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 2024-11-05.

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	22% 12% 4 8% 5% 4 4 3 5% 6% 27%	101977
	Night	22% 12% 4 8% 5% 4 4 3 4 6% 28%	97127
Transparency	Day	57% 7% 12% 16% 8%	105550
	Night	56% 7% 12% 16% 7% 1	101645
Seeing	Day	31% 20% 29% 15% 4 1	17457
	Night	29% 17% 31% 20% 4	24800

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% 18% 6% 14% 6% 4 4 3 5% 5% 22%	2526
	11th to 20th	Day	16% 18% 6% 18% 6% 5% 4 3 4 5% 17%	2530
	21st to end	Day	16% 13% 7% 13% 5% 4 4 4 5% 6% 23%	3120
February	1st to 10th	Day	20% 17% 4 11% 7% 3 3 3 4 4 23%	1710
	11th to 20th	Day	18% 15% 5% 14% 6% 4 3 4 4 5% 23%	1663
	21st to end	Day	13% 16% 6% 8% 7% 4 3 3 5% 6% 30%	1460
March	1st to 10th	Day	25% 14% 4 8% 6% 5% 3 2 4 4 25%	3355
	11th to 20th	Day	14% 26% 7% 10% 6% 4 4 3 3 4 20%	3480
	21st to end	Day	31% 17% 5% 9% 4 3 3 2 3 4 19%	3864
April	1st to 10th	Day	26% 17% 4 7% 4 4 3 3 4 4 24%	2280
	11th to 20th	Day	22% 12% 3 6% 5% 3 3 3 6% 6% 30%	2294
	21st to end	Day	24% 14% 3 6% 4 3 4 3 5% 5% 26%	2470
May	1st to 10th	Day	29% 18% 3 6% 3 3 3 3 4 5% 21%	4297
	11th to 20th	Day	21% 11% 3 8% 5% 4 4 3 5% 5% 30%	4398
	21st to end	Day	21% 10% 3 7% 5% 6% 5% 4 6% 8% 25%	4836
June	1st to 10th	Day	28% 11% 3 7% 5% 5% 4 3 5% 6% 22%	2660
	11th to 20th	Day	23% 9% 2 6% 4 4 3 3 5% 7% 33%	2714
	21st to end	Day	31% 9% 3 6% 5% 4 4 3 5% 6% 25%	2850
July	1st to 10th	Day	24% 9% 2 6% 4 4 4 4 5% 7% 30%	4550
	11th to 20th	Day	21% 8% 3 6% 4 4 5% 4 6% 8% 30%	4440
	21st to end	Day	18% 9% 3 8% 5% 4 5% 4 6% 8% 31%	4792
August	1st to 10th	Day	23% 10% 3 7% 6% 4 5% 4 6% 6% 26%	4105
	11th to 20th	Day	17% 8% 2 8% 5% 5% 5% 4 6% 8% 30%	4045
	21st to end	Day	12% 6% 2 5% 4 5% 4 4 6% 8% 43%	4316
September	1st to 10th	Day	19% 7% 3 6% 4 3 4 2 6% 7% 40%	2280
	11th to 20th	Day	20% 7% 3 6% 4 4 3 3 5% 6% 39%	2253
	21st to end	Day	21% 7% 2 6% 4 4 4 3 6% 6% 37%	2000
October	1st to 10th	Day	18% 8% 3 8% 5% 5% 4 3 6% 6% 35%	1900
	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 5% 27%	1581
	11th to 20th	Day	23% 14% 4 8% 4 4 4 3 3 5% 27%	1520
	21st to end	Day	19% 17% 6% 9% 6% 4 4 3 3 5% 24%	1520
December	1st to 10th	Day	25% 17% 6% 9% 4 4 3 2 3 5% 22%	1520
	11th to 20th	Day	32% 16% 5% 9% 4 2 3 2 4 4 20%	1906
	21st to end	Day	23% 16% 10% 12% 4 3 4 3 4 5% 16%	2783
January	1st to 10th	Night	15% 13% 6% 10% 6% 5% 5% 3 5% 6% 26%	4429
	11th to 20th	Night	21% 16% 5% 14% 5% 5% 4 3 4 5% 19%	4440
	21st to end	Night	18% 13% 5% 15% 6% 4 4 3 5% 5% 23%	4532
February	1st to 10th	Night	20% 16% 4 10% 7% 4 4 3 4 6% 23%	2470
	11th to 20th	Night	18% 16% 4 12% 6% 4 3 3 5% 6% 24%	2403
	21st to end	Night	13% 14% 4 11% 7% 5% 5% 3 5% 4 29%	1992
March	1st to 10th	Night	22% 13% 6% 12% 6% 5% 4 3 3 4 21%	3483
	11th to 20th	Night	21% 21% 7% 7% 4 4 4 3 4 5% 20%	3479
	21st to end	Night	30% 12% 7% 10% 5% 3 2 2 3 3 22%	3767
April	1st to 10th	Night	25% 15% 3 7% 4 4 3 2 4 4 27%	1900
	11th to 20th	Night	22% 13% 3 6% 4 3 4 3 4 5% 33%	1834
	21st to end	Night	25% 13% 3 6% 4 4 3 4 4 5% 29%	1710
May	1st to 10th	Night	26% 22% 3 6% 4 3 3 3 4 5% 22%	2648
	11th to 20th	Night	20% 10% 3 6% 5% 5% 5% 3 6% 7% 31%	2506
	21st to end	Night	23% 11% 4 8% 5% 4 4 4 5% 6% 26%	2749
June	1st to 10th	Night	28% 10% 3 7% 5% 3 3 2 5% 6% 27%	1520
	11th to 20th	Night	22% 9% 3 6% 3 3 4 3 5% 7% 36%	1476
	21st to end	Night	27% 9% 3 8% 5% 4 4 4 6% 5% 25%	1330
July	1st to 10th	Night	23% 10% 3 7% 5% 4 5% 3 6% 8% 25%	2365
	11th to 20th	Night	24% 8% 2 5% 4 4 4 5% 6% 7% 31%	2530
	21st to end	Night	18% 7% 2 6% 5% 4 3 4 4 8% 39%	2769
August	1st to 10th	Night	24% 10% 2 6% 4 4 5% 3 5% 8% 30%	2825
	11th to 20th	Night	19% 7% 2 7% 4 4 4 4 5% 8% 35%	2782
	21st to end	Night	15% 5% 2 5% 3 3 4 4 5% 6% 47%	3338
September	1st to 10th	Night	17% 6% 2 5% 3 3 4 3 5% 8% 43%	1900
	11th to 20th	Night	20% 7% 3 6% 3 3 3 3 4 6% 42%	1924
	21st to end	Night	21% 7% 3 6% 4 4 3 2 5% 6% 38%	2179
October	1st to 10th	Night	21% 8% 2 6% 4 4 4 4 5% 7% 36%	2280
	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%	2593
	11th to 20th	Night	22% 14% 3 7% 5% 3 4 4 6% 5% 27%	2660
	21st to end	Night	23% 16% 5% 8% 5% 4 3 3 4 6% 24%	2660
December	1st to 10th	Night	26% 14% 3 10% 5% 3 4 3 4 5% 22%	2660
	11th to 20th	Night	33% 15% 3 8% 4 3 3 2 3 4 22%	3293
	21st to end	Night	27% 16% 5% 11% 4 4 3 2 4 4 20%	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	62% 8% 18% 8% 3	2527
	11th to 20th	Day	64% 11% 9% 11% 5%	2530
	21st to end	Day	61% 7% 11% 11% 9%	3120
February	1st to 10th	Day	65% 5% 9% 11% 8% 1	1710
	11th to 20th	Day	65% 6% 10% 13% 6% 1	1671
	21st to end	Day	58% 4 10% 15% 10% 2	1460
March	1st to 10th	Day	62% 6% 9% 15% 7% 1	3359
	11th to 20th	Day	66% 9% 9% 10% 5% 1	3480
	21st to end	Day	72% 4 8% 10% 5% 1	3864
April	1st to 10th	Day	64% 5% 7% 13% 8% 2	2280
	11th to 20th	Day	54% 6% 11% 17% 11% 1	2294
	21st to end	Day	58% 7% 10% 15% 9% 1	2470
May	1st to 10th	Day	64% 6% 10% 14% 4	4311
	11th to 20th	Day	54% 10% 13% 17% 6%	4402
	21st to end	Day	50% 13% 17% 13% 6%	4836
June	1st to 10th	Day	62% 7% 10% 15% 4	2660
	11th to 20th	Day	51% 7% 13% 19% 9% 1	2714
	21st to end	Day	62% 6% 10% 13% 8% 1	2850
July	1st to 10th	Day	52% 7% 12% 17% 11%	4480
	11th to 20th	Day	52% 8% 13% 19% 7%	4364
	21st to end	Day	50% 9% 14% 20% 7%	4639
August	1st to 10th	Day	59% 8% 12% 17% 5%	4092
	11th to 20th	Day	49% 9% 12% 22% 7%	4084
	21st to end	Day	38% 7% 15% 26% 14%	4316
September	1st to 10th	Day	44% 7% 14% 23% 11% 1	2280
	11th to 20th	Day	47% 6% 14% 22% 12% 1	2253
	21st to end	Day	47% 6% 11% 23% 12%	2000
October	1st to 10th	Day	51% 8% 11% 21% 9%	3160
	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%	1581
	11th to 20th	Day	60% 6% 14% 13% 6% 1	1520
	21st to end	Day	63% 7% 13% 12% 4	1520
December	1st to 10th	Day	67% 5% 11% 13% 4	1520
	11th to 20th	Day	70% 6% 9% 11% 4	1905
	21st to end	Day	71% 7% 11% 7% 3	2783
January	1st to 10th	Night	55% 11% 18% 14% 3	4429
	11th to 20th	Night	62% 12% 10% 10% 6%	4440
	21st to end	Night	62% 7% 11% 15% 5% 1	4532
February	1st to 10th	Night	65% 5% 10% 12% 7% 1	2470
	11th to 20th	Night	63% 6% 11% 12% 7% 1	2406
	21st to end	Night	58% 7% 9% 16% 8% 1	1992
March	1st to 10th	Night	67% 5% 7% 13% 8%	3492
	11th to 20th	Night	65% 8% 9% 12% 6% 1	3480
	21st to end	Night	68% 5% 8% 12% 6% 1	3767
April	1st to 10th	Night	62% 5% 8% 13% 9% 3	1900
	11th to 20th	Night	54% 6% 9% 19% 11% 1	1834
	21st to end	Night	57% 6% 11% 14% 11% 1	1710
May	1st to 10th	Night	65% 8% 8% 13% 6%	2654
	11th to 20th	Night	50% 11% 15% 18% 6%	2507
	21st to end	Night	54% 11% 16% 14% 4 1	2750
June	1st to 10th	Night	59% 7% 11% 17% 6%	1520
	11th to 20th	Night	49% 7% 13% 21% 11%	1476
	21st to end	Night	59% 7% 14% 13% 6% 1	1330
July	1st to 10th	Night	57% 8% 14% 13% 7% 2	2326
	11th to 20th	Night	50% 9% 16% 16% 9%	2482
	21st to end	Night	46% 7% 14% 23% 10%	2682
August	1st to 10th	Night	53% 7% 15% 21% 4	2813
	11th to 20th	Night	47% 10% 15% 23% 5%	2809
	21st to end	Night	36% 8% 18% 25% 13% 1	3337
September	1st to 10th	Night	41% 7% 13% 27% 12% 1	1900
	11th to 20th	Night	44% 5% 13% 26% 11% 1	1924
	21st to end	Night	47% 7% 13% 23% 11% 1	2179
October	1st to 10th	Night	50% 6% 13% 22% 9%	3800
	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	55% 9% 12% 16% 7%	2593
	11th to 20th	Night	57% 9% 13% 13% 7%	2660
	21st to end	Night	62% 7% 14% 13% 3	2660
December	1st to 10th	Night	65% 7% 11% 13% 4	2660
	11th to 20th	Night	68% 5% 9% 12% 6%	3295
	21st to end	Night	68% 7% 12% 10% 4	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	37% 19% 29% 14% 1	370
	11th to 20th	Day	36% 23% 26% 11% 4	370
	21st to end	Day	27% 23% 34% 13% 3	407
February	1st to 10th	Day	34% 24% 27% 13% 2	370
	11th to 20th	Day	37% 27% 21% 13% 1	366
	21st to end	Day	30% 21% 34% 11% 4 1	307
March	1st to 10th	Day	28% 27% 32% 10% 2	436
	11th to 20th	Day	33% 26% 27% 10% 4	460
	21st to end	Day	37% 28% 23% 8% 3	506
April	1st to 10th	Day	45% 25% 22% 7% 2	460
	11th to 20th	Day	35% 27% 25% 10% 2	452
	21st to end	Day	34% 19% 30% 13% 3	460
May	1st to 10th	Day	39% 20% 27% 9% 3 1	576
	11th to 20th	Day	28% 20% 29% 17% 4 1	674
	21st to end	Day	31% 21% 31% 14% 2	735
June	1st to 10th	Day	38% 17% 28% 13% 4	680
	11th to 20th	Day	29% 17% 34% 17% 3 1	680
	21st to end	Day	36% 14% 26% 18% 5% 1	680
July	1st to 10th	Day	24% 14% 34% 21% 5%	670
	11th to 20th	Day	24% 11% 35% 21% 8%	668
	21st to end	Day	28% 12% 31% 22% 6%	729
August	1st to 10th	Day	18% 13% 38% 24% 5% 1	486
	11th to 20th	Day	24% 15% 32% 18% 9% 1	479
	21st to end	Day	17% 18% 32% 23% 8% 2	539
September	1st to 10th	Day	21% 18% 33% 21% 6% 1	490
	11th to 20th	Day	27% 19% 27% 20% 6% 1	490
	21st to end	Day	23% 19% 28% 19% 9% 1	443
October	1st to 10th	Day	23% 16% 32% 19% 8% 3	390
	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% 21% 27% 14% 5% 1	378
	11th to 20th	Day	37% 21% 24% 14% 4 1	370
	21st to end	Day	35% 24% 28% 10% 3 1	370
December	1st to 10th	Day	38% 21% 28% 12% 1	370
	11th to 20th	Day	44% 24% 23% 9% 1	370
	21st to end	Day	32% 25% 28% 11% 3 1	407
January	1st to 10th	Night	32% 20% 31% 15% 2	850
	11th to 20th	Night	34% 20% 29% 15% 3	850
	21st to end	Night	26% 22% 33% 15% 3	836
February	1st to 10th	Night	31% 19% 32% 18% 1	760
	11th to 20th	Night	33% 28% 25% 12% 2	743
	21st to end	Night	22% 23% 32% 18% 4	628
March	1st to 10th	Night	26% 23% 32% 17% 2	760
	11th to 20th	Night	27% 20% 27% 22% 5%	760
	21st to end	Night	30% 19% 29% 17% 4	812
April	1st to 10th	Night	35% 19% 30% 15% 1	570
	11th to 20th	Night	30% 18% 31% 16% 4	564
	21st to end	Night	32% 16% 32% 17% 3 1	570
May	1st to 10th	Night	35% 15% 30% 16% 5%	570
	11th to 20th	Night	28% 13% 33% 21% 4 1	562
	21st to end	Night	23% 16% 43% 15% 2	616
June	1st to 10th	Night	35% 14% 33% 16% 2	570
	11th to 20th	Night	26% 13% 37% 21% 2	526
	21st to end	Night	30% 8% 30% 26% 5%	380
July	1st to 10th	Night	24% 6% 36% 30% 5%	456
	11th to 20th	Night	22% 8% 36% 29% 6%	552
	21st to end	Night	22% 8% 31% 33% 6%	623
August	1st to 10th	Night	19% 7% 34% 32% 7% 1	568
	11th to 20th	Night	23% 8% 31% 30% 8% 1	562
	21st to end	Night	19% 7% 32% 33% 9%	627
September	1st to 10th	Night	20% 8% 36% 28% 7%	570
	11th to 20th	Night	25% 10% 29% 26% 9%	594
	21st to end	Night	24% 13% 31% 24% 7% 1	760
October	1st to 10th	Night	27% 11% 26% 27% 8% 1	760
	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% 18% 5% 1	854
	11th to 20th	Night	30% 22% 26% 18% 4	850
	21st to end	Night	34% 22% 26% 17% 2	850
December	1st to 10th	Night	35% 18% 30% 14% 2	850
	11th to 20th	Night	37% 23% 27% 11% 1	850
	21st to end	Night	33% 23% 26% 15% 2 1	935

Page generated at 2024-11-03 03:11:18. Version ac07

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