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 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	6% 3 1 3 3 2 3 3 4 6% 65%	78489
	Night	8% 4 2 3 3 2 3 3 4 6% 63%	80617
Transparency	Day	21% 3 8% 21% 26% 21%	78204
	Night	25% 3 10% 22% 24% 17%	80425
Seeing	Day	7% 2 14% 33% 31% 12%	14318
	Night	9% 2 7% 38% 38% 7%	24053

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	12% 10% 4 6% 5% 3 5% 5% 6% 7% 37%	2539
	11th to 20th	Day	9% 5% 4 7% 7% 4 5% 6% 7% 9% 38%	2738
	21st to end	Day	15% 8% 3 6% 6% 3 5% 6% 7% 8% 34%	3146
February	1st to 10th	Day	13% 7% 2 4 3 1 4 3 5% 6% 52%	1893
	11th to 20th	Day	10% 6% 2 3 3 2 3 4 5% 8% 53%	1791
	21st to end	Day	14% 7% 3 6% 4 3 3 4 5% 6% 47%	1480
March	1st to 10th	Day	12% 6% 2 5% 4 3 4 5% 6% 9% 45%	1800
	11th to 20th	Day	10% 5% 1 4 4 3 4 5% 6% 9% 49%	1894
	21st to end	Day	10% 4 2 4 5% 2 4 5% 5% 7% 53%	2357
April	1st to 10th	Day	8% 3 2 4 3 2 3 4 5% 6% 60%	2160
	11th to 20th	Day	7% 3 2 3 3 2 4 3 4 7% 61%	2129
	21st to end	Day	7% 2 1 3 3 1 3 3 4 6% 66%	2160
May	1st to 10th	Day	3 3 1 3 3 2 3 3 4 5% 72%	2220
	11th to 20th	Day	3 2 1 2 2 2 2 3 4 7% 73%	2301
	21st to end	Day	5% 2 1 2 2 2 3 3 4 5% 72%	2618
June	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 76%	2520
	11th to 20th	Day	1 1 1 1 1 1 1 3 3 88%	2520
	21st to end	Day	1 1 1 1 1 2 2 2 3 87%	2520
July	1st to 10th	Day	1 1 1 1 1 2 2 3 4 86%	2520
	11th to 20th	Day	1 1 1 1 2 1 2 2 3 4 82%	2520
	21st to end	Day	1 1 1 1 2 1 2 2 3 5% 79%	2574
August	1st to 10th	Day	1 1 1 1 1 1 2 2 2 3 85%	2149
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 87%	2122
	21st to end	Day	1 1 1 1 1 2 3 3 87%	2376
September	1st to 10th	Day	1 1 1 1 1 2 2 2 88%	2160
	11th to 20th	Day	1 1 1 1 2 2 2 3 87%	2080
	21st to end	Day	2 1 1 1 1 2 2 3 4 83%	1980
October	1st to 10th	Day	4 2 1 2 2 1 2 3 5% 6% 72%	1936
	11th to 20th	Day	2 2 1 3 2 2 3 3 3 5% 75%	1800
	21st to end	Day	4 2 1 2 3 1 3 3 3 5% 74%	1954
November	1st to 10th	Day	5% 4 1 4 2 2 3 3 4 8% 62%	1674
	11th to 20th	Day	6% 4 2 4 3 2 4 3 3 7% 62%	1710
	21st to end	Day	9% 5% 2 4 4 2 4 4 4 7% 56%	1710
December	1st to 10th	Day	10% 5% 2 5% 4 3 4 5% 5% 8% 51%	1710
	11th to 20th	Day	15% 7% 3 6% 5% 3 5% 5% 6% 9% 37%	1934
	21st to end	Day	12% 10% 4 7% 6% 3 5% 5% 6% 8% 33%	2794
January	1st to 10th	Night	16% 10% 3 5% 4 3 3 3 4 6% 43%	4406
	11th to 20th	Night	11% 7% 4 7% 6% 3 4 4 5% 8% 43%	4225
	21st to end	Night	18% 7% 3 5% 4 2 4 4 6% 8% 38%	4521
February	1st to 10th	Night	14% 5% 2 5% 4 2 3 3 3 7% 51%	2284
	11th to 20th	Night	12% 3 1 2 2 2 3 3 5% 8% 59%	2151
	21st to end	Night	16% 5% 2 3 3 2 3 3 3 5% 53%	1776
March	1st to 10th	Night	11% 4 2 5% 5% 2 4 4 5% 8% 49%	2160
	11th to 20th	Night	11% 5% 2 3 3 3 4 4 5% 7% 52%	2062
	21st to end	Night	13% 6% 2 3 3 2 3 3 5% 6% 54%	1998
April	1st to 10th	Night	9% 4 2 3 2 2 3 3 4 7% 60%	1800
	11th to 20th	Night	8% 3 1 3 4 2 3 3 4 6% 65%	1779
	21st to end	Night	7% 3 1 2 2 1 2 3 3 6% 71%	1800
May	1st to 10th	Night	6% 2 1 2 2 1 2 3 3 7% 70%	1738
	11th to 20th	Night	5% 2 1 1 2 2 2 3 3 6% 72%	1594
	21st to end	Night	5% 3 1 2 2 1 2 2 4 5% 73%	1653
June	1st to 10th	Night	3 2 1 2 2 1 2 2 3 6% 76%	1440
	11th to 20th	Night	1 1 1 2 1 1 2 2 3 87%	1440
	21st to end	Night	1 1 1 1 3 2 90%	1440
July	1st to 10th	Night	1 1 1 1 1 2 3 4 87%	1440
	11th to 20th	Night	1 1 1 1 1 1 2 2 2 5% 83%	1440
	21st to end	Night	1 1 1 2 3 1 2 3 2 4 80%	1736
August	1st to 10th	Night	2 1 1 1 2 1 1 1 2 3 86%	1790
	11th to 20th	Night	1 1 1 1 1 2 4 89%	1780
	21st to end	Night	1 1 1 1 2 2 2 88%	1980
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	1800
	11th to 20th	Night	1 1 1 1 1 2 3 3 86%	1879
	21st to end	Night	2 1 1 1 1 2 2 2 4 84%	1980
October	1st to 10th	Night	3 2 1 2 2 1 3 3 3 4 76%	2020
	11th to 20th	Night	3 1 1 2 2 1 2 3 4 6% 74%	2160
	21st to end	Night	4 2 1 2 2 1 1 2 3 4 78%	2402
November	1st to 10th	Night	7% 3 2 3 3 2 3 3 4 6% 65%	2405
	11th to 20th	Night	6% 3 1 3 3 2 3 3 4 7% 66%	2470
	21st to end	Night	9% 5% 2 4 3 2 3 3 4 7% 58%	2470
December	1st to 10th	Night	9% 5% 2 5% 4 2 3 4 4 6% 54%	2470
	11th to 20th	Night	15% 6% 2 6% 5% 3 4 5% 6% 8% 39%	3266
	21st to end	Night	10% 7% 3 6% 7% 3 6% 5% 6% 8% 38%	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	41% 6% 17% 24% 9% 2	2539
	11th to 20th	Day	39% 7% 18% 19% 10% 6%	2738
	21st to end	Day	46% 5% 16% 18% 12% 3	3146
February	1st to 10th	Day	34% 4 10% 26% 20% 7%	1893
	11th to 20th	Day	30% 3 11% 28% 20% 7%	1801
	21st to end	Day	40% 3 11% 27% 16% 3	1480
March	1st to 10th	Day	36% 4 13% 24% 18% 5%	1800
	11th to 20th	Day	31% 4 14% 26% 21% 4	1894
	21st to end	Day	31% 5% 14% 30% 15% 4	2357
April	1st to 10th	Day	26% 3 14% 30% 21% 7%	2160
	11th to 20th	Day	24% 3 12% 29% 23% 9%	2132
	21st to end	Day	20% 3 11% 33% 27% 6%	2160
May	1st to 10th	Day	16% 3 10% 26% 35% 10%	2220
	11th to 20th	Day	14% 1 8% 31% 33% 13%	2304
	21st to end	Day	16% 2 8% 33% 31% 9%	2618
June	1st to 10th	Day	13% 2 9% 22% 34% 20%	2520
	11th to 20th	Day	5% 1 5% 15% 38% 36%	2520
	21st to end	Day	6% 1 2 9% 32% 50%	2520
July	1st to 10th	Day	6% 1 2 9% 26% 57%	2451
	11th to 20th	Day	9% 2 3 10% 29% 49%	2445
	21st to end	Day	10% 2 3 14% 30% 40%	2431
August	1st to 10th	Day	8% 1 2 8% 33% 49%	2135
	11th to 20th	Day	6% 2 1 10% 34% 47%	2122
	21st to end	Day	6% 1 1 8% 38% 46%	2376
September	1st to 10th	Day	6% 1 2 9% 30% 52%	2160
	11th to 20th	Day	6% 2 2 17% 44% 30%	2080
	21st to end	Day	8% 1 2 18% 41% 29%	1980
October	1st to 10th	Day	15% 2 7% 21% 28% 28%	1936
	11th to 20th	Day	14% 2 5% 23% 29% 27%	1800
	21st to end	Day	15% 2 5% 23% 31% 24%	1954
November	1st to 10th	Day	22% 3 8% 23% 29% 15%	1674
	11th to 20th	Day	25% 3 10% 25% 28% 9%	1710
	21st to end	Day	30% 3 7% 22% 24% 14%	1710
December	1st to 10th	Day	32% 4 11% 22% 19% 11%	1710
	11th to 20th	Day	45% 5% 11% 18% 15% 6%	1934
	21st to end	Day	44% 9% 12% 22% 9% 3	2794
January	1st to 10th	Night	43% 4 14% 26% 11% 2	4406
	11th to 20th	Night	40% 5% 22% 18% 9% 6%	4225
	21st to end	Night	44% 4 16% 21% 12% 3	4521
February	1st to 10th	Night	36% 2 8% 28% 18% 7%	2284
	11th to 20th	Night	24% 4 12% 28% 25% 8%	2152
	21st to end	Night	36% 3 12% 29% 17% 4	1776
March	1st to 10th	Night	33% 4 14% 25% 19% 5%	2160
	11th to 20th	Night	32% 4 15% 25% 21% 4	2062
	21st to end	Night	33% 3 14% 31% 16% 4	1998
April	1st to 10th	Night	27% 2 13% 29% 19% 9%	1800
	11th to 20th	Night	22% 3 12% 28% 25% 9%	1779
	21st to end	Night	18% 2 11% 35% 30% 4	1800
May	1st to 10th	Night	18% 2 8% 26% 36% 11%	1738
	11th to 20th	Night	16% 2 6% 25% 39% 12%	1595
	21st to end	Night	16% 3 8% 25% 36% 12%	1653
June	1st to 10th	Night	14% 2 8% 22% 34% 21%	1440
	11th to 20th	Night	7% 1 4 16% 35% 37%	1440
	21st to end	Night	4 1 2 8% 35% 50%	1440
July	1st to 10th	Night	5% 1 2 8% 24% 60%	1400
	11th to 20th	Night	7% 1 3 10% 23% 56%	1392
	21st to end	Night	11% 3 3 13% 28% 43%	1640
August	1st to 10th	Night	8% 1 1 11% 30% 48%	1780
	11th to 20th	Night	4 1 2 12% 38% 43%	1780
	21st to end	Night	5% 1 1 9% 39% 44%	1980
September	1st to 10th	Night	6% 1 2 11% 29% 52%	1800
	11th to 20th	Night	6% 2 2 16% 46% 28%	1879
	21st to end	Night	7% 1 2 19% 40% 30%	1980
October	1st to 10th	Night	14% 2 6% 19% 30% 29%	2020
	11th to 20th	Night	15% 2 6% 22% 30% 26%	2160
	21st to end	Night	13% 2 4 26% 31% 24%	2402
November	1st to 10th	Night	22% 3 9% 24% 26% 16%	2405
	11th to 20th	Night	21% 3 12% 24% 29% 12%	2470
	21st to end	Night	28% 2 8% 22% 30% 10%	2470
December	1st to 10th	Night	31% 3 12% 23% 19% 12%	2470
	11th to 20th	Night	41% 6% 12% 20% 17% 4	3266
	21st to end	Night	41% 7% 16% 24% 9% 2	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	22% 2 10% 38% 22% 6%	180
	11th to 20th	Day	14% 1 10% 40% 24% 11%	300
	21st to end	Day	14% 3 11% 36% 27% 10%	407
February	1st to 10th	Day	16% 2 14% 33% 25% 10%	457
	11th to 20th	Day	13% 4 16% 37% 21% 9%	432
	21st to end	Day	15% 3 19% 37% 24% 3	346
March	1st to 10th	Day	13% 5% 21% 35% 21% 5%	420
	11th to 20th	Day	14% 3 19% 33% 25% 6%	420
	21st to end	Day	13% 6% 24% 37% 17% 4	462
April	1st to 10th	Day	10% 6% 22% 33% 22% 8%	420
	11th to 20th	Day	10% 7% 25% 31% 21% 6%	412
	21st to end	Day	5% 7% 24% 29% 24% 12%	420
May	1st to 10th	Day	4 4 26% 35% 23% 8%	423
	11th to 20th	Day	6% 2 22% 34% 26% 10%	448
	21st to end	Day	7% 4 18% 30% 31% 10%	485
June	1st to 10th	Day	2 3 16% 31% 35% 12%	450
	11th to 20th	Day	1 2 14% 30% 39% 13%	450
	21st to end	Day	1 1 12% 35% 38% 13%	450
July	1st to 10th	Day	1 6% 30% 44% 19%	446
	11th to 20th	Day	1 1 3 20% 49% 27%	444
	21st to end	Day	2 1 2 27% 46% 23%	487
August	1st to 10th	Day	2 1 10% 34% 35% 18%	447
	11th to 20th	Day	1 6% 32% 41% 19%	437
	21st to end	Day	1 5% 30% 44% 19%	495
September	1st to 10th	Day	3 1 8% 35% 36% 18%	450
	11th to 20th	Day	2 1 11% 42% 35% 10%	450
	21st to end	Day	2 2 13% 30% 38% 15%	450
October	1st to 10th	Day	3 3 17% 33% 29% 15%	450
	11th to 20th	Day	3 3 15% 33% 36% 11%	450
	21st to end	Day	3 3 12% 33% 38% 12%	469
November	1st to 10th	Day	8% 10% 30% 39% 13%	270
	11th to 20th	Day	8% 10% 32% 31% 19%	270
	21st to end	Day	11% 1 14% 28% 32% 14%	270
December	1st to 10th	Day	16% 3 10% 36% 26% 9%	270
	11th to 20th	Day	15% 2 8% 34% 26% 15%	183
	21st to end	Day	22% 2 11% 34% 23% 8%	198
January	1st to 10th	Night	18% 3 6% 37% 28% 8%	760
	11th to 20th	Night	11% 1 7% 43% 31% 6%	760
	21st to end	Night	17% 1 5% 44% 27% 5%	836
February	1st to 10th	Night	16% 2 6% 40% 29% 6%	760
	11th to 20th	Night	13% 2 8% 41% 28% 7%	722
	21st to end	Night	19% 2 8% 44% 24% 4	592
March	1st to 10th	Night	13% 2 10% 46% 26% 3	720
	11th to 20th	Night	13% 2 7% 40% 31% 7%	720
	21st to end	Night	16% 3 11% 41% 27% 3	792
April	1st to 10th	Night	11% 3 12% 43% 28% 3	720
	11th to 20th	Night	9% 4 13% 40% 30% 3	711
	21st to end	Night	9% 3 13% 35% 35% 5%	720
May	1st to 10th	Night	7% 2 15% 40% 32% 3	658
	11th to 20th	Night	6% 2 15% 36% 38% 3	531
	21st to end	Night	8% 5% 12% 42% 31% 3	491
June	1st to 10th	Night	5% 3 9% 43% 37% 4	360
	11th to 20th	Night	2 1 7% 50% 36% 4	360
	21st to end	Night	1 6% 44% 45% 4	360
July	1st to 10th	Night	1 2 32% 59% 6%	350
	11th to 20th	Night	1 1 24% 66% 8%	348
	21st to end	Night	1 1 22% 64% 12%	554
August	1st to 10th	Night	2 3 33% 56% 7%	716
	11th to 20th	Night	1 2 26% 61% 10%	712
	21st to end	Night	1 2 20% 65% 12%	792
September	1st to 10th	Night	2 2 26% 58% 13%	720
	11th to 20th	Night	2 3 36% 51% 8%	720
	21st to end	Night	2 4 33% 49% 11%	720
October	1st to 10th	Night	4 1 5% 40% 38% 11%	720
	11th to 20th	Night	4 1 5% 36% 43% 11%	720
	21st to end	Night	4 1 5% 37% 38% 15%	792
November	1st to 10th	Night	9% 1 6% 35% 38% 11%	740
	11th to 20th	Night	8% 1 4 42% 35% 10%	760
	21st to end	Night	13% 2 6% 39% 33% 8%	760
December	1st to 10th	Night	11% 2 6% 46% 28% 7%	760
	11th to 20th	Night	16% 1 7% 40% 30% 5%	760
	21st to end	Night	18% 1 6% 46% 24% 5%	836

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