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 2025-02-21.

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% 10% 4 9% 5% 5% 5% 4 6% 7% 33%	108561
	Night	14% 11% 4 9% 5% 4 5% 4 5% 7% 33%	110011
Transparency	Day	48% 6% 12% 20% 12% 1	112217
	Night	49% 6% 11% 20% 12% 1	114494
Seeing	Day	16% 7% 25% 29% 18% 6%	15687
	Night	19% 8% 27% 29% 15% 2	25730

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	14% 17% 7% 17% 6% 5% 4 3 5% 5% 17%	3617
	11th to 20th	Day	14% 18% 6% 15% 10% 6% 7% 3 3 4 15%	3837
	21st to end	Day	22% 16% 9% 13% 5% 5% 4 3 3 4 16%	4466
February	1st to 10th	Day	16% 16% 5% 13% 6% 5% 5% 3 5% 6% 21%	1800
	11th to 20th	Day	15% 12% 5% 12% 8% 6% 6% 3 5% 5% 24%	1901
	21st to end	Day	17% 14% 4 10% 6% 4 4 4 5% 7% 25%	1570
March	1st to 10th	Day	20% 14% 5% 10% 5% 4 4 3 4 6% 26%	3233
	11th to 20th	Day	12% 12% 4 8% 5% 5% 4 4 5% 6% 34%	3480
	21st to end	Day	19% 12% 5% 10% 5% 4 4 3 5% 6% 26%	3815
April	1st to 10th	Day	13% 12% 4 9% 7% 4 4 5% 6% 6% 30%	2175
	11th to 20th	Day	16% 8% 3 7% 5% 5% 4 3 6% 7% 34%	2406
	21st to end	Day	18% 10% 3 6% 4 4 4 4 5% 8% 35%	2470
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 4 5% 6% 33%	4082
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 34%	4081
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 6% 7% 42%	4476
June	1st to 10th	Day	11% 6% 2 7% 4 5% 5% 5% 8% 8% 39%	2532
	11th to 20th	Day	9% 5% 2 6% 6% 4 5% 4 6% 8% 44%	2660
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 5% 6% 9% 39%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 10% 38%	4440
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 5% 7% 10% 45%	4349
	21st to end	Day	7% 6% 2 7% 6% 6% 6% 5% 7% 9% 41%	4466
August	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 7% 9% 43%	4104
	11th to 20th	Day	8% 6% 2 6% 5% 5% 7% 6% 8% 11% 37%	4044
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 4 7% 10% 41%	4478
September	1st to 10th	Day	13% 5% 2 6% 5% 4 4 3 6% 7% 45%	2124
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 5% 9% 46%	2090
	21st to end	Day	17% 7% 3 6% 4 4 4 3 6% 6% 40%	2090
October	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 3 6% 7% 43%	2034
	11th to 20th	Day	17% 8% 2 7% 5% 4 4 3 5% 8% 38%	1900
	21st to end	Day	17% 10% 2 9% 5% 5% 5% 4 7% 7% 30%	1976
November	1st to 10th	Day	10% 10% 3 7% 4 4 5% 4 6% 7% 40%	1762
	11th to 20th	Day	17% 12% 3 8% 6% 5% 4 3 5% 5% 33%	1800
	21st to end	Day	17% 13% 5% 8% 5% 4 5% 3 6% 6% 26%	1795
December	1st to 10th	Day	14% 14% 6% 12% 6% 5% 5% 4 5% 7% 23%	2600
	11th to 20th	Day	19% 19% 7% 9% 5% 4 4 3 5% 5% 19%	3195
	21st to end	Day	24% 21% 6% 11% 6% 5% 3 3 3 4 14%	4053
January	1st to 10th	Night	14% 16% 6% 17% 6% 5% 5% 3 5% 5% 19%	6348
	11th to 20th	Night	12% 19% 8% 14% 7% 6% 6% 5% 4 5% 15%	6114
	21st to end	Night	21% 16% 8% 11% 4 4 4 3 4 5% 22%	6490
February	1st to 10th	Night	16% 16% 5% 10% 7% 5% 4 4 5% 5% 24%	2600
	11th to 20th	Night	14% 14% 4 11% 6% 5% 5% 4 5% 5% 27%	2383
	21st to end	Night	20% 12% 4 9% 7% 5% 4 3 4 6% 27%	1885
March	1st to 10th	Night	20% 13% 3 7% 4 3 3 2 4 5% 35%	3619
	11th to 20th	Night	11% 8% 2 6% 5% 4 5% 4 6% 7% 42%	3480
	21st to end	Night	16% 11% 4 7% 5% 5% 4 3 5% 7% 35%	3817
April	1st to 10th	Night	15% 11% 3 8% 5% 4 4 4 5% 6% 35%	2019
	11th to 20th	Night	16% 7% 3 7% 5% 5% 4 4 5% 7% 37%	1721
	21st to end	Night	16% 10% 2 7% 5% 4 4 4 6% 7% 36%	1710
May	1st to 10th	Night	18% 9% 3 7% 6% 4 5% 4 6% 7% 30%	2813
	11th to 20th	Night	17% 7% 3 7% 4 4 4 4 5% 7% 38%	2812
	21st to end	Night	9% 5% 2 6% 5% 4 4 4 5% 9% 47%	3079
June	1st to 10th	Night	12% 5% 3 5% 5% 4 5% 4 5% 8% 45%	1644
	11th to 20th	Night	8% 6% 3 7% 4 4 3 5% 7% 8% 44%	1520
	21st to end	Night	9% 5% 2 4 5% 4 5% 4 7% 10% 44%	1520
July	1st to 10th	Night	9% 4 2 5% 3 4 4 4 6% 9% 48%	2530
	11th to 20th	Night	7% 4 1 4 4 4 5% 5% 8% 9% 48%	2613
	21st to end	Night	5% 5% 3 6% 5% 4 5% 5% 8% 9% 45%	3087
August	1st to 10th	Night	12% 6% 3 6% 4 4 4 4 5% 10% 42%	2825
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 4 6% 8% 47%	2785
	21st to end	Night	6% 5% 3 6% 5% 5% 6% 4 6% 8% 46%	3174
September	1st to 10th	Night	10% 6% 2 6% 4 3 4 3 5% 7% 49%	2035
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 4 8% 53%	2090
	21st to end	Night	13% 7% 3 7% 5% 4 4 3 5% 6% 43%	2090
October	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 7% 47%	2144
	11th to 20th	Night	17% 9% 3 6% 4 4 3 4 5% 6% 39%	2280
	21st to end	Night	20% 8% 3 7% 5% 4 4 4 5% 6% 34%	2617
November	1st to 10th	Night	13% 8% 3 7% 4 4 4 3 4 7% 41%	2536
	11th to 20th	Night	15% 11% 3 8% 5% 4 5% 3 6% 7% 32%	2600
	21st to end	Night	20% 12% 4 8% 6% 5% 5% 4 5% 6% 26%	2600
December	1st to 10th	Night	12% 14% 5% 12% 6% 6% 5% 4 5% 6% 24%	4575
	11th to 20th	Night	17% 17% 6% 11% 5% 5% 4 3 4 6% 22%	4970
	21st to end	Night	25% 19% 5% 11% 5% 4 4 3 5% 5% 13%	6886

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% 5% 11% 13% 3	3618
	11th to 20th	Day	73% 5% 9% 5% 8%	3837
	21st to end	Day	74% 3 8% 9% 6%	4466
February	1st to 10th	Day	64% 6% 11% 13% 6%	1800
	11th to 20th	Day	62% 5% 10% 16% 6% 1	1911
	21st to end	Day	57% 7% 14% 14% 6% 2	1570
March	1st to 10th	Day	61% 5% 11% 18% 5%	3233
	11th to 20th	Day	48% 7% 15% 21% 8% 1	3480
	21st to end	Day	56% 8% 13% 15% 7%	3815
April	1st to 10th	Day	51% 7% 13% 16% 12% 1	2175
	11th to 20th	Day	48% 6% 12% 18% 13% 2	2406
	21st to end	Day	47% 5% 14% 21% 11% 2	2470
May	1st to 10th	Day	49% 8% 11% 22% 9% 1	4120
	11th to 20th	Day	45% 9% 14% 22% 9% 1	4085
	21st to end	Day	36% 9% 11% 19% 18% 5%	4489
June	1st to 10th	Day	39% 9% 11% 26% 12% 3	2532
	11th to 20th	Day	37% 7% 15% 22% 17% 3	2660
	21st to end	Day	39% 8% 14% 20% 16% 4	2660
July	1st to 10th	Day	41% 6% 11% 23% 16% 3	4371
	11th to 20th	Day	31% 7% 13% 30% 18% 1	4274
	21st to end	Day	37% 8% 14% 18% 21% 3	4324
August	1st to 10th	Day	35% 8% 15% 26% 15% 1	4091
	11th to 20th	Day	36% 8% 14% 30% 11% 1	4083
	21st to end	Day	36% 7% 13% 30% 12% 2	4477
September	1st to 10th	Day	38% 6% 14% 22% 17% 3	2124
	11th to 20th	Day	33% 7% 11% 28% 19% 1	2090
	21st to end	Day	42% 7% 12% 22% 16%	2090
October	1st to 10th	Day	29% 5% 13% 32% 20% 1	3385
	11th to 20th	Day	39% 4 10% 27% 19% 1	3140
	21st to end	Day	42% 5% 12% 23% 17% 1	3239
November	1st to 10th	Day	42% 6% 12% 24% 15% 1	1762
	11th to 20th	Day	52% 5% 12% 19% 11% 1	1800
	21st to end	Day	57% 7% 12% 17% 7%	1795
December	1st to 10th	Day	61% 5% 15% 15% 4	2600
	11th to 20th	Day	66% 5% 11% 15% 5%	3192
	21st to end	Day	76% 5% 6% 9% 4	4053
January	1st to 10th	Night	66% 6% 8% 15% 4	6348
	11th to 20th	Night	71% 5% 9% 9% 7%	6114
	21st to end	Night	65% 4 12% 10% 8%	6490
February	1st to 10th	Night	62% 6% 11% 15% 5% 1	2600
	11th to 20th	Night	58% 6% 10% 18% 8% 1	2384
	21st to end	Night	58% 6% 10% 17% 8% 1	1885
March	1st to 10th	Night	52% 4 12% 23% 8% 1	3619
	11th to 20th	Night	38% 7% 17% 21% 15% 1	3480
	21st to end	Night	47% 7% 12% 24% 9% 1	3817
April	1st to 10th	Night	49% 6% 12% 18% 14% 2	2019
	11th to 20th	Night	46% 6% 11% 18% 17% 2	1721
	21st to end	Night	45% 7% 11% 20% 15% 3	1710
May	1st to 10th	Night	51% 7% 11% 18% 12% 2	2840
	11th to 20th	Night	43% 9% 11% 23% 12% 1	2813
	21st to end	Night	32% 8% 12% 26% 20% 3	3087
June	1st to 10th	Night	37% 7% 11% 26% 16% 3	1644
	11th to 20th	Night	35% 8% 13% 25% 16% 3	1520
	21st to end	Night	32% 8% 14% 26% 18% 2	1520
July	1st to 10th	Night	32% 6% 11% 27% 18% 6%	2490
	11th to 20th	Night	28% 7% 16% 30% 16% 2	2565
	21st to end	Night	32% 8% 17% 21% 18% 4	2988
August	1st to 10th	Night	38% 8% 12% 25% 15% 2	2814
	11th to 20th	Night	33% 7% 12% 29% 17% 1	2812
	21st to end	Night	34% 7% 13% 28% 17% 2	3174
September	1st to 10th	Night	34% 7% 12% 25% 18% 4	2035
	11th to 20th	Night	30% 6% 11% 27% 25% 2	2090
	21st to end	Night	41% 7% 11% 22% 18% 2	2090
October	1st to 10th	Night	26% 4 9% 31% 29% 1	3573
	11th to 20th	Night	37% 4 11% 27% 20% 1	3779
	21st to end	Night	37% 5% 9% 31% 15% 3	4304
November	1st to 10th	Night	42% 6% 11% 24% 16% 1	2536
	11th to 20th	Night	49% 6% 12% 21% 11% 1	2600
	21st to end	Night	59% 5% 11% 18% 6%	2600
December	1st to 10th	Night	59% 5% 13% 17% 5%	4575
	11th to 20th	Night	62% 6% 12% 14% 5%	4972
	21st to end	Night	72% 6% 10% 9% 4	6886

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	21% 13% 33% 25% 6% 2	300
	11th to 20th	Day	22% 10% 45% 17% 6%	300
	21st to end	Day	25% 11% 30% 27% 6%	330
February	1st to 10th	Day	24% 12% 38% 21% 5%	300
	11th to 20th	Day	21% 17% 36% 23% 3	300
	21st to end	Day	21% 16% 44% 14% 4	225
March	1st to 10th	Day	20% 13% 34% 25% 8%	331
	11th to 20th	Day	20% 10% 34% 27% 7% 1	460
	21st to end	Day	25% 11% 31% 26% 6% 1	506
April	1st to 10th	Day	17% 10% 33% 27% 12% 1	460
	11th to 20th	Day	19% 11% 33% 24% 9% 4	452
	21st to end	Day	22% 9% 31% 27% 10% 2	460
May	1st to 10th	Day	21% 6% 28% 27% 15% 3	463
	11th to 20th	Day	17% 5% 26% 29% 18% 5%	487
	21st to end	Day	13% 3 20% 29% 26% 9%	529
June	1st to 10th	Day	11% 4 22% 37% 23% 3	552
	11th to 20th	Day	11% 4 18% 37% 25% 4	680
	21st to end	Day	12% 4 18% 39% 22% 5%	680
July	1st to 10th	Day	8% 3 14% 34% 29% 12%	671
	11th to 20th	Day	9% 3 15% 33% 29% 11%	616
	21st to end	Day	8% 2 13% 33% 33% 11%	532
August	1st to 10th	Day	10% 2 11% 33% 30% 13%	486
	11th to 20th	Day	11% 3 11% 34% 31% 10%	479
	21st to end	Day	10% 3 13% 28% 33% 12%	539
September	1st to 10th	Day	14% 2 16% 27% 29% 13%	490
	11th to 20th	Day	11% 1 11% 28% 30% 18%	490
	21st to end	Day	20% 4 22% 24% 20% 9%	490
October	1st to 10th	Day	11% 6% 21% 31% 21% 10%	490
	11th to 20th	Day	18% 12% 28% 27% 12% 3	490
	21st to end	Day	21% 12% 32% 26% 8% 2	425
November	1st to 10th	Day	13% 12% 27% 30% 14% 4	300
	11th to 20th	Day	22% 17% 27% 24% 9% 1	300
	21st to end	Day	25% 9% 37% 26% 3	297
December	1st to 10th	Day	18% 15% 36% 23% 9% 1	200
	11th to 20th	Day	21% 15% 39% 21% 4	247
	21st to end	Day	25% 12% 34% 25% 4	330
January	1st to 10th	Night	24% 11% 36% 22% 6%	1000
	11th to 20th	Night	26% 11% 36% 21% 5%	897
	21st to end	Night	23% 9% 36% 27% 4 1	880
February	1st to 10th	Night	24% 9% 36% 26% 5%	800
	11th to 20th	Night	23% 10% 35% 29% 4	783
	21st to end	Night	25% 12% 37% 23% 4	629
March	1st to 10th	Night	19% 9% 34% 29% 9%	760
	11th to 20th	Night	19% 11% 32% 28% 8% 1	760
	21st to end	Night	23% 8% 28% 31% 9% 1	836
April	1st to 10th	Night	20% 9% 33% 32% 7%	689
	11th to 20th	Night	20% 6% 33% 28% 10% 2	563
	21st to end	Night	21% 6% 30% 30% 11% 1	570
May	1st to 10th	Night	21% 4 28% 32% 12% 3	570
	11th to 20th	Night	21% 4 24% 32% 18% 2	561
	21st to end	Night	11% 2 20% 35% 25% 5%	617
June	1st to 10th	Night	13% 4 20% 39% 22% 2	570
	11th to 20th	Night	11% 3 19% 40% 25% 2	570
	21st to end	Night	11% 2 20% 39% 25% 3	570
July	1st to 10th	Night	10% 2 10% 37% 36% 5%	555
	11th to 20th	Night	9% 3 13% 41% 27% 6%	552
	21st to end	Night	8% 1 12% 42% 33% 4	624
August	1st to 10th	Night	9% 3 10% 41% 31% 6%	567
	11th to 20th	Night	10% 1 14% 35% 34% 7%	564
	21st to end	Night	10% 2 13% 33% 36% 6%	627
September	1st to 10th	Night	13% 3 16% 30% 31% 7%	705
	11th to 20th	Night	11% 3 13% 31% 35% 7%	760
	21st to end	Night	17% 6% 20% 31% 22% 5%	760
October	1st to 10th	Night	17% 7% 21% 32% 19% 5%	760
	11th to 20th	Night	22% 12% 27% 24% 12% 2	760
	21st to end	Night	26% 9% 29% 28% 7% 1	836
November	1st to 10th	Night	19% 11% 29% 27% 13% 2	780
	11th to 20th	Night	22% 13% 34% 22% 8% 1	800
	21st to end	Night	29% 11% 32% 22% 6%	800
December	1st to 10th	Night	25% 13% 35% 21% 6%	800
	11th to 20th	Night	25% 12% 37% 23% 4	800
	21st to end	Night	30% 11% 31% 24% 4	1055

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