Long Point Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Long Point Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	14% 9% 4 9% 5% 5% 5% 4 6% 7% 33%	126263
	Night	15% 10% 4 8% 5% 5% 5% 4 5% 7% 33%	122740
Transparency	Day	31% 40% 8% 14% 7% 1	129860
	Night	31% 39% 7% 13% 8% 1	127229
Seeing	Day	18% 8% 25% 28% 16% 5%	15597
	Night	20% 8% 27% 29% 14% 2	25100

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% 15% 7% 12% 7% 6% 4 3 4 5% 24%	3763
	11th to 20th	Day	10% 20% 8% 20% 9% 7% 5% 3 5% 4 9%	4026
	21st to end	Day	18% 15% 6% 16% 7% 6% 5% 4 4 4 16%	4466
February	1st to 10th	Day	17% 15% 5% 13% 8% 5% 5% 4 4 5% 19%	1800
	11th to 20th	Day	14% 13% 6% 13% 7% 6% 5% 5% 5% 5% 21%	1753
	21st to end	Day	18% 13% 5% 12% 6% 5% 5% 3 6% 5% 22%	1598
March	1st to 10th	Day	18% 9% 5% 12% 7% 5% 4 3 4 5% 27%	4922
	11th to 20th	Day	10% 12% 4 9% 6% 5% 4 4 5% 8% 32%	4980
	21st to end	Day	18% 10% 3 9% 7% 5% 5% 3 4 5% 32%	5465
April	1st to 10th	Day	16% 11% 4 8% 6% 5% 4 4 5% 7% 31%	2311
	11th to 20th	Day	15% 11% 3 8% 4 4 4 3 5% 7% 35%	2484
	21st to end	Day	17% 9% 3 7% 5% 4 5% 4 5% 6% 35%	2600
May	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 3 5% 7% 31%	5888
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 4 6% 7% 33%	5997
	21st to end	Day	15% 6% 3 7% 5% 4 5% 4 6% 7% 38%	6949
June	1st to 10th	Day	11% 6% 3 7% 5% 4 5% 4 7% 7% 42%	2800
	11th to 20th	Day	10% 6% 2 7% 5% 4 5% 5% 7% 9% 39%	2800
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 7% 10% 43%	2800
July	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 5% 6% 10% 45%	6360
	11th to 20th	Day	8% 5% 3 6% 5% 5% 5% 4 7% 9% 43%	6052
	21st to end	Day	7% 5% 2 5% 5% 5% 5% 4 6% 9% 48%	6424
August	1st to 10th	Day	15% 5% 2 5% 4 4 5% 4 7% 9% 40%	4234
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 9% 37%	4174
	21st to end	Day	9% 6% 2 7% 4 5% 5% 4 7% 9% 42%	4528
September	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 3 6% 8% 44%	2400
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 7% 8% 49%	2185
	21st to end	Day	15% 7% 3 6% 5% 5% 5% 4 6% 7% 37%	2090
October	1st to 10th	Day	15% 7% 2 7% 5% 4 5% 4 5% 7% 38%	1902
	11th to 20th	Day	18% 8% 4 8% 6% 5% 5% 3 6% 7% 30%	1900
	21st to end	Day	21% 11% 4 9% 6% 5% 4 4 6% 7% 24%	2090
November	1st to 10th	Day	14% 8% 2 8% 6% 5% 4 4 5% 7% 37%	1776
	11th to 20th	Day	15% 11% 4 8% 6% 6% 4 3 6% 8% 30%	1600
	21st to end	Day	20% 15% 3 8% 5% 5% 4 5% 7% 6% 20%	1600
December	1st to 10th	Day	19% 14% 5% 11% 7% 6% 7% 5% 7% 7% 14%	2600
	11th to 20th	Day	21% 16% 5% 10% 6% 4 5% 4 6% 5% 18%	2986
	21st to end	Day	25% 19% 5% 10% 5% 4 4 3 5% 6% 13%	3960
January	1st to 10th	Night	15% 13% 5% 11% 6% 5% 5% 4 5% 6% 26%	6165
	11th to 20th	Night	11% 16% 6% 14% 8% 5% 7% 5% 6% 7% 15%	5859
	21st to end	Night	20% 14% 6% 15% 6% 5% 4 3 4 5% 18%	6490
February	1st to 10th	Night	16% 16% 5% 10% 7% 5% 5% 4 5% 5% 23%	2600
	11th to 20th	Night	15% 13% 4 11% 6% 5% 5% 4 6% 7% 25%	2533
	21st to end	Night	17% 13% 4 11% 5% 5% 4 4 5% 8% 23%	2030
March	1st to 10th	Night	15% 9% 4 8% 7% 5% 4 4 5% 6% 33%	4918
	11th to 20th	Night	11% 9% 3 7% 4 4 5% 4 6% 7% 40%	4979
	21st to end	Night	17% 10% 4 7% 5% 4 4 3 5% 5% 38%	5467
April	1st to 10th	Night	19% 10% 4 7% 5% 5% 5% 4 5% 6% 31%	2087
	11th to 20th	Night	18% 8% 3 6% 5% 5% 4 5% 5% 6% 36%	1882
	21st to end	Night	15% 6% 3 7% 6% 6% 5% 4 5% 7% 37%	1800
May	1st to 10th	Night	21% 10% 4 8% 4 4 4 3 5% 5% 33%	4053
	11th to 20th	Night	13% 8% 4 8% 5% 5% 5% 4 5% 6% 37%	3895
	21st to end	Night	17% 5% 2 6% 4 3 4 4 6% 8% 41%	3926
June	1st to 10th	Night	10% 7% 2 6% 5% 5% 4 4 6% 8% 42%	1600
	11th to 20th	Night	9% 6% 2 7% 5% 5% 5% 5% 7% 9% 42%	1600
	21st to end	Night	11% 4 3 6% 5% 5% 5% 3 7% 8% 45%	1600
July	1st to 10th	Night	6% 6% 2 6% 4 4 5% 4 6% 10% 47%	3600
	11th to 20th	Night	7% 5% 2 6% 5% 5% 5% 4 6% 8% 48%	3890
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 6% 7% 50%	4428
August	1st to 10th	Night	10% 6% 2 7% 4 5% 4 4 5% 9% 44%	2915
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 6% 8% 46%	2875
	21st to end	Night	7% 4 1 5% 5% 4 4 4 6% 9% 49%	3322
September	1st to 10th	Night	11% 7% 2 6% 4 4 3 3 5% 7% 48%	2000
	11th to 20th	Night	9% 5% 2 5% 3 2 3 3 6% 6% 56%	2169
	21st to end	Night	16% 6% 3 7% 5% 4 5% 4 5% 6% 39%	2090
October	1st to 10th	Night	14% 7% 3 6% 4 3 4 4 6% 7% 43%	2276
	11th to 20th	Night	17% 9% 4 8% 4 5% 4 3 5% 6% 35%	2280
	21st to end	Night	21% 9% 3 7% 4 5% 4 4 5% 6% 33%	2508
November	1st to 10th	Night	15% 8% 2 6% 4 4 4 4 5% 7% 41%	2519
	11th to 20th	Night	15% 12% 3 7% 5% 6% 5% 5% 6% 8% 29%	2800
	21st to end	Night	19% 13% 4 7% 6% 5% 5% 4 5% 7% 26%	2800
December	1st to 10th	Night	15% 17% 4 10% 6% 5% 6% 5% 6% 6% 20%	4580
	11th to 20th	Night	19% 16% 5% 10% 6% 5% 5% 4 4 6% 20%	5208
	21st to end	Night	24% 17% 5% 10% 6% 5% 4 3 5% 5% 17%	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	46% 30% 7% 8% 8%	3764
	11th to 20th	Day	58% 31% 4 4 3	4060
	21st to end	Day	51% 32% 7% 6% 3 1	4466
February	1st to 10th	Day	26% 62% 4 6% 3	1800
	11th to 20th	Day	26% 61% 2 7% 3	1762
	21st to end	Day	21% 63% 4 8% 4 1	1598
March	1st to 10th	Day	43% 31% 9% 13% 4	4925
	11th to 20th	Day	34% 31% 14% 15% 5% 1	4980
	21st to end	Day	38% 31% 6% 16% 8% 1	5465
April	1st to 10th	Day	25% 61% 3 7% 3	2311
	11th to 20th	Day	19% 62% 4 9% 5% 1	2484
	21st to end	Day	19% 62% 5% 7% 5% 1	2600
May	1st to 10th	Day	38% 32% 8% 12% 6% 3	5897
	11th to 20th	Day	36% 32% 9% 16% 5% 1	6001
	21st to end	Day	34% 31% 7% 15% 11% 3	6949
June	1st to 10th	Day	17% 62% 3 10% 7% 1	2800
	11th to 20th	Day	17% 63% 6% 8% 5% 1	2800
	21st to end	Day	13% 61% 5% 12% 8%	2800
July	1st to 10th	Day	25% 28% 10% 22% 13% 2	6290
	11th to 20th	Day	25% 28% 11% 22% 13% 1	5979
	21st to end	Day	23% 28% 11% 20% 17% 1	6240
August	1st to 10th	Day	26% 37% 11% 15% 10% 1	4221
	11th to 20th	Day	28% 39% 11% 17% 4	4213
	21st to end	Day	22% 39% 8% 22% 8%	4528
September	1st to 10th	Day	15% 57% 8% 14% 7%	2400
	11th to 20th	Day	12% 58% 5% 14% 10%	2185
	21st to end	Day	18% 60% 6% 10% 6%	2090
October	1st to 10th	Day	20% 39% 10% 18% 13% 1	3162
	11th to 20th	Day	21% 38% 10% 20% 11%	3140
	21st to end	Day	26% 38% 12% 19% 5%	3428
November	1st to 10th	Day	25% 56% 6% 9% 3 1	1776
	11th to 20th	Day	23% 58% 5% 9% 5%	1600
	21st to end	Day	28% 58% 6% 5% 3	1600
December	1st to 10th	Day	45% 38% 10% 6% 1	2600
	11th to 20th	Day	46% 35% 8% 6% 5%	2986
	21st to end	Day	54% 29% 7% 8% 3	3960
January	1st to 10th	Night	40% 30% 8% 15% 5% 1	6165
	11th to 20th	Night	48% 36% 6% 7% 3	5900
	21st to end	Night	52% 31% 4 8% 5%	6490
February	1st to 10th	Night	24% 62% 4 7% 3	2600
	11th to 20th	Night	22% 61% 4 8% 5%	2535
	21st to end	Night	23% 62% 4 7% 3 1	2030
March	1st to 10th	Night	38% 30% 10% 13% 8% 1	4928
	11th to 20th	Night	28% 31% 13% 16% 11% 1	4980
	21st to end	Night	34% 30% 8% 13% 14% 1	5467
April	1st to 10th	Night	24% 61% 3 7% 4	2087
	11th to 20th	Night	19% 63% 3 8% 6% 1	1882
	21st to end	Night	16% 62% 4 11% 5% 2	1800
May	1st to 10th	Night	40% 30% 7% 9% 8% 7%	4060
	11th to 20th	Night	37% 31% 9% 18% 6%	3896
	21st to end	Night	32% 30% 8% 16% 13% 1	3927
June	1st to 10th	Night	16% 62% 3 10% 6% 3	1600
	11th to 20th	Night	15% 62% 6% 11% 5% 1	1600
	21st to end	Night	15% 60% 6% 8% 10% 1	1600
July	1st to 10th	Night	26% 28% 11% 18% 13% 3	3560
	11th to 20th	Night	25% 28% 9% 24% 13% 1	3840
	21st to end	Night	25% 28% 9% 16% 21% 2	4297
August	1st to 10th	Night	25% 37% 8% 17% 10% 2	2904
	11th to 20th	Night	26% 38% 10% 21% 5%	2902
	21st to end	Night	20% 38% 9% 24% 10%	3322
September	1st to 10th	Night	13% 57% 6% 14% 9%	2000
	11th to 20th	Night	11% 58% 5% 15% 11%	2169
	21st to end	Night	17% 60% 4 11% 6%	2090
October	1st to 10th	Night	17% 38% 4 21% 19% 1	3796
	11th to 20th	Night	25% 37% 8% 18% 12%	3779
	21st to end	Night	20% 38% 11% 19% 12%	4120
November	1st to 10th	Night	23% 57% 5% 9% 7% 1	2519
	11th to 20th	Night	21% 57% 5% 11% 6%	2800
	21st to end	Night	27% 58% 6% 7% 2 1	2800
December	1st to 10th	Night	40% 38% 11% 8% 2	4580
	11th to 20th	Night	45% 35% 6% 9% 5%	5208
	21st to end	Night	50% 29% 8% 8% 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	23% 16% 34% 16% 8% 2	237
	11th to 20th	Day	26% 13% 38% 17% 5% 2	300
	21st to end	Day	23% 16% 31% 24% 6%	330
February	1st to 10th	Day	21% 13% 42% 21% 3	300
	11th to 20th	Day	23% 19% 34% 21% 3	300
	21st to end	Day	22% 22% 36% 18% 3	249
March	1st to 10th	Day	21% 10% 32% 29% 8%	490
	11th to 20th	Day	19% 13% 33% 27% 7% 1	500
	21st to end	Day	27% 10% 33% 21% 8%	550
April	1st to 10th	Day	19% 10% 33% 29% 8% 2	500
	11th to 20th	Day	21% 11% 32% 23% 10% 4	492
	21st to end	Day	21% 6% 33% 29% 9% 2	500
May	1st to 10th	Day	25% 6% 26% 27% 11% 5%	503
	11th to 20th	Day	17% 5% 23% 29% 20% 6%	527
	21st to end	Day	14% 4 22% 31% 20% 9%	573
June	1st to 10th	Day	14% 3 25% 37% 16% 4	530
	11th to 20th	Day	14% 4 22% 35% 22% 4	530
	21st to end	Day	14% 2 23% 37% 18% 5%	530
July	1st to 10th	Day	10% 2 16% 33% 28% 12%	525
	11th to 20th	Day	15% 3 16% 34% 20% 11%	524
	21st to end	Day	8% 2 18% 38% 27% 7%	576
August	1st to 10th	Day	10% 2 13% 33% 30% 11%	526
	11th to 20th	Day	11% 4 14% 32% 28% 11%	519
	21st to end	Day	12% 3 17% 32% 27% 10%	583
September	1st to 10th	Day	12% 5% 18% 28% 25% 12%	530
	11th to 20th	Day	9% 1 14% 25% 33% 19%	522
	21st to end	Day	17% 7% 21% 29% 19% 7%	490
October	1st to 10th	Day	18% 6% 24% 26% 18% 9%	490
	11th to 20th	Day	20% 13% 27% 27% 9% 4	490
	21st to end	Day	28% 11% 28% 23% 10%	539
November	1st to 10th	Day	18% 14% 33% 23% 9% 3	322
	11th to 20th	Day	22% 21% 28% 20% 8% 2	200
	21st to end	Day	29% 13% 36% 19% 3	200
December	1st to 10th	Day	27% 17% 28% 21% 7% 2	200
	11th to 20th	Day	28% 20% 31% 17% 5%	200
	21st to end	Day	28% 17% 30% 19% 6%	220
January	1st to 10th	Night	26% 16% 34% 19% 6%	800
	11th to 20th	Night	29% 13% 34% 19% 5%	800
	21st to end	Night	21% 12% 35% 25% 6%	880
February	1st to 10th	Night	26% 12% 36% 24% 3	800
	11th to 20th	Night	23% 11% 35% 27% 4	783
	21st to end	Night	23% 13% 36% 24% 4	660
March	1st to 10th	Night	19% 9% 34% 31% 7%	800
	11th to 20th	Night	21% 7% 35% 28% 9% 1	800
	21st to end	Night	23% 8% 30% 29% 9% 1	880
April	1st to 10th	Night	23% 8% 33% 28% 7% 1	800
	11th to 20th	Night	22% 7% 31% 29% 10% 2	698
	21st to end	Night	18% 5% 30% 37% 11% 1	600
May	1st to 10th	Night	21% 4 26% 32% 16% 2	600
	11th to 20th	Night	17% 5% 21% 38% 17% 2	532
	21st to end	Night	15% 2 21% 33% 25% 4	433
June	1st to 10th	Night	14% 2 23% 42% 19% 2	400
	11th to 20th	Night	12% 2 22% 39% 25% 1	400
	21st to end	Night	12% 4 19% 40% 24% 2	400
July	1st to 10th	Night	8% 1 16% 43% 25% 7%	390
	11th to 20th	Night	12% 1 16% 38% 28% 5%	507
	21st to end	Night	11% 1 14% 41% 28% 4	657
August	1st to 10th	Night	9% 2 12% 39% 32% 6%	597
	11th to 20th	Night	11% 2 14% 34% 33% 6%	594
	21st to end	Night	11% 2 17% 35% 28% 7%	720
September	1st to 10th	Night	15% 4 17% 32% 26% 7%	800
	11th to 20th	Night	12% 2 14% 29% 34% 9%	793
	21st to end	Night	19% 5% 18% 31% 21% 6%	760
October	1st to 10th	Night	19% 6% 22% 28% 21% 4	760
	11th to 20th	Night	22% 10% 28% 26% 11% 2	760
	21st to end	Night	26% 9% 32% 25% 7% 1	836
November	1st to 10th	Night	21% 11% 27% 28% 12% 2	780
	11th to 20th	Night	23% 17% 31% 20% 8% 2	800
	21st to end	Night	28% 14% 30% 21% 6%	800
December	1st to 10th	Night	31% 15% 31% 18% 5% 1	800
	11th to 20th	Night	28% 15% 33% 20% 4	800
	21st to end	Night	30% 11% 32% 23% 3	880

Page generated at 2025-09-16 01:49:11. Version ac07

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