Robert, your graphics are stellar as usual. I don't grasp equations, but between the hodograph and that last table, the concept gels for me. After a while, though, one kind of wants to understand what this meters-squared-per-second-squared has to do with it. From a practical point of view, all I've needed to know for purposes of tornado forecasting is that >150 m2/s2 in the lowest kilometer of the BL is a Good Thing, with a similar threshold of 250 for 0-3 km helicity. Tim and Robert-thank you! I think my non-linear brain has grasped, or at least is coming close to grasping, the reasoning behind how helicity is expressed. So if you had a hodograph that exactly followed the edge of that box, the helicity would be 200 m^2/s^2 (twice the 'area' of the box). Now say the units of the x- and y-axes are m/s (or knots as plotted on the hodograph), the 'area' of a box is (10 m/s) * (10 m/s) = 100 m^2/s^2. The area of that box would be (10 m) * (10 m) = 100 m^2. Think of one of the boxes on the hodograph (doesn't matter which one), and pretend for a second that we're looking at a map and that the units of the x- and y- axes are meters (or feet or attoparsecs). It's not really an 'area,' though, because we're not talking about physical space, but it sort of works the same. The 'area' between your two lines and the hodograph is proportional to the helicity (it's half the helicity, I think). To find helicity, you would draw lines from the origin (the point where u and v are both 0) to two other points on the hodograph. Rather than lines of direction and magnitude, this one has lines of u and v wind components, which I think helps show the idea better. I'm kind of assuming you're familiar with them and how they're plotted. So it's kind of like area in physical space, except that 'space' in this sense is velocity. One of the definitions of helicity is the area between the hodograph and the origin, which is in 'velocity space,' if you could call it that. #a=v/t= (m/s)/s = (m/s)/(s/1) = m/s*1/s =m/(s*s) = m/s^2#Īnd the result is meters per second squared.One could possibly think of it is velocity (m/s) squared. #v=d/t=m/s# (meters per second or meters divided by seconds) We will use #m and s# (meters for distance and seconds for time) We can ensure the units check out: #v=#velocity #d=#distance We are multi-tasking to arrive sooner, so we have to multiply the time x time to calculate the correct numerical value for our acceleration. We are still moving across a distance over a time, but we are also increasing how fast we are doing it. We can think of acceleration as doing two things at once. And all of these changes take place over time.Īcceleration is the rate or speed at which an object is increasing or decreasing its velocity over a measurable time.

All of these changes are a form of acceleration. It is unusual to maintain a constant velocity in a given direction for very long at some point the speed will increase or decrease, or the direction of motion will change. Velocity is the rate or speed an object is moving from A to B over a measurable time. If the thing is moving in a particular direction, the speed can then be defined as velocity. It takes some time to complete that movement, so the change in location over the time is defined as speed, or its rate of change. We have already discovered that when something moves, it changes its location.

Meterrs squared persecond squared