Mont Hereford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mont Hereford clear sky chart from 2005-11-06 to 2024-01-22.

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
23%
22641
10%
74841
6%
22595
29%
22595
11%
58144

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
9%
760
5%
2649
2%
757
13%
757
3%
2346
January11th to 20thNight
10%
760
4%
2660
2%
760
12%
760
4%
2327
January21st to endNight
14%
800
7%
2798
3%
800
20%
800
6%
2315
February1st to 10thNight
10%
720
6%
2438
2%
720
16%
720
5%
2077
February11th to 20thNight
9%
703
8%
2100
2%
700
17%
700
6%
1787
February21st to endNight
14%
592
11%
1776
3%
592
23%
592
9%
1448
March1st to 10thNight
15%
720
11%
2160
6%
720
23%
720
9%
1751
March11th to 20thNight
18%
720
11%
2081
5%
720
25%
720
10%
1640
March21st to endNight
21%
792
15%
2031
6%
792
27%
792
13%
1606
April1st to 10thNight
18%
720
11%
1800
3%
720
23%
720
13%
1457
April11th to 20thNight
22%
665
12%
1729
4%
665
28%
665
16%
1317
April21st to endNight
19%
540
13%
1620
4%
540
26%
540
12%
1236
May1st to 10thNight
21%
540
13%
1620
6%
540
28%
540
14%
1202
May11th to 20thNight
27%
369
15%
1431
11%
369
38%
369
14%
986
May21st to endNight
34%
389
11%
1551
10%
388
44%
388
15%
1055
June1st to 10thNight
28%
360
10%
1440
9%
360
37%
360
12%
1037
June11th to 20thNight
40%
360
10%
1440
13%
360
45%
360
16%
954
June21st to endNight
39%
360
7%
1440
8%
360
44%
360
13%
1031
July1st to 10thNight
43%
350
11%
1400
15%
350
49%
350
17%
926
July11th to 20thNight
46%
348
13%
1392
17%
348
52%
348
18%
898
July21st to endNight
48%
394
11%
1486
13%
372
48%
372
17%
994
August1st to 10thNight
53%
384
13%
1448
14%
382
55%
382
21%
913
August11th to 20thNight
45%
532
14%
1596
9%
532
49%
532
21%
1113
August21st to endNight
44%
760
16%
1948
8%
760
46%
760
27%
1350
September1st to 10thNight
41%
720
11%
1800
5%
720
43%
720
26%
1272
September11th to 20thNight
43%
720
17%
1957
10%
720
47%
720
26%
1306
September21st to endNight
38%
720
19%
1980
14%
720
41%
720
25%
1348
October1st to 10thNight
32%
720
15%
1980
9%
720
37%
720
18%
1459
October11th to 20thNight
25%
720
11%
2275
6%
720
31%
720
11%
1749
October21st to endNight
16%
792
9%
2574
3%
792
23%
792
9%
2041
November1st to 10thNight
18%
740
11%
2407
7%
740
23%
740
11%
1861
November11th to 20thNight
18%
760
15%
2470
10%
760
24%
760
10%
1926
November21st to endNight
15%
760
8%
2592
5%
760
19%
760
6%
2148
December1st to 10thNight
10%
760
4%
2660
1%
760
13%
760
4%
2340
December11th to 20thNight
12%
760
6%
3264
4%
759
15%
759
4%
2840
December21st to endNight
8%
831
4%
4848
2%
817
13%
817
2%
4088

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
19%12%5%8%7%35%45%6%26%
74863
Night
18%13%5%10%7%35%445%25%
75072
TransparencyDay
59%6%10%16%7%1
74568
Night
61%6%10%15%7%1
74901
SeeingDay
29%19%27%16%7%2
13652
Night
27%18%26%21%7%1
22646

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 10thDay
14%17%9%16%11%6%5%43311%
1517
11th to 20thDay
14%17%8%13%12%35%43415%
1520
21st to endDay
10%13%10%14%11%47%6%4420%
1592
February1st to 10thDay
14%16%8%14%12%5%6%33314%
1519
11th to 20thDay
12%17%10%15%10%6%5%44315%
1748
21st to endDay
12%15%8%14%8%6%6%35%419%
1480
March1st to 10thDay
13%16%8%12%8%5%5%43422%
1797
11th to 20thDay
14%15%8%10%7%3443428%
1878
21st to endDay
16%18%6%11%7%3423426%
2322
April1st to 10thDay
20%17%6%9%5%3334425%
2160
11th to 20thDay
18%16%46%5%34345%33%
2179
21st to endDay
22%15%47%6%35%445%25%
2340
May1st to 10thDay
24%13%45%5%34345%30%
2340
11th to 20thDay
22%9%47%5%34446%32%
2478
21st to endDay
20%8%36%6%36%6%7%8%28%
2725
June1st to 10thDay
28%10%36%5%345%5%7%24%
2520
11th to 20thDay
20%7%35%6%35%5%6%7%33%
2520
21st to endDay
23%8%38%6%36%5%6%7%25%
2520
July1st to 10thDay
19%7%45%5%45%5%6%10%32%
2520
11th to 20thDay
16%7%36%7%46%6%7%9%28%
2520
21st to endDay
20%7%36%7%46%6%6%8%28%
2744
August1st to 10thDay
19%7%36%6%46%6%7%9%28%
2462
11th to 20thDay
16%7%37%6%46%6%6%9%30%
2304
21st to endDay
20%7%35%5%35%5%6%8%33%
2406
September1st to 10thDay
19%8%25%5%345%5%7%38%
2160
11th to 20thDay
20%7%25%434446%41%
2003
21st to endDay
18%9%35%5%345%6%8%33%
1980
October1st to 10thDay
23%13%45%5%346%5%6%26%
1980
11th to 20thDay
18%15%47%6%345%6%6%26%
1682
21st to endDay
24%22%5%6%5%3433421%
1782
November1st to 10thDay
18%18%5%9%6%34435%26%
1671
11th to 20thDay
18%16%7%11%6%34445%21%
1710
21st to endDay
17%16%7%11%8%35%45%5%19%
1586
December1st to 10thDay
18%21%9%13%7%4333314%
1520
11th to 20thDay
18%18%7%13%7%35%33419%
1906
21st to endDay
22%17%8%12%10%3443314%
2772
January1st to 10thNight
13%19%10%16%11%5%6%43310%
2649
11th to 20thNight
14%17%8%15%12%5%7%44312%
2660
21st to endNight
10%14%8%14%11%6%7%5%4417%
2798
February1st to 10thNight
12%16%8%15%12%5%7%33415%
2438
11th to 20thNight
11%16%9%15%11%6%7%43316%
2098
21st to endNight
13%15%6%12%10%5%6%33422%
1776
March1st to 10thNight
13%17%8%11%8%45%44422%
2150
11th to 20thNight
15%15%7%10%7%45%44327%
2080
21st to endNight
17%16%6%10%6%3433428%
2031
April1st to 10thNight
21%15%6%8%6%3445%5%24%
1800
11th to 20thNight
20%13%47%5%245%46%31%
1729
21st to endNight
22%14%36%5%35%45%5%27%
1620
May1st to 10thNight
23%11%36%6%3345%5%30%
1620
11th to 20thNight
19%9%36%5%35%5%5%6%33%
1430
21st to endNight
20%6%26%5%45%448%33%
1550
June1st to 10thNight
27%8%45%5%345%5%6%29%
1440
11th to 20thNight
20%6%36%5%26%5%48%36%
1440
21st to endNight
20%7%46%5%46%46%7%31%
1440
July1st to 10thNight
14%7%35%5%37%6%6%8%35%
1440
11th to 20thNight
14%7%36%5%45%7%6%9%36%
1440
21st to endNight
16%7%26%5%36%6%6%7%36%
1574
August1st to 10thNight
17%6%345%345%7%8%39%
1458
11th to 20thNight
16%7%37%6%35%5%6%7%36%
1596
21st to endNight
18%6%35%5%245%5%6%41%
1948
September1st to 10thNight
17%8%26%5%3445%6%40%
1800
11th to 20thNight
18%6%25%424446%44%
1957
21st to endNight
21%7%2342445%8%40%
1980
October1st to 10thNight
21%10%36%5%44447%33%
1980
11th to 20thNight
22%13%37%6%345%45%26%
2275
21st to endNight
26%17%46%5%34446%22%
2574
November1st to 10thNight
20%18%5%7%5%3345%5%24%
2407
11th to 20thNight
17%15%8%11%7%23335%25%
2470
21st to endNight
16%16%9%12%10%3433419%
2592
December1st to 10thNight
19%21%9%13%8%3433312%
2660
11th to 20thNight
18%20%8%15%10%4433313%
3288
21st to endNight
21%18%7%15%8%3433315%
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
79%47%8%2
1518
11th to 20thDay
72%5%7%10%5%1
1520
21st to endDay
69%5%7%12%7%1
1592
February1st to 10thDay
77%45%9%5%1
1519
11th to 20thDay
75%38%8%5%1
1757
21st to endDay
69%5%7%11%7%2
1480
March1st to 10thDay
67%49%12%6%2
1800
11th to 20thDay
61%410%12%11%2
1878
21st to endDay
64%37%15%9%2
2322
April1st to 10thDay
63%5%8%14%8%2
2160
11th to 20thDay
55%5%8%19%12%1
2179
21st to endDay
61%5%9%14%8%3
2340
May1st to 10thDay
57%47%19%11%1
2340
11th to 20thDay
54%5%10%18%10%1
2482
21st to endDay
51%11%13%17%9%
2725
June1st to 10thDay
60%7%11%17%5%1
2520
11th to 20thDay
49%7%11%21%10%1
2520
21st to endDay
56%8%12%17%6%
2520
July1st to 10thDay
48%7%15%21%9%
2450
11th to 20thDay
49%9%15%22%5%
2445
21st to endDay
53%8%13%20%6%
2591
August1st to 10thDay
50%9%17%19%5%
2448
11th to 20thDay
49%9%18%17%8%
2304
21st to endDay
47%8%13%21%11%
2406
September1st to 10thDay
45%7%14%23%10%
2160
11th to 20thDay
44%5%11%26%13%1
2003
21st to endDay
47%8%13%21%10%1
1980
October1st to 10thDay
56%6%11%17%8%
1980
11th to 20thDay
57%7%15%16%5%
1682
21st to endDay
68%49%11%7%1
1782
November1st to 10thDay
61%5%10%16%8%1
1671
11th to 20thDay
66%6%9%10%8%2
1710
21st to endDay
68%5%12%12%3
1586
December1st to 10thDay
76%47%8%4
1520
11th to 20thDay
70%47%12%6%1
1906
21st to endDay
75%6%5%9%4
2772
January1st to 10thNight
80%46%7%3
2649
11th to 20thNight
78%5%6%8%31
2660
21st to endNight
71%5%7%11%5%1
2798
February1st to 10thNight
74%5%6%9%5%
2438
11th to 20thNight
74%46%11%5%1
2100
21st to endNight
68%5%6%13%8%1
1776
March1st to 10thNight
66%5%7%14%7%1
2160
11th to 20thNight
62%5%8%13%10%2
2081
21st to endNight
62%46%17%9%2
2031
April1st to 10thNight
63%5%8%14%10%1
1800
11th to 20thNight
54%7%9%19%11%
1729
21st to endNight
58%6%10%15%9%2
1620
May1st to 10thNight
56%5%8%17%12%1
1620
11th to 20thNight
50%7%12%18%11%2
1431
21st to endNight
50%7%13%20%9%1
1551
June1st to 10thNight
56%7%10%20%7%
1440
11th to 20thNight
47%8%12%22%9%2
1440
21st to endNight
53%9%13%19%6%
1440
July1st to 10thNight
43%11%14%21%10%
1400
11th to 20thNight
42%10%15%26%7%
1392
21st to endNight
45%10%15%20%9%
1486
August1st to 10thNight
40%10%19%26%5%
1448
11th to 20thNight
44%10%17%20%8%
1596
21st to endNight
43%7%15%24%10%
1948
September1st to 10thNight
44%7%14%25%9%
1800
11th to 20thNight
42%6%14%27%11%1
1957
21st to endNight
43%7%13%25%10%2
1980
October1st to 10thNight
52%6%11%20%11%
1980
11th to 20thNight
59%7%11%17%5%
2275
21st to endNight
64%6%11%12%7%1
2574
November1st to 10thNight
62%5%11%14%7%1
2407
11th to 20thNight
63%5%9%14%7%2
2470
21st to endNight
70%6%9%11%4
2592
December1st to 10thNight
78%5%7%7%2
2660
11th to 20thNight
78%45%7%6%
3288
21st to endNight
75%5%6%9%4
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
29%22%36%11%2
180
11th to 20thDay
34%24%31%9%21
180
21st to endDay
27%26%31%12%4
178
February1st to 10thDay
45%26%21%7%1
211
11th to 20thDay
34%30%26%8%2
416
21st to endDay
32%28%25%12%1
346
March1st to 10thDay
32%22%31%12%3
420
11th to 20thDay
29%28%26%12%4
420
21st to endDay
32%23%24%14%6%1
462
April1st to 10thDay
40%18%23%13%5%1
420
11th to 20thDay
35%20%25%11%6%2
412
21st to endDay
34%24%25%12%5%1
420
May1st to 10thDay
43%14%25%11%5%1
423
11th to 20thDay
29%20%30%14%5%2
447
21st to endDay
25%21%29%18%6%1
485
June1st to 10thDay
34%17%29%13%6%
450
11th to 20thDay
24%20%26%21%7%2
450
21st to endDay
25%17%32%17%7%2
450
July1st to 10thDay
22%14%29%26%8%2
445
11th to 20thDay
18%13%36%23%8%2
444
21st to endDay
21%13%34%24%6%2
488
August1st to 10thDay
21%12%30%23%12%2
446
11th to 20thDay
18%11%29%24%14%4
439
21st to endDay
25%16%22%22%11%4
495
September1st to 10thDay
23%15%22%23%12%5%
450
11th to 20thDay
24%12%22%20%15%6%
450
21st to endDay
20%17%24%22%12%5%
450
October1st to 10thDay
30%18%20%19%10%2
450
11th to 20thDay
29%20%28%13%8%2
390
21st to endDay
36%22%23%14%4
396
November1st to 10thDay
35%19%26%16%4
365
11th to 20thDay
30%25%28%11%5%1
370
21st to endDay
34%21%28%13%3
246
December1st to 10thDay
35%24%25%11%5%
180
11th to 20thDay
36%30%26%6%3
180
21st to endDay
37%26%28%7%2
198
January1st to 10thNight
27%26%31%13%2
760
11th to 20thNight
27%28%26%15%3
760
21st to endNight
21%26%31%19%31
800
February1st to 10thNight
26%27%30%14%3
720
11th to 20thNight
24%28%32%13%3
703
21st to endNight
26%23%32%17%1
592
March1st to 10thNight
27%22%30%18%3
720
11th to 20thNight
31%19%27%16%6%
720
21st to endNight
32%19%22%18%7%1
792
April1st to 10thNight
34%21%25%17%4
720
11th to 20thNight
31%18%25%19%6%
665
21st to endNight
33%17%28%18%5%
540
May1st to 10thNight
30%16%29%19%6%1
540
11th to 20thNight
22%15%30%25%7%
369
21st to endNight
25%11%24%33%7%1
389
June1st to 10thNight
30%8%26%26%9%1
360
11th to 20thNight
22%9%24%33%13%
360
21st to endNight
20%8%27%36%8%
360
July1st to 10thNight
17%7%29%35%12%1
350
11th to 20thNight
17%6%25%35%16%1
348
21st to endNight
16%5%25%38%16%
394
August1st to 10thNight
18%5%18%39%19%1
384
11th to 20thNight
18%7%22%34%18%1
532
21st to endNight
21%7%21%32%16%2
760
September1st to 10thNight
21%8%23%33%14%1
720
11th to 20thNight
22%9%21%28%19%2
720
21st to endNight
25%12%23%25%14%2
720
October1st to 10thNight
29%12%23%24%12%1
720
11th to 20thNight
32%16%22%21%8%2
720
21st to endNight
36%19%26%17%3
792
November1st to 10thNight
32%17%27%17%6%1
740
11th to 20thNight
27%23%27%15%7%1
760
21st to endNight
31%24%24%15%5%1
760
December1st to 10thNight
34%25%24%13%3
760
11th to 20thNight
32%24%26%14%4
760
21st to endNight
36%22%26%14%2
836
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