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 2025-04-26.

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	18% 9% 3 7% 4 4 4 3 5% 6% 37%	117871
	Night	20% 10% 3 7% 4 4 4 3 4 5% 35%	119574
Transparency	Day	47% 5% 10% 18% 13% 5%	117620
	Night	50% 5% 11% 18% 11% 4	119414
Seeing	Day	22% 12% 24% 26% 12% 2	15340
	Night	24% 7% 18% 31% 18% 2	24461

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	36% 17% 4 9% 5% 5% 4 2 4 4 9%	3639
	11th to 20th	Day	27% 10% 2 6% 4 4 4 3 5% 8% 27%	3798
	21st to end	Day	23% 10% 3 7% 4 5% 5% 3 5% 6% 29%	4510
February	1st to 10th	Day	26% 14% 6% 10% 6% 5% 5% 4 5% 6% 14%	1800
	11th to 20th	Day	28% 15% 4 9% 6% 5% 5% 3 4 5% 18%	1755
	21st to end	Day	29% 17% 3 9% 5% 4 5% 3 3 4 18%	1550
March	1st to 10th	Day	19% 15% 4 9% 6% 6% 6% 4 6% 5% 18%	4883
	11th to 20th	Day	28% 14% 3 8% 5% 4 4 3 3 4 24%	5000
	21st to end	Day	26% 13% 3 11% 7% 7% 6% 4 5% 5% 13%	5502
April	1st to 10th	Day	26% 14% 4 11% 6% 6% 5% 3 4 5% 17%	2400
	11th to 20th	Day	23% 14% 4 7% 5% 4 4 3 5% 5% 25%	2371
	21st to end	Day	24% 10% 3 7% 5% 5% 4 4 6% 7% 26%	2538
May	1st to 10th	Day	20% 8% 3 8% 5% 5% 5% 3 5% 5% 34%	4193
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 5% 8% 51%	4377
	21st to end	Day	16% 6% 2 7% 5% 5% 6% 5% 7% 8% 32%	4813
June	1st to 10th	Day	21% 10% 3 7% 5% 4 4 3 5% 7% 32%	2660
	11th to 20th	Day	15% 8% 3 8% 6% 4 4 4 6% 8% 34%	2660
	21st to end	Day	9% 5% 2 4 3 3 3 3 5% 8% 55%	2660
July	1st to 10th	Day	3 2 1 3 2 2 2 2 2 5% 77%	4420
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 5% 72%	4420
	21st to end	Day	3 3 2 3 2 2 2 2 3 5% 74%	4612
August	1st to 10th	Day	2 4 1 4 3 3 3 2 5% 7% 66%	4091
	11th to 20th	Day	5% 5% 1 4 3 3 3 3 5% 7% 60%	4026
	21st to end	Day	7% 5% 2 4 3 3 3 3 5% 7% 59%	4314
September	1st to 10th	Day	6% 3 1 4 3 2 3 3 4 6% 66%	2280
	11th to 20th	Day	11% 6% 3 6% 4 3 4 3 5% 6% 47%	2228
	21st to end	Day	13% 5% 2 7% 3 4 4 3 5% 7% 46%	2028
October	1st to 10th	Day	12% 7% 2 7% 3 3 3 3 4 5% 50%	3180
	11th to 20th	Day	16% 7% 3 8% 5% 6% 6% 5% 6% 8% 30%	3161
	21st to end	Day	22% 11% 4 9% 5% 5% 6% 4 6% 8% 22%	3448
November	1st to 10th	Day	29% 11% 4 9% 5% 5% 4 3 6% 6% 18%	1702
	11th to 20th	Day	32% 11% 4 9% 5% 4 4 5% 4 6% 17%	1600
	21st to end	Day	26% 11% 3 8% 5% 5% 6% 4 5% 7% 20%	1600
December	1st to 10th	Day	19% 8% 3 8% 5% 4 5% 4 6% 8% 30%	2620
	11th to 20th	Day	30% 15% 5% 10% 6% 5% 6% 4 5% 5% 9%	3028
	21st to end	Day	34% 18% 5% 12% 7% 5% 3 2 3 4 7%	4004
January	1st to 10th	Night	37% 16% 4 10% 6% 4 4 3 4 3 10%	6306
	11th to 20th	Night	26% 12% 3 8% 4 4 4 3 4 5% 25%	6166
	21st to end	Night	28% 12% 3 7% 3 3 2 2 3 4 33%	6468
February	1st to 10th	Night	25% 13% 4 9% 6% 5% 5% 3 5% 6% 17%	2600
	11th to 20th	Night	26% 11% 4 8% 5% 5% 4 3 4 5% 25%	2532
	21st to end	Night	26% 15% 3 8% 4 4 5% 4 5% 6% 22%	2080
March	1st to 10th	Night	20% 16% 4 8% 6% 5% 5% 4 6% 7% 20%	4997
	11th to 20th	Night	24% 16% 3 8% 5% 5% 3 2 3 3 27%	5000
	21st to end	Night	26% 16% 4 10% 6% 5% 4 3 4 5% 17%	5460
April	1st to 10th	Night	26% 13% 4 9% 5% 5% 5% 2 4 5% 23%	2000
	11th to 20th	Night	18% 13% 3 7% 5% 4 4 3 5% 6% 33%	1974
	21st to end	Night	21% 10% 2 6% 4 4 4 5% 5% 6% 32%	1758
May	1st to 10th	Night	21% 10% 3 6% 4 3 4 3 3 4 39%	2774
	11th to 20th	Night	8% 6% 2 5% 3 3 3 2 4 5% 57%	2516
	21st to end	Night	14% 7% 2 5% 4 4 4 4 6% 8% 44%	2762
June	1st to 10th	Night	17% 7% 2 6% 5% 4 4 3 4 6% 41%	1520
	11th to 20th	Night	10% 7% 2 5% 3 3 3 3 6% 9% 49%	1520
	21st to end	Night	8% 3 2 3 2 2 4 2 5% 7% 63%	1520
July	1st to 10th	Night	2 2 1 2 1 1 1 1 2 3 84%	2540
	11th to 20th	Night	3 2 3 2 3 2 2 3 5% 75%	2540
	21st to end	Night	2 2 1 4 2 2 2 1 3 4 76%	2942
August	1st to 10th	Night	2 3 1 3 2 2 3 3 5% 7% 70%	2848
	11th to 20th	Night	3 4 2 6% 4 4 3 4 5% 7% 59%	2812
	21st to end	Night	4 4 2 4 2 2 3 2 3 6% 68%	3360
September	1st to 10th	Night	5% 4 1 3 2 2 3 2 4 7% 68%	1900
	11th to 20th	Night	8% 5% 2 6% 4 3 4 3 4 7% 55%	1949
	21st to end	Night	11% 6% 2 5% 3 3 3 3 5% 7% 54%	2151
October	1st to 10th	Night	9% 7% 2 5% 3 3 3 2 4 5% 57%	3800
	11th to 20th	Night	14% 8% 3 8% 4 4 4 4 6% 9% 36%	3777
	21st to end	Night	24% 13% 3 8% 5% 5% 4 3 4 5% 26%	4121
November	1st to 10th	Night	27% 10% 3 8% 5% 4 4 4 5% 6% 25%	2593
	11th to 20th	Night	31% 10% 3 9% 5% 5% 4 4 5% 5% 19%	2800
	21st to end	Night	22% 11% 3 8% 6% 5% 4 4 5% 6% 27%	2800
December	1st to 10th	Night	17% 8% 3 7% 4 5% 5% 4 6% 9% 33%	4560
	11th to 20th	Night	34% 14% 4 10% 5% 5% 4 3 4 4 12%	5176
	21st to end	Night	33% 14% 4 10% 6% 5% 5% 4 4 4 11%	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	78% 5% 6% 6% 3 1	3639
	11th to 20th	Day	57% 5% 6% 17% 12% 2	3798
	21st to end	Day	54% 6% 7% 11% 15% 7%	4510
February	1st to 10th	Day	69% 6% 9% 11% 5% 1	1800
	11th to 20th	Day	69% 6% 11% 9% 4 2	1764
	21st to end	Day	69% 6% 8% 9% 7% 1	1550
March	1st to 10th	Day	62% 6% 11% 17% 4	4883
	11th to 20th	Day	64% 5% 6% 16% 9% 1	5000
	21st to end	Day	68% 8% 12% 9% 2	5502
April	1st to 10th	Day	69% 5% 10% 11% 4	2400
	11th to 20th	Day	60% 6% 11% 13% 7% 2	2374
	21st to end	Day	56% 7% 13% 15% 8% 1	2538
May	1st to 10th	Day	50% 8% 13% 19% 10%	4193
	11th to 20th	Day	32% 5% 15% 33% 15% 1	4380
	21st to end	Day	44% 11% 25% 18% 2	4814
June	1st to 10th	Day	52% 5% 15% 21% 6% 1	2660
	11th to 20th	Day	46% 7% 17% 22% 7% 1	2660
	21st to end	Day	28% 4 17% 33% 16% 3	2660
July	1st to 10th	Day	13% 3 7% 17% 26% 34%	4351
	11th to 20th	Day	16% 3 5% 17% 29% 30%	4345
	21st to end	Day	14% 2 8% 26% 39% 11%	4466
August	1st to 10th	Day	19% 3 9% 26% 33% 11%	4076
	11th to 20th	Day	23% 4 11% 39% 18% 4	4065
	21st to end	Day	25% 5% 12% 25% 25% 8%	4313
September	1st to 10th	Day	20% 4 9% 30% 25% 13%	2280
	11th to 20th	Day	35% 6% 12% 23% 18% 6%	2228
	21st to end	Day	37% 6% 11% 24% 17% 6%	2028
October	1st to 10th	Day	36% 5% 7% 17% 22% 13%	3180
	11th to 20th	Day	48% 7% 14% 18% 12% 2	3161
	21st to end	Day	58% 6% 10% 15% 10% 2	3448
November	1st to 10th	Day	67% 5% 10% 12% 6%	1702
	11th to 20th	Day	67% 6% 9% 9% 6% 3	1600
	21st to end	Day	61% 7% 9% 8% 12% 3	1600
December	1st to 10th	Day	51% 5% 7% 14% 20% 4	2620
	11th to 20th	Day	75% 6% 7% 8% 2	3028
	21st to end	Day	81% 5% 6% 6% 2	4004
January	1st to 10th	Night	78% 5% 5% 7% 4 1	6306
	11th to 20th	Night	60% 6% 7% 16% 9% 2	6166
	21st to end	Night	57% 3 5% 12% 14% 8%	6468
February	1st to 10th	Night	67% 6% 9% 12% 4	2600
	11th to 20th	Night	62% 6% 12% 12% 6% 2	2534
	21st to end	Night	63% 7% 10% 12% 7% 2	2080
March	1st to 10th	Night	62% 6% 14% 13% 4 2	4997
	11th to 20th	Night	62% 5% 10% 15% 7% 1	5000
	21st to end	Night	68% 7% 13% 11% 2	5460
April	1st to 10th	Night	64% 6% 11% 15% 4	2000
	11th to 20th	Night	51% 6% 16% 17% 6% 3	1974
	21st to end	Night	52% 6% 14% 19% 7% 2	1758
May	1st to 10th	Night	48% 5% 15% 21% 10% 1	2774
	11th to 20th	Night	29% 4 13% 38% 14% 2	2516
	21st to end	Night	36% 8% 34% 19% 3	2762
June	1st to 10th	Night	43% 7% 16% 29% 4 1	1520
	11th to 20th	Night	32% 6% 22% 31% 8% 1	1520
	21st to end	Night	20% 4 20% 37% 15% 4	1520
July	1st to 10th	Night	9% 2 9% 21% 25% 35%	2500
	11th to 20th	Night	12% 3 7% 19% 30% 30%	2492
	21st to end	Night	12% 3 9% 28% 40% 8%	2850
August	1st to 10th	Night	14% 3 10% 30% 33% 10%	2839
	11th to 20th	Night	24% 4 16% 34% 19% 4	2839
	21st to end	Night	19% 4 10% 33% 27% 7%	3360
September	1st to 10th	Night	17% 4 9% 31% 26% 13%	1900
	11th to 20th	Night	29% 6% 13% 29% 16% 7%	1949
	21st to end	Night	30% 5% 14% 25% 17% 8%	2151
October	1st to 10th	Night	30% 5% 8% 23% 25% 10%	3800
	11th to 20th	Night	44% 6% 18% 22% 9% 2	3777
	21st to end	Night	60% 5% 8% 16% 9% 2	4121
November	1st to 10th	Night	59% 7% 11% 14% 9% 1	2593
	11th to 20th	Night	67% 6% 9% 10% 6% 2	2800
	21st to end	Night	57% 7% 10% 11% 11% 5%	2800
December	1st to 10th	Night	46% 5% 10% 19% 17% 2	4560
	11th to 20th	Night	75% 5% 9% 7% 3	5176
	21st to end	Night	74% 8% 8% 7% 2 1	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	43% 16% 23% 12% 6% 1	200
	11th to 20th	Day	39% 12% 22% 16% 9% 2	267
	21st to end	Day	30% 14% 20% 21% 13% 2	440
February	1st to 10th	Day	33% 14% 22% 20% 10% 2	400
	11th to 20th	Day	37% 19% 16% 19% 8% 1	395
	21st to end	Day	38% 15% 20% 20% 6% 1	364
March	1st to 10th	Day	34% 17% 26% 17% 5% 1	500
	11th to 20th	Day	34% 14% 21% 21% 8% 2	500
	21st to end	Day	35% 16% 21% 19% 7% 1	550
April	1st to 10th	Day	34% 17% 21% 19% 7% 2	500
	11th to 20th	Day	31% 17% 25% 17% 7% 3	492
	21st to end	Day	29% 16% 25% 20% 7% 2	481
May	1st to 10th	Day	23% 14% 29% 20% 11% 2	463
	11th to 20th	Day	26% 14% 27% 17% 13% 3	488
	21st to end	Day	21% 14% 23% 26% 13% 2	529
June	1st to 10th	Day	20% 14% 24% 30% 11% 1	490
	11th to 20th	Day	17% 19% 30% 26% 8% 1	490
	21st to end	Day	7% 10% 26% 40% 14% 2	490
July	1st to 10th	Day	7% 6% 27% 41% 16% 2	486
	11th to 20th	Day	4 7% 32% 39% 15% 3	484
	21st to end	Day	2 5% 25% 47% 18% 3	531
August	1st to 10th	Day	4 5% 27% 40% 21% 3	487
	11th to 20th	Day	4 5% 20% 40% 26% 4	477
	21st to end	Day	7% 8% 28% 37% 17% 3	539
September	1st to 10th	Day	7% 8% 28% 37% 17% 3	490
	11th to 20th	Day	14% 10% 27% 29% 16% 4	490
	21st to end	Day	17% 10% 31% 28% 9% 5%	490
October	1st to 10th	Day	19% 12% 27% 22% 17% 3	490
	11th to 20th	Day	21% 11% 21% 26% 18% 4	490
	21st to end	Day	29% 13% 22% 21% 10% 4	539
November	1st to 10th	Day	29% 19% 22% 19% 8% 2	288
	11th to 20th	Day	32% 17% 21% 20% 10% 2	200
	21st to end	Day	31% 15% 20% 22% 13% 1	200
December	1st to 10th	Day	31% 11% 22% 21% 14% 3	200
	11th to 20th	Day	40% 12% 23% 20% 5% 1	200
	21st to end	Day	43% 11% 20% 18% 7%	220
January	1st to 10th	Night	44% 9% 17% 19% 9% 3	800
	11th to 20th	Night	36% 9% 15% 23% 15% 2	800
	21st to end	Night	32% 6% 12% 31% 16% 3	880
February	1st to 10th	Night	30% 10% 16% 27% 15% 2	800
	11th to 20th	Night	33% 8% 18% 28% 13% 1	784
	21st to end	Night	32% 12% 16% 27% 13%	660
March	1st to 10th	Night	33% 8% 17% 30% 11% 1	800
	11th to 20th	Night	31% 7% 19% 29% 14% 2	800
	21st to end	Night	36% 8% 18% 25% 13%	880
April	1st to 10th	Night	32% 9% 18% 25% 15% 1	800
	11th to 20th	Night	24% 11% 22% 27% 15% 1	786
	21st to end	Night	25% 8% 17% 29% 19% 1	586
May	1st to 10th	Night	18% 8% 25% 30% 17% 2	523
	11th to 20th	Night	18% 5% 19% 33% 23% 2	374
	21st to end	Night	18% 4 23% 32% 22% 1	411
June	1st to 10th	Night	18% 5% 17% 40% 19% 1	380
	11th to 20th	Night	11% 5% 26% 40% 14% 3	380
	21st to end	Night	9% 2 19% 46% 24%	380
July	1st to 10th	Night	4 2 18% 47% 27% 1	370
	11th to 20th	Night	4 2 20% 54% 20% 1	368
	21st to end	Night	2 2 16% 47% 31% 2	522
August	1st to 10th	Night	3 2 15% 45% 31% 4	567
	11th to 20th	Night	4 2 12% 43% 35% 4	564
	21st to end	Night	6% 3 22% 41% 26% 2	750
September	1st to 10th	Night	7% 3 21% 40% 27% 1	760
	11th to 20th	Night	9% 5% 16% 32% 32% 5%	760
	21st to end	Night	14% 4 19% 36% 25% 3	760
October	1st to 10th	Night	17% 4 20% 33% 23% 3	760
	11th to 20th	Night	20% 4 16% 31% 24% 5%	760
	21st to end	Night	27% 8% 19% 25% 17% 4	836
November	1st to 10th	Night	31% 8% 15% 29% 17% 1	780
	11th to 20th	Night	34% 9% 19% 23% 13% 2	800
	21st to end	Night	26% 8% 19% 29% 16% 2	800
December	1st to 10th	Night	30% 7% 16% 28% 17% 3	800
	11th to 20th	Night	40% 6% 16% 22% 14% 1	800
	21st to end	Night	43% 10% 18% 23% 7%	880

Page generated at 2025-04-23 06:25:08. Version ac07

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