Longfellow Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Longfellow Observatory clear sky chart from 2005-11-05 to 2018-01-20.

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
72%
15833
30%
48485
28%
15797
75%
15797
45%
25126

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
65%
776
20%
2690
22%
776
67%
776
32%
1532
January16th to endNight
63%
788
25%
2561
27%
788
65%
788
29%
1540
February1st to 15thNight
62%
699
25%
2211
27%
697
64%
697
31%
1321
February16th to endNight
59%
632
23%
1895
25%
632
61%
632
30%
1188
March1st to 15thNight
59%
720
18%
2097
18%
720
61%
720
31%
1318
March16th to endNight
61%
768
21%
2031
16%
768
64%
768
37%
1257
April1st to 15thNight
58%
719
22%
1799
13%
719
61%
719
36%
1152
April16th to endNight
64%
720
25%
1800
13%
720
67%
720
46%
1043
May1st to 15thNight
69%
626
29%
1706
20%
626
72%
626
58%
824
May16th to endNight
72%
375
31%
1496
35%
374
76%
374
45%
634
June1st to 15thNight
79%
360
30%
1440
36%
360
83%
360
53%
559
June16th to endNight
90%
360
38%
1440
50%
360
91%
360
72%
453
July1st to 15thNight
94%
340
36%
1360
47%
340
95%
340
73%
427
July16th to endNight
89%
482
39%
1514
40%
452
90%
452
65%
591
August1st to 15thNight
91%
628
38%
1701
28%
625
91%
625
77%
728
August16th to endNight
93%
764
44%
1910
29%
764
94%
764
82%
856
September1st to 15thNight
93%
720
42%
1800
31%
720
93%
720
80%
814
September16th to endNight
87%
716
46%
1931
42%
716
89%
716
73%
858
October1st to 15thNight
79%
720
31%
2148
34%
720
81%
720
55%
1017
October16th to endNight
81%
768
41%
2304
44%
768
83%
768
57%
1068
November1st to 15thNight
75%
760
30%
2464
32%
760
76%
760
45%
1233
November16th to endNight
69%
780
29%
2545
31%
780
72%
780
37%
1382
December1st to 15thNight
66%
780
24%
2730
26%
780
69%
780
31%
1554
December16th to endNight
54%
832
23%
2912
24%
832
59%
832
25%
1777

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
9%312212235%64%
49040
Night
10%312212335%62%
48676
TransparencyDay
23%38%18%24%21%
48721
Night
24%38%19%24%18%
48495
SeeingDay
11%9%29%32%13%4
6233
Night
11%18%35%34%7%
15843

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
17%5%13323247%49%
1576
16th to endDay
19%6%13423336%44%
1766
February1st to 15thDay
18%6%13213345%47%
1642
16th to endDay
22%6%24413345%40%
1579
March1st to 15thDay
16%7%2442446%7%38%
1860
16th to endDay
16%5%24424447%44%
2193
April1st to 15thDay
16%6%23313336%47%
2149
16th to endDay
11%413312335%58%
2160
May1st to 15thDay
5%312312245%67%
2241
16th to endDay
7%313323345%61%
2615
June1st to 15thDay
3213312235%69%
2520
16th to endDay
1111112385%
2520
July1st to 15thDay
111122384%
2520
16th to endDay
11121122282%
2548
August1st to 15thDay
11112287%
2250
16th to endDay
1111122384%
2292
September1st to 15thDay
1111112384%
2160
16th to endDay
21211122379%
2028
October1st to 15thDay
5%211212345%69%
1809
16th to endDay
7%212213235%65%
1920
November1st to 15thDay
10%313413345%57%
1725
16th to endDay
13%413323447%51%
1743
December1st to 15thDay
17%6%23323236%49%
1560
16th to endDay
20%6%24423346%41%
1664
January1st to 15thNight
16%32322245%8%48%
2690
16th to endNight
18%422323446%47%
2561
February1st to 15thNight
16%5%24323346%47%
2209
16th to endNight
21%6%13212345%46%
1895
March1st to 15thNight
19%413323235%47%
2097
16th to endNight
17%313223347%50%
2031
April1st to 15thNight
14%423324346%50%
1799
16th to endNight
8%313312346%61%
1800
May1st to 15thNight
5%211112237%70%
1706
16th to endNight
5%313311245%67%
1496
June1st to 15thNight
321112236%73%
1440
16th to endNight
1111111286%
1440
July1st to 15thNight
111112285%
1440
16th to endNight
3111212380%
1618
August1st to 15thNight
112112284%
1710
16th to endNight
11122384%
1910
September1st to 15thNight
1111111286%
1800
16th to endNight
2111112381%
1931
October1st to 15thNight
5%21221323470%
2148
16th to endNight
6%12111335%70%
2304
November1st to 15thNight
11%312212236%62%
2464
16th to endNight
12%313224445%55%
2545
December1st to 15thNight
16%413213446%50%
2730
16th to endNight
21%5%23323346%43%
2912

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%412%20%19%7%
1576
16th to endDay
41%5%9%15%16%10%
1766
February1st to 15thDay
37%311%24%17%5%
1642
16th to endDay
44%39%24%16%
1579
March1st to 15thDay
41%6%11%18%16%5%
1860
16th to endDay
38%6%15%20%16%2
2193
April1st to 15thDay
37%410%24%16%5%
2149
16th to endDay
27%5%11%27%22%5%
2160
May1st to 15thDay
19%38%25%33%9%
2241
16th to endDay
23%5%9%26%25%7%
2619
June1st to 15thDay
17%49%19%27%22%
2520
16th to endDay
6%1311%33%43%
2520
July1st to 15thDay
6%127%22%60%
2371
16th to endDay
10%139%21%53%
2390
August1st to 15thDay
6%29%27%52%
2234
16th to endDay
6%138%36%43%
2292
September1st to 15thDay
7%1311%33%42%
2160
16th to endDay
10%1422%37%23%
2028
October1st to 15thDay
17%37%19%25%26%
1809
16th to endDay
20%27%22%27%19%
1920
November1st to 15thDay
29%312%24%23%7%
1725
16th to endDay
32%411%20%26%5%
1743
December1st to 15thDay
38%39%20%18%9%
1560
16th to endDay
42%310%18%17%6%
1664
January1st to 15thNight
33%5%12%21%19%7%
2690
16th to endNight
37%410%14%19%13%
2561
February1st to 15thNight
36%48%25%16%7%
2220
16th to endNight
40%311%25%15%3
1895
March1st to 15thNight
38%413%18%20%4
2097
16th to endNight
32%6%14%22%18%5%
2031
April1st to 15thNight
34%6%13%25%16%4
1799
16th to endNight
24%5%13%25%23%7%
1800
May1st to 15thNight
15%39%26%32%11%
1707
16th to endNight
19%39%30%28%8%
1496
June1st to 15thNight
13%411%20%24%25%
1440
16th to endNight
7%1211%33%43%
1440
July1st to 15thNight
7%125%24%59%
1360
16th to endNight
11%136%20%56%
1514
August1st to 15thNight
8%211%25%50%
1701
16th to endNight
6%129%34%45%
1910
September1st to 15thNight
7%139%32%44%
1800
16th to endNight
9%1320%41%23%
1931
October1st to 15thNight
17%37%20%26%23%
2148
16th to endNight
16%25%23%28%21%
2304
November1st to 15thNight
24%311%25%22%12%
2464
16th to endNight
29%48%23%26%7%
2545
December1st to 15thNight
34%49%22%18%10%
2730
16th to endNight
41%311%17%18%7%
2912

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
37%221%23%7%7%
114
16th to endDay
25%5%19%29%15%4
269
February1st to 15thDay
25%6%20%26%17%2
248
16th to endDay
24%13%25%24%11%
238
March1st to 15thDay
22%13%28%20%13%2
270
16th to endDay
19%14%31%22%11%
288
April1st to 15thDay
27%15%31%17%6%
269
16th to endDay
17%18%31%23%7%
270
May1st to 15thDay
12%15%38%24%6%3
288
16th to endDay
12%16%23%33%9%4
330
June1st to 15thDay
416%39%28%7%2
315
16th to endDay
18%32%39%11%6%
315
July1st to 15thDay
126%51%11%8%
305
16th to endDay
1129%47%11%7%
327
August1st to 15thDay
3335%40%11%5%
315
16th to endDay
2227%44%14%9%
332
September1st to 15thDay
1334%41%15%3
315
16th to endDay
39%30%37%16%1
315
October1st to 15thDay
6%13%34%29%13%2
315
16th to endDay
10%5%24%36%19%3
336
November1st to 15thDay
12%215%29%33%7%
141
16th to endDay
12%8%21%34%20%2
132
December1st to 15thDay
20%325%26%16%7%
90
16th to endDay
17%316%28%27%7%
96
January1st to 15thNight
16%18%37%26%9%
780
16th to endNight
21%18%35%25%7%
788
February1st to 15thNight
18%110%35%27%6%
704
16th to endNight
23%29%36%21%6%
632
March1st to 15thNight
18%412%34%24%5%
720
16th to endNight
18%212%34%27%4
768
April1st to 15thNight
15%315%37%24%3
719
16th to endNight
8%416%37%26%6%
720
May1st to 15thNight
7%216%39%28%5%
627
16th to endNight
7%111%36%38%3
375
June1st to 15thNight
6%9%40%39%3
360
16th to endNight
15%40%48%3
360
July1st to 15thNight
1131%61%4
340
16th to endNight
3227%54%11%
482
August1st to 15thNight
1335%49%10%
628
16th to endNight
233%55%8%
764
September1st to 15thNight
1232%51%10%
720
16th to endNight
26%33%45%11%
716
October1st to 15thNight
6%9%39%34%9%
720
16th to endNight
7%7%31%36%16%
768
November1st to 15thNight
12%16%37%31%10%
760
16th to endNight
14%19%36%29%8%
780
December1st to 15thNight
17%18%41%23%7%
780
16th to endNight
24%211%35%21%4
832
Page generated at 2018-01-21 09:19:24. Version ac04

Copyright 2018 A. Danko


Longfellow Observatory Clear Sky Chart |Charts Home