Montpelier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montpelier Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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
38%
23689
21%
74654
12%
23641
48%
23641
24%
46859

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 10thNight
19%
757
25%
2488
7%
757
39%
757
22%
1514
January11th to 20thNight
24%
725
22%
2354
8%
725
41%
725
18%
1541
January21st to endNight
21%
792
25%
2574
10%
792
40%
792
23%
1564
February1st to 10thNight
24%
720
19%
2340
9%
720
37%
720
21%
1475
February11th to 20thNight
16%
700
20%
2229
6%
699
34%
699
18%
1434
February21st to endNight
20%
592
26%
1758
9%
592
39%
592
22%
1135
March1st to 10thNight
25%
720
26%
1980
10%
720
41%
720
28%
1224
March11th to 20thNight
24%
720
20%
1980
9%
720
35%
720
23%
1316
March21st to endNight
24%
792
21%
2174
7%
792
35%
792
24%
1447
April1st to 10thNight
23%
720
21%
1800
5%
720
35%
720
27%
1195
April11th to 20thNight
32%
711
23%
1775
7%
711
40%
711
33%
1127
April21st to endNight
33%
720
22%
1800
8%
720
40%
720
30%
1201
May1st to 10thNight
32%
602
17%
1682
8%
602
38%
602
26%
1111
May11th to 20thNight
39%
525
15%
1587
8%
525
44%
525
22%
1078
May21st to endNight
52%
388
17%
1550
16%
388
57%
388
22%
940
June1st to 10thNight
58%
360
18%
1440
19%
360
63%
360
23%
855
June11th to 20thNight
58%
360
15%
1440
17%
360
60%
360
21%
882
June21st to endNight
59%
360
19%
1440
18%
360
66%
360
25%
820
July1st to 10thNight
58%
377
16%
1427
18%
377
61%
377
22%
884
July11th to 20thNight
71%
522
15%
1566
13%
522
73%
522
31%
983
July21st to endNight
63%
591
16%
1672
12%
558
66%
558
27%
1104
August1st to 10thNight
65%
537
15%
1600
12%
534
66%
534
26%
1084
August11th to 20thNight
59%
607
16%
1671
11%
607
61%
607
26%
1156
August21st to endNight
60%
792
17%
1980
11%
792
62%
792
29%
1360
September1st to 10thNight
60%
720
21%
1800
13%
720
63%
720
31%
1212
September11th to 20thNight
60%
720
28%
1802
21%
720
62%
720
35%
1164
September21st to endNight
44%
720
18%
2078
15%
720
48%
720
24%
1424
October1st to 10thNight
51%
720
22%
2160
20%
720
57%
720
30%
1343
October11th to 20thNight
40%
720
27%
2160
19%
720
51%
720
29%
1320
October21st to endNight
39%
792
29%
2376
19%
792
53%
792
31%
1418
November1st to 10thNight
40%
740
27%
2230
17%
740
52%
740
30%
1333
November11th to 20thNight
34%
760
25%
2472
14%
760
49%
760
26%
1463
November21st to endNight
36%
760
25%
2660
15%
760
51%
760
27%
1479
December1st to 10thNight
25%
760
21%
2660
9%
760
41%
760
19%
1675
December11th to 20thNight
31%
759
22%
3183
12%
759
46%
759
20%
1869
December21st to endNight
30%
828
15%
4766
12%
817
45%
817
13%
2729

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
14%5%25%5%35%5%5%8%43%
74630
Night
14%6%25%5%3445%7%44%
75077
TransparencyDay
39%7%12%23%17%4
74347
Night
39%7%11%22%18%4
74879
SeeingDay
19%11%27%25%14%5%
13945
Night
17%10%29%30%13%1
23705

By Month

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 10thDay
16%7%25%434346%46%
1672
11th to 20thDay
17%7%36%5%25%45%442%
1629
21st to endDay
16%7%25%5%2445%7%44%
1782
February1st to 10thDay
13%7%36%6%35%5%5%7%39%
1620
11th to 20thDay
21%6%345%24346%43%
1617
21st to endDay
19%8%36%6%335%5%7%36%
1494
March1st to 10thDay
14%5%16%423446%51%
1980
11th to 20thDay
17%6%35%5%3435%8%41%
1980
21st to endDay
17%7%45%5%3445%6%39%
2182
April1st to 10thDay
11%5%3442445%7%51%
2160
11th to 20thDay
12%5%25%43445%7%49%
2134
21st to endDay
16%6%25%435%45%7%42%
2160
May1st to 10thDay
15%6%25%5%45%6%5%8%40%
2293
11th to 20thDay
18%6%25%5%35%5%6%7%37%
2305
21st to endDay
13%5%25%5%35%5%7%9%41%
2725
June1st to 10thDay
11%435%5%35%5%7%8%44%
2520
11th to 20thDay
11%435%5%45%5%6%11%41%
2520
21st to endDay
8%325%5%46%7%7%10%42%
2520
July1st to 10thDay
9%5%36%5%47%6%7%9%40%
2486
11th to 20thDay
6%325%5%47%7%7%13%42%
2340
21st to endDay
8%5%26%7%47%6%7%11%37%
2547
August1st to 10thDay
10%6%26%7%46%6%7%10%35%
2327
11th to 20thDay
11%5%37%7%46%6%7%8%36%
2210
21st to endDay
10%5%25%7%45%6%6%10%41%
2376
September1st to 10thDay
14%5%25%435%5%5%8%45%
2160
11th to 20thDay
12%6%26%5%2446%7%46%
2155
21st to endDay
21%6%25%5%3445%6%40%
1882
October1st to 10thDay
17%425%5%345%46%47%
1800
11th to 20thDay
17%423433446%51%
1800
21st to endDay
15%5%25%423346%51%
1980
November1st to 10thDay
13%5%235%2434454%
1846
11th to 20thDay
14%6%24424346%51%
1706
21st to endDay
15%5%24433445%52%
1520
December1st to 10thDay
17%6%24435%46%8%40%
1520
11th to 20thDay
17%6%2432333452%
1910
21st to endDay
17%7%36%5%3445%6%40%
2772
January1st to 10thNight
16%7%24434346%47%
2488
11th to 20thNight
16%7%36%5%345%5%6%40%
2354
21st to endNight
16%7%25%43445%447%
2574
February1st to 10thNight
15%8%245%3445%7%42%
2340
11th to 20thNight
19%7%25%5%33345%43%
2228
21st to endNight
17%7%26%424345%46%
1758
March1st to 10thNight
13%434434446%50%
1980
11th to 20thNight
19%6%25%42347%6%41%
1980
21st to endNight
15%8%35%5%34347%43%
2174
April1st to 10thNight
14%7%25%434347%47%
1800
11th to 20thNight
11%5%345%3445%8%49%
1775
21st to endNight
15%6%25%42445%7%45%
1800
May1st to 10thNight
14%6%35%5%25%5%6%8%40%
1682
11th to 20thNight
15%7%25%5%45%5%5%9%37%
1586
21st to endNight
12%5%25%7%4446%9%41%
1550
June1st to 10thNight
10%5%35%5%35%5%5%11%42%
1440
11th to 20thNight
11%5%25%5%45%6%6%10%41%
1440
21st to endNight
8%5%345%45%6%8%11%43%
1440
July1st to 10thNight
9%6%37%5%35%5%6%10%41%
1468
11th to 20thNight
5%325%5%47%6%7%12%43%
1620
21st to endNight
7%426%7%5%7%7%8%9%37%
1771
August1st to 10thNight
8%5%27%6%46%6%8%11%37%
1611
11th to 20thNight
13%6%25%6%45%6%7%10%37%
1671
21st to endNight
9%6%36%6%45%6%8%10%37%
1980
September1st to 10thNight
13%5%25%5%345%6%8%44%
1800
11th to 20thNight
10%6%246%3445%7%48%
1802
21st to endNight
21%7%36%5%2345%6%39%
2078
October1st to 10thNight
16%6%24424446%48%
2160
11th to 20thNight
16%5%25%43345%6%47%
2160
21st to endNight
14%5%25%424446%51%
2376
November1st to 10thNight
13%6%24423345%53%
2230
11th to 20thNight
14%5%34424346%51%
2472
21st to endNight
13%4243245%6%6%50%
2660
December1st to 10thNight
17%7%35%43445%6%42%
2660
11th to 20thNight
18%6%2442445%6%46%
3285
21st to endNight
15%7%35%43446%7%41%
4884

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 10thDay
41%49%24%17%5%
1673
11th to 20thDay
44%5%9%21%18%3
1629
21st to endDay
40%49%23%18%5%
1782
February1st to 10thDay
43%6%10%26%12%3
1620
11th to 20thDay
44%47%21%20%4
1626
21st to endDay
48%5%10%16%17%5%
1494
March1st to 10thDay
35%6%9%23%22%5%
1980
11th to 20thDay
41%5%10%19%16%7%
1980
21st to endDay
45%6%9%18%17%4
2182
April1st to 10thDay
32%6%12%23%19%8%
2160
11th to 20thDay
35%6%12%18%23%6%
2134
21st to endDay
42%6%11%23%14%6%
2160
May1st to 10thDay
40%8%15%18%15%3
2293
11th to 20thDay
44%9%13%22%11%1
2309
21st to endDay
39%8%15%25%13%
2725
June1st to 10thDay
35%9%15%26%14%1
2520
11th to 20thDay
37%10%15%20%17%1
2520
21st to endDay
35%9%14%25%16%1
2520
July1st to 10thDay
38%10%14%20%15%3
2417
11th to 20thDay
30%11%20%26%12%2
2271
21st to endDay
37%10%14%23%16%1
2405
August1st to 10thDay
41%10%16%22%10%1
2314
11th to 20thDay
43%10%15%20%11%1
2210
21st to endDay
37%9%16%22%13%3
2376
September1st to 10thDay
37%8%14%17%20%4
2160
11th to 20thDay
36%7%9%27%18%4
2155
21st to endDay
45%6%9%20%17%3
1882
October1st to 10thDay
38%6%9%24%20%3
1800
11th to 20thDay
36%5%7%24%23%4
1800
21st to endDay
35%5%6%25%23%6%
1980
November1st to 10thDay
33%5%8%24%25%5%
1846
11th to 20thDay
35%410%22%23%7%
1706
21st to endDay
35%48%25%24%5%
1520
December1st to 10thDay
40%6%11%24%14%5%
1520
11th to 20thDay
37%47%30%16%6%
1906
21st to endDay
43%6%10%27%13%1
2772
January1st to 10thNight
40%48%21%20%7%
2488
11th to 20thNight
45%6%9%20%16%4
2354
21st to endNight
39%48%24%21%4
2574
February1st to 10thNight
41%6%10%24%16%3
2340
11th to 20thNight
44%47%20%18%7%
2230
21st to endNight
42%49%21%17%8%
1758
March1st to 10thNight
35%48%25%24%4
1980
11th to 20thNight
42%5%9%20%19%4
1980
21st to endNight
42%5%9%21%16%6%
2174
April1st to 10thNight
39%5%9%18%22%7%
1800
11th to 20thNight
33%6%12%21%22%6%
1775
21st to endNight
38%6%11%23%16%5%
1800
May1st to 10thNight
40%7%14%21%15%3
1682
11th to 20thNight
42%10%13%19%14%2
1587
21st to endNight
38%9%16%24%12%1
1551
June1st to 10thNight
36%9%13%25%15%1
1440
11th to 20thNight
37%11%17%19%15%2
1440
21st to endNight
34%8%14%22%18%3
1440
July1st to 10thNight
37%11%15%19%14%4
1427
11th to 20thNight
31%11%20%24%11%2
1566
21st to endNight
36%12%15%23%13%
1672
August1st to 10thNight
38%12%16%22%12%1
1600
11th to 20thNight
40%11%16%20%13%
1671
21st to endNight
37%11%15%23%11%2
1980
September1st to 10thNight
37%9%16%18%15%6%
1800
11th to 20thNight
35%7%10%25%20%3
1802
21st to endNight
46%5%10%18%17%4
2078
October1st to 10thNight
38%5%10%23%20%4
2160
11th to 20thNight
38%48%24%21%5%
2160
21st to endNight
35%5%8%24%23%6%
2376
November1st to 10thNight
34%6%9%23%23%6%
2230
11th to 20thNight
37%47%20%24%8%
2472
21st to endNight
33%5%10%25%22%5%
2660
December1st to 10thNight
43%6%9%23%16%4
2660
11th to 20thNight
39%5%10%20%20%5%
3288
21st to endNight
40%7%11%24%16%2
4884

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 10thDay
30%22%30%15%4
254
11th to 20thDay
27%26%32%13%2
243
21st to endDay
25%25%37%11%3
264
February1st to 10thDay
23%26%37%10%3
240
11th to 20thDay
35%25%30%10%
284
21st to endDay
30%20%30%17%3
346
March1st to 10thDay
21%20%36%18%4
420
11th to 20thDay
22%24%33%16%41
420
21st to endDay
25%18%36%16%5%1
462
April1st to 10thDay
21%20%32%20%6%1
420
11th to 20thDay
18%13%33%21%9%4
412
21st to endDay
24%15%34%19%8%1
420
May1st to 10thDay
20%15%31%24%7%3
423
11th to 20thDay
21%8%32%25%13%1
447
21st to endDay
19%423%29%19%4
485
June1st to 10thDay
13%5%26%31%22%4
450
11th to 20thDay
12%6%26%34%16%6%
450
21st to endDay
11%6%25%32%22%4
450
July1st to 10thDay
11%318%32%27%9%
445
11th to 20thDay
5%116%41%27%9%
444
21st to endDay
9%322%34%22%10%
488
August1st to 10thDay
13%318%30%27%9%
446
11th to 20thDay
13%317%31%27%10%
439
21st to endDay
11%417%30%24%14%
495
September1st to 10thDay
17%5%16%27%21%15%
450
11th to 20thDay
14%419%31%23%9%
450
21st to endDay
28%7%21%27%12%4
450
October1st to 10thDay
21%6%23%27%16%8%
450
11th to 20thDay
18%10%24%28%18%2
450
21st to endDay
17%13%33%25%8%3
495
November1st to 10thDay
18%10%31%26%11%4
449
11th to 20thDay
18%13%31%26%10%1
366
21st to endDay
21%16%33%19%9%2
180
December1st to 10thDay
22%19%37%15%6%1
180
11th to 20thDay
22%16%34%18%8%1
180
21st to endDay
27%15%31%19%7%1
198
January1st to 10thNight
22%17%39%19%3
760
11th to 20thNight
21%15%36%25%4
725
21st to endNight
21%16%39%23%2
792
February1st to 10thNight
20%14%38%24%3
720
11th to 20thNight
24%20%37%17%1
703
21st to endNight
22%18%37%20%3
592
March1st to 10thNight
18%18%37%25%2
720
11th to 20thNight
23%17%33%20%6%
720
21st to endNight
18%15%38%23%6%
792
April1st to 10thNight
19%18%38%21%4
720
11th to 20thNight
14%14%36%27%8%
711
21st to endNight
18%11%34%30%7%
720
May1st to 10thNight
18%11%34%28%8%
602
11th to 20thNight
17%5%34%31%13%1
525
21st to endNight
16%221%37%23%2
389
June1st to 10thNight
11%223%42%22%1
360
11th to 20thNight
12%5%18%42%22%2
360
21st to endNight
8%224%46%20%1
360
July1st to 10thNight
10%318%38%27%4
377
11th to 20thNight
6%112%41%34%7%
522
21st to endNight
8%217%43%27%3
591
August1st to 10thNight
10%212%44%30%3
537
11th to 20thNight
14%312%41%27%3
607
21st to endNight
10%314%38%30%5%
792
September1st to 10thNight
15%215%35%28%5%
720
11th to 20thNight
13%414%36%29%3
720
21st to endNight
26%319%33%18%1
720
October1st to 10thNight
19%421%31%21%3
720
11th to 20thNight
19%8%27%30%15%2
720
21st to endNight
16%10%31%30%13%1
792
November1st to 10thNight
17%8%30%30%13%2
740
11th to 20thNight
17%13%32%29%9%
760
21st to endNight
15%12%32%31%9%1
760
December1st to 10thNight
21%13%36%22%8%
760
11th to 20thNight
19%12%35%29%6%
760
21st to endNight
19%14%30%33%4
836
Page generated at 2024-01-08 22:31:27. Version ac07

Copyright 2024 Attilla Danko


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