Oxbow Trail Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oxbow Trail Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	13% 8% 3 7% 5% 4 4 4 6% 7% 38%	126728
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	122895
Transparency	Day	43% 6% 12% 24% 13% 1	130198
	Night	45% 6% 11% 23% 14% 2	127498
Seeing	Day	17% 8% 29% 26% 15% 5%	17243
	Night	18% 7% 30% 32% 12% 1	27030

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	8% 10% 6% 15% 9% 6% 4 3 5% 5% 28%	3617
	11th to 20th	Day	8% 12% 6% 10% 5% 4 8% 6% 6% 7% 27%	3620
	21st to end	Day	11% 16% 6% 11% 5% 4 3 3 4 5% 32%	4074
February	1st to 10th	Day	11% 13% 5% 11% 5% 5% 6% 3 5% 6% 31%	1800
	11th to 20th	Day	11% 12% 4 8% 6% 4 5% 4 5% 6% 35%	1876
	21st to end	Day	12% 12% 4 9% 7% 5% 5% 4 5% 6% 30%	1650
March	1st to 10th	Day	14% 7% 2 5% 4 3 4 3 5% 6% 47%	4480
	11th to 20th	Day	11% 7% 2 7% 5% 4 4 3 5% 8% 44%	4767
	21st to end	Day	24% 11% 4 8% 4 4 3 3 4 5% 29%	5920
April	1st to 10th	Day	17% 14% 3 8% 5% 4 4 4 5% 7% 31%	2400
	11th to 20th	Day	20% 10% 3 7% 5% 4 4 3 5% 6% 33%	2374
	21st to end	Day	20% 9% 3 7% 5% 4 5% 3 5% 6% 32%	2500
May	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 5% 7% 44%	6092
	11th to 20th	Day	14% 5% 3 7% 4 5% 4 4 6% 8% 42%	6319
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 6% 7% 39%	6935
June	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 4 5% 8% 44%	2800
	11th to 20th	Day	13% 6% 3 6% 5% 4 4 4 6% 8% 41%	2800
	21st to end	Day	12% 6% 2 6% 4 5% 5% 4 7% 9% 42%	2800
July	1st to 10th	Day	10% 5% 2 7% 4 5% 5% 5% 8% 10% 39%	6360
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 10% 48%	6360
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 8% 42%	6924
August	1st to 10th	Day	9% 5% 2 7% 5% 5% 6% 4 6% 9% 41%	4563
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 8% 45%	3961
	21st to end	Day	11% 6% 3 6% 5% 5% 5% 4 6% 9% 42%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 5% 7% 49%	2327
	21st to end	Day	13% 7% 3 6% 4 4 4 4 5% 7% 43%	2200
October	1st to 10th	Day	13% 6% 2 6% 5% 4 4 3 6% 7% 45%	1980
	11th to 20th	Day	19% 10% 3 8% 4 4 4 3 5% 7% 33%	1900
	21st to end	Day	18% 9% 3 8% 5% 4 3 3 5% 5% 37%	1886
November	1st to 10th	Day	13% 9% 2 6% 4 5% 4 3 6% 6% 42%	1762
	11th to 20th	Day	13% 13% 4 11% 6% 4 4 4 5% 7% 29%	1769
	21st to end	Day	14% 12% 5% 8% 4 4 5% 3 5% 6% 34%	1600
December	1st to 10th	Day	8% 11% 7% 11% 6% 6% 5% 4 6% 6% 30%	2610
	11th to 20th	Day	12% 15% 5% 13% 6% 5% 4 3 4 6% 27%	3008
	21st to end	Day	20% 20% 8% 12% 6% 4 4 3 4 6% 14%	3982
January	1st to 10th	Night	9% 12% 6% 13% 7% 4 3 3 4 5% 35%	6348
	11th to 20th	Night	11% 13% 6% 13% 6% 6% 6% 5% 5% 5% 24%	6360
	21st to end	Night	10% 17% 8% 10% 5% 4 3 3 4 5% 30%	6886
February	1st to 10th	Night	10% 15% 5% 10% 5% 5% 5% 4 4 5% 33%	2600
	11th to 20th	Night	10% 10% 4 9% 5% 4 5% 3 4 5% 40%	2411
	21st to end	Night	16% 11% 5% 8% 4 4 5% 3 5% 6% 34%	1980
March	1st to 10th	Night	15% 7% 2 7% 4 4 4 3 5% 5% 45%	5373
	11th to 20th	Night	10% 8% 2 6% 5% 5% 5% 4 6% 8% 43%	5224
	21st to end	Night	22% 11% 4 9% 4 4 4 3 3 5% 31%	5008
April	1st to 10th	Night	19% 11% 3 7% 4 3 4 3 4 5% 37%	2000
	11th to 20th	Night	20% 9% 3 7% 4 4 5% 4 4 6% 37%	1975
	21st to end	Night	16% 8% 2 6% 4 5% 4 4 5% 6% 39%	1900
May	1st to 10th	Night	14% 8% 3 7% 4 3 4 3 6% 8% 41%	3796
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 6% 8% 44%	3596
	21st to end	Night	17% 8% 3 7% 5% 5% 5% 3 5% 6% 37%	3942
June	1st to 10th	Night	9% 5% 2 6% 5% 4 4 4 6% 7% 48%	1600
	11th to 20th	Night	10% 7% 2 7% 5% 4 4 3 6% 9% 44%	1600
	21st to end	Night	10% 6% 2 5% 4 4 4 4 6% 8% 48%	1600
July	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 6% 9% 45%	3618
	11th to 20th	Night	7% 4 2 5% 4 4 4 3 5% 7% 54%	3617
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 6% 8% 50%	3947
August	1st to 10th	Night	10% 5% 2 6% 4 3 4 4 6% 8% 47%	2597
	11th to 20th	Night	10% 7% 2 7% 5% 5% 4 4 6% 7% 43%	3086
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 8% 49%	3586
September	1st to 10th	Night	10% 5% 3 5% 3 3 3 3 5% 7% 53%	2000
	11th to 20th	Night	13% 4 2 5% 3 3 3 4 5% 6% 52%	2052
	21st to end	Night	10% 7% 2 6% 4 3 4 3 4 7% 50%	2200
October	1st to 10th	Night	13% 6% 3 5% 4 3 4 3 4 7% 49%	2262
	11th to 20th	Night	17% 8% 2 6% 5% 4 4 4 5% 7% 39%	2280
	21st to end	Night	19% 9% 3 7% 5% 5% 4 3 5% 6% 35%	2710
November	1st to 10th	Night	13% 9% 3 6% 4 4 4 3 5% 5% 44%	2536
	11th to 20th	Night	14% 13% 4 8% 6% 5% 4 3 5% 5% 32%	2630
	21st to end	Night	15% 11% 4 7% 5% 4 4 4 5% 6% 36%	2800
December	1st to 10th	Night	9% 10% 5% 9% 5% 5% 5% 4 6% 8% 37%	4580
	11th to 20th	Night	14% 18% 7% 11% 6% 4 4 3 4 4 25%	5199
	21st to end	Night	20% 18% 7% 11% 6% 4 3 3 4 4 20%	6996

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	58% 5% 13% 18% 7%	3618
	11th to 20th	Day	53% 7% 9% 14% 15% 1	3620
	21st to end	Day	54% 5% 10% 19% 11%	4074
February	1st to 10th	Day	54% 5% 13% 18% 9% 1	1800
	11th to 20th	Day	49% 6% 12% 22% 10% 2	1885
	21st to end	Day	55% 6% 10% 17% 10% 2	1650
March	1st to 10th	Day	38% 5% 13% 26% 13% 5%	4480
	11th to 20th	Day	39% 4 14% 26% 15% 1	4767
	21st to end	Day	58% 5% 9% 16% 11% 1	5922
April	1st to 10th	Day	53% 6% 11% 19% 10% 1	2400
	11th to 20th	Day	51% 5% 9% 20% 12% 2	2374
	21st to end	Day	50% 7% 12% 19% 10% 1	2500
May	1st to 10th	Day	38% 6% 12% 24% 16% 3	6127
	11th to 20th	Day	38% 7% 11% 26% 17% 1	6323
	21st to end	Day	41% 7% 9% 26% 16% 1	6949
June	1st to 10th	Day	37% 7% 13% 26% 15% 1	2800
	11th to 20th	Day	40% 8% 13% 24% 15% 1	2800
	21st to end	Day	37% 8% 15% 25% 13% 2	2800
July	1st to 10th	Day	38% 7% 13% 32% 9% 1	6291
	11th to 20th	Day	30% 8% 14% 33% 15% 1	6285
	21st to end	Day	36% 8% 13% 24% 17% 2	6728
August	1st to 10th	Day	37% 8% 16% 26% 12% 1	4548
	11th to 20th	Day	35% 7% 12% 35% 12%	3999
	21st to end	Day	37% 6% 12% 28% 15% 1	4311
September	1st to 10th	Day	37% 6% 13% 26% 17% 1	2400
	11th to 20th	Day	35% 6% 14% 31% 13% 1	2327
	21st to end	Day	39% 6% 12% 31% 12%	2200
October	1st to 10th	Day	24% 3 7% 41% 24% 1	3266
	11th to 20th	Day	40% 5% 9% 28% 16% 2	3140
	21st to end	Day	43% 3 13% 27% 12% 1	3086
November	1st to 10th	Day	41% 6% 15% 22% 14% 2	1762
	11th to 20th	Day	54% 6% 13% 18% 8% 1	1769
	21st to end	Day	51% 6% 15% 21% 7%	1600
December	1st to 10th	Day	52% 8% 17% 18% 5%	2610
	11th to 20th	Day	58% 4 10% 18% 8% 1	3005
	21st to end	Day	72% 6% 7% 10% 6%	3982
January	1st to 10th	Night	53% 4 9% 23% 11%	6348
	11th to 20th	Night	60% 6% 8% 10% 15% 1	6360
	21st to end	Night	57% 4 11% 16% 11%	6886
February	1st to 10th	Night	53% 6% 10% 19% 10% 2	2600
	11th to 20th	Night	46% 6% 11% 24% 11% 2	2412
	21st to end	Night	49% 6% 12% 19% 11% 2	1980
March	1st to 10th	Night	42% 4 14% 24% 12% 5%	5373
	11th to 20th	Night	36% 6% 14% 26% 14% 3	5224
	21st to end	Night	56% 5% 8% 17% 12% 1	5008
April	1st to 10th	Night	48% 6% 9% 19% 16% 1	2000
	11th to 20th	Night	48% 5% 8% 20% 15% 3	1975
	21st to end	Night	44% 7% 12% 22% 13% 2	1900
May	1st to 10th	Night	41% 6% 12% 19% 20% 2	3823
	11th to 20th	Night	34% 10% 15% 21% 19% 2	3597
	21st to end	Night	46% 7% 9% 23% 13% 3	3950
June	1st to 10th	Night	33% 8% 13% 27% 17% 1	1600
	11th to 20th	Night	36% 7% 10% 27% 18% 2	1600
	21st to end	Night	33% 6% 17% 25% 15% 3	1600
July	1st to 10th	Night	35% 6% 15% 32% 10% 3	3580
	11th to 20th	Night	29% 6% 14% 33% 17% 2	3572
	21st to end	Night	30% 7% 15% 24% 21% 3	3832
August	1st to 10th	Night	34% 6% 16% 24% 19% 1	2588
	11th to 20th	Night	37% 7% 11% 30% 14% 1	3114
	21st to end	Night	31% 6% 14% 26% 20% 3	3586
September	1st to 10th	Night	31% 6% 12% 27% 22% 1	2000
	11th to 20th	Night	31% 6% 13% 34% 16% 1	2052
	21st to end	Night	34% 6% 10% 32% 17% 1	2200
October	1st to 10th	Night	27% 3 6% 36% 26% 2	3756
	11th to 20th	Night	33% 5% 12% 30% 19% 2	3779
	21st to end	Night	45% 5% 10% 24% 16%	4460
November	1st to 10th	Night	41% 6% 11% 26% 15% 2	2536
	11th to 20th	Night	52% 6% 12% 17% 11% 2	2630
	21st to end	Night	50% 6% 14% 23% 7% 1	2800
December	1st to 10th	Night	45% 7% 18% 22% 6% 2	4580
	11th to 20th	Night	62% 5% 9% 14% 10% 1	5201
	21st to end	Night	68% 5% 7% 13% 6%	6996

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% 10% 32% 28% 9% 3	300
	11th to 20th	Day	20% 11% 36% 26% 6% 2	300
	21st to end	Day	23% 8% 34% 28% 7%	330
February	1st to 10th	Day	15% 11% 42% 22% 8% 1	300
	11th to 20th	Day	18% 15% 38% 20% 7% 1	300
	21st to end	Day	18% 13% 44% 22% 2	249
March	1st to 10th	Day	21% 12% 37% 24% 5%	300
	11th to 20th	Day	22% 14% 34% 20% 6% 5%	300
	21st to end	Day	28% 9% 31% 23% 8% 2	529
April	1st to 10th	Day	26% 13% 32% 23% 6%	500
	11th to 20th	Day	28% 13% 28% 17% 11% 3	492
	21st to end	Day	22% 11% 31% 23% 11% 2	500
May	1st to 10th	Day	19% 8% 33% 28% 11% 2	603
	11th to 20th	Day	19% 9% 30% 24% 13% 5%	725
	21st to end	Day	20% 7% 25% 29% 14% 6%	789
June	1st to 10th	Day	11% 8% 30% 30% 17% 4	730
	11th to 20th	Day	16% 6% 30% 28% 15% 4	730
	21st to end	Day	16% 7% 25% 30% 17% 4	730
July	1st to 10th	Day	9% 2 17% 31% 29% 12%	721
	11th to 20th	Day	9% 4 24% 30% 24% 9%	718
	21st to end	Day	7% 2 23% 34% 25% 9%	794
August	1st to 10th	Day	8% 4 22% 30% 25% 12%	726
	11th to 20th	Day	15% 3 16% 28% 24% 14%	585
	21st to end	Day	13% 4 19% 31% 23% 9%	583
September	1st to 10th	Day	14% 5% 23% 28% 21% 8%	530
	11th to 20th	Day	13% 3 18% 29% 27% 10%	530
	21st to end	Day	16% 6% 26% 23% 22% 7%	530
October	1st to 10th	Day	15% 10% 23% 29% 19% 4	359
	11th to 20th	Day	24% 11% 33% 21% 9% 1	300
	21st to end	Day	24% 11% 35% 21% 7% 2	330
November	1st to 10th	Day	19% 10% 36% 24% 9% 2	300
	11th to 20th	Day	18% 15% 39% 21% 6% 1	300
	21st to end	Day	22% 14% 41% 19% 5%	300
December	1st to 10th	Day	17% 14% 42% 21% 6% 1	300
	11th to 20th	Day	19% 14% 37% 18% 10% 1	300
	21st to end	Day	27% 12% 38% 18% 5%	330
January	1st to 10th	Night	18% 11% 35% 27% 8% 1	1000
	11th to 20th	Night	22% 10% 34% 28% 6% 1	1000
	21st to end	Night	21% 9% 32% 31% 6%	1100
February	1st to 10th	Night	19% 8% 36% 29% 7% 1	1000
	11th to 20th	Night	16% 11% 37% 28% 8%	856
	21st to end	Night	22% 9% 37% 27% 4 1	660
March	1st to 10th	Night	22% 8% 35% 27% 8% 1	800
	11th to 20th	Night	18% 8% 38% 28% 7% 1	714
	21st to end	Night	24% 6% 30% 31% 7% 2	660
April	1st to 10th	Night	25% 9% 36% 24% 6%	600
	11th to 20th	Night	24% 9% 30% 30% 6% 1	593
	21st to end	Night	21% 8% 30% 34% 7% 1	600
May	1st to 10th	Night	21% 6% 32% 32% 8%	600
	11th to 20th	Night	18% 5% 33% 33% 10%	591
	21st to end	Night	15% 5% 27% 36% 16% 1	650
June	1st to 10th	Night	11% 3 29% 43% 14% 1	600
	11th to 20th	Night	12% 4 30% 42% 12% 1	600
	21st to end	Night	11% 4 28% 41% 16%	600
July	1st to 10th	Night	9% 3 17% 41% 28% 2	585
	11th to 20th	Night	9% 3 20% 43% 23% 2	582
	21st to end	Night	7% 2 15% 50% 24% 2	657
August	1st to 10th	Night	8% 2 16% 46% 26% 2	597
	11th to 20th	Night	11% 1 15% 40% 27% 4	594
	21st to end	Night	12% 3 17% 42% 24% 2	660
September	1st to 10th	Night	11% 4 23% 36% 21% 5%	600
	11th to 20th	Night	15% 4 20% 34% 24% 4	652
	21st to end	Night	14% 7% 23% 33% 21% 2	800
October	1st to 10th	Night	16% 6% 26% 33% 18% 1	773
	11th to 20th	Night	22% 10% 30% 29% 9%	760
	21st to end	Night	25% 7% 29% 31% 8% 1	836
November	1st to 10th	Night	17% 8% 36% 27% 10% 1	780
	11th to 20th	Night	22% 11% 38% 23% 6%	830
	21st to end	Night	22% 11% 35% 25% 7%	1000
December	1st to 10th	Night	21% 11% 40% 22% 6%	1000
	11th to 20th	Night	23% 10% 35% 26% 6% 1	1000
	21st to end	Night	29% 10% 31% 25% 4	1100

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