Jim Loudon Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jim Loudon Observatory clear sky chart from 2005-11-06 to 2024-03-18.

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	4 3 1 3 3 2 4 4 5% 6% 64%	79178
	Night	5% 3 1 3 3 2 3 4 4 6% 66%	80680
Transparency	Day	20% 4 5% 12% 26% 33%	78905
	Night	19% 3 6% 13% 27% 32%	80475
Seeing	Day	7% 9% 25% 32% 20% 7%	13987
	Night	6% 3 16% 40% 30% 5%	24222

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	7% 5% 3 3 3 2 3 4 4 5% 62%	2858
	11th to 20th	Day	5% 3 1 3 4 3 3 3 3 4 67%	2860
	21st to end	Day	11% 5% 2 3 3 2 4 4 5% 8% 54%	3146
February	1st to 10th	Day	6% 4 2 3 3 2 3 3 5% 5% 67%	1739
	11th to 20th	Day	7% 4 2 3 4 2 4 4 4 5% 61%	1850
	21st to end	Day	9% 4 1 3 3 2 3 3 4 5% 64%	1677
March	1st to 10th	Day	6% 3 1 3 3 2 4 4 5% 7% 62%	2090
	11th to 20th	Day	5% 4 1 3 3 2 2 4 4 5% 68%	2058
	21st to end	Day	4 1 1 3 3 2 2 3 4 5% 72%	2178
April	1st to 10th	Day	2 1 1 2 3 2 3 4 4 6% 74%	1980
	11th to 20th	Day	3 2 1 2 2 1 1 2 3 4 80%	2099
	21st to end	Day	2 1 1 1 2 1 2 2 3 5% 80%	2160
May	1st to 10th	Day	1 1 1 1 2 2 2 2 3 4 79%	2337
	11th to 20th	Day	1 1 1 2 2 2 3 3 3 4 78%	2301
	21st to end	Day	1 1 1 1 1 2 2 2 3 86%	2530
June	1st to 10th	Day	2 2 1 2 2 2 2 3 4 7% 74%	2340
	11th to 20th	Day	1 1 1 2 1 2 2 3 5% 82%	2340
	21st to end	Day	3 2 1 3 3 2 4 5% 6% 7% 66%	2340
July	1st to 10th	Day	3 2 1 5% 6% 3 5% 7% 7% 9% 51%	2340
	11th to 20th	Day	4 3 2 6% 6% 5% 8% 8% 11% 11% 37%	2340
	21st to end	Day	8% 5% 2 6% 7% 5% 8% 8% 9% 11% 32%	2545
August	1st to 10th	Day	5% 4 2 4 6% 4 6% 7% 8% 9% 44%	2328
	11th to 20th	Day	3 3 2 5% 5% 4 7% 7% 8% 10% 45%	2286
	21st to end	Day	3 3 2 4 5% 4 7% 6% 7% 9% 49%	2351
September	1st to 10th	Day	6% 5% 2 4 5% 4 4 5% 6% 8% 52%	1980
	11th to 20th	Day	3 2 1 3 3 3 4 5% 5% 5% 68%	1980
	21st to end	Day	3 2 1 3 3 2 3 4 5% 6% 69%	1980
October	1st to 10th	Day	2 1 1 1 2 2 4 3 5% 5% 73%	1895
	11th to 20th	Day	3 1 1 2 1 1 2 2 3 4 80%	1800
	21st to end	Day	2 1 1 3 2 2 2 4 5% 77%	1980
November	1st to 10th	Day	3 1 1 2 2 1 2 3 5% 6% 75%	1859
	11th to 20th	Day	2 2 1 2 3 1 3 4 4 5% 73%	1896
	21st to end	Day	6% 4 1 3 3 3 4 3 4 6% 63%	1710
December	1st to 10th	Day	7% 3 1 4 3 3 3 4 4 6% 62%	1710
	11th to 20th	Day	8% 5% 3 6% 4 2 3 4 6% 7% 52%	2169
	21st to end	Day	7% 4 2 4 4 2 4 4 4 5% 59%	3146
January	1st to 10th	Night	5% 4 1 4 3 2 4 4 4 6% 64%	4097
	11th to 20th	Night	3 2 1 2 2 1 2 3 4 5% 75%	4110
	21st to end	Night	14% 6% 2 4 3 2 3 3 5% 8% 49%	4521
February	1st to 10th	Night	6% 4 1 3 3 2 4 3 4 5% 66%	2437
	11th to 20th	Night	6% 4 2 4 3 2 4 3 4 5% 63%	2218
	21st to end	Night	7% 4 1 3 4 2 3 4 4 5% 63%	1775
March	1st to 10th	Night	4 3 2 3 3 3 3 3 5% 6% 64%	2090
	11th to 20th	Night	6% 3 1 2 2 1 3 3 4 6% 69%	2068
	21st to end	Night	5% 3 1 2 3 2 3 4 3 6% 68%	2178
April	1st to 10th	Night	2 2 1 2 2 2 2 3 4 6% 75%	1980
	11th to 20th	Night	2 2 1 2 2 1 2 3 2 4 78%	1808
	21st to end	Night	2 2 1 1 1 1 2 3 5% 82%	1799
May	1st to 10th	Night	1 1 1 1 2 1 2 3 3 5% 79%	1620
	11th to 20th	Night	1 1 1 2 2 1 2 3 4 6% 79%	1594
	21st to end	Night	1 1 2 1 1 1 2 2 3 88%	1746
June	1st to 10th	Night	3 1 1 2 1 1 3 3 4 6% 75%	1620
	11th to 20th	Night	1 1 1 2 1 2 2 2 5% 82%	1620
	21st to end	Night	1 2 2 3 3 2 3 4 5% 6% 69%	1620
July	1st to 10th	Night	2 2 2 5% 4 3 5% 7% 6% 9% 55%	1620
	11th to 20th	Night	2 2 2 4 6% 4 7% 9% 11% 15% 39%	1620
	21st to end	Night	4 3 2 6% 7% 5% 7% 9% 10% 16% 32%	1773
August	1st to 10th	Night	2 1 2 4 5% 3 5% 6% 7% 11% 54%	1611
	11th to 20th	Night	3 2 2 5% 6% 4 6% 6% 7% 10% 50%	1613
	21st to end	Night	3 3 2 4 5% 3 5% 7% 7% 11% 50%	2000
September	1st to 10th	Night	5% 3 2 5% 5% 3 4 5% 5% 9% 53%	1980
	11th to 20th	Night	3 2 1 3 3 2 3 4 4 5% 72%	1980
	21st to end	Night	2 3 1 3 3 2 3 3 5% 5% 69%	1980
October	1st to 10th	Night	2 2 1 3 2 2 4 3 4 5% 72%	2064
	11th to 20th	Night	2 2 1 1 2 1 2 2 3 4 80%	2160
	21st to end	Night	1 2 1 2 2 1 2 2 2 4 81%	2376
November	1st to 10th	Night	2 1 1 2 2 2 2 3 3 6% 75%	2220
	11th to 20th	Night	3 2 1 2 2 1 2 3 3 6% 74%	2280
	21st to end	Night	7% 4 1 3 3 2 3 4 4 6% 63%	2470
December	1st to 10th	Night	8% 3 1 3 2 2 3 3 4 7% 64%	2470
	11th to 20th	Night	8% 4 2 5% 4 3 4 3 4 6% 56%	3041
	21st to end	Night	9% 6% 2 3 4 2 5% 5% 4 5% 57%	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	25% 3 2 12% 33% 26%	2859
	11th to 20th	Day	22% 2 4 10% 38% 24%	2860
	21st to end	Day	28% 3 6% 18% 29% 16%	3146
February	1st to 10th	Day	21% 3 4 11% 23% 39%	1739
	11th to 20th	Day	25% 4 4 12% 25% 30%	1860
	21st to end	Day	23% 2 4 10% 29% 33%	1677
March	1st to 10th	Day	22% 2 3 8% 33% 32%	2090
	11th to 20th	Day	19% 2 4 10% 29% 37%	2058
	21st to end	Day	16% 2 3 9% 35% 34%	2178
April	1st to 10th	Day	12% 2 3 7% 35% 42%	1980
	11th to 20th	Day	12% 1 1 6% 27% 53%	2102
	21st to end	Day	9% 1 3 7% 30% 50%	2160
May	1st to 10th	Day	11% 1 3 6% 21% 58%	2337
	11th to 20th	Day	11% 2 4 8% 23% 52%	2304
	21st to end	Day	8% 1 1 4 24% 63%	2530
June	1st to 10th	Day	12% 1 2 5% 24% 56%	2340
	11th to 20th	Day	7% 2 1 3 17% 70%	2340
	21st to end	Day	18% 4 4 7% 19% 47%	2340
July	1st to 10th	Day	29% 6% 8% 14% 17% 25%	2276
	11th to 20th	Day	35% 11% 16% 21% 11% 6%	2271
	21st to end	Day	41% 11% 17% 19% 9% 3	2404
August	1st to 10th	Day	32% 8% 12% 17% 17% 14%	2313
	11th to 20th	Day	30% 10% 15% 18% 17% 11%	2286
	21st to end	Day	28% 7% 13% 20% 20% 13%	2351
September	1st to 10th	Day	30% 6% 8% 18% 17% 21%	1980
	11th to 20th	Day	18% 4 5% 13% 26% 34%	1980
	21st to end	Day	16% 4 5% 10% 32% 33%	1980
October	1st to 10th	Day	12% 3 7% 15% 26% 37%	1895
	11th to 20th	Day	11% 2 3 8% 29% 47%	1800
	21st to end	Day	11% 2 3 10% 28% 46%	1980
November	1st to 10th	Day	11% 2 3 11% 37% 36%	1859
	11th to 20th	Day	15% 3 4 11% 29% 39%	1896
	21st to end	Day	23% 3 4 11% 26% 33%	1710
December	1st to 10th	Day	22% 3 5% 14% 29% 26%	1710
	11th to 20th	Day	31% 2 5% 16% 26% 19%	2168
	21st to end	Day	26% 2 5% 15% 34% 19%	3146
January	1st to 10th	Night	21% 3 3 12% 41% 21%	4097
	11th to 20th	Night	14% 2 3 17% 42% 23%	4110
	21st to end	Night	34% 3 5% 16% 28% 14%	4521
February	1st to 10th	Night	21% 2 4 14% 21% 38%	2437
	11th to 20th	Night	24% 2 4 14% 26% 30%	2219
	21st to end	Night	22% 3 4 10% 28% 33%	1775
March	1st to 10th	Night	20% 2 4 13% 29% 32%	2090
	11th to 20th	Night	17% 2 4 11% 25% 41%	2068
	21st to end	Night	18% 3 4 12% 35% 29%	2178
April	1st to 10th	Night	13% 1 3 10% 35% 39%	1980
	11th to 20th	Night	13% 1 2 7% 28% 49%	1808
	21st to end	Night	9% 1 2 8% 30% 50%	1799
May	1st to 10th	Night	10% 1 4 7% 25% 53%	1620
	11th to 20th	Night	8% 2 4 9% 25% 51%	1595
	21st to end	Night	6% 1 2 4 26% 61%	1746
June	1st to 10th	Night	12% 2 2 6% 23% 56%	1620
	11th to 20th	Night	9% 2 2 4 15% 69%	1620
	21st to end	Night	17% 4 4 7% 18% 50%	1620
July	1st to 10th	Night	24% 6% 12% 16% 15% 27%	1575
	11th to 20th	Night	28% 12% 18% 25% 12% 5%	1566
	21st to end	Night	35% 10% 24% 19% 9% 3	1674
August	1st to 10th	Night	21% 10% 15% 22% 17% 14%	1602
	11th to 20th	Night	28% 7% 17% 18% 19% 11%	1613
	21st to end	Night	27% 6% 15% 21% 19% 13%	2000
September	1st to 10th	Night	28% 6% 10% 21% 17% 19%	1980
	11th to 20th	Night	15% 4 6% 14% 28% 33%	1980
	21st to end	Night	16% 4 4 13% 32% 31%	1980
October	1st to 10th	Night	14% 4 6% 15% 24% 37%	2064
	11th to 20th	Night	10% 2 3 11% 26% 48%	2160
	21st to end	Night	10% 1 3 10% 28% 47%	2376
November	1st to 10th	Night	12% 2 3 12% 35% 36%	2220
	11th to 20th	Night	14% 2 4 13% 30% 37%	2280
	21st to end	Night	22% 2 4 12% 28% 31%	2470
December	1st to 10th	Night	22% 3 5% 16% 26% 28%	2470
	11th to 20th	Night	29% 2 5% 15% 27% 20%	3041
	21st to end	Night	28% 3 5% 11% 31% 21%	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	16% 5% 34% 35% 10%	270
	11th to 20th	Day	11% 6% 34% 34% 13% 3	270
	21st to end	Day	10% 7% 34% 33% 14% 1	297
February	1st to 10th	Day	10% 11% 34% 28% 13% 4	299
	11th to 20th	Day	15% 16% 35% 26% 8%	455
	21st to end	Day	18% 19% 36% 21% 6% 1	382
March	1st to 10th	Day	12% 18% 33% 28% 8% 1	460
	11th to 20th	Day	11% 15% 33% 26% 12% 3	449
	21st to end	Day	9% 24% 35% 21% 9% 2	462
April	1st to 10th	Day	4 17% 36% 24% 15% 4	420
	11th to 20th	Day	7% 21% 34% 24% 12% 3	412
	21st to end	Day	5% 19% 30% 28% 13% 4	420
May	1st to 10th	Day	4 16% 33% 28% 16% 4	423
	11th to 20th	Day	2 11% 35% 33% 15% 5%	448
	21st to end	Day	3 11% 35% 34% 14% 3	485
June	1st to 10th	Day	4 7% 30% 36% 18% 7%	450
	11th to 20th	Day	2 6% 28% 40% 20% 5%	450
	21st to end	Day	4 2 25% 38% 25% 7%	450
July	1st to 10th	Day	3 1 11% 39% 34% 13%	446
	11th to 20th	Day	5% 2 5% 34% 37% 17%	444
	21st to end	Day	8% 2 3 29% 41% 16%	487
August	1st to 10th	Day	6% 1 4 33% 38% 18%	447
	11th to 20th	Day	5% 1 5% 36% 35% 17%	437
	21st to end	Day	4 2 5% 34% 39% 16%	495
September	1st to 10th	Day	9% 2 15% 38% 25% 11%	450
	11th to 20th	Day	4 2 15% 42% 26% 11%	450
	21st to end	Day	5% 9% 21% 33% 23% 9%	450
October	1st to 10th	Day	4 5% 26% 33% 24% 7%	365
	11th to 20th	Day	5% 5% 25% 35% 23% 7%	270
	21st to end	Day	3 5% 25% 35% 25% 7%	297
November	1st to 10th	Day	5% 9% 28% 41% 14% 2	270
	11th to 20th	Day	5% 6% 30% 37% 19% 2	270
	21st to end	Day	11% 13% 31% 31% 13% 2	270
December	1st to 10th	Day	10% 6% 33% 34% 17% 1	270
	11th to 20th	Day	7% 10% 41% 31% 11% 1	270
	21st to end	Day	14% 10% 33% 30% 9% 3	297
January	1st to 10th	Night	8% 3 23% 48% 15% 3	760
	11th to 20th	Night	8% 4 22% 43% 20% 2	760
	21st to end	Night	11% 4 21% 43% 18% 3	836
February	1st to 10th	Night	9% 4 22% 40% 23% 3	760
	11th to 20th	Night	9% 4 24% 45% 16% 2	744
	21st to end	Night	10% 5% 28% 42% 15% 1	628
March	1st to 10th	Night	6% 6% 28% 42% 16% 2	760
	11th to 20th	Night	8% 4 20% 36% 27% 5%	752
	21st to end	Night	6% 4 19% 44% 23% 4	792
April	1st to 10th	Night	3 4 19% 50% 22% 2	720
	11th to 20th	Night	4 6% 22% 43% 24% 1	564
	21st to end	Night	2 4 21% 47% 24% 3	540
May	1st to 10th	Night	2 5% 18% 49% 23% 4	540
	11th to 20th	Night	1 2 17% 47% 29% 5%	531
	21st to end	Night	1 3 15% 51% 28% 2	584
June	1st to 10th	Night	3 2 10% 45% 36% 3	540
	11th to 20th	Night	2 1 9% 47% 39% 3	540
	21st to end	Night	2 2 8% 36% 46% 6%	540
July	1st to 10th	Night	2 2 4 26% 57% 8%	525
	11th to 20th	Night	2 2 3 18% 63% 12%	522
	21st to end	Night	5% 2 2 20% 55% 16%	591
August	1st to 10th	Night	2 1 1 18% 61% 16%	537
	11th to 20th	Night	4 1 2 20% 58% 14%	534
	21st to end	Night	4 3 20% 58% 14%	614
September	1st to 10th	Night	7% 1 7% 28% 44% 13%	720
	11th to 20th	Night	3 1 8% 36% 43% 10%	720
	21st to end	Night	3 2 11% 41% 36% 8%	720
October	1st to 10th	Night	2 1 15% 46% 32% 3	720
	11th to 20th	Night	4 2 13% 43% 32% 6%	720
	21st to end	Night	3 3 14% 43% 29% 8%	792
November	1st to 10th	Night	5% 2 16% 42% 28% 6%	740
	11th to 20th	Night	4 3 18% 44% 29% 2	760
	21st to end	Night	10% 4 25% 39% 19% 4	760
December	1st to 10th	Night	11% 2 20% 45% 19% 3	760
	11th to 20th	Night	10% 5% 22% 44% 18% 2	760
	21st to end	Night	12% 4 21% 41% 18% 4	836

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