Andover
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Andover clear sky chart from 2005-11-06 to 2024-03-28.

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	17% 7% 3 5% 5% 3 4 5% 5% 7% 40%	78637
	Night	17% 7% 3 5% 5% 3 4 4 5% 6% 40%	81360
Transparency	Day	43% 6% 11% 22% 15% 2	78342
	Night	44% 5% 11% 22% 15% 2	81189
Seeing	Day	22% 19% 32% 19% 7% 2	14172
	Night	21% 15% 32% 24% 7% 1	23212

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	15% 7% 3 6% 4 2 4 4 5% 5% 44%	2517
	11th to 20th	Day	17% 6% 3 6% 4 2 4 5% 5% 5% 42%	2520
	21st to end	Day	20% 10% 3 5% 5% 3 5% 4 4 5% 35%	2772
February	1st to 10th	Day	19% 7% 3 5% 5% 2 5% 5% 6% 7% 36%	1755
	11th to 20th	Day	14% 7% 3 6% 5% 3 4 6% 5% 6% 41%	1848
	21st to end	Day	18% 9% 4 5% 5% 3 4 5% 4 5% 37%	1570
March	1st to 10th	Day	18% 8% 3 5% 5% 3 5% 4 6% 8% 36%	1897
	11th to 20th	Day	16% 7% 3 6% 5% 2 4 5% 5% 7% 39%	2009
	21st to end	Day	18% 9% 3 6% 5% 2 4 4 4 6% 39%	2364
April	1st to 10th	Day	19% 9% 3 6% 5% 3 3 4 5% 6% 38%	2160
	11th to 20th	Day	18% 6% 2 6% 4 3 6% 5% 6% 6% 39%	2160
	21st to end	Day	22% 6% 3 6% 5% 4 4 4 4 7% 34%	2340
May	1st to 10th	Day	24% 8% 2 6% 5% 4 4 4 4 6% 33%	2340
	11th to 20th	Day	17% 6% 3 5% 5% 3 4 4 5% 7% 40%	2336
	21st to end	Day	15% 6% 2 5% 5% 3 4 4 5% 8% 44%	2725
June	1st to 10th	Day	19% 6% 2 4 5% 3 6% 6% 5% 8% 34%	2520
	11th to 20th	Day	17% 6% 3 5% 5% 3 5% 5% 6% 8% 39%	2520
	21st to end	Day	18% 6% 2 6% 5% 4 6% 6% 5% 8% 34%	2520
July	1st to 10th	Day	14% 6% 2 5% 5% 3 5% 5% 7% 8% 41%	2520
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 6% 6% 10% 40%	2520
	21st to end	Day	12% 5% 2 5% 6% 3 5% 6% 7% 9% 40%	2744
August	1st to 10th	Day	10% 5% 3 6% 6% 4 6% 6% 7% 9% 38%	2355
	11th to 20th	Day	12% 6% 3 5% 5% 4 6% 6% 6% 9% 40%	2298
	21st to end	Day	12% 4 2 4 4 3 5% 5% 6% 9% 46%	2376
September	1st to 10th	Day	14% 5% 3 4 4 3 4 6% 5% 6% 46%	2151
	11th to 20th	Day	15% 6% 2 4 4 2 3 3 4 6% 51%	1980
	21st to end	Day	17% 5% 2 5% 4 3 3 3 5% 7% 45%	1980
October	1st to 10th	Day	23% 6% 2 6% 4 3 4 3 5% 6% 39%	1980
	11th to 20th	Day	15% 5% 2 4 5% 3 4 4 5% 7% 45%	1755
	21st to end	Day	22% 6% 3 5% 5% 3 3 4 5% 6% 39%	1782
November	1st to 10th	Day	16% 6% 2 5% 5% 2 3 3 4 5% 49%	1671
	11th to 20th	Day	19% 6% 3 4 4 2 4 3 4 5% 46%	1710
	21st to end	Day	19% 6% 3 6% 3 3 3 3 5% 6% 43%	1710
December	1st to 10th	Day	18% 7% 4 6% 4 3 5% 4 5% 5% 39%	1554
	11th to 20th	Day	22% 6% 2 3 4 2 4 3 4 4 44%	1906
	21st to end	Day	22% 10% 3 5% 5% 2 4 4 4 6% 33%	2772
January	1st to 10th	Night	18% 8% 2 4 3 2 3 4 4 6% 46%	4429
	11th to 20th	Night	15% 8% 3 7% 7% 3 4 4 4 5% 40%	4440
	21st to end	Night	20% 13% 3 7% 6% 3 4 4 4 5% 31%	4884
February	1st to 10th	Night	16% 9% 3 5% 5% 4 4 5% 5% 7% 38%	2421
	11th to 20th	Night	15% 8% 3 6% 5% 3 4 4 4 8% 40%	2218
	21st to end	Night	18% 9% 2 4 4 2 4 4 4 7% 42%	1884
March	1st to 10th	Night	17% 8% 3 6% 4 3 4 4 5% 5% 41%	2270
	11th to 20th	Night	16% 7% 3 6% 6% 4 5% 5% 4 5% 39%	2170
	21st to end	Night	19% 8% 3 5% 4 3 5% 3 5% 5% 41%	2145
April	1st to 10th	Night	19% 7% 3 6% 5% 3 4 5% 5% 6% 37%	1800
	11th to 20th	Night	20% 7% 2 4 5% 3 4 4 4 7% 40%	1749
	21st to end	Night	21% 6% 3 5% 5% 3 5% 5% 4 5% 37%	1620
May	1st to 10th	Night	22% 8% 3 6% 6% 3 4 4 5% 6% 32%	1620
	11th to 20th	Night	19% 5% 3 5% 5% 2 5% 6% 6% 8% 35%	1571
	21st to end	Night	17% 8% 3 6% 5% 3 4 4 5% 8% 38%	1550
June	1st to 10th	Night	18% 8% 3 5% 6% 3 5% 4 4 6% 37%	1440
	11th to 20th	Night	19% 8% 3 5% 4 3 4 5% 6% 8% 37%	1440
	21st to end	Night	16% 4 3 5% 4 3 6% 5% 6% 9% 39%	1440
July	1st to 10th	Night	15% 6% 2 6% 6% 3 5% 5% 6% 8% 40%	1440
	11th to 20th	Night	12% 6% 3 6% 6% 4 5% 5% 6% 10% 38%	1440
	21st to end	Night	14% 5% 2 6% 5% 3 4 6% 5% 9% 42%	1574
August	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 5% 7% 8% 46%	1566
	11th to 20th	Night	11% 6% 3 6% 6% 4 7% 6% 7% 8% 37%	1600
	21st to end	Night	13% 5% 2 4 4 2 4 4 6% 9% 47%	1980
September	1st to 10th	Night	13% 5% 2 5% 5% 2 3 4 5% 7% 47%	1808
	11th to 20th	Night	14% 5% 1 4 4 2 3 3 4 6% 53%	1980
	21st to end	Night	16% 6% 2 4 4 2 4 5% 5% 6% 45%	1980
October	1st to 10th	Night	21% 6% 2 5% 4 2 4 4 4 5% 41%	1980
	11th to 20th	Night	16% 5% 2 4 4 3 4 4 5% 6% 45%	2203
	21st to end	Night	22% 7% 2 5% 5% 3 4 4 4 6% 37%	2574
November	1st to 10th	Night	15% 5% 2 5% 5% 2 4 4 4 6% 48%	2407
	11th to 20th	Night	18% 6% 2 5% 3 3 4 4 4 5% 46%	2470
	21st to end	Night	16% 6% 2 5% 4 3 4 3 4 7% 44%	2470
December	1st to 10th	Night	19% 8% 4 6% 5% 3 4 4 5% 5% 38%	2625
	11th to 20th	Night	20% 6% 2 5% 4 2 4 4 5% 6% 41%	3288
	21st to end	Night	21% 9% 3 6% 5% 3 4 4 4 6% 36%	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% 4 9% 33% 12% 1	2518
	11th to 20th	Day	44% 5% 11% 25% 13% 2	2520
	21st to end	Day	51% 5% 9% 23% 9% 2	2772
February	1st to 10th	Day	47% 5% 12% 19% 14% 3	1755
	11th to 20th	Day	44% 6% 12% 20% 15% 3	1857
	21st to end	Day	48% 5% 10% 21% 13% 3	1570
March	1st to 10th	Day	46% 5% 11% 21% 13% 4	1900
	11th to 20th	Day	44% 4 11% 20% 16% 4	2009
	21st to end	Day	47% 4 7% 19% 16% 7%	2364
April	1st to 10th	Day	48% 5% 9% 18% 17% 4	2160
	11th to 20th	Day	44% 5% 11% 21% 16% 3	2160
	21st to end	Day	49% 5% 7% 18% 17% 4	2340
May	1st to 10th	Day	53% 4 8% 19% 14% 2	2340
	11th to 20th	Day	43% 7% 10% 19% 18% 4	2340
	21st to end	Day	39% 7% 15% 24% 13% 2	2725
June	1st to 10th	Day	48% 7% 12% 21% 11%	2520
	11th to 20th	Day	43% 7% 12% 19% 16% 2	2520
	21st to end	Day	46% 7% 12% 23% 11% 1	2520
July	1st to 10th	Day	39% 8% 11% 24% 16% 2	2450
	11th to 20th	Day	39% 7% 14% 26% 13% 1	2445
	21st to end	Day	38% 8% 14% 24% 13% 3	2591
August	1st to 10th	Day	39% 9% 14% 24% 12% 1	2341
	11th to 20th	Day	39% 8% 14% 25% 13% 1	2298
	21st to end	Day	34% 8% 12% 25% 19% 3	2376
September	1st to 10th	Day	36% 6% 12% 24% 20% 2	2151
	11th to 20th	Day	36% 4 8% 27% 23% 2	1980
	21st to end	Day	39% 5% 8% 23% 22% 2	1980
October	1st to 10th	Day	46% 6% 9% 21% 14% 4	1980
	11th to 20th	Day	38% 5% 12% 30% 14% 2	1755
	21st to end	Day	47% 5% 11% 20% 14% 3	1782
November	1st to 10th	Day	39% 4 12% 26% 16% 3	1671
	11th to 20th	Day	42% 4 8% 23% 21% 3	1710
	21st to end	Day	43% 5% 14% 22% 15% 2	1710
December	1st to 10th	Day	47% 5% 13% 17% 17% 1	1554
	11th to 20th	Day	45% 4 11% 19% 16% 5%	1906
	21st to end	Day	52% 5% 10% 21% 10% 1	2772
January	1st to 10th	Night	40% 3 12% 29% 12% 3	4429
	11th to 20th	Night	46% 5% 11% 23% 13% 3	4440
	21st to end	Night	55% 6% 9% 18% 11% 2	4884
February	1st to 10th	Night	46% 5% 13% 20% 14% 3	2421
	11th to 20th	Night	44% 5% 10% 21% 16% 4	2220
	21st to end	Night	43% 4 8% 25% 15% 4	1884
March	1st to 10th	Night	45% 4 8% 23% 15% 4	2280
	11th to 20th	Night	48% 5% 9% 17% 17% 4	2171
	21st to end	Night	46% 5% 7% 17% 19% 6%	2145
April	1st to 10th	Night	48% 5% 10% 18% 17% 3	1800
	11th to 20th	Night	45% 4 9% 21% 18% 3	1749
	21st to end	Night	48% 5% 10% 18% 15% 4	1620
May	1st to 10th	Night	53% 5% 8% 17% 13% 3	1620
	11th to 20th	Night	45% 6% 13% 18% 16% 3	1572
	21st to end	Night	45% 7% 13% 21% 13% 1	1551
June	1st to 10th	Night	49% 5% 11% 22% 12%	1440
	11th to 20th	Night	44% 7% 13% 19% 15% 2	1440
	21st to end	Night	39% 10% 14% 23% 14% 1	1440
July	1st to 10th	Night	40% 8% 14% 22% 16% 1	1400
	11th to 20th	Night	41% 8% 14% 26% 11% 1	1392
	21st to end	Night	38% 7% 12% 29% 11% 2	1486
August	1st to 10th	Night	35% 9% 14% 25% 15% 2	1556
	11th to 20th	Night	42% 9% 14% 21% 13% 1	1600
	21st to end	Night	35% 7% 10% 26% 21% 1	1980
September	1st to 10th	Night	36% 6% 12% 25% 19% 1	1808
	11th to 20th	Night	33% 5% 10% 24% 26% 2	1980
	21st to end	Night	38% 5% 8% 25% 21% 3	1980
October	1st to 10th	Night	46% 5% 11% 22% 12% 4	1980
	11th to 20th	Night	38% 6% 12% 28% 14% 1	2203
	21st to end	Night	48% 5% 11% 21% 14% 1	2574
November	1st to 10th	Night	38% 6% 11% 22% 19% 3	2407
	11th to 20th	Night	41% 4 9% 24% 19% 3	2470
	21st to end	Night	41% 4 13% 25% 15% 2	2470
December	1st to 10th	Night	48% 5% 11% 23% 13% 1	2625
	11th to 20th	Night	44% 4 13% 18% 19% 1	3288
	21st to end	Night	51% 4 8% 25% 12% 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	25% 27% 27% 19% 3	180
	11th to 20th	Day	24% 31% 30% 13% 2 1	180
	21st to end	Day	22% 29% 33% 12% 4	198
February	1st to 10th	Day	28% 25% 34% 12% 1	365
	11th to 20th	Day	28% 28% 32% 10% 2	456
	21st to end	Day	25% 23% 33% 15% 4	382
March	1st to 10th	Day	25% 22% 36% 13% 3	460
	11th to 20th	Day	25% 28% 33% 10% 3 1	460
	21st to end	Day	28% 24% 30% 12% 5% 2	490
April	1st to 10th	Day	27% 28% 24% 14% 5% 1	420
	11th to 20th	Day	26% 26% 30% 12% 6%	412
	21st to end	Day	28% 20% 33% 15% 2 1	420
May	1st to 10th	Day	31% 25% 26% 13% 3 2	423
	11th to 20th	Day	20% 20% 39% 17% 3 1	447
	21st to end	Day	17% 17% 36% 22% 7% 1	485
June	1st to 10th	Day	22% 13% 37% 18% 9% 1	450
	11th to 20th	Day	14% 22% 36% 22% 5% 2	450
	21st to end	Day	20% 14% 36% 21% 7% 2	450
July	1st to 10th	Day	18% 9% 33% 27% 11% 3	445
	11th to 20th	Day	16% 9% 31% 30% 13% 2	444
	21st to end	Day	11% 8% 38% 32% 9% 1	488
August	1st to 10th	Day	10% 11% 35% 30% 11% 2	446
	11th to 20th	Day	16% 8% 31% 30% 12% 3	439
	21st to end	Day	13% 11% 33% 28% 12% 3	495
September	1st to 10th	Day	19% 11% 28% 26% 14% 3	450
	11th to 20th	Day	20% 14% 29% 17% 12% 7%	450
	21st to end	Day	18% 15% 32% 22% 8% 5%	450
October	1st to 10th	Day	28% 14% 25% 21% 10% 3	450
	11th to 20th	Day	19% 20% 27% 23% 9% 2	394
	21st to end	Day	27% 16% 31% 19% 6% 1	396
November	1st to 10th	Day	17% 18% 34% 19% 10% 2	365
	11th to 20th	Day	27% 23% 28% 17% 5% 1	370
	21st to end	Day	26% 20% 31% 16% 6% 1	370
December	1st to 10th	Day	24% 29% 30% 14% 4	214
	11th to 20th	Day	33% 23% 33% 9% 2	180
	21st to end	Day	22% 30% 28% 14% 5% 2	198
January	1st to 10th	Night	20% 22% 38% 18% 3	760
	11th to 20th	Night	26% 21% 34% 18% 2	760
	21st to end	Night	20% 21% 36% 19% 3 1	836
February	1st to 10th	Night	22% 19% 35% 22% 2	760
	11th to 20th	Night	21% 21% 39% 16% 3	743
	21st to end	Night	25% 20% 32% 19% 3	628
March	1st to 10th	Night	25% 18% 35% 19% 3	760
	11th to 20th	Night	22% 20% 37% 16% 4	760
	21st to end	Night	24% 19% 33% 19% 5%	821
April	1st to 10th	Night	24% 19% 33% 20% 4	720
	11th to 20th	Night	24% 20% 32% 19% 6%	685
	21st to end	Night	23% 17% 33% 24% 2 1	540
May	1st to 10th	Night	28% 15% 31% 21% 4	540
	11th to 20th	Night	23% 14% 35% 23% 5%	510
	21st to end	Night	20% 11% 32% 28% 9%	389
June	1st to 10th	Night	21% 8% 33% 28% 9% 1	360
	11th to 20th	Night	21% 9% 32% 30% 8% 1	360
	21st to end	Night	19% 6% 33% 34% 8%	360
July	1st to 10th	Night	17% 4 27% 41% 9% 1	350
	11th to 20th	Night	13% 4 26% 43% 14% 1	348
	21st to end	Night	13% 5% 30% 41% 11%	394
August	1st to 10th	Night	11% 4 30% 39% 13% 1	492
	11th to 20th	Night	14% 7% 27% 37% 15%	536
	21st to end	Night	15% 5% 25% 36% 17% 2	792
September	1st to 10th	Night	15% 6% 32% 32% 14% 1	720
	11th to 20th	Night	17% 7% 28% 27% 18% 3	720
	21st to end	Night	18% 8% 28% 29% 15% 2	720
October	1st to 10th	Night	24% 11% 26% 25% 13% 2	720
	11th to 20th	Night	18% 13% 32% 25% 11% 1	720
	21st to end	Night	26% 15% 31% 22% 6%	792
November	1st to 10th	Night	17% 14% 34% 26% 8% 1	740
	11th to 20th	Night	22% 19% 31% 20% 7% 1	760
	21st to end	Night	21% 22% 33% 18% 5% 1	760
December	1st to 10th	Night	25% 17% 36% 17% 4	760
	11th to 20th	Night	23% 20% 32% 22% 3 1	760
	21st to end	Night	25% 19% 34% 19% 3	836

Page generated at 2024-03-24 23:37:00. Version ac07

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