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 2024-07-02.

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% 31%	89506
	Night	9% 11% 4 10% 6% 6% 5% 4 6% 7% 31%	89713
Transparency	Day	47% 9% 16% 19% 8% 1	89200
	Night	49% 8% 14% 19% 9% 1	89600
Seeing	Day	16% 10% 27% 28% 15% 5%	15578
	Night	15% 8% 28% 32% 14% 2	24039

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	4 15% 5% 13% 8% 6% 5% 4 7% 8% 25%	1931
	11th to 20th	Day	6% 10% 4 10% 6% 4 5% 6% 8% 9% 32%	2199
	21st to end	Day	6% 10% 4 10% 7% 5% 6% 5% 6% 8% 33%	2760
February	1st to 10th	Day	8% 15% 5% 13% 6% 7% 7% 4 7% 7% 21%	1520
	11th to 20th	Day	5% 15% 5% 11% 7% 5% 6% 4 7% 6% 28%	1539
	21st to end	Day	4 13% 5% 14% 8% 7% 6% 5% 6% 7% 25%	1541
March	1st to 10th	Day	3 11% 6% 14% 8% 6% 6% 5% 6% 8% 28%	3138
	11th to 20th	Day	5% 11% 4 8% 4 4 4 4 6% 8% 41%	3355
	21st to end	Day	7% 16% 7% 15% 6% 5% 5% 4 6% 6% 24%	4024
April	1st to 10th	Day	7% 15% 5% 10% 7% 5% 6% 6% 6% 9% 24%	2309
	11th to 20th	Day	11% 13% 4 9% 6% 6% 5% 4 7% 8% 27%	2440
	21st to end	Day	11% 12% 4 9% 6% 5% 5% 5% 7% 8% 29%	2529
May	1st to 10th	Day	16% 11% 4 9% 6% 6% 4 4 5% 7% 29%	4543
	11th to 20th	Day	11% 11% 4 9% 7% 6% 7% 5% 6% 8% 27%	4694
	21st to end	Day	13% 9% 3 10% 7% 6% 6% 4 7% 8% 25%	5161
June	1st to 10th	Day	16% 10% 3 7% 6% 5% 5% 4 6% 8% 30%	2945
	11th to 20th	Day	19% 9% 4 8% 7% 6% 6% 5% 6% 7% 21%	3040
	21st to end	Day	10% 6% 3 9% 5% 5% 7% 5% 8% 9% 33%	3040
July	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 5% 7% 7% 35%	2900
	11th to 20th	Day	12% 5% 3 6% 6% 6% 6% 6% 7% 10% 32%	2772
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 9% 40%	2920
August	1st to 10th	Day	11% 6% 3 8% 5% 5% 6% 5% 8% 8% 36%	2507
	11th to 20th	Day	8% 7% 3 6% 4 5% 5% 4 7% 10% 41%	2444
	21st to end	Day	14% 6% 3 6% 5% 5% 5% 5% 7% 8% 36%	2574
September	1st to 10th	Day	13% 6% 2 7% 6% 5% 5% 4 6% 8% 39%	2214
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 4 9% 42%	2030
	21st to end	Day	10% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	1980
October	1st to 10th	Day	10% 11% 4 10% 4 5% 5% 4 5% 6% 36%	1880
	11th to 20th	Day	10% 10% 3 8% 5% 4 5% 5% 6% 7% 35%	1620
	21st to end	Day	9% 15% 4 10% 6% 5% 5% 3 5% 5% 32%	1782
November	1st to 10th	Day	11% 17% 5% 9% 7% 4 4 3 7% 7% 24%	1491
	11th to 20th	Day	7% 12% 5% 11% 6% 6% 5% 4 5% 6% 34%	1330
	21st to end	Day	7% 13% 4 12% 7% 6% 6% 4 7% 7% 27%	1330
December	1st to 10th	Day	7% 13% 6% 12% 7% 5% 6% 4 5% 7% 28%	1330
	11th to 20th	Day	9% 13% 4 12% 6% 4 5% 4 5% 9% 30%	1604
	21st to end	Day	5% 11% 4 10% 6% 4 5% 4 6% 7% 38%	2090
January	1st to 10th	Night	4 14% 5% 15% 8% 5% 5% 4 5% 6% 29%	5012
	11th to 20th	Night	6% 12% 5% 11% 7% 6% 6% 5% 7% 7% 29%	4699
	21st to end	Night	7% 13% 5% 10% 6% 5% 5% 5% 6% 7% 31%	4926
February	1st to 10th	Night	8% 14% 5% 13% 7% 8% 5% 4 6% 7% 25%	2660
	11th to 20th	Night	5% 13% 6% 11% 8% 6% 5% 4 6% 8% 28%	2526
	21st to end	Night	5% 14% 5% 11% 7% 6% 5% 5% 6% 8% 29%	1909
March	1st to 10th	Night	3 10% 5% 13% 8% 7% 6% 5% 7% 9% 26%	3736
	11th to 20th	Night	5% 10% 3 8% 5% 5% 5% 5% 8% 10% 36%	3624
	21st to end	Night	7% 16% 7% 16% 5% 6% 4 4 5% 6% 23%	3626
April	1st to 10th	Night	7% 14% 4 11% 7% 6% 6% 5% 7% 7% 26%	1870
	11th to 20th	Night	10% 12% 4 10% 6% 6% 6% 5% 6% 7% 28%	1691
	21st to end	Night	11% 10% 3 8% 5% 5% 6% 5% 7% 9% 31%	1648
May	1st to 10th	Night	18% 13% 4 10% 5% 5% 4 3 5% 5% 29%	2417
	11th to 20th	Night	11% 9% 5% 9% 6% 6% 7% 5% 7% 9% 27%	2199
	21st to end	Night	19% 10% 3 9% 5% 6% 5% 5% 7% 7% 25%	2414
June	1st to 10th	Night	16% 11% 3 10% 5% 4 3 5% 6% 7% 29%	1230
	11th to 20th	Night	18% 8% 4 8% 7% 6% 6% 5% 7% 7% 22%	1140
	21st to end	Night	14% 7% 3 9% 5% 4 6% 5% 7% 10% 30%	1140
July	1st to 10th	Night	17% 7% 3 6% 5% 4 5% 4 7% 8% 36%	1088
	11th to 20th	Night	12% 5% 3 7% 8% 7% 6% 6% 9% 8% 29%	1188
	21st to end	Night	10% 6% 3 7% 6% 5% 5% 4 7% 10% 36%	1395
August	1st to 10th	Night	12% 6% 3 8% 5% 6% 5% 4 8% 8% 35%	1432
	11th to 20th	Night	8% 8% 3 6% 6% 5% 6% 4 7% 10% 38%	1455
	21st to end	Night	12% 7% 3 6% 4 6% 5% 4 7% 8% 38%	1782
September	1st to 10th	Night	12% 6% 3 6% 5% 5% 5% 4 6% 7% 41%	1743
	11th to 20th	Night	13% 8% 3 5% 4 4 4 4 5% 8% 43%	1929
	21st to end	Night	10% 7% 2 7% 5% 4 5% 4 6% 8% 41%	1980
October	1st to 10th	Night	13% 10% 3 8% 5% 4 5% 3 4 7% 38%	2076
	11th to 20th	Night	10% 9% 3 8% 6% 5% 4 4 6% 8% 36%	2340
	21st to end	Night	8% 12% 4 9% 7% 6% 5% 4 7% 8% 30%	2574
November	1st to 10th	Night	13% 16% 5% 9% 6% 4 5% 4 5% 6% 26%	2565
	11th to 20th	Night	9% 13% 6% 11% 7% 5% 5% 4 5% 6% 29%	2850
	21st to end	Night	7% 13% 5% 10% 8% 7% 6% 5% 7% 7% 27%	2850
December	1st to 10th	Night	6% 14% 6% 10% 6% 6% 5% 4 5% 6% 30%	2850
	11th to 20th	Night	8% 14% 5% 10% 7% 6% 5% 4 5% 8% 27%	3583
	21st to end	Night	4 10% 3 9% 6% 5% 6% 5% 6% 6% 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% 9% 14% 18% 6% 1	1931
	11th to 20th	Day	45% 6% 12% 13% 22% 3	2220
	21st to end	Day	45% 10% 20% 18% 6% 1	2760
February	1st to 10th	Day	58% 9% 13% 10% 9% 1	1520
	11th to 20th	Day	51% 8% 13% 18% 10%	1547
	21st to end	Day	55% 6% 13% 16% 9% 1	1541
March	1st to 10th	Day	52% 6% 15% 18% 9%	3138
	11th to 20th	Day	38% 8% 18% 20% 16% 1	3356
	21st to end	Day	57% 8% 13% 17% 6%	4024
April	1st to 10th	Day	51% 10% 14% 18% 7% 1	2309
	11th to 20th	Day	50% 8% 14% 20% 8% 1	2440
	21st to end	Day	48% 10% 13% 20% 7% 2	2529
May	1st to 10th	Day	51% 8% 10% 20% 10%	4543
	11th to 20th	Day	49% 10% 11% 22% 8%	4698
	21st to end	Day	49% 12% 15% 14% 9% 1	5162
June	1st to 10th	Day	49% 8% 16% 20% 7% 1	2945
	11th to 20th	Day	56% 11% 14% 15% 5%	3040
	21st to end	Day	41% 12% 19% 22% 6%	3040
July	1st to 10th	Day	43% 11% 18% 22% 5% 1	2821
	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	54% 7% 13% 16% 9% 2	5012
	11th to 20th	Night	52% 6% 11% 12% 16% 4	4740
	21st to end	Night	49% 10% 17% 16% 7% 1	4926
February	1st to 10th	Night	57% 7% 12% 15% 8% 1	2660
	11th to 20th	Night	53% 7% 13% 17% 9% 1	2528
	21st to end	Night	52% 6% 10% 20% 10% 1	1909
March	1st to 10th	Night	51% 8% 15% 18% 7%	3736
	11th to 20th	Night	39% 9% 18% 20% 14%	3624
	21st to end	Night	58% 8% 10% 18% 6%	3626
April	1st to 10th	Night	53% 9% 14% 15% 8% 1	1870
	11th to 20th	Night	51% 8% 11% 20% 9% 2	1691
	21st to end	Night	46% 8% 14% 22% 8% 1	1648
May	1st to 10th	Night	55% 7% 10% 17% 10%	2417
	11th to 20th	Night	48% 11% 13% 22% 7%	2199
	21st to end	Night	51% 11% 16% 13% 9% 1	2414
June	1st to 10th	Night	52% 8% 12% 20% 7% 1	1230
	11th to 20th	Night	56% 11% 14% 16% 2	1140
	21st to end	Night	46% 11% 18% 19% 6%	1140
July	1st to 10th	Night	43% 10% 19% 22% 6% 1	1058
	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	11% 15% 27% 29% 15% 3	180
	21st to end	Day	13% 12% 31% 28% 13% 4	198
February	1st to 10th	Day	18% 13% 30% 29% 8% 1	180
	11th to 20th	Day	13% 11% 33% 32% 8% 2	209
	21st to end	Day	9% 12% 35% 30% 12% 3	353
March	1st to 10th	Day	13% 15% 31% 27% 13% 2	460
	11th to 20th	Day	8% 11% 36% 28% 13% 4	460
	21st to end	Day	19% 10% 28% 26% 14% 2	506
April	1st to 10th	Day	17% 16% 32% 25% 7% 3	460
	11th to 20th	Day	18% 12% 27% 24% 14% 5%	452
	21st to end	Day	16% 13% 31% 26% 12% 2	460
May	1st to 10th	Day	18% 11% 28% 26% 14% 2	534
	11th to 20th	Day	14% 8% 26% 29% 18% 5%	675
	21st to end	Day	18% 8% 22% 29% 19% 5%	734
June	1st to 10th	Day	19% 9% 30% 28% 11% 3	775
	11th to 20th	Day	22% 8% 24% 27% 15% 4	870
	21st to end	Day	14% 7% 22% 29% 20% 9%	870
July	1st to 10th	Day	18% 5% 20% 33% 19% 6%	804
	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% 34% 30% 9% 1	950
	21st to end	Night	16% 16% 30% 28% 10% 1	874
February	1st to 10th	Night	15% 12% 25% 31% 14% 3	760
	11th to 20th	Night	11% 8% 35% 31% 12% 2	744
	21st to end	Night	10% 9% 32% 30% 15% 3	628
March	1st to 10th	Night	12% 8% 31% 32% 16% 2	760
	11th to 20th	Night	11% 8% 33% 33% 13% 2	760
	21st to end	Night	16% 7% 26% 33% 16% 2	836
April	1st to 10th	Night	13% 10% 32% 32% 12% 1	730
	11th to 20th	Night	15% 6% 32% 34% 13% 1	563
	21st to end	Night	16% 9% 29% 33% 11% 1	508
May	1st to 10th	Night	19% 9% 24% 32% 13% 3	380
	11th to 20th	Night	15% 6% 28% 34% 15% 3	374
	21st to end	Night	21% 7% 21% 30% 18% 2	411
June	1st to 10th	Night	20% 8% 24% 31% 16% 1	380
	11th to 20th	Night	19% 5% 25% 28% 19% 3	380
	21st to end	Night	16% 5% 18% 36% 23% 2	380
July	1st to 10th	Night	20% 3 14% 39% 22% 1	353
	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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