Oxford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oxford Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	17% 10% 3 8% 5% 5% 4 4 5% 6% 32%	126847
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	122525
Transparency	Day	53% 3 11% 22% 10% 1	130325
	Night	55% 3 11% 20% 10% 1	127134
Seeing	Day	25% 16% 30% 19% 8% 2	15674
	Night	24% 13% 30% 24% 8% 1	24717

By Month

Percentage Cloud Cover NE (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% 19% 6% 13% 5% 5% 5% 3 4 4 21%	3597
	11th to 20th	Day	12% 17% 7% 15% 7% 5% 4 3 5% 5% 21%	3570
	21st to end	Day	14% 17% 7% 14% 6% 7% 5% 3 4 5% 20%	4167
February	1st to 10th	Day	17% 15% 6% 12% 6% 5% 5% 3 4 4 24%	1800
	11th to 20th	Day	14% 15% 5% 11% 8% 4 5% 3 4 5% 26%	1772
	21st to end	Day	20% 15% 6% 9% 6% 6% 4 3 4 4 22%	1650
March	1st to 10th	Day	19% 14% 5% 10% 5% 4 4 4 4 5% 26%	4476
	11th to 20th	Day	11% 13% 4 9% 6% 6% 5% 4 5% 7% 31%	4975
	21st to end	Day	19% 10% 3 8% 5% 4 3 3 4 4 37%	5778
April	1st to 10th	Day	21% 13% 3 7% 5% 4 4 3 4 6% 31%	2400
	11th to 20th	Day	21% 10% 3 8% 4 3 4 3 4 5% 36%	2374
	21st to end	Day	23% 10% 3 8% 5% 4 4 3 4 5% 32%	2510
May	1st to 10th	Day	24% 11% 3 9% 5% 4 5% 3 4 6% 25%	6346
	11th to 20th	Day	18% 9% 3 9% 6% 5% 5% 4 5% 6% 30%	6318
	21st to end	Day	21% 8% 3 7% 4 4 4 4 5% 7% 34%	6949
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 4 6% 7% 31%	2800
	11th to 20th	Day	14% 6% 3 7% 5% 4 5% 4 6% 8% 38%	2800
	21st to end	Day	19% 8% 3 8% 4 5% 5% 4 6% 6% 32%	2800
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 4 7% 9% 35%	6360
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	6360
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 7% 9% 44%	6926
August	1st to 10th	Day	15% 8% 3 7% 5% 4 4 4 5% 7% 37%	4471
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 6% 9% 39%	4090
	21st to end	Day	16% 5% 3 6% 4 4 4 4 6% 9% 39%	4312
September	1st to 10th	Day	13% 6% 2 7% 4 5% 4 4 5% 8% 43%	2400
	11th to 20th	Day	13% 5% 2 5% 3 4 5% 3 5% 7% 47%	2396
	21st to end	Day	18% 6% 2 6% 4 4 5% 3 6% 7% 38%	2141
October	1st to 10th	Day	20% 8% 3 7% 4 4 4 2 5% 5% 37%	1930
	11th to 20th	Day	20% 10% 4 9% 5% 5% 4 4 4 6% 30%	1900
	21st to end	Day	26% 10% 3 8% 4 4 4 4 4 5% 28%	1942
November	1st to 10th	Day	16% 9% 3 7% 5% 5% 6% 3 5% 8% 33%	1761
	11th to 20th	Day	16% 14% 3 9% 6% 5% 5% 3 4 5% 30%	1630
	21st to end	Day	22% 14% 4 10% 5% 6% 5% 3 5% 6% 21%	1600
December	1st to 10th	Day	21% 19% 5% 11% 6% 4 4 4 4 5% 16%	2600
	11th to 20th	Day	18% 17% 5% 12% 6% 5% 4 3 4 4 23%	2986
	21st to end	Day	22% 16% 4 12% 6% 4 4 3 3 4 22%	3960
January	1st to 10th	Night	14% 20% 8% 13% 6% 4 4 2 4 4 21%	6349
	11th to 20th	Night	11% 17% 7% 16% 8% 6% 5% 3 4 4 17%	6315
	21st to end	Night	16% 17% 6% 13% 7% 6% 4 3 4 4 20%	6788
February	1st to 10th	Night	16% 16% 5% 12% 7% 5% 4 3 5% 5% 22%	2600
	11th to 20th	Night	14% 14% 5% 12% 6% 4 4 3 5% 5% 27%	2513
	21st to end	Night	16% 15% 5% 10% 6% 5% 5% 3 4 4 28%	1980
March	1st to 10th	Night	22% 13% 4 9% 5% 4 4 3 3 4 29%	5364
	11th to 20th	Night	14% 13% 4 7% 4 4 5% 4 6% 9% 31%	4980
	21st to end	Night	19% 11% 3 9% 5% 4 4 3 4 5% 33%	5153
April	1st to 10th	Night	20% 11% 4 8% 5% 4 4 3 4 6% 31%	2000
	11th to 20th	Night	21% 10% 3 7% 5% 3 2 3 4 5% 36%	1975
	21st to end	Night	22% 9% 3 7% 4 4 4 4 5% 5% 33%	1906
May	1st to 10th	Night	26% 9% 3 8% 4 4 5% 4 5% 6% 24%	3594
	11th to 20th	Night	16% 9% 3 8% 5% 3 4 3 5% 7% 38%	3575
	21st to end	Night	18% 9% 3 7% 4 4 5% 4 6% 8% 33%	3926
June	1st to 10th	Night	17% 7% 4 7% 6% 4 6% 4 5% 8% 32%	1600
	11th to 20th	Night	15% 6% 2 6% 4 5% 4 4 6% 9% 39%	1600
	21st to end	Night	19% 8% 3 6% 4 5% 4 5% 5% 7% 35%	1600
July	1st to 10th	Night	13% 6% 3 7% 5% 5% 5% 4 6% 9% 36%	3600
	11th to 20th	Night	13% 6% 2 6% 4 4 3 3 5% 8% 44%	3600
	21st to end	Night	10% 5% 2 7% 4 5% 5% 4 6% 8% 46%	3926
August	1st to 10th	Night	13% 7% 3 6% 4 4 4 4 6% 8% 41%	2674
	11th to 20th	Night	13% 7% 2 8% 4 4 4 4 6% 7% 41%	2925
	21st to end	Night	11% 5% 2 7% 5% 5% 5% 5% 6% 8% 42%	3586
September	1st to 10th	Night	12% 6% 3 6% 5% 4 3 3 5% 6% 47%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 6% 6% 51%	2003
	21st to end	Night	16% 6% 2 7% 4 4 4 3 5% 6% 43%	2255
October	1st to 10th	Night	17% 7% 3 7% 4 4 5% 4 5% 8% 37%	2317
	11th to 20th	Night	20% 9% 2 8% 5% 4 4 4 6% 7% 33%	2280
	21st to end	Night	25% 10% 3 7% 5% 4 4 3 4 7% 28%	2655
November	1st to 10th	Night	19% 10% 4 7% 4 4 4 3 5% 5% 36%	2537
	11th to 20th	Night	17% 12% 4 8% 5% 5% 5% 3 4 6% 32%	2765
	21st to end	Night	21% 14% 4 9% 5% 5% 4 4 5% 6% 23%	2800
December	1st to 10th	Night	18% 19% 6% 12% 7% 5% 4 3 4 5% 17%	4580
	11th to 20th	Night	23% 15% 4 10% 5% 3 3 3 3 4 26%	5208
	21st to end	Night	26% 16% 4 12% 5% 3 3 2 5% 4 19%	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	67% 3 11% 14% 4 1	3598
	11th to 20th	Day	68% 3 10% 11% 8% 1	3600
	21st to end	Day	69% 3 9% 10% 9% 1	4167
February	1st to 10th	Day	64% 4 7% 19% 5% 1	1800
	11th to 20th	Day	61% 4 9% 17% 8% 1	1781
	21st to end	Day	66% 3 9% 12% 9% 1	1650
March	1st to 10th	Day	62% 2 9% 13% 11% 3	4479
	11th to 20th	Day	53% 3 16% 15% 11% 1	4975
	21st to end	Day	51% 4 6% 18% 18% 3	5778
April	1st to 10th	Day	55% 4 6% 20% 13% 3	2400
	11th to 20th	Day	53% 3 6% 20% 16% 2	2374
	21st to end	Day	55% 3 8% 15% 15% 3	2510
May	1st to 10th	Day	62% 4 8% 17% 8% 2	6357
	11th to 20th	Day	55% 3 12% 19% 10% 1	6322
	21st to end	Day	50% 3 12% 23% 10% 1	6949
June	1st to 10th	Day	51% 5% 10% 21% 12% 1	2800
	11th to 20th	Day	43% 4 12% 26% 13% 2	2800
	21st to end	Day	51% 5% 12% 20% 11% 1	2800
July	1st to 10th	Day	46% 2 13% 28% 11%	6290
	11th to 20th	Day	39% 3 14% 30% 15%	6285
	21st to end	Day	36% 4 13% 38% 9%	6728
August	1st to 10th	Day	47% 3 15% 24% 10%	4457
	11th to 20th	Day	44% 3 14% 29% 9% 1	4129
	21st to end	Day	43% 3 16% 28% 11%	4312
September	1st to 10th	Day	40% 6% 13% 29% 12%	2400
	11th to 20th	Day	36% 4 12% 33% 14%	2396
	21st to end	Day	44% 6% 13% 26% 11%	2141
October	1st to 10th	Day	47% 2 10% 31% 9%	3190
	11th to 20th	Day	48% 3 15% 27% 7%	3140
	21st to end	Day	48% 3 16% 24% 8%	3180
November	1st to 10th	Day	50% 5% 14% 22% 8% 1	1761
	11th to 20th	Day	57% 5% 10% 16% 10% 1	1630
	21st to end	Day	64% 5% 13% 12% 6%	1600
December	1st to 10th	Day	72% 3 13% 11% 2	2600
	11th to 20th	Day	67% 3 10% 14% 6%	2986
	21st to end	Day	69% 2 8% 15% 6%	3960
January	1st to 10th	Night	68% 2 12% 13% 4	6349
	11th to 20th	Night	71% 4 9% 11% 5%	6360
	21st to end	Night	70% 2 7% 9% 11% 1	6788
February	1st to 10th	Night	66% 4 9% 15% 6% 1	2600
	11th to 20th	Night	60% 3 9% 20% 7% 1	2515
	21st to end	Night	61% 4 7% 18% 9% 1	1980
March	1st to 10th	Night	61% 2 7% 16% 11% 3	5374
	11th to 20th	Night	51% 3 19% 17% 8% 3	4981
	21st to end	Night	54% 3 5% 22% 13% 2	5153
April	1st to 10th	Night	55% 3 7% 20% 12% 2	2000
	11th to 20th	Night	52% 4 7% 20% 17% 1	1975
	21st to end	Night	53% 4 8% 17% 15% 4	1906
May	1st to 10th	Night	61% 3 9% 16% 8% 2	3600
	11th to 20th	Night	47% 3 14% 21% 14% 1	3576
	21st to end	Night	50% 4 13% 19% 14% 1	3927
June	1st to 10th	Night	49% 6% 12% 19% 12% 1	1600
	11th to 20th	Night	42% 5% 13% 23% 17% 2	1600
	21st to end	Night	47% 6% 12% 21% 13% 1	1600
July	1st to 10th	Night	46% 3 16% 20% 14% 1	3560
	11th to 20th	Night	40% 2 13% 32% 12%	3552
	21st to end	Night	38% 4 12% 31% 15%	3809
August	1st to 10th	Night	42% 5% 14% 27% 12%	2664
	11th to 20th	Night	44% 3 14% 30% 8% 1	2952
	21st to end	Night	41% 3 14% 29% 12% 1	3586
September	1st to 10th	Night	39% 5% 11% 31% 14% 1	2000
	11th to 20th	Night	33% 4 13% 36% 13% 1	2003
	21st to end	Night	42% 5% 11% 28% 13% 1	2255
October	1st to 10th	Night	43% 3 12% 31% 11%	3837
	11th to 20th	Night	45% 2 15% 28% 10%	3779
	21st to end	Night	49% 3 17% 22% 9%	4367
November	1st to 10th	Night	51% 4 13% 22% 10% 1	2537
	11th to 20th	Night	54% 4 11% 19% 10% 1	2765
	21st to end	Night	62% 5% 12% 15% 5%	2800
December	1st to 10th	Night	72% 3 11% 11% 3	4580
	11th to 20th	Night	64% 3 8% 14% 10% 1	5208
	21st to end	Night	70% 2 7% 12% 9%	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	26% 22% 36% 12% 5%	200
	11th to 20th	Day	35% 18% 31% 14% 2 1	200
	21st to end	Day	25% 28% 30% 12% 5%	265
February	1st to 10th	Day	35% 25% 30% 11%	300
	11th to 20th	Day	32% 28% 31% 9%	319
	21st to end	Day	35% 18% 34% 12% 1	414
March	1st to 10th	Day	29% 20% 31% 17% 3	500
	11th to 20th	Day	31% 20% 34% 12% 3 1	500
	21st to end	Day	33% 21% 25% 14% 5% 1	550
April	1st to 10th	Day	34% 21% 27% 13% 4 1	500
	11th to 20th	Day	33% 21% 25% 12% 7% 3	492
	21st to end	Day	32% 20% 30% 11% 5% 2	500
May	1st to 10th	Day	28% 18% 28% 18% 7% 1	503
	11th to 20th	Day	21% 17% 34% 18% 8% 2	527
	21st to end	Day	23% 17% 27% 23% 9% 1	573
June	1st to 10th	Day	25% 15% 31% 21% 7% 1	530
	11th to 20th	Day	16% 16% 35% 22% 9% 2	530
	21st to end	Day	23% 14% 33% 19% 9% 2	530
July	1st to 10th	Day	15% 8% 32% 30% 13% 2	525
	11th to 20th	Day	14% 11% 31% 27% 13% 3	524
	21st to end	Day	15% 9% 32% 31% 10% 3	576
August	1st to 10th	Day	17% 8% 27% 32% 14% 2	526
	11th to 20th	Day	17% 4 28% 29% 15% 7%	519
	21st to end	Day	16% 12% 31% 21% 14% 6%	583
September	1st to 10th	Day	16% 11% 30% 24% 12% 7%	530
	11th to 20th	Day	15% 5% 25% 27% 16% 11%	530
	21st to end	Day	19% 12% 30% 22% 14% 3	530
October	1st to 10th	Day	24% 16% 24% 18% 12% 5%	502
	11th to 20th	Day	28% 18% 30% 18% 4 1	490
	21st to end	Day	34% 15% 27% 19% 4	461
November	1st to 10th	Day	28% 20% 28% 17% 6% 1	395
	11th to 20th	Day	27% 20% 32% 14% 6% 1	230
	21st to end	Day	32% 22% 33% 11% 4	200
December	1st to 10th	Day	36% 17% 29% 15% 4	200
	11th to 20th	Day	36% 29% 24% 8% 3 1	200
	21st to end	Day	35% 15% 31% 13% 4 1	220
January	1st to 10th	Night	28% 20% 30% 18% 4	800
	11th to 20th	Night	32% 17% 26% 21% 3	800
	21st to end	Night	21% 17% 34% 23% 4	880
February	1st to 10th	Night	27% 20% 33% 18% 3	800
	11th to 20th	Night	24% 20% 39% 16% 2	783
	21st to end	Night	27% 18% 31% 20% 3	660
March	1st to 10th	Night	27% 16% 31% 22% 4	800
	11th to 20th	Night	26% 20% 31% 19% 4	800
	21st to end	Night	26% 16% 30% 21% 6%	880
April	1st to 10th	Night	28% 15% 31% 23% 3	800
	11th to 20th	Night	30% 17% 28% 19% 6%	791
	21st to end	Night	28% 13% 29% 23% 7%	706
May	1st to 10th	Night	27% 9% 27% 28% 8% 1	400
	11th to 20th	Night	18% 9% 37% 28% 7%	394
	21st to end	Night	16% 10% 37% 26% 11%	433
June	1st to 10th	Night	19% 9% 30% 36% 7% 1	400
	11th to 20th	Night	17% 10% 30% 30% 12% 1	400
	21st to end	Night	22% 6% 31% 29% 12%	400
July	1st to 10th	Night	10% 7% 31% 41% 11% 1	390
	11th to 20th	Night	15% 4 26% 40% 13% 1	388
	21st to end	Night	11% 4 31% 41% 12% 1	438
August	1st to 10th	Night	15% 4 24% 38% 18% 1	447
	11th to 20th	Night	15% 3 23% 36% 22% 1	618
	21st to end	Night	13% 7% 29% 34% 15% 2	880
September	1st to 10th	Night	16% 7% 30% 32% 15% 1	800
	11th to 20th	Night	17% 6% 22% 29% 23% 5%	800
	21st to end	Night	19% 9% 26% 25% 19% 1	800
October	1st to 10th	Night	21% 10% 30% 22% 16% 2	773
	11th to 20th	Night	26% 13% 30% 23% 8% 1	760
	21st to end	Night	31% 13% 28% 23% 5%	836
November	1st to 10th	Night	26% 16% 29% 21% 8%	780
	11th to 20th	Night	26% 17% 31% 18% 9%	800
	21st to end	Night	33% 17% 26% 18% 6%	800
December	1st to 10th	Night	33% 16% 30% 18% 4	800
	11th to 20th	Night	31% 19% 30% 16% 4 1	800
	21st to end	Night	35% 15% 28% 19% 3	880

Page generated at 2025-10-01 01:39:05. Version ac07

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