Tracy Airport
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tracy Airport clear sky chart from 2005-11-05 to 2017-09-19.

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 TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(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

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
69%
15376
32%
46917
26%
15339
75%
15339
50%
22731

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
61%
717
21%
2419
21%
716
66%
716
35%
1311
January16th to endNight
61%
768
26%
2496
26%
768
66%
768
34%
1402
February1st to 15thNight
59%
700
26%
2267
25%
700
66%
700
33%
1273
February16th to endNight
56%
632
27%
1895
21%
632
66%
632
39%
1053
March1st to 15thNight
58%
720
23%
2037
19%
720
63%
720
36%
1205
March16th to endNight
53%
768
25%
2067
16%
768
61%
768
41%
1177
April1st to 15thNight
51%
719
27%
1799
12%
719
58%
719
40%
1088
April16th to endNight
56%
720
30%
1800
12%
720
64%
720
56%
944
May1st to 15thNight
67%
631
33%
1711
19%
631
73%
631
68%
777
May16th to endNight
69%
374
36%
1496
32%
374
81%
374
62%
509
June1st to 15thNight
71%
360
33%
1440
33%
360
82%
360
61%
493
June16th to endNight
87%
360
40%
1440
50%
360
92%
360
80%
422
July1st to 15thNight
95%
340
37%
1360
48%
340
97%
340
83%
386
July16th to endNight
92%
560
40%
1589
34%
527
94%
527
79%
612
August1st to 15thNight
90%
580
41%
1653
30%
577
92%
577
81%
655
August16th to endNight
92%
764
46%
1910
30%
764
93%
764
88%
817
September1st to 15thNight
92%
720
45%
1800
31%
720
94%
720
86%
782
September16th to endNight
84%
671
47%
1821
40%
671
89%
671
77%
774
October1st to 15thNight
74%
660
35%
1980
33%
660
82%
660
60%
884
October16th to endNight
75%
704
40%
2112
39%
704
82%
704
60%
945
November1st to 15thNight
70%
700
32%
2217
31%
700
77%
700
52%
1039
November16th to endNight
70%
720
28%
2400
28%
720
75%
720
41%
1204
December1st to 15thNight
59%
720
22%
2520
19%
720
67%
720
29%
1445
December16th to endNight
53%
768
23%
2688
19%
768
61%
768
29%
1534

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 ForecastData
Points
Cloud CoverDay
5%212213335%67%
47896
Night
5%212212335%67%
47102
TransparencyDay
19%28%20%26%21%
47579
Night
20%29%20%24%22%
46918
SeeingDay
6%7%19%31%24%9%
5962
Night
6%315%33%33%7%
15385

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
11%4247%34448%43%
1516
16th to endDay
12%5%25%6%345%5%5%43%
1728
February1st to 15thDay
12%5%25%44435%5%45%
1596
16th to endDay
13%5%15%4145%5%7%45%
1579
March1st to 15thDay
8%5%25%435%5%6%10%43%
1920
16th to endDay
8%4145%35%6%5%8%47%
2156
April1st to 15thDay
9%5%2432436%7%50%
2149
16th to endDay
5%212213245%68%
2160
May1st to 15thDay
1211112236%74%
2235
16th to endDay
2112212246%71%
2615
June1st to 15thDay
211212236%74%
2520
16th to endDay
1112387%
2520
July1st to 15thDay
111290%
2520
16th to endDay
1211288%
2470
August1st to 15thDay
111289%
2295
16th to endDay
1111389%
2292
September1st to 15thDay
1112388%
2160
16th to endDay
2111122480%
1881
October1st to 15thDay
312112237%72%
1650
16th to endDay
4212312346%66%
1760
November1st to 15thDay
6%22342345%7%57%
1641
16th to endDay
7%4235%2446%7%48%
1557
December1st to 15thDay
10%7%35%5%2445%6%43%
1440
16th to endDay
12%5%35%425%35%9%41%
1536
January1st to 15thNight
8%5%1442445%7%50%
2419
16th to endNight
11%424323347%52%
2496
February1st to 15thNight
12%3134345%5%7%49%
2256
16th to endNight
11%32332335%7%53%
1895
March1st to 15thNight
9%424324447%51%
2037
16th to endNight
7%41332345%7%56%
2067
April1st to 15thNight
7%4134345%47%53%
1799
16th to endNight
4112312346%69%
1800
May1st to 15thNight
211211227%76%
1711
16th to endNight
111111235%78%
1496
June1st to 15thNight
11111136%78%
1440
16th to endNight
111290%
1440
July1st to 15thNight
111290%
1440
16th to endNight
1121386%
1696
August1st to 15thNight
1111189%
1662
16th to endNight
12389%
1910
September1st to 15thNight
111289%
1800
16th to endNight
111111485%
1821
October1st to 15thNight
321112236%72%
1980
16th to endNight
3111212346%71%
2112
November1st to 15thNight
6%212212347%66%
2217
16th to endNight
6%31232445%7%58%
2400
December1st to 15thNight
11%4243245%46%49%
2520
16th to endNight
13%5%23424446%49%
2688

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
35%5%15%21%19%3
1516
16th to endDay
38%410%19%20%7%
1728
February1st to 15thDay
38%312%23%18%3
1596
16th to endDay
38%412%28%15%2
1579
March1st to 15thDay
35%5%13%25%16%5%
1920
16th to endDay
34%5%16%21%18%3
2156
April1st to 15thDay
32%313%26%20%3
2149
16th to endDay
19%111%28%30%7%
2160
May1st to 15thDay
12%26%27%37%12%
2235
16th to endDay
15%28%29%30%12%
2619
June1st to 15thDay
13%27%23%27%24%
2520
16th to endDay
5%1113%32%45%
2520
July1st to 15thDay
3116%24%63%
2371
16th to endDay
5%127%28%54%
2314
August1st to 15thDay
5%111%30%50%
2279
16th to endDay
410%41%42%
2292
September1st to 15thDay
5%1213%33%44%
2160
16th to endDay
9%1323%38%23%
1881
October1st to 15thDay
14%19%25%28%20%
1650
16th to endDay
19%29%24%29%14%
1760
November1st to 15thDay
23%411%32%21%5%
1641
16th to endDay
29%314%28%22%3
1557
December1st to 15thDay
39%313%21%19%3
1440
16th to endDay
38%217%20%16%4
1536
January1st to 15thNight
31%313%25%18%7%
2419
16th to endNight
32%412%18%18%14%
2496
February1st to 15thNight
32%411%24%20%7%
2267
16th to endNight
30%214%29%18%4
1895
March1st to 15thNight
32%413%23%17%8%
2037
16th to endNight
27%414%24%20%8%
2067
April1st to 15thNight
29%411%27%20%6%
1799
16th to endNight
17%313%24%29%12%
1800
May1st to 15thNight
11%16%22%35%21%
1712
16th to endNight
9%28%29%32%17%
1496
June1st to 15thNight
9%28%19%28%30%
1440
16th to endNight
41112%25%55%
1440
July1st to 15thNight
315%17%70%
1360
16th to endNight
6%128%22%58%
1589
August1st to 15thNight
5%110%27%54%
1653
16th to endNight
318%35%49%
1910
September1st to 15thNight
41311%28%51%
1800
16th to endNight
7%1217%37%34%
1821
October1st to 15thNight
14%18%23%27%23%
1980
16th to endNight
14%27%24%30%20%
2112
November1st to 15thNight
19%212%30%25%9%
2217
16th to endNight
25%311%26%26%7%
2400
December1st to 15thNight
34%5%12%19%20%7%
2520
16th to endNight
35%314%20%19%6%
2688

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
19%422%29%17%7%
144
16th to endDay
14%7%21%23%20%12%
256
February1st to 15thDay
15%8%21%21%25%8%
240
16th to endDay
17%8%24%25%18%6%
238
March1st to 15thDay
12%8%25%26%20%5%
270
16th to endDay
13%13%23%30%14%3
288
April1st to 15thDay
13%18%27%21%14%3
269
16th to endDay
6%20%28%26%14%3
270
May1st to 15thDay
5%11%30%28%18%4
288
16th to endDay
310%22%27%26%10%
330
June1st to 15thDay
411%25%34%19%5%
315
16th to endDay
319%39%29%7%
315
July1st to 15thDay
14%36%35%12%
305
16th to endDay
10%42%32%13%
327
August1st to 15thDay
214%36%31%14%
315
16th to endDay
7%39%28%22%
332
September1st to 15thDay
10%42%35%11%
315
16th to endDay
26%16%32%31%10%
268
October1st to 15thDay
47%21%28%25%12%
255
16th to endDay
6%418%36%24%10%
272
November1st to 15thDay
10%314%13%38%19%
147
16th to endDay
8%8%13%36%25%6%
79
December1st to 15thDay
11%16%23%33%15%
60
16th to endDay
9%14%15%25%28%7%
64
January1st to 15thNight
11%213%30%29%11%
720
16th to endNight
12%316%30%27%9%
768
February1st to 15thNight
13%315%32%27%7%
704
16th to endNight
11%6%21%33%21%6%
632
March1st to 15thNight
11%5%17%31%28%5%
720
16th to endNight
10%6%24%31%24%3
768
April1st to 15thNight
8%8%24%34%19%3
719
16th to endNight
5%7%26%29%25%5%
720
May1st to 15thNight
3323%37%26%6%
632
16th to endNight
3321%35%34%2
375
June1st to 15thNight
2319%40%30%4
360
16th to endNight
10%45%37%5%
360
July1st to 15thNight
338%52%5%
340
16th to endNight
326%58%9%
560
August1st to 15thNight
15%37%46%9%
580
16th to endNight
434%51%7%
764
September1st to 15thNight
5%33%51%8%
720
16th to endNight
129%29%43%12%
671
October1st to 15thNight
5%412%33%34%10%
660
16th to endNight
3212%31%34%14%
704
November1st to 15thNight
6%214%31%35%10%
700
16th to endNight
7%313%34%32%9%
720
December1st to 15thNight
12%315%34%25%8%
720
16th to endNight
14%5%19%31%25%4
768
Page generated at 2017-09-20 01:49:52. Version ac04

Copyright 2017 A. Danko


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