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 2026-06-16.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	137274
	Night	14% 9% 3 8% 5% 4 4 4 5% 7% 36%	134810
Transparency	Day	45% 6% 11% 22% 14% 2	140912
	Night	45% 6% 10% 22% 14% 2	139244
Seeing	Day	18% 10% 27% 23% 15% 7%	17559
	Night	18% 8% 27% 30% 15% 2	27335

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	15% 15% 7% 11% 6% 4 4 3 5% 5% 24%	4691
	11th to 20th	Day	11% 12% 5% 16% 7% 7% 5% 3 4 5% 25%	4880
	21st to end	Day	18% 12% 7% 11% 6% 6% 5% 4 5% 5% 20%	5780
February	1st to 10th	Day	18% 13% 6% 9% 5% 5% 4 3 4 5% 28%	2011
	11th to 20th	Day	16% 11% 4 11% 6% 5% 4 3 4 6% 30%	2048
	21st to end	Day	17% 12% 4 10% 5% 4 3 3 4 5% 33%	1730
March	1st to 10th	Day	18% 13% 7% 10% 5% 4 4 3 4 5% 29%	4580
	11th to 20th	Day	11% 10% 2 7% 4 4 4 3 6% 7% 40%	5086
	21st to end	Day	16% 10% 4 8% 5% 4 5% 3 6% 7% 32%	5586
April	1st to 10th	Day	14% 11% 4 8% 4 4 4 3 5% 8% 33%	2506
	11th to 20th	Day	15% 9% 3 7% 5% 4 5% 3 5% 6% 38%	2540
	21st to end	Day	17% 8% 3 7% 5% 4 5% 4 5% 8% 35%	2730
May	1st to 10th	Day	18% 9% 3 8% 5% 5% 5% 4 5% 7% 31%	7616
	11th to 20th	Day	18% 8% 3 6% 4 4 4 3 5% 8% 38%	7626
	21st to end	Day	15% 7% 3 8% 5% 6% 6% 5% 8% 9% 28%	8391
June	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 5% 6% 8% 42%	2802
	11th to 20th	Day	8% 5% 3 6% 5% 5% 5% 4 6% 9% 43%	2871
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 10% 39%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 7% 11% 39%	6360
	11th to 20th	Day	8% 6% 2 7% 5% 5% 6% 5% 8% 11% 38%	6275
	21st to end	Day	9% 7% 3 8% 6% 6% 7% 5% 7% 9% 32%	6424
August	1st to 10th	Day	6% 5% 2 6% 5% 5% 5% 4 7% 10% 44%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 10% 41%	4174
	21st to end	Day	6% 4 2 6% 4 4 4 4 6% 8% 53%	4315
September	1st to 10th	Day	11% 5% 2 5% 3 4 5% 4 5% 7% 49%	2295
	11th to 20th	Day	9% 5% 2 5% 4 3 5% 3 7% 9% 49%	2200
	21st to end	Day	15% 6% 2 6% 4 4 3 3 5% 6% 47%	2200
October	1st to 10th	Day	10% 5% 2 6% 4 5% 4 3 6% 6% 49%	2200
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 46%	2045
	21st to end	Day	19% 9% 3 6% 4 3 4 3 5% 6% 37%	2020
November	1st to 10th	Day	9% 6% 2 6% 5% 4 4 3 4 6% 51%	1852
	11th to 20th	Day	14% 12% 4 9% 5% 4 3 2 5% 6% 36%	1890
	21st to end	Day	18% 11% 4 9% 5% 4 3 3 4 6% 33%	1890
December	1st to 10th	Day	16% 12% 3 8% 4 3 4 2 4 5% 38%	3020
	11th to 20th	Day	18% 13% 5% 11% 5% 5% 5% 3 4 5% 26%	3463
	21st to end	Day	23% 19% 6% 10% 5% 4 4 3 4 6% 17%	4143
January	1st to 10th	Night	13% 14% 5% 14% 8% 5% 5% 4 5% 6% 21%	8232
	11th to 20th	Night	10% 15% 7% 15% 7% 5% 5% 3 4 5% 22%	8068
	21st to end	Night	17% 12% 5% 8% 5% 5% 4 3 5% 5% 31%	8401
February	1st to 10th	Night	19% 12% 3 10% 5% 5% 5% 3 4 5% 28%	2604
	11th to 20th	Night	16% 10% 4 9% 6% 4 5% 4 5% 6% 32%	2458
	21st to end	Night	19% 11% 5% 10% 4 4 3 4 5% 5% 32%	2076
March	1st to 10th	Night	19% 12% 3 6% 4 4 3 3 4 5% 37%	5493
	11th to 20th	Night	11% 7% 3 7% 5% 5% 5% 4 5% 6% 44%	5094
	21st to end	Night	17% 9% 3 8% 4 4 5% 4 5% 6% 35%	5588
April	1st to 10th	Night	14% 9% 3 6% 5% 4 4 4 5% 7% 38%	2131
	11th to 20th	Night	14% 7% 2 6% 4 4 5% 5% 6% 7% 40%	2029
	21st to end	Night	15% 7% 3 7% 5% 4 4 4 7% 8% 37%	1890
May	1st to 10th	Night	19% 9% 3 8% 5% 4 4 3 6% 7% 30%	5237
	11th to 20th	Night	14% 7% 3 7% 4 4 4 4 6% 8% 38%	5234
	21st to end	Night	15% 7% 3 7% 5% 4 5% 4 6% 9% 35%	5761
June	1st to 10th	Night	10% 4 2 6% 5% 4 3 4 5% 8% 48%	1812
	11th to 20th	Night	8% 5% 2 5% 4 3 5% 4 7% 11% 45%	1640
	21st to end	Night	6% 4 2 5% 5% 5% 5% 5% 7% 9% 48%	1600
July	1st to 10th	Night	9% 5% 2 5% 4 3 5% 4 7% 8% 48%	3618
	11th to 20th	Night	7% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	3695
	21st to end	Night	8% 7% 2 7% 5% 5% 4 4 8% 11% 39%	4427
August	1st to 10th	Night	8% 6% 2 7% 5% 4 5% 4 6% 9% 43%	2915
	11th to 20th	Night	6% 6% 2 7% 5% 5% 5% 3 7% 7% 48%	2875
	21st to end	Night	3 3 1 5% 4 4 4 4 7% 8% 56%	3578
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 4 5% 7% 51%	2085
	11th to 20th	Night	8% 4 2 5% 3 4 4 4 5% 7% 55%	2200
	21st to end	Night	13% 7% 3 6% 4 3 3 2 4 6% 49%	2200
October	1st to 10th	Night	9% 3 2 4 4 4 4 3 5% 7% 55%	2200
	11th to 20th	Night	13% 6% 3 6% 4 4 4 3 5% 6% 46%	2355
	21st to end	Night	17% 8% 2 7% 4 4 4 3 5% 6% 41%	2818
November	1st to 10th	Night	12% 8% 2 6% 4 3 4 3 4 7% 47%	2666
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 3 5% 7% 40%	2730
	21st to end	Night	19% 12% 3 8% 5% 4 4 3 5% 6% 30%	2730
December	1st to 10th	Night	16% 11% 4 8% 5% 3 3 3 4 5% 37%	4380
	11th to 20th	Night	18% 13% 5% 10% 5% 5% 4 4 5% 5% 26%	4952
	21st to end	Night	24% 16% 7% 11% 5% 4 4 3 4 5% 17%	7038

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	60% 4 12% 18% 6%	4692
	11th to 20th	Day	61% 3 9% 16% 10% 1	4880
	21st to end	Day	65% 4 9% 15% 6% 2	5780
February	1st to 10th	Day	58% 4 10% 17% 9% 1	2011
	11th to 20th	Day	56% 4 10% 21% 9% 1	2058
	21st to end	Day	54% 4 11% 21% 8% 2	1730
March	1st to 10th	Day	57% 4 6% 19% 11% 2	4580
	11th to 20th	Day	41% 5% 10% 28% 15% 1	5086
	21st to end	Day	50% 6% 12% 19% 11% 2	5586
April	1st to 10th	Day	48% 6% 11% 22% 13% 1	2506
	11th to 20th	Day	46% 5% 10% 22% 14% 3	2540
	21st to end	Day	47% 6% 10% 23% 11% 2	2730
May	1st to 10th	Day	50% 8% 12% 20% 11%	7651
	11th to 20th	Day	45% 6% 11% 24% 14%	7630
	21st to end	Day	48% 9% 11% 20% 11% 2	8405
June	1st to 10th	Day	39% 8% 10% 23% 17% 3	2802
	11th to 20th	Day	36% 8% 13% 23% 18% 2	2871
	21st to end	Day	38% 8% 15% 19% 16% 3	2800
July	1st to 10th	Day	37% 7% 11% 26% 17% 1	6291
	11th to 20th	Day	36% 9% 14% 25% 15% 1	6200
	21st to end	Day	43% 9% 13% 19% 13% 2	6240
August	1st to 10th	Day	31% 9% 15% 31% 13% 1	4221
	11th to 20th	Day	34% 8% 12% 32% 14% 1	4213
	21st to end	Day	27% 7% 14% 30% 19% 3	4314
September	1st to 10th	Day	34% 7% 10% 25% 19% 5%	2295
	11th to 20th	Day	31% 6% 10% 27% 23% 3	2200
	21st to end	Day	38% 5% 9% 23% 23% 2	2200
October	1st to 10th	Day	24% 4 8% 30% 32% 2	3590
	11th to 20th	Day	29% 5% 9% 30% 25% 2	3311
	21st to end	Day	37% 4 10% 24% 23% 2	3244
November	1st to 10th	Day	35% 5% 9% 30% 19% 3	1852
	11th to 20th	Day	49% 3 10% 20% 15% 3	1890
	21st to end	Day	52% 4 10% 21% 12% 1	1890
December	1st to 10th	Day	49% 3 10% 26% 11%	3020
	11th to 20th	Day	60% 6% 8% 18% 7%	3460
	21st to end	Day	69% 4 10% 11% 5%	4143
January	1st to 10th	Night	62% 5% 10% 15% 8%	8233
	11th to 20th	Night	63% 5% 7% 13% 10% 2	8068
	21st to end	Night	54% 3 11% 21% 10% 1	8401
February	1st to 10th	Night	58% 5% 9% 20% 8% 1	2604
	11th to 20th	Night	53% 4 11% 20% 11% 1	2459
	21st to end	Night	53% 5% 12% 17% 10% 2	2076
March	1st to 10th	Night	50% 4 6% 26% 11% 2	5493
	11th to 20th	Night	38% 6% 11% 26% 17% 1	5094
	21st to end	Night	49% 5% 8% 25% 11% 2	5588
April	1st to 10th	Night	45% 5% 10% 25% 13% 2	2131
	11th to 20th	Night	40% 7% 11% 20% 17% 5%	2029
	21st to end	Night	43% 6% 11% 23% 13% 3	1890
May	1st to 10th	Night	51% 7% 10% 22% 8% 2	5264
	11th to 20th	Night	42% 7% 12% 25% 14% 1	5235
	21st to end	Night	43% 8% 14% 21% 10% 4	5771
June	1st to 10th	Night	34% 7% 9% 25% 22% 2	1812
	11th to 20th	Night	32% 9% 13% 22% 21% 4	1640
	21st to end	Night	30% 9% 16% 23% 18% 5%	1600
July	1st to 10th	Night	31% 8% 11% 25% 19% 5%	3580
	11th to 20th	Night	33% 9% 15% 26% 15% 1	3650
	21st to end	Night	36% 8% 13% 25% 14% 3	4297
August	1st to 10th	Night	35% 9% 13% 29% 13% 1	2904
	11th to 20th	Night	32% 8% 12% 31% 17% 1	2902
	21st to end	Night	22% 8% 15% 31% 21% 3	3578
September	1st to 10th	Night	32% 6% 12% 24% 19% 7%	2085
	11th to 20th	Night	27% 7% 10% 26% 27% 4	2200
	21st to end	Night	37% 4 9% 22% 26% 2	2200
October	1st to 10th	Night	21% 4 6% 29% 37% 3	3590
	11th to 20th	Night	28% 3 9% 25% 32% 2	3828
	21st to end	Night	34% 4 7% 24% 26% 5%	4544
November	1st to 10th	Night	37% 4 10% 23% 23% 3	2666
	11th to 20th	Night	43% 4 9% 23% 18% 3	2730
	21st to end	Night	54% 5% 10% 20% 10% 1	2730
December	1st to 10th	Night	49% 5% 11% 24% 10%	4380
	11th to 20th	Night	59% 5% 10% 18% 8% 1	4954
	21st to end	Night	69% 5% 10% 12% 4	7038

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% 15% 35% 15% 6%	330
	11th to 20th	Day	26% 22% 38% 11% 2	330
	21st to end	Day	30% 18% 31% 17% 3	363
February	1st to 10th	Day	25% 16% 39% 15% 4	330
	11th to 20th	Day	27% 24% 34% 13% 2	330
	21st to end	Day	24% 17% 47% 10% 3	273
March	1st to 10th	Day	25% 18% 40% 14% 2	330
	11th to 20th	Day	25% 16% 35% 18% 6% 1	536
	21st to end	Day	26% 17% 33% 17% 6%	594
April	1st to 10th	Day	22% 16% 31% 20% 7% 2	540
	11th to 20th	Day	21% 17% 29% 20% 11% 3	532
	21st to end	Day	21% 13% 33% 21% 9% 3	540
May	1st to 10th	Day	24% 12% 32% 20% 9% 3	543
	11th to 20th	Day	24% 10% 26% 22% 12% 7%	567
	21st to end	Day	16% 7% 24% 26% 17% 11%	617
June	1st to 10th	Day	13% 6% 24% 30% 19% 8%	642
	11th to 20th	Day	10% 5% 24% 32% 21% 7%	756
	21st to end	Day	10% 5% 25% 32% 18% 10%	730
July	1st to 10th	Day	12% 3 17% 26% 23% 19%	721
	11th to 20th	Day	10% 3 20% 26% 27% 13%	685
	21st to end	Day	13% 3 15% 27% 27% 16%	576
August	1st to 10th	Day	9% 3 13% 30% 25% 19%	526
	11th to 20th	Day	11% 4 17% 29% 23% 15%	519
	21st to end	Day	9% 3 13% 28% 30% 16%	583
September	1st to 10th	Day	12% 4 14% 25% 33% 12%	530
	11th to 20th	Day	10% 3 11% 27% 31% 17%	530
	21st to end	Day	18% 8% 18% 25% 22% 9%	530
October	1st to 10th	Day	11% 5% 23% 30% 23% 8%	530
	11th to 20th	Day	14% 12% 28% 27% 16% 3	530
	21st to end	Day	24% 13% 34% 21% 6% 1	403
November	1st to 10th	Day	12% 14% 36% 24% 11% 3	330
	11th to 20th	Day	22% 17% 32% 19% 8% 2	330
	21st to end	Day	29% 11% 39% 16% 5%	330
December	1st to 10th	Day	22% 15% 41% 17% 5% 1	330
	11th to 20th	Day	24% 16% 39% 15% 5%	330
	21st to end	Day	30% 14% 36% 16% 5%	363
January	1st to 10th	Night	21% 15% 33% 25% 5%	1050
	11th to 20th	Night	27% 13% 35% 21% 3	981
	21st to end	Night	23% 12% 37% 25% 3	924
February	1st to 10th	Night	25% 11% 34% 28% 2	840
	11th to 20th	Night	23% 14% 37% 23% 3	823
	21st to end	Night	26% 12% 36% 24% 2	692
March	1st to 10th	Night	23% 12% 34% 26% 5%	840
	11th to 20th	Night	19% 14% 35% 25% 7%	840
	21st to end	Night	24% 10% 31% 28% 7%	924
April	1st to 10th	Night	18% 9% 36% 28% 8%	661
	11th to 20th	Night	18% 9% 33% 28% 10% 1	623
	21st to end	Night	19% 7% 36% 27% 10% 1	630
May	1st to 10th	Night	23% 7% 29% 30% 10% 1	630
	11th to 20th	Night	19% 6% 22% 32% 18% 3	621
	21st to end	Night	11% 2 21% 37% 25% 2	683
June	1st to 10th	Night	12% 3 20% 40% 24% 1	630
	11th to 20th	Night	12% 3 17% 40% 25% 3	615
	21st to end	Night	7% 2 21% 38% 30% 3	600
July	1st to 10th	Night	13% 2 12% 31% 34% 8%	585
	11th to 20th	Night	9% 2 15% 35% 35% 5%	582
	21st to end	Night	10% 2 11% 40% 33% 4	657
August	1st to 10th	Night	8% 2 10% 37% 38% 5%	597
	11th to 20th	Night	8% 1 9% 45% 32% 6%	594
	21st to end	Night	6% 2 12% 35% 38% 7%	660
September	1st to 10th	Night	13% 3 14% 33% 33% 5%	685
	11th to 20th	Night	9% 2 13% 35% 36% 5%	800
	21st to end	Night	16% 4 20% 33% 25% 3	800
October	1st to 10th	Night	11% 5% 19% 34% 25% 5%	800
	11th to 20th	Night	17% 8% 25% 33% 15% 2	800
	21st to end	Night	21% 9% 33% 27% 10% 1	880
November	1st to 10th	Night	16% 8% 31% 29% 16% 1	820
	11th to 20th	Night	20% 13% 31% 28% 9%	840
	21st to end	Night	25% 11% 26% 30% 7% 1	840
December	1st to 10th	Night	26% 11% 33% 25% 5%	840
	11th to 20th	Night	25% 11% 33% 25% 6% 1	840
	21st to end	Night	31% 12% 30% 23% 3	1108

Page generated at 2026-06-13 02:31:44. Version ac07

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