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 2024-01-15.

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	8% 8% 4 7% 7% 4 5% 6% 6% 9% 37%	74671
	Night	7% 8% 4 8% 7% 4 6% 6% 7% 9% 36%	75106
Transparency	Day	42% 8% 17% 23% 9% 1	74352
	Night	42% 8% 16% 22% 10% 2	74943
Seeing	Day	11% 7% 26% 31% 20% 5%	14340
	Night	10% 6% 29% 37% 16% 2	23640

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% 11% 5% 9% 8% 5% 9% 7% 7% 9% 27%	1329
	11th to 20th	Day	5% 10% 5% 8% 8% 6% 6% 8% 7% 8% 28%	1288
	21st to end	Day	3 10% 5% 10% 10% 5% 7% 7% 8% 9% 25%	1503
February	1st to 10th	Day	6% 11% 6% 10% 10% 5% 7% 5% 6% 8% 27%	1508
	11th to 20th	Day	5% 11% 6% 10% 9% 5% 6% 8% 8% 10% 23%	1575
	21st to end	Day	3 12% 5% 10% 10% 6% 7% 7% 6% 9% 25%	1346
March	1st to 10th	Day	6% 10% 5% 10% 12% 6% 7% 5% 6% 9% 25%	1843
	11th to 20th	Day	7% 10% 4 8% 8% 5% 6% 7% 6% 8% 32%	1979
	21st to end	Day	7% 11% 6% 10% 7% 4 6% 5% 6% 7% 31%	2178
April	1st to 10th	Day	8% 10% 3 7% 8% 5% 6% 6% 7% 8% 32%	2222
	11th to 20th	Day	11% 10% 4 8% 7% 4 5% 6% 6% 7% 32%	2310
	21st to end	Day	12% 8% 4 7% 7% 4 5% 5% 6% 9% 32%	2340
May	1st to 10th	Day	11% 10% 4 8% 7% 5% 5% 6% 6% 8% 30%	2480
	11th to 20th	Day	8% 7% 3 6% 7% 4 6% 5% 7% 8% 39%	2653
	21st to end	Day	16% 7% 4 7% 7% 3 5% 6% 6% 8% 31%	2917
June	1st to 10th	Day	15% 6% 3 6% 6% 4 5% 6% 7% 9% 34%	2700
	11th to 20th	Day	17% 9% 3 7% 7% 3 6% 6% 6% 9% 27%	2700
	21st to end	Day	8% 4 3 5% 5% 4 5% 6% 8% 10% 43%	2700
July	1st to 10th	Day	7% 3 2 5% 6% 3 6% 6% 9% 10% 43%	2700
	11th to 20th	Day	8% 5% 2 4 5% 3 6% 5% 6% 11% 45%	2700
	21st to end	Day	4 3 2 3 4 3 4 5% 7% 10% 55%	2938
August	1st to 10th	Day	8% 5% 2 4 5% 3 4 5% 6% 9% 49%	2523
	11th to 20th	Day	7% 4 2 4 4 3 5% 5% 6% 10% 49%	2299
	21st to end	Day	7% 5% 3 4 4 3 5% 5% 7% 9% 49%	2574
September	1st to 10th	Day	10% 6% 2 5% 5% 3 4 4 5% 7% 49%	2260
	11th to 20th	Day	14% 7% 2 5% 4 2 4 4 5% 8% 44%	1990
	21st to end	Day	8% 5% 3 6% 4 3 4 4 6% 9% 48%	1980
October	1st to 10th	Day	9% 8% 3 8% 4 4 5% 5% 5% 7% 42%	1832
	11th to 20th	Day	7% 7% 4 7% 5% 3 5% 6% 6% 8% 43%	1751
	21st to end	Day	6% 10% 5% 7% 7% 4 5% 5% 7% 8% 36%	1751
November	1st to 10th	Day	9% 14% 6% 11% 8% 5% 5% 4 6% 7% 26%	1488
	11th to 20th	Day	6% 9% 4 9% 8% 4 6% 5% 7% 8% 34%	1520
	21st to end	Day	6% 10% 4 8% 7% 5% 7% 5% 6% 9% 33%	1340
December	1st to 10th	Day	5% 10% 5% 10% 10% 6% 7% 6% 7% 7% 26%	1330
	11th to 20th	Day	3 9% 7% 13% 8% 4 7% 5% 6% 9% 28%	1682
	21st to end	Day	4 7% 3 8% 6% 4 5% 4 6% 9% 44%	2442
January	1st to 10th	Night	4 9% 4 9% 9% 6% 7% 8% 9% 9% 25%	2836
	11th to 20th	Night	5% 8% 5% 9% 9% 5% 6% 5% 7% 12% 29%	2761
	21st to end	Night	5% 9% 4 9% 9% 5% 8% 7% 7% 10% 27%	2867
February	1st to 10th	Night	5% 9% 4 9% 9% 4 6% 6% 7% 9% 32%	2452
	11th to 20th	Night	5% 12% 5% 10% 8% 5% 8% 7% 8% 9% 24%	2272
	21st to end	Night	4 8% 4 8% 10% 4 7% 7% 7% 9% 31%	1906
March	1st to 10th	Night	4 8% 5% 8% 12% 5% 6% 5% 7% 8% 31%	2117
	11th to 20th	Night	5% 8% 4 8% 7% 4 8% 7% 7% 10% 33%	1980
	21st to end	Night	7% 12% 5% 8% 8% 4 6% 7% 7% 9% 28%	2178
April	1st to 10th	Night	8% 6% 5% 9% 8% 5% 6% 6% 7% 8% 33%	1735
	11th to 20th	Night	8% 10% 4 10% 7% 4 6% 5% 5% 8% 32%	1601
	21st to end	Night	10% 8% 3 6% 6% 4 5% 5% 7% 10% 36%	1620
May	1st to 10th	Night	12% 9% 4 6% 6% 4 6% 5% 6% 8% 35%	1478
	11th to 20th	Night	7% 7% 3 6% 6% 4 6% 6% 6% 8% 40%	1239
	21st to end	Night	16% 6% 2 6% 6% 4 6% 6% 7% 9% 31%	1358
June	1st to 10th	Night	15% 8% 3 6% 6% 4 5% 5% 6% 9% 33%	1260
	11th to 20th	Night	13% 7% 4 7% 7% 4 6% 7% 6% 9% 30%	1260
	21st to end	Night	9% 5% 3 4 6% 5% 5% 5% 6% 9% 44%	1260
July	1st to 10th	Night	6% 3 2 6% 6% 4 5% 5% 7% 10% 45%	1260
	11th to 20th	Night	7% 2 1 4 4 3 6% 6% 9% 12% 46%	1260
	21st to end	Night	4 3 2 3 3 2 4 4 6% 10% 58%	1379
August	1st to 10th	Night	5% 4 2 6% 6% 3 6% 5% 7% 12% 44%	1413
	11th to 20th	Night	6% 4 2 6% 6% 3 5% 5% 7% 11% 47%	1602
	21st to end	Night	7% 4 2 5% 4 3 4 5% 7% 9% 50%	1782
September	1st to 10th	Night	9% 5% 2 5% 5% 4 5% 6% 6% 8% 47%	1700
	11th to 20th	Night	12% 7% 2 5% 4 2 4 4 5% 8% 47%	1966
	21st to end	Night	7% 6% 2 5% 5% 3 4 5% 6% 9% 49%	1980
October	1st to 10th	Night	9% 9% 3 7% 5% 3 5% 5% 5% 8% 42%	2128
	11th to 20th	Night	6% 7% 3 7% 6% 4 5% 5% 7% 8% 41%	2208
	21st to end	Night	6% 8% 3 8% 8% 4 5% 6% 7% 9% 35%	2587
November	1st to 10th	Night	9% 11% 4 8% 6% 4 5% 6% 5% 8% 33%	2590
	11th to 20th	Night	4 8% 4 8% 7% 5% 6% 8% 7% 9% 34%	2660
	21st to end	Night	5% 9% 4 9% 7% 5% 6% 6% 7% 9% 32%	2837
December	1st to 10th	Night	6% 9% 4 12% 9% 4 6% 6% 6% 8% 28%	2850
	11th to 20th	Night	3 9% 4 10% 9% 4 6% 6% 8% 9% 31%	3510
	21st to end	Night	4 6% 3 6% 6% 4 6% 6% 6% 8% 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	52% 7% 14% 19% 7% 1	1329
	11th to 20th	Day	51% 10% 15% 17% 7% 1	1288
	21st to end	Day	51% 10% 16% 14% 8%	1503
February	1st to 10th	Day	54% 6% 14% 18% 8%	1508
	11th to 20th	Day	53% 10% 15% 14% 8%	1584
	21st to end	Day	54% 8% 11% 17% 10%	1346
March	1st to 10th	Day	55% 9% 15% 16% 5%	1843
	11th to 20th	Day	48% 8% 13% 21% 9% 1	1980
	21st to end	Day	51% 9% 17% 17% 6%	2178
April	1st to 10th	Day	46% 9% 16% 20% 8%	2222
	11th to 20th	Day	47% 8% 14% 22% 9% 1	2310
	21st to end	Day	46% 8% 15% 22% 7% 1	2340
May	1st to 10th	Day	49% 9% 14% 20% 8%	2480
	11th to 20th	Day	39% 8% 15% 25% 12% 1	2657
	21st to end	Day	48% 8% 15% 20% 8% 1	2918
June	1st to 10th	Day	44% 9% 18% 21% 9%	2700
	11th to 20th	Day	51% 9% 16% 17% 6% 1	2700
	21st to end	Day	33% 10% 23% 25% 7% 1	2700
July	1st to 10th	Day	29% 12% 22% 27% 8% 1	2626
	11th to 20th	Day	31% 11% 20% 27% 10% 1	2619
	21st to end	Day	22% 9% 21% 32% 14% 1	2775
August	1st to 10th	Day	31% 9% 21% 27% 11% 1	2505
	11th to 20th	Day	28% 9% 23% 28% 11% 1	2299
	21st to end	Day	29% 9% 19% 29% 13% 1	2574
September	1st to 10th	Day	33% 7% 16% 26% 13% 5%	2260
	11th to 20th	Day	39% 5% 15% 30% 10% 1	1990
	21st to end	Day	32% 8% 15% 30% 12% 3	1980
October	1st to 10th	Day	40% 8% 15% 26% 11% 1	1832
	11th to 20th	Day	38% 8% 17% 26% 10% 1	1751
	21st to end	Day	44% 6% 16% 22% 11% 1	1751
November	1st to 10th	Day	57% 6% 12% 17% 6% 1	1488
	11th to 20th	Day	48% 7% 17% 21% 7% 1	1520
	21st to end	Day	46% 8% 17% 21% 7%	1340
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	49% 10% 15% 17% 7% 2	2836
	11th to 20th	Night	47% 8% 16% 20% 7% 2	2761
	21st to end	Night	49% 9% 15% 16% 9% 1	2867
February	1st to 10th	Night	47% 8% 15% 19% 11%	2452
	11th to 20th	Night	53% 9% 13% 16% 8% 1	2273
	21st to end	Night	45% 9% 14% 20% 11% 1	1906
March	1st to 10th	Night	48% 9% 17% 20% 7%	2117
	11th to 20th	Night	43% 8% 15% 23% 10% 1	1980
	21st to end	Night	49% 10% 15% 17% 8% 1	2178
April	1st to 10th	Night	47% 7% 15% 18% 12% 1	1735
	11th to 20th	Night	48% 6% 13% 22% 10% 1	1601
	21st to end	Night	41% 9% 13% 25% 10% 2	1620
May	1st to 10th	Night	44% 9% 12% 22% 11% 2	1478
	11th to 20th	Night	38% 8% 13% 26% 14% 2	1239
	21st to end	Night	47% 9% 13% 20% 10% 2	1358
June	1st to 10th	Night	46% 8% 14% 22% 9% 1	1260
	11th to 20th	Night	47% 11% 17% 17% 8% 1	1260
	21st to end	Night	35% 8% 20% 27% 9% 1	1260
July	1st to 10th	Night	32% 10% 22% 25% 10% 1	1225
	11th to 20th	Night	27% 13% 21% 26% 11% 2	1218
	21st to end	Night	21% 7% 21% 33% 16% 2	1302
August	1st to 10th	Night	31% 9% 23% 27% 9% 1	1406
	11th to 20th	Night	29% 8% 18% 30% 13% 2	1602
	21st to end	Night	29% 7% 15% 30% 16% 3	1782
September	1st to 10th	Night	32% 9% 16% 22% 14% 7%	1700
	11th to 20th	Night	35% 6% 14% 28% 13% 3	1966
	21st to end	Night	30% 7% 17% 28% 16% 2	1980
October	1st to 10th	Night	40% 7% 15% 24% 14% 2	2128
	11th to 20th	Night	38% 7% 16% 27% 10% 2	2208
	21st to end	Night	43% 9% 16% 23% 8% 1	2587
November	1st to 10th	Night	48% 9% 14% 19% 10% 1	2590
	11th to 20th	Night	43% 9% 17% 20% 10%	2660
	21st to end	Night	46% 9% 18% 19% 6% 1	2837
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% 24% 36% 20% 5%	168
	21st to end	Day	7% 8% 34% 27% 17% 7%	176
February	1st to 10th	Day	12% 9% 34% 32% 9% 4	190
	11th to 20th	Day	13% 10% 27% 35% 13% 1	240
	21st to end	Day	5% 6% 31% 30% 21% 7%	212
March	1st to 10th	Day	9% 10% 34% 29% 14% 5%	420
	11th to 20th	Day	10% 6% 34% 30% 15% 5%	420
	21st to end	Day	8% 8% 34% 30% 15% 4	462
April	1st to 10th	Day	10% 12% 33% 30% 13% 3	420
	11th to 20th	Day	12% 12% 27% 32% 14% 3	412
	21st to end	Day	13% 9% 30% 31% 14% 2	420
May	1st to 10th	Day	14% 8% 22% 31% 20% 5%	563
	11th to 20th	Day	10% 5% 22% 34% 22% 6%	625
	21st to end	Day	15% 6% 20% 28% 22% 8%	679
June	1st to 10th	Day	15% 7% 22% 34% 18% 5%	630
	11th to 20th	Day	17% 7% 29% 26% 18% 3	630
	21st to end	Day	10% 4 22% 32% 24% 9%	630
July	1st to 10th	Day	7% 3 19% 35% 26% 9%	621
	11th to 20th	Day	10% 5% 22% 34% 22% 6%	618
	21st to end	Day	4 4 19% 32% 32% 10%	684
August	1st to 10th	Day	10% 6% 15% 38% 24% 9%	506
	11th to 20th	Day	8% 4 19% 32% 30% 8%	437
	21st to end	Day	8% 7% 19% 29% 28% 10%	495
September	1st to 10th	Day	12% 8% 24% 30% 22% 3	450
	11th to 20th	Day	17% 6% 21% 29% 23% 4	450
	21st to end	Day	11% 11% 29% 29% 16% 4	450
October	1st to 10th	Day	19% 4 26% 26% 20% 5%	302
	11th to 20th	Day	11% 7% 32% 29% 17% 4	270
	21st to end	Day	10% 11% 36% 31% 11% 1	297
November	1st to 10th	Day	17% 9% 28% 29% 15% 2	270
	11th to 20th	Day	12% 10% 36% 29% 11% 1	270
	21st to end	Day	6% 6% 34% 34% 11% 9%	185
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% 30% 39% 13% 1	921
	21st to end	Night	8% 9% 33% 35% 11% 2	887
February	1st to 10th	Night	9% 8% 26% 36% 17% 3	720
	11th to 20th	Night	10% 7% 32% 37% 14% 1	704
	21st to end	Night	6% 3 23% 41% 23% 4	592
March	1st to 10th	Night	8% 4 30% 40% 15% 3	720
	11th to 20th	Night	8% 5% 30% 40% 14% 3	720
	21st to end	Night	12% 6% 28% 35% 17% 2	792
April	1st to 10th	Night	10% 7% 32% 33% 17% 3	596
	11th to 20th	Night	12% 7% 32% 37% 11% 1	533
	21st to end	Night	14% 7% 26% 39% 13% 1	540
May	1st to 10th	Night	15% 6% 26% 36% 16% 2	540
	11th to 20th	Night	10% 4 27% 39% 20% 1	354
	21st to end	Night	17% 4 26% 29% 22% 2	389
June	1st to 10th	Night	16% 6% 28% 33% 14% 3	360
	11th to 20th	Night	14% 4 29% 33% 19%	360
	21st to end	Night	11% 5% 21% 38% 23% 3	360
July	1st to 10th	Night	7% 2 21% 48% 21% 1	350
	11th to 20th	Night	7% 4 24% 45% 20% 1	348
	21st to end	Night	4 3 24% 41% 26% 3	394
August	1st to 10th	Night	5% 4 23% 39% 24% 4	401
	11th to 20th	Night	7% 5% 20% 44% 24% 1	534
	21st to end	Night	9% 4 22% 39% 25% 1	594
September	1st to 10th	Night	10% 4 24% 42% 18% 2	540
	11th to 20th	Night	16% 3 23% 38% 18% 1	706
	21st to end	Night	8% 4 33% 37% 17% 2	720
October	1st to 10th	Night	13% 4 28% 37% 17% 1	720
	11th to 20th	Night	9% 5% 32% 36% 15% 3	720
	21st to end	Night	10% 10% 33% 35% 12%	805
November	1st to 10th	Night	13% 7% 35% 34% 10% 1	925
	11th to 20th	Night	7% 8% 38% 35% 8% 3	950
	21st to end	Night	9% 6% 35% 37% 12% 1	950
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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