Winchester
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winchester clear sky chart from 2005-11-06 to 2024-04-10.

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	18% 10% 4 7% 6% 3 5% 5% 5% 6% 32%	82968
	Night	18% 11% 4 7% 6% 3 4 4 5% 6% 31%	85807
Transparency	Day	52% 5% 12% 20% 10% 1	82673
	Night	54% 5% 11% 19% 10% 1	85636
Seeing	Day	22% 13% 30% 22% 10% 3	13705
	Night	23% 10% 29% 28% 9% 1	22959

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	18% 15% 6% 11% 8% 4 5% 4 4 4 21%	2517
	11th to 20th	Day	14% 14% 7% 10% 8% 5% 6% 6% 5% 5% 21%	2520
	21st to end	Day	16% 17% 6% 11% 8% 5% 5% 5% 4 5% 17%	2868
February	1st to 10th	Day	17% 14% 6% 10% 8% 4 5% 4 5% 5% 24%	1710
	11th to 20th	Day	15% 14% 5% 10% 7% 5% 4 4 4 6% 26%	1694
	21st to end	Day	19% 15% 5% 9% 6% 3 5% 4 4 5% 25%	1570
March	1st to 10th	Day	20% 12% 5% 9% 7% 3 4 4 4 5% 27%	3116
	11th to 20th	Day	13% 16% 5% 9% 6% 3 6% 4 4 6% 28%	3439
	21st to end	Day	14% 8% 3 6% 7% 3 4 4 5% 6% 41%	4059
April	1st to 10th	Day	22% 11% 3 6% 5% 2 4 4 5% 6% 31%	2270
	11th to 20th	Day	21% 8% 4 7% 5% 3 3 4 3 6% 36%	2134
	21st to end	Day	24% 9% 2 6% 5% 3 4 4 5% 6% 31%	2264
May	1st to 10th	Day	20% 8% 3 6% 6% 3 4 4 5% 6% 35%	2520
	11th to 20th	Day	17% 7% 3 6% 6% 3 4 4 5% 6% 39%	2478
	21st to end	Day	16% 6% 3 5% 5% 3 5% 5% 7% 7% 38%	2725
June	1st to 10th	Day	23% 7% 3 6% 5% 3 5% 6% 5% 7% 30%	2520
	11th to 20th	Day	16% 5% 2 5% 6% 3 5% 5% 6% 8% 39%	2520
	21st to end	Day	20% 7% 2 6% 6% 3 5% 5% 6% 7% 32%	2520
July	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	2520
	11th to 20th	Day	12% 5% 3 5% 6% 3 6% 7% 7% 10% 37%	2520
	21st to end	Day	12% 5% 2 5% 6% 4 6% 5% 7% 10% 38%	2744
August	1st to 10th	Day	12% 6% 3 5% 6% 4 5% 6% 6% 7% 39%	2489
	11th to 20th	Day	13% 5% 2 5% 5% 4 5% 6% 6% 7% 41%	2252
	21st to end	Day	15% 6% 2 4 5% 4 6% 5% 6% 7% 41%	2376
September	1st to 10th	Day	14% 5% 2 6% 5% 3 5% 5% 6% 7% 42%	2160
	11th to 20th	Day	15% 5% 2 4 5% 4 4 4 5% 7% 46%	2143
	21st to end	Day	19% 5% 2 5% 4 3 4 5% 6% 8% 39%	1949
October	1st to 10th	Day	21% 6% 3 7% 5% 3 5% 5% 4 6% 35%	1800
	11th to 20th	Day	21% 9% 3 8% 7% 4 5% 5% 5% 6% 27%	1800
	21st to end	Day	27% 10% 3 6% 6% 3 4 4 5% 5% 26%	1819
November	1st to 10th	Day	16% 9% 4 6% 6% 3 4 6% 5% 8% 32%	1671
	11th to 20th	Day	19% 12% 4 7% 7% 3 5% 4 5% 5% 29%	1563
	21st to end	Day	21% 14% 5% 9% 7% 4 5% 5% 4 6% 21%	1520
December	1st to 10th	Day	24% 15% 5% 10% 6% 4 5% 5% 4 5% 16%	1520
	11th to 20th	Day	23% 16% 5% 7% 5% 3 4 4 4 5% 23%	1906
	21st to end	Day	26% 17% 5% 10% 5% 3 4 3 3 3 20%	2772
January	1st to 10th	Night	19% 15% 7% 10% 7% 3 4 4 4 5% 21%	4429
	11th to 20th	Night	13% 15% 7% 12% 9% 5% 6% 5% 5% 4 19%	4440
	21st to end	Night	19% 18% 7% 9% 7% 4 4 3 4 4 21%	4784
February	1st to 10th	Night	16% 15% 6% 10% 7% 4 5% 4 5% 5% 22%	2470
	11th to 20th	Night	14% 13% 5% 10% 7% 4 4 4 5% 6% 28%	2371
	21st to end	Night	16% 14% 5% 8% 6% 4 5% 4 4 5% 29%	1884
March	1st to 10th	Night	21% 11% 4 7% 5% 3 4 4 4 5% 33%	3724
	11th to 20th	Night	19% 16% 5% 9% 6% 2 4 3 4 5% 27%	3516
	21st to end	Night	16% 10% 3 5% 5% 3 4 4 4 5% 41%	3572
April	1st to 10th	Night	21% 9% 4 7% 4 3 4 4 4 5% 33%	1900
	11th to 20th	Night	19% 9% 3 6% 5% 3 4 5% 5% 6% 35%	1775
	21st to end	Night	23% 9% 3 6% 5% 3 4 4 4 6% 34%	1712
May	1st to 10th	Night	22% 7% 3 5% 6% 2 5% 4 5% 7% 35%	1440
	11th to 20th	Night	17% 9% 3 6% 5% 3 3 5% 5% 5% 40%	1415
	21st to end	Night	12% 6% 2 5% 5% 3 5% 5% 5% 8% 44%	1550
June	1st to 10th	Night	19% 6% 3 7% 5% 4 6% 5% 7% 9% 30%	1440
	11th to 20th	Night	16% 6% 3 5% 4 2 5% 4 5% 9% 40%	1440
	21st to end	Night	18% 7% 3 6% 4 3 5% 4 6% 8% 36%	1440
July	1st to 10th	Night	10% 4 3 5% 5% 3 5% 5% 5% 10% 44%	1440
	11th to 20th	Night	12% 5% 2 5% 4 3 4 5% 7% 10% 43%	1440
	21st to end	Night	10% 5% 2 5% 5% 4 5% 6% 7% 8% 44%	1574
August	1st to 10th	Night	11% 6% 3 5% 6% 3 4 5% 5% 9% 42%	1447
	11th to 20th	Night	11% 7% 3 5% 6% 4 4 6% 5% 8% 43%	1629
	21st to end	Night	13% 6% 3 4 5% 3 4 5% 5% 7% 45%	1980
September	1st to 10th	Night	13% 5% 3 5% 6% 3 4 4 5% 6% 44%	1800
	11th to 20th	Night	14% 5% 2 3 4 3 3 4 5% 7% 49%	1816
	21st to end	Night	15% 6% 3 6% 5% 3 4 4 5% 6% 43%	2009
October	1st to 10th	Night	19% 7% 3 5% 5% 3 5% 4 5% 7% 36%	2160
	11th to 20th	Night	21% 8% 3 7% 5% 3 4 5% 5% 6% 32%	2160
	21st to end	Night	25% 9% 3 7% 6% 3 5% 4 5% 6% 27%	2535
November	1st to 10th	Night	20% 9% 4 6% 6% 3 4 4 3 7% 35%	2407
	11th to 20th	Night	18% 11% 3 7% 6% 3 5% 4 4 6% 32%	2616
	21st to end	Night	22% 12% 6% 8% 6% 3 4 4 4 7% 24%	2660
December	1st to 10th	Night	22% 16% 6% 9% 6% 4 5% 4 5% 5% 18%	2660
	11th to 20th	Night	23% 16% 4 8% 5% 3 4 3 3 4 27%	3288
	21st to end	Night	32% 15% 5% 9% 5% 3 3 3 3 3 18%	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	66% 5% 14% 9% 5% 1	2518
	11th to 20th	Day	63% 5% 13% 12% 7% 1	2520
	21st to end	Day	70% 5% 11% 10% 4	2868
February	1st to 10th	Day	63% 4 9% 20% 3 1	1710
	11th to 20th	Day	59% 5% 10% 17% 8% 1	1703
	21st to end	Day	62% 5% 11% 13% 9% 1	1570
March	1st to 10th	Day	60% 3 10% 19% 8% 1	3119
	11th to 20th	Day	57% 5% 12% 16% 9% 2	3439
	21st to end	Day	44% 4 8% 24% 17% 3	4059
April	1st to 10th	Day	55% 4 7% 20% 13% 2	2270
	11th to 20th	Day	51% 4 7% 21% 15% 2	2134
	21st to end	Day	54% 4 8% 16% 14% 3	2264
May	1st to 10th	Day	49% 5% 7% 20% 15% 3	2520
	11th to 20th	Day	45% 5% 9% 22% 16% 2	2482
	21st to end	Day	43% 6% 14% 21% 13% 2	2725
June	1st to 10th	Day	53% 6% 11% 18% 12% 1	2520
	11th to 20th	Day	43% 5% 12% 25% 13% 2	2520
	21st to end	Day	50% 6% 13% 20% 10% 1	2520
July	1st to 10th	Day	38% 6% 12% 28% 15% 1	2450
	11th to 20th	Day	39% 6% 16% 29% 10%	2445
	21st to end	Day	41% 7% 16% 25% 11%	2591
August	1st to 10th	Day	42% 7% 16% 26% 9%	2475
	11th to 20th	Day	40% 6% 17% 27% 10% 1	2252
	21st to end	Day	41% 6% 13% 28% 12%	2376
September	1st to 10th	Day	39% 7% 13% 29% 13%	2160
	11th to 20th	Day	38% 5% 11% 33% 13%	2143
	21st to end	Day	43% 6% 14% 28% 9%	1949
October	1st to 10th	Day	50% 5% 11% 22% 11%	1800
	11th to 20th	Day	57% 6% 14% 17% 6% 1	1800
	21st to end	Day	60% 4 11% 16% 8% 1	1819
November	1st to 10th	Day	50% 5% 15% 23% 7% 1	1671
	11th to 20th	Day	55% 5% 13% 17% 9% 1	1563
	21st to end	Day	63% 6% 13% 13% 5%	1520
December	1st to 10th	Day	69% 7% 11% 10% 2	1520
	11th to 20th	Day	64% 5% 13% 14% 4	1906
	21st to end	Day	72% 3 7% 15% 3	2772
January	1st to 10th	Night	66% 4 15% 11% 4	4429
	11th to 20th	Night	67% 6% 13% 10% 5%	4440
	21st to end	Night	68% 4 11% 11% 5% 1	4784
February	1st to 10th	Night	64% 4 10% 16% 6% 1	2470
	11th to 20th	Night	58% 5% 9% 20% 8%	2373
	21st to end	Night	58% 4 8% 18% 11% 1	1884
March	1st to 10th	Night	55% 3 9% 21% 9% 2	3734
	11th to 20th	Night	59% 4 14% 15% 7% 1	3517
	21st to end	Night	47% 4 5% 25% 17% 3	3572
April	1st to 10th	Night	54% 3 9% 19% 13% 1	1900
	11th to 20th	Night	49% 6% 7% 20% 16% 2	1775
	21st to end	Night	52% 4 9% 16% 16% 3	1712
May	1st to 10th	Night	49% 5% 8% 19% 16% 5%	1440
	11th to 20th	Night	45% 6% 7% 21% 18% 3	1416
	21st to end	Night	39% 5% 17% 20% 16% 3	1551
June	1st to 10th	Night	49% 7% 13% 19% 11% 1	1440
	11th to 20th	Night	41% 6% 12% 22% 17% 2	1440
	21st to end	Night	45% 6% 14% 21% 13% 1	1440
July	1st to 10th	Night	34% 7% 13% 27% 16% 2	1400
	11th to 20th	Night	36% 6% 14% 31% 13%	1392
	21st to end	Night	36% 8% 16% 26% 14% 1	1486
August	1st to 10th	Night	38% 6% 19% 24% 12% 1	1437
	11th to 20th	Night	41% 5% 15% 26% 13% 1	1629
	21st to end	Night	37% 6% 16% 28% 13% 1	1980
September	1st to 10th	Night	40% 6% 12% 27% 15% 1	1800
	11th to 20th	Night	36% 4 13% 34% 12% 1	1816
	21st to end	Night	41% 5% 11% 30% 11%	2009
October	1st to 10th	Night	47% 5% 11% 24% 12% 1	2160
	11th to 20th	Night	52% 4 12% 22% 9%	2160
	21st to end	Night	58% 4 13% 15% 8% 1	2535
November	1st to 10th	Night	51% 4 14% 21% 10% 1	2407
	11th to 20th	Night	54% 5% 12% 18% 10% 1	2616
	21st to end	Night	61% 5% 12% 16% 5%	2660
December	1st to 10th	Night	68% 5% 10% 14% 3	2660
	11th to 20th	Night	63% 3 10% 16% 7% 1	3288
	21st to end	Night	72% 4 8% 13% 3	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	23% 17% 31% 21% 8%	180
	11th to 20th	Day	28% 19% 32% 14% 6% 1	180
	21st to end	Day	23% 13% 36% 21% 7%	225
February	1st to 10th	Day	29% 20% 34% 12% 4	270
	11th to 20th	Day	23% 20% 41% 15% 2	301
	21st to end	Day	27% 15% 37% 19% 2	382
March	1st to 10th	Day	23% 13% 36% 20% 7%	460
	11th to 20th	Day	23% 16% 36% 19% 5% 1	460
	21st to end	Day	27% 16% 30% 18% 7% 2	506
April	1st to 10th	Day	26% 17% 32% 18% 6% 1	457
	11th to 20th	Day	27% 19% 25% 19% 7% 3	412
	21st to end	Day	29% 17% 31% 13% 9% 1	420
May	1st to 10th	Day	25% 13% 30% 21% 8% 2	423
	11th to 20th	Day	17% 15% 37% 19% 9% 2	447
	21st to end	Day	18% 15% 31% 21% 13% 2	485
June	1st to 10th	Day	27% 10% 27% 25% 9% 2	450
	11th to 20th	Day	16% 13% 29% 27% 10% 4	450
	21st to end	Day	22% 10% 30% 22% 11% 4	450
July	1st to 10th	Day	13% 7% 24% 36% 16% 4	445
	11th to 20th	Day	14% 8% 27% 33% 15% 3	444
	21st to end	Day	14% 7% 29% 33% 14% 2	488
August	1st to 10th	Day	16% 7% 24% 33% 17% 4	446
	11th to 20th	Day	14% 5% 23% 29% 20% 8%	439
	21st to end	Day	14% 11% 27% 24% 15% 8%	495
September	1st to 10th	Day	14% 8% 28% 26% 14% 10%	450
	11th to 20th	Day	14% 8% 25% 24% 18% 12%	450
	21st to end	Day	19% 9% 30% 19% 17% 5%	450
October	1st to 10th	Day	22% 12% 27% 18% 14% 6%	450
	11th to 20th	Day	25% 18% 26% 24% 6% 1	450
	21st to end	Day	32% 15% 25% 22% 6%	414
November	1st to 10th	Day	25% 19% 27% 20% 7% 1	365
	11th to 20th	Day	25% 18% 35% 14% 8%	223
	21st to end	Day	26% 22% 31% 13% 7% 1	180
December	1st to 10th	Day	31% 14% 32% 19% 4	180
	11th to 20th	Day	27% 25% 33% 11% 4 1	180
	21st to end	Day	34% 18% 29% 15% 3 2	198
January	1st to 10th	Night	27% 14% 31% 24% 4	760
	11th to 20th	Night	29% 13% 30% 23% 5% 1	760
	21st to end	Night	21% 10% 33% 29% 6%	836
February	1st to 10th	Night	21% 14% 36% 24% 4	760
	11th to 20th	Night	21% 15% 37% 23% 4	743
	21st to end	Night	24% 15% 32% 26% 4	628
March	1st to 10th	Night	25% 13% 32% 26% 5%	760
	11th to 20th	Night	24% 16% 32% 22% 5%	760
	21st to end	Night	23% 12% 30% 27% 8%	836
April	1st to 10th	Night	26% 12% 29% 28% 4	760
	11th to 20th	Night	24% 14% 31% 23% 8%	711
	21st to end	Night	27% 11% 28% 26% 8% 1	632
May	1st to 10th	Night	26% 7% 28% 27% 10% 2	360
	11th to 20th	Night	19% 7% 34% 31% 8%	354
	21st to end	Night	14% 6% 38% 28% 13% 1	389
June	1st to 10th	Night	19% 7% 29% 37% 9%	360
	11th to 20th	Night	18% 6% 29% 33% 13% 1	360
	21st to end	Night	17% 6% 30% 32% 14% 1	360
July	1st to 10th	Night	11% 5% 28% 42% 14%	350
	11th to 20th	Night	16% 3 22% 40% 19% 1	348
	21st to end	Night	11% 3 26% 48% 11% 1	394
August	1st to 10th	Night	15% 3 23% 40% 19% 1	373
	11th to 20th	Night	15% 2 18% 38% 25% 2	565
	21st to end	Night	15% 5% 26% 33% 19% 3	792
September	1st to 10th	Night	15% 7% 25% 35% 18% 1	720
	11th to 20th	Night	17% 6% 20% 33% 21% 4	720
	21st to end	Night	17% 7% 26% 28% 21% 2	720
October	1st to 10th	Night	21% 8% 29% 26% 15% 2	720
	11th to 20th	Night	24% 11% 30% 26% 9% 1	720
	21st to end	Night	29% 12% 27% 26% 7%	792
November	1st to 10th	Night	26% 12% 27% 26% 8% 1	740
	11th to 20th	Night	25% 15% 31% 21% 9%	760
	21st to end	Night	27% 15% 28% 23% 7%	760
December	1st to 10th	Night	30% 13% 28% 24% 4	760
	11th to 20th	Night	29% 13% 31% 21% 5% 1	760
	21st to end	Night	33% 12% 29% 22% 3	836

Page generated at 2024-04-07 09:28:13. Version ac07

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