Mount Washington Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Washington Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
28%
14604
12%
46461
7%
14572
34%
14572
14%
33554

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
19%
720
9%
2506
5%
716
25%
716
9%
1849
January16th to endNight
14%
768
9%
2688
3%
768
22%
768
8%
1975
February1st to 15thNight
13%
704
8%
2266
3%
701
20%
701
7%
1745
February16th to endNight
17%
631
11%
1893
4%
631
26%
631
11%
1404
March1st to 15thNight
18%
720
13%
2157
6%
720
27%
720
11%
1610
March16th to endNight
26%
768
16%
2028
7%
768
33%
768
15%
1494
April1st to 15thNight
22%
718
14%
1796
4%
718
28%
718
15%
1370
April16th to endNight
25%
568
14%
1648
5%
568
31%
568
16%
1175
May1st to 15thNight
25%
526
13%
1606
8%
526
32%
526
16%
1159
May16th to endNight
34%
375
10%
1497
9%
374
42%
374
12%
1055
June1st to 15thNight
28%
360
8%
1440
7%
360
36%
360
10%
1067
June16th to endNight
48%
360
7%
1440
10%
360
51%
360
16%
963
July1st to 15thNight
50%
340
11%
1360
17%
340
53%
340
20%
865
July16th to endNight
52%
376
11%
1418
14%
354
55%
354
19%
899
August1st to 15thNight
47%
453
11%
1502
8%
451
50%
451
20%
991
August16th to endNight
50%
628
15%
1678
9%
628
52%
628
29%
1095
September1st to 15thNight
45%
660
14%
1701
7%
660
48%
660
28%
1155
September16th to endNight
49%
657
21%
1815
16%
657
52%
657
30%
1150
October1st to 15thNight
33%
660
15%
1870
9%
660
39%
660
16%
1377
October16th to endNight
21%
704
9%
2288
4%
704
27%
704
10%
1733
November1st to 15thNight
31%
700
15%
2277
11%
700
37%
700
14%
1632
November16th to endNight
25%
720
13%
2379
9%
720
31%
720
12%
1707
December1st to 15thNight
16%
720
6%
2520
3%
720
20%
720
7%
1981
December16th to endNight
12%
768
7%
2688
3%
768
17%
768
7%
2103

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%11%48%6%35%45%6%25%
47516
Night
15%10%47%7%46%5%5%6%26%
46631
Cloud CoverDay
17%8%47%7%35%45%6%29%
47526
Night
15%8%36%6%35%5%5%7%32%
46636
TransparencyDay
56%5%11%16%8%1
47204
Night
53%5%14%17%8%1
46461
SeeingDay
24%15%27%19%9%3
5640
Night
23%16%26%23%10%1
14604

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
12%10%6%12%8%47%5%6%7%18%
1432
16th to endDay
10%10%7%7%9%46%7%6%6%23%
1536
February1st to 15thDay
11%13%7%10%10%46%6%5%5%19%
1591
16th to endDay
10%13%6%10%7%46%6%6%6%19%
1580
March1st to 15thDay
13%13%5%9%8%35%5%5%6%23%
1792
16th to endDay
15%12%48%6%25%44429%
2195
April1st to 15thDay
16%13%5%8%5%35%43428%
2152
16th to endDay
16%11%38%7%3445%429%
2311
May1st to 15thDay
24%13%5%8%5%3334423%
2355
16th to endDay
18%11%47%6%35%447%25%
2618
June1st to 15thDay
29%9%46%5%35%35%5%21%
2520
16th to endDay
21%9%38%6%35%5%5%6%24%
2520
July1st to 15thDay
16%9%37%7%46%6%6%7%24%
2520
16th to endDay
17%7%37%7%5%6%6%7%8%23%
2654
August1st to 15thDay
17%9%47%9%45%46%6%23%
2349
16th to endDay
14%7%36%6%36%5%6%8%30%
2171
September1st to 15thDay
17%6%35%5%35%446%37%
1928
16th to endDay
14%8%2443335%7%41%
1815
October1st to 15thDay
19%13%46%6%33335%29%
1758
16th to endDay
16%18%47%6%35%345%24%
1584
November1st to 15thDay
16%11%47%5%26%5%6%5%26%
1581
16th to endDay
15%9%5%8%7%5%5%6%5%6%24%
1578
December1st to 15thDay
17%15%5%9%8%5%5%445%19%
1440
16th to endDay
15%16%6%10%6%38%45%5%15%
1536
January1st to 15thNight
10%11%6%10%9%47%6%7%8%17%
2506
16th to endNight
10%11%6%8%8%5%7%7%7%7%18%
2688
February1st to 15thNight
10%11%6%9%9%5%8%7%7%6%16%
2265
16th to endNight
11%12%6%8%8%36%7%7%5%21%
1893
March1st to 15thNight
14%12%5%7%7%47%6%5%5%24%
2144
16th to endNight
12%9%5%8%7%36%5%5%429%
2028
April1st to 15thNight
15%12%46%7%37%5%4426%
1796
16th to endNight
18%10%46%5%44345%32%
1648
May1st to 15thNight
20%11%47%5%34345%28%
1606
16th to endNight
18%9%36%6%36%5%5%7%27%
1495
June1st to 15thNight
26%9%46%6%44436%24%
1440
16th to endNight
16%8%26%5%35%5%6%8%31%
1440
July1st to 15thNight
16%6%35%5%35%5%6%9%33%
1440
16th to endNight
14%5%26%6%45%46%9%34%
1514
August1st to 15thNight
15%6%37%5%34448%35%
1512
16th to endNight
14%5%2343446%9%39%
1678
September1st to 15thNight
15%6%245%3445%5%41%
1701
16th to endNight
14%5%2442446%7%43%
1815
October1st to 15thNight
18%11%36%5%45%445%30%
1870
16th to endNight
19%13%5%8%7%46%345%22%
2288
November1st to 15thNight
15%10%5%6%6%37%45%6%26%
2277
16th to endNight
15%9%5%7%7%47%7%45%25%
2379
December1st to 15thNight
17%13%5%10%8%5%7%5%45%16%
2520
16th to endNight
16%13%6%11%8%48%6%5%5%14%
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
13%7%48%8%5%6%5%5%8%25%
1432
16th to endDay
10%9%36%6%5%7%5%5%7%31%
1536
February1st to 15thDay
13%9%5%9%7%5%7%5%47%24%
1591
16th to endDay
11%11%5%8%8%36%5%5%5%27%
1580
March1st to 15thDay
14%10%47%8%45%45%6%28%
1802
16th to endDay
13%10%47%7%35%44434%
2195
April1st to 15thDay
16%10%5%7%6%45%345%31%
2152
16th to endDay
16%8%36%7%35%46%5%31%
2311
May1st to 15thDay
24%10%38%5%34346%24%
2355
16th to endDay
19%8%46%6%45%45%6%27%
2618
June1st to 15thDay
27%7%36%6%3446%6%23%
2520
16th to endDay
20%7%37%7%35%5%5%6%26%
2520
July1st to 15thDay
15%7%37%7%36%5%6%8%26%
2520
16th to endDay
16%5%36%6%47%7%6%9%25%
2654
August1st to 15thDay
16%7%38%8%46%5%6%7%25%
2349
16th to endDay
13%5%35%6%35%6%7%9%32%
2171
September1st to 15thDay
15%5%25%7%3435%7%38%
1928
16th to endDay
13%6%26%424347%43%
1815
October1st to 15thDay
21%9%37%6%34436%31%
1758
16th to endDay
16%10%48%7%35%5%45%27%
1584
November1st to 15thDay
16%6%36%7%3446%8%31%
1581
16th to endDay
17%8%35%6%35%5%6%5%31%
1578
December1st to 15thDay
15%11%47%9%5%5%44426%
1440
16th to endDay
17%10%5%9%7%26%6%5%6%23%
1536
January1st to 15thNight
11%9%36%7%48%6%6%8%27%
2506
16th to endNight
11%8%5%7%7%46%6%6%7%27%
2688
February1st to 15thNight
10%9%47%8%48%6%5%6%27%
2258
16th to endNight
12%10%36%7%45%5%5%8%30%
1893
March1st to 15thNight
15%9%37%5%36%5%36%31%
2157
16th to endNight
11%7%36%6%36%46%7%34%
2027
April1st to 15thNight
14%10%38%6%36%45%6%31%
1796
16th to endNight
19%6%346%3445%6%35%
1648
May1st to 15thNight
21%7%36%6%34247%30%
1606
16th to endNight
20%8%34435%5%6%8%28%
1495
June1st to 15thNight
25%7%246%34447%28%
1440
16th to endNight
16%5%245%345%6%9%34%
1440
July1st to 15thNight
15%5%345%235%6%9%36%
1440
16th to endNight
14%425%435%5%7%7%39%
1514
August1st to 15thNight
16%6%25%43346%7%38%
1512
16th to endNight
13%5%1443447%8%42%
1678
September1st to 15thNight
14%5%145%24446%44%
1701
16th to endNight
14%5%1343345%7%46%
1815
October1st to 15thNight
18%7%26%7%34436%33%
1870
16th to endNight
21%7%37%7%4445%7%27%
2288
November1st to 15thNight
15%7%26%6%5%5%5%5%8%32%
2277
16th to endNight
15%7%36%6%34447%34%
2379
December1st to 15thNight
16%10%46%7%5%6%6%5%6%24%
2520
16th to endNight
15%12%47%6%36%6%5%7%24%
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
61%6%13%12%5%
1433
16th to endDay
53%418%15%6%
1536
February1st to 15thDay
62%6%13%12%5%
1594
16th to endDay
58%5%14%11%8%2
1580
March1st to 15thDay
58%5%12%14%7%2
1802
16th to endDay
54%311%16%12%2
2195
April1st to 15thDay
57%39%16%11%2
2152
16th to endDay
53%49%19%9%2
2311
May1st to 15thDay
62%27%17%8%1
2355
16th to endDay
56%6%10%17%8%
2622
June1st to 15thDay
62%410%15%5%
2520
16th to endDay
57%5%10%16%8%
2520
July1st to 15thDay
54%6%12%17%8%
2371
16th to endDay
54%6%12%19%6%
2487
August1st to 15thDay
57%6%13%17%5%
2335
16th to endDay
47%6%14%21%10%
2171
September1st to 15thDay
46%5%9%26%11%
1928
16th to endDay
41%411%26%15%
1815
October1st to 15thDay
57%210%18%8%1
1758
16th to endDay
60%413%13%7%
1584
November1st to 15thDay
52%5%15%14%10%2
1581
16th to endDay
55%7%13%13%8%1
1578
December1st to 15thDay
64%413%11%6%
1440
16th to endDay
65%6%12%8%6%
1536
January1st to 15thNight
58%5%17%12%5%
2506
16th to endNight
56%5%16%14%5%1
2688
February1st to 15thNight
60%6%14%11%6%
2266
16th to endNight
55%5%14%13%7%2
1893
March1st to 15thNight
56%5%11%17%7%1
2157
16th to endNight
52%5%11%15%13%2
2028
April1st to 15thNight
55%48%19%10%1
1796
16th to endNight
51%5%10%18%12%2
1648
May1st to 15thNight
54%49%17%12%1
1606
16th to endNight
52%7%13%18%6%1
1497
June1st to 15thNight
59%5%10%17%5%
1440
16th to endNight
46%8%14%20%9%
1440
July1st to 15thNight
43%7%17%21%10%
1360
16th to endNight
43%7%17%21%9%
1418
August1st to 15thNight
45%5%17%22%7%
1502
16th to endNight
38%7%18%23%12%
1678
September1st to 15thNight
41%5%13%30%9%
1701
16th to endNight
37%5%12%29%13%1
1815
October1st to 15thNight
53%5%10%17%11%1
1870
16th to endNight
60%412%15%6%
2288
November1st to 15thNight
53%6%14%15%8%1
2277
16th to endNight
53%5%16%15%7%1
2379
December1st to 15thNight
64%5%13%11%4
2520
16th to endNight
62%6%16%8%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
23%30%23%18%5%
60
16th to endDay
17%26%35%14%6%
64
February1st to 15thDay
25%20%31%18%2
177
16th to endDay
19%26%25%22%5%
239
March1st to 15thDay
25%20%30%17%5%
270
16th to endDay
26%20%27%15%6%4
288
April1st to 15thDay
30%18%24%17%7%1
270
16th to endDay
27%18%26%14%9%3
270
May1st to 15thDay
40%16%21%10%7%3
288
16th to endDay
23%16%35%19%5%
331
June1st to 15thDay
33%15%28%14%7%
315
16th to endDay
25%13%30%20%8%
315
July1st to 15thDay
21%9%36%20%10%2
305
16th to endDay
18%7%34%24%11%2
326
August1st to 15thDay
20%9%28%23%15%3
297
16th to endDay
17%8%20%25%17%10%
268
September1st to 15thDay
16%12%19%23%19%8%
255
16th to endDay
18%12%22%16%18%10%
254
October1st to 15thDay
25%16%16%20%15%5%
251
16th to endDay
24%17%27%20%8%
240
November1st to 15thDay
23%15%25%24%9%1
235
16th to endDay
27%17%23%19%9%3
198
December1st to 15thDay
31%20%31%10%6%
60
16th to endDay
39%14%39%6%1
64
January1st to 15thNight
22%20%32%19%5%
720
16th to endNight
21%22%31%19%4
768
February1st to 15thNight
17%25%35%17%3
704
16th to endNight
22%24%30%17%4
631
March1st to 15thNight
26%18%31%19%4
720
16th to endNight
22%19%27%22%6%1
768
April1st to 15thNight
26%20%26%22%4
718
16th to endNight
25%18%26%20%8%
568
May1st to 15thNight
26%14%26%22%9%1
526
16th to endNight
21%12%26%27%11%1
375
June1st to 15thNight
31%9%24%24%9%
360
16th to endNight
18%5%20%39%15%
360
July1st to 15thNight
17%420%34%22%1
340
16th to endNight
14%320%37%22%1
376
August1st to 15thNight
19%320%36%17%2
453
16th to endNight
17%6%20%28%23%4
628
September1st to 15thNight
18%6%24%28%18%3
660
16th to endNight
17%9%18%27%22%4
657
October1st to 15thNight
26%16%19%20%13%4
660
16th to endNight
29%18%25%18%7%
704
November1st to 15thNight
21%16%25%24%11%2
700
16th to endNight
24%19%23%21%9%2
720
December1st to 15thNight
28%22%26%17%4
720
16th to endNight
27%22%29%17%2
768
Page generated at 2017-08-13 14:16:27. Version ac04

Copyright 2017 A. Danko


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