Sheet Harbour
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sheet Harbour clear sky chart from 2005-11-06 to 2024-01-16.

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	24% 10% 4 6% 5% 3 4 4 5% 6% 29%	74738
	Night	24% 10% 3 6% 5% 3 4 4 5% 6% 30%	75079
Transparency	Day	54% 7% 12% 17% 8% 1	74441
	Night	55% 6% 12% 18% 8% 1	74911
Seeing	Day	28% 9% 28% 25% 9% 1	16302
	Night	28% 10% 29% 24% 8% 1	24072

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	19% 13% 6% 9% 6% 4 4 5% 5% 5% 25%	1516
	11th to 20th	Day	23% 14% 5% 8% 7% 3 3 4 5% 5% 24%	1481
	21st to end	Day	15% 10% 4 8% 7% 3 5% 6% 4 7% 30%	1757
February	1st to 10th	Day	22% 12% 3 8% 7% 4 5% 4 4 5% 27%	1620
	11th to 20th	Day	19% 13% 5% 8% 5% 3 5% 4 3 5% 28%	1573
	21st to end	Day	13% 15% 5% 7% 7% 3 4 4 3 6% 33%	1415
March	1st to 10th	Day	17% 14% 6% 9% 8% 3 4 4 4 5% 27%	1890
	11th to 20th	Day	18% 12% 7% 7% 8% 3 4 3 4 4 29%	1980
	21st to end	Day	18% 14% 4 9% 8% 4 3 3 3 5% 29%	2225
April	1st to 10th	Day	27% 12% 4 6% 4 3 4 4 5% 5% 25%	2160
	11th to 20th	Day	25% 10% 3 6% 4 3 3 4 4 7% 31%	2134
	21st to end	Day	26% 12% 3 5% 5% 3 4 4 4 6% 28%	2324
May	1st to 10th	Day	30% 10% 4 5% 5% 3 5% 4 4 6% 24%	2470
	11th to 20th	Day	24% 11% 3 6% 5% 3 5% 4 4 6% 29%	2478
	21st to end	Day	27% 10% 3 7% 5% 3 4 4 4 6% 28%	2724
June	1st to 10th	Day	30% 9% 3 7% 5% 3 3 4 6% 7% 22%	2520
	11th to 20th	Day	26% 9% 3 6% 6% 3 4 4 5% 6% 30%	2520
	21st to end	Day	34% 10% 4 6% 4 2 4 4 4 6% 21%	2520
July	1st to 10th	Day	24% 7% 3 5% 4 3 5% 5% 7% 8% 29%	2520
	11th to 20th	Day	24% 8% 3 6% 5% 3 4 4 6% 8% 28%	2520
	21st to end	Day	29% 9% 4 5% 5% 4 4 4 5% 7% 25%	2745
August	1st to 10th	Day	20% 8% 3 6% 4 3 4 5% 5% 8% 32%	2345
	11th to 20th	Day	21% 7% 3 6% 5% 3 4 5% 6% 8% 31%	2304
	21st to end	Day	20% 6% 3 5% 4 3 4 4 5% 8% 37%	2417
September	1st to 10th	Day	20% 5% 2 5% 6% 3 4 4 5% 8% 36%	2160
	11th to 20th	Day	23% 7% 2 5% 5% 2 4 4 5% 7% 35%	2160
	21st to end	Day	21% 7% 2 6% 5% 3 4 4 6% 7% 35%	1886
October	1st to 10th	Day	20% 6% 2 6% 5% 4 4 5% 5% 7% 35%	1800
	11th to 20th	Day	23% 8% 2 5% 5% 3 5% 4 5% 8% 33%	1800
	21st to end	Day	23% 9% 3 6% 6% 4 4 4 5% 7% 29%	1926
November	1st to 10th	Day	25% 8% 3 7% 5% 3 4 4 4 7% 30%	1599
	11th to 20th	Day	25% 8% 3 7% 6% 3 3 5% 5% 6% 30%	1520
	21st to end	Day	20% 10% 5% 6% 5% 4 5% 4 5% 8% 28%	1520
December	1st to 10th	Day	25% 14% 4 6% 5% 3 4 4 5% 5% 25%	1520
	11th to 20th	Day	31% 12% 4 6% 5% 3 4 4 4 4 23%	1906
	21st to end	Day	24% 10% 7% 9% 5% 3 3 3 4 5% 26%	2783
January	1st to 10th	Night	20% 12% 4 8% 5% 3 4 5% 5% 7% 26%	2649
	11th to 20th	Night	23% 11% 5% 6% 5% 4 5% 4 5% 6% 26%	2601
	21st to end	Night	17% 9% 6% 8% 6% 4 5% 5% 5% 6% 31%	2596
February	1st to 10th	Night	21% 10% 4 8% 7% 3 5% 4 4 7% 26%	2340
	11th to 20th	Night	19% 11% 4 7% 7% 3 5% 5% 5% 6% 27%	2273
	21st to end	Night	12% 13% 6% 8% 6% 4 5% 6% 5% 6% 30%	1839
March	1st to 10th	Night	17% 13% 6% 7% 7% 4 5% 4 4 6% 27%	2054
	11th to 20th	Night	21% 9% 4 5% 5% 2 5% 5% 6% 6% 32%	1979
	21st to end	Night	21% 10% 4 7% 5% 3 4 4 4 6% 32%	2130
April	1st to 10th	Night	27% 11% 4 5% 5% 3 4 4 5% 5% 27%	1800
	11th to 20th	Night	23% 9% 3 7% 4 3 5% 4 4 6% 32%	1776
	21st to end	Night	27% 10% 3 5% 5% 3 3 4 5% 6% 29%	1634
May	1st to 10th	Night	30% 9% 4 5% 5% 2 3 4 4 6% 27%	1489
	11th to 20th	Night	27% 7% 3 5% 4 3 5% 5% 5% 6% 31%	1416
	21st to end	Night	23% 11% 3 6% 4 3 5% 4 6% 8% 27%	1550
June	1st to 10th	Night	32% 11% 2 6% 5% 2 4 3 4 7% 25%	1440
	11th to 20th	Night	27% 9% 2 5% 4 3 4 4 4 7% 32%	1440
	21st to end	Night	34% 11% 4 6% 5% 2 3 3 3 5% 23%	1440
July	1st to 10th	Night	24% 9% 3 5% 4 3 5% 4 6% 9% 28%	1440
	11th to 20th	Night	25% 11% 5% 7% 6% 3 4 5% 5% 6% 24%	1440
	21st to end	Night	27% 10% 3 6% 5% 3 4 5% 5% 6% 25%	1573
August	1st to 10th	Night	21% 9% 3 5% 5% 3 4 5% 5% 7% 34%	1591
	11th to 20th	Night	21% 10% 3 7% 4 3 4 5% 5% 8% 30%	1596
	21st to end	Night	21% 6% 2 5% 4 3 4 5% 4 7% 41%	1939
September	1st to 10th	Night	19% 6% 3 4 3 3 4 4 5% 8% 42%	1800
	11th to 20th	Night	22% 7% 2 5% 4 2 3 4 4 7% 41%	1800
	21st to end	Night	22% 9% 3 5% 5% 2 3 4 4 6% 37%	2069
October	1st to 10th	Night	20% 6% 2 4 5% 2 4 4 6% 7% 40%	2160
	11th to 20th	Night	23% 6% 2 4 4 2 4 4 6% 8% 37%	2160
	21st to end	Night	26% 8% 3 5% 5% 3 4 5% 4 5% 32%	2430
November	1st to 10th	Night	24% 9% 3 6% 5% 2 4 4 5% 6% 34%	2478
	11th to 20th	Night	22% 9% 3 6% 5% 3 5% 5% 6% 6% 30%	2660
	21st to end	Night	22% 10% 3 6% 5% 3 4 4 5% 6% 32%	2660
December	1st to 10th	Night	27% 12% 3 6% 5% 2 4 4 4 6% 26%	2660
	11th to 20th	Night	34% 10% 3 6% 5% 3 3 3 4 5% 25%	3293
	21st to end	Night	28% 10% 3 9% 5% 3 4 5% 4 5% 25%	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	60% 8% 12% 15% 5% 1	1517
	11th to 20th	Day	60% 8% 11% 16% 5% 1	1481
	21st to end	Day	50% 7% 13% 19% 10% 1	1757
February	1st to 10th	Day	59% 5% 11% 14% 10% 1	1620
	11th to 20th	Day	57% 6% 13% 14% 8% 2	1581
	21st to end	Day	50% 7% 11% 18% 11% 3	1415
March	1st to 10th	Day	55% 5% 10% 18% 10% 2	1894
	11th to 20th	Day	52% 6% 13% 17% 10% 2	1980
	21st to end	Day	54% 5% 11% 17% 11% 2	2225
April	1st to 10th	Day	57% 6% 8% 15% 11% 3	2160
	11th to 20th	Day	53% 5% 9% 19% 13% 1	2134
	21st to end	Day	56% 5% 11% 19% 8% 1	2324
May	1st to 10th	Day	59% 7% 10% 14% 8% 1	2470
	11th to 20th	Day	56% 6% 11% 18% 9%	2482
	21st to end	Day	59% 7% 11% 16% 8% 1	2724
June	1st to 10th	Day	63% 7% 11% 15% 4	2520
	11th to 20th	Day	54% 7% 11% 17% 9% 1	2520
	21st to end	Day	66% 8% 7% 12% 7%	2520
July	1st to 10th	Day	53% 8% 14% 16% 8%	2450
	11th to 20th	Day	58% 9% 12% 16% 6%	2444
	21st to end	Day	58% 9% 12% 15% 5%	2592
August	1st to 10th	Day	50% 9% 14% 20% 7% 1	2331
	11th to 20th	Day	49% 9% 16% 20% 6%	2304
	21st to end	Day	44% 7% 15% 24% 10% 1	2417
September	1st to 10th	Day	44% 7% 13% 21% 13% 2	2160
	11th to 20th	Day	44% 6% 13% 23% 13% 1	2160
	21st to end	Day	46% 6% 10% 23% 14%	1886
October	1st to 10th	Day	46% 7% 11% 24% 11% 1	1800
	11th to 20th	Day	48% 7% 15% 21% 8%	1800
	21st to end	Day	54% 6% 12% 20% 8%	1926
November	1st to 10th	Day	52% 6% 12% 21% 9%	1599
	11th to 20th	Day	53% 7% 13% 18% 9% 1	1520
	21st to end	Day	53% 8% 15% 16% 7% 1	1520
December	1st to 10th	Day	60% 7% 11% 17% 5%	1520
	11th to 20th	Day	63% 6% 11% 12% 7%	1905
	21st to end	Day	59% 6% 17% 12% 6%	2783
January	1st to 10th	Night	58% 8% 12% 17% 5%	2649
	11th to 20th	Night	59% 6% 15% 13% 6% 1	2601
	21st to end	Night	54% 8% 14% 17% 7%	2596
February	1st to 10th	Night	58% 7% 12% 12% 10% 1	2340
	11th to 20th	Night	58% 7% 13% 14% 7% 1	2276
	21st to end	Night	54% 5% 11% 19% 9% 1	1839
March	1st to 10th	Night	57% 6% 10% 16% 10% 1	2063
	11th to 20th	Night	50% 6% 13% 18% 11% 2	1980
	21st to end	Night	53% 4 11% 19% 10% 2	2130
April	1st to 10th	Night	58% 5% 10% 15% 10% 2	1800
	11th to 20th	Night	53% 5% 9% 19% 13% 2	1776
	21st to end	Night	54% 5% 12% 18% 10% 1	1634
May	1st to 10th	Night	59% 5% 8% 19% 7% 1	1489
	11th to 20th	Night	54% 6% 10% 21% 9% 1	1417
	21st to end	Night	56% 7% 12% 17% 7% 1	1551
June	1st to 10th	Night	61% 6% 9% 18% 6%	1440
	11th to 20th	Night	55% 5% 11% 15% 14%	1440
	21st to end	Night	68% 5% 8% 13% 6% 1	1440
July	1st to 10th	Night	54% 8% 14% 17% 7%	1401
	11th to 20th	Night	63% 6% 9% 14% 8%	1392
	21st to end	Night	59% 8% 13% 14% 6% 1	1486
August	1st to 10th	Night	52% 7% 12% 23% 6%	1580
	11th to 20th	Night	51% 8% 16% 18% 7%	1596
	21st to end	Night	44% 7% 14% 24% 11%	1939
September	1st to 10th	Night	42% 7% 13% 24% 13% 1	1800
	11th to 20th	Night	44% 4 12% 25% 14% 1	1800
	21st to end	Night	49% 6% 9% 24% 12% 1	2069
October	1st to 10th	Night	42% 6% 15% 24% 12% 1	2160
	11th to 20th	Night	45% 6% 14% 28% 7%	2160
	21st to end	Night	54% 6% 12% 19% 10%	2430
November	1st to 10th	Night	53% 7% 12% 18% 10% 1	2478
	11th to 20th	Night	52% 6% 14% 17% 10% 1	2660
	21st to end	Night	54% 6% 17% 17% 6%	2660
December	1st to 10th	Night	60% 7% 12% 15% 5%	2660
	11th to 20th	Night	64% 5% 10% 14% 7%	3295
	21st to end	Night	60% 7% 13% 15% 5%	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	25% 18% 34% 19% 4	370
	11th to 20th	Day	27% 15% 35% 17% 5%	356
	21st to end	Day	20% 14% 37% 23% 5%	374
February	1st to 10th	Day	28% 17% 34% 19% 2	340
	11th to 20th	Day	26% 17% 37% 17% 3	336
	21st to end	Day	18% 15% 42% 20% 5%	280
March	1st to 10th	Day	19% 15% 42% 21% 3	382
	11th to 20th	Day	22% 13% 34% 25% 6%	420
	21st to end	Day	23% 13% 37% 19% 6% 1	462
April	1st to 10th	Day	33% 11% 33% 18% 4	420
	11th to 20th	Day	28% 11% 36% 19% 6% 1	412
	21st to end	Day	27% 6% 29% 27% 8% 2	420
May	1st to 10th	Day	33% 6% 31% 22% 5% 2	553
	11th to 20th	Day	27% 4 30% 30% 6% 2	624
	21st to end	Day	34% 5% 26% 28% 7% 1	680
June	1st to 10th	Day	35% 5% 21% 28% 10% 1	630
	11th to 20th	Day	28% 5% 24% 32% 10% 1	630
	21st to end	Day	41% 4 17% 27% 10% 1	630
July	1st to 10th	Day	30% 5% 20% 32% 13%	620
	11th to 20th	Day	33% 4 16% 32% 14% 1	618
	21st to end	Day	34% 4 17% 31% 13% 1	685
August	1st to 10th	Day	21% 3 23% 35% 15% 3	464
	11th to 20th	Day	25% 5% 24% 27% 15% 4	439
	21st to end	Day	24% 5% 21% 31% 16% 4	495
September	1st to 10th	Day	21% 6% 25% 33% 14% 2	450
	11th to 20th	Day	25% 6% 29% 25% 12% 2	450
	21st to end	Day	24% 5% 29% 23% 18% 2	394
October	1st to 10th	Day	20% 8% 29% 25% 15% 3	360
	11th to 20th	Day	22% 8% 29% 26% 13% 2	360
	21st to end	Day	26% 12% 34% 19% 8% 1	396
November	1st to 10th	Day	23% 12% 31% 26% 7% 2	365
	11th to 20th	Day	26% 18% 29% 21% 6% 1	370
	21st to end	Day	26% 14% 38% 16% 5% 1	370
December	1st to 10th	Day	29% 16% 30% 20% 4	370
	11th to 20th	Day	33% 20% 30% 15% 2	370
	21st to end	Day	25% 17% 34% 19% 4 1	407
January	1st to 10th	Night	28% 15% 35% 19% 3	850
	11th to 20th	Night	29% 15% 35% 17% 3	829
	21st to end	Night	23% 15% 40% 18% 4	802
February	1st to 10th	Night	30% 15% 33% 20% 2	720
	11th to 20th	Night	25% 18% 38% 16% 2	703
	21st to end	Night	18% 13% 41% 23% 5%	592
March	1st to 10th	Night	23% 14% 41% 19% 3	720
	11th to 20th	Night	22% 14% 29% 25% 9%	720
	21st to end	Night	24% 13% 31% 25% 6%	744
April	1st to 10th	Night	33% 10% 29% 22% 6%	540
	11th to 20th	Night	27% 7% 36% 24% 6% 1	534
	21st to end	Night	33% 5% 26% 27% 8% 1	540
May	1st to 10th	Night	32% 6% 28% 23% 9% 2	540
	11th to 20th	Night	32% 4 22% 28% 12% 3	532
	21st to end	Night	30% 5% 26% 31% 8% 1	583
June	1st to 10th	Night	37% 4 19% 31% 8% 1	540
	11th to 20th	Night	33% 5% 14% 37% 11% 1	540
	21st to end	Night	40% 3 20% 23% 13% 1	540
July	1st to 10th	Night	29% 4 15% 38% 14% 1	526
	11th to 20th	Night	33% 4 16% 31% 15%	522
	21st to end	Night	35% 4 14% 29% 16% 1	590
August	1st to 10th	Night	24% 4 14% 36% 20% 2	538
	11th to 20th	Night	27% 3 14% 35% 19% 2	532
	21st to end	Night	24% 4 16% 31% 22% 3	594
September	1st to 10th	Night	21% 4 24% 33% 17% 2	540
	11th to 20th	Night	24% 5% 24% 29% 16% 2	540
	21st to end	Night	28% 5% 26% 29% 10% 2	697
October	1st to 10th	Night	21% 6% 28% 32% 11% 2	720
	11th to 20th	Night	25% 8% 31% 25% 10% 2	720
	21st to end	Night	30% 11% 31% 22% 6%	819
November	1st to 10th	Night	25% 11% 32% 25% 6% 1	830
	11th to 20th	Night	24% 16% 33% 20% 7%	850
	21st to end	Night	28% 14% 35% 18% 5%	850
December	1st to 10th	Night	33% 12% 35% 16% 4	850
	11th to 20th	Night	30% 18% 35% 13% 3	850
	21st to end	Night	29% 15% 35% 17% 4	935

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