The Williams' Ranch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Williams' Ranch clear sky chart from 2005-11-06 to 2024-11-06.

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	3 2 1 3 3 3 3 3 5% 7% 66%	104209
	Night	3 3 1 4 3 3 4 3 5% 7% 64%	103506
Transparency	Day	17% 5% 8% 17% 27% 27%	103978
	Night	19% 5% 7% 15% 27% 27%	103326
Seeing	Day	5% 7% 21% 29% 24% 14%	14465
	Night	5% 5% 26% 39% 23% 2	24776

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	2 3 1 3 3 2 3 2 4 6% 69%	2858
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 5% 76%	2860
	21st to end	Day	4 4 2 5% 3 4 4 3 6% 7% 59%	3222
February	1st to 10th	Day	3 3 1 4 3 2 3 3 4 7% 68%	1900
	11th to 20th	Day	4 4 1 4 4 3 4 3 4 6% 65%	1849
	21st to end	Day	2 4 1 3 3 2 3 3 4 5% 71%	1570
March	1st to 10th	Day	2 3 2 5% 3 3 4 3 6% 7% 63%	3280
	11th to 20th	Day	6% 5% 1 4 3 3 3 3 4 6% 62%	3490
	21st to end	Day	2 4 2 5% 3 3 4 3 5% 6% 63%	3992
April	1st to 10th	Day	2 2 1 3 2 3 3 3 5% 6% 71%	2280
	11th to 20th	Day	2 3 1 2 2 2 3 2 3 4 76%	2251
	21st to end	Day	2 2 1 2 2 1 2 3 4 4 78%	2280
May	1st to 10th	Day	2 2 1 1 2 2 2 3 4 82%	3800
	11th to 20th	Day	2 1 1 2 2 2 3 3 4 6% 74%	3765
	21st to end	Day	1 1 1 3 1 1 2 2 3 4 81%	4141
June	1st to 10th	Day	2 2 1 3 2 2 3 4 5% 7% 69%	2588
	11th to 20th	Day	1 2 3 2 2 3 3 5% 6% 73%	2660
	21st to end	Day	4 2 1 3 4 3 5% 4 7% 9% 59%	2477
July	1st to 10th	Day	3 3 1 5% 5% 5% 5% 5% 7% 9% 52%	4110
	11th to 20th	Day	3 2 1 3 3 4 4 5% 6% 10% 60%	4110
	21st to end	Day	6% 3 1 5% 4 5% 6% 6% 8% 9% 47%	4285
August	1st to 10th	Day	2 2 1 3 3 4 5% 5% 7% 11% 58%	3784
	11th to 20th	Day	3 2 1 4 4 4 4 3 6% 8% 61%	3726
	21st to end	Day	3 2 1 5% 4 4 5% 4 7% 12% 52%	4180
September	1st to 10th	Day	8% 4 2 4 3 3 4 3 5% 7% 56%	2280
	11th to 20th	Day	7% 4 2 5% 4 4 3 3 5% 7% 55%	2261
	21st to end	Day	4 2 1 3 3 2 3 3 4 6% 69%	2090
October	1st to 10th	Day	3 1 2 2 2 2 2 3 5% 79%	3246
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 4 76%	3150
	21st to end	Day	1 1 1 2 1 1 2 2 2 3 85%	3438
November	1st to 10th	Day	4 2 1 3 2 2 2 2 5% 6% 72%	1909
	11th to 20th	Day	2 2 1 3 2 3 3 3 5% 4 71%	1772
	21st to end	Day	5% 3 1 3 3 3 3 4 4 6% 65%	1710
December	1st to 10th	Day	5% 3 2 4 3 3 3 3 5% 7% 62%	1710
	11th to 20th	Day	2 2 2 5% 5% 4 4 4 5% 7% 60%	2121
	21st to end	Day	3 3 1 3 2 2 3 3 6% 7% 67%	3064
January	1st to 10th	Night	3 4 1 3 2 3 3 3 5% 6% 68%	4097
	11th to 20th	Night	2 3 1 2 2 2 2 2 5% 6% 73%	4110
	21st to end	Night	4 5% 2 4 3 4 4 3 5% 8% 56%	4428
February	1st to 10th	Night	3 3 1 3 2 2 3 3 5% 7% 69%	2280
	11th to 20th	Night	4 3 1 4 3 3 4 3 4 6% 64%	2218
	21st to end	Night	4 3 2 3 2 3 2 2 5% 6% 67%	1884
March	1st to 10th	Night	3 4 1 4 3 3 3 3 4 7% 65%	3584
	11th to 20th	Night	8% 7% 2 4 3 3 3 3 6% 6% 55%	3490
	21st to end	Night	2 3 1 4 2 3 4 3 7% 8% 63%	3653
April	1st to 10th	Night	2 3 1 3 2 2 2 3 4 6% 69%	1900
	11th to 20th	Night	4 3 1 2 2 3 3 3 4 6% 71%	1877
	21st to end	Night	2 2 1 2 2 2 2 2 4 5% 76%	1900
May	1st to 10th	Night	2 2 1 1 2 2 2 1 3 4 80%	3170
	11th to 20th	Night	4 2 1 3 3 3 3 3 5% 6% 69%	3140
	21st to end	Night	2 2 1 2 2 2 2 2 3 4 78%	3446
June	1st to 10th	Night	2 3 1 4 4 3 5% 4 7% 8% 59%	1572
	11th to 20th	Night	3 2 1 2 3 4 2 3 7% 6% 68%	1520
	21st to end	Night	3 4 1 5% 5% 4 5% 5% 7% 9% 52%	1690
July	1st to 10th	Night	3 4 1 6% 6% 6% 6% 5% 8% 11% 43%	2860
	11th to 20th	Night	2 2 1 3 3 3 5% 4 7% 11% 60%	2860
	21st to end	Night	4 3 2 7% 5% 7% 7% 6% 9% 9% 40%	3261
August	1st to 10th	Night	3 3 1 4 4 5% 7% 5% 8% 12% 49%	3155
	11th to 20th	Night	2 3 1 5% 4 3 4 4 7% 10% 56%	3113
	21st to end	Night	3 3 2 5% 5% 5% 6% 6% 9% 13% 43%	3487
September	1st to 10th	Night	5% 4 1 5% 3 4 5% 4 6% 9% 53%	1900
	11th to 20th	Night	7% 3 2 5% 3 4 5% 4 5% 8% 53%	1917
	21st to end	Night	5% 3 1 4 3 3 3 3 5% 6% 64%	2090
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 5% 76%	3704
	11th to 20th	Night	5% 3 1 4 3 2 2 2 3 5% 69%	3778
	21st to end	Night	2 1 1 2 2 1 2 2 3 4 82%	4120
November	1st to 10th	Night	4 3 1 2 1 2 2 2 4 6% 73%	2292
	11th to 20th	Night	2 3 1 3 2 3 3 3 4 5% 72%	2384
	21st to end	Night	6% 2 1 3 2 2 3 3 5% 6% 66%	2470
December	1st to 10th	Night	5% 5% 1 4 3 3 4 3 4 7% 63%	2470
	11th to 20th	Night	2 3 1 5% 4 3 3 3 6% 6% 63%	3069
	21st to end	Night	3 3 1 3 2 2 2 2 4 5% 72%	4617

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	14% 4 5% 14% 37% 26%	2859
	11th to 20th	Day	11% 3 5% 16% 45% 20%	2860
	21st to end	Day	23% 5% 8% 23% 27% 15%	3222
February	1st to 10th	Day	17% 3 7% 14% 27% 31%	1900
	11th to 20th	Day	21% 4 6% 15% 27% 27%	1859
	21st to end	Day	16% 3 4 11% 30% 36%	1570
March	1st to 10th	Day	18% 4 6% 10% 31% 31%	3280
	11th to 20th	Day	22% 5% 5% 18% 27% 23%	3490
	21st to end	Day	19% 5% 6% 10% 29% 31%	3992
April	1st to 10th	Day	14% 3 5% 12% 30% 37%	2280
	11th to 20th	Day	13% 3 4 9% 27% 45%	2254
	21st to end	Day	10% 3 5% 11% 28% 44%	2280
May	1st to 10th	Day	8% 3 4 7% 17% 62%	3800
	11th to 20th	Day	10% 3 5% 12% 25% 45%	3768
	21st to end	Day	8% 2 5% 10% 13% 62%	4141
June	1st to 10th	Day	13% 5% 8% 16% 28% 31%	2588
	11th to 20th	Day	12% 3 5% 12% 24% 45%	2660
	21st to end	Day	19% 6% 11% 18% 22% 24%	2477
July	1st to 10th	Day	25% 8% 14% 21% 20% 11%	4046
	11th to 20th	Day	17% 7% 15% 21% 28% 12%	4041
	21st to end	Day	27% 10% 14% 27% 20% 3	4149
August	1st to 10th	Day	17% 8% 11% 30% 27% 6%	3772
	11th to 20th	Day	20% 7% 10% 23% 29% 12%	3762
	21st to end	Day	23% 8% 15% 29% 20% 4	4179
September	1st to 10th	Day	27% 7% 11% 18% 20% 17%	2280
	11th to 20th	Day	27% 6% 10% 16% 24% 16%	2261
	21st to end	Day	16% 4 7% 21% 31% 20%	2090
October	1st to 10th	Day	11% 3 5% 9% 24% 48%	3246
	11th to 20th	Day	13% 3 6% 17% 25% 35%	3150
	21st to end	Day	8% 2 3 13% 37% 38%	3438
November	1st to 10th	Day	14% 3 5% 20% 31% 26%	1909
	11th to 20th	Day	15% 3 6% 20% 30% 25%	1772
	21st to end	Day	20% 3 6% 18% 32% 20%	1710
December	1st to 10th	Day	22% 4 8% 23% 30% 14%	1710
	11th to 20th	Day	23% 5% 9% 19% 30% 15%	2119
	21st to end	Day	15% 3 7% 17% 35% 23%	3064
January	1st to 10th	Night	17% 4 5% 13% 38% 24%	4097
	11th to 20th	Night	13% 3 5% 18% 41% 19%	4110
	21st to end	Night	25% 5% 7% 22% 28% 14%	4428
February	1st to 10th	Night	16% 2 7% 15% 26% 34%	2280
	11th to 20th	Night	21% 3 7% 12% 27% 30%	2218
	21st to end	Night	18% 2 5% 9% 27% 38%	1884
March	1st to 10th	Night	18% 5% 4 10% 27% 35%	3584
	11th to 20th	Night	26% 6% 7% 13% 26% 21%	3490
	21st to end	Night	16% 4 6% 13% 34% 27%	3653
April	1st to 10th	Night	16% 2 6% 9% 31% 37%	1900
	11th to 20th	Night	15% 3 6% 11% 22% 42%	1877
	21st to end	Night	12% 3 5% 12% 26% 43%	1900
May	1st to 10th	Night	10% 3 3 5% 15% 64%	3170
	11th to 20th	Night	14% 5% 7% 13% 18% 44%	3141
	21st to end	Night	10% 3 5% 7% 14% 59%	3447
June	1st to 10th	Night	21% 6% 9% 15% 21% 29%	1572
	11th to 20th	Night	15% 4 7% 11% 20% 43%	1520
	21st to end	Night	26% 7% 13% 14% 18% 22%	1690
July	1st to 10th	Night	30% 8% 17% 23% 13% 8%	2815
	11th to 20th	Night	17% 6% 14% 21% 31% 11%	2806
	21st to end	Night	33% 9% 14% 23% 18% 3	3158
August	1st to 10th	Night	23% 8% 12% 26% 23% 8%	3144
	11th to 20th	Night	21% 7% 11% 21% 31% 9%	3143
	21st to end	Night	26% 10% 15% 23% 19% 6%	3487
September	1st to 10th	Night	27% 8% 12% 17% 18% 17%	1900
	11th to 20th	Night	27% 8% 11% 19% 17% 18%	1917
	21st to end	Night	21% 5% 9% 16% 28% 21%	2090
October	1st to 10th	Night	13% 4 6% 8% 22% 48%	3704
	11th to 20th	Night	19% 3 6% 12% 19% 41%	3778
	21st to end	Night	9% 2 3 10% 35% 42%	4120
November	1st to 10th	Night	14% 2 5% 16% 33% 30%	2292
	11th to 20th	Night	15% 3 5% 18% 33% 27%	2384
	21st to end	Night	19% 3 6% 17% 32% 23%	2470
December	1st to 10th	Night	23% 4 5% 20% 30% 18%	2470
	11th to 20th	Night	21% 4 7% 21% 29% 19%	3070
	21st to end	Night	16% 3 5% 16% 39% 22%	4617

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	10% 12% 33% 26% 17% 3	270
	11th to 20th	Day	6% 10% 33% 31% 17% 3	270
	21st to end	Day	4 8% 32% 33% 17% 6%	297
February	1st to 10th	Day	7% 7% 34% 31% 15% 6%	270
	11th to 20th	Day	10% 11% 37% 29% 12% 2	270
	21st to end	Day	11% 17% 33% 28% 8% 3	225
March	1st to 10th	Day	8% 16% 34% 30% 10% 1	355
	11th to 20th	Day	11% 14% 32% 28% 12% 4	460
	21st to end	Day	7% 21% 38% 24% 8% 2	506
April	1st to 10th	Day	6% 14% 40% 26% 12% 2	460
	11th to 20th	Day	6% 17% 37% 28% 10% 2	452
	21st to end	Day	4 16% 37% 29% 12% 2	460
May	1st to 10th	Day	3 12% 37% 31% 14% 3	463
	11th to 20th	Day	1 7% 33% 36% 18% 5%	488
	21st to end	Day	2 7% 28% 40% 20% 3	529
June	1st to 10th	Day	2 2 18% 40% 28% 9%	656
	11th to 20th	Day	2 2 15% 39% 37% 6%	680
	21st to end	Day	3 1 11% 31% 32% 22%	497
July	1st to 10th	Day	3 1 6% 21% 39% 29%	486
	11th to 20th	Day	5% 1 3 22% 34% 35%	484
	21st to end	Day	3 1 5% 21% 36% 35%	531
August	1st to 10th	Day	4 2 17% 32% 44%	487
	11th to 20th	Day	6% 5% 21% 37% 31%	477
	21st to end	Day	6% 1 6% 20% 34% 34%	539
September	1st to 10th	Day	8% 1 7% 25% 32% 27%	490
	11th to 20th	Day	6% 1 7% 26% 36% 25%	471
	21st to end	Day	4 1 7% 26% 35% 27%	300
October	1st to 10th	Day	9% 1 12% 26% 33% 19%	300
	11th to 20th	Day	4 3 16% 31% 30% 15%	300
	21st to end	Day	4 8% 19% 29% 23% 17%	330
November	1st to 10th	Day	6% 8% 21% 31% 25% 9%	285
	11th to 20th	Day	3 6% 19% 30% 32% 10%	270
	21st to end	Day	10% 13% 28% 27% 16% 6%	270
December	1st to 10th	Day	9% 7% 26% 29% 24% 6%	270
	11th to 20th	Day	4 10% 28% 39% 17% 2	270
	21st to end	Day	8% 11% 30% 35% 12% 3	297
January	1st to 10th	Night	7% 5% 34% 39% 13% 2	760
	11th to 20th	Night	6% 8% 32% 38% 15% 2	760
	21st to end	Night	6% 9% 34% 36% 14% 1	836
February	1st to 10th	Night	4 9% 33% 38% 14% 1	760
	11th to 20th	Night	6% 7% 40% 36% 9% 1	744
	21st to end	Night	5% 11% 47% 30% 5%	628
March	1st to 10th	Night	6% 12% 37% 35% 9% 1	760
	11th to 20th	Night	6% 11% 32% 38% 11% 1	760
	21st to end	Night	3 10% 45% 34% 8%	738
April	1st to 10th	Night	4 11% 44% 31% 8%	570
	11th to 20th	Night	5% 10% 46% 29% 10%	563
	21st to end	Night	2 12% 42% 34% 10%	570
May	1st to 10th	Night	4 7% 43% 38% 7%	570
	11th to 20th	Night	3 6% 33% 44% 13% 1	561
	21st to end	Night	3 4 29% 51% 13%	617
June	1st to 10th	Night	4 2 22% 49% 22% 1	570
	11th to 20th	Night	3 1 19% 54% 23% 1	570
	21st to end	Night	4 2 12% 46% 35% 2	570
July	1st to 10th	Night	4 1 7% 39% 47% 2	555
	11th to 20th	Night	3 5% 33% 55% 4	552
	21st to end	Night	4 1 4 31% 54% 6%	624
August	1st to 10th	Night	5% 5% 25% 57% 7%	567
	11th to 20th	Night	3 1 5% 27% 54% 9%	564
	21st to end	Night	5% 1 4 29% 57% 5%	627
September	1st to 10th	Night	6% 1 5% 38% 47% 4	570
	11th to 20th	Night	8% 1 5% 43% 41% 3	570
	21st to end	Night	7% 1 11% 45% 34% 3	570
October	1st to 10th	Night	6% 1 16% 45% 30% 2	703
	11th to 20th	Night	6% 3 23% 42% 24% 2	760
	21st to end	Night	5% 5% 19% 43% 25% 4	836
November	1st to 10th	Night	5% 2 24% 39% 27% 3	764
	11th to 20th	Night	3 3 26% 43% 24% 1	864
	21st to end	Night	7% 7% 26% 39% 19% 2	950
December	1st to 10th	Night	7% 6% 28% 40% 17% 1	950
	11th to 20th	Night	5% 7% 34% 41% 12% 1	950
	21st to end	Night	5% 9% 31% 41% 12% 1	893

Page generated at 2024-11-03 11:19:02. Version ac07

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