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 2025-10-19.

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	12% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	126344
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	123591
Transparency	Day	41% 5% 12% 20% 14% 7%	130006
	Night	38% 5% 13% 20% 16% 9%	127992
Seeing	Day	13% 7% 20% 27% 22% 11%	16052
	Night	14% 5% 19% 32% 25% 4	25853

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	20% 12% 3 6% 4 3 3 3 3 5% 37%	4077
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 4 7% 50%	4080
	21st to end	Day	28% 9% 3 6% 3 3 3 2 5% 6% 32%	4488
February	1st to 10th	Day	19% 10% 3 7% 4 4 4 3 5% 4 35%	1931
	11th to 20th	Day	22% 9% 3 6% 4 4 4 3 4 5% 37%	1964
	21st to end	Day	21% 9% 3 5% 4 4 4 3 4 6% 38%	1815
March	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 4 6% 7% 38%	4936
	11th to 20th	Day	13% 6% 2 7% 5% 5% 4 3 4 5% 45%	4980
	21st to end	Day	16% 8% 3 7% 5% 4 4 3 5% 6% 40%	5462
April	1st to 10th	Day	14% 9% 3 7% 5% 4 5% 3 4 6% 39%	2200
	11th to 20th	Day	16% 8% 3 6% 5% 4 5% 3 5% 7% 37%	2307
	21st to end	Day	10% 7% 3 8% 6% 5% 5% 4 5% 6% 41%	2400
May	1st to 10th	Day	16% 8% 3 8% 6% 5% 5% 4 6% 8% 31%	5497
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 7% 45%	5862
	21st to end	Day	9% 6% 3 7% 6% 5% 5% 5% 7% 10% 36%	6434
June	1st to 10th	Day	6% 4 2 6% 6% 6% 7% 5% 8% 9% 41%	2600
	11th to 20th	Day	5% 4 1 6% 6% 7% 6% 5% 9% 11% 38%	2600
	21st to end	Day	7% 5% 3 7% 7% 8% 9% 8% 9% 9% 29%	2600
July	1st to 10th	Day	7% 6% 3 8% 7% 8% 7% 6% 9% 11% 28%	5900
	11th to 20th	Day	6% 6% 3 9% 8% 6% 7% 6% 8% 13% 30%	5900
	21st to end	Day	11% 6% 2 8% 6% 6% 6% 5% 7% 9% 34%	6422
August	1st to 10th	Day	2 2 1 3 4 5% 6% 5% 8% 11% 53%	4234
	11th to 20th	Day	3 2 1 5% 5% 6% 7% 6% 10% 13% 42%	4007
	21st to end	Day	7% 5% 2 8% 7% 6% 6% 6% 8% 10% 35%	4156
September	1st to 10th	Day	8% 6% 3 9% 7% 7% 7% 5% 7% 7% 34%	2200
	11th to 20th	Day	12% 7% 3 8% 6% 6% 6% 5% 6% 9% 33%	2200
	21st to end	Day	9% 6% 2 5% 5% 5% 5% 4 6% 9% 44%	2200
October	1st to 10th	Day	6% 5% 2 4 4 4 4 4 7% 6% 55%	2095
	11th to 20th	Day	9% 6% 2 5% 3 4 4 3 6% 6% 50%	1980
	21st to end	Day	11% 4 2 5% 4 4 3 4 4 6% 52%	2090
November	1st to 10th	Day	10% 7% 2 8% 5% 4 5% 4 6% 9% 41%	1958
	11th to 20th	Day	13% 6% 3 6% 5% 5% 3 3 5% 7% 45%	2000
	21st to end	Day	15% 8% 3 7% 5% 4 4 3 5% 5% 40%	1950
December	1st to 10th	Day	25% 9% 3 6% 4 3 3 3 4 7% 34%	2940
	11th to 20th	Day	12% 7% 2 7% 4 4 4 4 5% 8% 43%	3391
	21st to end	Day	14% 6% 2 7% 3 4 4 4 5% 6% 45%	4488
January	1st to 10th	Night	21% 7% 3 6% 4 4 4 3 5% 5% 40%	5888
	11th to 20th	Night	15% 8% 2 6% 3 3 3 3 4 6% 48%	5900
	21st to end	Night	29% 8% 3 6% 3 3 3 3 4 6% 32%	6490
February	1st to 10th	Night	19% 9% 3 7% 4 3 4 3 4 6% 38%	2485
	11th to 20th	Night	20% 8% 3 6% 4 2 3 2 3 6% 43%	2320
	21st to end	Night	21% 9% 3 5% 4 4 3 3 4 4 40%	1815
March	1st to 10th	Night	21% 9% 3 6% 3 4 3 3 4 6% 37%	4919
	11th to 20th	Night	18% 8% 3 5% 4 4 3 3 4 5% 45%	4980
	21st to end	Night	18% 10% 2 5% 4 3 3 3 4 5% 42%	5467
April	1st to 10th	Night	15% 7% 2 7% 4 4 4 4 5% 7% 41%	2200
	11th to 20th	Night	18% 6% 2 5% 4 3 3 2 5% 6% 45%	2040
	21st to end	Night	14% 7% 2 7% 5% 5% 4 3 6% 7% 41%	2000
May	1st to 10th	Night	25% 9% 3 6% 4 5% 4 4 5% 7% 29%	4372
	11th to 20th	Night	13% 7% 2 6% 4 3 4 3 6% 8% 43%	4053
	21st to end	Night	16% 8% 3 7% 4 5% 5% 4 8% 10% 30%	4443
June	1st to 10th	Night	5% 3 2 4 3 3 3 2 7% 11% 57%	1800
	11th to 20th	Night	5% 3 2 3 3 3 3 4 7% 11% 55%	1800
	21st to end	Night	5% 3 2 5% 4 5% 5% 6% 7% 11% 47%	1800
July	1st to 10th	Night	5% 5% 2 6% 5% 5% 5% 5% 7% 11% 44%	4078
	11th to 20th	Night	2 3 2 6% 5% 4 5% 5% 7% 11% 49%	4077
	21st to end	Night	7% 4 2 6% 4 5% 5% 4 6% 10% 47%	4449
August	1st to 10th	Night	1 1 1 1 1 2 3 4 9% 76%	2926
	11th to 20th	Night	2 2 1 2 2 3 3 3 6% 15% 59%	3043
	21st to end	Night	4 3 2 5% 4 4 4 5% 7% 11% 51%	3736
September	1st to 10th	Night	5% 4 2 6% 5% 5% 6% 4 7% 9% 47%	2200
	11th to 20th	Night	10% 6% 3 7% 4 4 4 3 5% 8% 46%	2200
	21st to end	Night	7% 4 2 5% 4 4 3 4 6% 8% 55%	2200
October	1st to 10th	Night	5% 4 1 4 3 3 3 3 5% 8% 61%	2284
	11th to 20th	Night	9% 3 1 3 2 2 3 2 5% 6% 63%	2377
	21st to end	Night	10% 5% 1 4 3 3 3 3 4 4 61%	2508
November	1st to 10th	Night	10% 4 1 6% 4 4 4 4 6% 6% 52%	2340
	11th to 20th	Night	12% 5% 2 5% 3 3 4 4 4 6% 50%	2400
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 5% 46%	2445
December	1st to 10th	Night	24% 11% 4 7% 4 4 3 2 4 6% 31%	4250
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 4 4 5% 44%	4816
	21st to end	Night	15% 6% 2 6% 3 3 3 3 4 6% 48%	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	51% 3 5% 18% 17% 6%	4078
	11th to 20th	Day	37% 3 6% 23% 21% 10%	4080
	21st to end	Day	55% 2 3 12% 15% 12%	4488
February	1st to 10th	Day	51% 5% 6% 13% 14% 12%	1931
	11th to 20th	Day	51% 3 8% 13% 16% 8%	1974
	21st to end	Day	48% 4 8% 16% 15% 8%	1815
March	1st to 10th	Day	43% 5% 10% 18% 11% 13%	4936
	11th to 20th	Day	40% 6% 7% 9% 21% 17%	4980
	21st to end	Day	45% 3 8% 19% 20% 5%	5464
April	1st to 10th	Day	46% 6% 8% 17% 14% 9%	2200
	11th to 20th	Day	46% 5% 10% 17% 14% 9%	2307
	21st to end	Day	42% 6% 9% 19% 18% 6%	2400
May	1st to 10th	Day	51% 5% 10% 20% 11% 3	5532
	11th to 20th	Day	38% 5% 11% 23% 19% 4	5866
	21st to end	Day	40% 6% 13% 32% 9% 1	6447
June	1st to 10th	Day	33% 9% 17% 25% 15% 1	2600
	11th to 20th	Day	35% 9% 14% 26% 15% 1	2600
	21st to end	Day	43% 11% 17% 21% 8%	2600
July	1st to 10th	Day	46% 5% 30% 17% 2	5836
	11th to 20th	Day	45% 6% 29% 18% 3	5831
	21st to end	Day	44% 7% 18% 25% 6%	6240
August	1st to 10th	Day	22% 7% 15% 43% 12%	4220
	11th to 20th	Day	28% 8% 32% 28% 4	4046
	21st to end	Day	43% 8% 16% 22% 11% 1	4155
September	1st to 10th	Day	45% 10% 14% 15% 13% 4	2200
	11th to 20th	Day	46% 8% 12% 19% 13% 3	2200
	21st to end	Day	37% 6% 13% 18% 16% 10%	2200
October	1st to 10th	Day	23% 3 6% 16% 17% 36%	3407
	11th to 20th	Day	25% 3 6% 19% 35% 12%	3220
	21st to end	Day	29% 3 4 18% 34% 12%	3428
November	1st to 10th	Day	38% 5% 11% 19% 19% 8%	1958
	11th to 20th	Day	40% 4 8% 15% 19% 14%	2000
	21st to end	Day	45% 6% 8% 15% 16% 10%	1950
December	1st to 10th	Day	53% 3 7% 17% 17% 4	2940
	11th to 20th	Day	40% 3 7% 21% 19% 10%	3389
	21st to end	Day	40% 4 5% 18% 21% 12%	4488
January	1st to 10th	Night	48% 3 6% 17% 16% 10%	5888
	11th to 20th	Night	41% 2 4 19% 21% 12%	5900
	21st to end	Night	55% 2 4 12% 15% 11%	6490
February	1st to 10th	Night	48% 4 7% 10% 17% 13%	2485
	11th to 20th	Night	44% 4 7% 15% 17% 12%	2320
	21st to end	Night	48% 5% 6% 13% 19% 9%	1815
March	1st to 10th	Night	49% 4 7% 15% 14% 11%	4919
	11th to 20th	Night	43% 4 7% 13% 17% 16%	4980
	21st to end	Night	45% 4 8% 15% 21% 8%	5467
April	1st to 10th	Night	43% 5% 10% 18% 15% 10%	2200
	11th to 20th	Night	40% 4 10% 18% 15% 13%	2040
	21st to end	Night	42% 5% 10% 20% 15% 7%	2000
May	1st to 10th	Night	56% 5% 8% 17% 10% 3	4402
	11th to 20th	Night	38% 5% 11% 26% 14% 5%	4054
	21st to end	Night	47% 7% 17% 22% 6% 1	4452
June	1st to 10th	Night	22% 7% 22% 31% 18% 1	1800
	11th to 20th	Night	23% 6% 19% 32% 17% 2	1800
	21st to end	Night	28% 10% 25% 26% 11%	1800
July	1st to 10th	Night	34% 4 35% 25% 2	4035
	11th to 20th	Night	29% 6% 34% 27% 4	4026
	21st to end	Night	34% 7% 24% 28% 6%	4320
August	1st to 10th	Night	9% 3 20% 53% 14%	2916
	11th to 20th	Night	16% 6% 37% 37% 4 1	3070
	21st to end	Night	27% 11% 24% 27% 9% 1	3736
September	1st to 10th	Night	33% 9% 22% 20% 12% 4	2200
	11th to 20th	Night	36% 7% 14% 24% 16% 3	2200
	21st to end	Night	27% 6% 15% 24% 19% 9%	2200
October	1st to 10th	Night	18% 3 7% 18% 19% 35%	3739
	11th to 20th	Night	16% 3 7% 25% 33% 16%	3876
	21st to end	Night	21% 2 5% 19% 36% 17%	4120
November	1st to 10th	Night	30% 5% 13% 22% 21% 9%	2340
	11th to 20th	Night	34% 6% 10% 19% 19% 13%	2400
	21st to end	Night	40% 5% 9% 20% 17% 10%	2445
December	1st to 10th	Night	56% 3 6% 16% 14% 5%	4250
	11th to 20th	Night	42% 4 10% 13% 17% 14%	4817
	21st to end	Night	40% 3 6% 15% 26% 10%	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	29% 11% 33% 20% 5% 2	300
	11th to 20th	Day	25% 11% 32% 24% 6% 1	300
	21st to end	Day	22% 19% 30% 22% 8%	330
February	1st to 10th	Day	22% 16% 31% 24% 7%	300
	11th to 20th	Day	30% 12% 32% 20% 6% 1	300
	21st to end	Day	28% 13% 40% 16% 4	249
March	1st to 10th	Day	23% 17% 36% 17% 6%	300
	11th to 20th	Day	17% 18% 43% 17% 3 2	300
	21st to end	Day	19% 17% 33% 21% 9% 1	330
April	1st to 10th	Day	22% 19% 32% 21% 6%	300
	11th to 20th	Day	22% 15% 30% 22% 11% 1	425
	21st to end	Day	15% 10% 35% 26% 12% 2	500
May	1st to 10th	Day	18% 9% 34% 25% 13% 1	543
	11th to 20th	Day	13% 7% 30% 32% 14% 4	725
	21st to end	Day	11% 4 25% 38% 19% 3	789
June	1st to 10th	Day	7% 2 14% 35% 31% 11%	730
	11th to 20th	Day	6% 2 15% 38% 30% 9%	730
	21st to end	Day	8% 2 10% 30% 34% 16%	730
July	1st to 10th	Day	10% 2 7% 28% 32% 21%	721
	11th to 20th	Day	5% 1 3 31% 35% 26%	718
	21st to end	Day	8% 1 4 28% 36% 24%	794
August	1st to 10th	Day	3 1 3 25% 39% 29%	726
	11th to 20th	Day	4 1 2 17% 41% 34%	616
	21st to end	Day	9% 2 7% 17% 33% 32%	492
September	1st to 10th	Day	12% 1 8% 26% 32% 21%	330
	11th to 20th	Day	9% 2 9% 28% 30% 22%	330
	21st to end	Day	8% 1 8% 31% 35% 16%	330
October	1st to 10th	Day	8% 3 9% 40% 32% 8%	330
	11th to 20th	Day	10% 4 13% 39% 24% 10%	324
	21st to end	Day	18% 7% 25% 25% 19% 6%	330
November	1st to 10th	Day	16% 5% 23% 35% 17% 4	300
	11th to 20th	Day	17% 8% 25% 33% 13% 4	300
	21st to end	Day	16% 16% 33% 20% 11% 3	300
December	1st to 10th	Day	21% 7% 32% 25% 9% 6%	300
	11th to 20th	Day	17% 14% 35% 24% 8% 2	300
	21st to end	Day	18% 9% 39% 21% 9% 2	330
January	1st to 10th	Night	21% 9% 30% 31% 9%	1000
	11th to 20th	Night	20% 10% 28% 33% 8% 1	1000
	21st to end	Night	24% 8% 27% 34% 7%	1100
February	1st to 10th	Night	24% 9% 28% 32% 6%	885
	11th to 20th	Night	25% 10% 26% 31% 7%	766
	21st to end	Night	25% 11% 34% 25% 5%	495
March	1st to 10th	Night	24% 8% 30% 31% 7%	600
	11th to 20th	Night	19% 9% 31% 34% 8%	600
	21st to end	Night	19% 7% 29% 34% 10% 1	660
April	1st to 10th	Night	16% 9% 32% 36% 7%	600
	11th to 20th	Night	21% 10% 25% 34% 11%	593
	21st to end	Night	17% 8% 28% 36% 11%	600
May	1st to 10th	Night	17% 5% 30% 39% 9%	600
	11th to 20th	Night	12% 3 24% 45% 15% 1	591
	21st to end	Night	9% 4 21% 42% 23% 1	650
June	1st to 10th	Night	5% 2 8% 42% 41% 2	600
	11th to 20th	Night	5% 1 9% 47% 35% 4	600
	21st to end	Night	5% 1 9% 28% 47% 9%	600
July	1st to 10th	Night	7% 3 27% 51% 10%	585
	11th to 20th	Night	4 1 2 22% 60% 12%	582
	21st to end	Night	4 1 3 23% 57% 11%	657
August	1st to 10th	Night	1 1 1 21% 65% 12%	597
	11th to 20th	Night	3 1 1 18% 66% 11%	594
	21st to end	Night	6% 1 3 19% 55% 16%	660
September	1st to 10th	Night	6% 2 5% 22% 51% 15%	600
	11th to 20th	Night	12% 1 4 24% 48% 10%	600
	21st to end	Night	8% 1 6% 29% 49% 8%	600
October	1st to 10th	Night	5% 2 8% 35% 43% 7%	684
	11th to 20th	Night	8% 2 19% 39% 29% 3	793
	21st to end	Night	11% 6% 18% 38% 24% 3	836
November	1st to 10th	Night	12% 3 18% 39% 26% 2	780
	11th to 20th	Night	14% 5% 21% 40% 20% 1	800
	21st to end	Night	18% 8% 25% 33% 15% 2	845
December	1st to 10th	Night	22% 5% 25% 30% 17% 1	1000
	11th to 20th	Night	19% 8% 26% 35% 10% 1	1000
	21st to end	Night	22% 7% 29% 32% 9% 1	1100

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