Drinking tea over cold over ice is nothing new in the United States. The first iced tea recipes date back to the Nineteenth Century. As refrigeration became common in American homes its popularity has grown to account for 80% of the tea consumed in the US. In the South, sweet tea, black tea sweetened with sugar, has become ubiquitous, and it's served practically everywhere.
American iced tea recipes often call for either preparing a strong concentrated tea, and diluting it before serving over ice, or steeping tea in water in the sun for several hours. Both of these methods have their pros and cons, but for my money, cold steeping tea yields the best results.
Mizudashi or cold-steeping is not a new concept in Japan, but when I introduced it to the Intelligentsia Coffee Bars in 2008, it was relatively unknown in the US. Many teas retain a large amount of astringency when steeped hot and diluted. Cold-steeping allowed us to steep a wider variety of teas since it curbed the astringency, and produced a sweet, nuanced cup. Really everything about the method is simple. Its one drawback is that it takes time to do.
Here is how you do it. In 1 quart (or liter) container, such as a Mason Jar, add 15-20 grams of loose tea, depending on how strong you enjoy your tea. I recommend 20 grams if serving over ice. Add cold water to fill the container, and refrigerate. Steep refrigerated for 12-24 hours. For green and white tea I like 12-16 hours. For black, oolong and herbals I suggest 24 hours. Don't worry about timing it precisely, the results will be delicious and refreshing regardless. When finished steeping, using a fine mesh strainer, strain the tea into a second container. It is now ready to serve and can be kept refrigerated for up to 72 hours.