Chatham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chatham clear sky chart from 2005-11-05 to 2017-09-18.

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%
15885
20%
46871
13%
15840
47%
15840
26%
30184

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
23%
899
17%
2506
7%
895
32%
895
22%
1737
January16th to endNight
22%
960
15%
2538
4%
960
29%
960
19%
1854
February1st to 15thNight
24%
822
13%
2230
4%
819
30%
819
18%
1617
February16th to endNight
25%
631
19%
1893
7%
631
35%
631
22%
1241
March1st to 15thNight
28%
693
16%
2133
8%
693
35%
693
18%
1474
March16th to endNight
31%
576
14%
2011
8%
576
39%
576
18%
1376
April1st to 15thNight
32%
539
20%
1797
12%
539
45%
539
24%
1104
April16th to endNight
36%
540
21%
1800
15%
540
46%
540
24%
1142
May1st to 15thNight
39%
540
17%
1614
11%
540
46%
540
23%
1103
May16th to endNight
50%
563
20%
1497
13%
561
55%
561
32%
987
June1st to 15thNight
63%
540
19%
1440
12%
540
67%
540
40%
856
June16th to endNight
57%
540
21%
1440
14%
540
61%
540
38%
882
July1st to 15thNight
68%
510
18%
1360
15%
510
69%
510
39%
820
July16th to endNight
74%
564
25%
1427
21%
531
75%
531
44%
816
August1st to 15thNight
78%
539
22%
1685
24%
536
80%
536
39%
912
August16th to endNight
74%
573
22%
1910
25%
573
77%
573
35%
1073
September1st to 15thNight
65%
540
28%
1800
29%
540
68%
540
34%
983
September16th to endNight
61%
633
31%
1812
23%
633
67%
633
38%
1046
October1st to 15thNight
51%
660
29%
1951
20%
660
59%
660
35%
1136
October16th to endNight
34%
704
24%
2164
15%
704
44%
704
27%
1319
November1st to 15thNight
35%
700
27%
2276
17%
700
48%
700
30%
1291
November16th to endNight
29%
759
21%
2379
13%
759
40%
759
24%
1500
December1st to 15thNight
24%
900
13%
2520
6%
900
31%
900
18%
1852
December16th to endNight
20%
960
13%
2688
4%
960
27%
960
15%
2063

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%3445%7%39%
47925
Night
13%6%35%424447%43%
47041
Cloud CoverDay
12%6%35%5%3445%7%39%
47933
Night
13%6%35%424446%43%
47046
TransparencyDay
42%6%12%22%13%2
47613
Night
40%5%12%23%15%2
46872
SeeingDay
13%6%24%27%20%7%
5907
Night
16%8%28%30%14%1
15886

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
14%10%5%9%7%45%445%27%
1432
16th to endDay
13%12%6%7%7%34345%31%
1683
February1st to 15thDay
13%10%49%6%34445%31%
1630
16th to endDay
12%11%47%6%34447%32%
1580
March1st to 15thDay
15%10%46%6%33346%33%
1826
16th to endDay
16%10%36%6%35%5%37%30%
2211
April1st to 15thDay
16%7%35%5%2445%7%38%
2151
16th to endDay
19%7%25%5%345%46%35%
2160
May1st to 15thDay
18%7%25%435%45%8%34%
2332
16th to endDay
12%6%35%6%245%5%7%39%
2617
June1st to 15thDay
8%415%4345%7%8%45%
2520
16th to endDay
8%425%5%36%5%5%8%43%
2520
July1st to 15thDay
8%314435%5%7%9%44%
2520
16th to endDay
5%3135%26%5%8%11%45%
2643
August1st to 15thDay
7%3246%345%7%10%44%
2265
16th to endDay
8%4246%3446%9%45%
2292
September1st to 15thDay
14%42443435%6%46%
2160
16th to endDay
9%435%424347%50%
1861
October1st to 15thDay
14%413433446%48%
1678
16th to endDay
16%5%23423445%46%
1708
November1st to 15thDay
15%5%25%412436%47%
1582
16th to endDay
17%8%46%423335%40%
1578
December1st to 15thDay
18%10%6%7%5%25%5%5%6%27%
1440
16th to endDay
18%10%5%7%6%2445%6%28%
1536
January1st to 15thNight
13%8%36%6%34345%38%
2506
16th to endNight
12%11%6%7%5%35%445%34%
2538
February1st to 15thNight
15%9%5%7%5%34445%34%
2228
16th to endNight
15%8%35%433345%41%
1893
March1st to 15thNight
19%8%35%5%34445%37%
2120
16th to endNight
18%8%45%5%2445%6%35%
2011
April1st to 15thNight
14%6%34424447%44%
1797
16th to endNight
17%6%25%5%24347%41%
1800
May1st to 15thNight
17%6%25%434447%38%
1614
16th to endNight
9%435%5%3446%9%43%
1496
June1st to 15thNight
7%42432445%8%52%
1440
16th to endNight
8%32442446%12%46%
1440
July1st to 15thNight
7%3334245%5%9%50%
1440
16th to endNight
431432446%11%53%
1523
August1st to 15thNight
6%3123246%5%9%54%
1695
16th to endNight
6%213435%5%6%7%52%
1910
September1st to 15thNight
10%424423446%53%
1800
16th to endNight
7%42442435%8%53%
1812
October1st to 15thNight
11%413324347%52%
1951
16th to endNight
16%6%24213345%49%
2164
November1st to 15thNight
12%425%323336%52%
2276
16th to endNight
16%7%24324336%44%
2379
December1st to 15thNight
21%10%36%423346%32%
2520
16th to endNight
20%10%47%5%3444428%
2688

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
14%10%5%9%8%35%445%27%
1432
16th to endDay
13%11%6%8%6%34345%31%
1683
February1st to 15thDay
13%11%5%9%6%3445%5%30%
1630
16th to endDay
11%11%5%6%6%34447%33%
1580
March1st to 15thDay
15%10%47%5%34446%33%
1836
16th to endDay
16%10%46%6%25%447%30%
2209
April1st to 15thDay
15%7%35%425%45%7%38%
2151
16th to endDay
19%6%25%5%345%5%6%35%
2160
May1st to 15thDay
18%6%35%425%45%7%34%
2332
16th to endDay
12%5%35%6%35%46%8%39%
2617
June1st to 15thDay
8%4245%345%6%9%45%
2520
16th to endDay
8%425%5%37%5%5%8%43%
2520
July1st to 15thDay
7%424435%5%7%9%44%
2520
16th to endDay
5%314435%5%8%11%45%
2643
August1st to 15thDay
7%415%6%345%7%10%43%
2265
16th to endDay
8%3246%345%6%9%45%
2292
September1st to 15thDay
13%4245%3435%6%46%
2160
16th to endDay
9%435%32345%6%50%
1861
October1st to 15thDay
14%423423346%48%
1678
16th to endDay
15%6%23423345%46%
1708
November1st to 15thDay
15%425%412446%47%
1582
16th to endDay
17%8%45%422335%40%
1578
December1st to 15thDay
18%10%6%7%5%25%5%5%6%27%
1440
16th to endDay
18%10%5%8%5%2445%6%28%
1536
January1st to 15thNight
13%8%36%6%34445%38%
2506
16th to endNight
13%11%5%7%5%34345%34%
2538
February1st to 15thNight
15%10%5%6%5%35%345%34%
2221
16th to endNight
15%8%345%24346%41%
1893
March1st to 15thNight
19%9%25%5%24445%37%
2133
16th to endNight
18%9%45%5%2345%6%35%
2011
April1st to 15thNight
14%6%24424447%44%
1797
16th to endNight
17%6%25%424347%41%
1800
May1st to 15thNight
17%6%35%425%45%7%38%
1613
16th to endNight
9%5%345%2446%9%43%
1496
June1st to 15thNight
7%42432345%8%52%
1440
16th to endNight
8%425%42446%11%46%
1440
July1st to 15thNight
7%4234245%5%8%50%
1440
16th to endNight
431332347%10%53%
1523
August1st to 15thNight
5%4123246%6%9%53%
1695
16th to endNight
6%2134345%6%7%52%
1910
September1st to 15thNight
10%5%24313446%53%
1800
16th to endNight
7%42442435%7%53%
1812
October1st to 15thNight
11%413423347%52%
1951
16th to endNight
16%6%24213345%49%
2164
November1st to 15thNight
12%415%313336%52%
2276
16th to endNight
16%7%24323336%44%
2379
December1st to 15thNight
21%10%36%423336%32%
2520
16th to endNight
20%11%38%5%3444429%
2688

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
58%49%17%9%
1433
16th to endDay
54%5%13%19%6%
1683
February1st to 15thDay
53%5%13%20%6%
1632
16th to endDay
51%5%13%20%8%
1580
March1st to 15thDay
51%5%13%21%7%
1836
16th to endDay
52%6%14%19%6%
2211
April1st to 15thDay
44%6%10%24%12%1
2151
16th to endDay
47%7%12%18%12%1
2160
May1st to 15thDay
47%8%14%21%8%
2332
16th to endDay
41%9%13%17%15%2
2621
June1st to 15thDay
34%8%13%23%18%1
2520
16th to endDay
35%9%13%21%16%3
2520
July1st to 15thDay
34%8%12%26%13%4
2371
16th to endDay
28%10%16%26%15%
2477
August1st to 15thDay
32%8%14%27%15%1
2251
16th to endDay
33%8%14%21%16%5%
2292
September1st to 15thDay
38%5%8%24%18%5%
2160
16th to endDay
33%5%8%30%18%2
1861
October1st to 15thDay
35%5%10%25%19%2
1678
16th to endDay
38%411%25%16%1
1708
November1st to 15thDay
37%312%22%18%4
1582
16th to endDay
46%412%25%10%
1578
December1st to 15thDay
55%7%13%15%8%
1440
16th to endDay
55%5%11%18%7%1
1536
January1st to 15thNight
47%412%22%13%
2506
16th to endNight
51%5%13%20%8%
2538
February1st to 15thNight
52%411%18%11%1
2230
16th to endNight
44%414%24%11%
1893
March1st to 15thNight
50%410%21%11%1
2133
16th to endNight
48%6%13%21%9%1
2011
April1st to 15thNight
39%410%25%16%2
1797
16th to endNight
43%411%22%16%3
1800
May1st to 15thNight
45%6%11%21%14%
1614
16th to endNight
36%7%15%19%17%3
1498
June1st to 15thNight
28%6%13%28%19%2
1440
16th to endNight
31%5%12%25%19%5%
1440
July1st to 15thNight
29%6%12%28%17%5%
1360
16th to endNight
24%7%16%30%18%2
1427
August1st to 15thNight
24%8%14%27%22%2
1685
16th to endNight
28%8%12%23%20%6%
1910
September1st to 15thNight
31%410%23%23%8%
1800
16th to endNight
28%6%9%25%24%5%
1812
October1st to 15thNight
31%49%26%21%5%
1951
16th to endNight
37%49%25%18%3
2164
November1st to 15thNight
33%410%23%20%6%
2276
16th to endNight
41%311%25%16%
2379
December1st to 15thNight
54%412%17%9%
2520
16th to endNight
58%411%15%8%1
2688

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
26%16%32%17%6%
90
16th to endDay
11%22%40%22%2
96
February1st to 15thDay
20%16%37%20%5%
90
16th to endDay
14%17%32%30%31
81
March1st to 15thDay
27%10%40%17%4
90
16th to endDay
24%17%28%21%7%
195
April1st to 15thDay
18%12%31%25%10%
270
16th to endDay
21%7%27%27%13%2
270
May1st to 15thDay
19%10%28%28%9%3
321
16th to endDay
14%7%32%26%14%4
517
June1st to 15thDay
9%224%30%26%7%
495
16th to endDay
9%329%29%21%5%
495
July1st to 15thDay
11%117%33%24%12%
475
16th to endDay
5%113%33%31%14%
515
August1st to 15thDay
8%11%30%32%15%
418
16th to endDay
7%116%24%34%15%
332
September1st to 15thDay
14%212%27%27%14%
315
16th to endDay
11%414%27%28%13%
262
October1st to 15thDay
11%8%21%28%26%4
118
16th to endDay
18%6%33%31%9%1
96
November1st to 15thDay
6%5%26%25%22%13%
90
16th to endDay
34%16%32%11%5%
90
December1st to 15thDay
15%13%36%24%5%4
90
16th to endDay
29%13%42%14%
96
January1st to 15thNight
19%13%38%23%5%
900
16th to endNight
17%15%36%25%3
960
February1st to 15thNight
19%12%36%27%3
822
16th to endNight
20%11%38%27%2
631
March1st to 15thNight
23%11%33%26%4
693
16th to endNight
22%9%31%26%9%
576
April1st to 15thNight
17%13%33%25%8%
539
16th to endNight
20%5%33%29%10%
540
May1st to 15thNight
18%6%29%34%9%1
540
16th to endNight
9%229%36%19%1
563
June1st to 15thNight
9%120%45%21%
540
16th to endNight
10%124%40%22%1
540
July1st to 15thNight
8%114%41%31%2
510
16th to endNight
412%40%36%4
564
August1st to 15thNight
7%8%41%37%5%
539
16th to endNight
5%12%34%41%4
573
September1st to 15thNight
10%313%37%30%5%
540
16th to endNight
8%417%32%30%5%
633
October1st to 15thNight
13%424%32%22%2
660
16th to endNight
20%9%33%26%9%
704
November1st to 15thNight
13%9%38%26%11%
700
16th to endNight
20%12%33%25%7%
759
December1st to 15thNight
26%11%33%22%6%
900
16th to endNight
26%11%34%23%4
960
Page generated at 2017-09-19 06:32:25. Version ac04

Copyright 2017 A. Danko


Chatham Clear Sky Chart | Charts Home