Lake Runnemede Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Runnemede Observatory 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	16% 8% 3 6% 6% 3 5% 5% 5% 7% 35%	74811
	Night	15% 8% 3 6% 6% 3 5% 5% 5% 7% 36%	74974
Transparency	Day	48% 7% 12% 19% 11% 2	74516
	Night	48% 7% 13% 20% 11% 1	74803
Seeing	Day	24% 17% 29% 19% 9% 2	13737
	Night	22% 14% 28% 27% 9% 1	22801

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	11% 9% 5% 7% 8% 4 6% 6% 8% 7% 29%	1517
	11th to 20th	Day	15% 10% 4 6% 6% 5% 5% 5% 6% 8% 29%	1478
	21st to end	Day	10% 10% 4 7% 8% 4 6% 6% 6% 7% 33%	1584
February	1st to 10th	Day	13% 12% 4 9% 7% 4 6% 6% 4 8% 27%	1607
	11th to 20th	Day	9% 11% 5% 9% 7% 5% 6% 5% 4 6% 32%	1748
	21st to end	Day	17% 11% 5% 8% 6% 4 4 5% 4 6% 30%	1480
March	1st to 10th	Day	12% 9% 4 8% 6% 4 6% 6% 7% 6% 32%	1797
	11th to 20th	Day	16% 11% 5% 7% 6% 2 4 6% 5% 5% 34%	1849
	21st to end	Day	15% 9% 4 7% 6% 3 5% 4 4 6% 37%	2362
April	1st to 10th	Day	17% 10% 3 7% 5% 3 4 4 5% 6% 36%	2160
	11th to 20th	Day	16% 7% 3 6% 6% 4 4 5% 5% 7% 37%	2134
	21st to end	Day	22% 9% 3 7% 5% 3 4 5% 4 6% 31%	2324
May	1st to 10th	Day	22% 10% 3 6% 6% 3 4 4 5% 6% 32%	2340
	11th to 20th	Day	17% 6% 3 6% 5% 4 5% 5% 5% 7% 38%	2460
	21st to end	Day	15% 6% 3 5% 6% 4 5% 5% 4 8% 40%	2725
June	1st to 10th	Day	18% 6% 2 7% 8% 4 7% 6% 6% 8% 30%	2520
	11th to 20th	Day	17% 6% 3 5% 5% 3 6% 5% 5% 8% 36%	2520
	21st to end	Day	17% 7% 3 7% 7% 3 6% 6% 5% 7% 31%	2520
July	1st to 10th	Day	15% 5% 3 6% 6% 4 5% 5% 6% 8% 38%	2520
	11th to 20th	Day	12% 6% 3 6% 6% 4 6% 6% 6% 10% 36%	2520
	21st to end	Day	13% 5% 3 5% 6% 4 6% 6% 7% 9% 37%	2744
August	1st to 10th	Day	10% 5% 3 7% 6% 4 6% 6% 8% 9% 36%	2453
	11th to 20th	Day	12% 5% 2 6% 6% 5% 7% 7% 6% 8% 35%	2240
	21st to end	Day	12% 4 2 5% 6% 4 5% 5% 7% 9% 42%	2376
September	1st to 10th	Day	12% 5% 2 5% 5% 3 6% 5% 6% 9% 43%	2160
	11th to 20th	Day	13% 5% 2 4 5% 3 4 4 4 6% 49%	2026
	21st to end	Day	14% 6% 2 4 5% 3 4 5% 5% 7% 45%	1980
October	1st to 10th	Day	21% 7% 3 6% 6% 3 4 5% 4 6% 35%	1958
	11th to 20th	Day	14% 7% 2 7% 5% 3 5% 4 6% 8% 38%	1733
	21st to end	Day	23% 10% 3 7% 6% 3 4 4 4 6% 30%	1782
November	1st to 10th	Day	12% 8% 3 5% 4 3 5% 5% 6% 8% 41%	1671
	11th to 20th	Day	17% 8% 4 6% 6% 3 4 5% 6% 6% 37%	1710
	21st to end	Day	18% 10% 3 6% 5% 3 5% 5% 6% 6% 33%	1615
December	1st to 10th	Day	18% 11% 4 10% 7% 3 5% 5% 5% 6% 25%	1520
	11th to 20th	Day	21% 11% 3 7% 5% 2 4 4 4 5% 34%	1906
	21st to end	Day	22% 13% 4 9% 7% 3 5% 4 4 4 25%	2772
January	1st to 10th	Night	14% 8% 4 7% 6% 3 5% 6% 6% 7% 33%	2649
	11th to 20th	Night	14% 11% 4 8% 7% 4 5% 6% 6% 7% 29%	2588
	21st to end	Night	10% 6% 4 9% 6% 5% 5% 6% 6% 8% 35%	2772
February	1st to 10th	Night	12% 10% 5% 8% 7% 4 6% 6% 5% 7% 28%	2350
	11th to 20th	Night	9% 9% 5% 6% 6% 4 7% 7% 7% 8% 32%	2098
	21st to end	Night	14% 9% 3 7% 6% 4 6% 6% 6% 5% 34%	1776
March	1st to 10th	Night	13% 10% 2 6% 7% 4 6% 5% 6% 6% 35%	2150
	11th to 20th	Night	16% 9% 3 6% 7% 3 5% 6% 5% 6% 34%	2108
	21st to end	Night	14% 9% 3 7% 5% 3 5% 5% 5% 6% 39%	1990
April	1st to 10th	Night	17% 8% 3 5% 6% 3 5% 5% 5% 7% 36%	1800
	11th to 20th	Night	17% 7% 3 6% 4 3 5% 4 6% 6% 38%	1775
	21st to end	Night	20% 7% 3 6% 6% 3 4 4 5% 7% 35%	1634
May	1st to 10th	Night	21% 9% 3 6% 5% 3 5% 3 5% 6% 32%	1620
	11th to 20th	Night	17% 7% 3 6% 6% 3 6% 4 5% 7% 37%	1449
	21st to end	Night	14% 6% 3 5% 5% 4 5% 5% 5% 9% 39%	1550
June	1st to 10th	Night	19% 5% 3 6% 6% 4 5% 4 6% 7% 36%	1440
	11th to 20th	Night	18% 6% 3 6% 5% 2 4 4 5% 8% 40%	1440
	21st to end	Night	14% 5% 2 6% 6% 4 6% 7% 6% 7% 37%	1440
July	1st to 10th	Night	14% 4 2 4 5% 3 6% 5% 7% 6% 44%	1440
	11th to 20th	Night	13% 4 2 3 5% 4 5% 4 6% 10% 43%	1440
	21st to end	Night	12% 4 2 5% 4 3 5% 5% 6% 9% 45%	1574
August	1st to 10th	Night	10% 5% 2 5% 6% 3 4 5% 5% 8% 46%	1468
	11th to 20th	Night	12% 6% 3 5% 5% 4 5% 4 6% 8% 42%	1658
	21st to end	Night	14% 6% 2 4 4 3 3 4 5% 8% 46%	1980
September	1st to 10th	Night	12% 5% 2 3 4 2 4 4 5% 7% 51%	1800
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 51%	1934
	21st to end	Night	14% 7% 1 4 4 2 4 3 5% 7% 47%	1980
October	1st to 10th	Night	20% 7% 3 6% 5% 3 4 4 4 6% 38%	1998
	11th to 20th	Night	15% 6% 2 5% 5% 4 5% 6% 7% 7% 37%	2227
	21st to end	Night	22% 8% 3 6% 6% 4 5% 5% 5% 5% 32%	2574
November	1st to 10th	Night	13% 8% 3 6% 6% 4 5% 4 4 7% 41%	2407
	11th to 20th	Night	15% 7% 4 6% 5% 4 5% 4 5% 7% 38%	2470
	21st to end	Night	13% 9% 3 7% 6% 3 6% 5% 5% 7% 36%	2563
December	1st to 10th	Night	18% 10% 5% 9% 7% 4 5% 5% 5% 7% 27%	2660
	11th to 20th	Night	20% 9% 5% 7% 5% 3 5% 5% 5% 6% 31%	3288
	21st to end	Night	22% 12% 6% 9% 6% 3 4 4 4 5% 24%	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	53% 6% 13% 19% 9%	1518
	11th to 20th	Day	53% 9% 13% 14% 8% 2	1478
	21st to end	Day	50% 6% 14% 18% 10% 2	1584
February	1st to 10th	Day	55% 8% 13% 16% 8% 1	1607
	11th to 20th	Day	53% 7% 11% 17% 10% 2	1757
	21st to end	Day	56% 4 11% 16% 12% 1	1480
March	1st to 10th	Day	53% 6% 12% 16% 10% 3	1800
	11th to 20th	Day	52% 6% 12% 13% 14% 3	1849
	21st to end	Day	50% 7% 9% 16% 15% 4	2362
April	1st to 10th	Day	51% 5% 10% 19% 12% 3	2160
	11th to 20th	Day	47% 8% 11% 18% 15% 2	2134
	21st to end	Day	54% 6% 9% 16% 12% 3	2324
May	1st to 10th	Day	56% 5% 7% 19% 11% 3	2340
	11th to 20th	Day	46% 7% 9% 20% 15% 3	2464
	21st to end	Day	44% 9% 13% 22% 11% 2	2725
June	1st to 10th	Day	52% 9% 12% 17% 9%	2520
	11th to 20th	Day	46% 7% 12% 18% 15% 2	2520
	21st to end	Day	51% 9% 13% 16% 10% 1	2520
July	1st to 10th	Day	42% 9% 13% 21% 15% 1	2450
	11th to 20th	Day	42% 10% 16% 23% 9% 1	2445
	21st to end	Day	41% 9% 16% 23% 10% 1	2591
August	1st to 10th	Day	41% 11% 17% 21% 10% 1	2439
	11th to 20th	Day	43% 10% 16% 21% 10% 1	2240
	21st to end	Day	37% 8% 15% 25% 14% 1	2376
September	1st to 10th	Day	38% 8% 14% 26% 14% 1	2160
	11th to 20th	Day	37% 6% 10% 27% 19% 1	2026
	21st to end	Day	38% 7% 15% 25% 14% 2	1980
October	1st to 10th	Day	51% 6% 10% 18% 13% 2	1958
	11th to 20th	Day	44% 7% 15% 25% 8% 1	1733
	21st to end	Day	58% 6% 10% 16% 8% 2	1782
November	1st to 10th	Day	41% 8% 15% 21% 14% 1	1671
	11th to 20th	Day	47% 7% 13% 17% 14% 2	1710
	21st to end	Day	51% 7% 18% 17% 7% 1	1615
December	1st to 10th	Day	60% 6% 15% 13% 6% 1	1520
	11th to 20th	Day	54% 7% 11% 16% 11% 1	1906
	21st to end	Day	63% 6% 9% 13% 8%	2772
January	1st to 10th	Night	51% 7% 13% 17% 11% 1	2649
	11th to 20th	Night	56% 8% 14% 15% 7% 1	2588
	21st to end	Night	49% 7% 12% 17% 13% 2	2772
February	1st to 10th	Night	54% 8% 12% 16% 9% 1	2350
	11th to 20th	Night	51% 7% 12% 19% 11% 1	2100
	21st to end	Night	49% 7% 14% 19% 8% 3	1776
March	1st to 10th	Night	49% 7% 12% 18% 11% 2	2160
	11th to 20th	Night	52% 6% 11% 17% 11% 3	2109
	21st to end	Night	45% 7% 10% 16% 18% 4	1990
April	1st to 10th	Night	50% 6% 10% 19% 13% 2	1800
	11th to 20th	Night	47% 7% 10% 21% 14% 1	1775
	21st to end	Night	50% 7% 10% 17% 12% 3	1634
May	1st to 10th	Night	55% 5% 8% 16% 14% 2	1620
	11th to 20th	Night	47% 7% 10% 20% 14% 2	1450
	21st to end	Night	43% 9% 14% 21% 11% 2	1551
June	1st to 10th	Night	48% 9% 13% 18% 11% 1	1440
	11th to 20th	Night	45% 8% 13% 19% 12% 2	1440
	21st to end	Night	45% 10% 13% 21% 11%	1440
July	1st to 10th	Night	37% 10% 16% 23% 13% 1	1400
	11th to 20th	Night	36% 9% 18% 25% 12%	1392
	21st to end	Night	34% 8% 18% 28% 11% 2	1486
August	1st to 10th	Night	35% 9% 15% 27% 13%	1458
	11th to 20th	Night	40% 8% 19% 22% 11% 1	1658
	21st to end	Night	36% 8% 14% 27% 15%	1980
September	1st to 10th	Night	34% 7% 17% 28% 15% 1	1800
	11th to 20th	Night	36% 6% 12% 28% 18% 1	1934
	21st to end	Night	36% 7% 13% 27% 16% 2	1980
October	1st to 10th	Night	48% 6% 13% 20% 11% 2	1998
	11th to 20th	Night	45% 9% 15% 22% 8% 1	2227
	21st to end	Night	54% 7% 12% 17% 8% 1	2574
November	1st to 10th	Night	45% 7% 14% 20% 12% 2	2407
	11th to 20th	Night	45% 6% 12% 21% 13% 2	2470
	21st to end	Night	50% 7% 15% 20% 8%	2563
December	1st to 10th	Night	59% 8% 12% 17% 5%	2660
	11th to 20th	Night	53% 8% 13% 14% 11% 1	3288
	21st to end	Night	62% 7% 10% 16% 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	27% 24% 24% 17% 7%	180
	11th to 20th	Day	32% 25% 26% 14% 2 1	170
	21st to end	Day	20% 29% 34% 11% 6%	176
February	1st to 10th	Day	31% 31% 30% 6% 2	252
	11th to 20th	Day	28% 27% 31% 12% 2	416
	21st to end	Day	29% 23% 29% 17% 2	346
March	1st to 10th	Day	28% 19% 34% 16% 3	420
	11th to 20th	Day	32% 23% 28% 12% 4	420
	21st to end	Day	31% 20% 26% 13% 6% 3	462
April	1st to 10th	Day	35% 18% 27% 12% 6% 2	420
	11th to 20th	Day	31% 22% 24% 13% 8% 1	412
	21st to end	Day	33% 20% 28% 12% 6% 1	420
May	1st to 10th	Day	32% 22% 26% 13% 5% 2	423
	11th to 20th	Day	23% 18% 34% 19% 4 2	447
	21st to end	Day	20% 14% 32% 24% 8% 1	485
June	1st to 10th	Day	24% 14% 37% 15% 9% 1	450
	11th to 20th	Day	21% 16% 33% 18% 10% 3	450
	21st to end	Day	21% 16% 32% 18% 10% 2	450
July	1st to 10th	Day	16% 12% 33% 24% 12% 3	445
	11th to 20th	Day	14% 9% 34% 28% 11% 3	444
	21st to end	Day	14% 8% 39% 25% 11% 2	488
August	1st to 10th	Day	12% 11% 31% 25% 17% 3	446
	11th to 20th	Day	15% 9% 30% 26% 14% 7%	439
	21st to end	Day	13% 16% 26% 26% 14% 6%	495
September	1st to 10th	Day	17% 12% 25% 24% 15% 6%	450
	11th to 20th	Day	17% 14% 26% 19% 16% 8%	450
	21st to end	Day	16% 14% 28% 23% 12% 6%	450
October	1st to 10th	Day	26% 12% 25% 20% 12% 4	450
	11th to 20th	Day	20% 18% 23% 27% 9% 2	417
	21st to end	Day	35% 15% 22% 21% 7%	396
November	1st to 10th	Day	24% 15% 25% 23% 13% 1	365
	11th to 20th	Day	31% 20% 23% 20% 6% 1	370
	21st to end	Day	31% 19% 28% 16% 5% 1	275
December	1st to 10th	Day	30% 25% 25% 14% 6%	180
	11th to 20th	Day	35% 22% 31% 8% 4	180
	21st to end	Day	34% 20% 28% 14% 4 1	198
January	1st to 10th	Night	22% 18% 35% 22% 3	760
	11th to 20th	Night	25% 18% 31% 22% 3	740
	21st to end	Night	19% 19% 36% 22% 4	792
February	1st to 10th	Night	23% 20% 36% 19% 3	720
	11th to 20th	Night	21% 19% 36% 21% 2	703
	21st to end	Night	23% 20% 32% 21% 3	592
March	1st to 10th	Night	24% 19% 30% 25% 3	720
	11th to 20th	Night	26% 19% 28% 22% 5%	720
	21st to end	Night	26% 17% 27% 23% 6% 1	792
April	1st to 10th	Night	26% 19% 27% 24% 5%	720
	11th to 20th	Night	22% 22% 27% 21% 7%	711
	21st to end	Night	27% 16% 30% 22% 5%	554
May	1st to 10th	Night	28% 14% 30% 22% 6%	540
	11th to 20th	Night	19% 13% 30% 31% 6%	388
	21st to end	Night	19% 8% 26% 36% 12% 1	389
June	1st to 10th	Night	21% 7% 30% 32% 10%	360
	11th to 20th	Night	20% 11% 25% 33% 11%	360
	21st to end	Night	15% 8% 26% 41% 10% 1	360
July	1st to 10th	Night	14% 5% 21% 45% 14% 1	350
	11th to 20th	Night	14% 3 25% 39% 18% 2	348
	21st to end	Night	13% 5% 21% 44% 16% 1	394
August	1st to 10th	Night	12% 4 19% 44% 20% 1	394
	11th to 20th	Night	12% 6% 26% 35% 17% 3	594
	21st to end	Night	16% 6% 20% 35% 21% 2	792
September	1st to 10th	Night	15% 7% 25% 32% 20% 1	720
	11th to 20th	Night	17% 9% 22% 30% 18% 4	720
	21st to end	Night	18% 8% 25% 30% 17% 2	720
October	1st to 10th	Night	27% 10% 18% 29% 15% 1	720
	11th to 20th	Night	21% 12% 25% 31% 11% 1	720
	21st to end	Night	28% 14% 27% 24% 6%	792
November	1st to 10th	Night	21% 16% 26% 29% 9%	740
	11th to 20th	Night	23% 14% 31% 23% 8% 1	760
	21st to end	Night	24% 17% 28% 23% 6%	760
December	1st to 10th	Night	24% 19% 34% 19% 4	760
	11th to 20th	Night	27% 18% 31% 20% 3	760
	21st to end	Night	29% 17% 33% 18% 3	836

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