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 2024-11-30.

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%	106135
	Night	9% 11% 4 10% 6% 5% 5% 4 6% 7% 32%	102011
Transparency	Day	45% 9% 16% 21% 8% 1	105871
	Night	48% 8% 15% 20% 9% 1	101922
Seeing	Day	16% 10% 27% 28% 15% 5%	16147
	Night	15% 8% 28% 32% 15% 2	24561

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	10% 5% 2 5% 4 4 4 4 6% 8% 46%	5060
	11th to 20th	Day	8% 4 2 5% 5% 5% 5% 4 6% 8% 46%	4854
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 10% 40%	5107
August	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 6% 7% 42%	4401
	11th to 20th	Day	10% 7% 2 6% 4 5% 5% 4 6% 9% 41%	4273
	21st to end	Day	14% 6% 3 6% 5% 5% 5% 4 7% 8% 38%	4521
September	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	2337
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 4 9% 42%	2142
	21st to end	Day	10% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	2090
October	1st to 10th	Day	8% 9% 4 9% 4 4 4 3 5% 6% 42%	3309
	11th to 20th	Day	12% 11% 4 10% 5% 5% 5% 4 6% 7% 31%	2842
	21st to end	Day	6% 13% 5% 11% 5% 4 4 3 5% 7% 36%	3101
November	1st to 10th	Day	12% 16% 5% 10% 7% 4 4 4 7% 7% 24%	1570
	11th to 20th	Day	7% 13% 5% 11% 6% 6% 5% 4 6% 6% 33%	1400
	21st to end	Day	7% 14% 4 12% 7% 6% 6% 4 6% 7% 26%	1396
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	12% 6% 3 7% 5% 5% 5% 4 6% 6% 41%	1900
	11th to 20th	Night	8% 4 2 7% 6% 6% 6% 5% 8% 10% 37%	2092
	21st to end	Night	12% 6% 3 7% 5% 5% 5% 4 6% 9% 40%	2443
August	1st to 10th	Night	15% 5% 2 6% 5% 5% 4 4 6% 6% 41%	2529
	11th to 20th	Night	14% 9% 2 6% 6% 4 5% 4 7% 9% 33%	2556
	21st to end	Night	13% 7% 2 6% 4 5% 5% 4 7% 8% 40%	3146
September	1st to 10th	Night	12% 6% 3 6% 5% 5% 5% 4 6% 7% 43%	1840
	11th to 20th	Night	12% 8% 3 6% 4 4 4 4 5% 8% 43%	2037
	21st to end	Night	10% 7% 2 7% 5% 4 6% 4 7% 8% 41%	2090
October	1st to 10th	Night	11% 9% 3 8% 5% 5% 5% 4 6% 8% 35%	3664
	11th to 20th	Night	10% 8% 3 9% 6% 6% 4 5% 6% 8% 35%	4086
	21st to end	Night	7% 9% 3 9% 6% 6% 5% 5% 7% 9% 33%	4457
November	1st to 10th	Night	13% 16% 5% 9% 6% 4 5% 4 5% 6% 26%	2705
	11th to 20th	Night	9% 13% 6% 12% 7% 5% 5% 4 5% 6% 29%	3000
	21st to end	Night	7% 13% 5% 11% 8% 6% 6% 4 6% 6% 27%	3000
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	42% 12% 18% 22% 6%	3040
July	1st to 10th	Day	32% 8% 14% 29% 14% 2	4981
	11th to 20th	Day	33% 9% 17% 32% 9%	4769
	21st to end	Day	36% 9% 22% 21% 11% 1	4946
August	1st to 10th	Day	38% 8% 20% 32% 2	4384
	11th to 20th	Day	37% 9% 19% 24% 10%	4315
	21st to end	Day	40% 10% 25% 22% 4	4520
September	1st to 10th	Day	40% 9% 19% 26% 6% 1	2337
	11th to 20th	Day	40% 8% 17% 27% 7%	2142
	21st to end	Day	41% 10% 14% 23% 10% 2	2090
October	1st to 10th	Day	41% 7% 24% 23% 5%	3309
	11th to 20th	Day	49% 7% 13% 23% 8%	2842
	21st to end	Day	48% 6% 18% 24% 5%	3101
November	1st to 10th	Day	58% 6% 14% 16% 5% 1	1570
	11th to 20th	Day	50% 8% 15% 17% 9% 1	1400
	21st to end	Day	54% 10% 13% 14% 7% 2	1396
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	37% 10% 15% 23% 15% 1	1870
	11th to 20th	Night	38% 11% 17% 26% 8%	2054
	21st to end	Night	38% 10% 20% 22% 8% 2	2366
August	1st to 10th	Night	41% 9% 20% 28% 2	2521
	11th to 20th	Night	44% 9% 16% 20% 10%	2580
	21st to end	Night	39% 9% 23% 25% 5%	3146
September	1st to 10th	Night	37% 8% 18% 27% 8% 2	1840
	11th to 20th	Night	38% 8% 16% 28% 9% 1	2037
	21st to end	Night	38% 8% 15% 24% 13% 2	2090
October	1st to 10th	Night	44% 6% 21% 21% 6%	3664
	11th to 20th	Night	43% 8% 14% 27% 8%	4086
	21st to end	Night	43% 9% 19% 22% 7%	4457
November	1st to 10th	Night	56% 7% 12% 18% 6%	2705
	11th to 20th	Night	54% 8% 12% 16% 9% 1	3000
	21st to end	Night	53% 10% 13% 16% 6% 2	3000
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% 8%	870
July	1st to 10th	Day	17% 5% 21% 34% 18% 6%	856
	11th to 20th	Day	14% 5% 26% 28% 19% 7%	739
	21st to end	Day	13% 7% 17% 33% 22% 8%	720
August	1st to 10th	Day	12% 7% 21% 29% 19% 11%	487
	11th to 20th	Day	9% 8% 26% 29% 21% 6%	477
	21st to end	Day	18% 8% 20% 28% 19% 7%	539
September	1st to 10th	Day	18% 9% 25% 26% 18% 4	490
	11th to 20th	Day	20% 9% 23% 28% 16% 5%	490
	21st to end	Day	17% 12% 27% 24% 15% 5%	490
October	1st to 10th	Day	15% 10% 28% 26% 18% 4	441
	11th to 20th	Day	15% 11% 30% 30% 9% 4	300
	21st to end	Day	17% 14% 33% 25% 7% 3	330
November	1st to 10th	Day	22% 8% 30% 27% 10% 3	275
	11th to 20th	Day	15% 13% 36% 27% 8% 3	200
	21st to end	Day	10% 12% 39% 24% 9% 6%	199
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% 24% 2	380
July	1st to 10th	Night	19% 3 15% 39% 22% 1	370
	11th to 20th	Night	15% 5% 18% 35% 27% 1	368
	21st to end	Night	14% 3 14% 41% 24% 3	416
August	1st to 10th	Night	13% 4 15% 33% 30% 5%	378
	11th to 20th	Night	13% 4 14% 41% 25% 3	409
	21st to end	Night	15% 5% 22% 37% 21% 1	627
September	1st to 10th	Night	15% 3 21% 40% 19% 1	700
	11th to 20th	Night	17% 3 21% 38% 20% 1	760
	21st to end	Night	14% 7% 29% 33% 15% 2	760
October	1st to 10th	Night	18% 7% 27% 33% 14% 1	760
	11th to 20th	Night	14% 9% 29% 33% 14% 2	760
	21st to end	Night	15% 10% 32% 31% 10% 2	836
November	1st to 10th	Night	23% 8% 28% 29% 11% 1	904
	11th to 20th	Night	14% 13% 38% 24% 10% 1	1000
	21st to end	Night	14% 9% 37% 29% 10% 2	1000
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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