Long Beach
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Long Beach clear sky chart from 2005-11-06 to 2025-12-03.

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	24% 12% 3 8% 5% 4 4 3 5% 7% 24%	131396
	Night	26% 14% 3 8% 4 4 4 3 4 5% 24%	128036
Transparency	Day	36% 40% 8% 10% 6% 1	131110
	Night	39% 39% 6% 10% 5% 1	127899
Seeing	Day	28% 7% 20% 26% 16% 3	16758
	Night	34% 7% 15% 23% 18% 3	25069

By Month

Percentage Cloud Cover NW (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	38% 12% 4 11% 7% 5% 4 3 4 3 10%	3179
	11th to 20th	Day	26% 10% 3 7% 4 4 3 3 4 5% 31%	3460
	21st to end	Day	36% 9% 3 7% 3 3 3 2 3 3 27%	4462
February	1st to 10th	Day	36% 12% 3 9% 5% 4 4 4 4 5% 15%	1800
	11th to 20th	Day	35% 13% 4 7% 4 4 3 3 4 3 20%	1755
	21st to end	Day	33% 15% 3 8% 5% 5% 4 3 4 5% 14%	1493
March	1st to 10th	Day	24% 16% 4 9% 6% 5% 5% 3 5% 6% 16%	4864
	11th to 20th	Day	28% 13% 4 8% 4 4 4 2 4 6% 23%	5000
	21st to end	Day	30% 14% 5% 10% 6% 6% 5% 4 4 5% 12%	5499
April	1st to 10th	Day	32% 13% 4 8% 6% 5% 5% 4 5% 6% 13%	2400
	11th to 20th	Day	28% 11% 3 7% 5% 4 4 3 5% 6% 24%	2465
	21st to end	Day	26% 11% 4 9% 5% 5% 5% 4 5% 7% 19%	2600
May	1st to 10th	Day	20% 9% 3 5% 4 4 4 3 5% 7% 36%	6139
	11th to 20th	Day	19% 14% 5% 9% 5% 6% 6% 5% 7% 7% 17%	6277
	21st to end	Day	24% 10% 3 8% 5% 4 5% 4 6% 9% 23%	6903
June	1st to 10th	Day	29% 12% 3 8% 4 4 4 4 4 6% 22%	2895
	11th to 20th	Day	26% 14% 4 9% 6% 5% 5% 4 6% 5% 17%	3000
	21st to end	Day	20% 12% 3 8% 6% 4 4 4 5% 7% 27%	3000
July	1st to 10th	Day	12% 14% 4 7% 5% 4 4 3 5% 8% 35%	6780
	11th to 20th	Day	9% 17% 3 7% 4 4 5% 4 6% 9% 32%	6606
	21st to end	Day	15% 19% 4 9% 5% 4 4 3 5% 7% 25%	6834
August	1st to 10th	Day	12% 17% 4 10% 5% 5% 5% 4 6% 8% 25%	4222
	11th to 20th	Day	16% 13% 4 9% 6% 5% 5% 5% 6% 8% 24%	4156
	21st to end	Day	16% 10% 3 9% 5% 4 4 3 5% 7% 33%	4551
September	1st to 10th	Day	13% 11% 3 8% 4 3 4 3 6% 9% 36%	2400
	11th to 20th	Day	17% 10% 4 8% 5% 5% 4 4 6% 8% 29%	2325
	21st to end	Day	21% 7% 2 6% 4 4 4 3 5% 6% 36%	2150
October	1st to 10th	Day	19% 7% 2 6% 3 3 4 3 5% 8% 41%	3280
	11th to 20th	Day	31% 9% 3 9% 4 4 4 2 4 6% 23%	3261
	21st to end	Day	23% 8% 3 8% 5% 6% 5% 4 5% 8% 24%	3519
November	1st to 10th	Day	37% 10% 3 9% 4 5% 4 4 4 5% 15%	1648
	11th to 20th	Day	38% 10% 3 7% 5% 3 4 3 5% 6% 17%	1680
	21st to end	Day	37% 10% 3 6% 4 4 4 3 3 5% 22%	1680
December	1st to 10th	Day	22% 9% 3 8% 5% 4 4 3 4 5% 35%	2636
	11th to 20th	Day	37% 12% 4 9% 6% 4 4 4 4 4 11%	2979
	21st to end	Day	36% 15% 4 10% 5% 4 4 3 4 4 11%	3498
January	1st to 10th	Night	37% 14% 4 10% 5% 5% 4 3 3 4 11%	6766
	11th to 20th	Night	24% 12% 3 7% 4 4 4 3 4 5% 29%	6421
	21st to end	Night	36% 12% 3 6% 2 3 2 2 3 5% 28%	6512
February	1st to 10th	Night	32% 13% 5% 9% 5% 5% 3 3 4 5% 16%	2600
	11th to 20th	Night	32% 15% 4 8% 5% 3 3 2 4 4 20%	2532
	21st to end	Night	32% 14% 3 9% 5% 4 4 3 4 5% 17%	2135
March	1st to 10th	Night	25% 15% 4 9% 5% 5% 5% 4 6% 6% 16%	5026
	11th to 20th	Night	24% 15% 4 8% 5% 4 4 3 4 5% 23%	4999
	21st to end	Night	33% 18% 3 9% 5% 4 4 4 4 4 11%	5475
April	1st to 10th	Night	29% 16% 4 10% 6% 4 3 3 5% 5% 16%	2000
	11th to 20th	Night	25% 13% 4 9% 5% 4 4 3 5% 6% 23%	1881
	21st to end	Night	27% 15% 4 8% 5% 4 5% 4 5% 6% 18%	1800
May	1st to 10th	Night	20% 9% 2 6% 4 3 3 3 4 6% 40%	3829
	11th to 20th	Night	21% 19% 3 10% 7% 6% 5% 4 4 4 16%	3616
	21st to end	Night	20% 15% 3 7% 5% 4 5% 4 6% 8% 23%	3972
June	1st to 10th	Night	26% 14% 4 6% 4 4 4 3 4 7% 24%	1500
	11th to 20th	Night	25% 17% 3 8% 5% 5% 4 3 5% 5% 19%	1400
	21st to end	Night	21% 15% 3 8% 4 3 4 3 4 4 32%	1400
July	1st to 10th	Night	10% 18% 3 5% 3 2 3 2 3 6% 45%	3180
	11th to 20th	Night	9% 22% 5% 8% 3 3 3 4 4 5% 35%	3354
	21st to end	Night	16% 31% 3 5% 2 2 2 2 2 3 31%	4007
August	1st to 10th	Night	14% 31% 4 8% 3 2 2 3 3 6% 24%	2938
	11th to 20th	Night	19% 16% 3 7% 3 3 3 3 5% 6% 31%	2902
	21st to end	Night	20% 9% 3 6% 3 3 3 2 4 6% 39%	3362
September	1st to 10th	Night	14% 13% 3 5% 3 3 3 3 4 7% 42%	2000
	11th to 20th	Night	18% 13% 4 8% 5% 3 4 3 4 6% 32%	2072
	21st to end	Night	22% 11% 2 6% 3 3 3 3 3 6% 38%	2250
October	1st to 10th	Night	17% 7% 3 6% 4 3 3 3 5% 9% 39%	3920
	11th to 20th	Night	27% 11% 3 8% 4 4 4 3 4 6% 26%	3897
	21st to end	Night	26% 9% 3 7% 5% 5% 5% 4 5% 6% 25%	4286
November	1st to 10th	Night	36% 9% 3 8% 5% 6% 4 3 5% 5% 17%	2870
	11th to 20th	Night	36% 11% 3 7% 5% 4 4 4 4 4 19%	2940
	21st to end	Night	30% 11% 3 8% 5% 4 4 3 4 5% 23%	2940
December	1st to 10th	Night	23% 9% 3 6% 4 3 4 3 6% 6% 32%	4588
	11th to 20th	Night	40% 14% 4 9% 5% 4 3 3 3 3 12%	5208
	21st to end	Night	39% 15% 4 9% 5% 4 3 3 3 4 10%	7458

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	56% 30% 5% 5% 4 1	3179
	11th to 20th	Day	40% 27% 6% 18% 9%	3484
	21st to end	Day	45% 29% 5% 10% 9% 2	4462
February	1st to 10th	Day	29% 63% 3 4 2	1800
	11th to 20th	Day	31% 61% 2 3 2 1	1764
	21st to end	Day	28% 63% 3 3 3	1493
March	1st to 10th	Day	46% 34% 10% 7% 3	4864
	11th to 20th	Day	43% 31% 7% 13% 7%	5000
	21st to end	Day	51% 34% 9% 5% 1	5499
April	1st to 10th	Day	29% 63% 4 3	2400
	11th to 20th	Day	23% 64% 4 7% 3	2465
	21st to end	Day	25% 63% 5% 6% 1	2600
May	1st to 10th	Day	30% 32% 13% 16% 10%	6139
	11th to 20th	Day	42% 37% 12% 7% 2	6280
	21st to end	Day	37% 35% 18% 9% 2	6904
June	1st to 10th	Day	21% 64% 7% 6% 3	2895
	11th to 20th	Day	23% 64% 5% 7% 2	3000
	21st to end	Day	24% 63% 6% 5% 2	3000
July	1st to 10th	Day	34% 28% 8% 8% 13% 10%	6706
	11th to 20th	Day	33% 29% 5% 16% 17%	6525
	21st to end	Day	46% 28% 7% 17% 2	6641
August	1st to 10th	Day	33% 40% 5% 14% 7% 1	4206
	11th to 20th	Day	40% 39% 9% 7% 3 1	4195
	21st to end	Day	31% 37% 8% 12% 12% 1	4550
September	1st to 10th	Day	24% 58% 6% 7% 3 1	2400
	11th to 20th	Day	23% 59% 4 6% 7% 1	2325
	21st to end	Day	18% 59% 7% 10% 6% 1	2150
October	1st to 10th	Day	24% 37% 8% 25% 6%	3280
	11th to 20th	Day	43% 37% 5% 9% 5%	3261
	21st to end	Day	37% 38% 9% 9% 7% 1	3519
November	1st to 10th	Day	34% 55% 4 6% 1	1648
	11th to 20th	Day	30% 56% 4 4 3 2	1680
	21st to end	Day	28% 55% 4 4 8% 1	1680
December	1st to 10th	Day	32% 36% 3 13% 13% 2	2636
	11th to 20th	Day	53% 33% 8% 3 2	2982
	21st to end	Day	57% 29% 9% 4 1	3498
January	1st to 10th	Night	57% 29% 4 5% 4 1	6766
	11th to 20th	Night	39% 28% 6% 20% 6%	6459
	21st to end	Night	45% 29% 3 11% 11% 1	6512
February	1st to 10th	Night	28% 63% 3 4 2	2600
	11th to 20th	Night	31% 61% 3 3 2 1	2534
	21st to end	Night	25% 63% 4 5% 2	2135
March	1st to 10th	Night	44% 35% 10% 9% 3	5026
	11th to 20th	Night	42% 31% 7% 14% 5%	5000
	21st to end	Night	55% 32% 7% 5% 1	5475
April	1st to 10th	Night	28% 63% 4 4 1	2000
	11th to 20th	Night	23% 62% 5% 6% 3	1881
	21st to end	Night	25% 62% 5% 6% 2	1800
May	1st to 10th	Night	29% 31% 13% 20% 7%	3826
	11th to 20th	Night	51% 32% 8% 4 4	3616
	21st to end	Night	39% 37% 14% 9% 1	3972
June	1st to 10th	Night	21% 63% 6% 7% 2	1500
	11th to 20th	Night	22% 63% 4 9% 2	1400
	21st to end	Night	25% 62% 6% 6% 1	1400
July	1st to 10th	Night	30% 25% 11% 9% 14% 11%	3145
	11th to 20th	Night	39% 25% 6% 15% 15%	3312
	21st to end	Night	47% 25% 6% 18% 3 1	3895
August	1st to 10th	Night	43% 36% 5% 11% 4 1	2929
	11th to 20th	Night	38% 38% 11% 8% 3 2	2929
	21st to end	Night	31% 36% 8% 13% 11% 1	3362
September	1st to 10th	Night	22% 58% 5% 10% 4 2	2000
	11th to 20th	Night	24% 58% 4 7% 6% 2	2072
	21st to end	Night	20% 57% 4 11% 6% 1	2250
October	1st to 10th	Night	24% 36% 9% 26% 4	3920
	11th to 20th	Night	43% 36% 6% 10% 4	3897
	21st to end	Night	39% 37% 6% 10% 7% 1	4286
November	1st to 10th	Night	34% 54% 5% 5% 2	2870
	11th to 20th	Night	32% 56% 3 5% 3 1	2940
	21st to end	Night	29% 54% 4 3 8% 2	2940
December	1st to 10th	Night	31% 36% 6% 13% 14% 1	4588
	11th to 20th	Night	56% 34% 4 4 2	5204
	21st to end	Night	55% 31% 8% 5% 1	7458

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	39% 20% 21% 14% 5% 2	200
	11th to 20th	Day	41% 9% 24% 16% 8% 4	200
	21st to end	Day	39% 6% 17% 20% 14% 3	418
February	1st to 10th	Day	42% 11% 16% 19% 12% 1	400
	11th to 20th	Day	43% 7% 21% 18% 10% 1	395
	21st to end	Day	32% 12% 26% 20% 9% 1	337
March	1st to 10th	Day	35% 11% 20% 20% 13% 1	500
	11th to 20th	Day	33% 8% 23% 21% 11% 4	500
	21st to end	Day	35% 11% 22% 20% 11% 2	550
April	1st to 10th	Day	35% 10% 18% 24% 11% 2	500
	11th to 20th	Day	30% 7% 26% 23% 10% 4	492
	21st to end	Day	30% 4 27% 25% 14% 1	500
May	1st to 10th	Day	23% 6% 25% 27% 15% 4	503
	11th to 20th	Day	31% 8% 19% 26% 14% 2	528
	21st to end	Day	30% 6% 18% 30% 13% 3	573
June	1st to 10th	Day	30% 4 18% 29% 17% 2	625
	11th to 20th	Day	25% 5% 27% 31% 11% 1	730
	21st to end	Day	20% 4 18% 36% 20% 2	730
July	1st to 10th	Day	23% 4 16% 33% 21% 2	721
	11th to 20th	Day	20% 3 17% 34% 24% 3	648
	21st to end	Day	19% 4 13% 35% 24% 5%	575
August	1st to 10th	Day	19% 4 14% 35% 24% 4	527
	11th to 20th	Day	19% 3 11% 27% 33% 8%	517
	21st to end	Day	18% 3 19% 34% 22% 3	583
September	1st to 10th	Day	18% 5% 17% 32% 24% 3	530
	11th to 20th	Day	20% 5% 18% 30% 23% 5%	530
	21st to end	Day	23% 4 19% 27% 23% 5%	530
October	1st to 10th	Day	25% 5% 18% 26% 23% 4	530
	11th to 20th	Day	30% 7% 19% 21% 18% 5%	530
	21st to end	Day	30% 10% 22% 22% 12% 4	572
November	1st to 10th	Day	36% 10% 29% 15% 8% 3	220
	11th to 20th	Day	41% 7% 15% 23% 12% 2	220
	21st to end	Day	37% 11% 16% 20% 13% 3	220
December	1st to 10th	Day	35% 14% 18% 16% 15% 3	204
	11th to 20th	Day	43% 13% 19% 20% 7% 1	200
	21st to end	Day	43% 11% 23% 17% 5% 1	220
January	1st to 10th	Night	46% 13% 18% 14% 7% 2	900
	11th to 20th	Night	42% 7% 14% 19% 15% 2	830
	21st to end	Night	39% 7% 13% 23% 14% 4	880
February	1st to 10th	Night	37% 9% 19% 22% 11% 2	800
	11th to 20th	Night	38% 9% 17% 22% 12% 1	784
	21st to end	Night	37% 9% 21% 23% 9% 1	660
March	1st to 10th	Night	37% 9% 16% 22% 15% 2	800
	11th to 20th	Night	37% 8% 16% 23% 13% 2	800
	21st to end	Night	40% 9% 17% 19% 13% 3	880
April	1st to 10th	Night	35% 8% 17% 23% 15% 2	800
	11th to 20th	Night	30% 5% 20% 29% 12% 3	693
	21st to end	Night	34% 6% 17% 24% 17% 3	600
May	1st to 10th	Night	29% 4 17% 28% 19% 4	485
	11th to 20th	Night	34% 6% 10% 23% 25% 3	394
	21st to end	Night	33% 6% 14% 21% 21% 4	433
June	1st to 10th	Night	32% 4 11% 26% 24% 4	400
	11th to 20th	Night	31% 5% 17% 29% 15% 3	400
	21st to end	Night	30% 3 10% 23% 29% 5%	400
July	1st to 10th	Night	27% 2 8% 23% 34% 7%	390
	11th to 20th	Night	26% 3 10% 26% 30% 5%	388
	21st to end	Night	25% 3 12% 23% 31% 7%	451
August	1st to 10th	Night	36% 2 8% 17% 27% 10%	597
	11th to 20th	Night	23% 2 6% 20% 35% 14%	594
	21st to end	Night	26% 2 11% 26% 29% 5%	725
September	1st to 10th	Night	22% 3 10% 25% 35% 6%	800
	11th to 20th	Night	23% 3 12% 23% 29% 11%	800
	21st to end	Night	26% 4 13% 30% 24% 3	800
October	1st to 10th	Night	24% 5% 14% 24% 29% 5%	800
	11th to 20th	Night	29% 5% 13% 27% 21% 5%	800
	21st to end	Night	33% 6% 19% 24% 16% 2	880
November	1st to 10th	Night	39% 8% 18% 22% 13% 1	820
	11th to 20th	Night	39% 8% 20% 21% 11% 1	840
	21st to end	Night	34% 10% 16% 24% 14% 2	840
December	1st to 10th	Night	32% 10% 17% 24% 14% 2	808
	11th to 20th	Night	42% 9% 19% 19% 9% 1	807
	21st to end	Night	49% 12% 18% 14% 7% 1	990

Page generated at 2025-11-29 22:00:53. Version ac07

Copyright © 2025 Attilla Danko.