Trout Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Trout Observatory clear sky chart from 2005-11-06 to 2024-01-31.

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	14% 9% 4 7% 6% 3 5% 5% 6% 7% 34%	74891
	Night	15% 10% 4 7% 6% 3 5% 5% 5% 7% 34%	75195
Transparency	Day	48% 7% 12% 20% 11% 1	74604
	Night	49% 7% 12% 20% 12% 2	75002
Seeing	Day	16% 7% 25% 28% 18% 6%	14268
	Night	20% 7% 27% 29% 15% 2	24310

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	18% 14% 7% 11% 9% 4 5% 4 4 5% 18%	1517
	11th to 20th	Day	17% 17% 8% 12% 9% 4 6% 3 4 4 16%	1612
	21st to end	Day	17% 17% 9% 11% 7% 4 4 4 5% 5% 17%	1872
February	1st to 10th	Day	17% 15% 6% 11% 8% 4 5% 4 5% 5% 21%	1620
	11th to 20th	Day	15% 11% 6% 11% 9% 5% 6% 5% 5% 5% 23%	1706
	21st to end	Day	17% 13% 6% 9% 7% 3 4 5% 5% 7% 25%	1480
March	1st to 10th	Day	18% 10% 5% 9% 7% 3 5% 5% 5% 6% 27%	1857
	11th to 20th	Day	15% 11% 4 7% 6% 3 4 4 5% 7% 33%	1980
	21st to end	Day	21% 13% 5% 8% 6% 3 4 4 5% 6% 26%	2178
April	1st to 10th	Day	13% 11% 5% 8% 8% 3 4 5% 6% 6% 30%	2060
	11th to 20th	Day	16% 8% 3 7% 7% 3 5% 4 6% 7% 34%	2277
	21st to end	Day	19% 10% 3 6% 5% 2 4 4 5% 8% 35%	2340
May	1st to 10th	Day	16% 8% 3 7% 6% 4 5% 6% 5% 7% 33%	2340
	11th to 20th	Day	16% 8% 4 8% 6% 4 5% 5% 5% 6% 33%	2301
	21st to end	Day	10% 5% 3 6% 6% 4 6% 6% 7% 8% 40%	2531
June	1st to 10th	Day	12% 6% 3 6% 5% 4 5% 6% 7% 8% 39%	2398
	11th to 20th	Day	10% 5% 2 6% 6% 3 5% 5% 6% 8% 43%	2520
	21st to end	Day	10% 5% 3 6% 6% 4 6% 6% 6% 9% 40%	2520
July	1st to 10th	Day	8% 5% 2 5% 6% 3 6% 5% 6% 9% 44%	2520
	11th to 20th	Day	8% 5% 2 4 5% 4 6% 6% 7% 9% 43%	2471
	21st to end	Day	8% 5% 2 5% 6% 4 5% 6% 7% 9% 43%	2547
August	1st to 10th	Day	7% 5% 2 4 5% 3 5% 6% 7% 11% 45%	2327
	11th to 20th	Day	9% 4 2 5% 6% 4 6% 6% 8% 11% 40%	2303
	21st to end	Day	10% 5% 2 6% 5% 3 6% 4 7% 9% 43%	2548
September	1st to 10th	Day	13% 5% 2 5% 6% 3 4 4 7% 7% 43%	2013
	11th to 20th	Day	10% 4 3 6% 4 3 5% 6% 5% 9% 44%	1980
	21st to end	Day	16% 6% 3 6% 4 3 4 4 6% 7% 42%	1980
October	1st to 10th	Day	12% 6% 3 6% 6% 4 5% 5% 6% 7% 41%	1927
	11th to 20th	Day	18% 7% 3 6% 6% 3 4 4 5% 8% 37%	1800
	21st to end	Day	18% 9% 3 8% 6% 4 5% 5% 6% 7% 30%	1872
November	1st to 10th	Day	11% 9% 3 7% 4 3 4 5% 6% 7% 41%	1672
	11th to 20th	Day	17% 10% 4 7% 6% 3 4 4 5% 6% 34%	1710
	21st to end	Day	17% 12% 5% 7% 6% 3 6% 4 6% 7% 27%	1706
December	1st to 10th	Day	16% 15% 7% 11% 8% 4 5% 4 5% 5% 20%	1520
	11th to 20th	Day	15% 14% 6% 9% 8% 4 6% 4 6% 6% 23%	2045
	21st to end	Day	24% 21% 7% 10% 7% 3 4 4 3 4 12%	2841
January	1st to 10th	Night	18% 14% 5% 9% 7% 4 5% 4 5% 5% 24%	2648
	11th to 20th	Night	19% 16% 7% 10% 7% 3 5% 4 4 5% 19%	2563
	21st to end	Night	17% 15% 7% 10% 7% 3 5% 3 4 5% 23%	2711
February	1st to 10th	Night	18% 14% 6% 9% 8% 4 5% 4 5% 5% 23%	2340
	11th to 20th	Night	15% 13% 5% 10% 6% 4 5% 5% 5% 5% 26%	2138
	21st to end	Night	21% 11% 4 8% 7% 3 4 4 4 6% 28%	1776
March	1st to 10th	Night	15% 10% 6% 9% 7% 3 4 4 4 5% 32%	2102
	11th to 20th	Night	15% 9% 3 6% 6% 3 4 4 6% 7% 38%	1980
	21st to end	Night	19% 11% 4 5% 5% 3 4 3 4 7% 33%	2178
April	1st to 10th	Night	14% 9% 4 7% 6% 3 5% 4 5% 6% 36%	1913
	11th to 20th	Night	16% 6% 4 6% 6% 4 5% 5% 5% 7% 37%	1630
	21st to end	Night	16% 9% 2 6% 6% 2 5% 4 6% 8% 36%	1620
May	1st to 10th	Night	18% 9% 4 6% 7% 4 6% 5% 6% 7% 30%	1620
	11th to 20th	Night	18% 6% 4 6% 5% 2 5% 4 5% 7% 37%	1592
	21st to end	Night	9% 5% 2 5% 7% 3 5% 6% 5% 10% 43%	1744
June	1st to 10th	Night	11% 5% 3 4 5% 3 5% 5% 5% 8% 45%	1558
	11th to 20th	Night	8% 6% 3 6% 5% 4 3 5% 7% 9% 44%	1440
	21st to end	Night	10% 4 2 4 4 3 5% 5% 7% 11% 45%	1440
July	1st to 10th	Night	9% 4 2 5% 4 3 5% 5% 5% 8% 50%	1440
	11th to 20th	Night	7% 4 2 4 5% 3 5% 5% 7% 10% 48%	1484
	21st to end	Night	6% 4 2 5% 4 3 5% 7% 7% 9% 49%	1771
August	1st to 10th	Night	6% 4 2 5% 5% 3 5% 5% 6% 11% 48%	1611
	11th to 20th	Night	7% 4 2 5% 5% 3 5% 5% 7% 9% 47%	1598
	21st to end	Night	7% 4 2 5% 5% 5% 5% 5% 5% 8% 48%	1804
September	1st to 10th	Night	10% 6% 2 5% 5% 3 4 5% 5% 8% 47%	1927
	11th to 20th	Night	9% 5% 3 5% 5% 3 4 4 4 8% 51%	1980
	21st to end	Night	12% 6% 3 6% 5% 3 4 5% 5% 6% 45%	1980
October	1st to 10th	Night	12% 7% 3 6% 4 3 4 4 5% 7% 45%	2031
	11th to 20th	Night	18% 9% 3 6% 5% 3 4 4 5% 6% 38%	2160
	21st to end	Night	21% 8% 4 7% 6% 3 4 4 5% 6% 33%	2479
November	1st to 10th	Night	13% 7% 4 6% 5% 3 4 5% 4 7% 41%	2406
	11th to 20th	Night	15% 10% 4 7% 6% 3 5% 4 6% 8% 32%	2470
	21st to end	Night	21% 11% 4 7% 7% 4 5% 4 5% 6% 27%	2470
December	1st to 10th	Night	16% 15% 5% 9% 7% 4 6% 4 6% 6% 23%	2655
	11th to 20th	Night	17% 14% 5% 8% 6% 4 5% 4 4 7% 26%	3134
	21st to end	Night	26% 19% 6% 9% 6% 3 4 4 4 5% 12%	4802

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	68% 6% 10% 14% 3	1518
	11th to 20th	Day	72% 6% 12% 7% 3	1612
	21st to end	Day	70% 5% 9% 10% 5% 1	1872
February	1st to 10th	Day	66% 5% 11% 13% 6%	1620
	11th to 20th	Day	62% 6% 10% 15% 6% 1	1716
	21st to end	Day	58% 7% 13% 13% 7% 2	1480
March	1st to 10th	Day	58% 5% 13% 18% 6%	1857
	11th to 20th	Day	53% 5% 11% 19% 10% 2	1980
	21st to end	Day	59% 6% 9% 19% 7% 1	2178
April	1st to 10th	Day	51% 7% 13% 16% 12% 1	2060
	11th to 20th	Day	49% 6% 12% 18% 13% 2	2277
	21st to end	Day	48% 5% 14% 20% 10% 2	2340
May	1st to 10th	Day	50% 7% 11% 19% 12% 1	2340
	11th to 20th	Day	50% 7% 10% 17% 14% 2	2305
	21st to end	Day	40% 8% 13% 24% 13% 2	2531
June	1st to 10th	Day	40% 8% 12% 25% 12% 3	2398
	11th to 20th	Day	37% 6% 16% 22% 16% 2	2520
	21st to end	Day	39% 8% 14% 20% 16% 4	2520
July	1st to 10th	Day	36% 7% 11% 25% 17% 4	2451
	11th to 20th	Day	33% 8% 15% 25% 17% 2	2396
	21st to end	Day	35% 8% 14% 23% 17% 2	2405
August	1st to 10th	Day	31% 9% 16% 29% 14%	2314
	11th to 20th	Day	35% 9% 17% 26% 12% 2	2303
	21st to end	Day	35% 8% 13% 24% 17% 3	2548
September	1st to 10th	Day	39% 6% 15% 21% 16% 3	2013
	11th to 20th	Day	35% 8% 12% 28% 17% 1	1980
	21st to end	Day	40% 7% 13% 23% 17%	1980
October	1st to 10th	Day	42% 7% 11% 25% 14% 2	1927
	11th to 20th	Day	46% 6% 12% 22% 14% 1	1800
	21st to end	Day	52% 7% 13% 17% 10% 1	1872
November	1st to 10th	Day	42% 6% 12% 23% 16% 1	1672
	11th to 20th	Day	51% 5% 12% 19% 11% 1	1710
	21st to end	Day	55% 7% 13% 17% 8%	1706
December	1st to 10th	Day	67% 6% 10% 13% 4	1520
	11th to 20th	Day	60% 6% 13% 15% 5%	2042
	21st to end	Day	77% 5% 8% 7% 3	2841
January	1st to 10th	Night	61% 6% 9% 16% 8%	2648
	11th to 20th	Night	67% 6% 11% 12% 4	2563
	21st to end	Night	65% 4 11% 14% 6% 1	2711
February	1st to 10th	Night	63% 6% 11% 15% 5% 1	2340
	11th to 20th	Night	59% 6% 10% 17% 8% 1	2139
	21st to end	Night	58% 5% 10% 18% 8% 1	1776
March	1st to 10th	Night	54% 5% 11% 20% 10% 1	2102
	11th to 20th	Night	45% 7% 11% 20% 16% 2	1980
	21st to end	Night	51% 6% 9% 24% 9% 2	2178
April	1st to 10th	Night	48% 6% 12% 18% 14% 2	1913
	11th to 20th	Night	46% 6% 11% 18% 17% 2	1630
	21st to end	Night	45% 7% 11% 20% 14% 3	1620
May	1st to 10th	Night	52% 7% 12% 16% 11% 2	1620
	11th to 20th	Night	46% 7% 8% 19% 17% 3	1593
	21st to end	Night	36% 8% 13% 26% 15% 2	1745
June	1st to 10th	Night	37% 7% 12% 26% 16% 3	1558
	11th to 20th	Night	35% 8% 13% 25% 16% 3	1440
	21st to end	Night	32% 9% 14% 26% 18% 2	1440
July	1st to 10th	Night	33% 7% 10% 25% 20% 4	1400
	11th to 20th	Night	29% 8% 15% 27% 19% 2	1436
	21st to end	Night	28% 9% 17% 24% 18% 4	1672
August	1st to 10th	Night	29% 8% 14% 31% 16% 2	1600
	11th to 20th	Night	31% 9% 15% 27% 15% 2	1598
	21st to end	Night	32% 7% 14% 23% 21% 3	1804
September	1st to 10th	Night	35% 7% 13% 24% 17% 4	1927
	11th to 20th	Night	31% 7% 11% 26% 24% 1	1980
	21st to end	Night	38% 7% 12% 23% 19% 2	1980
October	1st to 10th	Night	39% 6% 12% 22% 19% 2	2031
	11th to 20th	Night	46% 5% 11% 22% 15% 1	2160
	21st to end	Night	51% 6% 11% 21% 10% 1	2479
November	1st to 10th	Night	42% 6% 11% 23% 16% 2	2406
	11th to 20th	Night	49% 7% 12% 21% 11% 1	2470
	21st to end	Night	58% 6% 12% 18% 6%	2470
December	1st to 10th	Night	61% 6% 11% 14% 7%	2655
	11th to 20th	Night	56% 8% 15% 15% 5%	3136
	21st to end	Night	73% 7% 10% 7% 2	4802

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	22% 14% 34% 23% 5% 2	270
	11th to 20th	Day	23% 10% 44% 16% 7%	270
	21st to end	Day	27% 12% 29% 25% 6%	294
February	1st to 10th	Day	26% 13% 38% 20% 3	240
	11th to 20th	Day	23% 17% 39% 19% 3	240
	21st to end	Day	21% 17% 45% 14% 3	198
March	1st to 10th	Day	20% 12% 35% 26% 7%	297
	11th to 20th	Day	20% 10% 34% 28% 8% 1	420
	21st to end	Day	26% 10% 30% 26% 7% 1	462
April	1st to 10th	Day	18% 10% 30% 28% 13% 1	420
	11th to 20th	Day	18% 10% 32% 25% 10% 5%	412
	21st to end	Day	23% 9% 30% 26% 10% 2	420
May	1st to 10th	Day	22% 6% 26% 27% 15% 3	423
	11th to 20th	Day	17% 5% 28% 28% 17% 5%	447
	21st to end	Day	13% 2 20% 28% 26% 10%	485
June	1st to 10th	Day	11% 2 22% 38% 23% 3	508
	11th to 20th	Day	12% 4 20% 36% 24% 4	630
	21st to end	Day	12% 4 17% 39% 22% 6%	630
July	1st to 10th	Day	8% 3 14% 33% 29% 13%	621
	11th to 20th	Day	9% 3 16% 32% 30% 10%	569
	21st to end	Day	8% 2 14% 34% 32% 10%	488
August	1st to 10th	Day	10% 2 10% 33% 31% 14%	446
	11th to 20th	Day	11% 2 12% 34% 30% 10%	439
	21st to end	Day	11% 3 14% 29% 32% 11%	495
September	1st to 10th	Day	14% 2 17% 27% 28% 13%	450
	11th to 20th	Day	12% 1 12% 29% 30% 15%	450
	21st to end	Day	17% 5% 24% 24% 20% 10%	450
October	1st to 10th	Day	12% 6% 22% 30% 19% 11%	450
	11th to 20th	Day	19% 14% 28% 27% 11% 2	450
	21st to end	Day	23% 13% 32% 25% 6% 1	387
November	1st to 10th	Day	13% 13% 27% 29% 14% 4	270
	11th to 20th	Day	21% 19% 26% 23% 9% 1	270
	21st to end	Day	24% 10% 37% 27% 3	268
December	1st to 10th	Day	19% 16% 36% 22% 7% 1	180
	11th to 20th	Day	19% 15% 41% 20% 5%	222
	21st to end	Day	24% 13% 36% 24% 3	297
January	1st to 10th	Night	24% 11% 36% 21% 7%	950
	11th to 20th	Night	27% 12% 35% 21% 5%	853
	21st to end	Night	24% 10% 34% 27% 4 1	835
February	1st to 10th	Night	25% 8% 37% 26% 4	720
	11th to 20th	Night	24% 10% 34% 29% 4	703
	21st to end	Night	26% 11% 37% 23% 3	592
March	1st to 10th	Night	19% 9% 34% 28% 9%	720
	11th to 20th	Night	20% 9% 32% 29% 9% 2	720
	21st to end	Night	23% 8% 28% 31% 10% 1	792
April	1st to 10th	Night	20% 8% 32% 33% 7%	653
	11th to 20th	Night	20% 6% 33% 28% 11% 2	533
	21st to end	Night	22% 6% 30% 30% 12% 1	540
May	1st to 10th	Night	22% 4 27% 32% 12% 3	540
	11th to 20th	Night	21% 3 24% 32% 17% 3	531
	21st to end	Night	12% 2 20% 35% 26% 5%	584
June	1st to 10th	Night	13% 4 19% 40% 22% 2	540
	11th to 20th	Night	11% 3 19% 40% 25% 2	540
	21st to end	Night	11% 2 20% 39% 25% 3	540
July	1st to 10th	Night	10% 2 10% 36% 37% 5%	525
	11th to 20th	Night	9% 3 13% 41% 27% 6%	522
	21st to end	Night	9% 1 12% 42% 32% 4	591
August	1st to 10th	Night	8% 3 10% 42% 31% 6%	537
	11th to 20th	Night	10% 1 13% 33% 35% 7%	534
	21st to end	Night	10% 2 13% 33% 35% 6%	594
September	1st to 10th	Night	13% 3 16% 30% 30% 7%	667
	11th to 20th	Night	12% 3 14% 31% 34% 6%	720
	21st to end	Night	16% 6% 19% 32% 22% 6%	720
October	1st to 10th	Night	18% 6% 20% 31% 19% 5%	720
	11th to 20th	Night	23% 12% 27% 25% 11% 2	720
	21st to end	Night	27% 10% 30% 27% 6% 1	792
November	1st to 10th	Night	19% 11% 29% 26% 13% 2	740
	11th to 20th	Night	22% 13% 35% 21% 8% 1	760
	21st to end	Night	28% 11% 32% 23% 6%	760
December	1st to 10th	Night	26% 13% 34% 21% 6%	760
	11th to 20th	Night	25% 12% 37% 22% 4	760
	21st to end	Night	30% 11% 32% 24% 4	1002

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