Salisbury
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Salisbury clear sky chart from 2005-11-06 to 2024-01-13.

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% 6% 2 5% 5% 3 4 5% 6% 8% 44%	74723
	Night	14% 6% 2 5% 5% 3 4 5% 5% 7% 44%	75024
Transparency	Day	38% 7% 13% 23% 16% 3	74431
	Night	39% 6% 12% 23% 17% 3	74836
Seeing	Day	18% 14% 29% 23% 12% 4	14010
	Night	17% 11% 29% 30% 12% 1	23525

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% 8% 3 6% 5% 2 4 4 5% 7% 44%	1517
	11th to 20th	Day	16% 6% 2 6% 5% 3 4 4 4 6% 43%	1602
	21st to end	Day	16% 6% 2 6% 5% 3 5% 3 4 6% 44%	1782
February	1st to 10th	Day	15% 7% 3 5% 5% 3 5% 5% 6% 7% 38%	1620
	11th to 20th	Day	19% 6% 2 4 3 3 3 4 5% 6% 45%	1726
	21st to end	Day	18% 9% 4 5% 5% 3 4 4 5% 6% 37%	1480
March	1st to 10th	Day	14% 6% 2 4 5% 2 3 4 4 6% 50%	1836
	11th to 20th	Day	17% 7% 2 5% 3 2 4 4 5% 7% 45%	1980
	21st to end	Day	17% 7% 3 5% 5% 4 5% 4 4 6% 40%	2282
April	1st to 10th	Day	12% 5% 2 4 4 3 4 4 4 8% 49%	2160
	11th to 20th	Day	12% 5% 2 4 5% 2 4 5% 4 7% 50%	2134
	21st to end	Day	15% 6% 3 4 4 3 4 4 5% 7% 45%	2160
May	1st to 10th	Day	15% 6% 3 5% 4 2 4 4 5% 9% 42%	2186
	11th to 20th	Day	15% 6% 2 5% 4 3 4 5% 6% 8% 41%	2466
	21st to end	Day	13% 5% 2 4 5% 3 4 6% 6% 9% 43%	2725
June	1st to 10th	Day	12% 4 2 4 6% 2 5% 6% 7% 10% 42%	2520
	11th to 20th	Day	11% 5% 3 4 5% 3 5% 4 6% 10% 44%	2520
	21st to end	Day	9% 4 2 5% 5% 4 5% 5% 7% 11% 44%	2520
July	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 5% 6% 10% 43%	2520
	11th to 20th	Day	6% 3 1 5% 5% 3 7% 5% 9% 12% 44%	2520
	21st to end	Day	9% 4 2 5% 7% 3 7% 8% 9% 10% 37%	2582
August	1st to 10th	Day	10% 5% 2 6% 5% 4 6% 7% 8% 11% 36%	2306
	11th to 20th	Day	10% 7% 2 6% 7% 3 6% 6% 6% 9% 38%	2126
	21st to end	Day	10% 5% 3 6% 6% 4 5% 5% 6% 9% 40%	2376
September	1st to 10th	Day	13% 6% 2 5% 7% 3 5% 5% 5% 7% 42%	2160
	11th to 20th	Day	12% 6% 2 5% 5% 4 4 4 5% 7% 46%	2144
	21st to end	Day	20% 6% 3 6% 4 3 4 4 4 6% 39%	1966
October	1st to 10th	Day	16% 5% 3 5% 6% 3 4 4 5% 7% 44%	1800
	11th to 20th	Day	18% 5% 3 4 4 2 3 3 4 5% 50%	1800
	21st to end	Day	15% 6% 2 4 5% 3 3 4 5% 6% 47%	1980
November	1st to 10th	Day	14% 5% 1 4 4 2 4 3 5% 6% 52%	1671
	11th to 20th	Day	13% 5% 2 5% 5% 3 4 4 4 7% 48%	1710
	21st to end	Day	16% 4 3 5% 5% 3 3 3 4 7% 48%	1644
December	1st to 10th	Day	16% 7% 2 5% 5% 3 4 5% 5% 7% 41%	1520
	11th to 20th	Day	16% 7% 3 5% 4 3 3 3 3 5% 47%	1910
	21st to end	Day	16% 6% 2 4 5% 3 4 6% 5% 6% 43%	2772
January	1st to 10th	Night	15% 7% 3 4 4 3 3 4 4 6% 46%	2649
	11th to 20th	Night	17% 7% 2 6% 5% 3 4 4 5% 6% 41%	2418
	21st to end	Night	15% 7% 3 4 4 3 5% 3 5% 5% 46%	2574
February	1st to 10th	Night	17% 8% 2 5% 4 2 4 4 6% 7% 42%	2340
	11th to 20th	Night	19% 8% 2 4 4 2 4 4 4 7% 42%	2118
	21st to end	Night	19% 7% 2 5% 3 2 4 4 5% 5% 45%	1776
March	1st to 10th	Night	15% 6% 3 4 5% 3 3 4 5% 6% 48%	2120
	11th to 20th	Night	17% 7% 3 4 4 2 4 4 5% 7% 42%	1980
	21st to end	Night	16% 9% 3 6% 5% 2 4 4 4 6% 41%	2073
April	1st to 10th	Night	16% 7% 3 5% 5% 3 4 5% 5% 7% 41%	1800
	11th to 20th	Night	12% 5% 3 5% 5% 2 4 4 4 7% 49%	1775
	21st to end	Night	18% 6% 2 5% 5% 3 5% 5% 4 7% 42%	1800
May	1st to 10th	Night	15% 6% 3 6% 4 2 4 5% 7% 9% 38%	1772
	11th to 20th	Night	15% 6% 3 5% 4 3 5% 5% 5% 8% 39%	1442
	21st to end	Night	12% 7% 3 5% 5% 3 5% 5% 5% 8% 41%	1550
June	1st to 10th	Night	12% 5% 3 5% 4 3 5% 6% 6% 9% 43%	1440
	11th to 20th	Night	9% 5% 2 5% 7% 4 5% 5% 6% 10% 41%	1440
	21st to end	Night	8% 5% 2 4 6% 4 6% 7% 7% 11% 39%	1440
July	1st to 10th	Night	9% 5% 2 7% 5% 3 5% 6% 6% 9% 41%	1440
	11th to 20th	Night	5% 4 2 5% 6% 4 5% 6% 7% 10% 46%	1440
	21st to end	Night	7% 5% 3 6% 6% 3 6% 7% 7% 10% 41%	1732
August	1st to 10th	Night	7% 5% 3 6% 6% 4 6% 6% 7% 10% 40%	1615
	11th to 20th	Night	10% 5% 3 5% 6% 3 5% 6% 7% 9% 41%	1774
	21st to end	Night	9% 5% 2 5% 6% 4 4 5% 6% 9% 44%	1980
September	1st to 10th	Night	13% 8% 2 4 4 2 4 4 5% 7% 46%	1800
	11th to 20th	Night	10% 4 2 5% 4 3 4 5% 5% 8% 51%	1814
	21st to end	Night	18% 7% 3 4 5% 3 4 5% 6% 6% 39%	1990
October	1st to 10th	Night	17% 6% 2 5% 4 2 4 4 4 7% 45%	2160
	11th to 20th	Night	17% 5% 2 4 4 2 4 3 3 5% 50%	2160
	21st to end	Night	17% 7% 2 4 3 2 4 4 4 6% 47%	2376
November	1st to 10th	Night	13% 5% 3 5% 4 2 3 3 4 6% 51%	2405
	11th to 20th	Night	16% 6% 3 5% 4 3 3 3 4 6% 48%	2470
	21st to end	Night	13% 5% 2 3 5% 3 4 4 4 7% 50%	2532
December	1st to 10th	Night	17% 8% 3 5% 5% 3 4 4 5% 6% 40%	2660
	11th to 20th	Night	18% 6% 3 5% 5% 2 4 4 4 7% 43%	3285
	21st to end	Night	15% 7% 2 6% 5% 3 4 5% 5% 7% 40%	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	41% 5% 10% 23% 17% 5%	1518
	11th to 20th	Day	42% 5% 13% 21% 16% 3	1602
	21st to end	Day	43% 4 9% 25% 16% 3	1782
February	1st to 10th	Day	43% 7% 12% 24% 12% 2	1620
	11th to 20th	Day	41% 4 9% 24% 18% 3	1735
	21st to end	Day	47% 5% 12% 17% 15% 4	1480
March	1st to 10th	Day	36% 5% 12% 24% 20% 2	1836
	11th to 20th	Day	40% 5% 11% 21% 19% 4	1980
	21st to end	Day	45% 5% 9% 20% 17% 4	2282
April	1st to 10th	Day	35% 6% 11% 23% 19% 6%	2160
	11th to 20th	Day	33% 7% 11% 24% 19% 6%	2134
	21st to end	Day	38% 6% 13% 21% 18% 5%	2160
May	1st to 10th	Day	38% 8% 15% 21% 15% 3	2186
	11th to 20th	Day	39% 9% 14% 21% 14% 3	2470
	21st to end	Day	36% 9% 16% 27% 12% 1	2725
June	1st to 10th	Day	35% 9% 16% 24% 14% 2	2520
	11th to 20th	Day	34% 10% 16% 22% 15% 3	2520
	21st to end	Day	32% 8% 17% 24% 18% 1	2520
July	1st to 10th	Day	35% 9% 15% 23% 16% 2	2450
	11th to 20th	Day	28% 11% 20% 28% 11% 2	2445
	21st to end	Day	36% 11% 16% 21% 15% 2	2438
August	1st to 10th	Day	37% 10% 17% 24% 11% 1	2293
	11th to 20th	Day	41% 8% 16% 20% 14% 1	2126
	21st to end	Day	39% 10% 13% 21% 14% 3	2376
September	1st to 10th	Day	39% 8% 16% 18% 15% 4	2160
	11th to 20th	Day	37% 6% 11% 23% 20% 3	2144
	21st to end	Day	46% 6% 8% 19% 18% 3	1966
October	1st to 10th	Day	41% 5% 11% 22% 17% 4	1800
	11th to 20th	Day	37% 4 9% 26% 21% 3	1800
	21st to end	Day	38% 5% 7% 25% 21% 4	1980
November	1st to 10th	Day	33% 4 10% 29% 20% 4	1671
	11th to 20th	Day	37% 5% 11% 22% 21% 4	1710
	21st to end	Day	38% 4 10% 29% 16% 4	1644
December	1st to 10th	Day	43% 6% 14% 23% 12% 3	1520
	11th to 20th	Day	41% 4 9% 29% 14% 4	1906
	21st to end	Day	39% 7% 16% 25% 13%	2772
January	1st to 10th	Night	39% 5% 10% 23% 19% 5%	2649
	11th to 20th	Night	44% 5% 12% 22% 14% 2	2418
	21st to end	Night	40% 4 8% 27% 18% 3	2574
February	1st to 10th	Night	42% 5% 13% 23% 15% 2	2340
	11th to 20th	Night	44% 5% 8% 21% 18% 5%	2120
	21st to end	Night	41% 4 9% 23% 18% 4	1776
March	1st to 10th	Night	38% 5% 10% 25% 19% 4	2120
	11th to 20th	Night	41% 6% 10% 22% 18% 3	1980
	21st to end	Night	44% 5% 10% 19% 16% 6%	2073
April	1st to 10th	Night	42% 7% 11% 18% 19% 5%	1800
	11th to 20th	Night	35% 6% 13% 20% 20% 6%	1775
	21st to end	Night	41% 6% 11% 21% 17% 4	1800
May	1st to 10th	Night	40% 9% 13% 22% 14% 3	1772
	11th to 20th	Night	41% 9% 14% 18% 16% 2	1443
	21st to end	Night	41% 9% 15% 22% 12% 1	1551
June	1st to 10th	Night	37% 10% 13% 23% 16% 2	1440
	11th to 20th	Night	38% 8% 15% 23% 14% 2	1440
	21st to end	Night	36% 10% 16% 20% 15% 2	1440
July	1st to 10th	Night	37% 10% 16% 21% 12% 3	1400
	11th to 20th	Night	30% 11% 20% 25% 13% 1	1392
	21st to end	Night	34% 10% 17% 24% 15%	1636
August	1st to 10th	Night	36% 10% 19% 22% 14%	1604
	11th to 20th	Night	37% 11% 17% 21% 14% 1	1774
	21st to end	Night	36% 9% 16% 21% 15% 3	1980
September	1st to 10th	Night	37% 7% 15% 22% 15% 4	1800
	11th to 20th	Night	30% 6% 13% 27% 21% 3	1814
	21st to end	Night	45% 8% 10% 18% 17% 3	1990
October	1st to 10th	Night	40% 6% 11% 22% 18% 3	2160
	11th to 20th	Night	38% 5% 9% 24% 21% 3	2160
	21st to end	Night	38% 5% 9% 24% 21% 4	2376
November	1st to 10th	Night	36% 5% 9% 26% 18% 5%	2405
	11th to 20th	Night	39% 4 9% 20% 21% 5%	2470
	21st to end	Night	34% 5% 11% 25% 21% 4	2532
December	1st to 10th	Night	44% 5% 11% 23% 13% 3	2660
	11th to 20th	Night	42% 5% 9% 23% 19% 3	3288
	21st to end	Night	42% 6% 16% 24% 11% 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	27% 24% 33% 12% 4 1	180
	11th to 20th	Day	27% 34% 24% 13% 3	228
	21st to end	Day	25% 25% 33% 14% 2	264
February	1st to 10th	Day	25% 30% 31% 9% 4	240
	11th to 20th	Day	30% 22% 35% 11% 2	394
	21st to end	Day	30% 24% 32% 12% 3	346
March	1st to 10th	Day	26% 21% 30% 18% 4	420
	11th to 20th	Day	26% 23% 29% 18% 3 2	420
	21st to end	Day	22% 21% 35% 15% 5% 1	462
April	1st to 10th	Day	20% 24% 34% 15% 6% 1	420
	11th to 20th	Day	17% 16% 37% 21% 7% 2	412
	21st to end	Day	24% 16% 35% 17% 7% 1	420
May	1st to 10th	Day	24% 18% 34% 15% 8% 2	423
	11th to 20th	Day	19% 14% 36% 21% 8% 2	447
	21st to end	Day	17% 9% 26% 29% 14% 4	485
June	1st to 10th	Day	14% 11% 28% 30% 13% 3	450
	11th to 20th	Day	10% 12% 29% 28% 15% 5%	450
	21st to end	Day	11% 8% 28% 36% 13% 4	450
July	1st to 10th	Day	13% 3 23% 34% 19% 7%	445
	11th to 20th	Day	3 2 23% 36% 27% 8%	444
	21st to end	Day	9% 4 26% 35% 18% 8%	488
August	1st to 10th	Day	12% 3 20% 33% 23% 9%	446
	11th to 20th	Day	10% 4 21% 33% 24% 8%	439
	21st to end	Day	8% 5% 24% 27% 24% 11%	495
September	1st to 10th	Day	14% 6% 23% 25% 17% 14%	450
	11th to 20th	Day	12% 6% 24% 29% 18% 10%	450
	21st to end	Day	24% 11% 28% 22% 10% 5%	450
October	1st to 10th	Day	21% 7% 26% 25% 13% 8%	450
	11th to 20th	Day	19% 12% 29% 22% 15% 2	450
	21st to end	Day	18% 15% 34% 19% 11% 3	495
November	1st to 10th	Day	18% 13% 29% 22% 14% 4	365
	11th to 20th	Day	19% 19% 31% 21% 9% 1	370
	21st to end	Day	22% 19% 25% 22% 9% 3	304
December	1st to 10th	Day	27% 22% 33% 12% 6% 1	180
	11th to 20th	Day	28% 22% 29% 13% 7%	180
	21st to end	Day	22% 24% 27% 18% 9%	198
January	1st to 10th	Night	20% 19% 34% 23% 4	760
	11th to 20th	Night	21% 15% 33% 27% 5%	732
	21st to end	Night	20% 18% 36% 24% 2	792
February	1st to 10th	Night	21% 15% 37% 24% 3	720
	11th to 20th	Night	24% 22% 34% 18% 2	703
	21st to end	Night	22% 20% 36% 19% 3	592
March	1st to 10th	Night	21% 18% 35% 22% 5%	720
	11th to 20th	Night	23% 17% 34% 19% 5% 1	720
	21st to end	Night	20% 15% 35% 24% 5%	792
April	1st to 10th	Night	21% 15% 40% 20% 4	720
	11th to 20th	Night	15% 14% 36% 30% 5%	711
	21st to end	Night	21% 10% 35% 27% 7%	720
May	1st to 10th	Night	18% 12% 37% 25% 7% 1	692
	11th to 20th	Night	16% 7% 32% 38% 7%	381
	21st to end	Night	14% 5% 24% 39% 17% 1	389
June	1st to 10th	Night	13% 4 26% 46% 11% 1	360
	11th to 20th	Night	9% 3 26% 44% 18% 1	360
	21st to end	Night	9% 1 26% 45% 19%	360
July	1st to 10th	Night	10% 2 20% 45% 22% 2	350
	11th to 20th	Night	5% 2 14% 49% 30% 1	348
	21st to end	Night	7% 2 16% 44% 29% 2	552
August	1st to 10th	Night	8% 3 12% 45% 29% 3	541
	11th to 20th	Night	12% 3 16% 40% 26% 3	710
	21st to end	Night	9% 4 15% 39% 29% 4	792
September	1st to 10th	Night	16% 3 16% 36% 25% 4	720
	11th to 20th	Night	11% 3 18% 36% 27% 5%	720
	21st to end	Night	22% 6% 22% 32% 16% 2	720
October	1st to 10th	Night	19% 5% 21% 32% 17% 5%	720
	11th to 20th	Night	19% 11% 23% 31% 16% 1	720
	21st to end	Night	20% 11% 29% 27% 12% 2	792
November	1st to 10th	Night	16% 8% 31% 29% 13% 3	740
	11th to 20th	Night	18% 14% 29% 30% 8% 1	760
	21st to end	Night	17% 13% 29% 29% 11% 1	760
December	1st to 10th	Night	23% 14% 33% 21% 8% 1	760
	11th to 20th	Night	20% 15% 36% 24% 6%	760
	21st to end	Night	18% 15% 33% 30% 5%	836

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