Shell Point
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shell Point clear sky chart from 2005-11-06 to 2025-07-21.

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	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	120268
	Night	10% 5% 2 5% 4 4 4 4 6% 9% 46%	118857
Transparency	Day	32% 9% 16% 19% 16% 7%	124135
	Night	33% 8% 14% 19% 18% 8%	123127
Seeing	Day	10% 3 15% 26% 30% 16%	15962
	Night	11% 4 18% 29% 29% 10%	25956

By Month

Percentage Cloud Cover SE (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% 5% 2 5% 3 3 3 3 5% 6% 54%	4056
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 5% 7% 43%	4124
	21st to end	Day	13% 6% 2 5% 3 3 4 3 6% 9% 45%	4972
February	1st to 10th	Day	16% 8% 2 5% 5% 5% 4 3 5% 6% 41%	2000
	11th to 20th	Day	17% 7% 2 5% 3 3 4 3 4 6% 46%	1948
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 6% 45%	1650
March	1st to 10th	Day	20% 7% 2 5% 3 3 3 3 6% 8% 41%	4699
	11th to 20th	Day	7% 5% 2 6% 4 4 4 4 6% 8% 52%	4980
	21st to end	Day	16% 4 1 3 3 3 3 3 5% 7% 52%	5465
April	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 8% 47%	2200
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 5% 6% 52%	2337
	21st to end	Day	6% 4 1 4 3 4 4 3 5% 8% 57%	2456
May	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 6% 8% 48%	5862
	11th to 20th	Day	10% 5% 2 5% 3 3 4 4 6% 9% 49%	5861
	21st to end	Day	3 3 1 5% 4 5% 5% 5% 8% 11% 50%	6433
June	1st to 10th	Day	8% 5% 2 8% 6% 5% 6% 7% 10% 12% 32%	2600
	11th to 20th	Day	6% 4 2 5% 5% 6% 7% 8% 10% 14% 34%	2600
	21st to end	Day	6% 5% 2 7% 6% 7% 7% 8% 10% 12% 30%	2600
July	1st to 10th	Day	6% 4 2 7% 6% 6% 8% 8% 11% 15% 27%	4250
	11th to 20th	Day	4 4 2 5% 4 5% 6% 6% 10% 12% 42%	4250
	21st to end	Day	4 4 2 7% 7% 8% 9% 8% 12% 15% 25%	4466
August	1st to 10th	Day	8% 4 2 6% 6% 6% 7% 6% 9% 13% 32%	3851
	11th to 20th	Day	7% 4 2 5% 4 5% 7% 5% 9% 12% 39%	3729
	21st to end	Day	7% 5% 3 8% 5% 6% 6% 6% 8% 12% 33%	4098
September	1st to 10th	Day	10% 7% 2 7% 5% 7% 6% 7% 9% 12% 29%	2090
	11th to 20th	Day	9% 6% 3 7% 6% 6% 6% 6% 8% 10% 34%	2090
	21st to end	Day	10% 5% 2 6% 3 4 5% 4 6% 8% 47%	2090
October	1st to 10th	Day	8% 6% 2 6% 4 4 5% 3 5% 7% 49%	2090
	11th to 20th	Day	5% 4 1 3 2 3 3 3 4 6% 64%	2045
	21st to end	Day	7% 5% 2 4 4 4 4 4 5% 6% 56%	2090
November	1st to 10th	Day	7% 4 2 4 3 3 4 3 5% 6% 59%	1917
	11th to 20th	Day	10% 5% 2 4 3 4 3 3 5% 6% 55%	1800
	21st to end	Day	10% 5% 2 6% 3 4 4 4 6% 7% 50%	1800
December	1st to 10th	Day	10% 6% 2 6% 4 3 3 3 5% 6% 53%	2930
	11th to 20th	Day	11% 6% 2 6% 3 3 3 3 5% 6% 51%	3373
	21st to end	Day	18% 8% 3 6% 4 4 3 3 4 5% 43%	4466
January	1st to 10th	Night	12% 6% 2 4 3 3 3 3 5% 6% 53%	5889
	11th to 20th	Night	15% 7% 3 7% 5% 4 5% 4 6% 7% 37%	5831
	21st to end	Night	15% 8% 3 5% 3 4 3 3 6% 9% 41%	5984
February	1st to 10th	Night	15% 8% 2 5% 4 4 3 3 5% 6% 44%	2400
	11th to 20th	Night	15% 8% 3 6% 4 3 3 3 4 4 47%	2338
	21st to end	Night	14% 9% 2 6% 4 3 4 3 4 6% 45%	1980
March	1st to 10th	Night	16% 9% 3 6% 4 4 4 3 5% 7% 40%	5153
	11th to 20th	Night	8% 4 2 5% 4 3 4 3 6% 9% 52%	4980
	21st to end	Night	15% 5% 1 5% 4 4 3 3 5% 7% 49%	5467
April	1st to 10th	Night	9% 5% 2 5% 5% 5% 4 4 6% 7% 48%	2200
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 7% 51%	2025
	21st to end	Night	5% 3 1 3 4 4 4 4 6% 9% 57%	1940
May	1st to 10th	Night	6% 5% 2 4 3 3 4 4 6% 11% 53%	4033
	11th to 20th	Night	10% 4 2 5% 4 4 4 3 6% 10% 49%	4032
	21st to end	Night	3 2 1 4 3 4 4 4 5% 11% 59%	4421
June	1st to 10th	Night	6% 6% 2 5% 5% 5% 5% 5% 8% 13% 39%	1800
	11th to 20th	Night	4 5% 2 6% 5% 4 5% 5% 9% 13% 43%	1800
	21st to end	Night	5% 4 1 6% 5% 5% 6% 5% 9% 12% 41%	1800
July	1st to 10th	Night	4 5% 2 8% 6% 8% 8% 7% 12% 15% 25%	2930
	11th to 20th	Night	3 3 1 4 4 5% 5% 5% 8% 14% 48%	2930
	21st to end	Night	2 4 2 6% 6% 6% 7% 6% 11% 16% 35%	3087
August	1st to 10th	Night	6% 3 1 6% 5% 5% 5% 5% 10% 14% 39%	3072
	11th to 20th	Night	4 3 1 5% 4 5% 6% 6% 9% 14% 43%	3100
	21st to end	Night	7% 6% 2 8% 5% 6% 8% 6% 10% 13% 28%	3526
September	1st to 10th	Night	8% 7% 3 8% 7% 7% 7% 6% 9% 12% 28%	2090
	11th to 20th	Night	10% 6% 2 8% 6% 6% 7% 5% 8% 10% 33%	2090
	21st to end	Night	9% 4 2 6% 4 5% 5% 5% 6% 9% 46%	2090
October	1st to 10th	Night	8% 5% 3 7% 4 5% 3 3 6% 7% 49%	2090
	11th to 20th	Night	5% 3 1 4 4 4 3 3 5% 6% 61%	2133
	21st to end	Night	7% 4 1 5% 4 4 4 4 6% 6% 54%	2508
November	1st to 10th	Night	6% 5% 2 6% 3 4 4 4 6% 7% 52%	2376
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 4 6% 52%	2600
	21st to end	Night	11% 6% 2 6% 4 3 4 4 4 6% 51%	2600
December	1st to 10th	Night	10% 4 2 5% 4 4 4 3 5% 7% 54%	4250
	11th to 20th	Night	11% 6% 2 4 3 3 3 2 4 6% 54%	4822
	21st to end	Night	15% 7% 3 6% 4 4 4 3 5% 6% 44%	6490

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	30% 6% 7% 16% 27% 13%	4057
	11th to 20th	Day	41% 5% 7% 14% 21% 12%	4124
	21st to end	Day	35% 6% 11% 18% 20% 10%	4972
February	1st to 10th	Day	42% 6% 9% 14% 18% 10%	2000
	11th to 20th	Day	38% 5% 7% 16% 23% 12%	1958
	21st to end	Day	37% 7% 11% 15% 19% 11%	1650
March	1st to 10th	Day	39% 6% 11% 16% 20% 9%	4699
	11th to 20th	Day	27% 7% 13% 16% 20% 18%	4980
	21st to end	Day	31% 6% 12% 26% 16% 10%	5465
April	1st to 10th	Day	32% 8% 14% 18% 17% 12%	2200
	11th to 20th	Day	30% 7% 8% 23% 21% 10%	2337
	21st to end	Day	23% 7% 13% 26% 23% 9%	2456
May	1st to 10th	Day	31% 8% 18% 25% 14% 4	5897
	11th to 20th	Day	29% 7% 16% 32% 14% 2	5865
	21st to end	Day	22% 9% 21% 29% 17% 2	6447
June	1st to 10th	Day	34% 15% 21% 24% 6% 1	2600
	11th to 20th	Day	30% 16% 25% 20% 8% 1	2600
	21st to end	Day	36% 16% 24% 17% 7% 1	2600
July	1st to 10th	Day	37% 15% 37% 10% 1	4185
	11th to 20th	Day	26% 15% 45% 12% 2	4181
	21st to end	Day	37% 20% 31% 12% 1	4324
August	1st to 10th	Day	34% 15% 25% 18% 7% 1	3838
	11th to 20th	Day	28% 14% 24% 25% 9%	3765
	21st to end	Day	37% 14% 27% 18% 5%	4098
September	1st to 10th	Day	41% 15% 22% 14% 7% 2	2090
	11th to 20th	Day	39% 14% 20% 16% 9% 2	2090
	21st to end	Day	34% 8% 14% 22% 16% 6%	2090
October	1st to 10th	Day	42% 6% 9% 13% 17% 13%	3480
	11th to 20th	Day	13% 3 6% 20% 32% 26%	3376
	21st to end	Day	17% 4 5% 20% 39% 14%	3428
November	1st to 10th	Day	26% 5% 10% 20% 22% 15%	1917
	11th to 20th	Day	30% 5% 9% 16% 27% 13%	1800
	21st to end	Day	31% 6% 9% 21% 24% 9%	1800
December	1st to 10th	Day	32% 5% 9% 17% 27% 10%	2930
	11th to 20th	Day	34% 6% 8% 20% 23% 9%	3370
	21st to end	Day	42% 5% 8% 18% 24% 4	4466
January	1st to 10th	Night	31% 4 7% 15% 29% 15%	5889
	11th to 20th	Night	46% 4 6% 15% 21% 7%	5831
	21st to end	Night	40% 5% 9% 18% 19% 9%	5984
February	1st to 10th	Night	40% 6% 8% 14% 22% 11%	2400
	11th to 20th	Night	40% 5% 8% 13% 20% 14%	2339
	21st to end	Night	40% 5% 8% 16% 18% 12%	1980
March	1st to 10th	Night	43% 7% 8% 14% 18% 9%	5153
	11th to 20th	Night	26% 6% 10% 21% 18% 18%	4980
	21st to end	Night	35% 5% 8% 23% 18% 11%	5467
April	1st to 10th	Night	32% 8% 12% 17% 18% 12%	2200
	11th to 20th	Night	31% 6% 8% 20% 23% 12%	2025
	21st to end	Night	23% 7% 11% 28% 22% 9%	1940
May	1st to 10th	Night	24% 8% 22% 29% 13% 4	4060
	11th to 20th	Night	30% 8% 15% 29% 16% 2	4033
	21st to end	Night	18% 6% 20% 36% 17% 2	4429
June	1st to 10th	Night	33% 11% 22% 24% 9%	1800
	11th to 20th	Night	30% 11% 28% 22% 8%	1800
	21st to end	Night	31% 14% 25% 22% 7% 1	1800
July	1st to 10th	Night	43% 16% 29% 11% 1	2885
	11th to 20th	Night	24% 15% 45% 14% 2	2876
	21st to end	Night	32% 17% 37% 11% 2	2988
August	1st to 10th	Night	30% 13% 30% 18% 9%	3061
	11th to 20th	Night	26% 14% 27% 26% 6%	3130
	21st to end	Night	41% 15% 24% 14% 5%	3526
September	1st to 10th	Night	44% 14% 19% 14% 7% 2	2090
	11th to 20th	Night	44% 13% 18% 16% 8% 1	2090
	21st to end	Night	32% 9% 17% 20% 17% 6%	2090
October	1st to 10th	Night	45% 7% 9% 13% 15% 11%	3480
	11th to 20th	Night	15% 4 6% 21% 31% 23%	3541
	21st to end	Night	18% 5% 6% 18% 38% 14%	4120
November	1st to 10th	Night	30% 6% 9% 22% 19% 15%	2376
	11th to 20th	Night	32% 5% 7% 16% 24% 14%	2600
	21st to end	Night	33% 6% 10% 18% 21% 11%	2600
December	1st to 10th	Night	31% 4 9% 18% 23% 15%	4250
	11th to 20th	Night	31% 6% 10% 21% 21% 10%	4824
	21st to end	Night	40% 6% 7% 20% 22% 5%	6490

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	10% 5% 37% 36% 10% 1	300
	11th to 20th	Day	9% 8% 26% 38% 16% 3	300
	21st to end	Day	18% 8% 29% 38% 7%	330
February	1st to 10th	Day	16% 7% 25% 28% 21% 2	300
	11th to 20th	Day	22% 6% 25% 34% 13% 1	300
	21st to end	Day	12% 6% 29% 38% 14% 2	249
March	1st to 10th	Day	11% 6% 29% 40% 13% 1	389
	11th to 20th	Day	10% 8% 28% 34% 19% 2	500
	21st to end	Day	12% 8% 26% 32% 21% 1	550
April	1st to 10th	Day	11% 6% 27% 34% 20% 2	500
	11th to 20th	Day	11% 3 25% 32% 26% 3	492
	21st to end	Day	5% 3 19% 36% 32% 5%	500
May	1st to 10th	Day	6% 3 15% 39% 30% 8%	503
	11th to 20th	Day	8% 3 12% 33% 31% 13%	527
	21st to end	Day	9% 1 7% 26% 38% 19%	573
June	1st to 10th	Day	9% 6% 25% 43% 17%	530
	11th to 20th	Day	7% 1 7% 18% 38% 28%	530
	21st to end	Day	7% 1 4 13% 44% 30%	530
July	1st to 10th	Day	7% 1 6% 9% 34% 43%	525
	11th to 20th	Day	6% 1 3 10% 37% 44%	524
	21st to end	Day	4 5% 11% 41% 39%	532
August	1st to 10th	Day	10% 6% 12% 37% 35%	486
	11th to 20th	Day	9% 1 5% 6% 37% 42%	479
	21st to end	Day	11% 1 5% 15% 36% 32%	539
September	1st to 10th	Day	12% 2 7% 12% 35% 31%	490
	11th to 20th	Day	14% 2 5% 17% 40% 22%	490
	21st to end	Day	11% 1 7% 18% 39% 24%	490
October	1st to 10th	Day	13% 2 13% 30% 31% 11%	490
	11th to 20th	Day	5% 2 14% 27% 39% 14%	490
	21st to end	Day	7% 3 15% 35% 33% 7%	539
November	1st to 10th	Day	10% 4 14% 33% 31% 9%	455
	11th to 20th	Day	12% 3 19% 39% 21% 5%	300
	21st to end	Day	10% 3 23% 38% 19% 7%	300
December	1st to 10th	Day	14% 4 27% 31% 19% 5%	300
	11th to 20th	Day	17% 4 21% 37% 19% 3	300
	21st to end	Day	17% 6% 18% 35% 18% 7%	330
January	1st to 10th	Night	14% 10% 35% 30% 11% 1	800
	11th to 20th	Night	13% 7% 27% 34% 16% 2	800
	21st to end	Night	19% 6% 32% 33% 9% 1	880
February	1st to 10th	Night	17% 6% 25% 32% 19% 1	800
	11th to 20th	Night	18% 8% 24% 35% 14% 1	783
	21st to end	Night	16% 5% 30% 36% 13% 1	660
March	1st to 10th	Night	13% 6% 26% 37% 18% 1	800
	11th to 20th	Night	12% 5% 23% 36% 23% 2	800
	21st to end	Night	10% 6% 24% 36% 22% 1	880
April	1st to 10th	Night	10% 6% 23% 36% 23% 2	800
	11th to 20th	Night	10% 5% 23% 32% 27% 3	643
	21st to end	Night	5% 5% 18% 35% 33% 4	600
May	1st to 10th	Night	7% 2 16% 34% 34% 7%	600
	11th to 20th	Night	9% 1 14% 34% 34% 8%	591
	21st to end	Night	4 1 7% 29% 45% 14%	650
June	1st to 10th	Night	10% 7% 25% 47% 11%	600
	11th to 20th	Night	5% 1 6% 20% 42% 26%	600
	21st to end	Night	6% 2 4 15% 48% 26%	600
July	1st to 10th	Night	5% 2 5% 11% 44% 32%	585
	11th to 20th	Night	7% 1 2 6% 44% 40%	582
	21st to end	Night	4 4 8% 51% 34%	624
August	1st to 10th	Night	8% 3 10% 46% 33%	567
	11th to 20th	Night	7% 1 3 12% 44% 34%	564
	21st to end	Night	10% 2 5% 17% 39% 26%	651
September	1st to 10th	Night	10% 1 6% 19% 43% 21%	760
	11th to 20th	Night	12% 2 9% 21% 41% 15%	760
	21st to end	Night	10% 2 11% 24% 41% 12%	760
October	1st to 10th	Night	10% 2 18% 30% 31% 8%	760
	11th to 20th	Night	6% 3 17% 32% 35% 7%	760
	21st to end	Night	9% 3 16% 36% 31% 4	836
November	1st to 10th	Night	10% 3 20% 37% 26% 3	780
	11th to 20th	Night	12% 6% 22% 34% 23% 2	800
	21st to end	Night	12% 5% 24% 35% 21% 3	800
December	1st to 10th	Night	14% 6% 24% 32% 21% 4	800
	11th to 20th	Night	15% 5% 25% 34% 20% 2	800
	21st to end	Night	17% 6% 24% 33% 18% 2	880

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