Amherst
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Amherst 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
20%
14916
12%
46606
4%
14871
28%
14871
13%
34245

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
13%
750
11%
2644
2%
746
25%
746
8%
1950
January16th to endNight
16%
774
15%
2532
3%
774
30%
774
12%
1792
February1st to 15thNight
15%
704
9%
2286
2%
701
24%
701
10%
1722
February16th to endNight
15%
631
9%
2051
2%
631
24%
631
9%
1590
March1st to 15thNight
17%
720
15%
1980
5%
720
27%
720
15%
1502
March16th to endNight
18%
768
14%
2112
5%
768
27%
768
15%
1594
April1st to 15thNight
17%
548
12%
1793
5%
548
25%
548
12%
1356
April16th to endNight
20%
540
13%
1620
5%
540
26%
540
15%
1179
May1st to 15thNight
23%
540
9%
1515
4%
540
27%
540
13%
1172
May16th to endNight
18%
561
10%
1496
2%
560
24%
560
16%
1140
June1st to 15thNight
19%
439
6%
1339
3%
439
22%
439
12%
1038
June16th to endNight
26%
360
6%
1260
3%
360
31%
360
13%
927
July1st to 15thNight
29%
340
10%
1190
5%
340
35%
340
19%
771
July16th to endNight
26%
560
7%
1447
4%
527
29%
527
16%
1106
August1st to 15thNight
33%
539
9%
1608
6%
535
35%
535
19%
1153
August16th to endNight
31%
573
13%
1767
6%
573
37%
573
22%
1177
September1st to 15thNight
29%
525
14%
1749
8%
525
36%
525
21%
1141
September16th to endNight
33%
651
20%
1837
12%
651
40%
651
26%
1198
October1st to 15thNight
27%
660
17%
1980
7%
660
35%
660
21%
1338
October16th to endNight
16%
704
13%
2114
3%
704
25%
704
13%
1563
November1st to 15thNight
21%
729
11%
2447
6%
729
27%
729
11%
1858
November16th to endNight
20%
750
12%
2520
6%
750
28%
750
10%
1881
December1st to 15thNight
12%
750
12%
2520
2%
750
23%
750
8%
1915
December16th to endNight
10%
800
10%
2799
1%
800
20%
800
6%
2182

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
19%9%36%5%34446%31%
48023
Night
19%9%36%5%34446%31%
46771
TransparencyDay
54%6%12%18%8%
47706
Night
54%6%12%18%8%
46606
SeeingDay
27%18%30%17%5%
7260
Night
25%17%31%19%5%
14916

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
16%13%5%7%5%25%345%28%
1291
16th to endDay
14%11%47%7%24446%31%
1689
February1st to 15thDay
14%10%5%8%6%3445%6%29%
1575
16th to endDay
14%14%5%8%5%34447%25%
1422
March1st to 15thDay
17%13%5%8%6%33345%28%
1966
16th to endDay
16%12%7%7%5%34335%30%
2112
April1st to 15thDay
19%11%37%6%35%446%26%
2154
16th to endDay
22%9%25%5%34346%31%
2340
May1st to 15thDay
24%10%36%5%23446%27%
2437
16th to endDay
22%10%46%424446%28%
2618
June1st to 15thDay
29%8%35%42445%6%24%
2630
16th to endDay
24%7%36%434447%28%
2700
July1st to 15thDay
15%6%35%4245%6%9%36%
2694
16th to endDay
17%7%36%5%35%5%5%8%30%
2615
August1st to 15thDay
19%5%35%4345%6%7%34%
2340
16th to endDay
13%6%25%4245%6%7%39%
2435
September1st to 15thDay
19%7%245%3445%7%36%
2099
16th to endDay
16%6%24313237%47%
1789
October1st to 15thDay
19%8%25%423347%38%
1650
16th to endDay
21%9%36%6%34446%30%
1757
November1st to 15thDay
21%10%37%5%34245%30%
1411
16th to endDay
17%12%48%5%35%5%45%27%
1440
December1st to 15thDay
22%10%46%5%34346%27%
1440
16th to endDay
22%16%46%5%22335%26%
1419
January1st to 15thNight
17%11%48%5%33345%31%
2644
16th to endNight
16%11%45%6%25%345%35%
2532
February1st to 15thNight
16%12%38%5%35%5%47%28%
2285
16th to endNight
16%15%6%7%6%3444325%
2051
March1st to 15thNight
19%10%46%5%24445%29%
1968
16th to endNight
17%13%5%8%42334432%
2112
April1st to 15thNight
22%12%38%5%24335%26%
1793
16th to endNight
23%10%35%424446%32%
1620
May1st to 15thNight
26%7%35%4345%46%27%
1515
16th to endNight
18%8%46%6%35%5%47%29%
1494
June1st to 15thNight
27%9%27%6%33347%24%
1339
16th to endNight
21%7%35%5%345%6%9%27%
1260
July1st to 15thNight
11%7%36%5%35%5%7%10%33%
1260
16th to endNight
17%6%35%6%47%6%5%9%26%
1545
August1st to 15thNight
17%5%36%6%36%5%5%8%31%
1620
16th to endNight
14%6%26%5%25%46%8%37%
1767
September1st to 15thNight
18%5%25%424448%39%
1749
16th to endNight
13%7%24424345%46%
1837
October1st to 15thNight
19%8%245%23347%38%
1980
16th to endNight
25%9%35%423335%32%
2114
November1st to 15thNight
22%9%27%6%34346%29%
2447
16th to endNight
21%8%47%5%24446%30%
2520
December1st to 15thNight
23%10%46%5%24335%29%
2520
16th to endNight
24%14%5%6%5%2333425%
2799

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
59%6%11%13%7%1
1292
16th to endDay
54%6%10%16%11%
1689
February1st to 15thDay
56%6%12%15%7%1
1578
16th to endDay
59%6%10%13%8%1
1422
March1st to 15thDay
60%410%16%7%1
1977
16th to endDay
58%49%16%8%1
2112
April1st to 15thDay
59%6%7%14%10%1
2154
16th to endDay
54%6%10%17%10%1
2340
May1st to 15thDay
59%6%9%16%8%
2437
16th to endDay
58%6%11%18%5%
2622
June1st to 15thDay
60%7%10%14%5%
2630
16th to endDay
55%7%12%17%6%
2700
July1st to 15thDay
42%8%16%21%9%
2535
16th to endDay
52%8%14%18%5%
2451
August1st to 15thDay
47%9%14%22%7%
2327
16th to endDay
41%9%17%23%7%
2435
September1st to 15thDay
47%7%14%20%10%
2099
16th to endDay
39%411%29%13%1
1789
October1st to 15thDay
47%5%12%27%8%
1650
16th to endDay
55%6%11%17%7%
1757
November1st to 15thDay
56%6%12%14%9%
1411
16th to endDay
60%7%11%13%7%
1440
December1st to 15thDay
60%6%13%16%3
1440
16th to endDay
64%410%12%7%
1419
January1st to 15thNight
57%6%11%16%7%
2644
16th to endNight
52%5%12%21%7%
2532
February1st to 15thNight
57%6%10%13%10%
2286
16th to endNight
61%5%11%11%8%1
2051
March1st to 15thNight
57%410%17%9%1
1980
16th to endNight
57%48%18%9%1
2112
April1st to 15thNight
60%48%14%9%2
1793
16th to endNight
53%5%10%16%12%1
1620
May1st to 15thNight
57%7%10%16%6%
1515
16th to endNight
54%7%12%18%6%
1496
June1st to 15thNight
60%6%12%14%5%
1339
16th to endNight
54%8%13%16%5%
1260
July1st to 15thNight
42%9%18%21%7%
1190
16th to endNight
51%12%14%14%6%
1447
August1st to 15thNight
48%8%15%19%6%
1608
16th to endNight
42%8%19%23%6%
1767
September1st to 15thNight
42%7%13%26%10%
1749
16th to endNight
39%6%14%28%10%
1837
October1st to 15thNight
45%5%13%25%10%
1980
16th to endNight
54%6%10%19%8%
2114
November1st to 15thNight
57%8%11%15%7%
2447
16th to endNight
56%6%12%16%6%
2520
December1st to 15thNight
58%6%10%17%6%
2520
16th to endNight
64%49%13%7%
2799

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
30%21%37%8%2
98
16th to endDay
23%20%39%14%3
256
February1st to 15thDay
24%24%29%15%5%
236
16th to endDay
23%24%31%16%4
212
March1st to 15thDay
26%25%30%13%31
270
16th to endDay
26%29%23%13%42
288
April1st to 15thDay
28%28%28%10%2
270
16th to endDay
24%24%31%13%4
270
May1st to 15thDay
35%19%20%16%5%2
393
16th to endDay
31%18%30%16%3
519
June1st to 15thDay
34%18%25%16%5%
495
16th to endDay
28%12%32%20%5%
495
July1st to 15thDay
20%12%37%19%8%
475
16th to endDay
24%9%35%24%7%
476
August1st to 15thDay
21%12%31%25%8%
313
16th to endDay
19%12%32%25%8%
332
September1st to 15thDay
23%13%37%18%5%1
294
16th to endDay
21%12%33%23%6%2
249
October1st to 15thDay
24%13%30%17%10%3
225
16th to endDay
30%22%24%14%6%1
240
November1st to 15thDay
28%19%28%13%8%2
235
16th to endDay
32%16%25%18%5%1
240
December1st to 15thDay
31%25%32%8%2
240
16th to endDay
33%23%28%11%21
139
January1st to 15thNight
26%25%27%17%3
750
16th to endNight
25%22%32%15%3
774
February1st to 15thNight
25%19%35%15%2
704
16th to endNight
26%21%31%15%31
631
March1st to 15thNight
25%21%30%17%4
720
16th to endNight
26%18%28%18%6%
768
April1st to 15thNight
28%19%31%16%4
548
16th to endNight
29%17%30%17%4
540
May1st to 15thNight
29%13%29%20%6%
540
16th to endNight
24%15%37%20%2
561
June1st to 15thNight
30%10%33%22%3
439
16th to endNight
21%10%34%26%5%
360
July1st to 15thNight
15%347%25%7%
340
16th to endNight
18%8%40%26%5%
560
August1st to 15thNight
19%7%34%26%10%
539
16th to endNight
17%9%37%27%8%
573
September1st to 15thNight
21%9%37%25%5%
525
16th to endNight
17%13%32%27%7%
651
October1st to 15thNight
22%13%32%22%8%
660
16th to endNight
30%21%29%15%3
704
November1st to 15thNight
27%16%29%17%6%1
729
16th to endNight
28%21%24%19%5%
750
December1st to 15thNight
30%23%29%14%2
750
16th to endNight
31%24%25%14%2
800
Page generated at 2017-09-11 20:35:29. Version ac04

Copyright 2017 A. Danko


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