An Introduction to Marine Weather San Francisco Bay is one of the premier sailing areas in the world. What is it that makes the area special? Easy! The weather. Year round mild conditions allow sailing nearly every day throughout the year. And, consistent 25 to 30 knot winds pretty much every day from May to October have led to a cliché that if you can sail on San Francisco Bay, you can sail anywhere. The down side? Fog. Interestingly, both conditions are related, and probably would not exist without the other. It All Starts With: A high pressure area and a low pressure area and the wind they produce The…

Weather is based on the relationship of the Sun to the Earth. Warm air rises, cool air sinks, the Earth rotates beneath it, and revolves around the sun. As simple as it sounds, this sums up the processes driving the weather patterns we experience. We are going to start our weather education looking at a drawing of a large scale process which has been scaled down to size. In our drawing, rays from the sun pass through the atmosphere to heat the surface of the earth. Land warms more quickly than water. Warm Air Rises The warmed land then radiates its heat, warming the air near it. The warm air…

Lapse Rate Lapse Rate is the decrease of an atmospheric variable with height. In most cases, temperature is the variable the term is applied to. For our purposes, Lapse Rate may be defined as rate of temperature change with height, and is expressed officially as °C km-1. For simplicity sake, we will also use °F/1000′ Lapse Rate may be used to indicate either the environmental lapse rate or the process lapse rate, both of which are discussed below. Standard Atmosphere The Standard Atmosphere is a “hypothetical average” pressure, temperature and air density for various altitudes. The “U.S. Standard Atmosphere 1976″ is the most recent model used. Items of interest to a sailor include…

Sea Breeze Land warms and cools much quicker than nearby bodies of water. During the daytime, radiation from the sun creates warming of the land and the air above it. The resulting warmer air weighs less and begins to rise. A pressure gradient is formed between it and the cooler, denser air over the water, resulting in airflow from water to land. This effect is known as a sea breeze. Typically a sea breeze will start up mid to late morning, build throughout the day, and die out late afternoon to early evening. With a classic sea breeze, as the wind increases in speed, it will veer. This means that…

Use an anemometer An anemometer is generally the best way to determine wind speed. However, when one is not available, you may determine the speed of the wind through the use of the Beaufort Scale. A usable alternative The Beaufort Scale allows you to use the appearance of the sea surface to estimate wind speed. The state of the sea, the dimensions of the waves, the presence of white caps, foam, or spray, depends principally on three factors: Speed. The higher the speed of the wind, the greater is the disturbance. Duration. Disturbance increases the longer the wind blows at a given speed, until a maximum state of disturbance is…

A quick review The first half of this lesson is a review of pressure gradient, Coriolis Effect, and wind speeds. For a more complete discussion refer to the High and Low Pressure Systems lesson. Pressure Gradient Pressure gradient is the change in pressure measured across a given distance. Air flows from the high to the low because the pressure gradient creates a net force that starts the movement of air. The greater the pressure change over a given distance the greater the resulting net force, and the greater the air flow (wind). Coriolis Effect As air moves from higher pressure into lower pressure, the Coriolis Effect causes the air to…

What is Wind The title of this topic might have been “What causes the wind.” Quite simply, wind is moving air. Air doesn’t seem like much, however, it has weight and it has substance, and is actually considered a fluid. This becomes apparent when air moves. While moving, you can see many of the same characteristics you might find in a river. Things like moving as a “unit,” parting and coming back together as it flows around an obstacle, and eddies or back-flows. Air moves from an area of higher pressure into an area of lower pressure. The closer together the high and low pressure is and the greater the…

Temperature vs. Moisture Content An air mass is a large volume of air defined by two basic factors. What is the moisture content and is the air warm or cold. As a result, air masses are typically broken into five basic types. Moisture Content Moisture content is the primary defining characteristic. Air is either Continental (c) or Maritime (m). Continental air masses form over dry land and consequently have little moisture content. Maritime air masses on the other hand, form over oceans and are therefore moist. Temperature Each of these air mass types are then further divided by the temperature of the area they are formed. Tropical (T) air masses…

Weather is based on the relationship of the Sun to the Earth. Hadley Cells As seen in “Island in the Sun”, the rays of the sun heat the earth and the air above it. The more directly the rays of the sun impact the Earth, the more energy is transferred heating the Earth and the atmosphere above it. The sun is located more or less over the equator. Because of this, the air near the equator is heated more causing it to rise vertically. Heated air from the equator begins to move toward the poles and to cool. As the air cools, it begins to descend back to the Earth’s…