GCC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GCC Observatory clear sky chart from 2005-11-06 to 2024-01-22.

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	17% 12% 5% 8% 6% 3 4 5% 5% 7% 28%	74704
	Night	17% 13% 5% 8% 6% 3 4 4 5% 6% 28%	75236
Transparency	Day	56% 6% 11% 17% 9% 1	74418
	Night	58% 6% 10% 16% 9% 1	75041
Seeing	Day	25% 17% 29% 18% 9% 3	13474
	Night	25% 15% 27% 22% 9% 1	23189

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% 19% 10% 14% 10% 4 4 3 4 5% 11%	1621
	11th to 20th	Day	18% 22% 9% 14% 9% 4 4 3 3 4 11%	1710
	21st to end	Day	15% 18% 10% 18% 11% 4 4 4 3 3 10%	1791
February	1st to 10th	Day	17% 18% 9% 14% 11% 4 4 4 3 3 14%	1620
	11th to 20th	Day	15% 16% 10% 14% 10% 4 4 5% 4 4 15%	1573
	21st to end	Day	18% 18% 5% 13% 7% 2 3 4 4 4 20%	1445
March	1st to 10th	Day	18% 13% 8% 11% 6% 3 4 4 5% 7% 22%	1980
	11th to 20th	Day	16% 16% 7% 8% 5% 3 4 5% 4 5% 27%	1980
	21st to end	Day	21% 15% 6% 9% 5% 3 4 4 3 4 27%	2178
April	1st to 10th	Day	17% 13% 4 8% 6% 3 4 4 4 7% 30%	2083
	11th to 20th	Day	21% 12% 3 5% 5% 3 3 5% 4 6% 32%	2197
	21st to end	Day	22% 12% 3 6% 5% 3 3 3 4 6% 33%	2340
May	1st to 10th	Day	19% 10% 4 7% 6% 4 5% 5% 5% 6% 30%	2340
	11th to 20th	Day	20% 10% 4 6% 5% 3 4 4 5% 6% 32%	2322
	21st to end	Day	13% 7% 2 6% 5% 4 5% 5% 7% 8% 38%	2725
June	1st to 10th	Day	17% 6% 3 6% 5% 4 5% 5% 6% 9% 35%	2520
	11th to 20th	Day	15% 7% 3 6% 5% 4 5% 5% 5% 8% 36%	2520
	21st to end	Day	16% 5% 3 5% 5% 3 5% 6% 5% 10% 37%	2520
July	1st to 10th	Day	10% 6% 3 5% 6% 3 5% 5% 5% 9% 42%	2520
	11th to 20th	Day	14% 6% 3 5% 5% 3 5% 7% 6% 10% 37%	2368
	21st to end	Day	13% 5% 2 5% 7% 4 6% 6% 7% 9% 36%	2547
August	1st to 10th	Day	11% 6% 2 5% 6% 4 6% 6% 7% 9% 37%	2327
	11th to 20th	Day	13% 6% 3 7% 6% 4 7% 7% 8% 8% 31%	2303
	21st to end	Day	13% 5% 3 7% 6% 3 5% 6% 6% 9% 36%	2476
September	1st to 10th	Day	19% 7% 3 5% 6% 3 4 4 5% 9% 36%	2160
	11th to 20th	Day	13% 6% 3 6% 5% 3 5% 5% 6% 8% 39%	1995
	21st to end	Day	19% 8% 4 7% 4 4 4 4 6% 6% 34%	1976
October	1st to 10th	Day	19% 7% 3 6% 6% 4 4 5% 6% 7% 33%	1800
	11th to 20th	Day	23% 10% 3 8% 6% 4 5% 4 5% 5% 27%	1800
	21st to end	Day	27% 13% 3 8% 5% 3 5% 4 4 5% 22%	1980
November	1st to 10th	Day	14% 14% 4 8% 6% 2 5% 5% 5% 6% 32%	1740
	11th to 20th	Day	19% 20% 4 8% 5% 3 4 4 4 6% 23%	1525
	21st to end	Day	19% 26% 6% 8% 5% 3 4 4 4 6% 15%	1520
December	1st to 10th	Day	19% 27% 12% 10% 4 3 4 3 3 3 12%	1520
	11th to 20th	Day	18% 21% 10% 14% 6% 2 3 3 5% 5% 13%	1910
	21st to end	Day	27% 24% 9% 11% 5% 2 3 3 2 3 10%	2772
January	1st to 10th	Night	19% 19% 9% 15% 8% 3 4 3 3 3 14%	2540
	11th to 20th	Night	20% 18% 9% 15% 8% 3 4 3 3 4 13%	2470
	21st to end	Night	14% 21% 11% 13% 10% 3 4 4 3 4 13%	2599
February	1st to 10th	Night	19% 18% 7% 12% 10% 3 5% 3 3 4 16%	2340
	11th to 20th	Night	16% 17% 7% 14% 8% 4 5% 4 4 4 16%	2273
	21st to end	Night	20% 13% 7% 15% 7% 3 3 3 3 4 21%	1807
March	1st to 10th	Night	16% 14% 6% 11% 8% 4 4 4 4 6% 24%	1980
	11th to 20th	Night	18% 13% 6% 7% 5% 3 4 3 4 6% 30%	1980
	21st to end	Night	18% 15% 5% 8% 6% 3 5% 4 4 5% 28%	2178
April	1st to 10th	Night	18% 14% 5% 6% 4 3 4 5% 4 5% 30%	1876
	11th to 20th	Night	19% 9% 4 6% 6% 3 4 5% 5% 7% 32%	1727
	21st to end	Night	15% 12% 4 6% 5% 4 5% 4 5% 8% 33%	1620
May	1st to 10th	Night	19% 9% 2 6% 6% 3 5% 5% 4 6% 34%	1620
	11th to 20th	Night	15% 9% 4 6% 5% 4 5% 5% 6% 8% 35%	1567
	21st to end	Night	13% 5% 2 3 5% 3 4 4 6% 9% 45%	1550
June	1st to 10th	Night	11% 5% 2 6% 6% 4 5% 6% 7% 9% 40%	1440
	11th to 20th	Night	12% 6% 3 6% 6% 3 4 5% 5% 7% 42%	1440
	21st to end	Night	12% 6% 3 5% 5% 3 5% 5% 6% 8% 41%	1440
July	1st to 10th	Night	9% 5% 3 6% 5% 3 3 5% 5% 8% 48%	1440
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 6% 9% 47%	1588
	21st to end	Night	10% 5% 2 5% 5% 3 5% 6% 7% 9% 43%	1771
August	1st to 10th	Night	9% 5% 1 4 5% 3 5% 4 6% 8% 48%	1611
	11th to 20th	Night	10% 6% 3 5% 6% 4 5% 7% 6% 8% 40%	1598
	21st to end	Night	13% 5% 2 5% 5% 3 4 5% 6% 8% 45%	1860
September	1st to 10th	Night	15% 6% 3 5% 4 3 4 5% 6% 8% 43%	1800
	11th to 20th	Night	13% 5% 3 4 4 3 4 4 5% 8% 48%	1962
	21st to end	Night	17% 8% 3 6% 6% 3 4 5% 5% 7% 37%	1984
October	1st to 10th	Night	17% 7% 2 6% 5% 2 4 5% 5% 6% 41%	2160
	11th to 20th	Night	21% 9% 4 6% 6% 3 5% 5% 6% 6% 30%	2160
	21st to end	Night	27% 12% 3 6% 5% 4 4 4 4 5% 26%	2376
November	1st to 10th	Night	16% 11% 4 7% 4 3 4 4 6% 7% 33%	2338
	11th to 20th	Night	19% 18% 4 7% 6% 4 5% 4 5% 6% 23%	2652
	21st to end	Night	20% 21% 6% 7% 6% 3 4 4 4 5% 20%	2660
December	1st to 10th	Night	22% 23% 10% 9% 6% 4 3 3 3 4 13%	2660
	11th to 20th	Night	17% 23% 10% 12% 6% 3 3 3 3 4 15%	3285
	21st to end	Night	25% 24% 9% 10% 6% 3 4 4 4 4 9%	4884

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	77% 5% 7% 8% 3	1622
	11th to 20th	Day	79% 5% 8% 7% 2	1710
	21st to end	Day	80% 3 6% 8% 3	1791
February	1st to 10th	Day	78% 4 7% 8% 3	1620
	11th to 20th	Day	74% 4 6% 10% 5%	1582
	21st to end	Day	68% 4 9% 13% 5% 2	1445
March	1st to 10th	Day	62% 5% 12% 16% 5%	1980
	11th to 20th	Day	60% 5% 9% 16% 8% 2	1980
	21st to end	Day	62% 4 5% 17% 10% 1	2178
April	1st to 10th	Day	55% 5% 12% 17% 12% 1	2083
	11th to 20th	Day	52% 5% 9% 20% 11% 2	2197
	21st to end	Day	54% 4 10% 19% 10% 3	2340
May	1st to 10th	Day	54% 5% 10% 20% 10% 2	2340
	11th to 20th	Day	51% 6% 11% 18% 12% 2	2326
	21st to end	Day	41% 8% 16% 22% 12% 1	2725
June	1st to 10th	Day	45% 6% 13% 23% 12% 1	2520
	11th to 20th	Day	45% 7% 13% 22% 12% 1	2520
	21st to end	Day	42% 7% 12% 25% 14% 1	2520
July	1st to 10th	Day	39% 7% 12% 25% 16% 2	2450
	11th to 20th	Day	41% 7% 13% 24% 14% 1	2297
	21st to end	Day	42% 9% 14% 21% 13% 2	2405
August	1st to 10th	Day	41% 7% 14% 27% 10%	2314
	11th to 20th	Day	47% 9% 14% 22% 7% 2	2303
	21st to end	Day	43% 9% 11% 22% 14% 2	2476
September	1st to 10th	Day	46% 7% 12% 20% 13% 2	2160
	11th to 20th	Day	41% 8% 11% 25% 14% 1	1995
	21st to end	Day	49% 6% 11% 22% 11% 1	1976
October	1st to 10th	Day	49% 7% 14% 17% 12% 2	1800
	11th to 20th	Day	58% 5% 13% 16% 7% 1	1800
	21st to end	Day	65% 5% 10% 12% 7% 1	1980
November	1st to 10th	Day	53% 6% 10% 17% 11% 2	1740
	11th to 20th	Day	63% 5% 10% 14% 6% 2	1525
	21st to end	Day	72% 5% 9% 10% 4 1	1520
December	1st to 10th	Day	78% 5% 6% 9% 2	1520
	11th to 20th	Day	76% 4 7% 11% 2	1906
	21st to end	Day	82% 3 8% 7% 1	2772
January	1st to 10th	Night	77% 4 7% 8% 4	2540
	11th to 20th	Night	77% 4 8% 8% 2	2470
	21st to end	Night	77% 4 7% 9% 2 1	2599
February	1st to 10th	Night	75% 5% 8% 10% 3	2340
	11th to 20th	Night	73% 3 7% 12% 4	2275
	21st to end	Night	69% 3 9% 12% 7% 1	1807
March	1st to 10th	Night	62% 5% 8% 17% 7%	1980
	11th to 20th	Night	57% 4 10% 19% 9% 2	1980
	21st to end	Night	61% 4 5% 14% 14% 2	2178
April	1st to 10th	Night	56% 5% 11% 16% 11% 1	1876
	11th to 20th	Night	51% 6% 10% 18% 13% 3	1727
	21st to end	Night	50% 5% 11% 19% 10% 3	1620
May	1st to 10th	Night	51% 5% 9% 20% 12% 3	1620
	11th to 20th	Night	47% 7% 10% 18% 14% 3	1568
	21st to end	Night	36% 7% 16% 26% 13% 2	1551
June	1st to 10th	Night	39% 8% 12% 24% 15% 2	1440
	11th to 20th	Night	40% 7% 12% 23% 16% 2	1440
	21st to end	Night	39% 8% 12% 23% 16% 1	1440
July	1st to 10th	Night	36% 7% 11% 24% 20% 3	1400
	11th to 20th	Night	35% 7% 11% 26% 18% 3	1536
	21st to end	Night	35% 10% 14% 23% 18% 1	1672
August	1st to 10th	Night	34% 7% 13% 31% 14% 2	1600
	11th to 20th	Night	38% 9% 14% 26% 11% 1	1598
	21st to end	Night	35% 9% 14% 25% 15% 3	1860
September	1st to 10th	Night	38% 7% 13% 25% 14% 3	1800
	11th to 20th	Night	36% 6% 12% 26% 19% 1	1962
	21st to end	Night	46% 6% 13% 20% 14% 1	1984
October	1st to 10th	Night	43% 7% 12% 21% 16% 1	2160
	11th to 20th	Night	54% 7% 13% 15% 11% 1	2160
	21st to end	Night	61% 5% 10% 15% 8% 1	2376
November	1st to 10th	Night	50% 6% 12% 20% 10% 2	2338
	11th to 20th	Night	63% 5% 9% 14% 7% 3	2652
	21st to end	Night	67% 5% 10% 13% 3 1	2660
December	1st to 10th	Night	77% 4 8% 8% 3	2660
	11th to 20th	Night	75% 4 10% 9% 2	3288
	21st to end	Night	81% 4 8% 6% 1	4884

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	32% 28% 28% 9% 3	229
	11th to 20th	Day	34% 32% 25% 6% 3 1	270
	21st to end	Day	31% 32% 30% 5% 2	267
February	1st to 10th	Day	40% 24% 33% 3	240
	11th to 20th	Day	41% 26% 28% 5%	240
	21st to end	Day	43% 27% 25% 5%	208
March	1st to 10th	Day	32% 24% 31% 10% 2	420
	11th to 20th	Day	32% 20% 33% 10% 3 1	420
	21st to end	Day	37% 21% 26% 14% 3	462
April	1st to 10th	Day	28% 25% 29% 13% 5% 1	420
	11th to 20th	Day	31% 21% 25% 13% 8% 2	412
	21st to end	Day	30% 20% 30% 13% 5% 1	420
May	1st to 10th	Day	30% 18% 28% 17% 5% 2	423
	11th to 20th	Day	25% 17% 27% 20% 9% 1	447
	21st to end	Day	16% 11% 32% 23% 13% 4	485
June	1st to 10th	Day	18% 14% 32% 25% 7% 3	450
	11th to 20th	Day	23% 12% 27% 24% 13% 2	450
	21st to end	Day	17% 13% 32% 26% 10% 2	450
July	1st to 10th	Day	11% 9% 25% 29% 19% 7%	445
	11th to 20th	Day	16% 8% 29% 27% 15% 5%	444
	21st to end	Day	12% 9% 35% 26% 14% 4	488
August	1st to 10th	Day	13% 7% 27% 27% 18% 7%	446
	11th to 20th	Day	17% 5% 26% 30% 13% 8%	439
	21st to end	Day	14% 8% 28% 28% 16% 5%	495
September	1st to 10th	Day	20% 8% 26% 24% 16% 5%	450
	11th to 20th	Day	12% 6% 27% 29% 19% 7%	450
	21st to end	Day	22% 15% 28% 19% 12% 5%	450
October	1st to 10th	Day	21% 16% 26% 20% 14% 3	450
	11th to 20th	Day	28% 18% 34% 14% 5% 1	450
	21st to end	Day	37% 18% 30% 12% 2	495
November	1st to 10th	Day	28% 20% 28% 16% 5% 1	338
	11th to 20th	Day	37% 26% 25% 9% 4	183
	21st to end	Day	33% 24% 34% 8%	180
December	1st to 10th	Day	39% 28% 25% 3 4 1	180
	11th to 20th	Day	39% 24% 28% 7% 2	180
	21st to end	Day	36% 25% 33% 6% 1	198
January	1st to 10th	Night	34% 25% 29% 9% 3	760
	11th to 20th	Night	37% 21% 27% 13% 1	760
	21st to end	Night	25% 27% 32% 14% 2	800
February	1st to 10th	Night	36% 22% 30% 13% 1	720
	11th to 20th	Night	30% 25% 31% 12% 2	703
	21st to end	Night	32% 23% 31% 12% 2	592
March	1st to 10th	Night	27% 22% 33% 14% 4	720
	11th to 20th	Night	29% 18% 31% 18% 4	720
	21st to end	Night	28% 17% 31% 19% 4 1	792
April	1st to 10th	Night	28% 17% 32% 19% 5%	720
	11th to 20th	Night	27% 13% 31% 20% 7% 1	663
	21st to end	Night	22% 13% 34% 25% 5% 1	540
May	1st to 10th	Night	25% 10% 29% 27% 9% 1	540
	11th to 20th	Night	19% 9% 32% 31% 8% 1	506
	21st to end	Night	13% 7% 29% 29% 19% 2	389
June	1st to 10th	Night	13% 5% 34% 33% 14%	360
	11th to 20th	Night	13% 9% 26% 40% 11%	360
	21st to end	Night	15% 6% 29% 34% 15% 1	360
July	1st to 10th	Night	10% 4 20% 41% 20% 5%	350
	11th to 20th	Night	12% 4 22% 34% 25% 3	492
	21st to end	Night	12% 3 20% 39% 25% 1	591
August	1st to 10th	Night	11% 4 18% 39% 26% 2	537
	11th to 20th	Night	13% 4 18% 34% 28% 4	534
	21st to end	Night	15% 5% 22% 29% 25% 3	672
September	1st to 10th	Night	18% 8% 18% 31% 22% 3	720
	11th to 20th	Night	15% 6% 20% 32% 23% 4	720
	21st to end	Night	21% 11% 21% 27% 15% 4	720
October	1st to 10th	Night	21% 8% 28% 25% 16% 3	720
	11th to 20th	Night	28% 15% 28% 21% 7% 1	720
	21st to end	Night	36% 14% 29% 17% 2	792
November	1st to 10th	Night	25% 15% 28% 24% 8%	740
	11th to 20th	Night	32% 21% 28% 13% 5% 1	760
	21st to end	Night	33% 20% 29% 13% 4	760
December	1st to 10th	Night	38% 22% 27% 10% 3	760
	11th to 20th	Night	31% 29% 26% 12% 2	760
	21st to end	Night	36% 21% 30% 12% 1	836

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