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 2026-02-16.

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	10% 10% 4 9% 6% 5% 5% 4 6% 8% 34%	134882
	Night	9% 11% 4 10% 6% 5% 5% 4 6% 7% 32%	134638
Transparency	Day	45% 8% 16% 22% 9% 1	134574
	Night	48% 8% 15% 20% 9% 1	134526
Seeing	Day	16% 10% 27% 28% 15% 5%	17648
	Night	15% 9% 29% 32% 14% 2	26245

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% 15% 5% 12% 8% 6% 5% 4 6% 7% 27%	3587
	11th to 20th	Day	5% 8% 3 10% 5% 4 5% 5% 7% 9% 40%	4119
	21st to end	Day	7% 12% 5% 10% 6% 5% 6% 4 6% 7% 32%	5124
February	1st to 10th	Day	7% 14% 4 12% 7% 7% 7% 4 8% 7% 22%	1680
	11th to 20th	Day	5% 15% 4 11% 7% 5% 6% 4 8% 6% 29%	1663
	21st to end	Day	3 13% 5% 14% 7% 7% 6% 5% 7% 7% 26%	1620
March	1st to 10th	Day	3 10% 5% 11% 7% 6% 7% 5% 7% 8% 30%	4518
	11th to 20th	Day	6% 13% 5% 10% 5% 3 4 4 6% 8% 37%	4813
	21st to end	Day	10% 16% 6% 13% 6% 5% 5% 4 5% 6% 23%	5763
April	1st to 10th	Day	7% 15% 5% 10% 7% 5% 6% 6% 6% 9% 25%	2431
	11th to 20th	Day	11% 13% 4 9% 6% 6% 5% 4 7% 8% 28%	2570
	21st to end	Day	10% 12% 4 9% 6% 5% 5% 5% 7% 8% 29%	2662
May	1st to 10th	Day	12% 10% 3 8% 6% 6% 5% 4 5% 7% 34%	6499
	11th to 20th	Day	12% 11% 4 10% 7% 6% 7% 5% 7% 8% 24%	6734
	21st to end	Day	10% 7% 3 8% 6% 5% 5% 4 7% 8% 36%	7405
June	1st to 10th	Day	16% 10% 3 7% 6% 5% 5% 4 6% 8% 31%	3100
	11th to 20th	Day	19% 9% 4 8% 7% 6% 6% 5% 6% 7% 22%	3200
	21st to end	Day	10% 6% 3 9% 5% 5% 7% 5% 8% 9% 33%	3200
July	1st to 10th	Day	9% 5% 2 5% 4 4 5% 5% 7% 10% 45%	7240
	11th to 20th	Day	11% 5% 2 7% 6% 5% 6% 5% 7% 8% 39%	6950
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	7331
August	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	4541
	11th to 20th	Day	11% 7% 2 6% 4 5% 5% 4 6% 9% 40%	4411
	21st to end	Day	13% 5% 2 6% 5% 5% 5% 4 7% 8% 40%	4664
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 4 4 5% 8% 42%	2460
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 5% 9% 44%	2255
	21st to end	Day	10% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	2200
October	1st to 10th	Day	8% 9% 4 9% 5% 4 4 3 5% 6% 42%	3413
	11th to 20th	Day	12% 11% 4 10% 5% 5% 5% 4 6% 7% 31%	2932
	21st to end	Day	6% 13% 5% 11% 5% 4 4 3 5% 7% 36%	3200
November	1st to 10th	Day	11% 16% 5% 10% 7% 4 4 4 7% 7% 25%	1650
	11th to 20th	Day	6% 12% 5% 10% 6% 6% 5% 4 6% 6% 33%	1470
	21st to end	Day	7% 14% 4 12% 7% 7% 6% 4 6% 7% 26%	1470
December	1st to 10th	Day	9% 16% 6% 12% 7% 5% 6% 4 6% 7% 21%	2360
	11th to 20th	Day	9% 14% 4 11% 7% 5% 5% 4 6% 8% 25%	2589
	21st to end	Day	4 10% 3 9% 6% 4 5% 5% 6% 7% 41%	3058
January	1st to 10th	Night	6% 16% 5% 14% 9% 6% 6% 4 5% 5% 24%	9290
	11th to 20th	Night	3 10% 5% 11% 6% 5% 5% 4 7% 7% 37%	8779
	21st to end	Night	7% 13% 5% 10% 5% 5% 5% 4 6% 8% 32%	9114
February	1st to 10th	Night	8% 14% 5% 13% 7% 8% 5% 5% 6% 7% 25%	2940
	11th to 20th	Night	5% 12% 6% 12% 8% 6% 5% 4 6% 8% 28%	2732
	21st to end	Night	4 14% 5% 10% 7% 6% 5% 5% 6% 8% 29%	2006
March	1st to 10th	Night	4 10% 5% 11% 7% 6% 5% 4 7% 10% 32%	5376
	11th to 20th	Night	5% 13% 4 11% 6% 6% 5% 5% 7% 9% 28%	5186
	21st to end	Night	11% 17% 6% 14% 5% 5% 4 3 4 6% 24%	5209
April	1st to 10th	Night	7% 14% 4 11% 7% 6% 6% 5% 6% 7% 27%	1968
	11th to 20th	Night	10% 12% 4 10% 6% 6% 6% 5% 6% 7% 28%	1781
	21st to end	Night	11% 10% 3 9% 5% 5% 6% 5% 7% 9% 32%	1735
May	1st to 10th	Night	16% 11% 3 9% 5% 5% 4 3 5% 6% 35%	3461
	11th to 20th	Night	12% 9% 5% 11% 6% 7% 6% 5% 6% 8% 25%	3159
	21st to end	Night	14% 8% 3 7% 5% 5% 4 4 6% 7% 37%	3470
June	1st to 10th	Night	15% 11% 3 10% 5% 4 3 5% 6% 7% 29%	1295
	11th to 20th	Night	19% 9% 4 8% 7% 6% 7% 5% 7% 7% 22%	1200
	21st to end	Night	13% 7% 3 9% 5% 4 6% 5% 7% 10% 31%	1200
July	1st to 10th	Night	10% 6% 3 7% 5% 5% 5% 4 7% 7% 41%	2720
	11th to 20th	Night	13% 6% 3 8% 6% 6% 6% 5% 7% 9% 32%	2996
	21st to end	Night	13% 6% 3 7% 5% 5% 5% 3 6% 8% 40%	3510
August	1st to 10th	Night	16% 6% 3 6% 5% 5% 4 3 6% 6% 40%	2609
	11th to 20th	Night	14% 9% 3 7% 6% 4 5% 4 6% 9% 33%	2638
	21st to end	Night	13% 6% 2 6% 3 4 5% 4 7% 8% 41%	3245
September	1st to 10th	Night	11% 6% 2 5% 4 5% 5% 4 6% 7% 44%	1937
	11th to 20th	Night	12% 8% 3 5% 4 4 4 4 5% 8% 44%	2144
	21st to end	Night	10% 7% 2 7% 5% 4 6% 4 6% 8% 41%	2200
October	1st to 10th	Night	11% 9% 3 8% 5% 5% 6% 4 6% 8% 35%	3780
	11th to 20th	Night	10% 8% 3 9% 6% 6% 4 4 6% 8% 35%	4216
	21st to end	Night	7% 9% 3 9% 6% 6% 5% 5% 7% 9% 33%	4600
November	1st to 10th	Night	13% 16% 5% 9% 6% 4 5% 4 5% 6% 27%	2844
	11th to 20th	Night	8% 13% 5% 11% 7% 5% 5% 4 5% 6% 30%	3150
	21st to end	Night	7% 13% 5% 11% 8% 6% 5% 5% 7% 7% 27%	3150
December	1st to 10th	Night	8% 17% 5% 11% 7% 6% 5% 4 5% 6% 26%	5040
	11th to 20th	Night	9% 16% 6% 13% 8% 6% 5% 4 5% 7% 21%	5818
	21st to end	Night	3 9% 3 10% 7% 5% 6% 5% 6% 6% 39%	8140

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	55% 6% 11% 14% 14% 1	3587
	11th to 20th	Day	39% 5% 17% 18% 19% 3	4140
	21st to end	Day	46% 8% 20% 21% 4	5125
February	1st to 10th	Day	56% 9% 13% 11% 11% 1	1680
	11th to 20th	Day	50% 8% 13% 19% 10%	1671
	21st to end	Day	54% 6% 13% 17% 10% 1	1620
March	1st to 10th	Day	48% 6% 14% 18% 13%	4518
	11th to 20th	Day	43% 7% 14% 22% 14% 1	4814
	21st to end	Day	58% 7% 11% 15% 8% 1	5763
April	1st to 10th	Day	50% 9% 14% 19% 7% 1	2431
	11th to 20th	Day	49% 8% 14% 20% 9% 1	2570
	21st to end	Day	47% 10% 13% 21% 7% 1	2662
May	1st to 10th	Day	46% 7% 9% 22% 15% 1	6499
	11th to 20th	Day	51% 10% 12% 21% 6%	6738
	21st to end	Day	39% 10% 17% 25% 8% 1	7406
June	1st to 10th	Day	47% 8% 16% 21% 7% 1	3100
	11th to 20th	Day	56% 11% 15% 15% 4	3200
	21st to end	Day	42% 11% 18% 22% 6%	3200
July	1st to 10th	Day	30% 8% 20% 28% 11% 1	7161
	11th to 20th	Day	39% 9% 16% 30% 7%	6865
	21st to end	Day	37% 9% 21% 24% 8%	7125
August	1st to 10th	Day	39% 8% 20% 31% 2	4524
	11th to 20th	Day	38% 9% 19% 24% 10%	4453
	21st to end	Day	39% 9% 24% 23% 5%	4663
September	1st to 10th	Day	39% 8% 19% 27% 6% 1	2460
	11th to 20th	Day	38% 8% 18% 28% 8%	2255
	21st to end	Day	42% 9% 15% 23% 10% 2	2200
October	1st to 10th	Day	42% 7% 23% 24% 5%	3413
	11th to 20th	Day	49% 7% 13% 23% 8%	2932
	21st to end	Day	47% 6% 17% 25% 5%	3200
November	1st to 10th	Day	58% 6% 14% 16% 6% 1	1650
	11th to 20th	Day	49% 8% 15% 18% 9%	1470
	21st to end	Day	54% 10% 13% 14% 7% 2	1470
December	1st to 10th	Day	60% 8% 16% 10% 5% 1	2360
	11th to 20th	Day	57% 7% 15% 14% 6% 1	2591
	21st to end	Day	38% 7% 13% 25% 16% 2	3058
January	1st to 10th	Night	60% 5% 10% 14% 10% 1	9291
	11th to 20th	Night	44% 5% 16% 18% 15% 3	8820
	21st to end	Night	47% 8% 21% 18% 6%	9114
February	1st to 10th	Night	56% 7% 12% 15% 9% 1	2940
	11th to 20th	Night	53% 7% 12% 17% 10% 1	2734
	21st to end	Night	51% 6% 10% 20% 11% 1	2006
March	1st to 10th	Night	45% 7% 18% 20% 11%	5376
	11th to 20th	Night	47% 8% 15% 19% 11%	5186
	21st to end	Night	58% 7% 8% 17% 9%	5209
April	1st to 10th	Night	52% 9% 13% 16% 9% 1	1968
	11th to 20th	Night	51% 7% 11% 20% 9% 1	1781
	21st to end	Night	46% 8% 14% 23% 8% 1	1735
May	1st to 10th	Night	48% 6% 10% 20% 15% 2	3458
	11th to 20th	Night	53% 10% 14% 19% 5%	3159
	21st to end	Night	42% 9% 17% 23% 9% 1	3470
June	1st to 10th	Night	51% 8% 12% 20% 7% 1	1295
	11th to 20th	Night	57% 11% 14% 15% 2	1200
	21st to end	Night	45% 11% 18% 19% 6%	1200
July	1st to 10th	Night	36% 10% 19% 20% 14% 1	2690
	11th to 20th	Night	45% 9% 16% 24% 6%	2958
	21st to end	Night	39% 9% 23% 23% 6% 1	3412
August	1st to 10th	Night	41% 9% 19% 28% 2	2601
	11th to 20th	Night	44% 9% 17% 20% 10%	2662
	21st to end	Night	38% 8% 22% 26% 6%	3245
September	1st to 10th	Night	36% 8% 18% 27% 8% 2	1937
	11th to 20th	Night	37% 7% 16% 29% 10% 1	2144
	21st to end	Night	38% 8% 16% 24% 13% 2	2200
October	1st to 10th	Night	44% 7% 21% 22% 7%	3780
	11th to 20th	Night	43% 8% 14% 27% 8%	4216
	21st to end	Night	43% 9% 19% 22% 7%	4600
November	1st to 10th	Night	56% 7% 12% 18% 7% 1	2844
	11th to 20th	Night	53% 8% 12% 16% 9% 1	3150
	21st to end	Night	53% 9% 12% 17% 6% 2	3150
December	1st to 10th	Night	58% 7% 13% 15% 6% 1	5040
	11th to 20th	Night	61% 7% 14% 14% 4	5815
	21st to end	Night	41% 7% 11% 26% 13% 2	8140

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	11% 13% 26% 32% 13% 5%	220
	11th to 20th	Day	15% 15% 28% 27% 12% 3	220
	21st to end	Day	14% 13% 32% 26% 12% 3	242
February	1st to 10th	Day	16% 13% 31% 30% 9% 1	220
	11th to 20th	Day	12% 11% 36% 32% 7% 2	243
	21st to end	Day	9% 13% 36% 28% 11% 3	384
March	1st to 10th	Day	13% 14% 31% 27% 13% 2	500
	11th to 20th	Day	10% 11% 34% 28% 13% 4	500
	21st to end	Day	19% 11% 28% 25% 15% 2	550
April	1st to 10th	Day	16% 16% 31% 25% 9% 3	500
	11th to 20th	Day	18% 13% 27% 23% 13% 5%	492
	21st to end	Day	16% 13% 31% 26% 12% 2	500
May	1st to 10th	Day	18% 12% 28% 26% 14% 2	578
	11th to 20th	Day	14% 8% 25% 29% 19% 5%	725
	21st to end	Day	17% 8% 22% 29% 19% 6%	789
June	1st to 10th	Day	19% 10% 31% 27% 11% 2	830
	11th to 20th	Day	22% 8% 24% 27% 15% 4	930
	21st to end	Day	13% 6% 23% 29% 20% 8%	930
July	1st to 10th	Day	16% 6% 21% 33% 18% 5%	916
	11th to 20th	Day	15% 6% 26% 28% 18% 7%	793
	21st to end	Day	13% 7% 16% 33% 22% 9%	774
August	1st to 10th	Day	12% 7% 21% 29% 19% 11%	527
	11th to 20th	Day	11% 9% 27% 29% 20% 5%	517
	21st to end	Day	17% 8% 19% 28% 21% 8%	583
September	1st to 10th	Day	17% 9% 24% 26% 19% 5%	530
	11th to 20th	Day	18% 8% 22% 29% 16% 6%	530
	21st to end	Day	18% 12% 27% 24% 14% 4	530
October	1st to 10th	Day	15% 10% 28% 27% 17% 4	478
	11th to 20th	Day	15% 11% 31% 30% 9% 4	330
	21st to end	Day	17% 14% 36% 25% 7% 3	363
November	1st to 10th	Day	22% 8% 32% 26% 10% 2	302
	11th to 20th	Day	14% 12% 37% 25% 9% 2	220
	21st to end	Day	12% 12% 37% 24% 10% 5%	220
December	1st to 10th	Day	16% 13% 32% 25% 10% 4	220
	11th to 20th	Day	16% 15% 30% 27% 11% 2	220
	21st to end	Day	14% 7% 30% 29% 15% 4	242
January	1st to 10th	Night	13% 10% 34% 29% 12% 2	1146
	11th to 20th	Night	13% 13% 35% 28% 9% 1	1050
	21st to end	Night	16% 16% 31% 27% 10% 1	966
February	1st to 10th	Night	15% 12% 25% 31% 14% 3	840
	11th to 20th	Night	11% 8% 35% 32% 12% 2	804
	21st to end	Night	9% 10% 33% 29% 15% 3	660
March	1st to 10th	Night	12% 8% 30% 32% 16% 2	800
	11th to 20th	Night	11% 7% 33% 34% 13% 2	800
	21st to end	Night	16% 7% 26% 33% 16% 2	880
April	1st to 10th	Night	13% 9% 31% 33% 13% 1	768
	11th to 20th	Night	15% 7% 32% 33% 12% 1	593
	21st to end	Night	16% 9% 29% 34% 12% 1	535
May	1st to 10th	Night	19% 9% 24% 32% 12% 3	400
	11th to 20th	Night	15% 6% 28% 34% 15% 3	394
	21st to end	Night	21% 8% 21% 31% 18% 2	433
June	1st to 10th	Night	20% 9% 24% 32% 15% 1	400
	11th to 20th	Night	20% 5% 25% 28% 20% 3	400
	21st to end	Night	15% 6% 18% 36% 24% 2	400
July	1st to 10th	Night	19% 4 15% 39% 22% 1	390
	11th to 20th	Night	16% 5% 18% 35% 26% 1	388
	21st to end	Night	14% 3 14% 41% 24% 3	438
August	1st to 10th	Night	14% 4 15% 33% 31% 5%	398
	11th to 20th	Night	13% 4 15% 41% 25% 3	431
	21st to end	Night	14% 5% 21% 37% 23% 1	660
September	1st to 10th	Night	15% 3 20% 40% 21% 1	737
	11th to 20th	Night	16% 3 20% 39% 21% 1	800
	21st to end	Night	14% 7% 30% 33% 14% 2	800
October	1st to 10th	Night	18% 8% 27% 33% 14% 1	800
	11th to 20th	Night	14% 9% 29% 33% 14% 2	800
	21st to end	Night	14% 12% 32% 30% 9% 2	880
November	1st to 10th	Night	22% 9% 28% 29% 10% 1	950
	11th to 20th	Night	14% 13% 38% 25% 10% 1	1050
	21st to end	Night	14% 9% 37% 29% 10% 2	1050
December	1st to 10th	Night	14% 13% 33% 27% 11% 1	1050
	11th to 20th	Night	13% 12% 29% 33% 11% 1	1078
	21st to end	Night	12% 9% 33% 30% 14% 2	1276

Page generated at 2026-02-12 23:42:12. Version ac07

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