The Gravel Pit
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Gravel Pit clear sky chart from 2005-11-06 to 2024-01-15.

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	11% 9% 4 8% 7% 4 6% 6% 6% 8% 32%	74721
	Night	9% 11% 5% 8% 7% 4 6% 5% 6% 8% 31%	75039
Transparency	Day	47% 9% 16% 20% 7% 1	74394
	Night	49% 8% 14% 20% 8% 1	74885
Seeing	Day	16% 9% 26% 28% 16% 5%	14891
	Night	15% 8% 28% 32% 15% 2	23505

By Month

Percentage Cloud Cover NW (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	6% 12% 6% 10% 9% 4 6% 6% 6% 8% 27%	1158
	11th to 20th	Day	7% 11% 5% 11% 9% 3 5% 4 6% 9% 28%	1288
	21st to end	Day	7% 12% 6% 11% 8% 5% 6% 7% 7% 6% 26%	1564
February	1st to 10th	Day	8% 13% 6% 11% 8% 5% 7% 6% 7% 8% 22%	1440
	11th to 20th	Day	6% 14% 6% 9% 9% 4 6% 5% 7% 7% 27%	1456
	21st to end	Day	4 11% 7% 13% 9% 5% 6% 6% 7% 7% 25%	1453
March	1st to 10th	Day	5% 14% 6% 11% 9% 4 5% 6% 6% 7% 27%	1800
	11th to 20th	Day	7% 10% 5% 9% 6% 3 5% 7% 7% 8% 32%	1897
	21st to end	Day	9% 15% 6% 9% 8% 4 6% 5% 7% 7% 24%	2287
April	1st to 10th	Day	7% 12% 8% 9% 8% 4 6% 7% 6% 9% 24%	2187
	11th to 20th	Day	12% 13% 5% 7% 7% 4 6% 5% 6% 8% 27%	2310
	21st to end	Day	11% 11% 4 7% 7% 3 6% 6% 7% 9% 30%	2395
May	1st to 10th	Day	14% 11% 4 8% 7% 4 6% 6% 6% 7% 26%	2587
	11th to 20th	Day	10% 7% 4 6% 6% 4 6% 6% 5% 8% 39%	2654
	21st to end	Day	18% 8% 3 6% 7% 3 5% 5% 6% 7% 32%	2917
June	1st to 10th	Day	17% 9% 3 6% 7% 3 5% 5% 6% 9% 30%	2790
	11th to 20th	Day	21% 8% 4 7% 7% 4 7% 6% 6% 8% 21%	2880
	21st to end	Day	10% 6% 3 7% 6% 4 7% 7% 8% 9% 33%	2880
July	1st to 10th	Day	15% 7% 3 5% 5% 3 6% 6% 6% 8% 36%	2880
	11th to 20th	Day	12% 5% 3 6% 7% 5% 5% 7% 8% 10% 33%	2772
	21st to end	Day	10% 5% 2 5% 6% 4 5% 5% 8% 10% 40%	2920
August	1st to 10th	Day	11% 6% 3 6% 6% 4 6% 6% 8% 9% 36%	2507
	11th to 20th	Day	8% 7% 3 5% 5% 4 5% 6% 7% 10% 41%	2444
	21st to end	Day	14% 6% 3 5% 6% 4 5% 6% 7% 8% 36%	2574
September	1st to 10th	Day	13% 6% 2 6% 6% 4 5% 5% 6% 8% 39%	2214
	11th to 20th	Day	16% 7% 2 5% 5% 3 3 4 5% 9% 42%	2030
	21st to end	Day	10% 8% 3 7% 6% 4 5% 5% 5% 8% 39%	1980
October	1st to 10th	Day	10% 11% 4 8% 6% 4 5% 5% 5% 6% 36%	1880
	11th to 20th	Day	11% 10% 4 7% 7% 3 4 6% 6% 7% 35%	1620
	21st to end	Day	10% 14% 5% 8% 8% 4 5% 5% 5% 5% 32%	1782
November	1st to 10th	Day	12% 15% 7% 8% 8% 3 5% 4 7% 7% 24%	1491
	11th to 20th	Day	7% 11% 5% 10% 7% 5% 5% 5% 5% 6% 34%	1330
	21st to end	Day	7% 12% 5% 10% 8% 4 6% 6% 6% 8% 27%	1330
December	1st to 10th	Day	7% 11% 7% 11% 8% 5% 6% 5% 5% 7% 29%	1330
	11th to 20th	Day	9% 12% 4 10% 8% 3 5% 5% 5% 9% 30%	1604
	21st to end	Day	5% 10% 5% 8% 7% 3 4 6% 6% 7% 38%	2090
January	1st to 10th	Night	6% 14% 5% 11% 10% 5% 6% 5% 5% 7% 26%	3005
	11th to 20th	Night	7% 13% 6% 10% 7% 5% 6% 6% 6% 7% 28%	2761
	21st to end	Night	7% 14% 6% 10% 7% 4 6% 5% 5% 7% 29%	2808
February	1st to 10th	Night	8% 12% 6% 11% 9% 6% 5% 5% 6% 7% 25%	2520
	11th to 20th	Night	6% 13% 6% 11% 9% 5% 5% 5% 5% 8% 26%	2389
	21st to end	Night	5% 13% 7% 9% 8% 4 6% 5% 6% 8% 29%	1799
March	1st to 10th	Night	6% 12% 7% 9% 8% 5% 7% 6% 6% 8% 27%	2160
	11th to 20th	Night	6% 12% 5% 10% 7% 5% 6% 5% 7% 8% 29%	2062
	21st to end	Night	8% 14% 7% 10% 6% 5% 5% 6% 6% 9% 25%	2066
April	1st to 10th	Night	8% 13% 5% 10% 9% 4 6% 6% 6% 8% 25%	1772
	11th to 20th	Night	11% 11% 5% 9% 6% 4 6% 6% 6% 7% 28%	1601
	21st to end	Night	12% 9% 4 7% 5% 4 6% 5% 6% 9% 32%	1562
May	1st to 10th	Night	16% 10% 5% 9% 6% 3 5% 5% 6% 7% 27%	1373
	11th to 20th	Night	11% 8% 4 7% 4 3 6% 5% 6% 9% 36%	1239
	21st to end	Night	20% 8% 3 6% 6% 4 5% 5% 6% 6% 32%	1358
June	1st to 10th	Night	16% 10% 4 8% 6% 3 4 6% 6% 8% 29%	1166
	11th to 20th	Night	19% 8% 4 8% 8% 5% 6% 6% 7% 7% 21%	1080
	21st to end	Night	13% 6% 4 8% 6% 3 6% 6% 6% 11% 30%	1080
July	1st to 10th	Night	16% 6% 3 5% 6% 3 5% 5% 6% 8% 36%	1080
	11th to 20th	Night	12% 5% 3 5% 9% 5% 6% 7% 9% 9% 29%	1188
	21st to end	Night	11% 5% 3 6% 6% 4 5% 6% 7% 10% 36%	1395
August	1st to 10th	Night	12% 5% 3 7% 6% 5% 5% 5% 8% 8% 36%	1432
	11th to 20th	Night	8% 7% 3 6% 6% 3 6% 5% 7% 10% 38%	1455
	21st to end	Night	12% 7% 3 5% 4 4 6% 5% 7% 8% 39%	1782
September	1st to 10th	Night	12% 6% 3 5% 6% 4 5% 5% 6% 7% 41%	1743
	11th to 20th	Night	13% 7% 3 5% 4 3 4 5% 5% 7% 44%	1929
	21st to end	Night	10% 7% 2 6% 5% 3 6% 6% 6% 8% 41%	1980
October	1st to 10th	Night	13% 9% 3 7% 6% 3 4 5% 4 7% 38%	2076
	11th to 20th	Night	11% 8% 4 7% 7% 4 4 5% 6% 8% 36%	2340
	21st to end	Night	8% 12% 4 8% 8% 5% 5% 5% 7% 8% 30%	2574
November	1st to 10th	Night	14% 15% 6% 9% 7% 3 5% 5% 5% 7% 26%	2565
	11th to 20th	Night	9% 12% 6% 10% 8% 4 6% 5% 5% 6% 29%	2850
	21st to end	Night	7% 11% 6% 9% 10% 5% 6% 6% 6% 7% 28%	2850
December	1st to 10th	Night	7% 12% 7% 10% 7% 5% 6% 5% 5% 7% 30%	2850
	11th to 20th	Night	8% 14% 6% 9% 8% 4 5% 6% 5% 8% 27%	3583
	21st to end	Night	4 10% 4 8% 7% 4 6% 6% 6% 7% 39%	5566

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	53% 8% 12% 18% 7% 2	1158
	11th to 20th	Day	52% 7% 13% 16% 10% 2	1288
	21st to end	Day	53% 10% 14% 15% 7% 1	1564
February	1st to 10th	Day	57% 9% 13% 10% 9% 1	1440
	11th to 20th	Day	52% 9% 13% 18% 8%	1464
	21st to end	Day	55% 6% 14% 16% 9% 1	1453
March	1st to 10th	Day	56% 6% 13% 18% 7% 1	1800
	11th to 20th	Day	46% 8% 12% 21% 11% 1	1898
	21st to end	Day	54% 10% 15% 15% 5%	2287
April	1st to 10th	Day	51% 10% 14% 17% 7% 1	2187
	11th to 20th	Day	52% 7% 14% 19% 7% 1	2310
	21st to end	Day	47% 9% 14% 20% 8% 2	2395
May	1st to 10th	Day	53% 9% 13% 16% 8% 1	2587
	11th to 20th	Day	41% 7% 13% 28% 11%	2658
	21st to end	Day	48% 8% 15% 19% 10%	2918
June	1st to 10th	Day	49% 9% 16% 20% 6%	2790
	11th to 20th	Day	57% 11% 14% 14% 4	2880
	21st to end	Day	42% 12% 19% 22% 5%	2880
July	1st to 10th	Day	43% 10% 18% 23% 5% 1	2801
	11th to 20th	Day	42% 12% 24% 20% 2	2687
	21st to end	Day	35% 11% 27% 23% 3	2758
August	1st to 10th	Day	40% 12% 24% 21% 3	2490
	11th to 20th	Day	36% 10% 24% 24% 6%	2444
	21st to end	Day	41% 11% 22% 20% 6%	2574
September	1st to 10th	Day	41% 9% 19% 25% 5% 1	2214
	11th to 20th	Day	40% 8% 17% 27% 8%	2030
	21st to end	Day	41% 10% 15% 22% 10% 2	1980
October	1st to 10th	Day	47% 8% 16% 22% 7%	1880
	11th to 20th	Day	44% 9% 14% 23% 9%	1620
	21st to end	Day	52% 6% 12% 20% 8% 1	1782
November	1st to 10th	Day	58% 6% 13% 16% 5% 1	1491
	11th to 20th	Day	50% 8% 15% 17% 9% 1	1330
	21st to end	Day	52% 11% 14% 14% 7% 2	1330
December	1st to 10th	Day	54% 8% 12% 16% 9% 2	1330
	11th to 20th	Day	52% 8% 14% 17% 8% 1	1606
	21st to end	Day	41% 7% 14% 18% 18% 2	2090
January	1st to 10th	Night	55% 8% 12% 16% 7% 2	3005
	11th to 20th	Night	54% 7% 14% 16% 7% 2	2761
	21st to end	Night	54% 7% 13% 14% 10% 1	2808
February	1st to 10th	Night	56% 7% 13% 15% 8% 2	2520
	11th to 20th	Night	55% 7% 13% 17% 8%	2391
	21st to end	Night	52% 6% 10% 20% 11% 1	1799
March	1st to 10th	Night	52% 9% 14% 19% 7%	2160
	11th to 20th	Night	49% 7% 10% 20% 13% 1	2062
	21st to end	Night	54% 9% 12% 17% 7% 1	2066
April	1st to 10th	Night	53% 9% 14% 15% 8% 1	1772
	11th to 20th	Night	52% 7% 11% 20% 8% 2	1601
	21st to end	Night	47% 8% 13% 22% 9% 1	1562
May	1st to 10th	Night	53% 8% 13% 17% 9%	1373
	11th to 20th	Night	43% 8% 12% 25% 12%	1239
	21st to end	Night	51% 8% 12% 18% 10% 1	1358
June	1st to 10th	Night	52% 9% 12% 20% 6%	1166
	11th to 20th	Night	58% 12% 14% 14% 2	1080
	21st to end	Night	45% 12% 18% 19% 5%	1080
July	1st to 10th	Night	43% 10% 19% 22% 6% 1	1050
	11th to 20th	Night	46% 14% 22% 14% 4	1150
	21st to end	Night	37% 13% 24% 23% 3	1318
August	1st to 10th	Night	43% 12% 21% 22% 3	1424
	11th to 20th	Night	39% 11% 19% 24% 7%	1455
	21st to end	Night	40% 11% 18% 24% 8%	1782
September	1st to 10th	Night	39% 8% 18% 26% 7% 2	1743
	11th to 20th	Night	39% 7% 16% 28% 9% 2	1929
	21st to end	Night	38% 8% 15% 24% 13% 2	1980
October	1st to 10th	Night	46% 7% 13% 24% 9% 1	2076
	11th to 20th	Night	44% 7% 15% 25% 8%	2340
	21st to end	Night	49% 9% 14% 20% 8% 1	2574
November	1st to 10th	Night	57% 7% 11% 18% 6%	2565
	11th to 20th	Night	54% 8% 12% 16% 9% 1	2850
	21st to end	Night	52% 10% 13% 16% 6% 2	2850
December	1st to 10th	Night	52% 7% 11% 18% 10% 2	2850
	11th to 20th	Night	54% 8% 16% 17% 6%	3580
	21st to end	Night	41% 8% 12% 24% 13% 2	5566

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	9% 11% 26% 33% 16% 6%	180
	11th to 20th	Day	12% 15% 24% 30% 14% 5%	168
	21st to end	Day	13% 12% 31% 29% 11% 5%	176
February	1st to 10th	Day	18% 14% 31% 28% 9% 1	160
	11th to 20th	Day	15% 13% 33% 30% 8% 2	186
	21st to end	Day	8% 10% 34% 32% 13% 4	319
March	1st to 10th	Day	14% 15% 29% 26% 13% 2	420
	11th to 20th	Day	9% 11% 36% 28% 12% 5%	420
	21st to end	Day	20% 10% 29% 24% 15% 2	462
April	1st to 10th	Day	16% 15% 32% 26% 8% 3	420
	11th to 20th	Day	19% 12% 25% 24% 14% 6%	412
	21st to end	Day	15% 13% 31% 27% 11% 3	420
May	1st to 10th	Day	17% 12% 28% 26% 15% 3	490
	11th to 20th	Day	14% 8% 24% 30% 20% 5%	625
	21st to end	Day	18% 7% 22% 28% 19% 6%	679
June	1st to 10th	Day	19% 8% 30% 29% 11% 3	720
	11th to 20th	Day	22% 9% 22% 28% 15% 4	810
	21st to end	Day	14% 6% 22% 30% 20% 9%	810
July	1st to 10th	Day	18% 5% 20% 33% 19% 6%	796
	11th to 20th	Day	15% 6% 26% 28% 19% 6%	686
	21st to end	Day	12% 6% 16% 33% 23% 9%	666
August	1st to 10th	Day	13% 7% 21% 28% 19% 11%	447
	11th to 20th	Day	9% 9% 27% 28% 22% 5%	437
	21st to end	Day	18% 8% 20% 28% 20% 6%	495
September	1st to 10th	Day	19% 9% 26% 25% 16% 4	450
	11th to 20th	Day	20% 9% 23% 27% 16% 5%	450
	21st to end	Day	16% 12% 26% 25% 16% 5%	450
October	1st to 10th	Day	16% 10% 26% 25% 19% 4	404
	11th to 20th	Day	15% 11% 31% 30% 9% 4	270
	21st to end	Day	17% 15% 32% 26% 7% 3	297
November	1st to 10th	Day	22% 8% 29% 27% 10% 3	248
	11th to 20th	Day	14% 12% 36% 28% 7% 2	180
	21st to end	Day	11% 12% 39% 24% 8% 6%	180
December	1st to 10th	Day	14% 12% 31% 28% 10% 5%	180
	11th to 20th	Day	14% 15% 31% 26% 12% 2	180
	21st to end	Day	13% 7% 31% 28% 17% 4	198
January	1st to 10th	Night	13% 10% 34% 30% 12% 2	1029
	11th to 20th	Night	13% 12% 35% 29% 10% 1	921
	21st to end	Night	16% 16% 30% 28% 10% 1	828
February	1st to 10th	Night	16% 13% 25% 29% 14% 3	720
	11th to 20th	Night	12% 9% 35% 30% 12% 2	704
	21st to end	Night	10% 8% 32% 31% 16% 4	592
March	1st to 10th	Night	13% 8% 31% 31% 16% 2	720
	11th to 20th	Night	12% 8% 32% 34% 12% 2	720
	21st to end	Night	16% 7% 25% 33% 17% 2	792
April	1st to 10th	Night	14% 9% 31% 32% 13% 1	692
	11th to 20th	Night	15% 7% 32% 33% 13% 1	533
	21st to end	Night	17% 9% 29% 33% 12% 1	482
May	1st to 10th	Night	19% 9% 24% 32% 13% 3	360
	11th to 20th	Night	15% 5% 27% 35% 15% 3	354
	21st to end	Night	22% 6% 20% 31% 18% 2	389
June	1st to 10th	Night	20% 8% 22% 32% 16% 1	360
	11th to 20th	Night	20% 5% 24% 29% 19% 3	360
	21st to end	Night	15% 5% 18% 36% 24% 2	360
July	1st to 10th	Night	20% 3 14% 39% 22% 1	350
	11th to 20th	Night	16% 5% 18% 35% 26% 1	348
	21st to end	Night	13% 3 14% 42% 24% 4	394
August	1st to 10th	Night	13% 4 15% 32% 31% 5%	358
	11th to 20th	Night	13% 4 14% 43% 24% 3	387
	21st to end	Night	14% 4 22% 37% 22% 1	594
September	1st to 10th	Night	16% 3 21% 40% 19% 2	663
	11th to 20th	Night	17% 3 21% 39% 20% 1	720
	21st to end	Night	14% 7% 27% 34% 16% 3	720
October	1st to 10th	Night	19% 7% 26% 33% 15% 1	720
	11th to 20th	Night	14% 8% 29% 33% 14% 2	720
	21st to end	Night	15% 11% 31% 32% 9% 2	792
November	1st to 10th	Night	23% 8% 27% 30% 11% 1	857
	11th to 20th	Night	14% 13% 39% 24% 10% 1	950
	21st to end	Night	14% 8% 38% 29% 9% 2	950
December	1st to 10th	Night	13% 12% 32% 29% 12% 1	950
	11th to 20th	Night	12% 12% 29% 34% 12% 1	972
	21st to end	Night	12% 9% 32% 30% 14% 2	1144

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