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 2024-02-10.

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	13% 8% 3 6% 5% 3 4 5% 5% 7% 40%	78325
	Night	14% 9% 3 6% 5% 3 4 4 5% 6% 41%	80692
Transparency	Day	42% 7% 12% 23% 14% 2	78031
	Night	43% 6% 11% 23% 15% 3	80507
Seeing	Day	15% 9% 26% 27% 18% 5%	14611
	Night	16% 9% 30% 31% 13% 1	24624

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% 10% 7% 13% 7% 3 5% 5% 5% 6% 28%	2527
	11th to 20th	Day	13% 10% 5% 9% 9% 5% 8% 5% 6% 5% 25%	2658
	21st to end	Day	18% 19% 9% 8% 5% 3 3 3 4 4 25%	3135
February	1st to 10th	Day	16% 10% 6% 8% 5% 4 5% 4 4 7% 32%	1701
	11th to 20th	Day	14% 10% 4 7% 7% 4 5% 4 6% 8% 33%	1705
	21st to end	Day	16% 11% 4 8% 6% 3 4 3 5% 5% 34%	1480
March	1st to 10th	Day	16% 10% 5% 5% 6% 3 5% 4 4 6% 36%	1818
	11th to 20th	Day	16% 11% 3 6% 5% 3 4 4 5% 7% 36%	1980
	21st to end	Day	20% 11% 4 6% 6% 4 5% 4 5% 7% 28%	2294
April	1st to 10th	Day	14% 7% 3 6% 5% 3 4 4 5% 8% 41%	2160
	11th to 20th	Day	16% 9% 3 5% 5% 3 4 5% 5% 7% 38%	2134
	21st to end	Day	19% 5% 2 6% 6% 3 5% 5% 6% 6% 36%	2160
May	1st to 10th	Day	17% 8% 3 6% 5% 4 5% 5% 6% 8% 34%	2257
	11th to 20th	Day	17% 6% 3 6% 6% 3 3 5% 5% 7% 38%	2479
	21st to end	Day	12% 6% 3 6% 6% 4 6% 6% 6% 9% 37%	2725
June	1st to 10th	Day	7% 4 2 4 5% 3 5% 6% 7% 10% 46%	2520
	11th to 20th	Day	9% 4 2 5% 5% 3 5% 6% 6% 9% 45%	2520
	21st to end	Day	7% 4 2 5% 6% 3 6% 6% 7% 11% 43%	2520
July	1st to 10th	Day	8% 4 2 4 5% 3 5% 5% 8% 11% 45%	2520
	11th to 20th	Day	5% 4 2 4 5% 4 5% 6% 7% 11% 47%	2520
	21st to end	Day	8% 4 2 4 5% 3 4 5% 6% 8% 50%	2662
August	1st to 10th	Day	9% 5% 2 6% 6% 4 5% 6% 7% 10% 42%	2328
	11th to 20th	Day	9% 4 2 6% 6% 4 5% 6% 7% 10% 43%	2145
	21st to end	Day	8% 4 2 4 4 3 4 5% 7% 9% 50%	2376
September	1st to 10th	Day	13% 4 2 4 3 3 4 4 6% 7% 50%	2160
	11th to 20th	Day	9% 5% 2 3 5% 3 4 5% 5% 6% 52%	2138
	21st to end	Day	7% 4 2 4 5% 2 5% 4 6% 8% 53%	1969
October	1st to 10th	Day	12% 4 2 4 4 3 3 5% 5% 7% 51%	1800
	11th to 20th	Day	16% 8% 2 4 5% 2 3 3 4 7% 46%	1800
	21st to end	Day	19% 9% 3 5% 4 3 3 4 3 6% 43%	1939
November	1st to 10th	Day	11% 5% 2 5% 5% 3 4 4 5% 7% 51%	1672
	11th to 20th	Day	15% 7% 4 6% 4 3 3 4 4 6% 45%	1710
	21st to end	Day	15% 10% 5% 6% 6% 3 3 4 5% 4 39%	1592
December	1st to 10th	Day	14% 9% 5% 7% 6% 3 5% 3 4 6% 39%	1520
	11th to 20th	Day	14% 9% 4 5% 6% 3 5% 5% 6% 6% 36%	1918
	21st to end	Day	20% 16% 6% 10% 6% 2 3 4 4 5% 22%	2783
January	1st to 10th	Night	14% 12% 6% 9% 6% 3 4 5% 4 5% 33%	4428
	11th to 20th	Night	13% 17% 5% 8% 6% 3 7% 6% 4 4 26%	4308
	21st to end	Night	22% 17% 6% 9% 5% 3 3 3 4 6% 24%	4532
February	1st to 10th	Night	15% 11% 6% 7% 5% 3 4 5% 5% 6% 32%	2464
	11th to 20th	Night	14% 9% 5% 7% 5% 3 4 4 6% 6% 38%	2139
	21st to end	Night	17% 9% 3 6% 6% 3 4 3 4 5% 40%	1776
March	1st to 10th	Night	15% 10% 4 6% 5% 3 4 4 5% 5% 41%	2157
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 3 5% 6% 40%	1980
	21st to end	Night	21% 11% 4 6% 5% 2 5% 5% 5% 7% 29%	2062
April	1st to 10th	Night	13% 7% 3 5% 5% 4 4 4 5% 7% 45%	1800
	11th to 20th	Night	14% 8% 2 4 4 2 4 5% 5% 8% 44%	1775
	21st to end	Night	20% 7% 4 5% 4 3 4 4 5% 6% 40%	1800
May	1st to 10th	Night	15% 5% 2 6% 6% 3 5% 4 6% 9% 40%	1697
	11th to 20th	Night	16% 6% 3 5% 5% 3 4 4 5% 8% 42%	1416
	21st to end	Night	11% 4 3 6% 5% 3 5% 5% 7% 8% 42%	1551
June	1st to 10th	Night	7% 4 1 4 4 2 4 5% 6% 8% 53%	1440
	11th to 20th	Night	8% 5% 2 4 4 2 5% 5% 5% 10% 49%	1440
	21st to end	Night	7% 3 2 5% 5% 3 5% 5% 7% 10% 50%	1440
July	1st to 10th	Night	8% 4 2 5% 5% 3 5% 4 6% 10% 48%	1440
	11th to 20th	Night	5% 4 2 3 4 3 4 4 6% 10% 56%	1440
	21st to end	Night	6% 5% 2 4 4 3 5% 5% 6% 8% 53%	1653
August	1st to 10th	Night	7% 4 2 4 5% 3 4 5% 6% 10% 52%	1611
	11th to 20th	Night	7% 5% 2 5% 5% 3 4 4 6% 9% 51%	1754
	21st to end	Night	5% 3 2 4 4 3 4 5% 7% 8% 56%	1980
September	1st to 10th	Night	11% 3 2 4 4 2 4 4 5% 7% 55%	1800
	11th to 20th	Night	9% 4 2 4 4 2 4 4 5% 8% 54%	1802
	21st to end	Night	7% 4 2 4 5% 2 4 4 5% 6% 57%	1988
October	1st to 10th	Night	10% 4 2 4 4 3 4 3 5% 6% 55%	2160
	11th to 20th	Night	15% 5% 2 4 4 2 3 4 4 6% 50%	2160
	21st to end	Night	18% 8% 2 5% 4 3 3 4 4 6% 43%	2416
November	1st to 10th	Night	10% 5% 2 4 4 2 3 4 4 7% 54%	2406
	11th to 20th	Night	13% 6% 3 5% 4 3 3 3 4 6% 49%	2470
	21st to end	Night	16% 9% 3 5% 4 2 4 4 4 5% 42%	2584
December	1st to 10th	Night	16% 10% 4 6% 5% 3 5% 5% 5% 7% 34%	2660
	11th to 20th	Night	16% 9% 3 7% 5% 3 4 3 4 5% 41%	3279
	21st to end	Night	21% 19% 6% 9% 6% 3 4 4 3 4 21%	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% 2	1701
	11th to 20th	Day	50% 7% 12% 20% 11% 1	1715
	21st to end	Day	52% 4 11% 21% 10% 2	1480
March	1st to 10th	Day	48% 6% 11% 24% 10% 1	1818
	11th to 20th	Day	49% 6% 12% 21% 11% 2	1980
	21st to end	Day	56% 6% 13% 16% 8% 1	2294
April	1st to 10th	Day	42% 6% 12% 25% 14% 1	2160
	11th to 20th	Day	45% 6% 10% 24% 13% 2	2134
	21st to end	Day	47% 8% 11% 20% 13% 1	2160
May	1st to 10th	Day	47% 8% 13% 22% 9% 1	2257
	11th to 20th	Day	45% 7% 13% 23% 12% 1	2483
	21st to end	Day	42% 9% 12% 19% 14% 2	2725
June	1st to 10th	Day	31% 9% 15% 27% 16% 2	2520
	11th to 20th	Day	32% 8% 13% 23% 21% 3	2520
	21st to end	Day	34% 10% 14% 22% 17% 3	2520
July	1st to 10th	Day	32% 9% 15% 27% 13% 4	2451
	11th to 20th	Day	29% 8% 17% 26% 16% 3	2445
	21st to end	Day	31% 7% 14% 31% 16% 1	2514
August	1st to 10th	Day	35% 9% 16% 27% 12% 1	2314
	11th to 20th	Day	34% 10% 14% 27% 14% 1	2145
	21st to end	Day	27% 9% 14% 26% 19% 5%	2376
September	1st to 10th	Day	32% 7% 11% 25% 19% 6%	2160
	11th to 20th	Day	30% 6% 10% 28% 23% 4	2138
	21st to end	Day	29% 6% 11% 29% 23% 3	1969
October	1st to 10th	Day	33% 5% 12% 24% 23% 3	1800
	11th to 20th	Day	39% 5% 9% 23% 20% 3	1800
	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	2464
	11th to 20th	Night	46% 6% 12% 21% 14% 2	2140
	21st to end	Night	47% 5% 12% 22% 11% 3	1776
March	1st to 10th	Night	45% 5% 9% 24% 14% 2	2157
	11th to 20th	Night	46% 5% 12% 21% 13% 3	1980
	21st to end	Night	54% 7% 12% 18% 7% 1	2062
April	1st to 10th	Night	39% 6% 11% 25% 15% 4	1800
	11th to 20th	Night	38% 7% 10% 24% 17% 4	1775
	21st to end	Night	45% 5% 10% 21% 16% 3	1800
May	1st to 10th	Night	40% 8% 13% 24% 14% 1	1697
	11th to 20th	Night	41% 7% 10% 24% 16% 2	1417
	21st to end	Night	37% 9% 15% 19% 16% 4	1552
June	1st to 10th	Night	26% 8% 15% 29% 18% 4	1440
	11th to 20th	Night	29% 8% 12% 22% 25% 4	1440
	21st to end	Night	28% 8% 16% 24% 18% 5%	1440
July	1st to 10th	Night	31% 8% 14% 30% 14% 4	1400
	11th to 20th	Night	23% 7% 15% 33% 18% 3	1392
	21st to end	Night	27% 8% 14% 29% 20% 3	1561
August	1st to 10th	Night	26% 8% 16% 32% 15% 2	1601
	11th to 20th	Night	32% 5% 12% 29% 20% 2	1754
	21st to end	Night	24% 8% 15% 26% 22% 7%	1980
September	1st to 10th	Night	30% 5% 11% 24% 23% 7%	1800
	11th to 20th	Night	28% 6% 11% 25% 26% 5%	1802
	21st to end	Night	27% 6% 10% 25% 28% 5%	1988
October	1st to 10th	Night	30% 5% 11% 24% 25% 5%	2160
	11th to 20th	Night	35% 5% 8% 26% 22% 4	2160
	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	25% 13% 40% 16% 5%	267
	11th to 20th	Day	23% 17% 36% 19% 5%	240
	21st to end	Day	25% 15% 39% 19% 2	198
March	1st to 10th	Day	20% 19% 42% 16% 3	240
	11th to 20th	Day	27% 15% 33% 19% 5%	240
	21st to end	Day	30% 16% 31% 17% 5% 1	380
April	1st to 10th	Day	19% 13% 35% 25% 7% 1	420
	11th to 20th	Day	18% 16% 30% 22% 12% 1	412
	21st to end	Day	21% 13% 34% 24% 7% 1	420
May	1st to 10th	Day	19% 10% 34% 26% 9% 2	437
	11th to 20th	Day	19% 9% 28% 26% 15% 2	625
	21st to end	Day	14% 6% 30% 28% 16% 8%	679
June	1st to 10th	Day	7% 4 21% 33% 26% 8%	630
	11th to 20th	Day	10% 2 24% 33% 24% 7%	630
	21st to end	Day	8% 7% 25% 35% 21% 5%	630
July	1st to 10th	Day	9% 2 12% 32% 29% 16%	621
	11th to 20th	Day	7% 2 19% 34% 29% 10%	618
	21st to end	Day	10% 2 15% 33% 33% 7%	684
August	1st to 10th	Day	9% 3 13% 38% 29% 9%	626
	11th to 20th	Day	9% 2 14% 30% 30% 14%	459
	21st to end	Day	8% 1 14% 28% 37% 12%	495
September	1st to 10th	Day	14% 3 15% 30% 28% 11%	450
	11th to 20th	Day	9% 3 13% 31% 31% 13%	450
	21st to end	Day	8% 8% 21% 31% 25% 8%	439
October	1st to 10th	Day	13% 6% 28% 31% 18% 5%	270
	11th to 20th	Day	14% 23% 30% 22% 10% 1	270
	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	948
	11th to 20th	Night	20% 14% 37% 26% 3	744
	21st to end	Night	23% 16% 36% 22% 3	592
March	1st to 10th	Night	20% 13% 39% 23% 5%	717
	11th to 20th	Night	23% 10% 33% 28% 6%	540
	21st to end	Night	25% 12% 32% 24% 7%	594
April	1st to 10th	Night	17% 12% 36% 30% 5%	540
	11th to 20th	Night	16% 10% 37% 28% 9% 1	533
	21st to end	Night	22% 9% 34% 28% 7%	540
May	1st to 10th	Night	16% 5% 34% 35% 10% 1	540
	11th to 20th	Night	16% 5% 28% 38% 12% 1	531
	21st to end	Night	11% 3 27% 36% 20% 3	584
June	1st to 10th	Night	8% 4 20% 45% 22% 1	540
	11th to 20th	Night	8% 3 23% 44% 20% 2	540
	21st to end	Night	7% 2 23% 44% 22% 1	540
July	1st to 10th	Night	10% 2 10% 36% 37% 5%	525
	11th to 20th	Night	6% 2 15% 46% 28% 3	522
	21st to end	Night	6% 1 14% 44% 32% 2	591
August	1st to 10th	Night	8% 2 9% 46% 32% 3	537
	11th to 20th	Night	10% 1 10% 40% 33% 5%	534
	21st to end	Night	5% 1 13% 37% 41% 3	594
September	1st to 10th	Night	12% 3 16% 36% 30% 4	540
	11th to 20th	Night	10% 1 14% 40% 29% 5%	542
	21st to end	Night	9% 5% 24% 34% 24% 4	720
October	1st to 10th	Night	11% 4 29% 31% 22% 3	720
	11th to 20th	Night	17% 8% 33% 32% 10% 1	720
	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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