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 2024-04-10.

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% 3 5% 5% 3 4 5% 5% 7% 39%	82953
	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 40%	85824
Transparency	Day	43% 7% 12% 21% 14% 2	82658
	Night	44% 6% 12% 21% 14% 2	85653
Seeing	Day	23% 17% 32% 19% 7% 2	14207
	Night	21% 14% 33% 24% 7% 1	23267

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	15% 7% 3 6% 5% 3 4 4 5% 6% 44%	2517
	11th to 20th	Day	17% 7% 3 6% 5% 3 5% 4 5% 5% 41%	2520
	21st to end	Day	21% 10% 3 5% 5% 3 4 4 5% 5% 34%	2772
February	1st to 10th	Day	19% 8% 3 4 5% 4 5% 5% 5% 6% 36%	1757
	11th to 20th	Day	13% 8% 3 6% 5% 2 6% 5% 5% 8% 39%	1848
	21st to end	Day	18% 8% 3 7% 6% 4 5% 4 4 5% 36%	1570
March	1st to 10th	Day	24% 14% 3 6% 5% 3 4 3 4 4 30%	3116
	11th to 20th	Day	14% 7% 2 5% 5% 3 4 5% 6% 7% 41%	3337
	21st to end	Day	22% 9% 3 6% 4 3 4 5% 5% 6% 34%	4033
April	1st to 10th	Day	19% 9% 3 6% 6% 3 4 4 4 7% 36%	2270
	11th to 20th	Day	17% 6% 3 5% 4 3 5% 5% 6% 7% 38%	2148
	21st to end	Day	22% 5% 4 6% 5% 4 5% 4 5% 6% 34%	2340
May	1st to 10th	Day	24% 8% 2 5% 5% 3 4 4 5% 6% 32%	2340
	11th to 20th	Day	18% 6% 3 5% 6% 3 4 4 5% 7% 40%	2351
	21st to end	Day	14% 5% 2 4 5% 4 4 4 6% 7% 44%	2725
June	1st to 10th	Day	19% 6% 3 6% 5% 4 5% 5% 7% 7% 34%	2520
	11th to 20th	Day	17% 5% 3 5% 5% 3 5% 5% 5% 8% 38%	2520
	21st to end	Day	18% 5% 3 6% 5% 3 5% 5% 6% 9% 35%	2520
July	1st to 10th	Day	14% 5% 2 6% 5% 3 5% 5% 6% 8% 42%	2520
	11th to 20th	Day	12% 5% 2 5% 6% 4 5% 5% 6% 9% 40%	2520
	21st to end	Day	12% 5% 3 5% 5% 3 5% 6% 7% 9% 40%	2744
August	1st to 10th	Day	10% 5% 2 6% 6% 4 7% 7% 6% 9% 39%	2367
	11th to 20th	Day	13% 6% 2 5% 6% 3 6% 6% 7% 8% 39%	2281
	21st to end	Day	12% 5% 2 4 5% 3 5% 5% 7% 8% 45%	2376
September	1st to 10th	Day	14% 5% 2 4 4 3 4 6% 5% 7% 45%	2160
	11th to 20th	Day	16% 5% 2 4 4 2 3 3 4 5% 51%	1980
	21st to end	Day	17% 5% 3 4 4 3 3 4 5% 6% 46%	1980
October	1st to 10th	Day	22% 6% 3 6% 4 3 3 4 4 7% 39%	1980
	11th to 20th	Day	14% 5% 3 4 6% 2 4 5% 5% 7% 46%	1752
	21st to end	Day	23% 6% 2 5% 5% 3 3 4 5% 5% 39%	1782
November	1st to 10th	Day	16% 6% 3 4 5% 3 3 3 4 5% 48%	1671
	11th to 20th	Day	19% 6% 2 5% 4 2 3 4 4 5% 47%	1710
	21st to end	Day	19% 6% 3 4 4 3 3 5% 4 6% 43%	1710
December	1st to 10th	Day	18% 7% 4 7% 5% 4 4 4 4 6% 39%	1538
	11th to 20th	Day	22% 6% 2 4 4 2 4 4 4 4 44%	1906
	21st to end	Day	22% 10% 3 6% 5% 3 4 4 4 6% 33%	2772
January	1st to 10th	Night	18% 8% 2 4 4 2 3 3 4 6% 46%	4429
	11th to 20th	Night	14% 8% 4 8% 7% 3 4 4 4 5% 39%	4440
	21st to end	Night	22% 13% 3 6% 5% 3 4 4 4 5% 30%	4884
February	1st to 10th	Night	16% 9% 3 5% 6% 3 4 5% 5% 6% 38%	2420
	11th to 20th	Night	14% 9% 3 6% 5% 3 5% 4 5% 7% 40%	2218
	21st to end	Night	18% 9% 2 5% 5% 3 3 4 4 6% 41%	1884
March	1st to 10th	Night	27% 11% 2 5% 4 3 4 3 4 5% 32%	3724
	11th to 20th	Night	11% 8% 4 7% 6% 4 6% 5% 5% 6% 38%	3621
	21st to end	Night	25% 8% 3 5% 4 2 4 5% 5% 6% 33%	3596
April	1st to 10th	Night	19% 7% 3 6% 6% 3 5% 4 5% 7% 36%	1900
	11th to 20th	Night	20% 7% 2 5% 4 2 5% 4 5% 6% 40%	1759
	21st to end	Night	21% 7% 3 5% 6% 2 4 4 4 7% 38%	1620
May	1st to 10th	Night	23% 7% 3 6% 6% 3 5% 4 5% 7% 32%	1620
	11th to 20th	Night	19% 6% 2 6% 4 2 6% 6% 6% 7% 37%	1557
	21st to end	Night	17% 6% 3 6% 5% 3 5% 4 4 8% 39%	1550
June	1st to 10th	Night	19% 8% 3 6% 5% 3 4 4 6% 6% 37%	1440
	11th to 20th	Night	19% 6% 3 5% 4 3 4 6% 5% 8% 36%	1440
	21st to end	Night	14% 5% 3 5% 5% 3 5% 6% 6% 9% 39%	1440
July	1st to 10th	Night	14% 5% 3 6% 5% 4 5% 5% 6% 8% 40%	1440
	11th to 20th	Night	12% 6% 3 5% 6% 4 5% 5% 7% 9% 38%	1440
	21st to end	Night	13% 6% 2 5% 4 3 5% 5% 6% 8% 44%	1574
August	1st to 10th	Night	8% 6% 2 4 5% 3 5% 6% 6% 9% 45%	1553
	11th to 20th	Night	11% 5% 3 6% 6% 4 6% 7% 7% 8% 36%	1617
	21st to end	Night	14% 4 2 4 4 3 4 4 5% 8% 47%	1980
September	1st to 10th	Night	14% 5% 2 4 5% 3 3 4 5% 7% 48%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 3 3 5% 7% 52%	1980
	21st to end	Night	16% 5% 3 4 4 2 3 4 5% 6% 46%	1980
October	1st to 10th	Night	20% 7% 2 6% 4 3 3 3 4 5% 42%	1980
	11th to 20th	Night	16% 4 2 4 4 3 4 4 6% 6% 45%	2204
	21st to end	Night	23% 7% 2 6% 5% 2 4 4 4 5% 38%	2574
November	1st to 10th	Night	16% 5% 2 4 4 2 4 4 5% 5% 48%	2407
	11th to 20th	Night	19% 6% 2 5% 4 2 3 3 4 6% 46%	2470
	21st to end	Night	16% 6% 3 5% 5% 2 4 4 5% 6% 44%	2470
December	1st to 10th	Night	19% 9% 3 6% 5% 3 4 4 5% 5% 38%	2641
	11th to 20th	Night	19% 6% 2 5% 5% 3 3 4 5% 6% 41%	3288
	21st to end	Night	21% 10% 3 6% 6% 3 4 4 5% 6% 34%	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	40% 5% 11% 32% 10% 2	2518
	11th to 20th	Day	45% 7% 10% 25% 12% 2	2520
	21st to end	Day	50% 7% 9% 22% 9% 2	2772
February	1st to 10th	Day	47% 6% 12% 19% 13% 3	1757
	11th to 20th	Day	43% 7% 11% 21% 15% 3	1857
	21st to end	Day	49% 6% 10% 20% 13% 3	1570
March	1st to 10th	Day	57% 5% 10% 15% 10% 3	3119
	11th to 20th	Day	36% 9% 16% 22% 14% 3	3337
	21st to end	Day	49% 5% 8% 16% 16% 7%	4033
April	1st to 10th	Day	48% 5% 10% 17% 16% 3	2270
	11th to 20th	Day	43% 7% 11% 19% 17% 3	2148
	21st to end	Day	49% 6% 8% 17% 16% 4	2340
May	1st to 10th	Day	52% 6% 8% 19% 13% 2	2340
	11th to 20th	Day	43% 7% 11% 19% 17% 3	2355
	21st to end	Day	39% 8% 13% 23% 14% 3	2725
June	1st to 10th	Day	46% 8% 14% 21% 10%	2520
	11th to 20th	Day	42% 9% 12% 18% 16% 2	2520
	21st to end	Day	45% 9% 13% 21% 11% 1	2520
July	1st to 10th	Day	40% 8% 11% 23% 16% 2	2450
	11th to 20th	Day	38% 9% 15% 25% 12% 1	2445
	21st to end	Day	37% 9% 15% 22% 13% 3	2591
August	1st to 10th	Day	39% 10% 15% 23% 12% 1	2353
	11th to 20th	Day	40% 9% 14% 24% 12% 1	2281
	21st to end	Day	35% 8% 12% 24% 18% 2	2376
September	1st to 10th	Day	37% 7% 12% 24% 19% 2	2160
	11th to 20th	Day	36% 5% 9% 25% 23% 2	1980
	21st to end	Day	38% 6% 10% 22% 22% 2	1980
October	1st to 10th	Day	46% 6% 10% 21% 14% 4	1980
	11th to 20th	Day	35% 7% 13% 29% 14% 2	1752
	21st to end	Day	46% 6% 12% 20% 13% 3	1782
November	1st to 10th	Day	39% 4 12% 26% 16% 2	1671
	11th to 20th	Day	40% 4 10% 23% 20% 3	1710
	21st to end	Day	42% 6% 15% 23% 13% 2	1710
December	1st to 10th	Day	47% 5% 14% 16% 16% 1	1538
	11th to 20th	Day	43% 4 14% 19% 16% 5%	1906
	21st to end	Day	51% 7% 11% 20% 10% 1	2772
January	1st to 10th	Night	39% 5% 14% 28% 12% 3	4429
	11th to 20th	Night	46% 6% 13% 21% 12% 3	4440
	21st to end	Night	55% 7% 9% 17% 10% 2	4884
February	1st to 10th	Night	46% 6% 13% 20% 13% 2	2420
	11th to 20th	Night	44% 5% 10% 21% 16% 4	2220
	21st to end	Night	45% 4 9% 23% 15% 4	1884
March	1st to 10th	Night	53% 5% 9% 19% 10% 3	3734
	11th to 20th	Night	42% 9% 14% 18% 15% 2	3622
	21st to end	Night	50% 4 8% 13% 18% 7%	3596
April	1st to 10th	Night	47% 6% 11% 18% 15% 3	1900
	11th to 20th	Night	44% 5% 10% 20% 18% 3	1759
	21st to end	Night	47% 6% 12% 17% 14% 4	1620
May	1st to 10th	Night	52% 6% 10% 17% 13% 3	1620
	11th to 20th	Night	43% 9% 13% 16% 15% 3	1558
	21st to end	Night	45% 7% 13% 21% 12% 1	1551
June	1st to 10th	Night	47% 7% 12% 23% 11% 1	1440
	11th to 20th	Night	45% 7% 14% 18% 14% 2	1440
	21st to end	Night	38% 10% 16% 21% 14% 1	1440
July	1st to 10th	Night	40% 8% 14% 22% 15% 1	1400
	11th to 20th	Night	40% 8% 17% 25% 10% 1	1392
	21st to end	Night	36% 8% 15% 29% 11% 2	1486
August	1st to 10th	Night	33% 11% 15% 24% 15% 1	1543
	11th to 20th	Night	40% 10% 16% 21% 12%	1617
	21st to end	Night	35% 8% 11% 26% 20% 1	1980
September	1st to 10th	Night	37% 5% 14% 25% 19% 1	1800
	11th to 20th	Night	33% 6% 10% 23% 26% 2	1980
	21st to end	Night	37% 7% 10% 25% 20% 2	1980
October	1st to 10th	Night	45% 6% 12% 22% 12% 4	1980
	11th to 20th	Night	36% 7% 15% 27% 14% 1	2204
	21st to end	Night	48% 6% 12% 21% 13% 1	2574
November	1st to 10th	Night	38% 7% 11% 21% 20% 3	2407
	11th to 20th	Night	41% 5% 11% 22% 18% 3	2470
	21st to end	Night	40% 6% 14% 25% 14% 2	2470
December	1st to 10th	Night	47% 5% 13% 21% 12% 1	2641
	11th to 20th	Night	42% 7% 15% 17% 17% 1	3288
	21st to end	Night	50% 6% 10% 24% 10% 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	26% 24% 31% 14% 5%	180
	11th to 20th	Day	33% 21% 33% 9% 3 1	180
	21st to end	Day	25% 25% 35% 11% 4	198
February	1st to 10th	Day	31% 20% 35% 13% 1	358
	11th to 20th	Day	27% 29% 32% 11% 2	456
	21st to end	Day	29% 20% 35% 13% 3 1	382
March	1st to 10th	Day	28% 21% 35% 13% 3	460
	11th to 20th	Day	30% 22% 33% 12% 3 1	460
	21st to end	Day	31% 21% 30% 11% 6% 1	506
April	1st to 10th	Day	30% 23% 26% 14% 5% 1	457
	11th to 20th	Day	27% 25% 30% 11% 7%	412
	21st to end	Day	30% 19% 33% 15% 3 2	420
May	1st to 10th	Day	33% 20% 30% 12% 4 2	423
	11th to 20th	Day	22% 16% 42% 14% 4 1	447
	21st to end	Day	19% 13% 38% 21% 8% 1	485
June	1st to 10th	Day	18% 15% 39% 18% 9% 2	450
	11th to 20th	Day	17% 15% 40% 20% 6% 1	450
	21st to end	Day	18% 13% 37% 20% 8% 2	450
July	1st to 10th	Day	18% 10% 28% 29% 12% 2	445
	11th to 20th	Day	15% 7% 33% 30% 13% 2	444
	21st to end	Day	11% 9% 36% 32% 9% 1	488
August	1st to 10th	Day	11% 10% 32% 32% 13% 2	446
	11th to 20th	Day	17% 9% 29% 29% 14% 3	439
	21st to end	Day	13% 9% 34% 25% 14% 4	495
September	1st to 10th	Day	19% 10% 28% 23% 16% 4	450
	11th to 20th	Day	20% 14% 28% 19% 12% 7%	450
	21st to end	Day	19% 13% 33% 21% 10% 5%	450
October	1st to 10th	Day	28% 13% 25% 20% 10% 4	450
	11th to 20th	Day	18% 18% 29% 23% 11% 2	399
	21st to end	Day	28% 14% 30% 19% 8% 1	396
November	1st to 10th	Day	21% 15% 33% 20% 9% 2	365
	11th to 20th	Day	30% 19% 28% 18% 5% 1	370
	21st to end	Day	28% 18% 31% 17% 6% 1	370
December	1st to 10th	Day	21% 28% 31% 16% 3	198
	11th to 20th	Day	31% 25% 33% 9% 2	180
	21st to end	Day	22% 28% 30% 15% 4 2	198
January	1st to 10th	Night	21% 19% 39% 18% 3	760
	11th to 20th	Night	27% 17% 36% 18% 2	760
	21st to end	Night	20% 20% 37% 20% 3	836
February	1st to 10th	Night	23% 17% 38% 21% 2	760
	11th to 20th	Night	22% 19% 41% 17% 1	743
	21st to end	Night	25% 19% 33% 19% 3	628
March	1st to 10th	Night	25% 18% 35% 19% 3	760
	11th to 20th	Night	21% 20% 38% 16% 4	760
	21st to end	Night	26% 18% 33% 18% 4	836
April	1st to 10th	Night	24% 19% 34% 19% 4	760
	11th to 20th	Night	25% 18% 33% 19% 5%	695
	21st to end	Night	24% 17% 37% 20% 3	540
May	1st to 10th	Night	27% 13% 35% 21% 4	540
	11th to 20th	Night	21% 15% 38% 21% 5%	496
	21st to end	Night	18% 10% 35% 29% 7%	389
June	1st to 10th	Night	21% 6% 36% 28% 8% 1	360
	11th to 20th	Night	18% 10% 34% 30% 7% 1	360
	21st to end	Night	16% 6% 39% 34% 7%	360
July	1st to 10th	Night	15% 5% 29% 40% 11% 1	350
	11th to 20th	Night	13% 4 27% 44% 12% 1	348
	21st to end	Night	12% 5% 30% 44% 8% 1	394
August	1st to 10th	Night	10% 6% 29% 42% 11% 1	479
	11th to 20th	Night	12% 6% 29% 40% 13%	553
	21st to end	Night	14% 6% 25% 38% 16% 2	792
September	1st to 10th	Night	16% 5% 32% 33% 14%	720
	11th to 20th	Night	16% 9% 27% 28% 17% 2	720
	21st to end	Night	18% 8% 30% 29% 14% 2	720
October	1st to 10th	Night	24% 11% 26% 26% 11% 2	720
	11th to 20th	Night	20% 12% 32% 25% 11% 1	720
	21st to end	Night	27% 13% 32% 22% 5%	792
November	1st to 10th	Night	19% 14% 31% 27% 9% 1	740
	11th to 20th	Night	23% 17% 34% 20% 6% 1	760
	21st to end	Night	22% 19% 34% 19% 5% 1	760
December	1st to 10th	Night	25% 18% 35% 18% 4	760
	11th to 20th	Night	24% 17% 33% 23% 3	760
	21st to end	Night	25% 17% 36% 19% 3	836

Page generated at 2024-04-07 09:28:16. Version ac07

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