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 2024-03-01.

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 5% 5% 3 5% 5% 6% 8% 42%	78568
	Night	13% 6% 3 5% 5% 3 4 5% 5% 7% 44%	80824
Transparency	Day	39% 7% 12% 21% 17% 4	78286
	Night	39% 7% 11% 21% 18% 4	80625
Seeing	Day	17% 11% 26% 25% 16% 6%	14343
	Night	16% 9% 28% 31% 14% 2	24159

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	16% 6% 2 4 4 2 3 3 4 6% 51%	2786
	11th to 20th	Day	13% 7% 2 5% 4 2 4 3 5% 6% 49%	2840
	21st to end	Day	13% 11% 4 7% 4 2 4 4 4 6% 41%	3124
February	1st to 10th	Day	13% 7% 2 6% 5% 3 5% 4 5% 8% 43%	1710
	11th to 20th	Day	19% 7% 2 5% 4 3 4 3 4 5% 45%	1805
	21st to end	Day	16% 8% 2 6% 4 3 4 5% 5% 7% 40%	1570
March	1st to 10th	Day	13% 5% 3 5% 4 2 3 3 3 6% 52%	1944
	11th to 20th	Day	17% 6% 3 5% 5% 2 4 4 4 7% 43%	1980
	21st to end	Day	16% 6% 3 6% 5% 3 4 5% 4 7% 42%	2220
April	1st to 10th	Day	13% 5% 1 4 3 2 3 4 4 8% 53%	2160
	11th to 20th	Day	9% 4 2 4 6% 2 4 4 5% 8% 51%	2134
	21st to end	Day	10% 5% 2 5% 6% 3 5% 5% 5% 7% 47%	2160
May	1st to 10th	Day	11% 5% 2 7% 5% 3 6% 5% 6% 10% 41%	2160
	11th to 20th	Day	14% 6% 3 7% 6% 4 5% 5% 6% 8% 38%	2293
	21st to end	Day	11% 6% 2 6% 6% 3 5% 5% 7% 8% 40%	2725
June	1st to 10th	Day	9% 5% 3 5% 6% 4 6% 7% 7% 11% 37%	2520
	11th to 20th	Day	9% 6% 2 5% 5% 3 6% 7% 8% 10% 39%	2520
	21st to end	Day	7% 5% 2 6% 6% 4 7% 7% 9% 12% 36%	2520
July	1st to 10th	Day	8% 5% 2 5% 6% 4 7% 6% 9% 12% 37%	2474
	11th to 20th	Day	7% 5% 2 6% 6% 5% 7% 7% 8% 11% 35%	2340
	21st to end	Day	7% 5% 3 6% 8% 4 5% 7% 10% 12% 34%	2547
August	1st to 10th	Day	11% 5% 3 7% 6% 5% 8% 8% 7% 10% 31%	2315
	11th to 20th	Day	11% 6% 3 5% 8% 5% 6% 6% 7% 9% 33%	2126
	21st to end	Day	11% 6% 3 7% 6% 4 6% 6% 7% 10% 34%	2376
September	1st to 10th	Day	12% 6% 3 6% 5% 4 6% 6% 6% 10% 37%	2160
	11th to 20th	Day	13% 7% 3 6% 6% 3 5% 5% 5% 8% 39%	2160
	21st to end	Day	18% 8% 4 6% 5% 3 5% 5% 5% 7% 34%	1916
October	1st to 10th	Day	15% 7% 3 4 5% 2 4 5% 6% 7% 43%	1800
	11th to 20th	Day	12% 6% 2 6% 5% 2 4 4 5% 5% 50%	1800
	21st to end	Day	11% 5% 2 5% 5% 3 4 4 4 5% 52%	1980
November	1st to 10th	Day	15% 6% 2 4 4 2 3 4 5% 5% 50%	1828
	11th to 20th	Day	15% 6% 2 4 3 2 3 3 4 6% 52%	1710
	21st to end	Day	14% 5% 2 4 4 3 4 4 4 7% 50%	1663
December	1st to 10th	Day	17% 6% 2 6% 5% 3 4 5% 5% 8% 40%	1520
	11th to 20th	Day	14% 6% 2 4 3 2 2 3 4 5% 54%	1910
	21st to end	Day	15% 6% 3 5% 5% 2 4 4 5% 8% 42%	2772
January	1st to 10th	Night	14% 7% 2 5% 4 3 4 5% 4 6% 47%	4141
	11th to 20th	Night	15% 7% 3 6% 5% 3 4 5% 5% 5% 43%	4120
	21st to end	Night	15% 8% 3 6% 5% 3 4 4 4 5% 43%	4532
February	1st to 10th	Night	17% 6% 3 5% 5% 2 4 4 5% 7% 43%	2470
	11th to 20th	Night	19% 9% 3 5% 4 2 3 4 4 6% 41%	2259
	21st to end	Night	15% 8% 3 4 4 2 3 4 4 6% 45%	1884
March	1st to 10th	Night	14% 4 2 3 4 2 4 4 5% 5% 52%	2019
	11th to 20th	Night	16% 8% 3 5% 5% 3 5% 4 5% 7% 40%	1980
	21st to end	Night	16% 5% 3 5% 5% 3 4 4 4 7% 44%	2134
April	1st to 10th	Night	12% 5% 2 6% 4 3 4 5% 5% 6% 47%	1800
	11th to 20th	Night	11% 6% 3 5% 5% 3 4 4 4 7% 48%	1775
	21st to end	Night	13% 5% 2 5% 4 3 4 4 5% 7% 48%	1800
May	1st to 10th	Night	11% 6% 2 6% 4 4 4 5% 6% 9% 43%	1800
	11th to 20th	Night	13% 6% 2 5% 5% 3 4 5% 6% 9% 40%	1614
	21st to end	Night	10% 5% 2 5% 5% 3 4 6% 8% 10% 42%	1550
June	1st to 10th	Night	10% 5% 2 5% 5% 3 5% 6% 7% 12% 41%	1440
	11th to 20th	Night	8% 5% 4 6% 5% 3 4 6% 8% 11% 40%	1440
	21st to end	Night	8% 5% 3 5% 6% 3 7% 5% 10% 9% 40%	1440
July	1st to 10th	Night	5% 6% 3 8% 6% 3 7% 7% 8% 11% 36%	1481
	11th to 20th	Night	8% 4 3 5% 5% 4 6% 6% 7% 13% 40%	1620
	21st to end	Night	7% 5% 3 5% 6% 4 7% 6% 10% 13% 35%	1771
August	1st to 10th	Night	9% 8% 3 6% 6% 3 5% 6% 6% 10% 38%	1611
	11th to 20th	Night	8% 6% 4 7% 6% 4 6% 6% 7% 11% 35%	1769
	21st to end	Night	11% 7% 3 6% 7% 4 5% 6% 7% 9% 37%	1980
September	1st to 10th	Night	10% 5% 2 5% 4 4 5% 5% 7% 9% 44%	1800
	11th to 20th	Night	13% 6% 2 4 5% 3 4 4 6% 8% 45%	1800
	21st to end	Night	18% 7% 3 6% 6% 3 5% 5% 6% 7% 35%	2039
October	1st to 10th	Night	13% 6% 2 4 4 3 4 4 4 6% 48%	2160
	11th to 20th	Night	14% 6% 2 4 4 2 4 4 4 6% 50%	2160
	21st to end	Night	11% 5% 2 4 4 3 4 3 4 6% 53%	2376
November	1st to 10th	Night	16% 5% 3 4 3 2 3 4 4 6% 50%	2248
	11th to 20th	Night	15% 6% 2 4 3 2 4 4 4 6% 49%	2470
	21st to end	Night	12% 5% 2 4 5% 3 4 4 5% 6% 50%	2512
December	1st to 10th	Night	15% 8% 3 5% 5% 3 5% 4 5% 6% 39%	2660
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 4 6% 46%	3285
	21st to end	Night	14% 7% 2 4 4 3 4 4 5% 6% 46%	4884

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	36% 3 7% 27% 22% 5%	2787
	11th to 20th	Day	37% 4 6% 17% 29% 6%	2840
	21st to end	Day	45% 5% 8% 17% 18% 7%	3124
February	1st to 10th	Day	40% 5% 11% 24% 17% 4	1710
	11th to 20th	Day	43% 4 7% 18% 22% 6%	1814
	21st to end	Day	44% 6% 10% 17% 18% 6%	1570
March	1st to 10th	Day	36% 5% 9% 23% 22% 6%	1944
	11th to 20th	Day	42% 5% 9% 21% 16% 7%	1980
	21st to end	Day	43% 6% 11% 18% 19% 4	2220
April	1st to 10th	Day	31% 5% 11% 23% 23% 6%	2160
	11th to 20th	Day	32% 6% 11% 23% 21% 7%	2134
	21st to end	Day	36% 7% 11% 23% 18% 4	2160
May	1st to 10th	Day	38% 8% 15% 21% 15% 3	2160
	11th to 20th	Day	43% 9% 12% 23% 12% 2	2297
	21st to end	Day	39% 9% 14% 25% 12% 1	2725
June	1st to 10th	Day	40% 10% 13% 21% 15% 2	2520
	11th to 20th	Day	37% 9% 18% 21% 13% 1	2520
	21st to end	Day	38% 11% 16% 21% 12% 2	2520
July	1st to 10th	Day	36% 12% 17% 22% 11% 2	2406
	11th to 20th	Day	38% 10% 19% 23% 8% 2	2271
	21st to end	Day	37% 11% 20% 21% 10% 1	2405
August	1st to 10th	Day	44% 12% 18% 18% 7%	2302
	11th to 20th	Day	44% 10% 16% 19% 9% 1	2126
	21st to end	Day	42% 11% 16% 17% 11% 3	2376
September	1st to 10th	Day	40% 10% 15% 17% 15% 3	2160
	11th to 20th	Day	42% 6% 12% 20% 18% 2	2160
	21st to end	Day	49% 6% 11% 18% 14% 2	1916
October	1st to 10th	Day	39% 6% 9% 21% 22% 3	1800
	11th to 20th	Day	36% 5% 8% 24% 23% 5%	1800
	21st to end	Day	35% 5% 7% 20% 29% 6%	1980
November	1st to 10th	Day	36% 5% 10% 20% 24% 5%	1828
	11th to 20th	Day	35% 4 9% 22% 23% 7%	1710
	21st to end	Day	35% 5% 7% 25% 22% 5%	1663
December	1st to 10th	Day	43% 6% 12% 19% 15% 4	1520
	11th to 20th	Day	34% 4 7% 28% 18% 8%	1906
	21st to end	Day	40% 5% 8% 24% 21% 2	2772
January	1st to 10th	Night	38% 4 8% 23% 22% 5%	4141
	11th to 20th	Night	43% 5% 6% 19% 21% 5%	4120
	21st to end	Night	43% 6% 7% 17% 19% 7%	4532
February	1st to 10th	Night	41% 5% 10% 22% 17% 5%	2470
	11th to 20th	Night	45% 4 8% 17% 19% 6%	2261
	21st to end	Night	39% 5% 8% 21% 19% 7%	1884
March	1st to 10th	Night	34% 6% 10% 22% 21% 6%	2019
	11th to 20th	Night	44% 7% 9% 19% 16% 5%	1980
	21st to end	Night	40% 6% 12% 21% 17% 4	2134
April	1st to 10th	Night	37% 6% 10% 16% 24% 6%	1800
	11th to 20th	Night	36% 5% 10% 24% 19% 6%	1775
	21st to end	Night	35% 6% 11% 25% 18% 4	1800
May	1st to 10th	Night	37% 8% 14% 23% 14% 3	1800
	11th to 20th	Night	41% 9% 14% 20% 14% 2	1615
	21st to end	Night	33% 13% 17% 24% 11% 2	1551
June	1st to 10th	Night	34% 10% 14% 27% 13% 1	1440
	11th to 20th	Night	35% 11% 18% 22% 13% 1	1440
	21st to end	Night	35% 12% 16% 22% 13% 2	1440
July	1st to 10th	Night	39% 13% 18% 18% 11% 2	1439
	11th to 20th	Night	33% 13% 21% 24% 7% 1	1566
	21st to end	Night	34% 16% 18% 24% 8% 1	1672
August	1st to 10th	Night	39% 10% 22% 21% 8%	1600
	11th to 20th	Night	41% 11% 20% 17% 10% 1	1769
	21st to end	Night	41% 10% 17% 20% 9% 3	1980
September	1st to 10th	Night	34% 12% 18% 17% 15% 4	1800
	11th to 20th	Night	37% 8% 12% 22% 19% 2	1800
	21st to end	Night	48% 7% 12% 17% 15% 2	2039
October	1st to 10th	Night	37% 6% 10% 22% 23% 3	2160
	11th to 20th	Night	35% 5% 9% 22% 24% 5%	2160
	21st to end	Night	33% 5% 8% 22% 26% 7%	2376
November	1st to 10th	Night	36% 5% 9% 20% 25% 6%	2248
	11th to 20th	Night	38% 4 10% 15% 24% 8%	2470
	21st to end	Night	35% 5% 8% 24% 23% 5%	2512
December	1st to 10th	Night	45% 6% 10% 17% 18% 4	2660
	11th to 20th	Night	39% 4 10% 20% 22% 6%	3288
	21st to end	Night	37% 6% 11% 23% 21% 2	4884

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% 24% 38% 12% 4	258
	11th to 20th	Day	19% 26% 40% 13% 2	270
	21st to end	Day	24% 22% 37% 14% 2 1	297
February	1st to 10th	Day	20% 24% 36% 16% 4	270
	11th to 20th	Day	27% 22% 34% 15% 1	413
	21st to end	Day	26% 15% 37% 18% 3 1	382
March	1st to 10th	Day	18% 20% 36% 19% 6%	420
	11th to 20th	Day	24% 19% 35% 15% 5% 2	420
	21st to end	Day	21% 17% 37% 19% 6% 1	462
April	1st to 10th	Day	20% 18% 34% 19% 7% 2	420
	11th to 20th	Day	15% 15% 33% 23% 11% 3	412
	21st to end	Day	15% 14% 32% 27% 10% 2	420
May	1st to 10th	Day	14% 15% 35% 26% 8% 2	423
	11th to 20th	Day	17% 7% 25% 32% 17% 3	447
	21st to end	Day	18% 6% 20% 28% 21% 7%	485
June	1st to 10th	Day	13% 4 25% 30% 22% 5%	450
	11th to 20th	Day	13% 6% 20% 30% 23% 8%	450
	21st to end	Day	8% 6% 18% 34% 27% 7%	450
July	1st to 10th	Day	9% 3 14% 31% 30% 12%	445
	11th to 20th	Day	8% 3 10% 34% 35% 11%	444
	21st to end	Day	9% 3 15% 35% 29% 9%	488
August	1st to 10th	Day	12% 3 17% 30% 26% 11%	446
	11th to 20th	Day	12% 3 13% 23% 35% 16%	439
	21st to end	Day	13% 2 15% 27% 27% 15%	495
September	1st to 10th	Day	14% 4 17% 26% 24% 16%	450
	11th to 20th	Day	15% 4 18% 30% 22% 12%	450
	21st to end	Day	21% 7% 23% 24% 20% 6%	450
October	1st to 10th	Day	20% 5% 21% 25% 20% 8%	450
	11th to 20th	Day	15% 7% 28% 28% 17% 5%	450
	21st to end	Day	13% 11% 29% 25% 16% 5%	495
November	1st to 10th	Day	20% 9% 29% 25% 13% 3	441
	11th to 20th	Day	19% 14% 31% 24% 11% 1	370
	21st to end	Day	17% 15% 25% 28% 12% 2	323
December	1st to 10th	Day	20% 18% 41% 14% 7% 1	180
	11th to 20th	Day	18% 17% 35% 20% 9% 1	180
	21st to end	Day	23% 18% 27% 18% 12% 3	198
January	1st to 10th	Night	19% 17% 33% 25% 5%	760
	11th to 20th	Night	19% 14% 34% 25% 7%	760
	21st to end	Night	18% 15% 41% 24% 3	836
February	1st to 10th	Night	20% 13% 37% 26% 3	760
	11th to 20th	Night	25% 16% 38% 18% 2	743
	21st to end	Night	19% 15% 39% 24% 3	628
March	1st to 10th	Night	17% 13% 39% 26% 5%	723
	11th to 20th	Night	20% 15% 38% 20% 6% 1	720
	21st to end	Night	18% 13% 39% 24% 5%	792
April	1st to 10th	Night	17% 13% 40% 26% 4	720
	11th to 20th	Night	14% 11% 37% 29% 9%	711
	21st to end	Night	15% 8% 36% 33% 7%	720
May	1st to 10th	Night	14% 11% 36% 33% 7%	720
	11th to 20th	Night	16% 7% 26% 39% 12% 1	553
	21st to end	Night	11% 2 16% 47% 23% 1	389
June	1st to 10th	Night	11% 3 21% 45% 20% 1	360
	11th to 20th	Night	10% 3 16% 48% 23% 1	360
	21st to end	Night	10% 1 19% 49% 20% 1	360
July	1st to 10th	Night	8% 2 15% 46% 26% 3	389
	11th to 20th	Night	9% 2 9% 40% 35% 5%	522
	21st to end	Night	8% 2 16% 42% 30% 3	591
August	1st to 10th	Night	14% 1 13% 37% 30% 4	537
	11th to 20th	Night	12% 1 12% 39% 31% 4	705
	21st to end	Night	12% 3 14% 33% 34% 5%	792
September	1st to 10th	Night	12% 2 12% 37% 30% 7%	720
	11th to 20th	Night	17% 3 15% 35% 26% 4	720
	21st to end	Night	21% 5% 18% 32% 23% 2	720
October	1st to 10th	Night	16% 4 21% 30% 24% 5%	720
	11th to 20th	Night	18% 6% 27% 30% 18% 1	720
	21st to end	Night	15% 7% 29% 29% 18% 3	792
November	1st to 10th	Night	19% 6% 25% 32% 15% 4	740
	11th to 20th	Night	18% 9% 31% 29% 11% 1	760
	21st to end	Night	14% 10% 31% 29% 15% 1	760
December	1st to 10th	Night	20% 12% 34% 27% 7% 1	760
	11th to 20th	Night	19% 8% 36% 29% 8%	760
	21st to end	Night	18% 12% 32% 31% 7%	836

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