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 2025-06-10.

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 5% 7% 38%	119327
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 41%	118886
Transparency	Day	45% 3 11% 23% 15% 3	122888
	Night	45% 3 11% 23% 16% 3	123486
Seeing	Day	25% 16% 31% 19% 7% 2	15696
	Night	23% 14% 32% 24% 7% 1	24810

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% 7% 3 8% 6% 5% 4 4 5% 5% 42%	3597
	11th to 20th	Day	14% 8% 3 8% 5% 4 4 4 4 5% 40%	3570
	21st to end	Day	17% 10% 3 6% 4 4 4 4 4 5% 40%	4080
February	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 6% 36%	1849
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	1948
	21st to end	Day	20% 10% 3 6% 4 4 4 3 4 5% 38%	1650
March	1st to 10th	Day	21% 9% 2 5% 4 3 4 3 4 6% 38%	4476
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 4 6% 8% 41%	4681
	21st to end	Day	20% 8% 3 6% 4 4 4 3 6% 7% 36%	5970
April	1st to 10th	Day	19% 9% 2 5% 5% 4 4 4 5% 6% 36%	2400
	11th to 20th	Day	18% 6% 2 5% 4 4 5% 4 5% 6% 40%	2374
	21st to end	Day	21% 6% 2 6% 4 4 4 3 5% 7% 39%	2505
May	1st to 10th	Day	31% 8% 2 5% 4 4 4 4 5% 7% 26%	5886
	11th to 20th	Day	21% 8% 3 7% 4 4 4 4 5% 7% 33%	6194
	21st to end	Day	21% 7% 2 6% 4 4 4 4 6% 8% 34%	6949
June	1st to 10th	Day	17% 6% 2 6% 4 6% 5% 5% 7% 9% 35%	2786
	11th to 20th	Day	15% 7% 2 6% 5% 5% 4 5% 7% 9% 36%	2660
	21st to end	Day	14% 6% 3 6% 5% 5% 6% 4 6% 9% 36%	2660
July	1st to 10th	Day	12% 6% 3 6% 6% 6% 6% 5% 7% 8% 36%	4440
	11th to 20th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 9% 39%	4440
	21st to end	Day	17% 9% 3 7% 5% 5% 5% 4 6% 7% 32%	4814
August	1st to 10th	Day	20% 6% 3 7% 5% 5% 5% 4 6% 8% 30%	4283
	11th to 20th	Day	12% 6% 3 6% 4 5% 5% 4 6% 9% 39%	3739
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	4180
September	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 4 6% 44%	2280
	11th to 20th	Day	13% 6% 2 4 4 4 4 3 4 7% 50%	2157
	21st to end	Day	19% 6% 2 6% 3 4 4 3 5% 6% 41%	2090
October	1st to 10th	Day	22% 7% 2 6% 3 4 4 2 4 5% 40%	2045
	11th to 20th	Day	13% 7% 2 5% 3 4 5% 3 5% 7% 46%	1891
	21st to end	Day	22% 6% 2 5% 4 4 4 3 4 5% 40%	1881
November	1st to 10th	Day	12% 7% 2 5% 4 3 3 3 5% 6% 49%	1761
	11th to 20th	Day	18% 6% 1 4 3 3 4 3 5% 7% 47%	1800
	21st to end	Day	18% 8% 3 5% 3 4 3 3 5% 6% 42%	1745
December	1st to 10th	Day	15% 7% 2 7% 3 4 3 4 5% 6% 44%	2600
	11th to 20th	Day	21% 8% 1 4 3 3 3 3 4 6% 44%	2986
	21st to end	Day	18% 11% 4 7% 4 3 3 3 4 6% 37%	3960
January	1st to 10th	Night	14% 7% 2 7% 4 4 4 3 5% 6% 44%	6349
	11th to 20th	Night	14% 8% 3 8% 5% 3 3 3 5% 5% 42%	6315
	21st to end	Night	17% 10% 3 6% 5% 4 4 3 4 6% 38%	6861
February	1st to 10th	Night	16% 10% 3 6% 4 4 4 3 4 5% 40%	2550
	11th to 20th	Night	15% 8% 3 6% 4 5% 5% 3 6% 6% 40%	2338
	21st to end	Night	18% 8% 2 7% 4 4 4 3 5% 5% 40%	1980
March	1st to 10th	Night	20% 10% 2 5% 3 4 4 3 4 6% 40%	5364
	11th to 20th	Night	16% 9% 3 6% 4 4 4 3 5% 6% 41%	5273
	21st to end	Night	21% 10% 2 6% 4 3 3 3 5% 5% 38%	4962
April	1st to 10th	Night	20% 7% 3 5% 4 5% 4 4 5% 6% 37%	2000
	11th to 20th	Night	19% 6% 2 6% 4 3 5% 4 5% 6% 39%	1975
	21st to end	Night	20% 7% 2 6% 4 4 3 2 4 6% 41%	1895
May	1st to 10th	Night	31% 9% 3 7% 4 3 3 3 4 6% 27%	4054
	11th to 20th	Night	21% 9% 3 7% 4 4 3 4 5% 7% 32%	3741
	21st to end	Night	22% 8% 2 6% 4 4 4 3 6% 7% 34%	3926
June	1st to 10th	Night	19% 7% 3 5% 4 4 4 4 4 6% 41%	1598
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 5% 9% 39%	1520
	21st to end	Night	15% 6% 1 6% 5% 3 3 3 7% 9% 41%	1520
July	1st to 10th	Night	13% 6% 2 6% 5% 4 5% 5% 8% 9% 38%	2520
	11th to 20th	Night	16% 6% 2 6% 4 4 3 4 7% 10% 37%	2520
	21st to end	Night	18% 9% 3 7% 5% 5% 4 4 5% 5% 35%	2739
August	1st to 10th	Night	21% 6% 3 7% 5% 5% 5% 3 6% 7% 31%	2602
	11th to 20th	Night	15% 7% 3 7% 4 4 4 3 5% 7% 41%	3085
	21st to end	Night	10% 5% 1 4 3 3 4 3 5% 8% 53%	3476
September	1st to 10th	Night	15% 6% 2 6% 4 4 3 3 5% 6% 45%	1900
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 6% 52%	2023
	21st to end	Night	17% 7% 2 5% 4 3 4 4 5% 5% 44%	2090
October	1st to 10th	Night	19% 7% 2 6% 4 3 3 3 4 6% 42%	2133
	11th to 20th	Night	15% 5% 2 6% 3 4 4 3 4 6% 48%	2289
	21st to end	Night	23% 6% 2 4 3 3 4 4 4 6% 41%	2717
November	1st to 10th	Night	13% 6% 2 5% 4 4 3 3 5% 6% 48%	2537
	11th to 20th	Night	18% 7% 2 5% 3 3 4 3 4 6% 45%	2600
	21st to end	Night	16% 8% 2 4 5% 4 3 3 5% 6% 45%	2650
December	1st to 10th	Night	16% 8% 2 6% 4 3 3 2 4 5% 47%	4580
	11th to 20th	Night	24% 8% 2 4 3 3 4 3 4 4 41%	5208
	21st to end	Night	19% 10% 3 7% 4 3 3 3 4 5% 40%	6996

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	43% 3 6% 29% 15% 3	3598
	11th to 20th	Day	47% 2 9% 23% 13% 7%	3600
	21st to end	Day	48% 2 8% 24% 17% 1	4080
February	1st to 10th	Day	48% 3 12% 21% 15% 2	1849
	11th to 20th	Day	44% 4 9% 25% 16% 3	1957
	21st to end	Day	49% 2 10% 22% 15% 3	1650
March	1st to 10th	Day	49% 2 10% 17% 13% 10%	4479
	11th to 20th	Day	42% 2 12% 32% 10% 2	4681
	21st to end	Day	49% 2 7% 15% 21% 6%	5970
April	1st to 10th	Day	49% 4 9% 20% 15% 4	2400
	11th to 20th	Day	44% 4 10% 22% 18% 3	2374
	21st to end	Day	46% 3 9% 21% 18% 4	2505
May	1st to 10th	Day	59% 3 13% 15% 9% 1	5897
	11th to 20th	Day	52% 2 12% 20% 12% 1	6198
	21st to end	Day	49% 3 10% 24% 13% 1	6949
June	1st to 10th	Day	46% 5% 11% 24% 13% 1	2786
	11th to 20th	Day	44% 4 14% 24% 12% 2	2660
	21st to end	Day	43% 5% 13% 24% 14% 1	2660
July	1st to 10th	Day	46% 5% 11% 21% 16% 1	4370
	11th to 20th	Day	42% 4 16% 24% 13% 1	4365
	21st to end	Day	51% 3 12% 18% 15% 1	4661
August	1st to 10th	Day	51% 4 15% 21% 9%	4269
	11th to 20th	Day	41% 3 14% 30% 11% 1	3775
	21st to end	Day	38% 5% 12% 28% 16% 2	4180
September	1st to 10th	Day	41% 5% 9% 24% 19% 2	2280
	11th to 20th	Day	35% 4 9% 25% 24% 3	2157
	21st to end	Day	44% 3 8% 24% 19% 3	2090
October	1st to 10th	Day	41% 3 7% 20% 25% 4	3396
	11th to 20th	Day	30% 2 8% 26% 29% 4	3119
	21st to end	Day	34% 2 13% 25% 22% 4	3081
November	1st to 10th	Day	38% 3 11% 25% 20% 3	1761
	11th to 20th	Day	39% 2 10% 25% 21% 4	1800
	21st to end	Day	44% 2 12% 27% 12% 3	1745
December	1st to 10th	Day	41% 2 15% 28% 12% 1	2600
	11th to 20th	Day	42% 2 15% 20% 18% 3	2986
	21st to end	Day	50% 3 7% 21% 15% 5%	3960
January	1st to 10th	Night	40% 3 8% 28% 18% 3	6349
	11th to 20th	Night	45% 2 12% 21% 14% 6%	6360
	21st to end	Night	49% 2 9% 18% 18% 4	6861
February	1st to 10th	Night	47% 3 12% 23% 12% 2	2550
	11th to 20th	Night	45% 4 9% 21% 18% 4	2340
	21st to end	Night	47% 3 7% 24% 14% 5%	1980
March	1st to 10th	Night	47% 2 10% 19% 16% 6%	5374
	11th to 20th	Night	46% 2 15% 22% 12% 4	5274
	21st to end	Night	49% 2 5% 14% 25% 5%	4962
April	1st to 10th	Night	48% 4 8% 22% 15% 3	2000
	11th to 20th	Night	45% 4 9% 20% 20% 1	1975
	21st to end	Night	46% 2 9% 23% 17% 4	1895
May	1st to 10th	Night	61% 2 13% 13% 10% 1	4060
	11th to 20th	Night	53% 3 13% 17% 11% 3	3742
	21st to end	Night	51% 3 13% 22% 11% 1	3927
June	1st to 10th	Night	46% 4 10% 24% 16% 1	1598
	11th to 20th	Night	43% 3 15% 27% 11% 1	1520
	21st to end	Night	39% 5% 15% 26% 15% 1	1520
July	1st to 10th	Night	42% 5% 17% 20% 15% 1	2480
	11th to 20th	Night	43% 4 16% 24% 12% 1	2472
	21st to end	Night	51% 4 13% 15% 15% 2	2651
August	1st to 10th	Night	53% 5% 13% 18% 9% 1	2592
	11th to 20th	Night	44% 4 13% 30% 9% 1	3115
	21st to end	Night	31% 4 13% 35% 17% 1	3476
September	1st to 10th	Night	41% 3 10% 26% 18% 1	1900
	11th to 20th	Night	35% 3 10% 25% 25% 2	2023
	21st to end	Night	42% 4 8% 24% 20% 2	2090
October	1st to 10th	Night	40% 3 13% 22% 20% 2	3562
	11th to 20th	Night	28% 3 8% 28% 29% 4	3800
	21st to end	Night	34% 2 13% 30% 19% 2	4467
November	1st to 10th	Night	38% 3 11% 23% 22% 4	2537
	11th to 20th	Night	42% 3 9% 22% 20% 4	2600
	21st to end	Night	41% 3 13% 30% 12% 1	2650
December	1st to 10th	Night	41% 2 12% 32% 12% 1	4580
	11th to 20th	Night	48% 2 12% 17% 18% 2	5208
	21st to end	Night	49% 2 8% 21% 16% 4	6996

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	33% 24% 29% 14% 2	200
	11th to 20th	Day	31% 30% 28% 10% 2	200
	21st to end	Day	26% 30% 33% 7% 4 1	247
February	1st to 10th	Day	33% 24% 32% 9% 1	349
	11th to 20th	Day	31% 25% 33% 11% 1	496
	21st to end	Day	33% 20% 32% 12% 3	414
March	1st to 10th	Day	28% 24% 32% 14% 3	500
	11th to 20th	Day	28% 21% 36% 11% 3 1	500
	21st to end	Day	33% 20% 33% 9% 3 1	550
April	1st to 10th	Day	34% 22% 26% 11% 6% 1	500
	11th to 20th	Day	34% 20% 29% 11% 4 1	492
	21st to end	Day	31% 18% 33% 12% 4 1	500
May	1st to 10th	Day	36% 17% 27% 13% 6% 1	503
	11th to 20th	Day	25% 17% 36% 15% 6%	527
	21st to end	Day	24% 12% 31% 22% 8% 2	573
June	1st to 10th	Day	21% 13% 33% 22% 9% 2	526
	11th to 20th	Day	19% 13% 35% 23% 9% 2	490
	21st to end	Day	18% 13% 34% 24% 8% 2	490
July	1st to 10th	Day	17% 7% 31% 27% 14% 3	485
	11th to 20th	Day	14% 8% 32% 29% 15% 2	484
	21st to end	Day	14% 11% 36% 28% 10% 2	532
August	1st to 10th	Day	15% 7% 30% 33% 9% 4	486
	11th to 20th	Day	15% 9% 31% 27% 13% 5%	479
	21st to end	Day	14% 11% 29% 25% 16% 6%	539
September	1st to 10th	Day	19% 13% 25% 22% 15% 6%	490
	11th to 20th	Day	17% 11% 27% 24% 12% 10%	490
	21st to end	Day	23% 13% 32% 20% 8% 5%	490
October	1st to 10th	Day	27% 12% 26% 17% 13% 4	490
	11th to 20th	Day	17% 20% 33% 22% 7% 2	485
	21st to end	Day	29% 17% 25% 21% 7% 1	429
November	1st to 10th	Day	21% 16% 31% 22% 9% 1	395
	11th to 20th	Day	30% 20% 27% 18% 5% 1	400
	21st to end	Day	30% 17% 28% 19% 5% 1	345
December	1st to 10th	Day	26% 23% 33% 14% 5%	200
	11th to 20th	Day	37% 23% 27% 11% 3	200
	21st to end	Day	29% 22% 29% 16% 4 1	220
January	1st to 10th	Night	24% 20% 37% 16% 3	800
	11th to 20th	Night	28% 19% 32% 19% 2	800
	21st to end	Night	21% 20% 36% 20% 2	880
February	1st to 10th	Night	25% 19% 33% 22% 2	800
	11th to 20th	Night	25% 21% 37% 15% 1	783
	21st to end	Night	28% 18% 35% 16% 3	660
March	1st to 10th	Night	25% 21% 34% 17% 3	800
	11th to 20th	Night	23% 20% 36% 19% 3 1	800
	21st to end	Night	26% 20% 33% 17% 4	880
April	1st to 10th	Night	27% 19% 34% 18% 3	800
	11th to 20th	Night	25% 18% 35% 18% 4	791
	21st to end	Night	23% 21% 35% 17% 4	695
May	1st to 10th	Night	29% 15% 33% 19% 4	600
	11th to 20th	Night	23% 14% 38% 21% 4	469
	21st to end	Night	24% 7% 30% 30% 7% 1	433
June	1st to 10th	Night	20% 9% 31% 30% 9% 1	400
	11th to 20th	Night	20% 6% 33% 30% 10%	380
	21st to end	Night	18% 5% 30% 37% 9%	380
July	1st to 10th	Night	15% 4 27% 35% 18% 1	370
	11th to 20th	Night	15% 4 23% 43% 14% 2	368
	21st to end	Night	15% 5% 29% 38% 13% 1	416
August	1st to 10th	Night	14% 4 28% 40% 13% 1	435
	11th to 20th	Night	15% 5% 25% 36% 17% 1	738
	21st to end	Night	16% 5% 24% 35% 19% 1	836
September	1st to 10th	Night	18% 4 31% 31% 13% 2	760
	11th to 20th	Night	16% 8% 24% 32% 17% 3	760
	21st to end	Night	22% 7% 29% 26% 15% 2	760
October	1st to 10th	Night	23% 9% 28% 25% 11% 3	760
	11th to 20th	Night	20% 13% 31% 27% 9% 1	760
	21st to end	Night	28% 11% 33% 21% 6%	836
November	1st to 10th	Night	18% 15% 33% 25% 8% 1	780
	11th to 20th	Night	24% 16% 35% 19% 6% 1	800
	21st to end	Night	25% 16% 32% 22% 5% 1	800
December	1st to 10th	Night	26% 16% 33% 20% 5% 1	800
	11th to 20th	Night	25% 19% 33% 20% 3	800
	21st to end	Night	28% 18% 33% 19% 2	880

Page generated at 2025-06-07 10:28:18. Version ac07

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