Worcester
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Worcester clear sky chart from 2005-11-06 to 2024-03-23.

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 5% 5% 5% 7% 39%	78564
	Night	17% 7% 3 5% 5% 3 4 4 5% 6% 41%	81314
Transparency	Day	44% 4 11% 23% 15% 2	78269
	Night	44% 3 11% 24% 15% 2	81143
Seeing	Day	24% 14% 33% 19% 7% 2	14133
	Night	23% 15% 34% 22% 6% 1	23232

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	13% 7% 3 5% 5% 3 4 4 4 6% 44%	2517
	11th to 20th	Day	16% 7% 4 6% 5% 3 4 5% 5% 5% 39%	2520
	21st to end	Day	20% 11% 3 5% 5% 3 4 5% 4 5% 34%	2772
February	1st to 10th	Day	17% 7% 3 5% 5% 3 6% 6% 5% 7% 36%	1775
	11th to 20th	Day	12% 7% 3 6% 6% 3 6% 5% 5% 8% 39%	1848
	21st to end	Day	18% 10% 2 6% 6% 3 4 3 4 6% 37%	1570
March	1st to 10th	Day	16% 8% 3 5% 4 3 5% 5% 5% 7% 39%	1897
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 5% 5% 9% 38%	1981
	21st to end	Day	18% 8% 3 5% 5% 3 5% 4 4 6% 40%	2348
April	1st to 10th	Day	17% 9% 3 6% 5% 4 5% 4 4 7% 38%	2160
	11th to 20th	Day	17% 6% 2 5% 5% 3 5% 4 5% 8% 40%	2134
	21st to end	Day	22% 6% 2 6% 5% 3 4 5% 5% 7% 35%	2326
May	1st to 10th	Day	24% 9% 2 5% 5% 3 5% 5% 5% 7% 30%	2340
	11th to 20th	Day	19% 6% 3 6% 5% 3 3 3 5% 8% 40%	2364
	21st to end	Day	15% 6% 2 5% 5% 3 5% 4 5% 8% 42%	2725
June	1st to 10th	Day	18% 5% 3 6% 6% 4 5% 5% 5% 8% 34%	2520
	11th to 20th	Day	17% 5% 3 6% 5% 3 5% 5% 6% 8% 36%	2520
	21st to end	Day	17% 6% 3 6% 5% 4 5% 6% 7% 8% 34%	2520
July	1st to 10th	Day	14% 6% 3 5% 6% 3 5% 5% 6% 8% 40%	2520
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 6% 6% 10% 38%	2520
	21st to end	Day	12% 5% 3 6% 6% 4 6% 6% 7% 8% 39%	2744
August	1st to 10th	Day	10% 6% 2 6% 6% 5% 6% 6% 7% 9% 37%	2381
	11th to 20th	Day	13% 6% 1 5% 6% 4 5% 5% 6% 8% 39%	2241
	21st to end	Day	12% 4 2 5% 5% 3 5% 5% 6% 9% 43%	2376
September	1st to 10th	Day	14% 6% 2 4 5% 4 5% 4 5% 7% 43%	2160
	11th to 20th	Day	16% 6% 2 3 3 2 3 4 4 6% 50%	1994
	21st to end	Day	17% 5% 2 5% 4 3 4 4 4 7% 46%	1980
October	1st to 10th	Day	23% 6% 3 5% 4 2 4 3 4 6% 39%	1980
	11th to 20th	Day	14% 6% 2 4 5% 2 4 4 5% 8% 45%	1755
	21st to end	Day	20% 7% 3 6% 4 2 4 5% 4 5% 40%	1782
November	1st to 10th	Day	14% 7% 3 5% 5% 3 3 4 4 5% 48%	1671
	11th to 20th	Day	19% 6% 2 4 3 2 3 3 6% 6% 45%	1710
	21st to end	Day	16% 7% 2 5% 4 3 5% 5% 5% 5% 43%	1710
December	1st to 10th	Day	17% 8% 3 6% 5% 3 6% 4 6% 6% 37%	1525
	11th to 20th	Day	22% 6% 2 4 4 2 3 3 4 6% 44%	1906
	21st to end	Day	20% 11% 3 6% 5% 3 4 4 4 7% 33%	2772
January	1st to 10th	Night	17% 7% 2 4 4 2 3 3 5% 6% 46%	4429
	11th to 20th	Night	14% 9% 4 7% 6% 2 3 4 5% 6% 40%	4440
	21st to end	Night	21% 12% 4 6% 5% 3 4 4 4 5% 31%	4884
February	1st to 10th	Night	16% 8% 4 6% 5% 3 4 5% 5% 6% 40%	2399
	11th to 20th	Night	13% 7% 3 7% 4 3 5% 4 4 7% 41%	2218
	21st to end	Night	17% 9% 3 6% 5% 3 3 3 5% 7% 40%	1884
March	1st to 10th	Night	16% 9% 2 4 6% 3 4 5% 5% 6% 40%	2270
	11th to 20th	Night	14% 8% 3 6% 5% 4 5% 4 5% 7% 39%	2198
	21st to end	Night	15% 10% 3 6% 4 3 4 4 4 5% 41%	2052
April	1st to 10th	Night	16% 9% 3 6% 6% 4 5% 5% 5% 7% 36%	1800
	11th to 20th	Night	18% 6% 3 5% 5% 3 4 4 4 6% 41%	1775
	21st to end	Night	21% 7% 3 6% 5% 2 4 4 4 6% 39%	1632
May	1st to 10th	Night	24% 8% 3 6% 5% 2 4 5% 5% 5% 32%	1620
	11th to 20th	Night	18% 6% 3 5% 5% 4 4 4 6% 6% 38%	1543
	21st to end	Night	17% 7% 4 5% 4 3 5% 4 5% 8% 39%	1550
June	1st to 10th	Night	17% 7% 3 6% 6% 4 4 4 4 8% 37%	1440
	11th to 20th	Night	19% 7% 2 6% 6% 2 4 5% 5% 8% 36%	1440
	21st to end	Night	13% 6% 3 4 4 3 5% 6% 6% 8% 42%	1440
July	1st to 10th	Night	14% 5% 3 5% 5% 3 5% 5% 6% 9% 40%	1440
	11th to 20th	Night	15% 6% 3 5% 5% 4 4 5% 8% 9% 36%	1440
	21st to end	Night	12% 5% 2 4 5% 4 6% 6% 5% 6% 45%	1574
August	1st to 10th	Night	8% 5% 3 6% 6% 4 6% 6% 6% 9% 43%	1540
	11th to 20th	Night	12% 6% 1 6% 6% 4 5% 7% 7% 9% 38%	1656
	21st to end	Night	14% 4 2 4 4 2 4 4 5% 7% 48%	1980
September	1st to 10th	Night	13% 6% 3 5% 6% 3 4 5% 6% 6% 45%	1800
	11th to 20th	Night	13% 6% 2 4 4 3 4 3 5% 5% 52%	1964
	21st to end	Night	16% 6% 3 5% 4 2 4 4 5% 6% 45%	1980
October	1st to 10th	Night	21% 6% 2 4 4 2 4 4 4 6% 42%	1980
	11th to 20th	Night	15% 5% 2 4 4 4 4 4 5% 7% 46%	2203
	21st to end	Night	22% 8% 3 5% 5% 3 4 3 4 6% 38%	2574
November	1st to 10th	Night	14% 6% 3 4 5% 3 4 4 5% 5% 48%	2407
	11th to 20th	Night	18% 7% 2 4 4 3 4 3 5% 6% 46%	2470
	21st to end	Night	15% 6% 2 4 4 3 4 4 5% 7% 45%	2470
December	1st to 10th	Night	18% 8% 3 5% 5% 3 4 4 4 6% 39%	2650
	11th to 20th	Night	19% 6% 2 4 4 2 4 5% 5% 7% 40%	3288
	21st to end	Night	19% 10% 4 7% 4 3 4 4 5% 5% 35%	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	42% 2 7% 35% 12% 2	2518
	11th to 20th	Day	48% 3 9% 26% 13% 2	2520
	21st to end	Day	53% 3 9% 22% 11% 2	2772
February	1st to 10th	Day	48% 3 13% 21% 13% 2	1775
	11th to 20th	Day	44% 5% 10% 23% 16% 3	1857
	21st to end	Day	50% 2 9% 21% 15% 3	1570
March	1st to 10th	Day	45% 3 11% 22% 14% 4	1900
	11th to 20th	Day	44% 2 11% 22% 16% 4	1981
	21st to end	Day	47% 3 7% 19% 18% 6%	2348
April	1st to 10th	Day	47% 3 10% 21% 16% 4	2160
	11th to 20th	Day	43% 3 10% 22% 19% 3	2134
	21st to end	Day	50% 3 8% 19% 17% 3	2326
May	1st to 10th	Day	54% 3 7% 19% 14% 2	2340
	11th to 20th	Day	44% 3 10% 22% 18% 3	2368
	21st to end	Day	41% 4 14% 25% 14% 2	2725
June	1st to 10th	Day	48% 5% 12% 24% 11% 1	2520
	11th to 20th	Day	45% 5% 12% 21% 16% 1	2520
	21st to end	Day	46% 6% 11% 24% 12% 1	2520
July	1st to 10th	Day	42% 5% 10% 25% 16% 2	2450
	11th to 20th	Day	41% 5% 14% 27% 12% 1	2445
	21st to end	Day	40% 5% 14% 23% 15% 3	2591
August	1st to 10th	Day	42% 5% 13% 26% 13% 1	2367
	11th to 20th	Day	43% 4 13% 25% 13% 1	2241
	21st to end	Day	38% 4 11% 27% 18% 2	2376
September	1st to 10th	Day	39% 4 11% 25% 18% 2	2160
	11th to 20th	Day	36% 3 8% 27% 24% 3	1994
	21st to end	Day	39% 3 9% 23% 23% 2	1980
October	1st to 10th	Day	47% 4 8% 23% 14% 4	1980
	11th to 20th	Day	38% 3 11% 30% 16% 2	1755
	21st to end	Day	47% 4 12% 20% 14% 4	1782
November	1st to 10th	Day	40% 3 11% 26% 17% 3	1671
	11th to 20th	Day	40% 3 9% 24% 21% 2	1710
	21st to end	Day	42% 4 13% 26% 13% 2	1710
December	1st to 10th	Day	47% 3 13% 18% 16% 2	1525
	11th to 20th	Day	43% 2 13% 19% 19% 3	1906
	21st to end	Day	53% 3 10% 21% 11% 1	2772
January	1st to 10th	Night	39% 2 11% 31% 13% 3	4429
	11th to 20th	Night	47% 4 10% 24% 13% 3	4440
	21st to end	Night	57% 2 9% 18% 12% 2	4884
February	1st to 10th	Night	46% 4 13% 21% 14% 2	2399
	11th to 20th	Night	44% 4 10% 21% 18% 4	2220
	21st to end	Night	46% 2 8% 25% 15% 4	1884
March	1st to 10th	Night	45% 3 8% 25% 15% 3	2280
	11th to 20th	Night	46% 4 10% 19% 17% 4	2199
	21st to end	Night	46% 2 8% 19% 19% 6%	2052
April	1st to 10th	Night	48% 3 9% 20% 17% 3	1800
	11th to 20th	Night	44% 3 10% 22% 19% 2	1775
	21st to end	Night	48% 3 9% 22% 14% 5%	1632
May	1st to 10th	Night	53% 3 9% 20% 12% 3	1620
	11th to 20th	Night	46% 4 11% 20% 16% 3	1544
	21st to end	Night	46% 4 14% 21% 13% 1	1551
June	1st to 10th	Night	46% 6% 11% 25% 12% 1	1440
	11th to 20th	Night	47% 4 13% 23% 13% 1	1440
	21st to end	Night	38% 5% 16% 26% 15% 1	1440
July	1st to 10th	Night	40% 5% 13% 25% 15% 1	1400
	11th to 20th	Night	43% 5% 16% 27% 9% 1	1392
	21st to end	Night	37% 5% 15% 27% 13% 2	1486
August	1st to 10th	Night	37% 5% 16% 26% 14% 2	1530
	11th to 20th	Night	41% 5% 16% 24% 13%	1656
	21st to end	Night	36% 4 12% 28% 20% 1	1980
September	1st to 10th	Night	40% 4 13% 25% 18% 1	1800
	11th to 20th	Night	34% 3 10% 25% 26% 2	1964
	21st to end	Night	41% 3 8% 26% 20% 3	1980
October	1st to 10th	Night	44% 4 11% 24% 13% 4	1980
	11th to 20th	Night	39% 3 14% 28% 15% 1	2203
	21st to end	Night	49% 3 11% 21% 14% 2	2574
November	1st to 10th	Night	39% 4 11% 22% 21% 4	2407
	11th to 20th	Night	41% 3 10% 23% 20% 3	2470
	21st to end	Night	40% 3 14% 27% 14% 2	2470
December	1st to 10th	Night	47% 3 12% 24% 14% 1	2650
	11th to 20th	Night	43% 3 14% 20% 17% 2	3288
	21st to end	Night	51% 3 7% 26% 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	27% 24% 28% 17% 4	180
	11th to 20th	Day	33% 26% 30% 7% 3	180
	21st to end	Day	28% 19% 36% 13% 4	198
February	1st to 10th	Day	33% 17% 35% 13% 1	357
	11th to 20th	Day	28% 27% 35% 9% 1	456
	21st to end	Day	33% 15% 35% 12% 4 1	382
March	1st to 10th	Day	31% 19% 36% 12% 3	460
	11th to 20th	Day	28% 25% 30% 13% 3 1	460
	21st to end	Day	29% 19% 34% 11% 6% 1	470
April	1st to 10th	Day	34% 19% 30% 11% 6% 1	420
	11th to 20th	Day	28% 18% 37% 13% 4	412
	21st to end	Day	34% 15% 32% 16% 3 1	420
May	1st to 10th	Day	34% 18% 30% 12% 5% 1	423
	11th to 20th	Day	23% 16% 40% 16% 4 1	447
	21st to end	Day	19% 12% 36% 21% 10% 2	485
June	1st to 10th	Day	19% 10% 38% 22% 10% 2	450
	11th to 20th	Day	18% 13% 38% 22% 7% 2	450
	21st to end	Day	20% 9% 36% 23% 10% 3	450
July	1st to 10th	Day	19% 7% 30% 26% 14% 4	445
	11th to 20th	Day	14% 5% 32% 34% 12% 3	444
	21st to end	Day	12% 8% 38% 32% 9% 2	488
August	1st to 10th	Day	11% 8% 32% 35% 11% 2	446
	11th to 20th	Day	18% 6% 30% 31% 12% 3	439
	21st to end	Day	14% 9% 32% 27% 15% 3	495
September	1st to 10th	Day	21% 9% 27% 24% 15% 5%	450
	11th to 20th	Day	21% 7% 30% 21% 14% 7%	450
	21st to end	Day	21% 11% 35% 20% 9% 4	450
October	1st to 10th	Day	30% 10% 26% 21% 10% 3	450
	11th to 20th	Day	19% 18% 32% 22% 8% 2	412
	21st to end	Day	31% 14% 32% 16% 7% 1	396
November	1st to 10th	Day	22% 17% 32% 19% 7% 1	365
	11th to 20th	Day	30% 19% 28% 18% 3 1	370
	21st to end	Day	29% 19% 32% 15% 5%	370
December	1st to 10th	Day	24% 27% 30% 15% 4	185
	11th to 20th	Day	39% 18% 36% 5% 2	180
	21st to end	Day	33% 20% 32% 11% 3 1	198
January	1st to 10th	Night	24% 21% 39% 15% 1	760
	11th to 20th	Night	29% 20% 35% 15% 2	760
	21st to end	Night	22% 22% 34% 19% 3	836
February	1st to 10th	Night	25% 18% 39% 17% 2	760
	11th to 20th	Night	22% 26% 38% 12% 1	743
	21st to end	Night	29% 21% 32% 16% 2	628
March	1st to 10th	Night	25% 21% 34% 18% 3	760
	11th to 20th	Night	22% 24% 35% 17% 2	760
	21st to end	Night	25% 19% 32% 21% 3	801
April	1st to 10th	Night	24% 19% 35% 19% 3	720
	11th to 20th	Night	23% 19% 34% 18% 5%	711
	21st to end	Night	26% 17% 35% 19% 2	552
May	1st to 10th	Night	34% 14% 30% 19% 3 1	540
	11th to 20th	Night	22% 16% 38% 19% 5%	482
	21st to end	Night	21% 8% 38% 28% 5%	389
June	1st to 10th	Night	22% 5% 34% 30% 9%	360
	11th to 20th	Night	22% 8% 37% 27% 6% 1	360
	21st to end	Night	14% 4 42% 32% 8%	360
July	1st to 10th	Night	16% 6% 33% 35% 9% 1	350
	11th to 20th	Night	18% 3 30% 37% 12%	348
	21st to end	Night	13% 3 36% 39% 9%	394
August	1st to 10th	Night	11% 3 39% 36% 11%	466
	11th to 20th	Night	14% 5% 31% 35% 14% 1	592
	21st to end	Night	16% 5% 26% 38% 14% 2	792
September	1st to 10th	Night	18% 4 33% 32% 12% 1	720
	11th to 20th	Night	18% 7% 28% 29% 17% 2	720
	21st to end	Night	19% 8% 30% 29% 11% 2	720
October	1st to 10th	Night	27% 10% 29% 22% 10% 2	720
	11th to 20th	Night	20% 13% 34% 21% 11% 1	720
	21st to end	Night	29% 15% 32% 18% 6%	792
November	1st to 10th	Night	19% 15% 36% 21% 7% 1	740
	11th to 20th	Night	25% 21% 31% 16% 6% 1	760
	21st to end	Night	24% 20% 35% 16% 5% 1	760
December	1st to 10th	Night	27% 18% 35% 15% 4	760
	11th to 20th	Night	27% 19% 34% 17% 2	760
	21st to end	Night	27% 20% 36% 16% 2	836

Page generated at 2024-03-19 23:31:10. Version ac07

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