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 2024-04-10.

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	12% 6% 3 6% 6% 4 5% 5% 6% 8% 38%	83631
	Night	13% 6% 2 5% 4 3 4 4 5% 7% 46%	85202
Transparency	Day	42% 7% 12% 19% 14% 6%	83356
	Night	37% 6% 13% 20% 16% 8%	84997
Seeing	Day	13% 6% 20% 27% 22% 12%	13913
	Night	14% 5% 20% 33% 25% 4	23966

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 5% 4 3 4 4 4 6% 39%	2847
	11th to 20th	Day	15% 6% 2 3 3 1 2 3 3 7% 53%	2850
	21st to end	Day	23% 8% 3 6% 4 2 3 4 7% 7% 33%	3135
February	1st to 10th	Day	20% 9% 3 7% 5% 3 4 4 4 4 35%	1834
	11th to 20th	Day	21% 9% 3 5% 4 3 4 3 4 5% 37%	1863
	21st to end	Day	20% 8% 3 4 5% 3 4 4 4 6% 39%	1727
March	1st to 10th	Day	17% 8% 3 7% 6% 4 5% 6% 6% 7% 30%	3436
	11th to 20th	Day	18% 8% 3 8% 7% 5% 6% 5% 5% 6% 29%	3480
	21st to end	Day	16% 6% 2 6% 5% 2 4 3 5% 7% 43%	3814
April	1st to 10th	Day	15% 8% 4 6% 6% 3 5% 4 4 6% 39%	2079
	11th to 20th	Day	17% 8% 4 5% 5% 3 5% 4 4 8% 37%	2073
	21st to end	Day	10% 6% 3 7% 6% 4 5% 5% 5% 7% 43%	2160
May	1st to 10th	Day	9% 7% 3 7% 6% 3 5% 6% 6% 8% 40%	2196
	11th to 20th	Day	10% 6% 3 6% 7% 3 5% 5% 7% 8% 40%	2302
	21st to end	Day	8% 6% 3 7% 8% 5% 7% 7% 8% 10% 30%	2531
June	1st to 10th	Day	6% 4 2 6% 7% 4 8% 7% 7% 10% 40%	2340
	11th to 20th	Day	5% 3 2 6% 7% 5% 7% 7% 9% 12% 38%	2340
	21st to end	Day	7% 5% 3 7% 8% 5% 10% 10% 9% 9% 28%	2340
July	1st to 10th	Day	10% 7% 3 6% 8% 5% 7% 7% 8% 11% 28%	2340
	11th to 20th	Day	5% 4 3 7% 8% 5% 8% 8% 9% 11% 33%	2340
	21st to end	Day	5% 4 3 6% 7% 6% 7% 9% 9% 11% 33%	2546
August	1st to 10th	Day	3 3 2 4 6% 5% 7% 7% 9% 12% 41%	2328
	11th to 20th	Day	5% 3 2 6% 7% 6% 7% 8% 9% 11% 37%	2212
	21st to end	Day	9% 5% 3 6% 8% 5% 7% 7% 8% 10% 33%	2295
September	1st to 10th	Day	7% 5% 3 8% 8% 5% 8% 6% 7% 8% 34%	1980
	11th to 20th	Day	14% 6% 3 8% 7% 5% 6% 6% 6% 8% 31%	1980
	21st to end	Day	10% 6% 2 5% 6% 4 6% 5% 6% 9% 42%	1980
October	1st to 10th	Day	7% 5% 2 4 4 3 5% 5% 7% 6% 52%	1886
	11th to 20th	Day	10% 6% 2 4 4 4 4 5% 6% 7% 48%	1800
	21st to end	Day	12% 4 3 4 5% 3 4 4 3 6% 53%	1980
November	1st to 10th	Day	9% 6% 3 6% 6% 3 5% 5% 6% 9% 43%	1858
	11th to 20th	Day	13% 6% 3 5% 6% 3 4 4 5% 6% 44%	1900
	21st to end	Day	16% 7% 3 6% 6% 4 4 4 5% 6% 39%	1853
December	1st to 10th	Day	17% 7% 4 5% 5% 3 4 4 4 8% 40%	1710
	11th to 20th	Day	14% 7% 3 6% 5% 3 5% 5% 5% 7% 40%	2161
	21st to end	Day	16% 6% 2 5% 4 3 4 5% 5% 6% 44%	3135
January	1st to 10th	Night	21% 8% 3 5% 5% 3 4 3 4 5% 39%	4108
	11th to 20th	Night	17% 6% 2 5% 4 2 3 3 3 6% 49%	4120
	21st to end	Night	25% 8% 3 5% 4 2 3 3 5% 7% 36%	4532
February	1st to 10th	Night	18% 9% 3 6% 5% 2 4 3 4 6% 38%	2361
	11th to 20th	Night	19% 7% 3 5% 4 2 3 3 3 6% 44%	2201
	21st to end	Night	21% 8% 3 4 4 3 4 4 4 5% 41%	1727
March	1st to 10th	Night	25% 11% 3 6% 4 2 3 4 4 7% 30%	3419
	11th to 20th	Night	23% 9% 4 6% 6% 4 4 4 5% 6% 29%	3480
	21st to end	Night	13% 6% 2 5% 5% 3 4 4 5% 6% 47%	3817
April	1st to 10th	Night	15% 7% 3 6% 5% 3 4 5% 5% 7% 41%	2090
	11th to 20th	Night	18% 6% 2 5% 4 2 3 3 5% 7% 44%	1834
	21st to end	Night	13% 6% 3 6% 6% 3 5% 4 5% 7% 42%	1800
May	1st to 10th	Night	12% 6% 3 4 5% 2 5% 5% 6% 9% 43%	1760
	11th to 20th	Night	9% 5% 2 5% 5% 2 5% 5% 6% 9% 47%	1593
	21st to end	Night	6% 5% 3 4 4 4 4 6% 8% 11% 44%	1745
June	1st to 10th	Night	4 3 2 4 4 3 4 3 6% 11% 57%	1620
	11th to 20th	Night	5% 3 2 3 3 2 3 4 7% 11% 56%	1620
	21st to end	Night	5% 3 2 5% 5% 4 5% 7% 7% 11% 45%	1620
July	1st to 10th	Night	5% 5% 3 5% 5% 3 6% 5% 6% 11% 46%	1620
	11th to 20th	Night	2 2 1 3 3 2 4 5% 6% 11% 60%	1620
	21st to end	Night	3 2 2 3 4 2 5% 5% 6% 12% 55%	1772
August	1st to 10th	Night	1 1 1 2 2 3 4 5% 11% 70%	1611
	11th to 20th	Night	3 3 2 3 4 3 4 5% 6% 14% 54%	1685
	21st to end	Night	4 3 2 5% 5% 3 5% 6% 7% 11% 49%	2056
September	1st to 10th	Night	5% 4 2 5% 6% 3 6% 5% 8% 10% 48%	1980
	11th to 20th	Night	11% 6% 3 6% 5% 3 4 4 5% 8% 44%	1980
	21st to end	Night	7% 4 2 4 5% 3 3 4 5% 8% 53%	1980
October	1st to 10th	Night	5% 4 1 4 3 2 3 4 5% 8% 61%	2055
	11th to 20th	Night	9% 3 1 3 3 2 3 3 5% 6% 60%	2160
	21st to end	Night	10% 5% 2 3 3 2 4 3 3 4 60%	2376
November	1st to 10th	Night	9% 4 1 5% 4 2 4 4 5% 6% 55%	2220
	11th to 20th	Night	13% 5% 3 4 4 2 4 5% 5% 7% 49%	2280
	21st to end	Night	15% 7% 3 5% 5% 3 4 4 5% 5% 45%	2322
December	1st to 10th	Night	17% 7% 3 5% 4 3 4 4 5% 7% 42%	2470
	11th to 20th	Night	15% 7% 3 6% 6% 3 4 5% 5% 6% 40%	3036
	21st to end	Night	16% 7% 2 6% 4 3 4 4 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%	2079
	11th to 20th	Day	46% 5% 9% 17% 14% 9%	2073
	21st to end	Day	41% 6% 9% 19% 19% 7%	2160
May	1st to 10th	Day	42% 7% 10% 19% 15% 7%	2196
	11th to 20th	Day	40% 9% 10% 21% 17% 3	2306
	21st to end	Day	44% 9% 15% 19% 13% 1	2531
June	1st to 10th	Day	34% 9% 17% 23% 15% 1	2340
	11th to 20th	Day	35% 10% 14% 27% 14% 1	2340
	21st to end	Day	45% 12% 16% 19% 8%	2340
July	1st to 10th	Day	42% 11% 20% 21% 5%	2276
	11th to 20th	Day	38% 12% 23% 21% 6%	2271
	21st to end	Day	35% 15% 21% 25% 3 1	2405
August	1st to 10th	Day	29% 12% 21% 28% 8%	2314
	11th to 20th	Day	35% 14% 21% 23% 7% 1	2212
	21st to end	Day	43% 12% 18% 19% 7% 2	2295
September	1st to 10th	Day	44% 11% 14% 15% 12% 4	1980
	11th to 20th	Day	48% 9% 12% 16% 12% 3	1980
	21st to end	Day	38% 7% 14% 16% 17% 8%	1980
October	1st to 10th	Day	30% 5% 9% 26% 18% 12%	1886
	11th to 20th	Day	35% 5% 8% 19% 20% 14%	1800
	21st to end	Day	34% 5% 6% 17% 22% 17%	1980
November	1st to 10th	Day	36% 5% 11% 20% 19% 8%	1858
	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	43% 6% 10% 18% 15% 9%	2090
	11th to 20th	Night	41% 4 10% 18% 15% 13%	1834
	21st to end	Night	42% 5% 9% 20% 16% 8%	1800
May	1st to 10th	Night	38% 7% 11% 19% 16% 9%	1760
	11th to 20th	Night	31% 8% 13% 25% 19% 4	1594
	21st to end	Night	32% 9% 19% 28% 12% 1	1746
June	1st to 10th	Night	22% 7% 21% 30% 20%	1620
	11th to 20th	Night	22% 6% 19% 34% 16% 2	1620
	21st to end	Night	30% 10% 23% 25% 10%	1620
July	1st to 10th	Night	33% 9% 26% 27% 5%	1575
	11th to 20th	Night	18% 10% 35% 30% 8%	1566
	21st to end	Night	21% 9% 32% 33% 5%	1673
August	1st to 10th	Night	11% 6% 31% 42% 9%	1601
	11th to 20th	Night	21% 10% 30% 32% 6% 1	1685
	21st to end	Night	28% 11% 23% 29% 7% 2	2056
September	1st to 10th	Night	30% 10% 24% 20% 12% 4	1980
	11th to 20th	Night	37% 8% 15% 23% 14% 3	1980
	21st to end	Night	28% 7% 16% 22% 20% 8%	1980
October	1st to 10th	Night	23% 5% 12% 29% 19% 12%	2055
	11th to 20th	Night	24% 5% 10% 25% 23% 13%	2160
	21st to end	Night	29% 4 7% 16% 24% 19%	2376
November	1st to 10th	Night	28% 6% 13% 22% 22% 10%	2220
	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	22% 18% 33% 21% 6%	267
	11th to 20th	Day	23% 14% 28% 21% 12% 1	351
	21st to end	Day	15% 10% 34% 27% 13% 2	420
May	1st to 10th	Day	16% 8% 31% 29% 15% 1	459
	11th to 20th	Day	12% 6% 28% 33% 16% 4	625
	21st to end	Day	10% 4 23% 39% 21% 3	679
June	1st to 10th	Day	7% 2 12% 34% 33% 11%	630
	11th to 20th	Day	5% 2 14% 37% 32% 10%	630
	21st to end	Day	9% 2 10% 29% 34% 17%	630
July	1st to 10th	Day	10% 1 6% 28% 34% 21%	621
	11th to 20th	Day	4 1 3 30% 35% 28%	618
	21st to end	Day	6% 1 4 27% 37% 25%	684
August	1st to 10th	Day	3 1 4 26% 38% 28%	626
	11th to 20th	Day	4 1 2 16% 41% 35%	526
	21st to end	Day	9% 3 7% 15% 33% 33%	414
September	1st to 10th	Day	9% 6% 26% 34% 25%	270
	11th to 20th	Day	11% 2 7% 25% 31% 22%	270
	21st to end	Day	9% 1 6% 29% 36% 19%	270
October	1st to 10th	Day	9% 3 8% 37% 33% 10%	270
	11th to 20th	Day	10% 5% 15% 35% 23% 11%	270
	21st to end	Day	19% 8% 26% 24% 19% 5%	297
November	1st to 10th	Day	14% 6% 21% 36% 19% 4	270
	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% 36% 7%	570
	11th to 20th	Night	21% 9% 25% 34% 12%	533
	21st to end	Night	16% 8% 29% 36% 11%	540
May	1st to 10th	Night	15% 5% 29% 41% 10%	540
	11th to 20th	Night	11% 3 23% 47% 16% 1	531
	21st to end	Night	8% 4 19% 43% 24% 1	584
June	1st to 10th	Night	4 2 7% 42% 43% 3	540
	11th to 20th	Night	5% 1 7% 47% 36% 4	540
	21st to end	Night	6% 1 9% 28% 47% 9%	540
July	1st to 10th	Night	7% 4 26% 51% 11%	525
	11th to 20th	Night	3 1 2 21% 60% 13%	522
	21st to end	Night	3 1 3 23% 57% 12%	591
August	1st to 10th	Night	1 1 1 22% 65% 11%	537
	11th to 20th	Night	4 1 18% 65% 11%	534
	21st to end	Night	6% 1 3 20% 54% 15%	594
September	1st to 10th	Night	5% 2 4 18% 55% 16%	540
	11th to 20th	Night	12% 1 4 23% 48% 10%	540
	21st to end	Night	9% 1 6% 27% 49% 8%	540
October	1st to 10th	Night	6% 2 8% 33% 43% 8%	615
	11th to 20th	Night	9% 2 19% 38% 29% 3	720
	21st to end	Night	11% 6% 18% 38% 24% 3	792
November	1st to 10th	Night	11% 2 18% 40% 27% 3	740
	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

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