Whiskey Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whiskey Bay Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	14% 9% 4 7% 6% 3 5% 5% 5% 7% 36%	74594
	Night	16% 11% 4 7% 6% 3 5% 5% 5% 6% 31%	75153
Transparency	Day	31% 39% 9% 13% 6% 1	74293
	Night	35% 39% 8% 12% 5% 1	74989
Seeing	Day	17% 9% 28% 30% 14% 3	14352
	Night	20% 9% 29% 28% 12% 2	23915

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% 16% 6% 11% 8% 5% 5% 5% 6% 6% 20%	1441
	11th to 20th	Day	15% 15% 6% 11% 9% 4 5% 5% 6% 5% 20%	1461
	21st to end	Day	13% 15% 6% 10% 8% 5% 6% 5% 4 7% 21%	1584
February	1st to 10th	Day	9% 13% 6% 11% 9% 4 7% 5% 6% 7% 23%	1583
	11th to 20th	Day	12% 11% 5% 10% 8% 4 5% 5% 6% 5% 30%	1645
	21st to end	Day	11% 12% 6% 9% 7% 4 5% 6% 5% 6% 28%	1480
March	1st to 10th	Day	11% 11% 4 7% 5% 5% 5% 3 5% 7% 38%	1798
	11th to 20th	Day	12% 9% 5% 7% 6% 3 5% 4 5% 7% 38%	1979
	21st to end	Day	15% 8% 4 6% 5% 4 5% 4 4 7% 39%	2179
April	1st to 10th	Day	17% 10% 5% 6% 6% 3 5% 5% 6% 6% 31%	2160
	11th to 20th	Day	18% 9% 3 5% 5% 3 5% 5% 5% 5% 36%	2303
	21st to end	Day	17% 7% 4 8% 5% 4 5% 5% 5% 7% 34%	2340
May	1st to 10th	Day	16% 7% 2 6% 5% 3 5% 5% 6% 7% 39%	2340
	11th to 20th	Day	17% 7% 3 5% 5% 2 3 5% 5% 7% 42%	2345
	21st to end	Day	11% 5% 2 4 5% 3 5% 5% 7% 7% 46%	2902
June	1st to 10th	Day	13% 5% 2 5% 5% 2 4 6% 6% 8% 43%	2700
	11th to 20th	Day	12% 6% 2 5% 5% 3 5% 6% 5% 8% 42%	2700
	21st to end	Day	13% 7% 3 4 4 3 5% 4 6% 8% 43%	2700
July	1st to 10th	Day	7% 4 2 4 4 2 4 5% 6% 9% 52%	2700
	11th to 20th	Day	10% 5% 2 5% 5% 3 4 5% 6% 8% 47%	2535
	21st to end	Day	10% 4 2 4 4 3 4 5% 6% 10% 47%	2743
August	1st to 10th	Day	10% 5% 2 4 6% 3 4 4 6% 10% 46%	2373
	11th to 20th	Day	10% 5% 2 4 5% 3 5% 5% 6% 7% 48%	2303
	21st to end	Day	12% 5% 2 5% 5% 3 3 4 6% 8% 47%	2574
September	1st to 10th	Day	14% 7% 3 5% 5% 2 4 4 5% 7% 44%	2102
	11th to 20th	Day	14% 4 2 6% 5% 3 5% 4 6% 9% 43%	1980
	21st to end	Day	15% 7% 3 6% 6% 3 4 5% 6% 8% 37%	1980
October	1st to 10th	Day	20% 7% 3 6% 6% 2 4 5% 5% 7% 35%	1922
	11th to 20th	Day	21% 9% 4 8% 7% 5% 5% 5% 6% 8% 21%	1751
	21st to end	Day	22% 9% 4 8% 7% 4 5% 5% 5% 7% 23%	1782
November	1st to 10th	Day	19% 10% 4 7% 7% 3 6% 5% 5% 8% 25%	1672
	11th to 20th	Day	13% 15% 7% 10% 8% 5% 5% 5% 6% 6% 20%	1599
	21st to end	Day	21% 16% 7% 10% 7% 3 5% 4 4 6% 16%	1506
December	1st to 10th	Day	15% 17% 6% 10% 8% 4 5% 5% 5% 6% 20%	1330
	11th to 20th	Day	16% 17% 6% 11% 9% 4 5% 4 4 6% 18%	1671
	21st to end	Day	25% 22% 6% 12% 6% 3 3 3 3 3 13%	2431
January	1st to 10th	Night	16% 14% 6% 10% 7% 4 6% 5% 6% 6% 20%	2723
	11th to 20th	Night	16% 16% 7% 9% 7% 4 5% 5% 5% 6% 20%	2561
	21st to end	Night	14% 15% 6% 10% 9% 4 6% 4 5% 6% 21%	2772
February	1st to 10th	Night	10% 14% 6% 10% 9% 5% 6% 6% 6% 6% 22%	2376
	11th to 20th	Night	11% 13% 4 10% 9% 4 5% 6% 5% 6% 27%	2197
	21st to end	Night	13% 14% 5% 10% 7% 4 6% 6% 4 6% 25%	1776
March	1st to 10th	Night	12% 10% 4 6% 7% 3 5% 4 5% 6% 38%	2149
	11th to 20th	Night	13% 9% 4 6% 6% 3 5% 6% 6% 7% 35%	1979
	21st to end	Night	14% 9% 3 6% 5% 3 5% 5% 5% 6% 38%	2178
April	1st to 10th	Night	19% 8% 4 8% 6% 4 5% 4 5% 6% 31%	1814
	11th to 20th	Night	18% 7% 4 6% 4 2 4 4 5% 7% 38%	1605
	21st to end	Night	19% 8% 4 6% 5% 2 5% 4 5% 6% 35%	1620
May	1st to 10th	Night	13% 8% 3 7% 6% 4 5% 6% 5% 8% 36%	1620
	11th to 20th	Night	16% 7% 4 7% 6% 3 4 5% 6% 6% 36%	1547
	21st to end	Night	12% 6% 2 6% 5% 2 4 5% 5% 8% 45%	1374
June	1st to 10th	Night	14% 5% 3 4 4 3 5% 5% 6% 9% 42%	1260
	11th to 20th	Night	10% 6% 3 5% 5% 2 6% 5% 7% 9% 43%	1260
	21st to end	Night	12% 7% 2 5% 5% 3 5% 4 5% 8% 43%	1260
July	1st to 10th	Night	9% 4 2 4 3 2 5% 4 6% 9% 52%	1260
	11th to 20th	Night	6% 4 3 6% 6% 3 6% 6% 6% 7% 47%	1422
	21st to end	Night	9% 5% 2 5% 5% 3 4 4 5% 9% 48%	1575
August	1st to 10th	Night	8% 4 2 5% 5% 3 5% 4 6% 8% 51%	1563
	11th to 20th	Night	9% 4 2 5% 5% 4 5% 4 5% 8% 50%	1598
	21st to end	Night	11% 5% 3 5% 5% 3 4 5% 5% 6% 47%	1782
September	1st to 10th	Night	13% 7% 2 4 5% 4 4 4 5% 6% 45%	1839
	11th to 20th	Night	15% 5% 2 5% 4 3 3 4 5% 7% 46%	1980
	21st to end	Night	18% 9% 3 6% 5% 3 4 6% 5% 6% 35%	1980
October	1st to 10th	Night	19% 8% 3 6% 5% 2 4 4 5% 8% 37%	2038
	11th to 20th	Night	21% 11% 4 7% 6% 4 5% 5% 5% 7% 26%	2204
	21st to end	Night	28% 12% 3 6% 6% 4 4 4 5% 6% 23%	2574
November	1st to 10th	Night	17% 11% 3 8% 7% 3 5% 4 6% 7% 29%	2406
	11th to 20th	Night	19% 15% 5% 9% 8% 4 5% 5% 5% 6% 20%	2579
	21st to end	Night	19% 18% 7% 10% 7% 3 4 4 4 5% 19%	2670
December	1st to 10th	Night	15% 18% 6% 9% 7% 3 6% 5% 5% 5% 21%	2850
	11th to 20th	Night	18% 15% 5% 9% 8% 3 5% 5% 5% 5% 22%	3526
	21st to end	Night	26% 23% 7% 10% 5% 3 3 3 3 3 14%	5236

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	43% 43% 8% 6% 1	1442
	11th to 20th	Day	41% 45% 6% 6% 3	1461
	21st to end	Day	41% 40% 7% 8% 4	1584
February	1st to 10th	Day	41% 38% 11% 8% 2	1583
	11th to 20th	Day	36% 39% 9% 11% 4 1	1654
	21st to end	Day	37% 39% 8% 12% 5%	1480
March	1st to 10th	Day	30% 39% 10% 11% 9% 1	1800
	11th to 20th	Day	29% 38% 10% 17% 5% 1	1979
	21st to end	Day	32% 39% 9% 13% 6% 2	2179
April	1st to 10th	Day	32% 39% 8% 13% 7% 1	2160
	11th to 20th	Day	31% 39% 8% 14% 8%	2303
	21st to end	Day	32% 38% 7% 10% 10% 3	2340
May	1st to 10th	Day	27% 38% 8% 15% 10% 1	2340
	11th to 20th	Day	27% 39% 7% 13% 12% 1	2349
	21st to end	Day	24% 39% 9% 16% 10% 2	2902
June	1st to 10th	Day	24% 40% 10% 17% 8% 1	2700
	11th to 20th	Day	24% 39% 10% 17% 10%	2700
	21st to end	Day	27% 39% 10% 17% 7% 1	2700
July	1st to 10th	Day	20% 39% 11% 19% 11%	2626
	11th to 20th	Day	22% 40% 9% 19% 10% 1	2459
	21st to end	Day	22% 41% 12% 18% 7%	2591
August	1st to 10th	Day	25% 39% 13% 16% 7% 1	2358
	11th to 20th	Day	20% 40% 12% 19% 8% 1	2303
	21st to end	Day	22% 39% 11% 19% 8% 1	2574
September	1st to 10th	Day	25% 38% 10% 19% 7% 1	2102
	11th to 20th	Day	26% 39% 13% 18% 4	1980
	21st to end	Day	29% 40% 12% 14% 5%	1980
October	1st to 10th	Day	30% 40% 11% 15% 5% 1	1922
	11th to 20th	Day	40% 41% 9% 9% 1	1751
	21st to end	Day	38% 41% 10% 7% 3	1782
November	1st to 10th	Day	35% 43% 8% 10% 3	1672
	11th to 20th	Day	37% 44% 9% 7% 3	1599
	21st to end	Day	43% 44% 8% 5% 1	1506
December	1st to 10th	Day	41% 44% 8% 5% 2	1330
	11th to 20th	Day	48% 36% 8% 7% 1	1671
	21st to end	Day	60% 28% 5% 4 3	2431
January	1st to 10th	Night	41% 43% 7% 7% 2	2723
	11th to 20th	Night	42% 44% 6% 6% 3	2561
	21st to end	Night	41% 39% 7% 9% 3	2772
February	1st to 10th	Night	43% 37% 9% 8% 3	2376
	11th to 20th	Night	40% 40% 9% 8% 3	2199
	21st to end	Night	40% 39% 8% 10% 3	1776
March	1st to 10th	Night	31% 37% 10% 13% 9%	2160
	11th to 20th	Night	31% 39% 9% 14% 5% 1	1980
	21st to end	Night	31% 38% 9% 13% 7% 2	2178
April	1st to 10th	Night	33% 38% 8% 11% 10%	1814
	11th to 20th	Night	30% 37% 6% 17% 9% 1	1605
	21st to end	Night	31% 38% 6% 12% 10% 3	1620
May	1st to 10th	Night	29% 39% 8% 14% 9% 2	1620
	11th to 20th	Night	31% 39% 6% 14% 9% 2	1548
	21st to end	Night	24% 39% 8% 14% 13% 3	1375
June	1st to 10th	Night	26% 39% 8% 15% 11% 1	1260
	11th to 20th	Night	21% 40% 10% 17% 11% 1	1260
	21st to end	Night	27% 39% 10% 17% 7% 1	1260
July	1st to 10th	Night	19% 39% 11% 20% 11% 1	1225
	11th to 20th	Night	23% 39% 10% 17% 10% 1	1375
	21st to end	Night	24% 41% 10% 16% 9%	1487
August	1st to 10th	Night	23% 38% 11% 20% 6% 2	1554
	11th to 20th	Night	19% 39% 13% 19% 10% 1	1598
	21st to end	Night	24% 39% 11% 18% 9% 1	1782
September	1st to 10th	Night	25% 38% 9% 19% 8% 1	1839
	11th to 20th	Night	26% 38% 12% 20% 4	1980
	21st to end	Night	31% 39% 11% 13% 5%	1980
October	1st to 10th	Night	29% 38% 10% 16% 6%	2038
	11th to 20th	Night	38% 40% 9% 10% 3	2204
	21st to end	Night	42% 40% 8% 8% 2	2574
November	1st to 10th	Night	31% 44% 8% 13% 4	2406
	11th to 20th	Night	41% 42% 7% 6% 3	2579
	21st to end	Night	42% 44% 9% 5%	2670
December	1st to 10th	Night	41% 43% 8% 6% 1	2850
	11th to 20th	Night	43% 37% 8% 7% 3 2	3525
	21st to end	Night	60% 27% 5% 6% 1	5236

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	20% 16% 30% 23% 11% 1	234
	11th to 20th	Day	23% 8% 35% 26% 7%	248
	21st to end	Day	24% 8% 29% 29% 9% 1	264
February	1st to 10th	Day	18% 19% 40% 18% 5% 1	240
	11th to 20th	Day	18% 13% 45% 21% 2 1	240
	21st to end	Day	16% 22% 40% 20% 3	198
March	1st to 10th	Day	16% 18% 33% 25% 7% 1	240
	11th to 20th	Day	16% 12% 36% 27% 8% 1	419
	21st to end	Day	23% 11% 29% 27% 9% 1	462
April	1st to 10th	Day	20% 10% 32% 27% 9% 1	420
	11th to 20th	Day	23% 12% 30% 23% 11% 2	412
	21st to end	Day	21% 11% 33% 21% 11% 2	420
May	1st to 10th	Day	20% 8% 31% 27% 12% 1	423
	11th to 20th	Day	18% 5% 33% 29% 12% 4	491
	21st to end	Day	13% 6% 29% 35% 15% 3	679
June	1st to 10th	Day	11% 5% 31% 37% 14% 2	630
	11th to 20th	Day	13% 6% 29% 37% 13% 3	630
	21st to end	Day	16% 7% 22% 38% 14% 2	630
July	1st to 10th	Day	9% 3 25% 41% 18% 4	621
	11th to 20th	Day	11% 5% 24% 36% 20% 4	618
	21st to end	Day	11% 4 23% 37% 22% 4	684
August	1st to 10th	Day	13% 2 17% 37% 25% 5%	492
	11th to 20th	Day	12% 3 17% 33% 26% 10%	439
	21st to end	Day	12% 7% 23% 32% 23% 4	495
September	1st to 10th	Day	16% 4 24% 27% 23% 6%	450
	11th to 20th	Day	14% 6% 18% 35% 21% 6%	450
	21st to end	Day	19% 9% 21% 31% 17% 4	450
October	1st to 10th	Day	22% 11% 26% 21% 16% 4	450
	11th to 20th	Day	21% 15% 30% 26% 7% 1	401
	21st to end	Day	27% 12% 34% 21% 5%	297
November	1st to 10th	Day	19% 16% 33% 25% 7% 1	270
	11th to 20th	Day	20% 16% 41% 18% 4	217
	21st to end	Day	22% 22% 36% 17% 3	180
December	1st to 10th	Day	24% 14% 32% 21% 7% 1	180
	11th to 20th	Day	25% 22% 33% 14% 6% 1	180
	21st to end	Day	29% 19% 35% 14% 3	198
January	1st to 10th	Night	22% 13% 33% 24% 7%	950
	11th to 20th	Night	23% 12% 32% 24% 8% 1	915
	21st to end	Night	22% 8% 33% 29% 8% 1	990
February	1st to 10th	Night	16% 12% 40% 25% 7%	756
	11th to 20th	Night	17% 12% 35% 27% 8% 1	703
	21st to end	Night	19% 12% 35% 27% 6% 1	592
March	1st to 10th	Night	17% 9% 38% 28% 8% 1	720
	11th to 20th	Night	19% 11% 34% 28% 8% 1	720
	21st to end	Night	19% 8% 32% 30% 9% 1	792
April	1st to 10th	Night	22% 9% 30% 32% 6% 1	554
	11th to 20th	Night	21% 7% 27% 31% 12% 1	533
	21st to end	Night	23% 5% 30% 28% 13% 1	540
May	1st to 10th	Night	13% 6% 28% 38% 13% 2	540
	11th to 20th	Night	19% 5% 24% 33% 18% 1	531
	21st to end	Night	13% 4 26% 37% 19% 1	407
June	1st to 10th	Night	14% 4 24% 39% 17%	360
	11th to 20th	Night	11% 3 26% 38% 20% 2	360
	21st to end	Night	14% 4 22% 38% 20% 2	360
July	1st to 10th	Night	11% 1 18% 48% 21% 1	350
	11th to 20th	Night	8% 2 19% 40% 26% 5%	505
	21st to end	Night	11% 3 19% 39% 26% 3	591
August	1st to 10th	Night	11% 3 18% 40% 25% 3	537
	11th to 20th	Night	11% 2 18% 31% 31% 7%	534
	21st to end	Night	13% 3 19% 36% 26% 3	594
September	1st to 10th	Night	14% 4 23% 31% 23% 6%	594
	11th to 20th	Night	15% 5% 24% 31% 20% 5%	720
	21st to end	Night	21% 5% 23% 27% 19% 4	720
October	1st to 10th	Night	20% 8% 25% 28% 16% 3	720
	11th to 20th	Night	25% 14% 27% 26% 8% 1	720
	21st to end	Night	33% 9% 28% 22% 6% 1	792
November	1st to 10th	Night	23% 13% 31% 24% 9%	740
	11th to 20th	Night	23% 16% 32% 21% 6% 1	760
	21st to end	Night	30% 17% 29% 19% 4	770
December	1st to 10th	Night	23% 17% 33% 20% 6% 1	950
	11th to 20th	Night	26% 14% 38% 15% 6% 1	950
	21st to end	Night	31% 13% 31% 19% 6%	1045

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