Torrance Barrens Dark-Sky Preserve
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Torrance Barrens Dark-Sky Preserve clear sky chart from 2005-11-06 to 2025-08-05.

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	17% 11% 4 8% 5% 4 4 3 5% 6% 32%	126820
	Night	17% 13% 5% 10% 5% 4 4 3 4 6% 29%	121074
Transparency	Day	53% 3 11% 21% 11% 1	130402
	Night	57% 3 10% 19% 10% 1	125581
Seeing	Day	23% 15% 30% 20% 9% 3	15863
	Night	23% 13% 28% 25% 10% 1	24564

By Month

Percentage Cloud Cover NE (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	14% 16% 9% 10% 5% 5% 4 3 4 6% 24%	3457
	11th to 20th	Day	11% 20% 8% 18% 9% 7% 5% 4 4 4 11%	3775
	21st to end	Day	14% 19% 8% 15% 6% 5% 5% 3 5% 4 16%	4466
February	1st to 10th	Day	13% 21% 6% 16% 8% 6% 3 2 3 4 19%	1800
	11th to 20th	Day	14% 16% 6% 14% 6% 6% 4 3 4 4 24%	1753
	21st to end	Day	17% 18% 6% 12% 6% 5% 5% 3 4 5% 20%	1509
March	1st to 10th	Day	18% 14% 5% 10% 5% 4 4 3 4 5% 29%	4932
	11th to 20th	Day	13% 14% 5% 10% 6% 5% 6% 4 5% 6% 26%	4980
	21st to end	Day	19% 15% 5% 8% 3 3 3 2 3 5% 34%	5465
April	1st to 10th	Day	20% 14% 4 8% 5% 3 4 3 4 6% 30%	2370
	11th to 20th	Day	22% 12% 2 6% 4 3 4 3 4 4 36%	2482
	21st to end	Day	22% 11% 4 6% 4 3 4 3 4 5% 34%	2600
May	1st to 10th	Day	20% 9% 2 6% 4 5% 5% 4 5% 7% 34%	5970
	11th to 20th	Day	16% 9% 3 7% 4 4 4 3 5% 7% 40%	6318
	21st to end	Day	22% 9% 2 6% 4 4 4 3 5% 6% 35%	6949
June	1st to 10th	Day	19% 7% 3 6% 5% 4 4 4 5% 7% 36%	2800
	11th to 20th	Day	14% 6% 2 5% 5% 4 5% 5% 7% 9% 38%	2982
	21st to end	Day	19% 7% 2 6% 4 4 4 4 5% 7% 36%	3000
July	1st to 10th	Day	15% 5% 2 6% 4 4 5% 4 6% 9% 38%	6819
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 48%	6438
	21st to end	Day	9% 5% 2 5% 4 3 4 4 6% 8% 50%	6635
August	1st to 10th	Day	14% 7% 2 8% 4 4 5% 4 6% 8% 39%	4156
	11th to 20th	Day	17% 7% 2 6% 4 4 5% 4 6% 9% 36%	4044
	21st to end	Day	9% 4 2 6% 5% 5% 5% 4 6% 9% 45%	4386
September	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 4 5% 7% 40%	2280
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 6% 7% 44%	2174
	21st to end	Day	18% 8% 3 7% 4 4 5% 4 5% 6% 35%	2048
October	1st to 10th	Day	21% 8% 2 7% 5% 4 5% 3 5% 6% 34%	1900
	11th to 20th	Day	25% 11% 3 9% 6% 4 5% 3 5% 6% 23%	1900
	21st to end	Day	30% 11% 4 8% 5% 4 4 3 4 5% 21%	2090
November	1st to 10th	Day	18% 15% 4 8% 5% 4 4 3 5% 7% 26%	1620
	11th to 20th	Day	19% 14% 5% 10% 6% 5% 5% 3 6% 5% 22%	1600
	21st to end	Day	19% 22% 5% 9% 7% 5% 5% 4 4 5% 15%	1600
December	1st to 10th	Day	17% 28% 6% 9% 5% 4 4 4 5% 5% 14%	2600
	11th to 20th	Day	18% 23% 8% 14% 4 3 3 2 3 3 17%	2986
	21st to end	Day	23% 20% 6% 14% 5% 4 3 2 3 4 17%	3936
January	1st to 10th	Night	14% 20% 8% 12% 5% 6% 5% 3 4 5% 18%	6349
	11th to 20th	Night	12% 17% 7% 20% 8% 5% 5% 3 5% 4 14%	6105
	21st to end	Night	13% 19% 8% 14% 6% 6% 4 3 3 5% 19%	6490
February	1st to 10th	Night	12% 18% 6% 14% 8% 5% 4 3 5% 5% 19%	2600
	11th to 20th	Night	13% 16% 6% 12% 6% 6% 4 4 4 5% 23%	2533
	21st to end	Night	17% 16% 5% 10% 6% 5% 5% 3 5% 5% 23%	2117
March	1st to 10th	Night	17% 13% 4 10% 4 4 4 3 5% 6% 29%	4908
	11th to 20th	Night	11% 14% 7% 8% 5% 5% 5% 4 5% 6% 29%	4979
	21st to end	Night	21% 12% 5% 7% 4 3 4 3 5% 5% 32%	5467
April	1st to 10th	Night	21% 13% 4 8% 5% 4 4 3 4 4 30%	2030
	11th to 20th	Night	20% 10% 3 7% 4 4 4 3 5% 6% 36%	1884
	21st to end	Night	19% 10% 3 8% 6% 5% 4 4 5% 6% 31%	1800
May	1st to 10th	Night	19% 8% 3 6% 5% 5% 5% 4 5% 7% 35%	3970
	11th to 20th	Night	14% 7% 2 6% 5% 3 4 4 5% 7% 43%	3575
	21st to end	Night	19% 8% 2 6% 4 4 4 3 4 7% 37%	3926
June	1st to 10th	Night	16% 9% 2 7% 5% 4 4 4 6% 8% 36%	1600
	11th to 20th	Night	12% 7% 2 5% 5% 4 4 5% 4 9% 43%	1412
	21st to end	Night	15% 8% 2 7% 5% 4 4 4 5% 8% 38%	1400
July	1st to 10th	Night	13% 6% 2 7% 4 5% 4 4 5% 8% 41%	3160
	11th to 20th	Night	15% 5% 2 5% 4 3 4 3 5% 6% 48%	3509
	21st to end	Night	11% 5% 2 5% 4 4 3 3 5% 8% 51%	4213
August	1st to 10th	Night	11% 6% 2 5% 4 5% 5% 4 5% 7% 46%	2861
	11th to 20th	Night	17% 6% 3 6% 4 4 4 3 4 6% 45%	2785
	21st to end	Night	15% 5% 2 5% 3 4 4 3 5% 7% 48%	3238
September	1st to 10th	Night	12% 6% 2 6% 4 3 5% 3 5% 8% 45%	1900
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 7% 49%	2004
	21st to end	Night	18% 7% 2 6% 4 5% 3 3 6% 6% 41%	2131
October	1st to 10th	Night	18% 8% 3 6% 4 5% 5% 3 5% 7% 36%	2280
	11th to 20th	Night	26% 11% 3 8% 4 4 4 4 5% 6% 27%	2280
	21st to end	Night	25% 13% 4 9% 4 5% 4 3 4 5% 23%	2508
November	1st to 10th	Night	20% 14% 4 7% 5% 4 3 3 5% 5% 31%	2676
	11th to 20th	Night	17% 19% 4 8% 5% 4 4 4 5% 6% 25%	2800
	21st to end	Night	23% 19% 6% 8% 6% 4 5% 4 4 6% 15%	2800
December	1st to 10th	Night	18% 27% 6% 11% 5% 4 4 3 4 5% 12%	4580
	11th to 20th	Night	16% 22% 9% 16% 6% 5% 4 3 4 4 12%	5208
	21st to end	Night	24% 20% 6% 15% 5% 4 3 2 3 4 14%	6996

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	62% 3 9% 17% 9%	3458
	11th to 20th	Day	77% 2 7% 9% 5%	3807
	21st to end	Day	74% 2 7% 9% 8%	4466
February	1st to 10th	Day	73% 2 6% 14% 4 1	1800
	11th to 20th	Day	66% 2 8% 14% 8% 1	1762
	21st to end	Day	69% 2 7% 15% 6%	1509
March	1st to 10th	Day	61% 2 12% 11% 14% 1	4935
	11th to 20th	Day	59% 2 13% 16% 7% 2	4980
	21st to end	Day	57% 1 7% 20% 14% 2	5465
April	1st to 10th	Day	58% 2 8% 20% 11% 1	2370
	11th to 20th	Day	54% 2 7% 24% 13% 1	2482
	21st to end	Day	54% 3 8% 19% 13% 3	2600
May	1st to 10th	Day	52% 2 9% 21% 10% 5%	5979
	11th to 20th	Day	47% 2 10% 21% 18% 1	6322
	21st to end	Day	51% 3 11% 20% 13% 3	6949
June	1st to 10th	Day	49% 2 10% 25% 14%	2800
	11th to 20th	Day	41% 4 12% 27% 15% 1	2982
	21st to end	Day	47% 3 14% 23% 12% 1	3000
July	1st to 10th	Day	42% 4 15% 27% 11%	6746
	11th to 20th	Day	37% 4 12% 33% 14%	6361
	21st to end	Day	33% 4 13% 35% 15%	6445
August	1st to 10th	Day	44% 4 15% 26% 11% 1	4143
	11th to 20th	Day	45% 4 16% 26% 8%	4083
	21st to end	Day	35% 4 13% 37% 10% 1	4386
September	1st to 10th	Day	44% 3 12% 29% 11% 1	2280
	11th to 20th	Day	40% 3 13% 30% 13%	2174
	21st to end	Day	50% 5% 11% 25% 9%	2048
October	1st to 10th	Day	47% 4 12% 26% 10% 1	3160
	11th to 20th	Day	53% 4 13% 23% 7%	3140
	21st to end	Day	59% 4 14% 17% 6%	3428
November	1st to 10th	Day	59% 3 12% 19% 6%	1620
	11th to 20th	Day	64% 4 12% 13% 6% 1	1600
	21st to end	Day	73% 3 9% 12% 3	1600
December	1st to 10th	Day	72% 4 12% 9% 3	2600
	11th to 20th	Day	74% 2 6% 13% 5%	2986
	21st to end	Day	75% 2 6% 11% 7%	3936
January	1st to 10th	Night	72% 2 6% 13% 7%	6349
	11th to 20th	Night	75% 2 8% 9% 5% 1	6148
	21st to end	Night	70% 1 9% 10% 9% 1	6490
February	1st to 10th	Night	70% 2 8% 15% 5% 1	2600
	11th to 20th	Night	64% 3 9% 16% 8%	2535
	21st to end	Night	64% 2 9% 18% 7%	2117
March	1st to 10th	Night	58% 2 10% 14% 12% 4	4918
	11th to 20th	Night	57% 2 15% 17% 8% 1	4980
	21st to end	Night	57% 1 6% 20% 14% 2	5467
April	1st to 10th	Night	59% 2 7% 19% 11% 1	2030
	11th to 20th	Night	51% 3 6% 23% 15% 2	1884
	21st to end	Night	55% 3 8% 17% 16% 2	1800
May	1st to 10th	Night	50% 2 9% 21% 14% 4	3977
	11th to 20th	Night	42% 2 11% 24% 19% 1	3576
	21st to end	Night	48% 2 10% 25% 14% 1	3927
June	1st to 10th	Night	47% 3 10% 23% 16% 1	1600
	11th to 20th	Night	39% 3 13% 25% 19% 1	1412
	21st to end	Night	46% 5% 12% 24% 12% 1	1400
July	1st to 10th	Night	42% 4 13% 26% 14% 2	3125
	11th to 20th	Night	37% 4 11% 32% 15%	3464
	21st to end	Night	34% 3 8% 37% 17% 1	4088
August	1st to 10th	Night	37% 3 15% 27% 16% 1	2850
	11th to 20th	Night	42% 4 17% 25% 13%	2812
	21st to end	Night	37% 4 11% 35% 12% 1	3238
September	1st to 10th	Night	38% 4 16% 27% 13% 1	1900
	11th to 20th	Night	37% 3 14% 32% 14%	2004
	21st to end	Night	46% 2 12% 28% 11%	2131
October	1st to 10th	Night	40% 4 10% 28% 17%	3800
	11th to 20th	Night	51% 5% 13% 24% 8%	3779
	21st to end	Night	55% 4 15% 19% 7%	4120
November	1st to 10th	Night	57% 3 11% 21% 8%	2676
	11th to 20th	Night	62% 3 11% 16% 7% 1	2800
	21st to end	Night	71% 4 10% 12% 3	2800
December	1st to 10th	Night	75% 3 12% 8% 2	4580
	11th to 20th	Night	78% 2 8% 8% 3	5208
	21st to end	Night	78% 2 5% 9% 5%	6996

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	19% 26% 37% 14% 6%	200
	11th to 20th	Day	23% 24% 34% 13% 4 1	245
	21st to end	Day	26% 17% 36% 17% 4	330
February	1st to 10th	Day	29% 28% 32% 7% 4	300
	11th to 20th	Day	27% 24% 38% 9% 1	300
	21st to end	Day	40% 21% 33% 7%	251
March	1st to 10th	Day	25% 20% 35% 15% 4	500
	11th to 20th	Day	24% 20% 33% 18% 4	500
	21st to end	Day	34% 15% 29% 16% 6%	550
April	1st to 10th	Day	29% 21% 28% 15% 7% 1	500
	11th to 20th	Day	36% 16% 22% 16% 7% 3	492
	21st to end	Day	32% 14% 30% 15% 6% 2	500
May	1st to 10th	Day	27% 12% 30% 22% 7% 3	503
	11th to 20th	Day	20% 14% 30% 21% 12% 3	527
	21st to end	Day	20% 14% 30% 21% 11% 3	573
June	1st to 10th	Day	20% 15% 29% 28% 7% 1	530
	11th to 20th	Day	17% 12% 32% 26% 11% 3	712
	21st to end	Day	21% 11% 35% 23% 8% 2	730
July	1st to 10th	Day	13% 9% 33% 27% 15% 3	721
	11th to 20th	Day	15% 10% 27% 29% 13% 5%	561
	21st to end	Day	13% 8% 33% 28% 14% 5%	576
August	1st to 10th	Day	14% 8% 24% 31% 15% 8%	502
	11th to 20th	Day	16% 6% 24% 28% 16% 9%	479
	21st to end	Day	15% 9% 29% 22% 18% 6%	539
September	1st to 10th	Day	17% 9% 28% 26% 13% 7%	490
	11th to 20th	Day	13% 7% 27% 23% 20% 10%	490
	21st to end	Day	20% 12% 26% 24% 15% 4	490
October	1st to 10th	Day	24% 16% 27% 18% 11% 5%	490
	11th to 20th	Day	27% 19% 30% 17% 6%	490
	21st to end	Day	35% 19% 29% 14% 4	539
November	1st to 10th	Day	32% 17% 31% 15% 5%	233
	11th to 20th	Day	28% 23% 34% 13% 3	200
	21st to end	Day	28% 22% 38% 12% 1	200
December	1st to 10th	Day	30% 21% 30% 16% 4	200
	11th to 20th	Day	35% 29% 20% 13% 3 1	200
	21st to end	Day	34% 22% 30% 11% 2	220
January	1st to 10th	Night	30% 16% 32% 18% 4 1	800
	11th to 20th	Night	30% 16% 31% 18% 5% 1	800
	21st to end	Night	22% 11% 34% 25% 7% 1	880
February	1st to 10th	Night	23% 20% 32% 22% 3	800
	11th to 20th	Night	22% 18% 34% 21% 6%	783
	21st to end	Night	27% 19% 29% 21% 4	660
March	1st to 10th	Night	23% 18% 33% 22% 3	800
	11th to 20th	Night	23% 16% 34% 22% 5% 1	800
	21st to end	Night	22% 14% 31% 24% 9%	880
April	1st to 10th	Night	28% 16% 29% 22% 4	800
	11th to 20th	Night	26% 13% 28% 22% 9% 1	700
	21st to end	Night	25% 11% 29% 27% 9% 1	600
May	1st to 10th	Night	22% 7% 25% 33% 12% 1	569
	11th to 20th	Night	19% 9% 31% 31% 10%	394
	21st to end	Night	15% 5% 29% 35% 15%	433
June	1st to 10th	Night	18% 7% 32% 33% 10% 1	400
	11th to 20th	Night	13% 10% 28% 38% 12%	400
	21st to end	Night	18% 8% 24% 37% 13%	400
July	1st to 10th	Night	10% 5% 19% 46% 18% 1	390
	11th to 20th	Night	14% 6% 24% 36% 21% 1	388
	21st to end	Night	12% 3 26% 36% 23% 1	559
August	1st to 10th	Night	12% 4 18% 37% 25% 3	579
	11th to 20th	Night	11% 5% 18% 36% 28% 2	564
	21st to end	Night	16% 7% 24% 30% 22% 2	689
September	1st to 10th	Night	14% 7% 26% 31% 18% 4	760
	11th to 20th	Night	15% 6% 23% 29% 22% 5%	760
	21st to end	Night	21% 8% 19% 33% 17% 2	760
October	1st to 10th	Night	21% 12% 28% 24% 13% 2	760
	11th to 20th	Night	30% 14% 24% 22% 8% 1	760
	21st to end	Night	31% 17% 29% 19% 4	836
November	1st to 10th	Night	29% 14% 26% 22% 8% 1	780
	11th to 20th	Night	26% 19% 31% 18% 5% 1	800
	21st to end	Night	35% 14% 30% 18% 3	800
December	1st to 10th	Night	34% 20% 28% 14% 4	800
	11th to 20th	Night	28% 19% 34% 15% 4 1	800
	21st to end	Night	33% 16% 30% 18% 3	880

Page generated at 2025-08-02 01:32:27. Version ac07

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