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Word of the Month - Orographic

Orographic cloud
Orographic cloud formation

By Ian Brown, Editor - Bits and Pieces, EAA 657159

Even though you might not fly in mountainous areas often, itís worth remembering that even small mountains or hills produce unique weather patterns. Glider pilots love this effect, but if we donít keep in mind the big picture we can run into trouble with orographic uplift or downdraft.

In my area, we often see orographic clouds forming over local hills. The effect is caused by lower, moist air being forced up over the hill. The resulting temperature drop causes the air to reach its dew point (100 percent humidity).

We need to keep an eye on our altitude when passing a hill or mountain because of potential rising air currents upwind of a mountain. The downwind side of a mountain can easily cause you to lose altitude unexpectedly as the air tries to regain its original level.

As the air cools at the approach of dusk, the heavier cold air can slide down the side of the mountainous terrain, causing downdrafts, so be especially alert in the sunset hours.

Traveling cross-country, the threat of unexpectedly changing your planned altitude is always present, and never more than when passing a change in ground level.


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