This page made possible by the sponsorship of: Jeff Alfeld.

Tomstown
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tomstown clear sky chart from 2005-11-06 to 2024-02-20.

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
33%
23485
19%
80209
10%
23379
44%
23379
19%
51369

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 10thNight
14%
755
18%
4307
6%
737
34%
737
8%
2615
January11th to 20thNight
17%
760
21%
4002
5%
740
34%
740
9%
2494
January21st to endNight
15%
831
13%
4480
5%
814
34%
814
9%
3031
February1st to 10thNight
19%
760
18%
2470
7%
760
33%
760
16%
1633
February11th to 20thNight
11%
742
17%
2382
2%
739
29%
739
12%
1604
February21st to endNight
13%
592
20%
1776
5%
592
30%
592
13%
1223
March1st to 10thNight
18%
720
23%
1999
7%
720
35%
720
21%
1276
March11th to 20thNight
20%
720
20%
1980
7%
720
33%
720
17%
1341
March21st to endNight
17%
792
19%
2156
6%
792
29%
792
19%
1483
April1st to 10thNight
18%
720
15%
1800
3%
720
27%
720
17%
1321
April11th to 20thNight
30%
711
19%
1775
6%
711
36%
711
26%
1183
April21st to endNight
27%
720
18%
1800
4%
720
35%
720
24%
1235
May1st to 10thNight
37%
541
17%
1621
9%
541
43%
541
21%
1084
May11th to 20thNight
37%
410
16%
1472
11%
410
44%
410
18%
971
May21st to endNight
43%
388
16%
1551
14%
388
51%
388
20%
942
June1st to 10thNight
49%
360
20%
1440
19%
360
58%
360
24%
821
June11th to 20thNight
48%
360
16%
1440
16%
360
54%
360
20%
883
June21st to endNight
56%
360
18%
1440
19%
360
64%
360
26%
811
July1st to 10thNight
58%
350
14%
1400
17%
350
62%
350
21%
845
July11th to 20thNight
64%
400
18%
1444
17%
400
67%
400
29%
801
July21st to endNight
63%
591
17%
1672
12%
558
68%
558
33%
1002
August1st to 10thNight
64%
537
17%
1600
14%
534
66%
534
30%
1004
August11th to 20thNight
61%
573
19%
1637
14%
573
63%
573
28%
1080
August21st to endNight
60%
792
20%
1980
11%
792
62%
792
34%
1305
September1st to 10thNight
50%
720
22%
1800
10%
720
55%
720
32%
1188
September11th to 20thNight
55%
720
30%
1800
20%
720
60%
720
39%
1107
September21st to endNight
44%
720
21%
2074
15%
720
51%
720
24%
1359
October1st to 10thNight
45%
720
18%
2160
15%
720
51%
720
26%
1383
October11th to 20thNight
37%
720
23%
2160
15%
720
50%
720
26%
1340
October21st to endNight
31%
792
26%
2376
14%
792
45%
792
24%
1498
November1st to 10thNight
35%
740
27%
2320
15%
740
50%
740
28%
1347
November11th to 20thNight
26%
760
23%
2546
11%
760
42%
760
19%
1553
November21st to endNight
25%
760
21%
2660
9%
760
42%
760
17%
1613
December1st to 10thNight
18%
760
16%
2660
5%
760
32%
760
13%
1801
December11th to 20thNight
20%
760
17%
3207
5%
759
35%
759
13%
2019
December21st to endNight
20%
828
12%
4822
6%
817
34%
817
8%
3173

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
14%7%36%6%35%5%6%7%38%
78403
Night
14%7%36%5%35%5%5%7%40%
80789
TransparencyDay
44%6%12%22%14%2
78113
Night
43%6%12%23%14%2
80598
SeeingDay
22%17%27%20%11%4
13858
Night
19%16%27%26%11%1
23504

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 10thDay
15%8%38%5%36%446%37%
2558
11th to 20thDay
13%13%5%9%7%5%5%5%5%5%29%
2840
21st to endDay
17%13%47%6%35%5%5%6%30%
3124
February1st to 10thDay
14%9%47%7%45%5%5%7%35%
1710
11th to 20thDay
15%8%37%8%45%45%6%36%
1669
21st to endDay
19%10%47%434345%36%
1480
March1st to 10thDay
12%7%35%5%35%45%6%44%
1957
11th to 20thDay
16%9%36%5%45%45%6%36%
1980
21st to endDay
18%9%37%6%35%45%7%34%
2200
April1st to 10thDay
14%8%37%5%35%5%6%7%36%
2160
11th to 20thDay
14%6%36%5%3445%7%42%
2134
21st to endDay
19%6%36%5%345%5%6%38%
2173
May1st to 10thDay
17%7%36%6%35%5%6%8%34%
2340
11th to 20thDay
19%6%46%5%35%5%6%7%35%
2419
21st to endDay
13%7%26%6%35%5%6%8%37%
2725
June1st to 10thDay
12%5%35%5%46%5%7%10%39%
2520
11th to 20thDay
12%6%35%6%36%7%6%9%37%
2520
21st to endDay
11%435%5%35%5%6%10%43%
2520
July1st to 10thDay
9%5%26%6%37%6%7%9%40%
2520
11th to 20thDay
7%5%346%46%6%7%11%42%
2464
21st to endDay
9%6%47%6%46%6%7%9%37%
2547
August1st to 10thDay
10%6%36%6%47%7%7%10%35%
2327
11th to 20thDay
11%6%35%5%45%5%7%9%39%
2245
21st to endDay
11%5%35%6%36%5%7%8%40%
2376
September1st to 10thDay
16%5%25%5%3445%5%45%
2160
11th to 20thDay
12%4244335%7%9%47%
2160
21st to endDay
17%7%35%5%4445%7%39%
1882
October1st to 10thDay
14%5%25%5%35%5%5%7%44%
1800
11th to 20thDay
16%6%245%345%5%7%43%
1800
21st to endDay
15%7%25%5%235%6%6%43%
1980
November1st to 10thDay
11%6%45%42445%7%48%
1758
11th to 20thDay
15%5%25%6%3445%6%43%
1633
21st to endDay
15%6%35%6%345%5%7%39%
1520
December1st to 10thDay
18%8%46%6%346%7%8%30%
1520
11th to 20thDay
15%9%37%6%35%5%47%35%
1910
21st to endDay
18%9%37%6%5%6%6%6%7%28%
2772
January1st to 10thNight
12%9%38%7%45%5%6%7%35%
4383
11th to 20thNight
16%8%47%6%35%5%45%37%
4120
21st to endNight
20%11%47%6%3445%6%31%
4532
February1st to 10thNight
15%7%47%6%35%46%7%37%
2470
11th to 20thNight
17%8%36%5%35%5%5%7%35%
2380
21st to endNight
17%10%47%5%25%5%5%6%35%
1776
March1st to 10thNight
14%6%25%5%345%6%6%44%
1999
11th to 20thNight
17%7%35%5%245%5%7%40%
1980
21st to endNight
16%6%38%5%3445%6%39%
2156
April1st to 10thNight
15%7%48%8%45%5%5%6%34%
1800
11th to 20thNight
12%7%26%5%3446%9%42%
1775
21st to endNight
16%6%345%35%5%6%6%41%
1800
May1st to 10thNight
16%7%35%5%445%5%8%38%
1621
11th to 20thNight
16%7%46%6%35%5%5%8%36%
1471
21st to endNight
13%6%35%5%35%5%7%9%41%
1550
June1st to 10thNight
11%5%25%4345%5%9%47%
1440
11th to 20thNight
12%5%36%5%45%5%7%9%39%
1440
21st to endNight
8%5%25%6%45%5%6%11%44%
1440
July1st to 10thNight
9%5%35%5%36%6%6%10%41%
1440
11th to 20thNight
6%4345%25%5%7%10%47%
1492
21st to endNight
9%424446%6%7%12%43%
1771
August1st to 10thNight
10%5%35%6%45%6%6%10%41%
1611
11th to 20thNight
10%5%27%6%45%6%5%7%43%
1637
21st to endNight
11%4346%4445%11%44%
1980
September1st to 10thNight
14%5%25%5%345%6%6%46%
1800
11th to 20thNight
10%6%35%42435%7%52%
1800
21st to endNight
16%6%25%4445%5%6%43%
2074
October1st to 10thNight
17%5%35%5%3445%6%43%
2160
11th to 20thNight
13%5%35%435%446%48%
2160
21st to endNight
17%5%25%4345%5%7%44%
2376
November1st to 10thNight
10%6%35%5%3445%6%52%
2320
11th to 20thNight
13%7%35%5%4445%7%44%
2546
21st to endNight
14%5%36%6%445%5%8%40%
2660
December1st to 10thNight
15%8%47%6%35%5%47%35%
2660
11th to 20thNight
17%7%36%6%36%5%6%6%36%
3285
21st to endNight
16%9%36%6%46%7%7%8%29%
4884

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 10thDay
47%412%26%9%3
2559
11th to 20thDay
54%6%10%16%12%2
2840
21st to endDay
54%5%10%22%7%1
3124
February1st to 10thDay
49%5%12%22%11%1
1710
11th to 20thDay
49%5%11%22%12%2
1678
21st to endDay
51%311%20%11%4
1480
March1st to 10thDay
40%5%12%25%17%2
1957
11th to 20thDay
48%413%17%14%5%
1980
21st to endDay
50%5%9%19%15%2
2200
April1st to 10thDay
45%5%13%18%16%3
2160
11th to 20thDay
41%6%10%21%18%3
2134
21st to endDay
46%6%9%21%16%3
2173
May1st to 10thDay
47%7%11%19%14%2
2340
11th to 20thDay
49%7%10%18%13%3
2423
21st to endDay
43%7%15%21%12%2
2725
June1st to 10thDay
39%7%14%23%15%1
2520
11th to 20thDay
42%9%14%21%13%1
2520
21st to endDay
35%7%15%28%13%1
2520
July1st to 10thDay
38%9%12%22%15%3
2450
11th to 20thDay
33%8%15%26%16%2
2389
21st to endDay
41%8%12%21%15%2
2405
August1st to 10thDay
40%10%15%24%11%
2314
11th to 20thDay
38%8%14%24%15%1
2245
21st to endDay
39%6%14%24%13%3
2376
September1st to 10thDay
41%5%12%20%18%3
2160
11th to 20thDay
34%6%11%26%20%3
2160
21st to endDay
45%49%19%20%2
1882
October1st to 10thDay
40%6%12%24%16%2
1800
11th to 20thDay
39%5%12%25%18%1
1800
21st to endDay
41%5%11%24%17%2
1980
November1st to 10thDay
36%413%24%19%4
1758
11th to 20thDay
41%5%11%22%18%3
1633
21st to endDay
44%412%29%10%1
1520
December1st to 10thDay
50%6%14%21%8%1
1520
11th to 20thDay
49%5%12%27%7%1
1906
21st to endDay
51%8%15%17%9%
2772
January1st to 10thNight
46%5%11%24%11%2
4383
11th to 20thNight
48%413%23%10%2
4120
21st to endNight
53%49%24%8%1
4532
February1st to 10thNight
46%413%23%11%1
2470
11th to 20thNight
48%5%9%19%17%2
2382
21st to endNight
49%411%19%12%4
1776
March1st to 10thNight
39%411%27%16%2
1999
11th to 20thNight
44%5%10%22%16%3
1980
21st to endNight
45%6%10%20%15%4
2156
April1st to 10thNight
50%5%9%18%15%3
1800
11th to 20thNight
39%5%11%22%17%5%
1775
21st to endNight
41%6%11%22%15%5%
1800
May1st to 10thNight
45%5%11%21%14%4
1621
11th to 20thNight
46%7%11%18%17%2
1472
21st to endNight
39%9%15%25%11%2
1551
June1st to 10thNight
35%8%11%27%16%3
1440
11th to 20thNight
41%7%13%24%13%2
1440
21st to endNight
35%6%14%27%17%1
1440
July1st to 10thNight
37%10%15%22%14%3
1400
11th to 20thNight
30%9%16%25%17%3
1444
21st to endNight
32%8%20%23%16%2
1672
August1st to 10thNight
36%7%17%25%13%1
1600
11th to 20thNight
39%8%14%22%17%1
1637
21st to endNight
36%7%14%24%16%3
1980
September1st to 10thNight
38%6%12%22%18%4
1800
11th to 20thNight
32%411%28%22%3
1800
21st to endNight
41%6%10%21%19%2
2074
October1st to 10thNight
41%5%12%24%16%2
2160
11th to 20thNight
38%5%9%28%18%2
2160
21st to endNight
40%5%8%28%18%2
2376
November1st to 10thNight
34%5%10%27%20%4
2320
11th to 20thNight
41%49%25%18%4
2546
21st to endNight
41%5%13%25%15%2
2660
December1st to 10thNight
48%6%12%23%11%1
2660
11th to 20thNight
45%413%21%14%2
3288
21st to endNight
49%8%15%19%9%
4884

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 10thDay
33%28%28%8%22
193
11th to 20thDay
38%27%29%6%1
270
21st to endDay
27%34%29%8%2
297
February1st to 10thDay
31%31%29%6%2
270
11th to 20thDay
42%28%28%2
282
21st to endDay
34%28%23%13%1
346
March1st to 10thDay
30%25%27%15%3
420
11th to 20thDay
31%27%25%12%41
420
21st to endDay
31%23%28%13%41
462
April1st to 10thDay
28%24%30%13%41
420
11th to 20thDay
22%21%29%16%10%3
412
21st to endDay
29%20%31%12%6%2
420
May1st to 10thDay
27%22%28%12%7%3
423
11th to 20thDay
25%18%29%17%10%2
447
21st to endDay
19%10%24%26%16%6%
485
June1st to 10thDay
15%16%29%20%16%4
450
11th to 20thDay
20%12%29%24%12%3
450
21st to endDay
13%13%32%25%14%3
450
July1st to 10thDay
12%6%27%27%19%10%
445
11th to 20thDay
8%6%25%32%22%8%
444
21st to endDay
14%6%26%31%16%6%
488
August1st to 10thDay
12%5%22%31%21%9%
446
11th to 20thDay
13%7%20%31%19%10%
439
21st to endDay
12%7%24%31%18%10%
495
September1st to 10thDay
21%7%19%27%18%8%
450
11th to 20thDay
14%6%21%31%19%8%
450
21st to endDay
26%13%24%21%12%4
450
October1st to 10thDay
20%10%26%25%15%4
450
11th to 20thDay
20%19%25%25%9%2
450
21st to endDay
21%18%36%18%7%1
495
November1st to 10thDay
19%18%27%24%10%2
408
11th to 20thDay
26%23%27%17%6%
293
21st to endDay
26%25%34%11%4
180
December1st to 10thDay
31%30%27%8%41
180
11th to 20thDay
31%25%29%13%3
180
21st to endDay
26%28%29%15%3
198
January1st to 10thNight
19%29%33%16%3
760
11th to 20thNight
23%22%31%21%3
760
21st to endNight
20%26%34%18%2
836
February1st to 10thNight
21%22%33%22%2
760
11th to 20thNight
27%28%31%13%1
742
21st to endNight
26%26%31%15%2
592
March1st to 10thNight
22%24%33%18%3
720
11th to 20thNight
27%21%28%18%5%1
720
21st to endNight
21%21%37%16%5%1
792
April1st to 10thNight
23%25%30%18%4
720
11th to 20thNight
17%18%31%26%7%
711
21st to endNight
23%17%30%23%8%1
720
May1st to 10thNight
19%14%26%28%13%1
541
11th to 20thNight
21%9%28%31%10%
410
21st to endNight
15%6%26%31%20%2
389
June1st to 10thNight
13%328%33%23%1
360
11th to 20thNight
16%7%22%39%16%1
360
21st to endNight
9%5%23%47%16%1
360
July1st to 10thNight
14%316%37%27%3
350
11th to 20thNight
8%416%41%28%3
400
21st to endNight
10%318%40%26%3
591
August1st to 10thNight
12%211%43%29%2
537
11th to 20thNight
12%5%12%40%28%4
573
21st to endNight
13%316%39%25%5%
792
September1st to 10thNight
17%321%31%24%3
720
11th to 20thNight
13%6%19%32%27%4
720
21st to endNight
20%10%20%28%20%3
720
October1st to 10thNight
21%9%18%28%20%3
720
11th to 20thNight
17%15%27%29%10%1
720
21st to endNight
21%16%28%27%8%1
792
November1st to 10thNight
14%15%30%29%11%1
740
11th to 20thNight
18%23%29%23%7%1
760
21st to endNight
18%22%31%23%6%1
760
December1st to 10thNight
22%23%30%18%6%
760
11th to 20thNight
20%23%32%23%3
760
21st to endNight
20%21%33%25%2
836
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