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 2024-11-06.

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% 70%	105048
	Night	6% 3 1 3 2 2 3 3 4 5% 67%	102757
Transparency	Day	19% 2 7% 19% 26% 27%	104801
	Night	21% 2 8% 19% 24% 26%	102593
Seeing	Day	6% 7% 20% 31% 26% 10%	15349
	Night	7% 4 17% 35% 31% 6%	24704

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	12% 8% 3 7% 6% 5% 5% 3 5% 8% 37%	2603
	11th to 20th	Day	11% 8% 3 9% 7% 6% 4 4 5% 6% 36%	2860
	21st to end	Day	16% 7% 3 7% 5% 4 5% 4 6% 8% 33%	3146
February	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 5% 48%	1710
	11th to 20th	Day	8% 6% 2 5% 4 5% 5% 4 5% 7% 50%	1793
	21st to end	Day	10% 5% 1 5% 3 3 4 4 6% 6% 53%	1570
March	1st to 10th	Day	7% 7% 3 8% 6% 7% 6% 5% 8% 8% 35%	3293
	11th to 20th	Day	6% 4 2 4 3 3 3 3 4 6% 61%	3490
	21st to end	Day	8% 6% 2 6% 5% 4 5% 6% 7% 9% 43%	3940
April	1st to 10th	Day	8% 6% 1 4 4 3 3 3 6% 7% 53%	2280
	11th to 20th	Day	7% 4 1 3 3 3 3 2 5% 6% 63%	2249
	21st to end	Day	4 3 1 2 2 3 3 2 5% 6% 70%	2280
May	1st to 10th	Day	4 2 1 1 1 1 2 2 3 5% 78%	3822
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 85%	4274
	21st to end	Day	1 1 1 1 1 2 1 3 4 84%	4813
June	1st to 10th	Day	2 2 1 2 2 2 2 2 5% 6% 73%	2660
	11th to 20th	Day	1 1 1 1 1 3 5% 86%	2660
	21st to end	Day	1 1 1 1 1 1 1 2 2 88%	2660
July	1st to 10th	Day	1 1 1 2 95%	4420
	11th to 20th	Day	1 1 1 1 1 2 1 2 3 87%	4267
	21st to end	Day	1 1 1 1 2 2 92%	4455
August	1st to 10th	Day	1 1 1 1 1 2 1 3 4 84%	4091
	11th to 20th	Day	1 1 1 1 1 2 93%	3760
	21st to end	Day	1 1 1 1 1 1 1 1 2 90%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 2 91%	2280
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 5% 81%	2280
	21st to end	Day	2 1 1 1 1 2 2 2 4 83%	2044
October	1st to 10th	Day	1 1 2 1 2 2 2 4 5% 81%	3180
	11th to 20th	Day	2 1 2 1 1 2 2 3 4 82%	3161
	21st to end	Day	2 2 1 2 2 3 3 2 4 7% 71%	3448
November	1st to 10th	Day	5% 3 1 4 4 3 4 4 5% 7% 59%	1816
	11th to 20th	Day	6% 3 1 5% 5% 4 5% 3 6% 8% 54%	1710
	21st to end	Day	7% 6% 2 4 4 4 3 3 4 8% 55%	1611
December	1st to 10th	Day	8% 7% 2 5% 5% 4 4 4 6% 7% 49%	1520
	11th to 20th	Day	16% 9% 3 9% 6% 5% 4 3 6% 6% 32%	1928
	21st to end	Day	9% 7% 3 7% 5% 5% 6% 5% 7% 9% 37%	2794
January	1st to 10th	Night	9% 7% 2 5% 4 3 4 3 5% 7% 49%	4340
	11th to 20th	Night	15% 9% 2 7% 5% 5% 5% 4 6% 7% 37%	4110
	21st to end	Night	14% 6% 2 7% 4 4 5% 4 6% 8% 39%	4521
February	1st to 10th	Night	12% 5% 1 4 3 3 4 4 4 6% 53%	2470
	11th to 20th	Night	7% 3 2 4 5% 3 3 4 5% 8% 55%	2274
	21st to end	Night	9% 4 1 5% 3 3 4 3 4 6% 58%	1884
March	1st to 10th	Night	9% 6% 2 6% 5% 4 6% 4 6% 7% 45%	3564
	11th to 20th	Night	8% 4 2 4 3 3 2 2 3 5% 64%	3490
	21st to end	Night	8% 6% 2 5% 4 4 5% 4 6% 7% 49%	3711
April	1st to 10th	Night	7% 5% 1 4 4 4 4 4 5% 7% 57%	1900
	11th to 20th	Night	5% 3 1 3 2 3 3 3 5% 6% 64%	1879
	21st to end	Night	3 2 1 2 3 2 3 2 5% 7% 72%	1900
May	1st to 10th	Night	1 1 1 1 1 1 2 2 5% 83%	3152
	11th to 20th	Night	1 1 1 1 1 1 1 2 5% 87%	2621
	21st to end	Night	1 1 1 1 1 1 2 2 4 87%	2762
June	1st to 10th	Night	2 1 1 2 2 2 2 2 3 7% 76%	1520
	11th to 20th	Night	1 1 1 2 3 4 88%	1520
	21st to end	Night	1 1 1 1 2 2 2 90%	1520
July	1st to 10th	Night	1 1 1 1 95%	2540
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 88%	2692
	21st to end	Night	1 1 1 1 1 1 1 2 3 89%	3109
August	1st to 10th	Night	2 1 1 1 1 1 1 3 4 84%	2848
	11th to 20th	Night	1 1 1 1 2 92%	3087
	21st to end	Night	1 1 1 1 1 1 1 1 2 91%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 92%	1900
	11th to 20th	Night	1 1 1 1 1 2 1 2 3 86%	1900
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	2132
October	1st to 10th	Night	2 1 1 1 1 2 2 2 4 83%	3800
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 82%	3777
	21st to end	Night	3 1 1 2 2 3 2 3 4 6% 73%	4121
November	1st to 10th	Night	6% 3 1 2 2 2 2 2 5% 8% 67%	2382
	11th to 20th	Night	6% 3 1 4 2 3 3 3 5% 8% 60%	2470
	21st to end	Night	6% 4 1 3 3 3 4 4 5% 6% 61%	2565
December	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 4 6% 53%	2660
	11th to 20th	Night	14% 7% 2 6% 5% 5% 4 4 6% 8% 37%	3276
	21st to end	Night	13% 7% 2 7% 5% 5% 5% 4 6% 7% 39%	4862

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	43% 3 14% 31% 8% 2	2603
	11th to 20th	Day	48% 2 14% 18% 13% 4	2860
	21st to end	Day	46% 2 14% 22% 13% 2	3146
February	1st to 10th	Day	37% 3 9% 25% 20% 6%	1710
	11th to 20th	Day	34% 3 11% 23% 23% 6%	1803
	21st to end	Day	32% 3 10% 27% 22% 5%	1570
March	1st to 10th	Day	44% 3 14% 20% 16% 3	3293
	11th to 20th	Day	26% 2 10% 32% 18% 12%	3490
	21st to end	Day	38% 3 12% 31% 13% 3	3940
April	1st to 10th	Day	31% 3 13% 26% 21% 7%	2280
	11th to 20th	Day	24% 2 12% 29% 26% 7%	2252
	21st to end	Day	18% 1 10% 32% 32% 7%	2280
May	1st to 10th	Day	12% 2 5% 31% 31% 19%	3822
	11th to 20th	Day	8% 1 3 21% 43% 23%	4277
	21st to end	Day	8% 2 6% 28% 44% 13%	4814
June	1st to 10th	Day	14% 2 8% 23% 35% 17%	2660
	11th to 20th	Day	5% 1 5% 20% 37% 32%	2660
	21st to end	Day	7% 1 1 10% 28% 53%	2660
July	1st to 10th	Day	2 1 1 4 13% 80%	4351
	11th to 20th	Day	7% 1 1 5% 15% 70%	4192
	21st to end	Day	3 1 4 26% 65%	4315
August	1st to 10th	Day	8% 1 1 6% 28% 56%	4076
	11th to 20th	Day	3 1 5% 33% 58%	3796
	21st to end	Day	5% 1 2 7% 34% 51%	4179
September	1st to 10th	Day	5% 1 1 8% 29% 57%	2280
	11th to 20th	Day	8% 2 3 20% 41% 26%	2280
	21st to end	Day	8% 1 3 19% 39% 29%	2044
October	1st to 10th	Day	9% 1 3 13% 26% 47%	3180
	11th to 20th	Day	9% 1 5% 26% 37% 21%	3161
	21st to end	Day	15% 1 6% 24% 39% 16%	3448
November	1st to 10th	Day	23% 4 8% 28% 26% 11%	1816
	11th to 20th	Day	25% 3 12% 31% 24% 5%	1710
	21st to end	Day	26% 2 12% 25% 27% 8%	1611
December	1st to 10th	Day	32% 4 13% 23% 19% 8%	1520
	11th to 20th	Day	50% 3 12% 16% 15% 3	1928
	21st to end	Day	43% 2 20% 22% 12% 2	2794
January	1st to 10th	Night	34% 2 20% 29% 12% 3	4340
	11th to 20th	Night	47% 2 16% 17% 10% 7%	4110
	21st to end	Night	44% 2 14% 21% 13% 5%	4521
February	1st to 10th	Night	32% 3 8% 24% 23% 10%	2470
	11th to 20th	Night	27% 3 11% 25% 25% 9%	2275
	21st to end	Night	28% 2 11% 29% 24% 7%	1884
March	1st to 10th	Night	38% 2 17% 21% 18% 4	3564
	11th to 20th	Night	25% 2 9% 26% 21% 16%	3490
	21st to end	Night	34% 2 14% 30% 15% 5%	3711
April	1st to 10th	Night	28% 4 12% 24% 22% 10%	1900
	11th to 20th	Night	22% 3 10% 31% 22% 13%	1879
	21st to end	Night	15% 2 10% 28% 34% 10%	1900
May	1st to 10th	Night	8% 1 9% 25% 37% 21%	3152
	11th to 20th	Night	6% 1 3 16% 40% 34%	2621
	21st to end	Night	5% 1 6% 26% 42% 20%	2762
June	1st to 10th	Night	11% 2 10% 19% 32% 26%	1520
	11th to 20th	Night	4 1 5% 16% 33% 42%	1520
	21st to end	Night	5% 1 1 9% 26% 58%	1520
July	1st to 10th	Night	2 1 3 10% 85%	2500
	11th to 20th	Night	6% 1 2 4 15% 71%	2644
	21st to end	Night	5% 1 6% 21% 67%	3011
August	1st to 10th	Night	8% 1 6% 28% 56%	2839
	11th to 20th	Night	3 5% 26% 65%	3117
	21st to end	Night	5% 1 1 8% 29% 56%	3498
September	1st to 10th	Night	3 1 2 6% 25% 62%	1900
	11th to 20th	Night	7% 1 2 17% 37% 35%	1900
	21st to end	Night	6% 1 2 17% 36% 37%	2132
October	1st to 10th	Night	9% 1 4 12% 18% 57%	3800
	11th to 20th	Night	8% 1 5% 25% 33% 27%	3777
	21st to end	Night	15% 1 4 21% 35% 24%	4121
November	1st to 10th	Night	18% 2 10% 29% 25% 16%	2382
	11th to 20th	Night	23% 3 10% 27% 30% 7%	2470
	21st to end	Night	23% 3 11% 24% 28% 11%	2565
December	1st to 10th	Night	31% 4 12% 22% 18% 12%	2660
	11th to 20th	Night	45% 4 12% 18% 18% 4	3276
	21st to end	Night	43% 2 17% 22% 12% 4	4862

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	24% 6% 21% 31% 16% 3	218
	11th to 20th	Day	13% 6% 23% 31% 20% 7%	370
	21st to end	Day	11% 8% 21% 29% 24% 8%	407
February	1st to 10th	Day	16% 9% 22% 23% 20% 10%	370
	11th to 20th	Day	12% 13% 28% 24% 18% 6%	428
	21st to end	Day	12% 13% 26% 29% 18% 3	382
March	1st to 10th	Day	13% 11% 26% 33% 14% 3	460
	11th to 20th	Day	13% 13% 24% 24% 20% 5%	460
	21st to end	Day	15% 14% 26% 28% 15% 2	506
April	1st to 10th	Day	13% 13% 28% 23% 19% 4	460
	11th to 20th	Day	8% 13% 26% 29% 17% 7%	452
	21st to end	Day	4 15% 26% 28% 20% 6%	460
May	1st to 10th	Day	6% 14% 28% 29% 17% 6%	463
	11th to 20th	Day	3 9% 26% 29% 23% 9%	488
	21st to end	Day	3 8% 20% 30% 27% 12%	529
June	1st to 10th	Day	4 8% 21% 32% 25% 11%	490
	11th to 20th	Day	2 7% 22% 38% 24% 7%	490
	21st to end	Day	1 4 17% 37% 30% 10%	490
July	1st to 10th	Day	1 12% 37% 36% 13%	486
	11th to 20th	Day	11% 36% 40% 13%	484
	21st to end	Day	9% 39% 34% 17%	531
August	1st to 10th	Day	3 2 12% 35% 34% 14%	487
	11th to 20th	Day	1 1 16% 35% 30% 16%	477
	21st to end	Day	1 1 8% 38% 34% 19%	539
September	1st to 10th	Day	1 1 12% 38% 36% 13%	490
	11th to 20th	Day	2 3 18% 35% 30% 13%	490
	21st to end	Day	2 5% 14% 33% 37% 9%	490
October	1st to 10th	Day	1 5% 21% 26% 34% 13%	490
	11th to 20th	Day	4 7% 16% 26% 34% 13%	490
	21st to end	Day	4 6% 17% 31% 30% 12%	539
November	1st to 10th	Day	8% 7% 20% 23% 31% 12%	382
	11th to 20th	Day	5% 6% 19% 27% 33% 11%	270
	21st to end	Day	8% 6% 17% 33% 30% 7%	223
December	1st to 10th	Day	10% 10% 21% 27% 21% 12%	180
	11th to 20th	Day	9% 14% 20% 24% 22% 11%	180
	21st to end	Day	12% 13% 23% 28% 18% 6%	198
January	1st to 10th	Night	14% 4 18% 33% 23% 9%	760
	11th to 20th	Night	13% 4 15% 35% 27% 6%	760
	21st to end	Night	12% 4 19% 34% 25% 6%	836
February	1st to 10th	Night	15% 6% 17% 33% 25% 4	760
	11th to 20th	Night	9% 9% 20% 32% 24% 7%	744
	21st to end	Night	10% 7% 25% 38% 17% 3	628
March	1st to 10th	Night	12% 7% 20% 34% 23% 4	760
	11th to 20th	Night	12% 6% 22% 32% 24% 5%	760
	21st to end	Night	10% 7% 27% 33% 21% 2	836
April	1st to 10th	Night	10% 7% 24% 36% 20% 3	760
	11th to 20th	Night	6% 9% 26% 34% 21% 4	751
	21st to end	Night	3 8% 29% 30% 24% 6%	760
May	1st to 10th	Night	3 6% 28% 37% 21% 6%	745
	11th to 20th	Night	3 3 18% 41% 32% 4	440
	21st to end	Night	3 4 19% 39% 33% 2	411
June	1st to 10th	Night	2 4 20% 42% 29% 4	380
	11th to 20th	Night	1 3 18% 42% 33% 3	380
	21st to end	Night	1 9% 49% 36% 5%	380
July	1st to 10th	Night	1 5% 43% 48% 3	370
	11th to 20th	Night	3 35% 53% 8%	455
	21st to end	Night	1 1 3 29% 58% 9%	624
August	1st to 10th	Night	2 4 37% 49% 9%	567
	11th to 20th	Night	1 10% 41% 42% 6%	725
	21st to end	Night	1 5% 37% 49% 8%	836
September	1st to 10th	Night	1 8% 36% 48% 7%	760
	11th to 20th	Night	1 2 12% 37% 41% 7%	760
	21st to end	Night	2 3 10% 33% 42% 10%	760
October	1st to 10th	Night	3 3 13% 36% 36% 8%	760
	11th to 20th	Night	3 5% 14% 34% 37% 8%	760
	21st to end	Night	4 3 15% 34% 31% 13%	836
November	1st to 10th	Night	6% 4 16% 34% 33% 9%	764
	11th to 20th	Night	8% 3 15% 36% 32% 7%	760
	21st to end	Night	8% 3 16% 37% 29% 6%	760
December	1st to 10th	Night	11% 4 20% 34% 24% 7%	760
	11th to 20th	Night	15% 4 17% 34% 25% 5%	760
	21st to end	Night	13% 7% 22% 33% 22% 3	836

Page generated at 2024-11-03 10:19:00. Version ac07

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