Porterville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Porterville 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	5% 3 1 3 2 2 2 2 4 5% 70%	146278
	Night	7% 4 1 3 2 2 2 2 4 5% 67%	144246
Transparency	Day	18% 3 7% 19% 28% 25%	141353
	Night	21% 3 9% 21% 26% 20%	140148
Seeing	Day	7% 3 14% 33% 31% 12%	17650
	Night	9% 2 7% 39% 37% 7%	27211

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	11% 9% 3 6% 4 4 4 4 6% 7% 42%	4722
	11th to 20th	Day	5% 3 2 5% 3 3 3 3 5% 7% 62%	5134
	21st to end	Day	9% 6% 2 6% 4 4 4 3 6% 8% 48%	5843
February	1st to 10th	Day	13% 7% 2 5% 3 3 3 3 5% 6% 50%	2093
	11th to 20th	Day	11% 7% 2 4 3 3 4 4 7% 8% 49%	2050
	21st to end	Day	12% 7% 2 6% 3 3 4 3 5% 6% 48%	1730
March	1st to 10th	Day	15% 7% 2 6% 3 4 4 4 5% 9% 41%	4618
	11th to 20th	Day	10% 5% 2 6% 3 3 4 4 5% 7% 52%	4902
	21st to end	Day	10% 6% 2 5% 4 4 4 4 5% 8% 48%	6055
April	1st to 10th	Day	7% 4 1 4 3 3 3 3 5% 6% 60%	2520
	11th to 20th	Day	7% 4 1 4 3 3 3 3 4 7% 61%	2489
	21st to end	Day	7% 2 1 3 3 3 3 3 5% 6% 64%	2520
May	1st to 10th	Day	3 3 1 3 2 2 2 2 3 4 76%	7259
	11th to 20th	Day	2 1 1 1 2 1 2 3 4 82%	7596
	21st to end	Day	3 1 1 2 1 2 2 2 3 4 81%	8653
June	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 77%	2940
	11th to 20th	Day	1 1 1 1 1 1 2 3 89%	2940
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	2940
July	1st to 10th	Day	1 1 1 2 94%	6459
	11th to 20th	Day	1 1 1 1 1 2 1 2 2 89%	6460
	21st to end	Day	1 1 1 1 1 1 1 2 3 90%	6592
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 88%	5346
	11th to 20th	Day	1 1 1 95%	5364
	21st to end	Day	1 1 1 1 2 1 3 3 87%	5984
September	1st to 10th	Day	1 1 1 1 1 1 2 2 89%	2400
	11th to 20th	Day	1 1 1 1 2 1 1 2 3 86%	2310
	21st to end	Day	1 1 1 1 1 1 2 2 3 4 83%	2200
October	1st to 10th	Day	2 1 1 1 1 2 2 4 5% 79%	3514
	11th to 20th	Day	2 2 1 2 2 2 3 2 3 4 77%	3261
	21st to end	Day	4 2 1 3 2 2 3 3 5% 7% 67%	3504
November	1st to 10th	Day	5% 4 1 4 2 3 3 3 4 8% 63%	1854
	11th to 20th	Day	8% 5% 2 4 3 4 3 3 4 7% 59%	1890
	21st to end	Day	9% 6% 2 6% 3 4 4 3 4 6% 53%	1890
December	1st to 10th	Day	6% 4 2 6% 4 4 4 3 5% 7% 56%	3040
	11th to 20th	Day	12% 6% 2 7% 5% 4 4 5% 6% 8% 41%	3114
	21st to end	Day	14% 12% 4 9% 4 4 5% 4 6% 8% 30%	4092
January	1st to 10th	Night	14% 9% 3 6% 4 3 3 2 4 6% 46%	8172
	11th to 20th	Night	6% 4 2 4 3 2 2 2 4 6% 64%	7843
	21st to end	Night	11% 6% 2 6% 4 3 4 3 5% 7% 49%	8371
February	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 4 7% 51%	2524
	11th to 20th	Night	13% 4 1 3 2 2 2 3 5% 7% 55%	2458
	21st to end	Night	14% 6% 1 4 3 3 3 3 3 5% 55%	2076
March	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 5% 8% 45%	5496
	11th to 20th	Night	10% 6% 2 4 3 4 4 3 5% 7% 54%	5301
	21st to end	Night	12% 6% 3 6% 4 4 4 3 5% 6% 48%	5161
April	1st to 10th	Night	9% 5% 2 4 2 3 2 3 4 7% 59%	2100
	11th to 20th	Night	7% 3 1 3 3 3 3 3 4 6% 65%	2079
	21st to end	Night	8% 4 1 3 2 2 2 3 3 5% 68%	2100
May	1st to 10th	Night	6% 2 1 3 2 1 2 2 3 4 75%	5705
	11th to 20th	Night	3 1 1 1 1 1 1 3 5% 82%	5296
	21st to end	Night	3 2 1 2 1 1 1 1 2 2 84%	5520
June	1st to 10th	Night	3 2 2 1 1 2 1 3 6% 78%	1680
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 88%	1680
	21st to end	Night	1 1 1 1 1 2 2 91%	1680
July	1st to 10th	Night	1 1 2 94%	3720
	11th to 20th	Night	1 1 1 1 1 1 2 3 89%	3720
	21st to end	Night	1 1 1 1 1 1 2 3 89%	4478
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	4486
	11th to 20th	Night	1 2 95%	4506
	21st to end	Night	1 1 1 1 2 1 2 3 88%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	2000
	11th to 20th	Night	1 1 2 1 1 1 1 3 3 84%	2089
	21st to end	Night	2 1 1 1 1 2 2 2 4 83%	2200
October	1st to 10th	Night	2 1 1 1 1 1 1 2 3 85%	3681
	11th to 20th	Night	2 1 2 1 1 2 2 4 5% 80%	3897
	21st to end	Night	3 3 1 3 1 2 2 1 3 5% 76%	4305
November	1st to 10th	Night	7% 3 2 3 3 2 2 3 4 6% 67%	2665
	11th to 20th	Night	7% 3 1 4 2 2 3 2 4 7% 64%	2730
	21st to end	Night	9% 6% 1 4 3 2 3 3 4 7% 56%	2730
December	1st to 10th	Night	5% 4 1 5% 3 3 3 3 5% 6% 61%	4370
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 7% 10% 43%	5306
	21st to end	Night	10% 9% 3 8% 5% 5% 5% 4 7% 9% 34%	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	38% 6% 18% 20% 15% 2	4723
	11th to 20th	Day	22% 4 10% 16% 26% 21%	5134
	21st to end	Day	33% 4 11% 20% 25% 7%	5843
February	1st to 10th	Day	34% 4 10% 26% 19% 7%	2093
	11th to 20th	Day	33% 4 12% 27% 18% 6%	2060
	21st to end	Day	37% 3 13% 28% 16% 3	1730
March	1st to 10th	Day	38% 5% 13% 19% 20% 4	4618
	11th to 20th	Day	30% 4 14% 35% 12% 5%	4902
	21st to end	Day	33% 6% 14% 30% 15% 3	6055
April	1st to 10th	Day	25% 3 13% 30% 22% 7%	2520
	11th to 20th	Day	23% 3 12% 30% 24% 8%	2492
	21st to end	Day	21% 3 11% 33% 26% 6%	2520
May	1st to 10th	Day	15% 2 8% 24% 39% 12%	7259
	11th to 20th	Day	8% 2 4 25% 43% 18%	7599
	21st to end	Day	9% 2 5% 23% 48% 13%	8654
June	1st to 10th	Day	11% 2 8% 23% 36% 20%	2940
	11th to 20th	Day	5% 1 5% 13% 39% 38%	2940
	21st to end	Day	6% 1 2 8% 31% 52%	2940
July	1st to 10th	Day	2 1 5% 18% 74%	6391
	11th to 20th	Day	5% 1 1 5% 22% 65%	6385
	21st to end	Day	4 1 1 14% 37% 43%	6405
August	1st to 10th	Day	7% 1 2 6% 36% 49%	3890
	11th to 20th	Day	3 1 1 6% 28% 62%	3880
	21st to end	Day	4 1 1 9% 35% 51%	4311
September	1st to 10th	Day	6% 1 2 9% 32% 51%	2400
	11th to 20th	Day	7% 2 2 17% 44% 28%	2310
	21st to end	Day	8% 2 3 18% 40% 30%	2200
October	1st to 10th	Day	9% 1 5% 13% 22% 50%	3514
	11th to 20th	Day	13% 2 5% 24% 35% 21%	3261
	21st to end	Day	17% 2 5% 18% 35% 23%	3504
November	1st to 10th	Day	21% 3 9% 24% 29% 15%	1854
	11th to 20th	Day	27% 3 10% 25% 27% 9%	1890
	21st to end	Day	32% 4 7% 22% 23% 13%	1890
December	1st to 10th	Day	26% 4 11% 30% 20% 8%	3040
	11th to 20th	Day	37% 7% 13% 20% 18% 6%	3114
	21st to end	Day	47% 9% 14% 21% 7% 2	4092
January	1st to 10th	Night	39% 5% 18% 23% 14% 1	8172
	11th to 20th	Night	22% 3 12% 19% 31% 13%	7843
	21st to end	Night	35% 4 10% 28% 19% 5%	8371
February	1st to 10th	Night	37% 2 8% 28% 18% 7%	2524
	11th to 20th	Night	28% 4 13% 27% 22% 7%	2459
	21st to end	Night	33% 3 12% 31% 16% 4	2076
March	1st to 10th	Night	38% 4 12% 22% 19% 5%	5496
	11th to 20th	Night	29% 5% 16% 34% 11% 5%	5301
	21st to end	Night	37% 4 19% 24% 15% 2	5161
April	1st to 10th	Night	27% 3 12% 30% 20% 8%	2100
	11th to 20th	Night	22% 3 11% 30% 24% 8%	2079
	21st to end	Night	20% 2 10% 34% 30% 4	2100
May	1st to 10th	Night	16% 2 7% 25% 38% 11%	5703
	11th to 20th	Night	8% 1 2 24% 43% 21%	5297
	21st to end	Night	10% 1 3 21% 50% 15%	5521
June	1st to 10th	Night	12% 1 7% 22% 37% 20%	1680
	11th to 20th	Night	6% 1 4 14% 35% 41%	1680
	21st to end	Night	4 1 1 7% 33% 54%	1680
July	1st to 10th	Night	2 1 4 18% 76%	3680
	11th to 20th	Night	5% 1 2 5% 16% 72%	3672
	21st to end	Night	5% 1 1 12% 35% 46%	4354
August	1st to 10th	Night	7% 1 1 8% 33% 50%	3256
	11th to 20th	Night	2 1 1 7% 27% 62%	3256
	21st to end	Night	4 1 1 9% 30% 56%	3608
September	1st to 10th	Night	6% 1 2 10% 31% 52%	2000
	11th to 20th	Night	7% 2 3 16% 45% 27%	2089
	21st to end	Night	8% 2 3 18% 38% 31%	2200
October	1st to 10th	Night	8% 1 4 11% 25% 50%	3681
	11th to 20th	Night	10% 1 4 24% 41% 21%	3897
	21st to end	Night	14% 2 4 20% 36% 24%	4305
November	1st to 10th	Night	21% 2 9% 25% 27% 16%	2665
	11th to 20th	Night	22% 3 12% 24% 27% 11%	2730
	21st to end	Night	29% 3 8% 23% 28% 9%	2730
December	1st to 10th	Night	24% 3 14% 32% 20% 7%	4370
	11th to 20th	Night	34% 6% 16% 23% 19% 3	5306
	21st to end	Night	43% 7% 18% 23% 7% 2	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	21% 1 13% 36% 21% 7%	220
	11th to 20th	Day	12% 1 12% 40% 24% 11%	354
	21st to end	Day	13% 3 11% 35% 28% 10%	473
February	1st to 10th	Day	15% 2 14% 35% 24% 9%	537
	11th to 20th	Day	15% 4 19% 35% 20% 6%	535
	21st to end	Day	13% 2 18% 41% 23% 4	446
March	1st to 10th	Day	14% 5% 23% 34% 21% 4	540
	11th to 20th	Day	13% 2 19% 34% 26% 6%	540
	21st to end	Day	12% 6% 23% 38% 17% 5%	594
April	1st to 10th	Day	9% 5% 20% 34% 22% 10%	540
	11th to 20th	Day	10% 6% 23% 30% 24% 8%	532
	21st to end	Day	5% 6% 23% 29% 25% 11%	540
May	1st to 10th	Day	5% 5% 23% 34% 24% 8%	543
	11th to 20th	Day	5% 3 21% 33% 25% 12%	568
	21st to end	Day	6% 4 16% 31% 32% 10%	617
June	1st to 10th	Day	1 2 15% 32% 37% 12%	570
	11th to 20th	Day	1 2 16% 33% 36% 12%	570
	21st to end	Day	1 2 13% 34% 37% 13%	570
July	1st to 10th	Day	1 7% 33% 43% 16%	566
	11th to 20th	Day	1 1 4 22% 47% 25%	564
	21st to end	Day	1 1 2 27% 47% 22%	616
August	1st to 10th	Day	2 1 9% 35% 35% 18%	527
	11th to 20th	Day	1 6% 33% 41% 18%	517
	21st to end	Day	1 6% 31% 42% 20%	583
September	1st to 10th	Day	2 1 8% 36% 34% 19%	530
	11th to 20th	Day	2 1 13% 41% 34% 9%	530
	21st to end	Day	2 2 15% 31% 36% 15%	530
October	1st to 10th	Day	3 3 16% 33% 31% 15%	530
	11th to 20th	Day	3 3 15% 34% 33% 12%	530
	21st to end	Day	3 3 13% 33% 36% 11%	553
November	1st to 10th	Day	8% 11% 29% 36% 16%	330
	11th to 20th	Day	11% 2 11% 33% 28% 16%	330
	21st to end	Day	13% 1 12% 30% 32% 12%	330
December	1st to 10th	Day	14% 3 10% 34% 28% 12%	330
	11th to 20th	Day	13% 2 8% 34% 27% 16%	223
	21st to end	Day	23% 2 12% 35% 21% 7%	242
January	1st to 10th	Night	18% 3 8% 39% 26% 7%	840
	11th to 20th	Night	10% 1 8% 43% 32% 6%	840
	21st to end	Night	16% 1 5% 44% 28% 5%	924
February	1st to 10th	Night	16% 3 7% 42% 27% 6%	840
	11th to 20th	Night	15% 3 10% 40% 25% 6%	824
	21st to end	Night	17% 2 10% 45% 22% 3	692
March	1st to 10th	Night	13% 2 13% 45% 24% 3	840
	11th to 20th	Night	12% 2 8% 41% 30% 6%	840
	21st to end	Night	15% 3 12% 41% 27% 2	924
April	1st to 10th	Night	11% 3 13% 43% 28% 3	840
	11th to 20th	Night	9% 3 13% 42% 30% 3	831
	21st to end	Night	10% 2 13% 37% 32% 5%	840
May	1st to 10th	Night	7% 2 15% 41% 32% 3	767
	11th to 20th	Night	5% 3 15% 38% 37% 3	621
	21st to end	Night	7% 4 13% 44% 30% 2	574
June	1st to 10th	Night	4 3 8% 45% 36% 3	420
	11th to 20th	Night	1 1 7% 54% 33% 4	420
	21st to end	Night	7% 45% 44% 4	420
July	1st to 10th	Night	1 2 36% 56% 5%	410
	11th to 20th	Night	1 25% 67% 7%	408
	21st to end	Night	1 1 24% 63% 11%	644
August	1st to 10th	Night	2 3 34% 55% 6%	796
	11th to 20th	Night	1 2 29% 59% 9%	792
	21st to end	Night	1 2 23% 63% 11%	880
September	1st to 10th	Night	2 2 27% 58% 11%	800
	11th to 20th	Night	2 1 4 38% 49% 7%	800
	21st to end	Night	3 5% 35% 46% 10%	800
October	1st to 10th	Night	4 1 5% 41% 39% 10%	800
	11th to 20th	Night	4 1 6% 37% 42% 10%	800
	21st to end	Night	4 1 6% 38% 38% 13%	880
November	1st to 10th	Night	9% 1 6% 36% 38% 11%	820
	11th to 20th	Night	9% 1 5% 43% 33% 9%	840
	21st to end	Night	13% 2 7% 38% 33% 8%	840
December	1st to 10th	Night	10% 2 6% 46% 29% 7%	840
	11th to 20th	Night	15% 1 8% 40% 30% 6%	840
	21st to end	Night	18% 2 7% 46% 23% 4	924

Page generated at 2026-07-28 00:25:22. Version ac07

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