Logan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Logan Observatory clear sky chart from 2005-11-06 to 2024-01-17.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	3 4 2 5% 5% 3 4 5% 5% 7% 58%	75040
	Night	2 3 1 3 3 2 4 4 5% 7% 65%	74822
Transparency	Day	25% 6% 8% 16% 27% 18%	74766
	Night	18% 6% 9% 18% 29% 19%	74617
Seeing	Day	9% 18% 26% 24% 16% 6%	13518
	Night	4 14% 25% 30% 22% 6%	23675

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	1 3 2 4 4 2 5% 4 5% 7% 63%	1708
	11th to 20th	Day	1 3 2 3 2 1 3 5% 5% 6% 67%	1674
	21st to end	Day	2 3 2 4 3 2 4 3 6% 5% 66%	1782
February	1st to 10th	Day	1 4 1 4 5% 1 3 4 4 6% 67%	1620
	11th to 20th	Day	2 3 2 4 4 3 5% 5% 4 7% 63%	1634
	21st to end	Day	4 2 4 3 2 3 3 6% 7% 66%	1628
March	1st to 10th	Day	1 3 1 3 4 2 4 5% 5% 9% 62%	1980
	11th to 20th	Day	3 4 3 4 3 3 3 4 6% 7% 61%	1980
	21st to end	Day	1 3 2 3 3 2 3 4 5% 8% 65%	2178
April	1st to 10th	Day	1 2 2 3 3 2 4 4 5% 7% 68%	2052
	11th to 20th	Day	1 5% 2 3 4 2 3 3 4 5% 68%	2131
	21st to end	Day	2 3 1 3 3 2 3 4 4 7% 68%	2160
May	1st to 10th	Day	2 3 2 3 3 2 4 3 3 6% 69%	2272
	11th to 20th	Day	2 3 2 4 5% 3 5% 5% 5% 6% 62%	2301
	21st to end	Day	3 2 1 3 3 2 2 3 3 6% 72%	2530
June	1st to 10th	Day	2 2 1 3 3 3 4 4 4 6% 68%	2340
	11th to 20th	Day	3 2 1 3 4 2 4 3 3 5% 71%	2340
	21st to end	Day	3 4 2 4 6% 3 5% 6% 6% 8% 52%	2340
July	1st to 10th	Day	7% 5% 3 8% 8% 5% 7% 7% 7% 6% 37%	2340
	11th to 20th	Day	7% 6% 3 8% 9% 6% 9% 9% 8% 7% 29%	2340
	21st to end	Day	10% 6% 3 9% 9% 6% 9% 8% 8% 8% 24%	2545
August	1st to 10th	Day	8% 7% 4 8% 10% 6% 8% 7% 7% 7% 27%	2328
	11th to 20th	Day	8% 5% 3 10% 9% 5% 7% 7% 7% 7% 31%	2235
	21st to end	Day	8% 6% 3 8% 8% 4 7% 7% 7% 9% 35%	2376
September	1st to 10th	Day	7% 6% 3 7% 7% 4 5% 6% 6% 7% 42%	2152
	11th to 20th	Day	5% 5% 3 6% 6% 4 5% 4 4 5% 54%	1980
	21st to end	Day	4 3 2 4 4 3 4 3 4 6% 63%	1980
October	1st to 10th	Day	3 4 2 4 4 2 3 4 4 6% 65%	1806
	11th to 20th	Day	2 2 1 1 2 1 2 2 3 5% 79%	1800
	21st to end	Day	1 3 1 2 2 2 2 2 3 5% 76%	1980
November	1st to 10th	Day	1 1 1 2 2 2 3 3 4 7% 75%	1859
	11th to 20th	Day	1 3 1 3 3 2 3 4 5% 6% 71%	1900
	21st to end	Day	1 5% 2 4 3 2 3 4 4 7% 66%	1744
December	1st to 10th	Day	2 3 1 3 2 2 4 4 5% 7% 67%	1710
	11th to 20th	Day	2 5% 3 5% 5% 3 5% 4 5% 7% 57%	2169
	21st to end	Day	3 2 5% 5% 3 5% 5% 5% 7% 61%	3146
January	1st to 10th	Night	3 1 4 4 2 4 5% 6% 7% 62%	2457
	11th to 20th	Night	1 2 2 3 3 2 4 4 5% 6% 69%	2424
	21st to end	Night	1 3 2 3 4 2 4 5% 6% 6% 64%	2574
February	1st to 10th	Night	1 3 2 2 3 2 3 4 5% 7% 68%	2340
	11th to 20th	Night	1 3 1 4 3 3 4 4 5% 6% 65%	2207
	21st to end	Night	2 1 3 3 3 3 4 5% 8% 69%	1628
March	1st to 10th	Night	3 2 3 2 2 4 3 4 7% 69%	1980
	11th to 20th	Night	2 3 2 3 3 2 3 3 4 6% 70%	1980
	21st to end	Night	1 2 1 3 3 2 4 4 5% 8% 67%	2178
April	1st to 10th	Night	1 2 1 2 2 2 4 4 6% 7% 71%	1906
	11th to 20th	Night	1 3 1 3 3 2 3 3 5% 5% 72%	1777
	21st to end	Night	1 2 1 1 2 1 3 3 4 7% 76%	1800
May	1st to 10th	Night	1 1 1 3 2 2 4 3 4 5% 75%	1685
	11th to 20th	Night	2 2 2 3 3 2 3 3 5% 7% 70%	1594
	21st to end	Night	1 1 1 2 2 1 2 2 3 4 80%	1746
June	1st to 10th	Night	1 2 1 3 3 2 4 4 4 5% 70%	1620
	11th to 20th	Night	2 1 1 2 2 1 2 3 4 5% 78%	1620
	21st to end	Night	1 2 2 4 4 3 5% 7% 6% 7% 60%	1620
July	1st to 10th	Night	3 3 2 5% 6% 4 6% 7% 8% 9% 46%	1620
	11th to 20th	Night	3 3 1 6% 7% 5% 7% 10% 9% 13% 36%	1620
	21st to end	Night	5% 5% 2 6% 6% 4 8% 7% 12% 15% 31%	1773
August	1st to 10th	Night	3 3 2 4 5% 4 6% 7% 8% 11% 46%	1611
	11th to 20th	Night	3 5% 2 6% 6% 4 8% 7% 8% 10% 42%	1664
	21st to end	Night	4 3 2 5% 5% 4 4 6% 9% 11% 47%	1980
September	1st to 10th	Night	3 4 2 5% 6% 3 5% 6% 6% 8% 50%	1804
	11th to 20th	Night	3 2 1 4 3 2 3 4 4 5% 67%	1980
	21st to end	Night	2 1 1 3 3 2 3 4 4 6% 72%	1980
October	1st to 10th	Night	2 2 1 2 3 2 3 4 4 6% 70%	2152
	11th to 20th	Night	1 1 1 2 1 1 2 2 3 4 81%	2160
	21st to end	Night	1 2 1 2 2 1 2 2 3 4 79%	2376
November	1st to 10th	Night	1 1 2 2 2 3 3 4 6% 75%	2220
	11th to 20th	Night	1 3 1 2 2 1 2 3 4 6% 74%	2280
	21st to end	Night	1 3 1 3 3 2 3 3 5% 7% 69%	2434
December	1st to 10th	Night	2 3 2 3 3 2 3 4 5% 7% 68%	2470
	11th to 20th	Night	1 4 3 5% 4 3 3 4 5% 8% 59%	3041
	21st to end	Night	1 3 1 4 4 3 4 5% 5% 7% 63%	4521

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	20% 6% 8% 20% 29% 17%	1709
	11th to 20th	Day	18% 5% 7% 19% 35% 16%	1674
	21st to end	Day	21% 5% 6% 17% 35% 16%	1782
February	1st to 10th	Day	20% 3 8% 19% 26% 24%	1620
	11th to 20th	Day	22% 4 8% 20% 25% 21%	1643
	21st to end	Day	19% 5% 4 16% 30% 25%	1628
March	1st to 10th	Day	19% 6% 7% 16% 32% 20%	1980
	11th to 20th	Day	22% 4 6% 15% 36% 18%	1980
	21st to end	Day	18% 6% 7% 18% 37% 15%	2178
April	1st to 10th	Day	16% 3 7% 17% 38% 18%	2052
	11th to 20th	Day	20% 3 4 14% 34% 25%	2134
	21st to end	Day	18% 3 7% 19% 36% 16%	2160
May	1st to 10th	Day	19% 4 6% 14% 35% 23%	2272
	11th to 20th	Day	22% 6% 8% 15% 31% 18%	2304
	21st to end	Day	16% 3 4 13% 36% 27%	2530
June	1st to 10th	Day	17% 4 6% 14% 30% 29%	2340
	11th to 20th	Day	18% 3 4 8% 24% 43%	2340
	21st to end	Day	29% 6% 9% 12% 24% 21%	2340
July	1st to 10th	Day	42% 9% 11% 12% 13% 13%	2276
	11th to 20th	Day	47% 13% 17% 13% 6% 5%	2271
	21st to end	Day	51% 13% 19% 12% 3 2	2404
August	1st to 10th	Day	51% 11% 13% 11% 11% 4	2313
	11th to 20th	Day	46% 11% 13% 15% 10% 5%	2235
	21st to end	Day	44% 9% 12% 17% 14% 3	2376
September	1st to 10th	Day	39% 7% 9% 15% 19% 11%	2152
	11th to 20th	Day	32% 5% 7% 15% 27% 15%	1980
	21st to end	Day	23% 4 7% 19% 29% 18%	1980
October	1st to 10th	Day	22% 5% 8% 17% 24% 24%	1806
	11th to 20th	Day	11% 3 4 12% 35% 35%	1800
	21st to end	Day	14% 2 6% 16% 42% 20%	1980
November	1st to 10th	Day	11% 3 6% 24% 34% 22%	1859
	11th to 20th	Day	16% 3 5% 16% 37% 23%	1900
	21st to end	Day	20% 5% 5% 14% 34% 21%	1744
December	1st to 10th	Day	18% 4 7% 23% 28% 19%	1710
	11th to 20th	Day	27% 5% 10% 19% 22% 17%	2168
	21st to end	Day	21% 7% 8% 16% 27% 20%	3146
January	1st to 10th	Night	20% 7% 7% 21% 29% 16%	2457
	11th to 20th	Night	17% 4 8% 21% 35% 15%	2424
	21st to end	Night	20% 5% 9% 16% 36% 15%	2574
February	1st to 10th	Night	16% 5% 8% 22% 27% 22%	2340
	11th to 20th	Night	19% 5% 8% 21% 27% 20%	2208
	21st to end	Night	15% 4 7% 17% 31% 27%	1628
March	1st to 10th	Night	16% 5% 7% 17% 37% 18%	1980
	11th to 20th	Night	16% 5% 6% 18% 39% 17%	1980
	21st to end	Night	17% 5% 7% 24% 33% 14%	2178
April	1st to 10th	Night	13% 5% 7% 24% 37% 15%	1906
	11th to 20th	Night	15% 4 5% 18% 35% 22%	1777
	21st to end	Night	10% 4 7% 23% 40% 16%	1800
May	1st to 10th	Night	13% 3 7% 17% 39% 21%	1685
	11th to 20th	Night	15% 6% 8% 19% 35% 18%	1595
	21st to end	Night	10% 3 5% 16% 39% 26%	1746
June	1st to 10th	Night	16% 4 6% 11% 33% 30%	1620
	11th to 20th	Night	11% 3 3 12% 30% 41%	1620
	21st to end	Night	21% 7% 9% 14% 24% 25%	1620
July	1st to 10th	Night	30% 9% 14% 16% 14% 17%	1575
	11th to 20th	Night	32% 16% 24% 16% 7% 5%	1566
	21st to end	Night	35% 16% 31% 11% 4 2	1674
August	1st to 10th	Night	28% 14% 22% 14% 15% 7%	1602
	11th to 20th	Night	33% 13% 19% 16% 12% 6%	1664
	21st to end	Night	28% 11% 18% 21% 18% 4	1980
September	1st to 10th	Night	30% 8% 14% 18% 19% 11%	1804
	11th to 20th	Night	19% 5% 10% 20% 31% 15%	1980
	21st to end	Night	15% 4 7% 21% 32% 21%	1980
October	1st to 10th	Night	16% 5% 10% 21% 26% 23%	2152
	11th to 20th	Night	9% 2 5% 16% 34% 35%	2160
	21st to end	Night	11% 3 5% 19% 42% 20%	2376
November	1st to 10th	Night	11% 4 5% 22% 37% 22%	2220
	11th to 20th	Night	13% 3 7% 18% 33% 27%	2280
	21st to end	Night	16% 3 7% 20% 32% 22%	2434
December	1st to 10th	Night	18% 5% 6% 23% 26% 21%	2470
	11th to 20th	Night	22% 7% 11% 18% 26% 16%	3041
	21st to end	Night	20% 8% 10% 17% 23% 23%	4521

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	8% 27% 35% 19% 8% 3	270
	11th to 20th	Day	15% 24% 26% 16% 14% 5%	258
	21st to end	Day	7% 23% 29% 24% 14% 3	264
February	1st to 10th	Day	5% 25% 33% 21% 13% 4	240
	11th to 20th	Day	17% 30% 29% 18% 5% 1	284
	21st to end	Day	18% 36% 30% 12% 5%	346
March	1st to 10th	Day	17% 34% 28% 16% 4 1	420
	11th to 20th	Day	17% 25% 26% 22% 9% 2	420
	21st to end	Day	15% 37% 27% 16% 4 2	462
April	1st to 10th	Day	7% 28% 37% 17% 6% 4	420
	11th to 20th	Day	15% 30% 36% 13% 5% 1	412
	21st to end	Day	13% 30% 33% 17% 6% 2	420
May	1st to 10th	Day	7% 24% 40% 21% 7% 2	423
	11th to 20th	Day	5% 21% 37% 25% 10% 2	448
	21st to end	Day	9% 23% 37% 19% 10% 2	485
June	1st to 10th	Day	4 14% 35% 31% 14% 3	450
	11th to 20th	Day	4 14% 31% 32% 14% 6%	450
	21st to end	Day	5% 8% 28% 38% 17% 4	450
July	1st to 10th	Day	9% 4 17% 30% 30% 10%	446
	11th to 20th	Day	10% 2 9% 28% 31% 19%	444
	21st to end	Day	11% 1 10% 24% 32% 21%	487
August	1st to 10th	Day	9% 3 12% 25% 33% 19%	447
	11th to 20th	Day	11% 2 14% 30% 29% 14%	437
	21st to end	Day	7% 2 13% 30% 31% 15%	495
September	1st to 10th	Day	10% 6% 15% 30% 29% 9%	450
	11th to 20th	Day	6% 7% 27% 33% 21% 6%	450
	21st to end	Day	8% 12% 25% 28% 20% 6%	450
October	1st to 10th	Day	6% 17% 28% 26% 16% 7%	276
	11th to 20th	Day	8% 14% 23% 29% 18% 7%	270
	21st to end	Day	8% 12% 29% 24% 22% 5%	297
November	1st to 10th	Day	7% 25% 27% 26% 12% 3	270
	11th to 20th	Day	3 16% 30% 24% 20% 7%	270
	21st to end	Day	10% 26% 26% 21% 13% 4	270
December	1st to 10th	Day	8% 24% 23% 28% 15% 3	270
	11th to 20th	Day	8% 27% 29% 23% 12% 3	270
	21st to end	Day	13% 28% 29% 20% 8% 1	297
January	1st to 10th	Night	4 18% 22% 34% 16% 7%	760
	11th to 20th	Night	5% 16% 27% 30% 18% 4	746
	21st to end	Night	4 19% 27% 29% 15% 5%	792
February	1st to 10th	Night	3 20% 27% 31% 16% 4	720
	11th to 20th	Night	5% 26% 31% 24% 12% 2	704
	21st to end	Night	9% 26% 32% 23% 9% 2	592
March	1st to 10th	Night	8% 26% 28% 25% 10% 2	720
	11th to 20th	Night	6% 20% 32% 25% 14% 4	720
	21st to end	Night	7% 22% 32% 25% 13% 2	792
April	1st to 10th	Night	4 24% 36% 25% 11% 1	646
	11th to 20th	Night	5% 26% 35% 24% 9% 2	533
	21st to end	Night	2 23% 31% 28% 14% 3	540
May	1st to 10th	Night	2 16% 36% 31% 14% 2	540
	11th to 20th	Night	3 9% 35% 32% 17% 3	531
	21st to end	Night	3 15% 34% 31% 15% 2	584
June	1st to 10th	Night	2 7% 31% 38% 20% 2	540
	11th to 20th	Night	2 7% 32% 41% 17% 2	540
	21st to end	Night	1 2 23% 41% 28% 5%	540
July	1st to 10th	Night	6% 1 13% 33% 39% 8%	525
	11th to 20th	Night	4 8% 28% 46% 14%	522
	21st to end	Night	6% 1 7% 23% 48% 16%	591
August	1st to 10th	Night	4 1 7% 22% 49% 16%	537
	11th to 20th	Night	5% 1 8% 30% 43% 13%	534
	21st to end	Night	5% 8% 27% 47% 13%	594
September	1st to 10th	Night	4 1 16% 31% 41% 8%	544
	11th to 20th	Night	4 2 21% 34% 31% 8%	720
	21st to end	Night	2 11% 17% 34% 28% 8%	720
October	1st to 10th	Night	3 13% 30% 32% 18% 4	720
	11th to 20th	Night	3 9% 24% 37% 21% 6%	720
	21st to end	Night	4 11% 23% 30% 24% 8%	792
November	1st to 10th	Night	2 15% 23% 34% 18% 8%	740
	11th to 20th	Night	2 14% 24% 32% 22% 5%	760
	21st to end	Night	5% 21% 23% 26% 19% 6%	760
December	1st to 10th	Night	6% 18% 19% 32% 20% 4	760
	11th to 20th	Night	4 21% 29% 26% 15% 3	760
	21st to end	Night	6% 22% 27% 30% 11% 4	836

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