Berkeley High School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Berkeley High School clear sky chart from 2005-11-06 to 2026-07-16.

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% 5% 5% 5% 4 6% 8% 39%	137983
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	134720
Transparency	Day	40% 8% 12% 18% 16% 6%	141602
	Night	37% 7% 12% 18% 18% 7%	139172
Seeing	Day	14% 11% 26% 25% 17% 7%	17823
	Night	14% 9% 24% 31% 18% 3	27902

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% 4 2 5% 3 3 4 3 5% 7% 52%	5258
	11th to 20th	Day	13% 5% 2 5% 3 3 3 2 4 7% 52%	5280
	21st to end	Day	17% 8% 2 7% 4 4 4 3 5% 6% 41%	5780
February	1st to 10th	Day	14% 7% 2 5% 4 5% 4 4 6% 6% 43%	1890
	11th to 20th	Day	17% 7% 3 5% 4 3 4 3 4 7% 44%	1998
	21st to end	Day	15% 8% 2 6% 3 4 4 3 5% 5% 45%	1884
March	1st to 10th	Day	21% 9% 2 6% 4 4 4 3 5% 5% 37%	5045
	11th to 20th	Day	9% 4 2 5% 3 3 4 3 5% 7% 54%	5090
	21st to end	Day	13% 6% 2 6% 4 4 4 3 4 5% 48%	5586
April	1st to 10th	Day	8% 5% 2 5% 4 4 5% 4 6% 8% 49%	2336
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 3 6% 6% 51%	2494
	21st to end	Day	7% 5% 1 5% 4 5% 4 4 6% 9% 49%	2520
May	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 5% 6% 9% 36%	7554
	11th to 20th	Day	12% 6% 2 7% 4 5% 5% 5% 7% 8% 40%	7626
	21st to end	Day	8% 6% 3 8% 6% 7% 7% 5% 8% 9% 34%	8391
June	1st to 10th	Day	10% 6% 3 8% 7% 6% 6% 6% 8% 9% 31%	2730
	11th to 20th	Day	9% 4 1 6% 6% 6% 7% 6% 8% 10% 36%	2730
	21st to end	Day	5% 5% 2 6% 7% 6% 7% 7% 10% 12% 32%	2730
July	1st to 10th	Day	11% 8% 3 9% 7% 7% 9% 6% 8% 10% 21%	6030
	11th to 20th	Day	5% 5% 2 8% 8% 8% 8% 7% 10% 13% 25%	5965
	21st to end	Day	8% 6% 3 9% 7% 7% 8% 6% 10% 12% 24%	6424
August	1st to 10th	Day	21% 8% 3 9% 6% 7% 6% 6% 7% 9% 18%	4234
	11th to 20th	Day	9% 6% 2 8% 6% 7% 8% 6% 8% 10% 29%	4059
	21st to end	Day	8% 7% 3 8% 6% 6% 7% 5% 7% 11% 33%	4312
September	1st to 10th	Day	9% 7% 2 6% 5% 6% 6% 5% 6% 10% 38%	2292
	11th to 20th	Day	14% 6% 2 7% 5% 6% 7% 5% 7% 7% 34%	2200
	21st to end	Day	16% 7% 3 6% 4 5% 5% 4 6% 7% 37%	2200
October	1st to 10th	Day	13% 7% 2 6% 5% 5% 4 4 5% 6% 43%	2034
	11th to 20th	Day	10% 4 2 5% 3 3 3 3 4 6% 57%	2000
	21st to end	Day	10% 6% 2 5% 4 3 3 3 4 5% 56%	2200
November	1st to 10th	Day	11% 6% 2 4 3 4 3 2 5% 6% 55%	2057
	11th to 20th	Day	13% 8% 2 5% 4 3 2 3 4 6% 51%	2100
	21st to end	Day	13% 5% 2 6% 3 3 3 3 5% 7% 51%	1906
December	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 6% 6% 51%	3020
	11th to 20th	Day	14% 7% 3 5% 4 4 4 3 4 5% 47%	3463
	21st to end	Day	18% 8% 3 5% 3 3 4 3 4 6% 43%	4565
January	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 4 6% 53%	7635
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 48%	7680
	21st to end	Night	15% 8% 3 7% 4 4 4 3 4 5% 43%	8401
February	1st to 10th	Night	15% 7% 2 6% 4 3 4 4 6% 6% 44%	2730
	11th to 20th	Night	17% 7% 3 6% 4 4 3 3 5% 5% 42%	2505
	21st to end	Night	13% 7% 2 5% 4 4 4 4 5% 6% 45%	1919
March	1st to 10th	Night	18% 10% 3 7% 4 4 3 2 4 5% 39%	5028
	11th to 20th	Night	11% 4 1 4 3 3 3 3 5% 9% 52%	5090
	21st to end	Night	12% 5% 2 5% 3 4 4 3 5% 7% 51%	5588
April	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 5% 8% 49%	2281
	11th to 20th	Night	10% 4 2 4 4 3 4 3 6% 8% 53%	2075
	21st to end	Night	7% 5% 2 5% 4 4 3 3 7% 8% 53%	2100
May	1st to 10th	Night	10% 6% 2 7% 5% 4 5% 4 6% 9% 42%	5342
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 6% 10% 44%	5234
	21st to end	Night	9% 7% 3 8% 5% 5% 6% 5% 7% 9% 37%	5761
June	1st to 10th	Night	10% 6% 3 7% 5% 5% 6% 5% 8% 11% 35%	1890
	11th to 20th	Night	8% 4 2 5% 5% 6% 5% 5% 7% 13% 39%	1890
	21st to end	Night	5% 5% 3 7% 5% 5% 6% 6% 10% 14% 37%	1890
July	1st to 10th	Night	10% 7% 3 7% 6% 6% 7% 6% 8% 9% 31%	4148
	11th to 20th	Night	7% 5% 3 7% 6% 5% 6% 6% 10% 14% 32%	4102
	21st to end	Night	8% 7% 3 9% 6% 6% 6% 5% 8% 11% 30%	4427
August	1st to 10th	Night	20% 8% 2 6% 4 5% 6% 4 8% 11% 25%	2915
	11th to 20th	Night	8% 6% 2 8% 5% 6% 6% 6% 9% 10% 34%	2941
	21st to end	Night	7% 4 2 6% 5% 5% 6% 5% 6% 8% 46%	3586
September	1st to 10th	Night	9% 4 2 7% 6% 6% 5% 4 8% 9% 41%	2102
	11th to 20th	Night	9% 6% 3 8% 5% 5% 5% 5% 7% 8% 40%	2200
	21st to end	Night	16% 7% 3 7% 5% 4 4 4 6% 7% 39%	2200
October	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 5% 6% 50%	2365
	11th to 20th	Night	11% 4 1 4 3 3 3 3 5% 6% 59%	2400
	21st to end	Night	10% 5% 2 5% 2 3 3 3 5% 5% 57%	2640
November	1st to 10th	Night	11% 5% 2 6% 4 3 3 3 5% 5% 55%	2461
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 5% 6% 51%	2520
	21st to end	Night	11% 5% 2 5% 4 4 4 4 6% 7% 48%	2709
December	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 55%	4380
	11th to 20th	Night	17% 8% 3 7% 4 4 3 3 3 4 43%	4952
	21st to end	Night	16% 6% 2 6% 3 4 4 4 5% 6% 44%	6633

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	31% 4 9% 22% 28% 6%	5259
	11th to 20th	Day	33% 3 5% 15% 31% 13%	5280
	21st to end	Day	44% 4 6% 18% 17% 11%	5780
February	1st to 10th	Day	40% 6% 7% 22% 20% 6%	1890
	11th to 20th	Day	42% 4 7% 16% 23% 8%	2008
	21st to end	Day	40% 4 8% 18% 18% 11%	1884
March	1st to 10th	Day	48% 5% 7% 13% 18% 10%	5045
	11th to 20th	Day	29% 5% 9% 16% 24% 17%	5090
	21st to end	Day	38% 5% 8% 17% 21% 11%	5586
April	1st to 10th	Day	32% 5% 12% 20% 22% 8%	2336
	11th to 20th	Day	32% 5% 7% 24% 20% 12%	2494
	21st to end	Day	30% 7% 10% 26% 22% 5%	2520
May	1st to 10th	Day	42% 8% 14% 23% 10% 2	7588
	11th to 20th	Day	40% 7% 16% 22% 14% 1	7630
	21st to end	Day	42% 10% 14% 16% 15% 1	8405
June	1st to 10th	Day	44% 10% 16% 18% 11% 1	2730
	11th to 20th	Day	38% 10% 18% 24% 8% 1	2730
	21st to end	Day	36% 13% 22% 21% 7% 1	2730
July	1st to 10th	Day	51% 13% 18% 14% 3 1	5965
	11th to 20th	Day	44% 17% 29% 8% 2	5896
	21st to end	Day	45% 14% 17% 20% 4	6240
August	1st to 10th	Day	59% 10% 18% 11% 2	4221
	11th to 20th	Day	43% 13% 17% 20% 8%	4098
	21st to end	Day	42% 10% 14% 28% 5% 1	4312
September	1st to 10th	Day	40% 10% 17% 16% 14% 4	2292
	11th to 20th	Day	45% 9% 12% 18% 13% 4	2200
	21st to end	Day	45% 7% 12% 18% 14% 4	2200
October	1st to 10th	Day	41% 6% 7% 14% 26% 6%	3307
	11th to 20th	Day	19% 3 5% 15% 31% 28%	3240
	21st to end	Day	25% 4 5% 17% 37% 11%	3538
November	1st to 10th	Day	31% 4 8% 19% 27% 11%	2057
	11th to 20th	Day	36% 3 7% 17% 24% 12%	2100
	21st to end	Day	34% 3 9% 22% 24% 8%	1906
December	1st to 10th	Day	33% 5% 9% 17% 23% 14%	3020
	11th to 20th	Day	38% 5% 9% 20% 22% 5%	3460
	21st to end	Day	43% 4 7% 22% 21% 3	4565
January	1st to 10th	Night	33% 4 7% 21% 28% 6%	7636
	11th to 20th	Night	36% 4 5% 15% 29% 11%	7680
	21st to end	Night	44% 4 6% 13% 23% 10%	8401
February	1st to 10th	Night	39% 4 8% 20% 20% 8%	2730
	11th to 20th	Night	44% 4 8% 16% 19% 9%	2506
	21st to end	Night	39% 5% 10% 17% 17% 12%	1919
March	1st to 10th	Night	48% 4 8% 12% 20% 8%	5028
	11th to 20th	Night	29% 5% 9% 13% 27% 17%	5090
	21st to end	Night	33% 5% 11% 20% 20% 11%	5588
April	1st to 10th	Night	33% 6% 13% 21% 19% 8%	2281
	11th to 20th	Night	29% 5% 9% 23% 22% 11%	2075
	21st to end	Night	28% 6% 13% 27% 21% 5%	2100
May	1st to 10th	Night	37% 8% 18% 25% 10% 2	5369
	11th to 20th	Night	34% 8% 19% 23% 13% 2	5235
	21st to end	Night	41% 11% 15% 19% 13% 1	5771
June	1st to 10th	Night	39% 10% 19% 20% 11% 1	1890
	11th to 20th	Night	34% 11% 22% 26% 8%	1890
	21st to end	Night	32% 13% 25% 23% 5% 1	1890
July	1st to 10th	Night	45% 12% 24% 16% 3 1	4105
	11th to 20th	Night	38% 15% 35% 8% 3	4051
	21st to end	Night	44% 13% 18% 22% 3	4297
August	1st to 10th	Night	50% 12% 24% 12% 2	2904
	11th to 20th	Night	39% 13% 20% 22% 6%	2968
	21st to end	Night	33% 9% 17% 35% 5%	3586
September	1st to 10th	Night	37% 10% 17% 21% 11% 4	2102
	11th to 20th	Night	39% 11% 14% 20% 14% 2	2200
	21st to end	Night	43% 7% 13% 20% 14% 3	2200
October	1st to 10th	Night	36% 5% 9% 20% 25% 6%	3872
	11th to 20th	Night	17% 3 5% 14% 26% 34%	3899
	21st to end	Night	24% 4 7% 21% 28% 16%	4252
November	1st to 10th	Night	32% 3 9% 16% 26% 13%	2461
	11th to 20th	Night	34% 3 8% 17% 26% 12%	2520
	21st to end	Night	35% 5% 9% 18% 22% 11%	2709
December	1st to 10th	Night	32% 4 8% 14% 30% 11%	4380
	11th to 20th	Night	44% 5% 9% 15% 19% 8%	4954
	21st to end	Night	40% 5% 7% 21% 23% 3	6633

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	18% 22% 45% 13% 2	330
	11th to 20th	Day	18% 22% 38% 16% 6%	330
	21st to end	Day	27% 23% 40% 8% 2	363
February	1st to 10th	Day	18% 21% 42% 17% 2 1	330
	11th to 20th	Day	29% 25% 35% 9% 1	330
	21st to end	Day	23% 18% 39% 17% 3	427
March	1st to 10th	Day	21% 18% 31% 21% 8% 1	540
	11th to 20th	Day	17% 22% 36% 18% 6% 1	540
	21st to end	Day	15% 21% 39% 15% 10% 1	594
April	1st to 10th	Day	17% 20% 32% 20% 9% 1	540
	11th to 20th	Day	11% 16% 33% 22% 13% 6%	532
	21st to end	Day	9% 12% 30% 26% 15% 8%	540
May	1st to 10th	Day	10% 16% 32% 25% 12% 5%	543
	11th to 20th	Day	12% 7% 28% 27% 18% 7%	567
	21st to end	Day	11% 6% 21% 33% 18% 10%	617
June	1st to 10th	Day	12% 6% 20% 30% 23% 9%	570
	11th to 20th	Day	9% 5% 20% 29% 24% 13%	570
	21st to end	Day	6% 3 16% 35% 27% 13%	570
July	1st to 10th	Day	7% 3 9% 31% 34% 16%	565
	11th to 20th	Day	9% 2 7% 28% 36% 18%	544
	21st to end	Day	6% 2 13% 34% 32% 13%	576
August	1st to 10th	Day	15% 4 11% 30% 25% 15%	526
	11th to 20th	Day	9% 3 9% 29% 31% 18%	519
	21st to end	Day	15% 4 14% 25% 27% 15%	583
September	1st to 10th	Day	11% 3 17% 30% 25% 13%	530
	11th to 20th	Day	17% 4 15% 35% 22% 8%	530
	21st to end	Day	19% 2 20% 28% 23% 8%	530
October	1st to 10th	Day	16% 6% 29% 25% 15% 8%	530
	11th to 20th	Day	12% 7% 26% 29% 19% 7%	530
	21st to end	Day	13% 11% 25% 30% 17% 4	583
November	1st to 10th	Day	17% 9% 31% 24% 16% 3	535
	11th to 20th	Day	19% 14% 34% 20% 10% 2	540
	21st to end	Day	15% 15% 36% 26% 8% 1	346
December	1st to 10th	Day	21% 16% 38% 20% 5%	330
	11th to 20th	Day	18% 18% 38% 18% 8% 1	330
	21st to end	Day	23% 18% 35% 18% 6% 1	363
January	1st to 10th	Night	16% 17% 34% 28% 5% 1	840
	11th to 20th	Night	17% 17% 31% 28% 7%	840
	21st to end	Night	18% 18% 39% 23% 2	924
February	1st to 10th	Night	20% 14% 33% 28% 6%	840
	11th to 20th	Night	20% 19% 33% 25% 3	823
	21st to end	Night	18% 18% 36% 21% 5%	692
March	1st to 10th	Night	16% 14% 35% 28% 7%	840
	11th to 20th	Night	16% 15% 36% 26% 7%	840
	21st to end	Night	14% 16% 37% 26% 8%	924
April	1st to 10th	Night	12% 17% 34% 27% 9%	840
	11th to 20th	Night	12% 13% 34% 28% 12% 1	831
	21st to end	Night	10% 11% 31% 36% 12% 1	840
May	1st to 10th	Night	9% 9% 29% 39% 13%	670
	11th to 20th	Night	11% 6% 23% 38% 20% 2	621
	21st to end	Night	9% 3 17% 38% 29% 4	683
June	1st to 10th	Night	12% 2 17% 43% 23% 3	630
	11th to 20th	Night	10% 1 14% 41% 28% 7%	630
	21st to end	Night	7% 1 15% 37% 35% 6%	630
July	1st to 10th	Night	9% 2 8% 39% 37% 6%	615
	11th to 20th	Night	11% 1 7% 32% 42% 8%	597
	21st to end	Night	5% 3 8% 39% 38% 7%	657
August	1st to 10th	Night	12% 2 5% 34% 37% 9%	597
	11th to 20th	Night	10% 2 6% 32% 40% 10%	634
	21st to end	Night	13% 2 10% 31% 33% 10%	880
September	1st to 10th	Night	10% 2 13% 33% 37% 5%	800
	11th to 20th	Night	12% 4 11% 37% 30% 7%	800
	21st to end	Night	20% 2 11% 34% 29% 4	800
October	1st to 10th	Night	15% 5% 21% 33% 23% 4	800
	11th to 20th	Night	12% 7% 23% 32% 24% 3	800
	21st to end	Night	13% 9% 24% 31% 21% 2	880
November	1st to 10th	Night	12% 8% 26% 36% 14% 3	820
	11th to 20th	Night	16% 10% 30% 30% 13% 1	840
	21st to end	Night	13% 11% 30% 30% 14% 1	840
December	1st to 10th	Night	18% 14% 29% 29% 9% 1	840
	11th to 20th	Night	20% 12% 26% 29% 12% 1	840
	21st to end	Night	22% 14% 28% 27% 9%	924

Page generated at 2026-07-13 03:14:54. Version ac07

Copyright © 2026 Attilla Danko.