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Stanford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stanford clear sky chart from 2005-11-05 to 2017-05-24.

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
67%
15095
25%
45905
23%
15059
70%
15059
37%
25833

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
62%
716
18%
2369
18%
716
65%
716
28%
1439
January16th to endNight
59%
768
24%
2496
25%
768
62%
768
25%
1570
February1st to 15thNight
57%
700
22%
2288
22%
700
61%
700
25%
1438
February16th to endNight
53%
632
21%
1898
18%
632
59%
632
26%
1233
March1st to 15thNight
56%
720
18%
1999
16%
720
58%
720
28%
1325
March16th to endNight
54%
768
19%
2092
14%
768
58%
768
29%
1373
April1st to 15thNight
52%
719
20%
1799
11%
719
56%
719
32%
1191
April16th to endNight
61%
720
24%
1800
12%
720
64%
720
45%
1049
May1st to 15thNight
70%
617
25%
1697
17%
617
72%
617
48%
910
May16th to endNight
69%
394
23%
1475
20%
394
75%
394
36%
726
June1st to 15thNight
75%
330
25%
1320
27%
330
82%
330
39%
610
June16th to endNight
85%
330
33%
1320
43%
330
88%
330
61%
468
July1st to 15thNight
89%
362
34%
1292
38%
362
91%
362
62%
494
July16th to endNight
86%
522
31%
1467
29%
489
87%
489
56%
663
August1st to 15thNight
86%
509
33%
1492
28%
506
86%
506
54%
697
August16th to endNight
85%
700
34%
1750
22%
700
85%
700
62%
889
September1st to 15thNight
86%
660
39%
1650
28%
660
86%
660
67%
810
September16th to endNight
81%
656
36%
1802
34%
656
82%
656
57%
889
October1st to 15thNight
74%
660
27%
1980
28%
660
78%
660
44%
1041
October16th to endNight
71%
704
30%
2112
32%
704
74%
704
41%
1144
November1st to 15thNight
70%
700
26%
2172
28%
700
72%
700
38%
1211
November16th to endNight
66%
720
23%
2427
25%
720
69%
720
33%
1365
December1st to 15thNight
56%
720
21%
2520
19%
720
62%
720
23%
1571
December16th to endNight
48%
768
19%
2688
16%
768
54%
768
22%
1727

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
6%423323347%58%
46366
Night
7%424323347%55%
46094
TransparencyDay
27%5%10%19%23%13%
46055
Night
28%6%11%19%20%13%
45906
SeeingDay
7%9%24%33%19%5%
5585
Night
8%313%33%32%7%
15105

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
12%5%25%42435%7%46%
1566
16th to endDay
13%6%245%3335%6%44%
1728
February1st to 15thDay
13%6%35%425%45%6%41%
1576
16th to endDay
15%6%35%5%3445%7%39%
1576
March1st to 15thDay
12%5%25%5%35%5%6%10%37%
1959
16th to endDay
10%425%6%45%5%5%7%42%
2130
April1st to 15thDay
12%6%35%324448%44%
2149
16th to endDay
7%423323347%59%
2160
May1st to 15thDay
3313213247%65%
2251
16th to endDay
6%5%23213247%59%
2480
June1st to 15thDay
5%422213347%62%
2310
16th to endDay
111112136%76%
2310
July1st to 15thDay
1112112236%76%
2250
16th to endDay
11212336%74%
2286
August1st to 15thDay
212112236%73%
2129
16th to endDay
212212347%72%
2100
September1st to 15thDay
212212235%76%
1980
16th to endDay
3213211235%72%
1826
October1st to 15thDay
6%313322237%62%
1650
16th to endDay
5%5%13323346%58%
1760
November1st to 15thDay
7%5%2442345%8%50%
1684
16th to endDay
9%6%25%5%24346%47%
1530
December1st to 15thDay
11%7%38%7%2345%5%40%
1440
16th to endDay
14%7%46%5%2435%7%39%
1536
January1st to 15thNight
11%7%2442445%8%43%
2369
16th to endNight
14%5%24424446%46%
2496
February1st to 15thNight
9%6%26%5%35%45%7%43%
2276
16th to endNight
13%5%245%335%5%7%42%
1898
March1st to 15thNight
12%426%5%34447%43%
1999
16th to endNight
9%6%35%334446%47%
2092
April1st to 15thNight
8%5%2443447%8%46%
1799
16th to endNight
6%213314347%61%
1800
May1st to 15thNight
131331335%9%62%
1697
16th to endNight
2424213447%60%
1474
June1st to 15thNight
221332125%8%65%
1320
16th to endNight
11112225%78%
1320
July1st to 15thNight
11112248%74%
1377
16th to endNight
21321236%12%63%
1575
August1st to 15thNight
2132223410%65%
1501
16th to endNight
2321235%10%66%
1750
September1st to 15thNight
121111248%72%
1650
16th to endNight
2212213346%69%
1802
October1st to 15thNight
432321345%7%61%
1980
16th to endNight
5%5%1342235%7%58%
2112
November1st to 15thNight
7%423323347%56%
2172
16th to endNight
7%5%23434437%52%
2427
December1st to 15thNight
11%6%35%434447%45%
2520
16th to endNight
14%6%34424346%43%
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
37%5%13%18%15%9%
1566
16th to endDay
40%7%8%12%18%12%
1728
February1st to 15thDay
42%5%9%17%20%4
1576
16th to endDay
45%5%9%22%12%3
1576
March1st to 15thDay
43%7%11%18%13%5%
1959
16th to endDay
40%8%13%19%15%2
2130
April1st to 15thDay
39%6%15%18%17%2
2149
16th to endDay
26%6%13%26%21%6%
2160
May1st to 15thDay
19%6%12%26%26%7%
2251
16th to endDay
27%6%15%24%21%5%
2484
June1st to 15thDay
22%8%13%20%20%14%
2310
16th to endDay
10%37%19%36%22%
2310
July1st to 15thDay
11%47%15%23%37%
2106
16th to endDay
10%38%14%31%31%
2131
August1st to 15thDay
12%47%14%31%29%
2113
16th to endDay
10%46%19%36%21%
2100
September1st to 15thDay
11%36%18%28%31%
1980
16th to endDay
15%36%25%28%19%
1826
October1st to 15thDay
24%410%19%24%16%
1650
16th to endDay
26%5%10%22%21%12%
1760
November1st to 15thDay
33%5%11%25%16%7%
1684
16th to endDay
37%6%9%16%25%4
1530
December1st to 15thDay
44%5%12%15%18%4
1440
16th to endDay
44%6%13%15%16%4
1536
January1st to 15thNight
37%413%18%15%8%
2369
16th to endNight
40%6%9%12%16%15%
2496
February1st to 15thNight
39%6%10%18%18%7%
2288
16th to endNight
41%6%12%25%11%2
1898
March1st to 15thNight
41%6%13%19%14%5%
1999
16th to endNight
38%7%13%18%17%5%
2092
April1st to 15thNight
35%7%16%20%16%2
1799
16th to endNight
23%7%14%26%20%8%
1800
May1st to 15thNight
19%7%11%24%25%10%
1698
16th to endNight
23%7%16%26%20%6%
1475
June1st to 15thNight
19%8%14%21%20%15%
1320
16th to endNight
10%36%17%35%26%
1320
July1st to 15thNight
9%45%14%29%36%
1292
16th to endNight
13%6%10%17%25%26%
1467
August1st to 15thNight
16%5%7%17%28%25%
1492
16th to endNight
12%5%8%19%28%24%
1750
September1st to 15thNight
11%47%16%26%33%
1650
16th to endNight
15%46%21%28%22%
1802
October1st to 15thNight
23%5%10%20%22%17%
1980
16th to endNight
26%410%21%23%13%
2112
November1st to 15thNight
28%5%14%26%17%7%
2172
16th to endNight
31%5%11%22%20%8%
2427
December1st to 15thNight
39%7%9%15%19%7%
2520
16th to endNight
43%5%12%15%16%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
18%8%21%33%13%5%
195
16th to endDay
13%8%24%27%20%5%
256
February1st to 15thDay
15%12%23%25%17%4
236
16th to endDay
18%12%25%27%14%1
237
March1st to 15thDay
17%14%25%23%15%3
270
16th to endDay
10%15%38%25%7%2
288
April1st to 15thDay
15%19%33%21%8%2
269
16th to endDay
7%21%31%26%10%2
270
May1st to 15thDay
5%13%36%31%10%2
288
16th to endDay
11%12%24%31%14%4
301
June1st to 15thDay
39%32%38%14%2
255
16th to endDay
1421%45%20%7%
255
July1st to 15thDay
2114%52%24%4
245
16th to endDay
114%43%32%7%
271
August1st to 15thDay
21%45%24%7%
255
16th to endDay
116%47%25%8%
268
September1st to 15thDay
213%44%27%10%
255
16th to endDay
35%18%34%27%10%
255
October1st to 15thDay
5%8%20%37%21%6%
255
16th to endDay
9%7%25%31%20%5%
272
November1st to 15thDay
7%324%23%27%13%
190
16th to endDay
5%13%24%29%26%1
75
December1st to 15thDay
10%13%31%28%16%
60
16th to endDay
12%12%15%26%26%6%
64
January1st to 15thNight
15%310%33%29%7%
720
16th to endNight
16%412%31%25%9%
768
February1st to 15thNight
11%7%15%34%25%6%
704
16th to endNight
15%6%17%34%21%5%
632
March1st to 15thNight
14%6%15%29%29%4
720
16th to endNight
11%6%21%34%22%3
768
April1st to 15thNight
10%7%22%35%20%3
719
16th to endNight
6%5%22%34%24%6%
720
May1st to 15thNight
3218%39%30%5%
618
16th to endNight
3417%34%33%6%
395
June1st to 15thNight
2313%45%33%2
330
16th to endNight
9%43%43%2
330
July1st to 15thNight
432%52%9%
362
16th to endNight
1422%55%14%
522
August1st to 15thNight
21429%51%10%
509
16th to endNight
116%32%49%9%
700
September1st to 15thNight
36%25%51%12%
660
16th to endNight
319%27%43%13%
656
October1st to 15thNight
6%210%32%36%10%
660
16th to endNight
7%39%30%33%14%
704
November1st to 15thNight
8%13%34%33%9%
700
16th to endNight
9%312%35%29%9%
720
December1st to 15thNight
13%417%33%23%7%
720
16th to endNight
17%6%19%32%20%3
768
Page generated at 2017-05-25 10:27:30. Version ac04

Copyright 2017 A. Danko


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