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 2025-02-21.

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	14% 9% 3 8% 5% 4 5% 4 6% 7% 36%	108855
	Night	14% 10% 3 8% 5% 4 4 3 5% 6% 36%	109771
Transparency	Day	45% 6% 11% 22% 14% 2	112537
	Night	45% 6% 10% 22% 15% 2	114228
Seeing	Day	18% 10% 27% 23% 15% 7%	15745
	Night	18% 8% 26% 30% 15% 2	25624

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	14% 16% 8% 11% 6% 4 4 3 5% 5% 25%	3617
	11th to 20th	Day	13% 13% 5% 14% 6% 7% 5% 3 4 4 25%	3751
	21st to end	Day	21% 12% 8% 10% 5% 4 5% 3 4 5% 24%	4466
February	1st to 10th	Day	18% 13% 6% 9% 5% 5% 4 3 4 5% 28%	1915
	11th to 20th	Day	16% 11% 3 11% 6% 5% 4 3 4 6% 30%	1948
	21st to end	Day	19% 12% 4 10% 5% 3 3 3 4 5% 31%	1570
March	1st to 10th	Day	21% 16% 5% 10% 5% 4 4 3 3 5% 25%	3120
	11th to 20th	Day	13% 9% 2 7% 4 4 4 3 6% 7% 41%	3476
	21st to end	Day	17% 9% 4 8% 5% 4 5% 3 6% 7% 33%	3815
April	1st to 10th	Day	13% 12% 4 9% 4 4 4 3 6% 7% 34%	2267
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 3 5% 6% 38%	2296
	21st to end	Day	17% 9% 3 7% 5% 4 5% 4 5% 7% 34%	2470
May	1st to 10th	Day	21% 10% 3 7% 5% 5% 4 4 5% 7% 29%	4082
	11th to 20th	Day	20% 8% 3 6% 4 3 4 3 5% 7% 37%	4081
	21st to end	Day	9% 6% 3 8% 5% 5% 6% 5% 9% 10% 33%	4476
June	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 5% 6% 8% 41%	2535
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 44%	2660
	21st to end	Day	9% 7% 2 6% 5% 5% 5% 5% 7% 10% 39%	2660
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 10% 35%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 5% 5% 7% 11% 46%	4383
	21st to end	Day	9% 6% 3 8% 6% 7% 6% 5% 7% 10% 33%	4466
August	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 4 7% 10% 44%	4104
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	4044
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 8% 53%	4183
September	1st to 10th	Day	12% 5% 2 6% 3 4 5% 4 5% 7% 48%	2180
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 3 7% 9% 48%	2090
	21st to end	Day	14% 6% 2 6% 4 4 3 3 5% 6% 47%	2090
October	1st to 10th	Day	10% 6% 2 7% 4 5% 4 3 6% 6% 48%	2090
	11th to 20th	Day	12% 6% 2 6% 4 4 3 4 5% 7% 46%	1943
	21st to end	Day	18% 9% 3 7% 4 4 4 3 5% 6% 37%	1919
November	1st to 10th	Day	9% 6% 2 6% 5% 4 4 3 4 6% 52%	1762
	11th to 20th	Day	14% 11% 4 9% 5% 4 4 3 5% 6% 36%	1800
	21st to end	Day	18% 11% 4 8% 5% 4 3 3 4 6% 34%	1800
December	1st to 10th	Day	17% 12% 3 8% 4 3 4 2 4 5% 39%	2930
	11th to 20th	Day	18% 13% 5% 11% 5% 5% 5% 3 4 4 26%	3373
	21st to end	Day	23% 19% 6% 10% 4 4 4 3 4 6% 17%	4053
January	1st to 10th	Night	14% 15% 5% 14% 8% 5% 4 3 5% 6% 19%	6348
	11th to 20th	Night	13% 16% 7% 14% 7% 5% 5% 3 4 4 20%	6204
	21st to end	Night	20% 13% 5% 7% 4 4 4 3 4 5% 30%	6490
February	1st to 10th	Night	20% 12% 3 10% 5% 5% 5% 3 4 5% 28%	2480
	11th to 20th	Night	16% 10% 4 9% 6% 4 5% 4 5% 6% 30%	2338
	21st to end	Night	20% 11% 5% 10% 4 4 3 4 5% 4 31%	1885
March	1st to 10th	Night	21% 12% 2 6% 4 4 4 3 4 5% 33%	3733
	11th to 20th	Night	11% 8% 2 6% 4 4 4 3 5% 6% 45%	3484
	21st to end	Night	17% 8% 3 7% 4 4 4 3 4 6% 40%	3817
April	1st to 10th	Night	13% 10% 3 6% 5% 4 4 4 6% 7% 38%	1929
	11th to 20th	Night	15% 8% 2 6% 3 3 4 5% 6% 7% 41%	1833
	21st to end	Night	16% 7% 3 7% 5% 4 4 4 7% 7% 36%	1710
May	1st to 10th	Night	21% 12% 4 8% 6% 4 4 3 5% 6% 27%	2813
	11th to 20th	Night	18% 6% 2 6% 4 4 4 3 6% 8% 38%	2812
	21st to end	Night	9% 5% 2 7% 4 3 5% 4 7% 10% 45%	3079
June	1st to 10th	Night	9% 5% 2 6% 5% 4 4 4 5% 8% 48%	1640
	11th to 20th	Night	8% 5% 2 5% 4 3 5% 4 7% 11% 46%	1520
	21st to end	Night	6% 4 2 5% 5% 5% 5% 5% 7% 9% 48%	1520
July	1st to 10th	Night	11% 6% 2 5% 3 3 4 4 7% 6% 49%	2530
	11th to 20th	Night	5% 3 1 5% 4 4 5% 3 6% 9% 53%	2582
	21st to end	Night	10% 7% 2 6% 5% 5% 4 4 8% 11% 37%	3087
August	1st to 10th	Night	8% 6% 2 8% 5% 4 5% 4 6% 9% 43%	2825
	11th to 20th	Night	6% 6% 2 7% 5% 5% 5% 3 7% 7% 48%	2785
	21st to end	Night	3 3 1 5% 4 4 4 4 7% 8% 56%	3468
September	1st to 10th	Night	12% 5% 2 5% 3 3 4 4 5% 7% 50%	1981
	11th to 20th	Night	8% 4 2 5% 4 3 4 4 6% 8% 54%	2090
	21st to end	Night	13% 7% 2 6% 4 4 3 2 4 5% 49%	2090
October	1st to 10th	Night	9% 3 2 4 4 4 4 3 5% 7% 54%	2090
	11th to 20th	Night	13% 7% 3 6% 4 4 4 3 5% 6% 45%	2237
	21st to end	Night	16% 8% 2 7% 4 4 4 3 5% 6% 41%	2677
November	1st to 10th	Night	12% 8% 2 6% 4 3 4 3 4 7% 47%	2536
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 3 5% 7% 39%	2600
	21st to end	Night	19% 12% 3 8% 5% 4 4 3 5% 7% 30%	2600
December	1st to 10th	Night	16% 11% 3 8% 5% 3 3 3 4 5% 38%	4250
	11th to 20th	Night	19% 13% 5% 10% 5% 5% 4 4 4 5% 26%	4822
	21st to end	Night	24% 16% 7% 11% 5% 4 4 3 4 5% 17%	6886

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	61% 4 8% 20% 7%	3618
	11th to 20th	Day	62% 3 9% 14% 10% 1	3751
	21st to end	Day	63% 4 9% 15% 7% 2	4466
February	1st to 10th	Day	59% 4 10% 16% 10% 1	1915
	11th to 20th	Day	56% 4 10% 20% 9% 1	1958
	21st to end	Day	56% 4 10% 21% 8% 2	1570
March	1st to 10th	Day	62% 4 7% 18% 8% 1	3120
	11th to 20th	Day	41% 6% 11% 26% 16% 1	3476
	21st to end	Day	50% 7% 12% 22% 10% 1	3815
April	1st to 10th	Day	48% 6% 10% 22% 13% 1	2267
	11th to 20th	Day	46% 5% 10% 22% 14% 3	2296
	21st to end	Day	49% 6% 10% 22% 12% 2	2470
May	1st to 10th	Day	52% 7% 12% 20% 8% 1	4120
	11th to 20th	Day	46% 7% 12% 23% 11% 1	4085
	21st to end	Day	40% 10% 13% 20% 15% 2	4489
June	1st to 10th	Day	39% 8% 10% 24% 16% 3	2535
	11th to 20th	Day	35% 8% 13% 23% 19% 2	2660
	21st to end	Day	38% 8% 15% 19% 16% 4	2660
July	1st to 10th	Day	42% 7% 11% 21% 18% 2	4371
	11th to 20th	Day	29% 9% 12% 30% 20% 1	4308
	21st to end	Day	43% 10% 14% 17% 14% 3	4324
August	1st to 10th	Day	31% 9% 16% 30% 13% 1	4091
	11th to 20th	Day	34% 7% 12% 32% 15% 1	4083
	21st to end	Day	27% 7% 14% 29% 19% 3	4182
September	1st to 10th	Day	35% 7% 10% 23% 19% 5%	2180
	11th to 20th	Day	32% 6% 11% 26% 22% 3	2090
	21st to end	Day	37% 5% 10% 24% 22% 2	2090
October	1st to 10th	Day	24% 5% 8% 30% 32% 2	3480
	11th to 20th	Day	29% 5% 9% 30% 26% 2	3209
	21st to end	Day	37% 4 10% 24% 23% 2	3143
November	1st to 10th	Day	34% 5% 9% 30% 19% 3	1762
	11th to 20th	Day	48% 3 10% 20% 15% 3	1800
	21st to end	Day	51% 4 10% 21% 12% 1	1800
December	1st to 10th	Day	49% 3 10% 27% 11%	2930
	11th to 20th	Day	61% 6% 8% 18% 7%	3370
	21st to end	Day	69% 4 10% 12% 5%	4053
January	1st to 10th	Night	64% 5% 7% 14% 9%	6348
	11th to 20th	Night	66% 5% 8% 11% 8% 2	6204
	21st to end	Night	57% 3 9% 20% 10% 1	6490
February	1st to 10th	Night	59% 4 9% 19% 8% 1	2480
	11th to 20th	Night	54% 4 11% 19% 12% 1	2339
	21st to end	Night	55% 5% 12% 16% 10% 2	1885
March	1st to 10th	Night	52% 5% 7% 23% 11% 1	3733
	11th to 20th	Night	37% 7% 12% 25% 18% 2	3484
	21st to end	Night	46% 5% 10% 28% 11% 1	3817
April	1st to 10th	Night	45% 5% 10% 25% 13% 2	1929
	11th to 20th	Night	41% 7% 10% 20% 18% 5%	1833
	21st to end	Night	44% 6% 11% 22% 13% 3	1710
May	1st to 10th	Night	56% 7% 8% 20% 6% 2	2840
	11th to 20th	Night	43% 7% 13% 27% 9% 1	2813
	21st to end	Night	32% 8% 15% 27% 14% 4	3087
June	1st to 10th	Night	34% 7% 9% 27% 21% 2	1640
	11th to 20th	Night	30% 9% 13% 23% 21% 4	1520
	21st to end	Night	30% 9% 16% 23% 18% 5%	1520
July	1st to 10th	Night	32% 8% 11% 25% 17% 6%	2490
	11th to 20th	Night	25% 7% 14% 32% 21% 2	2534
	21st to end	Night	38% 8% 13% 24% 14% 4	2988
August	1st to 10th	Night	35% 9% 13% 29% 13% 2	2814
	11th to 20th	Night	32% 8% 12% 30% 17% 1	2812
	21st to end	Night	23% 8% 15% 30% 21% 3	3468
September	1st to 10th	Night	32% 6% 13% 23% 19% 7%	1981
	11th to 20th	Night	27% 7% 10% 26% 26% 4	2090
	21st to end	Night	37% 4 9% 22% 25% 2	2090
October	1st to 10th	Night	21% 4 6% 29% 37% 3	3480
	11th to 20th	Night	28% 3 10% 25% 32% 2	3710
	21st to end	Night	34% 4 7% 24% 26% 5%	4403
November	1st to 10th	Night	37% 4 10% 22% 23% 3	2536
	11th to 20th	Night	44% 4 9% 22% 18% 3	2600
	21st to end	Night	54% 5% 10% 20% 10% 1	2600
December	1st to 10th	Night	48% 5% 11% 25% 10%	4250
	11th to 20th	Night	59% 5% 10% 17% 8% 1	4824
	21st to end	Night	69% 5% 10% 12% 4	6886

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% 16% 35% 15% 6%	300
	11th to 20th	Day	26% 20% 40% 12% 2	300
	21st to end	Day	31% 17% 32% 17% 3	330
February	1st to 10th	Day	26% 17% 38% 15% 3	300
	11th to 20th	Day	28% 25% 33% 13% 2	300
	21st to end	Day	28% 16% 44% 9% 3	225
March	1st to 10th	Day	25% 19% 39% 14% 2	270
	11th to 20th	Day	24% 15% 35% 19% 6% 1	456
	21st to end	Day	27% 16% 33% 16% 7%	506
April	1st to 10th	Day	23% 14% 31% 21% 8% 3	460
	11th to 20th	Day	21% 16% 27% 20% 12% 4	452
	21st to end	Day	22% 12% 35% 19% 9% 3	460
May	1st to 10th	Day	24% 11% 29% 22% 9% 4	463
	11th to 20th	Day	24% 8% 25% 23% 13% 7%	487
	21st to end	Day	12% 5% 25% 26% 18% 12%	529
June	1st to 10th	Day	13% 6% 23% 31% 19% 9%	555
	11th to 20th	Day	10% 5% 24% 33% 21% 8%	680
	21st to end	Day	11% 5% 26% 32% 17% 9%	680
July	1st to 10th	Day	13% 3 17% 25% 23% 19%	671
	11th to 20th	Day	9% 3 22% 26% 26% 14%	637
	21st to end	Day	13% 3 15% 27% 27% 15%	532
August	1st to 10th	Day	10% 3 14% 30% 25% 18%	486
	11th to 20th	Day	12% 4 17% 30% 23% 14%	479
	21st to end	Day	10% 3 13% 27% 30% 17%	539
September	1st to 10th	Day	13% 2 15% 25% 34% 11%	490
	11th to 20th	Day	11% 3 12% 28% 31% 14%	490
	21st to end	Day	17% 8% 19% 26% 22% 8%	490
October	1st to 10th	Day	11% 5% 24% 31% 21% 8%	490
	11th to 20th	Day	14% 13% 29% 25% 16% 3	490
	21st to end	Day	24% 12% 35% 21% 7% 1	368
November	1st to 10th	Day	11% 13% 36% 24% 11% 4	300
	11th to 20th	Day	21% 16% 33% 19% 9% 2	300
	21st to end	Day	28% 11% 39% 17% 6%	300
December	1st to 10th	Day	22% 15% 39% 18% 5% 1	300
	11th to 20th	Day	24% 16% 39% 15% 5%	300
	21st to end	Day	29% 15% 34% 16% 5%	330
January	1st to 10th	Night	21% 15% 34% 24% 6%	1000
	11th to 20th	Night	28% 12% 34% 21% 4	935
	21st to end	Night	24% 12% 36% 26% 3	880
February	1st to 10th	Night	26% 12% 34% 27% 2	800
	11th to 20th	Night	23% 14% 37% 23% 3	783
	21st to end	Night	27% 11% 35% 24% 3	629
March	1st to 10th	Night	22% 12% 34% 26% 5% 1	760
	11th to 20th	Night	19% 14% 35% 25% 7%	760
	21st to end	Night	24% 9% 31% 28% 8%	836
April	1st to 10th	Night	17% 9% 35% 30% 9% 1	599
	11th to 20th	Night	19% 9% 32% 28% 11% 1	563
	21st to end	Night	19% 7% 37% 26% 10% 1	570
May	1st to 10th	Night	22% 7% 28% 31% 11% 1	570
	11th to 20th	Night	19% 5% 21% 32% 20% 3	561
	21st to end	Night	9% 2 20% 38% 27% 3	617
June	1st to 10th	Night	11% 2 20% 41% 24% 1	570
	11th to 20th	Night	10% 3 18% 41% 26% 3	570
	21st to end	Night	7% 2 21% 38% 28% 3	570
July	1st to 10th	Night	13% 1 12% 32% 34% 8%	555
	11th to 20th	Night	8% 2 15% 35% 35% 5%	552
	21st to end	Night	11% 2 11% 41% 32% 3	624
August	1st to 10th	Night	8% 2 10% 37% 39% 5%	567
	11th to 20th	Night	8% 1 9% 46% 30% 6%	564
	21st to end	Night	6% 2 12% 34% 40% 7%	627
September	1st to 10th	Night	13% 3 14% 33% 32% 5%	651
	11th to 20th	Night	9% 2 14% 34% 36% 5%	760
	21st to end	Night	16% 4 21% 32% 25% 3	760
October	1st to 10th	Night	11% 5% 20% 35% 24% 5%	760
	11th to 20th	Night	18% 8% 25% 31% 15% 2	760
	21st to end	Night	21% 9% 32% 27% 11% 1	836
November	1st to 10th	Night	16% 8% 30% 29% 16% 1	780
	11th to 20th	Night	20% 12% 31% 28% 9%	800
	21st to end	Night	25% 11% 27% 30% 7% 1	800
December	1st to 10th	Night	26% 11% 33% 25% 5%	800
	11th to 20th	Night	25% 11% 33% 25% 6% 1	800
	21st to end	Night	31% 12% 30% 24% 3	1055

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