Prevost
Clear Sky Chart
History

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

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
26%
23509
13%
98855
7%
23269
36%
23269
13%
67104

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
15%
754
10%
4392
3%
737
24%
737
5%
3111
January11th to 20thNight
16%
754
12%
4389
4%
740
26%
740
7%
3014
January21st to endNight
20%
826
9%
4869
7%
814
31%
814
7%
3599
February1st to 10thNight
16%
760
14%
2504
4%
760
28%
760
13%
1838
February11th to 20thNight
16%
743
14%
2329
4%
740
30%
740
14%
1652
February21st to endNight
19%
628
16%
1884
6%
628
32%
628
15%
1354
March1st to 10thNight
19%
753
17%
3699
8%
743
30%
743
9%
2493
March11th to 20thNight
20%
756
15%
3628
5%
740
32%
740
9%
2575
March21st to endNight
22%
834
14%
3361
6%
814
33%
814
14%
2211
April1st to 10thNight
21%
760
17%
1900
4%
760
28%
760
20%
1413
April11th to 20thNight
23%
751
17%
1875
4%
751
30%
751
21%
1345
April21st to endNight
21%
615
17%
1755
6%
615
31%
615
19%
1223
May1st to 10thNight
24%
401
15%
2543
10%
398
35%
398
9%
1644
May11th to 20thNight
27%
367
14%
2469
15%
364
44%
364
10%
1479
May21st to endNight
26%
408
10%
2729
8%
399
41%
399
11%
1635
June1st to 10thNight
28%
380
10%
1520
7%
380
40%
380
16%
995
June11th to 20thNight
35%
380
10%
1520
10%
380
43%
380
18%
949
June21st to endNight
37%
380
10%
1520
12%
380
43%
380
17%
993
July1st to 10thNight
42%
365
10%
2422
14%
360
54%
360
13%
1462
July11th to 20thNight
45%
363
10%
2268
18%
358
52%
358
15%
1285
July21st to endNight
44%
409
10%
2381
14%
383
51%
383
15%
1330
August1st to 10thNight
52%
372
15%
2275
14%
365
60%
365
13%
1419
August11th to 20thNight
47%
541
13%
2636
11%
533
54%
533
18%
1616
August21st to endNight
43%
824
15%
3256
8%
814
48%
814
20%
2100
September1st to 10thNight
45%
760
18%
1900
7%
760
50%
760
35%
1232
September11th to 20thNight
48%
760
21%
1966
10%
760
53%
760
36%
1222
September21st to endNight
42%
760
24%
2090
15%
760
49%
760
30%
1332
October1st to 10thNight
33%
760
13%
3165
12%
740
43%
740
14%
2157
October11th to 20thNight
26%
760
12%
3003
7%
740
37%
740
14%
2147
October21st to endNight
21%
836
9%
3748
6%
814
31%
814
11%
2482
November1st to 10thNight
24%
780
17%
2537
8%
780
38%
780
16%
1752
November11th to 20thNight
22%
800
18%
2710
9%
800
33%
800
16%
1853
November21st to endNight
18%
800
13%
2800
5%
800
29%
800
11%
2029
December1st to 10thNight
16%
783
11%
2737
3%
783
26%
783
9%
2104
December11th to 20thNight
15%
760
13%
3244
5%
759
27%
759
8%
2394
December21st to endNight
13%
826
8%
4831
2%
817
23%
817
5%
3665

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%10%38%5%5%5%46%6%32%
101837
Night
16%11%48%5%4435%6%34%
97900
TransparencyDay
50%5%13%21%10%1
105300
Night
50%5%12%21%11%1
102526
SeeingDay
23%16%32%20%7%2
14602
Night
22%13%31%25%8%1
23609

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%16%6%15%6%4434424%
2517
11th to 20thDay
13%14%6%15%8%44345%25%
2520
21st to endDay
11%16%7%11%6%5%6%46%5%23%
2772
February1st to 10thDay
13%17%411%7%5%4445%27%
1675
11th to 20thDay
11%17%412%6%5%5%36%5%27%
1739
21st to endDay
16%15%5%10%6%5%434427%
1570
March1st to 10thDay
18%15%411%6%5%334426%
3116
11th to 20thDay
11%18%5%12%6%4434428%
3310
21st to endDay
14%10%28%44435%6%38%
4151
April1st to 10thDay
20%13%48%5%43345%32%
2280
11th to 20thDay
21%8%37%5%3435%6%35%
2254
21st to endDay
21%10%38%5%4435%5%32%
2424
May1st to 10thDay
20%10%38%6%5%435%6%30%
4421
11th to 20thDay
19%8%38%5%5%436%8%31%
4398
21st to endDay
17%6%27%5%5%5%46%7%36%
4837
June1st to 10thDay
21%6%38%6%5%5%47%7%28%
2660
11th to 20thDay
15%7%27%5%5%5%47%7%34%
2660
21st to endDay
21%9%28%5%6%5%46%6%27%
2660
July1st to 10thDay
13%6%37%6%6%6%5%7%9%34%
4440
11th to 20thDay
15%5%25%5%6%6%6%8%8%34%
4440
21st to endDay
12%5%26%5%45%46%7%45%
4814
August1st to 10thDay
15%5%27%5%6%6%5%8%8%32%
4423
11th to 20thDay
15%6%37%5%5%5%5%6%8%36%
4021
21st to endDay
16%5%27%445%46%7%39%
4180
September1st to 10thDay
14%6%26%44446%8%41%
2280
11th to 20thDay
13%6%26%34435%6%48%
2214
21st to endDay
18%7%26%44435%7%41%
2090
October1st to 10thDay
21%7%36%445%447%36%
1938
11th to 20thDay
20%10%39%5%445%5%6%29%
1868
21st to endDay
23%11%37%5%44346%29%
1881
November1st to 10thDay
14%10%38%5%45%46%7%34%
1761
11th to 20thDay
17%11%49%5%4446%6%30%
1685
21st to endDay
21%13%49%5%6%5%35%6%24%
1600
December1st to 10thDay
21%16%6%11%5%5%434421%
1560
11th to 20thDay
19%15%49%6%44345%27%
1906
21st to endDay
24%18%5%10%5%4324422%
2772
January1st to 10thNight
14%15%5%11%6%5%5%35%5%25%
4429
11th to 20thNight
10%13%6%14%7%5%435%6%28%
4440
21st to endNight
12%15%6%12%7%6%5%45%6%25%
4884
February1st to 10thNight
12%13%5%11%7%6%5%45%6%27%
2504
11th to 20thNight
11%13%411%6%45%45%6%31%
2327
21st to endNight
14%12%5%9%7%5%5%35%5%31%
1884
March1st to 10thNight
18%13%47%5%5%4345%32%
3724
11th to 20thNight
12%20%5%10%5%44345%29%
3644
21st to endNight
13%8%49%6%5%4345%39%
3480
April1st to 10thNight
22%11%37%5%4325%6%31%
1900
11th to 20thNight
19%6%37%5%44446%37%
1875
21st to endNight
20%8%37%44435%5%36%
1755
May1st to 10thNight
19%10%38%5%45%446%31%
2549
11th to 20thNight
21%8%36%43335%7%37%
2495
21st to endNight
18%6%37%5%5%5%46%7%35%
2738
June1st to 10thNight
20%7%36%5%4445%8%33%
1520
11th to 20thNight
18%7%26%445%35%7%39%
1520
21st to endNight
20%7%16%5%4436%7%37%
1520
July1st to 10thNight
12%5%37%6%5%6%5%7%7%37%
2520
11th to 20thNight
15%5%26%44445%8%44%
2520
21st to endNight
11%5%15%445%47%7%46%
2739
August1st to 10thNight
13%6%26%5%5%5%46%8%40%
2506
11th to 20thNight
15%7%36%34435%7%43%
2774
21st to endNight
17%5%26%44435%6%43%
3476
September1st to 10thNight
10%5%26%44446%7%47%
1900
11th to 20thNight
13%5%25%33335%7%50%
1966
21st to endNight
18%5%25%33335%6%46%
2090
October1st to 10thNight
17%7%37%45%4345%41%
2238
11th to 20thNight
19%8%27%44445%5%37%
2310
21st to endNight
21%10%37%5%4435%7%32%
2717
November1st to 10thNight
16%10%37%5%5%436%7%32%
2537
11th to 20thNight
16%11%38%5%5%445%6%34%
2710
21st to endNight
19%14%5%7%5%4445%6%29%
2800
December1st to 10thNight
17%15%5%10%6%5%5%46%7%21%
2737
11th to 20thNight
20%15%48%5%44345%27%
3288
21st to endNight
24%18%48%44324424%
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
63%6%9%15%5%1
2518
11th to 20thDay
60%411%16%8%1
2520
21st to endDay
62%5%11%14%7%1
2772
February1st to 10thDay
60%38%18%8%2
1675
11th to 20thDay
58%411%15%11%1
1748
21st to endDay
60%49%15%10%3
1570
March1st to 10thDay
62%38%16%9%1
3119
11th to 20thDay
58%410%13%12%3
3310
21st to endDay
47%48%20%19%3
4151
April1st to 10thDay
55%38%19%12%3
2280
11th to 20thDay
49%48%23%14%2
2254
21st to endDay
55%48%17%13%4
2424
May1st to 10thDay
54%5%8%22%10%2
4435
11th to 20thDay
49%6%17%16%9%2
4402
21st to endDay
45%5%11%22%15%1
4837
June1st to 10thDay
53%5%10%18%12%2
2660
11th to 20thDay
46%6%12%20%12%3
2660
21st to endDay
56%6%11%17%9%1
2660
July1st to 10thDay
44%7%15%24%9%
4370
11th to 20thDay
41%8%17%27%7%
4365
21st to endDay
37%6%14%32%10%
4661
August1st to 10thDay
46%6%18%21%8%
4409
11th to 20thDay
43%7%17%27%6%
4063
21st to endDay
42%7%16%20%14%1
4180
September1st to 10thDay
39%6%14%28%13%
2280
11th to 20thDay
36%414%32%13%1
2214
21st to endDay
43%414%27%12%
2090
October1st to 10thDay
44%6%15%26%9%
3224
11th to 20thDay
44%5%15%27%8%
3084
21st to endDay
44%423%19%10%1
3081
November1st to 10thDay
48%5%14%22%10%1
1761
11th to 20thDay
53%5%12%18%12%1
1685
21st to endDay
59%6%13%14%6%1
1600
December1st to 10thDay
67%410%12%6%
1560
11th to 20thDay
60%410%17%8%
1906
21st to endDay
66%5%8%14%6%
2772
January1st to 10thNight
60%5%14%15%5%
4429
11th to 20thNight
56%6%13%19%5%1
4440
21st to endNight
61%5%11%14%8%1
4884
February1st to 10thNight
58%49%20%9%1
2504
11th to 20thNight
53%5%10%20%11%1
2329
21st to endNight
55%49%17%14%2
1884
March1st to 10thNight
56%410%17%10%3
3734
11th to 20thNight
57%5%8%15%13%2
3645
21st to endNight
49%46%22%15%4
3480
April1st to 10thNight
53%38%19%15%2
1900
11th to 20thNight
48%48%23%15%2
1875
21st to endNight
50%48%21%15%3
1755
May1st to 10thNight
53%412%20%8%3
2555
11th to 20thNight
46%6%14%20%13%2
2496
21st to endNight
47%5%10%20%16%2
2739
June1st to 10thNight
48%6%11%22%13%1
1520
11th to 20thNight
45%5%11%22%14%3
1520
21st to endNight
47%6%13%20%13%1
1520
July1st to 10thNight
44%6%17%20%12%1
2480
11th to 20thNight
38%6%19%30%7%
2472
21st to endNight
35%7%18%27%13%
2651
August1st to 10thNight
42%7%15%27%9%
2496
11th to 20thNight
42%5%19%27%8%
2798
21st to endNight
41%6%14%24%13%1
3476
September1st to 10thNight
35%6%15%28%16%1
1900
11th to 20thNight
34%415%34%12%1
1966
21st to endNight
39%5%12%30%15%
2090
October1st to 10thNight
43%5%14%25%12%1
3732
11th to 20thNight
42%5%13%29%12%
3833
21st to endNight
43%5%19%18%13%2
4467
November1st to 10thNight
50%5%12%23%9%1
2537
11th to 20thNight
51%413%20%11%2
2710
21st to endNight
56%6%11%18%8%1
2800
December1st to 10thNight
63%5%11%17%5%
2737
11th to 20thNight
59%5%8%19%8%1
3288
21st to endNight
65%47%17%6%
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
18%21%37%20%3
180
11th to 20thDay
31%14%37%12%41
180
21st to endDay
20%21%39%16%41
198
February1st to 10thDay
34%22%33%9%1
254
11th to 20thDay
23%24%39%12%2
346
21st to endDay
29%20%32%16%3
382
March1st to 10thDay
28%18%32%17%4
460
11th to 20thDay
25%23%32%15%31
460
21st to endDay
29%22%27%15%5%1
506
April1st to 10thDay
30%17%33%15%6%1
460
11th to 20thDay
33%19%26%14%6%1
452
21st to endDay
30%19%30%16%41
460
May1st to 10thDay
26%17%36%15%5%1
463
11th to 20thDay
23%18%31%22%5%1
487
21st to endDay
23%20%31%20%6%1
529
June1st to 10thDay
24%16%35%18%6%1
490
11th to 20thDay
19%17%34%19%9%1
490
21st to endDay
27%12%32%20%7%2
490
July1st to 10thDay
13%12%36%26%10%2
485
11th to 20thDay
16%11%36%27%9%1
484
21st to endDay
14%9%35%28%12%1
532
August1st to 10thDay
15%9%31%30%13%1
486
11th to 20thDay
13%7%31%29%13%7%
479
21st to endDay
18%11%30%23%12%6%
539
September1st to 10thDay
18%7%36%23%11%5%
490
11th to 20thDay
12%11%28%26%17%6%
490
21st to endDay
22%12%27%22%12%5%
490
October1st to 10thDay
24%17%27%19%11%1
490
11th to 20thDay
24%17%30%21%7%1
473
21st to endDay
31%15%31%18%4
429
November1st to 10thDay
29%17%31%17%5%1
395
11th to 20thDay
26%20%33%17%5%
285
21st to endDay
30%20%33%14%4
200
December1st to 10thDay
27%16%33%19%5%1
190
11th to 20thDay
30%21%37%10%21
180
21st to endDay
36%21%25%13%5%1
198
January1st to 10thNight
23%16%34%22%5%
760
11th to 20thNight
23%17%32%23%4
760
21st to endNight
16%15%39%24%6%
836
February1st to 10thNight
18%17%39%22%41
760
11th to 20thNight
20%18%39%18%4
743
21st to endNight
21%16%34%26%2
628
March1st to 10thNight
23%17%33%24%3
760
11th to 20thNight
24%16%34%19%6%
760
21st to endNight
22%14%35%23%5%
836
April1st to 10thNight
28%14%32%23%4
760
11th to 20thNight
25%15%33%23%5%1
751
21st to endNight
26%16%33%18%7%
615
May1st to 10thNight
24%13%32%23%6%2
402
11th to 20thNight
23%12%28%29%7%1
374
21st to endNight
20%9%40%23%8%
411
June1st to 10thNight
22%11%33%25%8%
380
11th to 20thNight
24%8%28%29%11%
380
21st to endNight
23%6%28%29%14%1
380
July1st to 10thNight
10%9%33%35%12%1
370
11th to 20thNight
11%6%28%41%14%
368
21st to endNight
12%6%29%39%12%2
416
August1st to 10thNight
15%5%21%42%16%1
378
11th to 20thNight
14%426%32%21%2
550
21st to endNight
15%7%28%31%17%2
836
September1st to 10thNight
13%7%31%37%12%1
760
11th to 20thNight
15%7%25%31%20%1
760
21st to endNight
19%9%25%30%15%3
760
October1st to 10thNight
22%11%27%29%10%1
760
11th to 20thNight
24%12%29%25%8%1
760
21st to endNight
27%12%32%24%5%
836
November1st to 10thNight
24%15%29%23%8%1
780
11th to 20thNight
25%14%32%21%8%1
800
21st to endNight
29%17%28%20%5%1
800
December1st to 10thNight
27%15%32%19%7%
783
11th to 20thNight
28%18%31%19%41
760
21st to endNight
32%16%31%17%4
836
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