Oregon Trail Overlook
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oregon Trail Overlook clear sky chart from 2005-11-06 to 2025-05-26.

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	5% 5% 2 5% 4 4 4 3 5% 7% 57%	117744
	Night	6% 6% 2 6% 4 4 4 4 5% 7% 52%	120327
Transparency	Day	28% 3 10% 23% 21% 15%	117490
	Night	32% 3 12% 23% 18% 11%	120171
Seeing	Day	8% 9% 24% 29% 21% 10%	15153
	Night	9% 5% 21% 42% 21% 2	24682

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% 12% 5% 11% 6% 5% 5% 4 7% 8% 30%	3639
	11th to 20th	Day	5% 8% 4 12% 7% 5% 5% 3 5% 6% 39%	3640
	21st to end	Day	7% 8% 3 7% 5% 5% 4 3 5% 5% 48%	4075
February	1st to 10th	Day	6% 8% 3 8% 5% 5% 4 4 6% 9% 41%	1810
	11th to 20th	Day	7% 9% 3 8% 6% 4 5% 3 6% 9% 41%	1950
	21st to end	Day	5% 8% 4 7% 6% 4 4 5% 7% 7% 42%	1650
March	1st to 10th	Day	7% 9% 3 9% 5% 4 5% 3 6% 7% 41%	4518
	11th to 20th	Day	5% 8% 2 6% 4 3 3 3 5% 6% 54%	4947
	21st to end	Day	5% 7% 3 7% 6% 6% 6% 5% 7% 9% 41%	5529
April	1st to 10th	Day	5% 8% 2 6% 5% 5% 4 4 6% 9% 45%	2400
	11th to 20th	Day	5% 6% 2 6% 4 4 4 4 6% 7% 52%	2460
	21st to end	Day	6% 5% 3 5% 4 4 3 3 6% 8% 52%	2600
May	1st to 10th	Day	6% 6% 2 7% 4 4 4 3 5% 7% 52%	4240
	11th to 20th	Day	3 3 1 4 2 3 3 3 5% 6% 68%	4201
	21st to end	Day	5% 3 1 4 3 3 5% 3 6% 8% 59%	4784
June	1st to 10th	Day	6% 3 1 4 4 4 5% 4 7% 9% 53%	2660
	11th to 20th	Day	4 3 1 4 3 3 3 4 4 8% 64%	2660
	21st to end	Day	2 2 1 2 2 2 3 2 4 6% 74%	2660
July	1st to 10th	Day	1 1 1 2 2 1 2 2 3 4 81%	4420
	11th to 20th	Day	2 2 1 3 2 2 3 3 4 5% 73%	4420
	21st to end	Day	2 2 1 3 2 3 3 3 4 5% 72%	4740
August	1st to 10th	Day	3 3 1 4 3 3 4 3 5% 7% 64%	4092
	11th to 20th	Day	1 1 1 2 2 2 3 2 4 5% 77%	4028
	21st to end	Day	2 3 1 3 2 2 2 2 3 4 77%	4305
September	1st to 10th	Day	1 2 1 3 3 2 3 2 4 4 75%	2236
	11th to 20th	Day	3 3 1 4 2 2 3 3 4 5% 68%	2090
	21st to end	Day	6% 3 1 3 2 3 2 3 4 6% 68%	2090
October	1st to 10th	Day	4 3 1 4 3 3 3 2 3 5% 69%	3490
	11th to 20th	Day	4 4 1 4 3 3 3 2 4 7% 66%	3112
	21st to end	Day	3 3 1 4 3 3 4 3 6% 10% 59%	3101
November	1st to 10th	Day	7% 7% 2 7% 4 5% 4 3 5% 8% 48%	1764
	11th to 20th	Day	7% 6% 3 5% 5% 5% 5% 5% 7% 8% 45%	1800
	21st to end	Day	6% 8% 2 7% 6% 5% 3 4 6% 9% 43%	1800
December	1st to 10th	Day	5% 5% 2 6% 3 3 3 3 4 7% 58%	2801
	11th to 20th	Day	10% 9% 4 8% 5% 5% 5% 5% 8% 8% 33%	3028
	21st to end	Day	9% 10% 5% 11% 6% 5% 5% 4 7% 6% 33%	4004
January	1st to 10th	Night	9% 12% 4 12% 6% 6% 5% 4 7% 7% 27%	6306
	11th to 20th	Night	7% 10% 4 10% 6% 5% 5% 4 5% 7% 37%	6320
	21st to end	Night	7% 8% 3 8% 4 4 4 3 5% 6% 48%	6881
February	1st to 10th	Night	8% 9% 4 9% 6% 4 5% 4 6% 8% 37%	2585
	11th to 20th	Night	7% 8% 3 6% 4 5% 4 4 5% 8% 46%	2338
	21st to end	Night	5% 9% 3 6% 5% 3 4 5% 8% 8% 43%	1980
March	1st to 10th	Night	8% 9% 3 6% 4 4 4 4 6% 8% 45%	5376
	11th to 20th	Night	6% 7% 2 6% 5% 5% 4 4 5% 7% 48%	5053
	21st to end	Night	4 6% 2 6% 5% 5% 6% 5% 7% 8% 45%	5431
April	1st to 10th	Night	7% 7% 2 6% 4 5% 5% 4 6% 7% 47%	2000
	11th to 20th	Night	5% 8% 2 5% 5% 4 4 4 5% 6% 51%	1886
	21st to end	Night	4 5% 2 6% 4 4 4 4 7% 8% 52%	1800
May	1st to 10th	Night	9% 5% 2 5% 3 3 4 4 7% 8% 50%	2950
	11th to 20th	Night	4 3 1 3 3 2 3 3 5% 7% 66%	2924
	21st to end	Night	5% 5% 2 5% 4 3 5% 3 6% 7% 55%	2906
June	1st to 10th	Night	6% 4 2 5% 4 5% 4 4 6% 8% 52%	1520
	11th to 20th	Night	3 3 2 4 3 3 3 3 4 8% 65%	1520
	21st to end	Night	1 1 1 2 2 2 3 3 5% 6% 73%	1520
July	1st to 10th	Night	1 1 1 2 2 3 2 3 4 79%	2540
	11th to 20th	Night	1 1 2 3 2 3 3 4 6% 76%	2540
	21st to end	Night	2 3 1 4 2 2 3 4 5% 7% 67%	2813
August	1st to 10th	Night	3 3 1 4 3 4 4 3 5% 7% 62%	2847
	11th to 20th	Night	2 1 1 2 2 3 3 3 4 6% 73%	2811
	21st to end	Night	2 2 1 3 2 3 3 2 3 4 74%	3365
September	1st to 10th	Night	3 2 1 3 2 3 3 2 3 5% 74%	1943
	11th to 20th	Night	4 3 1 3 2 4 4 3 4 7% 65%	2090
	21st to end	Night	5% 2 1 4 3 2 3 2 4 4 70%	2090
October	1st to 10th	Night	4 2 1 3 2 2 2 1 4 5% 74%	3490
	11th to 20th	Night	5% 3 2 5% 4 4 4 2 4 6% 61%	3814
	21st to end	Night	4 4 1 5% 4 4 5% 4 6% 8% 55%	4457
November	1st to 10th	Night	9% 6% 2 5% 4 4 5% 4 6% 7% 49%	2535
	11th to 20th	Night	7% 6% 2 7% 5% 4 5% 5% 7% 8% 45%	2600
	21st to end	Night	6% 7% 3 6% 4 5% 5% 4 6% 7% 48%	2600
December	1st to 10th	Night	6% 5% 2 5% 3 3 3 3 4 8% 59%	4368
	11th to 20th	Night	9% 10% 4 10% 5% 5% 5% 4 7% 9% 31%	5176
	21st to end	Night	9% 8% 4 9% 6% 6% 6% 5% 6% 7% 35%	6952

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	51% 5% 20% 20% 3	3639
	11th to 20th	Day	47% 4 20% 23% 6% 1	3640
	21st to end	Day	39% 2 19% 27% 13%	4075
February	1st to 10th	Day	41% 5% 22% 24% 7% 1	1810
	11th to 20th	Day	41% 5% 19% 25% 10% 1	1960
	21st to end	Day	39% 4 15% 28% 12% 1	1650
March	1st to 10th	Day	43% 2 10% 31% 12% 3	4518
	11th to 20th	Day	31% 2 9% 37% 19% 1	4947
	21st to end	Day	40% 3 16% 28% 11% 1	5529
April	1st to 10th	Day	36% 3 12% 26% 19% 5%	2400
	11th to 20th	Day	30% 3 9% 30% 22% 5%	2463
	21st to end	Day	31% 2 9% 30% 24% 4	2600
May	1st to 10th	Day	34% 1 5% 24% 28% 7%	4240
	11th to 20th	Day	19% 2 5% 28% 33% 13%	4204
	21st to end	Day	25% 2 8% 31% 29% 5%	4784
June	1st to 10th	Day	28% 3 12% 29% 24% 5%	2660
	11th to 20th	Day	21% 2 6% 22% 34% 15%	2660
	21st to end	Day	13% 1 3 18% 35% 29%	2660
July	1st to 10th	Day	9% 1 1 9% 19% 61%	4351
	11th to 20th	Day	14% 2 1 6% 20% 57%	4345
	21st to end	Day	15% 2 1 8% 28% 46%	4590
August	1st to 10th	Day	20% 3 3 12% 25% 37%	4078
	11th to 20th	Day	11% 2 2 12% 35% 39%	4067
	21st to end	Day	13% 2 1 12% 36% 35%	4304
September	1st to 10th	Day	14% 2 3 15% 32% 34%	2236
	11th to 20th	Day	20% 2 6% 26% 27% 19%	2090
	21st to end	Day	19% 2 5% 23% 38% 14%	2090
October	1st to 10th	Day	21% 2 5% 19% 36% 17%	3490
	11th to 20th	Day	21% 2 7% 34% 27% 8%	3112
	21st to end	Day	21% 2 12% 35% 24% 5%	3101
November	1st to 10th	Day	36% 4 14% 28% 17% 2	1764
	11th to 20th	Day	35% 4 20% 32% 8% 1	1800
	21st to end	Day	38% 2 18% 32% 9%	1800
December	1st to 10th	Day	28% 4 24% 37% 7%	2801
	11th to 20th	Day	47% 3 26% 22% 2	3028
	21st to end	Day	51% 3 17% 23% 6%	4004
January	1st to 10th	Night	55% 4 20% 17% 3	6306
	11th to 20th	Night	47% 4 19% 24% 6% 1	6320
	21st to end	Night	39% 3 18% 28% 11% 1	6881
February	1st to 10th	Night	44% 5% 21% 23% 6% 1	2585
	11th to 20th	Night	36% 4 21% 27% 11% 1	2339
	21st to end	Night	36% 4 18% 29% 12% 1	1980
March	1st to 10th	Night	38% 3 10% 33% 15% 1	5376
	11th to 20th	Night	36% 2 10% 30% 21% 1	5053
	21st to end	Night	36% 3 17% 30% 13% 2	5431
April	1st to 10th	Night	36% 3 11% 27% 19% 5%	2000
	11th to 20th	Night	34% 3 7% 28% 24% 4	1886
	21st to end	Night	28% 3 11% 29% 26% 4	1800
May	1st to 10th	Night	33% 1 5% 25% 26% 10%	2950
	11th to 20th	Night	19% 2 5% 22% 38% 13%	2925
	21st to end	Night	29% 2 8% 22% 35% 4	2906
June	1st to 10th	Night	30% 2 9% 28% 25% 6%	1520
	11th to 20th	Night	20% 2 6% 19% 37% 15%	1520
	21st to end	Night	13% 2 3 16% 32% 34%	1520
July	1st to 10th	Night	11% 2 1 6% 18% 62%	2500
	11th to 20th	Night	11% 2 2 5% 17% 63%	2492
	21st to end	Night	17% 2 1 7% 30% 43%	2725
August	1st to 10th	Night	21% 3 3 10% 21% 43%	2838
	11th to 20th	Night	12% 3 2 13% 28% 42%	2838
	21st to end	Night	15% 2 2 10% 32% 39%	3365
September	1st to 10th	Night	15% 1 3 12% 31% 38%	1943
	11th to 20th	Night	21% 2 4 21% 29% 23%	2090
	21st to end	Night	20% 1 3 18% 35% 21%	2090
October	1st to 10th	Night	16% 1 5% 17% 33% 28%	3490
	11th to 20th	Night	26% 2 4 28% 30% 9%	3814
	21st to end	Night	27% 3 12% 33% 23% 3	4457
November	1st to 10th	Night	34% 3 13% 28% 19% 3	2535
	11th to 20th	Night	36% 3 16% 32% 13% 1	2600
	21st to end	Night	34% 3 19% 34% 10%	2600
December	1st to 10th	Night	27% 3 26% 36% 8% 1	4368
	11th to 20th	Night	49% 4 22% 22% 2	5176
	21st to end	Night	48% 3 20% 21% 7%	6952

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	18% 9% 28% 29% 14% 3	200
	11th to 20th	Day	8% 6% 36% 28% 17% 6%	200
	21st to end	Day	13% 6% 25% 28% 24% 4	237
February	1st to 10th	Day	16% 11% 33% 26% 13% 1	310
	11th to 20th	Day	13% 14% 30% 27% 13% 3	495
	21st to end	Day	10% 16% 31% 31% 11% 1	414
March	1st to 10th	Day	10% 12% 33% 31% 12% 2	500
	11th to 20th	Day	10% 13% 30% 29% 15% 3	500
	21st to end	Day	11% 17% 32% 25% 13% 3	550
April	1st to 10th	Day	15% 15% 25% 23% 17% 6%	500
	11th to 20th	Day	11% 18% 30% 24% 13% 4	492
	21st to end	Day	10% 15% 31% 26% 14% 4	500
May	1st to 10th	Day	8% 15% 27% 29% 14% 8%	503
	11th to 20th	Day	13% 12% 25% 25% 17% 9%	528
	21st to end	Day	9% 8% 22% 23% 24% 14%	550
June	1st to 10th	Day	6% 8% 24% 27% 23% 12%	490
	11th to 20th	Day	7% 13% 29% 28% 17% 6%	490
	21st to end	Day	3 6% 24% 34% 23% 10%	490
July	1st to 10th	Day	3 4 21% 33% 27% 13%	486
	11th to 20th	Day	2 3 17% 36% 27% 14%	484
	21st to end	Day	3 2 12% 34% 34% 16%	531
August	1st to 10th	Day	6% 3 18% 32% 26% 14%	487
	11th to 20th	Day	2 4 14% 29% 30% 20%	477
	21st to end	Day	3 5% 17% 29% 26% 20%	539
September	1st to 10th	Day	3 4 16% 23% 32% 22%	490
	11th to 20th	Day	6% 3 19% 29% 27% 17%	490
	21st to end	Day	6% 8% 19% 26% 30% 12%	490
October	1st to 10th	Day	8% 7% 17% 24% 27% 18%	490
	11th to 20th	Day	6% 11% 21% 21% 27% 14%	333
	21st to end	Day	8% 8% 19% 40% 19% 5%	330
November	1st to 10th	Day	16% 6% 22% 32% 19% 5%	300
	11th to 20th	Day	9% 10% 19% 36% 19% 6%	300
	21st to end	Day	13% 7% 21% 32% 21% 7%	300
December	1st to 10th	Day	15% 7% 21% 30% 16% 11%	257
	11th to 20th	Day	15% 9% 28% 30% 16% 4	200
	21st to end	Day	16% 10% 25% 30% 14% 5%	220
January	1st to 10th	Night	14% 5% 23% 41% 17% 1	800
	11th to 20th	Night	13% 6% 23% 39% 18% 2	800
	21st to end	Night	12% 5% 19% 42% 19% 2	880
February	1st to 10th	Night	12% 5% 31% 39% 12% 1	800
	11th to 20th	Night	10% 7% 25% 42% 15% 1	784
	21st to end	Night	11% 8% 27% 39% 15%	660
March	1st to 10th	Night	10% 6% 28% 44% 12% 1	800
	11th to 20th	Night	9% 5% 28% 39% 18% 2	800
	21st to end	Night	11% 9% 29% 37% 13% 1	880
April	1st to 10th	Night	12% 7% 22% 41% 17% 2	800
	11th to 20th	Night	9% 10% 30% 39% 10% 1	698
	21st to end	Night	6% 9% 29% 39% 16% 2	600
May	1st to 10th	Night	9% 8% 27% 37% 18% 2	600
	11th to 20th	Night	8% 7% 18% 40% 24% 3	591
	21st to end	Night	10% 7% 19% 33% 28% 3	485
June	1st to 10th	Night	8% 5% 16% 40% 28% 3	380
	11th to 20th	Night	5% 6% 23% 43% 23% 1	380
	21st to end	Night	3 3 21% 48% 25% 1	380
July	1st to 10th	Night	2 1 11% 55% 30% 1	370
	11th to 20th	Night	1 1 14% 56% 28%	368
	21st to end	Night	2 2 8% 51% 36% 1	444
August	1st to 10th	Night	5% 1 11% 52% 29% 1	567
	11th to 20th	Night	2 1 12% 52% 31% 1	564
	21st to end	Night	3 2 15% 46% 31% 3	755
September	1st to 10th	Night	3 2 12% 45% 34% 5%	760
	11th to 20th	Night	6% 3 12% 41% 33% 5%	760
	21st to end	Night	7% 4 12% 39% 32% 6%	760
October	1st to 10th	Night	7% 4 16% 43% 27% 4	760
	11th to 20th	Night	10% 4 17% 38% 28% 3	760
	21st to end	Night	7% 4 23% 40% 23% 3	836
November	1st to 10th	Night	12% 4 19% 41% 23% 1	780
	11th to 20th	Night	11% 5% 25% 40% 16% 2	800
	21st to end	Night	10% 6% 22% 38% 22% 3	800
December	1st to 10th	Night	13% 5% 24% 39% 17% 3	800
	11th to 20th	Night	17% 5% 18% 43% 16% 2	800
	21st to end	Night	18% 7% 25% 39% 10% 1	880

Page generated at 2025-05-23 07:21:03. Version ac07

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