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 2025-03-12.

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% 5% 5% 5% 4 6% 8% 40%	109337
	Night	12% 6% 2 6% 4 4 4 4 6% 7% 44%	109669
Transparency	Day	40% 7% 11% 18% 17% 6%	113000
	Night	36% 7% 12% 18% 19% 7%	114144
Seeing	Day	15% 11% 26% 25% 17% 7%	15951
	Night	14% 9% 24% 31% 18% 3	26123

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	13% 4 1 4 3 3 3 3 5% 6% 53%	4056
	11th to 20th	Day	13% 6% 2 5% 4 3 3 2 4 7% 52%	4060
	21st to end	Day	15% 8% 2 7% 3 3 3 3 5% 6% 44%	4466
February	1st to 10th	Day	14% 7% 2 5% 4 5% 4 4 6% 7% 42%	1800
	11th to 20th	Day	18% 7% 3 4 4 3 4 3 4 7% 44%	1900
	21st to end	Day	14% 7% 2 6% 3 4 4 4 5% 5% 46%	1797
March	1st to 10th	Day	22% 10% 3 7% 4 3 4 3 5% 5% 34%	3545
	11th to 20th	Day	9% 4 2 5% 4 3 4 3 5% 7% 53%	3491
	21st to end	Day	18% 6% 2 6% 4 3 3 3 4 6% 44%	3815
April	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 5% 8% 49%	2114
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 3 6% 6% 48%	2254
	21st to end	Day	8% 5% 1 4 4 4 4 4 7% 9% 51%	2280
May	1st to 10th	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 10% 39%	4048
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 6% 6% 37%	4081
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 9% 43%	4476
June	1st to 10th	Day	11% 7% 3 8% 7% 6% 7% 6% 7% 9% 30%	2470
	11th to 20th	Day	9% 4 1 6% 6% 6% 7% 6% 8% 10% 36%	2470
	21st to end	Day	5% 6% 3 7% 7% 6% 7% 8% 10% 12% 30%	2470
July	1st to 10th	Day	12% 8% 3 8% 6% 7% 8% 7% 8% 10% 22%	4120
	11th to 20th	Day	7% 6% 3 9% 8% 9% 8% 6% 10% 12% 23%	4120
	21st to end	Day	8% 7% 4 10% 8% 8% 8% 7% 10% 11% 20%	4466
August	1st to 10th	Day	21% 7% 3 9% 6% 7% 6% 6% 7% 9% 19%	4104
	11th to 20th	Day	9% 6% 2 8% 6% 7% 8% 6% 8% 10% 29%	3932
	21st to end	Day	8% 6% 3 8% 6% 6% 7% 5% 7% 11% 33%	4180
September	1st to 10th	Day	9% 7% 2 6% 6% 6% 6% 5% 6% 10% 37%	2178
	11th to 20th	Day	15% 7% 2 8% 5% 6% 7% 5% 7% 7% 33%	2090
	21st to end	Day	16% 7% 3 6% 4 5% 5% 4 6% 7% 37%	2090
October	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 5% 6% 44%	1933
	11th to 20th	Day	9% 4 2 5% 4 3 3 3 4 6% 57%	1900
	21st to end	Day	9% 6% 2 5% 4 3 3 3 4 5% 56%	2090
November	1st to 10th	Day	11% 6% 2 4 3 4 3 3 5% 6% 54%	1957
	11th to 20th	Day	13% 8% 2 5% 4 3 3 3 4 6% 49%	2000
	21st to end	Day	14% 6% 2 5% 3 3 3 2 5% 7% 50%	1815
December	1st to 10th	Day	9% 5% 2 5% 4 4 4 4 6% 6% 52%	2930
	11th to 20th	Day	14% 7% 3 5% 4 4 4 3 4 6% 47%	3373
	21st to end	Day	19% 8% 3 5% 3 3 4 3 4 6% 42%	4466
January	1st to 10th	Night	11% 5% 1 4 3 3 3 3 4 6% 56%	5889
	11th to 20th	Night	13% 7% 2 5% 4 4 4 3 5% 7% 44%	5900
	21st to end	Night	13% 8% 2 7% 4 4 4 3 5% 5% 46%	6490
February	1st to 10th	Night	15% 6% 2 6% 4 3 4 4 6% 6% 44%	2600
	11th to 20th	Night	18% 7% 3 6% 4 4 3 3 4 5% 43%	2383
	21st to end	Night	13% 7% 2 5% 4 4 4 4 5% 6% 45%	1830
March	1st to 10th	Night	21% 12% 3 7% 5% 4 3 2 4 5% 33%	3528
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 6% 8% 50%	3498
	21st to end	Night	15% 5% 2 5% 3 4 4 3 5% 8% 45%	3817
April	1st to 10th	Night	9% 5% 2 6% 5% 4 3 4 5% 8% 50%	2064
	11th to 20th	Night	11% 5% 2 4 4 3 4 3 6% 8% 51%	1875
	21st to end	Night	8% 5% 1 4 4 4 3 3 6% 7% 53%	1900
May	1st to 10th	Night	6% 5% 2 6% 4 4 5% 4 7% 10% 49%	2862
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 3 6% 9% 44%	2812
	21st to end	Night	6% 4 2 6% 4 4 4 4 6% 9% 51%	3079
June	1st to 10th	Night	9% 6% 3 7% 4 5% 6% 5% 8% 11% 36%	1710
	11th to 20th	Night	9% 5% 2 5% 4 5% 5% 5% 7% 13% 40%	1710
	21st to end	Night	5% 5% 3 7% 5% 5% 6% 6% 10% 13% 35%	1710
July	1st to 10th	Night	10% 8% 3 8% 6% 6% 7% 5% 8% 9% 31%	2840
	11th to 20th	Night	8% 5% 3 8% 7% 5% 6% 6% 11% 13% 30%	2840
	21st to end	Night	8% 7% 3 9% 6% 7% 7% 6% 9% 11% 26%	3087
August	1st to 10th	Night	19% 8% 2 6% 4 5% 6% 4 9% 11% 26%	2825
	11th to 20th	Night	8% 6% 2 8% 5% 6% 6% 6% 9% 10% 34%	2849
	21st to end	Night	7% 4 2 6% 5% 5% 6% 5% 6% 8% 46%	3476
September	1st to 10th	Night	9% 5% 2 7% 6% 6% 5% 4 8% 10% 40%	1997
	11th to 20th	Night	9% 6% 3 8% 5% 5% 5% 5% 6% 8% 39%	2090
	21st to end	Night	16% 7% 3 7% 4 4 4 3 6% 7% 39%	2090
October	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 5% 6% 50%	2247
	11th to 20th	Night	9% 4 1 4 3 3 3 3 5% 6% 59%	2280
	21st to end	Night	9% 5% 2 5% 2 3 3 3 5% 5% 57%	2508
November	1st to 10th	Night	11% 5% 2 6% 4 3 3 3 5% 5% 54%	2341
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 5% 6% 50%	2400
	21st to end	Night	12% 5% 2 5% 4 4 4 3 6% 7% 48%	2580
December	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 5% 56%	4250
	11th to 20th	Night	18% 8% 3 7% 4 4 3 3 3 4 43%	4822
	21st to end	Night	17% 6% 2 6% 3 4 4 4 5% 6% 44%	6490

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 7% 17% 35% 7%	4057
	11th to 20th	Day	34% 3 5% 16% 32% 10%	4060
	21st to end	Day	41% 4 5% 18% 20% 11%	4466
February	1st to 10th	Day	41% 6% 8% 21% 19% 6%	1800
	11th to 20th	Day	42% 4 7% 16% 23% 8%	1910
	21st to end	Day	39% 5% 8% 19% 19% 11%	1797
March	1st to 10th	Day	51% 5% 7% 14% 14% 8%	3545
	11th to 20th	Day	29% 6% 10% 18% 24% 14%	3491
	21st to end	Day	42% 5% 7% 17% 23% 6%	3815
April	1st to 10th	Day	32% 5% 12% 19% 23% 9%	2114
	11th to 20th	Day	35% 5% 8% 21% 19% 12%	2254
	21st to end	Day	29% 7% 10% 27% 23% 5%	2280
May	1st to 10th	Day	37% 9% 15% 23% 13% 3	4085
	11th to 20th	Day	44% 7% 12% 21% 14% 1	4085
	21st to end	Day	31% 10% 15% 23% 19% 2	4489
June	1st to 10th	Day	45% 10% 16% 17% 10% 1	2470
	11th to 20th	Day	38% 10% 18% 25% 8% 1	2470
	21st to end	Day	38% 13% 21% 20% 7% 1	2470
July	1st to 10th	Day	50% 12% 16% 17% 4 1	4055
	11th to 20th	Day	48% 15% 24% 9% 3	4051
	21st to end	Day	50% 15% 17% 15% 3	4324
August	1st to 10th	Day	58% 10% 18% 11% 2	4091
	11th to 20th	Day	43% 13% 17% 20% 8%	3971
	21st to end	Day	42% 10% 14% 29% 5% 1	4180
September	1st to 10th	Day	41% 10% 18% 14% 14% 4	2178
	11th to 20th	Day	46% 9% 12% 16% 13% 4	2090
	21st to end	Day	44% 7% 12% 18% 15% 4	2090
October	1st to 10th	Day	40% 5% 7% 14% 26% 6%	3206
	11th to 20th	Day	19% 3 5% 14% 31% 29%	3140
	21st to end	Day	25% 4 6% 18% 37% 11%	3428
November	1st to 10th	Day	32% 4 8% 19% 26% 11%	1957
	11th to 20th	Day	38% 3 7% 17% 24% 11%	2000
	21st to end	Day	35% 3 8% 22% 23% 9%	1815
December	1st to 10th	Day	32% 5% 9% 17% 23% 15%	2930
	11th to 20th	Day	38% 5% 9% 20% 22% 5%	3370
	21st to end	Day	44% 4 7% 22% 21% 3	4466
January	1st to 10th	Night	30% 4 6% 16% 36% 8%	5889
	11th to 20th	Night	39% 4 5% 16% 29% 7%	5900
	21st to end	Night	41% 5% 6% 12% 26% 10%	6490
February	1st to 10th	Night	40% 4 8% 20% 19% 8%	2600
	11th to 20th	Night	44% 4 8% 15% 19% 9%	2384
	21st to end	Night	38% 5% 10% 17% 18% 12%	1830
March	1st to 10th	Night	54% 5% 9% 12% 13% 7%	3528
	11th to 20th	Night	32% 6% 10% 15% 24% 14%	3498
	21st to end	Night	37% 6% 11% 20% 19% 8%	3817
April	1st to 10th	Night	33% 6% 12% 19% 21% 9%	2064
	11th to 20th	Night	31% 5% 10% 22% 21% 11%	1875
	21st to end	Night	28% 6% 13% 27% 22% 5%	1900
May	1st to 10th	Night	29% 9% 19% 26% 13% 3	2889
	11th to 20th	Night	35% 9% 16% 23% 17%	2813
	21st to end	Night	26% 11% 17% 27% 18% 2	3087
June	1st to 10th	Night	38% 10% 20% 20% 11% 1	1710
	11th to 20th	Night	33% 10% 21% 27% 8%	1710
	21st to end	Night	34% 13% 25% 22% 5% 1	1710
July	1st to 10th	Night	45% 11% 18% 21% 4 1	2795
	11th to 20th	Night	41% 16% 29% 10% 4	2786
	21st to end	Night	46% 15% 19% 18% 2	2988
August	1st to 10th	Night	49% 12% 25% 12% 2	2814
	11th to 20th	Night	39% 13% 20% 22% 6%	2876
	21st to end	Night	33% 10% 17% 35% 5%	3476
September	1st to 10th	Night	38% 11% 17% 19% 11% 4	1997
	11th to 20th	Night	40% 11% 14% 19% 14% 2	2090
	21st to end	Night	43% 7% 13% 20% 14% 3	2090
October	1st to 10th	Night	35% 5% 8% 20% 26% 6%	3754
	11th to 20th	Night	16% 3 5% 14% 26% 35%	3779
	21st to end	Night	23% 4 7% 21% 28% 16%	4120
November	1st to 10th	Night	33% 4 9% 16% 26% 13%	2341
	11th to 20th	Night	36% 4 7% 16% 25% 12%	2400
	21st to end	Night	35% 5% 9% 18% 23% 10%	2580
December	1st to 10th	Night	31% 4 9% 14% 30% 12%	4250
	11th to 20th	Night	45% 5% 9% 15% 19% 7%	4824
	21st to end	Night	41% 5% 7% 20% 23% 3	6490

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	19% 23% 45% 11% 2	300
	11th to 20th	Day	16% 23% 38% 17% 6%	300
	21st to end	Day	26% 24% 39% 9% 2	330
February	1st to 10th	Day	17% 21% 42% 18% 2 1	300
	11th to 20th	Day	29% 25% 35% 10% 1	300
	21st to end	Day	20% 18% 41% 17% 4	396
March	1st to 10th	Day	22% 19% 32% 21% 6% 1	500
	11th to 20th	Day	16% 23% 36% 17% 6% 1	464
	21st to end	Day	16% 21% 39% 13% 10% 1	506
April	1st to 10th	Day	18% 21% 31% 19% 9% 1	460
	11th to 20th	Day	12% 17% 32% 20% 13% 6%	452
	21st to end	Day	10% 13% 31% 25% 13% 8%	460
May	1st to 10th	Day	9% 16% 30% 25% 13% 6%	463
	11th to 20th	Day	13% 6% 27% 27% 19% 7%	487
	21st to end	Day	11% 4 20% 34% 20% 11%	529
June	1st to 10th	Day	13% 6% 19% 31% 23% 9%	490
	11th to 20th	Day	10% 4 20% 28% 22% 15%	490
	21st to end	Day	7% 3 18% 35% 25% 13%	490
July	1st to 10th	Day	8% 4 8% 30% 34% 16%	485
	11th to 20th	Day	9% 2 8% 28% 36% 17%	484
	21st to end	Day	7% 2 13% 34% 32% 11%	532
August	1st to 10th	Day	14% 3 11% 30% 26% 16%	486
	11th to 20th	Day	10% 4 8% 30% 32% 17%	479
	21st to end	Day	14% 4 13% 25% 28% 17%	539
September	1st to 10th	Day	11% 3 15% 31% 26% 14%	490
	11th to 20th	Day	18% 4 14% 34% 22% 8%	490
	21st to end	Day	19% 1 20% 28% 23% 9%	490
October	1st to 10th	Day	16% 6% 29% 25% 15% 9%	490
	11th to 20th	Day	11% 7% 27% 28% 19% 7%	490
	21st to end	Day	12% 10% 23% 32% 19% 4	539
November	1st to 10th	Day	17% 9% 31% 24% 17% 3	495
	11th to 20th	Day	21% 14% 34% 20% 11% 2	500
	21st to end	Day	16% 14% 35% 26% 8% 1	315
December	1st to 10th	Day	18% 17% 39% 20% 6%	300
	11th to 20th	Day	17% 18% 39% 18% 7% 1	300
	21st to end	Day	24% 18% 35% 16% 6% 1	330
January	1st to 10th	Night	16% 17% 34% 27% 5% 1	800
	11th to 20th	Night	17% 17% 31% 28% 7%	800
	21st to end	Night	18% 18% 39% 23% 3	880
February	1st to 10th	Night	19% 14% 32% 28% 6%	800
	11th to 20th	Night	21% 19% 32% 25% 3	783
	21st to end	Night	18% 17% 37% 22% 6%	660
March	1st to 10th	Night	16% 14% 36% 28% 6%	800
	11th to 20th	Night	16% 15% 35% 26% 7%	767
	21st to end	Night	14% 16% 36% 26% 8%	836
April	1st to 10th	Night	12% 17% 35% 26% 8%	760
	11th to 20th	Night	13% 13% 33% 27% 12% 1	751
	21st to end	Night	10% 11% 32% 34% 12% 1	760
May	1st to 10th	Night	8% 9% 30% 38% 14%	606
	11th to 20th	Night	10% 5% 22% 39% 21% 2	561
	21st to end	Night	9% 2 16% 39% 30% 4	617
June	1st to 10th	Night	11% 2 16% 43% 24% 3	570
	11th to 20th	Night	9% 1 13% 39% 29% 7%	570
	21st to end	Night	7% 1 16% 38% 32% 6%	570
July	1st to 10th	Night	9% 2 7% 38% 37% 6%	555
	11th to 20th	Night	11% 1 7% 32% 41% 8%	552
	21st to end	Night	5% 3 8% 40% 37% 7%	624
August	1st to 10th	Night	11% 1 5% 35% 38% 10%	567
	11th to 20th	Night	10% 2 6% 33% 40% 9%	602
	21st to end	Night	13% 2 9% 31% 34% 11%	836
September	1st to 10th	Night	10% 2 12% 33% 38% 5%	760
	11th to 20th	Night	12% 4 11% 36% 30% 7%	760
	21st to end	Night	19% 2 12% 35% 29% 4	760
October	1st to 10th	Night	15% 4 20% 33% 24% 4	760
	11th to 20th	Night	11% 7% 24% 31% 24% 3	760
	21st to end	Night	12% 8% 24% 32% 22% 2	836
November	1st to 10th	Night	13% 8% 25% 36% 15% 3	780
	11th to 20th	Night	17% 10% 30% 29% 13% 2	800
	21st to end	Night	14% 12% 29% 30% 14% 2	800
December	1st to 10th	Night	17% 13% 30% 30% 10% 1	800
	11th to 20th	Night	20% 12% 26% 29% 12% 1	800
	21st to end	Night	23% 14% 28% 27% 8%	880

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