Grant O. Gale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grant O. Gale Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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	13% 8% 3 6% 6% 3 5% 5% 5% 7% 38%	74736
	Night	12% 8% 3 6% 5% 3 4 4 5% 7% 42%	75092
Transparency	Day	45% 5% 13% 23% 13% 2	74445
	Night	42% 4 12% 25% 15% 2	74903
Seeing	Day	17% 9% 30% 27% 14% 3	14677
	Night	16% 9% 33% 30% 11% 1	24675

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	9% 11% 6% 10% 9% 6% 5% 5% 4 7% 29%	1517
	11th to 20th	Day	11% 14% 6% 11% 8% 5% 4 5% 4 6% 26%	1603
	21st to end	Day	13% 13% 6% 10% 8% 4 5% 5% 5% 6% 26%	1782
February	1st to 10th	Day	12% 13% 5% 10% 7% 4 3 4 5% 6% 30%	1620
	11th to 20th	Day	13% 12% 4 6% 6% 3 5% 6% 5% 8% 33%	1574
	21st to end	Day	14% 11% 5% 9% 7% 3 4 4 3 6% 33%	1456
March	1st to 10th	Day	15% 10% 4 8% 6% 3 5% 3 5% 8% 34%	1943
	11th to 20th	Day	14% 9% 4 8% 7% 3 4 4 5% 6% 34%	1980
	21st to end	Day	23% 12% 4 7% 6% 3 4 4 5% 6% 28%	2204
April	1st to 10th	Day	18% 11% 4 6% 5% 3 4 4 5% 7% 33%	2160
	11th to 20th	Day	18% 9% 3 7% 6% 3 4 4 5% 7% 35%	2134
	21st to end	Day	22% 8% 3 5% 6% 3 4 5% 5% 6% 33%	2230
May	1st to 10th	Day	18% 7% 3 8% 6% 3 5% 5% 6% 7% 32%	2353
	11th to 20th	Day	18% 8% 2 6% 5% 3 6% 4 5% 6% 36%	2479
	21st to end	Day	16% 7% 3 7% 6% 3 4 5% 6% 7% 36%	2725
June	1st to 10th	Day	10% 6% 2 5% 5% 3 5% 5% 6% 9% 44%	2520
	11th to 20th	Day	9% 4 2 5% 5% 3 5% 5% 5% 8% 47%	2520
	21st to end	Day	10% 6% 3 5% 6% 3 6% 6% 6% 10% 38%	2520
July	1st to 10th	Day	8% 4 2 5% 5% 3 5% 4 7% 9% 49%	2520
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 5% 6% 9% 47%	2520
	21st to end	Day	6% 4 2 5% 5% 3 6% 6% 7% 10% 45%	2591
August	1st to 10th	Day	8% 4 2 5% 6% 4 7% 7% 7% 10% 41%	2328
	11th to 20th	Day	10% 5% 2 5% 6% 3 5% 5% 6% 9% 45%	2301
	21st to end	Day	10% 5% 2 5% 5% 3 4 3 4 8% 51%	2384
September	1st to 10th	Day	13% 5% 2 4 4 2 3 5% 5% 7% 51%	2160
	11th to 20th	Day	10% 4 2 4 4 3 4 5% 5% 7% 51%	2160
	21st to end	Day	11% 5% 2 5% 4 2 5% 4 5% 8% 49%	1877
October	1st to 10th	Day	16% 6% 2 5% 5% 3 4 4 5% 5% 46%	1800
	11th to 20th	Day	16% 8% 3 6% 6% 3 4 4 3 6% 43%	1800
	21st to end	Day	16% 9% 4 6% 4 3 4 3 4 6% 41%	1980
November	1st to 10th	Day	12% 6% 2 5% 4 3 5% 5% 6% 7% 46%	1707
	11th to 20th	Day	14% 9% 5% 7% 5% 3 3 4 5% 8% 38%	1547
	21st to end	Day	16% 10% 4 5% 5% 2 3 4 4 8% 38%	1520
December	1st to 10th	Day	13% 13% 4 9% 6% 3 4 4 4 7% 33%	1520
	11th to 20th	Day	12% 10% 5% 9% 7% 4 4 4 5% 7% 34%	1918
	21st to end	Day	22% 17% 8% 11% 6% 2 4 3 4 5% 17%	2783
January	1st to 10th	Night	9% 11% 5% 6% 6% 4 5% 5% 5% 7% 39%	2648
	11th to 20th	Night	11% 14% 6% 11% 7% 4 5% 4 4 6% 28%	2475
	21st to end	Night	13% 12% 4 9% 6% 4 5% 4 5% 7% 31%	2574
February	1st to 10th	Night	11% 13% 5% 8% 7% 3 5% 4 4 5% 33%	2340
	11th to 20th	Night	12% 8% 4 7% 5% 3 5% 4 5% 7% 39%	2273
	21st to end	Night	14% 10% 4 7% 6% 3 4 3 4 5% 40%	1798
March	1st to 10th	Night	15% 9% 4 5% 4 2 4 3 4 6% 43%	2034
	11th to 20th	Night	13% 8% 4 7% 5% 4 4 5% 5% 6% 39%	1980
	21st to end	Night	17% 12% 5% 8% 6% 3 5% 5% 5% 5% 28%	2152
April	1st to 10th	Night	15% 9% 4 7% 4 2 3 4 5% 7% 41%	1800
	11th to 20th	Night	14% 9% 3 6% 6% 3 4 4 4 6% 40%	1775
	21st to end	Night	18% 7% 2 5% 5% 2 4 4 5% 7% 41%	1728
May	1st to 10th	Night	16% 7% 2 4 5% 3 5% 5% 7% 7% 40%	1604
	11th to 20th	Night	16% 6% 3 6% 5% 4 5% 4 5% 6% 41%	1416
	21st to end	Night	14% 5% 2 5% 6% 3 5% 4 6% 7% 44%	1551
June	1st to 10th	Night	7% 5% 2 4 5% 4 4 5% 6% 8% 51%	1440
	11th to 20th	Night	6% 4 3 3 4 2 5% 5% 7% 11% 49%	1440
	21st to end	Night	10% 4 2 4 4 3 5% 5% 5% 9% 48%	1440
July	1st to 10th	Night	5% 3 1 3 4 3 5% 5% 7% 10% 54%	1440
	11th to 20th	Night	4 2 1 3 3 2 4 4 6% 9% 61%	1440
	21st to end	Night	5% 3 1 5% 4 3 5% 5% 6% 9% 55%	1723
August	1st to 10th	Night	6% 4 2 5% 5% 3 4 5% 7% 9% 50%	1611
	11th to 20th	Night	7% 3 2 4 5% 2 4 4 5% 7% 56%	1600
	21st to end	Night	9% 3 2 3 5% 2 4 3 5% 7% 57%	1970
September	1st to 10th	Night	10% 5% 2 4 4 2 4 3 4 6% 55%	1800
	11th to 20th	Night	11% 5% 1 4 3 2 3 4 4 7% 55%	1800
	21st to end	Night	9% 4 2 3 4 2 4 3 4 7% 57%	2060
October	1st to 10th	Night	13% 6% 2 5% 3 2 3 4 4 6% 52%	2160
	11th to 20th	Night	15% 7% 3 5% 4 2 4 4 5% 6% 48%	2160
	21st to end	Night	16% 8% 3 6% 5% 3 3 4 4 6% 43%	2376
November	1st to 10th	Night	11% 5% 2 4 4 2 3 4 4 7% 54%	2370
	11th to 20th	Night	13% 8% 3 6% 6% 3 4 4 4 6% 42%	2631
	21st to end	Night	17% 8% 3 6% 4 3 4 4 4 6% 41%	2660
December	1st to 10th	Night	11% 12% 5% 8% 6% 3 4 5% 4 7% 36%	2660
	11th to 20th	Night	12% 10% 4 9% 6% 3 4 4 4 6% 37%	3279
	21st to end	Night	19% 15% 7% 11% 7% 3 4 4 4 4 21%	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	57% 3 11% 16% 11% 1	1518
	11th to 20th	Day	61% 3 12% 17% 6%	1603
	21st to end	Day	59% 4 13% 15% 8% 1	1782
February	1st to 10th	Day	56% 3 14% 19% 8% 1	1620
	11th to 20th	Day	49% 5% 12% 22% 10% 2	1583
	21st to end	Day	54% 3 12% 19% 9% 2	1456
March	1st to 10th	Day	50% 5% 14% 21% 9% 1	1943
	11th to 20th	Day	51% 5% 12% 21% 10% 1	1980
	21st to end	Day	58% 4 13% 17% 7% 1	2204
April	1st to 10th	Day	51% 4 11% 24% 10%	2160
	11th to 20th	Day	50% 5% 11% 22% 11% 1	2134
	21st to end	Day	51% 5% 10% 21% 12% 1	2230
May	1st to 10th	Day	52% 5% 13% 20% 9% 1	2353
	11th to 20th	Day	48% 5% 13% 23% 11% 1	2483
	21st to end	Day	46% 6% 13% 21% 12% 2	2725
June	1st to 10th	Day	36% 6% 16% 25% 15% 2	2520
	11th to 20th	Day	35% 5% 13% 23% 21% 3	2520
	21st to end	Day	39% 7% 15% 23% 14% 2	2520
July	1st to 10th	Day	31% 6% 14% 32% 14% 3	2451
	11th to 20th	Day	32% 6% 15% 27% 16% 3	2445
	21st to end	Day	31% 6% 18% 26% 17% 2	2447
August	1st to 10th	Day	35% 8% 17% 27% 13% 1	2314
	11th to 20th	Day	34% 6% 13% 29% 16% 2	2301
	21st to end	Day	34% 4 12% 27% 20% 3	2384
September	1st to 10th	Day	33% 5% 13% 24% 22% 4	2160
	11th to 20th	Day	32% 4 12% 32% 18% 1	2160
	21st to end	Day	34% 3 10% 34% 17% 1	1877
October	1st to 10th	Day	40% 4 10% 26% 18% 2	1800
	11th to 20th	Day	45% 3 8% 23% 19% 2	1800
	21st to end	Day	45% 3 12% 24% 14% 2	1980
November	1st to 10th	Day	37% 5% 14% 25% 17% 2	1707
	11th to 20th	Day	45% 4 13% 22% 15% 1	1547
	21st to end	Day	46% 4 14% 25% 10% 1	1520
December	1st to 10th	Day	52% 3 14% 21% 8% 2	1520
	11th to 20th	Day	51% 4 15% 20% 9% 1	1915
	21st to end	Day	70% 4 9% 12% 4 1	2783
January	1st to 10th	Night	46% 3 13% 22% 15% 1	2648
	11th to 20th	Night	58% 3 15% 16% 8%	2475
	21st to end	Night	53% 4 13% 20% 10% 1	2574
February	1st to 10th	Night	54% 3 11% 21% 11% 1	2340
	11th to 20th	Night	45% 4 11% 26% 12% 2	2274
	21st to end	Night	48% 3 11% 20% 15% 3	1798
March	1st to 10th	Night	44% 4 12% 28% 12% 1	2034
	11th to 20th	Night	45% 5% 11% 23% 14% 2	1980
	21st to end	Night	57% 5% 13% 16% 8% 1	2152
April	1st to 10th	Night	44% 4 10% 26% 14% 2	1800
	11th to 20th	Night	47% 3 10% 22% 16% 3	1775
	21st to end	Night	44% 4 13% 24% 13% 3	1728
May	1st to 10th	Night	41% 6% 15% 22% 14% 1	1604
	11th to 20th	Night	44% 4 13% 25% 12% 1	1417
	21st to end	Night	41% 6% 12% 23% 15% 3	1552
June	1st to 10th	Night	30% 6% 15% 29% 16% 3	1440
	11th to 20th	Night	28% 6% 16% 23% 23% 5%	1440
	21st to end	Night	32% 7% 13% 28% 17% 4	1440
July	1st to 10th	Night	23% 6% 17% 34% 17% 4	1400
	11th to 20th	Night	19% 6% 18% 35% 18% 4	1392
	21st to end	Night	25% 6% 16% 31% 20% 3	1627
August	1st to 10th	Night	29% 5% 18% 32% 13% 3	1601
	11th to 20th	Night	27% 5% 13% 34% 18% 2	1600
	21st to end	Night	28% 4 12% 25% 24% 7%	1970
September	1st to 10th	Night	31% 4 9% 26% 26% 3	1800
	11th to 20th	Night	31% 3 10% 34% 21% 2	1800
	21st to end	Night	29% 3 10% 32% 23% 3	2060
October	1st to 10th	Night	35% 3 10% 27% 23% 3	2160
	11th to 20th	Night	38% 3 11% 24% 18% 5%	2160
	21st to end	Night	43% 4 11% 22% 17% 2	2376
November	1st to 10th	Night	32% 4 14% 28% 19% 4	2370
	11th to 20th	Night	44% 3 12% 26% 14% 2	2631
	21st to end	Night	45% 3 11% 27% 13% 1	2660
December	1st to 10th	Night	48% 4 12% 25% 9% 1	2660
	11th to 20th	Night	48% 4 12% 26% 8% 1	3281
	21st to end	Night	67% 5% 7% 14% 6%	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	24% 12% 34% 20% 9% 1	270
	11th to 20th	Day	25% 11% 44% 16% 4	255
	21st to end	Day	21% 13% 38% 26% 2 1	264
February	1st to 10th	Day	24% 16% 41% 13% 5%	240
	11th to 20th	Day	25% 14% 36% 18% 6% 1	240
	21st to end	Day	28% 12% 45% 11% 3 1	198
March	1st to 10th	Day	17% 23% 39% 16% 5%	240
	11th to 20th	Day	27% 13% 30% 21% 8%	240
	21st to end	Day	38% 12% 27% 19% 3 1	290
April	1st to 10th	Day	27% 13% 37% 18% 5%	420
	11th to 20th	Day	23% 15% 30% 23% 8% 1	412
	21st to end	Day	25% 12% 36% 17% 8% 2	490
May	1st to 10th	Day	23% 11% 35% 21% 7% 2	603
	11th to 20th	Day	23% 10% 29% 24% 10% 2	625
	21st to end	Day	18% 8% 28% 27% 15% 5%	679
June	1st to 10th	Day	10% 4 27% 31% 23% 6%	630
	11th to 20th	Day	11% 5% 29% 34% 18% 3	630
	21st to end	Day	12% 8% 29% 31% 16% 4	630
July	1st to 10th	Day	9% 2 18% 36% 22% 14%	621
	11th to 20th	Day	9% 4 26% 35% 21% 6%	618
	21st to end	Day	6% 2 23% 40% 24% 4	684
August	1st to 10th	Day	8% 3 21% 38% 25% 5%	626
	11th to 20th	Day	10% 4 20% 34% 27% 5%	615
	21st to end	Day	11% 2 20% 38% 23% 7%	503
September	1st to 10th	Day	19% 6% 19% 30% 21% 6%	450
	11th to 20th	Day	10% 2 27% 34% 22% 6%	450
	21st to end	Day	13% 14% 29% 25% 16% 3	270
October	1st to 10th	Day	21% 7% 27% 30% 11% 4	270
	11th to 20th	Day	20% 22% 34% 16% 8%	270
	21st to end	Day	24% 13% 33% 20% 7% 2	297
November	1st to 10th	Day	17% 11% 41% 21% 7% 2	270
	11th to 20th	Day	21% 17% 36% 17% 7% 1	270
	21st to end	Day	24% 13% 40% 19% 4	270
December	1st to 10th	Day	14% 13% 44% 20% 8% 1	270
	11th to 20th	Day	18% 15% 35% 22% 8% 2	270
	21st to end	Day	29% 14% 40% 13% 3	297
January	1st to 10th	Night	14% 14% 41% 24% 7%	950
	11th to 20th	Night	18% 12% 38% 25% 6% 1	930
	21st to end	Night	19% 12% 36% 27% 6%	990
February	1st to 10th	Night	19% 11% 38% 26% 5%	900
	11th to 20th	Night	18% 13% 39% 26% 5%	879
	21st to end	Night	20% 14% 37% 24% 4 1	614
March	1st to 10th	Night	19% 13% 39% 22% 8% 1	594
	11th to 20th	Night	19% 9% 34% 27% 10%	540
	21st to end	Night	23% 10% 37% 23% 5% 1	594
April	1st to 10th	Night	18% 15% 39% 24% 3	540
	11th to 20th	Night	19% 12% 37% 25% 6%	533
	21st to end	Night	21% 8% 37% 24% 8% 1	540
May	1st to 10th	Night	21% 6% 34% 31% 7% 2	540
	11th to 20th	Night	19% 6% 33% 32% 10%	531
	21st to end	Night	16% 4 30% 35% 15%	584
June	1st to 10th	Night	10% 4 23% 44% 18% 1	540
	11th to 20th	Night	8% 3 28% 45% 14% 1	540
	21st to end	Night	12% 3 29% 41% 15% 1	540
July	1st to 10th	Night	7% 1 13% 46% 30% 4	525
	11th to 20th	Night	6% 1 24% 41% 25% 3	522
	21st to end	Night	6% 2 17% 50% 23% 2	591
August	1st to 10th	Night	7% 2 16% 47% 26% 3	537
	11th to 20th	Night	8% 2 16% 42% 29% 3	534
	21st to end	Night	11% 1 18% 41% 27% 2	594
September	1st to 10th	Night	13% 4 19% 40% 21% 4	540
	11th to 20th	Night	14% 2 21% 39% 21% 3	540
	21st to end	Night	11% 7% 30% 35% 16% 1	625
October	1st to 10th	Night	15% 5% 33% 31% 14% 1	720
	11th to 20th	Night	18% 10% 34% 30% 7%	720
	21st to end	Night	21% 10% 35% 26% 8%	792
November	1st to 10th	Night	15% 10% 38% 28% 8% 1	740
	11th to 20th	Night	17% 13% 41% 22% 7% 1	921
	21st to end	Night	21% 11% 37% 22% 8%	950
December	1st to 10th	Night	17% 13% 38% 27% 5% 1	950
	11th to 20th	Night	21% 10% 36% 27% 6% 1	950
	21st to end	Night	25% 12% 37% 22% 5%	1045

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