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 2024-10-20.

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% 43%	100572
	Night	13% 7% 2 5% 4 4 4 3 5% 7% 47%	98135
Transparency	Day	39% 6% 11% 23% 18% 4	100374
	Night	37% 5% 10% 22% 21% 5%	98005
Seeing	Day	16% 10% 23% 25% 19% 7%	15174
	Night	16% 9% 24% 29% 19% 4	24897

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	16% 14% 3 6% 3 3 3 2 3 4 43%	2656
	11th to 20th	Day	21% 8% 2 5% 4 3 3 2 4 4 42%	2850
	21st to end	Day	22% 10% 2 4 3 4 3 3 3 5% 42%	3169
February	1st to 10th	Day	19% 9% 4 6% 4 4 3 3 4 6% 39%	1900
	11th to 20th	Day	21% 10% 3 5% 4 3 4 3 4 4 40%	1848
	21st to end	Day	21% 9% 3 6% 4 3 4 2 4 5% 38%	1570
March	1st to 10th	Day	21% 11% 4 8% 5% 3 3 2 4 5% 33%	3120
	11th to 20th	Day	13% 8% 2 5% 4 4 5% 5% 6% 8% 41%	3463
	21st to end	Day	13% 7% 3 7% 5% 6% 5% 3 5% 5% 41%	4087
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% 57%	1890
	21st to end	Day	18% 6% 2 4 3 3 2 3 4 5% 51%	1980
November	1st to 10th	Day	12% 5% 2 4 2 3 3 2 3 4 60%	1711
	11th to 20th	Day	14% 5% 3 4 3 3 3 2 4 7% 53%	1710
	21st to end	Day	19% 8% 3 5% 3 3 3 3 5% 5% 45%	1710
December	1st to 10th	Day	20% 10% 3 6% 4 4 4 3 4 5% 37%	1710
	11th to 20th	Day	15% 8% 3 7% 5% 4 3 3 4 4 44%	1994
	21st to end	Day	13% 10% 3 7% 5% 4 5% 4 5% 6% 37%	2783
January	1st to 10th	Night	18% 11% 3 6% 3 4 3 3 4 4 42%	4296
	11th to 20th	Night	18% 9% 3 6% 3 3 3 2 3 4 46%	4120
	21st to end	Night	23% 10% 2 4 3 3 2 2 3 5% 44%	4494
February	1st to 10th	Night	20% 8% 3 6% 4 3 3 2 4 5% 42%	2280
	11th to 20th	Night	19% 9% 3 6% 4 3 3 2 4 4 43%	2218
	21st to end	Night	21% 9% 2 6% 3 3 3 2 4 5% 41%	1884
March	1st to 10th	Night	19% 10% 3 6% 4 3 3 2 5% 6% 38%	3733
	11th to 20th	Night	13% 8% 2 5% 4 4 3 4 4 7% 46%	3525
	21st to end	Night	15% 7% 3 7% 5% 4 4 3 5% 5% 42%	3543
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%	2275
	21st to end	Night	16% 7% 2 5% 2 2 3 2 3 5% 53%	2376
November	1st to 10th	Night	11% 4 1 4 3 3 3 3 4 6% 58%	2365
	11th to 20th	Night	10% 6% 2 5% 3 3 4 3 5% 6% 53%	2470
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 49%	2470
December	1st to 10th	Night	19% 9% 2 6% 4 3 3 2 4 6% 43%	2470
	11th to 20th	Night	16% 7% 3 5% 4 4 3 2 4 4 47%	3198
	21st to end	Night	16% 9% 3 7% 4 4 5% 4 6% 7% 35%	4884

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	47% 2 5% 20% 22% 3	2657
	11th to 20th	Day	46% 4 5% 23% 21% 2	2850
	21st to end	Day	46% 3 7% 19% 21% 4	3169
February	1st to 10th	Day	47% 4 7% 20% 18% 4	1900
	11th to 20th	Day	49% 4 5% 19% 18% 6%	1858
	21st to end	Day	49% 4 7% 15% 17% 7%	1570
March	1st to 10th	Day	55% 4 6% 11% 15% 9%	3120
	11th to 20th	Day	39% 6% 9% 23% 17% 6%	3463
	21st to end	Day	44% 5% 7% 22% 19% 3	4087
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	27% 5% 9% 24% 24% 13%	2022
	11th to 20th	Day	30% 3 6% 22% 29% 10%	1890
	21st to end	Day	37% 4 5% 21% 26% 8%	1980
November	1st to 10th	Day	29% 2 9% 20% 30% 10%	1711
	11th to 20th	Day	33% 3 7% 20% 27% 9%	1710
	21st to end	Day	42% 3 6% 20% 23% 5%	1710
December	1st to 10th	Day	50% 5% 6% 17% 16% 6%	1710
	11th to 20th	Day	44% 3 6% 20% 22% 5%	1991
	21st to end	Day	45% 3 10% 20% 17% 3	2783
January	1st to 10th	Night	47% 2 5% 17% 26% 3	4296
	11th to 20th	Night	43% 4 5% 22% 21% 6%	4120
	21st to end	Night	46% 2 5% 21% 21% 4	4494
February	1st to 10th	Night	47% 3 8% 19% 20% 4	2280
	11th to 20th	Night	45% 4 6% 20% 18% 7%	2219
	21st to end	Night	47% 3 6% 16% 20% 8%	1884
March	1st to 10th	Night	48% 4 8% 13% 17% 10%	3733
	11th to 20th	Night	38% 6% 7% 21% 20% 8%	3525
	21st to end	Night	44% 4 7% 19% 23% 2	3543
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	20% 4 10% 25% 26% 14%	2154
	11th to 20th	Night	29% 4 7% 18% 28% 13%	2275
	21st to end	Night	37% 3 5% 18% 29% 8%	2376
November	1st to 10th	Night	28% 4 7% 22% 29% 10%	2365
	11th to 20th	Night	32% 4 7% 19% 27% 11%	2470
	21st to end	Night	38% 5% 6% 19% 24% 8%	2470
December	1st to 10th	Night	45% 3 6% 22% 20% 5%	2470
	11th to 20th	Night	41% 2 7% 19% 26% 5%	3200
	21st to end	Night	46% 4 10% 18% 17% 4	4884

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	29% 21% 34% 13% 3	270
	11th to 20th	Day	32% 18% 33% 13% 2 1	270
	21st to end	Day	26% 26% 35% 9% 3	297
February	1st to 10th	Day	27% 19% 33% 13% 7%	270
	11th to 20th	Day	39% 19% 30% 9% 2	270
	21st to end	Day	39% 26% 27% 6% 2	225
March	1st to 10th	Day	24% 26% 31% 14% 4	270
	11th to 20th	Day	24% 25% 39% 12%	270
	21st to end	Day	24% 19% 35% 17% 6%	454
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% 33% 9% 1	297
	21st to end	Day	24% 13% 31% 23% 8% 1	297
November	1st to 10th	Day	20% 10% 25% 29% 15% 2	270
	11th to 20th	Day	13% 18% 40% 25% 4 1	270
	21st to end	Day	26% 14% 33% 14% 11% 2	270
December	1st to 10th	Day	28% 20% 33% 15% 4	270
	11th to 20th	Day	22% 20% 34% 17% 6% 1	270
	21st to end	Day	24% 21% 37% 14% 3 1	297
January	1st to 10th	Night	24% 17% 31% 23% 4	950
	11th to 20th	Night	25% 18% 30% 21% 5%	950
	21st to end	Night	22% 16% 37% 23% 2	1020
February	1st to 10th	Night	26% 14% 33% 23% 4	760
	11th to 20th	Night	24% 19% 31% 24% 2	743
	21st to end	Night	25% 22% 31% 20% 2	628
March	1st to 10th	Night	21% 14% 30% 30% 5%	760
	11th to 20th	Night	21% 12% 33% 27% 6%	602
	21st to end	Night	19% 12% 35% 25% 9%	627
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	759
	21st to end	Night	20% 8% 23% 29% 18% 2	792
November	1st to 10th	Night	12% 8% 23% 35% 19% 3	740
	11th to 20th	Night	14% 11% 30% 33% 12% 1	760
	21st to end	Night	21% 10% 30% 27% 11% 1	760
December	1st to 10th	Night	25% 12% 27% 28% 8%	760
	11th to 20th	Night	21% 14% 29% 29% 7%	869
	21st to end	Night	27% 15% 27% 26% 4	1045

Page generated at 2024-10-17 12:09:43. Version ac07

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