Martinez
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Martinez clear sky chart from 2005-11-06 to 2025-11-18.

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%	126815
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	123550
Transparency	Day	37% 8% 12% 19% 18% 6%	130513
	Night	36% 7% 11% 17% 20% 8%	127921
Seeing	Day	15% 9% 23% 24% 20% 9%	16613
	Night	15% 8% 23% 32% 20% 3	26640

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	15% 4 2 4 3 3 3 3 6% 6% 52%	4056
	11th to 20th	Day	12% 5% 2 5% 3 3 2 2 3 6% 57%	4060
	21st to end	Day	17% 6% 2 5% 4 4 4 3 4 6% 44%	4466
February	1st to 10th	Day	14% 6% 3 6% 5% 4 4 4 6% 8% 40%	1916
	11th to 20th	Day	20% 8% 2 5% 3 3 3 3 4 5% 44%	1948
	21st to end	Day	17% 6% 2 5% 4 4 4 3 5% 5% 45%	1651
March	1st to 10th	Day	20% 8% 3 7% 4 3 4 3 5% 5% 39%	4935
	11th to 20th	Day	9% 4 2 4 3 3 4 3 5% 8% 55%	4980
	21st to end	Day	16% 6% 2 6% 4 4 4 3 4 5% 46%	5465
April	1st to 10th	Day	11% 5% 2 5% 5% 4 4 4 5% 7% 50%	2200
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 5% 6% 50%	2283
	21st to end	Day	8% 4 1 5% 4 4 4 4 6% 9% 51%	2600
May	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 6% 10% 41%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 5% 7% 8% 32%	5861
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 42%	6433
June	1st to 10th	Day	10% 6% 3 7% 5% 4 5% 6% 7% 11% 36%	2600
	11th to 20th	Day	6% 4 1 5% 5% 5% 5% 5% 9% 11% 43%	2600
	21st to end	Day	5% 4 2 7% 5% 5% 7% 6% 9% 12% 38%	2600
July	1st to 10th	Day	11% 7% 3 9% 7% 6% 8% 6% 9% 10% 25%	5900
	11th to 20th	Day	4 4 2 7% 6% 6% 8% 7% 10% 14% 32%	5900
	21st to end	Day	7% 6% 3 8% 7% 7% 8% 7% 10% 13% 25%	6424
August	1st to 10th	Day	18% 7% 2 7% 5% 6% 6% 6% 8% 11% 23%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 6% 9% 11% 34%	4174
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 6% 10% 45%	4197
September	1st to 10th	Day	8% 5% 2 5% 6% 5% 6% 4 7% 10% 42%	2200
	11th to 20th	Day	14% 5% 2 5% 4 5% 5% 5% 5% 9% 40%	2200
	21st to end	Day	15% 7% 2 6% 4 4 4 3 5% 6% 42%	2200
October	1st to 10th	Day	13% 6% 3 6% 4 3 4 3 4 5% 49%	2155
	11th to 20th	Day	11% 5% 1 4 2 2 3 2 4 5% 60%	2000
	21st to end	Day	12% 4 2 5% 4 3 4 3 3 4 57%	2200
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 4 6% 52%	2057
	11th to 20th	Day	13% 6% 2 3 2 3 3 2 4 6% 55%	1889
	21st to end	Day	14% 5% 2 5% 4 3 3 3 4 6% 49%	1800
December	1st to 10th	Day	12% 5% 2 6% 4 3 4 3 5% 6% 50%	2930
	11th to 20th	Day	15% 6% 2 5% 4 4 3 3 5% 6% 47%	3373
	21st to end	Day	21% 7% 2 4 3 2 3 3 4 6% 45%	4466
January	1st to 10th	Night	13% 5% 1 4 3 3 3 3 4 5% 56%	5889
	11th to 20th	Night	13% 5% 2 5% 3 3 4 4 6% 7% 49%	5900
	21st to end	Night	16% 8% 2 6% 4 4 3 3 5% 5% 46%	6490
February	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 5% 6% 45%	2480
	11th to 20th	Night	18% 7% 3 5% 4 3 4 2 4 5% 44%	2338
	21st to end	Night	13% 6% 2 6% 4 4 3 3 5% 6% 47%	1976
March	1st to 10th	Night	20% 7% 2 6% 4 3 4 3 5% 6% 39%	4918
	11th to 20th	Night	10% 3 2 3 3 3 4 3 6% 7% 55%	4980
	21st to end	Night	15% 5% 2 5% 4 3 4 4 5% 6% 48%	5467
April	1st to 10th	Night	11% 5% 1 5% 5% 4 5% 4 6% 7% 49%	2200
	11th to 20th	Night	11% 4 2 5% 4 4 3 3 5% 7% 52%	2075
	21st to end	Night	8% 4 2 6% 4 4 5% 4 6% 7% 53%	1806
May	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 10% 40%	4033
	11th to 20th	Night	18% 5% 2 6% 3 5% 5% 4 7% 10% 35%	4032
	21st to end	Night	11% 4 2 5% 4 4 5% 4 7% 8% 46%	4421
June	1st to 10th	Night	11% 7% 3 6% 6% 4 6% 6% 7% 11% 34%	1800
	11th to 20th	Night	7% 5% 2 7% 5% 5% 5% 5% 8% 11% 41%	1800
	21st to end	Night	6% 5% 1 6% 6% 6% 6% 7% 10% 11% 36%	1800
July	1st to 10th	Night	14% 8% 4 9% 6% 6% 6% 5% 8% 10% 25%	4058
	11th to 20th	Night	5% 4 2 7% 6% 6% 7% 7% 11% 15% 30%	4057
	21st to end	Night	11% 7% 3 9% 7% 6% 7% 6% 8% 11% 25%	4427
August	1st to 10th	Night	18% 7% 3 8% 7% 7% 6% 6% 8% 9% 22%	2915
	11th to 20th	Night	10% 6% 2 7% 5% 6% 5% 6% 8% 11% 35%	2875
	21st to end	Night	7% 5% 2 5% 5% 3 5% 4 6% 11% 47%	3647
September	1st to 10th	Night	10% 5% 2 6% 6% 5% 4 5% 7% 9% 41%	2200
	11th to 20th	Night	12% 7% 2 8% 5% 5% 5% 3 5% 7% 41%	2200
	21st to end	Night	14% 5% 2 7% 4 5% 4 4 5% 6% 45%	2200
October	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 4 6% 51%	2242
	11th to 20th	Night	12% 4 1 4 3 3 3 2 4 4 59%	2400
	21st to end	Night	11% 4 2 4 4 3 3 2 4 5% 57%	2640
November	1st to 10th	Night	12% 5% 2 4 3 3 4 2 4 5% 55%	2461
	11th to 20th	Night	14% 5% 2 4 4 3 3 2 4 6% 53%	2661
	21st to end	Night	13% 4 2 5% 3 4 4 3 4 7% 50%	2600
December	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 5% 55%	4250
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 4 5% 47%	4822
	21st to end	Night	21% 6% 2 5% 4 4 4 3 5% 5% 42%	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	30% 6% 7% 19% 31% 7%	4057
	11th to 20th	Day	30% 4 4 18% 33% 10%	4060
	21st to end	Day	41% 3 5% 19% 22% 10%	4466
February	1st to 10th	Day	41% 5% 10% 20% 18% 7%	1916
	11th to 20th	Day	43% 4 5% 19% 22% 7%	1958
	21st to end	Day	40% 5% 8% 15% 21% 10%	1651
March	1st to 10th	Day	46% 5% 7% 10% 23% 9%	4935
	11th to 20th	Day	27% 5% 8% 15% 22% 21%	4980
	21st to end	Day	41% 4 6% 19% 21% 9%	5465
April	1st to 10th	Day	33% 6% 10% 21% 22% 8%	2200
	11th to 20th	Day	33% 6% 11% 19% 21% 10%	2283
	21st to end	Day	28% 7% 12% 25% 23% 4	2600
May	1st to 10th	Day	36% 9% 18% 23% 13% 2	5897
	11th to 20th	Day	47% 9% 16% 17% 11% 1	5865
	21st to end	Day	35% 9% 15% 18% 22% 2	6447
June	1st to 10th	Day	38% 12% 18% 19% 12% 2	2600
	11th to 20th	Day	30% 12% 20% 24% 13% 1	2600
	21st to end	Day	33% 13% 19% 20% 13% 3	2600
July	1st to 10th	Day	47% 12% 17% 18% 4 1	5835
	11th to 20th	Day	34% 17% 29% 17% 3 1	5831
	21st to end	Day	43% 16% 19% 17% 5%	6240
August	1st to 10th	Day	50% 12% 18% 16% 3	4221
	11th to 20th	Day	37% 12% 17% 21% 12% 1	4213
	21st to end	Day	32% 9% 13% 28% 16% 2	4197
September	1st to 10th	Day	35% 12% 15% 17% 16% 5%	2200
	11th to 20th	Day	39% 9% 13% 17% 19% 4	2200
	21st to end	Day	42% 6% 11% 17% 17% 7%	2200
October	1st to 10th	Day	33% 4 6% 14% 32% 10%	3506
	11th to 20th	Day	17% 2 5% 15% 30% 31%	3240
	21st to end	Day	25% 4 4 21% 33% 14%	3538
November	1st to 10th	Day	34% 5% 8% 15% 25% 13%	2057
	11th to 20th	Day	31% 4 7% 21% 25% 12%	1889
	21st to end	Day	36% 4 9% 19% 22% 9%	1800
December	1st to 10th	Day	35% 5% 7% 16% 28% 9%	2930
	11th to 20th	Day	38% 4 10% 21% 20% 7%	3370
	21st to end	Day	41% 4 8% 22% 23% 2	4466
January	1st to 10th	Night	29% 5% 6% 19% 34% 8%	5889
	11th to 20th	Night	33% 5% 6% 17% 30% 8%	5900
	21st to end	Night	41% 5% 5% 14% 28% 8%	6490
February	1st to 10th	Night	39% 4 9% 18% 21% 9%	2480
	11th to 20th	Night	42% 5% 6% 16% 21% 9%	2339
	21st to end	Night	38% 6% 9% 15% 19% 13%	1976
March	1st to 10th	Night	44% 5% 10% 12% 21% 8%	4918
	11th to 20th	Night	27% 4 7% 13% 28% 20%	4980
	21st to end	Night	36% 6% 7% 17% 20% 14%	5467
April	1st to 10th	Night	33% 7% 9% 20% 21% 10%	2200
	11th to 20th	Night	31% 6% 8% 21% 23% 11%	2075
	21st to end	Night	29% 7% 12% 24% 22% 6%	1806
May	1st to 10th	Night	36% 10% 16% 24% 12% 3	4060
	11th to 20th	Night	42% 10% 19% 18% 12% 1	4033
	21st to end	Night	32% 9% 14% 20% 22% 3	4429
June	1st to 10th	Night	42% 11% 16% 19% 12% 1	1800
	11th to 20th	Night	34% 11% 19% 24% 12% 1	1800
	21st to end	Night	35% 14% 21% 16% 11% 3	1800
July	1st to 10th	Night	50% 13% 18% 13% 4 1	4015
	11th to 20th	Night	36% 17% 31% 14% 2 1	4006
	21st to end	Night	49% 14% 17% 16% 4	4297
August	1st to 10th	Night	53% 13% 17% 14% 3	2904
	11th to 20th	Night	38% 11% 21% 19% 8% 1	2902
	21st to end	Night	30% 8% 17% 30% 14% 1	3647
September	1st to 10th	Night	37% 12% 16% 18% 13% 5%	2200
	11th to 20th	Night	41% 8% 12% 18% 17% 4	2200
	21st to end	Night	39% 7% 12% 19% 17% 5%	2200
October	1st to 10th	Night	34% 5% 9% 14% 29% 9%	3671
	11th to 20th	Night	18% 2 6% 14% 24% 35%	3899
	21st to end	Night	23% 3 6% 17% 32% 19%	4252
November	1st to 10th	Night	31% 5% 8% 16% 25% 14%	2461
	11th to 20th	Night	34% 4 7% 16% 25% 15%	2661
	21st to end	Night	34% 5% 9% 17% 25% 11%	2600
December	1st to 10th	Night	32% 4 7% 16% 33% 8%	4250
	11th to 20th	Night	40% 4 11% 18% 19% 8%	4824
	21st to end	Night	43% 5% 5% 22% 22% 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	20% 22% 41% 15% 2	300
	11th to 20th	Day	21% 22% 31% 20% 5%	300
	21st to end	Day	22% 27% 34% 13% 3 1	330
February	1st to 10th	Day	17% 22% 38% 19% 5%	300
	11th to 20th	Day	35% 17% 32% 14% 2	300
	21st to end	Day	24% 21% 34% 16% 5%	250
March	1st to 10th	Day	20% 17% 30% 23% 9%	500
	11th to 20th	Day	17% 15% 39% 21% 8%	500
	21st to end	Day	18% 15% 38% 20% 8% 1	550
April	1st to 10th	Day	16% 15% 31% 26% 11% 1	500
	11th to 20th	Day	13% 13% 28% 26% 15% 3	492
	21st to end	Day	11% 9% 25% 29% 19% 6%	500
May	1st to 10th	Day	10% 13% 28% 28% 15% 6%	503
	11th to 20th	Day	17% 5% 20% 29% 19% 11%	527
	21st to end	Day	15% 6% 16% 27% 23% 13%	573
June	1st to 10th	Day	13% 3 16% 30% 26% 12%	530
	11th to 20th	Day	7% 3 18% 26% 25% 21%	530
	21st to end	Day	6% 3 12% 31% 29% 18%	530
July	1st to 10th	Day	8% 2 9% 25% 31% 26%	525
	11th to 20th	Day	7% 2 6% 25% 36% 24%	524
	21st to end	Day	6% 1 7% 28% 37% 21%	576
August	1st to 10th	Day	15% 3 13% 25% 28% 16%	526
	11th to 20th	Day	10% 4 10% 21% 36% 20%	519
	21st to end	Day	12% 3 11% 26% 28% 20%	583
September	1st to 10th	Day	8% 5% 15% 27% 30% 15%	530
	11th to 20th	Day	17% 3 13% 25% 29% 12%	530
	21st to end	Day	18% 4 16% 28% 24% 11%	530
October	1st to 10th	Day	16% 6% 21% 29% 20% 8%	530
	11th to 20th	Day	11% 5% 23% 29% 23% 8%	530
	21st to end	Day	15% 8% 26% 26% 21% 4	583
November	1st to 10th	Day	20% 7% 26% 25% 19% 4	535
	11th to 20th	Day	16% 14% 40% 19% 10% 1	347
	21st to end	Day	13% 15% 35% 24% 12% 2	300
December	1st to 10th	Day	24% 16% 34% 18% 7% 1	300
	11th to 20th	Day	20% 14% 40% 14% 9% 2	300
	21st to end	Day	29% 16% 33% 15% 5% 2	330
January	1st to 10th	Night	20% 14% 34% 27% 6%	800
	11th to 20th	Night	18% 13% 32% 29% 7%	800
	21st to end	Night	20% 13% 39% 25% 3	880
February	1st to 10th	Night	17% 12% 34% 30% 7%	800
	11th to 20th	Night	23% 13% 34% 27% 3	783
	21st to end	Night	18% 15% 34% 29% 4	660
March	1st to 10th	Night	15% 12% 35% 32% 7%	800
	11th to 20th	Night	17% 11% 35% 29% 8% 1	800
	21st to end	Night	15% 13% 36% 28% 9%	880
April	1st to 10th	Night	12% 14% 33% 31% 11%	800
	11th to 20th	Night	14% 11% 28% 33% 13% 1	791
	21st to end	Night	10% 5% 26% 43% 16%	606
May	1st to 10th	Night	10% 7% 28% 38% 17% 1	600
	11th to 20th	Night	11% 5% 26% 36% 19% 3	591
	21st to end	Night	11% 3 12% 35% 35% 4	650
June	1st to 10th	Night	13% 3 17% 36% 28% 4	600
	11th to 20th	Night	8% 2 13% 34% 34% 10%	600
	21st to end	Night	7% 2 12% 36% 39% 6%	600
July	1st to 10th	Night	11% 2 8% 31% 39% 9%	585
	11th to 20th	Night	8% 2 5% 31% 46% 8%	582
	21st to end	Night	8% 1 8% 35% 40% 8%	657
August	1st to 10th	Night	11% 2 10% 31% 38% 8%	597
	11th to 20th	Night	13% 3 6% 29% 41% 7%	594
	21st to end	Night	13% 2 11% 30% 37% 7%	775
September	1st to 10th	Night	9% 3 12% 32% 37% 6%	800
	11th to 20th	Night	14% 2 12% 30% 39% 3	800
	21st to end	Night	17% 3 14% 30% 32% 4	800
October	1st to 10th	Night	15% 4 17% 34% 25% 5%	800
	11th to 20th	Night	13% 5% 21% 34% 25% 3	800
	21st to end	Night	13% 7% 24% 32% 23% 2	880
November	1st to 10th	Night	14% 6% 23% 35% 19% 3	820
	11th to 20th	Night	17% 10% 28% 31% 13% 1	829
	21st to end	Night	15% 11% 26% 31% 16% 1	800
December	1st to 10th	Night	18% 11% 29% 31% 10% 1	800
	11th to 20th	Night	20% 8% 28% 31% 12% 1	800
	21st to end	Night	26% 11% 26% 29% 8%	880

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