Arrowhead Beach
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arrowhead Beach clear sky chart from 2005-11-06 to 2026-09-14.

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	12% 6% 2 7% 4 4 5% 4 6% 8% 41%	148160
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 43%	142887
Transparency	Day	39% 7% 13% 21% 17% 4	151758
	Night	39% 6% 12% 20% 18% 5%	147360
Seeing	Day	16% 11% 26% 25% 16% 6%	17789
	Night	16% 9% 28% 31% 14% 2	27445

By Month

Percentage Cloud Cover SE (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% 5% 2 4 3 3 3 2 4 6% 56%	5181
	11th to 20th	Day	14% 6% 2 4 3 2 3 2 4 4 56%	5280
	21st to end	Day	12% 10% 3 8% 4 4 4 3 4 5% 42%	5780
February	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 5% 7% 43%	1890
	11th to 20th	Day	19% 7% 2 5% 4 4 3 3 4 5% 43%	2001
	21st to end	Day	16% 8% 2 7% 4 3 4 4 5% 7% 40%	1730
March	1st to 10th	Day	19% 8% 3 6% 4 3 4 3 4 6% 40%	4954
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 4 6% 53%	5090
	21st to end	Day	12% 5% 2 6% 4 4 4 3 4 6% 49%	5713
April	1st to 10th	Day	12% 5% 1 5% 3 3 3 3 4 8% 51%	2520
	11th to 20th	Day	9% 5% 2 5% 5% 4 4 3 5% 7% 52%	2494
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 5% 7% 47%	2520
May	1st to 10th	Day	12% 6% 2 8% 5% 5% 5% 4 7% 9% 38%	7021
	11th to 20th	Day	14% 6% 2 7% 4 4 4 4 6% 8% 41%	7619
	21st to end	Day	13% 7% 3 8% 5% 4 4 4 6% 7% 38%	9045
June	1st to 10th	Day	9% 6% 2 6% 6% 5% 6% 5% 6% 10% 39%	2940
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 5% 9% 10% 40%	2940
	21st to end	Day	6% 5% 2 6% 5% 6% 6% 5% 9% 12% 38%	2940
July	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 5% 9% 13% 36%	8108
	11th to 20th	Day	10% 6% 3 8% 6% 6% 6% 5% 8% 12% 29%	7680
	21st to end	Day	13% 7% 2 8% 5% 5% 5% 5% 8% 11% 31%	8340
August	1st to 10th	Day	15% 8% 3 9% 6% 6% 6% 5% 8% 11% 21%	5974
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 5% 8% 12% 35%	5488
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 5% 6% 10% 40%	6080
September	1st to 10th	Day	12% 6% 2 6% 4 5% 6% 4 5% 10% 40%	2520
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	2436
	21st to end	Day	18% 8% 3 8% 4 4 5% 3 5% 7% 34%	2127
October	1st to 10th	Day	14% 7% 3 5% 4 3 4 3 6% 7% 44%	2000
	11th to 20th	Day	12% 6% 2 6% 4 3 3 3 5% 6% 51%	2000
	21st to end	Day	11% 5% 2 6% 3 3 4 3 4 5% 53%	2200
November	1st to 10th	Day	14% 6% 2 5% 3 3 4 3 6% 5% 49%	2024
	11th to 20th	Day	14% 6% 2 4 3 3 3 3 5% 6% 52%	1890
	21st to end	Day	12% 5% 2 5% 4 3 3 3 5% 6% 51%	1837
December	1st to 10th	Day	12% 6% 1 5% 4 3 4 3 5% 6% 51%	2680
	11th to 20th	Day	13% 8% 3 7% 4 4 3 3 5% 5% 45%	3070
	21st to end	Day	13% 5% 2 5% 4 3 4 3 5% 8% 47%	4048
January	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 51%	7694
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 4 4 55%	7680
	21st to end	Night	15% 8% 3 7% 4 4 3 3 4 5% 44%	8401
February	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 5% 7% 43%	2730
	11th to 20th	Night	19% 9% 3 5% 4 3 3 3 5% 6% 40%	2503
	21st to end	Night	15% 8% 2 5% 4 3 3 4 5% 6% 45%	2076
March	1st to 10th	Night	19% 7% 3 5% 4 4 4 3 4 5% 41%	5102
	11th to 20th	Night	12% 6% 2 5% 3 4 4 3 4 6% 50%	5090
	21st to end	Night	14% 5% 2 5% 4 4 4 3 4 6% 50%	5459
April	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 6% 7% 47%	2100
	11th to 20th	Night	10% 7% 2 5% 4 4 4 3 4 8% 50%	2075
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 8% 48%	2100
May	1st to 10th	Night	11% 7% 3 8% 4 6% 6% 4 6% 9% 35%	5833
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	5268
	21st to end	Night	16% 8% 3 7% 5% 4 5% 4 6% 7% 36%	5108
June	1st to 10th	Night	9% 6% 2 6% 4 4 5% 5% 8% 11% 40%	1680
	11th to 20th	Night	8% 5% 3 7% 4 5% 5% 5% 8% 10% 39%	1680
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 4 9% 9% 42%	1680
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 7% 5% 8% 13% 37%	4831
	11th to 20th	Night	12% 7% 3 8% 6% 6% 6% 5% 9% 12% 27%	5277
	21st to end	Night	16% 7% 3 7% 5% 6% 5% 5% 8% 9% 30%	5736
August	1st to 10th	Night	14% 9% 3 8% 5% 5% 5% 4 7% 10% 29%	4135
	11th to 20th	Night	8% 7% 3 8% 6% 6% 6% 5% 6% 10% 34%	4552
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 5% 6% 9% 40%	5045
September	1st to 10th	Night	10% 5% 2 5% 4 5% 4 4 6% 8% 47%	2100
	11th to 20th	Night	15% 7% 2 5% 4 4 4 3 6% 8% 42%	2031
	21st to end	Night	18% 8% 3 7% 5% 4 5% 4 6% 7% 34%	2267
October	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 4 6% 48%	2400
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 6% 51%	2400
	21st to end	Night	12% 5% 2 5% 3 4 3 2 4 6% 54%	2640
November	1st to 10th	Night	15% 6% 2 4 3 3 3 3 4 6% 50%	2492
	11th to 20th	Night	14% 7% 2 5% 3 3 4 3 4 6% 49%	2730
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 6% 50%	2777
December	1st to 10th	Night	10% 6% 2 5% 4 4 5% 3 5% 5% 52%	4720
	11th to 20th	Night	16% 7% 3 8% 5% 5% 5% 4 5% 6% 38%	5345
	21st to end	Night	11% 6% 2 5% 3 4 4 3 5% 7% 50%	7150

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	32% 3 11% 24% 20% 10%	5182
	11th to 20th	Day	33% 2 4 14% 37% 9%	5280
	21st to end	Day	45% 4 6% 21% 17% 7%	5780
February	1st to 10th	Day	40% 5% 11% 23% 17% 4	1890
	11th to 20th	Day	44% 4 8% 18% 21% 5%	2010
	21st to end	Day	43% 5% 9% 18% 19% 5%	1730
March	1st to 10th	Day	45% 4 8% 17% 16% 9%	4954
	11th to 20th	Day	32% 4 7% 19% 25% 12%	5090
	21st to end	Day	36% 5% 12% 20% 21% 5%	5713
April	1st to 10th	Day	32% 5% 12% 22% 23% 6%	2520
	11th to 20th	Day	31% 6% 12% 25% 20% 6%	2494
	21st to end	Day	36% 7% 12% 25% 18% 4	2520
May	1st to 10th	Day	41% 8% 16% 20% 12% 2	7053
	11th to 20th	Day	40% 6% 14% 24% 16% 1	7623
	21st to end	Day	43% 7% 12% 22% 13% 2	9061
June	1st to 10th	Day	38% 10% 12% 22% 17% 2	2940
	11th to 20th	Day	35% 9% 19% 23% 13% 1	2940
	21st to end	Day	35% 11% 17% 24% 12% 2	2940
July	1st to 10th	Day	35% 11% 19% 26% 8% 1	8040
	11th to 20th	Day	45% 11% 21% 17% 6% 1	7611
	21st to end	Day	45% 10% 14% 19% 12%	8156
August	1st to 10th	Day	54% 11% 21% 10% 3	5961
	11th to 20th	Day	38% 11% 22% 21% 8%	5524
	21st to end	Day	37% 8% 15% 27% 11% 1	6080
September	1st to 10th	Day	39% 10% 15% 19% 16% 2	2520
	11th to 20th	Day	44% 6% 11% 19% 17% 2	2436
	21st to end	Day	49% 6% 11% 18% 13% 2	2127
October	1st to 10th	Day	32% 6% 9% 23% 26% 3	3260
	11th to 20th	Day	25% 3 6% 20% 32% 14%	3240
	21st to end	Day	24% 3 6% 26% 32% 9%	3538
November	1st to 10th	Day	35% 5% 10% 21% 24% 4	2024
	11th to 20th	Day	35% 4 9% 23% 23% 7%	1890
	21st to end	Day	34% 5% 8% 25% 23% 5%	1837
December	1st to 10th	Day	34% 5% 9% 24% 23% 5%	2680
	11th to 20th	Day	42% 4 10% 22% 16% 6%	3066
	21st to end	Day	37% 4 11% 26% 21% 1	4048
January	1st to 10th	Night	34% 3 11% 21% 22% 9%	7695
	11th to 20th	Night	34% 4 5% 16% 32% 10%	7680
	21st to end	Night	43% 4 7% 20% 20% 7%	8401
February	1st to 10th	Night	41% 5% 10% 22% 17% 5%	2730
	11th to 20th	Night	45% 4 8% 18% 19% 6%	2505
	21st to end	Night	39% 5% 8% 21% 19% 8%	2076
March	1st to 10th	Night	45% 5% 9% 15% 17% 8%	5102
	11th to 20th	Night	36% 5% 8% 14% 23% 14%	5090
	21st to end	Night	37% 5% 11% 24% 17% 6%	5459
April	1st to 10th	Night	36% 6% 12% 18% 22% 6%	2100
	11th to 20th	Night	33% 5% 11% 27% 19% 5%	2075
	21st to end	Night	35% 7% 11% 26% 18% 3	2100
May	1st to 10th	Night	43% 9% 16% 22% 9% 1	5863
	11th to 20th	Night	41% 6% 13% 26% 13% 1	5269
	21st to end	Night	45% 9% 13% 20% 12% 2	5115
June	1st to 10th	Night	34% 11% 14% 25% 15% 2	1680
	11th to 20th	Night	36% 11% 17% 23% 12% 1	1680
	21st to end	Night	34% 10% 16% 26% 12% 2	1680
July	1st to 10th	Night	37% 11% 19% 23% 10% 1	4791
	11th to 20th	Night	47% 12% 20% 13% 6% 1	5226
	21st to end	Night	48% 11% 14% 16% 11%	5606
August	1st to 10th	Night	49% 10% 22% 15% 4	4124
	11th to 20th	Night	42% 10% 21% 19% 7%	4582
	21st to end	Night	39% 8% 14% 25% 13% 2	5045
September	1st to 10th	Night	33% 11% 18% 20% 16% 3	2100
	11th to 20th	Night	40% 8% 11% 21% 18% 2	2031
	21st to end	Night	48% 7% 12% 17% 14% 2	2267
October	1st to 10th	Night	37% 5% 9% 22% 24% 3	3920
	11th to 20th	Night	26% 4 6% 18% 28% 18%	3899
	21st to end	Night	23% 3 7% 28% 25% 14%	4252
November	1st to 10th	Night	35% 4 9% 21% 25% 5%	2492
	11th to 20th	Night	37% 4 10% 17% 24% 8%	2730
	21st to end	Night	34% 5% 8% 25% 23% 5%	2777
December	1st to 10th	Night	35% 5% 8% 19% 26% 8%	4720
	11th to 20th	Night	47% 4 11% 15% 17% 6%	5348
	21st to end	Night	33% 6% 14% 27% 19% 2	7150

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	20% 26% 39% 12% 3	316
	11th to 20th	Day	19% 26% 40% 13% 2	330
	21st to end	Day	23% 21% 40% 14% 1 1	363
February	1st to 10th	Day	21% 24% 36% 16% 3	330
	11th to 20th	Day	28% 21% 34% 15% 1	489
	21st to end	Day	25% 16% 36% 19% 3 1	446
March	1st to 10th	Day	19% 19% 35% 21% 6%	540
	11th to 20th	Day	21% 18% 36% 17% 5% 2	540
	21st to end	Day	18% 17% 39% 20% 5% 1	594
April	1st to 10th	Day	18% 16% 35% 21% 7% 2	540
	11th to 20th	Day	14% 14% 34% 26% 9% 3	532
	21st to end	Day	13% 14% 32% 28% 12% 2	540
May	1st to 10th	Day	14% 14% 36% 26% 8% 2	543
	11th to 20th	Day	16% 8% 26% 31% 16% 3	567
	21st to end	Day	17% 8% 23% 27% 19% 6%	617
June	1st to 10th	Day	12% 5% 26% 30% 21% 5%	570
	11th to 20th	Day	11% 5% 21% 31% 24% 7%	570
	21st to end	Day	7% 5% 16% 34% 28% 10%	570
July	1st to 10th	Day	9% 4 14% 30% 30% 14%	565
	11th to 20th	Day	8% 3 11% 34% 32% 11%	564
	21st to end	Day	10% 3 15% 34% 28% 10%	620
August	1st to 10th	Day	14% 4 16% 31% 26% 9%	566
	11th to 20th	Day	11% 3 13% 26% 32% 14%	559
	21st to end	Day	11% 3 15% 30% 26% 15%	627
September	1st to 10th	Day	13% 4 17% 27% 24% 15%	570
	11th to 20th	Day	17% 5% 18% 28% 21% 11%	542
	21st to end	Day	22% 7% 21% 25% 20% 6%	530
October	1st to 10th	Day	19% 6% 20% 26% 20% 9%	530
	11th to 20th	Day	15% 9% 27% 28% 16% 5%	530
	21st to end	Day	13% 11% 29% 25% 15% 7%	583
November	1st to 10th	Day	18% 10% 30% 26% 12% 3	517
	11th to 20th	Day	18% 14% 31% 25% 10% 1	430
	21st to end	Day	16% 16% 27% 28% 12% 2	377
December	1st to 10th	Day	21% 16% 40% 16% 5% 1	220
	11th to 20th	Day	18% 16% 36% 21% 8%	220
	21st to end	Day	21% 18% 33% 16% 10% 2	242
January	1st to 10th	Night	18% 18% 34% 24% 5%	840
	11th to 20th	Night	18% 16% 36% 24% 6%	840
	21st to end	Night	18% 15% 42% 23% 2	924
February	1st to 10th	Night	21% 13% 38% 25% 3	840
	11th to 20th	Night	26% 16% 38% 19% 1	823
	21st to end	Night	19% 14% 41% 23% 2	692
March	1st to 10th	Night	17% 13% 37% 27% 5%	840
	11th to 20th	Night	19% 16% 39% 21% 5% 1	840
	21st to end	Night	18% 14% 41% 23% 5%	924
April	1st to 10th	Night	16% 14% 40% 26% 4	840
	11th to 20th	Night	13% 11% 38% 29% 8%	831
	21st to end	Night	14% 8% 36% 35% 7%	840
May	1st to 10th	Night	13% 11% 36% 33% 7%	840
	11th to 20th	Night	16% 7% 27% 38% 11%	644
	21st to end	Night	12% 3 18% 46% 21%	455
June	1st to 10th	Night	10% 3 21% 46% 20%	420
	11th to 20th	Night	10% 3 16% 47% 22% 1	420
	21st to end	Night	9% 1 19% 49% 22% 1	420
July	1st to 10th	Night	9% 2 14% 46% 27% 3	457
	11th to 20th	Night	9% 2 9% 40% 35% 6%	612
	21st to end	Night	9% 3 15% 41% 29% 4	690
August	1st to 10th	Night	15% 1 13% 37% 29% 4	627
	11th to 20th	Night	11% 2 12% 39% 32% 4	825
	21st to end	Night	12% 3 13% 35% 33% 4	924
September	1st to 10th	Night	11% 3 13% 37% 30% 6%	840
	11th to 20th	Night	19% 3 16% 33% 26% 4	813
	21st to end	Night	20% 6% 18% 32% 23% 2	800
October	1st to 10th	Night	15% 4 20% 31% 24% 5%	800
	11th to 20th	Night	19% 6% 27% 30% 17% 1	800
	21st to end	Night	15% 7% 28% 29% 18% 3	880
November	1st to 10th	Night	17% 7% 25% 33% 14% 3	820
	11th to 20th	Night	18% 10% 32% 29% 10% 1	840
	21st to end	Night	13% 11% 32% 29% 13% 1	840
December	1st to 10th	Night	20% 12% 33% 28% 7%	840
	11th to 20th	Night	19% 9% 35% 30% 8%	840
	21st to end	Night	17% 11% 33% 31% 7%	924

Page generated at 2026-09-11 04:11:56. Version ac07

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