Winchester
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winchester clear sky chart from 2005-11-05 to 2024-10-14.

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% 8% 3 7% 5% 4 4 4 6% 8% 40%	101093
	Night	13% 9% 3 6% 4 4 4 3 5% 7% 42%	97529
Transparency	Day	40% 7% 13% 24% 14% 2	100883
	Night	41% 6% 12% 24% 15% 2	97416
Seeing	Day	14% 9% 27% 27% 18% 5%	15635
	Night	16% 9% 30% 31% 13% 1	25422

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	11% 11% 6% 14% 6% 4 5% 4 5% 6% 28%	2527
	11th to 20th	Day	13% 10% 4 10% 7% 7% 7% 4 6% 5% 25%	2658
	21st to end	Day	18% 20% 8% 9% 5% 3 2 3 4 4 25%	3135
February	1st to 10th	Day	15% 11% 5% 9% 5% 5% 5% 3 4 6% 32%	1710
	11th to 20th	Day	13% 11% 3 6% 6% 5% 5% 3 6% 7% 34%	1802
	21st to end	Day	15% 11% 4 8% 5% 4 4 2 5% 5% 36%	1570
March	1st to 10th	Day	17% 13% 3 6% 4 3 3 2 4 5% 40%	3152
	11th to 20th	Day	10% 8% 2 5% 3 3 4 3 5% 8% 48%	3480
	21st to end	Day	16% 11% 4 8% 5% 5% 4 3 5% 7% 31%	4027
April	1st to 10th	Day	14% 8% 2 7% 5% 4 4 3 5% 7% 40%	2280
	11th to 20th	Day	15% 9% 3 5% 4 4 4 4 5% 7% 39%	2254
	21st to end	Day	19% 6% 2 8% 5% 4 5% 4 6% 6% 36%	2280
May	1st to 10th	Day	14% 8% 3 8% 5% 5% 5% 4 7% 10% 32%	3946
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 3 5% 7% 37%	4399
	21st to end	Day	9% 6% 2 6% 5% 4 5% 4 5% 8% 47%	4824
June	1st to 10th	Day	7% 5% 2 5% 5% 4 5% 5% 7% 10% 46%	2660
	11th to 20th	Day	9% 4 2 5% 4 4 5% 5% 7% 9% 47%	2660
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	2660
July	1st to 10th	Day	13% 6% 2 5% 5% 4 4 4 7% 10% 39%	4440
	11th to 20th	Day	5% 4 2 6% 5% 4 6% 5% 7% 10% 47%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 11% 40%	4665
August	1st to 10th	Day	5% 4 2 6% 4 4 4 5% 7% 10% 49%	4104
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 7% 10% 41%	3762
	21st to end	Day	5% 4 2 5% 4 4 4 5% 7% 10% 50%	4180
September	1st to 10th	Day	12% 4 2 4 3 4 4 3 6% 6% 52%	2280
	11th to 20th	Day	9% 5% 2 5% 4 4 4 5% 6% 6% 51%	2256
	21st to end	Day	9% 5% 2 5% 4 3 5% 3 6% 8% 51%	2078
October	1st to 10th	Day	12% 4 2 5% 3 3 3 4 5% 7% 53%	1900
	11th to 20th	Day	15% 9% 2 5% 4 3 3 3 4 7% 47%	1830
	21st to end	Day	19% 9% 3 6% 3 3 3 3 3 6% 42%	1939
November	1st to 10th	Day	11% 5% 1 5% 4 3 3 4 5% 7% 51%	1672
	11th to 20th	Day	15% 8% 3 6% 3 4 3 3 5% 6% 45%	1710
	21st to end	Day	15% 11% 5% 7% 5% 4 3 3 5% 4 39%	1592
December	1st to 10th	Day	13% 10% 4 8% 6% 4 4 3 4 6% 38%	1520
	11th to 20th	Day	14% 10% 3 6% 6% 4 5% 4 6% 6% 36%	1918
	21st to end	Day	20% 17% 6% 11% 5% 3 4 3 5% 5% 22%	2783
January	1st to 10th	Night	13% 13% 6% 9% 5% 4 4 4 4 5% 33%	4428
	11th to 20th	Night	13% 17% 5% 9% 5% 5% 7% 5% 4 4 26%	4308
	21st to end	Night	22% 17% 6% 9% 4 3 3 2 4 5% 24%	4532
February	1st to 10th	Night	15% 12% 6% 8% 5% 4 5% 4 5% 6% 32%	2470
	11th to 20th	Night	13% 9% 4 7% 5% 3 4 3 6% 6% 39%	2262
	21st to end	Night	16% 9% 4 7% 5% 3 4 3 4 5% 41%	1884
March	1st to 10th	Night	20% 11% 3 5% 4 3 3 2 4 5% 40%	3716
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 5% 52%	3480
	21st to end	Night	20% 10% 3 8% 4 3 5% 4 5% 7% 32%	3604
April	1st to 10th	Night	13% 7% 2 6% 5% 5% 4 3 5% 7% 43%	1900
	11th to 20th	Night	14% 8% 2 5% 4 3 4 4 5% 8% 45%	1875
	21st to end	Night	19% 7% 3 6% 4 4 4 3 5% 6% 40%	1900
May	1st to 10th	Night	17% 8% 2 7% 5% 4 4 3 6% 8% 36%	2944
	11th to 20th	Night	15% 6% 2 6% 4 4 5% 4 6% 8% 41%	2506
	21st to end	Night	9% 4 2 6% 4 4 5% 4 6% 8% 48%	2743
June	1st to 10th	Night	7% 4 2 5% 3 3 4 5% 6% 8% 52%	1520
	11th to 20th	Night	7% 5% 2 5% 4 3 5% 4 5% 10% 50%	1520
	21st to end	Night	6% 3 1 6% 4 4 5% 4 7% 9% 49%	1520
July	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 6% 9% 41%	2530
	11th to 20th	Night	4 3 2 4 4 4 4 3 6% 9% 58%	2530
	21st to end	Night	8% 6% 2 6% 5% 5% 5% 5% 7% 10% 41%	2895
August	1st to 10th	Night	4 3 1 4 4 3 4 4 6% 9% 58%	2836
	11th to 20th	Night	8% 5% 2 5% 4 5% 4 3 6% 8% 50%	3066
	21st to end	Night	4 3 1 5% 3 4 4 4 7% 10% 54%	3476
September	1st to 10th	Night	11% 3 2 4 3 3 3 3 5% 7% 56%	1900
	11th to 20th	Night	8% 4 2 5% 4 3 5% 3 6% 8% 53%	1903
	21st to end	Night	8% 5% 2 5% 4 3 4 3 5% 6% 55%	2099
October	1st to 10th	Night	10% 4 2 5% 3 3 3 3 5% 6% 56%	2280
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 4 6% 50%	2203
	21st to end	Night	18% 9% 2 6% 3 4 3 3 4 5% 43%	2416
November	1st to 10th	Night	10% 6% 2 5% 4 3 3 3 4 7% 54%	2406
	11th to 20th	Night	12% 7% 3 6% 4 3 4 2 4 7% 48%	2470
	21st to end	Night	16% 10% 3 5% 4 3 4 3 5% 5% 42%	2584
December	1st to 10th	Night	15% 11% 4 7% 5% 4 4 4 5% 7% 34%	2660
	11th to 20th	Night	16% 10% 3 7% 4 4 3 3 4 5% 41%	3279
	21st to end	Night	21% 19% 6% 10% 5% 4 4 3 4 4 20%	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	54% 7% 11% 19% 9%	2528
	11th to 20th	Day	56% 7% 15% 16% 6%	2658
	21st to end	Day	63% 5% 9% 14% 8% 1	3135
February	1st to 10th	Day	53% 5% 11% 20% 9% 1	1710
	11th to 20th	Day	48% 7% 12% 20% 13% 1	1812
	21st to end	Day	50% 5% 11% 22% 11% 2	1570
March	1st to 10th	Day	47% 5% 9% 21% 17% 2	3152
	11th to 20th	Day	34% 7% 15% 24% 18% 2	3480
	21st to end	Day	50% 8% 18% 18% 5% 1	4027
April	1st to 10th	Day	43% 6% 12% 24% 15% 1	2280
	11th to 20th	Day	44% 6% 10% 24% 14% 2	2254
	21st to end	Day	46% 7% 12% 20% 13% 1	2280
May	1st to 10th	Day	44% 10% 14% 22% 9% 1	3981
	11th to 20th	Day	45% 7% 11% 27% 9% 1	4403
	21st to end	Day	34% 8% 10% 23% 23% 1	4837
June	1st to 10th	Day	31% 9% 15% 27% 16% 3	2660
	11th to 20th	Day	31% 8% 12% 24% 21% 3	2660
	21st to end	Day	34% 9% 14% 22% 17% 3	2660
July	1st to 10th	Day	38% 7% 13% 26% 13% 2	4371
	11th to 20th	Day	29% 8% 15% 27% 19% 2	4365
	21st to end	Day	34% 10% 19% 25% 12% 1	4518
August	1st to 10th	Day	27% 8% 18% 34% 12% 1	4090
	11th to 20th	Day	37% 7% 14% 27% 14% 1	3798
	21st to end	Day	26% 9% 17% 30% 15% 4	4179
September	1st to 10th	Day	31% 7% 10% 25% 21% 6%	2280
	11th to 20th	Day	30% 6% 10% 27% 23% 4	2256
	21st to end	Day	31% 6% 11% 28% 22% 2	2078
October	1st to 10th	Day	31% 5% 12% 23% 26% 3	1900
	11th to 20th	Day	39% 5% 8% 24% 21% 3	1830
	21st to end	Day	44% 5% 9% 24% 15% 3	1939
November	1st to 10th	Day	33% 5% 13% 25% 19% 4	1672
	11th to 20th	Day	41% 4 11% 23% 18% 3	1710
	21st to end	Day	48% 5% 11% 24% 12%	1592
December	1st to 10th	Day	48% 6% 14% 21% 11% 1	1520
	11th to 20th	Day	46% 7% 11% 24% 10% 2	1915
	21st to end	Day	62% 7% 10% 14% 7% 1	2783
January	1st to 10th	Night	52% 7% 11% 21% 8%	4428
	11th to 20th	Night	61% 5% 9% 17% 8%	4308
	21st to end	Night	63% 5% 9% 16% 6% 1	4532
February	1st to 10th	Night	52% 7% 10% 19% 10% 2	2470
	11th to 20th	Night	44% 6% 12% 21% 15% 2	2263
	21st to end	Night	45% 5% 12% 22% 13% 3	1884
March	1st to 10th	Night	47% 5% 9% 24% 13% 1	3716
	11th to 20th	Night	35% 6% 9% 27% 19% 4	3480
	21st to end	Night	51% 8% 12% 24% 6% 1	3604
April	1st to 10th	Night	40% 6% 11% 25% 15% 4	1900
	11th to 20th	Night	37% 7% 11% 25% 18% 4	1875
	21st to end	Night	45% 5% 10% 21% 16% 2	1900
May	1st to 10th	Night	43% 9% 15% 21% 11% 1	2974
	11th to 20th	Night	39% 10% 12% 27% 11% 1	2507
	21st to end	Night	31% 9% 13% 22% 22% 4	2751
June	1st to 10th	Night	27% 8% 14% 28% 18% 4	1520
	11th to 20th	Night	28% 8% 11% 23% 26% 4	1520
	21st to end	Night	28% 8% 16% 24% 18% 5%	1520
July	1st to 10th	Night	38% 7% 11% 29% 12% 3	2490
	11th to 20th	Night	20% 7% 14% 35% 22% 2	2482
	21st to end	Night	34% 10% 17% 23% 15% 2	2803
August	1st to 10th	Night	21% 8% 18% 35% 16% 1	2826
	11th to 20th	Night	32% 6% 13% 28% 20% 1	3096
	21st to end	Night	24% 7% 18% 31% 16% 4	3476
September	1st to 10th	Night	29% 5% 11% 24% 24% 7%	1900
	11th to 20th	Night	28% 6% 11% 24% 26% 5%	1903
	21st to end	Night	29% 6% 10% 25% 26% 4	2099
October	1st to 10th	Night	28% 5% 11% 24% 27% 5%	2280
	11th to 20th	Night	34% 5% 9% 26% 22% 4	2208
	21st to end	Night	43% 5% 8% 21% 20% 3	2416
November	1st to 10th	Night	31% 6% 13% 25% 20% 6%	2406
	11th to 20th	Night	36% 5% 10% 27% 19% 3	2470
	21st to end	Night	44% 4 10% 25% 15% 1	2584
December	1st to 10th	Night	49% 6% 13% 22% 10% 1	2660
	11th to 20th	Night	46% 4 12% 25% 13% 1	3281
	21st to end	Night	67% 6% 8% 12% 6% 1	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	21% 17% 32% 22% 7% 1	270
	11th to 20th	Day	25% 18% 36% 18% 3	270
	21st to end	Day	17% 15% 40% 22% 5%	297
February	1st to 10th	Day	26% 13% 40% 16% 5%	270
	11th to 20th	Day	22% 16% 37% 19% 6%	270
	21st to end	Day	23% 15% 41% 19% 2	225
March	1st to 10th	Day	19% 19% 42% 17% 3	270
	11th to 20th	Day	24% 18% 35% 19% 5%	270
	21st to end	Day	29% 17% 31% 18% 5% 1	420
April	1st to 10th	Day	19% 14% 35% 24% 7% 1	460
	11th to 20th	Day	18% 17% 31% 22% 12% 1	452
	21st to end	Day	21% 13% 35% 23% 7% 1	460
May	1st to 10th	Day	17% 10% 35% 27% 9% 2	478
	11th to 20th	Day	18% 9% 27% 27% 15% 3	675
	21st to end	Day	13% 6% 30% 28% 15% 7%	734
June	1st to 10th	Day	7% 5% 23% 33% 25% 8%	680
	11th to 20th	Day	9% 3 24% 34% 24% 7%	680
	21st to end	Day	8% 7% 27% 35% 20% 4	680
July	1st to 10th	Day	10% 3 13% 32% 27% 15%	671
	11th to 20th	Day	7% 2 19% 34% 28% 9%	668
	21st to end	Day	10% 2 15% 32% 33% 7%	739
August	1st to 10th	Day	8% 3 14% 38% 28% 8%	676
	11th to 20th	Day	10% 2 17% 29% 29% 13%	500
	21st to end	Day	8% 2 13% 28% 38% 11%	539
September	1st to 10th	Day	13% 3 14% 30% 28% 12%	490
	11th to 20th	Day	9% 3 13% 31% 31% 14%	490
	21st to end	Day	10% 9% 21% 30% 23% 7%	478
October	1st to 10th	Day	12% 6% 27% 29% 19% 7%	300
	11th to 20th	Day	14% 23% 29% 23% 10% 1	279
	21st to end	Day	30% 14% 31% 19% 6%	297
November	1st to 10th	Day	9% 8% 39% 28% 11% 5%	270
	11th to 20th	Day	17% 18% 31% 20% 14% 1	270
	21st to end	Day	21% 15% 32% 23% 8% 1	270
December	1st to 10th	Day	13% 15% 41% 20% 7% 4	270
	11th to 20th	Day	21% 14% 36% 23% 6%	270
	21st to end	Day	23% 20% 39% 14% 4	297
January	1st to 10th	Night	17% 13% 40% 24% 5%	950
	11th to 20th	Night	19% 16% 37% 21% 6%	950
	21st to end	Night	21% 13% 37% 26% 3	1045
February	1st to 10th	Night	22% 12% 37% 25% 4	950
	11th to 20th	Night	19% 14% 37% 26% 4	787
	21st to end	Night	22% 16% 37% 22% 3	628
March	1st to 10th	Night	20% 13% 39% 23% 5%	756
	11th to 20th	Night	22% 11% 34% 28% 6%	570
	21st to end	Night	25% 13% 32% 24% 7%	627
April	1st to 10th	Night	18% 12% 37% 29% 5%	570
	11th to 20th	Night	15% 11% 37% 27% 9% 1	563
	21st to end	Night	21% 9% 35% 28% 7%	570
May	1st to 10th	Night	16% 6% 34% 34% 9% 1	570
	11th to 20th	Night	15% 5% 29% 39% 12% 1	561
	21st to end	Night	11% 3 28% 36% 20% 3	617
June	1st to 10th	Night	8% 5% 21% 44% 21% 1	570
	11th to 20th	Night	8% 2 23% 45% 20% 2	570
	21st to end	Night	7% 3 24% 43% 22% 1	570
July	1st to 10th	Night	11% 2 11% 36% 35% 5%	555
	11th to 20th	Night	5% 2 16% 46% 28% 3	552
	21st to end	Night	6% 1 13% 44% 32% 2	624
August	1st to 10th	Night	8% 2 10% 46% 31% 3	567
	11th to 20th	Night	10% 2 11% 41% 32% 5%	564
	21st to end	Night	5% 1 13% 37% 41% 3	627
September	1st to 10th	Night	12% 3 15% 36% 30% 4	570
	11th to 20th	Night	10% 1 14% 40% 30% 5%	573
	21st to end	Night	11% 5% 23% 34% 24% 4	760
October	1st to 10th	Night	11% 4 29% 31% 23% 3	760
	11th to 20th	Night	16% 8% 33% 32% 10% 1	735
	21st to end	Night	23% 12% 32% 26% 7%	792
November	1st to 10th	Night	13% 9% 39% 29% 10% 1	740
	11th to 20th	Night	16% 13% 37% 28% 6% 1	760
	21st to end	Night	20% 12% 35% 27% 6%	874
December	1st to 10th	Night	20% 12% 37% 24% 6% 1	950
	11th to 20th	Night	23% 9% 31% 29% 6%	950
	21st to end	Night	25% 15% 35% 23% 2	1045

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