Tracy Airport
Clear Sky Chart
History

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

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	4 3 1 3 2 2 3 2 4 5% 71%	135229
	Night	5% 3 1 3 2 2 3 2 4 5% 69%	132256
Transparency	Day	18% 2 7% 20% 27% 27%	130179
	Night	20% 2 9% 20% 25% 25%	128289
Seeing	Day	6% 7% 20% 31% 26% 10%	16653
	Night	7% 4 17% 36% 31% 6%	25991

By Month

Percentage Cloud Cover SW (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	10% 6% 2 6% 5% 4 4 3 5% 9% 47%	3731
	11th to 20th	Day	8% 6% 2 6% 5% 4 3 3 4 5% 54%	4100
	21st to end	Day	12% 6% 2 6% 5% 4 4 4 6% 7% 44%	4510
February	1st to 10th	Day	12% 6% 2 6% 4 5% 4 4 6% 5% 47%	1800
	11th to 20th	Day	8% 6% 2 5% 4 5% 5% 4 5% 8% 49%	1890
	21st to end	Day	10% 5% 1 5% 3 3 4 4 6% 7% 52%	1650
March	1st to 10th	Day	8% 6% 2 7% 5% 5% 5% 4 7% 8% 41%	4738
	11th to 20th	Day	7% 5% 2 5% 3 4 4 4 5% 7% 55%	5000
	21st to end	Day	7% 5% 2 5% 4 4 5% 5% 7% 10% 47%	5653
April	1st to 10th	Day	8% 6% 1 4 4 3 3 3 6% 7% 54%	2400
	11th to 20th	Day	6% 4 1 3 3 3 3 2 5% 6% 63%	2369
	21st to end	Day	4 3 1 3 2 3 3 2 5% 6% 70%	2400
May	1st to 10th	Day	3 2 1 1 1 1 2 1 3 4 81%	5475
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 85%	6135
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	6902
June	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 75%	2800
	11th to 20th	Day	1 1 1 1 1 3 5% 87%	2800
	21st to end	Day	1 1 1 1 1 1 1 2 2 88%	2800
July	1st to 10th	Day	1 1 1 2 95%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 90%	6102
	21st to end	Day	1 1 1 1 1 2 3 90%	6402
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 88%	5777
	11th to 20th	Day	1 1 1 1 2 94%	5413
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	5984
September	1st to 10th	Day	1 1 1 1 1 1 2 2 90%	2400
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 5% 81%	2400
	21st to end	Day	3 1 1 1 1 1 2 2 2 4 82%	2151
October	1st to 10th	Day	1 1 2 1 2 2 2 3 5% 81%	3280
	11th to 20th	Day	2 1 2 1 1 2 2 3 4 81%	3261
	21st to end	Day	2 2 1 3 2 3 3 2 4 7% 71%	3558
November	1st to 10th	Day	5% 3 1 4 4 3 4 4 5% 7% 60%	1882
	11th to 20th	Day	6% 3 1 5% 5% 4 5% 3 6% 9% 53%	1800
	21st to end	Day	8% 6% 2 4 4 3 3 3 5% 8% 54%	1695
December	1st to 10th	Day	5% 4 2 4 4 3 4 3 6% 10% 55%	2620
	11th to 20th	Day	13% 8% 3 10% 7% 6% 6% 4 7% 7% 28%	3028
	21st to end	Day	11% 8% 3 8% 6% 5% 6% 5% 7% 9% 33%	4004
January	1st to 10th	Night	7% 5% 2 4 3 3 3 3 6% 9% 55%	6201
	11th to 20th	Night	10% 6% 2 5% 3 3 3 3 4 5% 55%	5880
	21st to end	Night	10% 5% 2 5% 4 3 4 4 4 7% 51%	6468
February	1st to 10th	Night	12% 5% 2 4 3 3 4 4 4 6% 53%	2600
	11th to 20th	Night	8% 4 2 4 5% 3 3 4 5% 8% 55%	2397
	21st to end	Night	8% 4 1 5% 3 3 4 3 4 6% 58%	1980
March	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 6% 7% 48%	5139
	11th to 20th	Night	9% 4 2 4 3 4 3 3 4 6% 58%	5000
	21st to end	Night	8% 6% 2 4 3 4 4 3 6% 8% 52%	5320
April	1st to 10th	Night	6% 4 1 4 4 4 4 4 5% 7% 58%	2000
	11th to 20th	Night	5% 3 1 3 2 3 3 3 5% 6% 65%	1979
	21st to end	Night	3 2 1 2 3 2 3 3 5% 6% 71%	2000
May	1st to 10th	Night	1 1 1 1 1 1 1 3 5% 84%	4518
	11th to 20th	Night	1 1 1 1 1 1 1 2 5% 86%	3763
	21st to end	Night	1 1 1 1 1 1 1 2 3 89%	3972
June	1st to 10th	Night	2 1 1 2 2 2 2 2 3 7% 77%	1600
	11th to 20th	Night	1 1 1 2 3 4 89%	1600
	21st to end	Night	1 1 1 1 1 2 2 91%	1600
July	1st to 10th	Night	1 1 97%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 91%	3862
	21st to end	Night	1 1 1 1 1 1 2 3 89%	4472
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 89%	4035
	11th to 20th	Night	1 1 1 1 2 95%	4453
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	5015
September	1st to 10th	Night	1 1 1 1 1 2 2 91%	2000
	11th to 20th	Night	1 1 1 1 1 2 1 2 3 86%	2000
	21st to end	Night	2 1 1 1 1 1 1 2 3 86%	2245
October	1st to 10th	Night	2 1 1 1 1 2 2 2 4 83%	3920
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 4 82%	3897
	21st to end	Night	2 2 1 2 2 3 2 3 4 6% 73%	4253
November	1st to 10th	Night	5% 3 1 2 2 2 2 2 5% 8% 68%	2459
	11th to 20th	Night	6% 3 1 4 3 3 3 3 5% 8% 60%	2600
	21st to end	Night	7% 4 1 3 3 2 4 4 5% 7% 60%	2700
December	1st to 10th	Night	6% 3 1 4 3 3 3 3 5% 6% 63%	4560
	11th to 20th	Night	13% 6% 2 6% 6% 6% 5% 4 7% 10% 34%	5176
	21st to end	Night	15% 7% 3 8% 5% 5% 5% 4 6% 7% 35%	6952

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% 3 19% 25% 15% 3	3731
	11th to 20th	Day	34% 2 10% 18% 24% 12%	4100
	21st to end	Day	39% 2 10% 18% 21% 10%	4510
February	1st to 10th	Day	38% 3 9% 25% 19% 7%	1800
	11th to 20th	Day	34% 2 12% 23% 22% 6%	1900
	21st to end	Day	32% 3 10% 27% 22% 6%	1650
March	1st to 10th	Day	39% 3 10% 22% 23% 3	4738
	11th to 20th	Day	30% 2 12% 32% 15% 9%	5000
	21st to end	Day	34% 3 12% 36% 13% 2	5653
April	1st to 10th	Day	30% 3 12% 27% 21% 6%	2400
	11th to 20th	Day	24% 2 12% 29% 26% 8%	2372
	21st to end	Day	18% 1 10% 32% 32% 7%	2400
May	1st to 10th	Day	10% 1 4 27% 34% 24%	5475
	11th to 20th	Day	7% 1 3 25% 44% 19%	6138
	21st to end	Day	6% 1 4 22% 49% 17%	6904
June	1st to 10th	Day	14% 2 8% 23% 36% 17%	2800
	11th to 20th	Day	5% 1 4 19% 37% 34%	2800
	21st to end	Day	6% 1 1 10% 29% 54%	2800
July	1st to 10th	Day	2 1 4 16% 78%	6251
	11th to 20th	Day	5% 1 1 4 13% 76%	6027
	21st to end	Day	4 1 1 11% 31% 51%	6217
August	1st to 10th	Day	7% 1 1 6% 29% 55%	4206
	11th to 20th	Day	3 1 6% 33% 58%	3917
	21st to end	Day	5% 1 2 7% 35% 51%	4311
September	1st to 10th	Day	5% 1 2 10% 29% 54%	2400
	11th to 20th	Day	9% 2 3 20% 41% 25%	2400
	21st to end	Day	9% 1 3 19% 39% 29%	2151
October	1st to 10th	Day	9% 1 4 13% 27% 46%	3280
	11th to 20th	Day	10% 1 5% 26% 37% 21%	3261
	21st to end	Day	16% 1 6% 24% 39% 15%	3558
November	1st to 10th	Day	23% 4 7% 28% 27% 11%	1882
	11th to 20th	Day	26% 3 12% 31% 23% 5%	1800
	21st to end	Day	27% 2 12% 25% 26% 7%	1695
December	1st to 10th	Day	25% 2 10% 36% 22% 5%	2620
	11th to 20th	Day	52% 2 11% 20% 13% 2	3028
	21st to end	Day	46% 2 20% 20% 10% 2	4004
January	1st to 10th	Night	27% 2 22% 30% 16% 3	6201
	11th to 20th	Night	33% 2 11% 19% 26% 10%	5880
	21st to end	Night	34% 2 11% 17% 25% 12%	6468
February	1st to 10th	Night	33% 3 8% 23% 23% 10%	2600
	11th to 20th	Night	28% 3 11% 25% 24% 9%	2398
	21st to end	Night	28% 2 11% 29% 24% 7%	1980
March	1st to 10th	Night	34% 2 13% 22% 23% 5%	5139
	11th to 20th	Night	29% 2 13% 27% 17% 12%	5000
	21st to end	Night	31% 2 15% 32% 16% 4	5320
April	1st to 10th	Night	28% 4 12% 24% 23% 10%	2000
	11th to 20th	Night	21% 3 10% 30% 23% 13%	1979
	21st to end	Night	15% 2 10% 29% 34% 10%	2000
May	1st to 10th	Night	7% 1 7% 26% 34% 25%	4515
	11th to 20th	Night	6% 1 4 20% 42% 27%	3763
	21st to end	Night	5% 1 4 20% 43% 27%	3972
June	1st to 10th	Night	11% 2 9% 20% 33% 26%	1600
	11th to 20th	Night	4 1 5% 15% 32% 44%	1600
	21st to end	Night	5% 1 1 9% 26% 59%	1600
July	1st to 10th	Night	1 2 13% 83%	3600
	11th to 20th	Night	4 1 1 3 16% 75%	3814
	21st to end	Night	5% 1 15% 26% 53%	4345
August	1st to 10th	Night	8% 1 6% 29% 56%	2929
	11th to 20th	Night	3 5% 27% 64%	3216
	21st to end	Night	5% 1 1 7% 29% 56%	3608
September	1st to 10th	Night	4 1 2 8% 26% 60%	2000
	11th to 20th	Night	8% 1 2 17% 38% 34%	2000
	21st to end	Night	7% 1 2 17% 35% 37%	2245
October	1st to 10th	Night	9% 1 4 12% 19% 56%	3920
	11th to 20th	Night	9% 1 4 26% 33% 26%	3897
	21st to end	Night	15% 1 5% 21% 35% 24%	4253
November	1st to 10th	Night	18% 2 9% 29% 26% 16%	2459
	11th to 20th	Night	23% 3 11% 28% 29% 7%	2600
	21st to end	Night	25% 3 11% 24% 27% 11%	2700
December	1st to 10th	Night	23% 3 9% 31% 25% 10%	4560
	11th to 20th	Night	45% 4 13% 22% 14% 2	5176
	21st to end	Night	48% 2 17% 21% 10% 3	6952

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	23% 6% 20% 32% 17% 3	240
	11th to 20th	Day	12% 7% 23% 30% 21% 8%	400
	21st to end	Day	11% 7% 21% 29% 24% 8%	440
February	1st to 10th	Day	17% 9% 23% 24% 19% 9%	400
	11th to 20th	Day	11% 13% 28% 25% 17% 5%	465
	21st to end	Day	12% 13% 25% 29% 18% 3	414
March	1st to 10th	Day	13% 11% 27% 32% 14% 3	500
	11th to 20th	Day	14% 14% 25% 23% 20% 5%	500
	21st to end	Day	15% 13% 25% 28% 15% 3	550
April	1st to 10th	Day	12% 13% 28% 24% 19% 5%	500
	11th to 20th	Day	8% 13% 25% 29% 18% 7%	492
	21st to end	Day	4 14% 25% 29% 20% 7%	500
May	1st to 10th	Day	6% 13% 28% 28% 18% 7%	503
	11th to 20th	Day	3 10% 27% 28% 22% 9%	528
	21st to end	Day	3 8% 21% 30% 27% 12%	573
June	1st to 10th	Day	4 7% 20% 31% 26% 12%	530
	11th to 20th	Day	2 7% 22% 38% 23% 7%	530
	21st to end	Day	1 5% 18% 35% 29% 12%	530
July	1st to 10th	Day	2 13% 38% 36% 12%	526
	11th to 20th	Day	10% 35% 40% 15%	524
	21st to end	Day	9% 38% 35% 18%	575
August	1st to 10th	Day	3 2 12% 35% 33% 16%	527
	11th to 20th	Day	1 1 16% 35% 29% 18%	517
	21st to end	Day	1 1 7% 38% 33% 20%	583
September	1st to 10th	Day	1 1 12% 38% 35% 13%	530
	11th to 20th	Day	2 3 17% 34% 30% 15%	530
	21st to end	Day	3 5% 14% 34% 36% 9%	530
October	1st to 10th	Day	2 5% 20% 26% 34% 13%	530
	11th to 20th	Day	5% 7% 16% 25% 34% 13%	530
	21st to end	Day	5% 5% 16% 30% 31% 12%	583
November	1st to 10th	Day	7% 6% 20% 22% 31% 14%	406
	11th to 20th	Day	6% 5% 21% 27% 30% 10%	300
	21st to end	Day	9% 6% 17% 34% 28% 6%	247
December	1st to 10th	Day	9% 9% 21% 26% 22% 14%	200
	11th to 20th	Day	10% 13% 19% 25% 24% 10%	200
	21st to end	Day	12% 12% 26% 29% 16% 5%	220
January	1st to 10th	Night	13% 4 18% 34% 23% 9%	800
	11th to 20th	Night	12% 4 15% 36% 27% 6%	800
	21st to end	Night	11% 4 18% 35% 25% 6%	880
February	1st to 10th	Night	15% 7% 18% 33% 23% 4	800
	11th to 20th	Night	9% 9% 20% 33% 22% 6%	784
	21st to end	Night	9% 6% 25% 39% 17% 3	660
March	1st to 10th	Night	12% 8% 21% 34% 22% 3	800
	11th to 20th	Night	12% 6% 22% 32% 23% 4	800
	21st to end	Night	10% 7% 26% 34% 21% 2	880
April	1st to 10th	Night	9% 7% 25% 36% 20% 3	800
	11th to 20th	Night	6% 8% 26% 34% 21% 4	791
	21st to end	Night	3 8% 28% 31% 24% 5%	800
May	1st to 10th	Night	3 5% 28% 37% 21% 6%	784
	11th to 20th	Night	2 3 20% 41% 31% 4	463
	21st to end	Night	3 4 19% 41% 32% 2	433
June	1st to 10th	Night	2 4 19% 41% 30% 4	400
	11th to 20th	Night	1 3 17% 44% 33% 3	400
	21st to end	Night	1 10% 47% 38% 5%	400
July	1st to 10th	Night	1 5% 44% 48% 3	390
	11th to 20th	Night	3 35% 53% 8%	480
	21st to end	Night	1 1 3 29% 58% 9%	657
August	1st to 10th	Night	2 4 37% 48% 10%	597
	11th to 20th	Night	1 10% 40% 42% 7%	764
	21st to end	Night	1 6% 37% 48% 8%	880
September	1st to 10th	Night	1 9% 36% 48% 6%	800
	11th to 20th	Night	1 2 12% 37% 41% 7%	800
	21st to end	Night	3 3 10% 34% 41% 10%	800
October	1st to 10th	Night	3 3 13% 36% 37% 8%	800
	11th to 20th	Night	4 5% 14% 34% 36% 9%	800
	21st to end	Night	4 3 15% 35% 31% 13%	880
November	1st to 10th	Night	6% 3 15% 33% 34% 9%	788
	11th to 20th	Night	7% 3 16% 37% 31% 7%	800
	21st to end	Night	9% 4 16% 37% 28% 6%	800
December	1st to 10th	Night	10% 4 19% 34% 25% 8%	800
	11th to 20th	Night	15% 4 17% 34% 25% 5%	800
	21st to end	Night	14% 7% 23% 34% 20% 3	880

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