Distant Thunder Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Distant Thunder Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	11% 5% 3 5% 6% 4 6% 6% 6% 9% 40%	74708
	Night	11% 5% 2 5% 5% 3 4 5% 6% 8% 47%	75018
Transparency	Day	39% 8% 11% 18% 17% 6%	74444
	Night	35% 7% 13% 19% 18% 8%	74802
Seeing	Day	13% 6% 19% 25% 24% 12%	13556
	Night	13% 5% 19% 31% 25% 7%	23262

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 7% 2 5% 5% 3 3 5% 5% 6% 43%	1722
	11th to 20th	Day	15% 6% 3 5% 4 3 5% 4 4 6% 46%	1699
	21st to end	Day	19% 5% 3 5% 5% 3 3 4 4 5% 45%	1980
February	1st to 10th	Day	17% 8% 3 5% 5% 2 5% 4 4 6% 40%	1800
	11th to 20th	Day	17% 8% 2 4 5% 2 3 4 4 7% 44%	1748
	21st to end	Day	17% 8% 3 7% 7% 3 4 4 5% 6% 37%	1480
March	1st to 10th	Day	15% 6% 3 5% 5% 3 4 4 5% 7% 43%	1872
	11th to 20th	Day	13% 7% 4 6% 6% 3 4 5% 6% 6% 40%	1980
	21st to end	Day	10% 6% 3 5% 5% 3 4 5% 5% 8% 47%	2288
April	1st to 10th	Day	10% 5% 3 5% 5% 3 6% 6% 7% 9% 41%	2160
	11th to 20th	Day	14% 6% 3 6% 6% 3 5% 4 5% 9% 40%	2134
	21st to end	Day	9% 5% 2 4 5% 3 5% 4 7% 7% 49%	2160
May	1st to 10th	Day	11% 4 2 4 4 3 5% 6% 6% 9% 46%	2160
	11th to 20th	Day	9% 5% 2 4 5% 3 6% 6% 8% 9% 43%	2124
	21st to end	Day	7% 4 2 5% 6% 3 6% 7% 9% 11% 39%	2337
June	1st to 10th	Day	9% 5% 3 5% 8% 5% 7% 7% 9% 10% 33%	2160
	11th to 20th	Day	4 4 3 6% 7% 5% 7% 9% 8% 12% 35%	2267
	21st to end	Day	7% 6% 2 7% 7% 6% 8% 9% 9% 11% 28%	2340
July	1st to 10th	Day	9% 6% 3 7% 8% 5% 8% 8% 9% 11% 26%	2340
	11th to 20th	Day	7% 6% 4 7% 7% 6% 9% 9% 10% 11% 24%	2278
	21st to end	Day	8% 5% 3 7% 7% 5% 9% 8% 9% 11% 28%	2350
August	1st to 10th	Day	5% 3 3 6% 8% 6% 8% 7% 9% 11% 34%	2148
	11th to 20th	Day	9% 5% 3 6% 8% 6% 8% 8% 8% 11% 27%	2125
	21st to end	Day	10% 5% 3 7% 7% 5% 7% 7% 8% 10% 31%	2376
September	1st to 10th	Day	9% 5% 3 6% 7% 3 7% 7% 8% 11% 34%	2160
	11th to 20th	Day	10% 5% 3 5% 6% 4 7% 5% 7% 9% 39%	2098
	21st to end	Day	8% 4 2 5% 5% 4 6% 5% 7% 8% 46%	1980
October	1st to 10th	Day	6% 3 2 4 4 3 5% 4 6% 8% 54%	1918
	11th to 20th	Day	6% 4 2 4 5% 3 5% 5% 6% 8% 55%	1800
	21st to end	Day	10% 4 2 4 4 2 4 3 4 7% 56%	1980
November	1st to 10th	Day	8% 4 2 4 4 3 4 5% 5% 6% 55%	1858
	11th to 20th	Day	11% 5% 1 3 4 2 4 4 4 7% 55%	1805
	21st to end	Day	13% 5% 2 5% 5% 3 3 5% 5% 7% 46%	1710
December	1st to 10th	Day	15% 6% 3 5% 5% 2 5% 3 5% 7% 44%	1710
	11th to 20th	Day	15% 8% 3 5% 5% 4 4 5% 5% 7% 39%	2185
	21st to end	Day	18% 7% 3 5% 5% 3 5% 5% 5% 6% 38%	3476
January	1st to 10th	Night	17% 6% 3 4 4 2 3 4 4 7% 46%	2429
	11th to 20th	Night	16% 6% 2 4 4 2 3 4 4 6% 48%	2329
	21st to end	Night	18% 7% 3 5% 4 2 3 4 4 5% 44%	2376
February	1st to 10th	Night	17% 7% 2 5% 4 3 4 4 4 6% 43%	2160
	11th to 20th	Night	18% 7% 3 4 4 2 4 3 5% 7% 43%	2098
	21st to end	Night	20% 7% 3 6% 4 3 3 4 5% 7% 38%	1776
March	1st to 10th	Night	14% 7% 2 4 4 3 4 5% 5% 8% 44%	2102
	11th to 20th	Night	14% 6% 3 5% 5% 4 5% 4 5% 8% 41%	1980
	21st to end	Night	12% 5% 2 4 4 2 3 4 5% 7% 53%	2066
April	1st to 10th	Night	14% 7% 2 5% 4 3 4 4 5% 6% 47%	1800
	11th to 20th	Night	13% 7% 2 5% 4 3 4 6% 5% 6% 46%	1775
	21st to end	Night	6% 4 2 3 3 2 4 5% 6% 7% 57%	1800
May	1st to 10th	Night	8% 3 2 4 3 3 4 4 6% 9% 54%	1800
	11th to 20th	Night	6% 5% 2 5% 4 3 5% 5% 6% 8% 49%	1769
	21st to end	Night	6% 4 2 4 4 3 5% 6% 8% 11% 48%	1938
June	1st to 10th	Night	7% 6% 2 4 5% 3 5% 5% 8% 11% 42%	1800
	11th to 20th	Night	3 3 2 4 5% 3 7% 8% 8% 12% 46%	1685
	21st to end	Night	6% 5% 3 5% 7% 3 6% 7% 8% 10% 40%	1620
July	1st to 10th	Night	6% 4 3 6% 5% 4 6% 6% 8% 12% 42%	1620
	11th to 20th	Night	5% 3 2 5% 5% 4 6% 7% 10% 15% 38%	1674
	21st to end	Night	5% 4 2 5% 6% 5% 5% 6% 8% 14% 41%	1968
August	1st to 10th	Night	3 2 2 3 5% 3 6% 7% 8% 13% 48%	1790
	11th to 20th	Night	5% 3 2 5% 6% 3 7% 8% 10% 14% 37%	1776
	21st to end	Night	8% 4 2 5% 6% 3 6% 7% 8% 10% 40%	1980
September	1st to 10th	Night	7% 5% 2 5% 6% 5% 6% 6% 7% 11% 42%	1800
	11th to 20th	Night	8% 5% 2 5% 5% 3 6% 6% 6% 10% 44%	1842
	21st to end	Night	7% 5% 2 4 5% 2 4 5% 4 9% 52%	1980
October	1st to 10th	Night	6% 4 1 4 3 3 4 4 4 7% 60%	2038
	11th to 20th	Night	6% 4 1 4 4 2 4 4 5% 7% 60%	2160
	21st to end	Night	11% 4 2 4 4 2 3 4 4 6% 56%	2376
November	1st to 10th	Night	10% 5% 2 4 4 2 4 3 3 6% 58%	2220
	11th to 20th	Night	11% 4 1 4 4 3 4 3 4 7% 55%	2370
	21st to end	Night	12% 5% 3 5% 4 4 4 4 4 7% 48%	2470
December	1st to 10th	Night	15% 6% 2 5% 5% 3 3 4 4 7% 46%	2470
	11th to 20th	Night	14% 7% 2 5% 5% 3 5% 5% 5% 6% 43%	3001
	21st to end	Night	18% 10% 3 5% 5% 3 4 4 4 5% 40%	4180

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	41% 5% 7% 16% 22% 9%	1723
	11th to 20th	Day	40% 4 6% 15% 25% 9%	1699
	21st to end	Day	43% 4 5% 17% 20% 11%	1980
February	1st to 10th	Day	45% 4 8% 13% 19% 10%	1800
	11th to 20th	Day	41% 5% 8% 18% 18% 10%	1758
	21st to end	Day	48% 5% 8% 14% 17% 9%	1480
March	1st to 10th	Day	40% 5% 8% 20% 18% 7%	1872
	11th to 20th	Day	43% 7% 10% 12% 20% 8%	1980
	21st to end	Day	36% 6% 12% 20% 20% 6%	2288
April	1st to 10th	Day	38% 8% 10% 17% 19% 8%	2160
	11th to 20th	Day	42% 5% 8% 19% 18% 8%	2134
	21st to end	Day	32% 7% 9% 22% 24% 6%	2160
May	1st to 10th	Day	33% 7% 10% 24% 19% 7%	2160
	11th to 20th	Day	34% 10% 13% 23% 19% 2	2128
	21st to end	Day	34% 11% 14% 24% 15% 2	2337
June	1st to 10th	Day	41% 12% 12% 24% 10% 1	2160
	11th to 20th	Day	35% 12% 15% 24% 12% 2	2267
	21st to end	Day	45% 12% 16% 18% 9%	2340
July	1st to 10th	Day	47% 14% 18% 17% 4 1	2276
	11th to 20th	Day	46% 13% 21% 16% 3 1	2209
	21st to end	Day	44% 13% 18% 20% 5% 1	2219
August	1st to 10th	Day	38% 13% 23% 18% 8%	2136
	11th to 20th	Day	46% 13% 18% 17% 5% 1	2125
	21st to end	Day	43% 12% 17% 17% 9% 2	2376
September	1st to 10th	Day	39% 11% 17% 16% 12% 4	2160
	11th to 20th	Day	39% 9% 15% 16% 14% 7%	2098
	21st to end	Day	34% 8% 11% 21% 18% 9%	1980
October	1st to 10th	Day	27% 6% 11% 20% 22% 14%	1918
	11th to 20th	Day	27% 7% 6% 19% 28% 14%	1800
	21st to end	Day	30% 4 5% 17% 28% 16%	1980
November	1st to 10th	Day	28% 5% 8% 20% 27% 12%	1858
	11th to 20th	Day	30% 4 6% 19% 28% 12%	1805
	21st to end	Day	38% 5% 7% 19% 20% 11%	1710
December	1st to 10th	Day	40% 5% 9% 19% 18% 8%	1710
	11th to 20th	Day	44% 5% 7% 14% 22% 8%	2182
	21st to end	Day	45% 6% 8% 16% 21% 4	3476
January	1st to 10th	Night	39% 3 8% 16% 23% 10%	2429
	11th to 20th	Night	38% 5% 6% 17% 23% 11%	2329
	21st to end	Night	44% 3 5% 13% 25% 10%	2376
February	1st to 10th	Night	43% 3 8% 18% 18% 10%	2160
	11th to 20th	Night	42% 5% 6% 15% 20% 12%	2099
	21st to end	Night	45% 6% 10% 14% 13% 12%	1776
March	1st to 10th	Night	38% 6% 9% 16% 21% 10%	2102
	11th to 20th	Night	42% 5% 13% 15% 18% 7%	1980
	21st to end	Night	32% 5% 11% 21% 23% 7%	2066
April	1st to 10th	Night	38% 6% 10% 18% 18% 10%	1800
	11th to 20th	Night	39% 5% 9% 21% 17% 9%	1775
	21st to end	Night	26% 5% 12% 23% 26% 8%	1800
May	1st to 10th	Night	27% 5% 12% 29% 21% 6%	1800
	11th to 20th	Night	31% 8% 16% 22% 20% 2	1770
	21st to end	Night	28% 9% 18% 29% 14% 3	1939
June	1st to 10th	Night	33% 10% 19% 28% 10% 1	1800
	11th to 20th	Night	26% 11% 21% 29% 11% 2	1685
	21st to end	Night	35% 11% 22% 22% 9%	1620
July	1st to 10th	Night	33% 14% 28% 21% 4	1575
	11th to 20th	Night	30% 17% 32% 17% 3 1	1620
	21st to end	Night	31% 12% 27% 25% 5%	1858
August	1st to 10th	Night	23% 12% 35% 22% 7%	1778
	11th to 20th	Night	31% 18% 28% 18% 5% 1	1776
	21st to end	Night	35% 14% 22% 18% 8% 4	1980
September	1st to 10th	Night	34% 10% 20% 18% 12% 6%	1800
	11th to 20th	Night	34% 8% 18% 17% 14% 8%	1842
	21st to end	Night	30% 6% 14% 25% 17% 9%	1980
October	1st to 10th	Night	25% 4 11% 21% 22% 16%	2038
	11th to 20th	Night	24% 5% 9% 22% 25% 15%	2160
	21st to end	Night	30% 3 6% 16% 28% 17%	2376
November	1st to 10th	Night	29% 3 8% 20% 25% 14%	2220
	11th to 20th	Night	31% 4 7% 17% 28% 15%	2370
	21st to end	Night	36% 4 6% 20% 20% 13%	2470
December	1st to 10th	Night	39% 4 9% 21% 20% 7%	2470
	11th to 20th	Night	42% 5% 8% 13% 19% 14%	3003
	21st to end	Night	47% 5% 7% 17% 19% 6%	4180

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	23% 11% 39% 19% 7% 1	270
	11th to 20th	Day	20% 14% 36% 22% 7%	249
	21st to end	Day	22% 19% 42% 14% 3	264
February	1st to 10th	Day	26% 17% 34% 18% 5%	240
	11th to 20th	Day	28% 15% 35% 18% 5%	240
	21st to end	Day	16% 24% 44% 15% 2	198
March	1st to 10th	Day	15% 14% 45% 21% 5%	240
	11th to 20th	Day	18% 20% 37% 20% 5%	240
	21st to end	Day	18% 11% 34% 26% 10% 1	374
April	1st to 10th	Day	15% 12% 33% 27% 12%	420
	11th to 20th	Day	17% 13% 31% 24% 12% 2	412
	21st to end	Day	11% 8% 29% 34% 15% 3	420
May	1st to 10th	Day	13% 7% 28% 28% 21% 4	423
	11th to 20th	Day	12% 2 18% 33% 25% 10%	447
	21st to end	Day	8% 2 12% 34% 29% 16%	485
June	1st to 10th	Day	13% 2 8% 32% 26% 18%	450
	11th to 20th	Day	7% 2 10% 29% 37% 15%	557
	21st to end	Day	10% 2 9% 26% 33% 21%	630
July	1st to 10th	Day	10% 2 4 23% 36% 26%	621
	11th to 20th	Day	8% 1 3 17% 42% 28%	556
	21st to end	Day	11% 2 3 18% 40% 27%	488
August	1st to 10th	Day	4 3 3 18% 39% 32%	446
	11th to 20th	Day	8% 3 2 14% 42% 32%	439
	21st to end	Day	12% 1 7% 19% 34% 27%	495
September	1st to 10th	Day	8% 1 3 23% 41% 23%	450
	11th to 20th	Day	12% 2 6% 25% 39% 16%	450
	21st to end	Day	9% 2 8% 30% 37% 14%	450
October	1st to 10th	Day	6% 4 16% 40% 27% 8%	388
	11th to 20th	Day	6% 5% 22% 36% 25% 6%	270
	21st to end	Day	19% 8% 18% 37% 12% 6%	297
November	1st to 10th	Day	15% 4 20% 38% 20% 2	270
	11th to 20th	Day	17% 12% 27% 29% 13% 2	270
	21st to end	Day	15% 10% 38% 25% 9% 3	270
December	1st to 10th	Day	21% 8% 29% 27% 11% 3	270
	11th to 20th	Day	23% 11% 37% 24% 6%	270
	21st to end	Day	25% 11% 33% 22% 8% 1	297
January	1st to 10th	Night	20% 9% 31% 33% 7% 1	921
	11th to 20th	Night	20% 8% 30% 33% 9% 1	857
	21st to end	Night	22% 10% 30% 32% 5%	792
February	1st to 10th	Night	21% 8% 35% 29% 6%	720
	11th to 20th	Night	21% 10% 30% 31% 7%	703
	21st to end	Night	23% 11% 35% 28% 3	592
March	1st to 10th	Night	16% 10% 36% 30% 8%	662
	11th to 20th	Night	16% 6% 28% 36% 13% 1	540
	21st to end	Night	15% 7% 30% 35% 12% 1	594
April	1st to 10th	Night	16% 9% 28% 34% 13%	540
	11th to 20th	Night	15% 7% 30% 31% 16% 1	533
	21st to end	Night	10% 3 24% 44% 18% 1	540
May	1st to 10th	Night	10% 3 22% 40% 24% 1	540
	11th to 20th	Night	8% 2 15% 41% 29% 5%	531
	21st to end	Night	5% 2 11% 36% 38% 8%	584
June	1st to 10th	Night	10% 1 4 35% 40% 10%	540
	11th to 20th	Night	2 1 6% 28% 49% 14%	540
	21st to end	Night	7% 2 5% 26% 43% 18%	540
July	1st to 10th	Night	6% 2 2 16% 52% 22%	525
	11th to 20th	Night	5% 2 1 11% 50% 32%	522
	21st to end	Night	5% 1 2 14% 54% 24%	591
August	1st to 10th	Night	3 1 1 17% 49% 30%	537
	11th to 20th	Night	5% 1 1 12% 51% 31%	534
	21st to end	Night	8% 2 5% 16% 45% 24%	594
September	1st to 10th	Night	9% 1 3 19% 48% 21%	540
	11th to 20th	Night	11% 3 5% 24% 47% 11%	582
	21st to end	Night	10% 2 6% 33% 41% 8%	720
October	1st to 10th	Night	7% 2 14% 37% 35% 5%	720
	11th to 20th	Night	8% 4 18% 39% 28% 4	720
	21st to end	Night	12% 5% 19% 36% 24% 3	792
November	1st to 10th	Night	13% 4 15% 43% 21% 3	740
	11th to 20th	Night	14% 6% 25% 36% 17% 2	760
	21st to end	Night	15% 7% 26% 34% 17% 1	760
December	1st to 10th	Night	16% 8% 26% 33% 15% 1	760
	11th to 20th	Night	18% 7% 30% 35% 10% 1	760
	21st to end	Night	26% 8% 28% 30% 7%	836

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