Rue Lambert
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rue Lambert clear sky chart from 2005-11-06 to 2026-05-17.

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	17% 12% 4 10% 6% 5% 5% 4 5% 6% 26%	131822
	Night	16% 14% 5% 12% 7% 5% 4 3 4 5% 25%	131007
Transparency	Day	57% 7% 11% 17% 8% 1	135277
	Night	59% 7% 10% 16% 8% 1	135626
Seeing	Day	26% 18% 29% 18% 7% 2	16539
	Night	24% 16% 28% 23% 8% 1	25526

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	12% 17% 10% 20% 9% 9% 4 3 3 3 12%	4661
	11th to 20th	Day	8% 18% 8% 22% 14% 6% 5% 3 3 3 9%	4650
	21st to end	Day	7% 15% 7% 15% 9% 14% 6% 4 5% 4 12%	5124
February	1st to 10th	Day	12% 19% 6% 16% 9% 6% 5% 3 4 3 18%	1769
	11th to 20th	Day	12% 17% 8% 17% 8% 5% 5% 3 5% 4 16%	2034
	21st to end	Day	12% 17% 6% 14% 8% 6% 4 4 4 5% 19%	1730
March	1st to 10th	Day	18% 15% 7% 16% 6% 5% 5% 3 3 3 18%	4576
	11th to 20th	Day	12% 17% 6% 12% 5% 4 4 3 5% 5% 26%	4759
	21st to end	Day	19% 14% 6% 12% 6% 4 3 2 3 4 26%	6073
April	1st to 10th	Day	20% 18% 5% 11% 5% 4 3 3 3 5% 24%	2520
	11th to 20th	Day	19% 15% 5% 8% 4 3 4 2 4 3 32%	2520
	21st to end	Day	22% 13% 3 8% 4 4 4 3 5% 6% 27%	2730
May	1st to 10th	Day	28% 14% 4 8% 5% 4 4 3 5% 6% 20%	6154
	11th to 20th	Day	19% 9% 3 8% 6% 6% 5% 4 6% 7% 27%	6402
	21st to end	Day	20% 10% 3 7% 5% 5% 5% 4 6% 7% 29%	6949
June	1st to 10th	Day	25% 9% 3 7% 4 4 4 4 6% 6% 27%	2800
	11th to 20th	Day	16% 8% 2 7% 5% 5% 5% 4 6% 9% 34%	2800
	21st to end	Day	23% 8% 3 8% 5% 4 5% 4 6% 6% 28%	2800
July	1st to 10th	Day	15% 8% 3 8% 6% 5% 6% 6% 7% 9% 27%	6360
	11th to 20th	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	6360
	21st to end	Day	14% 8% 2 5% 4 4 5% 4 6% 9% 40%	6926
August	1st to 10th	Day	17% 8% 3 8% 5% 5% 6% 4 7% 8% 28%	4544
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 4 7% 9% 30%	4154
	21st to end	Day	18% 7% 2 6% 4 4 4 4 6% 8% 36%	4312
September	1st to 10th	Day	14% 8% 2 6% 4 4 5% 4 5% 7% 40%	2400
	11th to 20th	Day	17% 7% 3 5% 4 3 3 3 5% 7% 44%	2255
	21st to end	Day	19% 8% 3 5% 4 4 4 4 5% 9% 35%	2200
October	1st to 10th	Day	22% 10% 2 7% 4 4 5% 3 6% 6% 31%	2155
	11th to 20th	Day	19% 13% 3 9% 5% 4 5% 3 5% 6% 28%	1894
	21st to end	Day	25% 19% 4 8% 4 4 3 2 3 4 24%	1980
November	1st to 10th	Day	18% 19% 4 7% 5% 4 4 3 4 5% 26%	1851
	11th to 20th	Day	15% 20% 7% 12% 5% 3 4 4 3 5% 22%	1890
	21st to end	Day	18% 20% 8% 13% 5% 4 3 3 5% 5% 16%	1696
December	1st to 10th	Day	14% 24% 8% 19% 9% 4 4 2 3 4 10%	2680
	11th to 20th	Day	17% 18% 7% 16% 5% 4 4 3 4 4 19%	3066
	21st to end	Day	18% 17% 6% 15% 6% 4 5% 3 4 5% 18%	4048
January	1st to 10th	Night	10% 17% 9% 19% 10% 7% 4 3 3 3 14%	8234
	11th to 20th	Night	8% 18% 9% 21% 12% 7% 5% 3 3 3 10%	8235
	21st to end	Night	8% 14% 6% 15% 10% 12% 6% 4 5% 4 17%	9057
February	1st to 10th	Night	12% 17% 6% 16% 8% 6% 5% 3 4 5% 18%	2846
	11th to 20th	Night	11% 15% 6% 17% 9% 6% 5% 3 5% 4 20%	2471
	21st to end	Night	12% 17% 6% 12% 7% 6% 4 4 4 4 25%	2076
March	1st to 10th	Night	18% 15% 6% 12% 5% 6% 5% 3 3 5% 23%	5484
	11th to 20th	Night	13% 15% 5% 12% 6% 5% 5% 3 5% 5% 24%	5419
	21st to end	Night	15% 16% 6% 13% 7% 5% 4 3 3 3 25%	5086
April	1st to 10th	Night	22% 14% 4 9% 5% 4 4 3 4 5% 26%	2100
	11th to 20th	Night	20% 13% 4 7% 4 5% 4 2 4 5% 31%	2049
	21st to end	Night	21% 10% 3 8% 5% 4 3 3 5% 6% 32%	1890
May	1st to 10th	Night	25% 14% 4 9% 5% 4 4 3 4 5% 23%	4050
	11th to 20th	Night	23% 8% 3 7% 5% 5% 5% 4 5% 6% 30%	3623
	21st to end	Night	19% 10% 3 8% 5% 4 5% 3 5% 6% 32%	3926
June	1st to 10th	Night	26% 8% 2 7% 3 4 4 4 6% 7% 30%	1600
	11th to 20th	Night	18% 6% 3 6% 4 4 5% 4 7% 8% 36%	1600
	21st to end	Night	19% 9% 3 6% 5% 5% 5% 4 7% 7% 30%	1600
July	1st to 10th	Night	13% 8% 3 8% 6% 6% 6% 5% 7% 9% 29%	3600
	11th to 20th	Night	17% 9% 2 6% 4 4 5% 4 5% 7% 37%	3600
	21st to end	Night	12% 7% 2 7% 5% 5% 5% 4 7% 8% 39%	3926
August	1st to 10th	Night	21% 9% 3 6% 5% 3 4 3 5% 7% 33%	2586
	11th to 20th	Night	14% 7% 3 8% 6% 4 6% 4 7% 8% 33%	2876
	21st to end	Night	16% 7% 2 6% 4 4 5% 4 5% 7% 40%	3586
September	1st to 10th	Night	15% 7% 2 7% 4 4 4 3 5% 7% 42%	2000
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 7% 47%	2145
	21st to end	Night	20% 7% 2 5% 4 4 4 2 5% 7% 41%	2200
October	1st to 10th	Night	20% 9% 3 6% 4 4 4 4 5% 5% 36%	2240
	11th to 20th	Night	20% 11% 4 8% 6% 4 4 3 4 5% 30%	2505
	21st to end	Night	24% 14% 4 8% 4 4 4 3 4 5% 25%	2860
November	1st to 10th	Night	18% 15% 5% 8% 5% 5% 4 4 4 6% 26%	2667
	11th to 20th	Night	17% 14% 6% 11% 6% 5% 3 3 4 5% 26%	2730
	21st to end	Night	17% 18% 7% 13% 6% 4 3 2 4 4 21%	2922
December	1st to 10th	Night	15% 23% 9% 20% 7% 4 4 2 3 3 9%	4720
	11th to 20th	Night	18% 19% 6% 17% 7% 4 3 2 3 3 16%	5348
	21st to end	Night	18% 18% 5% 13% 6% 5% 5% 3 3 5% 20%	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	78% 4 8% 8% 1	4662
	11th to 20th	Day	76% 6% 7% 7% 3	4680
	21st to end	Day	71% 6% 8% 9% 7%	5124
February	1st to 10th	Day	72% 5% 6% 12% 5% 1	1769
	11th to 20th	Day	71% 4 9% 10% 5% 1	2043
	21st to end	Day	66% 5% 9% 13% 6% 2	1730
March	1st to 10th	Day	69% 6% 7% 8% 7% 3	4579
	11th to 20th	Day	58% 5% 13% 12% 10% 1	4759
	21st to end	Day	61% 4 7% 13% 14% 1	6073
April	1st to 10th	Day	63% 4 8% 16% 8% 1	2520
	11th to 20th	Day	56% 4 7% 18% 13% 1	2520
	21st to end	Day	57% 5% 9% 17% 9% 2	2730
May	1st to 10th	Day	64% 6% 8% 15% 6% 1	6165
	11th to 20th	Day	53% 10% 13% 16% 7% 1	6406
	21st to end	Day	50% 8% 11% 20% 10% 1	6949
June	1st to 10th	Day	54% 7% 11% 18% 9% 1	2800
	11th to 20th	Day	46% 7% 11% 23% 12% 1	2800
	21st to end	Day	54% 8% 11% 19% 8% 1	2800
July	1st to 10th	Day	48% 9% 15% 20% 7%	6290
	11th to 20th	Day	41% 10% 14% 28% 7%	6285
	21st to end	Day	39% 8% 13% 32% 8%	6728
August	1st to 10th	Day	50% 9% 15% 20% 6%	4530
	11th to 20th	Day	48% 9% 18% 19% 6%	4193
	21st to end	Day	44% 8% 15% 20% 12% 2	4312
September	1st to 10th	Day	41% 7% 13% 29% 10% 1	2400
	11th to 20th	Day	40% 5% 11% 29% 14%	2255
	21st to end	Day	45% 7% 13% 24% 11% 1	2200
October	1st to 10th	Day	46% 7% 14% 25% 8%	3506
	11th to 20th	Day	51% 5% 14% 23% 6%	3058
	21st to end	Day	53% 4 14% 20% 8% 1	3180
November	1st to 10th	Day	59% 6% 10% 17% 7%	1851
	11th to 20th	Day	65% 6% 9% 12% 7% 1	1890
	21st to end	Day	69% 6% 12% 10% 2	1696
December	1st to 10th	Day	80% 4 8% 7% 2	2680
	11th to 20th	Day	68% 4 8% 15% 5%	3066
	21st to end	Day	69% 5% 7% 10% 9%	4048
January	1st to 10th	Night	74% 6% 9% 9% 3	8234
	11th to 20th	Night	77% 5% 8% 7% 3	8280
	21st to end	Night	66% 7% 8% 10% 8%	9057
February	1st to 10th	Night	68% 5% 8% 13% 5%	2846
	11th to 20th	Night	67% 6% 8% 13% 6%	2473
	21st to end	Night	63% 4 9% 14% 9% 1	2076
March	1st to 10th	Night	63% 6% 8% 12% 9% 2	5494
	11th to 20th	Night	60% 6% 13% 12% 8% 1	5420
	21st to end	Night	63% 5% 5% 15% 10% 1	5086
April	1st to 10th	Night	59% 5% 9% 16% 11% 1	2100
	11th to 20th	Night	55% 5% 8% 18% 12% 1	2049
	21st to end	Night	53% 6% 8% 18% 13% 2	1890
May	1st to 10th	Night	62% 7% 9% 15% 6% 1	4056
	11th to 20th	Night	53% 9% 11% 18% 7% 1	3624
	21st to end	Night	51% 8% 10% 18% 12% 1	3927
June	1st to 10th	Night	52% 6% 11% 22% 9% 1	1600
	11th to 20th	Night	44% 8% 12% 22% 11% 3	1600
	21st to end	Night	50% 8% 13% 19% 9% 1	1600
July	1st to 10th	Night	47% 10% 17% 19% 7%	3560
	11th to 20th	Night	43% 8% 13% 28% 8%	3552
	21st to end	Night	40% 9% 16% 27% 9%	3809
August	1st to 10th	Night	48% 7% 15% 24% 6%	2576
	11th to 20th	Night	45% 11% 17% 19% 7%	2903
	21st to end	Night	42% 8% 13% 23% 13% 1	3586
September	1st to 10th	Night	40% 8% 12% 27% 12%	2000
	11th to 20th	Night	36% 7% 15% 29% 12% 1	2145
	21st to end	Night	43% 7% 12% 24% 12% 1	2200
October	1st to 10th	Night	48% 7% 11% 24% 9%	3656
	11th to 20th	Night	49% 7% 12% 24% 9%	4080
	21st to end	Night	51% 6% 14% 19% 9% 1	4610
November	1st to 10th	Night	58% 6% 12% 17% 6% 1	2667
	11th to 20th	Night	60% 6% 10% 14% 8% 2	2730
	21st to end	Night	66% 7% 10% 13% 4	2922
December	1st to 10th	Night	80% 6% 6% 6% 2	4720
	11th to 20th	Night	73% 4 6% 10% 6%	5348
	21st to end	Night	69% 4 7% 13% 7%	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% 24% 30% 17% 3	220
	11th to 20th	Day	25% 30% 31% 10% 3 1	220
	21st to end	Day	26% 23% 32% 13% 5% 1	242
February	1st to 10th	Day	33% 27% 27% 10% 2	267
	11th to 20th	Day	29% 26% 32% 11% 2	522
	21st to end	Day	30% 23% 31% 15% 1	446
March	1st to 10th	Day	32% 21% 31% 14% 3	540
	11th to 20th	Day	32% 24% 28% 12% 4 1	540
	21st to end	Day	34% 24% 23% 12% 6% 1	594
April	1st to 10th	Day	37% 19% 27% 13% 4 1	540
	11th to 20th	Day	36% 19% 25% 13% 5% 2	532
	21st to end	Day	33% 19% 29% 11% 6% 1	540
May	1st to 10th	Day	34% 19% 25% 16% 5% 1	543
	11th to 20th	Day	24% 17% 31% 20% 6% 2	551
	21st to end	Day	21% 20% 32% 21% 5% 1	573
June	1st to 10th	Day	29% 14% 32% 18% 5% 1	530
	11th to 20th	Day	21% 15% 32% 22% 9% 1	530
	21st to end	Day	26% 14% 35% 16% 6% 2	530
July	1st to 10th	Day	17% 13% 35% 23% 10% 2	525
	11th to 20th	Day	14% 12% 40% 24% 7% 2	524
	21st to end	Day	18% 11% 36% 25% 9% 1	576
August	1st to 10th	Day	16% 12% 28% 29% 14% 2	526
	11th to 20th	Day	17% 9% 28% 27% 15% 3	519
	21st to end	Day	19% 15% 26% 24% 11% 6%	583
September	1st to 10th	Day	18% 12% 29% 22% 13% 5%	530
	11th to 20th	Day	17% 12% 22% 23% 19% 8%	530
	21st to end	Day	19% 15% 26% 22% 14% 4	530
October	1st to 10th	Day	25% 15% 26% 18% 12% 4	530
	11th to 20th	Day	28% 20% 24% 18% 10% 1	471
	21st to end	Day	36% 19% 24% 17% 4	462
November	1st to 10th	Day	33% 21% 27% 15% 4	425
	11th to 20th	Day	28% 24% 29% 13% 6% 1	430
	21st to end	Day	34% 22% 31% 12% 2	236
December	1st to 10th	Day	30% 23% 26% 16% 4 1	220
	11th to 20th	Day	33% 25% 30% 10%	220
	21st to end	Day	35% 22% 27% 12% 3 1	242
January	1st to 10th	Night	27% 22% 31% 18% 3	840
	11th to 20th	Night	25% 22% 32% 16% 4	840
	21st to end	Night	20% 20% 33% 21% 5% 1	924
February	1st to 10th	Night	22% 20% 33% 20% 4	840
	11th to 20th	Night	21% 22% 35% 18% 3	823
	21st to end	Night	26% 17% 33% 22% 2	692
March	1st to 10th	Night	28% 20% 28% 20% 3	840
	11th to 20th	Night	30% 18% 27% 20% 6%	840
	21st to end	Night	27% 19% 27% 20% 7% 1	924
April	1st to 10th	Night	31% 16% 28% 20% 4	840
	11th to 20th	Night	29% 17% 27% 20% 7%	805
	21st to end	Night	30% 16% 28% 19% 7%	630
May	1st to 10th	Night	26% 11% 31% 25% 6% 1	588
	11th to 20th	Night	22% 12% 30% 27% 9% 1	406
	21st to end	Night	20% 12% 29% 31% 8%	433
June	1st to 10th	Night	28% 8% 28% 27% 10%	400
	11th to 20th	Night	20% 8% 27% 32% 13%	400
	21st to end	Night	19% 9% 28% 34% 10% 1	400
July	1st to 10th	Night	13% 7% 29% 36% 14%	390
	11th to 20th	Night	13% 4 25% 40% 17% 1	388
	21st to end	Night	13% 6% 26% 41% 14%	438
August	1st to 10th	Night	19% 5% 15% 40% 20%	398
	11th to 20th	Night	15% 7% 22% 35% 20% 1	583
	21st to end	Night	16% 7% 24% 34% 18% 2	880
September	1st to 10th	Night	17% 8% 25% 34% 14% 1	800
	11th to 20th	Night	18% 8% 19% 31% 22% 2	800
	21st to end	Night	22% 9% 28% 25% 16% 1	800
October	1st to 10th	Night	25% 12% 23% 26% 13% 1	800
	11th to 20th	Night	27% 14% 26% 23% 9% 1	800
	21st to end	Night	30% 16% 28% 23% 3	880
November	1st to 10th	Night	29% 17% 29% 20% 5% 1	820
	11th to 20th	Night	26% 19% 29% 16% 9% 1	840
	21st to end	Night	28% 24% 27% 15% 5% 1	840
December	1st to 10th	Night	30% 25% 26% 15% 4	840
	11th to 20th	Night	30% 20% 29% 16% 4	840
	21st to end	Night	32% 22% 25% 18% 3	924

Page generated at 2026-05-14 01:58:57. Version ac07

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