Leominster
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Leominster clear sky chart from 2005-11-06 to 2025-10-19.

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% 2 6% 4 4 4 4 5% 7% 39%	126379
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 41%	123323
Transparency	Day	43% 6% 12% 22% 15% 2	129899
	Night	42% 6% 12% 22% 16% 3	127890
Seeing	Day	23% 17% 32% 19% 8% 2	16231
	Night	22% 15% 34% 23% 6%	25103

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	11% 7% 2 9% 5% 5% 5% 4 5% 6% 40%	3597
	11th to 20th	Day	13% 7% 3 6% 4 4 4 3 4 6% 46%	3570
	21st to end	Day	16% 9% 3 5% 4 3 4 3 4 6% 43%	3960
February	1st to 10th	Day	17% 7% 2 7% 6% 4 6% 3 5% 7% 36%	1853
	11th to 20th	Day	12% 8% 3 7% 4 4 6% 4 6% 7% 38%	1948
	21st to end	Day	18% 10% 2 6% 5% 4 4 3 6% 4 38%	1650
March	1st to 10th	Day	19% 11% 2 6% 4 4 4 4 5% 5% 35%	4476
	11th to 20th	Day	15% 8% 2 5% 4 4 5% 4 5% 7% 40%	4736
	21st to end	Day	23% 9% 3 5% 3 3 4 3 6% 7% 33%	5832
April	1st to 10th	Day	17% 10% 2 6% 6% 4 4 3 4 6% 36%	2400
	11th to 20th	Day	17% 6% 2 6% 4 3 5% 4 5% 7% 39%	2374
	21st to end	Day	20% 6% 2 7% 5% 4 5% 3 5% 6% 37%	2595
May	1st to 10th	Day	30% 10% 2 7% 4 4 4 4 5% 6% 25%	5886
	11th to 20th	Day	21% 8% 3 8% 4 5% 4 4 4 6% 32%	6058
	21st to end	Day	19% 6% 2 6% 5% 4 5% 4 6% 8% 34%	6949
June	1st to 10th	Day	19% 5% 2 6% 4 4 5% 4 7% 8% 34%	2800
	11th to 20th	Day	17% 5% 2 5% 5% 4 4 5% 7% 9% 37%	2800
	21st to end	Day	17% 7% 3 7% 5% 4 5% 4 6% 8% 35%	2800
July	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 9% 34%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 7% 11% 39%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 8% 43%	6926
August	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 5% 7% 9% 35%	4339
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 4 5% 9% 38%	4077
	21st to end	Day	8% 5% 2 6% 4 4 5% 5% 7% 10% 46%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 4 5% 3 6% 7% 46%	2400
	11th to 20th	Day	15% 5% 2 5% 3 3 4 2 5% 6% 51%	2215
	21st to end	Day	16% 5% 2 6% 4 5% 4 3 5% 7% 43%	2200
October	1st to 10th	Day	22% 6% 1 5% 4 4 4 3 4 5% 42%	2200
	11th to 20th	Day	14% 6% 2 5% 4 3 4 3 5% 8% 46%	1923
	21st to end	Day	21% 6% 3 6% 4 3 4 3 4 6% 40%	1881
November	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 5% 4 50%	1761
	11th to 20th	Day	18% 7% 2 4 4 3 3 3 4 5% 48%	1800
	21st to end	Day	17% 7% 2 5% 5% 4 4 3 5% 5% 42%	1795
December	1st to 10th	Day	15% 8% 2 5% 3 4 4 4 5% 6% 45%	2600
	11th to 20th	Day	19% 7% 2 4 3 3 4 3 5% 6% 45%	2986
	21st to end	Day	18% 11% 3 6% 4 3 4 3 4 6% 38%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 6% 7% 47%	6349
	11th to 20th	Night	12% 9% 3 9% 5% 4 4 3 5% 6% 41%	6315
	21st to end	Night	15% 9% 3 7% 5% 5% 4 3 5% 6% 39%	6996
February	1st to 10th	Night	15% 9% 3 6% 4 4 4 4 6% 6% 39%	2543
	11th to 20th	Night	12% 8% 3 6% 4 4 5% 4 6% 7% 40%	2338
	21st to end	Night	15% 9% 3 6% 5% 4 4 3 5% 6% 40%	1980
March	1st to 10th	Night	22% 9% 2 5% 3 3 3 3 4 6% 39%	5364
	11th to 20th	Night	15% 9% 3 6% 4 4 4 3 5% 7% 39%	5222
	21st to end	Night	24% 9% 2 6% 3 3 4 3 5% 6% 35%	5082
April	1st to 10th	Night	18% 8% 2 6% 5% 5% 4 4 6% 7% 36%	2000
	11th to 20th	Night	20% 6% 2 6% 4 3 4 4 5% 7% 39%	1975
	21st to end	Night	20% 7% 2 7% 4 3 3 3 5% 6% 40%	1805
May	1st to 10th	Night	33% 9% 3 6% 3 4 3 3 4 7% 26%	4054
	11th to 20th	Night	17% 9% 3 8% 6% 4 5% 5% 6% 7% 30%	3864
	21st to end	Night	20% 8% 3 7% 4 5% 4 4 6% 6% 33%	3926
June	1st to 10th	Night	19% 7% 3 7% 5% 4 2 3 5% 7% 38%	1600
	11th to 20th	Night	19% 7% 2 6% 5% 4 4 3 6% 8% 37%	1600
	21st to end	Night	15% 6% 2 6% 4 4 5% 3 7% 8% 42%	1600
July	1st to 10th	Night	12% 6% 2 7% 4 4 5% 4 7% 10% 39%	3600
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 4 6% 9% 42%	3600
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 4 7% 48%	3926
August	1st to 10th	Night	19% 6% 2 7% 4 4 6% 4 6% 8% 34%	2778
	11th to 20th	Night	12% 6% 3 7% 5% 6% 6% 4 6% 7% 37%	2966
	21st to end	Night	9% 4 1 5% 3 3 3 3 5% 7% 58%	3586
September	1st to 10th	Night	13% 6% 2 4 4 4 4 3 6% 7% 49%	2000
	11th to 20th	Night	12% 6% 2 4 3 4 3 2 5% 6% 53%	2185
	21st to end	Night	17% 5% 2 6% 3 3 4 3 5% 6% 45%	2200
October	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 4 5% 45%	2200
	11th to 20th	Night	15% 5% 1 5% 3 3 4 3 5% 7% 47%	2430
	21st to end	Night	22% 6% 2 5% 4 4 3 3 5% 7% 39%	2717
November	1st to 10th	Night	14% 6% 1 6% 4 4 3 4 4 5% 49%	2537
	11th to 20th	Night	17% 7% 2 4 4 3 4 3 4 5% 47%	2600
	21st to end	Night	15% 7% 2 5% 4 4 4 4 5% 7% 45%	2601
December	1st to 10th	Night	16% 9% 2 5% 4 4 4 3 5% 6% 43%	4580
	11th to 20th	Night	22% 8% 2 6% 3 3 3 3 5% 5% 41%	5208
	21st to end	Night	18% 9% 3 5% 4 4 3 3 5% 5% 41%	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	41% 6% 9% 27% 15% 2	3598
	11th to 20th	Day	40% 5% 10% 22% 18% 5%	3600
	21st to end	Day	42% 5% 10% 21% 20% 3	3960
February	1st to 10th	Day	46% 6% 13% 20% 12% 2	1853
	11th to 20th	Day	44% 6% 11% 20% 15% 3	1957
	21st to end	Day	48% 5% 9% 20% 15% 3	1650
March	1st to 10th	Day	50% 4 12% 13% 11% 9%	4479
	11th to 20th	Day	41% 6% 15% 26% 11% 2	4736
	21st to end	Day	50% 4 7% 16% 16% 6%	5832
April	1st to 10th	Day	48% 5% 9% 19% 16% 4	2400
	11th to 20th	Day	42% 7% 11% 20% 18% 3	2374
	21st to end	Day	48% 5% 9% 18% 17% 3	2595
May	1st to 10th	Day	58% 7% 13% 13% 8% 1	5897
	11th to 20th	Day	51% 7% 14% 17% 10% 1	6062
	21st to end	Day	45% 7% 12% 21% 13% 1	6949
June	1st to 10th	Day	44% 8% 13% 23% 11% 1	2800
	11th to 20th	Day	42% 9% 13% 20% 14% 2	2800
	21st to end	Day	45% 8% 12% 21% 13% 1	2800
July	1st to 10th	Day	45% 9% 12% 21% 13% 1	6290
	11th to 20th	Day	37% 9% 17% 25% 12% 1	6285
	21st to end	Day	39% 5% 12% 27% 15% 1	6728
August	1st to 10th	Day	44% 9% 16% 21% 9% 1	4325
	11th to 20th	Day	42% 8% 15% 25% 9% 1	4116
	21st to end	Day	31% 8% 13% 31% 17% 1	4312
September	1st to 10th	Day	37% 7% 11% 25% 19% 2	2400
	11th to 20th	Day	35% 4 9% 26% 23% 3	2215
	21st to end	Day	41% 5% 9% 22% 20% 2	2200
October	1st to 10th	Day	39% 7% 9% 22% 20% 3	3590
	11th to 20th	Day	30% 4 9% 28% 25% 4	3113
	21st to end	Day	33% 5% 11% 30% 18% 3	3081
November	1st to 10th	Day	38% 5% 13% 25% 17% 2	1761
	11th to 20th	Day	38% 5% 10% 22% 23% 3	1800
	21st to end	Day	43% 6% 14% 24% 12% 2	1795
December	1st to 10th	Day	39% 5% 15% 28% 12% 1	2600
	11th to 20th	Day	40% 5% 14% 18% 20% 3	2986
	21st to end	Day	48% 5% 9% 19% 15% 5%	3960
January	1st to 10th	Night	35% 5% 12% 31% 14% 2	6349
	11th to 20th	Night	44% 5% 13% 20% 13% 4	6360
	21st to end	Night	47% 5% 10% 16% 17% 5%	6996
February	1st to 10th	Night	45% 6% 14% 20% 13% 2	2543
	11th to 20th	Night	43% 6% 11% 20% 16% 3	2340
	21st to end	Night	45% 5% 9% 23% 15% 4	1980
March	1st to 10th	Night	47% 4 10% 17% 15% 6%	5374
	11th to 20th	Night	44% 6% 17% 18% 12% 2	5223
	21st to end	Night	50% 4 6% 13% 22% 6%	5082
April	1st to 10th	Night	47% 6% 10% 19% 15% 3	2000
	11th to 20th	Night	44% 6% 10% 19% 19% 2	1975
	21st to end	Night	45% 6% 10% 19% 16% 4	1805
May	1st to 10th	Night	58% 7% 13% 12% 8% 1	4060
	11th to 20th	Night	51% 10% 13% 15% 9% 2	3865
	21st to end	Night	50% 7% 13% 18% 12%	3927
June	1st to 10th	Night	46% 6% 12% 22% 13% 1	1600
	11th to 20th	Night	45% 7% 13% 20% 13% 1	1600
	21st to end	Night	38% 10% 14% 23% 14% 1	1600
July	1st to 10th	Night	41% 7% 13% 23% 16%	3560
	11th to 20th	Night	38% 9% 19% 19% 14% 1	3552
	21st to end	Night	36% 6% 12% 30% 15% 1	3809
August	1st to 10th	Night	48% 9% 12% 20% 10% 1	2768
	11th to 20th	Night	42% 9% 14% 24% 10%	2993
	21st to end	Night	25% 7% 15% 32% 20% 1	3586
September	1st to 10th	Night	36% 6% 13% 25% 19% 1	2000
	11th to 20th	Night	32% 6% 10% 24% 26% 2	2185
	21st to end	Night	40% 6% 8% 25% 18% 3	2200
October	1st to 10th	Night	36% 6% 16% 24% 16% 2	3590
	11th to 20th	Night	27% 4 11% 27% 28% 4	3979
	21st to end	Night	34% 4 12% 32% 16% 2	4467
November	1st to 10th	Night	37% 6% 11% 21% 21% 4	2537
	11th to 20th	Night	40% 4 11% 21% 20% 4	2600
	21st to end	Night	40% 6% 14% 25% 14% 1	2601
December	1st to 10th	Night	43% 5% 12% 29% 11%	4580
	11th to 20th	Night	45% 5% 14% 15% 20% 1	5208
	21st to end	Night	45% 5% 9% 21% 16% 3	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	31% 26% 26% 15% 3	200
	11th to 20th	Day	33% 25% 31% 9% 3	200
	21st to end	Day	29% 25% 32% 11% 3	220
February	1st to 10th	Day	32% 25% 30% 11% 1	381
	11th to 20th	Day	30% 30% 30% 9% 1	496
	21st to end	Day	32% 19% 35% 10% 3	414
March	1st to 10th	Day	31% 23% 33% 10% 3	500
	11th to 20th	Day	28% 26% 29% 13% 3 1	500
	21st to end	Day	32% 21% 32% 10% 4 1	550
April	1st to 10th	Day	33% 22% 28% 11% 5% 1	500
	11th to 20th	Day	32% 22% 30% 10% 5%	492
	21st to end	Day	29% 19% 32% 15% 4 2	500
May	1st to 10th	Day	32% 21% 27% 15% 4 1	503
	11th to 20th	Day	22% 17% 40% 15% 4 1	527
	21st to end	Day	18% 15% 36% 21% 8% 1	573
June	1st to 10th	Day	18% 14% 37% 21% 8% 2	530
	11th to 20th	Day	19% 14% 35% 23% 7% 1	530
	21st to end	Day	19% 14% 37% 21% 7% 3	530
July	1st to 10th	Day	17% 11% 29% 29% 12% 3	525
	11th to 20th	Day	14% 7% 32% 31% 13% 3	524
	21st to end	Day	13% 9% 37% 30% 10% 2	576
August	1st to 10th	Day	10% 10% 32% 31% 14% 2	526
	11th to 20th	Day	17% 7% 28% 29% 16% 3	519
	21st to end	Day	10% 10% 35% 27% 14% 4	583
September	1st to 10th	Day	17% 12% 29% 24% 13% 6%	530
	11th to 20th	Day	17% 9% 28% 22% 16% 8%	530
	21st to end	Day	19% 12% 31% 23% 9% 6%	530
October	1st to 10th	Day	25% 14% 23% 22% 12% 4	530
	11th to 20th	Day	20% 18% 29% 23% 8% 2	473
	21st to end	Day	28% 14% 32% 18% 7% 1	429
November	1st to 10th	Day	22% 18% 31% 19% 9% 1	395
	11th to 20th	Day	30% 21% 27% 17% 4 1	400
	21st to end	Day	30% 19% 30% 14% 6%	395
December	1st to 10th	Day	23% 30% 28% 16% 4	200
	11th to 20th	Day	36% 26% 30% 7% 3	200
	21st to end	Day	29% 20% 34% 11% 5%	220
January	1st to 10th	Night	24% 21% 39% 15% 1	800
	11th to 20th	Night	27% 20% 34% 17% 2	800
	21st to end	Night	20% 21% 39% 17% 3	880
February	1st to 10th	Night	24% 20% 38% 17% 1	800
	11th to 20th	Night	23% 23% 40% 14% 1	783
	21st to end	Night	26% 23% 34% 15% 2	660
March	1st to 10th	Night	25% 22% 35% 17% 2	800
	11th to 20th	Night	22% 23% 35% 17% 3	800
	21st to end	Night	25% 20% 33% 18% 4	880
April	1st to 10th	Night	26% 20% 32% 19% 3	800
	11th to 20th	Night	24% 21% 33% 18% 5%	791
	21st to end	Night	25% 21% 32% 20% 2	605
May	1st to 10th	Night	30% 15% 33% 20% 2	600
	11th to 20th	Night	19% 18% 38% 21% 4	527
	21st to end	Night	20% 9% 37% 29% 5%	433
June	1st to 10th	Night	20% 7% 37% 28% 7%	400
	11th to 20th	Night	19% 11% 34% 31% 5% 1	400
	21st to end	Night	15% 4 42% 35% 6%	400
July	1st to 10th	Night	15% 6% 31% 38% 9% 1	390
	11th to 20th	Night	12% 5% 31% 40% 11% 1	388
	21st to end	Night	12% 4 34% 41% 9%	438
August	1st to 10th	Night	12% 4 31% 41% 12%	512
	11th to 20th	Night	13% 5% 30% 37% 14% 1	647
	21st to end	Night	16% 5% 26% 37% 15% 1	880
September	1st to 10th	Night	16% 6% 32% 33% 13% 1	800
	11th to 20th	Night	15% 8% 27% 29% 19% 2	800
	21st to end	Night	19% 10% 29% 29% 12% 2	800
October	1st to 10th	Night	23% 11% 27% 25% 11% 2	800
	11th to 20th	Night	19% 14% 32% 23% 11% 1	793
	21st to end	Night	27% 15% 32% 20% 6%	836
November	1st to 10th	Night	18% 16% 34% 23% 7% 1	780
	11th to 20th	Night	24% 20% 32% 18% 6% 1	800
	21st to end	Night	23% 20% 35% 17% 4 1	800
December	1st to 10th	Night	26% 20% 34% 16% 4	800
	11th to 20th	Night	26% 19% 34% 18% 2	800
	21st to end	Night	26% 18% 36% 16% 3	880

Page generated at 2025-10-16 01:50:25. Version ac07

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