Foresthill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Foresthill clear sky chart from 2005-11-05 to 2017-09-18.

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 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
65%
15208
27%
46880
24%
15171
69%
15171
39%
25656

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
54%
717
19%
2491
18%
716
60%
716
28%
1462
January16th to endNight
49%
768
25%
2496
20%
768
58%
768
26%
1533
February1st to 15thNight
50%
699
23%
2255
19%
696
59%
696
25%
1421
February16th to endNight
51%
632
24%
1895
21%
632
57%
632
26%
1230
March1st to 15thNight
48%
720
15%
2076
13%
720
52%
720
23%
1426
March16th to endNight
52%
768
19%
2046
14%
768
56%
768
27%
1381
April1st to 15thNight
52%
719
18%
1799
10%
719
54%
719
28%
1250
April16th to endNight
62%
720
24%
1800
12%
720
65%
720
41%
1089
May1st to 15thNight
65%
557
27%
1637
21%
557
69%
557
46%
826
May16th to endNight
74%
374
24%
1496
30%
374
75%
374
36%
711
June1st to 15thNight
75%
360
27%
1440
34%
360
80%
360
41%
639
June16th to endNight
88%
360
35%
1440
48%
360
90%
360
63%
497
July1st to 15thNight
96%
340
34%
1360
48%
340
96%
340
73%
426
July16th to endNight
93%
471
39%
1503
42%
441
94%
441
73%
540
August1st to 15thNight
89%
580
37%
1653
30%
577
89%
577
71%
717
August16th to endNight
92%
764
43%
1910
28%
764
92%
764
81%
859
September1st to 15thNight
91%
720
42%
1800
30%
720
91%
720
80%
815
September16th to endNight
83%
667
42%
1804
38%
667
86%
667
67%
841
October1st to 15thNight
71%
660
30%
1980
31%
660
75%
660
47%
999
October16th to endNight
69%
704
34%
2112
33%
704
74%
704
47%
1087
November1st to 15thNight
66%
700
27%
2267
26%
700
71%
700
37%
1236
November16th to endNight
57%
720
25%
2412
25%
720
62%
720
31%
1358
December1st to 15thNight
50%
720
21%
2520
17%
720
59%
720
23%
1548
December16th to endNight
41%
768
21%
2688
17%
768
49%
768
19%
1765

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
11%31221223462%
47911
Night
13%312212335%58%
47071
TransparencyDay
26%37%18%24%19%
47591
Night
28%38%18%22%17%
46890
SeeingDay
14%11%30%29%10%3
5836
Night
14%311%32%31%6%
15218

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
19%5%12314235%49%
1444
16th to endDay
23%7%1221323445%
1728
February1st to 15thDay
21%7%2321324447%
1600
16th to endDay
25%9%22212335%39%
1579
March1st to 15thDay
24%7%23423347%37%
1881
16th to endDay
22%5%23314437%40%
2178
April1st to 15thDay
20%6%233235%46%41%
2149
16th to endDay
14%313312336%56%
2160
May1st to 15thDay
7%312112247%64%
2310
16th to endDay
9%423313346%57%
2615
June1st to 15thDay
7%322213236%63%
2520
16th to endDay
2111112383%
2520
July1st to 15thDay
1111112285%
2520
16th to endDay
1111112284%
2559
August1st to 15thDay
211122285%
2295
16th to endDay
1112287%
2292
September1st to 15thDay
111123381%
2160
16th to endDay
41111122477%
1878
October1st to 15thDay
8%22213435%64%
1650
16th to endDay
10%21331333361%
1760
November1st to 15thDay
11%312424347%52%
1592
16th to endDay
17%41332434450%
1545
December1st to 15thDay
21%6%23313346%42%
1440
16th to endDay
25%7%13422235%39%
1536
January1st to 15thNight
19%413223348%46%
2491
16th to endNight
20%5%13313335%47%
2496
February1st to 15thNight
20%6%13423347%42%
2253
16th to endNight
23%6%12212445%43%
1895
March1st to 15thNight
23%6%23423345%39%
2076
16th to endNight
21%413323446%45%
2046
April1st to 15thNight
18%6%25%424346%41%
1799
16th to endNight
12%413222336%58%
1800
May1st to 15thNight
8%111312247%64%
1637
16th to endNight
9%312313246%61%
1496
June1st to 15thNight
6%31221223468%
1440
16th to endNight
11111222282%
1440
July1st to 15thNight
222385%
1440
16th to endNight
11112384%
1607
August1st to 15thNight
111221122379%
1662
16th to endNight
111122284%
1910
September1st to 15thNight
111113384%
1800
16th to endNight
31221222377%
1804
October1st to 15thNight
7%21321333466%
1980
16th to endNight
9%22322346%62%
2112
November1st to 15thNight
12%312212446%57%
2267
16th to endNight
14%424323345%52%
2412
December1st to 15thNight
23%412223345%45%
2520
16th to endNight
28%6%13212345%40%
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
39%39%20%19%7%
1444
16th to endDay
43%37%14%17%12%
1728
February1st to 15thDay
41%310%17%20%6%
1600
16th to endDay
48%38%25%12%3
1579
March1st to 15thDay
47%5%11%14%16%5%
1881
16th to endDay
44%5%14%16%15%2
2178
April1st to 15thDay
42%6%12%20%15%2
2149
16th to endDay
29%5%12%23%22%5%
2160
May1st to 15thDay
20%310%31%26%7%
2310
16th to endDay
28%6%11%27%20%6%
2619
June1st to 15thDay
23%49%20%23%18%
2520
16th to endDay
9%1315%39%30%
2520
July1st to 15thDay
7%138%28%51%
2371
16th to endDay
9%139%23%52%
2400
August1st to 15thDay
7%1211%30%46%
2279
16th to endDay
6%1211%38%39%
2292
September1st to 15thDay
7%2412%33%39%
2160
16th to endDay
13%15%23%33%22%
1878
October1st to 15thDay
22%36%25%26%15%
1650
16th to endDay
27%47%21%23%15%
1760
November1st to 15thDay
30%5%12%20%21%9%
1592
16th to endDay
38%47%19%23%6%
1545
December1st to 15thDay
43%5%7%18%19%4
1440
16th to endDay
47%48%14%17%8%
1536
January1st to 15thNight
36%411%19%17%10%
2491
16th to endNight
40%47%13%18%15%
2496
February1st to 15thNight
42%36%20%17%9%
2264
16th to endNight
42%49%23%16%4
1895
March1st to 15thNight
46%49%16%15%6%
2076
16th to endNight
41%6%9%19%16%6%
2046
April1st to 15thNight
44%413%19%13%4
1799
16th to endNight
29%412%26%21%6%
1800
May1st to 15thNight
21%410%26%26%10%
1638
16th to endNight
24%414%29%19%6%
1496
June1st to 15thNight
20%5%9%23%23%18%
1440
16th to endNight
10%2415%33%34%
1440
July1st to 15thNight
5%237%25%54%
1360
16th to endNight
7%248%23%54%
1503
August1st to 15thNight
10%1314%25%44%
1653
16th to endNight
6%1313%31%44%
1910
September1st to 15thNight
7%1411%37%37%
1800
16th to endNight
12%25%19%35%24%
1804
October1st to 15thNight
21%37%22%27%16%
1980
16th to endNight
23%36%22%29%15%
2112
November1st to 15thNight
28%5%11%21%21%11%
2267
16th to endNight
34%38%21%23%9%
2412
December1st to 15thNight
41%49%18%19%7%
2520
16th to endNight
47%47%12%19%8%
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
50%419%18%6%1
72
16th to endDay
30%7%23%23%12%1
256
February1st to 15thDay
27%10%27%24%9%
241
16th to endDay
32%15%23%21%6%
238
March1st to 15thDay
29%19%20%18%9%2
270
16th to endDay
25%20%26%20%5%
288
April1st to 15thDay
26%23%26%17%41
269
16th to endDay
18%24%30%21%5%
270
May1st to 15thDay
12%20%37%18%7%3
288
16th to endDay
14%15%24%27%11%6%
330
June1st to 15thDay
10%16%33%28%5%5%
315
16th to endDay
48%39%31%10%6%
315
July1st to 15thDay
234%44%8%9%
305
16th to endDay
1240%37%12%5%
327
August1st to 15thDay
2437%38%9%6%
315
16th to endDay
429%42%16%6%
332
September1st to 15thDay
25%38%34%14%3
315
16th to endDay
38%35%36%14%1
264
October1st to 15thDay
8%14%35%28%9%3
255
16th to endDay
15%9%32%30%11%
272
November1st to 15thDay
10%6%13%30%32%7%
98
16th to endDay
19%12%23%28%14%1
77
December1st to 15thDay
33%15%21%25%5%
60
16th to endDay
25%115%35%17%4
64
January1st to 15thNight
21%215%35%19%6%
720
16th to endNight
23%417%29%20%3
768
February1st to 15thNight
22%414%36%19%2
704
16th to endNight
26%6%12%30%20%3
632
March1st to 15thNight
24%5%13%30%22%2
720
16th to endNight
23%5%13%30%23%2
768
April1st to 15thNight
19%5%14%36%21%2
719
16th to endNight
14%5%12%34%25%6%
720
May1st to 15thNight
9%317%35%28%5%
558
16th to endNight
10%37%33%40%5%
375
June1st to 15thNight
8%28%36%37%6%
360
16th to endNight
16%38%47%6%
360
July1st to 15thNight
30%62%5%
340
16th to endNight
1125%58%12%
471
August1st to 15thNight
2333%51%9%
580
16th to endNight
333%53%8%
764
September1st to 15thNight
2330%53%10%
720
16th to endNight
418%33%36%16%
667
October1st to 15thNight
9%49%34%33%9%
660
16th to endNight
12%210%30%34%8%
704
November1st to 15thNight
13%212%33%28%9%
700
16th to endNight
15%5%14%36%21%6%
720
December1st to 15thNight
24%5%14%32%18%3
720
16th to endNight
30%6%15%29%15%2
768
Page generated at 2017-09-19 04:32:29. Version ac04

Copyright 2017 A. Danko


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