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 2024-01-16.

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	17% 6% 3 5% 5% 3 5% 5% 5% 7% 40%	74813
	Night	17% 6% 2 5% 5% 3 4 4 5% 6% 42%	74989
Transparency	Day	43% 5% 12% 22% 15% 2	74518
	Night	43% 5% 12% 22% 15% 2	74818
Seeing	Day	22% 14% 30% 21% 9% 3	13860
	Night	21% 11% 31% 28% 8% 1	22999

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% 3 4 5% 2 4 6% 5% 7% 43%	1517
	11th to 20th	Day	18% 7% 3 6% 5% 3 4 6% 6% 7% 36%	1486
	21st to end	Day	13% 8% 2 5% 3 3 5% 5% 5% 7% 43%	1607
February	1st to 10th	Day	18% 8% 3 5% 6% 4 5% 5% 6% 6% 34%	1673
	11th to 20th	Day	14% 7% 3 6% 6% 3 5% 4 5% 7% 41%	1748
	21st to end	Day	21% 9% 3 5% 4 2 3 5% 4 5% 38%	1480
March	1st to 10th	Day	15% 7% 3 5% 5% 3 4 4 4 7% 42%	1797
	11th to 20th	Day	16% 6% 2 6% 5% 3 5% 5% 6% 7% 37%	1846
	21st to end	Day	20% 8% 2 5% 5% 3 3 4 4 5% 41%	2376
April	1st to 10th	Day	18% 7% 3 5% 5% 2 5% 4 6% 6% 39%	2160
	11th to 20th	Day	16% 5% 3 6% 5% 3 4 4 5% 7% 42%	2134
	21st to end	Day	22% 6% 2 5% 4 3 4 4 6% 6% 39%	2272
May	1st to 10th	Day	24% 7% 3 6% 5% 3 4 4 4 8% 32%	2340
	11th to 20th	Day	18% 6% 3 4 4 3 4 4 5% 7% 41%	2417
	21st to end	Day	15% 5% 2 5% 6% 3 4 5% 6% 7% 42%	2725
June	1st to 10th	Day	18% 6% 2 6% 5% 4 5% 5% 7% 9% 35%	2520
	11th to 20th	Day	16% 5% 2 6% 6% 3 5% 5% 6% 9% 38%	2520
	21st to end	Day	14% 6% 3 5% 6% 3 5% 5% 7% 8% 38%	2520
July	1st to 10th	Day	11% 6% 3 6% 5% 3 6% 5% 6% 8% 41%	2520
	11th to 20th	Day	12% 5% 2 4 5% 3 6% 6% 7% 9% 41%	2520
	21st to end	Day	12% 5% 3 6% 6% 3 6% 5% 7% 8% 39%	2744
August	1st to 10th	Day	11% 5% 3 5% 6% 3 6% 6% 7% 9% 39%	2427
	11th to 20th	Day	13% 5% 2 5% 5% 3 6% 4 6% 8% 41%	2161
	21st to end	Day	12% 5% 2 5% 5% 3 5% 6% 5% 8% 44%	2376
September	1st to 10th	Day	14% 5% 2 5% 6% 4 5% 5% 5% 6% 44%	2160
	11th to 20th	Day	15% 5% 2 4 3 2 3 4 4 6% 51%	2034
	21st to end	Day	18% 5% 2 4 4 2 4 4 5% 7% 45%	1980
October	1st to 10th	Day	24% 6% 2 4 4 2 3 4 4 5% 41%	1954
	11th to 20th	Day	15% 6% 2 4 5% 2 5% 4 5% 8% 45%	1774
	21st to end	Day	23% 6% 2 4 4 3 4 4 4 5% 40%	1782
November	1st to 10th	Day	14% 5% 3 5% 4 3 4 3 5% 5% 48%	1671
	11th to 20th	Day	19% 5% 2 4 3 2 3 4 5% 7% 45%	1710
	21st to end	Day	19% 6% 2 5% 4 3 4 4 5% 6% 43%	1664
December	1st to 10th	Day	18% 6% 3 6% 7% 3 4 6% 4 5% 38%	1520
	11th to 20th	Day	23% 5% 2 3 3 2 3 4 5% 6% 44%	1906
	21st to end	Day	19% 11% 5% 7% 6% 3 4 4 5% 6% 31%	2772
January	1st to 10th	Night	15% 7% 3 4 4 3 4 4 5% 6% 45%	2649
	11th to 20th	Night	19% 8% 3 5% 5% 3 4 5% 5% 7% 37%	2602
	21st to end	Night	13% 8% 3 5% 5% 3 5% 5% 6% 6% 42%	2745
February	1st to 10th	Night	17% 10% 3 4 5% 3 4 4 5% 6% 38%	2286
	11th to 20th	Night	15% 7% 3 6% 6% 3 5% 5% 5% 6% 40%	2098
	21st to end	Night	19% 6% 3 6% 5% 2 4 4 5% 5% 40%	1776
March	1st to 10th	Night	15% 7% 2 7% 5% 3 4 4 5% 8% 41%	2150
	11th to 20th	Night	17% 7% 2 5% 4 3 4 4 6% 6% 41%	2110
	21st to end	Night	17% 7% 2 5% 5% 2 4 4 6% 5% 42%	1980
April	1st to 10th	Night	19% 7% 3 6% 4 3 4 5% 6% 7% 38%	1800
	11th to 20th	Night	19% 6% 2 6% 5% 3 3 5% 4 6% 40%	1775
	21st to end	Night	23% 6% 2 5% 5% 3 3 4 4 6% 40%	1685
May	1st to 10th	Night	22% 8% 3 6% 6% 3 5% 4 4 6% 33%	1620
	11th to 20th	Night	20% 4 2 5% 5% 3 4 6% 6% 7% 38%	1490
	21st to end	Night	17% 5% 3 5% 4 4 4 5% 5% 9% 42%	1550
June	1st to 10th	Night	19% 4 2 5% 4 3 4 4 5% 7% 42%	1440
	11th to 20th	Night	18% 5% 2 5% 5% 3 4 5% 5% 7% 40%	1440
	21st to end	Night	15% 4 2 4 5% 3 4 4 6% 8% 45%	1440
July	1st to 10th	Night	13% 4 2 4 4 4 5% 5% 6% 9% 43%	1440
	11th to 20th	Night	14% 4 3 4 7% 3 5% 4 7% 10% 40%	1440
	21st to end	Night	10% 5% 2 4 5% 4 6% 5% 6% 9% 44%	1574
August	1st to 10th	Night	9% 5% 3 5% 5% 3 5% 4 7% 8% 46%	1493
	11th to 20th	Night	13% 5% 3 5% 5% 4 6% 5% 6% 8% 40%	1735
	21st to end	Night	15% 5% 2 4 4 3 4 4 5% 7% 47%	1980
September	1st to 10th	Night	15% 5% 3 5% 4 3 4 4 5% 6% 45%	1800
	11th to 20th	Night	14% 5% 2 4 3 2 3 3 3 5% 54%	1926
	21st to end	Night	18% 6% 2 5% 3 2 3 4 6% 6% 46%	1980
October	1st to 10th	Night	23% 5% 2 5% 4 2 3 4 4 6% 42%	2004
	11th to 20th	Night	16% 5% 2 5% 5% 3 4 4 5% 6% 47%	2186
	21st to end	Night	25% 6% 2 4 4 3 4 4 4 5% 39%	2574
November	1st to 10th	Night	14% 6% 2 4 5% 2 4 3 5% 6% 48%	2407
	11th to 20th	Night	20% 6% 2 4 3 2 4 4 5% 6% 45%	2470
	21st to end	Night	17% 5% 2 4 4 2 5% 4 4 5% 46%	2512
December	1st to 10th	Night	19% 7% 2 6% 4 3 4 5% 5% 6% 40%	2660
	11th to 20th	Night	20% 7% 2 4 4 2 4 4 5% 7% 42%	3288
	21st to end	Night	22% 10% 3 6% 5% 3 4 4 4 5% 35%	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	39% 4 12% 29% 13% 3	1518
	11th to 20th	Day	47% 6% 15% 20% 11% 2	1486
	21st to end	Day	40% 4 14% 25% 15% 2	1607
February	1st to 10th	Day	49% 5% 13% 19% 13% 2	1673
	11th to 20th	Day	44% 5% 10% 23% 15% 3	1757
	21st to end	Day	49% 4 10% 20% 15% 3	1480
March	1st to 10th	Day	44% 4 13% 22% 14% 3	1800
	11th to 20th	Day	45% 4 12% 22% 14% 3	1846
	21st to end	Day	47% 4 8% 19% 18% 5%	2376
April	1st to 10th	Day	45% 5% 9% 22% 15% 4	2160
	11th to 20th	Day	42% 4 10% 22% 19% 3	2134
	21st to end	Day	46% 4 9% 20% 16% 4	2272
May	1st to 10th	Day	51% 5% 10% 19% 13% 3	2340
	11th to 20th	Day	43% 6% 9% 22% 18% 3	2421
	21st to end	Day	40% 7% 14% 22% 14% 3	2725
June	1st to 10th	Day	45% 6% 13% 22% 12% 1	2520
	11th to 20th	Day	43% 7% 13% 22% 14% 2	2520
	21st to end	Day	42% 7% 13% 23% 15% 1	2520
July	1st to 10th	Day	41% 7% 11% 21% 17% 2	2450
	11th to 20th	Day	39% 7% 13% 26% 14% 2	2445
	21st to end	Day	40% 7% 12% 21% 17% 2	2591
August	1st to 10th	Day	40% 8% 14% 24% 14% 1	2413
	11th to 20th	Day	40% 7% 14% 25% 13% 1	2161
	21st to end	Day	37% 6% 10% 24% 20% 2	2376
September	1st to 10th	Day	41% 5% 9% 24% 17% 2	2160
	11th to 20th	Day	34% 4 9% 25% 24% 3	2034
	21st to end	Day	40% 5% 9% 24% 19% 3	1980
October	1st to 10th	Day	47% 3 10% 22% 15% 3	1954
	11th to 20th	Day	38% 5% 13% 28% 15% 2	1774
	21st to end	Day	46% 5% 12% 20% 13% 3	1782
November	1st to 10th	Day	39% 4 13% 24% 18% 3	1671
	11th to 20th	Day	40% 4 12% 23% 19% 3	1710
	21st to end	Day	43% 4 12% 27% 11% 3	1664
December	1st to 10th	Day	49% 5% 13% 17% 15% 1	1520
	11th to 20th	Day	42% 4 16% 21% 15% 2	1906
	21st to end	Day	54% 5% 11% 20% 9% 1	2772
January	1st to 10th	Night	42% 3 12% 24% 16% 3	2649
	11th to 20th	Night	48% 5% 13% 21% 10% 2	2602
	21st to end	Night	41% 4 12% 22% 18% 3	2745
February	1st to 10th	Night	48% 4 13% 22% 12% 2	2286
	11th to 20th	Night	45% 6% 10% 20% 16% 3	2100
	21st to end	Night	46% 4 9% 24% 12% 4	1776
March	1st to 10th	Night	43% 5% 11% 23% 15% 3	2160
	11th to 20th	Night	45% 4 12% 21% 15% 3	2111
	21st to end	Night	44% 4 10% 18% 19% 5%	1980
April	1st to 10th	Night	46% 4 10% 21% 15% 3	1800
	11th to 20th	Night	45% 5% 9% 20% 20% 2	1775
	21st to end	Night	46% 4 10% 21% 14% 5%	1685
May	1st to 10th	Night	52% 6% 9% 16% 13% 3	1620
	11th to 20th	Night	44% 7% 10% 20% 16% 3	1491
	21st to end	Night	41% 7% 17% 21% 13% 1	1551
June	1st to 10th	Night	43% 7% 12% 23% 15% 1	1440
	11th to 20th	Night	43% 6% 15% 23% 12% 1	1440
	21st to end	Night	36% 7% 16% 26% 14%	1440
July	1st to 10th	Night	37% 7% 15% 23% 16% 1	1400
	11th to 20th	Night	40% 8% 16% 27% 9% 1	1392
	21st to end	Night	36% 8% 16% 22% 15% 3	1486
August	1st to 10th	Night	35% 8% 15% 26% 14% 2	1483
	11th to 20th	Night	41% 7% 14% 24% 12% 1	1735
	21st to end	Night	37% 5% 11% 26% 19% 1	1980
September	1st to 10th	Night	41% 5% 10% 24% 18% 1	1800
	11th to 20th	Night	35% 5% 10% 22% 26% 2	1926
	21st to end	Night	38% 5% 10% 23% 22% 2	1980
October	1st to 10th	Night	45% 5% 12% 22% 13% 3	2004
	11th to 20th	Night	38% 5% 14% 27% 15% 2	2186
	21st to end	Night	48% 4 12% 21% 13% 1	2574
November	1st to 10th	Night	38% 5% 11% 22% 20% 4	2407
	11th to 20th	Night	42% 4 11% 21% 18% 3	2470
	21st to end	Night	41% 4 15% 27% 12% 1	2512
December	1st to 10th	Night	46% 4 14% 22% 13% 1	2660
	11th to 20th	Night	44% 4 15% 19% 16% 2	3288
	21st to end	Night	53% 4 9% 23% 10% 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	27% 16% 31% 21% 6%	180
	11th to 20th	Day	28% 25% 30% 13% 4	172
	21st to end	Day	19% 22% 40% 12% 6% 1	187
February	1st to 10th	Day	33% 18% 32% 14% 3	293
	11th to 20th	Day	27% 20% 36% 15% 2	416
	21st to end	Day	30% 16% 33% 18% 3	346
March	1st to 10th	Day	24% 19% 35% 19% 3	420
	11th to 20th	Day	26% 21% 33% 15% 4 1	420
	21st to end	Day	27% 19% 32% 13% 6% 2	462
April	1st to 10th	Day	27% 20% 32% 12% 7% 1	420
	11th to 20th	Day	26% 19% 31% 16% 8% 1	412
	21st to end	Day	32% 19% 27% 15% 6% 1	420
May	1st to 10th	Day	34% 18% 26% 13% 8% 1	423
	11th to 20th	Day	20% 17% 37% 17% 6% 1	447
	21st to end	Day	19% 15% 29% 24% 10% 3	485
June	1st to 10th	Day	19% 14% 30% 24% 10% 3	450
	11th to 20th	Day	18% 14% 35% 22% 8% 2	450
	21st to end	Day	15% 14% 30% 26% 11% 4	450
July	1st to 10th	Day	14% 9% 28% 29% 16% 4	445
	11th to 20th	Day	14% 4 27% 35% 16% 5%	444
	21st to end	Day	14% 10% 32% 30% 12% 3	488
August	1st to 10th	Day	12% 9% 28% 33% 13% 6%	446
	11th to 20th	Day	14% 6% 31% 26% 16% 7%	439
	21st to end	Day	13% 8% 29% 24% 19% 6%	495
September	1st to 10th	Day	20% 8% 23% 24% 17% 8%	450
	11th to 20th	Day	17% 10% 27% 23% 12% 10%	450
	21st to end	Day	20% 12% 27% 24% 10% 8%	450
October	1st to 10th	Day	30% 8% 24% 19% 13% 6%	450
	11th to 20th	Day	19% 14% 29% 25% 11% 3	437
	21st to end	Day	31% 12% 25% 23% 8% 1	396
November	1st to 10th	Day	20% 11% 29% 26% 11% 3	365
	11th to 20th	Day	28% 16% 28% 18% 8% 1	370
	21st to end	Day	25% 13% 35% 19% 8% 1	324
December	1st to 10th	Day	22% 19% 37% 14% 8% 1	180
	11th to 20th	Day	32% 20% 33% 12% 3	180
	21st to end	Day	27% 15% 38% 15% 6%	198
January	1st to 10th	Night	20% 14% 39% 24% 3	760
	11th to 20th	Night	26% 16% 30% 25% 3	744
	21st to end	Night	19% 14% 38% 27% 3	792
February	1st to 10th	Night	24% 13% 35% 27% 2	720
	11th to 20th	Night	22% 17% 39% 21% 2	703
	21st to end	Night	25% 14% 38% 21% 3	592
March	1st to 10th	Night	22% 16% 38% 22% 2	720
	11th to 20th	Night	22% 15% 39% 20% 4	720
	21st to end	Night	22% 15% 35% 22% 6%	792
April	1st to 10th	Night	25% 16% 33% 22% 4	720
	11th to 20th	Night	24% 14% 34% 22% 6%	711
	21st to end	Night	27% 15% 32% 21% 4	605
May	1st to 10th	Night	28% 16% 31% 21% 4	540
	11th to 20th	Night	22% 12% 40% 21% 4	429
	21st to end	Night	19% 9% 27% 37% 7% 1	389
June	1st to 10th	Night	19% 8% 29% 34% 10%	360
	11th to 20th	Night	20% 8% 29% 34% 9%	360
	21st to end	Night	16% 3 31% 39% 11%	360
July	1st to 10th	Night	14% 4 28% 38% 16%	350
	11th to 20th	Night	16% 2 23% 42% 16% 1	348
	21st to end	Night	11% 4 24% 46% 14%	394
August	1st to 10th	Night	12% 3 27% 43% 14%	419
	11th to 20th	Night	14% 4 22% 38% 21% 1	671
	21st to end	Night	15% 4 23% 36% 20% 2	792
September	1st to 10th	Night	17% 4 29% 34% 14% 2	720
	11th to 20th	Night	17% 5% 25% 33% 17% 3	720
	21st to end	Night	20% 6% 26% 30% 16% 2	720
October	1st to 10th	Night	26% 6% 25% 29% 12% 3	720
	11th to 20th	Night	18% 11% 30% 30% 10% 1	720
	21st to end	Night	29% 10% 30% 24% 7%	792
November	1st to 10th	Night	18% 11% 30% 31% 9% 1	740
	11th to 20th	Night	24% 10% 33% 25% 7% 1	760
	21st to end	Night	22% 13% 31% 28% 5% 1	760
December	1st to 10th	Night	26% 11% 34% 24% 6% 1	760
	11th to 20th	Night	24% 12% 32% 28% 4	760
	21st to end	Night	27% 12% 33% 25% 3	836

Page generated at 2024-01-13 19:28:30. Version ac07

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