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 2025-03-27.

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 4 4 5% 7% 44%	108852
	Night	14% 7% 2 5% 3 3 3 3 5% 7% 47%	110611
Transparency	Day	38% 5% 10% 23% 19% 5%	112570
	Night	36% 4 9% 22% 21% 7%	115034
Seeing	Day	16% 10% 24% 25% 18% 7%	15651
	Night	16% 9% 24% 29% 18% 3	25559

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% 11% 3 9% 3 3 3 2 3 3 45%	3816
	11th to 20th	Day	17% 7% 2 7% 3 3 3 2 4 4 49%	4080
	21st to end	Day	19% 10% 2 5% 3 4 3 3 3 4 43%	4548
February	1st to 10th	Day	19% 10% 4 6% 4 4 3 3 4 5% 38%	2000
	11th to 20th	Day	21% 10% 3 5% 4 3 4 3 4 4 40%	1948
	21st to end	Day	20% 9% 3 6% 4 3 4 3 4 5% 39%	1650
March	1st to 10th	Day	21% 11% 4 9% 5% 3 3 2 4 5% 33%	3220
	11th to 20th	Day	13% 8% 2 5% 4 4 5% 5% 6% 8% 42%	3572
	21st to end	Day	12% 7% 3 7% 5% 6% 4 3 5% 6% 41%	4156
April	1st to 10th	Day	14% 6% 2 6% 4 4 3 4 5% 8% 44%	2280
	11th to 20th	Day	16% 6% 2 6% 4 3 4 3 5% 6% 44%	2254
	21st to end	Day	11% 7% 2 6% 5% 5% 4 4 5% 6% 45%	2280
May	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 5% 6% 9% 40%	3765
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 4 6% 9% 39%	3985
	21st to end	Day	13% 6% 3 7% 5% 5% 5% 4 6% 9% 37%	4531
June	1st to 10th	Day	8% 6% 2 6% 6% 5% 5% 5% 7% 9% 41%	2660
	11th to 20th	Day	5% 4 1 5% 5% 5% 5% 5% 8% 10% 47%	2660
	21st to end	Day	5% 4 2 6% 4 5% 6% 5% 7% 9% 47%	2660
July	1st to 10th	Day	5% 5% 2 7% 6% 5% 7% 5% 8% 11% 38%	4440
	11th to 20th	Day	9% 5% 2 6% 5% 5% 7% 6% 9% 13% 31%	4249
	21st to end	Day	15% 9% 3 8% 6% 6% 6% 5% 7% 8% 28%	4372
August	1st to 10th	Day	3 3 1 4 3 4 5% 3 6% 8% 61%	3785
	11th to 20th	Day	6% 4 2 6% 4 4 6% 4 7% 10% 47%	3729
	21st to end	Day	7% 4 2 6% 4 4 5% 4 7% 8% 50%	4180
September	1st to 10th	Day	8% 5% 3 6% 5% 5% 5% 5% 7% 8% 45%	2280
	11th to 20th	Day	12% 5% 3 7% 4 5% 4 3 5% 7% 44%	2199
	21st to end	Day	10% 5% 1 5% 3 4 4 3 4 8% 52%	2090
October	1st to 10th	Day	7% 4 2 4 3 3 3 3 5% 6% 58%	2022
	11th to 20th	Day	10% 5% 1 6% 3 4 4 2 4 5% 58%	1900
	21st to end	Day	17% 6% 2 4 3 3 2 3 4 5% 52%	2090
November	1st to 10th	Day	13% 6% 2 4 2 3 3 2 3 4 58%	1803
	11th to 20th	Day	14% 5% 2 4 3 3 3 2 4 7% 54%	1800
	21st to end	Day	18% 8% 3 4 3 3 3 3 4 5% 46%	1800
December	1st to 10th	Day	17% 10% 2 5% 3 3 4 3 4 4 44%	2930
	11th to 20th	Day	21% 8% 3 6% 4 3 3 3 3 4 42%	3136
	21st to end	Day	13% 10% 3 6% 4 3 5% 4 5% 6% 40%	3982
January	1st to 10th	Night	16% 10% 4 8% 3 3 3 2 3 4 43%	6146
	11th to 20th	Night	16% 8% 3 7% 3 2 2 2 3 3 50%	5900
	21st to end	Night	23% 9% 2 4 2 3 2 3 4 5% 43%	6424
February	1st to 10th	Night	20% 9% 3 6% 4 3 3 2 4 5% 41%	2400
	11th to 20th	Night	20% 9% 3 6% 4 3 3 2 5% 4 41%	2338
	21st to end	Night	20% 9% 2 6% 3 4 3 2 4 5% 42%	1980
March	1st to 10th	Night	19% 10% 3 6% 5% 3 3 2 5% 6% 38%	3853
	11th to 20th	Night	13% 8% 2 5% 4 4 3 4 5% 7% 46%	3636
	21st to end	Night	15% 7% 3 7% 5% 4 4 3 5% 5% 43%	3611
April	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 8% 47%	1900
	11th to 20th	Night	14% 6% 2 5% 2 3 3 3 5% 7% 50%	1875
	21st to end	Night	10% 6% 3 6% 4 4 4 3 4 6% 50%	1900
May	1st to 10th	Night	18% 7% 3 5% 4 3 3 3 6% 8% 41%	3130
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 45%	2890
	21st to end	Night	10% 5% 3 7% 5% 5% 5% 5% 7% 10% 39%	3020
June	1st to 10th	Night	7% 4 2 7% 5% 4 4 3 7% 12% 46%	1520
	11th to 20th	Night	3 4 1 3 4 3 4 5% 7% 11% 56%	1520
	21st to end	Night	4 4 1 4 4 4 4 5% 6% 10% 54%	1520
July	1st to 10th	Night	4 3 2 7% 6% 5% 5% 5% 8% 13% 43%	2530
	11th to 20th	Night	8% 4 2 5% 3 4 4 4 7% 12% 47%	2715
	21st to end	Night	11% 9% 3 8% 5% 5% 5% 5% 7% 8% 35%	3188
August	1st to 10th	Night	2 1 1 2 3 3 3 4 6% 9% 67%	3144
	11th to 20th	Night	3 2 2 4 4 4 5% 5% 7% 11% 53%	3100
	21st to end	Night	4 4 2 4 3 3 4 4 7% 10% 55%	3476
September	1st to 10th	Night	8% 5% 2 6% 4 5% 4 4 6% 7% 51%	1900
	11th to 20th	Night	10% 5% 2 6% 3 4 4 4 4 8% 49%	1962
	21st to end	Night	11% 4 1 4 3 4 3 3 6% 6% 56%	2090
October	1st to 10th	Night	7% 2 1 4 2 3 3 2 5% 7% 64%	2154
	11th to 20th	Night	11% 5% 2 5% 3 2 3 2 5% 5% 57%	2280
	21st to end	Night	16% 7% 2 5% 2 2 3 2 3 5% 55%	2508
November	1st to 10th	Night	12% 5% 1 4 3 3 4 3 4 6% 56%	2493
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 5% 6% 53%	2600
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 50%	2600
December	1st to 10th	Night	19% 8% 2 4 3 2 2 2 3 6% 48%	4250
	11th to 20th	Night	21% 8% 3 5% 3 3 3 2 4 4 44%	5062
	21st to end	Night	18% 8% 2 5% 4 4 5% 4 5% 7% 38%	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	46% 2 5% 26% 19% 2	3817
	11th to 20th	Day	40% 3 4 25% 25% 3	4080
	21st to end	Day	46% 3 5% 18% 21% 7%	4548
February	1st to 10th	Day	48% 4 7% 20% 18% 4	2000
	11th to 20th	Day	50% 4 5% 19% 17% 5%	1958
	21st to end	Day	48% 4 7% 15% 19% 8%	1650
March	1st to 10th	Day	55% 4 6% 11% 15% 9%	3220
	11th to 20th	Day	39% 6% 9% 23% 18% 7%	3572
	21st to end	Day	43% 5% 7% 22% 20% 3	4156
April	1st to 10th	Day	39% 5% 10% 22% 20% 5%	2280
	11th to 20th	Day	40% 4 7% 22% 20% 6%	2254
	21st to end	Day	38% 5% 10% 24% 21% 2	2280
May	1st to 10th	Day	39% 8% 11% 28% 13% 2	3800
	11th to 20th	Day	40% 7% 15% 24% 13% 1	3989
	21st to end	Day	42% 9% 12% 27% 10% 1	4544
June	1st to 10th	Day	37% 9% 13% 22% 19% 2	2660
	11th to 20th	Day	29% 8% 14% 28% 18% 3	2660
	21st to end	Day	30% 8% 15% 25% 17% 4	2660
July	1st to 10th	Day	36% 9% 20% 23% 10% 2	4371
	11th to 20th	Day	38% 11% 17% 27% 6% 2	4175
	21st to end	Day	51% 8% 13% 23% 5% 1	4234
August	1st to 10th	Day	20% 7% 13% 34% 22% 3	3773
	11th to 20th	Day	31% 7% 17% 27% 16% 2	3765
	21st to end	Day	31% 6% 12% 23% 25% 4	4179
September	1st to 10th	Day	34% 8% 10% 24% 17% 6%	2280
	11th to 20th	Day	39% 7% 10% 15% 22% 7%	2199
	21st to end	Day	31% 5% 9% 23% 23% 9%	2090
October	1st to 10th	Day	17% 4 5% 15% 28% 30%	3360
	11th to 20th	Day	18% 2 4 20% 38% 18%	3140
	21st to end	Day	26% 3 5% 25% 31% 10%	3428
November	1st to 10th	Day	32% 2 9% 19% 28% 9%	1803
	11th to 20th	Day	32% 3 7% 20% 28% 9%	1800
	21st to end	Day	41% 3 5% 21% 24% 5%	1800
December	1st to 10th	Day	44% 4 4 23% 22% 4	2930
	11th to 20th	Day	48% 2 5% 22% 18% 5%	3133
	21st to end	Day	44% 3 10% 24% 17% 2	3982
January	1st to 10th	Night	47% 2 6% 21% 22% 2	6146
	11th to 20th	Night	41% 3 3 20% 27% 6%	5900
	21st to end	Night	45% 2 5% 19% 22% 6%	6424
February	1st to 10th	Night	47% 3 7% 19% 20% 4	2400
	11th to 20th	Night	46% 4 6% 20% 18% 7%	2339
	21st to end	Night	46% 3 6% 17% 21% 8%	1980
March	1st to 10th	Night	48% 4 8% 13% 17% 10%	3853
	11th to 20th	Night	38% 6% 7% 21% 20% 9%	3636
	21st to end	Night	44% 4 7% 19% 23% 3	3611
April	1st to 10th	Night	36% 4 11% 23% 19% 7%	1900
	11th to 20th	Night	34% 4 6% 22% 25% 8%	1875
	21st to end	Night	35% 5% 9% 26% 21% 4	1900
May	1st to 10th	Night	41% 6% 12% 20% 18% 3	3160
	11th to 20th	Night	38% 8% 12% 28% 13% 1	2891
	21st to end	Night	36% 10% 12% 26% 14% 2	3028
June	1st to 10th	Night	32% 7% 14% 24% 21% 2	1520
	11th to 20th	Night	21% 6% 17% 28% 23% 5%	1520
	21st to end	Night	23% 7% 16% 25% 23% 5%	1520
July	1st to 10th	Night	32% 8% 25% 23% 10% 2	2490
	11th to 20th	Night	28% 7% 20% 35% 8% 2	2666
	21st to end	Night	45% 8% 14% 23% 10% 1	3087
August	1st to 10th	Night	13% 6% 13% 39% 28% 1	3132
	11th to 20th	Night	23% 7% 23% 26% 19% 2	3130
	21st to end	Night	24% 7% 13% 23% 28% 5%	3476
September	1st to 10th	Night	31% 6% 12% 21% 22% 8%	1900
	11th to 20th	Night	33% 6% 12% 17% 26% 6%	1962
	21st to end	Night	29% 4 10% 23% 25% 9%	2090
October	1st to 10th	Night	14% 4 6% 18% 28% 30%	3596
	11th to 20th	Night	19% 3 4 15% 32% 27%	3779
	21st to end	Night	25% 3 5% 24% 27% 16%	4120
November	1st to 10th	Night	30% 4 8% 21% 27% 10%	2493
	11th to 20th	Night	32% 4 7% 18% 27% 12%	2600
	21st to end	Night	38% 4 6% 20% 24% 8%	2600
December	1st to 10th	Night	41% 2 4 27% 23% 3	4250
	11th to 20th	Night	45% 2 7% 20% 23% 4	5064
	21st to end	Night	45% 3 10% 23% 16% 3	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	28% 23% 35% 11% 3	300
	11th to 20th	Day	31% 20% 33% 13% 2 1	300
	21st to end	Day	26% 26% 37% 8% 2	330
February	1st to 10th	Day	29% 19% 33% 12% 7%	300
	11th to 20th	Day	39% 21% 29% 9% 2	300
	21st to end	Day	35% 24% 31% 7% 2	249
March	1st to 10th	Day	26% 25% 32% 13% 4	300
	11th to 20th	Day	25% 25% 38% 11%	300
	21st to end	Day	23% 18% 36% 17% 6%	475
April	1st to 10th	Day	23% 18% 29% 24% 5%	460
	11th to 20th	Day	23% 17% 29% 20% 8% 2	452
	21st to end	Day	20% 15% 30% 24% 10% 2	460
May	1st to 10th	Day	17% 13% 29% 27% 11% 2	463
	11th to 20th	Day	12% 9% 29% 30% 15% 5%	487
	21st to end	Day	11% 5% 27% 30% 20% 8%	558
June	1st to 10th	Day	10% 4 20% 31% 24% 10%	680
	11th to 20th	Day	6% 3 19% 34% 26% 13%	680
	21st to end	Day	8% 2 18% 36% 26% 10%	680
July	1st to 10th	Day	5% 3 13% 34% 31% 14%	671
	11th to 20th	Day	9% 2 11% 29% 32% 18%	668
	21st to end	Day	9% 2 10% 29% 33% 17%	682
August	1st to 10th	Day	7% 3 8% 31% 37% 14%	486
	11th to 20th	Day	13% 2 13% 28% 33% 12%	479
	21st to end	Day	13% 2 10% 22% 35% 18%	539
September	1st to 10th	Day	9% 2 13% 31% 33% 12%	490
	11th to 20th	Day	12% 2 14% 25% 31% 15%	490
	21st to end	Day	12% 4 19% 31% 23% 11%	490
October	1st to 10th	Day	9% 3 24% 30% 25% 9%	422
	11th to 20th	Day	10% 11% 35% 34% 9% 1	300
	21st to end	Day	22% 12% 30% 25% 9% 2	330
November	1st to 10th	Day	22% 10% 25% 28% 14% 2	300
	11th to 20th	Day	14% 17% 40% 25% 4 1	300
	21st to end	Day	25% 15% 33% 15% 10% 2	300
December	1st to 10th	Day	28% 20% 33% 16% 4	300
	11th to 20th	Day	25% 20% 33% 16% 5% 1	300
	21st to end	Day	24% 21% 36% 15% 3 1	330
January	1st to 10th	Night	24% 17% 31% 24% 4	1000
	11th to 20th	Night	25% 18% 31% 22% 5%	1000
	21st to end	Night	23% 16% 37% 23% 2	1073
February	1st to 10th	Night	27% 13% 33% 23% 4	800
	11th to 20th	Night	25% 19% 30% 23% 2	783
	21st to end	Night	24% 22% 30% 22% 2	660
March	1st to 10th	Night	22% 14% 30% 29% 5%	800
	11th to 20th	Night	21% 13% 33% 27% 6%	633
	21st to end	Night	19% 12% 35% 25% 9%	647
April	1st to 10th	Night	17% 12% 34% 27% 10%	570
	11th to 20th	Night	16% 11% 34% 26% 12% 1	563
	21st to end	Night	13% 11% 36% 26% 13% 1	570
May	1st to 10th	Night	15% 7% 26% 35% 13% 3	570
	11th to 20th	Night	12% 5% 27% 33% 20% 4	561
	21st to end	Night	8% 2 20% 37% 26% 7%	617
June	1st to 10th	Night	7% 3 13% 39% 31% 6%	570
	11th to 20th	Night	4 2 12% 36% 39% 7%	570
	21st to end	Night	5% 2 11% 35% 39% 8%	570
July	1st to 10th	Night	4 1 10% 30% 43% 11%	555
	11th to 20th	Night	8% 1 8% 28% 38% 16%	552
	21st to end	Night	7% 1 7% 28% 45% 12%	624
August	1st to 10th	Night	4 1 4 28% 49% 14%	567
	11th to 20th	Night	6% 1 8% 38% 37% 10%	564
	21st to end	Night	7% 2 8% 28% 44% 11%	627
September	1st to 10th	Night	10% 2 9% 33% 40% 6%	570
	11th to 20th	Night	12% 1 13% 31% 35% 8%	632
	21st to end	Night	13% 3 15% 32% 33% 4	760
October	1st to 10th	Night	8% 4 19% 32% 32% 6%	760
	11th to 20th	Night	14% 7% 22% 36% 18% 2	760
	21st to end	Night	20% 8% 22% 30% 18% 2	836
November	1st to 10th	Night	14% 7% 24% 34% 18% 3	780
	11th to 20th	Night	14% 11% 30% 33% 11% 1	800
	21st to end	Night	21% 11% 30% 26% 11% 1	800
December	1st to 10th	Night	25% 13% 27% 28% 7%	800
	11th to 20th	Night	21% 14% 30% 28% 6%	915
	21st to end	Night	27% 15% 27% 26% 5%	1100

Page generated at 2025-03-24 08:04:30. Version ac07

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