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 2026-03-18.

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	15% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	130932
	Night	15% 9% 3 8% 5% 5% 5% 4 5% 6% 34%	130579
Transparency	Day	48% 8% 12% 19% 11% 1	134426
	Night	50% 8% 12% 18% 11% 1	135172
Seeing	Day	25% 18% 29% 18% 8% 2	16493
	Night	22% 15% 28% 26% 8% 1	25558

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	8% 8% 6% 17% 9% 6% 5% 4 5% 6% 25%	4661
	11th to 20th	Day	9% 13% 5% 13% 8% 6% 5% 3 5% 5% 28%	4650
	21st to end	Day	10% 11% 4 10% 6% 5% 5% 4 6% 6% 34%	5124
February	1st to 10th	Day	11% 12% 3 10% 6% 6% 5% 5% 5% 7% 30%	1875
	11th to 20th	Day	9% 11% 4 10% 7% 6% 5% 4 5% 6% 32%	2048
	21st to end	Day	15% 13% 5% 9% 5% 5% 4 4 4 6% 31%	1730
March	1st to 10th	Day	17% 13% 4 11% 5% 5% 5% 3 5% 4 29%	4576
	11th to 20th	Day	14% 10% 4 8% 6% 4 4 4 5% 6% 34%	4723
	21st to end	Day	21% 11% 3 7% 5% 4 4 3 4 6% 32%	5956
April	1st to 10th	Day	17% 12% 3 8% 4 4 4 3 5% 6% 35%	2400
	11th to 20th	Day	16% 8% 3 7% 5% 5% 4 4 6% 6% 36%	2374
	21st to end	Day	21% 9% 3 7% 5% 4 4 4 4 6% 33%	2583
May	1st to 10th	Day	31% 10% 3 8% 5% 4 4 3 4 6% 21%	5886
	11th to 20th	Day	17% 9% 4 10% 5% 6% 5% 4 6% 7% 28%	6272
	21st to end	Day	18% 7% 2 7% 5% 5% 5% 5% 6% 8% 32%	6949
June	1st to 10th	Day	18% 7% 2 8% 6% 5% 6% 5% 6% 8% 30%	2800
	11th to 20th	Day	16% 7% 3 6% 4 5% 5% 5% 6% 9% 36%	2800
	21st to end	Day	17% 7% 3 8% 6% 5% 5% 5% 6% 8% 31%	2800
July	1st to 10th	Day	13% 6% 3 7% 5% 6% 5% 5% 7% 10% 34%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 6% 5% 6% 8% 11% 36%	6360
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 6% 9% 40%	6926
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 6% 4 7% 9% 34%	4466
	11th to 20th	Day	13% 7% 3 7% 5% 5% 6% 5% 7% 8% 34%	4051
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 8% 10% 42%	4312
September	1st to 10th	Day	12% 5% 2 5% 5% 4 5% 3 6% 8% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 4 5% 4 3 4 7% 49%	2250
	21st to end	Day	15% 6% 2 5% 4 4 4 4 6% 7% 44%	2200
October	1st to 10th	Day	20% 7% 3 6% 5% 3 5% 4 4 5% 37%	2175
	11th to 20th	Day	15% 8% 2 7% 4 4 4 3 6% 7% 39%	1925
	21st to end	Day	21% 10% 3 8% 5% 4 4 4 4 6% 32%	1980
November	1st to 10th	Day	12% 9% 3 6% 4 4 5% 4 6% 8% 41%	1851
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 4 5% 6% 39%	1890
	21st to end	Day	18% 11% 3 7% 4 4 5% 4 6% 6% 32%	1785
December	1st to 10th	Day	14% 13% 5% 11% 5% 5% 4 4 6% 7% 25%	2680
	11th to 20th	Day	20% 11% 3 8% 5% 4 4 3 5% 6% 32%	3066
	21st to end	Day	18% 11% 4 10% 6% 4 4 3 4 5% 30%	4048
January	1st to 10th	Night	11% 8% 5% 15% 8% 6% 5% 4 5% 6% 28%	8234
	11th to 20th	Night	10% 12% 5% 13% 8% 6% 6% 4 6% 6% 24%	8235
	21st to end	Night	11% 8% 3 9% 6% 7% 7% 4 6% 6% 33%	9057
February	1st to 10th	Night	11% 10% 4 10% 6% 5% 5% 5% 6% 7% 31%	2741
	11th to 20th	Night	9% 9% 4 7% 6% 5% 7% 5% 8% 8% 31%	2458
	21st to end	Night	13% 10% 3 8% 6% 5% 5% 4 6% 5% 34%	2076
March	1st to 10th	Night	19% 11% 3 7% 6% 4 5% 4 6% 6% 29%	5484
	11th to 20th	Night	14% 10% 3 8% 6% 5% 5% 4 5% 6% 34%	5389
	21st to end	Night	20% 12% 3 9% 4 4 4 3 5% 5% 31%	4972
April	1st to 10th	Night	17% 9% 3 6% 6% 5% 4 4 6% 7% 34%	2000
	11th to 20th	Night	17% 9% 3 7% 4 4 5% 3 5% 6% 37%	1975
	21st to end	Night	19% 8% 3 7% 5% 3 4 3 5% 6% 37%	1815
May	1st to 10th	Night	32% 10% 2 7% 4 4 4 3 5% 6% 22%	4054
	11th to 20th	Night	15% 11% 3 10% 5% 4 5% 3 5% 6% 30%	3651
	21st to end	Night	19% 7% 2 8% 6% 5% 5% 4 5% 8% 32%	3926
June	1st to 10th	Night	19% 6% 3 6% 5% 5% 4 4 6% 7% 37%	1600
	11th to 20th	Night	17% 6% 2 7% 5% 3 4 3 5% 8% 39%	1600
	21st to end	Night	15% 6% 2 7% 5% 5% 5% 6% 7% 7% 36%	1600
July	1st to 10th	Night	11% 5% 2 5% 5% 5% 5% 5% 7% 8% 42%	3600
	11th to 20th	Night	14% 7% 2 6% 4 5% 5% 4 6% 9% 38%	3600
	21st to end	Night	12% 5% 2 5% 3 4 4 4 5% 7% 49%	3926
August	1st to 10th	Night	19% 8% 3 7% 5% 4 5% 3 5% 7% 36%	2652
	11th to 20th	Night	16% 6% 3 6% 4 4 4 3 6% 8% 40%	2994
	21st to end	Night	8% 4 1 4 3 4 3 4 6% 10% 50%	3586
September	1st to 10th	Night	12% 6% 2 4 4 3 4 3 5% 6% 52%	2000
	11th to 20th	Night	12% 5% 2 5% 4 4 3 2 5% 6% 53%	2150
	21st to end	Night	15% 7% 1 5% 4 3 4 3 5% 7% 46%	2200
October	1st to 10th	Night	18% 7% 3 6% 5% 4 4 3 4 6% 39%	2221
	11th to 20th	Night	16% 6% 2 6% 5% 5% 5% 5% 6% 7% 38%	2475
	21st to end	Night	21% 8% 2 7% 5% 4 4 4 6% 5% 33%	2860
November	1st to 10th	Night	13% 8% 2 7% 5% 5% 4 3 5% 7% 41%	2667
	11th to 20th	Night	14% 8% 3 7% 4 5% 5% 4 6% 7% 38%	2730
	21st to end	Night	13% 10% 3 8% 5% 5% 5% 4 6% 6% 34%	2833
December	1st to 10th	Night	15% 11% 4 11% 6% 5% 5% 4 5% 7% 27%	4720
	11th to 20th	Night	22% 10% 4 8% 4 4 4 3 4 4 31%	5348
	21st to end	Night	19% 13% 6% 10% 4 4 4 3 4 5% 28%	7150

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	58% 7% 11% 18% 5%	4662
	11th to 20th	Day	54% 8% 10% 16% 10% 2	4680
	21st to end	Day	48% 8% 10% 19% 13% 2	5124
February	1st to 10th	Day	52% 7% 12% 18% 9% 2	1875
	11th to 20th	Day	53% 7% 11% 18% 9% 2	2057
	21st to end	Day	54% 5% 11% 17% 12% 1	1730
March	1st to 10th	Day	60% 6% 8% 12% 9% 6%	4579
	11th to 20th	Day	45% 10% 16% 18% 8% 2	4723
	21st to end	Day	54% 6% 7% 13% 17% 3	5956
April	1st to 10th	Day	52% 5% 9% 19% 12% 3	2400
	11th to 20th	Day	48% 8% 10% 18% 15% 2	2374
	21st to end	Day	52% 7% 9% 17% 13% 3	2583
May	1st to 10th	Day	64% 7% 8% 14% 6% 1	5897
	11th to 20th	Day	53% 10% 13% 15% 8% 1	6276
	21st to end	Day	46% 10% 13% 21% 9% 1	6949
June	1st to 10th	Day	51% 9% 12% 17% 10%	2800
	11th to 20th	Day	45% 8% 13% 18% 14% 2	2800
	21st to end	Day	50% 9% 12% 17% 10% 1	2800
July	1st to 10th	Day	44% 8% 15% 20% 12%	6290
	11th to 20th	Day	39% 11% 16% 22% 12% 1	6285
	21st to end	Day	40% 8% 14% 28% 11% 1	6728
August	1st to 10th	Day	45% 10% 15% 19% 10%	4452
	11th to 20th	Day	45% 10% 15% 23% 6%	4090
	21st to end	Day	34% 9% 17% 25% 13% 1	4312
September	1st to 10th	Day	36% 7% 13% 28% 15% 1	2400
	11th to 20th	Day	36% 6% 11% 27% 19% 1	2250
	21st to end	Day	39% 7% 14% 25% 14% 2	2200
October	1st to 10th	Day	45% 7% 10% 22% 16% 1	3539
	11th to 20th	Day	43% 5% 13% 25% 14% 1	3115
	21st to end	Day	41% 6% 9% 31% 12% 2	3180
November	1st to 10th	Day	41% 8% 15% 21% 14% 1	1851
	11th to 20th	Day	46% 7% 12% 18% 15% 2	1890
	21st to end	Day	53% 7% 17% 17% 6% 1	1785
December	1st to 10th	Day	59% 7% 14% 14% 5%	2680
	11th to 20th	Day	55% 7% 9% 16% 12% 1	3066
	21st to end	Day	57% 5% 7% 15% 13% 3	4048
January	1st to 10th	Night	58% 8% 11% 15% 7% 1	8234
	11th to 20th	Night	58% 10% 11% 11% 8% 2	8280
	21st to end	Night	50% 9% 9% 16% 13% 3	9057
February	1st to 10th	Night	52% 8% 11% 19% 10% 1	2741
	11th to 20th	Night	50% 8% 13% 19% 10% 1	2460
	21st to end	Night	49% 7% 13% 19% 8% 3	2076
March	1st to 10th	Night	54% 6% 10% 14% 13% 3	5494
	11th to 20th	Night	50% 8% 16% 15% 9% 2	5390
	21st to end	Night	54% 7% 7% 13% 18% 2	4972
April	1st to 10th	Night	52% 6% 10% 18% 13% 2	2000
	11th to 20th	Night	49% 6% 10% 20% 14% 1	1975
	21st to end	Night	49% 7% 10% 17% 14% 3	1815
May	1st to 10th	Night	63% 8% 10% 11% 7% 1	4060
	11th to 20th	Night	53% 10% 11% 16% 8% 1	3652
	21st to end	Night	50% 10% 13% 16% 9% 1	3927
June	1st to 10th	Night	47% 10% 13% 18% 12% 1	1600
	11th to 20th	Night	45% 9% 14% 19% 12% 2	1600
	21st to end	Night	45% 9% 13% 21% 11% 1	1600
July	1st to 10th	Night	39% 8% 18% 24% 10%	3560
	11th to 20th	Night	44% 9% 13% 21% 13%	3552
	21st to end	Night	36% 7% 14% 32% 11% 1	3809
August	1st to 10th	Night	49% 8% 14% 20% 10%	2642
	11th to 20th	Night	42% 9% 19% 20% 10%	3021
	21st to end	Night	30% 8% 20% 29% 13% 1	3586
September	1st to 10th	Night	33% 7% 16% 28% 16% 1	2000
	11th to 20th	Night	34% 5% 12% 29% 18% 1	2150
	21st to end	Night	37% 7% 13% 26% 16% 2	2200
October	1st to 10th	Night	45% 8% 14% 22% 10% 1	3637
	11th to 20th	Night	39% 6% 13% 24% 15% 2	4024
	21st to end	Night	44% 6% 10% 29% 10% 1	4610
November	1st to 10th	Night	45% 6% 13% 21% 12% 2	2667
	11th to 20th	Night	46% 6% 12% 21% 14% 2	2730
	21st to end	Night	51% 8% 15% 19% 8%	2833
December	1st to 10th	Night	61% 8% 12% 16% 3	4720
	11th to 20th	Night	57% 6% 9% 13% 14% 1	5348
	21st to end	Night	59% 6% 9% 15% 8% 3	7150

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	26% 27% 24% 17% 6%	220
	11th to 20th	Day	30% 27% 29% 12% 1 1	220
	21st to end	Day	26% 26% 31% 11% 5%	242
February	1st to 10th	Day	29% 30% 31% 8% 2	340
	11th to 20th	Day	29% 26% 32% 12% 1	536
	21st to end	Day	28% 24% 30% 15% 2	446
March	1st to 10th	Day	32% 20% 32% 14% 2	540
	11th to 20th	Day	34% 23% 28% 11% 4	528
	21st to end	Day	34% 21% 25% 12% 5% 3	550
April	1st to 10th	Day	37% 20% 25% 11% 6% 1	500
	11th to 20th	Day	34% 23% 24% 11% 7% 1	492
	21st to end	Day	31% 22% 28% 12% 6% 1	500
May	1st to 10th	Day	34% 20% 26% 13% 4 2	503
	11th to 20th	Day	23% 18% 33% 20% 5% 2	527
	21st to end	Day	21% 15% 31% 24% 7% 1	573
June	1st to 10th	Day	23% 15% 36% 16% 9% 1	530
	11th to 20th	Day	21% 16% 33% 19% 9% 2	530
	21st to end	Day	22% 17% 31% 18% 9% 2	530
July	1st to 10th	Day	16% 12% 34% 24% 10% 3	525
	11th to 20th	Day	13% 11% 34% 29% 11% 3	524
	21st to end	Day	14% 10% 38% 26% 10% 2	576
August	1st to 10th	Day	13% 10% 31% 26% 17% 3	526
	11th to 20th	Day	16% 9% 29% 24% 14% 7%	519
	21st to end	Day	13% 15% 26% 27% 14% 5%	583
September	1st to 10th	Day	16% 12% 25% 24% 15% 7%	530
	11th to 20th	Day	15% 13% 25% 21% 17% 10%	530
	21st to end	Day	17% 14% 28% 23% 12% 6%	530
October	1st to 10th	Day	25% 12% 25% 20% 13% 5%	530
	11th to 20th	Day	23% 19% 21% 26% 8% 2	489
	21st to end	Day	34% 15% 24% 20% 6%	462
November	1st to 10th	Day	27% 18% 24% 20% 11%	425
	11th to 20th	Day	31% 20% 23% 19% 6% 1	430
	21st to end	Day	35% 19% 27% 14% 4 1	325
December	1st to 10th	Day	30% 25% 26% 15% 5%	220
	11th to 20th	Day	36% 23% 30% 7% 4	220
	21st to end	Day	33% 19% 30% 12% 5%	242
January	1st to 10th	Night	23% 20% 34% 21% 3	840
	11th to 20th	Night	25% 19% 32% 21% 3 1	840
	21st to end	Night	20% 20% 36% 20% 4 1	924
February	1st to 10th	Night	22% 21% 35% 20% 3	840
	11th to 20th	Night	22% 20% 36% 20% 2	823
	21st to end	Night	23% 20% 33% 21% 3	692
March	1st to 10th	Night	25% 19% 30% 23% 2	840
	11th to 20th	Night	27% 19% 28% 22% 5%	829
	21st to end	Night	28% 16% 27% 22% 6% 1	880
April	1st to 10th	Night	27% 19% 27% 23% 4	800
	11th to 20th	Night	23% 23% 27% 20% 7%	791
	21st to end	Night	27% 17% 29% 22% 5%	615
May	1st to 10th	Night	28% 14% 29% 22% 6%	600
	11th to 20th	Night	19% 12% 30% 32% 6%	431
	21st to end	Night	19% 8% 26% 35% 11%	433
June	1st to 10th	Night	21% 8% 29% 33% 9%	400
	11th to 20th	Night	19% 12% 27% 33% 11%	400
	21st to end	Night	16% 9% 26% 39% 10% 1	400
July	1st to 10th	Night	14% 6% 21% 45% 13% 1	390
	11th to 20th	Night	15% 4 25% 37% 18% 2	388
	21st to end	Night	13% 6% 22% 43% 15% 1	438
August	1st to 10th	Night	14% 4 18% 44% 19%	438
	11th to 20th	Night	13% 7% 26% 33% 18% 3	662
	21st to end	Night	15% 6% 21% 35% 20% 1	880
September	1st to 10th	Night	14% 8% 26% 31% 19% 1	800
	11th to 20th	Night	15% 8% 21% 32% 20% 5%	800
	21st to end	Night	18% 8% 24% 31% 18% 2	800
October	1st to 10th	Night	25% 10% 20% 28% 16% 1	800
	11th to 20th	Night	21% 13% 25% 30% 11% 1	800
	21st to end	Night	28% 14% 27% 25% 6%	880
November	1st to 10th	Night	22% 17% 26% 27% 8%	820
	11th to 20th	Night	24% 15% 30% 23% 7% 1	840
	21st to end	Night	25% 18% 29% 23% 6%	840
December	1st to 10th	Night	24% 19% 33% 19% 4	840
	11th to 20th	Night	28% 18% 31% 20% 3	840
	21st to end	Night	29% 18% 32% 18% 3	924

Page generated at 2026-03-15 02:23:57. Version ac07

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