George Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the George Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	12% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	100971
	Night	13% 6% 2 6% 4 4 4 3 5% 8% 45%	98138
Transparency	Day	41% 6% 12% 20% 14% 6%	104672
	Night	36% 6% 13% 20% 16% 8%	102564
Seeing	Day	13% 6% 20% 27% 22% 12%	14774
	Night	14% 5% 19% 33% 25% 4	24624

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	18% 11% 3 6% 4 4 3 3 4 6% 38%	2847
	11th to 20th	Day	15% 7% 2 4 3 2 2 2 3 7% 53%	2850
	21st to end	Day	23% 9% 2 7% 3 3 3 3 7% 7% 33%	3135
February	1st to 10th	Day	20% 10% 3 8% 4 4 4 3 4 4 35%	1834
	11th to 20th	Day	21% 10% 3 6% 4 4 4 3 4 5% 37%	1863
	21st to end	Day	20% 9% 3 5% 4 4 4 3 4 6% 38%	1727
March	1st to 10th	Day	17% 9% 3 8% 6% 5% 6% 4 6% 7% 29%	3436
	11th to 20th	Day	18% 8% 3 9% 6% 6% 5% 4 5% 6% 29%	3480
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 7% 43%	3814
April	1st to 10th	Day	14% 9% 3 7% 6% 4 5% 3 4 6% 38%	2090
	11th to 20th	Day	17% 8% 3 6% 5% 4 5% 3 4 7% 37%	2190
	21st to end	Day	9% 7% 3 8% 6% 5% 5% 4 5% 6% 42%	2280
May	1st to 10th	Day	16% 8% 3 7% 6% 5% 5% 4 6% 7% 33%	3829
	11th to 20th	Day	15% 6% 3 7% 5% 4 4 4 5% 7% 40%	4082
	21st to end	Day	8% 6% 2 7% 6% 5% 5% 5% 8% 10% 38%	4477
June	1st to 10th	Day	6% 4 2 6% 6% 6% 7% 6% 8% 9% 41%	2470
	11th to 20th	Day	5% 4 1 7% 6% 7% 6% 5% 9% 11% 38%	2470
	21st to end	Day	7% 5% 3 7% 7% 8% 9% 7% 9% 9% 29%	2470
July	1st to 10th	Day	8% 6% 2 6% 7% 8% 7% 6% 8% 11% 31%	4120
	11th to 20th	Day	6% 7% 3 11% 9% 8% 8% 6% 8% 10% 24%	4120
	21st to end	Day	14% 8% 3 8% 7% 7% 6% 6% 8% 9% 25%	4465
August	1st to 10th	Day	2 2 1 3 4 5% 6% 5% 8% 11% 54%	4104
	11th to 20th	Day	3 2 1 5% 5% 6% 7% 6% 10% 13% 43%	3882
	21st to end	Day	7% 5% 2 8% 7% 6% 6% 6% 8% 10% 35%	4029
September	1st to 10th	Day	8% 6% 3 9% 7% 7% 7% 5% 7% 7% 33%	2090
	11th to 20th	Day	13% 7% 3 9% 6% 6% 6% 5% 6% 9% 32%	2090
	21st to end	Day	9% 6% 2 5% 5% 5% 5% 4 6% 9% 43%	2090
October	1st to 10th	Day	6% 5% 2 4 4 4 4 4 7% 6% 53%	1990
	11th to 20th	Day	10% 6% 2 5% 3 4 4 3 6% 7% 50%	1900
	21st to end	Day	11% 4 2 5% 4 4 3 4 4 6% 52%	2090
November	1st to 10th	Day	10% 6% 2 7% 5% 4 5% 4 6% 9% 42%	1898
	11th to 20th	Day	13% 7% 3 6% 5% 5% 3 3 5% 6% 44%	1900
	21st to end	Day	15% 8% 3 7% 5% 4 4 3 5% 5% 38%	1853
December	1st to 10th	Day	16% 8% 3 6% 5% 4 4 3 4 8% 40%	1710
	11th to 20th	Day	13% 8% 2 7% 4 4 4 4 5% 7% 40%	2161
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 6% 43%	3135
January	1st to 10th	Night	21% 8% 2 6% 4 4 4 2 4 5% 39%	4108
	11th to 20th	Night	17% 7% 2 5% 3 3 2 2 3 6% 49%	4120
	21st to end	Night	25% 8% 3 5% 4 3 3 3 5% 7% 35%	4532
February	1st to 10th	Night	18% 9% 3 7% 4 3 4 3 4 6% 38%	2361
	11th to 20th	Night	19% 8% 3 5% 4 2 3 2 3 6% 44%	2201
	21st to end	Night	21% 9% 3 5% 4 4 3 3 4 4 40%	1727
March	1st to 10th	Night	25% 11% 3 6% 3 3 3 3 4 7% 30%	3419
	11th to 20th	Night	23% 10% 4 7% 5% 5% 4 4 5% 6% 29%	3480
	21st to end	Night	13% 7% 2 6% 4 4 4 3 5% 6% 47%	3817
April	1st to 10th	Night	14% 7% 2 7% 4 4 4 4 5% 7% 41%	2090
	11th to 20th	Night	19% 6% 2 6% 4 3 3 2 5% 6% 44%	1937
	21st to end	Night	14% 7% 2 7% 5% 5% 4 3 5% 7% 42%	1900
May	1st to 10th	Night	26% 9% 2 6% 4 4 4 3 5% 6% 31%	3051
	11th to 20th	Night	13% 7% 3 7% 4 4 4 4 6% 7% 42%	2823
	21st to end	Night	14% 8% 2 6% 4 4 4 4 8% 10% 35%	3090
June	1st to 10th	Night	4 3 2 4 3 3 3 2 6% 11% 57%	1710
	11th to 20th	Night	5% 3 2 3 3 3 3 4 7% 11% 56%	1710
	21st to end	Night	5% 3 2 5% 4 5% 5% 6% 7% 11% 46%	1710
July	1st to 10th	Night	6% 4 2 5% 4 4 4 4 6% 11% 50%	2850
	11th to 20th	Night	3 4 2 6% 5% 5% 6% 6% 8% 11% 44%	2850
	21st to end	Night	9% 6% 3 7% 5% 5% 6% 4 6% 10% 39%	3098
August	1st to 10th	Night	1 1 1 1 1 2 3 4 9% 76%	2836
	11th to 20th	Night	2 2 1 2 2 3 3 4 6% 16% 59%	2948
	21st to end	Night	4 3 2 5% 4 4 4 5% 7% 12% 51%	3622
September	1st to 10th	Night	5% 4 2 6% 5% 5% 5% 4 7% 10% 46%	2090
	11th to 20th	Night	10% 6% 3 7% 4 4 4 3 5% 8% 45%	2090
	21st to end	Night	7% 5% 2 5% 3 4 3 4 6% 8% 54%	2090
October	1st to 10th	Night	5% 4 1 5% 3 3 3 3 5% 8% 61%	2170
	11th to 20th	Night	9% 3 1 4 2 2 3 3 5% 6% 62%	2280
	21st to end	Night	10% 5% 1 4 3 3 3 3 4 4 61%	2508
November	1st to 10th	Night	9% 4 1 6% 3 4 4 3 6% 6% 54%	2280
	11th to 20th	Night	13% 6% 2 5% 3 3 4 4 5% 7% 49%	2280
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 5% 45%	2322
December	1st to 10th	Night	17% 8% 2 6% 4 3 4 3 5% 7% 41%	2470
	11th to 20th	Night	15% 7% 3 7% 5% 4 4 4 5% 6% 40%	3036
	21st to end	Night	16% 7% 3 7% 3 3 4 3 4 6% 45%	4532

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	47% 4 6% 16% 18% 9%	2848
	11th to 20th	Day	33% 4 6% 28% 19% 11%	2850
	21st to end	Day	51% 3 4 14% 18% 9%	3135
February	1st to 10th	Day	52% 5% 6% 13% 14% 11%	1834
	11th to 20th	Day	50% 3 8% 13% 17% 9%	1873
	21st to end	Day	47% 4 9% 16% 15% 9%	1727
March	1st to 10th	Day	50% 8% 13% 15% 11% 3	3436
	11th to 20th	Day	53% 8% 9% 9% 15% 5%	3480
	21st to end	Day	41% 5% 10% 19% 20% 6%	3814
April	1st to 10th	Day	46% 6% 9% 17% 14% 9%	2090
	11th to 20th	Day	46% 5% 10% 16% 14% 9%	2190
	21st to end	Day	41% 6% 9% 19% 18% 6%	2280
May	1st to 10th	Day	48% 7% 11% 19% 10% 4	3864
	11th to 20th	Day	42% 8% 9% 17% 20% 5%	4086
	21st to end	Day	36% 9% 13% 33% 8% 1	4489
June	1st to 10th	Day	33% 9% 18% 24% 16% 1	2470
	11th to 20th	Day	35% 9% 13% 26% 15% 1	2470
	21st to end	Day	44% 11% 16% 21% 8%	2470
July	1st to 10th	Day	43% 6% 28% 20% 3	4056
	11th to 20th	Day	52% 8% 20% 16% 4	4051
	21st to end	Day	51% 10% 18% 20% 2	4325
August	1st to 10th	Day	22% 7% 15% 44% 12%	4090
	11th to 20th	Day	27% 8% 32% 27% 4	3921
	21st to end	Day	42% 8% 16% 22% 11% 1	4028
September	1st to 10th	Day	46% 10% 13% 15% 12% 3	2090
	11th to 20th	Day	47% 8% 12% 17% 13% 3	2090
	21st to end	Day	37% 7% 14% 16% 17% 10%	2090
October	1st to 10th	Day	23% 3 6% 16% 15% 37%	3302
	11th to 20th	Day	25% 3 6% 19% 35% 12%	3140
	21st to end	Day	29% 3 4 18% 34% 12%	3428
November	1st to 10th	Day	37% 5% 11% 19% 19% 8%	1898
	11th to 20th	Day	41% 4 8% 15% 19% 12%	1900
	21st to end	Day	45% 6% 8% 15% 16% 10%	1853
December	1st to 10th	Day	45% 5% 9% 20% 15% 7%	1710
	11th to 20th	Day	43% 5% 8% 15% 19% 10%	2159
	21st to end	Day	41% 6% 6% 17% 19% 11%	3135
January	1st to 10th	Night	47% 5% 7% 13% 15% 13%	4108
	11th to 20th	Night	38% 3 6% 17% 24% 12%	4120
	21st to end	Night	50% 3 5% 14% 18% 10%	4532
February	1st to 10th	Night	48% 4 7% 11% 17% 13%	2361
	11th to 20th	Night	43% 4 8% 15% 17% 13%	2201
	21st to end	Night	47% 5% 6% 14% 18% 9%	1727
March	1st to 10th	Night	54% 6% 10% 15% 11% 5%	3419
	11th to 20th	Night	56% 6% 10% 13% 11% 5%	3480
	21st to end	Night	37% 5% 10% 15% 25% 8%	3817
April	1st to 10th	Night	42% 6% 10% 18% 15% 9%	2090
	11th to 20th	Night	42% 4 10% 17% 14% 13%	1937
	21st to end	Night	42% 5% 9% 21% 16% 7%	1900
May	1st to 10th	Night	53% 7% 9% 16% 10% 5%	3081
	11th to 20th	Night	38% 7% 11% 21% 16% 7%	2824
	21st to end	Night	41% 10% 16% 26% 7% 1	3099
June	1st to 10th	Night	22% 7% 22% 30% 19% 1	1710
	11th to 20th	Night	23% 6% 19% 33% 17% 3	1710
	21st to end	Night	29% 10% 24% 26% 11%	1710
July	1st to 10th	Night	30% 5% 34% 27% 3	2805
	11th to 20th	Night	32% 7% 31% 24% 6%	2796
	21st to end	Night	40% 9% 26% 22% 3	2999
August	1st to 10th	Night	9% 4 19% 54% 14%	2826
	11th to 20th	Night	16% 6% 37% 36% 4 1	2975
	21st to end	Night	26% 11% 24% 28% 9% 1	3622
September	1st to 10th	Night	32% 9% 23% 20% 13% 4	2090
	11th to 20th	Night	37% 8% 14% 23% 15% 3	2090
	21st to end	Night	28% 6% 15% 23% 19% 9%	2090
October	1st to 10th	Night	18% 3 7% 18% 18% 36%	3625
	11th to 20th	Night	16% 3 7% 23% 34% 16%	3779
	21st to end	Night	21% 2 5% 19% 36% 17%	4120
November	1st to 10th	Night	29% 6% 13% 22% 21% 9%	2280
	11th to 20th	Night	34% 6% 11% 20% 18% 11%	2280
	21st to end	Night	41% 5% 9% 20% 17% 9%	2322
December	1st to 10th	Night	42% 6% 8% 20% 15% 9%	2470
	11th to 20th	Night	43% 6% 9% 12% 16% 15%	3037
	21st to end	Night	42% 5% 7% 14% 24% 8%	4532

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	26% 10% 35% 22% 6% 2	270
	11th to 20th	Day	24% 10% 33% 25% 7% 1	270
	21st to end	Day	20% 20% 30% 23% 8%	297
February	1st to 10th	Day	25% 17% 31% 21% 6%	270
	11th to 20th	Day	29% 11% 33% 21% 6% 1	270
	21st to end	Day	26% 15% 42% 15% 2	225
March	1st to 10th	Day	23% 16% 37% 17% 6%	270
	11th to 20th	Day	17% 17% 42% 18% 3 2	270
	21st to end	Day	20% 18% 32% 22% 8%	297
April	1st to 10th	Day	21% 19% 32% 21% 6%	270
	11th to 20th	Day	22% 15% 30% 21% 11% 1	388
	21st to end	Day	14% 10% 35% 26% 13% 2	460
May	1st to 10th	Day	18% 8% 32% 27% 14% 1	501
	11th to 20th	Day	13% 7% 28% 33% 15% 4	675
	21st to end	Day	10% 4 25% 39% 19% 3	734
June	1st to 10th	Day	6% 2 13% 35% 32% 11%	680
	11th to 20th	Day	6% 2 15% 36% 31% 10%	680
	21st to end	Day	9% 2 10% 30% 34% 16%	680
July	1st to 10th	Day	10% 1 6% 28% 33% 22%	671
	11th to 20th	Day	5% 1 3 30% 35% 27%	668
	21st to end	Day	8% 1 4 27% 36% 24%	739
August	1st to 10th	Day	3 1 3 25% 38% 29%	676
	11th to 20th	Day	4 1 2 17% 41% 35%	571
	21st to end	Day	9% 3 7% 16% 34% 31%	453
September	1st to 10th	Day	12% 1 8% 25% 32% 22%	300
	11th to 20th	Day	10% 2 9% 26% 30% 23%	300
	21st to end	Day	8% 1 8% 29% 36% 18%	300
October	1st to 10th	Day	8% 3 9% 40% 31% 9%	300
	11th to 20th	Day	9% 4 14% 38% 23% 11%	300
	21st to end	Day	18% 7% 25% 25% 19% 6%	330
November	1st to 10th	Day	14% 6% 22% 35% 18% 4	282
	11th to 20th	Day	18% 8% 26% 31% 13% 4	270
	21st to end	Day	18% 17% 32% 21% 10% 3	270
December	1st to 10th	Day	19% 5% 31% 27% 10% 7%	270
	11th to 20th	Day	18% 15% 35% 24% 6% 1	270
	21st to end	Day	19% 9% 39% 21% 9% 3	297
January	1st to 10th	Night	21% 9% 29% 32% 10%	950
	11th to 20th	Night	20% 10% 27% 34% 9% 1	950
	21st to end	Night	23% 8% 27% 35% 7%	1045
February	1st to 10th	Night	23% 10% 29% 32% 6%	841
	11th to 20th	Night	24% 10% 26% 33% 7%	727
	21st to end	Night	25% 11% 35% 25% 4	471
March	1st to 10th	Night	23% 8% 30% 32% 7%	570
	11th to 20th	Night	19% 9% 29% 35% 8%	570
	21st to end	Night	17% 8% 30% 34% 10% 1	627
April	1st to 10th	Night	15% 9% 33% 37% 7%	570
	11th to 20th	Night	22% 10% 24% 33% 11%	563
	21st to end	Night	17% 8% 28% 35% 11%	570
May	1st to 10th	Night	17% 5% 30% 39% 9%	570
	11th to 20th	Night	11% 3 23% 46% 15% 1	561
	21st to end	Night	8% 5% 21% 42% 23% 1	617
June	1st to 10th	Night	4 2 8% 41% 42% 2	570
	11th to 20th	Night	5% 1 8% 46% 36% 4	570
	21st to end	Night	5% 1 10% 29% 46% 9%	570
July	1st to 10th	Night	7% 3 27% 51% 11%	555
	11th to 20th	Night	4 1 1 21% 61% 13%	552
	21st to end	Night	4 1 3 23% 57% 12%	624
August	1st to 10th	Night	1 1 1 21% 65% 11%	567
	11th to 20th	Night	4 1 18% 66% 11%	564
	21st to end	Night	6% 1 3 20% 55% 15%	627
September	1st to 10th	Night	6% 2 5% 19% 52% 15%	570
	11th to 20th	Night	12% 1 4 24% 48% 10%	570
	21st to end	Night	8% 1 6% 28% 49% 8%	570
October	1st to 10th	Night	6% 2 8% 35% 43% 7%	650
	11th to 20th	Night	9% 2 20% 39% 28% 3	760
	21st to end	Night	11% 6% 18% 38% 24% 3	836
November	1st to 10th	Night	11% 2 18% 40% 26% 3	760
	11th to 20th	Night	14% 5% 21% 40% 20% 1	760
	21st to end	Night	18% 7% 25% 33% 15% 2	802
December	1st to 10th	Night	21% 5% 24% 31% 17% 1	950
	11th to 20th	Night	20% 8% 26% 36% 10% 1	950
	21st to end	Night	22% 8% 30% 30% 9% 1	1045

Page generated at 2024-11-03 01:11:21. Version ac07

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