Hi Alexis,

Solar storms, sometimes referred to as "Coronal Mass Ejections" or CMEs, can be seen exploding away from the solar surface in white-light data (the same type of observations seen when an eclipse occurs, allowing us to see the faint corona). So we are able to measure how fast these CMEs are traveling by following a point in the data and measuring its increase in height (measured from the Sun) in consequtive images (which we know the exact time of). Once we know the distance the CME has traveled from one image to the next, and we know the difference in time, we can divide that distance by the time to get a velocity (measured in units of km/s). There are some uncertainties involved in calculating the velocities of those CMEs directed at or away from the Earth, because measuring the distance traveled in those events headed toward or away from us becomes less certain due to what we call "projection effects". We still make estimations for the velocities of those events, but we aren't as certain about how accurate they are.

Holly