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 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
28%
15592
18%
49088
8%
15560
39%
15560
20%
32482

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
19%
780
19%
2716
7%
776
34%
776
17%
1744
January16th to endNight
16%
832
17%
2912
5%
832
32%
832
15%
1926
February1st to 15thNight
14%
764
15%
2380
3%
761
29%
761
16%
1656
February16th to endNight
17%
683
20%
2049
6%
683
32%
683
16%
1411
March1st to 15thNight
16%
764
17%
2291
6%
764
30%
764
15%
1605
March16th to endNight
21%
768
22%
2010
7%
768
34%
768
22%
1367
April1st to 15thNight
20%
718
16%
1796
3%
718
28%
718
19%
1308
April16th to endNight
23%
629
18%
1709
5%
629
32%
629
21%
1172
May1st to 15thNight
21%
529
16%
1609
7%
529
30%
529
16%
1131
May16th to endNight
34%
375
12%
1497
11%
374
41%
374
16%
981
June1st to 15thNight
35%
360
13%
1440
11%
360
45%
360
16%
946
June16th to endNight
42%
360
12%
1440
12%
360
51%
360
21%
880
July1st to 15thNight
52%
340
13%
1360
17%
340
57%
340
22%
832
July16th to endNight
53%
376
15%
1418
17%
354
57%
354
22%
848
August1st to 15thNight
47%
473
17%
1544
12%
471
53%
471
27%
941
August16th to endNight
55%
764
19%
1910
10%
764
57%
764
32%
1258
September1st to 15thNight
42%
720
20%
1830
9%
720
47%
720
31%
1203
September16th to endNight
48%
717
26%
1980
18%
717
54%
717
32%
1216
October1st to 15thNight
37%
720
21%
2049
12%
720
47%
720
26%
1343
October16th to endNight
26%
768
17%
2466
8%
768
37%
768
20%
1638
November1st to 15thNight
28%
760
20%
2472
10%
760
42%
760
21%
1582
November16th to endNight
23%
780
20%
2568
9%
780
39%
780
18%
1678
December1st to 15thNight
21%
780
15%
2730
5%
780
34%
780
15%
1848
December16th to endNight
16%
832
18%
2912
5%
832
32%
832
15%
1968

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%6%25%43445%7%38%
49548
Night
17%6%25%42445%6%40%
49258
Cloud CoverDay
17%6%25%43445%7%38%
49558
Night
17%6%25%42445%6%40%
49263
TransparencyDay
45%5%11%21%13%1
49236
Night
45%5%11%21%14%1
49088
SeeingDay
23%12%28%21%10%3
6333
Night
22%12%32%24%7%
15592

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%6%35%4335%6%7%37%
1552
16th to endDay
14%8%25%42445%7%40%
1664
February1st to 15thDay
15%7%47%6%35%45%7%32%
1806
16th to endDay
16%8%36%5%33446%37%
1710
March1st to 15thDay
20%8%35%424447%33%
1896
16th to endDay
17%6%44433345%42%
2211
April1st to 15thDay
19%6%45%434346%37%
2152
16th to endDay
17%5%14425%45%6%42%
2250
May1st to 15thDay
23%6%36%424446%32%
2352
16th to endDay
18%5%25%5%345%47%37%
2618
June1st to 15thDay
23%5%3443445%8%33%
2520
16th to endDay
15%4246%36%5%7%9%35%
2520
July1st to 15thDay
12%436%6%36%5%7%8%36%
2520
16th to endDay
13%425%5%35%5%7%8%38%
2654
August1st to 15thDay
12%5%25%6%46%5%6%7%38%
2393
16th to endDay
11%3244345%6%7%44%
2292
September1st to 15thDay
15%4245%2345%5%44%
2130
16th to endDay
14%5%24422436%50%
1980
October1st to 15thDay
19%5%245%34347%40%
1908
16th to endDay
17%6%35%5%34445%39%
1758
November1st to 15thDay
17%6%33333446%41%
1716
16th to endDay
19%5%24423446%42%
1722
December1st to 15thDay
20%6%35%424445%37%
1560
16th to endDay
21%8%34434447%33%
1664
January1st to 15thNight
16%7%35%424445%40%
2716
16th to endNight
14%8%35%5%3445%5%37%
2912
February1st to 15thNight
13%9%36%6%35%45%6%35%
2379
16th to endNight
16%7%37%424445%39%
2049
March1st to 15thNight
19%7%34435%446%36%
2278
16th to endNight
15%6%34423446%43%
2010
April1st to 15thNight
18%7%36%434335%38%
1796
16th to endNight
22%5%24323345%41%
1709
May1st to 15thNight
20%7%25%5%2345%5%36%
1609
16th to endNight
19%425%4335%6%9%35%
1495
June1st to 15thNight
22%5%34433446%37%
1440
16th to endNight
16%42442435%7%42%
1440
July1st to 15thNight
14%424435%46%8%42%
1440
16th to endNight
11%414435%5%7%8%43%
1514
August1st to 15thNight
10%425%5%3335%8%46%
1554
16th to endNight
13%41443445%8%45%
1910
September1st to 15thNight
15%424334446%47%
1830
16th to endNight
13%424223336%51%
1980
October1st to 15thNight
19%42442324446%
2049
16th to endNight
20%6%2442345%5%39%
2466
November1st to 15thNight
17%6%24424345%44%
2472
16th to endNight
18%6%23424445%41%
2568
December1st to 15thNight
21%7%25%423446%36%
2730
16th to endNight
21%7%2442446%5%35%
2912

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
16%7%244245%6%7%38%
1552
16th to endDay
15%6%35%5%2346%5%40%
1664
February1st to 15thDay
16%8%38%5%3445%8%31%
1806
16th to endDay
17%8%46%5%34345%37%
1710
March1st to 15thDay
20%9%36%424346%33%
1906
16th to endDay
18%6%35%5%34335%41%
2211
April1st to 15thDay
19%7%35%424446%37%
2152
16th to endDay
17%6%23424447%42%
2250
May1st to 15thDay
23%7%36%424445%32%
2352
16th to endDay
17%6%25%5%25%45%7%37%
2618
June1st to 15thDay
22%5%35%5%2345%8%33%
2520
16th to endDay
15%5%246%35%5%6%8%34%
2520
July1st to 15thDay
13%5%25%5%35%6%6%9%36%
2520
16th to endDay
13%425%5%35%6%7%7%37%
2654
August1st to 15thDay
12%5%146%45%5%5%8%38%
2393
16th to endDay
11%414435%46%8%44%
2292
September1st to 15thDay
14%5%245%34446%44%
2130
16th to endDay
14%5%14323345%50%
1980
October1st to 15thDay
19%6%14433347%40%
1908
16th to endDay
18%6%35%5%23446%39%
1758
November1st to 15thDay
18%7%24433446%41%
1716
16th to endDay
19%5%23324436%43%
1722
December1st to 15thDay
20%7%35%434435%38%
1560
16th to endDay
22%7%35%425%447%32%
1664
January1st to 15thNight
16%7%34433446%39%
2716
16th to endNight
16%9%25%5%3445%5%37%
2912
February1st to 15thNight
14%8%36%5%35%45%6%35%
2372
16th to endNight
17%8%36%42444438%
2049
March1st to 15thNight
20%8%35%424446%35%
2291
16th to endNight
16%7%34423346%43%
2009
April1st to 15thNight
19%8%35%5%24345%37%
1796
16th to endNight
22%5%24423345%41%
1709
May1st to 15thNight
20%6%35%6%33446%35%
1609
16th to endNight
19%5%235%335%6%8%36%
1495
June1st to 15thNight
22%425%4235%5%5%37%
1440
16th to endNight
16%6%2433445%8%41%
1440
July1st to 15thNight
14%5%14435%5%6%7%41%
1440
16th to endNight
11%335%425%47%9%41%
1514
August1st to 15thNight
10%5%25%6%24448%46%
1554
16th to endNight
12%41443445%8%45%
1910
September1st to 15thNight
14%5%2442345%6%46%
1830
16th to endNight
13%5%24323246%51%
1980
October1st to 15thNight
18%6%24423235%45%
2049
16th to endNight
20%6%25%42345%5%39%
2466
November1st to 15thNight
18%6%25%423345%44%
2472
16th to endNight
18%7%24424446%40%
2568
December1st to 15thNight
21%6%35%424446%35%
2730
16th to endNight
21%7%25%42446%5%35%
2912

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
43%413%24%11%2
1553
16th to endDay
43%5%15%22%12%1
1664
February1st to 15thDay
52%411%18%10%
1809
16th to endDay
49%39%21%13%2
1710
March1st to 15thDay
51%312%19%10%2
1906
16th to endDay
45%39%19%16%4
2211
April1st to 15thDay
49%310%19%14%2
2152
16th to endDay
42%5%8%21%19%3
2250
May1st to 15thDay
54%46%19%12%1
2352
16th to endDay
46%6%12%19%13%1
2622
June1st to 15thDay
50%5%12%20%9%
2520
16th to endDay
45%7%13%20%12%
2520
July1st to 15thDay
43%7%11%21%13%1
2371
16th to endDay
40%7%15%20%12%2
2487
August1st to 15thDay
43%6%13%23%11%1
2379
16th to endDay
36%5%10%27%16%1
2292
September1st to 15thDay
40%5%11%23%17%1
2130
16th to endDay
36%38%24%25%2
1980
October1st to 15thDay
45%49%24%12%3
1908
16th to endDay
45%413%20%13%2
1758
November1st to 15thDay
44%5%13%21%13%1
1716
16th to endDay
43%312%23%15%1
1722
December1st to 15thDay
48%415%17%12%
1560
16th to endDay
51%414%17%10%
1664
January1st to 15thNight
45%412%21%13%2
2716
16th to endNight
48%412%19%13%2
2912
February1st to 15thNight
49%5%13%18%11%1
2380
16th to endNight
48%410%20%12%2
2049
March1st to 15thNight
50%410%20%12%1
2291
16th to endNight
43%49%19%17%5%
2010
April1st to 15thNight
51%38%18%16%1
1796
16th to endNight
46%37%20%16%4
1709
May1st to 15thNight
50%5%8%17%14%3
1609
16th to endNight
45%8%14%19%10%1
1497
June1st to 15thNight
48%5%12%22%9%
1440
16th to endNight
40%6%13%24%14%
1440
July1st to 15thNight
39%7%13%26%13%
1360
16th to endNight
38%8%13%25%12%1
1418
August1st to 15thNight
37%6%13%25%15%1
1544
16th to endNight
36%6%10%27%17%
1910
September1st to 15thNight
38%5%11%24%18%
1830
16th to endNight
35%47%25%24%1
1980
October1st to 15thNight
42%411%23%15%3
2049
16th to endNight
46%413%23%11%
2466
November1st to 15thNight
43%411%20%17%2
2472
16th to endNight
45%412%21%13%1
2568
December1st to 15thNight
49%5%14%20%10%
2730
16th to endNight
49%5%12%19%12%1
2912

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%32%26%13%5%
90
16th to endDay
28%17%39%14%
96
February1st to 15thDay
28%23%31%15%1
234
16th to endDay
24%22%32%18%2
291
March1st to 15thDay
32%16%31%14%4
312
16th to endDay
23%20%34%12%7%2
288
April1st to 15thDay
27%13%32%17%8%
270
16th to endDay
23%14%28%21%6%4
270
May1st to 15thDay
34%14%23%17%9%1
288
16th to endDay
22%9%33%21%11%2
331
June1st to 15thDay
24%12%26%22%11%2
315
16th to endDay
17%14%27%25%11%3
315
July1st to 15thDay
19%8%28%27%12%3
305
16th to endDay
15%5%27%28%18%3
326
August1st to 15thDay
13%6%26%28%18%6%
313
16th to endDay
15%6%24%24%20%9%
332
September1st to 15thDay
21%7%25%24%13%8%
315
16th to endDay
18%6%24%18%15%15%
314
October1st to 15thDay
22%6%26%23%14%6%
315
16th to endDay
23%15%29%22%8%
295
November1st to 15thDay
25%7%31%24%8%3
280
16th to endDay
27%12%30%20%7%1
252
December1st to 15thDay
26%21%35%8%7%
90
16th to endDay
37%20%31%10%
96
January1st to 15thNight
22%17%35%20%2
780
16th to endNight
23%15%39%19%2
832
February1st to 15thNight
21%17%41%17%1
764
16th to endNight
25%17%34%19%2
683
March1st to 15thNight
27%18%33%18%1
764
16th to endNight
22%17%34%20%5%
768
April1st to 15thNight
25%16%34%19%3
718
16th to endNight
27%15%29%20%5%
629
May1st to 15thNight
24%13%35%20%5%
529
16th to endNight
20%7%33%27%9%
375
June1st to 15thNight
25%6%25%31%10%
360
16th to endNight
19%330%37%8%
360
July1st to 15thNight
14%225%43%13%
340
16th to endNight
11%425%40%17%
376
August1st to 15thNight
13%326%42%13%1
473
16th to endNight
14%321%37%20%2
764
September1st to 15thNight
18%429%32%14%
720
16th to endNight
18%5%23%29%18%4
717
October1st to 15thNight
22%8%25%29%11%1
720
16th to endNight
25%12%31%23%7%
768
November1st to 15thNight
21%14%30%24%8%1
760
16th to endNight
21%14%36%19%7%
780
December1st to 15thNight
27%12%35%19%5%
780
16th to endNight
27%16%36%18%1
832
Page generated at 2018-03-12 13:28:04. Version ac04

Copyright 2018 A. Danko


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