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 2024-12-09.

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 8% 5% 4 4 4 5% 7% 38%	101730
	Night	12% 9% 3 8% 4 4 4 3 5% 6% 41%	98132
Transparency	Day	44% 4 12% 24% 14% 2	105367
	Night	42% 4 12% 24% 16% 2	102635
Seeing	Day	17% 9% 30% 26% 14% 3	15952
	Night	16% 9% 33% 30% 11% 1	25843

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	8% 11% 6% 23% 7% 7% 5% 4 4 5% 21%	2527
	11th to 20th	Day	9% 14% 5% 11% 8% 7% 6% 4 6% 6% 24%	2736
	21st to end	Day	14% 16% 7% 17% 6% 5% 4 3 4 5% 19%	3135
February	1st to 10th	Day	11% 13% 4 10% 6% 5% 4 4 6% 7% 30%	1710
	11th to 20th	Day	12% 12% 3 7% 5% 5% 4 5% 6% 7% 35%	1664
	21st to end	Day	13% 11% 4 11% 5% 4 4 3 4 6% 35%	1544
March	1st to 10th	Day	18% 10% 3 8% 4 3 3 2 4 5% 40%	3374
	11th to 20th	Day	10% 7% 3 8% 5% 4 4 3 5% 6% 45%	3480
	21st to end	Day	23% 15% 5% 9% 5% 4 3 3 4 6% 22%	3867
April	1st to 10th	Day	18% 11% 4 7% 4 4 4 3 5% 7% 33%	2280
	11th to 20th	Day	17% 9% 3 8% 4 4 4 4 5% 6% 35%	2254
	21st to end	Day	22% 8% 3 7% 4 4 5% 4 6% 6% 33%	2354
May	1st to 10th	Day	19% 7% 2 7% 5% 4 4 4 6% 7% 33%	4109
	11th to 20th	Day	16% 8% 2 6% 4 4 5% 4 5% 7% 40%	4399
	21st to end	Day	14% 6% 2 6% 4 4 4 3 6% 7% 44%	4824
June	1st to 10th	Day	10% 6% 2 5% 5% 4 5% 4 7% 9% 44%	2660
	11th to 20th	Day	9% 4 2 5% 5% 5% 5% 4 6% 9% 46%	2660
	21st to end	Day	10% 6% 2 6% 6% 4 6% 5% 7% 10% 39%	2660
July	1st to 10th	Day	10% 7% 3 8% 5% 5% 5% 4 7% 9% 35%	4440
	11th to 20th	Day	6% 5% 2 5% 3 4 4 4 6% 8% 54%	4440
	21st to end	Day	5% 4 2 6% 5% 5% 6% 5% 8% 11% 41%	4538
August	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 44%	4104
	11th to 20th	Day	11% 6% 2 7% 5% 4 5% 4 6% 9% 40%	4041
	21st to end	Day	8% 5% 2 6% 4 5% 5% 3 5% 9% 49%	4188
September	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 5% 7% 52%	2280
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 7% 50%	2280
	21st to end	Day	11% 5% 2 5% 4 4 4 3 5% 8% 49%	1981
October	1st to 10th	Day	15% 6% 2 6% 4 3 4 3 4 5% 48%	1900
	11th to 20th	Day	15% 8% 3 7% 4 4 4 2 3 5% 44%	1900
	21st to end	Day	15% 10% 3 6% 4 4 4 3 4 6% 42%	2090
November	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 4 6% 7% 45%	1798
	11th to 20th	Day	14% 10% 4 7% 5% 4 3 3 5% 7% 37%	1628
	21st to end	Day	15% 11% 4 7% 4 3 3 3 6% 7% 37%	1600
December	1st to 10th	Day	12% 13% 4 10% 5% 4 3 3 5% 6% 35%	1584
	11th to 20th	Day	12% 10% 5% 10% 6% 5% 4 3 5% 6% 33%	1918
	21st to end	Day	22% 17% 8% 12% 5% 3 3 3 4 5% 17%	2783
January	1st to 10th	Night	10% 12% 5% 19% 6% 5% 4 3 4 5% 26%	4428
	11th to 20th	Night	10% 17% 6% 11% 6% 4 6% 3 8% 6% 23%	4216
	21st to end	Night	15% 15% 6% 14% 5% 4 4 3 4 6% 22%	4532
February	1st to 10th	Night	10% 14% 4 9% 6% 5% 4 3 5% 6% 34%	2470
	11th to 20th	Night	12% 9% 3 7% 4 4 4 4 5% 7% 40%	2403
	21st to end	Night	13% 10% 3 8% 5% 4 3 2 4 5% 41%	1908
March	1st to 10th	Night	16% 9% 3 6% 3 3 3 3 4 5% 46%	3497
	11th to 20th	Night	10% 7% 3 5% 4 4 3 4 5% 6% 50%	3480
	21st to end	Night	20% 12% 5% 13% 5% 5% 4 4 4 5% 24%	3764
April	1st to 10th	Night	15% 9% 3 7% 4 3 4 3 5% 6% 41%	1900
	11th to 20th	Night	14% 9% 3 7% 5% 4 3 3 4 6% 41%	1875
	21st to end	Night	18% 7% 2 6% 4 3 4 3 5% 6% 41%	1823
May	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 6% 6% 38%	2793
	11th to 20th	Night	12% 7% 2 7% 5% 5% 3 3 5% 5% 46%	2506
	21st to end	Night	11% 6% 2 6% 4 4 4 4 5% 7% 47%	2743
June	1st to 10th	Night	8% 5% 2 5% 4 5% 4 4 6% 8% 50%	1520
	11th to 20th	Night	7% 5% 2 4 4 4 4 4 8% 11% 49%	1520
	21st to end	Night	11% 4 2 5% 3 5% 4 5% 5% 8% 47%	1520
July	1st to 10th	Night	9% 4 2 6% 4 4 5% 5% 8% 9% 43%	2530
	11th to 20th	Night	4 2 1 4 3 3 4 3 5% 8% 62%	2530
	21st to end	Night	6% 4 1 6% 4 4 5% 4 6% 9% 50%	3021
August	1st to 10th	Night	5% 4 2 5% 4 5% 4 3 7% 9% 54%	2836
	11th to 20th	Night	9% 4 2 5% 4 3 3 3 5% 7% 56%	2798
	21st to end	Night	9% 4 2 5% 4 3 4 3 5% 8% 53%	3466
September	1st to 10th	Night	9% 5% 2 5% 3 3 4 2 4 6% 57%	1900
	11th to 20th	Night	11% 5% 1 4 3 4 3 3 5% 7% 54%	1900
	21st to end	Night	9% 4 2 4 3 3 3 3 4 7% 57%	2175
October	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 53%	2280
	11th to 20th	Night	14% 6% 3 5% 3 4 3 3 4 6% 50%	2280
	21st to end	Night	16% 8% 3 6% 4 4 4 3 5% 6% 43%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 6% 53%	2498
	11th to 20th	Night	13% 9% 3 7% 5% 3 4 3 5% 6% 42%	2770
	21st to end	Night	16% 9% 3 7% 4 4 4 3 5% 6% 40%	2800
December	1st to 10th	Night	10% 12% 4 9% 5% 4 4 4 5% 6% 37%	2779
	11th to 20th	Night	12% 10% 4 10% 5% 4 4 3 4 6% 37%	3279
	21st to end	Night	19% 16% 7% 13% 6% 4 4 3 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	65% 5% 9% 13% 7% 1	2528
	11th to 20th	Day	61% 3 10% 19% 7%	2736
	21st to end	Day	69% 4 10% 12% 5%	3135
February	1st to 10th	Day	54% 4 13% 19% 9% 1	1710
	11th to 20th	Day	48% 5% 12% 23% 11% 2	1673
	21st to end	Day	52% 3 12% 20% 10% 3	1544
March	1st to 10th	Day	48% 3 10% 24% 14% 1	3374
	11th to 20th	Day	39% 5% 12% 26% 16% 2	3480
	21st to end	Day	63% 5% 13% 14% 4 1	3867
April	1st to 10th	Day	51% 4 11% 24% 9%	2280
	11th to 20th	Day	49% 5% 11% 21% 12% 1	2254
	21st to end	Day	51% 5% 10% 21% 12% 1	2354
May	1st to 10th	Day	48% 6% 13% 19% 13% 1	4147
	11th to 20th	Day	43% 6% 12% 27% 12% 1	4403
	21st to end	Day	40% 6% 10% 27% 16% 1	4837
June	1st to 10th	Day	36% 6% 16% 24% 15% 3	2660
	11th to 20th	Day	35% 5% 13% 23% 21% 3	2660
	21st to end	Day	39% 7% 15% 23% 14% 2	2660
July	1st to 10th	Day	45% 4 13% 28% 8% 2	4371
	11th to 20th	Day	28% 4 14% 33% 19% 2	4365
	21st to end	Day	35% 6% 19% 21% 18% 1	4395
August	1st to 10th	Day	34% 5% 17% 28% 15% 1	4090
	11th to 20th	Day	41% 4 13% 27% 14% 1	4080
	21st to end	Day	34% 3 16% 29% 16% 2	4187
September	1st to 10th	Day	32% 4 13% 24% 23% 4	2280
	11th to 20th	Day	33% 4 12% 31% 18% 1	2280
	21st to end	Day	34% 3 10% 34% 18% 1	1981
October	1st to 10th	Day	23% 2 6% 28% 39% 2	3160
	11th to 20th	Day	30% 2 8% 30% 25% 4	3140
	21st to end	Day	40% 2 9% 29% 15% 3	3428
November	1st to 10th	Day	38% 5% 15% 25% 16% 2	1798
	11th to 20th	Day	46% 4 13% 22% 15% 1	1628
	21st to end	Day	47% 4 14% 25% 10% 1	1600
December	1st to 10th	Day	51% 3 14% 23% 8% 1	1584
	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	60% 4 11% 16% 9% 1	4428
	11th to 20th	Night	59% 3 12% 14% 10% 1	4216
	21st to end	Night	64% 3 10% 15% 6%	4532
February	1st to 10th	Night	52% 3 11% 22% 11% 1	2470
	11th to 20th	Night	44% 4 11% 27% 12% 2	2404
	21st to end	Night	46% 4 11% 21% 15% 3	1908
March	1st to 10th	Night	42% 3 9% 28% 17% 2	3497
	11th to 20th	Night	34% 5% 11% 32% 15% 4	3480
	21st to end	Night	61% 7% 15% 10% 5% 1	3764
April	1st to 10th	Night	44% 4 10% 27% 13% 2	1900
	11th to 20th	Night	45% 3 10% 22% 17% 3	1875
	21st to end	Night	43% 4 12% 24% 14% 3	1823
May	1st to 10th	Night	45% 5% 10% 19% 19% 2	2820
	11th to 20th	Night	40% 5% 10% 29% 14% 2	2507
	21st to end	Night	36% 7% 10% 26% 18% 3	2751
June	1st to 10th	Night	31% 6% 15% 28% 16% 4	1520
	11th to 20th	Night	28% 6% 15% 23% 23% 5%	1520
	21st to end	Night	33% 6% 13% 27% 17% 4	1520
July	1st to 10th	Night	36% 4 14% 32% 13% 2	2490
	11th to 20th	Night	21% 4 18% 34% 20% 3	2482
	21st to end	Night	30% 5% 18% 26% 19% 2	2925
August	1st to 10th	Night	27% 4 17% 34% 16% 2	2826
	11th to 20th	Night	30% 3 16% 34% 15% 1	2825
	21st to end	Night	31% 3 14% 28% 18% 6%	3466
September	1st to 10th	Night	29% 4 9% 28% 27% 3	1900
	11th to 20th	Night	32% 2 10% 34% 20% 2	1900
	21st to end	Night	29% 3 10% 32% 23% 3	2175
October	1st to 10th	Night	23% 2 6% 26% 41% 3	3800
	11th to 20th	Night	27% 2 9% 25% 29% 8%	3779
	21st to end	Night	44% 2 9% 24% 17% 3	4120
November	1st to 10th	Night	33% 4 14% 27% 18% 4	2498
	11th to 20th	Night	44% 3 12% 26% 14% 2	2770
	21st to end	Night	46% 3 11% 27% 12% 1	2800
December	1st to 10th	Night	48% 4 12% 26% 9% 1	2779
	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	24% 11% 44% 17% 4	270
	21st to end	Day	21% 13% 37% 26% 2 1	297
February	1st to 10th	Day	23% 16% 39% 15% 6%	270
	11th to 20th	Day	23% 14% 36% 20% 6% 1	270
	21st to end	Day	26% 14% 44% 12% 4 1	225
March	1st to 10th	Day	19% 21% 39% 17% 5%	270
	11th to 20th	Day	26% 15% 31% 20% 7%	270
	21st to end	Day	36% 14% 28% 19% 3 1	325
April	1st to 10th	Day	27% 15% 36% 17% 5%	460
	11th to 20th	Day	25% 15% 30% 22% 7% 1	452
	21st to end	Day	26% 12% 36% 17% 7% 2	534
May	1st to 10th	Day	24% 11% 35% 21% 7% 2	653
	11th to 20th	Day	23% 10% 30% 25% 11% 2	675
	21st to end	Day	17% 8% 29% 28% 14% 4	734
June	1st to 10th	Day	10% 5% 28% 30% 22% 5%	680
	11th to 20th	Day	11% 6% 31% 32% 17% 3	680
	21st to end	Day	12% 8% 31% 31% 15% 4	680
July	1st to 10th	Day	9% 2 20% 35% 20% 13%	671
	11th to 20th	Day	8% 3 26% 34% 21% 6%	668
	21st to end	Day	6% 2 24% 39% 25% 5%	739
August	1st to 10th	Day	9% 3 22% 37% 25% 5%	676
	11th to 20th	Day	12% 4 21% 33% 25% 5%	665
	21st to end	Day	10% 2 20% 39% 23% 6%	547
September	1st to 10th	Day	17% 6% 19% 30% 22% 5%	490
	11th to 20th	Day	10% 2 26% 35% 21% 6%	490
	21st to end	Day	13% 14% 29% 26% 16% 3	300
October	1st to 10th	Day	19% 7% 29% 29% 12% 4	300
	11th to 20th	Day	19% 20% 35% 18% 8%	300
	21st to end	Day	24% 13% 34% 20% 7% 2	330
November	1st to 10th	Day	19% 11% 40% 20% 7% 2	300
	11th to 20th	Day	22% 18% 36% 17% 7% 1	300
	21st to end	Day	23% 14% 40% 19% 4	300
December	1st to 10th	Day	14% 13% 46% 19% 7% 1	294
	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	950
	21st to end	Night	20% 11% 36% 27% 6%	1045
February	1st to 10th	Night	18% 12% 39% 25% 5%	950
	11th to 20th	Night	17% 13% 39% 26% 5%	929
	21st to end	Night	20% 14% 38% 24% 3 1	652
March	1st to 10th	Night	19% 13% 40% 21% 7%	626
	11th to 20th	Night	19% 10% 35% 26% 9%	570
	21st to end	Night	24% 10% 38% 22% 5% 1	627
April	1st to 10th	Night	19% 16% 39% 23% 3	570
	11th to 20th	Night	19% 12% 37% 25% 6%	563
	21st to end	Night	21% 9% 37% 24% 8% 1	570
May	1st to 10th	Night	21% 6% 35% 31% 6% 2	570
	11th to 20th	Night	18% 6% 33% 32% 11%	561
	21st to end	Night	16% 4 30% 35% 15%	617
June	1st to 10th	Night	10% 5% 24% 43% 17% 1	570
	11th to 20th	Night	9% 3 29% 44% 14% 1	570
	21st to end	Night	13% 4 29% 40% 14% 1	570
July	1st to 10th	Night	7% 1 14% 46% 28% 4	555
	11th to 20th	Night	6% 1 23% 41% 26% 3	552
	21st to end	Night	6% 2 17% 50% 23% 3	624
August	1st to 10th	Night	7% 2 17% 47% 25% 3	567
	11th to 20th	Night	9% 2 16% 43% 27% 3	564
	21st to end	Night	11% 1 17% 42% 26% 2	627
September	1st to 10th	Night	12% 4 19% 41% 20% 4	570
	11th to 20th	Night	14% 2 21% 40% 21% 2	570
	21st to end	Night	11% 7% 30% 35% 16% 1	660
October	1st to 10th	Night	15% 6% 32% 32% 14% 1	760
	11th to 20th	Night	18% 10% 35% 30% 7%	760
	21st to end	Night	21% 11% 34% 26% 8%	836
November	1st to 10th	Night	17% 10% 37% 28% 8% 1	780
	11th to 20th	Night	18% 14% 40% 21% 6% 1	970
	21st to end	Night	21% 12% 38% 22% 8%	1000
December	1st to 10th	Night	17% 13% 38% 27% 5% 1	993
	11th to 20th	Night	21% 10% 36% 27% 6% 1	950
	21st to end	Night	25% 12% 37% 22% 5%	1045

Page generated at 2024-12-06 02:54:48. Version ac07

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