Night	4 4 2 5% 5% 5% 4 5% 8% 8% 51%	1520
July	1st to 10th	Night	2 3 1 6% 5% 4 5% 5% 7% 8% 52%	2618
	11th to 20th	Night	3 3 2 5% 5% 5% 7% 7% 10% 13% 40%	2860
	21st to end	Night	6% 5% 1 6% 5% 5% 6% 5% 9% 13% 39%	3109
August	1st to 10th	Night	2 4 2 10% 7% 7% 8% 7% 9% 11% 33%	2958
	11th to 20th	Night	3 4 2 5% 5% 6% 7% 6% 8% 11% 44%	3113
	21st to end	Night	3 3 2 7% 6% 8% 8% 7% 10% 12% 33%	3487
September	1st to 10th	Night	3 3 1 4 5% 5% 5% 5% 8% 11% 49%	1900
	11th to 20th	Night	4 3 2 5% 3 4 4 4 7% 7% 58%	1900
	21st to end	Night	3 3 1 3 3 3 3 3 4 6% 67%	2109
October	1st to 10th	Night	5% 3 1 4 3 3 3 3 5% 5% 63%	2280
	11th to 20th	Night	3 2 2 2 2 3 3 4 5% 75%	2280
	21st to end	Night	4 2 1 2 3 2 2 2 3 6% 73%	2508
November	1st to 10th	Night	1 2 1 2 2 2 2 3 4 5% 77%	2254
	11th to 20th	Night	1 2 1 3 2 2 2 2 5% 6% 74%	2470
	21st to end	Night	4 4 2 4 4 3 3 3 4 6% 65%	2470
December	1st to 10th	Night	3 4 1 3 2 2 3 3 5% 7% 66%	2615
	11th to 20th	Night	3 3 1 3 3 4 4 3 6% 9% 60%	3271
	21st to end	Night	3 5% 2 5% 4 4 3 3 4 7% 63%	4873

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	19% 4 10% 32% 30% 4	2809
	11th to 20th	Day	14% 5% 12% 38% 27% 4	2860
	21st to end	Day	24% 7% 10% 24% 29% 7%	3146
February	1st to 10th	Day	21% 4 12% 25% 28% 11%	1734
	11th to 20th	Day	21% 5% 7% 26% 29% 13%	1859
	21st to end	Day	18% 4 6% 21% 33% 19%	1570
March	1st to 10th	Day	15% 4 5% 17% 39% 20%	3308
	11th to 20th	Day	23% 10% 9% 25% 25% 9%	3490
	21st to end	Day	17% 5% 8% 23% 36% 12%	3946
April	1st to 10th	Day	15% 4 7% 19% 34% 20%	2280
	11th to 20th	Day	19% 4 6% 19% 30% 23%	2254
	21st to end	Day	15% 4 8% 22% 35% 17%	2280
May	1st to 10th	Day	10% 4 8% 17% 33% 28%	3800
	11th to 20th	Day	18% 7% 9% 19% 30% 17%	3924
	21st to end	Day	9% 4 6% 12% 21% 48%	4825
June	1st to 10th	Day	14% 5% 10% 22% 26% 23%	2660
	11th to 20th	Day	13% 3 5% 13% 23% 42%	2660
	21st to end	Day	22% 6% 9% 20% 21% 23%	2660
July	1st to 10th	Day	21% 9% 12% 15% 17% 25%	4279
	11th to 20th	Day	21% 8% 18% 30% 18% 5%	4041
	21st to end	Day	27% 10% 18% 31% 12% 2	4314
August	1st to 10th	Day	23% 11% 20% 24% 15% 7%	3965
	11th to 20th	Day	19% 6% 12% 25% 23% 15%	3762
	21st to end	Day	29% 11% 17% 22% 18% 3	4179
September	1st to 10th	Day	24% 7% 11% 22% 24% 11%	2280
	11th to 20th	Day	23% 6% 9% 18% 26% 19%	2280
	21st to end	Day	15% 4 6% 23% 33% 20%	2067
October	1st to 10th	Day	23% 6% 7% 19% 26% 20%	1900
	11th to 20th	Day	14% 3 4 14% 34% 31%	1900
	21st to end	Day	15% 3 6% 21% 36% 19%	2090
November	1st to 10th	Day	11% 2 7% 22% 39% 18%	1809
	11th to 20th	Day	13% 3 8% 25% 39% 12%	1710
	21st to end	Day	18% 4 6% 23% 33% 15%	1710
December	1st to 10th	Day	17% 4 11% 33% 26% 8%	1562
	11th to 20th	Day	23% 4 13% 23% 29% 8%	1924
	21st to end	Day	19% 3 9% 21% 32% 16%	2794
January	1st to 10th	Night	23% 3 13% 27% 26% 7%	4162
	11th to 20th	Night	15% 5% 10% 37% 27% 7%	4110
	21st to end	Night	24% 6% 12% 24% 29% 6%	4521
February	1st to 10th	Night	19% 4 11% 26% 27% 13%	2443
	11th to 20th	Night	22% 4 9% 24% 27% 14%	2218
	21st to end	Night	18% 4 6% 16% 34% 21%	1884
March	1st to 10th	Night	16% 5% 4 17% 40% 19%	3560
	11th to 20th	Night	29% 8% 8% 21% 24% 10%	3490
	21st to end	Night	17% 5% 8% 26% 34% 12%	3701
April	1st to 10th	Night	17% 4 6% 19% 31% 22%	1900
	11th to 20th	Night	20% 3 6% 19% 26% 25%	1877
	21st to end	Night	17% 4 8% 20% 34% 16%	1900
May	1st to 10th	Night	13% 6% 10% 15% 28% 28%	3170
	11th to 20th	Night	24% 9% 11% 21% 19% 15%	2979
	21st to end	Night	12% 5% 6% 11% 18% 48%	2762
June	1st to 10th	Night	21% 7% 13% 17% 19% 23%	1520
	11th to 20th	Night	16% 4 6% 14% 21% 40%	1520
	21st to end	Night	26% 9% 12% 15% 16% 22%	1520
July	1st to 10th	Night	27% 8% 12% 19% 12% 22%	2576
	11th to 20th	Night	27% 11% 21% 27% 9% 5%	2806
	21st to end	Night	34% 10% 18% 27% 10% 1	3011
August	1st to 10th	Night	37% 12% 16% 17% 12% 6%	2949
	11th to 20th	Night	28% 11% 16% 17% 18% 11%	3143
	21st to end	Night	37% 10% 17% 22% 9% 4	3487
September	1st to 10th	Night	25% 10% 15% 20% 20% 10%	1900
	11th to 20th	Night	23% 8% 11% 17% 22% 20%	1900
	21st to end	Night	19% 5% 8% 18% 30% 21%	2109
October	1st to 10th	Night	22% 5% 9% 19% 21% 24%	2280
	11th to 20th	Night	13% 4 5% 14% 34% 31%	2280
	21st to end	Night	15% 3 5% 19% 38% 20%	2508
November	1st to 10th	Night	10% 3 6% 22% 41% 18%	2254
	11th to 20th	Night	12% 3 6% 23% 40% 15%	2470
	21st to end	Night	21% 4 6% 20% 32% 17%	2470
December	1st to 10th	Night	17% 4 10% 31% 29% 8%	2615
	11th to 20th	Night	21% 4 11% 26% 30% 9%	3272
	21st to end	Night	21% 3 10% 22% 32% 12%	4873

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	5% 15% 38% 30% 11%	270
	11th to 20th	Day	9% 16% 36% 27% 12%	270
	21st to end	Day	3 9% 35% 37% 13% 2	297
February	1st to 10th	Day	4 21% 37% 27% 11%	270
	11th to 20th	Day	7% 20% 41% 24% 7%	270
	21st to end	Day	12% 24% 40% 21% 4	225
March	1st to 10th	Day	5% 24% 33% 28% 10%	370
	11th to 20th	Day	10% 26% 31% 23% 9% 1	460
	21st to end	Day	11% 33% 30% 19% 7%	506
April	1st to 10th	Day	9% 23% 37% 22% 8% 2	460
	11th to 20th	Day	9% 27% 38% 17% 8% 1	452
	21st to end	Day	8% 23% 38% 25% 6% 1	460
May	1st to 10th	Day	6% 21% 41% 24% 8% 1	463
	11th to 20th	Day	5% 14% 32% 30% 15% 3	511
	21st to end	Day	4 13% 31% 36% 14% 1	734
June	1st to 10th	Day	2 6% 21% 40% 26% 4	680
	11th to 20th	Day	3 7% 24% 36% 27% 3	680
	21st to end	Day	6% 4 21% 34% 29% 7%	680
July	1st to 10th	Day	3 2 15% 35% 33% 12%	628
	11th to 20th	Day	3 7% 28% 40% 22%	484
	21st to end	Day	2 1 8% 24% 42% 24%	531
August	1st to 10th	Day	3 3 10% 28% 37% 19%	487
	11th to 20th	Day	3 1 16% 32% 35% 14%	477
	21st to end	Day	5% 1 11% 33% 35% 15%	539
September	1st to 10th	Day	4 3 16% 31% 30% 14%	490
	11th to 20th	Day	3 2 25% 36% 26% 8%	490
	21st to end	Day	4 6% 22% 32% 27% 9%	324
October	1st to 10th	Day	6% 9% 25% 24% 27% 9%	300
	11th to 20th	Day	7% 12% 25% 28% 22% 7%	300
	21st to end	Day	9% 12% 26% 28% 21% 5%	330
November	1st to 10th	Day	4 16% 33% 32% 14% 1	270
	11th to 20th	Day	4 8% 29% 36% 21% 1	270
	21st to end	Day	9% 16% 34% 27% 14% 1	270
December	1st to 10th	Day	4 16% 32% 34% 13% 1	270
	11th to 20th	Day	8% 12% 36% 34% 10% 1	270
	21st to end	Day	9% 20% 35% 32% 4	297
January	1st to 10th	Night	5% 6% 33% 41% 15% 1	799
	11th to 20th	Night	4 6% 34% 41% 14% 1	760
	21st to end	Night	5% 8% 31% 41% 13% 2	836
February	1st to 10th	Night	4 9% 38% 40% 9% 1	760
	11th to 20th	Night	6% 10% 40% 36% 8%	744
	21st to end	Night	6% 10% 48% 33% 3	628
March	1st to 10th	Night	5% 10% 40% 35% 8%	760
	11th to 20th	Night	6% 12% 32% 38% 11%	760
	21st to end	Night	4 13% 38% 39% 7%	760
April	1st to 10th	Night	5% 13% 41% 36% 4 1	570
	11th to 20th	Night	4 17% 36% 36% 7%	563
	21st to end	Night	5% 13% 41% 34% 7% 1	570
May	1st to 10th	Night	4 9% 37% 39% 11%	570
	11th to 20th	Night	6% 9% 30% 40% 14% 1	561
	21st to end	Night	4 8% 35% 41% 12%	617
June	1st to 10th	Night	2 6% 25% 46% 20% 1	570
	11th to 20th	Night	2 4 29% 49% 16% 1	570
	21st to end	Night	4 4 18% 47% 26% 1	570
July	1st to 10th	Night	4 2 15% 45% 32% 3	555
	11th to 20th	Night	4 1 9% 36% 46% 4	552
	21st to end	Night	6% 1 8% 32% 47% 6%	624
August	1st to 10th	Night	3 1 9% 32% 47% 8%	567
	11th to 20th	Night	3 2 10% 45% 37% 3	564
	21st to end	Night	3 8% 44% 40% 5%	627
September	1st to 10th	Night	4 3 12% 49% 31% 2	570
	11th to 20th	Night	5% 3 14% 49% 29% 1	570
	21st to end	Night	4 4 21% 46% 23% 2	613
October	1st to 10th	Night	5% 3 22% 48% 20% 1	760
	11th to 20th	Night	4 5% 26% 46% 17% 2	760
	21st to end	Night	5% 6% 26% 40% 21% 3	836
November	1st to 10th	Night	2 5% 25% 46% 20% 1	770
	11th to 20th	Night	2 6% 28% 45% 18% 2	950
	21st to end	Night	5% 9% 28% 39% 17% 2	950
December	1st to 10th	Night	5% 10% 30% 43% 12%	950
	11th to 20th	Night	6% 10% 32% 41% 11%	950
	21st to end	Night	6% 9% 37% 41% 7%	1045

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