Sheerness Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sheerness Lake clear sky chart from 2005-11-06 to 2024-11-30.

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	7% 8% 3 8% 5% 5% 5% 4 7% 8% 39%	105815
	Night	6% 8% 3 9% 6% 5% 6% 5% 7% 9% 37%	102351
Transparency	Day	40% 8% 16% 25% 10% 1	105556
	Night	41% 8% 15% 23% 10% 1	102256
Seeing	Day	11% 7% 25% 31% 20% 6%	15577
	Night	10% 6% 29% 37% 16% 2	24716

By Month

Percentage Cloud Cover NW (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	5% 12% 4 12% 7% 5% 6% 5% 7% 9% 28%	2219
	11th to 20th	Day	3 10% 4 10% 6% 7% 6% 9% 10% 9% 27%	2199
	21st to end	Day	3 8% 4 9% 7% 6% 6% 5% 8% 9% 35%	2643
February	1st to 10th	Day	6% 13% 5% 12% 8% 7% 4 4 6% 8% 26%	1591
	11th to 20th	Day	5% 13% 4 12% 7% 6% 7% 6% 7% 9% 24%	1665
	21st to end	Day	3 13% 4 11% 8% 8% 7% 5% 7% 8% 25%	1428
March	1st to 10th	Day	4 10% 4 12% 10% 7% 6% 4 5% 8% 30%	3220
	11th to 20th	Day	6% 9% 3 7% 5% 5% 5% 4 6% 8% 41%	3489
	21st to end	Day	6% 16% 6% 13% 6% 6% 5% 4 5% 6% 28%	3825
April	1st to 10th	Day	8% 12% 3 8% 7% 6% 5% 4 7% 7% 32%	2346
	11th to 20th	Day	10% 10% 4 9% 6% 5% 5% 4 6% 8% 32%	2440
	21st to end	Day	12% 8% 3 9% 6% 5% 5% 4 7% 9% 32%	2470
May	1st to 10th	Day	16% 13% 5% 8% 5% 4 4 4 5% 7% 28%	4362
	11th to 20th	Day	8% 8% 3 8% 6% 6% 6% 6% 8% 8% 31%	4693
	21st to end	Day	12% 8% 4 10% 6% 5% 6% 5% 6% 8% 28%	5161
June	1st to 10th	Day	14% 7% 3 6% 5% 5% 5% 5% 7% 9% 34%	2850
	11th to 20th	Day	16% 9% 3 8% 6% 5% 5% 5% 6% 9% 28%	2850
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 8% 10% 43%	2850
July	1st to 10th	Day	5% 4 1 5% 4 4 5% 4 8% 9% 49%	4740
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 6% 10% 51%	4740
	21st to end	Day	3 3 1 3 3 3 4 4 6% 9% 61%	5138
August	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 4 6% 9% 48%	4420
	11th to 20th	Day	5% 4 2 5% 4 5% 4 4 6% 11% 50%	4028
	21st to end	Day	7% 4 2 4 3 3 4 4 7% 9% 53%	4521
September	1st to 10th	Day	10% 6% 2 5% 4 4 3 4 5% 6% 51%	2385
	11th to 20th	Day	14% 7% 3 6% 3 3 4 3 5% 8% 43%	2100
	21st to end	Day	8% 6% 3 7% 3 4 4 4 6% 9% 48%	2090
October	1st to 10th	Day	5% 5% 3 7% 4 6% 5% 4 6% 8% 47%	3216
	11th to 20th	Day	4 6% 3 7% 4 5% 6% 5% 7% 10% 44%	3064
	21st to end	Day	6% 7% 3 6% 4 5% 5% 4 6% 8% 45%	3044
November	1st to 10th	Day	8% 15% 5% 12% 7% 6% 4 4 6% 7% 27%	1568
	11th to 20th	Day	5% 10% 3 11% 7% 6% 6% 4 7% 8% 33%	1600
	21st to end	Day	5% 12% 3 9% 8% 6% 6% 4 5% 9% 31%	1406
December	1st to 10th	Day	5% 11% 4 12% 9% 8% 6% 5% 7% 7% 26%	1330
	11th to 20th	Day	3 10% 7% 15% 7% 6% 6% 4 6% 9% 28%	1682
	21st to end	Day	4 7% 3 9% 5% 5% 5% 3 6% 8% 44%	2442
January	1st to 10th	Night	3 10% 5% 12% 8% 7% 7% 6% 8% 9% 25%	4726
	11th to 20th	Night	3 7% 4 12% 9% 7% 6% 5% 7% 9% 31%	4699
	21st to end	Night	3 9% 4 10% 6% 6% 6% 5% 7% 9% 35%	5041
February	1st to 10th	Night	6% 10% 4 11% 8% 5% 6% 5% 7% 8% 32%	2588
	11th to 20th	Night	4 12% 4 11% 7% 6% 7% 5% 8% 8% 26%	2402
	21st to end	Night	4 9% 4 10% 8% 5% 7% 5% 8% 8% 31%	2022
March	1st to 10th	Night	3 9% 4 13% 10% 7% 7% 4 7% 7% 29%	3654
	11th to 20th	Night	6% 7% 2 7% 5% 5% 6% 5% 8% 9% 39%	3490
	21st to end	Night	6% 16% 6% 17% 8% 6% 5% 5% 6% 7% 19%	3828
April	1st to 10th	Night	8% 8% 4 11% 7% 7% 6% 4 6% 7% 32%	1831
	11th to 20th	Night	8% 11% 4 11% 6% 5% 6% 4 6% 7% 32%	1691
	21st to end	Night	9% 9% 3 8% 5% 5% 5% 4 7% 10% 35%	1710
May	1st to 10th	Night	20% 13% 4 8% 5% 4 4 3 5% 6% 27%	2592
	11th to 20th	Night	8% 6% 3 7% 7% 6% 7% 6% 9% 10% 33%	2199
	21st to end	Night	12% 8% 4 11% 7% 7% 6% 5% 7% 9% 25%	2414
June	1st to 10th	Night	15% 9% 3 7% 6% 6% 4 4 6% 9% 32%	1330
	11th to 20th	Night	13% 8% 3 7% 7% 5% 6% 6% 7% 8% 30%	1330
	21st to end	Night	9% 5% 2 5% 6% 5% 5% 3 6% 8% 44%	1330
July	1st to 10th	Night	5% 3 2 7% 5% 5% 5% 5% 7% 10% 45%	2220
	11th to 20th	Night	5% 3 1 5% 4 4 5% 5% 8% 14% 46%	2220
	21st to end	Night	3 3 2 5% 4 4 4 4 6% 9% 56%	2414
August	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 6% 9% 46%	2510
	11th to 20th	Night	4 3 2 6% 5% 5% 5% 5% 7% 12% 45%	2811
	21st to end	Night	8% 3 1 5% 3 3 4 3 6% 10% 53%	3146
September	1st to 10th	Night	9% 5% 2 6% 4 5% 5% 5% 6% 8% 48%	1795
	11th to 20th	Night	12% 8% 2 6% 3 3 4 3 5% 8% 46%	2076
	21st to end	Night	6% 6% 2 6% 4 4 5% 3 7% 9% 49%	2090
October	1st to 10th	Night	6% 8% 2 7% 5% 4 5% 4 6% 8% 45%	3755
	11th to 20th	Night	4 5% 2 7% 4 4 5% 4 7% 9% 47%	3863
	21st to end	Night	5% 7% 2 7% 5% 5% 5% 5% 7% 9% 41%	4483
November	1st to 10th	Night	8% 11% 4 9% 6% 6% 5% 5% 5% 8% 33%	2730
	11th to 20th	Night	4 9% 4 9% 7% 6% 6% 6% 7% 9% 33%	2800
	21st to end	Night	5% 11% 3 10% 7% 7% 6% 5% 7% 9% 30%	2987
December	1st to 10th	Night	5% 11% 4 13% 8% 6% 5% 5% 6% 8% 28%	2850
	11th to 20th	Night	3 9% 4 12% 7% 6% 6% 4 8% 9% 30%	3510
	21st to end	Night	3 7% 3 7% 5% 6% 5% 4 6% 7% 46%	5214

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	50% 8% 14% 18% 9% 1	2219
	11th to 20th	Day	46% 10% 15% 12% 16% 1	2220
	21st to end	Day	40% 11% 19% 24% 6%	2643
February	1st to 10th	Day	55% 6% 14% 17% 8%	1591
	11th to 20th	Day	52% 10% 14% 15% 9%	1674
	21st to end	Day	53% 8% 11% 17% 10%	1428
March	1st to 10th	Day	50% 8% 15% 18% 9%	3220
	11th to 20th	Day	39% 7% 20% 22% 12%	3490
	21st to end	Day	54% 9% 13% 17% 6%	3825
April	1st to 10th	Day	47% 8% 16% 20% 9%	2346
	11th to 20th	Day	47% 8% 14% 22% 8% 1	2440
	21st to end	Day	46% 9% 15% 22% 7% 1	2470
May	1st to 10th	Day	53% 8% 12% 21% 5%	4362
	11th to 20th	Day	41% 11% 15% 23% 10%	4697
	21st to end	Day	47% 12% 15% 19% 6% 1	5162
June	1st to 10th	Day	43% 9% 18% 20% 9% 1	2850
	11th to 20th	Day	50% 8% 15% 18% 7% 1	2850
	21st to end	Day	33% 10% 23% 25% 8% 1	2850
July	1st to 10th	Day	27% 10% 18% 28% 15% 1	4666
	11th to 20th	Day	27% 8% 17% 33% 14% 1	4659
	21st to end	Day	19% 6% 15% 31% 23% 5%	4976
August	1st to 10th	Day	32% 7% 17% 33% 10% 1	4402
	11th to 20th	Day	27% 8% 22% 35% 8% 1	4067
	21st to end	Day	25% 6% 23% 34% 11% 1	4520
September	1st to 10th	Day	32% 7% 16% 26% 13% 5%	2385
	11th to 20th	Day	40% 5% 16% 29% 9% 1	2100
	21st to end	Day	32% 8% 15% 30% 12% 3	2090
October	1st to 10th	Day	35% 6% 19% 30% 10%	3216
	11th to 20th	Day	34% 6% 18% 31% 9% 1	3064
	21st to end	Day	37% 5% 13% 34% 10% 1	3044
November	1st to 10th	Day	57% 6% 13% 18% 6% 1	1568
	11th to 20th	Day	48% 7% 17% 20% 7% 1	1600
	21st to end	Day	49% 8% 16% 20% 7%	1406
December	1st to 10th	Day	54% 9% 14% 18% 5% 2	1330
	11th to 20th	Day	52% 9% 14% 21% 3 1	1684
	21st to end	Day	36% 6% 13% 26% 15% 4	2442
January	1st to 10th	Night	48% 11% 14% 20% 5% 1	4726
	11th to 20th	Night	48% 7% 13% 17% 13% 3	4740
	21st to end	Night	42% 9% 15% 26% 8% 1	5041
February	1st to 10th	Night	48% 7% 14% 19% 11%	2588
	11th to 20th	Night	51% 9% 13% 15% 10% 1	2403
	21st to end	Night	45% 9% 13% 21% 10% 1	2022
March	1st to 10th	Night	47% 9% 17% 20% 7%	3654
	11th to 20th	Night	36% 9% 21% 24% 9% 1	3490
	21st to end	Night	60% 11% 11% 12% 6%	3828
April	1st to 10th	Night	48% 7% 15% 18% 12%	1831
	11th to 20th	Night	48% 6% 13% 21% 11% 1	1691
	21st to end	Night	42% 9% 13% 25% 10% 2	1710
May	1st to 10th	Night	56% 8% 10% 19% 7% 1	2592
	11th to 20th	Night	36% 12% 18% 24% 10% 1	2199
	21st to end	Night	50% 13% 13% 17% 6% 1	2414
June	1st to 10th	Night	46% 9% 13% 21% 9% 1	1330
	11th to 20th	Night	46% 10% 16% 17% 8% 1	1330
	21st to end	Night	35% 8% 19% 27% 10% 1	1330
July	1st to 10th	Night	31% 9% 21% 21% 16% 2	2185
	11th to 20th	Night	25% 9% 17% 35% 12% 2	2178
	21st to end	Night	23% 5% 15% 31% 22% 4	2337
August	1st to 10th	Night	33% 7% 18% 30% 12% 1	2503
	11th to 20th	Night	29% 7% 17% 38% 8% 1	2838
	21st to end	Night	26% 5% 18% 35% 13% 2	3146
September	1st to 10th	Night	32% 9% 16% 21% 15% 7%	1795
	11th to 20th	Night	36% 6% 15% 28% 12% 3	2076
	21st to end	Night	30% 7% 17% 28% 16% 2	2090
October	1st to 10th	Night	36% 5% 19% 26% 13% 1	3755
	11th to 20th	Night	32% 5% 17% 33% 12% 2	3863
	21st to end	Night	37% 7% 15% 33% 7% 1	4483
November	1st to 10th	Night	48% 8% 14% 20% 9% 1	2730
	11th to 20th	Night	44% 9% 17% 19% 10% 1	2800
	21st to end	Night	48% 9% 18% 18% 5% 1	2987
December	1st to 10th	Night	52% 8% 12% 19% 8% 1	2850
	11th to 20th	Night	46% 11% 20% 17% 5%	3507
	21st to end	Night	35% 7% 13% 24% 17% 4	5214

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	7% 5% 32% 28% 21% 8%	180
	11th to 20th	Day	6% 9% 26% 34% 21% 4	180
	21st to end	Day	7% 8% 31% 29% 19% 6%	198
February	1st to 10th	Day	11% 10% 32% 32% 11% 3	213
	11th to 20th	Day	12% 9% 27% 34% 14% 3	270
	21st to end	Day	6% 8% 30% 29% 21% 7%	240
March	1st to 10th	Day	8% 11% 33% 29% 15% 4	460
	11th to 20th	Day	10% 6% 33% 30% 15% 5%	460
	21st to end	Day	9% 9% 34% 30% 15% 4	506
April	1st to 10th	Day	10% 13% 33% 29% 12% 3	460
	11th to 20th	Day	12% 11% 29% 32% 13% 3	452
	21st to end	Day	13% 10% 29% 32% 14% 2	460
May	1st to 10th	Day	15% 8% 22% 31% 20% 5%	611
	11th to 20th	Day	9% 6% 23% 34% 21% 6%	675
	21st to end	Day	14% 6% 21% 29% 22% 8%	734
June	1st to 10th	Day	14% 9% 23% 33% 17% 5%	680
	11th to 20th	Day	17% 7% 30% 26% 18% 4	680
	21st to end	Day	9% 4 23% 33% 23% 8%	680
July	1st to 10th	Day	7% 3 18% 35% 27% 9%	671
	11th to 20th	Day	10% 4 21% 34% 23% 7%	668
	21st to end	Day	4 4 19% 32% 32% 10%	739
August	1st to 10th	Day	9% 5% 15% 37% 24% 10%	549
	11th to 20th	Day	8% 3 18% 33% 30% 8%	477
	21st to end	Day	8% 7% 19% 29% 27% 10%	539
September	1st to 10th	Day	12% 7% 23% 29% 23% 5%	490
	11th to 20th	Day	18% 7% 20% 29% 23% 3	490
	21st to end	Day	11% 11% 30% 29% 16% 3	490
October	1st to 10th	Day	17% 4 27% 26% 20% 5%	333
	11th to 20th	Day	10% 6% 33% 29% 17% 4	300
	21st to end	Day	10% 11% 34% 32% 12% 2	330
November	1st to 10th	Day	16% 8% 28% 31% 15% 2	300
	11th to 20th	Day	11% 10% 38% 28% 11% 2	300
	21st to end	Day	7% 7% 31% 35% 12% 8%	204
December	1st to 10th	Day	8% 5% 32% 32% 19% 4	180
	11th to 20th	Day	7% 12% 31% 37% 12% 2	180
	21st to end	Day	8% 7% 22% 36% 23% 5%	198
January	1st to 10th	Night	9% 7% 33% 35% 15% 1	950
	11th to 20th	Night	9% 8% 31% 39% 13% 1	950
	21st to end	Night	8% 9% 32% 35% 13% 2	937
February	1st to 10th	Night	10% 8% 25% 37% 17% 3	760
	11th to 20th	Night	10% 6% 31% 37% 15% 1	744
	21st to end	Night	6% 4 24% 40% 22% 4	628
March	1st to 10th	Night	8% 5% 30% 40% 14% 3	760
	11th to 20th	Night	8% 5% 31% 39% 14% 3	760
	21st to end	Night	12% 7% 28% 35% 17% 2	836
April	1st to 10th	Night	10% 7% 32% 32% 16% 3	629
	11th to 20th	Night	12% 7% 33% 36% 11% 1	563
	21st to end	Night	14% 7% 27% 39% 13% 1	570
May	1st to 10th	Night	16% 7% 25% 35% 15% 2	569
	11th to 20th	Night	10% 4 27% 39% 20% 1	374
	21st to end	Night	16% 4 27% 29% 22% 2	411
June	1st to 10th	Night	16% 7% 28% 32% 14% 3	380
	11th to 20th	Night	14% 5% 29% 33% 18% 1	380
	21st to end	Night	11% 5% 22% 38% 22% 2	380
July	1st to 10th	Night	7% 2 20% 48% 21% 1	370
	11th to 20th	Night	7% 4 23% 44% 21% 1	368
	21st to end	Night	3 3 24% 40% 26% 3	416
August	1st to 10th	Night	6% 4 23% 38% 25% 4	424
	11th to 20th	Night	6% 5% 20% 45% 24% 1	564
	21st to end	Night	10% 4 22% 39% 25% 1	627
September	1st to 10th	Night	10% 4 23% 41% 20% 2	570
	11th to 20th	Night	16% 3 24% 38% 18% 1	746
	21st to end	Night	8% 5% 33% 37% 16% 2	760
October	1st to 10th	Night	13% 5% 28% 36% 17% 1	760
	11th to 20th	Night	9% 5% 32% 36% 15% 3	760
	21st to end	Night	10% 9% 32% 36% 13%	850
November	1st to 10th	Night	13% 7% 34% 35% 9% 1	975
	11th to 20th	Night	7% 8% 39% 34% 9% 3	1000
	21st to end	Night	9% 6% 35% 37% 12% 2	1000
December	1st to 10th	Night	10% 8% 27% 36% 16% 3	950
	11th to 20th	Night	8% 7% 36% 36% 12% 2	950
	21st to end	Night	9% 4 33% 37% 14% 3	1045

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