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 2025-05-11.

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 6% 4 4 5% 4 6% 8% 42%	113660
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	115274
Transparency	Day	38% 7% 12% 21% 18% 5%	117301
	Night	38% 6% 11% 20% 19% 6%	119772
Seeing	Day	16% 11% 27% 25% 16% 6%	15956
	Night	16% 9% 28% 31% 14% 2	25763

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	14% 5% 2 4 3 3 3 2 4 5% 56%	3992
	11th to 20th	Day	12% 7% 2 5% 3 2 3 2 4 5% 54%	4060
	21st to end	Day	10% 10% 3 8% 4 4 4 3 4 5% 44%	4466
February	1st to 10th	Day	14% 7% 2 6% 4 4 5% 4 5% 7% 42%	1800
	11th to 20th	Day	20% 7% 2 5% 4 4 3 3 4 5% 44%	1903
	21st to end	Day	15% 8% 2 6% 4 4 4 4 5% 7% 41%	1650
March	1st to 10th	Day	19% 8% 3 6% 4 3 4 3 4 6% 40%	4846
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 4 6% 53%	4980
	21st to end	Day	12% 5% 3 6% 4 4 4 3 4 6% 49%	5590
April	1st to 10th	Day	12% 6% 1 5% 3 3 3 3 4 8% 52%	2400
	11th to 20th	Day	9% 5% 2 5% 5% 3 4 3 5% 7% 51%	2374
	21st to end	Day	10% 5% 2 6% 5% 4 5% 4 6% 7% 46%	2400
May	1st to 10th	Day	10% 6% 3 9% 5% 6% 6% 4 7% 10% 35%	3885
	11th to 20th	Day	17% 8% 2 8% 5% 5% 5% 4 6% 7% 32%	4087
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 8% 46%	4823
June	1st to 10th	Day	9% 6% 3 6% 6% 5% 6% 5% 7% 10% 37%	2660
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 5% 9% 9% 40%	2660
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 5% 9% 11% 37%	2660
July	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 6% 9% 13% 36%	4352
	11th to 20th	Day	10% 7% 3 9% 6% 6% 6% 5% 7% 10% 30%	4120
	21st to end	Day	10% 8% 3 10% 7% 6% 6% 5% 8% 10% 29%	4466
August	1st to 10th	Day	17% 9% 4 9% 6% 7% 6% 5% 7% 9% 21%	4079
	11th to 20th	Day	9% 6% 2 7% 6% 6% 6% 5% 7% 11% 35%	3730
	21st to end	Day	7% 4 2 7% 5% 5% 6% 5% 7% 11% 41%	4180
September	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 4 6% 10% 37%	2280
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	2280
	21st to end	Day	18% 8% 3 7% 4 4 5% 4 5% 7% 34%	2021
October	1st to 10th	Day	14% 7% 3 4 4 3 4 3 6% 7% 44%	1900
	11th to 20th	Day	11% 6% 2 6% 4 3 4 3 5% 5% 51%	1900
	21st to end	Day	11% 5% 2 6% 4 4 4 3 4 5% 52%	2090
November	1st to 10th	Day	14% 6% 2 5% 3 3 4 4 6% 6% 48%	1926
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 5% 6% 51%	1800
	21st to end	Day	13% 5% 2 5% 4 3 3 3 5% 6% 51%	1750
December	1st to 10th	Day	11% 6% 1 6% 4 3 4 3 5% 6% 51%	2600
	11th to 20th	Day	13% 8% 3 7% 4 4 3 3 5% 5% 45%	2990
	21st to end	Day	14% 5% 3 5% 4 3 4 3 5% 7% 47%	3960
January	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 4 5% 53%	5935
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 5% 52%	5900
	21st to end	Night	13% 8% 3 6% 4 4 4 3 4 5% 46%	6490
February	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 5% 7% 42%	2600
	11th to 20th	Night	19% 9% 3 6% 4 3 2 3 5% 6% 41%	2381
	21st to end	Night	15% 8% 2 5% 3 3 3 3 5% 6% 47%	1980
March	1st to 10th	Night	19% 7% 3 6% 4 4 4 3 4 5% 41%	4990
	11th to 20th	Night	12% 6% 2 5% 3 4 4 3 4 6% 50%	4980
	21st to end	Night	14% 5% 2 5% 4 4 4 3 4 6% 50%	5340
April	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 6% 6% 46%	2000
	11th to 20th	Night	10% 7% 2 5% 5% 3 4 3 4 7% 49%	1975
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 8% 48%	2000
May	1st to 10th	Night	11% 7% 3 9% 5% 6% 5% 3 6% 8% 37%	3230
	11th to 20th	Night	18% 8% 2 6% 5% 4 5% 4 6% 7% 35%	2841
	21st to end	Night	8% 5% 3 6% 4 4 5% 5% 7% 9% 43%	2732
June	1st to 10th	Night	9% 6% 2 6% 5% 4 5% 5% 8% 12% 39%	1520
	11th to 20th	Night	7% 5% 3 7% 4 4 4 5% 9% 11% 41%	1520
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 4 9% 9% 41%	1520
July	1st to 10th	Night	6% 6% 2 8% 5% 5% 6% 5% 7% 13% 36%	2603
	11th to 20th	Night	11% 6% 3 7% 5% 5% 6% 4 8% 11% 33%	2840
	21st to end	Night	16% 8% 3 7% 5% 6% 5% 5% 8% 8% 29%	3087
August	1st to 10th	Night	17% 9% 3 8% 5% 5% 5% 4 6% 9% 28%	2825
	11th to 20th	Night	8% 8% 4 9% 6% 6% 5% 4 6% 9% 35%	3092
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 9% 43%	3476
September	1st to 10th	Night	10% 5% 2 5% 4 5% 5% 4 7% 8% 44%	1900
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 6% 8% 43%	1900
	21st to end	Night	18% 7% 3 7% 5% 4 5% 4 6% 7% 34%	2153
October	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 4 6% 48%	2280
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 6% 52%	2280
	21st to end	Night	11% 5% 2 5% 3 4 3 3 4 6% 55%	2508
November	1st to 10th	Night	15% 6% 2 4 3 3 3 3 4 6% 50%	2370
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 4 6% 48%	2600
	21st to end	Night	12% 5% 2 5% 4 4 4 3 6% 6% 50%	2645
December	1st to 10th	Night	9% 6% 2 5% 4 4 4 3 5% 5% 53%	4580
	11th to 20th	Night	16% 7% 3 8% 5% 5% 5% 4 5% 6% 37%	5205
	21st to end	Night	12% 6% 2 5% 3 4 4 3 5% 7% 50%	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	32% 3 5% 23% 24% 13%	3993
	11th to 20th	Day	33% 3 5% 16% 35% 8%	4060
	21st to end	Day	42% 5% 7% 21% 19% 6%	4466
February	1st to 10th	Day	41% 5% 10% 23% 16% 4	1800
	11th to 20th	Day	44% 4 7% 18% 21% 6%	1912
	21st to end	Day	42% 6% 9% 18% 20% 5%	1650
March	1st to 10th	Day	46% 4 8% 17% 16% 9%	4846
	11th to 20th	Day	32% 4 7% 18% 25% 13%	4980
	21st to end	Day	37% 5% 12% 20% 21% 5%	5590
April	1st to 10th	Day	32% 5% 11% 22% 23% 6%	2400
	11th to 20th	Day	31% 6% 11% 24% 20% 6%	2374
	21st to end	Day	36% 7% 12% 24% 18% 4	2400
May	1st to 10th	Day	41% 11% 16% 20% 10% 2	3920
	11th to 20th	Day	47% 8% 12% 21% 10% 1	4091
	21st to end	Day	32% 9% 14% 28% 15% 2	4837
June	1st to 10th	Day	39% 10% 13% 21% 16% 2	2660
	11th to 20th	Day	35% 9% 18% 23% 14% 1	2660
	21st to end	Day	37% 11% 16% 22% 13% 2	2660
July	1st to 10th	Day	36% 12% 20% 21% 10% 2	4284
	11th to 20th	Day	47% 10% 21% 17% 4 1	4051
	21st to end	Day	48% 10% 15% 16% 10% 1	4324
August	1st to 10th	Day	57% 9% 16% 12% 4	4066
	11th to 20th	Day	40% 10% 16% 23% 11% 1	3766
	21st to end	Day	34% 9% 16% 26% 13% 2	4180
September	1st to 10th	Day	40% 10% 15% 16% 16% 3	2280
	11th to 20th	Day	43% 6% 12% 19% 18% 2	2280
	21st to end	Day	49% 6% 11% 18% 14% 2	2021
October	1st to 10th	Day	32% 6% 10% 23% 26% 4	3160
	11th to 20th	Day	24% 3 6% 20% 33% 14%	3140
	21st to end	Day	23% 3 6% 26% 32% 9%	3428
November	1st to 10th	Day	36% 5% 10% 21% 23% 4	1926
	11th to 20th	Day	35% 4 9% 22% 23% 7%	1800
	21st to end	Day	34% 5% 8% 25% 23% 5%	1750
December	1st to 10th	Day	34% 5% 9% 24% 23% 5%	2600
	11th to 20th	Day	42% 4 10% 22% 15% 6%	2986
	21st to end	Day	37% 4 11% 26% 21% 1	3960
January	1st to 10th	Night	33% 3 6% 19% 27% 11%	5935
	11th to 20th	Night	36% 4 5% 18% 31% 7%	5900
	21st to end	Night	40% 4 8% 20% 22% 6%	6490
February	1st to 10th	Night	41% 5% 10% 22% 17% 5%	2600
	11th to 20th	Night	45% 4 8% 17% 19% 6%	2383
	21st to end	Night	38% 5% 8% 22% 19% 8%	1980
March	1st to 10th	Night	45% 5% 9% 15% 17% 9%	4990
	11th to 20th	Night	36% 5% 8% 14% 23% 14%	4980
	21st to end	Night	37% 5% 10% 24% 17% 7%	5340
April	1st to 10th	Night	37% 7% 11% 16% 23% 6%	2000
	11th to 20th	Night	35% 5% 10% 26% 19% 6%	1975
	21st to end	Night	35% 7% 12% 26% 18% 4	2000
May	1st to 10th	Night	43% 10% 16% 21% 9% 1	3260
	11th to 20th	Night	47% 8% 11% 23% 10% 1	2842
	21st to end	Night	32% 12% 19% 24% 12% 2	2739
June	1st to 10th	Night	35% 11% 14% 26% 13% 1	1520
	11th to 20th	Night	34% 11% 18% 23% 13% 1	1520
	21st to end	Night	34% 11% 16% 24% 12% 2	1520
July	1st to 10th	Night	38% 13% 19% 18% 10% 2	2561
	11th to 20th	Night	43% 11% 24% 17% 4 1	2786
	21st to end	Night	50% 12% 11% 15% 11% 1	2988
August	1st to 10th	Night	51% 8% 20% 17% 4	2814
	11th to 20th	Night	43% 10% 17% 19% 10% 1	3122
	21st to end	Night	34% 8% 14% 27% 15% 2	3476
September	1st to 10th	Night	34% 11% 18% 17% 16% 4	1900
	11th to 20th	Night	39% 8% 12% 21% 19% 2	1900
	21st to end	Night	48% 6% 12% 17% 15% 2	2153
October	1st to 10th	Night	37% 5% 9% 22% 24% 3	3800
	11th to 20th	Night	25% 4 6% 18% 29% 18%	3779
	21st to end	Night	22% 3 7% 28% 25% 14%	4120
November	1st to 10th	Night	36% 5% 9% 21% 25% 5%	2370
	11th to 20th	Night	38% 4 10% 16% 24% 8%	2600
	21st to end	Night	34% 5% 8% 25% 23% 5%	2645
December	1st to 10th	Night	34% 5% 8% 19% 27% 8%	4580
	11th to 20th	Night	48% 4 11% 15% 16% 6%	5208
	21st to end	Night	33% 6% 14% 27% 19% 2	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	21% 26% 39% 11% 3	287
	11th to 20th	Day	19% 25% 40% 13% 2	300
	21st to end	Day	22% 22% 39% 14% 2 1	330
February	1st to 10th	Day	21% 24% 36% 15% 4	300
	11th to 20th	Day	29% 22% 34% 14% 1	451
	21st to end	Day	24% 15% 37% 20% 3 1	414
March	1st to 10th	Day	19% 20% 36% 19% 6%	500
	11th to 20th	Day	22% 18% 35% 17% 5% 2	500
	21st to end	Day	19% 18% 39% 18% 5% 1	550
April	1st to 10th	Day	19% 17% 37% 19% 6% 2	500
	11th to 20th	Day	15% 15% 34% 24% 9% 3	492
	21st to end	Day	13% 14% 32% 28% 11% 2	500
May	1st to 10th	Day	13% 15% 35% 26% 9% 3	503
	11th to 20th	Day	17% 8% 25% 31% 15% 2	487
	21st to end	Day	17% 6% 22% 28% 20% 7%	529
June	1st to 10th	Day	12% 4 25% 31% 22% 5%	490
	11th to 20th	Day	12% 5% 20% 31% 23% 8%	490
	21st to end	Day	8% 6% 17% 34% 27% 8%	490
July	1st to 10th	Day	9% 3 15% 30% 30% 13%	485
	11th to 20th	Day	8% 3 11% 35% 33% 10%	484
	21st to end	Day	9% 3 15% 36% 28% 8%	532
August	1st to 10th	Day	14% 4 17% 30% 25% 10%	486
	11th to 20th	Day	11% 3 14% 25% 33% 15%	479
	21st to end	Day	12% 2 14% 28% 28% 16%	539
September	1st to 10th	Day	14% 4 18% 26% 23% 15%	490
	11th to 20th	Day	16% 4 18% 29% 22% 11%	490
	21st to end	Day	21% 7% 22% 24% 20% 6%	490
October	1st to 10th	Day	20% 5% 20% 26% 21% 8%	490
	11th to 20th	Day	13% 8% 27% 30% 16% 5%	490
	21st to end	Day	12% 11% 29% 25% 16% 7%	539
November	1st to 10th	Day	19% 9% 30% 26% 13% 3	479
	11th to 20th	Day	19% 14% 32% 25% 11% 1	400
	21st to end	Day	16% 16% 26% 27% 12% 2	350
December	1st to 10th	Day	19% 18% 42% 15% 6% 1	200
	11th to 20th	Day	18% 18% 36% 21% 8% 1	200
	21st to end	Day	21% 18% 30% 17% 11% 2	220
January	1st to 10th	Night	19% 18% 34% 24% 5%	800
	11th to 20th	Night	19% 14% 35% 25% 7%	800
	21st to end	Night	18% 14% 42% 23% 3	880
February	1st to 10th	Night	21% 13% 38% 26% 3	800
	11th to 20th	Night	26% 16% 38% 18% 2	783
	21st to end	Night	18% 15% 40% 24% 2	660
March	1st to 10th	Night	17% 14% 38% 26% 5%	800
	11th to 20th	Night	20% 15% 39% 20% 5% 1	800
	21st to end	Night	18% 14% 40% 23% 5%	880
April	1st to 10th	Night	16% 14% 40% 26% 4	800
	11th to 20th	Night	14% 11% 38% 28% 8%	791
	21st to end	Night	14% 8% 36% 35% 7%	800
May	1st to 10th	Night	13% 11% 36% 33% 7%	800
	11th to 20th	Night	16% 7% 27% 38% 11% 1	586
	21st to end	Night	10% 2 17% 48% 23%	411
June	1st to 10th	Night	11% 3 21% 46% 20% 1	380
	11th to 20th	Night	9% 3 15% 48% 23% 1	380
	21st to end	Night	9% 1 19% 50% 20% 1	380
July	1st to 10th	Night	9% 1 15% 45% 27% 3	412
	11th to 20th	Night	9% 2 9% 40% 35% 5%	552
	21st to end	Night	9% 2 15% 42% 29% 3	624
August	1st to 10th	Night	14% 2 13% 37% 29% 4	567
	11th to 20th	Night	11% 2 12% 40% 31% 4	745
	21st to end	Night	12% 3 13% 33% 35% 5%	836
September	1st to 10th	Night	11% 3 13% 36% 30% 7%	760
	11th to 20th	Night	18% 3 15% 34% 26% 4	760
	21st to end	Night	20% 6% 18% 32% 23% 2	760
October	1st to 10th	Night	15% 4 20% 31% 24% 5%	760
	11th to 20th	Night	18% 6% 27% 30% 17% 1	760
	21st to end	Night	14% 6% 28% 30% 19% 3	836
November	1st to 10th	Night	18% 6% 25% 33% 15% 3	780
	11th to 20th	Night	19% 9% 32% 30% 10% 1	800
	21st to end	Night	13% 11% 32% 29% 14% 1	800
December	1st to 10th	Night	19% 12% 34% 28% 7% 1	800
	11th to 20th	Night	19% 8% 36% 30% 8%	800
	21st to end	Night	18% 12% 32% 31% 7%	880

Page generated at 2025-05-08 09:33:54. Version ac07

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