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 2018-03-11.

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
57%
16149
24%
48971
20%
16105
61%
16105
32%
29146

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
42%
974
25%
2522
13%
970
49%
970
32%
1641
January16th to endNight
40%
1040
26%
2704
12%
1040
46%
1040
32%
1762
February1st to 15thNight
40%
861
23%
2389
11%
858
47%
858
30%
1549
February16th to endNight
31%
612
25%
1978
11%
612
42%
612
24%
1258
March1st to 15thNight
43%
573
24%
2101
20%
573
51%
573
24%
1256
March16th to endNight
52%
576
23%
2112
20%
576
59%
576
31%
1144
April1st to 15thNight
47%
539
25%
1812
19%
539
57%
539
34%
993
April16th to endNight
51%
540
25%
1800
22%
540
59%
540
31%
1038
May1st to 15thNight
54%
539
19%
1800
19%
539
61%
539
31%
1038
May16th to endNight
69%
562
24%
1758
26%
561
71%
561
36%
985
June1st to 15thNight
80%
540
19%
1620
22%
540
82%
540
42%
895
June16th to endNight
75%
540
19%
1620
21%
540
76%
540
33%
975
July1st to 15thNight
81%
510
17%
1530
20%
510
82%
510
37%
886
July16th to endNight
78%
564
17%
1594
20%
531
79%
531
32%
978
August1st to 15thNight
84%
539
17%
1736
21%
536
84%
536
39%
917
August16th to endNight
70%
573
15%
1910
19%
573
70%
573
23%
1259
September1st to 15thNight
76%
540
22%
1800
25%
540
76%
540
31%
1068
September16th to endNight
74%
594
31%
1972
32%
594
75%
594
34%
1106
October1st to 15thNight
71%
720
30%
2160
26%
720
73%
720
43%
1130
October16th to endNight
64%
768
38%
2304
30%
768
69%
768
45%
1196
November1st to 15thNight
59%
760
31%
2280
26%
760
63%
760
39%
1274
November16th to endNight
54%
780
34%
2340
27%
780
61%
780
37%
1321
December1st to 15thNight
43%
865
25%
2425
17%
865
48%
865
27%
1572
December16th to endNight
38%
1040
20%
2704
11%
1040
41%
1040
25%
1905

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%6%35%5%35%5%5%7%39%
49642
Night
11%5%2442445%7%47%
49169
TransparencyDay
42%410%19%17%5%
49343
Night
35%412%20%19%8%
48973
SeeingDay
12%312%22%30%19%
6016
Night
13%318%33%25%5%
16152

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 15thDay
19%5%33422335%45%
1746
16th to endDay
19%8%3442334342%
1872
February1st to 15thDay
18%7%36%42333441%
1800
16th to endDay
16%7%36%5%24446%37%
1791
March1st to 15thDay
19%7%35%434445%38%
2095
16th to endDay
10%6%38%6%34446%41%
2110
April1st to 15thDay
12%6%26%5%34447%41%
2134
16th to endDay
13%5%36%5%36%5%6%6%37%
2160
May1st to 15thDay
10%8%37%5%36%5%5%8%35%
2151
16th to endDay
8%425%7%47%7%7%9%35%
2352
June1st to 15thDay
6%437%9%5%7%5%7%9%33%
2340
16th to endDay
6%5%26%7%5%9%7%9%10%29%
2340
July1st to 15thDay
9%436%7%47%7%8%11%28%
2340
16th to endDay
8%5%36%7%5%8%8%9%10%27%
2466
August1st to 15thDay
6%425%6%46%7%7%11%36%
2211
16th to endDay
11%6%36%6%47%6%8%7%28%
2292
September1st to 15thDay
10%5%35%6%35%5%6%8%38%
2160
16th to endDay
9%5%25%5%45%45%7%44%
1970
October1st to 15thDay
7%423424446%54%
1800
16th to endDay
10%5%1442223458%
1920
November1st to 15thDay
9%435%424437%49%
1908
16th to endDay
13%7%24413345%50%
1950
December1st to 15thDay
16%10%25%5%24345%40%
1862
16th to endDay
21%8%35%5%2335%5%35%
1872
January1st to 15thNight
17%5%24323346%45%
2522
16th to endNight
17%6%2432434446%
2704
February1st to 15thNight
15%7%35%423346%44%
2380
16th to endNight
19%6%25%323335%43%
1978
March1st to 15thNight
16%5%24424446%43%
2101
16th to endNight
11%424323447%50%
2112
April1st to 15thNight
8%415%33345%8%48%
1812
16th to endNight
9%425%43446%7%47%
1800
May1st to 15thNight
8%5%25%5%345%5%8%45%
1799
16th to endNight
6%3345%3445%9%49%
1758
June1st to 15thNight
3313435%5%7%11%51%
1620
16th to endNight
5%333445%6%8%11%43%
1620
July1st to 15thNight
4424325%6%8%12%46%
1620
16th to endNight
6%225%436%6%7%12%41%
1702
August1st to 15thNight
3213335%5%7%13%49%
1746
16th to endNight
8%426%6%45%7%8%9%35%
1910
September1st to 15thNight
7%415%5%35%5%6%8%46%
1800
16th to endNight
7%5%235%345%45%52%
1972
October1st to 15thNight
6%213434436%58%
2160
16th to endNight
10%31321233462%
2304
November1st to 15thNight
9%5%2332335%7%53%
2280
16th to endNight
11%425%322335%55%
2340
December1st to 15thNight
16%7%35%423336%42%
2425
16th to endNight
20%8%36%5%24235%37%
2704

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
42%1419%22%9%
1747
16th to endDay
46%2413%23%9%
1872
February1st to 15thDay
47%25%16%19%8%
1802
16th to endDay
47%36%17%18%7%
1791
March1st to 15thDay
48%37%16%18%6%
2095
16th to endDay
44%27%17%24%4
2112
April1st to 15thDay
41%37%21%18%7%
2134
16th to endDay
43%48%20%17%4
2160
May1st to 15thDay
44%411%17%15%6%
2151
16th to endDay
40%413%26%13%1
2356
June1st to 15thDay
43%413%22%13%1
2340
16th to endDay
43%8%14%21%11%
2340
July1st to 15thDay
43%7%18%20%8%1
2201
16th to endDay
45%7%18%21%6%
2311
August1st to 15thDay
38%5%19%25%9%
2197
16th to endDay
49%6%17%14%9%1
2292
September1st to 15thDay
41%5%15%18%14%4
2160
16th to endDay
37%310%21%20%6%
1970
October1st to 15thDay
30%37%21%21%14%
1800
16th to endDay
30%2216%32%16%
1920
November1st to 15thDay
35%28%19%25%7%
1908
16th to endDay
36%25%14%28%12%
1950
December1st to 15thDay
46%28%19%15%6%
1862
16th to endDay
51%25%17%16%5%
1872
January1st to 15thNight
40%25%16%23%10%
2522
16th to endNight
40%25%17%22%11%
2704
February1st to 15thNight
42%26%18%20%10%
2389
16th to endNight
44%25%18%16%11%
1978
March1st to 15thNight
41%38%15%21%9%
2101
16th to endNight
32%412%20%22%7%
2112
April1st to 15thNight
32%312%23%18%11%
1812
16th to endNight
35%410%22%18%8%
1800
May1st to 15thNight
34%414%18%17%9%
1800
16th to endNight
30%415%26%20%2
1760
June1st to 15thNight
24%7%18%31%16%1
1620
16th to endNight
31%8%18%26%14%
1620
July1st to 15thNight
26%9%28%24%10%1
1530
16th to endNight
30%10%25%24%8%
1594
August1st to 15thNight
24%7%29%26%11%
1736
16th to endNight
39%10%20%15%11%1
1910
September1st to 15thNight
33%7%16%22%13%6%
1800
16th to endNight
32%310%21%20%10%
1972
October1st to 15thNight
26%38%22%24%15%
2160
16th to endNight
25%2415%28%23%
2304
November1st to 15thNight
30%310%19%26%9%
2280
16th to endNight
32%26%15%26%16%
2340
December1st to 15thNight
43%28%19%17%8%
2425
16th to endNight
50%16%17%17%6%
2704

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
22%235%22%14%1
135
16th to endDay
17%8%46%19%7%
144
February1st to 15thDay
23%6%28%32%7%
135
16th to endDay
18%5%40%32%2
120
March1st to 15thDay
24%7%34%28%4
121
16th to endDay
13%8%26%30%17%4
96
April1st to 15thDay
16%6%24%31%18%3
253
16th to endDay
17%5%15%34%24%2
270
May1st to 15thDay
13%415%37%22%6%
288
16th to endDay
12%411%29%29%11%
439
June1st to 15thDay
6%3228%42%16%
495
16th to endDay
8%4321%37%25%
495
July1st to 15thDay
5%2117%40%33%
475
16th to endDay
7%110%41%38%
515
August1st to 15thDay
8%117%41%39%
364
16th to endDay
13%2112%34%36%
332
September1st to 15thDay
8%1411%41%32%
315
16th to endDay
9%312%42%31%
196
October1st to 15thDay
5%11%24%33%25%
135
16th to endDay
13%419%20%26%15%
144
November1st to 15thDay
14%122%33%25%2
135
16th to endDay
11%425%30%20%8%
135
December1st to 15thDay
22%225%26%14%8%
135
16th to endDay
27%10%29%26%5%
144
January1st to 15thNight
19%427%37%10%
975
16th to endNight
20%429%36%7%
1040
February1st to 15thNight
19%7%27%37%7%
861
16th to endNight
22%6%34%31%4
612
March1st to 15thNight
18%428%41%7%
573
16th to endNight
12%227%40%16%
576
April1st to 15thNight
10%5%31%37%13%
539
16th to endNight
12%427%39%15%
540
May1st to 15thNight
10%322%43%19%1
540
16th to endNight
6%212%40%35%3
563
June1st to 15thNight
5%236%48%6%
540
16th to endNight
7%2427%44%13%
540
July1st to 15thNight
41125%51%15%
510
16th to endNight
6%215%54%20%
564
August1st to 15thNight
4118%56%18%
539
16th to endNight
10%1218%48%19%
573
September1st to 15thNight
9%1323%47%15%
540
16th to endNight
9%1228%47%9%
594
October1st to 15thNight
8%110%30%41%6%
720
16th to endNight
12%215%36%27%4
768
November1st to 15thNight
11%218%41%23%2
760
16th to endNight
13%5%20%39%19%1
780
December1st to 15thNight
19%5%24%35%13%
865
16th to endNight
23%5%25%33%10%
1040
Page generated at 2018-03-12 12:28:07. Version ac04

Copyright 2018 A. Danko


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