Irma
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Irma clear sky chart from 2005-11-05 to 2017-09-17.

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
41%
15299
11%
46768
7%
15274
45%
15274
20%
32700

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
34%
895
7%
2685
5%
895
36%
895
14%
2052
January16th to endNight
29%
945
14%
2865
6%
945
37%
945
17%
2068
February1st to 15thNight
38%
696
9%
2374
6%
696
43%
696
16%
1697
February16th to endNight
44%
632
13%
2052
12%
632
47%
632
17%
1453
March1st to 15thNight
38%
720
12%
2084
8%
720
42%
720
19%
1514
March16th to endNight
38%
768
10%
2111
6%
767
40%
767
18%
1605
April1st to 15thNight
40%
557
12%
1680
7%
557
44%
557
19%
1185
April16th to endNight
40%
540
14%
1602
12%
540
43%
540
21%
1119
May1st to 15thNight
45%
455
10%
1355
7%
455
48%
455
25%
889
May16th to endNight
41%
374
2%
1309
2%
374
41%
374
17%
894
June1st to 15thNight
43%
360
0%
1260
0%
360
43%
360
16%
882
June16th to endNight
52%
360
0%
1260
0%
360
52%
360
20%
821
July1st to 15thNight
59%
340
0%
1190
0%
340
59%
340
28%
696
July16th to endNight
59%
376
4%
1239
4%
354
60%
354
29%
711
August1st to 15thNight
65%
434
7%
1410
8%
432
66%
432
26%
872
August16th to endNight
59%
573
16%
1719
12%
573
62%
573
34%
1026
September1st to 15thNight
56%
567
14%
1763
11%
567
58%
567
30%
1065
September16th to endNight
54%
664
21%
1837
14%
664
61%
664
39%
1078
October1st to 15thNight
42%
660
17%
1974
11%
660
48%
660
24%
1341
October16th to endNight
39%
748
15%
2309
10%
748
44%
748
21%
1599
November1st to 15thNight
36%
875
15%
2450
9%
875
41%
875
20%
1762
November16th to endNight
32%
900
10%
2660
5%
900
36%
900
16%
1986
December1st to 15thNight
27%
900
7%
2700
4%
900
30%
900
11%
2169
December16th to endNight
32%
960
10%
2880
5%
960
37%
960
14%
2216

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
11%9%47%6%35%5%6%8%31%
48050
Night
9%9%48%7%45%5%6%7%31%
46920
TransparencyDay
49%6%18%19%5%
47700
Night
49%6%16%20%6%
46774
SeeingDay
16%6%24%29%18%5%
6132
Night
13%5%26%35%16%2
15314

By Month

Percentage Cloud Cover NW (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
8%11%6%8%9%6%6%6%6%6%23%
1253
16th to endDay
6%12%6%10%9%35%45%7%27%
1371
February1st to 15thDay
9%11%5%11%7%35%5%5%7%25%
1485
16th to endDay
8%13%7%12%9%45%6%5%8%18%
1422
March1st to 15thDay
8%12%5%9%8%5%6%5%6%7%23%
1876
16th to endDay
12%15%6%9%6%3445%7%22%
2111
April1st to 15thDay
11%12%5%7%6%45%5%6%7%28%
2266
16th to endDay
16%11%36%5%35%5%47%29%
2358
May1st to 15thDay
9%8%36%7%46%5%6%7%34%
2594
16th to endDay
17%8%26%5%36%5%6%8%29%
2802
June1st to 15thDay
16%9%37%7%47%5%6%7%25%
2700
16th to endDay
15%6%26%6%36%6%7%9%28%
2700
July1st to 15thDay
10%425%6%35%6%6%10%38%
2700
16th to endDay
8%5%25%5%45%5%7%9%39%
2846
August1st to 15thDay
12%5%25%5%45%5%6%9%35%
2541
16th to endDay
8%6%24435%46%8%43%
2483
September1st to 15thDay
11%7%27%5%335%45%42%
2196
16th to endDay
11%424424449%47%
1828
October1st to 15thDay
13%11%36%5%345%45%36%
1656
16th to endDay
8%12%45%5%345%5%8%35%
1551
November1st to 15thDay
10%11%6%7%6%345%6%8%30%
1408
16th to endDay
8%11%6%11%7%35%449%25%
1299
December1st to 15thDay
9%14%7%11%8%48%6%7%5%16%
1260
16th to endDay
6%13%5%10%11%5%7%6%5%6%20%
1344
January1st to 15thNight
8%10%6%9%8%5%7%7%6%6%22%
2685
16th to endNight
5%10%5%10%9%46%45%7%30%
2865
February1st to 15thNight
6%9%411%10%46%6%6%6%28%
2367
16th to endNight
6%10%6%8%7%35%6%6%8%27%
2052
March1st to 15thNight
7%8%410%8%47%6%6%9%27%
2084
16th to endNight
10%12%6%8%7%445%6%7%25%
2109
April1st to 15thNight
10%9%37%7%46%6%6%8%29%
1680
16th to endNight
13%11%36%5%2445%6%34%
1602
May1st to 15thNight
11%7%37%6%36%46%7%35%
1355
16th to endNight
17%8%25%5%35%5%5%7%31%
1309
June1st to 15thNight
13%8%37%8%36%7%6%6%28%
1260
16th to endNight
11%425%7%5%6%6%7%8%34%
1260
July1st to 15thNight
8%425%5%247%6%11%41%
1260
16th to endNight
8%323426%47%10%44%
1323
August1st to 15thNight
8%425%5%35%6%8%10%39%
1417
16th to endNight
6%426%5%45%5%6%9%45%
1719
September1st to 15thNight
11%5%25%425%45%6%46%
1763
16th to endNight
8%41342345%9%51%
1837
October1st to 15thNight
11%9%46%5%3445%7%36%
1974
16th to endNight
12%9%37%5%45%5%6%7%32%
2309
November1st to 15thNight
11%10%47%6%35%5%5%6%31%
2450
16th to endNight
7%11%6%9%8%45%45%7%28%
2660
December1st to 15thNight
9%13%6%12%9%46%5%5%7%18%
2700
16th to endNight
410%6%12%10%5%7%6%6%7%21%
2880

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
57%7%15%14%4
1253
16th to endDay
55%5%12%16%7%1
1371
February1st to 15thDay
56%5%15%18%4
1485
16th to endDay
61%413%13%6%
1422
March1st to 15thDay
57%5%19%13%3
1876
16th to endDay
59%5%13%16%4
2112
April1st to 15thDay
52%5%16%18%6%
2266
16th to endDay
52%5%14%18%7%1
2358
May1st to 15thDay
46%414%23%9%
2594
16th to endDay
51%5%14%17%9%
2807
June1st to 15thDay
55%6%15%18%4
2700
16th to endDay
48%7%22%17%3
2700
July1st to 15thDay
38%7%25%24%4
2541
16th to endDay
38%7%25%24%3
2667
August1st to 15thDay
42%8%26%18%4
2523
16th to endDay
35%5%23%27%6%1
2483
September1st to 15thDay
41%419%23%8%2
2196
16th to endDay
34%7%17%31%8%1
1828
October1st to 15thDay
48%416%24%5%
1656
16th to endDay
46%6%16%23%6%
1551
November1st to 15thDay
50%5%17%20%4
1408
16th to endDay
55%6%17%15%3
1299
December1st to 15thDay
66%7%11%12%2
1260
16th to endDay
62%5%11%15%5%
1344
January1st to 15thNight
57%6%16%14%4
2685
16th to endNight
52%5%15%18%6%1
2865
February1st to 15thNight
53%5%16%16%7%
2378
16th to endNight
52%5%14%19%7%
2052
March1st to 15thNight
51%6%17%18%5%
2084
16th to endNight
56%5%13%19%6%
2112
April1st to 15thNight
50%5%15%19%8%1
1680
16th to endNight
48%5%11%19%11%3
1602
May1st to 15thNight
45%311%26%12%
1356
16th to endNight
51%49%24%7%2
1309
June1st to 15thNight
51%6%15%20%5%
1260
16th to endNight
43%8%21%20%6%
1260
July1st to 15thNight
33%8%23%27%6%
1190
16th to endNight
31%8%26%27%6%
1239
August1st to 15thNight
35%10%26%23%4
1410
16th to endNight
33%5%19%31%9%
1719
September1st to 15thNight
36%5%18%24%11%4
1763
16th to endNight
28%6%16%34%10%2
1837
October1st to 15thNight
46%415%25%8%
1974
16th to endNight
47%6%16%24%4
2309
November1st to 15thNight
50%5%15%20%6%1
2450
16th to endNight
55%6%18%13%6%
2660
December1st to 15thNight
64%5%13%12%3
2700
16th to endNight
59%6%15%14%3
2880

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
13%326%36%16%3
60
16th to endDay
10%6%20%35%21%4
64
February1st to 15thDay
13%228%37%13%5%
75
16th to endDay
27%325%18%20%3
81
March1st to 15thDay
15%7%38%28%7%1
252
16th to endDay
20%8%30%27%10%1
288
April1st to 15thDay
18%10%35%23%9%1
269
16th to endDay
25%9%22%23%15%2
288
May1st to 15thDay
13%8%24%31%17%4
468
16th to endDay
18%7%20%26%17%8%
517
June1st to 15thDay
17%6%25%30%15%5%
495
16th to endDay
16%6%21%29%18%7%
495
July1st to 15thDay
13%416%30%28%6%
475
16th to endDay
11%5%22%30%25%4
515
August1st to 15thDay
12%214%30%29%10%
410
16th to endDay
8%5%25%32%25%2
332
September1st to 15thDay
19%6%25%29%14%5%
315
16th to endDay
15%8%27%30%14%4
260
October1st to 15thDay
29%321%35%7%3
96
16th to endDay
17%5%27%26%18%5%
96
November1st to 15thDay
16%424%44%8%1
90
16th to endDay
20%5%26%26%7%11%
67
December1st to 15thDay
13%6%30%23%20%6%
60
16th to endDay
17%6%25%26%18%6%
64
January1st to 15thNight
13%6%29%35%13%1
900
16th to endNight
11%9%33%34%10%1
945
February1st to 15thNight
9%6%25%37%17%2
704
16th to endNight
11%422%38%19%3
632
March1st to 15thNight
10%430%36%14%2
720
16th to endNight
15%429%31%16%3
768
April1st to 15thNight
14%7%26%35%14%1
557
16th to endNight
20%6%22%34%15%1
540
May1st to 15thNight
16%5%21%35%18%2
456
16th to endNight
20%5%24%25%21%2
375
June1st to 15thNight
19%5%20%34%17%3
360
16th to endNight
14%122%32%25%2
360
July1st to 15thNight
10%219%40%26%1
340
16th to endNight
9%419%40%22%3
376
August1st to 15thNight
11%310%38%30%5%
434
16th to endNight
7%122%42%23%1
573
September1st to 15thNight
15%418%37%22%1
567
16th to endNight
10%426%40%17%1
664
October1st to 15thNight
15%328%31%18%3
660
16th to endNight
18%427%36%11%1
748
November1st to 15thNight
17%5%29%32%12%2
875
16th to endNight
13%7%31%34%10%2
900
December1st to 15thNight
16%7%28%27%16%3
900
16th to endNight
9%430%34%17%3
960
Page generated at 2017-09-18 06:38:29. Version ac04

Copyright 2017 A. Danko


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