Weather Essentials Concept Map Sun Moisture Temperature Density / pressure Highs, lows, and fronts Sun Sun Jet streams, horizontal Pressure Gradient Force (PGF) Wind/air motion (horizontal & vertical) Other influences Clouds Climate Daily weather Sun

What is a cloud? Clouds Precipitation occurs when cloud droplets grow large and heavy enough to overcome the rising air creating the cloud Clouds are part of the global water transport cycle Clouds form when: 1) there is enough water vapor (water in gas form) to change to liquid droplets or ice; 2) the air is cooled enough from vertical lifting; and 3) there are particles (sand, dust, salt) onto which water vapor can condense.

How is the atmosphere heated How is the atmosphere heated? (Solar Radiation/Differential Heating of Water and Land) Why does air rise or sink? Why are the poles cold and the equator hot? Density differences create areas of high and low pressure. Winds blow from high to low pressure and create air mass characteristics and boundaries (fronts). Uneven heating of Earth’s surface creates air temperature differences, which result in air density differences. Air that is less dense than the surrounding air will rise. Air that is more dense that the surrounding air will sink. Over a year, the tropical latitudes (0–30º) receive the most sun’s energy, the mid-latitudes (30–60º) receive less energy, and the polar latitudes (60–90º) even less energy. The end result is that the poles are colder than the equator. Uneven heating causes weather processes like high/low pressure, winds, temperature differences, water transport (water cycle), and unstable/stable air conditions (density differences). Although the sun’s energy is constant, Earth’s surface is heated unevenly because of its tilt. Solar energy is the driving force that causes weather.

Where does water come from and how does it travel? Clouds and Winds in Connection to the Global Water Cycle Some of the water in clouds falls as precipitation (rain and snow, etc.) over oceans and land. Precipitation reaching land may evaporate again, soak into the soil as groundwater, or run off into rivers and eventually back to the ocean. If land Clouds are also transported by the wind. The moist air (water vapor in the air) condenses to form clouds. Plants may take up some of the groundwater and release water vapor through evapotranspiration. Glaciers may grow or shrink based on the accumulation or sublimation occurring at their surfaces. If oceans Winds transport moist air to other regions. Sun evaporates water from oceans, land, and plants, creating water vapor in the atmosphere.

What is wind? (High and low pressure systems) Does the Coriolis Force affect how water flushes down the toilet? Mention fronts in the context of high/low pressure systems The polar-front jet stream influences U.S. weather, especially in winter by steering weather systems across the country. Near the Earth’s surface, the PGF, Coriolis Force (which results from Earth’s rotation), and surface roughness (which creates friction) cause winds in the northern hemisphere to circulate counter-clockwise, inward and upward for low pressure areas; and clockwise, outward, and downward for high pressure areas. Locations poleward of these jet streams tend to be cooler than locations equatorward of them. Jet streams are narrow bands of strong winds (fastest moving air aloft). Jet streams occur 6 to 9 miles (9 to 14 kilometers) above sea level. The U.S. is influenced by 2 major jet streams: the southern subtropical jet and the northern polar jet. Winds above the Earth’s surface (e.g., jet stream) are generally faster than winds near the Earth’s surface because the PGF increases with altitude (is stronger). Earth’s rotation causes these winds to blow from west to east. (The Coriolis Force balances the PGF.) Wind at surface Upper air wind Winds are the result of trying to equalize pressure and density differences caused by unequal heating. A pressure gradient between high and low pressure centers causes winds to blow from high to low pressure due to the horizontal pressure gradient force (PGF).

What is wind? (High and low pressure systems) Because the Coriolis effect is weak near the equator, hurricanes develop slightly poleward of the equator so that air rotation can be achieved For people in the Northern Hemisphere, winds appears to be deflected to the right of the wind direction. In the Southern Hemisphere, winds appear to be deflected to the left of the wind direction. The veering of the air is called the Coriolis Effect. At the Earth’s surface, air moving over relatively long distances appears to veer (be deflected) from its intended path. Earth rotates from west to east. Because the atmosphere is not generally solid like the Earth, air movement is influenced by Earth’s rotation.

What’s the weather going to be like What’s the weather going to be like? (Understanding pressure, temperature and wind) Low pressure areas tend to have stormy weather and stronger winds (rising air and clouds). The lowest pressure areas on Earth are the centers of hurricanes and tornadoes. High pressure areas tend to have fair weather and light winds (descending air and clear skies). At Earth’s surface, air moving towards (converging) a low pressure system must rise, leading to cooling and cloud development. At Earth’s surface, air moving outward (diverging) from a high pressure system sinks, leading to warming and cloud dissipation. The boundaries between air masses are called fronts. Cold fronts occur where warm air is being replaced by cold air. Warm fronts occur where cold air is being replaced by warm air. Warm and cold fronts are commonly linked by warm pressure systems. Air masses are blobs of air identified by their pressure, temperature, and moisture characteristics, and they represent the conditions from where they originated (e.g. cold and dry from Canada in the winter, warm and moist air from southeast U.S.)

Factors influencing climate What is the temperature profile (daily maximum and minimum)? How does the temperature profile vary daily and seasonally? How do these profiles compare? What is the typical wind direction and speed in summer and winter? Do oceans influence the temperature and winds at the location? What do the daily and seasonal precipitation profiles look like – monsoonal, consistent, or very little precipitation?