North Ashburnham Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the North Ashburnham Observatory 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% 3 7% 5% 4 4 4 6% 7% 36%	119814
	Night	17% 8% 3 7% 4 4 4 3 5% 6% 38%	119310
Transparency	Day	45% 5% 11% 22% 14% 2	123366
	Night	44% 5% 12% 22% 15% 2	123906
Seeing	Day	26% 17% 31% 17% 6% 2	15836
	Night	24% 18% 32% 20% 6% 1	24760

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	10% 8% 3 10% 7% 6% 5% 4 6% 6% 35%	3597
	11th to 20th	Day	12% 9% 3 7% 4 4 4 4 5% 6% 41%	3570
	21st to end	Day	14% 11% 3 7% 3 4 4 3 5% 5% 40%	3960
February	1st to 10th	Day	16% 8% 3 7% 6% 5% 5% 5% 5% 7% 33%	1840
	11th to 20th	Day	12% 7% 3 8% 6% 5% 5% 4 6% 7% 36%	1948
	21st to end	Day	17% 11% 3 7% 5% 5% 4 3 4 4 37%	1650
March	1st to 10th	Day	18% 12% 3 7% 5% 4 4 2 5% 5% 33%	4476
	11th to 20th	Day	15% 8% 2 7% 5% 5% 6% 4 5% 7% 37%	4699
	21st to end	Day	23% 10% 3 5% 4 3 4 3 5% 7% 33%	5873
April	1st to 10th	Day	17% 11% 2 8% 6% 4 3 4 5% 5% 35%	2400
	11th to 20th	Day	16% 9% 2 5% 5% 4 4 5% 6% 7% 36%	2374
	21st to end	Day	19% 8% 3 8% 5% 5% 4 4 5% 6% 35%	2587
May	1st to 10th	Day	30% 10% 3 7% 4 3 4 4 5% 6% 23%	5886
	11th to 20th	Day	19% 10% 3 9% 5% 4 4 3 5% 6% 30%	6121
	21st to end	Day	19% 7% 3 7% 4 4 5% 4 6% 8% 32%	6949
June	1st to 10th	Day	18% 7% 2 5% 5% 5% 5% 4 7% 9% 33%	2800
	11th to 20th	Day	18% 7% 2 6% 5% 4 5% 4 7% 8% 35%	2800
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 6% 8% 33%	2800
July	1st to 10th	Day	12% 6% 3 6% 5% 5% 6% 5% 7% 9% 35%	4580
	11th to 20th	Day	12% 6% 2 6% 4 5% 6% 5% 7% 11% 37%	4580
	21st to end	Day	18% 8% 3 7% 4 4 4 4 6% 8% 33%	4814
August	1st to 10th	Day	16% 6% 3 7% 5% 5% 5% 5% 6% 8% 34%	4238
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 5% 7% 8% 34%	3927
	21st to end	Day	9% 4 2 6% 5% 5% 5% 4 8% 9% 43%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 5% 5% 5% 7% 44%	2280
	11th to 20th	Day	15% 6% 2 5% 3 3 3 3 5% 6% 50%	2114
	21st to end	Day	17% 6% 3 5% 4 4 4 3 5% 8% 42%	2090
October	1st to 10th	Day	23% 7% 2 7% 5% 4 3 3 4 5% 37%	2090
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 5% 7% 44%	1833
	21st to end	Day	20% 9% 3 7% 4 3 3 3 5% 6% 38%	1881
November	1st to 10th	Day	13% 8% 2 7% 4 3 3 3 4 5% 49%	1761
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 4 5% 46%	1800
	21st to end	Day	18% 9% 2 6% 4 4 4 4 5% 6% 40%	1770
December	1st to 10th	Day	14% 10% 3 7% 4 4 4 3 5% 6% 41%	2600
	11th to 20th	Day	19% 9% 3 5% 3 4 3 4 4 5% 41%	2986
	21st to end	Day	18% 11% 3 7% 4 4 4 3 4 6% 35%	3960
January	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	6349
	11th to 20th	Night	11% 9% 3 10% 6% 5% 5% 3 5% 7% 37%	6315
	21st to end	Night	15% 9% 3 7% 5% 5% 5% 4 5% 6% 38%	6996
February	1st to 10th	Night	15% 9% 3 7% 4 4 4 5% 6% 6% 37%	2555
	11th to 20th	Night	12% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	2338
	21st to end	Night	15% 9% 3 7% 4 5% 4 3 5% 7% 38%	1980
March	1st to 10th	Night	20% 9% 2 5% 4 4 4 3 5% 7% 36%	5364
	11th to 20th	Night	15% 8% 3 7% 5% 5% 5% 4 5% 7% 38%	5258
	21st to end	Night	23% 10% 3 5% 4 3 4 4 6% 5% 34%	5054
April	1st to 10th	Night	18% 9% 2 8% 5% 4 4 4 7% 6% 34%	2000
	11th to 20th	Night	19% 9% 2 5% 4 4 4 4 5% 6% 38%	1975
	21st to end	Night	19% 8% 3 7% 4 4 3 3 4 6% 39%	1811
May	1st to 10th	Night	30% 10% 3 7% 4 4 4 3 5% 6% 24%	4054
	11th to 20th	Night	15% 12% 4 9% 6% 5% 4 5% 6% 6% 28%	3815
	21st to end	Night	21% 9% 3 7% 5% 4 4 3 5% 7% 32%	3926
June	1st to 10th	Night	19% 8% 3 6% 5% 3 5% 4 4 6% 38%	1600
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 7% 6% 37%	1600
	21st to end	Night	16% 7% 2 5% 4 3 4 4 7% 8% 41%	1600
July	1st to 10th	Night	12% 7% 2 7% 5% 5% 5% 4 7% 9% 37%	2600
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 5% 8% 40%	2600
	21st to end	Night	16% 8% 3 7% 5% 4 4 3 5% 7% 39%	2739
August	1st to 10th	Night	22% 7% 2 6% 4 4 4 4 5% 6% 34%	2661
	11th to 20th	Night	13% 8% 3 9% 6% 4 4 4 5% 6% 36%	2896
	21st to end	Night	9% 4 1 4 3 3 3 3 6% 8% 57%	3476
September	1st to 10th	Night	13% 7% 2 5% 3 4 3 3 5% 7% 48%	1900
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 6% 52%	2065
	21st to end	Night	15% 6% 3 6% 5% 4 4 2 5% 6% 45%	2090
October	1st to 10th	Night	20% 7% 2 6% 4 4 4 3 4 6% 40%	2090
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 6% 6% 45%	2340
	21st to end	Night	20% 8% 3 6% 3 4 4 3 4 6% 38%	2717
November	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 4 5% 48%	2537
	11th to 20th	Night	16% 6% 2 5% 4 4 4 3 4 5% 45%	2600
	21st to end	Night	14% 8% 2 6% 4 4 4 3 5% 7% 43%	2625
December	1st to 10th	Night	16% 11% 3 6% 4 4 4 3 6% 6% 38%	4580
	11th to 20th	Night	22% 9% 3 6% 3 3 3 3 4 5% 38%	5208
	21st to end	Night	18% 10% 4 7% 4 3 3 3 5% 5% 38%	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	45% 5% 7% 27% 15% 2	3598
	11th to 20th	Day	42% 4 10% 22% 18% 3	3600
	21st to end	Day	43% 5% 8% 23% 19% 3	3960
February	1st to 10th	Day	49% 4 13% 19% 13% 3	1840
	11th to 20th	Day	45% 6% 10% 21% 15% 3	1957
	21st to end	Day	50% 3 9% 20% 15% 3	1650
March	1st to 10th	Day	51% 4 10% 14% 12% 9%	4479
	11th to 20th	Day	43% 4 15% 24% 11% 2	4699
	21st to end	Day	51% 3 6% 15% 19% 6%	5873
April	1st to 10th	Day	50% 4 8% 19% 16% 3	2400
	11th to 20th	Day	45% 4 10% 22% 16% 3	2374
	21st to end	Day	49% 4 7% 19% 17% 3	2587
May	1st to 10th	Day	60% 7% 10% 14% 8% 1	5897
	11th to 20th	Day	53% 6% 13% 17% 10% 1	6125
	21st to end	Day	47% 7% 11% 22% 12% 1	6949
June	1st to 10th	Day	46% 6% 12% 23% 12% 1	2800
	11th to 20th	Day	44% 6% 12% 21% 15% 2	2800
	21st to end	Day	48% 6% 11% 22% 13% 1	2800
July	1st to 10th	Day	42% 7% 13% 21% 16% 1	4510
	11th to 20th	Day	38% 7% 16% 25% 12% 1	4505
	21st to end	Day	47% 5% 12% 20% 14% 1	4661
August	1st to 10th	Day	46% 7% 15% 22% 10% 1	4224
	11th to 20th	Day	44% 7% 16% 25% 8% 1	3966
	21st to end	Day	34% 7% 12% 30% 15% 1	4180
September	1st to 10th	Day	38% 5% 12% 26% 18% 2	2280
	11th to 20th	Day	36% 4 7% 27% 24% 2	2114
	21st to end	Day	41% 5% 8% 23% 20% 3	2090
October	1st to 10th	Day	44% 6% 8% 23% 17% 2	3480
	11th to 20th	Day	33% 4 9% 28% 23% 4	3010
	21st to end	Day	35% 4 10% 31% 17% 4	3081
November	1st to 10th	Day	39% 4 13% 25% 17% 3	1761
	11th to 20th	Day	39% 4 9% 22% 21% 4	1800
	21st to end	Day	45% 4 14% 24% 12% 2	1770
December	1st to 10th	Day	42% 5% 14% 28% 10% 1	2600
	11th to 20th	Day	43% 4 12% 19% 20% 2	2986
	21st to end	Day	49% 4 8% 19% 14% 6%	3960
January	1st to 10th	Night	38% 6% 11% 29% 13% 2	6349
	11th to 20th	Night	46% 5% 12% 19% 14% 3	6360
	21st to end	Night	47% 4 9% 17% 18% 5%	6996
February	1st to 10th	Night	46% 5% 13% 21% 13% 2	2555
	11th to 20th	Night	43% 5% 11% 21% 16% 3	2340
	21st to end	Night	45% 3 11% 23% 14% 4	1980
March	1st to 10th	Night	47% 5% 9% 18% 17% 6%	5374
	11th to 20th	Night	45% 6% 16% 19% 12% 2	5259
	21st to end	Night	49% 4 6% 14% 22% 5%	5054
April	1st to 10th	Night	48% 4 10% 20% 16% 3	2000
	11th to 20th	Night	45% 5% 10% 21% 18% 2	1975
	21st to end	Night	47% 4 10% 19% 17% 4	1811
May	1st to 10th	Night	59% 6% 12% 14% 8% 1	4060
	11th to 20th	Night	52% 8% 12% 16% 9% 2	3816
	21st to end	Night	51% 6% 12% 18% 11% 1	3927
June	1st to 10th	Night	47% 5% 10% 25% 12% 1	1600
	11th to 20th	Night	44% 7% 13% 21% 13% 2	1600
	21st to end	Night	40% 8% 14% 23% 15% 1	1600
July	1st to 10th	Night	41% 7% 14% 23% 14% 1	2560
	11th to 20th	Night	40% 6% 15% 23% 15%	2552
	21st to end	Night	44% 5% 12% 22% 16% 1	2651
August	1st to 10th	Night	50% 6% 13% 20% 11% 1	2651
	11th to 20th	Night	46% 6% 14% 24% 10%	2923
	21st to end	Night	25% 5% 16% 34% 19% 1	3476
September	1st to 10th	Night	36% 5% 12% 28% 19% 1	1900
	11th to 20th	Night	34% 4 10% 26% 24% 2	2065
	21st to end	Night	40% 4 9% 27% 17% 3	2090
October	1st to 10th	Night	40% 4 15% 24% 15% 2	3480
	11th to 20th	Night	28% 5% 12% 26% 26% 4	3889
	21st to end	Night	35% 3 12% 33% 14% 2	4467
November	1st to 10th	Night	38% 5% 10% 23% 20% 3	2537
	11th to 20th	Night	41% 3 10% 22% 20% 3	2600
	21st to end	Night	41% 4 16% 25% 14% 1	2625
December	1st to 10th	Night	44% 4 12% 29% 10% 1	4580
	11th to 20th	Night	47% 4 12% 15% 20% 2	5208
	21st to end	Night	46% 4 8% 21% 16% 4	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	32% 28% 27% 13% 1	200
	11th to 20th	Day	36% 23% 31% 8% 2 1	200
	21st to end	Day	31% 27% 33% 8% 2	220
February	1st to 10th	Day	35% 32% 24% 8% 1	365
	11th to 20th	Day	39% 26% 27% 8%	496
	21st to end	Day	35% 22% 31% 10% 2	414
March	1st to 10th	Day	34% 22% 34% 9% 2	500
	11th to 20th	Day	31% 27% 30% 8% 3 1	500
	21st to end	Day	33% 23% 30% 9% 3 1	550
April	1st to 10th	Day	37% 21% 28% 9% 5%	500
	11th to 20th	Day	33% 21% 32% 10% 3 1	492
	21st to end	Day	32% 18% 31% 13% 4 1	500
May	1st to 10th	Day	38% 18% 26% 14% 4 1	503
	11th to 20th	Day	22% 19% 37% 16% 5% 1	527
	21st to end	Day	21% 13% 35% 23% 7% 1	573
June	1st to 10th	Day	22% 15% 35% 19% 7% 2	530
	11th to 20th	Day	22% 12% 37% 21% 7% 1	530
	21st to end	Day	21% 13% 37% 19% 7% 3	530
July	1st to 10th	Day	18% 10% 32% 28% 10% 2	525
	11th to 20th	Day	15% 6% 34% 32% 10% 2	524
	21st to end	Day	15% 10% 36% 28% 8% 2	532
August	1st to 10th	Day	14% 10% 33% 28% 13% 2	486
	11th to 20th	Day	18% 10% 30% 27% 12% 3	479
	21st to end	Day	14% 11% 33% 28% 10% 4	539
September	1st to 10th	Day	19% 12% 29% 23% 14% 4	490
	11th to 20th	Day	20% 11% 26% 22% 13% 8%	490
	21st to end	Day	20% 12% 33% 21% 8% 5%	490
October	1st to 10th	Day	30% 12% 26% 18% 10% 3	490
	11th to 20th	Day	24% 15% 32% 21% 5% 2	447
	21st to end	Day	31% 15% 32% 16% 4 1	429
November	1st to 10th	Day	26% 22% 28% 15% 8% 1	395
	11th to 20th	Day	34% 21% 28% 14% 2 1	400
	21st to end	Day	34% 19% 29% 14% 4	370
December	1st to 10th	Day	30% 29% 27% 12% 3	200
	11th to 20th	Day	41% 22% 31% 6% 1	200
	21st to end	Day	32% 23% 31% 11% 3	220
January	1st to 10th	Night	25% 28% 34% 11% 2	800
	11th to 20th	Night	31% 24% 30% 13% 3	800
	21st to end	Night	24% 24% 35% 16% 2	880
February	1st to 10th	Night	26% 25% 35% 14% 1	800
	11th to 20th	Night	25% 28% 36% 11% 1	783
	21st to end	Night	30% 26% 30% 12% 2	660
March	1st to 10th	Night	28% 23% 33% 14% 2	800
	11th to 20th	Night	26% 23% 33% 15% 3	800
	21st to end	Night	28% 21% 32% 15% 4	880
April	1st to 10th	Night	29% 19% 32% 17% 2	800
	11th to 20th	Night	27% 21% 31% 17% 4	791
	21st to end	Night	27% 19% 34% 17% 2	611
May	1st to 10th	Night	30% 13% 34% 20% 3	600
	11th to 20th	Night	21% 17% 36% 22% 4	504
	21st to end	Night	21% 9% 38% 27% 6%	433
June	1st to 10th	Night	22% 12% 34% 24% 8%	400
	11th to 20th	Night	20% 10% 33% 31% 5% 1	400
	21st to end	Night	17% 8% 37% 31% 7%	400
July	1st to 10th	Night	16% 7% 35% 33% 9%	390
	11th to 20th	Night	17% 4 29% 38% 10% 2	388
	21st to end	Night	13% 5% 35% 37% 10%	416
August	1st to 10th	Night	13% 4 34% 35% 13%	468
	11th to 20th	Night	16% 6% 30% 36% 11% 1	624
	21st to end	Night	17% 5% 27% 35% 15% 1	836
September	1st to 10th	Night	16% 8% 34% 31% 11% 1	760
	11th to 20th	Night	17% 8% 30% 27% 16% 3	760
	21st to end	Night	19% 11% 30% 26% 12% 2	760
October	1st to 10th	Night	25% 13% 28% 22% 10% 1	760
	11th to 20th	Night	21% 15% 32% 23% 8% 1	760
	21st to end	Night	28% 18% 30% 19% 5%	836
November	1st to 10th	Night	20% 22% 33% 17% 7% 1	780
	11th to 20th	Night	24% 25% 29% 15% 6% 1	800
	21st to end	Night	27% 22% 33% 14% 4	800
December	1st to 10th	Night	30% 22% 32% 12% 4	800
	11th to 20th	Night	29% 24% 30% 14% 3	800
	21st to end	Night	31% 24% 28% 15% 2	880

Page generated at 2025-07-18 01:10:03. Version ac07

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