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 2025-08-05.

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% 8% 2 6% 4 4 4 4 5% 7% 38%	125475
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	122577
Transparency	Day	44% 4 11% 23% 15% 3	128995
	Night	44% 3 11% 23% 16% 3	127144
Seeing	Day	25% 15% 33% 19% 7% 2	15957
	Night	23% 15% 34% 22% 6% 1	24850

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% 3 9% 5% 4 4 4 5% 6% 42%	3597
	11th to 20th	Day	13% 7% 4 6% 4 4 4 3 5% 5% 46%	3570
	21st to end	Day	16% 9% 2 5% 3 4 5% 3 5% 5% 43%	3960
February	1st to 10th	Day	17% 8% 3 6% 5% 5% 6% 4 5% 7% 35%	1869
	11th to 20th	Day	13% 7% 2 6% 5% 5% 6% 4 6% 8% 39%	1948
	21st to end	Day	17% 10% 2 7% 5% 4 4 2 4 6% 38%	1650
March	1st to 10th	Day	19% 11% 3 6% 4 4 4 3 5% 6% 36%	4476
	11th to 20th	Day	14% 7% 2 5% 4 5% 5% 5% 6% 8% 39%	4722
	21st to end	Day	22% 10% 3 6% 3 4 4 3 6% 8% 32%	5850
April	1st to 10th	Day	17% 9% 3 7% 5% 5% 4 4 4 7% 36%	2400
	11th to 20th	Day	17% 6% 2 6% 5% 4 4 3 5% 7% 40%	2374
	21st to end	Day	21% 7% 2 6% 4 4 4 4 5% 7% 36%	2586
May	1st to 10th	Day	29% 10% 3 7% 4 4 4 4 5% 7% 24%	5886
	11th to 20th	Day	21% 9% 3 8% 4 4 4 3 5% 7% 31%	6036
	21st to end	Day	19% 7% 3 7% 4 4 5% 4 6% 8% 34%	6949
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 4 6% 8% 35%	2800
	11th to 20th	Day	17% 6% 3 6% 4 4 5% 4 6% 8% 37%	2800
	21st to end	Day	17% 6% 2 7% 4 5% 4 4 7% 8% 35%	2800
July	1st to 10th	Day	16% 7% 2 6% 5% 5% 6% 4 7% 8% 33%	6360
	11th to 20th	Day	9% 7% 3 7% 5% 6% 5% 5% 7% 11% 36%	6360
	21st to end	Day	15% 6% 3 6% 4 4 4 3 5% 7% 43%	6926
August	1st to 10th	Day	15% 6% 3 7% 6% 5% 6% 5% 6% 9% 33%	4242
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 4 6% 7% 38%	3926
	21st to end	Day	7% 4 2 6% 5% 5% 6% 4 7% 9% 46%	4180
September	1st to 10th	Day	13% 6% 2 5% 4 5% 5% 3 5% 7% 45%	2280
	11th to 20th	Day	16% 6% 2 4 2 3 3 3 4 6% 50%	2104
	21st to end	Day	18% 6% 2 6% 4 4 4 3 4 7% 43%	2090
October	1st to 10th	Day	22% 6% 3 5% 4 4 4 3 4 6% 40%	2090
	11th to 20th	Day	13% 7% 2 5% 4 3 3 3 5% 8% 46%	1851
	21st to end	Day	19% 7% 2 7% 4 3 4 3 5% 5% 41%	1881
November	1st to 10th	Day	13% 8% 2 6% 4 3 3 3 4 5% 49%	1761
	11th to 20th	Day	18% 7% 2 4 3 3 3 2 6% 6% 47%	1800
	21st to end	Day	16% 8% 2 5% 4 5% 3 5% 5% 5% 42%	1800
December	1st to 10th	Day	14% 9% 2 5% 3 4 4 3 5% 7% 45%	2605
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 4 6% 46%	2986
	21st to end	Day	18% 11% 3 7% 4 4 4 3 4 6% 37%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 6% 7% 46%	6349
	11th to 20th	Night	12% 9% 3 8% 5% 4 4 3 5% 6% 41%	6315
	21st to end	Night	15% 9% 3 7% 5% 4 4 3 5% 6% 39%	6996
February	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 5% 5% 39%	2525
	11th to 20th	Night	13% 8% 3 7% 4 5% 4 4 5% 7% 40%	2338
	21st to end	Night	16% 9% 3 6% 5% 4 4 3 4 7% 39%	1980
March	1st to 10th	Night	21% 10% 2 4 4 3 4 3 5% 6% 39%	5364
	11th to 20th	Night	15% 9% 3 7% 4 4 4 3 5% 7% 40%	5237
	21st to end	Night	22% 10% 3 5% 3 3 4 3 5% 6% 34%	5067
April	1st to 10th	Night	17% 9% 2 7% 5% 5% 4 4 5% 6% 35%	2000
	11th to 20th	Night	18% 7% 2 6% 4 5% 3 4 5% 6% 39%	1975
	21st to end	Night	20% 7% 3 6% 4 3 3 3 5% 6% 40%	1812
May	1st to 10th	Night	32% 9% 3 6% 4 3 3 3 4 6% 26%	4054
	11th to 20th	Night	17% 9% 3 9% 6% 6% 5% 4 6% 6% 30%	3899
	21st to end	Night	20% 8% 3 7% 4 4 5% 3 5% 7% 33%	3926
June	1st to 10th	Night	17% 7% 3 7% 5% 5% 4 3 5% 8% 37%	1600
	11th to 20th	Night	18% 7% 1 6% 5% 4 4 4 5% 9% 36%	1600
	21st to end	Night	14% 7% 2 5% 4 4 5% 4 6% 7% 41%	1600
July	1st to 10th	Night	14% 7% 3 7% 4 4 5% 4 7% 9% 37%	3600
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 7% 9% 39%	3600
	21st to end	Night	12% 7% 3 6% 4 4 5% 4 4 6% 46%	3926
August	1st to 10th	Night	20% 6% 3 8% 5% 5% 6% 4 5% 7% 32%	2731
	11th to 20th	Night	12% 7% 3 8% 5% 6% 5% 4 6% 7% 36%	2898
	21st to end	Night	8% 4 2 5% 3 3 3 3 5% 7% 58%	3476
September	1st to 10th	Night	13% 6% 3 6% 5% 3 4 4 6% 6% 45%	1900
	11th to 20th	Night	13% 6% 2 4 3 3 4 2 5% 5% 52%	2074
	21st to end	Night	17% 7% 3 6% 4 3 4 3 5% 6% 43%	2090
October	1st to 10th	Night	20% 6% 2 5% 4 3 3 4 4 6% 42%	2090
	11th to 20th	Night	15% 5% 2 5% 3 4 4 3 5% 6% 47%	2327
	21st to end	Night	21% 8% 3 6% 4 4 4 3 4 6% 40%	2717
November	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 5% 49%	2537
	11th to 20th	Night	17% 7% 2 4 4 4 3 3 5% 5% 47%	2600
	21st to end	Night	15% 7% 2 5% 4 3 5% 3 5% 6% 45%	2600
December	1st to 10th	Night	15% 10% 2 6% 4 3 3 2 4 6% 44%	4570
	11th to 20th	Night	22% 7% 2 6% 3 3 3 3 4 6% 41%	5208
	21st to end	Night	17% 11% 3 7% 4 3 3 3 4 4 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	42% 2 7% 29% 17% 3	3598
	11th to 20th	Day	41% 3 10% 22% 19% 6%	3600
	21st to end	Day	43% 2 10% 23% 20% 2	3960
February	1st to 10th	Day	47% 3 14% 20% 13% 2	1869
	11th to 20th	Day	44% 5% 10% 23% 16% 3	1957
	21st to end	Day	49% 2 9% 21% 16% 3	1650
March	1st to 10th	Day	50% 2 11% 16% 12% 9%	4479
	11th to 20th	Day	42% 3 13% 29% 11% 2	4722
	21st to end	Day	51% 3 6% 16% 18% 6%	5850
April	1st to 10th	Day	49% 3 9% 20% 16% 3	2400
	11th to 20th	Day	44% 3 10% 21% 19% 3	2374
	21st to end	Day	48% 3 8% 19% 18% 3	2586
May	1st to 10th	Day	61% 3 13% 13% 9% 1	5897
	11th to 20th	Day	52% 3 14% 18% 11% 1	6040
	21st to end	Day	48% 4 10% 24% 13% 1	6949
June	1st to 10th	Day	46% 5% 12% 25% 12% 1	2800
	11th to 20th	Day	44% 5% 13% 22% 15% 2	2800
	21st to end	Day	46% 5% 11% 23% 14% 1	2800
July	1st to 10th	Day	46% 7% 11% 21% 13% 1	6290
	11th to 20th	Day	40% 6% 17% 25% 11% 1	6285
	21st to end	Day	40% 4 12% 27% 16% 1	6728
August	1st to 10th	Day	47% 6% 15% 22% 9% 1	4228
	11th to 20th	Day	43% 5% 15% 27% 10% 1	3965
	21st to end	Day	33% 5% 12% 32% 18% 1	4180
September	1st to 10th	Day	38% 4 11% 25% 20% 2	2280
	11th to 20th	Day	35% 3 8% 28% 23% 3	2104
	21st to end	Day	41% 4 9% 22% 21% 2	2090
October	1st to 10th	Day	41% 4 9% 23% 20% 3	3480
	11th to 20th	Day	31% 3 7% 28% 27% 4	3041
	21st to end	Day	33% 3 12% 29% 19% 3	3081
November	1st to 10th	Day	39% 3 12% 26% 18% 2	1761
	11th to 20th	Day	38% 3 9% 23% 24% 3	1800
	21st to end	Day	43% 4 12% 26% 13% 2	1800
December	1st to 10th	Day	40% 3 15% 28% 13% 1	2605
	11th to 20th	Day	41% 2 14% 18% 21% 3	2986
	21st to end	Day	50% 3 8% 20% 16% 5%	3960
January	1st to 10th	Night	37% 2 10% 33% 16% 2	6349
	11th to 20th	Night	45% 3 12% 21% 15% 4	6360
	21st to end	Night	48% 2 10% 17% 18% 5%	6996
February	1st to 10th	Night	46% 4 12% 22% 14% 2	2525
	11th to 20th	Night	44% 4 9% 21% 18% 4	2340
	21st to end	Night	47% 2 7% 24% 15% 4	1980
March	1st to 10th	Night	48% 3 9% 18% 17% 6%	5374
	11th to 20th	Night	46% 3 16% 20% 13% 3	5238
	21st to end	Night	52% 2 4 13% 23% 6%	5067
April	1st to 10th	Night	49% 3 9% 20% 16% 3	2000
	11th to 20th	Night	45% 3 9% 21% 19% 2	1975
	21st to end	Night	46% 3 9% 21% 16% 5%	1812
May	1st to 10th	Night	61% 2 13% 14% 9% 1	4060
	11th to 20th	Night	54% 5% 13% 17% 9% 2	3900
	21st to end	Night	52% 4 13% 19% 12% 1	3927
June	1st to 10th	Night	46% 5% 11% 25% 13% 1	1600
	11th to 20th	Night	46% 4 13% 23% 13% 1	1600
	21st to end	Night	40% 5% 15% 25% 15% 1	1600
July	1st to 10th	Night	43% 6% 14% 23% 15%	3560
	11th to 20th	Night	41% 5% 19% 22% 13% 1	3552
	21st to end	Night	39% 4 11% 28% 16% 1	3809
August	1st to 10th	Night	51% 6% 13% 20% 10% 1	2721
	11th to 20th	Night	45% 6% 15% 25% 10%	2925
	21st to end	Night	27% 4 15% 32% 21% 1	3476
September	1st to 10th	Night	39% 3 12% 26% 19% 1	1900
	11th to 20th	Night	34% 3 10% 25% 26% 2	2074
	21st to end	Night	43% 3 7% 25% 19% 3	2090
October	1st to 10th	Night	39% 4 14% 24% 17% 2	3480
	11th to 20th	Night	27% 3 11% 27% 29% 4	3876
	21st to end	Night	35% 3 12% 32% 17% 2	4467
November	1st to 10th	Night	38% 4 11% 23% 21% 4	2537
	11th to 20th	Night	40% 3 10% 22% 21% 4	2600
	21st to end	Night	40% 3 14% 27% 14% 2	2600
December	1st to 10th	Night	43% 2 12% 30% 12% 1	4570
	11th to 20th	Night	46% 3 14% 16% 20% 2	5208
	21st to end	Night	47% 3 6% 23% 17% 4	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% 28% 15% 4	200
	11th to 20th	Day	34% 26% 31% 7% 3	200
	21st to end	Day	30% 21% 34% 12% 3	220
February	1st to 10th	Day	33% 19% 35% 12% 1	391
	11th to 20th	Day	31% 27% 33% 9% 1	496
	21st to end	Day	31% 17% 37% 11% 3	414
March	1st to 10th	Day	31% 21% 34% 11% 2	500
	11th to 20th	Day	28% 25% 30% 13% 3 1	500
	21st to end	Day	31% 19% 34% 9% 5% 1	550
April	1st to 10th	Day	35% 19% 30% 10% 5% 1	500
	11th to 20th	Day	33% 19% 34% 11% 4	492
	21st to end	Day	32% 16% 32% 16% 3 2	500
May	1st to 10th	Day	34% 18% 29% 14% 4 1	503
	11th to 20th	Day	24% 16% 38% 16% 5% 1	527
	21st to end	Day	21% 12% 37% 20% 9% 1	573
June	1st to 10th	Day	20% 9% 38% 23% 9% 2	530
	11th to 20th	Day	19% 13% 35% 24% 7% 2	530
	21st to end	Day	19% 10% 37% 22% 8% 3	530
July	1st to 10th	Day	18% 8% 31% 26% 13% 3	525
	11th to 20th	Day	12% 6% 32% 34% 13% 3	524
	21st to end	Day	13% 8% 38% 31% 9% 2	576
August	1st to 10th	Day	12% 8% 31% 34% 12% 2	502
	11th to 20th	Day	17% 7% 31% 30% 12% 3	479
	21st to end	Day	13% 9% 32% 28% 15% 3	539
September	1st to 10th	Day	20% 9% 28% 24% 14% 5%	490
	11th to 20th	Day	20% 7% 28% 21% 15% 9%	490
	21st to end	Day	22% 10% 35% 20% 8% 4	490
October	1st to 10th	Day	28% 11% 26% 21% 10% 3	490
	11th to 20th	Day	19% 19% 31% 21% 8% 2	447
	21st to end	Day	29% 13% 33% 16% 7% 1	429
November	1st to 10th	Day	23% 17% 32% 19% 7% 1	395
	11th to 20th	Day	29% 20% 30% 17% 3 1	400
	21st to end	Day	30% 20% 31% 15% 4	400
December	1st to 10th	Day	24% 26% 31% 15% 4	205
	11th to 20th	Day	40% 19% 34% 5% 3	200
	21st to end	Day	32% 19% 32% 13% 4 1	220
January	1st to 10th	Night	24% 23% 38% 14% 1	800
	11th to 20th	Night	28% 20% 35% 15% 2	800
	21st to end	Night	22% 22% 34% 19% 3	880
February	1st to 10th	Night	25% 18% 38% 17% 2	800
	11th to 20th	Night	24% 25% 38% 12% 1	783
	21st to end	Night	28% 22% 33% 15% 2	660
March	1st to 10th	Night	26% 22% 33% 17% 2	800
	11th to 20th	Night	22% 23% 35% 17% 2	800
	21st to end	Night	26% 18% 33% 20% 3	880
April	1st to 10th	Night	25% 18% 37% 18% 3	800
	11th to 20th	Night	25% 19% 35% 17% 5%	791
	21st to end	Night	25% 19% 35% 19% 2	612
May	1st to 10th	Night	34% 13% 30% 19% 3 1	600
	11th to 20th	Night	21% 15% 38% 20% 5%	536
	21st to end	Night	22% 8% 37% 28% 5%	433
June	1st to 10th	Night	21% 6% 35% 30% 8%	400
	11th to 20th	Night	22% 8% 37% 27% 5% 1	400
	21st to end	Night	14% 6% 41% 31% 8%	400
July	1st to 10th	Night	16% 6% 33% 37% 9% 1	390
	11th to 20th	Night	17% 5% 30% 36% 12%	388
	21st to end	Night	13% 4 37% 37% 9%	438
August	1st to 10th	Night	13% 3 38% 36% 10%	501
	11th to 20th	Night	14% 5% 32% 34% 14% 1	626
	21st to end	Night	16% 5% 26% 38% 15% 2	836
September	1st to 10th	Night	17% 4 34% 31% 13% 1	760
	11th to 20th	Night	17% 6% 27% 29% 19% 2	760
	21st to end	Night	21% 8% 30% 28% 11% 2	760
October	1st to 10th	Night	25% 10% 30% 23% 10% 2	760
	11th to 20th	Night	20% 14% 33% 21% 11% 1	760
	21st to end	Night	28% 15% 32% 19% 6%	836
November	1st to 10th	Night	19% 15% 36% 21% 7% 1	780
	11th to 20th	Night	25% 21% 31% 17% 6% 1	800
	21st to end	Night	25% 20% 35% 15% 5% 1	800
December	1st to 10th	Night	27% 19% 35% 15% 4	800
	11th to 20th	Night	28% 19% 34% 17% 2	800
	21st to end	Night	26% 19% 35% 17% 2	880

Page generated at 2025-08-02 01:32:23. Version ac07

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