Hammond Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hammond Bay Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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% 10% 4 8% 6% 3 5% 5% 5% 7% 32%	74580
	Night	15% 12% 5% 9% 7% 4 5% 5% 5% 6% 29%	75171
Transparency	Day	50% 6% 12% 20% 10% 1	74283
	Night	58% 6% 11% 17% 8% 1	75004
Seeing	Day	18% 5% 19% 32% 22% 6%	14370
	Night	21% 7% 24% 30% 15% 3	23998

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% 15% 9% 16% 11% 6% 7% 5% 4 5% 9%	1498
	11th to 20th	Day	16% 17% 8% 13% 13% 7% 5% 5% 4 3 10%	1464
	21st to end	Day	13% 17% 8% 15% 11% 5% 5% 4 4 5% 14%	1584
February	1st to 10th	Day	10% 15% 8% 13% 12% 6% 6% 5% 4 6% 15%	1593
	11th to 20th	Day	12% 14% 7% 12% 10% 4 5% 4 3 6% 23%	1666
	21st to end	Day	12% 16% 8% 11% 9% 4 5% 5% 4 5% 22%	1480
March	1st to 10th	Day	13% 11% 4 8% 7% 4 5% 4 5% 5% 32%	1798
	11th to 20th	Day	12% 12% 5% 8% 6% 3 3 5% 5% 7% 35%	1975
	21st to end	Day	16% 11% 4 7% 5% 4 6% 5% 4 5% 33%	2186
April	1st to 10th	Day	19% 13% 4 8% 5% 3 5% 4 5% 6% 28%	2160
	11th to 20th	Day	18% 12% 4 6% 5% 2 5% 4 4 6% 33%	2289
	21st to end	Day	17% 10% 3 7% 5% 3 5% 4 5% 7% 33%	2340
May	1st to 10th	Day	16% 9% 3 5% 6% 2 5% 5% 5% 6% 37%	2340
	11th to 20th	Day	16% 9% 4 6% 5% 3 4 5% 4 6% 39%	2327
	21st to end	Day	12% 6% 2 5% 5% 3 5% 5% 6% 7% 45%	2844
June	1st to 10th	Day	13% 6% 2 5% 4 3 5% 4 6% 8% 44%	2700
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 5% 6% 9% 41%	2700
	21st to end	Day	12% 6% 2 6% 5% 3 4 5% 6% 9% 42%	2700
July	1st to 10th	Day	9% 4 2 4 4 2 4 6% 6% 9% 50%	2646
	11th to 20th	Day	12% 5% 3 5% 5% 3 5% 5% 6% 8% 43%	2520
	21st to end	Day	9% 6% 2 6% 6% 4 4 5% 5% 8% 45%	2743
August	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 5% 7% 10% 41%	2362
	11th to 20th	Day	10% 5% 2 4 5% 3 5% 5% 7% 7% 46%	2303
	21st to end	Day	12% 6% 2 5% 5% 3 4 4 5% 7% 45%	2565
September	1st to 10th	Day	14% 7% 3 6% 5% 3 5% 5% 6% 8% 40%	2090
	11th to 20th	Day	14% 5% 3 4 5% 3 5% 5% 6% 7% 43%	1980
	21st to end	Day	17% 8% 3 5% 6% 3 4 5% 7% 8% 34%	1980
October	1st to 10th	Day	19% 10% 3 6% 5% 3 4 5% 5% 7% 33%	1936
	11th to 20th	Day	24% 11% 4 9% 6% 4 5% 5% 5% 7% 20%	1751
	21st to end	Day	23% 16% 5% 8% 7% 4 5% 4 4 7% 18%	1782
November	1st to 10th	Day	16% 15% 6% 8% 6% 3 4 4 5% 8% 24%	1672
	11th to 20th	Day	14% 17% 6% 10% 8% 5% 5% 5% 6% 7% 16%	1648
	21st to end	Day	19% 20% 7% 11% 7% 4 4 4 6% 4 15%	1520
December	1st to 10th	Day	14% 18% 10% 17% 9% 4 6% 4 4 3 10%	1336
	11th to 20th	Day	16% 15% 7% 13% 11% 5% 6% 5% 5% 5% 12%	1671
	21st to end	Day	22% 24% 10% 15% 8% 3 3 4 2 4 6%	2431
January	1st to 10th	Night	14% 15% 8% 16% 11% 5% 5% 4 4 5% 13%	2663
	11th to 20th	Night	17% 17% 7% 12% 10% 5% 5% 4 5% 5% 14%	2569
	21st to end	Night	12% 14% 7% 13% 11% 5% 7% 5% 5% 5% 17%	2772
February	1st to 10th	Night	10% 15% 7% 12% 10% 5% 6% 5% 6% 5% 19%	2366
	11th to 20th	Night	12% 13% 5% 12% 8% 4 6% 5% 5% 7% 24%	2175
	21st to end	Night	16% 15% 6% 11% 7% 4 5% 5% 4 5% 23%	1776
March	1st to 10th	Night	11% 10% 4 8% 7% 4 4 4 4 6% 37%	2149
	11th to 20th	Night	12% 12% 5% 8% 7% 4 4 4 4 6% 35%	1984
	21st to end	Night	15% 9% 4 7% 5% 3 5% 5% 5% 7% 35%	2178
April	1st to 10th	Night	20% 11% 3 6% 5% 3 4 6% 5% 7% 29%	1809
	11th to 20th	Night	19% 9% 4 6% 5% 2 3 4 5% 7% 35%	1620
	21st to end	Night	17% 9% 4 7% 6% 4 5% 4 5% 6% 35%	1620
May	1st to 10th	Night	13% 8% 4 7% 6% 4 6% 7% 5% 5% 35%	1620
	11th to 20th	Night	17% 8% 3 6% 5% 3 4 4 5% 7% 38%	1564
	21st to end	Night	11% 6% 3 4 4 3 3 4 7% 7% 48%	1428
June	1st to 10th	Night	14% 6% 3 5% 4 3 5% 3 5% 9% 42%	1260
	11th to 20th	Night	10% 5% 2 6% 5% 3 6% 6% 7% 9% 42%	1260
	21st to end	Night	13% 7% 2 6% 4 3 3 5% 4 7% 46%	1260
July	1st to 10th	Night	9% 4 2 4 4 2 3 4 6% 9% 54%	1309
	11th to 20th	Night	8% 5% 2 5% 6% 3 5% 6% 5% 8% 47%	1440
	21st to end	Night	7% 4 3 6% 4 4 5% 5% 6% 7% 49%	1575
August	1st to 10th	Night	10% 4 2 4 4 3 3 5% 5% 9% 52%	1575
	11th to 20th	Night	12% 4 2 4 5% 3 4 4 5% 8% 51%	1598
	21st to end	Night	13% 6% 2 6% 5% 3 5% 4 5% 7% 46%	1789
September	1st to 10th	Night	15% 6% 2 5% 5% 3 3 4 4 7% 47%	1851
	11th to 20th	Night	16% 5% 2 5% 4 3 5% 4 5% 7% 44%	1980
	21st to end	Night	18% 7% 2 6% 5% 3 5% 4 5% 8% 37%	1980
October	1st to 10th	Night	18% 8% 3 6% 5% 3 4 4 5% 6% 38%	2024
	11th to 20th	Night	23% 11% 4 7% 5% 4 5% 5% 5% 7% 25%	2204
	21st to end	Night	26% 15% 4 7% 6% 3 4 4 4 6% 22%	2574
November	1st to 10th	Night	15% 14% 5% 7% 6% 3 5% 5% 6% 8% 27%	2406
	11th to 20th	Night	15% 15% 6% 10% 7% 5% 5% 5% 6% 6% 19%	2531
	21st to end	Night	18% 20% 7% 11% 6% 3 5% 5% 5% 5% 16%	2660
December	1st to 10th	Night	13% 17% 9% 14% 7% 5% 6% 6% 4 6% 14%	2840
	11th to 20th	Night	16% 17% 7% 15% 9% 4 5% 4 5% 5% 13%	3526
	21st to end	Night	23% 22% 9% 13% 7% 4 4 4 3 3 8%	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	76% 7% 7% 9%	1499
	11th to 20th	Day	81% 7% 6% 5% 1	1464
	21st to end	Day	73% 7% 9% 8% 4	1584
February	1st to 10th	Day	67% 7% 11% 12% 4	1593
	11th to 20th	Day	61% 6% 12% 13% 8%	1675
	21st to end	Day	60% 5% 11% 18% 6%	1480
March	1st to 10th	Day	50% 5% 14% 19% 10% 2	1800
	11th to 20th	Day	49% 4 14% 23% 8% 3	1975
	21st to end	Day	49% 6% 12% 21% 10% 2	2186
April	1st to 10th	Day	54% 5% 12% 17% 11% 1	2160
	11th to 20th	Day	50% 4 10% 19% 15% 2	2289
	21st to end	Day	47% 5% 13% 15% 15% 4	2340
May	1st to 10th	Day	44% 4 11% 23% 14% 3	2340
	11th to 20th	Day	43% 4 10% 24% 15% 3	2331
	21st to end	Day	38% 6% 15% 25% 14% 3	2844
June	1st to 10th	Day	37% 5% 12% 28% 14% 3	2700
	11th to 20th	Day	38% 7% 13% 24% 16% 2	2700
	21st to end	Day	37% 7% 14% 25% 14% 3	2700
July	1st to 10th	Day	29% 6% 15% 28% 20% 2	2575
	11th to 20th	Day	33% 7% 14% 29% 16% 1	2445
	21st to end	Day	34% 6% 15% 28% 16% 1	2591
August	1st to 10th	Day	34% 7% 17% 28% 12% 1	2347
	11th to 20th	Day	33% 8% 17% 29% 12% 1	2303
	21st to end	Day	36% 6% 16% 28% 13% 2	2565
September	1st to 10th	Day	40% 6% 14% 27% 13% 1	2090
	11th to 20th	Day	36% 7% 17% 30% 9% 1	1980
	21st to end	Day	47% 7% 17% 22% 8%	1980
October	1st to 10th	Day	51% 6% 11% 23% 9%	1936
	11th to 20th	Day	63% 8% 11% 14% 4	1751
	21st to end	Day	68% 6% 10% 12% 4	1782
November	1st to 10th	Day	60% 5% 10% 19% 5%	1672
	11th to 20th	Day	69% 7% 12% 8% 4	1648
	21st to end	Day	74% 7% 10% 8% 1	1520
December	1st to 10th	Day	81% 4 8% 5% 2	1336
	11th to 20th	Day	77% 7% 8% 7% 1	1671
	21st to end	Day	86% 4 6% 3 2	2431
January	1st to 10th	Night	77% 5% 8% 8% 2	2663
	11th to 20th	Night	77% 7% 7% 7% 2	2569
	21st to end	Night	71% 6% 9% 10% 3	2772
February	1st to 10th	Night	66% 7% 10% 11% 5%	2366
	11th to 20th	Night	62% 6% 11% 15% 6%	2177
	21st to end	Night	63% 5% 10% 15% 6%	1776
March	1st to 10th	Night	49% 5% 13% 20% 12% 1	2160
	11th to 20th	Night	52% 3 11% 22% 10% 2	1985
	21st to end	Night	48% 5% 12% 20% 13% 2	2178
April	1st to 10th	Night	55% 5% 10% 18% 11% 1	1809
	11th to 20th	Night	48% 4 9% 24% 13% 3	1620
	21st to end	Night	50% 5% 9% 18% 14% 4	1620
May	1st to 10th	Night	47% 6% 11% 19% 14% 3	1620
	11th to 20th	Night	46% 4 9% 22% 16% 3	1565
	21st to end	Night	36% 5% 14% 25% 16% 4	1429
June	1st to 10th	Night	39% 4 11% 27% 17% 2	1260
	11th to 20th	Night	38% 6% 12% 26% 15% 3	1260
	21st to end	Night	39% 5% 12% 27% 13% 3	1260
July	1st to 10th	Night	29% 5% 13% 30% 21% 3	1272
	11th to 20th	Night	33% 6% 13% 28% 19% 1	1392
	21st to end	Night	35% 6% 16% 25% 18% 1	1487
August	1st to 10th	Night	30% 7% 16% 33% 12% 2	1566
	11th to 20th	Night	31% 7% 18% 32% 12% 1	1598
	21st to end	Night	39% 6% 15% 25% 14% 1	1789
September	1st to 10th	Night	39% 5% 12% 27% 15% 2	1851
	11th to 20th	Night	40% 6% 14% 30% 9% 1	1980
	21st to end	Night	46% 6% 16% 24% 8%	1980
October	1st to 10th	Night	50% 5% 10% 23% 12%	2024
	11th to 20th	Night	62% 6% 12% 15% 4	2204
	21st to end	Night	68% 6% 9% 13% 5%	2574
November	1st to 10th	Night	58% 6% 12% 18% 6%	2406
	11th to 20th	Night	68% 7% 11% 10% 5%	2531
	21st to end	Night	75% 5% 9% 8% 2	2660
December	1st to 10th	Night	77% 7% 8% 7% 1	2840
	11th to 20th	Night	76% 6% 10% 6% 2	3525
	21st to end	Night	85% 5% 5% 4 2	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	18% 9% 29% 29% 14% 1	261
	11th to 20th	Day	20% 7% 31% 30% 9% 2	249
	21st to end	Day	23% 7% 26% 29% 13% 2	264
February	1st to 10th	Day	20% 10% 34% 27% 8%	240
	11th to 20th	Day	15% 8% 43% 25% 9%	240
	21st to end	Day	14% 15% 32% 33% 6%	198
March	1st to 10th	Day	18% 7% 28% 34% 13%	240
	11th to 20th	Day	16% 8% 31% 30% 12% 3	415
	21st to end	Day	26% 5% 23% 31% 15% 2	462
April	1st to 10th	Day	21% 5% 25% 36% 11% 2	420
	11th to 20th	Day	24% 6% 23% 28% 15% 3	412
	21st to end	Day	20% 4 25% 29% 18% 4	420
May	1st to 10th	Day	22% 1 22% 32% 17% 5%	423
	11th to 20th	Day	17% 2 16% 39% 20% 7%	473
	21st to end	Day	14% 1 13% 38% 26% 8%	679
June	1st to 10th	Day	12% 2 14% 37% 28% 6%	630
	11th to 20th	Day	14% 3 13% 39% 27% 5%	630
	21st to end	Day	16% 3 9% 37% 30% 5%	630
July	1st to 10th	Day	10% 1 7% 36% 38% 8%	621
	11th to 20th	Day	11% 1 12% 34% 32% 10%	618
	21st to end	Day	10% 2 10% 36% 32% 9%	684
August	1st to 10th	Day	12% 1 9% 33% 33% 11%	481
	11th to 20th	Day	11% 2 7% 29% 36% 15%	439
	21st to end	Day	15% 3 11% 33% 29% 8%	495
September	1st to 10th	Day	15% 3 14% 29% 27% 11%	450
	11th to 20th	Day	15% 1 13% 28% 31% 12%	450
	21st to end	Day	22% 4 14% 28% 25% 8%	450
October	1st to 10th	Day	26% 4 19% 24% 21% 6%	450
	11th to 20th	Day	25% 5% 25% 32% 11% 2	401
	21st to end	Day	32% 6% 22% 27% 12% 1	297
November	1st to 10th	Day	20% 11% 21% 30% 14% 3	270
	11th to 20th	Day	20% 13% 32% 27% 8% 1	240
	21st to end	Day	29% 12% 26% 25% 8%	180
December	1st to 10th	Day	22% 8% 31% 29% 9% 1	180
	11th to 20th	Day	24% 18% 30% 20% 7% 1	180
	21st to end	Day	30% 11% 32% 21% 4 3	198
January	1st to 10th	Night	23% 11% 31% 26% 7% 1	950
	11th to 20th	Night	26% 9% 30% 26% 9% 1	918
	21st to end	Night	19% 8% 31% 32% 9% 2	990
February	1st to 10th	Night	17% 9% 38% 27% 8% 1	746
	11th to 20th	Night	20% 9% 32% 27% 11% 1	703
	21st to end	Night	24% 6% 32% 31% 7%	592
March	1st to 10th	Night	18% 7% 30% 32% 12% 2	720
	11th to 20th	Night	18% 8% 31% 31% 10% 2	720
	21st to end	Night	20% 5% 25% 35% 13% 1	792
April	1st to 10th	Night	26% 5% 24% 35% 9% 1	549
	11th to 20th	Night	22% 5% 20% 39% 12% 2	533
	21st to end	Night	19% 4 23% 34% 17% 1	540
May	1st to 10th	Night	17% 3 19% 39% 20% 2	540
	11th to 20th	Night	21% 3 15% 37% 22% 3	531
	21st to end	Night	14% 2 13% 38% 28% 5%	461
June	1st to 10th	Night	18% 1 16% 36% 26% 3	360
	11th to 20th	Night	13% 1 11% 43% 28% 4	360
	21st to end	Night	15% 2 12% 38% 28% 5%	360
July	1st to 10th	Night	11% 1 10% 39% 34% 5%	397
	11th to 20th	Night	9% 2 10% 38% 31% 9%	522
	21st to end	Night	10% 2 10% 39% 33% 5%	591
August	1st to 10th	Night	11% 2 11% 41% 32% 4	537
	11th to 20th	Night	13% 2 12% 30% 35% 9%	534
	21st to end	Night	16% 3 12% 35% 32% 3	594
September	1st to 10th	Night	18% 3 15% 30% 25% 8%	591
	11th to 20th	Night	18% 2 15% 32% 25% 7%	720
	21st to end	Night	22% 4 14% 29% 23% 7%	720
October	1st to 10th	Night	25% 5% 18% 31% 18% 3	720
	11th to 20th	Night	29% 9% 24% 28% 9% 1	720
	21st to end	Night	36% 10% 20% 25% 7% 1	792
November	1st to 10th	Night	24% 9% 26% 29% 10% 2	740
	11th to 20th	Night	28% 11% 30% 23% 7%	760
	21st to end	Night	32% 11% 29% 23% 4	760
December	1st to 10th	Night	23% 13% 34% 23% 6% 1	940
	11th to 20th	Night	26% 15% 34% 19% 6% 1	950
	21st to end	Night	33% 10% 32% 20% 5%	1045

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