What type of airspace may contain a high volume of pilot training or unusual aerial activity, often depicted with an 'A'?

Alert Areas are specifically designated airspace that indicates a high volume of pilot training or unusual aerial activity. These areas are marked on aeronautical charts with an “A” to warn pilots that they may encounter unusual activities, and they should exercise caution when flying in those regions. The purpose of alert areas is to inform pilots of potential risks, especially since they may include student pilot training, aerobatic activities, or other aviation activities that differ from standard flight operations.

In contrast, Military Operations Areas (MOAs) are designed for military training flights and may be active at various times but are not specifically labeled with an "A". Restricted Areas (R-###) denote airspace wherein flight is prohibited without special permission, typically to protect sensitive military operations or airspace. Similarly, Warning Areas (W-###) are designated over international waters where activities may be hazardous to non-participating aircraft, but they do not involve the same level of training or unusual activity indicated in alert areas.