Truro
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Truro clear sky chart from 2005-11-06 to 2025-10-04.

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	21% 13% 4 8% 5% 4 4 3 5% 6% 27%	127397
	Night	21% 13% 4 9% 5% 4 4 3 4 6% 28%	122115
Transparency	Day	57% 7% 12% 17% 8% 1	130952
	Night	57% 7% 12% 17% 7% 1	126649
Seeing	Day	31% 20% 29% 15% 4 1	18748
	Night	29% 16% 30% 20% 4	26015

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	14% 25% 7% 14% 6% 4 3 2 4 4 17%	3616
	11th to 20th	Day	14% 19% 9% 16% 5% 4 4 3 4 5% 16%	3591
	21st to end	Day	13% 13% 6% 11% 6% 5% 4 3 5% 7% 27%	4462
February	1st to 10th	Day	19% 17% 4 11% 7% 4 3 3 5% 4 23%	1800
	11th to 20th	Day	18% 15% 5% 15% 6% 4 3 3 4 5% 24%	1753
	21st to end	Day	13% 16% 6% 8% 7% 4 3 3 4 6% 30%	1535
March	1st to 10th	Day	24% 14% 4 12% 5% 4 3 2 3 4 24%	4813
	11th to 20th	Day	14% 20% 6% 9% 5% 4 4 3 4 6% 24%	4980
	21st to end	Day	27% 18% 5% 10% 4 3 4 3 4 5% 18%	5528
April	1st to 10th	Day	26% 17% 4 7% 4 4 3 3 4 4 24%	2400
	11th to 20th	Day	22% 13% 3 6% 5% 3 3 3 6% 6% 29%	2416
	21st to end	Day	24% 14% 3 7% 4 3 4 3 5% 5% 27%	2600
May	1st to 10th	Day	32% 16% 3 6% 3 3 4 3 5% 5% 20%	6161
	11th to 20th	Day	23% 12% 3 8% 5% 4 4 3 4 5% 28%	6318
	21st to end	Day	22% 12% 3 7% 5% 5% 4 3 5% 7% 27%	6948
June	1st to 10th	Day	27% 11% 3 7% 5% 5% 4 4 6% 6% 23%	2800
	11th to 20th	Day	23% 9% 3 6% 4 4 4 3 5% 7% 33%	2857
	21st to end	Day	30% 9% 3 6% 5% 4 4 3 5% 6% 26%	3000
July	1st to 10th	Day	23% 9% 3 6% 5% 5% 5% 4 6% 8% 27%	6526
	11th to 20th	Day	20% 8% 3 6% 4 4 4 3 5% 8% 34%	6360
	21st to end	Day	15% 8% 3 7% 4 4 5% 4 6% 8% 35%	6890
August	1st to 10th	Day	23% 10% 3 7% 6% 4 5% 4 6% 6% 27%	4235
	11th to 20th	Day	17% 8% 2 8% 5% 5% 5% 4 6% 8% 31%	4175
	21st to end	Day	12% 6% 2 5% 4 5% 4 4 6% 8% 43%	4452
September	1st to 10th	Day	18% 7% 3 6% 4 4 4 2 6% 7% 39%	2400
	11th to 20th	Day	20% 7% 3 6% 4 4 3 3 5% 6% 39%	2371
	21st to end	Day	21% 7% 2 6% 4 4 4 3 5% 6% 38%	2105
October	1st to 10th	Day	18% 8% 3 8% 5% 5% 4 3 6% 6% 35%	1930
	11th to 20th	Day	22% 10% 2 7% 4 4 4 3 6% 6% 30%	1900
	21st to end	Day	23% 12% 3 7% 6% 4 4 3 4 5% 29%	2059
November	1st to 10th	Day	23% 13% 3 8% 5% 5% 4 2 3 6% 27%	1628
	11th to 20th	Day	24% 16% 4 8% 4 4 4 3 3 5% 26%	1600
	21st to end	Day	20% 18% 6% 9% 6% 4 4 3 3 5% 23%	1600
December	1st to 10th	Day	19% 15% 7% 11% 4 3 4 3 4 6% 25%	2610
	11th to 20th	Day	28% 16% 5% 8% 4 3 3 2 3 4 24%	2996
	21st to end	Day	21% 16% 10% 12% 4 3 4 3 4 5% 19%	3982
January	1st to 10th	Night	14% 21% 7% 10% 6% 5% 5% 3 4 5% 20%	6349
	11th to 20th	Night	17% 16% 8% 16% 6% 5% 4 3 3 4 17%	6314
	21st to end	Night	13% 12% 5% 16% 7% 4 4 3 4 6% 25%	6490
February	1st to 10th	Night	19% 16% 4 11% 7% 4 4 3 4 5% 23%	2600
	11th to 20th	Night	18% 16% 4 12% 6% 4 3 3 5% 6% 24%	2533
	21st to end	Night	14% 14% 4 11% 7% 4 5% 3 5% 4 29%	2093
March	1st to 10th	Night	25% 14% 5% 12% 6% 4 3 3 3 4 20%	5011
	11th to 20th	Night	21% 18% 5% 6% 4 4 4 3 4 5% 26%	4979
	21st to end	Night	29% 15% 6% 10% 4 3 3 2 2 3 22%	5403
April	1st to 10th	Night	26% 15% 3 7% 4 4 3 2 4 4 27%	2000
	11th to 20th	Night	23% 13% 3 6% 4 3 4 3 4 5% 33%	1932
	21st to end	Night	25% 13% 3 6% 4 4 4 4 4 5% 29%	1800
May	1st to 10th	Night	31% 19% 3 6% 4 3 3 3 3 4 19%	3790
	11th to 20th	Night	22% 11% 2 7% 5% 4 5% 3 5% 6% 29%	3596
	21st to end	Night	21% 13% 3 7% 4 4 4 3 5% 7% 28%	3948
June	1st to 10th	Night	27% 9% 3 7% 5% 3 3 2 5% 6% 28%	1600
	11th to 20th	Night	22% 9% 3 5% 3 3 4 3 5% 7% 37%	1553
	21st to end	Night	27% 9% 3 7% 5% 4 4 4 6% 6% 25%	1400
July	1st to 10th	Night	22% 10% 3 8% 5% 5% 5% 4 6% 8% 23%	3385
	11th to 20th	Night	21% 8% 2 5% 4 4 4 4 5% 7% 36%	3620
	21st to end	Night	18% 7% 2 5% 5% 4 3 4 5% 8% 38%	3980
August	1st to 10th	Night	24% 9% 2 6% 4 4 4 3 5% 7% 32%	2915
	11th to 20th	Night	18% 7% 2 7% 4 4 4 4 6% 8% 36%	2872
	21st to end	Night	15% 5% 2 5% 3 4 4 4 5% 6% 47%	3444
September	1st to 10th	Night	17% 6% 2 5% 3 3 4 3 5% 8% 43%	2000
	11th to 20th	Night	19% 8% 3 6% 3 3 3 3 4 6% 42%	2025
	21st to end	Night	20% 7% 3 6% 4 4 3 2 5% 6% 39%	2294
October	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 5% 7% 37%	2317
	11th to 20th	Night	20% 8% 3 6% 4 4 4 4 4 7% 36%	2280
	21st to end	Night	27% 11% 3 7% 5% 4 3 3 4 5% 29%	2537
November	1st to 10th	Night	23% 11% 4 7% 4 4 4 3 4 5% 30%	2666
	11th to 20th	Night	24% 14% 3 7% 4 3 4 4 5% 5% 26%	2800
	21st to end	Night	24% 17% 5% 8% 5% 3 3 3 4 5% 23%	2800
December	1st to 10th	Night	18% 13% 4 11% 5% 4 3 3 4 6% 29%	4580
	11th to 20th	Night	26% 15% 4 8% 4 2 2 2 3 5% 28%	5213
	21st to end	Night	23% 15% 6% 12% 4 4 3 2 4 4 22%	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	69% 7% 13% 7% 3	3617
	11th to 20th	Day	67% 9% 10% 10% 3	3620
	21st to end	Day	56% 6% 11% 11% 11% 4	4462
February	1st to 10th	Day	65% 5% 9% 11% 9% 1	1800
	11th to 20th	Day	64% 6% 10% 13% 6% 1	1761
	21st to end	Day	59% 4 10% 15% 10% 2	1535
March	1st to 10th	Day	65% 5% 8% 12% 8% 2	4817
	11th to 20th	Day	59% 9% 14% 14% 4 1	4980
	21st to end	Day	71% 4 7% 13% 5% 1	5528
April	1st to 10th	Day	64% 5% 7% 13% 8% 2	2400
	11th to 20th	Day	55% 6% 10% 17% 11% 1	2416
	21st to end	Day	58% 7% 10% 15% 9% 1	2600
May	1st to 10th	Day	66% 6% 9% 13% 6%	6172
	11th to 20th	Day	57% 8% 11% 18% 5%	6322
	21st to end	Day	53% 10% 14% 15% 8%	6948
June	1st to 10th	Day	61% 7% 10% 16% 5% 1	2800
	11th to 20th	Day	51% 7% 13% 19% 9% 1	2857
	21st to end	Day	62% 6% 10% 13% 9% 1	3000
July	1st to 10th	Day	55% 7% 13% 16% 9%	6456
	11th to 20th	Day	49% 7% 13% 23% 8%	6284
	21st to end	Day	44% 8% 16% 24% 7%	6693
August	1st to 10th	Day	57% 7% 11% 17% 6%	4222
	11th to 20th	Day	49% 9% 12% 23% 7%	4214
	21st to end	Day	38% 7% 14% 27% 14%	4452
September	1st to 10th	Day	44% 6% 13% 23% 12% 1	2400
	11th to 20th	Day	46% 6% 13% 22% 12% 1	2371
	21st to end	Day	46% 6% 11% 23% 13%	2105
October	1st to 10th	Day	50% 8% 12% 21% 9%	3190
	11th to 20th	Day	49% 6% 10% 23% 11%	3140
	21st to end	Day	50% 4 15% 20% 11%	3375
November	1st to 10th	Day	60% 8% 9% 16% 7%	1628
	11th to 20th	Day	62% 6% 13% 12% 6% 1	1600
	21st to end	Day	65% 7% 12% 12% 4	1600
December	1st to 10th	Day	60% 4 12% 21% 3	2610
	11th to 20th	Day	66% 5% 8% 12% 9%	2995
	21st to end	Day	66% 6% 10% 12% 7%	3982
January	1st to 10th	Night	63% 8% 14% 13% 2	6349
	11th to 20th	Night	67% 10% 9% 9% 4	6360
	21st to end	Night	59% 6% 10% 14% 9% 1	6490
February	1st to 10th	Night	65% 4 10% 12% 7% 2	2600
	11th to 20th	Night	63% 6% 11% 13% 7% 1	2536
	21st to end	Night	58% 7% 9% 16% 8% 1	2093
March	1st to 10th	Night	69% 4 6% 10% 10% 1	5020
	11th to 20th	Night	59% 7% 11% 14% 8% 1	4980
	21st to end	Night	70% 4 6% 13% 6% 1	5403
April	1st to 10th	Night	62% 5% 8% 13% 10% 2	2000
	11th to 20th	Night	54% 6% 8% 19% 11% 1	1932
	21st to end	Night	57% 6% 11% 14% 11% 1	1800
May	1st to 10th	Night	69% 6% 7% 13% 5%	3796
	11th to 20th	Night	54% 9% 12% 19% 5% 1	3597
	21st to end	Night	53% 10% 15% 17% 5%	3949
June	1st to 10th	Night	58% 7% 10% 18% 7%	1600
	11th to 20th	Night	48% 7% 12% 22% 11%	1553
	21st to end	Night	58% 7% 14% 13% 7% 1	1400
July	1st to 10th	Night	58% 7% 14% 15% 5% 1	3346
	11th to 20th	Night	47% 7% 14% 21% 10% 1	3572
	21st to end	Night	45% 7% 16% 23% 8%	3863
August	1st to 10th	Night	51% 7% 15% 22% 5%	2903
	11th to 20th	Night	47% 10% 15% 23% 6%	2899
	21st to end	Night	36% 8% 18% 25% 13% 1	3443
September	1st to 10th	Night	40% 6% 13% 27% 12% 1	2000
	11th to 20th	Night	44% 5% 13% 26% 11% 1	2025
	21st to end	Night	46% 6% 12% 24% 11% 1	2294
October	1st to 10th	Night	50% 6% 13% 22% 9%	3837
	11th to 20th	Night	41% 6% 13% 28% 12%	3779
	21st to end	Night	49% 4 15% 21% 10%	4173
November	1st to 10th	Night	56% 9% 12% 16% 7%	2666
	11th to 20th	Night	59% 9% 13% 13% 7%	2800
	21st to end	Night	64% 7% 14% 13% 3	2800
December	1st to 10th	Night	55% 6% 12% 21% 6%	4580
	11th to 20th	Night	61% 4 8% 16% 9% 1	5215
	21st to end	Night	63% 7% 11% 12% 6%	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	39% 19% 29% 13% 1	400
	11th to 20th	Day	35% 22% 26% 12% 4	400
	21st to end	Day	27% 24% 34% 12% 3	440
February	1st to 10th	Day	34% 24% 28% 12% 2	400
	11th to 20th	Day	37% 27% 22% 12% 1	396
	21st to end	Day	31% 20% 33% 12% 3 1	331
March	1st to 10th	Day	29% 27% 32% 9% 2	473
	11th to 20th	Day	32% 26% 28% 11% 4	500
	21st to end	Day	37% 29% 23% 8% 3	550
April	1st to 10th	Day	45% 25% 21% 7% 2	500
	11th to 20th	Day	37% 26% 24% 10% 2	492
	21st to end	Day	36% 20% 29% 12% 3	500
May	1st to 10th	Day	40% 20% 27% 9% 3 1	622
	11th to 20th	Day	30% 20% 28% 16% 4 1	724
	21st to end	Day	31% 21% 31% 14% 2 1	790
June	1st to 10th	Day	37% 17% 28% 14% 4	730
	11th to 20th	Day	28% 17% 34% 17% 3 1	730
	21st to end	Day	37% 15% 26% 17% 5% 1	730
July	1st to 10th	Day	25% 14% 34% 21% 5%	720
	11th to 20th	Day	24% 11% 36% 22% 7%	718
	21st to end	Day	27% 12% 32% 22% 7%	783
August	1st to 10th	Day	17% 13% 39% 24% 5% 1	526
	11th to 20th	Day	23% 14% 33% 19% 9% 1	519
	21st to end	Day	17% 19% 32% 22% 8% 2	583
September	1st to 10th	Day	21% 17% 34% 21% 6% 1	530
	11th to 20th	Day	26% 20% 28% 19% 6% 2	530
	21st to end	Day	23% 19% 29% 19% 8% 1	478
October	1st to 10th	Day	22% 17% 31% 19% 8% 3	399
	11th to 20th	Day	29% 19% 28% 13% 9% 1	390
	21st to end	Day	31% 25% 26% 12% 6%	429
November	1st to 10th	Day	32% 22% 27% 14% 5% 1	395
	11th to 20th	Day	40% 21% 23% 13% 3 1	400
	21st to end	Day	37% 23% 27% 10% 3 1	400
December	1st to 10th	Day	37% 21% 28% 12% 2	400
	11th to 20th	Day	43% 23% 23% 10% 1	400
	21st to end	Day	32% 25% 29% 11% 3 1	440
January	1st to 10th	Night	32% 21% 31% 14% 2	900
	11th to 20th	Night	33% 19% 29% 16% 3	900
	21st to end	Night	26% 23% 34% 15% 3	880
February	1st to 10th	Night	31% 20% 31% 18% 1	800
	11th to 20th	Night	34% 27% 25% 12% 2	783
	21st to end	Night	22% 23% 32% 18% 4	660
March	1st to 10th	Night	27% 23% 31% 17% 2	800
	11th to 20th	Night	27% 20% 27% 21% 5%	800
	21st to end	Night	30% 20% 29% 17% 4	855
April	1st to 10th	Night	36% 19% 29% 15% 1	600
	11th to 20th	Night	31% 18% 31% 16% 4	594
	21st to end	Night	33% 16% 31% 17% 3 1	600
May	1st to 10th	Night	36% 15% 30% 16% 4	600
	11th to 20th	Night	29% 13% 33% 21% 4 1	592
	21st to end	Night	23% 16% 43% 15% 3	649
June	1st to 10th	Night	34% 14% 33% 17% 3	600
	11th to 20th	Night	26% 13% 38% 21% 2	553
	21st to end	Night	31% 8% 30% 26% 5%	400
July	1st to 10th	Night	24% 6% 36% 30% 4	481
	11th to 20th	Night	21% 8% 36% 29% 5%	582
	21st to end	Night	22% 8% 31% 33% 6%	656
August	1st to 10th	Night	19% 7% 34% 33% 7% 1	598
	11th to 20th	Night	22% 8% 32% 30% 8% 1	592
	21st to end	Night	20% 7% 33% 32% 8%	660
September	1st to 10th	Night	21% 8% 35% 28% 8%	600
	11th to 20th	Night	25% 10% 29% 27% 9%	625
	21st to end	Night	24% 13% 31% 24% 7% 1	800
October	1st to 10th	Night	26% 11% 26% 27% 8% 1	773
	11th to 20th	Night	25% 14% 33% 20% 8% 1	760
	21st to end	Night	34% 17% 28% 16% 4	852
November	1st to 10th	Night	33% 15% 29% 17% 5% 1	880
	11th to 20th	Night	33% 21% 25% 17% 4	900
	21st to end	Night	35% 21% 25% 16% 2	900
December	1st to 10th	Night	34% 17% 30% 16% 3	900
	11th to 20th	Night	37% 23% 27% 11% 2	900
	21st to end	Night	33% 23% 26% 16% 2 1	990

Page generated at 2025-10-01 02:39:07. Version ac07

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