Leslie C. Peltier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Leslie C. Peltier Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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% 7% 2 6% 4 4 4 4 6% 7% 38%	149252
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 41%	142478
Transparency	Day	45% 3 12% 23% 15% 2	152770
	Night	45% 3 11% 23% 16% 3	147049
Seeing	Day	25% 16% 31% 19% 7% 2	17487
	Night	23% 14% 32% 24% 7% 1	26459

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% 8% 3 9% 6% 5% 4 3 5% 5% 40%	4661
	11th to 20th	Day	15% 8% 3 8% 4 4 4 3 4 5% 42%	4650
	21st to end	Day	15% 9% 2 5% 3 3 4 3 5% 7% 43%	5286
February	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 4 5% 6% 37%	1942
	11th to 20th	Day	13% 7% 3 6% 5% 5% 5% 4 6% 7% 39%	2048
	21st to end	Day	19% 10% 3 6% 4 4 4 3 4 5% 38%	1730
March	1st to 10th	Day	21% 9% 2 5% 4 3 4 3 4 6% 38%	4576
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 4 6% 8% 40%	4784
	21st to end	Day	20% 8% 3 6% 4 4 4 3 6% 7% 36%	6102
April	1st to 10th	Day	19% 9% 2 5% 4 4 4 4 5% 6% 37%	2520
	11th to 20th	Day	18% 6% 2 5% 4 4 5% 4 5% 7% 40%	2494
	21st to end	Day	20% 6% 2 6% 4 4 4 4 5% 7% 39%	2630
May	1st to 10th	Day	28% 8% 2 5% 4 4 4 3 5% 7% 31%	7639
	11th to 20th	Day	19% 7% 2 7% 4 4 4 3 5% 7% 37%	8056
	21st to end	Day	23% 7% 2 6% 4 4 5% 4 6% 8% 30%	9060
June	1st to 10th	Day	16% 6% 2 6% 4 6% 5% 5% 7% 9% 35%	2940
	11th to 20th	Day	16% 7% 2 5% 5% 5% 4 4 7% 9% 36%	2940
	21st to end	Day	14% 6% 2 6% 5% 5% 6% 4 6% 9% 37%	2940
July	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 5% 7% 9% 35%	8280
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 7% 10% 41%	8280
	21st to end	Day	15% 7% 3 6% 4 4 4 3 6% 7% 41%	8993
August	1st to 10th	Day	16% 6% 3 8% 5% 6% 6% 5% 7% 10% 29%	6283
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 42%	5497
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 10% 39%	6080
September	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 4 6% 44%	2520
	11th to 20th	Day	13% 6% 2 4 3 3 4 3 4 7% 50%	2384
	21st to end	Day	19% 6% 2 6% 4 5% 4 4 5% 7% 39%	2310
October	1st to 10th	Day	20% 7% 2 5% 3 4 4 2 4 5% 44%	2260
	11th to 20th	Day	13% 7% 2 5% 3 4 5% 3 5% 8% 44%	2020
	21st to end	Day	21% 6% 2 5% 4 4 4 3 4 5% 40%	1980
November	1st to 10th	Day	12% 6% 2 5% 4 4 3 3 5% 6% 48%	1851
	11th to 20th	Day	17% 6% 1 4 3 3 4 3 6% 7% 46%	1890
	21st to end	Day	18% 9% 3 5% 4 4 3 3 5% 6% 41%	1832
December	1st to 10th	Day	15% 7% 2 6% 3 4 3 4 5% 6% 44%	2680
	11th to 20th	Day	21% 7% 1 4 3 3 3 3 4 6% 44%	3066
	21st to end	Day	18% 11% 4 7% 4 3 4 3 4 6% 36%	4048
January	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 5% 6% 42%	8234
	11th to 20th	Night	15% 9% 3 8% 5% 3 3 3 4 5% 40%	8235
	21st to end	Night	15% 8% 3 6% 5% 4 4 3 4 6% 41%	8880
February	1st to 10th	Night	15% 9% 3 6% 4 4 4 3 4 6% 40%	2677
	11th to 20th	Night	14% 9% 3 6% 4 5% 5% 3 6% 6% 39%	2458
	21st to end	Night	18% 8% 2 7% 4 4 4 3 5% 5% 40%	2076
March	1st to 10th	Night	20% 10% 2 5% 3 4 4 3 4 6% 40%	5484
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 6% 41%	5390
	21st to end	Night	21% 10% 2 6% 4 3 4 3 5% 5% 38%	5072
April	1st to 10th	Night	20% 7% 2 5% 4 5% 4 4 5% 6% 37%	2100
	11th to 20th	Night	19% 7% 2 5% 5% 4 5% 4 5% 7% 39%	2075
	21st to end	Night	20% 7% 2 6% 4 4 3 2 4 6% 42%	1990
May	1st to 10th	Night	27% 10% 3 6% 4 3 3 3 4 6% 31%	5259
	11th to 20th	Night	20% 9% 3 7% 4 4 3 4 5% 7% 34%	4859
	21st to end	Night	23% 8% 3 6% 4 4 4 3 6% 6% 32%	5114
June	1st to 10th	Night	18% 6% 3 5% 4 4 4 4 4 6% 42%	1680
	11th to 20th	Night	17% 6% 3 6% 4 4 5% 3 6% 9% 38%	1680
	21st to end	Night	15% 6% 1 6% 4 3 3 3 7% 9% 42%	1680
July	1st to 10th	Night	17% 6% 2 6% 4 4 4 4 7% 9% 37%	4680
	11th to 20th	Night	14% 7% 3 6% 4 4 4 4 6% 9% 39%	4680
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 6% 44%	5084
August	1st to 10th	Night	18% 6% 3 8% 5% 5% 5% 4 6% 8% 32%	3807
	11th to 20th	Night	14% 7% 3 6% 4 4 4 4 5% 7% 42%	4531
	21st to end	Night	14% 5% 2 4 3 3 4 3 5% 8% 48%	5045
September	1st to 10th	Night	15% 6% 2 6% 4 4 3 3 5% 6% 44%	2100
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 6% 52%	2236
	21st to end	Night	18% 7% 2 5% 4 3 4 4 5% 5% 43%	2310
October	1st to 10th	Night	18% 7% 2 5% 4 3 3 3 4 6% 45%	2357
	11th to 20th	Night	16% 5% 2 6% 3 4 4 3 4 6% 47%	2447
	21st to end	Night	22% 6% 2 4 3 3 4 4 4 6% 41%	2860
November	1st to 10th	Night	13% 6% 2 5% 4 4 3 3 5% 6% 48%	2667
	11th to 20th	Night	17% 7% 2 5% 3 3 4 3 4 6% 45%	2730
	21st to end	Night	15% 8% 2 4 5% 4 3 3 5% 6% 45%	2783
December	1st to 10th	Night	15% 8% 2 6% 4 3 3 2 4 5% 46%	4720
	11th to 20th	Night	24% 8% 2 4 3 3 4 3 4 4 41%	5348
	21st to end	Night	19% 10% 3 7% 4 3 3 3 4 5% 40%	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	45% 3 8% 27% 15% 2	4662
	11th to 20th	Day	45% 2 8% 22% 15% 8%	4680
	21st to end	Day	43% 2 10% 26% 19% 1	5286
February	1st to 10th	Day	47% 3 11% 21% 16% 2	1942
	11th to 20th	Day	44% 4 9% 25% 16% 3	2057
	21st to end	Day	48% 2 10% 22% 15% 3	1730
March	1st to 10th	Day	49% 2 10% 17% 13% 10%	4579
	11th to 20th	Day	42% 2 12% 32% 10% 2	4784
	21st to end	Day	49% 2 7% 15% 21% 6%	6102
April	1st to 10th	Day	48% 3 9% 21% 16% 4	2520
	11th to 20th	Day	44% 3 11% 22% 18% 3	2494
	21st to end	Day	46% 3 9% 21% 18% 4	2630
May	1st to 10th	Day	55% 3 12% 17% 12% 1	7651
	11th to 20th	Day	49% 2 13% 22% 13% 1	8060
	21st to end	Day	52% 3 11% 21% 12% 1	9061
June	1st to 10th	Day	44% 4 11% 24% 15% 1	2940
	11th to 20th	Day	44% 4 14% 24% 13% 2	2940
	21st to end	Day	43% 5% 13% 24% 14% 1	2940
July	1st to 10th	Day	46% 5% 14% 20% 15% 1	8210
	11th to 20th	Day	38% 3 17% 28% 13% 1	8205
	21st to end	Day	43% 3 11% 25% 17% 1	8795
August	1st to 10th	Day	50% 4 18% 19% 10%	6269
	11th to 20th	Day	39% 3 17% 33% 9% 1	5533
	21st to end	Day	40% 4 14% 26% 14% 2	6080
September	1st to 10th	Day	41% 4 9% 25% 19% 1	2520
	11th to 20th	Day	35% 3 10% 25% 24% 3	2384
	21st to end	Day	45% 3 8% 23% 18% 3	2310
October	1st to 10th	Day	40% 3 8% 20% 26% 4	3611
	11th to 20th	Day	31% 2 8% 26% 28% 4	3248
	21st to end	Day	34% 2 13% 26% 22% 3	3180
November	1st to 10th	Day	38% 3 11% 25% 20% 2	1851
	11th to 20th	Day	39% 3 10% 25% 20% 3	1890
	21st to end	Day	45% 2 12% 27% 12% 3	1832
December	1st to 10th	Day	41% 2 15% 28% 12% 1	2680
	11th to 20th	Day	42% 2 14% 20% 18% 3	3066
	21st to end	Day	50% 2 7% 21% 15% 5%	4048
January	1st to 10th	Night	43% 2 10% 26% 16% 2	8234
	11th to 20th	Night	47% 3 10% 19% 15% 6%	8280
	21st to end	Night	46% 2 8% 20% 20% 3	8880
February	1st to 10th	Night	46% 3 12% 24% 13% 2	2677
	11th to 20th	Night	45% 4 9% 21% 18% 4	2460
	21st to end	Night	47% 3 7% 25% 14% 5%	2076
March	1st to 10th	Night	47% 2 10% 19% 16% 6%	5494
	11th to 20th	Night	46% 2 15% 22% 12% 4	5391
	21st to end	Night	49% 2 5% 14% 25% 5%	5072
April	1st to 10th	Night	48% 4 9% 21% 16% 3	2100
	11th to 20th	Night	46% 4 9% 21% 20% 1	2075
	21st to end	Night	46% 2 9% 22% 17% 4	1990
May	1st to 10th	Night	56% 2 12% 16% 12% 1	5265
	11th to 20th	Night	52% 2 13% 21% 10% 2	4860
	21st to end	Night	53% 3 12% 22% 10% 1	5115
June	1st to 10th	Night	45% 4 9% 24% 17% 1	1680
	11th to 20th	Night	44% 3 14% 27% 11% 1	1680
	21st to end	Night	39% 4 15% 26% 15% 1	1680
July	1st to 10th	Night	44% 4 16% 20% 14% 1	4640
	11th to 20th	Night	43% 3 15% 25% 13% 1	4632
	21st to end	Night	41% 3 12% 26% 17% 1	4967
August	1st to 10th	Night	50% 5% 15% 18% 11% 1	3797
	11th to 20th	Night	42% 3 16% 32% 7%	4561
	21st to end	Night	36% 3 13% 32% 15% 1	5045
September	1st to 10th	Night	42% 3 10% 26% 18% 1	2100
	11th to 20th	Night	35% 3 10% 24% 25% 2	2236
	21st to end	Night	43% 3 8% 23% 20% 2	2310
October	1st to 10th	Night	39% 3 12% 23% 21% 2	3786
	11th to 20th	Night	29% 3 9% 28% 28% 4	3958
	21st to end	Night	34% 2 13% 30% 19% 2	4610
November	1st to 10th	Night	38% 3 11% 23% 21% 4	2667
	11th to 20th	Night	41% 3 9% 23% 21% 4	2730
	21st to end	Night	41% 3 13% 30% 13% 1	2783
December	1st to 10th	Night	41% 2 12% 32% 12% 1	4720
	11th to 20th	Night	48% 2 12% 17% 18% 3	5348
	21st to end	Night	49% 2 7% 21% 16% 5%	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	32% 24% 30% 13% 2	220
	11th to 20th	Day	32% 31% 26% 9% 1	220
	21st to end	Day	28% 31% 31% 7% 3 1	272
February	1st to 10th	Day	32% 25% 32% 10% 1	382
	11th to 20th	Day	30% 24% 34% 11% 1	536
	21st to end	Day	33% 20% 33% 11% 3	446
March	1st to 10th	Day	28% 23% 32% 14% 3	540
	11th to 20th	Day	30% 20% 35% 11% 3 1	540
	21st to end	Day	34% 21% 32% 9% 3 1	594
April	1st to 10th	Day	35% 21% 27% 11% 5% 1	540
	11th to 20th	Day	33% 20% 30% 11% 4 1	532
	21st to end	Day	31% 18% 34% 13% 4 1	540
May	1st to 10th	Day	35% 19% 28% 12% 6% 1	543
	11th to 20th	Day	25% 18% 36% 16% 5%	567
	21st to end	Day	25% 12% 32% 21% 8% 2	617
June	1st to 10th	Day	20% 13% 32% 24% 9% 2	570
	11th to 20th	Day	21% 13% 35% 22% 7% 2	570
	21st to end	Day	18% 13% 34% 25% 8% 2	570
July	1st to 10th	Day	18% 7% 32% 27% 13% 3	565
	11th to 20th	Day	14% 9% 33% 28% 14% 2	564
	21st to end	Day	14% 10% 35% 28% 10% 2	620
August	1st to 10th	Day	14% 7% 31% 32% 11% 5%	566
	11th to 20th	Day	14% 8% 31% 28% 13% 5%	559
	21st to end	Day	14% 11% 31% 24% 14% 5%	627
September	1st to 10th	Day	20% 12% 28% 21% 14% 5%	570
	11th to 20th	Day	16% 9% 28% 26% 13% 8%	570
	21st to end	Day	24% 13% 30% 20% 8% 4	570
October	1st to 10th	Day	25% 13% 26% 19% 12% 5%	570
	11th to 20th	Day	19% 20% 32% 21% 6% 2	536
	21st to end	Day	29% 18% 26% 19% 7% 1	462
November	1st to 10th	Day	21% 18% 31% 21% 8% 1	425
	11th to 20th	Day	29% 21% 27% 18% 5%	430
	21st to end	Day	31% 17% 28% 18% 4 1	372
December	1st to 10th	Day	27% 21% 33% 14% 5%	220
	11th to 20th	Day	38% 23% 26% 11% 2	220
	21st to end	Day	31% 21% 29% 15% 3 1	242
January	1st to 10th	Night	24% 19% 38% 16% 3	840
	11th to 20th	Night	28% 20% 32% 18% 2	840
	21st to end	Night	22% 21% 36% 19% 2	924
February	1st to 10th	Night	24% 19% 33% 22% 2	840
	11th to 20th	Night	25% 21% 37% 16% 1	823
	21st to end	Night	28% 18% 36% 15% 2	692
March	1st to 10th	Night	25% 21% 34% 18% 2	840
	11th to 20th	Night	23% 20% 35% 18% 2	840
	21st to end	Night	26% 21% 33% 16% 4	924
April	1st to 10th	Night	28% 18% 33% 18% 3	840
	11th to 20th	Night	26% 18% 35% 18% 4	831
	21st to end	Night	23% 20% 36% 16% 4	730
May	1st to 10th	Night	28% 16% 33% 19% 3	630
	11th to 20th	Night	23% 14% 38% 22% 4	493
	21st to end	Night	25% 8% 30% 29% 7% 1	455
June	1st to 10th	Night	20% 9% 31% 30% 10%	420
	11th to 20th	Night	20% 8% 33% 29% 10%	420
	21st to end	Night	18% 5% 30% 37% 9%	420
July	1st to 10th	Night	16% 4 28% 35% 16%	410
	11th to 20th	Night	15% 5% 24% 42% 13% 1	408
	21st to end	Night	15% 5% 28% 38% 13% 1	460
August	1st to 10th	Night	14% 4 27% 40% 14% 1	481
	11th to 20th	Night	15% 5% 25% 36% 17% 1	817
	21st to end	Night	16% 5% 26% 35% 18% 1	924
September	1st to 10th	Night	19% 5% 30% 32% 13% 2	840
	11th to 20th	Night	15% 7% 25% 33% 16% 3	840
	21st to end	Night	23% 7% 28% 26% 15% 1	840
October	1st to 10th	Night	22% 9% 29% 25% 13% 2	840
	11th to 20th	Night	21% 13% 30% 27% 9% 1	813
	21st to end	Night	28% 12% 33% 21% 6%	880
November	1st to 10th	Night	19% 15% 33% 25% 7% 1	820
	11th to 20th	Night	23% 17% 34% 19% 6% 1	840
	21st to end	Night	24% 16% 32% 22% 5%	840
December	1st to 10th	Night	26% 17% 32% 20% 5%	840
	11th to 20th	Night	26% 18% 33% 20% 3	840
	21st to end	Night	29% 18% 32% 19% 2	924

Page generated at 2026-10-11 05:06:26. Version ac07

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