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 2026-09-29.

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 5% 2 5% 4 3 4 3 5% 7% 58%	157596
	Night	5% 5% 2 6% 4 4 4 4 6% 7% 53%	151217
Transparency	Day	26% 3 9% 23% 23% 17%	152380
	Night	31% 3 12% 23% 19% 13%	147433
Seeing	Day	8% 9% 23% 29% 21% 10%	16943
	Night	9% 5% 21% 42% 21% 2	26290

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% 11% 5% 10% 6% 5% 5% 4 7% 8% 32%	4722
	11th to 20th	Day	4 6% 4 11% 6% 4 4 3 4 7% 47%	4740
	21st to end	Day	5% 7% 3 8% 5% 6% 5% 4 6% 7% 44%	5286
February	1st to 10th	Day	6% 8% 3 8% 5% 5% 4 4 6% 9% 42%	1901
	11th to 20th	Day	7% 10% 3 8% 6% 4 5% 3 6% 9% 40%	2050
	21st to end	Day	5% 8% 4 7% 6% 4 5% 5% 7% 7% 43%	1730
March	1st to 10th	Day	7% 9% 3 8% 5% 4 5% 3 6% 7% 41%	4618
	11th to 20th	Day	5% 8% 2 6% 3 3 3 3 5% 6% 54%	5055
	21st to end	Day	5% 7% 3 7% 6% 6% 6% 5% 7% 9% 41%	5651
April	1st to 10th	Day	5% 7% 2 6% 5% 5% 4 4 6% 9% 46%	2520
	11th to 20th	Day	4 6% 2 6% 4 4 4 4 6% 8% 51%	2585
	21st to end	Day	6% 5% 3 6% 4 3 3 3 6% 8% 53%	2730
May	1st to 10th	Day	4 4 2 5% 3 3 3 3 4 7% 62%	7626
	11th to 20th	Day	3 3 2 4 3 3 3 3 5% 7% 65%	7596
	21st to end	Day	3 3 1 4 3 3 4 4 6% 8% 61%	8815
June	1st to 10th	Day	5% 3 1 4 4 4 5% 4 7% 8% 54%	2940
	11th to 20th	Day	4 3 1 3 3 3 3 3 4 7% 65%	2940
	21st to end	Day	2 2 1 2 2 2 3 2 4 6% 74%	2940
July	1st to 10th	Day	1 1 1 2 2 2 2 3 4 5% 76%	8219
	11th to 20th	Day	1 2 1 3 2 3 3 3 4 5% 73%	8220
	21st to end	Day	1 2 1 3 2 3 3 3 5% 7% 69%	8820
August	1st to 10th	Day	2 2 1 3 2 3 3 2 4 6% 72%	7549
	11th to 20th	Day	2 2 1 3 3 3 3 3 5% 6% 69%	7565
	21st to end	Day	4 4 2 4 3 3 4 3 5% 6% 63%	7976
September	1st to 10th	Day	1 2 1 3 3 2 3 2 4 5% 73%	2471
	11th to 20th	Day	3 3 1 4 2 2 3 3 4 5% 69%	2310
	21st to end	Day	5% 3 1 4 2 3 3 2 4 6% 67%	2289
October	1st to 10th	Day	3 3 1 4 3 3 3 2 3 5% 69%	3600
	11th to 20th	Day	4 4 2 4 3 3 3 2 4 7% 65%	3211
	21st to end	Day	3 3 1 3 3 3 4 3 6% 9% 59%	3200
November	1st to 10th	Day	7% 6% 2 7% 4 4 4 3 5% 8% 49%	1854
	11th to 20th	Day	7% 6% 3 5% 5% 5% 5% 5% 7% 8% 45%	1890
	21st to end	Day	6% 8% 2 7% 6% 5% 3 4 6% 9% 44%	1890
December	1st to 10th	Day	5% 5% 2 6% 4 3 4 3 4 7% 58%	2887
	11th to 20th	Day	10% 9% 3 8% 5% 5% 5% 5% 8% 8% 33%	3108
	21st to end	Day	9% 10% 5% 11% 6% 5% 5% 4 7% 6% 33%	4092
January	1st to 10th	Night	9% 11% 4 12% 7% 6% 6% 5% 7% 7% 27%	8172
	11th to 20th	Night	6% 7% 3 8% 5% 4 4 4 5% 7% 47%	8220
	21st to end	Night	6% 7% 3 8% 4 4 4 3 6% 8% 47%	8897
February	1st to 10th	Night	8% 9% 4 8% 6% 4 5% 4 6% 8% 38%	2714
	11th to 20th	Night	7% 8% 3 7% 4 5% 5% 4 5% 8% 45%	2458
	21st to end	Night	5% 9% 3 6% 5% 3 4 5% 8% 9% 43%	2076
March	1st to 10th	Night	8% 9% 3 6% 4 3 4 4 6% 8% 45%	5496
	11th to 20th	Night	6% 7% 2 6% 5% 5% 4 4 5% 7% 49%	5164
	21st to end	Night	4 5% 2 6% 5% 5% 6% 5% 7% 8% 45%	5551
April	1st to 10th	Night	7% 7% 2 5% 4 5% 4 4 6% 7% 48%	2100
	11th to 20th	Night	5% 8% 2 5% 4 4 5% 4 6% 6% 50%	1981
	21st to end	Night	4 5% 2 6% 4 4 4 4 7% 8% 53%	1890
May	1st to 10th	Night	5% 4 1 4 3 3 4 3 6% 8% 59%	5321
	11th to 20th	Night	4 4 1 5% 4 3 4 3 5% 7% 61%	5296
	21st to end	Night	3 3 1 5% 4 4 5% 3 6% 7% 58%	5363
June	1st to 10th	Night	5% 4 2 4 3 5% 4 4 6% 8% 54%	1680
	11th to 20th	Night	3 3 1 4 3 3 3 3 4 8% 65%	1680
	21st to end	Night	2 1 1 3 3 2 3 3 5% 6% 72%	1680
July	1st to 10th	Night	2 1 1 2 2 3 4 3 4 5% 74%	4740
	11th to 20th	Night	1 2 1 3 3 3 3 3 5% 6% 73%	4740
	21st to end	Night	2 2 1 4 3 3 4 4 6% 8% 63%	5247
August	1st to 10th	Night	2 2 1 2 2 3 3 2 4 6% 74%	5273
	11th to 20th	Night	2 2 1 3 2 3 4 4 5% 7% 67%	5291
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 7% 58%	6257
September	1st to 10th	Night	3 2 1 3 2 3 3 2 4 6% 72%	2148
	11th to 20th	Night	4 2 1 3 2 4 4 3 4 7% 65%	2310
	21st to end	Night	5% 3 1 4 3 2 3 2 4 5% 69%	2288
October	1st to 10th	Night	4 2 1 3 2 3 2 1 4 5% 73%	3600
	11th to 20th	Night	5% 3 2 6% 4 4 4 3 5% 6% 60%	3935
	21st to end	Night	4 4 1 6% 4 4 4 4 6% 8% 55%	4600
November	1st to 10th	Night	8% 6% 2 5% 4 4 5% 4 6% 7% 49%	2665
	11th to 20th	Night	7% 6% 3 7% 5% 4 5% 5% 7% 8% 44%	2730
	21st to end	Night	6% 7% 3 6% 4 5% 5% 4 6% 8% 48%	2730
December	1st to 10th	Night	6% 6% 2 5% 3 3 3 3 4 8% 58%	4502
	11th to 20th	Night	9% 10% 4 10% 5% 5% 5% 4 7% 9% 31%	5316
	21st to end	Night	9% 8% 4 9% 6% 6% 6% 5% 6% 7% 35%	7106

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	49% 5% 23% 19% 4	4723
	11th to 20th	Day	38% 4 20% 30% 7%	4740
	21st to end	Day	40% 3 20% 27% 10%	5286
February	1st to 10th	Day	40% 5% 23% 25% 7% 1	1901
	11th to 20th	Day	40% 5% 18% 26% 10% 1	2060
	21st to end	Day	39% 4 15% 28% 13% 1	1730
March	1st to 10th	Day	43% 2 10% 31% 11% 3	4618
	11th to 20th	Day	31% 2 10% 36% 19% 2	5055
	21st to end	Day	40% 3 16% 28% 12% 1	5651
April	1st to 10th	Day	35% 3 12% 27% 19% 4	2520
	11th to 20th	Day	31% 3 10% 29% 22% 5%	2588
	21st to end	Day	30% 2 9% 30% 25% 4	2730
May	1st to 10th	Day	24% 2 4 22% 37% 10%	7626
	11th to 20th	Day	20% 3 5% 27% 34% 12%	7599
	21st to end	Day	21% 2 7% 29% 30% 10%	8816
June	1st to 10th	Day	26% 3 11% 27% 26% 7%	2940
	11th to 20th	Day	20% 2 6% 22% 35% 15%	2940
	21st to end	Day	14% 1 3 18% 35% 28%	2940
July	1st to 10th	Day	11% 2 2 11% 23% 51%	8151
	11th to 20th	Day	14% 2 1 8% 23% 51%	8145
	21st to end	Day	14% 3 2 12% 31% 38%	8622
August	1st to 10th	Day	15% 2 2 9% 23% 49%	5978
	11th to 20th	Day	18% 3 4 16% 31% 28%	5964
	21st to end	Day	18% 3 4 17% 32% 26%	6255
September	1st to 10th	Day	15% 2 4 16% 32% 32%	2471
	11th to 20th	Day	19% 2 6% 27% 28% 18%	2310
	21st to end	Day	20% 2 5% 22% 37% 14%	2289
October	1st to 10th	Day	20% 2 6% 20% 36% 17%	3600
	11th to 20th	Day	22% 2 7% 34% 26% 8%	3211
	21st to end	Day	21% 2 12% 36% 24% 5%	3200
November	1st to 10th	Day	36% 4 14% 29% 16% 2	1854
	11th to 20th	Day	35% 4 21% 32% 8% 1	1890
	21st to end	Day	38% 2 20% 32% 9%	1890
December	1st to 10th	Day	28% 4 24% 36% 7%	2887
	11th to 20th	Day	47% 3 26% 22% 2	3108
	21st to end	Day	50% 3 17% 23% 6%	4092
January	1st to 10th	Night	55% 4 22% 16% 3	8172
	11th to 20th	Night	38% 3 21% 30% 7% 1	8220
	21st to end	Night	36% 4 20% 30% 9% 1	8897
February	1st to 10th	Night	44% 5% 22% 23% 5% 1	2714
	11th to 20th	Night	37% 4 20% 27% 10% 1	2459
	21st to end	Night	36% 4 17% 30% 12% 1	2076
March	1st to 10th	Night	38% 3 11% 33% 15% 1	5496
	11th to 20th	Night	35% 2 10% 30% 21% 1	5164
	21st to end	Night	35% 3 17% 30% 13% 2	5551
April	1st to 10th	Night	35% 3 11% 27% 19% 5%	2100
	11th to 20th	Night	33% 3 8% 27% 24% 4	1981
	21st to end	Night	28% 3 10% 28% 26% 4	1890
May	1st to 10th	Night	24% 2 6% 20% 37% 11%	5319
	11th to 20th	Night	23% 2 5% 19% 38% 12%	5297
	21st to end	Night	24% 3 7% 20% 37% 8%	5364
June	1st to 10th	Night	29% 2 9% 26% 26% 8%	1680
	11th to 20th	Night	20% 2 6% 19% 36% 17%	1680
	21st to end	Night	14% 2 3 15% 33% 33%	1680
July	1st to 10th	Night	14% 2 1 9% 20% 53%	4700
	11th to 20th	Night	13% 2 2 6% 21% 55%	4692
	21st to end	Night	17% 3 3 11% 31% 35%	5133
August	1st to 10th	Night	15% 2 2 7% 18% 56%	4168
	11th to 20th	Night	18% 4 6% 15% 25% 32%	4168
	21st to end	Night	22% 3 4 14% 28% 28%	4902
September	1st to 10th	Night	16% 2 3 13% 30% 36%	2148
	11th to 20th	Night	21% 2 5% 21% 30% 22%	2310
	21st to end	Night	21% 1 4 18% 35% 21%	2288
October	1st to 10th	Night	16% 1 5% 17% 33% 28%	3600
	11th to 20th	Night	26% 2 4 28% 30% 9%	3935
	21st to end	Night	27% 3 11% 33% 23% 3	4600
November	1st to 10th	Night	34% 3 13% 29% 19% 3	2665
	11th to 20th	Night	36% 4 16% 31% 12% 1	2730
	21st to end	Night	34% 3 20% 33% 9%	2730
December	1st to 10th	Night	28% 3 26% 35% 8% 1	4502
	11th to 20th	Night	49% 4 22% 23% 2	5316
	21st to end	Night	48% 3 20% 21% 7%	7106

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% 10% 28% 28% 14% 4	220
	11th to 20th	Day	7% 5% 32% 29% 19% 7%	220
	21st to end	Day	12% 5% 25% 31% 24% 3	261
February	1st to 10th	Day	15% 11% 31% 26% 15% 2	341
	11th to 20th	Day	13% 14% 30% 27% 13% 3	535
	21st to end	Day	10% 16% 32% 30% 11% 1	446
March	1st to 10th	Day	10% 14% 32% 31% 12% 2	540
	11th to 20th	Day	9% 14% 30% 29% 16% 3	540
	21st to end	Day	11% 16% 32% 26% 13% 2	594
April	1st to 10th	Day	14% 15% 24% 23% 17% 7%	540
	11th to 20th	Day	10% 18% 30% 24% 13% 4	532
	21st to end	Day	10% 15% 31% 26% 14% 4	540
May	1st to 10th	Day	7% 14% 28% 28% 15% 8%	543
	11th to 20th	Day	13% 12% 26% 24% 16% 9%	568
	21st to end	Day	9% 9% 22% 24% 22% 15%	617
June	1st to 10th	Day	5% 9% 25% 27% 22% 12%	570
	11th to 20th	Day	6% 12% 28% 29% 18% 6%	570
	21st to end	Day	4 7% 23% 34% 24% 9%	570
July	1st to 10th	Day	3 4 20% 33% 28% 13%	566
	11th to 20th	Day	2 4 18% 36% 26% 14%	564
	21st to end	Day	3 2 12% 33% 33% 16%	619
August	1st to 10th	Day	5% 3 19% 32% 26% 14%	567
	11th to 20th	Day	3 4 14% 29% 30% 20%	557
	21st to end	Day	4 5% 18% 29% 26% 19%	627
September	1st to 10th	Day	4 4 16% 24% 32% 21%	570
	11th to 20th	Day	5% 4 19% 30% 26% 16%	570
	21st to end	Day	6% 8% 19% 26% 29% 11%	563
October	1st to 10th	Day	7% 6% 19% 24% 27% 17%	530
	11th to 20th	Day	7% 11% 21% 22% 26% 13%	365
	21st to end	Day	9% 8% 19% 39% 20% 5%	363
November	1st to 10th	Day	15% 6% 22% 32% 20% 5%	330
	11th to 20th	Day	8% 10% 18% 36% 20% 7%	330
	21st to end	Day	12% 7% 21% 32% 21% 7%	330
December	1st to 10th	Day	14% 8% 21% 31% 16% 10%	283
	11th to 20th	Day	16% 8% 26% 29% 17% 4	220
	21st to end	Day	18% 10% 24% 31% 13% 5%	242
January	1st to 10th	Night	13% 5% 23% 42% 17% 1	840
	11th to 20th	Night	12% 5% 22% 39% 20% 2	840
	21st to end	Night	12% 5% 19% 43% 19% 2	924
February	1st to 10th	Night	12% 5% 30% 40% 13% 1	840
	11th to 20th	Night	10% 7% 25% 42% 15% 1	824
	21st to end	Night	11% 8% 27% 39% 15%	692
March	1st to 10th	Night	10% 7% 27% 44% 12% 1	840
	11th to 20th	Night	8% 5% 28% 40% 17% 2	840
	21st to end	Night	10% 9% 29% 38% 13% 1	924
April	1st to 10th	Night	11% 7% 22% 40% 18% 1	840
	11th to 20th	Night	9% 10% 30% 40% 10% 1	733
	21st to end	Night	6% 9% 29% 39% 16% 1	630
May	1st to 10th	Night	8% 8% 27% 37% 19% 1	630
	11th to 20th	Night	7% 7% 19% 41% 23% 2	621
	21st to end	Night	10% 7% 20% 32% 29% 3	521
June	1st to 10th	Night	8% 5% 16% 41% 27% 3	420
	11th to 20th	Night	5% 5% 22% 45% 22% 1	420
	21st to end	Night	3 3 20% 49% 24% 1	420
July	1st to 10th	Night	2 12% 55% 29% 1	410
	11th to 20th	Night	1 14% 56% 28%	408
	21st to end	Night	3 2 8% 51% 36% 1	491
August	1st to 10th	Night	5% 2 13% 52% 28% 1	627
	11th to 20th	Night	2 1 12% 53% 30% 1	624
	21st to end	Night	4 2 15% 46% 31% 3	835
September	1st to 10th	Night	4 2 12% 44% 33% 5%	840
	11th to 20th	Night	5% 3 13% 40% 34% 5%	840
	21st to end	Night	7% 4 13% 39% 31% 6%	832
October	1st to 10th	Night	7% 4 16% 43% 26% 4	800
	11th to 20th	Night	10% 3 19% 38% 27% 3	800
	21st to end	Night	8% 4 22% 40% 23% 3	880
November	1st to 10th	Night	11% 4 19% 42% 23% 1	820
	11th to 20th	Night	11% 5% 25% 40% 17% 2	840
	21st to end	Night	10% 5% 22% 38% 23% 3	840
December	1st to 10th	Night	13% 5% 24% 40% 17% 2	840
	11th to 20th	Night	17% 5% 18% 43% 16% 2	840
	21st to end	Night	18% 7% 26% 38% 10% 1	924

Page generated at 2026-09-26 02:18:43. Version ac07

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