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 2017-04-30.

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%
14815
27%
45647
24%
14778
69%
14778
38%
25141

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%
716
23%
1790
12%
716
64%
716
41%
1085
May1st to 15thNight
64%
510
28%
1500
21%
510
68%
510
46%
759
May16th to endNight
73%
342
21%
1368
26%
342
74%
342
35%
663
June1st to 15thNight
76%
330
28%
1320
35%
330
80%
330
41%
584
June16th to endNight
87%
330
33%
1320
44%
330
89%
330
62%
462
July1st to 15thNight
95%
310
36%
1240
51%
310
95%
310
72%
394
July16th to endNight
93%
435
37%
1383
40%
405
93%
405
72%
500
August1st to 15thNight
90%
531
40%
1514
32%
528
90%
528
73%
646
August16th to endNight
92%
700
41%
1750
27%
700
92%
700
81%
785
September1st to 15thNight
91%
660
44%
1650
32%
660
92%
660
82%
737
September16th to endNight
83%
656
42%
1775
37%
656
86%
656
67%
830
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%312212235%61%
46077
Night
13%312212335%58%
45838
TransparencyDay
27%38%18%24%18%
45757
Night
29%38%18%22%17%
45657
SeeingDay
15%12%31%28%9%3
5280
Night
14%311%33%30%6%
14825

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%
2150
May1st to 15thDay
7%312112347%64%
2117
16th to endDay
9%424213346%55%
2391
June1st to 15thDay
6%322213236%63%
2310
16th to endDay
3111112381%
2310
July1st to 15thDay
1111112285%
2310
16th to endDay
2111112284%
2371
August1st to 15thDay
111121286%
2104
16th to endDay
1112287%
2100
September1st to 15thDay
111122383%
1980
16th to endDay
41111122478%
1852
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%
1790
May1st to 15thNight
8%111312347%63%
1500
16th to endNight
9%312324346%60%
1368
June1st to 15thNight
6%31221323468%
1320
16th to endNight
11111222281%
1320
July1st to 15thNight
1222384%
1320
16th to endNight
1111112384%
1487
August1st to 15thNight
1121122380%
1523
16th to endNight
111122285%
1750
September1st to 15thNight
11113286%
1650
16th to endNight
31221222377%
1775
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
30%5%12%23%22%5%
2150
May1st to 15thDay
21%39%30%26%7%
2117
16th to endDay
28%6%12%26%18%6%
2395
June1st to 15thDay
22%49%19%22%20%
2310
16th to endDay
9%2315%41%27%
2310
July1st to 15thDay
7%138%29%48%
2161
16th to endDay
9%1310%25%49%
2212
August1st to 15thDay
7%1210%30%48%
2088
16th to endDay
6%1211%38%39%
2100
September1st to 15thDay
7%1410%34%41%
1980
16th to endDay
12%15%23%33%23%
1852
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%
1790
May1st to 15thNight
22%410%24%27%10%
1501
16th to endNight
25%5%15%29%16%7%
1368
June1st to 15thNight
19%5%10%21%22%20%
1320
16th to endNight
10%2415%35%31%
1320
July1st to 15thNight
6%248%27%51%
1240
16th to endNight
7%249%24%51%
1383
August1st to 15thNight
9%1314%25%45%
1514
16th to endNight
6%1212%32%44%
1750
September1st to 15thNight
6%149%39%38%
1650
16th to endNight
12%25%19%35%24%
1775
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%29%20%5%
267
May1st to 15thDay
14%21%39%17%5%2
228
16th to endDay
15%18%25%26%8%6%
266
June1st to 15thDay
9%17%35%29%44
255
16th to endDay
5%10%43%29%6%5%
255
July1st to 15thDay
339%42%6%7%
245
16th to endDay
1142%38%10%5%
271
August1st to 15thDay
25%39%38%8%5%
255
16th to endDay
435%43%10%5%
268
September1st to 15thDay
25%39%36%11%3
255
16th to endDay
39%34%36%14%1
255
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%
716
May1st to 15thNight
9%317%34%28%5%
511
16th to endNight
10%37%32%39%5%
343
June1st to 15thNight
7%28%38%36%6%
330
16th to endNight
26%40%45%5%
330
July1st to 15thNight
32%60%6%
310
16th to endNight
1126%58%12%
435
August1st to 15thNight
2334%50%9%
531
16th to endNight
334%53%7%
700
September1st to 15thNight
2329%52%11%
660
16th to endNight
418%33%35%16%
656
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 2017-04-30 17:40:11. Version ac04

Copyright 2017 A. Danko


Foresthill Clear Sky Chart | Charts Home