The Oxbow
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Oxbow clear sky chart from 2005-11-06 to 2026-08-30.

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	15% 8% 3 7% 4 4 4 3 5% 6% 41%	153261
	Night	18% 10% 3 7% 4 4 4 3 4 6% 38%	146351
Transparency	Day	44% 5% 11% 20% 14% 6%	148051
	Night	48% 5% 11% 19% 12% 5%	142560
Seeing	Day	22% 12% 25% 27% 12% 2	17117
	Night	24% 7% 18% 31% 18% 2	26019

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	33% 16% 4 11% 6% 5% 4 3 4 5% 8%	4722
	11th to 20th	Day	21% 8% 2 5% 3 3 4 4 6% 9% 36%	4954
	21st to end	Day	21% 10% 4 9% 5% 6% 5% 4 5% 6% 27%	5843
February	1st to 10th	Day	26% 13% 6% 10% 6% 5% 5% 4 5% 6% 15%	1890
	11th to 20th	Day	27% 15% 4 9% 6% 5% 5% 3 4 5% 17%	1845
	21st to end	Day	30% 17% 3 9% 5% 4 5% 3 3 4 18%	1625
March	1st to 10th	Day	19% 15% 4 9% 6% 6% 6% 4 6% 5% 18%	4991
	11th to 20th	Day	28% 14% 3 8% 5% 4 4 3 3 4 24%	5110
	21st to end	Day	26% 13% 3 11% 7% 7% 6% 4 5% 5% 13%	5623
April	1st to 10th	Day	25% 14% 3 11% 6% 6% 5% 3 4 5% 17%	2520
	11th to 20th	Day	23% 14% 4 8% 6% 5% 4 3 5% 5% 24%	2491
	21st to end	Day	23% 10% 3 8% 5% 5% 5% 4 5% 7% 26%	2730
May	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 3 5% 7% 42%	7773
	11th to 20th	Day	11% 8% 3 7% 5% 4 5% 4 6% 8% 40%	8160
	21st to end	Day	12% 6% 2 7% 4 5% 6% 4 6% 8% 41%	8992
June	1st to 10th	Day	20% 9% 3 7% 5% 4 4 3 5% 7% 34%	2940
	11th to 20th	Day	14% 8% 3 8% 6% 4 4 4 6% 8% 37%	2940
	21st to end	Day	9% 6% 2 5% 3 4 3 3 5% 7% 53%	2940
July	1st to 10th	Day	2 2 1 3 2 2 2 2 3 5% 78%	8219
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 4 75%	8220
	21st to end	Day	3 3 2 4 2 2 2 2 3 5% 71%	8588
August	1st to 10th	Day	2 4 2 4 3 3 3 2 4 7% 65%	5907
	11th to 20th	Day	5% 6% 2 4 3 3 3 3 5% 7% 60%	5926
	21st to end	Day	5% 4 2 5% 3 4 3 3 5% 8% 58%	6284
September	1st to 10th	Day	6% 3 1 4 3 2 3 2 4 6% 65%	2400
	11th to 20th	Day	11% 6% 2 6% 4 3 4 3 5% 6% 48%	2345
	21st to end	Day	13% 5% 2 7% 3 4 4 3 5% 7% 47%	2135
October	1st to 10th	Day	12% 8% 2 7% 3 3 3 3 4 5% 49%	3280
	11th to 20th	Day	16% 7% 3 8% 5% 6% 6% 5% 6% 8% 31%	3261
	21st to end	Day	22% 10% 3 9% 5% 5% 6% 4 6% 8% 22%	3558
November	1st to 10th	Day	29% 11% 4 9% 5% 5% 3 3 6% 6% 18%	1789
	11th to 20th	Day	31% 11% 4 10% 5% 4 4 4 4 5% 17%	1680
	21st to end	Day	26% 11% 3 9% 5% 5% 6% 3 5% 7% 20%	1680
December	1st to 10th	Day	20% 8% 3 8% 5% 4 5% 4 6% 8% 30%	2700
	11th to 20th	Day	30% 15% 5% 10% 6% 5% 6% 4 5% 5% 9%	3108
	21st to end	Day	34% 18% 5% 11% 7% 5% 3 3 3 4 8%	4092
January	1st to 10th	Night	35% 17% 4 10% 6% 4 4 2 4 3 10%	8172
	11th to 20th	Night	20% 9% 3 7% 4 4 4 3 4 5% 37%	8014
	21st to end	Night	24% 12% 3 8% 4 3 3 2 3 4 31%	8371
February	1st to 10th	Night	25% 13% 4 9% 6% 5% 5% 3 5% 6% 18%	2730
	11th to 20th	Night	26% 11% 4 8% 5% 5% 4 3 4 5% 24%	2662
	21st to end	Night	26% 14% 3 8% 4 4 4 4 5% 5% 21%	2181
March	1st to 10th	Night	20% 16% 4 8% 6% 5% 5% 4 6% 7% 20%	5109
	11th to 20th	Night	24% 16% 3 8% 5% 5% 3 2 3 3 27%	5110
	21st to end	Night	26% 15% 4 10% 6% 5% 4 3 4 6% 17%	5581
April	1st to 10th	Night	25% 12% 4 9% 5% 5% 5% 3 4 5% 23%	2100
	11th to 20th	Night	18% 13% 3 7% 5% 4 4 3 5% 7% 32%	2074
	21st to end	Night	21% 10% 3 7% 4 4 4 4 5% 6% 33%	1890
May	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 4 6% 46%	5167
	11th to 20th	Night	10% 10% 4 7% 5% 4 4 3 5% 6% 43%	4700
	21st to end	Night	11% 6% 2 5% 4 4 3 3 6% 8% 47%	5181
June	1st to 10th	Night	16% 7% 2 6% 5% 4 4 3 4 6% 44%	1680
	11th to 20th	Night	9% 6% 2 5% 3 3 3 3 6% 9% 50%	1680
	21st to end	Night	8% 3 2 3 2 2 4 2 5% 6% 62%	1680
July	1st to 10th	Night	1 1 2 1 2 2 3 4 5% 79%	4740
	11th to 20th	Night	2 3 1 3 2 2 2 2 3 4 76%	4740
	21st to end	Night	3 3 1 5% 3 2 3 2 3 6% 69%	5502
August	1st to 10th	Night	2 4 1 3 2 3 3 3 5% 7% 66%	4125
	11th to 20th	Night	6% 4 2 5% 4 4 3 3 5% 6% 58%	4142
	21st to end	Night	3 3 1 4 2 3 4 3 4 7% 66%	4906
September	1st to 10th	Night	5% 4 1 4 2 2 3 2 4 7% 65%	2000
	11th to 20th	Night	8% 5% 2 5% 4 3 4 3 4 6% 55%	2052
	21st to end	Night	10% 6% 2 5% 3 3 3 3 5% 7% 55%	2264
October	1st to 10th	Night	9% 7% 2 5% 3 3 3 2 4 5% 56%	3920
	11th to 20th	Night	14% 8% 3 8% 4 4 4 4 6% 9% 36%	3897
	21st to end	Night	24% 12% 3 8% 5% 5% 4 3 5% 5% 26%	4253
November	1st to 10th	Night	27% 10% 3 8% 5% 4 4 4 5% 6% 25%	2726
	11th to 20th	Night	31% 11% 3 9% 5% 5% 4 3 5% 5% 19%	2940
	21st to end	Night	21% 11% 3 8% 6% 5% 5% 4 5% 6% 26%	2940
December	1st to 10th	Night	18% 8% 3 7% 4 5% 5% 4 6% 8% 32%	4700
	11th to 20th	Night	34% 14% 4 10% 5% 5% 4 3 4 4 12%	5316
	21st to end	Night	33% 13% 4 10% 6% 5% 5% 4 4 4 11%	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	79% 5% 6% 7% 3	4723
	11th to 20th	Day	45% 4 6% 17% 16% 12%	4954
	21st to end	Day	57% 5% 7% 11% 13% 7%	5843
February	1st to 10th	Day	67% 6% 9% 11% 5% 1	1890
	11th to 20th	Day	69% 6% 11% 9% 4 2	1854
	21st to end	Day	69% 5% 8% 9% 7% 1	1625
March	1st to 10th	Day	63% 6% 11% 16% 4	4991
	11th to 20th	Day	64% 5% 6% 16% 9% 1	5110
	21st to end	Day	68% 8% 12% 9% 3	5623
April	1st to 10th	Day	68% 5% 11% 12% 4	2520
	11th to 20th	Day	61% 6% 11% 13% 7% 2	2494
	21st to end	Day	56% 7% 13% 16% 7% 1	2730
May	1st to 10th	Day	40% 6% 11% 28% 15%	7773
	11th to 20th	Day	39% 7% 18% 28% 8% 1	8163
	21st to end	Day	37% 9% 24% 24% 6%	8994
June	1st to 10th	Day	49% 5% 15% 23% 7% 1	2940
	11th to 20th	Day	44% 6% 16% 24% 9% 1	2940
	21st to end	Day	29% 4 16% 32% 16% 3	2940
July	1st to 10th	Day	11% 3 9% 33% 26% 19%	8151
	11th to 20th	Day	15% 2 5% 14% 34% 30%	8145
	21st to end	Day	16% 2 10% 36% 29% 7%	8396
August	1st to 10th	Day	18% 3 9% 26% 33% 11%	4336
	11th to 20th	Day	23% 4 11% 39% 18% 4	4325
	21st to end	Day	25% 5% 12% 26% 25% 8%	4563
September	1st to 10th	Day	21% 4 10% 30% 24% 12%	2400
	11th to 20th	Day	34% 6% 12% 24% 18% 6%	2345
	21st to end	Day	36% 6% 11% 24% 17% 6%	2135
October	1st to 10th	Day	36% 5% 7% 17% 22% 13%	3280
	11th to 20th	Day	48% 7% 14% 18% 12% 2	3261
	21st to end	Day	58% 5% 10% 15% 10% 2	3558
November	1st to 10th	Day	67% 5% 10% 12% 6%	1789
	11th to 20th	Day	68% 6% 9% 9% 6% 3	1680
	21st to end	Day	62% 7% 9% 8% 11% 3	1680
December	1st to 10th	Day	52% 5% 7% 13% 19% 4	2700
	11th to 20th	Day	75% 6% 7% 8% 3	3108
	21st to end	Day	81% 5% 6% 6% 2	4092
January	1st to 10th	Night	79% 5% 5% 7% 3 1	8172
	11th to 20th	Night	49% 5% 6% 15% 14% 11%	8014
	21st to end	Night	58% 3 5% 11% 15% 7%	8371
February	1st to 10th	Night	67% 6% 9% 12% 5%	2730
	11th to 20th	Night	62% 6% 12% 12% 6% 2	2664
	21st to end	Night	63% 7% 10% 12% 7% 2	2181
March	1st to 10th	Night	62% 6% 13% 13% 4 2	5109
	11th to 20th	Night	62% 5% 10% 15% 7% 2	5110
	21st to end	Night	68% 7% 13% 11% 2	5581
April	1st to 10th	Night	63% 6% 12% 15% 4	2100
	11th to 20th	Night	52% 6% 16% 17% 6% 3	2074
	21st to end	Night	51% 6% 14% 21% 7% 2	1890
May	1st to 10th	Night	39% 4 14% 30% 13% 1	5165
	11th to 20th	Night	42% 5% 15% 29% 7% 1	4700
	21st to end	Night	34% 6% 31% 24% 5%	5182
June	1st to 10th	Night	41% 7% 15% 30% 6% 1	1680
	11th to 20th	Night	30% 6% 20% 32% 10% 2	1680
	21st to end	Night	22% 4 19% 36% 15% 4	1680
July	1st to 10th	Night	10% 3 13% 34% 22% 19%	4700
	11th to 20th	Night	13% 2 7% 17% 34% 27%	4692
	21st to end	Night	16% 3 13% 34% 28% 5%	5382
August	1st to 10th	Night	15% 3 10% 30% 32% 11%	3019
	11th to 20th	Night	24% 4 16% 34% 18% 4	3019
	21st to end	Night	19% 4 10% 34% 26% 7%	3551
September	1st to 10th	Night	19% 4 9% 31% 25% 12%	2000
	11th to 20th	Night	29% 6% 13% 30% 16% 7%	2052
	21st to end	Night	30% 5% 14% 26% 17% 8%	2264
October	1st to 10th	Night	30% 5% 8% 23% 25% 10%	3920
	11th to 20th	Night	44% 6% 18% 21% 10% 2	3897
	21st to end	Night	59% 5% 8% 17% 8% 2	4253
November	1st to 10th	Night	59% 7% 11% 14% 8% 1	2726
	11th to 20th	Night	67% 6% 9% 11% 6% 2	2940
	21st to end	Night	58% 7% 10% 11% 11% 4	2940
December	1st to 10th	Night	47% 5% 11% 19% 17% 2	4700
	11th to 20th	Night	75% 5% 9% 7% 3	5316
	21st to end	Night	74% 8% 8% 7% 2 1	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	42% 15% 24% 12% 5%	220
	11th to 20th	Day	35% 11% 23% 19% 9% 2	291
	21st to end	Day	29% 14% 22% 21% 12% 2	473
February	1st to 10th	Day	32% 14% 22% 21% 10% 1	430
	11th to 20th	Day	36% 19% 18% 19% 7% 1	425
	21st to end	Day	38% 15% 21% 19% 5% 1	391
March	1st to 10th	Day	34% 17% 26% 17% 5% 1	540
	11th to 20th	Day	34% 14% 23% 21% 8% 1	540
	21st to end	Day	34% 18% 22% 19% 6% 1	594
April	1st to 10th	Day	32% 16% 21% 20% 9% 2	540
	11th to 20th	Day	31% 17% 26% 17% 7% 3	532
	21st to end	Day	27% 16% 27% 21% 8% 2	540
May	1st to 10th	Day	21% 14% 29% 22% 12% 2	543
	11th to 20th	Day	26% 14% 28% 17% 11% 3	568
	21st to end	Day	19% 14% 26% 26% 12% 2	617
June	1st to 10th	Day	19% 14% 26% 28% 11% 2	570
	11th to 20th	Day	15% 18% 31% 27% 8% 1	570
	21st to end	Day	8% 11% 26% 40% 14% 2	570
July	1st to 10th	Day	6% 6% 27% 42% 17% 2	566
	11th to 20th	Day	4 8% 34% 37% 15% 2	564
	21st to end	Day	3 5% 26% 44% 18% 4	619
August	1st to 10th	Day	4 5% 27% 40% 20% 4	567
	11th to 20th	Day	4 5% 21% 40% 25% 4	557
	21st to end	Day	7% 8% 28% 36% 17% 4	620
September	1st to 10th	Day	7% 8% 28% 36% 17% 3	530
	11th to 20th	Day	13% 10% 28% 29% 17% 3	530
	21st to end	Day	17% 9% 30% 29% 10% 4	530
October	1st to 10th	Day	19% 12% 28% 21% 16% 3	530
	11th to 20th	Day	21% 12% 21% 26% 17% 3	530
	21st to end	Day	29% 13% 23% 21% 10% 3	583
November	1st to 10th	Day	30% 19% 21% 19% 9% 2	315
	11th to 20th	Day	31% 16% 22% 19% 9% 2	220
	21st to end	Day	30% 14% 20% 23% 13%	220
December	1st to 10th	Day	34% 11% 21% 20% 12% 2	220
	11th to 20th	Day	40% 12% 22% 19% 6%	220
	21st to end	Day	40% 14% 21% 18% 7%	242
January	1st to 10th	Night	43% 9% 17% 20% 8% 3	840
	11th to 20th	Night	34% 8% 15% 25% 15% 2	840
	21st to end	Night	30% 7% 13% 31% 15% 2	924
February	1st to 10th	Night	30% 10% 16% 28% 15% 2	840
	11th to 20th	Night	33% 8% 18% 28% 13% 1	824
	21st to end	Night	33% 12% 16% 27% 12%	692
March	1st to 10th	Night	32% 9% 18% 29% 11%	840
	11th to 20th	Night	31% 7% 19% 29% 13% 1	840
	21st to end	Night	36% 9% 18% 25% 12%	924
April	1st to 10th	Night	32% 9% 18% 25% 16% 1	840
	11th to 20th	Night	24% 11% 22% 27% 15% 1	826
	21st to end	Night	25% 8% 18% 29% 19% 1	630
May	1st to 10th	Night	16% 7% 25% 32% 18% 2	578
	11th to 20th	Night	18% 6% 20% 32% 22% 2	414
	21st to end	Night	17% 4 23% 34% 22% 1	455
June	1st to 10th	Night	17% 5% 19% 40% 18% 1	420
	11th to 20th	Night	10% 5% 26% 41% 14% 3	420
	21st to end	Night	9% 3 19% 46% 23%	420
July	1st to 10th	Night	4 2 18% 49% 27% 1	410
	11th to 20th	Night	4 2 22% 53% 19%	408
	21st to end	Night	2 2 16% 47% 31% 2	578
August	1st to 10th	Night	3 2 17% 44% 31% 4	627
	11th to 20th	Night	4 2 11% 44% 34% 4	624
	21st to end	Night	6% 3 22% 42% 25% 2	821
September	1st to 10th	Night	8% 3 22% 39% 27% 2	800
	11th to 20th	Night	9% 5% 16% 33% 32% 5%	800
	21st to end	Night	13% 4 19% 37% 25% 3	800
October	1st to 10th	Night	17% 4 20% 34% 23% 3	800
	11th to 20th	Night	20% 5% 16% 31% 23% 5%	800
	21st to end	Night	26% 9% 20% 25% 16% 3	880
November	1st to 10th	Night	31% 8% 15% 29% 16% 1	820
	11th to 20th	Night	34% 10% 19% 23% 12% 2	840
	21st to end	Night	26% 8% 19% 30% 15% 2	840
December	1st to 10th	Night	31% 7% 16% 28% 16% 3	840
	11th to 20th	Night	40% 7% 16% 22% 14% 1	840
	21st to end	Night	42% 9% 18% 23% 7%	924

Page generated at 2026-08-27 01:07:14. Version ac07

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