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 2024-12-24.

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 5% 4 6% 7% 39%	101677
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 40%	98453
Transparency	Day	43% 5% 12% 22% 15% 2	105214
	Night	44% 5% 12% 23% 15% 2	103005
Seeing	Day	22% 14% 30% 21% 9% 3	15100
	Night	21% 11% 32% 28% 8% 1	24175

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	14% 7% 2 5% 4 3 4 4 5% 5% 45%	2517
	11th to 20th	Day	16% 8% 3 10% 4 4 5% 4 5% 5% 35%	2520
	21st to end	Day	21% 12% 4 8% 4 4 3 2 4 4 33%	2809
February	1st to 10th	Day	17% 9% 2 6% 6% 5% 5% 4 6% 6% 35%	1766
	11th to 20th	Day	13% 8% 3 6% 5% 4 4 3 6% 7% 41%	1848
	21st to end	Day	20% 9% 3 6% 4 3 4 3 4 5% 38%	1570
March	1st to 10th	Day	25% 11% 3 6% 4 3 3 3 3 5% 34%	3116
	11th to 20th	Day	11% 6% 2 7% 4 5% 6% 5% 7% 7% 40%	3248
	21st to end	Day	21% 7% 2 6% 5% 3 3 4 5% 5% 39%	4166
April	1st to 10th	Day	18% 9% 3 6% 5% 4 5% 4 5% 6% 37%	2280
	11th to 20th	Day	17% 6% 3 6% 5% 4 4 3 5% 6% 41%	2254
	21st to end	Day	21% 6% 2 6% 4 4 5% 3 6% 6% 39%	2399
May	1st to 10th	Day	26% 8% 3 6% 4 4 4 3 5% 8% 29%	4106
	11th to 20th	Day	22% 8% 3 7% 4 4 4 3 5% 6% 35%	4295
	21st to end	Day	14% 5% 2 5% 4 4 4 4 6% 8% 45%	4837
June	1st to 10th	Day	17% 6% 2 6% 4 5% 4 5% 7% 8% 35%	2660
	11th to 20th	Day	15% 5% 2 7% 5% 4 5% 4 6% 9% 37%	2660
	21st to end	Day	14% 7% 3 6% 5% 4 5% 5% 7% 8% 37%	2660
July	1st to 10th	Day	10% 6% 2 7% 6% 6% 6% 5% 7% 9% 37%	4440
	11th to 20th	Day	12% 5% 2 6% 5% 5% 6% 5% 7% 10% 39%	4440
	21st to end	Day	17% 8% 3 7% 5% 4 5% 3 6% 6% 35%	4814
August	1st to 10th	Day	18% 6% 3 7% 5% 5% 5% 4 7% 8% 31%	4275
	11th to 20th	Day	13% 5% 2 7% 5% 4 5% 3 6% 8% 41%	3791
	21st to end	Day	9% 5% 2 6% 4 5% 6% 5% 7% 9% 43%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 3 5% 6% 45%	2280
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 5% 6% 51%	2147
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 7% 42%	2090
October	1st to 10th	Day	23% 6% 2 5% 3 3 4 3 5% 5% 41%	2062
	11th to 20th	Day	14% 6% 2 4 4 3 5% 3 5% 7% 46%	1872
	21st to end	Day	22% 6% 2 5% 4 3 5% 3 4 5% 41%	1881
November	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 5% 49%	1761
	11th to 20th	Day	18% 6% 2 4 3 3 4 3 5% 6% 47%	1800
	21st to end	Day	19% 7% 2 5% 4 4 4 3 5% 5% 42%	1751
December	1st to 10th	Day	18% 7% 3 7% 6% 4 5% 4 5% 5% 38%	1600
	11th to 20th	Day	23% 5% 2 4 3 2 3 3 5% 6% 44%	1986
	21st to end	Day	19% 12% 4 8% 5% 4 4 3 5% 6% 31%	2796
January	1st to 10th	Night	17% 7% 2 5% 4 4 3 3 5% 5% 46%	4429
	11th to 20th	Night	15% 8% 3 8% 5% 4 4 3 4 5% 41%	4440
	21st to end	Night	22% 13% 4 7% 4 4 4 3 4 5% 31%	4843
February	1st to 10th	Night	16% 9% 3 5% 4 4 4 4 5% 6% 40%	2413
	11th to 20th	Night	14% 8% 2 7% 5% 4 4 3 5% 6% 40%	2218
	21st to end	Night	18% 7% 3 7% 4 3 3 3 4 5% 41%	1884
March	1st to 10th	Night	25% 10% 3 6% 3 3 3 2 4 6% 33%	3724
	11th to 20th	Night	13% 8% 2 6% 4 4 5% 3 6% 7% 42%	3708
	21st to end	Night	21% 8% 2 6% 4 4 4 3 6% 5% 38%	3466
April	1st to 10th	Night	18% 7% 2 6% 4 4 4 4 6% 7% 36%	1900
	11th to 20th	Night	20% 7% 2 6% 5% 4 4 4 4 6% 39%	1875
	21st to end	Night	22% 7% 2 6% 4 3 3 3 4 5% 41%	1778
May	1st to 10th	Night	24% 9% 3 7% 5% 4 4 3 5% 6% 30%	2834
	11th to 20th	Night	22% 7% 3 7% 5% 4 5% 4 5% 7% 31%	2626
	21st to end	Night	17% 5% 2 5% 3 4 4 4 5% 8% 43%	2738
June	1st to 10th	Night	18% 5% 2 6% 4 5% 4 3 5% 7% 42%	1520
	11th to 20th	Night	17% 6% 2 6% 5% 4 3 4 5% 7% 41%	1520
	21st to end	Night	16% 5% 2 5% 4 4 4 3 6% 8% 43%	1520
July	1st to 10th	Night	13% 5% 2 5% 4 5% 5% 5% 7% 9% 39%	2520
	11th to 20th	Night	15% 6% 2 6% 5% 4 4 3 7% 9% 40%	2520
	21st to end	Night	16% 9% 3 6% 4 4 5% 4 5% 7% 36%	2739
August	1st to 10th	Night	21% 5% 3 8% 4 4 4 3 6% 7% 33%	2623
	11th to 20th	Night	15% 7% 3 6% 4 4 5% 3 6% 7% 41%	3032
	21st to end	Night	10% 5% 1 4 3 3 4 3 5% 8% 53%	3476
September	1st to 10th	Night	15% 5% 3 6% 3 4 4 3 5% 6% 46%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 5% 54%	2033
	21st to end	Night	18% 6% 1 5% 3 3 3 3 6% 6% 45%	2090
October	1st to 10th	Night	22% 5% 2 5% 4 4 3 3 4 6% 42%	2116
	11th to 20th	Night	16% 5% 2 6% 4 3 3 3 5% 6% 48%	2308
	21st to end	Night	24% 6% 2 4 3 4 4 3 5% 5% 41%	2717
November	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 6% 49%	2537
	11th to 20th	Night	19% 6% 2 4 3 3 4 3 5% 6% 45%	2600
	21st to end	Night	17% 6% 2 5% 3 4 4 3 4 6% 45%	2645
December	1st to 10th	Night	19% 7% 2 6% 3 4 4 4 5% 6% 40%	2800
	11th to 20th	Night	20% 7% 2 4 3 3 3 4 5% 6% 42%	3428
	21st to end	Night	22% 10% 3 7% 4 4 3 3 4 5% 35%	4933

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	40% 4 11% 33% 10% 2	2518
	11th to 20th	Day	50% 5% 11% 24% 10% 1	2520
	21st to end	Day	55% 4 10% 21% 9% 1	2809
February	1st to 10th	Day	47% 5% 13% 20% 13% 2	1766
	11th to 20th	Day	44% 5% 10% 23% 15% 3	1857
	21st to end	Day	48% 4 10% 20% 15% 3	1570
March	1st to 10th	Day	55% 3 11% 17% 11% 2	3119
	11th to 20th	Day	39% 5% 14% 28% 11% 2	3248
	21st to end	Day	47% 3 6% 18% 21% 5%	4166
April	1st to 10th	Day	46% 5% 9% 22% 14% 3	2280
	11th to 20th	Day	43% 4 10% 22% 19% 3	2254
	21st to end	Day	46% 5% 9% 20% 17% 4	2399
May	1st to 10th	Day	53% 5% 14% 15% 12% 1	4120
	11th to 20th	Day	50% 6% 13% 19% 10% 1	4299
	21st to end	Day	36% 7% 12% 25% 19% 2	4837
June	1st to 10th	Day	44% 6% 13% 23% 13% 1	2660
	11th to 20th	Day	42% 7% 14% 22% 14% 2	2660
	21st to end	Day	42% 7% 13% 22% 15% 1	2660
July	1st to 10th	Day	43% 8% 11% 20% 17% 1	4370
	11th to 20th	Day	39% 6% 15% 24% 13% 2	4365
	21st to end	Day	48% 6% 11% 19% 15% 2	4661
August	1st to 10th	Day	49% 7% 15% 20% 9%	4261
	11th to 20th	Day	39% 6% 13% 30% 11% 1	3829
	21st to end	Day	35% 7% 12% 28% 17% 2	4180
September	1st to 10th	Day	40% 5% 9% 25% 19% 2	2280
	11th to 20th	Day	34% 4 9% 26% 24% 3	2147
	21st to end	Day	42% 5% 9% 23% 19% 3	2090
October	1st to 10th	Day	40% 4 9% 19% 24% 3	3426
	11th to 20th	Day	30% 3 8% 27% 28% 3	3088
	21st to end	Day	33% 4 13% 26% 19% 4	3081
November	1st to 10th	Day	38% 4 13% 24% 18% 2	1761
	11th to 20th	Day	38% 4 11% 23% 21% 3	1800
	21st to end	Day	44% 4 12% 27% 11% 3	1751
December	1st to 10th	Day	48% 5% 14% 18% 14% 1	1600
	11th to 20th	Day	42% 4 16% 20% 16% 2	1986
	21st to end	Day	54% 5% 11% 20% 9% 1	2796
January	1st to 10th	Night	41% 3 12% 30% 12% 3	4429
	11th to 20th	Night	46% 5% 12% 23% 12% 3	4440
	21st to end	Night	57% 3 10% 19% 10% 1	4843
February	1st to 10th	Night	46% 4 14% 23% 12% 2	2413
	11th to 20th	Night	44% 6% 10% 20% 17% 3	2220
	21st to end	Night	46% 4 10% 25% 12% 4	1884
March	1st to 10th	Night	54% 4 10% 19% 11% 2	3734
	11th to 20th	Night	41% 6% 14% 23% 14% 2	3709
	21st to end	Night	48% 3 7% 14% 23% 5%	3466
April	1st to 10th	Night	47% 4 10% 21% 15% 3	1900
	11th to 20th	Night	46% 5% 9% 20% 19% 2	1875
	21st to end	Night	46% 4 9% 21% 15% 5%	1778
May	1st to 10th	Night	54% 8% 13% 14% 10% 2	2840
	11th to 20th	Night	53% 7% 13% 16% 9% 2	2627
	21st to end	Night	39% 7% 16% 23% 14% 1	2739
June	1st to 10th	Night	42% 7% 12% 23% 15% 1	1520
	11th to 20th	Night	42% 6% 17% 24% 12% 1	1520
	21st to end	Night	37% 7% 16% 25% 14%	1520
July	1st to 10th	Night	40% 8% 16% 21% 15% 1	2480
	11th to 20th	Night	42% 7% 15% 24% 12% 1	2472
	21st to end	Night	48% 6% 13% 16% 16% 2	2651
August	1st to 10th	Night	51% 8% 13% 19% 9% 1	2613
	11th to 20th	Night	42% 7% 14% 28% 9%	3060
	21st to end	Night	30% 5% 15% 33% 16% 1	3476
September	1st to 10th	Night	40% 5% 10% 25% 19% 1	1900
	11th to 20th	Night	34% 4 10% 23% 26% 2	2033
	21st to end	Night	39% 6% 10% 22% 21% 2	2090
October	1st to 10th	Night	40% 5% 13% 22% 19% 2	3532
	11th to 20th	Night	27% 3 9% 28% 29% 3	3831
	21st to end	Night	34% 3 13% 31% 18% 2	4467
November	1st to 10th	Night	37% 5% 11% 23% 20% 4	2537
	11th to 20th	Night	42% 4 11% 20% 20% 4	2600
	21st to end	Night	41% 4 15% 27% 12% 1	2645
December	1st to 10th	Night	46% 4 14% 23% 12% 1	2800
	11th to 20th	Night	44% 4 15% 19% 16% 2	3428
	21st to end	Night	53% 4 9% 23% 11% 1	4933

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% 16% 31% 21% 6%	180
	11th to 20th	Day	27% 25% 31% 13% 4	180
	21st to end	Day	20% 21% 37% 14% 7% 1	210
February	1st to 10th	Day	30% 19% 33% 14% 3	326
	11th to 20th	Day	27% 21% 35% 15% 2	456
	21st to end	Day	30% 16% 33% 18% 4	382
March	1st to 10th	Day	25% 19% 34% 18% 4	460
	11th to 20th	Day	26% 22% 32% 15% 4 1	460
	21st to end	Day	28% 20% 33% 12% 6% 2	506
April	1st to 10th	Day	27% 20% 32% 12% 7% 2	460
	11th to 20th	Day	27% 20% 30% 15% 7% 1	452
	21st to end	Day	31% 18% 28% 15% 6% 2	460
May	1st to 10th	Day	33% 19% 25% 14% 8% 2	463
	11th to 20th	Day	21% 17% 36% 17% 7% 2	487
	21st to end	Day	19% 15% 31% 23% 9% 3	529
June	1st to 10th	Day	19% 13% 30% 24% 10% 3	490
	11th to 20th	Day	17% 13% 36% 22% 9% 3	490
	21st to end	Day	16% 15% 31% 25% 10% 3	490
July	1st to 10th	Day	14% 9% 29% 28% 16% 4	485
	11th to 20th	Day	13% 5% 29% 34% 15% 4	484
	21st to end	Day	15% 10% 32% 29% 12% 3	532
August	1st to 10th	Day	13% 9% 29% 32% 13% 5%	486
	11th to 20th	Day	14% 7% 30% 27% 15% 7%	479
	21st to end	Day	12% 8% 29% 26% 19% 6%	539
September	1st to 10th	Day	19% 9% 23% 24% 17% 8%	490
	11th to 20th	Day	16% 10% 26% 23% 13% 12%	490
	21st to end	Day	20% 11% 28% 24% 9% 7%	490
October	1st to 10th	Day	28% 9% 24% 19% 13% 7%	490
	11th to 20th	Day	18% 15% 29% 24% 10% 3	475
	21st to end	Day	29% 12% 25% 23% 9% 2	429
November	1st to 10th	Day	20% 12% 30% 25% 10% 3	395
	11th to 20th	Day	28% 17% 29% 18% 8% 1	400
	21st to end	Day	26% 14% 34% 18% 7% 1	351
December	1st to 10th	Day	21% 20% 37% 14% 8% 1	200
	11th to 20th	Day	33% 21% 30% 14% 3	200
	21st to end	Day	26% 15% 38% 15% 6%	204
January	1st to 10th	Night	20% 14% 39% 24% 3	760
	11th to 20th	Night	26% 16% 31% 24% 3	760
	21st to end	Night	20% 14% 37% 26% 3	836
February	1st to 10th	Night	23% 12% 35% 28% 2	760
	11th to 20th	Night	21% 17% 40% 21% 1	743
	21st to end	Night	25% 14% 37% 21% 3	628
March	1st to 10th	Night	23% 15% 37% 22% 3	760
	11th to 20th	Night	21% 16% 39% 21% 4	760
	21st to end	Night	23% 16% 34% 21% 6%	836
April	1st to 10th	Night	24% 16% 34% 22% 4	760
	11th to 20th	Night	25% 15% 34% 21% 6%	751
	21st to end	Night	26% 16% 32% 21% 5%	638
May	1st to 10th	Night	28% 15% 31% 22% 4	570
	11th to 20th	Night	22% 13% 40% 22% 4	453
	21st to end	Night	19% 9% 28% 37% 6% 1	411
June	1st to 10th	Night	18% 8% 30% 34% 10%	380
	11th to 20th	Night	19% 8% 30% 34% 9%	380
	21st to end	Night	16% 5% 30% 38% 11%	380
July	1st to 10th	Night	13% 5% 29% 38% 15%	370
	11th to 20th	Night	16% 2 23% 43% 15% 1	368
	21st to end	Night	12% 5% 24% 45% 14%	416
August	1st to 10th	Night	13% 3 27% 42% 14%	443
	11th to 20th	Night	14% 4 23% 37% 21% 1	709
	21st to end	Night	14% 4 23% 37% 21% 2	836
September	1st to 10th	Night	17% 5% 28% 34% 15% 2	760
	11th to 20th	Night	16% 5% 24% 34% 18% 3	760
	21st to end	Night	20% 6% 26% 31% 15% 2	760
October	1st to 10th	Night	25% 7% 25% 29% 12% 2	760
	11th to 20th	Night	18% 11% 30% 30% 10% 1	760
	21st to end	Night	28% 10% 30% 26% 7%	836
November	1st to 10th	Night	18% 11% 32% 30% 9% 1	780
	11th to 20th	Night	23% 11% 33% 25% 7% 1	800
	21st to end	Night	23% 13% 31% 27% 5% 1	800
December	1st to 10th	Night	26% 11% 34% 24% 5% 1	800
	11th to 20th	Night	25% 12% 32% 28% 4	800
	21st to end	Night	27% 13% 33% 25% 3	851

Page generated at 2024-12-21 04:13:48. Version ac07

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