Saint-Lazare
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Saint-Lazare 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% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	131977
	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 30%	130851
Transparency	Day	34% 36% 8% 15% 7% 1	135422
	Night	37% 35% 8% 13% 7% 1	135493
Seeing	Day	20% 10% 28% 25% 13% 4	16419
	Night	22% 9% 28% 29% 10% 1	25511

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	15% 18% 6% 13% 5% 4 4 3 4 5% 21%	4661
	11th to 20th	Day	12% 15% 7% 12% 7% 6% 5% 3 6% 5% 23%	4650
	21st to end	Day	10% 13% 5% 12% 9% 8% 6% 4 5% 5% 23%	5124
February	1st to 10th	Day	13% 16% 4 11% 6% 5% 5% 4 6% 5% 26%	1873
	11th to 20th	Day	14% 13% 4 10% 7% 6% 5% 4 5% 7% 26%	1935
	21st to end	Day	18% 14% 5% 10% 6% 5% 5% 3 4 5% 26%	1730
March	1st to 10th	Day	18% 12% 5% 12% 6% 5% 4 3 4 5% 25%	4576
	11th to 20th	Day	11% 13% 4 11% 6% 5% 5% 4 5% 6% 31%	4850
	21st to end	Day	19% 8% 4 9% 5% 5% 4 3 5% 5% 34%	6055
April	1st to 10th	Day	21% 11% 3 7% 5% 4 4 3 4 6% 31%	2520
	11th to 20th	Day	20% 9% 3 7% 5% 5% 4 3 5% 5% 34%	2494
	21st to end	Day	21% 10% 3 6% 5% 5% 4 3 5% 5% 33%	2666
May	1st to 10th	Day	25% 10% 4 9% 5% 5% 5% 4 5% 6% 23%	6439
	11th to 20th	Day	19% 9% 4 8% 5% 5% 4 4 5% 7% 31%	6402
	21st to end	Day	21% 8% 3 6% 4 4 4 4 6% 7% 32%	6949
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 5% 4 5% 7% 31%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 4 6% 4 6% 8% 38%	2800
	21st to end	Day	20% 7% 3 6% 4 5% 6% 4 7% 8% 31%	2800
July	1st to 10th	Day	12% 6% 3 8% 6% 5% 6% 5% 7% 8% 34%	6360
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	6360
	21st to end	Day	11% 5% 2 4 4 4 4 4 6% 9% 46%	6926
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 4 7% 9% 35%	4563
	11th to 20th	Day	13% 6% 3 7% 5% 4 4 3 5% 9% 40%	4093
	21st to end	Day	16% 4 2 5% 4 4 5% 4 7% 9% 40%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 3 6% 8% 44%	2400
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 5% 7% 50%	2331
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 7% 42%	2200
October	1st to 10th	Day	20% 6% 3 7% 4 4 3 3 5% 6% 39%	2040
	11th to 20th	Day	18% 9% 3 8% 7% 5% 5% 4 5% 5% 32%	1977
	21st to end	Day	24% 10% 3 7% 4 3 4 3 6% 6% 29%	1980
November	1st to 10th	Day	17% 10% 4 7% 4 5% 4 4 6% 7% 32%	1851
	11th to 20th	Day	17% 11% 4 9% 5% 6% 5% 3 5% 6% 30%	1786
	21st to end	Day	22% 12% 3 8% 6% 6% 5% 4 5% 7% 22%	1680
December	1st to 10th	Day	20% 20% 5% 10% 5% 5% 5% 4 6% 5% 17%	2680
	11th to 20th	Day	19% 12% 5% 10% 6% 5% 5% 4 5% 4 26%	3066
	21st to end	Day	23% 14% 4 10% 5% 4 5% 3 4 5% 23%	4048
January	1st to 10th	Night	14% 20% 8% 13% 5% 4 4 3 4 5% 21%	8234
	11th to 20th	Night	12% 17% 6% 17% 8% 6% 5% 3 5% 5% 16%	8235
	21st to end	Night	11% 15% 5% 12% 8% 6% 5% 4 5% 6% 25%	9057
February	1st to 10th	Night	13% 14% 6% 12% 7% 5% 5% 4 5% 6% 23%	2746
	11th to 20th	Night	14% 11% 4 11% 6% 5% 6% 4 5% 6% 28%	2570
	21st to end	Night	14% 15% 4 11% 5% 4 4 3 4 4 31%	2076
March	1st to 10th	Night	20% 13% 3 8% 5% 4 4 3 4 5% 29%	5484
	11th to 20th	Night	16% 13% 4 8% 5% 5% 5% 4 6% 6% 30%	5324
	21st to end	Night	16% 11% 3 9% 5% 5% 5% 4 4 5% 32%	5119
April	1st to 10th	Night	24% 9% 3 7% 5% 5% 4 3 5% 6% 31%	2100
	11th to 20th	Night	23% 9% 3 7% 4 4 4 3 4 6% 33%	2075
	21st to end	Night	21% 9% 3 7% 4 4 4 3 4 6% 35%	1953
May	1st to 10th	Night	25% 10% 3 8% 5% 5% 5% 4 5% 6% 25%	3750
	11th to 20th	Night	22% 8% 3 7% 5% 5% 4 2 5% 7% 33%	3623
	21st to end	Night	18% 9% 3 9% 5% 4 4 3 5% 7% 32%	3926
June	1st to 10th	Night	20% 7% 2 7% 5% 5% 6% 4 6% 8% 30%	1600
	11th to 20th	Night	16% 6% 3 6% 4 3 5% 5% 6% 7% 40%	1600
	21st to end	Night	18% 7% 3 6% 5% 4 4 4 6% 7% 35%	1600
July	1st to 10th	Night	13% 6% 3 7% 5% 5% 6% 5% 7% 8% 34%	3600
	11th to 20th	Night	14% 5% 2 6% 5% 4 5% 4 5% 8% 42%	3600
	21st to end	Night	10% 6% 2 6% 5% 4 4 4 6% 8% 45%	3926
August	1st to 10th	Night	14% 8% 2 8% 4 4 4 4 6% 7% 40%	2586
	11th to 20th	Night	15% 7% 3 7% 4 4 4 4 6% 6% 41%	2920
	21st to end	Night	16% 5% 2 7% 5% 4 4 4 6% 7% 40%	3586
September	1st to 10th	Night	12% 5% 2 8% 5% 4 4 4 6% 8% 44%	2000
	11th to 20th	Night	15% 6% 2 4 3 3 3 2 4 6% 51%	2068
	21st to end	Night	17% 6% 2 5% 3 4 4 3 5% 7% 43%	2200
October	1st to 10th	Night	17% 6% 2 6% 4 4 5% 3 5% 7% 39%	2357
	11th to 20th	Night	19% 7% 3 8% 5% 4 5% 4 4 6% 35%	2421
	21st to end	Night	23% 8% 3 8% 5% 5% 5% 3 5% 6% 31%	2860
November	1st to 10th	Night	20% 9% 3 7% 5% 4 4 3 5% 7% 32%	2667
	11th to 20th	Night	18% 10% 4 8% 4 5% 5% 4 5% 5% 32%	2830
	21st to end	Night	21% 12% 4 10% 5% 5% 5% 3 5% 5% 25%	2940
December	1st to 10th	Night	18% 18% 6% 11% 6% 5% 5% 4 5% 5% 19%	4720
	11th to 20th	Night	23% 15% 4 10% 5% 4 4 3 5% 4 23%	5348
	21st to end	Night	25% 16% 4 9% 4 4 3 4 5% 5% 22%	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	51% 21% 11% 13% 3	4662
	11th to 20th	Day	49% 22% 8% 12% 8% 1	4680
	21st to end	Day	50% 23% 9% 9% 8% 1	5124
February	1st to 10th	Day	24% 59% 3 10% 3 1	1873
	11th to 20th	Day	24% 57% 4 9% 5%	1944
	21st to end	Day	25% 58% 3 6% 6% 1	1730
March	1st to 10th	Day	45% 29% 5% 10% 8% 3	4579
	11th to 20th	Day	38% 29% 13% 11% 9% 1	4850
	21st to end	Day	39% 28% 4 13% 14% 1	6055
April	1st to 10th	Day	22% 59% 3 8% 6% 1	2520
	11th to 20th	Day	22% 59% 3 10% 6% 1	2494
	21st to end	Day	22% 58% 4 9% 7%	2666
May	1st to 10th	Day	48% 29% 6% 14% 3	6450
	11th to 20th	Day	41% 28% 11% 14% 5% 1	6406
	21st to end	Day	40% 28% 9% 15% 7%	6949
June	1st to 10th	Day	20% 61% 3 9% 6% 1	2800
	11th to 20th	Day	17% 61% 5% 9% 7% 1	2800
	21st to end	Day	19% 60% 4 11% 5% 1	2800
July	1st to 10th	Day	36% 27% 10% 20% 6%	6290
	11th to 20th	Day	28% 27% 11% 25% 10%	6285
	21st to end	Day	25% 26% 11% 32% 6%	6728
August	1st to 10th	Day	29% 37% 13% 16% 6%	4549
	11th to 20th	Day	30% 36% 11% 20% 3	4135
	21st to end	Day	26% 37% 12% 15% 9%	4312
September	1st to 10th	Day	14% 58% 7% 16% 6%	2400
	11th to 20th	Day	13% 57% 6% 16% 7% 1	2331
	21st to end	Day	16% 57% 6% 13% 7% 1	2200
October	1st to 10th	Day	28% 36% 9% 21% 6%	3326
	11th to 20th	Day	23% 36% 14% 22% 5%	3193
	21st to end	Day	27% 36% 16% 17% 5%	3180
November	1st to 10th	Day	27% 54% 6% 9% 4	1851
	11th to 20th	Day	23% 54% 5% 12% 5% 1	1786
	21st to end	Day	29% 55% 6% 7% 3	1680
December	1st to 10th	Day	47% 35% 6% 11% 1	2680
	11th to 20th	Day	43% 32% 6% 11% 7%	3066
	21st to end	Day	48% 26% 8% 13% 5%	4048
January	1st to 10th	Night	54% 20% 10% 11% 4	8234
	11th to 20th	Night	56% 22% 8% 10% 4	8280
	21st to end	Night	48% 22% 7% 12% 10%	9057
February	1st to 10th	Night	26% 58% 4 9% 3	2746
	11th to 20th	Night	23% 58% 3 10% 5%	2572
	21st to end	Night	21% 58% 3 9% 8%	2076
March	1st to 10th	Night	42% 28% 5% 11% 10% 4	5494
	11th to 20th	Night	38% 28% 10% 12% 9% 2	5325
	21st to end	Night	41% 28% 4 16% 9% 1	5119
April	1st to 10th	Night	24% 58% 3 8% 6%	2100
	11th to 20th	Night	25% 59% 2 8% 6% 1	2075
	21st to end	Night	21% 59% 3 7% 8% 1	1953
May	1st to 10th	Night	47% 29% 9% 13% 3	3756
	11th to 20th	Night	40% 29% 9% 16% 6%	3624
	21st to end	Night	42% 28% 8% 13% 9%	3927
June	1st to 10th	Night	20% 62% 3 8% 7% 1	1600
	11th to 20th	Night	16% 62% 4 9% 8% 1	1600
	21st to end	Night	20% 60% 4 9% 6% 1	1600
July	1st to 10th	Night	36% 27% 14% 16% 7%	3560
	11th to 20th	Night	30% 27% 12% 23% 7%	3552
	21st to end	Night	27% 26% 12% 24% 10%	3809
August	1st to 10th	Night	30% 37% 10% 19% 5%	2576
	11th to 20th	Night	33% 37% 12% 14% 4	2944
	21st to end	Night	29% 37% 10% 14% 10% 1	3586
September	1st to 10th	Night	14% 57% 6% 15% 8%	2000
	11th to 20th	Night	14% 56% 7% 16% 6% 1	2068
	21st to end	Night	17% 57% 5% 13% 9%	2200
October	1st to 10th	Night	26% 36% 11% 20% 6% 1	3851
	11th to 20th	Night	25% 36% 9% 23% 7%	3944
	21st to end	Night	29% 36% 13% 14% 9%	4610
November	1st to 10th	Night	27% 53% 6% 10% 4 1	2667
	11th to 20th	Night	25% 54% 5% 10% 5%	2830
	21st to end	Night	28% 55% 6% 7% 3	2940
December	1st to 10th	Night	45% 35% 8% 9% 3	4720
	11th to 20th	Night	45% 31% 5% 9% 9%	5348
	21st to end	Night	48% 26% 6% 12% 8%	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	24% 13% 31% 22% 10%	220
	11th to 20th	Day	28% 15% 32% 19% 6%	220
	21st to end	Day	19% 15% 34% 23% 9% 1	242
February	1st to 10th	Day	26% 16% 36% 17% 5%	322
	11th to 20th	Day	22% 12% 41% 21% 3	422
	21st to end	Day	26% 11% 31% 28% 3	446
March	1st to 10th	Day	23% 12% 35% 22% 7%	540
	11th to 20th	Day	22% 13% 35% 22% 6% 2	540
	21st to end	Day	24% 13% 31% 18% 11% 2	594
April	1st to 10th	Day	28% 13% 27% 21% 9% 1	540
	11th to 20th	Day	26% 14% 27% 20% 11% 3	532
	21st to end	Day	26% 12% 29% 21% 10% 2	540
May	1st to 10th	Day	23% 10% 29% 26% 9% 3	543
	11th to 20th	Day	19% 7% 30% 24% 15% 4	551
	21st to end	Day	20% 9% 28% 26% 13% 5%	573
June	1st to 10th	Day	20% 9% 25% 29% 12% 4	530
	11th to 20th	Day	15% 8% 26% 30% 17% 2	530
	21st to end	Day	22% 7% 28% 25% 14% 4	530
July	1st to 10th	Day	10% 8% 27% 30% 19% 7%	525
	11th to 20th	Day	12% 6% 28% 30% 18% 5%	524
	21st to end	Day	12% 7% 23% 35% 19% 4	576
August	1st to 10th	Day	14% 4 21% 32% 21% 8%	526
	11th to 20th	Day	12% 4 21% 32% 22% 9%	519
	21st to end	Day	15% 7% 25% 24% 20% 8%	583
September	1st to 10th	Day	15% 7% 24% 25% 19% 10%	530
	11th to 20th	Day	12% 6% 21% 25% 23% 13%	530
	21st to end	Day	18% 8% 25% 23% 19% 6%	530
October	1st to 10th	Day	20% 9% 26% 26% 14% 6%	530
	11th to 20th	Day	20% 12% 31% 23% 12% 2	516
	21st to end	Day	28% 12% 25% 25% 10%	462
November	1st to 10th	Day	27% 15% 23% 25% 9% 1	425
	11th to 20th	Day	23% 14% 34% 21% 8% 1	326
	21st to end	Day	29% 15% 31% 18% 6%	220
December	1st to 10th	Day	30% 10% 28% 23% 7% 1	220
	11th to 20th	Day	25% 14% 40% 16% 5% 1	220
	21st to end	Day	29% 12% 32% 19% 5% 2	242
January	1st to 10th	Night	24% 12% 32% 27% 5%	840
	11th to 20th	Night	28% 10% 31% 25% 6%	840
	21st to end	Night	19% 10% 35% 28% 8% 1	924
February	1st to 10th	Night	19% 11% 36% 27% 7% 1	840
	11th to 20th	Night	20% 11% 36% 27% 5%	823
	21st to end	Night	22% 9% 34% 30% 5%	692
March	1st to 10th	Night	24% 12% 31% 27% 6%	840
	11th to 20th	Night	25% 11% 30% 27% 6%	840
	21st to end	Night	23% 11% 28% 29% 9%	924
April	1st to 10th	Night	28% 11% 27% 29% 5%	840
	11th to 20th	Night	25% 11% 30% 27% 6%	831
	21st to end	Night	26% 9% 27% 28% 9%	693
May	1st to 10th	Night	26% 6% 29% 29% 9% 2	457
	11th to 20th	Night	22% 7% 27% 33% 10% 1	406
	21st to end	Night	16% 6% 30% 37% 10%	433
June	1st to 10th	Night	25% 4 27% 34% 10% 1	400
	11th to 20th	Night	17% 6% 25% 35% 15% 1	400
	21st to end	Night	19% 6% 27% 32% 17% 1	400
July	1st to 10th	Night	12% 4 24% 40% 19% 1	390
	11th to 20th	Night	11% 3 25% 43% 17% 2	388
	21st to end	Night	13% 5% 22% 42% 17% 1	438
August	1st to 10th	Night	15% 3 16% 43% 21% 1	398
	11th to 20th	Night	14% 3 20% 35% 25% 3	610
	21st to end	Night	15% 4 23% 35% 21% 2	880
September	1st to 10th	Night	13% 5% 23% 39% 19% 1	800
	11th to 20th	Night	17% 5% 17% 35% 24% 3	800
	21st to end	Night	19% 6% 23% 31% 18% 2	800
October	1st to 10th	Night	21% 6% 27% 30% 15% 1	800
	11th to 20th	Night	22% 9% 29% 29% 11% 1	800
	21st to end	Night	26% 10% 30% 28% 6%	880
November	1st to 10th	Night	25% 12% 28% 27% 8%	820
	11th to 20th	Night	23% 13% 33% 22% 9% 1	840
	21st to end	Night	29% 13% 30% 20% 7% 1	840
December	1st to 10th	Night	29% 11% 30% 24% 5%	840
	11th to 20th	Night	28% 15% 30% 22% 5% 1	840
	21st to end	Night	35% 11% 27% 22% 5%	924

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

Copyright © 2026 Attilla Danko.