Chelmsford High School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chelmsford High School clear sky chart from 2005-11-05 to

2017-09-12
.

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
28%
14858
18%
46764
8%
14826
39%
14826
20%
30614

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
18%
720
19%
2506
6%
716
33%
716
16%
1618
January16th to endNight
14%
768
19%
2688
4%
768
33%
768
15%
1731
February1st to 15thNight
17%
704
16%
2200
4%
701
31%
701
16%
1488
February16th to endNight
18%
631
22%
1893
7%
631
37%
631
18%
1251
March1st to 15thNight
19%
720
17%
2157
6%
720
33%
720
16%
1489
March16th to endNight
21%
768
22%
2028
7%
768
34%
768
21%
1386
April1st to 15thNight
20%
718
18%
1796
4%
718
29%
718
22%
1268
April16th to endNight
22%
589
17%
1669
4%
589
31%
589
20%
1145
May1st to 15thNight
25%
540
14%
1620
6%
540
33%
540
19%
1127
May16th to endNight
25%
408
11%
1530
7%
407
33%
407
15%
1021
June1st to 15thNight
29%
360
11%
1440
8%
360
41%
360
15%
966
June16th to endNight
37%
360
11%
1440
12%
360
46%
360
21%
888
July1st to 15thNight
47%
340
12%
1360
15%
340
52%
340
22%
823
July16th to endNight
48%
376
15%
1418
15%
354
54%
354
21%
859
August1st to 15thNight
47%
500
16%
1571
12%
498
51%
498
27%
991
August16th to endNight
51%
720
20%
1866
10%
720
54%
720
33%
1206
September1st to 15thNight
39%
707
20%
1824
8%
707
46%
707
33%
1170
September16th to endNight
46%
657
25%
1815
16%
657
54%
657
35%
1099
October1st to 15thNight
33%
660
22%
1842
10%
660
45%
660
25%
1199
October16th to endNight
24%
704
16%
2288
6%
704
38%
704
20%
1500
November1st to 15thNight
31%
700
20%
2277
11%
700
44%
700
22%
1446
November16th to endNight
25%
720
20%
2340
8%
720
39%
720
18%
1518
December1st to 15thNight
22%
720
17%
2508
5%
720
37%
720
17%
1621
December16th to endNight
16%
768
18%
2688
5%
768
32%
768
15%
1804

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%5%25%43445%6%39%
47905
Night
17%6%24424446%41%
46934
Cloud CoverDay
17%6%25%43445%6%39%
47915
Night
17%6%25%424446%41%
46939
TransparencyDay
43%6%11%21%14%2
47593
Night
43%5%12%21%14%2
46764
SeeingDay
22%13%32%20%9%2
5860
Night
20%12%33%24%7%
14858

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
17%6%25%5%25%446%40%
1432
16th to endDay
16%5%2443435%5%43%
1536
February1st to 15thDay
16%7%25%5%36%5%5%7%34%
1659
16th to endDay
14%9%26%5%34345%39%
1580
March1st to 15thDay
21%7%35%324345%36%
1793
16th to endDay
18%6%26%433445%40%
2193
April1st to 15thDay
17%7%35%5%23446%38%
2152
16th to endDay
19%32443445%7%39%
2289
May1st to 15thDay
24%7%25%5%33446%32%
2332
16th to endDay
17%5%245%4445%7%39%
2594
June1st to 15thDay
24%5%2443445%7%32%
2520
16th to endDay
16%425%5%35%5%6%8%35%
2520
July1st to 15thDay
12%425%5%36%5%7%8%38%
2520
16th to endDay
12%5%25%5%35%6%6%8%38%
2654
August1st to 15thDay
12%425%5%35%5%6%8%38%
2367
16th to endDay
11%4145%35%47%7%44%
2335
September1st to 15thDay
14%5%23423446%46%
2056
16th to endDay
14%424322345%51%
1815
October1st to 15thDay
20%425%423446%40%
1785
16th to endDay
17%5%25%5%2345%5%42%
1584
November1st to 15thDay
20%6%24423345%43%
1581
16th to endDay
17%6%25%423435%44%
1620
December1st to 15thDay
18%5%36%424335%40%
1452
16th to endDay
23%8%24324436%34%
1536
January1st to 15thNight
16%8%345%24346%40%
2506
16th to endNight
15%9%35%423445%40%
2688
February1st to 15thNight
14%9%35%5%345%5%5%37%
2199
16th to endNight
16%9%25%423446%40%
1893
March1st to 15thNight
18%7%26%335%346%37%
2144
16th to endNight
16%7%35%32443444%
2028
April1st to 15thNight
17%7%26%5%24346%38%
1796
16th to endNight
21%6%245%24336%40%
1669
May1st to 15thNight
21%5%25%5%25%446%35%
1620
16th to endNight
20%425%425%448%35%
1528
June1st to 15thNight
23%6%2443335%7%34%
1440
16th to endNight
15%5%344345%48%40%
1440
July1st to 15thNight
12%5%35%43445%8%41%
1440
16th to endNight
11%5%25%5%45%5%6%7%41%
1514
August1st to 15thNight
9%425%5%45%5%6%7%43%
1581
16th to endNight
13%32443445%8%45%
1866
September1st to 15thNight
14%424423346%48%
1824
16th to endNight
14%423313346%52%
1815
October1st to 15thNight
18%6%1342335%5%45%
1842
16th to endNight
20%5%2442445%5%39%
2288
November1st to 15thNight
17%5%24423345%44%
2277
16th to endNight
19%6%345%23346%41%
2340
December1st to 15thNight
19%7%25%42345%5%39%
2508
16th to endNight
19%8%3443335%6%36%
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
17%6%35%425%446%40%
1432
16th to endDay
16%5%2442435%6%43%
1536
February1st to 15thDay
17%7%35%435%5%5%6%34%
1659
16th to endDay
14%9%35%5%35%345%39%
1580
March1st to 15thDay
20%8%35%425%35%5%35%
1803
16th to endDay
18%6%25%423436%40%
2193
April1st to 15thDay
17%7%25%5%34447%37%
2152
16th to endDay
19%415%5%2445%6%39%
2289
May1st to 15thDay
24%8%26%42345%5%32%
2332
16th to endDay
17%5%25%435%35%6%39%
2594
June1st to 15thDay
24%5%145%35%45%7%32%
2520
16th to endDay
16%435%5%35%5%6%7%35%
2520
July1st to 15thDay
12%5%25%5%35%5%7%9%37%
2520
16th to endDay
12%5%25%5%35%5%7%8%38%
2654
August1st to 15thDay
11%5%25%5%45%5%7%9%37%
2367
16th to endDay
11%424435%5%7%7%44%
2335
September1st to 15thDay
14%5%2343345%6%46%
2056
16th to endDay
14%424323245%50%
1815
October1st to 15thDay
20%5%25%423446%40%
1785
16th to endDay
17%5%25%42345%5%42%
1584
November1st to 15thDay
20%7%24323345%43%
1581
16th to endDay
17%6%25%433336%44%
1620
December1st to 15thDay
19%6%36%5%23345%39%
1452
16th to endDay
23%8%25%325%346%34%
1536
January1st to 15thNight
16%8%25%434345%40%
2506
16th to endNight
15%10%25%423445%40%
2688
February1st to 15thNight
14%9%35%5%3445%6%37%
2192
16th to endNight
16%9%25%423436%41%
1893
March1st to 15thNight
18%7%35%426%346%37%
2157
16th to endNight
16%7%35%32433444%
2027
April1st to 15thNight
17%7%35%5%2445%5%38%
1796
16th to endNight
22%5%245%24336%40%
1669
May1st to 15thNight
21%5%35%425%446%36%
1620
16th to endNight
20%425%435%45%7%35%
1528
June1st to 15thNight
23%6%2443335%6%34%
1440
16th to endNight
15%5%3443445%8%40%
1440
July1st to 15thNight
13%5%25%4245%5%9%40%
1440
16th to endNight
11%6%25%5%35%5%5%7%40%
1514
August1st to 15thNight
9%425%5%35%5%6%7%43%
1581
16th to endNight
12%224425%45%8%45%
1866
September1st to 15thNight
14%424423346%48%
1824
16th to endNight
14%5%13323346%52%
1815
October1st to 15thNight
18%6%13423445%45%
1842
16th to endNight
20%6%14424446%39%
2288
November1st to 15thNight
17%5%25%423345%44%
2277
16th to endNight
18%5%345%23346%41%
2340
December1st to 15thNight
19%7%25%433445%39%
2508
16th to endNight
19%8%3442445%6%36%
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
44%6%10%23%11%3
1433
16th to endDay
40%5%12%27%13%1
1536
February1st to 15thDay
47%6%13%18%12%1
1662
16th to endDay
46%5%9%21%14%3
1580
March1st to 15thDay
49%411%19%11%3
1803
16th to endDay
44%49%19%14%6%
2193
April1st to 15thDay
46%410%18%15%3
2152
16th to endDay
43%5%8%20%18%3
2289
May1st to 15thDay
52%5%8%16%13%2
2332
16th to endDay
43%7%12%20%12%2
2598
June1st to 15thDay
50%6%13%19%9%
2520
16th to endDay
43%9%13%18%14%
2520
July1st to 15thDay
42%8%11%21%14%1
2371
16th to endDay
39%9%14%21%12%2
2487
August1st to 15thDay
40%8%15%22%12%1
2353
16th to endDay
35%8%11%25%16%1
2335
September1st to 15thDay
36%6%12%22%20%1
2056
16th to endDay
35%5%8%23%24%2
1815
October1st to 15thDay
43%5%10%23%14%3
1785
16th to endDay
41%6%12%23%14%1
1584
November1st to 15thDay
44%412%23%14%1
1581
16th to endDay
41%5%13%21%16%1
1620
December1st to 15thDay
46%415%16%16%
1452
16th to endDay
50%6%11%19%10%
1536
January1st to 15thNight
45%412%21%12%2
2506
16th to endNight
45%411%20%15%2
2688
February1st to 15thNight
46%5%13%20%11%2
2200
16th to endNight
45%410%22%15%2
1893
March1st to 15thNight
48%5%10%19%13%3
2157
16th to endNight
43%48%17%19%5%
2028
April1st to 15thNight
46%410%18%17%2
1796
16th to endNight
46%5%10%17%16%4
1669
May1st to 15thNight
47%5%11%17%14%3
1620
16th to endNight
46%8%14%18%11%1
1530
June1st to 15thNight
48%6%12%20%9%1
1440
16th to endNight
40%7%13%21%15%1
1440
July1st to 15thNight
38%7%13%25%14%
1360
16th to endNight
40%6%13%26%10%2
1418
August1st to 15thNight
35%9%13%23%15%1
1571
16th to endNight
35%6%12%26%18%
1866
September1st to 15thNight
36%5%13%24%19%
1824
16th to endNight
33%48%24%27%1
1815
October1st to 15thNight
41%412%21%15%3
1842
16th to endNight
44%5%13%23%12%
2288
November1st to 15thNight
41%5%12%20%17%2
2277
16th to endNight
44%5%12%21%14%2
2340
December1st to 15thNight
46%415%21%12%
2508
16th to endNight
48%5%12%21%12%
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
21%26%26%21%3
60
16th to endDay
28%21%37%12%
64
February1st to 15thDay
25%18%36%16%2
216
16th to endDay
20%20%35%17%4
239
March1st to 15thDay
28%19%32%15%3
270
16th to endDay
25%19%34%10%9%1
288
April1st to 15thDay
25%17%32%14%8%
270
16th to endDay
24%14%33%18%6%2
270
May1st to 15thDay
35%14%26%14%6%2
288
16th to endDay
21%10%38%22%6%
331
June1st to 15thDay
24%12%33%19%8%
315
16th to endDay
18%10%42%22%42
315
July1st to 15thDay
19%9%30%27%11%1
305
16th to endDay
13%6%31%32%13%2
326
August1st to 15thDay
15%8%29%27%16%2
313
16th to endDay
12%5%31%25%17%6%
332
September1st to 15thDay
20%8%29%22%14%5%
299
16th to endDay
17%8%27%18%13%13%
254
October1st to 15thDay
24%9%23%20%16%5%
254
16th to endDay
19%16%30%25%7%
240
November1st to 15thDay
28%8%31%18%10%2
235
16th to endDay
24%15%29%18%10%1
240
December1st to 15thDay
22%33%26%13%4
72
16th to endDay
25%28%29%17%
64
January1st to 15thNight
22%17%38%19%2
720
16th to endNight
23%17%37%18%2
768
February1st to 15thNight
19%18%39%21%1
704
16th to endNight
22%19%35%18%3
631
March1st to 15thNight
24%18%35%18%3
720
16th to endNight
20%18%35%19%6%
768
April1st to 15thNight
21%17%37%18%4
718
16th to endNight
24%16%33%19%6%
589
May1st to 15thNight
24%12%34%22%5%
540
16th to endNight
21%8%40%24%5%
408
June1st to 15thNight
24%5%35%26%7%
360
16th to endNight
16%437%33%6%
360
July1st to 15thNight
13%329%41%11%
340
16th to endNight
12%328%42%12%
376
August1st to 15thNight
11%430%42%11%
500
16th to endNight
13%425%37%17%1
720
September1st to 15thNight
17%5%32%29%14%
707
16th to endNight
15%5%27%27%20%2
657
October1st to 15thNight
23%10%28%25%11%1
660
16th to endNight
25%11%33%23%5%
704
November1st to 15thNight
20%12%31%25%10%1
700
16th to endNight
22%15%31%21%7%1
720
December1st to 15thNight
24%15%33%21%4
720
16th to endNight
24%18%36%17%2
768
Page generated at 2017-09-13 07:39:07. Version ac04

Copyright 2017 A. Danko


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