Northfield Mountain Environmental Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northfield Mountain Environmental Center clear sky chart from 2005-11-05 to 2017-09-16.

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
32%
14864
17%
46811
9%
14832
41%
14832
20%
30964

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
23%
720
18%
2506
8%
716
37%
716
16%
1662
January16th to endNight
20%
768
17%
2688
7%
768
35%
768
15%
1781
February1st to 15thNight
19%
704
15%
2204
4%
701
32%
701
15%
1547
February16th to endNight
21%
631
20%
1893
8%
631
36%
631
17%
1288
March1st to 15thNight
20%
720
18%
2160
6%
720
34%
720
16%
1496
March16th to endNight
24%
768
22%
2010
7%
768
35%
768
23%
1366
April1st to 15thNight
23%
718
16%
1796
4%
718
31%
718
20%
1298
April16th to endNight
26%
624
18%
1704
5%
624
35%
624
21%
1160
May1st to 15thNight
25%
525
16%
1605
8%
525
33%
525
18%
1109
May16th to endNight
36%
375
12%
1497
12%
374
42%
374
16%
995
June1st to 15thNight
36%
360
13%
1440
10%
360
47%
360
16%
951
June16th to endNight
45%
360
11%
1440
14%
360
51%
360
21%
880
July1st to 15thNight
53%
340
13%
1360
17%
340
58%
340
22%
810
July16th to endNight
59%
376
16%
1418
18%
354
63%
354
25%
809
August1st to 15thNight
53%
461
17%
1532
13%
459
57%
459
27%
918
August16th to endNight
56%
764
20%
1910
11%
764
59%
764
34%
1248
September1st to 15thNight
45%
720
21%
1830
10%
720
50%
720
34%
1183
September16th to endNight
49%
658
26%
1818
18%
658
55%
658
35%
1107
October1st to 15thNight
39%
660
21%
1878
13%
660
49%
660
25%
1252
October16th to endNight
26%
704
14%
2263
7%
704
36%
704
20%
1521
November1st to 15thNight
33%
700
20%
2277
12%
700
45%
700
22%
1452
November16th to endNight
28%
720
19%
2374
10%
720
40%
720
17%
1583
December1st to 15thNight
23%
720
15%
2520
6%
720
35%
720
16%
1703
December16th to endNight
19%
768
17%
2688
5%
768
33%
768
13%
1845

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
17%6%25%5%3445%7%37%
47936
Night
16%6%35%424446%39%
46981
Cloud CoverDay
17%6%25%5%3445%6%37%
47946
Night
16%6%25%42445%6%39%
46986
TransparencyDay
45%6%11%21%13%1
47624
Night
44%5%11%22%13%1
46811
SeeingDay
21%12%27%22%11%4
5796
Night
20%11%29%28%9%
14864

By Month

Percentage Cloud Cover NE (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
16%8%245%35%5%6%7%35%
1432
16th to endDay
13%7%25%5%34447%39%
1536
February1st to 15thDay
16%8%47%6%36%5%6%6%30%
1655
16th to endDay
15%8%37%5%33445%37%
1580
March1st to 15thDay
20%9%36%433346%33%
1790
16th to endDay
18%6%35%423245%41%
2211
April1st to 15thDay
19%7%35%6%23435%36%
2152
16th to endDay
16%5%25%42445%6%40%
2256
May1st to 15thDay
25%6%35%5%34346%31%
2356
16th to endDay
19%5%2442445%8%37%
2618
June1st to 15thDay
23%6%25%5%2445%8%32%
2520
16th to endDay
16%5%25%5%35%5%6%9%33%
2520
July1st to 15thDay
12%6%25%6%36%5%6%7%36%
2520
16th to endDay
13%5%247%35%46%8%37%
2654
August1st to 15thDay
11%5%35%6%35%45%9%37%
2403
16th to endDay
10%425%4345%6%8%42%
2292
September1st to 15thDay
14%425%425%5%5%7%42%
2129
16th to endDay
15%5%23222336%50%
1815
October1st to 15thDay
20%5%35%6%23335%38%
1749
16th to endDay
18%7%26%5%24446%37%
1607
November1st to 15thDay
16%8%34323446%42%
1581
16th to endDay
19%6%24424345%42%
1584
December1st to 15thDay
19%6%36%6%25%345%34%
1440
16th to endDay
21%11%2433445%5%31%
1536
January1st to 15thNight
15%8%35%325%45%5%37%
2506
16th to endNight
14%9%36%5%35%446%36%
2688
February1st to 15thNight
12%9%5%6%5%36%446%35%
2203
16th to endNight
15%8%36%5%24345%38%
1893
March1st to 15thNight
18%8%26%424446%37%
2147
16th to endNight
15%7%3442445%6%42%
2010
April1st to 15thNight
19%6%35%5%33436%37%
1796
16th to endNight
21%5%25%423345%41%
1704
May1st to 15thNight
19%6%36%5%34446%34%
1605
16th to endNight
18%5%25%5%35%5%5%8%35%
1495
June1st to 15thNight
21%6%35%5%23346%36%
1440
16th to endNight
15%5%24425%5%5%7%41%
1440
July1st to 15thNight
12%5%345%35%5%5%8%42%
1440
16th to endNight
10%4244345%5%9%44%
1514
August1st to 15thNight
10%315%5%3445%7%46%
1542
16th to endNight
12%42442446%8%44%
1910
September1st to 15thNight
14%42432335%6%47%
1830
16th to endNight
13%5%13323247%51%
1818
October1st to 15thNight
19%5%24423335%43%
1878
16th to endNight
20%6%25%433446%37%
2263
November1st to 15thNight
16%6%25%423346%43%
2277
16th to endNight
18%6%25%5%25%445%38%
2374
December1st to 15thNight
20%7%25%5%24446%36%
2520
16th to endNight
22%8%3442445%6%32%
2688

Percentage Cloud Cover SE (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
16%7%245%35%5%7%7%34%
1432
16th to endDay
14%7%346%335%5%7%39%
1536
February1st to 15thDay
15%7%48%6%26%5%5%6%29%
1655
16th to endDay
15%8%47%5%33345%37%
1580
March1st to 15thDay
20%9%35%5%33446%33%
1800
16th to endDay
17%7%36%424335%41%
2211
April1st to 15thDay
19%7%37%5%24445%36%
2152
16th to endDay
17%5%25%4345%5%6%40%
2256
May1st to 15thDay
25%7%25%5%34346%31%
2356
16th to endDay
18%6%24435%45%8%36%
2618
June1st to 15thDay
22%6%245%245%5%7%31%
2520
16th to endDay
15%5%25%5%35%5%7%8%33%
2520
July1st to 15thDay
12%6%26%6%36%6%6%7%35%
2520
16th to endDay
12%5%25%5%35%5%7%7%37%
2654
August1st to 15thDay
12%426%6%36%5%5%9%36%
2403
16th to endDay
10%425%5%345%5%7%43%
2292
September1st to 15thDay
14%5%15%425%5%5%6%42%
2129
16th to endDay
15%5%23323435%50%
1815
October1st to 15thDay
20%5%25%5%34345%38%
1749
16th to endDay
17%7%26%5%2445%5%37%
1607
November1st to 15thDay
16%9%24323346%42%
1581
16th to endDay
19%6%34423346%41%
1584
December1st to 15thDay
18%7%36%6%34446%34%
1440
16th to endDay
22%10%343245%45%32%
1536
January1st to 15thNight
16%8%35%42446%5%38%
2506
16th to endNight
14%10%36%5%245%46%36%
2688
February1st to 15thNight
12%10%46%5%36%446%35%
2196
16th to endNight
15%9%36%5%2345%5%38%
1893
March1st to 15thNight
18%7%35%5%24346%36%
2160
16th to endNight
15%7%35%424446%41%
2009
April1st to 15thNight
19%7%35%5%34445%37%
1796
16th to endNight
21%6%2443334441%
1704
May1st to 15thNight
20%6%27%5%34446%34%
1605
16th to endNight
19%425%5%246%5%8%35%
1495
June1st to 15thNight
22%6%25%5%23446%35%
1440
16th to endNight
14%5%245%245%5%8%41%
1440
July1st to 15thNight
12%5%25%335%45%10%41%
1440
16th to endNight
10%5%25%4345%6%9%44%
1514
August1st to 15thNight
10%415%5%3446%7%46%
1542
16th to endNight
12%415%5%2446%7%44%
1910
September1st to 15thNight
14%424324446%47%
1830
16th to endNight
13%424323246%51%
1818
October1st to 15thNight
19%6%24422346%42%
1878
16th to endNight
20%6%25%42345%6%37%
2263
November1st to 15thNight
16%6%35%323346%43%
2277
16th to endNight
17%7%25%5%35%345%39%
2374
December1st to 15thNight
20%7%35%5%24346%36%
2520
16th to endNight
21%8%3443445%6%33%
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
45%6%12%23%10%1
1433
16th to endDay
43%414%23%11%1
1536
February1st to 15thDay
52%7%10%17%11%
1658
16th to endDay
48%49%20%14%1
1580
March1st to 15thDay
51%412%18%10%2
1800
16th to endDay
45%48%19%16%5%
2211
April1st to 15thDay
49%5%9%19%13%2
2152
16th to endDay
42%5%8%21%18%3
2256
May1st to 15thDay
54%5%7%19%12%1
2356
16th to endDay
44%7%13%19%13%1
2622
June1st to 15thDay
50%7%12%18%10%
2520
16th to endDay
44%8%12%20%12%
2520
July1st to 15thDay
43%8%11%22%13%1
2371
16th to endDay
41%7%15%21%13%1
2487
August1st to 15thDay
42%6%13%24%11%1
2389
16th to endDay
36%7%11%27%15%1
2292
September1st to 15thDay
39%6%11%22%18%1
2129
16th to endDay
35%47%25%23%2
1815
October1st to 15thDay
47%5%8%23%12%2
1749
16th to endDay
46%6%13%18%12%2
1607
November1st to 15thDay
43%414%22%13%1
1581
16th to endDay
44%412%21%14%1
1584
December1st to 15thDay
50%5%14%16%12%
1440
16th to endDay
52%6%13%18%9%
1536
January1st to 15thNight
46%6%12%20%12%2
2506
16th to endNight
46%5%11%20%13%2
2688
February1st to 15thNight
48%7%13%19%10%1
2204
16th to endNight
46%5%10%22%12%2
1893
March1st to 15thNight
48%5%9%22%12%1
2160
16th to endNight
42%5%10%18%17%5%
2010
April1st to 15thNight
48%5%10%18%16%1
1796
16th to endNight
45%5%8%19%17%4
1704
May1st to 15thNight
49%410%18%14%2
1605
16th to endNight
46%7%12%20%11%1
1497
June1st to 15thNight
48%5%12%22%9%
1440
16th to endNight
38%9%13%23%14%
1440
July1st to 15thNight
36%9%13%27%12%
1360
16th to endNight
35%6%15%28%11%1
1418
August1st to 15thNight
35%8%14%24%16%1
1532
16th to endNight
35%6%11%28%16%
1910
September1st to 15thNight
36%5%12%25%19%1
1830
16th to endNight
34%48%27%23%1
1818
October1st to 15thNight
42%5%11%21%15%3
1878
16th to endNight
46%6%14%21%11%
2263
November1st to 15thNight
42%5%12%21%16%2
2277
16th to endNight
47%412%21%12%1
2374
December1st to 15thNight
49%5%14%20%10%
2520
16th to endNight
50%5%12%19%10%
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
21%21%35%21%
60
16th to endDay
32%18%29%17%1
64
February1st to 15thDay
26%20%31%18%2
179
16th to endDay
22%20%30%22%2
239
March1st to 15thDay
31%14%30%19%3
270
16th to endDay
24%18%29%16%6%4
288
April1st to 15thDay
28%16%27%16%9%1
270
16th to endDay
20%17%30%21%7%4
270
May1st to 15thDay
34%14%23%16%7%3
288
16th to endDay
21%10%33%20%11%2
331
June1st to 15thDay
23%13%26%22%11%2
315
16th to endDay
16%14%31%20%12%4
315
July1st to 15thDay
17%6%30%26%16%2
305
16th to endDay
14%6%27%29%18%3
326
August1st to 15thDay
11%7%28%29%16%6%
313
16th to endDay
12%6%22%28%21%9%
332
September1st to 15thDay
17%8%23%24%17%8%
315
16th to endDay
17%5%21%22%14%17%
254
October1st to 15thDay
21%9%19%25%16%7%
255
16th to endDay
22%14%25%29%8%
244
November1st to 15thDay
22%8%28%26%10%2
235
16th to endDay
28%8%25%24%10%1
204
December1st to 15thDay
25%21%31%15%6%
60
16th to endDay
39%23%26%9%1
64
January1st to 15thNight
21%15%33%25%3
720
16th to endNight
21%15%34%25%2
768
February1st to 15thNight
19%16%38%24%1
704
16th to endNight
22%18%32%22%3
631
March1st to 15thNight
26%17%31%22%2
720
16th to endNight
21%15%32%23%7%
768
April1st to 15thNight
23%15%33%23%3
718
16th to endNight
26%14%28%21%8%
624
May1st to 15thNight
23%13%32%24%5%
525
16th to endNight
20%6%29%31%11%
375
June1st to 15thNight
22%6%25%35%10%
360
16th to endNight
14%5%30%38%10%
360
July1st to 15thNight
13%324%43%14%
340
16th to endNight
10%321%45%18%
376
August1st to 15thNight
11%223%45%16%1
461
16th to endNight
14%317%36%24%2
764
September1st to 15thNight
17%5%25%32%18%1
720
16th to endNight
15%5%23%31%21%3
658
October1st to 15thNight
21%10%21%32%12%1
660
16th to endNight
25%9%32%24%7%
704
November1st to 15thNight
20%12%28%27%9%1
700
16th to endNight
22%12%29%26%8%
720
December1st to 15thNight
23%12%35%22%5%
720
16th to endNight
27%16%33%21%1
768
Page generated at 2017-09-16 17:07:00. Version ac04

Copyright 2017 A. Danko


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