Oxford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oxford clear sky chart from 2005-11-06 to 2026-07-31.

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	12% 7% 2 6% 4 4 5% 4 6% 7% 42%	137385
	Night	14% 7% 2 6% 4 3 4 3 5% 7% 46%	135783
Transparency	Day	39% 5% 10% 22% 18% 5%	141064
	Night	37% 4 10% 22% 20% 7%	140184
Seeing	Day	16% 10% 24% 25% 18% 7%	17567
	Night	16% 9% 24% 29% 18% 3	27252

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	17% 12% 3 8% 3 3 3 2 3 4 41%	4946
	11th to 20th	Day	14% 6% 2 6% 3 3 3 2 4 4 55%	5310
	21st to end	Day	21% 9% 3 6% 5% 3 3 3 3 4 41%	5885
February	1st to 10th	Day	19% 9% 4 6% 4 4 3 3 4 5% 39%	2100
	11th to 20th	Day	21% 10% 3 5% 4 3 4 2 4 4 39%	2048
	21st to end	Day	20% 8% 3 6% 4 3 4 3 4 5% 40%	1730
March	1st to 10th	Day	19% 12% 4 9% 5% 4 4 2 4 4 34%	4580
	11th to 20th	Day	12% 8% 2 6% 4 4 4 4 5% 6% 46%	5058
	21st to end	Day	12% 8% 3 7% 5% 5% 4 3 5% 6% 42%	5976
April	1st to 10th	Day	13% 6% 2 6% 4 4 3 4 5% 8% 44%	2520
	11th to 20th	Day	14% 6% 2 5% 4 3 4 3 5% 7% 46%	2494
	21st to end	Day	11% 7% 2 6% 5% 5% 5% 4 5% 7% 43%	2520
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 5% 6% 8% 34%	7021
	11th to 20th	Day	11% 7% 3 8% 5% 5% 5% 4 6% 9% 36%	7448
	21st to end	Day	17% 9% 3 8% 5% 5% 5% 4 7% 7% 29%	8502
June	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 10% 41%	2940
	11th to 20th	Day	6% 5% 1 6% 5% 6% 5% 5% 7% 10% 44%	2940
	21st to end	Day	5% 4 1 6% 4 5% 6% 5% 7% 9% 47%	2940
July	1st to 10th	Day	5% 5% 2 6% 6% 6% 7% 6% 9% 11% 38%	6500
	11th to 20th	Day	7% 4 2 5% 5% 5% 6% 6% 10% 14% 36%	6219
	21st to end	Day	10% 6% 2 7% 5% 6% 6% 6% 8% 11% 32%	6415
August	1st to 10th	Day	3 3 1 4 3 4 5% 4 6% 8% 61%	3905
	11th to 20th	Day	6% 4 2 6% 4 4 6% 4 7% 10% 46%	3849
	21st to end	Day	7% 4 2 6% 4 4 5% 4 7% 8% 50%	4312
September	1st to 10th	Day	8% 5% 3 7% 5% 5% 5% 5% 7% 8% 45%	2400
	11th to 20th	Day	12% 5% 3 6% 4 5% 4 3 5% 7% 45%	2314
	21st to end	Day	10% 5% 1 5% 3 4 4 3 4 8% 52%	2200
October	1st to 10th	Day	8% 4 2 4 3 3 3 3 5% 6% 59%	2128
	11th to 20th	Day	9% 5% 1 5% 3 4 4 2 4 5% 59%	2000
	21st to end	Day	17% 6% 2 4 3 3 2 3 4 5% 52%	2200
November	1st to 10th	Day	12% 6% 2 4 3 3 3 2 3 4 58%	1895
	11th to 20th	Day	13% 5% 2 4 3 3 3 2 4 7% 54%	1890
	21st to end	Day	18% 8% 3 4 3 3 3 3 4 5% 46%	1890
December	1st to 10th	Day	16% 11% 3 6% 3 3 4 3 4 4 43%	3020
	11th to 20th	Day	20% 8% 3 6% 4 3 3 2 3 4 43%	3220
	21st to end	Day	13% 10% 3 6% 4 3 5% 4 5% 6% 40%	4070
January	1st to 10th	Night	17% 9% 3 8% 3 3 3 3 4 5% 42%	7974
	11th to 20th	Night	13% 7% 3 6% 3 2 3 2 3 4 54%	7680
	21st to end	Night	24% 8% 2 5% 4 3 3 3 4 5% 40%	8308
February	1st to 10th	Night	20% 8% 3 6% 4 3 3 2 4 5% 42%	2520
	11th to 20th	Night	20% 10% 3 6% 4 3 3 2 5% 5% 41%	2458
	21st to end	Night	20% 9% 2 6% 4 4 3 2 4 5% 42%	2076
March	1st to 10th	Night	18% 10% 3 7% 5% 3 4 2 5% 5% 39%	5493
	11th to 20th	Night	12% 7% 2 5% 3 4 3 3 5% 7% 49%	5151
	21st to end	Night	14% 7% 3 8% 5% 4 4 3 4 5% 44%	5180
April	1st to 10th	Night	13% 6% 2 5% 4 4 5% 4 5% 8% 47%	2100
	11th to 20th	Night	13% 5% 2 5% 2 3 3 3 5% 7% 51%	2075
	21st to end	Night	11% 7% 3 6% 4 4 4 3 4 6% 48%	2100
May	1st to 10th	Night	21% 9% 3 6% 4 3 3 3 6% 7% 36%	5832
	11th to 20th	Night	15% 7% 3 8% 5% 5% 4 3 5% 6% 38%	5394
	21st to end	Night	14% 9% 4 9% 6% 5% 5% 4 6% 8% 29%	5650
June	1st to 10th	Night	7% 4 2 6% 5% 4 4 3 7% 11% 46%	1680
	11th to 20th	Night	3 4 1 3 4 3 4 5% 7% 11% 54%	1680
	21st to end	Night	4 4 1 4 4 4 4 5% 7% 10% 53%	1680
July	1st to 10th	Night	3 3 2 6% 5% 5% 5% 6% 8% 13% 45%	3698
	11th to 20th	Night	6% 4 1 4 3 4 4 4 8% 12% 50%	3972
	21st to end	Night	8% 6% 2 6% 5% 5% 5% 5% 9% 13% 37%	4670
August	1st to 10th	Night	2 1 1 2 3 3 3 4 6% 9% 67%	3244
	11th to 20th	Night	3 2 2 4 4 4 5% 5% 7% 11% 53%	3200
	21st to end	Night	4 4 2 4 3 3 4 4 7% 10% 55%	3586
September	1st to 10th	Night	8% 5% 2 5% 4 5% 4 4 6% 7% 51%	2000
	11th to 20th	Night	10% 5% 2 6% 3 4 4 4 4 8% 50%	2065
	21st to end	Night	10% 4 1 4 3 4 3 3 6% 6% 56%	2200
October	1st to 10th	Night	7% 2 1 4 2 3 3 2 5% 7% 64%	2268
	11th to 20th	Night	11% 5% 2 4 3 2 3 2 4 5% 59%	2400
	21st to end	Night	16% 7% 2 5% 2 2 3 2 3 5% 54%	2640
November	1st to 10th	Night	11% 5% 1 4 3 3 3 3 4 6% 56%	2621
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 5% 6% 54%	2730
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 5% 49%	2730
December	1st to 10th	Night	19% 8% 2 4 3 3 2 2 3 6% 47%	4380
	11th to 20th	Night	20% 8% 3 5% 3 3 3 2 4 4 45%	5198
	21st to end	Night	18% 8% 2 5% 4 4 5% 3 5% 7% 39%	7150

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% 2 7% 26% 14% 2	4947
	11th to 20th	Day	35% 3 4 22% 31% 6%	5310
	21st to end	Day	49% 3 4 18% 19% 7%	5885
February	1st to 10th	Day	47% 3 7% 20% 18% 4	2100
	11th to 20th	Day	50% 3 5% 19% 17% 5%	2058
	21st to end	Day	47% 4 7% 15% 19% 8%	1730
March	1st to 10th	Day	55% 3 5% 14% 14% 9%	4580
	11th to 20th	Day	38% 5% 7% 19% 22% 10%	5058
	21st to end	Day	43% 4 7% 23% 20% 2	5978
April	1st to 10th	Day	39% 5% 9% 23% 19% 4	2520
	11th to 20th	Day	38% 4 8% 23% 20% 6%	2494
	21st to end	Day	40% 5% 10% 23% 20% 2	2520
May	1st to 10th	Day	46% 6% 10% 24% 12% 2	7056
	11th to 20th	Day	43% 6% 15% 19% 15% 1	7452
	21st to end	Day	52% 7% 12% 18% 10% 1	8516
June	1st to 10th	Day	36% 8% 13% 22% 19% 2	2940
	11th to 20th	Day	32% 8% 14% 26% 17% 3	2940
	21st to end	Day	31% 8% 16% 26% 16% 4	2940
July	1st to 10th	Day	35% 9% 19% 24% 11% 1	6431
	11th to 20th	Day	34% 10% 21% 30% 4 1	6145
	21st to end	Day	42% 8% 20% 27% 4	6235
August	1st to 10th	Day	20% 7% 14% 34% 22% 3	3893
	11th to 20th	Day	31% 7% 17% 27% 16% 2	3885
	21st to end	Day	30% 6% 11% 23% 25% 4	4311
September	1st to 10th	Day	34% 8% 11% 24% 17% 6%	2400
	11th to 20th	Day	38% 7% 10% 17% 21% 7%	2314
	21st to end	Day	31% 5% 10% 23% 23% 9%	2200
October	1st to 10th	Day	17% 4 5% 15% 29% 30%	3466
	11th to 20th	Day	18% 2 4 20% 38% 18%	3240
	21st to end	Day	27% 3 5% 25% 31% 10%	3538
November	1st to 10th	Day	32% 2 9% 19% 29% 9%	1895
	11th to 20th	Day	32% 3 8% 22% 27% 9%	1890
	21st to end	Day	41% 3 5% 21% 24% 5%	1890
December	1st to 10th	Day	45% 4 4 22% 22% 3	3020
	11th to 20th	Day	47% 2 5% 22% 18% 6%	3217
	21st to end	Day	44% 3 10% 24% 17% 2	4070
January	1st to 10th	Night	47% 2 7% 24% 17% 2	7975
	11th to 20th	Night	36% 3 3 19% 32% 8%	7680
	21st to end	Night	48% 2 4 18% 19% 7%	8308
February	1st to 10th	Night	46% 3 7% 20% 20% 4	2520
	11th to 20th	Night	46% 4 6% 20% 17% 6%	2459
	21st to end	Night	46% 3 6% 16% 21% 8%	2076
March	1st to 10th	Night	48% 4 7% 14% 19% 9%	5493
	11th to 20th	Night	35% 5% 5% 19% 19% 17%	5151
	21st to end	Night	43% 4 7% 22% 20% 4	5180
April	1st to 10th	Night	37% 4 11% 23% 19% 7%	2100
	11th to 20th	Night	33% 4 7% 23% 24% 8%	2075
	21st to end	Night	37% 5% 10% 26% 20% 4	2100
May	1st to 10th	Night	47% 5% 13% 18% 15% 2	5862
	11th to 20th	Night	46% 6% 11% 23% 11% 4	5395
	21st to end	Night	51% 8% 10% 17% 11% 2	5660
June	1st to 10th	Night	32% 7% 13% 24% 22% 2	1680
	11th to 20th	Night	23% 6% 17% 27% 22% 4	1680
	21st to end	Night	24% 7% 17% 25% 22% 4	1680
July	1st to 10th	Night	29% 9% 23% 25% 12% 2	3660
	11th to 20th	Night	25% 7% 23% 37% 6% 2	3926
	21st to end	Night	36% 7% 23% 26% 7%	4539
August	1st to 10th	Night	14% 6% 13% 39% 27% 1	3232
	11th to 20th	Night	23% 7% 24% 27% 18% 2	3230
	21st to end	Night	24% 6% 13% 23% 28% 5%	3586
September	1st to 10th	Night	30% 6% 12% 22% 22% 8%	2000
	11th to 20th	Night	32% 6% 11% 19% 26% 6%	2065
	21st to end	Night	29% 4 10% 22% 25% 9%	2200
October	1st to 10th	Night	14% 4 6% 19% 28% 30%	3710
	11th to 20th	Night	19% 3 4 16% 33% 26%	3899
	21st to end	Night	26% 3 5% 24% 27% 15%	4252
November	1st to 10th	Night	30% 4 8% 21% 28% 9%	2621
	11th to 20th	Night	31% 4 7% 20% 27% 11%	2730
	21st to end	Night	39% 4 6% 19% 24% 8%	2730
December	1st to 10th	Night	41% 2 4 27% 22% 3	4380
	11th to 20th	Night	45% 2 7% 19% 23% 4	5200
	21st to end	Night	45% 3 10% 23% 16% 3	7150

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	28% 22% 36% 12% 2	330
	11th to 20th	Day	29% 21% 34% 12% 2 1	330
	21st to end	Day	26% 27% 36% 8% 2	363
February	1st to 10th	Day	28% 20% 34% 12% 6%	330
	11th to 20th	Day	38% 21% 30% 9% 2	330
	21st to end	Day	36% 25% 29% 8% 3	273
March	1st to 10th	Day	27% 24% 33% 12% 4	330
	11th to 20th	Day	25% 25% 39% 11%	330
	21st to end	Day	24% 18% 35% 18% 5%	536
April	1st to 10th	Day	23% 18% 31% 22% 5% 1	540
	11th to 20th	Day	21% 19% 31% 20% 7% 2	532
	21st to end	Day	19% 16% 29% 25% 10% 1	540
May	1st to 10th	Day	19% 15% 29% 25% 10% 2	543
	11th to 20th	Day	13% 10% 31% 28% 13% 4	567
	21st to end	Day	13% 6% 27% 29% 18% 7%	650
June	1st to 10th	Day	10% 6% 21% 32% 23% 9%	780
	11th to 20th	Day	7% 3 23% 32% 24% 11%	780
	21st to end	Day	7% 2 18% 35% 27% 11%	780
July	1st to 10th	Day	5% 3 12% 33% 33% 15%	771
	11th to 20th	Day	8% 2 10% 30% 34% 17%	768
	21st to end	Day	8% 2 10% 31% 33% 16%	782
August	1st to 10th	Day	7% 3 8% 31% 37% 13%	526
	11th to 20th	Day	12% 2 12% 26% 34% 14%	519
	21st to end	Day	13% 2 12% 22% 34% 17%	583
September	1st to 10th	Day	9% 2 14% 32% 32% 11%	530
	11th to 20th	Day	12% 2 14% 25% 31% 15%	530
	21st to end	Day	12% 4 20% 31% 22% 11%	530
October	1st to 10th	Day	10% 3 23% 32% 24% 8%	458
	11th to 20th	Day	10% 11% 34% 34% 10% 1	330
	21st to end	Day	23% 12% 31% 24% 8% 1	363
November	1st to 10th	Day	22% 11% 25% 27% 13% 2	330
	11th to 20th	Day	14% 15% 41% 26% 4 1	330
	21st to end	Day	25% 16% 32% 14% 10% 2	330
December	1st to 10th	Day	27% 20% 34% 15% 3	330
	11th to 20th	Day	23% 20% 35% 17% 5% 1	330
	21st to end	Day	24% 21% 36% 16% 2 1	363
January	1st to 10th	Night	24% 17% 31% 24% 4	1050
	11th to 20th	Night	24% 18% 32% 22% 5%	1050
	21st to end	Night	23% 16% 37% 23% 2	1126
February	1st to 10th	Night	26% 13% 35% 23% 4	840
	11th to 20th	Night	26% 19% 30% 22% 2	823
	21st to end	Night	25% 22% 30% 21% 2	692
March	1st to 10th	Night	21% 14% 31% 29% 5%	840
	11th to 20th	Night	22% 13% 33% 26% 6%	665
	21st to end	Night	18% 13% 35% 25% 9%	693
April	1st to 10th	Night	17% 12% 34% 27% 10%	630
	11th to 20th	Night	16% 11% 34% 26% 12% 1	623
	21st to end	Night	14% 11% 34% 27% 13% 1	630
May	1st to 10th	Night	17% 7% 27% 35% 12% 3	630
	11th to 20th	Night	13% 5% 28% 32% 18% 3	621
	21st to end	Night	10% 3 21% 36% 24% 6%	683
June	1st to 10th	Night	7% 3 15% 39% 30% 6%	630
	11th to 20th	Night	4 2 14% 37% 36% 7%	630
	21st to end	Night	5% 2 12% 34% 39% 8%	630
July	1st to 10th	Night	4 1 9% 29% 46% 11%	615
	11th to 20th	Night	8% 2 8% 27% 40% 16%	612
	21st to end	Night	6% 2 7% 29% 45% 11%	687
August	1st to 10th	Night	4 2 4 29% 48% 13%	597
	11th to 20th	Night	6% 1 8% 37% 38% 11%	594
	21st to end	Night	8% 2 9% 29% 42% 10%	660
September	1st to 10th	Night	10% 2 10% 34% 39% 6%	600
	11th to 20th	Night	11% 2 12% 31% 36% 8%	665
	21st to end	Night	13% 3 15% 33% 32% 3	800
October	1st to 10th	Night	8% 4 19% 33% 31% 6%	800
	11th to 20th	Night	14% 7% 21% 37% 19% 2	800
	21st to end	Night	20% 8% 23% 30% 17% 1	880
November	1st to 10th	Night	14% 7% 24% 34% 18% 3	820
	11th to 20th	Night	13% 10% 31% 33% 11%	840
	21st to end	Night	21% 11% 30% 26% 11% 1	840
December	1st to 10th	Night	25% 13% 27% 28% 7%	840
	11th to 20th	Night	21% 14% 30% 28% 6%	961
	21st to end	Night	27% 15% 27% 26% 4	1155

Page generated at 2026-07-28 02:25:22. Version ac07

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