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 2024-04-03.

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	16% 7% 3 6% 5% 3 5% 5% 5% 7% 38%	78680
	Night	16% 8% 3 6% 5% 3 4 4 5% 7% 39%	81432
Transparency	Day	45% 5% 11% 23% 14% 2	78385
	Night	44% 5% 12% 23% 14% 2	81261
Seeing	Day	26% 17% 31% 18% 6% 2	14119
	Night	24% 18% 32% 20% 6% 1	23222

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% 8% 3 7% 6% 3 4 4 5% 6% 41%	2517
	11th to 20th	Day	15% 8% 4 8% 6% 3 5% 5% 4 6% 36%	2520
	21st to end	Day	18% 14% 4 7% 4 3 4 4 5% 5% 32%	2772
February	1st to 10th	Day	17% 7% 4 6% 6% 4 6% 6% 5% 7% 34%	1747
	11th to 20th	Day	12% 7% 3 7% 7% 3 5% 5% 5% 8% 37%	1848
	21st to end	Day	17% 11% 4 6% 6% 4 4 4 4 4 36%	1570
March	1st to 10th	Day	15% 9% 3 5% 7% 4 5% 4 5% 7% 37%	1897
	11th to 20th	Day	15% 8% 3 6% 6% 4 5% 5% 4 6% 37%	1971
	21st to end	Day	18% 8% 4 6% 5% 2 4 3 4 6% 39%	2465
April	1st to 10th	Day	16% 10% 2 7% 6% 3 4 5% 5% 5% 36%	2184
	11th to 20th	Day	16% 8% 3 5% 6% 3 5% 5% 6% 7% 37%	2134
	21st to end	Day	20% 7% 3 7% 6% 4 4 4 5% 5% 33%	2327
May	1st to 10th	Day	23% 8% 3 6% 5% 3 4 5% 4 7% 31%	2340
	11th to 20th	Day	18% 7% 3 6% 5% 3 4 3 5% 6% 40%	2393
	21st to end	Day	16% 6% 2 5% 5% 3 4 5% 5% 8% 40%	2725
June	1st to 10th	Day	19% 6% 3 4 6% 4 5% 5% 7% 8% 33%	2520
	11th to 20th	Day	18% 6% 3 5% 5% 3 5% 5% 6% 9% 35%	2520
	21st to end	Day	17% 6% 3 7% 6% 4 5% 5% 6% 9% 32%	2520
July	1st to 10th	Day	14% 6% 3 5% 5% 4 6% 5% 6% 8% 38%	2520
	11th to 20th	Day	13% 5% 2 6% 5% 3 6% 6% 6% 9% 38%	2520
	21st to end	Day	13% 6% 3 6% 5% 3 5% 6% 7% 8% 37%	2744
August	1st to 10th	Day	12% 5% 2 5% 6% 4 6% 7% 7% 8% 38%	2404
	11th to 20th	Day	13% 5% 4 6% 7% 4 5% 5% 6% 9% 36%	2242
	21st to end	Day	13% 5% 2 5% 5% 3 5% 5% 7% 8% 42%	2376
September	1st to 10th	Day	13% 6% 2 5% 5% 3 5% 5% 5% 8% 43%	2160
	11th to 20th	Day	15% 6% 2 4 3 2 3 3 4 6% 50%	2003
	21st to end	Day	17% 5% 3 4 4 3 4 4 4 8% 44%	1980
October	1st to 10th	Day	24% 7% 2 6% 5% 2 4 3 4 5% 37%	1980
	11th to 20th	Day	14% 6% 2 5% 5% 3 4 4 6% 7% 43%	1738
	21st to end	Day	21% 8% 3 7% 5% 2 3 3 5% 6% 37%	1782
November	1st to 10th	Day	14% 7% 3 6% 4 2 3 4 4 5% 48%	1671
	11th to 20th	Day	19% 6% 2 4 4 2 4 3 4 6% 45%	1710
	21st to end	Day	17% 8% 2 6% 5% 3 4 4 5% 6% 40%	1682
December	1st to 10th	Day	18% 9% 3 7% 6% 4 5% 4 5% 6% 35%	1520
	11th to 20th	Day	22% 7% 2 4 4 3 3 5% 4 6% 40%	1906
	21st to end	Day	20% 12% 3 6% 5% 3 5% 4 5% 7% 30%	2772
January	1st to 10th	Night	16% 7% 3 5% 4 3 4 4 5% 6% 44%	4429
	11th to 20th	Night	13% 8% 3 8% 8% 3 5% 4 4 7% 36%	4440
	21st to end	Night	21% 11% 4 7% 6% 3 5% 4 4 5% 29%	4884
February	1st to 10th	Night	15% 9% 3 6% 5% 3 5% 5% 6% 6% 37%	2428
	11th to 20th	Night	12% 7% 4 6% 5% 4 5% 5% 6% 7% 39%	2218
	21st to end	Night	16% 8% 4 6% 5% 3 4 4 5% 7% 39%	1884
March	1st to 10th	Night	16% 8% 3 5% 6% 2 5% 4 4 7% 41%	2270
	11th to 20th	Night	14% 7% 3 6% 6% 4 6% 6% 5% 7% 37%	2206
	21st to end	Night	16% 9% 3 5% 4 2 4 5% 6% 5% 40%	2128
April	1st to 10th	Night	17% 8% 2 6% 6% 3 5% 5% 6% 6% 36%	1821
	11th to 20th	Night	19% 6% 3 4 4 3 4 4 5% 7% 39%	1775
	21st to end	Night	20% 7% 3 6% 5% 3 4 3 4 6% 38%	1631
May	1st to 10th	Night	20% 9% 4 6% 5% 4 5% 4 5% 6% 30%	1620
	11th to 20th	Night	18% 7% 3 6% 5% 3 4 4 6% 7% 37%	1515
	21st to end	Night	17% 7% 3 5% 6% 3 4 4 5% 8% 38%	1550
June	1st to 10th	Night	20% 7% 4 6% 5% 3 4 5% 4 6% 37%	1440
	11th to 20th	Night	19% 7% 3 5% 5% 3 4 4 6% 6% 37%	1440
	21st to end	Night	15% 6% 2 4 5% 2 3 5% 6% 8% 43%	1440
July	1st to 10th	Night	14% 5% 2 6% 5% 3 6% 4 6% 8% 41%	1440
	11th to 20th	Night	14% 6% 2 5% 5% 3 5% 6% 6% 9% 39%	1440
	21st to end	Night	12% 4 2 4 5% 3 5% 4 5% 9% 45%	1574
August	1st to 10th	Night	10% 6% 2 5% 5% 3 5% 6% 6% 7% 45%	1517
	11th to 20th	Night	14% 5% 2 6% 8% 3 4 6% 6% 7% 36%	1654
	21st to end	Night	15% 6% 2 4 4 2 3 3 6% 9% 46%	1980
September	1st to 10th	Night	13% 6% 2 5% 4 2 4 4 5% 8% 47%	1800
	11th to 20th	Night	13% 5% 2 5% 4 2 4 3 4 6% 52%	1956
	21st to end	Night	15% 5% 3 5% 5% 3 3 4 5% 6% 46%	1980
October	1st to 10th	Night	21% 7% 3 5% 5% 3 5% 3 4 5% 40%	1980
	11th to 20th	Night	15% 6% 2 4 5% 3 3 5% 6% 6% 44%	2216
	21st to end	Night	21% 8% 3 6% 4 3 4 4 4 7% 37%	2574
November	1st to 10th	Night	15% 6% 3 5% 4 3 4 4 4 5% 47%	2407
	11th to 20th	Night	17% 6% 3 5% 5% 3 4 4 4 6% 44%	2470
	21st to end	Night	15% 7% 3 5% 4 3 4 4 5% 7% 43%	2493
December	1st to 10th	Night	18% 10% 3 5% 5% 2 4 5% 5% 7% 36%	2660
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 5% 5% 6% 38%	3288
	21st to end	Night	20% 11% 4 7% 5% 3 4 4 5% 6% 33%	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	43% 3 8% 35% 11% 2	2518
	11th to 20th	Day	48% 5% 10% 25% 11% 2	2520
	21st to end	Day	53% 5% 9% 22% 10% 2	2772
February	1st to 10th	Day	50% 4 13% 19% 12% 3	1747
	11th to 20th	Day	45% 6% 10% 21% 16% 3	1857
	21st to end	Day	51% 3 9% 20% 15% 3	1570
March	1st to 10th	Day	47% 4 12% 20% 14% 4	1900
	11th to 20th	Day	46% 3 12% 20% 15% 4	1971
	21st to end	Day	48% 3 7% 18% 18% 6%	2465
April	1st to 10th	Day	49% 4 8% 20% 16% 3	2184
	11th to 20th	Day	44% 4 10% 22% 16% 3	2134
	21st to end	Day	51% 4 7% 18% 16% 3	2327
May	1st to 10th	Day	53% 3 8% 21% 13% 2	2340
	11th to 20th	Day	46% 4 8% 21% 18% 3	2397
	21st to end	Day	41% 6% 14% 24% 13% 2	2725
June	1st to 10th	Day	47% 7% 13% 23% 11% 1	2520
	11th to 20th	Day	44% 6% 12% 20% 15% 2	2520
	21st to end	Day	48% 6% 11% 22% 11% 1	2520
July	1st to 10th	Day	42% 6% 11% 24% 16% 2	2450
	11th to 20th	Day	41% 7% 13% 27% 12% 1	2445
	21st to end	Day	41% 6% 14% 23% 15% 2	2591
August	1st to 10th	Day	40% 7% 15% 24% 12% 1	2390
	11th to 20th	Day	42% 6% 14% 24% 12% 2	2242
	21st to end	Day	37% 7% 11% 26% 16% 2	2376
September	1st to 10th	Day	39% 5% 12% 25% 17% 2	2160
	11th to 20th	Day	36% 4 7% 26% 24% 2	2003
	21st to end	Day	39% 5% 9% 24% 21% 3	1980
October	1st to 10th	Day	50% 4 8% 21% 13% 4	1980
	11th to 20th	Day	38% 4 12% 30% 13% 2	1738
	21st to end	Day	49% 4 10% 20% 13% 3	1782
November	1st to 10th	Day	40% 3 12% 25% 17% 3	1671
	11th to 20th	Day	41% 4 10% 23% 19% 3	1710
	21st to end	Day	44% 4 15% 23% 12% 2	1682
December	1st to 10th	Day	49% 4 14% 16% 15% 1	1520
	11th to 20th	Day	44% 3 12% 20% 19% 2	1906
	21st to end	Day	53% 5% 11% 19% 11% 1	2772
January	1st to 10th	Night	40% 5% 12% 28% 12% 3	4429
	11th to 20th	Night	47% 5% 11% 22% 12% 3	4440
	21st to end	Night	58% 4 9% 17% 11% 2	4884
February	1st to 10th	Night	46% 5% 13% 21% 13% 2	2428
	11th to 20th	Night	43% 5% 11% 21% 16% 3	2220
	21st to end	Night	45% 3 11% 24% 13% 4	1884
March	1st to 10th	Night	44% 4 9% 25% 14% 3	2280
	11th to 20th	Night	46% 5% 11% 18% 16% 4	2207
	21st to end	Night	44% 4 10% 17% 20% 5%	2128
April	1st to 10th	Night	46% 5% 10% 21% 16% 2	1821
	11th to 20th	Night	44% 4 11% 22% 18% 2	1775
	21st to end	Night	48% 4 10% 20% 14% 4	1631
May	1st to 10th	Night	53% 4 9% 21% 11% 3	1620
	11th to 20th	Night	46% 5% 12% 19% 15% 3	1516
	21st to end	Night	45% 6% 12% 23% 13% 1	1551
June	1st to 10th	Night	47% 6% 10% 25% 12% 1	1440
	11th to 20th	Night	46% 7% 12% 21% 13% 2	1440
	21st to end	Night	38% 8% 15% 23% 15% 1	1440
July	1st to 10th	Night	39% 7% 14% 24% 15% 1	1400
	11th to 20th	Night	40% 6% 16% 27% 11%	1392
	21st to end	Night	35% 6% 16% 28% 13% 2	1486
August	1st to 10th	Night	35% 7% 16% 25% 16% 1	1507
	11th to 20th	Night	43% 7% 15% 22% 12%	1654
	21st to end	Night	35% 5% 11% 29% 19% 1	1980
September	1st to 10th	Night	37% 5% 13% 27% 18% 1	1800
	11th to 20th	Night	34% 4 10% 26% 24% 2	1956
	21st to end	Night	38% 4 9% 27% 18% 3	1980
October	1st to 10th	Night	46% 3 12% 23% 12% 3	1980
	11th to 20th	Night	38% 5% 15% 28% 14% 1	2216
	21st to end	Night	49% 3 12% 22% 11% 2	2574
November	1st to 10th	Night	39% 5% 10% 22% 20% 3	2407
	11th to 20th	Night	42% 3 11% 23% 19% 3	2470
	21st to end	Night	41% 3 16% 25% 14% 1	2493
December	1st to 10th	Night	47% 4 13% 23% 13% 1	2660
	11th to 20th	Night	45% 5% 13% 20% 16% 2	3288
	21st to end	Night	50% 5% 10% 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	31% 25% 28% 14% 1	180
	11th to 20th	Day	37% 22% 30% 8% 2 1	180
	21st to end	Day	27% 27% 35% 9% 2	198
February	1st to 10th	Day	34% 31% 25% 9% 1	332
	11th to 20th	Day	37% 26% 29% 8%	456
	21st to end	Day	36% 21% 31% 10% 2	382
March	1st to 10th	Day	32% 21% 35% 9% 2	460
	11th to 20th	Day	31% 27% 29% 8% 3 1	460
	21st to end	Day	32% 23% 30% 9% 4 1	506
April	1st to 10th	Day	35% 21% 29% 10% 5%	428
	11th to 20th	Day	29% 20% 34% 11% 4 1	412
	21st to end	Day	34% 17% 31% 13% 5% 1	420
May	1st to 10th	Day	37% 19% 25% 13% 5% 1	423
	11th to 20th	Day	22% 19% 38% 15% 4 2	447
	21st to end	Day	21% 12% 35% 24% 8% 1	485
June	1st to 10th	Day	22% 14% 34% 20% 7% 2	450
	11th to 20th	Day	21% 12% 38% 21% 7% 1	450
	21st to end	Day	19% 13% 37% 20% 7% 3	450
July	1st to 10th	Day	19% 8% 30% 29% 11% 2	445
	11th to 20th	Day	17% 6% 33% 32% 10% 2	444
	21st to end	Day	15% 9% 36% 30% 8% 2	488
August	1st to 10th	Day	13% 10% 33% 28% 14% 2	446
	11th to 20th	Day	17% 9% 31% 28% 12% 3	439
	21st to end	Day	15% 11% 32% 29% 9% 4	495
September	1st to 10th	Day	20% 11% 28% 24% 14% 4	450
	11th to 20th	Day	22% 11% 27% 21% 13% 6%	450
	21st to end	Day	20% 12% 33% 22% 8% 5%	450
October	1st to 10th	Day	32% 12% 25% 18% 11% 3	450
	11th to 20th	Day	22% 15% 34% 22% 6% 1	412
	21st to end	Day	33% 15% 30% 16% 5% 1	396
November	1st to 10th	Day	25% 21% 28% 16% 8% 1	365
	11th to 20th	Day	34% 21% 27% 15% 2 1	370
	21st to end	Day	31% 20% 30% 14% 4	342
December	1st to 10th	Day	30% 29% 26% 12% 3	180
	11th to 20th	Day	39% 22% 33% 5% 1	180
	21st to end	Day	34% 23% 31% 8% 3 1	198
January	1st to 10th	Night	25% 27% 35% 11% 2	760
	11th to 20th	Night	31% 24% 29% 13% 2	760
	21st to end	Night	24% 23% 35% 16% 2	836
February	1st to 10th	Night	25% 24% 35% 14% 1	760
	11th to 20th	Night	24% 28% 36% 11% 1	743
	21st to end	Night	29% 26% 29% 13% 2	628
March	1st to 10th	Night	27% 22% 34% 14% 2	760
	11th to 20th	Night	26% 24% 34% 14% 2	760
	21st to end	Night	28% 22% 31% 15% 4	836
April	1st to 10th	Night	28% 19% 32% 19% 3	729
	11th to 20th	Night	25% 21% 32% 18% 4	711
	21st to end	Night	28% 18% 34% 17% 2	551
May	1st to 10th	Night	29% 13% 35% 20% 3	540
	11th to 20th	Night	21% 17% 36% 22% 4	454
	21st to end	Night	20% 8% 39% 28% 6% 1	389
June	1st to 10th	Night	23% 11% 35% 24% 8%	360
	11th to 20th	Night	21% 9% 34% 31% 5% 1	360
	21st to end	Night	16% 6% 39% 32% 7%	360
July	1st to 10th	Night	17% 5% 34% 34% 10%	350
	11th to 20th	Night	17% 4 29% 39% 9% 2	348
	21st to end	Night	11% 5% 36% 38% 10%	394
August	1st to 10th	Night	12% 4 35% 36% 13%	443
	11th to 20th	Night	16% 5% 30% 37% 11% 1	590
	21st to end	Night	18% 6% 27% 34% 15% 1	792
September	1st to 10th	Night	16% 8% 33% 31% 10% 1	720
	11th to 20th	Night	17% 8% 31% 26% 15% 3	720
	21st to end	Night	18% 10% 31% 26% 12% 2	720
October	1st to 10th	Night	26% 13% 28% 21% 10% 2	720
	11th to 20th	Night	21% 15% 33% 23% 8% 1	720
	21st to end	Night	29% 18% 29% 19% 5%	792
November	1st to 10th	Night	21% 21% 33% 17% 7% 1	740
	11th to 20th	Night	24% 25% 29% 15% 6% 1	760
	21st to end	Night	26% 23% 33% 14% 4	760
December	1st to 10th	Night	30% 22% 32% 12% 3	760
	11th to 20th	Night	28% 24% 31% 14% 3	760
	21st to end	Night	32% 24% 28% 15% 1	836

Page generated at 2024-03-31 06:08:53. Version ac07

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