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 2018-02-14.

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
27%
15341
12%
48787
7%
15309
33%
15309
14%
35154

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%
780
9%
2716
5%
776
24%
776
9%
1994
January16th to endNight
14%
832
8%
2912
2%
832
22%
832
8%
2153
February1st to 15thNight
13%
760
9%
2446
3%
757
21%
757
7%
1878
February16th to endNight
16%
631
11%
1893
4%
631
26%
631
11%
1404
March1st to 15thNight
17%
720
13%
2157
6%
720
26%
720
11%
1610
March16th to endNight
24%
768
16%
2028
7%
768
31%
768
15%
1494
April1st to 15thNight
21%
718
14%
1796
4%
718
27%
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
31%
526
16%
1159
May16th to endNight
33%
375
10%
1497
9%
374
40%
374
12%
1055
June1st to 15thNight
27%
360
8%
1440
7%
360
35%
360
10%
1067
June16th to endNight
47%
360
7%
1440
10%
360
50%
360
16%
963
July1st to 15thNight
50%
340
11%
1360
17%
340
53%
340
20%
865
July16th to endNight
49%
376
11%
1418
13%
354
52%
354
19%
899
August1st to 15thNight
46%
464
11%
1535
8%
462
50%
462
20%
1012
August16th to endNight
49%
686
17%
1832
10%
686
51%
686
30%
1184
September1st to 15thNight
44%
720
14%
1856
7%
720
47%
720
28%
1257
September16th to endNight
50%
717
23%
1980
18%
717
53%
717
31%
1240
October1st to 15thNight
34%
720
15%
2040
9%
720
40%
720
17%
1484
October16th to endNight
21%
768
9%
2496
5%
768
27%
768
11%
1881
November1st to 15thNight
29%
760
14%
2472
11%
760
35%
760
14%
1776
November16th to endNight
26%
780
13%
2577
9%
780
32%
780
13%
1835
December1st to 15thNight
16%
780
6%
2730
3%
780
20%
780
7%
2146
December16th to endNight
11%
832
7%
2912
3%
832
17%
832
7%
2253

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%5%5%6%25%
49293
Night
15%10%47%7%46%5%5%6%26%
48957
Cloud CoverDay
16%8%37%7%35%45%6%29%
49303
Night
15%8%36%6%35%5%5%7%32%
48962
TransparencyDay
56%5%11%16%8%1
48981
Night
53%5%13%17%8%1
48787
SeeingDay
24%15%27%19%9%3
6197
Night
22%16%25%23%10%1
15341

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%11%6%13%8%47%5%6%7%18%
1552
16th to endDay
10%9%6%8%9%46%7%6%6%23%
1664
February1st to 15thDay
12%13%6%10%9%46%6%5%5%19%
1712
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%
2401
16th to endDay
14%7%36%6%46%5%6%8%31%
2369
September1st to 15thDay
17%6%36%6%34336%37%
2103
16th to endDay
13%8%2443335%7%41%
1980
October1st to 15thDay
19%12%45%6%33335%30%
1918
16th to endDay
16%17%47%6%35%335%25%
1728
November1st to 15thDay
16%11%5%7%6%36%5%6%5%25%
1716
16th to endDay
15%9%5%8%7%5%5%6%5%5%24%
1709
December1st to 15thDay
16%15%5%9%7%45%5%45%19%
1560
16th to endDay
15%16%7%10%6%38%45%5%15%
1664
January1st to 15thNight
10%10%6%10%9%47%6%7%8%18%
2716
16th to endNight
10%11%6%9%9%5%7%7%7%6%17%
2912
February1st to 15thNight
10%12%6%10%9%5%8%7%7%6%16%
2445
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%7%37%5%34448%35%
1545
16th to endNight
14%5%235%3446%8%41%
1832
September1st to 15thNight
15%6%245%35%45%5%41%
1856
16th to endNight
13%5%2342446%7%44%
1980
October1st to 15thNight
17%10%36%5%45%435%31%
2040
16th to endNight
19%13%5%7%6%46%445%22%
2496
November1st to 15thNight
15%10%5%6%6%46%46%6%26%
2472
16th to endNight
14%9%5%8%7%47%7%45%25%
2577
December1st to 15thNight
17%13%5%9%8%5%7%5%5%5%16%
2730
16th to endNight
15%13%6%11%7%48%6%5%5%15%
2912

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
14%7%48%8%46%5%5%8%25%
1552
16th to endDay
11%8%36%6%47%5%5%7%31%
1664
February1st to 15thDay
13%9%5%9%7%5%7%5%46%24%
1712
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%
2401
16th to endDay
13%5%346%35%6%7%9%33%
2369
September1st to 15thDay
15%5%26%7%35%35%7%38%
2103
16th to endDay
12%6%25%42435%7%44%
1980
October1st to 15thDay
20%8%36%6%34336%33%
1918
16th to endDay
17%10%47%7%34445%29%
1728
November1st to 15thDay
16%7%36%7%3446%8%30%
1716
16th to endDay
17%7%35%6%35%6%6%5%31%
1709
December1st to 15thDay
15%11%48%9%5%5%44426%
1560
16th to endDay
16%10%49%7%35%6%46%23%
1664
January1st to 15thNight
11%8%36%8%48%6%6%7%28%
2716
16th to endNight
11%8%5%7%7%46%6%6%7%27%
2912
February1st to 15thNight
10%9%47%8%48%6%5%6%26%
2438
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%
1545
16th to endNight
13%5%1443436%8%44%
1832
September1st to 15thNight
14%4145%2445%7%44%
1856
16th to endNight
13%5%1343345%8%46%
1980
October1st to 15thNight
17%7%16%7%345%46%35%
2040
16th to endNight
21%7%37%7%44447%27%
2496
November1st to 15thNight
15%8%26%6%45%45%7%32%
2472
16th to endNight
15%7%26%6%345%5%7%35%
2577
December1st to 15thNight
17%10%47%7%45%6%5%6%24%
2730
16th to endNight
14%11%38%7%46%6%5%7%24%
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
60%5%13%13%5%
1553
16th to endDay
53%417%16%6%
1664
February1st to 15thDay
62%5%13%12%5%
1715
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%
2387
16th to endDay
46%5%14%21%10%
2369
September1st to 15thDay
47%5%9%25%11%
2103
16th to endDay
40%5%11%26%16%1
1980
October1st to 15thDay
55%311%19%10%1
1918
16th to endDay
59%412%15%7%
1728
November1st to 15thDay
53%5%14%14%9%2
1716
16th to endDay
55%7%13%14%7%1
1709
December1st to 15thDay
64%413%11%6%
1560
16th to endDay
65%5%12%9%6%1
1664
January1st to 15thNight
57%5%17%12%6%
2716
16th to endNight
57%5%16%13%5%1
2912
February1st to 15thNight
60%6%14%12%6%
2446
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%23%7%
1535
16th to endNight
37%6%17%25%12%
1832
September1st to 15thNight
41%5%13%28%10%
1856
16th to endNight
36%5%12%29%14%1
1980
October1st to 15thNight
52%5%11%18%10%1
2040
16th to endNight
59%411%16%6%
2496
November1st to 15thNight
53%5%14%16%7%1
2472
16th to endNight
53%5%16%15%7%1
2577
December1st to 15thNight
64%5%13%11%4
2730
16th to endNight
61%6%15%9%6%
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
23%24%30%17%4
90
16th to endDay
20%25%34%15%4
96
February1st to 15thDay
27%21%31%16%2
218
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%29%23%14%3
313
16th to endDay
17%11%20%24%16%9%
332
September1st to 15thDay
17%13%21%22%17%7%
315
16th to endDay
17%11%21%18%20%11%
314
October1st to 15thDay
25%13%19%21%15%5%
309
16th to endDay
26%18%26%19%8%
288
November1st to 15thDay
23%15%25%24%9%1
280
16th to endDay
28%16%24%19%8%2
239
December1st to 15thDay
26%23%31%13%5%
90
16th to endDay
40%16%35%6%1
96
January1st to 15thNight
21%21%31%19%5%
780
16th to endNight
20%23%31%20%4
832
February1st to 15thNight
18%25%35%18%3
760
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%37%17%2
464
16th to endNight
17%6%19%28%23%4
686
September1st to 15thNight
18%6%23%30%17%2
720
16th to endNight
16%9%17%26%23%5%
717
October1st to 15thNight
25%15%19%22%14%3
720
16th to endNight
29%18%24%18%8%
768
November1st to 15thNight
21%16%24%24%11%1
760
16th to endNight
23%19%23%22%9%2
780
December1st to 15thNight
28%22%26%18%4
780
16th to endNight
27%23%28%17%2
832
Page generated at 2018-02-15 10:58:43. Version ac04

Copyright 2018 A. Danko


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