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 2025-07-21.

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% 8% 2 6% 4 4 4 4 5% 7% 38%	119724
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	119391
Transparency	Day	44% 5% 11% 22% 15% 3	123289
	Night	44% 5% 11% 21% 16% 3	123987
Seeing	Day	22% 19% 32% 19% 7% 2	15915
	Night	21% 15% 32% 24% 7% 1	24777

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	12% 7% 3 9% 4 4 5% 4 6% 6% 40%	3597
	11th to 20th	Day	14% 7% 3 6% 3 3 4 3 5% 6% 47%	3570
	21st to end	Day	16% 9% 3 5% 4 4 4 3 4 5% 45%	3960
February	1st to 10th	Day	18% 7% 3 6% 4 4 5% 3 6% 7% 36%	1849
	11th to 20th	Day	14% 8% 3 7% 4 4 5% 4 6% 6% 40%	1948
	21st to end	Day	17% 10% 3 6% 5% 5% 4 4 4 5% 38%	1650
March	1st to 10th	Day	21% 10% 2 6% 4 4 4 3 5% 5% 34%	4476
	11th to 20th	Day	16% 9% 3 7% 4 4 4 4 6% 6% 38%	4791
	21st to end	Day	26% 10% 3 6% 3 3 4 4 5% 6% 30%	5739
April	1st to 10th	Day	19% 11% 2 7% 5% 3 4 3 4 6% 37%	2400
	11th to 20th	Day	18% 7% 2 6% 4 4 5% 4 6% 6% 39%	2404
	21st to end	Day	20% 7% 3 7% 5% 4 4 3 5% 7% 36%	2600
May	1st to 10th	Day	29% 10% 2 7% 4 4 4 3 5% 6% 24%	5886
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 3 5% 7% 31%	5952
	21st to end	Day	19% 7% 2 6% 4 4 4 4 5% 8% 36%	6949
June	1st to 10th	Day	19% 7% 2 5% 4 5% 5% 5% 6% 8% 34%	2800
	11th to 20th	Day	16% 7% 2 5% 5% 4 4 4 6% 8% 39%	2800
	21st to end	Day	17% 6% 2 7% 4 5% 5% 4 5% 8% 35%	2800
July	1st to 10th	Day	13% 7% 2 7% 5% 4 5% 5% 8% 9% 35%	4580
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 7% 10% 40%	4580
	21st to end	Day	16% 8% 3 7% 5% 4 5% 4 6% 8% 35%	4814
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 9% 33%	4147
	11th to 20th	Day	13% 8% 3 7% 4 4 5% 4 6% 7% 38%	4025
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 7% 10% 45%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 6% 6% 47%	2270
	11th to 20th	Day	15% 7% 2 4 3 3 2 3 4 6% 51%	2090
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 43%	2090
October	1st to 10th	Day	22% 6% 2 6% 4 4 4 3 5% 6% 39%	2090
	11th to 20th	Day	15% 5% 2 4 4 4 4 3 5% 7% 46%	1851
	21st to end	Day	21% 6% 2 6% 4 4 3 3 5% 6% 40%	1881
November	1st to 10th	Day	15% 6% 2 6% 4 3 3 3 4 5% 50%	1761
	11th to 20th	Day	18% 7% 2 5% 3 3 4 2 4 5% 48%	1800
	21st to end	Day	19% 7% 2 6% 3 4 3 3 5% 6% 43%	1800
December	1st to 10th	Day	14% 9% 2 5% 3 4 3 3 4 6% 47%	2648
	11th to 20th	Day	20% 5% 2 5% 3 3 3 3 4 5% 47%	2986
	21st to end	Day	20% 10% 3 6% 4 3 4 3 4 6% 37%	3960
January	1st to 10th	Night	14% 7% 2 5% 3 4 4 3 5% 6% 47%	6349
	11th to 20th	Night	13% 8% 3 9% 6% 4 4 3 4 6% 40%	6315
	21st to end	Night	15% 10% 3 7% 5% 5% 4 3 5% 5% 38%	6996
February	1st to 10th	Night	16% 9% 3 5% 4 5% 4 4 5% 6% 37%	2547
	11th to 20th	Night	15% 9% 3 6% 4 4 4 4 5% 7% 40%	2338
	21st to end	Night	17% 9% 2 6% 4 3 3 3 5% 7% 41%	1980
March	1st to 10th	Night	24% 8% 2 5% 4 3 4 3 5% 5% 37%	5364
	11th to 20th	Night	16% 9% 3 7% 4 5% 5% 3 5% 5% 37%	5168
	21st to end	Night	26% 9% 3 6% 4 4 4 3 5% 5% 32%	5178
April	1st to 10th	Night	20% 8% 3 6% 5% 4 5% 4 4 6% 35%	2000
	11th to 20th	Night	21% 8% 2 5% 4 4 4 3 4 7% 39%	1945
	21st to end	Night	20% 7% 3 5% 4 5% 4 4 4 5% 39%	1800
May	1st to 10th	Night	32% 10% 3 7% 4 3 4 3 4 6% 25%	4054
	11th to 20th	Night	16% 9% 4 8% 5% 6% 5% 5% 7% 8% 28%	3969
	21st to end	Night	21% 8% 3 8% 5% 4 4 4 5% 6% 33%	3926
June	1st to 10th	Night	17% 9% 3 6% 4 5% 4 3 5% 6% 38%	1600
	11th to 20th	Night	18% 9% 2 5% 4 4 4 4 6% 8% 37%	1600
	21st to end	Night	16% 6% 2 6% 4 4 5% 3 6% 8% 38%	1600
July	1st to 10th	Night	12% 7% 2 8% 5% 5% 5% 5% 7% 9% 34%	2600
	11th to 20th	Night	10% 7% 3 8% 5% 5% 5% 4 6% 9% 39%	2600
	21st to end	Night	15% 8% 4 7% 4 4 4 4 5% 8% 36%	2739
August	1st to 10th	Night	19% 7% 3 7% 5% 5% 5% 4 6% 6% 33%	2752
	11th to 20th	Night	10% 7% 3 8% 6% 5% 6% 5% 7% 8% 35%	2800
	21st to end	Night	8% 4 2 5% 3 4 4 4 6% 9% 52%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 3 4 3 5% 7% 48%	1909
	11th to 20th	Night	14% 5% 1 4 4 3 3 2 4 6% 53%	2090
	21st to end	Night	17% 7% 2 5% 4 4 4 4 5% 6% 43%	2090
October	1st to 10th	Night	20% 6% 2 6% 4 3 4 3 4 5% 41%	2090
	11th to 20th	Night	16% 5% 2 5% 4 3 4 3 5% 6% 47%	2327
	21st to end	Night	21% 7% 2 6% 4 4 4 3 4 6% 39%	2717
November	1st to 10th	Night	14% 5% 2 5% 4 3 4 3 5% 6% 49%	2537
	11th to 20th	Night	17% 6% 2 5% 3 4 3 3 4 5% 47%	2600
	21st to end	Night	16% 7% 2 5% 4 4 4 3 5% 7% 44%	2600
December	1st to 10th	Night	18% 8% 2 6% 4 4 4 3 5% 5% 40%	4531
	11th to 20th	Night	23% 6% 3 5% 3 3 4 3 4 5% 41%	5208
	21st to end	Night	18% 10% 3 6% 4 3 3 3 4 5% 40%	6996

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	42% 4 7% 27% 17% 1	3598
	11th to 20th	Day	39% 4 11% 23% 20% 3	3600
	21st to end	Day	42% 4 8% 23% 19% 3	3960
February	1st to 10th	Day	47% 5% 12% 19% 14% 2	1849
	11th to 20th	Day	44% 5% 12% 20% 15% 3	1957
	21st to end	Day	48% 4 10% 21% 14% 3	1650
March	1st to 10th	Day	51% 3 9% 15% 12% 10%	4479
	11th to 20th	Day	45% 5% 13% 25% 12% 2	4791
	21st to end	Day	54% 4 6% 15% 14% 7%	5739
April	1st to 10th	Day	49% 4 8% 17% 17% 4	2400
	11th to 20th	Day	45% 5% 11% 20% 16% 3	2404
	21st to end	Day	48% 5% 7% 19% 18% 4	2600
May	1st to 10th	Day	60% 6% 11% 14% 8% 1	5897
	11th to 20th	Day	51% 8% 13% 17% 10% 2	5956
	21st to end	Day	45% 8% 12% 21% 13% 1	6949
June	1st to 10th	Day	47% 7% 12% 21% 12%	2800
	11th to 20th	Day	42% 7% 14% 20% 16% 3	2800
	21st to end	Day	45% 7% 12% 22% 13% 1	2800
July	1st to 10th	Day	43% 7% 11% 21% 15% 1	4510
	11th to 20th	Day	38% 6% 16% 25% 14% 1	4505
	21st to end	Day	46% 6% 12% 21% 13% 2	4661
August	1st to 10th	Day	45% 8% 16% 22% 8% 1	4133
	11th to 20th	Day	43% 7% 14% 27% 9% 1	4064
	21st to end	Day	32% 7% 12% 30% 18% 2	4180
September	1st to 10th	Day	36% 6% 12% 24% 21% 2	2270
	11th to 20th	Day	36% 4 8% 27% 22% 3	2090
	21st to end	Day	41% 5% 8% 23% 22% 2	2090
October	1st to 10th	Day	43% 5% 9% 22% 19% 2	3480
	11th to 20th	Day	32% 3 7% 30% 25% 3	3041
	21st to end	Day	34% 4 9% 30% 20% 3	3081
November	1st to 10th	Day	38% 4 12% 27% 16% 2	1761
	11th to 20th	Day	41% 4 7% 22% 23% 3	1800
	21st to end	Day	43% 5% 14% 23% 14% 2	1800
December	1st to 10th	Day	40% 4 14% 29% 13% 1	2648
	11th to 20th	Day	42% 3 12% 19% 19% 5%	2986
	21st to end	Day	49% 4 8% 20% 14% 5%	3960
January	1st to 10th	Night	38% 3 10% 32% 15% 2	6349
	11th to 20th	Night	46% 5% 11% 21% 14% 3	6360
	21st to end	Night	48% 5% 8% 17% 17% 5%	6996
February	1st to 10th	Night	46% 5% 12% 20% 14% 3	2547
	11th to 20th	Night	45% 5% 10% 21% 16% 4	2340
	21st to end	Night	44% 4 8% 25% 15% 4	1980
March	1st to 10th	Night	50% 4 8% 16% 15% 8%	5374
	11th to 20th	Night	48% 5% 14% 17% 13% 2	5169
	21st to end	Night	54% 4 4 13% 20% 6%	5178
April	1st to 10th	Night	49% 5% 9% 17% 16% 3	2000
	11th to 20th	Night	47% 4 8% 20% 19% 3	1945
	21st to end	Night	47% 5% 10% 18% 17% 4	1800
May	1st to 10th	Night	61% 6% 11% 12% 8% 2	4060
	11th to 20th	Night	51% 9% 14% 16% 9% 1	3970
	21st to end	Night	52% 7% 12% 18% 11% 1	3927
June	1st to 10th	Night	48% 5% 11% 22% 13% 1	1600
	11th to 20th	Night	43% 7% 14% 20% 14% 2	1600
	21st to end	Night	41% 9% 12% 22% 14% 1	1600
July	1st to 10th	Night	43% 7% 13% 21% 15%	2560
	11th to 20th	Night	41% 7% 15% 22% 15% 1	2552
	21st to end	Night	47% 6% 11% 22% 15% 1	2651
August	1st to 10th	Night	50% 7% 12% 19% 11% 1	2742
	11th to 20th	Night	44% 9% 13% 23% 10%	2827
	21st to end	Night	28% 7% 14% 28% 23% 1	3476
September	1st to 10th	Night	36% 6% 11% 26% 20% 1	1909
	11th to 20th	Night	33% 5% 10% 24% 26% 2	2090
	21st to end	Night	40% 5% 8% 25% 19% 2	2090
October	1st to 10th	Night	41% 5% 14% 23% 15% 2	3480
	11th to 20th	Night	28% 4 10% 28% 28% 3	3876
	21st to end	Night	35% 3 11% 31% 18% 2	4467
November	1st to 10th	Night	37% 6% 11% 23% 20% 3	2537
	11th to 20th	Night	40% 4 9% 23% 21% 4	2600
	21st to end	Night	41% 4 13% 25% 15% 2	2600
December	1st to 10th	Night	44% 4 10% 29% 12% 1	4531
	11th to 20th	Night	47% 3 13% 15% 21% 1	5208
	21st to end	Night	47% 4 7% 22% 17% 2	6996

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	29% 26% 26% 17% 3	200
	11th to 20th	Day	25% 31% 30% 13% 2 1	200
	21st to end	Day	24% 28% 34% 10% 4	220
February	1st to 10th	Day	29% 25% 34% 11% 1	399
	11th to 20th	Day	30% 27% 32% 10% 1	496
	21st to end	Day	25% 24% 33% 14% 4	414
March	1st to 10th	Day	27% 23% 35% 12% 3	500
	11th to 20th	Day	25% 29% 32% 11% 3 1	500
	21st to end	Day	31% 23% 29% 11% 4 1	550
April	1st to 10th	Day	31% 27% 24% 13% 5% 1	500
	11th to 20th	Day	29% 27% 28% 11% 5%	492
	21st to end	Day	26% 20% 34% 16% 3 1	500
May	1st to 10th	Day	32% 24% 28% 12% 3 1	503
	11th to 20th	Day	21% 20% 38% 17% 4 1	527
	21st to end	Day	18% 17% 37% 22% 6% 1	573
June	1st to 10th	Day	22% 13% 38% 18% 8% 1	530
	11th to 20th	Day	15% 21% 36% 22% 5% 1	530
	21st to end	Day	20% 16% 36% 20% 7% 2	530
July	1st to 10th	Day	18% 10% 35% 26% 10% 2	525
	11th to 20th	Day	15% 9% 32% 30% 13% 2	524
	21st to end	Day	12% 8% 38% 31% 9% 1	532
August	1st to 10th	Day	11% 12% 35% 30% 11% 2	486
	11th to 20th	Day	16% 9% 31% 28% 13% 4	479
	21st to end	Day	12% 10% 33% 27% 14% 4	539
September	1st to 10th	Day	18% 12% 28% 25% 14% 3	490
	11th to 20th	Day	19% 13% 28% 18% 13% 9%	490
	21st to end	Day	18% 16% 32% 22% 8% 5%	490
October	1st to 10th	Day	26% 15% 24% 21% 10% 3	490
	11th to 20th	Day	20% 19% 26% 22% 9% 2	427
	21st to end	Day	26% 15% 33% 19% 6% 1	429
November	1st to 10th	Day	18% 19% 34% 18% 9% 2	395
	11th to 20th	Day	27% 23% 29% 16% 5% 1	400
	21st to end	Day	28% 21% 30% 15% 6% 1	400
December	1st to 10th	Day	23% 28% 31% 14% 4	235
	11th to 20th	Day	35% 23% 32% 9% 3	200
	21st to end	Day	21% 29% 28% 16% 5% 2	220
January	1st to 10th	Night	21% 22% 37% 17% 3	800
	11th to 20th	Night	26% 20% 34% 18% 2	800
	21st to end	Night	19% 21% 37% 19% 3 1	880
February	1st to 10th	Night	23% 19% 35% 22% 2	800
	11th to 20th	Night	23% 21% 38% 16% 2	783
	21st to end	Night	25% 20% 33% 19% 3	660
March	1st to 10th	Night	26% 19% 34% 19% 3	800
	11th to 20th	Night	23% 20% 37% 17% 4	800
	21st to end	Night	26% 18% 33% 19% 5%	880
April	1st to 10th	Night	25% 19% 33% 19% 3	800
	11th to 20th	Night	25% 21% 32% 18% 5%	761
	21st to end	Night	23% 18% 33% 23% 2 1	600
May	1st to 10th	Night	29% 15% 31% 21% 4	600
	11th to 20th	Night	22% 15% 35% 23% 5% 1	567
	21st to end	Night	21% 10% 33% 28% 8%	433
June	1st to 10th	Night	21% 8% 35% 27% 8% 1	400
	11th to 20th	Night	20% 10% 33% 30% 8% 1	400
	21st to end	Night	20% 6% 33% 34% 8%	400
July	1st to 10th	Night	17% 4 29% 40% 9% 1	390
	11th to 20th	Night	12% 6% 26% 42% 13% 1	388
	21st to end	Night	14% 5% 30% 40% 11%	416
August	1st to 10th	Night	13% 5% 29% 39% 13% 1	520
	11th to 20th	Night	13% 8% 28% 36% 15% 1	567
	21st to end	Night	15% 5% 25% 36% 17% 2	836
September	1st to 10th	Night	15% 6% 32% 32% 14% 1	760
	11th to 20th	Night	17% 7% 27% 27% 18% 4	760
	21st to end	Night	19% 7% 28% 29% 14% 2	760
October	1st to 10th	Night	23% 11% 26% 25% 13% 2	760
	11th to 20th	Night	18% 13% 31% 25% 11% 1	760
	21st to end	Night	25% 15% 32% 22% 7%	836
November	1st to 10th	Night	17% 14% 35% 25% 8% 1	780
	11th to 20th	Night	22% 20% 31% 20% 7% 1	800
	21st to end	Night	22% 22% 33% 18% 5% 1	800
December	1st to 10th	Night	25% 17% 36% 17% 4	800
	11th to 20th	Night	24% 20% 32% 21% 3 1	800
	21st to end	Night	24% 19% 34% 20% 3	880

Page generated at 2025-07-18 01:10:00. Version ac07

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