December 18, 2010

Calcuating Trigger Time

The last post covered the process of calculate trigger temperature and the mechanics of the atmosphere that are underway as the day heats up and thermals start. Today's post will cover how to estimate what time the surface temperature will reach trigger temperature and will also cover a few other tips related to trigger temperature.

To find the estimated trigger time first recall this plot from the last post:
It turns out that the heat required to eliminate the ground inversion is proportional to the area of the triangle formed by the temperature profile (shown above as the area shaded with dashed lines). So, first calculate the area under the temperature profile, this can be done by copying the plot onto graph paper and counting the squares or by simple geometry; the area of a triangle is 1/2 * base * height. For plots of this type the y axis might be in milibars, you will want to convert milibars into feet. The x axis is temperature. That means the area calculation you have done will result in units of (degree*ft). Which is handy because the chart below, which shows the available heating from the sun just happens to be in degree*ft!

This chart is for a latitude of 45deg, if your latitude is greater/less move/down up 1/2 hour for every 5deg difference in latitude. This chart allows us to predict when the air will have been heated enough to over power the ground inversion. Note that the above chart is for clear skies, cirrus, fog, smoke or any other sun blocker can reduce the about of energy reaching the ground and should be accounted for.

The other method, which is much more user friendly is to rely on your local weather station predictions for the temperature as throughout the day. You can always supplement this information with temperature readings of you own. I glued a thermometer to my wing wheel so when i'm waiting to launch I can monitor the rise in temperature and know if it is rising faster or slower than predicted and adjust my launch time accordingly.

All of these methods help you become more aware of how the soaring day starts and how to position yourself to start your flight on time and get the most of the soaring day for those long tasks. Until next time...

Keep soaring,

December 10, 2010

Calculating Trigger Temperature

If you want to fly long distances you need to get going early. The best way to make that happen is to be ready when the thermals start to pop; but how do you know when that will be? Today's post will address how to determine Trigger temperature. My next post will cover how to estimate when trigger temperature will be reached. Like my previous post, the information for this post comes from Dennis Pagen's book, "Understanding the Sky" which is required reading for anyone serious about soaring XC.

The first step to this process is to find the skew-T for the location you plan on launching from. The easiest way to get a skew-T is thought XCSkies point forecast tool or use the tools provided by Dr Jack.To find the trigger temperature you will need to determine the height of the ground inversion layer and the lapse rate of the air layer above it. In the figure below, taken from Understanding the Sky, a notional skew-T chart is shown with only the relevant information displayed.

The ground inversion layer, usually about 1000'-2000' thick, is caused by the relatively cold temperature of the ground at night cooling the air layer closest to the ground. This is the same mechanism that forms the superadiabatic layer near the ground when the surface heats up and warms the lower layer of air. It isn't until the ground inversion layer is warmed to match the lapse rate of the air above it that thermals will escape the ground inversion layer; this warming process is shown in the figure above as the dashed lines. To find trigger temperature, draw a line starting where the temperature profile begins to decrease and follow the dry adiabatic lapse rate (DALR) lines on the skew-T to ground level. The temperature where the DALR line intersects ground level is trigger temperature, represented in the figure as the far right dashed line.

We now know that for trigger temperature to be reached the early morning solar heating must eliminate the cold ground inversion layer, this means that on days following cold clear nights where the ground inversion is thick trigger temperature will occur later in the day.

Hopefully this helps take the mystery out of calculating trigger temperature. Until next time..

Keep soaring,

December 5, 2010

My Six Month Leave is Over, and Unvailing a New Posting Formatt

Gosh, has it really been 6 months since my last post? It's been a busy 6 months at work and with my flying. The end of the soaring season left me wanting a little bit more; the weather just didn't stack up this year like it did last year so to get my fix I started paragliding. I know, not the safest decision but it really has brought to light a new understating of low level wind and thermal development phenomena and a strong desire to study more about micro-meteorology. It also means I can get my feet off the ground more often, even if it's only for half an hour after work.

The major challenge I faced with my blog over the last 6 months is that I rarely had time to put forth the effort to collate information for one of my longer posts. However as winter sets in and any hope of flying - glider, paraglider, or otherwise - gets covered over with snow I have once again re-dedicated myself to studying all topics of Soaring. This means I can continue to share what I am learning in hopes that others can benefit. In a effort to have a more consistent presence and give all my dedicated readers (or soon to be dedicated readers) a reason to stop in regularly I will be posting shorter articles covering some of the details of the work I'm doing with longer more detailed post when I have the time. In this spirit, below is a short snippet of of information gleaned from an amazing book recently purchased called, "Understanding the Sky" by Dennis Pagen.

In Chapter 7 Local Winds, Dennis addresses a phenomenon common to pilots in the North West US and on the East Coast; Sea Breeze Fronts. Here are some of the more important notes. If we imagine we are on the east coast with a north/south coast line, for winds coming from the south west the sea breeze front is most likely to push far inland. Winds from northwest will promote a sea breeze front which will likely form off shore and slowly push inland as heating increases. Winds from the north east likely means a high pressure inland which my set up a sea breeze later in the day but will be dependent on heating and will not push as far inland as general off shore winds. South east winds are indicative of a low over land increasing the onshore component of the wind and driving cool stable air far inland; No traditional sea breeze will be present. This is also the least favorable for soaring conditions. A more detailed explanation of why this is the case is covered in the book and worth a detailed read if you fly near a large body of water.

Also addressed in Chapter 7 is the effect of a sea breeze. For a soaring pilot sea breezes bring various different kinds of lift. Stable on shore winds bring smooth ridge lift to coastal sites and sea breezes flow around mountains and through passes colliding on the other side to create convergence lift bands familiar to the folks flying out of Hollister in nor-cal and Lake Elsinore in so-cal. Additional the formation of sea breeze fronts forms lift bands ahead of the cool airmass much like pre-frontal lift found ahead of an arriving cold front. Sea breeze frontal lift is characterized by a small band of good lift marked by good clouds with dying cu on the far side. Pilots are advised to stay ahead of the front to not suffer the same fate. The lift associated with the sea breeze front usually tops out around 3000', for inland covection levels below that the sea breeze front will be weak, convection levels greater will likly cause over development and localized showers and thunderstorms.

Hopefully you learned a little bit about sea breeze winds and front and are encourages to go out and get a great weather resource in the form of Dennis Pagen's book, "Understanding the Sky."

I'll take any comments or advice on the new format or anything else I've written about.

Keep soaring, even when you're close to the beach,