Chelmsford High School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chelmsford High School 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% 38%	126336
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	123382
Transparency	Day	43% 7% 12% 21% 15% 2	129856
	Night	43% 6% 12% 21% 16% 2	127936
Seeing	Day	23% 17% 32% 19% 8% 2	16252
	Night	21% 14% 33% 24% 7% 1	25095

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% 6% 2 8% 5% 5% 5% 4 6% 7% 39%	3597
	11th to 20th	Day	13% 8% 2 6% 3 3 5% 3 4 5% 46%	3570
	21st to end	Day	16% 8% 2 5% 4 4 4 3 4 5% 44%	3960
February	1st to 10th	Day	18% 8% 3 5% 4 5% 5% 5% 5% 6% 36%	1850
	11th to 20th	Day	13% 8% 2 7% 5% 4 5% 4 5% 8% 38%	1948
	21st to end	Day	17% 9% 3 7% 5% 5% 4 3 4 5% 37%	1650
March	1st to 10th	Day	21% 11% 3 6% 4 4 4 3 5% 4 35%	4476
	11th to 20th	Day	16% 8% 2 7% 4 4 4 4 6% 6% 39%	4767
	21st to end	Day	25% 9% 2 6% 4 4 4 4 6% 6% 31%	5781
April	1st to 10th	Day	18% 10% 3 6% 5% 4 4 3 4 6% 36%	2400
	11th to 20th	Day	18% 7% 2 5% 4 4 5% 4 6% 7% 38%	2390
	21st to end	Day	21% 6% 3 7% 4 5% 4 3 5% 6% 36%	2600
May	1st to 10th	Day	30% 10% 3 6% 4 4 4 4 5% 6% 24%	5886
	11th to 20th	Day	20% 8% 3 9% 5% 5% 4 3 5% 7% 32%	5995
	21st to end	Day	19% 6% 2 6% 4 5% 4 4 6% 7% 36%	6949
June	1st to 10th	Day	18% 6% 2 6% 4 5% 4 5% 7% 7% 35%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	2800
	21st to end	Day	17% 6% 2 7% 4 4 4 4 6% 9% 36%	2800
July	1st to 10th	Day	16% 6% 2 7% 5% 4 5% 4 7% 9% 34%	6360
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 5% 7% 11% 39%	6360
	21st to end	Day	14% 6% 2 6% 4 4 4 3 6% 8% 43%	6926
August	1st to 10th	Day	14% 6% 2 7% 5% 5% 6% 5% 7% 9% 34%	4305
	11th to 20th	Day	14% 8% 3 7% 4 4 5% 4 7% 7% 38%	4122
	21st to end	Day	7% 4 2 6% 4 5% 6% 5% 8% 9% 45%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 6% 6% 46%	2399
	11th to 20th	Day	15% 5% 2 5% 3 3 3 3 5% 5% 51%	2200
	21st to end	Day	17% 5% 3 6% 4 4 4 3 5% 7% 43%	2200
October	1st to 10th	Day	21% 6% 2 6% 3 4 3 4 4 6% 41%	2200
	11th to 20th	Day	15% 5% 2 4 5% 3 4 3 5% 7% 47%	1927
	21st to end	Day	21% 6% 2 6% 4 4 3 4 4 5% 41%	1881
November	1st to 10th	Day	15% 6% 3 5% 4 3 3 2 4 5% 49%	1761
	11th to 20th	Day	18% 6% 2 5% 3 3 3 3 4 5% 48%	1800
	21st to end	Day	19% 7% 2 4 4 4 3 4 4 6% 43%	1800
December	1st to 10th	Day	15% 7% 2 6% 4 4 3 3 4 6% 47%	2618
	11th to 20th	Day	20% 6% 2 5% 3 3 3 2 4 5% 46%	2986
	21st to end	Day	20% 10% 2 6% 4 4 3 3 4 5% 38%	3960
January	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 46%	6349
	11th to 20th	Night	13% 8% 3 9% 6% 4 4 4 5% 5% 39%	6315
	21st to end	Night	15% 10% 3 7% 5% 4 5% 3 5% 5% 37%	6996
February	1st to 10th	Night	16% 9% 2 6% 5% 4 4 4 5% 5% 38%	2547
	11th to 20th	Night	14% 9% 3 6% 5% 5% 4 3 5% 7% 39%	2338
	21st to end	Night	17% 9% 2 6% 4 4 3 3 5% 6% 40%	1980
March	1st to 10th	Night	23% 9% 2 5% 4 4 3 3 4 6% 37%	5364
	11th to 20th	Night	15% 10% 3 7% 5% 4 5% 3 5% 6% 37%	5191
	21st to end	Night	26% 9% 2 5% 4 3 5% 3 5% 5% 33%	5148
April	1st to 10th	Night	19% 7% 3 7% 5% 5% 4 3 5% 7% 35%	2000
	11th to 20th	Night	20% 8% 2 5% 4 4 4 3 4 5% 39%	1957
	21st to end	Night	20% 7% 3 6% 4 3 4 3 4 7% 39%	1800
May	1st to 10th	Night	33% 9% 2 6% 4 3 4 3 4 6% 25%	4054
	11th to 20th	Night	17% 8% 3 8% 4 6% 5% 5% 6% 8% 29%	3941
	21st to end	Night	20% 8% 4 7% 5% 4 5% 3 5% 7% 33%	3926
June	1st to 10th	Night	18% 8% 3 6% 5% 4 4 3 6% 5% 38%	1600
	11th to 20th	Night	18% 7% 2 6% 4 4 5% 4 5% 8% 36%	1600
	21st to end	Night	15% 7% 2 6% 5% 4 5% 4 6% 8% 38%	1600
July	1st to 10th	Night	12% 7% 3 6% 5% 5% 4 4 6% 10% 37%	3600
	11th to 20th	Night	10% 7% 3 7% 5% 5% 6% 4 7% 9% 39%	3600
	21st to end	Night	11% 7% 3 6% 3 4 4 4 5% 6% 47%	3926
August	1st to 10th	Night	18% 6% 3 7% 5% 5% 5% 4 6% 7% 34%	2812
	11th to 20th	Night	10% 7% 3 8% 5% 5% 6% 5% 7% 7% 36%	2922
	21st to end	Night	9% 4 1 5% 3 4 4 3 5% 8% 54%	3586
September	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 49%	2000
	11th to 20th	Night	13% 5% 2 4 4 3 3 2 4 6% 52%	2200
	21st to end	Night	17% 6% 3 6% 3 3 3 4 5% 6% 44%	2200
October	1st to 10th	Night	19% 6% 2 6% 3 5% 3 3 5% 5% 43%	2200
	11th to 20th	Night	16% 4 2 5% 3 3 4 3 6% 6% 47%	2425
	21st to end	Night	22% 7% 2 6% 4 3 4 3 4 5% 39%	2717
November	1st to 10th	Night	15% 5% 2 5% 4 3 4 3 5% 5% 49%	2537
	11th to 20th	Night	19% 6% 2 5% 3 3 3 3 5% 5% 47%	2600
	21st to end	Night	17% 7% 2 5% 5% 3 4 3 5% 6% 44%	2600
December	1st to 10th	Night	17% 9% 2 5% 5% 4 4 4 5% 5% 40%	4547
	11th to 20th	Night	22% 8% 2 5% 4 3 3 3 5% 5% 41%	5208
	21st to end	Night	18% 10% 3 7% 5% 4 3 3 5% 5% 39%	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% 5% 9% 26% 16% 2	3598
	11th to 20th	Day	39% 6% 11% 22% 19% 4	3600
	21st to end	Day	41% 6% 9% 22% 19% 3	3960
February	1st to 10th	Day	47% 6% 13% 19% 13% 3	1850
	11th to 20th	Day	44% 7% 11% 20% 14% 3	1957
	21st to end	Day	48% 6% 10% 20% 14% 3	1650
March	1st to 10th	Day	51% 4 10% 15% 10% 10%	4479
	11th to 20th	Day	41% 8% 14% 25% 11% 2	4767
	21st to end	Day	52% 4 8% 15% 15% 7%	5781
April	1st to 10th	Day	49% 5% 10% 17% 16% 4	2400
	11th to 20th	Day	43% 7% 11% 19% 17% 3	2390
	21st to end	Day	47% 6% 8% 18% 17% 4	2600
May	1st to 10th	Day	58% 8% 12% 13% 8% 1	5897
	11th to 20th	Day	51% 8% 14% 16% 10% 1	5999
	21st to end	Day	44% 8% 12% 21% 14% 1	6949
June	1st to 10th	Day	45% 8% 14% 22% 11%	2800
	11th to 20th	Day	41% 9% 14% 18% 16% 2	2800
	21st to end	Day	44% 8% 13% 21% 13% 1	2800
July	1st to 10th	Day	46% 8% 11% 20% 14% 1	6290
	11th to 20th	Day	37% 8% 17% 25% 13% 1	6285
	21st to end	Day	39% 6% 12% 26% 16% 1	6728
August	1st to 10th	Day	44% 8% 16% 22% 9% 1	4291
	11th to 20th	Day	42% 8% 14% 25% 10% 1	4161
	21st to end	Day	32% 8% 13% 29% 18% 1	4312
September	1st to 10th	Day	36% 7% 12% 24% 20% 2	2399
	11th to 20th	Day	35% 5% 9% 26% 23% 3	2200
	21st to end	Day	40% 6% 10% 21% 21% 2	2200
October	1st to 10th	Day	41% 6% 9% 22% 19% 3	3590
	11th to 20th	Day	31% 5% 8% 28% 24% 3	3117
	21st to end	Day	33% 5% 10% 31% 19% 3	3081
November	1st to 10th	Day	38% 4 13% 27% 16% 2	1761
	11th to 20th	Day	39% 4 10% 22% 22% 3	1800
	21st to end	Day	42% 6% 15% 23% 13% 2	1800
December	1st to 10th	Day	40% 4 15% 28% 12% 1	2618
	11th to 20th	Day	41% 4 14% 19% 19% 5%	2986
	21st to end	Day	48% 5% 8% 19% 14% 5%	3960
January	1st to 10th	Night	36% 5% 12% 31% 14% 2	6349
	11th to 20th	Night	45% 5% 13% 20% 13% 4	6360
	21st to end	Night	47% 6% 9% 17% 17% 5%	6996
February	1st to 10th	Night	46% 6% 13% 20% 13% 2	2547
	11th to 20th	Night	44% 5% 10% 21% 16% 4	2340
	21st to end	Night	45% 4 9% 23% 16% 3	1980
March	1st to 10th	Night	48% 5% 9% 16% 15% 7%	5374
	11th to 20th	Night	45% 7% 16% 17% 12% 2	5192
	21st to end	Night	52% 4 6% 12% 20% 6%	5148
April	1st to 10th	Night	48% 5% 11% 18% 15% 3	2000
	11th to 20th	Night	46% 5% 10% 19% 18% 2	1957
	21st to end	Night	45% 6% 11% 17% 17% 4	1800
May	1st to 10th	Night	59% 7% 13% 12% 8% 2	4060
	11th to 20th	Night	50% 10% 15% 15% 9% 2	3942
	21st to end	Night	50% 8% 13% 18% 10% 1	3927
June	1st to 10th	Night	46% 7% 12% 22% 12% 1	1600
	11th to 20th	Night	44% 7% 14% 19% 14% 2	1600
	21st to end	Night	40% 10% 14% 21% 14% 1	1600
July	1st to 10th	Night	42% 7% 13% 23% 16%	3560
	11th to 20th	Night	40% 7% 19% 19% 14% 1	3552
	21st to end	Night	37% 6% 11% 30% 15% 1	3809
August	1st to 10th	Night	48% 8% 12% 19% 11% 1	2802
	11th to 20th	Night	42% 10% 14% 24% 10%	2949
	21st to end	Night	27% 7% 15% 29% 22% 1	3586
September	1st to 10th	Night	36% 6% 13% 26% 19% 1	2000
	11th to 20th	Night	33% 6% 10% 24% 26% 2	2200
	21st to end	Night	40% 7% 10% 24% 18% 2	2200
October	1st to 10th	Night	38% 6% 15% 23% 16% 2	3590
	11th to 20th	Night	27% 4 12% 27% 27% 3	3961
	21st to end	Night	34% 4 12% 31% 17% 2	4467
November	1st to 10th	Night	37% 7% 11% 22% 20% 3	2537
	11th to 20th	Night	40% 4 10% 22% 20% 4	2600
	21st to end	Night	40% 6% 14% 25% 14% 2	2600
December	1st to 10th	Night	44% 4 12% 27% 12% 1	4547
	11th to 20th	Night	45% 5% 14% 14% 20% 1	5208
	21st to end	Night	46% 5% 9% 22% 15% 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	29% 26% 29% 13% 5%	200
	11th to 20th	Day	33% 21% 34% 9% 3 1	200
	21st to end	Day	26% 25% 35% 10% 4	220
February	1st to 10th	Day	32% 20% 36% 12% 1	391
	11th to 20th	Day	29% 28% 31% 11% 2	496
	21st to end	Day	28% 21% 35% 12% 3	414
March	1st to 10th	Day	29% 23% 34% 12% 3	500
	11th to 20th	Day	29% 23% 32% 12% 3 1	500
	21st to end	Day	32% 21% 30% 10% 5% 1	550
April	1st to 10th	Day	33% 23% 26% 13% 5% 1	500
	11th to 20th	Day	31% 26% 27% 10% 5%	492
	21st to end	Day	28% 18% 34% 15% 3 2	500
May	1st to 10th	Day	33% 20% 29% 13% 4 1	503
	11th to 20th	Day	23% 17% 41% 15% 5% 1	527
	21st to end	Day	19% 14% 38% 21% 7% 1	573
June	1st to 10th	Day	18% 16% 38% 18% 8% 2	530
	11th to 20th	Day	18% 15% 39% 21% 6% 1	530
	21st to end	Day	18% 14% 37% 20% 8% 2	530
July	1st to 10th	Day	18% 11% 30% 27% 11% 2	525
	11th to 20th	Day	13% 8% 33% 29% 14% 2	524
	21st to end	Day	12% 10% 36% 31% 9% 1	576
August	1st to 10th	Day	11% 10% 33% 31% 14% 2	526
	11th to 20th	Day	17% 9% 27% 29% 14% 3	519
	21st to end	Day	11% 10% 36% 25% 14% 4	583
September	1st to 10th	Day	18% 11% 30% 22% 15% 5%	530
	11th to 20th	Day	19% 12% 27% 20% 14% 9%	530
	21st to end	Day	19% 12% 33% 21% 10% 5%	530
October	1st to 10th	Day	25% 14% 25% 19% 12% 4	530
	11th to 20th	Day	20% 18% 28% 23% 10% 2	461
	21st to end	Day	27% 14% 31% 20% 7% 1	429
November	1st to 10th	Day	21% 16% 34% 19% 9% 2	395
	11th to 20th	Day	30% 19% 29% 18% 5% 1	400
	21st to end	Day	29% 19% 30% 16% 6% 1	400
December	1st to 10th	Day	22% 28% 30% 18% 3	218
	11th to 20th	Day	32% 26% 31% 10% 2	200
	21st to end	Day	21% 26% 30% 17% 4 1	220
January	1st to 10th	Night	22% 20% 39% 18% 3	800
	11th to 20th	Night	27% 17% 36% 18% 2	800
	21st to end	Night	19% 20% 37% 20% 3	880
February	1st to 10th	Night	23% 17% 38% 20% 2	800
	11th to 20th	Night	23% 19% 40% 17% 1	783
	21st to end	Night	25% 20% 34% 18% 3	660
March	1st to 10th	Night	25% 19% 35% 19% 3	800
	11th to 20th	Night	22% 20% 38% 17% 4	800
	21st to end	Night	26% 18% 33% 19% 4	880
April	1st to 10th	Night	25% 19% 34% 19% 4	800
	11th to 20th	Night	26% 19% 33% 18% 5%	773
	21st to end	Night	23% 18% 37% 20% 3	600
May	1st to 10th	Night	28% 13% 35% 20% 4	600
	11th to 20th	Night	20% 14% 39% 21% 5%	552
	21st to end	Night	19% 10% 36% 29% 6%	433
June	1st to 10th	Night	21% 7% 37% 28% 7% 1	400
	11th to 20th	Night	17% 11% 35% 30% 7% 1	400
	21st to end	Night	16% 7% 38% 32% 7%	400
July	1st to 10th	Night	15% 5% 29% 41% 10% 1	390
	11th to 20th	Night	12% 4 29% 43% 11% 1	388
	21st to end	Night	12% 5% 32% 43% 8%	438
August	1st to 10th	Night	12% 5% 29% 42% 11% 1	533
	11th to 20th	Night	11% 6% 30% 40% 13%	616
	21st to end	Night	13% 6% 27% 38% 15% 2	880
September	1st to 10th	Night	15% 6% 32% 33% 14% 1	800
	11th to 20th	Night	15% 9% 26% 30% 18% 3	800
	21st to end	Night	19% 7% 29% 30% 13% 2	800
October	1st to 10th	Night	22% 11% 26% 27% 12% 2	800
	11th to 20th	Night	20% 12% 31% 25% 11% 1	793
	21st to end	Night	26% 14% 32% 23% 5%	836
November	1st to 10th	Night	18% 14% 32% 26% 8% 1	780
	11th to 20th	Night	22% 17% 34% 20% 6% 1	800
	21st to end	Night	23% 19% 34% 19% 5% 1	800
December	1st to 10th	Night	25% 18% 35% 18% 5%	800
	11th to 20th	Night	25% 17% 33% 23% 3	800
	21st to end	Night	24% 17% 36% 20% 3	880

Page generated at 2025-10-16 01:50:23. Version ac07

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