# Background and Basics

#### 1.1  Climate and Weather

Weather is considered to be the atmospheric conditions that are happening now or within a few days. On the other hand, Climate represents the long term atmospheric patterns within which weather occurs. As the popular saying goes, "Climate is what you expect, and weather is what you get!".

#### 1.2  Tilt and Latitude

Latitude is a measure of the distance you are located from the equator.  It is commonly shown as an imaginary horizontal line that goes across the earth on maps and is used along with longitude as a reference point to determine location. The tilt of the earth affects the seasons we experience throughout the year.

## Atmospheric Properties

#### 1.3a  Temperature

Temperature is the measure of thermal or internal energy of the molecules within an object or gas.    We can measure temperature of an object using either direct contact or remote sensing.  Temperature of air is closely related to other atmospheric properties, such as pressure, volume and density.

#### 1.3b  Density

Density measures the ‘heaviness’ of an object or how closely ‘packed’ the substance is.  Density is related to both the type of material that an object is made of and how closely packed the material is.

#### 1.3c  Pressure

Pressure is the force exerted over a given area or object, either because of gravity pulling on it or other motion the object has.  Molecules in the air produce pressure through both their weight and movement, and this pressure is connected to other properties of the atmosphere.

#### 1.3d  Humidity

Humidity is a measure of the amount of moisture in the air.  It tells you how comfortable it is to be outside, and if there is enough moisture to create clouds and rain.

## Energy Transfer

#### 1.4a  Conduction

Conduction is the transfer of heat through a solid, liquid or gas by direct contact.

#### 1.4b  Convection

Convection is the transfer of heat through the movement of liquids and gases.

Radiation is the transfer of energy through air and space by light waves (visible, ultraviolet and infrared waves).

#### 1.4d  Latent and Sensible Heat

Latent and sensible heat are types of energy released or absorbed in the atmosphere.  Latent heat is related to changes in phase between liquids, gases, and solids.  Sensible heat is related to changes in temperature of a gas or object with no change in phase.

#### 1.4e  Evapotranspiration

Evapotranspiration is the process of evaporating water from leaves through plant transpiration during photosynthesis.  It varies because of a multitude of factors like wind, temperature, humidity, and water availability.

#### 1.4f  Longwave and Shortwave Radiation

Everything that has a temperature gives off electromagnetic radiation (light).  The sun is extremely hot and has a lot of energy to give, so it gives off shortwave radiation because shortwave radiation contains higher amounts of energy  The earth is much cooler, but still emits radiation.  Earth’s radiation is emitted as longwave because longwave radiation contains a smaller amount of energy.

#### 1.4g  Albedo

Albedo (al-bee-doh) is a measure of how much light that hits a surface is reflected without being absorbed.  Something that appears white reflects most of the light that hits it and has a high albedo, while something that looks dark absorbs most of the light that hits it, indicating a low albedo.

#### 1.4h  Earth's Energy Balance

Earth's Energy balance describes how the incoming energy from the sun is used and returned to space.  If incoming and outgoing energy are in balance, the earth's temperature remains constant.

## The Atmosphere

#### 1.5a  Structure of the Atmosphere

The atmosphere has 4 layers: the troposphere that we live in near the surface of the earth; the stratosphere that houses the ozone layer; the mesosphere, a colder and lower density layer with about 0.1% of the atmosphere; and the thermosphere, the top layer, where the air is hot but very thin.

#### 1.5b  Composition of the Atmosphere

The atmosphere contains many gases, most in small amounts, including some pollutants and greenhouse gases.  The most abundant gas in the atmosphere is nitrogen, with oxygen second.  Argon, an inert gas, is the third most abundant gas in the atmosphere.

## Understanding Weather Patterns

#### 1.6a  Isobars and Isotherms

Isobars and isotherms are lines on weather maps which represent patterns of pressure and temperature, respectively. They show how temperature and pressure are changing over space and so help describe the large-scale weather patterns across a region in the map.

#### 1.6b  Convergence and Divergence

Convergence and divergence roughly determine where air will be sinking or rising.  Rising air is associated with clouds and precipitation; sinking air is associated with clear, calm conditions and good weather.

#### 1.6c  Air Masses

An air mass is a large uniform volume of air that has consistent properties of density, temperature, and humidity, compared to the air surrounding this air mass which may have different properties.

#### 1.6d  Fronts

A front describes the boundary between two air masses containing different temperature and/or water vapor content.

#### 1.6e  Jet Streams

Jets are fast moving ribbons of air high up in the atmosphere.  They are responsible for transporting highs and lows.  They affect precipitation and temperatures, and they mark boundaries between massive air masses.

## Weather Forces

#### 1.7a  Coriolis Effect

The Coriolis Effect describes the turn of the wind to the right in the Northern Hemisphere caused by earth's rotation.

#### 1.7b  What Drives Weather

Weather is ultimately caused by temperature differences due to variations in solar energy input.  These temperature differences cause pressure variations in the atmosphere.  The wind blows to try and correct these imbalances in pressure.  Regions of low and high pressure moving through an area cause changes in weather as different air masses pass by.

#### 1.7c  Friction and Turbulence

Friction slows the wind and changes its direction.  This effect can cause turbulence in winds near the surface or boundaries between areas with different surface roughnesses.  Turbulence mixes the lower part of the atmosphere, stabilizing the temperature.  Turbulence causes eddies in the atmosphere, which are swirls of wind responsible for wind gusts and bumpy plane rides.

#### 1.7d  Wind Shear

Wind shear is the change of wind speed and/or wind direction over a certain height.

## The Water Cycle

#### 1.8a  The Water Cycle

About 70% of the earth’s surface is covered with water.  This water evaporates and condenses into clouds.  The clouds cause precipitation and this water falls back to the earth’s surface, ready to start the cycle again.

#### 1.8b  Stability

Atmospheric stability tells you how likely it is air will rise and form clouds and precipitation.  Unstable  air is likely to rise and form tall clouds and often precipitation.  Stable air means any air that rises up will sink back to where it came from.

#### 1.8c  Lifting Mechanisms

Lifting mechanisms are forms of lift that cause air to rise. In this topic we cover orographic lift, frontal lift, convergence, and convective lift.

#### 1.8d  How Clouds Form

Clouds form when moist, warm rising air cools and expands in the atmosphere.  The water vapor in the air condenses to form tiny water droplets which are the basis of clouds.

#### 1.8e  Precipitation Types

There are many types of precipitation.  Rain and drizzle are the only liquid types; the freezing types are snow, sleet, freezing rain, and hail.  Hail is unique because it is frozen precipitation that generally only falls from thunderstorms in the summer.

#### 1.8f  Cloud Seeding

Cloud seeding is the process where substances like dry ice and silver iodide are put into clouds in an attempt to make precipitation fall.  Cloud seeding has also been used to dissipate fog and weaken some storms.

## Severe Weather

#### 1.9a  Severe Weather Hazards

Severe weather hazards are dangerous weather phenomena that threaten life and property. Within this topic, we discuss tornadoes, lightning, hail, flash flooding, and downbursts.

#### 1.9b  Life Cycle of a Thunderstorm

A basic thunderstorm (single cell) goes through three phases during its lifetime: cumulus, mature, and dissipating. This can last between 30 minutes to an hour.

#### 1.9c  Thunderstorm Classification

Although thunderstorms can vary in size and shape, we can classify them into four main groups: single cell, multi-cell clusters, multi-cell lines, and supercells.

#### 1.9d  Watches and Warnings

The National Weather Service (NWS) issues watches and warnings all across the United States to warn people of impending, dangerous weather.

## Measuring Weather and Climate

#### 1.10a  Instruments

Weather instruments are used to take measurements of various atmospheric parameters at local weather stations.

#### 1.10b  Averages and Normals

Average weather and climate conditions describe the expected weather a location is likely to get. Normal conditions describe what is normal for an area using the most recent statistically adjusted 30-year interval of the average weather conditions.

#### 1.10c  Extremes

Extremes of weather describe the range of possible weather and climate conditions that you can expect to encounter at your location. These can be expressed in a variety of ways, from the highest or lowest temperature ever reported at a site to a probability of occurrence for rare events like heavy rainstorms.

#### 1.10d  Trends

Trends in climate can be caused by natural changes or variability in ocean currents, land use, solar radiation, or other factors. They can also be the result of manmade influences on climate. While some trends in climate variables reflect local changes in climate at a single location, trends across regions and the globe indicate significant movements away from "average" conditions which could cause disruptions in society and global ecosystems.