Oakland
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oakland clear sky chart from 2005-11-06 to 2026-01-17.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	127435
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 43%	124240
Transparency	Day	43% 4 11% 23% 16% 3	130949
	Night	42% 4 11% 23% 17% 3	128813
Seeing	Day	22% 14% 30% 21% 10% 3	16425
	Night	21% 12% 30% 28% 9% 1	25666

By Month

Percentage Cloud Cover NE (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% 5% 2 5% 4 5% 4 4 5% 6% 47%	3677
	11th to 20th	Day	14% 8% 3 6% 5% 6% 4 3 5% 5% 40%	3618
	21st to end	Day	18% 9% 3 8% 4 3 4 3 4 4 39%	4325
February	1st to 10th	Day	17% 7% 3 7% 4 4 5% 4 5% 7% 38%	1800
	11th to 20th	Day	15% 8% 2 6% 4 4 4 4 4 6% 43%	1938
	21st to end	Day	19% 9% 2 5% 4 5% 5% 3 5% 6% 38%	1650
March	1st to 10th	Day	21% 6% 2 5% 4 4 4 3 4 5% 42%	4476
	11th to 20th	Day	12% 6% 3 6% 5% 4 4 4 6% 8% 42%	4796
	21st to end	Day	18% 7% 3 6% 4 3 4 3 4 6% 42%	5914
April	1st to 10th	Day	18% 9% 2 6% 4 4 4 3 5% 6% 38%	2400
	11th to 20th	Day	17% 7% 3 6% 5% 4 4 3 5% 7% 40%	2374
	21st to end	Day	18% 7% 2 6% 4 4 4 3 6% 6% 40%	2415
May	1st to 10th	Day	30% 7% 2 5% 4 4 4 3 5% 7% 28%	5903
	11th to 20th	Day	24% 8% 2 7% 5% 4 3 3 5% 6% 33%	6318
	21st to end	Day	21% 6% 2 6% 4 4 5% 4 6% 8% 35%	6949
June	1st to 10th	Day	16% 6% 2 6% 5% 5% 5% 5% 7% 9% 35%	2800
	11th to 20th	Day	18% 6% 3 6% 5% 5% 6% 4 6% 8% 35%	2800
	21st to end	Day	12% 5% 3 7% 5% 5% 5% 5% 6% 9% 39%	2800
July	1st to 10th	Day	11% 7% 2 8% 5% 5% 6% 4 7% 9% 36%	6360
	11th to 20th	Day	10% 7% 3 7% 5% 5% 7% 6% 8% 11% 32%	6360
	21st to end	Day	13% 6% 3 6% 5% 5% 5% 4 6% 8% 39%	6926
August	1st to 10th	Day	21% 5% 3 7% 5% 5% 5% 4 6% 7% 32%	4316
	11th to 20th	Day	13% 6% 2 6% 5% 5% 4 4 6% 8% 42%	3850
	21st to end	Day	12% 6% 2 5% 3 4 4 3 5% 8% 48%	4312
September	1st to 10th	Day	15% 5% 2 6% 5% 4 5% 4 5% 6% 43%	2400
	11th to 20th	Day	13% 6% 2 4 3 4 4 3 4 7% 50%	2324
	21st to end	Day	19% 6% 3 7% 4 5% 4 3 5% 6% 39%	2200
October	1st to 10th	Day	19% 6% 2 5% 3 4 3 3 5% 6% 43%	2070
	11th to 20th	Day	16% 6% 1 5% 3 4 4 4 5% 6% 47%	2000
	21st to end	Day	20% 5% 2 4 4 4 4 4 4 7% 43%	2049
November	1st to 10th	Day	14% 6% 3 5% 3 3 3 3 4 6% 52%	1851
	11th to 20th	Day	17% 6% 1 3 3 4 3 3 5% 7% 47%	1890
	21st to end	Day	17% 6% 3 5% 4 4 4 5% 5% 7% 43%	1780
December	1st to 10th	Day	16% 7% 2 5% 3 4 4 3 6% 7% 43%	2680
	11th to 20th	Day	22% 6% 2 4 3 3 3 3 4 6% 44%	3066
	21st to end	Day	17% 7% 3 7% 4 4 4 3 5% 7% 37%	4048
January	1st to 10th	Night	13% 5% 2 5% 3 4 4 4 5% 6% 48%	6489
	11th to 20th	Night	13% 7% 3 7% 4 3 4 3 4 6% 45%	6400
	21st to end	Night	18% 9% 2 5% 3 4 4 3 5% 6% 41%	6628
February	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 5% 6% 41%	2600
	11th to 20th	Night	16% 7% 2 6% 4 4 4 4 5% 5% 41%	2346
	21st to end	Night	18% 7% 3 7% 4 5% 4 3 4 5% 41%	1980
March	1st to 10th	Night	20% 8% 2 5% 3 3 3 2 4 6% 43%	5364
	11th to 20th	Night	14% 9% 3 5% 3 3 3 3 5% 6% 47%	5145
	21st to end	Night	17% 10% 3 6% 4 3 3 3 4 5% 41%	5018
April	1st to 10th	Night	19% 9% 3 6% 4 4 5% 4 4 7% 38%	2000
	11th to 20th	Night	19% 6% 2 6% 3 4 4 3 5% 7% 40%	1975
	21st to end	Night	19% 7% 1 6% 3 4 3 3 5% 6% 42%	1982
May	1st to 10th	Night	30% 8% 3 7% 4 3 3 3 5% 6% 28%	4054
	11th to 20th	Night	23% 9% 3 6% 4 4 4 3 5% 5% 35%	3587
	21st to end	Night	22% 7% 2 6% 4 4 4 3 5% 8% 36%	3926
June	1st to 10th	Night	16% 5% 2 4 4 4 4 4 5% 8% 43%	1600
	11th to 20th	Night	15% 7% 2 6% 4 4 4 5% 6% 10% 40%	1600
	21st to end	Night	12% 4 2 6% 5% 4 4 4 6% 9% 43%	1600
July	1st to 10th	Night	11% 6% 2 7% 5% 5% 5% 4 6% 9% 40%	3600
	11th to 20th	Night	16% 8% 3 7% 4 4 5% 5% 7% 9% 33%	3600
	21st to end	Night	13% 6% 2 5% 4 3 4 3 6% 7% 46%	3926
August	1st to 10th	Night	22% 5% 2 7% 4 5% 5% 3 5% 8% 34%	2795
	11th to 20th	Night	14% 7% 3 7% 5% 5% 5% 4 5% 6% 40%	3197
	21st to end	Night	11% 4 2 4 3 3 3 3 5% 7% 53%	3586
September	1st to 10th	Night	15% 6% 2 7% 4 4 3 4 5% 6% 45%	2000
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 7% 52%	2075
	21st to end	Night	19% 6% 2 5% 4 3 3 3 4 6% 45%	2200
October	1st to 10th	Night	19% 8% 2 5% 3 3 3 3 4 6% 44%	2325
	11th to 20th	Night	17% 5% 2 4 3 3 3 2 5% 6% 50%	2400
	21st to end	Night	20% 6% 2 5% 2 2 3 3 4 6% 46%	2790
November	1st to 10th	Night	12% 5% 2 5% 3 3 4 4 5% 6% 51%	2667
	11th to 20th	Night	19% 6% 2 4 3 3 3 3 4 5% 48%	2730
	21st to end	Night	15% 5% 2 6% 4 4 4 4 5% 7% 46%	2837
December	1st to 10th	Night	14% 7% 2 6% 3 3 4 3 5% 6% 48%	4720
	11th to 20th	Night	25% 7% 2 5% 3 3 3 2 4 4 43%	5348
	21st to end	Night	19% 9% 3 6% 4 4 4 3 4 5% 39%	7150

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	35% 4 9% 31% 18% 3	3678
	11th to 20th	Day	46% 3 9% 22% 14% 6%	3648
	21st to end	Day	49% 3 9% 23% 16% 1	4325
February	1st to 10th	Day	46% 4 13% 22% 14% 2	1800
	11th to 20th	Day	43% 4 10% 25% 16% 3	1947
	21st to end	Day	46% 4 10% 23% 14% 3	1650
March	1st to 10th	Day	45% 3 10% 20% 14% 8%	4479
	11th to 20th	Day	40% 4 12% 29% 11% 4	4796
	21st to end	Day	44% 4 9% 13% 24% 6%	5914
April	1st to 10th	Day	47% 3 9% 20% 15% 5%	2400
	11th to 20th	Day	43% 4 11% 22% 17% 3	2374
	21st to end	Day	44% 3 10% 22% 17% 4	2415
May	1st to 10th	Day	55% 5% 14% 15% 11% 1	5914
	11th to 20th	Day	52% 3 10% 20% 14% 2	6322
	21st to end	Day	46% 5% 10% 23% 14% 1	6949
June	1st to 10th	Day	43% 6% 13% 21% 15% 1	2800
	11th to 20th	Day	45% 6% 12% 23% 11% 2	2800
	21st to end	Day	40% 6% 14% 23% 16% 1	2800
July	1st to 10th	Day	43% 8% 11% 21% 14% 1	6290
	11th to 20th	Day	42% 8% 17% 22% 10% 1	6285
	21st to end	Day	41% 5% 10% 21% 21% 2	6728
August	1st to 10th	Day	49% 6% 13% 21% 10% 1	4302
	11th to 20th	Day	38% 5% 14% 29% 13% 1	3886
	21st to end	Day	35% 5% 9% 30% 20% 2	4312
September	1st to 10th	Day	41% 6% 9% 23% 19% 3	2400
	11th to 20th	Day	34% 4 11% 24% 25% 3	2324
	21st to end	Day	45% 6% 10% 19% 18% 3	2200
October	1st to 10th	Day	38% 5% 8% 19% 24% 6%	3369
	11th to 20th	Day	28% 3 10% 25% 30% 5%	3240
	21st to end	Day	27% 3 14% 26% 26% 4	3287
November	1st to 10th	Day	35% 3 11% 25% 23% 3	1851
	11th to 20th	Day	38% 3 10% 25% 20% 4	1890
	21st to end	Day	41% 3 14% 29% 12% 2	1780
December	1st to 10th	Day	40% 3 15% 29% 11% 2	2680
	11th to 20th	Day	42% 3 14% 24% 14% 2	3066
	21st to end	Day	47% 3 10% 22% 15% 2	4048
January	1st to 10th	Night	37% 3 11% 25% 21% 4	6489
	11th to 20th	Night	41% 3 14% 21% 16% 5%	6445
	21st to end	Night	45% 3 10% 20% 17% 5%	6628
February	1st to 10th	Night	45% 4 13% 24% 13% 2	2600
	11th to 20th	Night	44% 4 9% 20% 18% 5%	2348
	21st to end	Night	46% 3 6% 25% 17% 3	1980
March	1st to 10th	Night	44% 3 10% 21% 13% 8%	5374
	11th to 20th	Night	40% 3 14% 25% 13% 4	5146
	21st to end	Night	46% 4 6% 16% 23% 5%	5018
April	1st to 10th	Night	47% 4 10% 21% 15% 4	2000
	11th to 20th	Night	44% 5% 9% 20% 19% 2	1975
	21st to end	Night	43% 3 9% 22% 18% 4	1982
May	1st to 10th	Night	57% 6% 11% 14% 11% 1	4060
	11th to 20th	Night	52% 5% 13% 14% 15% 1	3588
	21st to end	Night	47% 5% 13% 22% 10% 2	3927
June	1st to 10th	Night	40% 5% 10% 22% 20% 1	1600
	11th to 20th	Night	40% 6% 15% 26% 12% 1	1600
	21st to end	Night	35% 8% 14% 26% 16% 2	1600
July	1st to 10th	Night	41% 7% 14% 19% 15% 3	3560
	11th to 20th	Night	47% 7% 13% 21% 11% 1	3552
	21st to end	Night	36% 6% 13% 23% 21% 1	3809
August	1st to 10th	Night	49% 6% 15% 18% 10% 1	2785
	11th to 20th	Night	43% 6% 12% 27% 10% 1	3227
	21st to end	Night	31% 5% 10% 33% 20% 1	3586
September	1st to 10th	Night	40% 5% 9% 25% 19% 1	2000
	11th to 20th	Night	34% 4 10% 23% 25% 3	2075
	21st to end	Night	42% 4 10% 22% 22% 2	2200
October	1st to 10th	Night	38% 4 10% 24% 21% 3	3806
	11th to 20th	Night	26% 4 9% 29% 29% 4	3899
	21st to end	Night	31% 3 11% 32% 21% 2	4502
November	1st to 10th	Night	34% 4 11% 25% 22% 4	2667
	11th to 20th	Night	40% 3 8% 24% 21% 5%	2730
	21st to end	Night	39% 4 13% 29% 15% 1	2837
December	1st to 10th	Night	39% 3 13% 31% 12% 2	4720
	11th to 20th	Night	46% 3 10% 22% 15% 3	5348
	21st to end	Night	48% 4 10% 19% 16% 4	7150

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	26% 25% 32% 14% 2	220
	11th to 20th	Day	30% 24% 36% 8% 2	212
	21st to end	Day	22% 24% 36% 12% 4 1	299
February	1st to 10th	Day	30% 21% 37% 10% 1	300
	11th to 20th	Day	29% 20% 36% 13% 1	486
	21st to end	Day	26% 20% 36% 15% 3	414
March	1st to 10th	Day	28% 21% 31% 16% 4	500
	11th to 20th	Day	23% 23% 34% 14% 5% 1	500
	21st to end	Day	28% 23% 28% 15% 5% 1	550
April	1st to 10th	Day	29% 22% 30% 13% 5% 1	500
	11th to 20th	Day	25% 22% 32% 13% 6% 2	492
	21st to end	Day	31% 16% 32% 16% 4 2	500
May	1st to 10th	Day	35% 16% 28% 14% 6% 1	503
	11th to 20th	Day	24% 15% 32% 20% 8% 1	527
	21st to end	Day	23% 9% 27% 28% 12% 2	573
June	1st to 10th	Day	19% 11% 32% 24% 12% 2	530
	11th to 20th	Day	22% 12% 29% 26% 10% 1	530
	21st to end	Day	14% 12% 32% 27% 13% 2	530
July	1st to 10th	Day	14% 7% 26% 30% 18% 4	525
	11th to 20th	Day	11% 6% 27% 34% 19% 3	524
	21st to end	Day	14% 8% 29% 32% 14% 2	576
August	1st to 10th	Day	15% 5% 25% 32% 16% 8%	526
	11th to 20th	Day	16% 5% 22% 34% 18% 5%	519
	21st to end	Day	13% 9% 29% 27% 16% 7%	583
September	1st to 10th	Day	18% 11% 25% 25% 15% 7%	530
	11th to 20th	Day	16% 6% 27% 24% 15% 11%	530
	21st to end	Day	22% 9% 30% 24% 11% 5%	530
October	1st to 10th	Day	24% 9% 27% 21% 14% 6%	530
	11th to 20th	Day	19% 17% 29% 21% 11% 2	530
	21st to end	Day	26% 12% 32% 19% 10% 1	499
November	1st to 10th	Day	20% 14% 32% 24% 8% 3	425
	11th to 20th	Day	26% 17% 30% 22% 4 1	430
	21st to end	Day	24% 21% 33% 17% 4 2	320
December	1st to 10th	Day	27% 22% 30% 15% 5%	220
	11th to 20th	Day	33% 20% 30% 14% 3	220
	21st to end	Day	29% 16% 36% 14% 5%	242
January	1st to 10th	Night	21% 17% 39% 20% 3	840
	11th to 20th	Night	25% 16% 32% 23% 3	825
	21st to end	Night	21% 18% 37% 22% 3	880
February	1st to 10th	Night	24% 13% 38% 24% 2	800
	11th to 20th	Night	24% 18% 39% 17% 1	783
	21st to end	Night	25% 16% 36% 20% 3	660
March	1st to 10th	Night	24% 16% 37% 19% 4	800
	11th to 20th	Night	23% 17% 34% 22% 4 1	800
	21st to end	Night	22% 17% 36% 20% 5%	880
April	1st to 10th	Night	24% 17% 35% 19% 4	800
	11th to 20th	Night	23% 16% 34% 23% 5%	791
	21st to end	Night	23% 14% 34% 24% 5%	782
May	1st to 10th	Night	27% 12% 34% 20% 7%	600
	11th to 20th	Night	22% 14% 29% 29% 6%	406
	21st to end	Night	21% 6% 23% 36% 13% 1	433
June	1st to 10th	Night	18% 5% 28% 36% 12% 1	400
	11th to 20th	Night	17% 6% 28% 37% 13%	400
	21st to end	Night	12% 5% 27% 45% 12%	400
July	1st to 10th	Night	12% 5% 24% 37% 21% 1	390
	11th to 20th	Night	13% 4 21% 42% 19% 1	388
	21st to end	Night	13% 4 21% 45% 15% 1	438
August	1st to 10th	Night	13% 2 20% 42% 21% 2	516
	11th to 20th	Night	13% 3 20% 39% 23% 1	790
	21st to end	Night	14% 5% 19% 40% 20% 2	880
September	1st to 10th	Night	17% 4 24% 36% 17% 2	800
	11th to 20th	Night	17% 5% 19% 36% 20% 4	800
	21st to end	Night	22% 4 24% 34% 15% 2	800
October	1st to 10th	Night	24% 7% 27% 24% 16% 3	800
	11th to 20th	Night	21% 12% 27% 30% 10% 1	800
	21st to end	Night	24% 11% 33% 24% 8%	880
November	1st to 10th	Night	17% 11% 31% 29% 11% 1	820
	11th to 20th	Night	23% 14% 32% 24% 7% 1	840
	21st to end	Night	19% 16% 33% 26% 6% 1	840
December	1st to 10th	Night	24% 15% 31% 25% 5% 1	840
	11th to 20th	Night	24% 14% 35% 25% 3	840
	21st to end	Night	25% 14% 34% 24% 3	924

Page generated at 2026-01-14 01:14:26. Version ac07

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