Alexandria
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alexandria clear sky chart from 2005-11-05 to 2017-09-10.

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
37%
16146
15%
46661
9%
16099
43%
16099
24%
31634

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
26%
900
12%
2685
5%
895
33%
895
17%
1965
January16th to endNight
21%
960
12%
2541
4%
960
28%
960
17%
1947
February1st to 15thNight
35%
879
12%
2288
4%
875
40%
875
22%
1647
February16th to endNight
32%
768
14%
2030
6%
768
38%
768
22%
1461
March1st to 15thNight
33%
567
13%
2007
9%
567
38%
567
18%
1388
March16th to endNight
34%
576
15%
2112
13%
576
41%
576
16%
1458
April1st to 15thNight
33%
539
16%
1791
11%
539
43%
539
23%
1167
April16th to endNight
31%
540
16%
1620
11%
540
40%
540
20%
1140
May1st to 15thNight
34%
540
14%
1545
7%
540
39%
540
23%
1067
May16th to endNight
42%
563
12%
1497
5%
561
46%
561
30%
998
June1st to 15thNight
47%
540
14%
1273
5%
540
51%
540
36%
832
June16th to endNight
56%
540
15%
1260
7%
540
59%
540
46%
766
July1st to 15thNight
60%
510
17%
1190
8%
510
62%
510
48%
715
July16th to endNight
65%
564
20%
1422
10%
531
67%
531
49%
777
August1st to 15thNight
64%
539
21%
1610
17%
536
68%
536
39%
900
August16th to endNight
56%
573
24%
1728
22%
573
64%
573
40%
950
September1st to 15thNight
55%
525
21%
1780
19%
525
63%
525
35%
958
September16th to endNight
43%
492
21%
1815
15%
492
54%
492
29%
995
October1st to 15thNight
35%
657
21%
1977
14%
657
41%
657
27%
1247
October16th to endNight
32%
739
15%
2147
7%
739
38%
739
20%
1500
November1st to 15thNight
33%
875
19%
2417
9%
875
41%
875
24%
1631
November16th to endNight
29%
900
15%
2520
9%
900
33%
900
19%
1821
December1st to 15thNight
24%
900
8%
2526
4%
900
27%
900
15%
1950
December16th to endNight
19%
960
10%
2880
3%
960
27%
960
10%
2354

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
12%8%36%5%3445%7%36%
47975
Night
11%8%36%5%3445%7%38%
46825
Cloud CoverDay
12%8%36%5%35%45%7%36%
47985
Night
11%8%36%5%3445%7%38%
46828
Cloud CoverDay
12%8%36%5%35%45%7%36%
47986
Night
11%8%36%5%3445%7%38%
46829
Cloud CoverDay
12%8%36%5%3445%7%36%
47984
Night
11%8%36%5%3445%7%38%
46829
TransparencyDay
45%6%14%22%9%
47656
Night
44%6%14%23%10%1
46662
SeeingDay
15%7%30%27%16%3
6142
Night
15%8%32%31%11%1
16147

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
7%13%5%11%5%5%7%6%6%8%22%
1253
16th to endDay
9%13%6%11%9%46%6%6%7%20%
1680
February1st to 15thDay
7%10%5%8%7%48%5%6%8%27%
1575
16th to endDay
8%11%410%9%5%5%7%6%8%21%
1455
March1st to 15thDay
14%8%48%7%34446%31%
1942
16th to endDay
15%11%5%8%6%3446%6%28%
2112
April1st to 15thDay
13%9%35%5%3445%7%38%
2155
16th to endDay
22%11%35%434446%30%
2340
May1st to 15thDay
20%8%46%5%24446%31%
2403
16th to endDay
15%6%35%5%35%46%8%35%
2617
June1st to 15thDay
15%7%36%6%35%46%7%35%
2685
16th to endDay
8%425%5%25%5%6%9%43%
2700
July1st to 15thDay
6%3234345%6%9%50%
2700
16th to endDay
5%314425%6%6%9%50%
2644
August1st to 15thDay
9%5%245%35%5%7%8%42%
2340
16th to endDay
12%5%1432345%8%48%
2474
September1st to 15thDay
10%42342435%7%50%
2064
16th to endDay
14%5%1442345%7%46%
1807
October1st to 15thDay
15%9%2442445%7%40%
1650
16th to endDay
20%10%35%423346%34%
1724
November1st to 15thDay
11%11%5%6%424446%38%
1440
16th to endDay
14%10%5%8%9%35%25%6%27%
1440
December1st to 15thDay
12%12%5%11%7%47%45%6%22%
1431
16th to endDay
14%14%7%12%9%34345%20%
1344
January1st to 15thNight
8%9%5%10%8%47%5%5%7%27%
2685
16th to endNight
9%12%5%9%8%36%5%6%8%23%
2541
February1st to 15thNight
6%11%6%8%5%45%46%8%32%
2285
16th to endNight
7%10%38%5%46%5%6%7%32%
2030
March1st to 15thNight
11%9%48%6%3445%5%35%
1995
16th to endNight
15%8%48%5%25%45%6%33%
2112
April1st to 15thNight
14%8%36%434445%42%
1791
16th to endNight
19%9%36%6%23345%35%
1620
May1st to 15thNight
17%7%35%43335%6%38%
1545
16th to endNight
10%5%245%26%6%5%8%42%
1496
June1st to 15thNight
13%41232336%9%47%
1273
16th to endNight
7%2133145%6%10%53%
1260
July1st to 15thNight
43123235%6%8%57%
1260
16th to endNight
321232335%10%60%
1522
August1st to 15thNight
7%31332346%8%54%
1620
16th to endNight
7%313313347%58%
1728
September1st to 15thNight
9%324322347%55%
1780
16th to endNight
11%5%23323346%52%
1815
October1st to 15thNight
11%8%25%5%23346%45%
1977
16th to endNight
17%10%35%42335%6%36%
2147
November1st to 15thNight
10%8%36%5%3446%6%40%
2417
16th to endNight
12%9%47%7%3446%6%33%
2520
December1st to 15thNight
12%11%5%9%8%45%5%5%7%25%
2526
16th to endNight
14%15%7%10%8%4445%6%18%
2880

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
7%11%6%10%7%5%6%6%6%7%22%
1254
16th to endDay
9%12%6%11%9%47%6%6%7%20%
1680
February1st to 15thDay
7%11%5%8%7%47%6%5%8%26%
1575
16th to endDay
8%11%5%8%9%6%5%6%6%7%22%
1455
March1st to 15thDay
14%8%49%8%3445%7%31%
1953
16th to endDay
15%11%5%9%6%2445%6%27%
2110
April1st to 15thDay
14%9%25%4245%47%38%
2155
16th to endDay
22%12%35%43444431%
2340
May1st to 15thDay
20%9%36%5%34446%32%
2403
16th to endDay
15%6%25%5%25%46%8%35%
2617
June1st to 15thDay
16%6%35%5%35%5%5%7%35%
2685
16th to endDay
8%4244346%6%9%44%
2700
July1st to 15thDay
6%323435%5%5%9%51%
2700
16th to endDay
5%313435%5%6%9%51%
2644
August1st to 15thDay
9%425%445%5%7%8%42%
2340
16th to endDay
12%42422335%7%50%
2474
September1st to 15thDay
11%423424348%50%
2064
16th to endDay
13%5%1342435%8%46%
1807
October1st to 15thDay
15%9%34424446%41%
1650
16th to endDay
20%10%36%423345%35%
1724
November1st to 15thDay
11%11%47%42445%5%38%
1440
16th to endDay
15%11%47%9%44346%27%
1440
December1st to 15thDay
11%11%7%9%8%46%5%5%6%23%
1431
16th to endDay
14%14%7%13%8%35%345%20%
1344
January1st to 15thNight
8%9%5%9%8%46%5%6%7%26%
2685
16th to endNight
9%11%5%10%8%45%5%6%8%23%
2541
February1st to 15thNight
6%11%6%8%5%36%46%8%32%
2278
16th to endNight
7%9%47%7%46%6%6%7%33%
2030
March1st to 15thNight
11%8%49%6%35%45%6%34%
2007
16th to endNight
14%8%5%8%6%2435%6%33%
2110
April1st to 15thNight
14%8%35%423446%41%
1791
16th to endNight
18%10%45%5%23335%36%
1620
May1st to 15thNight
17%7%25%5%2345%6%38%
1544
16th to endNight
10%6%24435%47%8%42%
1497
June1st to 15thNight
13%5%1322347%9%47%
1273
16th to endNight
7%1132235%6%10%54%
1260
July1st to 15thNight
42124245%6%7%58%
1260
16th to endNight
32133223411%60%
1522
August1st to 15thNight
7%31432445%8%53%
1620
16th to endNight
8%313313337%59%
1728
September1st to 15thNight
9%31431335%7%56%
1780
16th to endNight
10%5%2332335%5%53%
1815
October1st to 15thNight
12%6%345%23446%46%
1977
16th to endNight
17%10%35%423346%37%
2147
November1st to 15thNight
10%8%36%5%245%5%5%41%
2417
16th to endNight
12%9%47%7%3445%7%33%
2520
December1st to 15thNight
11%11%5%9%9%35%5%5%6%25%
2526
16th to endNight
14%15%6%10%9%35%5%5%6%18%
2880

Percentage Cloud Cover SW (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
7%12%5%11%5%5%7%6%5%7%23%
1253
16th to endDay
9%13%6%10%9%46%6%6%6%20%
1680
February1st to 15thDay
7%10%5%9%7%48%5%6%8%27%
1575
16th to endDay
9%11%410%9%5%5%7%6%8%21%
1455
March1st to 15thDay
14%8%48%7%34446%31%
1953
16th to endDay
15%11%5%8%6%3446%6%28%
2112
April1st to 15thDay
13%9%35%5%3445%7%38%
2155
16th to endDay
22%11%35%434446%30%
2340
May1st to 15thDay
20%8%46%5%24446%31%
2403
16th to endDay
15%6%345%35%46%8%35%
2617
June1st to 15thDay
15%6%36%5%35%45%7%35%
2685
16th to endDay
8%425%5%25%5%7%9%43%
2700
July1st to 15thDay
6%313435%5%6%9%50%
2700
16th to endDay
5%3134346%6%9%50%
2644
August1st to 15thDay
8%5%24435%5%6%8%42%
2340
16th to endDay
12%5%1432345%8%48%
2474
September1st to 15thDay
10%42342335%8%50%
2064
16th to endDay
14%5%1442345%7%47%
1807
October1st to 15thDay
15%9%2442435%7%40%
1650
16th to endDay
20%10%35%423346%34%
1724
November1st to 15thDay
11%11%5%6%424446%37%
1440
16th to endDay
14%10%5%8%9%35%25%6%27%
1440
December1st to 15thDay
12%12%5%11%7%47%5%5%6%22%
1431
16th to endDay
14%13%7%12%9%34335%20%
1344
January1st to 15thNight
8%9%5%10%8%47%5%5%7%27%
2685
16th to endNight
9%11%6%9%8%36%5%6%8%23%
2541
February1st to 15thNight
6%11%6%7%5%45%5%6%8%32%
2278
16th to endNight
7%9%48%5%46%5%6%7%32%
2030
March1st to 15thNight
11%9%48%6%35%45%5%34%
2007
16th to endNight
15%8%48%5%35%45%6%33%
2112
April1st to 15thNight
14%8%35%434445%42%
1791
16th to endNight
19%9%45%6%23345%35%
1620
May1st to 15thNight
17%7%35%43335%7%38%
1544
16th to endNight
10%6%245%35%6%5%8%42%
1496
June1st to 15thNight
13%41232346%9%47%
1273
16th to endNight
7%2123145%6%9%53%
1260
July1st to 15thNight
43123235%6%8%57%
1260
16th to endNight
321233335%10%60%
1522
August1st to 15thNight
7%31332346%8%53%
1620
16th to endNight
7%313323347%58%
1728
September1st to 15thNight
9%324322347%55%
1780
16th to endNight
11%6%23322446%52%
1815
October1st to 15thNight
11%8%15%5%24346%45%
1977
16th to endNight
17%10%35%42345%6%36%
2147
November1st to 15thNight
10%8%36%5%3446%6%40%
2417
16th to endNight
12%9%48%7%345%5%6%33%
2520
December1st to 15thNight
12%12%5%9%8%35%5%5%7%25%
2526
16th to endNight
14%15%7%10%8%4445%6%18%
2880

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
7%13%5%11%5%5%7%6%6%8%22%
1253
16th to endDay
9%13%6%10%9%46%6%7%7%20%
1680
February1st to 15thDay
7%10%5%8%7%48%5%6%8%27%
1575
16th to endDay
9%10%410%9%5%5%6%6%8%21%
1455
March1st to 15thDay
14%8%48%7%34446%31%
1953
16th to endDay
15%11%5%8%5%3446%6%28%
2110
April1st to 15thDay
14%9%35%5%3445%7%38%
2155
16th to endDay
22%11%35%424445%30%
2340
May1st to 15thDay
20%8%46%5%24446%31%
2403
16th to endDay
15%6%345%35%5%6%8%35%
2617
June1st to 15thDay
15%6%36%5%4446%7%34%
2685
16th to endDay
8%425%5%25%5%6%9%43%
2700
July1st to 15thDay
6%3234345%6%9%50%
2700
16th to endDay
5%314425%6%6%8%50%
2644
August1st to 15thDay
9%5%245%35%5%7%8%42%
2340
16th to endDay
12%5%1442335%8%48%
2474
September1st to 15thDay
10%42343335%7%50%
2064
16th to endDay
14%5%2342345%7%47%
1807
October1st to 15thDay
15%9%2442435%6%40%
1650
16th to endDay
20%10%35%433346%34%
1724
November1st to 15thDay
11%10%5%7%434446%38%
1440
16th to endDay
14%10%6%8%8%35%25%6%27%
1440
December1st to 15thDay
12%12%5%11%6%47%5%5%6%21%
1431
16th to endDay
14%14%6%13%9%34345%20%
1344
January1st to 15thNight
8%9%5%10%8%47%5%5%8%27%
2685
16th to endNight
9%11%5%9%8%45%5%6%8%23%
2541
February1st to 15thNight
6%11%6%7%5%45%46%8%32%
2278
16th to endNight
7%10%47%6%46%5%6%7%32%
2030
March1st to 15thNight
11%9%48%6%35%45%6%34%
2007
16th to endNight
15%8%47%5%3445%6%33%
2112
April1st to 15thNight
14%8%35%424445%42%
1791
16th to endNight
19%9%45%6%23345%35%
1620
May1st to 15thNight
17%7%35%43335%7%38%
1544
16th to endNight
10%6%145%35%6%5%8%42%
1496
June1st to 15thNight
13%41232336%9%47%
1273
16th to endNight
7%2133145%6%9%54%
1260
July1st to 15thNight
43123245%6%8%57%
1260
16th to endNight
32123233410%60%
1522
August1st to 15thNight
7%41332346%8%54%
1620
16th to endNight
7%413323347%58%
1728
September1st to 15thNight
9%324313346%56%
1780
16th to endNight
11%6%23322346%52%
1815
October1st to 15thNight
11%8%25%5%23346%45%
1977
16th to endNight
17%10%35%42335%6%36%
2147
November1st to 15thNight
10%8%36%5%3445%6%40%
2417
16th to endNight
12%9%48%7%3446%6%33%
2520
December1st to 15thNight
12%12%410%8%45%5%5%7%25%
2526
16th to endNight
14%15%7%10%9%35%45%6%18%
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%5%11%18%6%
1254
16th to endDay
60%6%13%15%3
1680
February1st to 15thDay
53%7%15%17%5%
1576
16th to endDay
57%7%10%19%4
1455
March1st to 15thDay
52%6%13%20%6%
1953
16th to endDay
54%7%15%19%3
2112
April1st to 15thDay
44%5%14%21%12%1
2155
16th to endDay
55%5%13%14%9%1
2340
May1st to 15thDay
53%6%10%22%6%
2403
16th to endDay
44%8%14%23%8%
2621
June1st to 15thDay
46%7%14%22%7%
2685
16th to endDay
34%8%18%26%11%
2700
July1st to 15thDay
28%7%16%29%15%1
2541
16th to endDay
27%9%19%27%13%2
2481
August1st to 15thDay
35%8%15%27%10%2
2326
16th to endDay
33%6%15%29%13%2
2474
September1st to 15thDay
32%5%19%31%10%
2064
16th to endDay
36%5%16%32%8%
1807
October1st to 15thDay
43%5%14%23%11%1
1650
16th to endDay
51%5%13%21%8%
1724
November1st to 15thDay
46%5%12%21%12%1
1440
16th to endDay
57%5%15%12%7%1
1440
December1st to 15thDay
61%6%13%13%4
1431
16th to endDay
67%39%12%6%
1344
January1st to 15thNight
54%5%11%19%8%
2685
16th to endNight
56%5%13%17%6%
2541
February1st to 15thNight
47%6%15%20%9%
2288
16th to endNight
47%7%14%19%10%
2030
March1st to 15thNight
49%6%14%20%8%
2007
16th to endNight
50%6%14%22%5%
2112
April1st to 15thNight
44%5%12%24%11%1
1791
16th to endNight
51%49%19%12%2
1620
May1st to 15thNight
46%5%12%23%10%1
1545
16th to endNight
37%8%15%24%12%2
1498
June1st to 15thNight
32%8%16%29%12%1
1273
16th to endNight
24%7%18%29%17%3
1260
July1st to 15thNight
22%7%16%33%16%2
1190
16th to endNight
20%6%16%34%17%4
1422
August1st to 15thNight
26%7%15%32%13%4
1610
16th to endNight
25%6%14%30%19%3
1728
September1st to 15thNight
28%5%16%32%15%
1780
16th to endNight
33%415%33%12%
1815
October1st to 15thNight
39%412%27%13%2
1977
16th to endNight
47%6%13%22%9%
2147
November1st to 15thNight
42%6%12%23%12%1
2417
16th to endNight
49%7%17%16%7%2
2520
December1st to 15thNight
57%6%13%16%5%
2526
16th to endNight
65%6%9%12%5%
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
14%11%36%22%13%2
90
16th to endDay
14%7%37%28%12%
96
February1st to 15thDay
8%21%50%15%4
90
16th to endDay
17%12%34%27%8%
81
March1st to 15thDay
17%14%31%28%7%
90
16th to endDay
19%10%29%25%9%6%
96
April1st to 15thDay
17%13%32%23%12%
274
16th to endDay
30%7%28%20%11%2
450
May1st to 15thDay
21%10%31%24%10%1
468
16th to endDay
20%7%34%24%10%3
517
June1st to 15thDay
14%7%31%28%16%1
495
16th to endDay
8%6%38%26%16%3
495
July1st to 15thDay
7%426%31%25%4
475
16th to endDay
6%325%35%25%4
515
August1st to 15thDay
11%122%33%26%3
493
16th to endDay
13%6%22%28%23%6%
514
September1st to 15thDay
11%5%27%36%16%3
261
16th to endDay
24%430%22%15%3
90
October1st to 15thDay
10%12%37%24%10%5%
90
16th to endDay
27%6%35%14%13%3
96
November1st to 15thDay
14%228%41%11%2
90
16th to endDay
40%5%34%16%3
90
December1st to 15thDay
15%445%26%7%
90
16th to endDay
26%10%37%21%4
96
January1st to 15thNight
12%11%38%27%8%1
900
16th to endNight
15%11%40%26%5%
960
February1st to 15thNight
11%9%34%32%10%1
880
16th to endNight
12%9%37%28%10%1
768
March1st to 15thNight
16%8%34%31%9%
567
16th to endNight
18%7%33%28%10%1
576
April1st to 15thNight
19%7%35%28%9%
539
16th to endNight
24%7%30%27%8%2
540
May1st to 15thNight
19%8%33%29%7%
540
16th to endNight
12%7%33%33%13%
563
June1st to 15thNight
15%429%38%12%
540
16th to endNight
7%330%43%15%
540
July1st to 15thNight
7%226%40%21%1
510
16th to endNight
4123%46%21%2
564
August1st to 15thNight
9%118%44%23%2
539
16th to endNight
9%426%34%23%2
573
September1st to 15thNight
10%325%40%17%2
525
16th to endNight
13%8%31%27%16%2
492
October1st to 15thNight
19%8%32%28%10%
657
16th to endNight
23%7%32%29%6%
739
November1st to 15thNight
15%11%34%29%8%1
875
16th to endNight
16%15%33%25%9%
900
December1st to 15thNight
17%11%35%28%6%
900
16th to endNight
22%9%33%26%6%1
960
Page generated at 2017-09-11 18:35:20. Version ac04

Copyright 2017 A. Danko


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