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 2024-04-25.

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% 4 7% 6% 3 5% 5% 5% 6% 32%	83157
	Night	18% 11% 4 8% 6% 3 4 4 4 6% 31%	85945
Transparency	Day	53% 5% 11% 20% 10% 1	82862
	Night	55% 4 11% 19% 10% 1	85774
Seeing	Day	25% 16% 30% 19% 8% 2	13762
	Night	24% 13% 30% 24% 8% 1	23043

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% 18% 5% 10% 8% 4 4 4 5% 4 23%	2517
	11th to 20th	Day	14% 15% 6% 11% 8% 5% 5% 5% 5% 5% 21%	2520
	21st to end	Day	18% 18% 7% 10% 8% 5% 5% 4 3 5% 16%	2909
February	1st to 10th	Day	17% 14% 6% 11% 7% 3 5% 5% 4 4 24%	1710
	11th to 20th	Day	14% 15% 5% 9% 8% 4 5% 4 4 6% 27%	1681
	21st to end	Day	20% 14% 6% 7% 7% 4 5% 4 4 5% 23%	1570
March	1st to 10th	Day	21% 12% 5% 8% 6% 3 4 4 4 6% 27%	3116
	11th to 20th	Day	13% 16% 5% 8% 6% 4 5% 4 4 5% 29%	3475
	21st to end	Day	14% 8% 3 7% 7% 3 4 3 4 6% 41%	4030
April	1st to 10th	Day	22% 11% 3 6% 5% 2 5% 4 4 6% 32%	2280
	11th to 20th	Day	22% 8% 3 7% 5% 2 3 4 4 5% 36%	2254
	21st to end	Day	23% 9% 3 7% 6% 3 4 4 4 5% 32%	2308
May	1st to 10th	Day	20% 8% 3 6% 5% 3 4 4 4 6% 36%	2520
	11th to 20th	Day	17% 7% 3 7% 5% 3 4 4 4 7% 39%	2478
	21st to end	Day	16% 6% 3 5% 5% 3 5% 6% 6% 6% 39%	2725
June	1st to 10th	Day	21% 7% 3 6% 6% 4 4 6% 6% 7% 30%	2520
	11th to 20th	Day	15% 6% 3 6% 5% 3 5% 4 6% 8% 39%	2520
	21st to end	Day	19% 7% 3 6% 5% 3 6% 5% 6% 7% 32%	2520
July	1st to 10th	Day	12% 5% 3 5% 5% 3 5% 5% 6% 9% 42%	2520
	11th to 20th	Day	12% 6% 2 5% 6% 3 5% 6% 6% 10% 39%	2520
	21st to end	Day	12% 5% 3 5% 5% 4 6% 6% 8% 9% 38%	2744
August	1st to 10th	Day	12% 6% 3 6% 7% 4 5% 5% 6% 7% 39%	2459
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 5% 6% 10% 39%	2263
	21st to end	Day	14% 5% 3 5% 5% 4 4 5% 7% 8% 39%	2376
September	1st to 10th	Day	14% 6% 2 6% 4 3 5% 6% 5% 8% 41%	2160
	11th to 20th	Day	15% 5% 2 5% 4 3 4 5% 5% 7% 45%	2157
	21st to end	Day	18% 6% 2 5% 4 3 4 4 6% 7% 39%	1927
October	1st to 10th	Day	21% 7% 4 6% 5% 3 4 4 5% 6% 36%	1800
	11th to 20th	Day	20% 10% 4 8% 6% 4 4 5% 5% 6% 29%	1800
	21st to end	Day	27% 10% 3 7% 5% 3 4 5% 4 5% 27%	1840
November	1st to 10th	Day	16% 8% 3 6% 6% 3 6% 5% 5% 8% 33%	1671
	11th to 20th	Day	17% 13% 4 7% 8% 4 5% 4 4 5% 29%	1549
	21st to end	Day	22% 13% 5% 9% 6% 4 5% 5% 5% 6% 21%	1520
December	1st to 10th	Day	25% 15% 6% 10% 7% 3 4 4 4 5% 17%	1520
	11th to 20th	Day	22% 17% 5% 8% 6% 3 4 3 4 4 24%	1906
	21st to end	Day	27% 17% 6% 10% 6% 3 3 3 3 3 20%	2772
January	1st to 10th	Night	18% 15% 7% 10% 7% 3 5% 4 4 5% 22%	4429
	11th to 20th	Night	13% 15% 7% 13% 9% 5% 6% 5% 5% 5% 18%	4440
	21st to end	Night	19% 18% 6% 10% 8% 4 5% 3 3 4 20%	4746
February	1st to 10th	Night	16% 15% 6% 11% 8% 4 5% 4 5% 5% 21%	2470
	11th to 20th	Night	14% 14% 6% 10% 7% 3 5% 4 4 5% 29%	2384
	21st to end	Night	17% 13% 5% 8% 7% 4 5% 4 4 4 29%	1884
March	1st to 10th	Night	21% 11% 4 7% 7% 3 4 3 3 5% 32%	3724
	11th to 20th	Night	19% 16% 5% 8% 5% 3 3 4 3 6% 27%	3480
	21st to end	Night	16% 10% 3 6% 6% 3 4 4 4 5% 40%	3601
April	1st to 10th	Night	20% 10% 4 7% 6% 3 4 4 4 6% 32%	1900
	11th to 20th	Night	21% 9% 3 6% 6% 2 3 4 4 5% 36%	1875
	21st to end	Night	22% 9% 3 7% 4 3 4 4 4 6% 33%	1768
May	1st to 10th	Night	22% 7% 2 6% 4 3 4 5% 5% 7% 35%	1440
	11th to 20th	Night	15% 9% 3 5% 6% 2 4 5% 5% 7% 40%	1415
	21st to end	Night	13% 5% 2 5% 5% 2 5% 4 6% 7% 45%	1550
June	1st to 10th	Night	18% 6% 4 6% 6% 3 5% 5% 5% 9% 32%	1440
	11th to 20th	Night	16% 6% 3 4 5% 3 5% 5% 5% 8% 40%	1440
	21st to end	Night	17% 8% 3 5% 5% 4 4 6% 5% 7% 36%	1440
July	1st to 10th	Night	10% 4 3 5% 6% 3 6% 4 6% 8% 45%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 3 5% 4 7% 10% 42%	1440
	21st to end	Night	10% 4 2 5% 6% 3 4 6% 7% 9% 44%	1574
August	1st to 10th	Night	11% 6% 3 5% 6% 4 4 6% 6% 9% 41%	1475
	11th to 20th	Night	11% 6% 2 7% 5% 3 5% 4 6% 7% 43%	1619
	21st to end	Night	13% 5% 3 5% 6% 3 5% 5% 6% 7% 45%	1980
September	1st to 10th	Night	13% 6% 3 6% 5% 3 4 4 5% 6% 46%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 3 4 5% 6% 50%	1802
	21st to end	Night	16% 6% 2 6% 5% 3 4 4 5% 6% 43%	2029
October	1st to 10th	Night	18% 7% 3 6% 6% 3 4 5% 5% 8% 35%	2160
	11th to 20th	Night	20% 9% 3 7% 6% 3 4 5% 5% 7% 32%	2160
	21st to end	Night	25% 9% 4 6% 6% 3 4 4 4 7% 28%	2515
November	1st to 10th	Night	19% 9% 4 6% 4 3 4 4 5% 6% 36%	2407
	11th to 20th	Night	17% 11% 4 7% 6% 3 5% 4 4 6% 32%	2626
	21st to end	Night	22% 13% 5% 8% 6% 3 5% 4 4 6% 23%	2660
December	1st to 10th	Night	20% 18% 6% 10% 7% 4 5% 4 4 5% 18%	2660
	11th to 20th	Night	22% 16% 4 8% 6% 3 4 3 3 4 28%	3288
	21st to end	Night	31% 15% 4 10% 6% 3 3 3 4 4 17%	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	65% 4 14% 10% 5% 1	2518
	11th to 20th	Day	66% 4 11% 11% 8% 1	2520
	21st to end	Day	73% 4 10% 8% 4 1	2909
February	1st to 10th	Day	64% 4 7% 20% 4 2	1710
	11th to 20th	Day	60% 4 9% 17% 9% 1	1690
	21st to end	Day	65% 3 10% 12% 9% 1	1570
March	1st to 10th	Day	59% 3 10% 18% 8% 1	3119
	11th to 20th	Day	57% 4 12% 15% 10% 2	3475
	21st to end	Day	44% 5% 7% 22% 18% 4	4030
April	1st to 10th	Day	54% 4 6% 20% 13% 3	2280
	11th to 20th	Day	52% 3 7% 20% 16% 2	2254
	21st to end	Day	55% 3 9% 15% 15% 4	2308
May	1st to 10th	Day	50% 4 8% 19% 15% 3	2520
	11th to 20th	Day	46% 4 9% 23% 15% 2	2482
	21st to end	Day	43% 6% 15% 21% 13% 2	2725
June	1st to 10th	Day	52% 5% 11% 21% 11% 1	2520
	11th to 20th	Day	44% 5% 11% 26% 13% 2	2520
	21st to end	Day	51% 6% 13% 19% 11% 1	2520
July	1st to 10th	Day	37% 6% 12% 29% 15% 1	2450
	11th to 20th	Day	39% 6% 17% 28% 10%	2445
	21st to end	Day	41% 7% 16% 25% 10%	2591
August	1st to 10th	Day	43% 5% 15% 26% 10%	2445
	11th to 20th	Day	40% 5% 17% 26% 11% 1	2263
	21st to end	Day	41% 5% 14% 28% 12%	2376
September	1st to 10th	Day	41% 6% 13% 27% 13%	2160
	11th to 20th	Day	38% 5% 12% 32% 14%	2157
	21st to end	Day	44% 6% 14% 28% 9%	1927
October	1st to 10th	Day	51% 4 11% 23% 12%	1800
	11th to 20th	Day	56% 5% 14% 18% 7% 1	1800
	21st to end	Day	60% 5% 11% 16% 8% 1	1840
November	1st to 10th	Day	49% 6% 15% 22% 8% 1	1671
	11th to 20th	Day	57% 5% 11% 15% 10% 1	1549
	21st to end	Day	64% 5% 13% 12% 6%	1520
December	1st to 10th	Day	70% 5% 11% 11% 3 1	1520
	11th to 20th	Day	66% 4 13% 13% 3	1906
	21st to end	Day	73% 3 7% 14% 4	2772
January	1st to 10th	Night	65% 3 16% 11% 4	4429
	11th to 20th	Night	69% 5% 11% 10% 4	4440
	21st to end	Night	71% 3 10% 10% 5% 1	4746
February	1st to 10th	Night	66% 4 9% 14% 6% 1	2470
	11th to 20th	Night	59% 4 9% 21% 7%	2386
	21st to end	Night	61% 4 8% 18% 9% 1	1884
March	1st to 10th	Night	56% 3 10% 21% 9% 1	3734
	11th to 20th	Night	60% 3 14% 14% 8% 1	3481
	21st to end	Night	47% 5% 5% 23% 17% 3	3601
April	1st to 10th	Night	54% 4 7% 20% 13% 2	1900
	11th to 20th	Night	51% 4 7% 19% 17% 1	1875
	21st to end	Night	53% 4 8% 16% 15% 4	1768
May	1st to 10th	Night	49% 4 8% 19% 16% 5%	1440
	11th to 20th	Night	43% 5% 8% 23% 18% 3	1416
	21st to end	Night	38% 5% 16% 21% 16% 3	1551
June	1st to 10th	Night	48% 6% 13% 20% 11% 1	1440
	11th to 20th	Night	42% 5% 12% 22% 17% 2	1440
	21st to end	Night	45% 7% 13% 21% 13% 1	1440
July	1st to 10th	Night	35% 6% 12% 28% 18% 2	1400
	11th to 20th	Night	37% 5% 14% 31% 13%	1392
	21st to end	Night	36% 8% 16% 26% 14% 1	1486
August	1st to 10th	Night	38% 8% 16% 24% 14% 1	1465
	11th to 20th	Night	40% 5% 14% 27% 13% 1	1619
	21st to end	Night	39% 6% 14% 28% 13% 1	1980
September	1st to 10th	Night	39% 6% 11% 29% 14% 1	1800
	11th to 20th	Night	35% 4 14% 35% 12% 1	1802
	21st to end	Night	41% 5% 11% 30% 11% 1	2029
October	1st to 10th	Night	48% 5% 10% 22% 13% 1	2160
	11th to 20th	Night	51% 4 13% 22% 9%	2160
	21st to end	Night	57% 4 14% 15% 9% 1	2515
November	1st to 10th	Night	50% 4 13% 21% 10% 1	2407
	11th to 20th	Night	54% 4 12% 19% 9% 2	2626
	21st to end	Night	62% 5% 13% 15% 5%	2660
December	1st to 10th	Night	69% 5% 9% 13% 3	2660
	11th to 20th	Night	63% 3 10% 16% 8% 1	3288
	21st to end	Night	73% 2 9% 13% 2	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	27% 22% 33% 13% 5%	180
	11th to 20th	Day	36% 19% 28% 14% 2 1	180
	21st to end	Day	24% 26% 31% 13% 6%	238
February	1st to 10th	Day	34% 26% 29% 11%	270
	11th to 20th	Day	30% 27% 34% 8%	288
	21st to end	Day	35% 19% 32% 13% 1	382
March	1st to 10th	Day	28% 20% 32% 17% 3	460
	11th to 20th	Day	31% 20% 35% 11% 3 1	460
	21st to end	Day	32% 22% 25% 13% 6% 1	506
April	1st to 10th	Day	33% 19% 28% 14% 5% 1	460
	11th to 20th	Day	33% 21% 25% 12% 7% 3	452
	21st to end	Day	32% 20% 29% 12% 5% 2	437
May	1st to 10th	Day	27% 19% 28% 17% 7% 1	423
	11th to 20th	Day	22% 18% 33% 18% 8% 2	447
	21st to end	Day	22% 17% 28% 21% 10% 1	485
June	1st to 10th	Day	26% 14% 31% 20% 8% 1	450
	11th to 20th	Day	17% 15% 32% 24% 9% 2	450
	21st to end	Day	21% 14% 33% 20% 11% 2	450
July	1st to 10th	Day	15% 7% 29% 32% 15% 2	445
	11th to 20th	Day	15% 11% 30% 27% 14% 3	444
	21st to end	Day	15% 8% 33% 32% 9% 3	488
August	1st to 10th	Day	16% 9% 26% 32% 14% 3	446
	11th to 20th	Day	16% 4 28% 28% 16% 7%	439
	21st to end	Day	15% 12% 32% 20% 14% 7%	495
September	1st to 10th	Day	15% 12% 31% 23% 12% 7%	450
	11th to 20th	Day	17% 6% 27% 25% 14% 10%	450
	21st to end	Day	19% 13% 29% 22% 13% 4	450
October	1st to 10th	Day	26% 16% 23% 18% 12% 5%	450
	11th to 20th	Day	28% 18% 30% 18% 4 1	450
	21st to end	Day	35% 15% 26% 20% 4	425
November	1st to 10th	Day	27% 20% 28% 18% 6% 1	365
	11th to 20th	Day	29% 20% 31% 14% 6% 1	209
	21st to end	Day	29% 22% 34% 11% 4	180
December	1st to 10th	Day	35% 18% 30% 14% 3	180
	11th to 20th	Day	35% 30% 24% 9% 2 1	180
	21st to end	Day	37% 17% 31% 11% 3 1	198
January	1st to 10th	Night	28% 20% 29% 19% 4	760
	11th to 20th	Night	33% 17% 26% 21% 3	760
	21st to end	Night	22% 16% 34% 24% 5%	836
February	1st to 10th	Night	26% 19% 33% 18% 3	760
	11th to 20th	Night	23% 20% 39% 16% 2	743
	21st to end	Night	26% 19% 31% 20% 3	628
March	1st to 10th	Night	26% 16% 32% 23% 4	760
	11th to 20th	Night	27% 20% 30% 18% 4	760
	21st to end	Night	25% 16% 30% 22% 6%	836
April	1st to 10th	Night	28% 14% 31% 24% 3	760
	11th to 20th	Night	29% 17% 28% 19% 6%	751
	21st to end	Night	28% 13% 29% 22% 7%	658
May	1st to 10th	Night	26% 9% 27% 28% 9% 1	360
	11th to 20th	Night	18% 10% 38% 27% 7%	354
	21st to end	Night	16% 11% 37% 24% 12% 1	389
June	1st to 10th	Night	19% 8% 29% 36% 7% 1	360
	11th to 20th	Night	17% 10% 30% 30% 13% 1	360
	21st to end	Night	21% 6% 30% 31% 13%	360
July	1st to 10th	Night	10% 7% 30% 41% 12% 1	350
	11th to 20th	Night	14% 5% 25% 41% 14% 1	348
	21st to end	Night	11% 4 32% 41% 12% 1	394
August	1st to 10th	Night	15% 5% 24% 37% 18% 1	401
	11th to 20th	Night	14% 3 23% 37% 21% 2	555
	21st to end	Night	13% 8% 29% 33% 16% 2	792
September	1st to 10th	Night	16% 7% 29% 31% 15% 2	720
	11th to 20th	Night	18% 6% 23% 28% 20% 4	720
	21st to end	Night	19% 10% 26% 24% 19% 1	720
October	1st to 10th	Night	22% 11% 29% 21% 15% 2	720
	11th to 20th	Night	26% 13% 30% 23% 8% 1	720
	21st to end	Night	31% 13% 28% 23% 5%	792
November	1st to 10th	Night	26% 16% 29% 20% 9%	740
	11th to 20th	Night	26% 18% 29% 18% 9%	760
	21st to end	Night	33% 16% 26% 19% 6%	760
December	1st to 10th	Night	33% 17% 29% 18% 3	760
	11th to 20th	Night	30% 19% 31% 15% 4 1	760
	21st to end	Night	36% 15% 27% 19% 2	836

Page generated at 2024-04-22 09:25:40. Version ac07

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