Gebbie Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gebbie Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 7% 2 6% 4 4 4 4 6% 7% 39%	131223
	Night	18% 7% 3 6% 4 4 4 3 5% 6% 41%	130957
Transparency	Day	44% 5% 11% 22% 15% 2	134742
	Night	44% 4 12% 22% 16% 3	135525
Seeing	Day	23% 15% 30% 21% 9% 3	16758
	Night	21% 11% 31% 28% 8% 1	25894

By Month

Percentage Cloud Cover NE (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	11% 9% 4 9% 5% 4 4 4 5% 5% 40%	4661
	11th to 20th	Day	14% 9% 3 7% 4 3 4 3 5% 5% 42%	4650
	21st to end	Day	15% 9% 3 6% 3 3 3 3 5% 6% 44%	5217
February	1st to 10th	Day	16% 8% 2 6% 6% 5% 5% 4 6% 6% 37%	1952
	11th to 20th	Day	13% 7% 3 7% 5% 4 4 4 6% 7% 40%	2048
	21st to end	Day	19% 10% 3 6% 4 3 4 3 4 5% 39%	1730
March	1st to 10th	Day	21% 9% 3 5% 4 4 3 3 4 5% 39%	4576
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 4 6% 7% 40%	4753
	21st to end	Day	21% 8% 2 6% 4 3 4 4 5% 6% 37%	6102
April	1st to 10th	Day	19% 8% 3 6% 5% 3 4 3 5% 6% 38%	2520
	11th to 20th	Day	17% 6% 3 7% 5% 4 4 3 5% 6% 40%	2446
	21st to end	Day	20% 6% 2 6% 4 4 4 3 6% 6% 40%	2526
May	1st to 10th	Day	31% 7% 2 6% 4 4 4 3 5% 8% 25%	5886
	11th to 20th	Day	20% 8% 3 7% 4 5% 4 3 5% 6% 34%	6173
	21st to end	Day	20% 7% 2 6% 4 4 4 4 6% 8% 34%	6949
June	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 5% 7% 9% 35%	2800
	11th to 20th	Day	16% 5% 2 7% 5% 4 5% 4 6% 9% 37%	2800
	21st to end	Day	14% 7% 3 6% 5% 4 5% 5% 7% 8% 38%	2800
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 9% 35%	6360
	11th to 20th	Day	10% 6% 2 6% 5% 5% 6% 5% 8% 11% 36%	6360
	21st to end	Day	15% 7% 2 6% 4 4 4 3 5% 7% 42%	6926
August	1st to 10th	Day	17% 6% 3 7% 5% 5% 5% 4 7% 8% 32%	4410
	11th to 20th	Day	12% 5% 2 7% 4 4 5% 3 6% 8% 42%	3913
	21st to end	Day	8% 5% 2 6% 4 5% 6% 5% 7% 9% 44%	4312
September	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 3 5% 6% 45%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 6% 52%	2260
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 7% 42%	2200
October	1st to 10th	Day	22% 6% 2 5% 3 3 4 3 5% 5% 43%	2170
	11th to 20th	Day	15% 6% 2 5% 4 3 5% 3 5% 8% 45%	1970
	21st to end	Day	21% 6% 2 5% 4 3 5% 3 4 5% 41%	1980
November	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 5% 48%	1851
	11th to 20th	Day	17% 6% 2 4 3 3 4 3 6% 6% 46%	1890
	21st to end	Day	19% 7% 2 6% 4 4 3 3 5% 5% 41%	1838
December	1st to 10th	Day	16% 7% 2 6% 5% 4 4 3 4 6% 45%	2680
	11th to 20th	Day	22% 6% 2 4 3 3 3 3 5% 6% 45%	3066
	21st to end	Day	19% 10% 3 8% 5% 3 3 2 5% 5% 36%	4048
January	1st to 10th	Night	14% 7% 3 8% 4 4 4 3 5% 6% 41%	8234
	11th to 20th	Night	15% 10% 3 8% 4 4 3 3 4 6% 40%	8235
	21st to end	Night	14% 9% 3 7% 4 4 5% 3 5% 6% 41%	8960
February	1st to 10th	Night	16% 9% 3 5% 4 4 4 4 5% 6% 40%	2667
	11th to 20th	Night	14% 8% 2 7% 5% 4 5% 4 5% 6% 40%	2458
	21st to end	Night	18% 8% 2 7% 5% 4 4 3 4 5% 40%	2076
March	1st to 10th	Night	21% 8% 3 6% 3 3 3 3 4 6% 39%	5484
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 7% 41%	5423
	21st to end	Night	22% 8% 2 6% 4 4 3 3 6% 5% 37%	5072
April	1st to 10th	Night	20% 7% 2 6% 4 5% 4 4 5% 6% 36%	2100
	11th to 20th	Night	20% 7% 2 6% 5% 4 4 4 4 6% 39%	2036
	21st to end	Night	21% 7% 2 6% 4 3 3 3 4 5% 41%	1871
May	1st to 10th	Night	32% 9% 2 6% 4 3 3 3 4 6% 27%	4054
	11th to 20th	Night	21% 8% 3 8% 4 4 5% 4 6% 7% 31%	3762
	21st to end	Night	22% 7% 3 7% 4 5% 5% 4 5% 7% 33%	3926
June	1st to 10th	Night	18% 5% 2 6% 4 5% 4 4 5% 7% 42%	1600
	11th to 20th	Night	17% 6% 2 6% 5% 4 4 4 6% 8% 39%	1600
	21st to end	Night	16% 5% 2 5% 4 4 4 3 6% 8% 44%	1600
July	1st to 10th	Night	14% 6% 3 6% 4 4 5% 4 7% 9% 39%	3600
	11th to 20th	Night	15% 7% 2 6% 5% 4 5% 3 7% 9% 37%	3600
	21st to end	Night	12% 7% 2 5% 4 4 4 4 5% 6% 47%	3926
August	1st to 10th	Night	21% 6% 3 8% 4 4 4 3 6% 7% 34%	2707
	11th to 20th	Night	14% 7% 3 6% 4 4 5% 3 5% 7% 41%	3130
	21st to end	Night	10% 5% 1 4 3 3 4 3 5% 8% 53%	3586
September	1st to 10th	Night	15% 5% 3 6% 3 4 4 3 5% 6% 47%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 3 2 4 5% 55%	2140
	21st to end	Night	18% 6% 2 5% 3 3 3 3 5% 6% 45%	2200
October	1st to 10th	Night	21% 5% 2 5% 4 4 3 3 4 6% 44%	2228
	11th to 20th	Night	16% 5% 2 6% 4 3 3 3 5% 6% 48%	2430
	21st to end	Night	23% 6% 2 5% 3 4 4 3 5% 5% 41%	2860
November	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 6% 49%	2667
	11th to 20th	Night	18% 6% 2 4 3 3 4 3 5% 6% 45%	2730
	21st to end	Night	16% 6% 2 5% 3 4 4 3 4 6% 45%	2777
December	1st to 10th	Night	18% 7% 2 6% 3 3 3 3 4 5% 45%	4720
	11th to 20th	Night	24% 8% 2 4 3 3 3 3 4 5% 42%	5348
	21st to end	Night	19% 10% 3 7% 4 4 3 3 4 4 40%	7150

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	46% 4 10% 26% 13% 2	4662
	11th to 20th	Day	44% 4 9% 21% 16% 7%	4680
	21st to end	Day	42% 3 12% 24% 19% 1	5217
February	1st to 10th	Day	46% 5% 13% 20% 14% 2	1952
	11th to 20th	Day	44% 5% 10% 23% 15% 3	2057
	21st to end	Day	47% 4 10% 21% 16% 2	1730
March	1st to 10th	Day	49% 3 11% 17% 12% 9%	4579
	11th to 20th	Day	42% 5% 14% 28% 9% 2	4753
	21st to end	Day	48% 3 6% 16% 20% 6%	6102
April	1st to 10th	Day	47% 5% 9% 22% 14% 4	2520
	11th to 20th	Day	43% 5% 11% 21% 18% 2	2446
	21st to end	Day	45% 4 9% 20% 17% 4	2526
May	1st to 10th	Day	57% 5% 14% 14% 9% 1	5897
	11th to 20th	Day	50% 6% 12% 19% 12% 1	6177
	21st to end	Day	46% 7% 11% 22% 13% 2	6949
June	1st to 10th	Day	45% 6% 12% 23% 13% 1	2800
	11th to 20th	Day	43% 6% 14% 22% 14% 2	2800
	21st to end	Day	42% 6% 12% 22% 16% 1	2800
July	1st to 10th	Day	44% 8% 11% 20% 15% 1	6290
	11th to 20th	Day	39% 7% 16% 24% 11% 2	6285
	21st to end	Day	41% 5% 10% 25% 18% 1	6728
August	1st to 10th	Day	48% 7% 14% 20% 9% 1	4396
	11th to 20th	Day	39% 6% 13% 30% 11% 1	3951
	21st to end	Day	35% 7% 12% 28% 18% 1	4312
September	1st to 10th	Day	40% 5% 9% 25% 19% 2	2400
	11th to 20th	Day	34% 4 9% 25% 25% 3	2260
	21st to end	Day	42% 5% 9% 23% 18% 3	2200
October	1st to 10th	Day	39% 4 10% 19% 25% 3	3534
	11th to 20th	Day	30% 3 9% 27% 28% 3	3186
	21st to end	Day	33% 4 13% 26% 20% 4	3180
November	1st to 10th	Day	39% 4 13% 24% 18% 2	1851
	11th to 20th	Day	38% 4 11% 23% 20% 3	1890
	21st to end	Day	45% 4 12% 27% 11% 2	1838
December	1st to 10th	Day	41% 4 15% 27% 11% 1	2680
	11th to 20th	Day	41% 4 15% 21% 17% 3	3066
	21st to end	Day	50% 4 8% 20% 14% 4	4048
January	1st to 10th	Night	44% 3 11% 25% 15% 2	8234
	11th to 20th	Night	47% 4 11% 19% 14% 6%	8280
	21st to end	Night	45% 3 9% 20% 19% 4	8960
February	1st to 10th	Night	45% 4 13% 23% 13% 2	2667
	11th to 20th	Night	45% 6% 10% 20% 17% 3	2460
	21st to end	Night	46% 4 9% 25% 12% 4	2076
March	1st to 10th	Night	47% 3 11% 18% 16% 5%	5494
	11th to 20th	Night	45% 5% 16% 21% 11% 3	5424
	21st to end	Night	50% 3 6% 13% 23% 5%	5072
April	1st to 10th	Night	48% 4 10% 20% 15% 3	2100
	11th to 20th	Night	46% 5% 9% 19% 19% 2	2036
	21st to end	Night	46% 4 9% 21% 16% 5%	1871
May	1st to 10th	Night	58% 6% 14% 11% 10% 1	4060
	11th to 20th	Night	52% 6% 12% 16% 11% 3	3763
	21st to end	Night	50% 7% 13% 19% 10% 1	3927
June	1st to 10th	Night	43% 7% 12% 23% 15% 1	1600
	11th to 20th	Night	42% 6% 16% 24% 11% 1	1600
	21st to end	Night	37% 7% 16% 25% 14% 1	1600
July	1st to 10th	Night	42% 7% 14% 21% 14% 1	3560
	11th to 20th	Night	43% 7% 17% 21% 11% 2	3552
	21st to end	Night	38% 5% 12% 26% 18% 1	3809
August	1st to 10th	Night	50% 8% 12% 19% 10% 1	2697
	11th to 20th	Night	42% 7% 14% 28% 9%	3158
	21st to end	Night	30% 5% 15% 34% 16% 1	3586
September	1st to 10th	Night	40% 5% 10% 26% 19% 1	2000
	11th to 20th	Night	34% 4 10% 24% 26% 2	2140
	21st to end	Night	40% 5% 10% 22% 21% 2	2200
October	1st to 10th	Night	39% 5% 12% 23% 19% 2	3644
	11th to 20th	Night	28% 3 9% 28% 28% 3	3953
	21st to end	Night	34% 3 12% 31% 18% 2	4610
November	1st to 10th	Night	37% 5% 11% 23% 20% 3	2667
	11th to 20th	Night	41% 4 11% 21% 20% 4	2730
	21st to end	Night	41% 4 14% 27% 12% 1	2777
December	1st to 10th	Night	43% 3 12% 30% 11% 1	4720
	11th to 20th	Night	46% 3 14% 17% 18% 2	5348
	21st to end	Night	49% 3 8% 21% 15% 4	7150

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	27% 20% 29% 20% 5%	220
	11th to 20th	Day	27% 27% 30% 13% 3	220
	21st to end	Day	25% 22% 35% 12% 5% 1	258
February	1st to 10th	Day	29% 20% 33% 14% 4	392
	11th to 20th	Day	29% 20% 35% 15% 2	536
	21st to end	Day	29% 18% 33% 16% 4	446
March	1st to 10th	Day	27% 19% 33% 17% 3	540
	11th to 20th	Day	28% 21% 31% 15% 4 1	540
	21st to end	Day	30% 22% 31% 11% 5% 2	594
April	1st to 10th	Day	31% 20% 30% 12% 6% 2	540
	11th to 20th	Day	29% 21% 28% 14% 7% 1	516
	21st to end	Day	31% 20% 27% 15% 6% 1	500
May	1st to 10th	Day	34% 18% 25% 14% 8% 1	503
	11th to 20th	Day	20% 18% 36% 17% 7% 2	527
	21st to end	Day	20% 14% 31% 22% 9% 3	573
June	1st to 10th	Day	20% 13% 30% 24% 10% 3	530
	11th to 20th	Day	18% 13% 35% 23% 8% 2	530
	21st to end	Day	16% 15% 31% 26% 10% 3	530
July	1st to 10th	Day	14% 9% 30% 28% 16% 3	525
	11th to 20th	Day	13% 5% 29% 34% 15% 4	524
	21st to end	Day	15% 9% 32% 29% 13% 3	576
August	1st to 10th	Day	12% 8% 28% 32% 14% 6%	526
	11th to 20th	Day	14% 7% 29% 27% 17% 7%	519
	21st to end	Day	11% 8% 30% 26% 18% 5%	583
September	1st to 10th	Day	19% 9% 23% 25% 17% 8%	530
	11th to 20th	Day	15% 9% 26% 25% 14% 12%	530
	21st to end	Day	20% 11% 28% 25% 10% 6%	530
October	1st to 10th	Day	26% 9% 24% 20% 13% 7%	530
	11th to 20th	Day	19% 16% 28% 24% 10% 3	513
	21st to end	Day	28% 13% 26% 23% 8% 2	462
November	1st to 10th	Day	21% 13% 30% 24% 9% 3	425
	11th to 20th	Day	27% 18% 29% 18% 7% 1	430
	21st to end	Day	28% 15% 33% 17% 7% 1	378
December	1st to 10th	Day	22% 20% 35% 14% 8% 1	220
	11th to 20th	Day	33% 21% 30% 13% 3	220
	21st to end	Day	29% 14% 37% 14% 6%	242
January	1st to 10th	Night	21% 15% 38% 23% 3	840
	11th to 20th	Night	26% 17% 30% 23% 3	840
	21st to end	Night	20% 14% 37% 26% 3	924
February	1st to 10th	Night	22% 13% 35% 27% 2	840
	11th to 20th	Night	23% 16% 38% 21% 1	823
	21st to end	Night	24% 14% 38% 20% 3	692
March	1st to 10th	Night	23% 17% 36% 22% 3	840
	11th to 20th	Night	23% 15% 38% 21% 3	840
	21st to end	Night	24% 16% 35% 20% 5%	924
April	1st to 10th	Night	26% 16% 34% 20% 4	840
	11th to 20th	Night	25% 16% 33% 20% 5%	816
	21st to end	Night	26% 16% 32% 21% 4	671
May	1st to 10th	Night	29% 15% 31% 21% 4	600
	11th to 20th	Night	22% 14% 39% 21% 4	477
	21st to end	Night	20% 8% 28% 36% 6% 1	433
June	1st to 10th	Night	19% 8% 30% 34% 10%	400
	11th to 20th	Night	19% 8% 30% 34% 9%	400
	21st to end	Night	17% 5% 29% 38% 12%	400
July	1st to 10th	Night	13% 5% 28% 39% 15%	390
	11th to 20th	Night	15% 3 23% 42% 15% 1	388
	21st to end	Night	11% 5% 25% 45% 14%	438
August	1st to 10th	Night	13% 3 27% 42% 14% 1	467
	11th to 20th	Night	14% 5% 23% 37% 21% 1	747
	21st to end	Night	14% 4 24% 36% 20% 2	880
September	1st to 10th	Night	16% 5% 28% 34% 15% 2	800
	11th to 20th	Night	15% 5% 24% 35% 19% 3	800
	21st to end	Night	20% 6% 26% 32% 15% 2	800
October	1st to 10th	Night	24% 7% 25% 29% 13% 2	800
	11th to 20th	Night	19% 12% 30% 30% 9% 1	800
	21st to end	Night	27% 10% 30% 26% 7%	880
November	1st to 10th	Night	19% 11% 31% 30% 9% 1	820
	11th to 20th	Night	23% 12% 34% 24% 7% 1	840
	21st to end	Night	22% 13% 31% 28% 5%	840
December	1st to 10th	Night	25% 11% 34% 24% 5%	840
	11th to 20th	Night	25% 12% 32% 28% 3	840
	21st to end	Night	27% 13% 32% 25% 3	924

Page generated at 2026-04-14 02:44:28. Version ac07

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