Well Jason got it right when he said that derecho is a spanish word that means "straight ahead" or "direct". However, there are some scientific publications that set some criteria for derecho events. It is not necessarily just any old windstorm that blows through in the summer...
This is what was published in Johns and Hirt (1986) as their sort of criteria checklist for determining derecho events to use in their study:
1.There must be a concentrated area of reports consisting of convectively induced wind damage and/or convective gusts in excess of 50 knots (58 mph). This area must have a major axis length of at least 400 km (250 nm).
2. The reports within this area must also exhibit a nonrandom pattern of occurrence. That is, the reports must show a pattern of chronological progression either as a singular swath (progressive) or as a series of swaths (serial).
3. Within the area there must be at least three reports, seperated by 64 km (40 nm) or more, of either F1-equivalent damage and/or convective gusts of 65 knots (74 mph) or greater.
4. No more than 3 hours can elapse between successive wind damage (gust) events.
The derecho is the actual windstorm. It is defined as a "widespread convectively induced windstorm".
Now to answer one of the questions you had initially. A
bow echo is merely "a radar echo which is linear but bent outward in a bow shape" (
NWS OUN). I linked a picture of it too. A
derecho can be caused by a bowed line of storms, but the storms do not necessarily
have to be bowed. Any line of storms, whether it is perfectly linear or bowed like a semi-circle, can produce a derecho, as long as the wind report pattern fits the criteria listed above.
Here is a radar image from the May 30-31 1998 derecho event. You can see that the line of severe storms was more or less pretty linear, with perhaps some individual bowed segments.
So basically just remember that a bow echo is just what its name sounds like - a bowed radar echo, and a derecho is a major windstorm caused by a line of storms. I hear people use derecho to describe any wind event now, but it really was intended to describe the more widespread and extreme events - the type of wind event the SPC would issue a High Risk for.
Hope that helps!
AJL
